Mechanical Properties of PDMS and the Use of Hybrid ...

Mechanical Properties of PDMS and the Use of Hybrid

Polymer-Grafted Carbon Microspheres as Stimuli-Responsive

Lubricating Particles

AN ABSTRACT SUBMITTED ON THE TWENTY FOURTH DAY OF

APRIL 2020

TO THE DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

OF THE SCHOOL OF SCIENCE AND ENGINEERING

OF TULANE UNIVERSITY

FOR THE DEGREE

OF

DOCTOR OF PHILOSOPHY

BY: ___________________________

Shreyas Oak

APPROVED: ________________________

Noshir Pesika, Ph.D.

Director

_______________________

Julie Albert, Ph.D.

_______________________

Vijay John, Ph.D.

_______________________

Damir Khismatullin, Ph.D.

DocuSign Envelope ID: 5FE42D38-0CCB-4EE5-A968-41F5C1EE5913DocuSign Envelope ID: 21E9D8C1-FDBE-4FAB-B264-5AC453A4855F

ii

Abstract

Polymers are one of the widely used materials within a short time after their discovery.

They are used in various industries such as automobiles, food, personal care, etc. Polymers

can be categorized in various ways, depending on their structure, stimulus responsiveness,

toughness, etc. Therefore, it is important to characterize the properties of polymers.

In the first part of my thesis, two different methods of measuring the modulus of PDMS

are described. The two tests viz Compression test and Hertz Indentation are used to study

the correlation between the degree of polymer cross-linking and the resulting Young’s

modulus. The polymer used in this study, PDMS can be made softer or harder, depending

on the ratio of the base to the cross-linker. We show that PDMS has a high Young’s

modulus for smaller ratios of the base to cross-linker and vice versa. This observation holds

for both types of tests.

In the second part of my thesis, we explore the use of PNIPAm-grafted carbon

microspheres (CM) dispersed in water as a stimulus responsive lubricant. It is found that a

critical concentration between 3 and 5 mg/mL PNIPAm-grafted CM is needed to achieve

low friction (coefficient of friction ∼ 0.04) at room temperature. An increase in the

temperature of the system above the lower critical solution temperature (LCST) causes the

aggregation of PNIPAm-grafted CM which leads to an increase in friction forces for all

concentrations of PNIPAm grafted CM in water. A mechanism to explain the lubrication

properties of PNIPAm-grafted CM is proposed which points toward the need of particle

singlets at sufficiently high concentrations within the confined region to achieve low

friction through a rolling mechanism.


iii

Mechanical Properties of PDMS and the Use of Hybrid

Polymer-Grafted Carbon Microspheres as Stimuli-Responsive

Lubricating Particles

A DISSERTATION SUBMITTED ON THE TWENTY FOURTH DAY OF

APRIL 2020

TO THE DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

OF THE SCHOOL OF SCIENCE AND ENGINEERING

OF TULANE UNIVERSITY

FOR THE DEGREE

OF

DOCTOR OF PHILOSOPHY

BY: ___________________________

Shreyas Oak

APPROVED: ________________________

Noshir Pesika, Ph.D.

Director

_______________________

Julie Albert, Ph.D.

_______________________

Vijay John, Ph.D.

_______________________

Damir Khismatullin, Ph.D


ii

Acknowledgments

This work is dedicated to my family and my mentors, who have supported me throughout

my academic career and encouraged me to see learning as a life-long pursuit. I want to

thank with sincere gratitude the Chemical and Biomolecular Engineering Department at

Tulane University for providing me with a positive and intellectual environment and a

wealth of resources and infrastructure to make this work possible. The faculty and staff

have been immensely encouraging throughout the program at Tulane University. I would

like to send a special acknowledgment to my adviser Dr. Noshir Pesika, whose support and

guidance were instrumental in the completion of this project. I would also like to thank Dr.

Vijay John, Dr. Julie Albert, and Dr. Damir Khismatullin for serving on my committee and

providing me with numerous insights and feedbacks. And last but not least, a special thanks

to all my friends, classmates, and colleagues who have trained me, advised me and with

whom I shared many memorable experiences during my Ph.D.

iii

CONTENTS

Chapter 1. Introduction to PDMS Mechanical Properties .................................................. 1

1.1 Introduction to Polymers Mechanical Properties ...................................................... 1

1.1.1 Young’s Modulus ............................................................................................... 1

1.1.2 Viscosity of Fluids .............................................................................................. 4

1.1.3 Viscoelasticity .................................................................................................... 5

1.2 Introduction to PDMS ............................................................................................... 6

1.2.1 Microstructure of PDMS .................................................................................... 6

1.2.2 Samples for Research ......................................................................................... 7

1.3 The Objectives and Challenges for This Research .................................................... 8

Chapter 2. Modulus measurements ................................................................................... 10

2.1 Introduction to Tensile and Compression Tests ...................................................... 10

2.1.1 Compression Test ................................................................................................. 11

2.1.2 Hertz indentation .............................................................................................. 12

2.2 Samples and Instrumentation .................................................................................. 13

2.2.1 Samples Preparation for Compression Study ................................................... 15

2.2.2. Sample preparation for Hertz Indentation ....................................................... 17

2.3 Analysis Methods .................................................................................................... 18

2.3.1 Compression Study Analysis ............................................................................ 18

2.3.2 Hertz Indentation Analysis ............................................................................... 19

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2.4 Results and discussion ............................................................................................. 20

2.4.1 Compression test............................................................................................... 20

2.4.2 Hertz Indentation .............................................................................................. 21

2.4 Conclusions ............................................................................................................. 22

Chapter 3. Friction and wear ............................................................................................. 24

3.1 Mechanisms of friction............................................................................................ 25

3.1.1 Static friction .................................................................................................... 25

3.1.2 Sliding friction .................................................................................................. 27

3.1.3 Fluid friction ..................................................................................................... 27

3.1.4 Rolling friction ................................................................................................. 28

3.2 Lubrication and Wear .............................................................................................. 30

3.2.1 Biotribology ...................................................................................................... 30

3.2.2 Green Tribology ............................................................................................... 31

3.2.3 Nanotribology ................................................................................................... 31

Chapter 4 Carbon And Its Forms ...................................................................................... 33

4.1 Nanotubes: ............................................................................................................... 33

4.2 Carbon Nano onions ................................................................................................ 35

4.3 Fullerenes ................................................................................................................ 36

4.4 Nanofibres ............................................................................................................... 38

4.5 Carbon spheres ........................................................................................................ 40

v

Chapter 5 Stimuli responsive polymers ............................................................................ 42

5.1 Temperature Responsiveness .................................................................................. 43

5.2 pH responsiveness ................................................................................................... 44

5.3 Electromagnetic responsiveness .............................................................................. 45

Chapter 6. Tunable Friction Through Stimuli Responsive Hybrid Carbon Microspheres 47

6.1 Experimental Section .............................................................................................. 49

6.1.1 Carbon Microsphere (CM) Synthesis. .............................................................. 49

6.1.2 Grafting of Carbon Microspheres with PNIPAm. ............................................ 49

6.1.3 Friction Measurements. .................................................................................... 53

6.2 Results and Discussion ............................................................................................ 55

6.3 Conclusions ............................................................................................................. 70

6.4 Future Direction ...................................................................................................... 71

Biography .......................................................................................................................... 73

Bibliography ..................................................................................................................... 74

vi

List of Equations

Equation [1] Young’s Modulus

Equation [2] Strain

Equation [3] Viscosity according to Newton’s law

Equation [4] Complex Young’s modulus

Equation [5] Stress applied on a cylindrical sample

Equation [6] Hertz equation for the contact between a rigid sphere and elastic flat solid

Equation [7] Effective Young’s Modulus

Equation [8] Coefficient of static friction

Equation [9] Modified Amontons’ Equation

List of Tables

Table 1 Summary of compression study

Table 2 Summary of Hertz Indentation

vii

List of figures

Figure 1 Hooke’s law depiction for an elastic solid. Hooke’s law is obeyed in the straight

segment of the plot, and the slope is 1/k. As higher force is applied, the plot becomes

curved, but the deformation is still elastic, meaning the object will return to its original

shape. Application of higher forces will lead to permanent deformation and ultimately, a

fracture

Figure 2 PDMS chemical formula, where n is the number of repeating monomer units

[SiO[CH3]2]

Figure 3 Mild steel tensile test regimes, plotted as stress vs. strain

Figure 4 Schematic of Hertz Indentation. A solid sphere of Radius R indents an elastic

substrate up to a depth δ. The radius of the indent is α, and the modulus of the substrate is

E

Figure 5 Experimental setup for compression test. A steel disc (12 mm diameter) bigger

than the size of the PDMS disc was used to compress the samples. DFM force sensor was

used with the force range of 50-500 g

Figure 6 Experimental setup for Hertz indentation. A Stainless-Steel sphere 6.3 mm in

diameter was used the probe. Sylgard 184 samples of varying thickness were used to

measure and compare their modulus

Figure 7 Plot of Stress (σ) vs. Strain (ε) for Compression study. The slope of the line

gives us the elastic modulus. Sylgard 184 sample with 10:1 ratio of base: curing agent

was used for this particular sample

viii

Figure 8. Plot of Force (P) vs. Displacement (δ) for Hertz indentation

Figure 9 Plot of Modulus values using compression testing for various values of PDMS

base/curing agent ratios. The modulus value is high for smaller mixing ration and decreases

for higher base/curing agent ratios

Figure 10 Plot of Modulus values using Hertz Indentation for various values of PDMS

base/curing agent ratios. The modulus value is high for smaller mixing ration and

decreases for higher base/curing agent ratios

Figure 11 Summary of Modulus Experiments

Figure 12. Schematic of a typical friction coefficient for two bodies moving against each

other under dry conditions

Figure 13. Hard wheel rolling on and deforming a soft surface, resulting in the reaction

force R from the surface having a component that opposes the motion. (W is some vertical

load on the axle, F is some towing force applied to the axle, r is the wheel radius, and both

friction with the ground and friction at the axle are assumed to be negligible and so are not

shown. The wheel is rolling to the left at a constant speed.) Note that R is the resultant

force from non-uniform pressure at the wheel-roadbed contact surface. This pressure is

greater towards the front of the wheel due to hysteresis.

Figure 14 Single Walled and Multi-Walled Carbon Nanotubes. Single-walled CNT have 1

Graphene cylinder whereas Multi-Walled CNT have multiple graphene cylinders

Figure 15. HRTEM image of Carbon Nano-onions (left) and schematic of Carbon nano-

onions formed by 3 concentric layers (C60, C240, C540)

ix

Figure 16. Varieties of fullerenes found in natural shungite with different numbers of

carbon atoms: C24, C28, C32, C36, C50, C60, C70

Figure 17. Carbon Nano-Fibres

Figure 18. Carbon Spheres

Figure 19. Temperature Responsive Polymers

Figure 20. pH-Responsive Polymers

Figure 21 Electromagnetic Responsive Polymers

Figure 22. Schematic illustration of the rolling mechanism of grafted CM confined

between a silica lens (green) and a silicon wafer substrate (grey). The grafted particles

(black spheres) with PNIPAm coating (orange) get trapped in the contact region and

lower friction due to rolling

Figure 23. Schematic illustration of the grafting process. (a, b) Macroinitiator preparation,

(c) polymerization of PNIPAm from the surface of the carbon microsphere (CM).

Figure 24. SEM images of (A) bare carbon microspheres after pyrolysis, (B) PNIPAm-

grafted carbon microspheres. The scale bars are 500 and 300 nm for the low and high

magnification images, respectively

Figure 25. DLS measurements of average particle size of PNIPAm-grafted CM dispersed

in deionized water within a temperature range from 25 to 50 °C. The concentration of

particles is 0.1 mg/ml. Each data point is an average of 6 individual measurements along

with their corresponding standard deviations

x

Figure 26. Thermogravimetry (TG) and derivative thermogravimetry (DTG) data. Thermal

gravimetric analysis (TGA) of CM and PNIPAm-grafted CM were carried out by using a

thermogravimetric analyzer (Seiko, SII EXSTAR6000) under dry air at a flow rate of 150

ml/min and a heating rate of 15 °C/min. Approximately 4 mg of the sample was placed in

ceramic crucibles and the weight loss was recorded over the temperature range 50-900 °C

Figure 27. FTIR spectra of CM and PNIPAm-grafted CM

Figure 28. Plot of friction force versus applied load between a shearing borosilicate

spherical lens and a flat silicon wafer using an aqueous solution containing PNIPAm-

grafted CM at (A) 1 mg/mL, (B) 3 mg/mL, (C) 5 mg/mL, and (D) 10 mg/mL

concentrations. Data points marked by open circles (○) and filled circles (●) correspond to

measurements at 22 and 40 °C, respectively. Data points were fitted to a straight line with

the slope corresponding to the coefficient of friction (CoF). The shear velocity is 0.5 mm/s

over a distance of 10 mm. Error bars represent the standard deviation of friction force

obtained from at least 5 trials.


spherical lens and a flat silicon wafer using DI water only at room temperature. Data points

were fitted to a straight line with the slope corresponding to the coefficient of friction

(CoF). The shear velocity is 0.5 mm/s over a distance of 10 mm. Error bars represent the

standard deviation of friction force obtaining from at least 5 trials.

Figure 30. Plot of Friction force Fx vs. Applied load L while shearing a spherical probe on

Silicon wafer in presence of: a)10 mg/mL of bare Carbon Microspheres (CM) in DI water

b)Bare CM in presence of 0.36M SDS

xi

Figure 31. Cryo-SEM images PNIMAm-coated CM. (A) Low temperature, and (B) high-

temperature samples with a concentration of 1 mg/ml PNIMAm-coated CM particles.

Figure 32. Bar chart summarizing the CoF values for the PNIPAm-grafted CM-based

aqueous lubricants at various concentrations below and above the LCST

Figure 33. Plot of the friction force as a function of time using (A) 1 mg/mL and (B) 10

mg/mL concentrations of PNIPAm-grafted CM-based aqueous lubricant at room

temperature (gray data) and high temperature (black data). The stepwise increase in the

applied load as a function of time is superimposed on the graph. The shear distance is 10

mm, and the shear velocity is 0.5 mm/s. The friction force data for the room temperature

experiments have been offset by 2 s to facilitate the visualization of the stiction spikes

Figure 34. Schematic illustration of the proposed mechanism by which PNIPAm-grafted

CM affect the lubrication between two shearing surfaces at low and high temperatures and

under low and high concentrations of PNIPAm-grafted CM dispersed in water. The upper

surface moves in the x-direction relative to the bottom surface at a velocity Vx.

1

CHAPTER 1. INTRODUCTION TO PDMS MECHANICAL

PROPERTIES

1.1 Introduction to Polymers Mechanical Properties

It is of great importance to be familiar with some basic mechanical properties of the

material before its application in any field. These properties determine the range of

usefulness of a material, its behavior upon application of repeated applied load, life

expentancy and so on. In this chapter, we will discuss some common terminologies used

to describe the mechanical properties of a material, their stress-strain dependence, and also

talk about the polymer used in this research work, Polydimethylsiloxane (PDMS).

1.1.1 Young’s Modulus

Force applied on a body can result in the motion of the body (such as friction and drag) or

can affect its shape1. A change in shape of an object due to the application of a force is

called deformation. The applied force, irrespective of its magnitude, is known to cause

some deformation. If a small force is applied, two important characteristics are observed

for small deformation:

1) The object returns to its original shape and size when the force is removed. This

deformation is called “elastic deformation”2.

2) For such small deformations, the stress is proportional to the strain, and Hooke’s

law is obeyed [equation 1]. The coefficient of proportionality is called Young’s

Modulus.

2

Young’s modulus is named after a 19th-century British scientist named Thomas Young,

and it describes “stiffness” of material, i.e., resistance to deformation upon applied force.

For an ideal elastic solid, Young’s modulus E is expressed according to Hooke’s law3 as,

E,

E=σ/ε [1]

where,

σ is the stress (Force per unit area or F/A) and

ε is the strain.

The strain ε is the change in length per unit original length, that is

ε=(L-L0)/L0 or ΔL/L0 [2]

where,

L0 corresponds to the original length of the object,

L corresponds to the new or changed length of the object.

The applied stress and strain can be either tensile or compressive. From Equation [1], one

can get Young’s modulus E.

Figure 1 shows a generic plot of deformation ΔL versus applied force F. Hooke’s law is

obeyed in the straight segment of the linear region. The slope of the straight segment is 1/E

and E is the Young’s modulus. Upon application of larger forces, the graph becomes

curved, but the deformation is still elastic, which means ΔL will return to zero if the force

is removed. When even greater are applied, the object permanently deforms until it finally

3

fractures. Note that in the plot, the slope increases drastically just before fracture, which

indicates that a small increase in applied force F is producing a large increase in L near the

fracture.

Figure 1. Hooke’s law depiction for an elastic solid4. Hooke’s law is obeyed in the

straight segment of the plot, and the slope is 1/k. As higher force is applied, the plot

becomes curved, but the deformation is still elastic, meaning the object will return to

its original shape. The application of higher forces will lead to permanent deformation

and, ultimately, a fracture.

From Figure 1, it can be seen that for the same strain value, the larger the stress, the larger

is Young’s modulus (stiffer material) and vice versa (softer material).

4

1.1.2 Viscosity of Fluids

Equation 1,2 and Figure 1 describes the behavior of a solid object upon application of a

force, but a similar analogy can be applied for fluids as well. For fluids, their resistance to

deformation at a given rate is often called “thickness” in common language. For example:

Honey is considered thicker than water. Viscosity arises due to the internal friction between

two adjacent layers of fluid that are in motion. For an ideal viscous liquid, Newton’s law5

expresses the shear viscosity, η, defined as:

η=τ /(dγ/dt) [3]

where,

τ represents the shear stress or force per unit area (F/A),

γ represents the shear strain,

and t is the time.

Equation 3 describes viscosity, for simple liquids such as water or toluene, especially at

low shear rates,. For larger values of shear viscosity η, the flow is slower at constant shear

stress6. While Equation 1 describes the mechanical properties of ideal elastic solids and

Equation 3 is used for ideal viscous liquids, neither of these equations can accurately

describe the mechanical behavior of polymers. Polymers tend to show both elastic as well

as viscous behavior. Thus a different term called Complex Young’s Modulus7 is used.

Complex Young’s Modulus is defined as:

E*= E’ + iE'' [4]

Where,

5

E' is the storage modulus and

E'' is the loss modulus.

Note that E=│E*│ and The quantity i represents the square root of minus one.

The storage modulus E’ is a measure of the energy stored elastically during deformation,

and the loss modulus E” is a measure of the energy converted to heat8. Similar definitions

hold true for Complex Shear Modulus G* and other mechanical quantities. When

molecules deform, they store a portion of the energy elastically and dissipate a portion in

the form of heat. The quantity E' is a measure of the energy stored elastically, whereas E''

is a measure of the energy lost as heat.

1.1.3 Viscoelasticity

A material that exhibits both viscous and elastic characteristics when undergoing

deformation is called a Viscoelastic material. A typical viscous material strains linearly

with time when stress is applied, and an elastic material changes its shape and comes back

to its original shape when the stress is removed. As viscoelastic materials show both

viscous as well as elastic properties, the complex modulus is used to describe its

mechanical properties. In order to study their stress-strain behavior, there are two basic

models, namely Maxwell and Kelvin-Voigt9 models. Viscoelasticity results in a lot of

interesting phenomena in polymers. For example, creep and stress relaxation represent the

static viscoelasticity, while lag and internal friction can describe the dynamic

viscoelasticity. Hysitron TriboIndentor can be used to study viscoelasticity can be studied

with dynamic mechanical analysis (DMA) utilizing.

6

1.2 Introduction to PDMS

The material used in this research work is a silicon-based polymer called

Polydimethylsiloxane (PDMS), also known as dimethicone. PDMS is one of the most

widely used silicon-based organic polymer and is particularly known for its unusual

rheological (or flow) properties10. It is used in various applications, ranging from contact

lenses and medical devices to elastomers. It is also found in shampoos (makes hair shiny

and slippery), caulk to seal joints, lubricating oils, and heat resistant tiles11. PDMS is

considered to be non-reactive (inert) and is optically clear, non-toxic and non-flammable.

Methyltrichlorosilane, which is a Silane precursor with more acid-forming groups and

fewer methyl groups, can be used to introduce branches or cross-links in the polymer

chains12. Ideally, each molecule of such a compound becomes a branch point. This can be

used to produce hard silicone resins. PDMS network can also be used as a substrate to grow

cells. As the crosslink density in the polymer network can be controlled/tuned, it is used to

mimic living tissues. The main focus of the first of this thesis is to characterize the local

surface mechanical properties of a series of PDMS network samples, which are cured to

different crosslink densities, which results in different mechanical properties.

1.2.1 Microstructure of PDMS

The chemical formula for PDMS is shown below

(H3C)3SiO[Si(CH3)2O]nSi(CH3)3,

where n represents the number of repeating monomer [SiO(CH3)2] units. Its brief molecular

structure is shown in Figure 2.

7

The industrial synthesis starts from dimethylchlorosilane and water following the

reaction:

Si(CH3)2Cl2 + n H2O → [Si(CH3)2O]n + 2n HCl.

Figure 2. PDMS chemical formula13 , where n is the number of repeating monomer

units [SiO[CH3]2]

The long PDMS polymer chains usually have vinyl groups at each end. In order to cross-

link these short chains, a cross-linker, usually polymethylhydrosiloxane, is used. A

network of PDMS polymer is assembled by crosslinking these polymer chains. This

reaction can be catalyzed by Platinum.

1.2.2 Samples for Research

PDMS samples for this research were synthesized using Dow Corning Sylgard 184 silicone

elastomer base and Sylgard 184 silicone elastomer curing agent. Sylgard 184 has dispersed

Silica particles to give additional mechanical stability; thus, it is often called “reinforced

PDMS.” For this research, we will use the words “Sylgard” and “PDMS” interchangeably.

The prepared samples have different base/agent ratios, which means different degrees of

cross-linking. The lower the weight of cross-linker, the softer the PDMS network is due to

8

low degree polymerization. Conversely, the higher the degree of cross-linking, the stiffer

the sample will be. The stiffness of the sample was varied by changing the ratio of

crosslinker to base polymer in this research. The most common type of PDMS network in

literature is PDMS 10:1, which has ten units by weight of Sylgard 184 silicone elastomer

base to 1 unit by weight of Sylgard 184 silicone elastomer cross-linker. For PDMS network

with different base/agent ratios, sdifferent degree of cross-linking can be found. For this

research, a series of PDMS network samples with increasing ratios of base/agent were used

to explore the relationship between Young’s modulus and the ratio of mixing. The samples

used for this research had the following base:curing agent ratios: PDMS network 7:1,

PDMS network 10:1, PDMS network 12:1, PDMS network 15:1, PDMS network 20:1 and

PDMS network 25:1. By changing the ratio of base to cross-linker, one tune the mechanical

properties of PDMS. During the tests, one can get the elastic modulus, compare the data

within different test methods, and obtain the relationship between PDMS network

mechanical properties and its amount of crosslinking.

1.3 The Objectives and Challenges for This Research

The goals of this first part of the research are:

1) Measuring mechanical properties of PDMS network for varying degree of cross-

linking

2) Using two different types of compression tests to compare the results between 2

different measurement methods.

The testing of PDMS network mechanical properties, although common, depends heavily

on the instrument used methods, human errors. These challenges are mostly about two

9

aspects. PDMS being a soft material, shows compatibility issues with industrial type

mechanical property testing machines. Typical instruments cannot provide a low force

control system and cannot easily measure the significant displacement during polymer

testing. The conventional DMA instrument is complicated to control, and the testing

process depends too much on the testing temperature. Second, the PDMS network is mostly

soft, and even for the stiffest sample, its elastic modulus is less than 5 MPa. To make things

further complicated, making a standard specimen for mechanical property testing gets

challenging as the polymer is soft. This can lead to not developing full contact at the

beginning of the experiments, which can result in errors in modulus measurements.

In our lab, we did not have a setup to measure the modulus of soft materials. Thus, we had

to get creative to modify the already existing instrument, Universal Material Tester

(UMT)(CETR, nanotribometer, Campbell, CA). A UMT is an instrument used to measure

surface-surface interaction on a macroscopic level. It is used to measure adhesion,

repulsion, friction between 2 interacting bodies, or surfaces. The feedback mechanism of

UMT allows us to measure forces (compressive, adhesive, friction) with respect to time or

displacement. By using this to our advantage, we were able to modify UMT to enable us

to measure Young’s modulus. A detailed explanation is given in chapter 2.

10

CHAPTER 2. MODULUS MEASUREMENTS

2.1 Introduction to Tensile and Compression Tests

For most of the metals, their Young’s modulus and yield stress can be obtained by a simple

stress-strain plot for a tensile test. The mild steel tensile test result is shown in Figure 3.

Figure 3. Mild steel tensile test regimes plotted as stress vs. strain14

In Figure 3. Stress σ is the applied normal force per unit area of the sample cross-section.

The SI units of stress are Pascals (Pa). Strain ε is the change in length per unit original

length.

Point A corresponds to the proportional limit, which is the upper-stress limit to the linear

relationship.

11

Point B corresponds to the elastic stress. Past this point, the material is yielding, and the

corresponding deformation is called plastic deformation. The rise in the curve is called

strain hardening,

Point E corresponds to the ultimate stress. At this point, the cross-sectional area of the

sample begins to decrease in a localized region of the specimen rather than its entire length.

This is called necking.

Point F corresponds to the point when the specimen breaks. The material’s rupture strength

and the stress corresponding to this point is called fracture stress. The tensile test curve is

different for different materials. For example, for more ductile materials, the proportional

limit is lower, while for brittle materials, there will be no necking. The compression testing

is the opposite of tensile testing but can describe the same properties of materials. For this

research, the modulus was measured in 2 different ways viz: Compression test and Hertz

indentation

2.1.1 Compression Test

Hooke’s law states that Young’s modulus on an elastic solid is a ratio of applied stress and

resultant strain.

E=σ/ε,

Where σ= Force per unit area or stress and ε= (L-L0)/L0 or strain and E= material stiffness

or Young’s modulus. For the same amounts of strain, if the applied stress is high, higher is

Young’s modulus, and stiffer the material becomes. In our study, we have used

Polydimethylsiloxane (PDMS) as our test candidate to measure its modulus. PDMS

samples used in this study were cylindrical (disc) in shape. Hence while calculating stress,

12

sample geometry had to be taken into consideration. The formula for stress (σ) changes as

follows:

σ= (9.8*4*Fz)/ (πD2) for compression test [5]

Where Fz= force applied on the sample, D= diameter of the sample

2.1.2 Hertz indentation

According to Hertz theory, contact between a rigid sphere and elastic material can be used

to measure material modulus (Figure 4). The indentation can be described by the following

equation

P = 4/3 E*R1/2δ3/2 [6]

Where P = applied force,

R = radius of the probe,

δ = displacement (indentation depth),

E* = effective Young’s modulus

E*= E/ (1-𝜈2) [7]

E= Young’s modulus of the soft material,

𝜈=Poisson’s ratio of the soft material

13

Figure 4. Schematic of Hertz Indentation15. A solid sphere of Radius R indents an

elastic substrate up to a depth δ. The radius of the indent is α, and the modulus of

the substrate is E.

Equation 6 assumes that the sample is infinitely thick. In reality, the samples can have a

finite thickness, and it has to be expressed in Equation 6. Many researchers have shown

that for samples with finite thicknesses, the standard Hertzian model can result in large

errors16,17. To address this issue, Dimitriadis et al18. derived a correction factor that can be

added to the Hertzian equation used for a semi-infitine substrate. For samples with a

finite thickness, equation 8 is used:

P= 16

9 ER1/2δ3/2[1+1.133 χ +1.283 χ2 +0.769 χ 3+0.0975 χ4] [8]

Where P = applied force,

R = radius of the probe,

δ = displacement (indentation depth),

E = Young’s modulus

h=sample thickness

χ= √𝑅δ/h

14

In Equation 8, we can see a linear dependence on Young’s modulus E and nonlinear

dependence on Poisson’s ratio 𝜈. For most polymers, it is safe to assume that Poisson’s

ratio 𝜈 is 0.5. Equation 8 can be used for films, biological samples19 or coating of

micrometer size. As the samples were subjected to smaller strain values and were

relatively thicker than the sphere/probe diameter, we have focused primarily on the

standard Hertzian model (Equation 6) and compared the values with modified Hertz

Theory (Equation 8) in plot 11.

15

2.2 Samples and Instrumentation

2.2.1 Samples Preparation for Compression Study

PDMS samples were made by pouring 20 g of PDMS in a plastic petri dish of 9 cm in

diameter. The samples were cured for 24 hours at 65˚C.

After curing, the samples were allowed to sit outside for 2 hours to cool down. Once cooled

off, cylindrical discs were cut out from cured PDMS using a manual punch. These discs

Steel disk

Force Sensor

PDMS Sample

Figure 5. Experimental setup for compression test. A steel disc (12 mm diameter)

bigger than the size of PDMS disc was used to compress the samples. DFM force

sensor was used with the force range of 50-500 g

16

were around 6 mm in diameter and around 4 mm in thickness. Exact diameter and thickness

were measured using digital calipers. CETR Universal Materials Tester (UMT) was used

to conduct compression analysis. DFM sensor with a force range of 50-500g was used

without the spring to avoid additional deflection.

For the compression study, PDMS discs were compressed by a flat steel disc attached to

the sensor (figure 5). The diameter of the steel disc was always bigger than the sample

diameter; thus, the sample was uniformly compressed by the disc. An initial force of 10 g

was applied to the sample to make sure the sample is in complete contact with the probe.

Once complete contact was achieved, the force was increased by increments of 1 g till the

maximum value of 150 g was reached

17

2.2.2. Sample preparation for Hertz Indentation

Sylgard samples were made with the same protocol mentioned for the compression study.

Once they were cured, a metallic ball of 6.3 mm in diameter was pressed on the sample.

In this method, displacement of the probe was controlled instead of the force, and resultant

force value was recorded. The probe was brought down till the force exerted on the sample

was 2 g. The displacement was then increased by 0.01 mm in each step, and the

corresponding force was recorded using the in-built UMT software.

Figure 6. Experimental setup for Hertz indentation. A Stainless-Steel sphere 6.3

mm in diameter was used the probe. Sylgard 184 samples of varying thickness

were used to measure and compare their modulus

Force Sensor

Spherical indenter

Sylgard sample

18

2.3 Analysis Methods

2.3.1 Compression Study Analysis

With the designed instrument setup, one can get the stress and strain of the sample using

equation [5]:

Stress σ = Fx*9.8/(π*(D/2)2)

Strain ε = dL/L0

where Fx is the force applied by UMT, which applies the force to the sample. D is the

diameter of the sample. dL is the change in the sample’s length under compressive force.

From the above equations, it is easy to see the slope of the stress-strain curve is the elastic

modulus of the sample. More details are shown in figure 7.

y = 2,111,988.25x + 4,881.03

0

20000

40000

60000

80000

100000

120000

0 0.01 0.02 0.03 0.04 0.05

Str

ess,

Pa

Strain

19

Figure 7. Plot of Stress (σ) vs. Strain (ε) for Compression study. The slope of the line

gives us the elastic modulus. Sylgard 184 sample with 10:1 ratio of base: curing

agent was used for this particular sample

2.3.2 Hertz Indentation Analysis

For Hertz Indentation, we use equation [6]:

P = 4/3 E*R1/2δ3/2

Where P = applied force

E* = effective Young’s modulus= E*= E/ (1-𝜈2), E= Young’s modulus of the soft material,

𝜈=Poisson’s ratio of the soft material

R = radius of the probe, δ = displacement (indentation depth)

An example of a plot of Force (P) vs. Displacement (δ) is shown in figure 8.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003

Ap

plie

d f

orc

e P

(N

)

Displacement δ (m)

20

Figure 8. Plot of Force (P) vs. Displacement (δ) for Hertz indentation

By plotting P vs. δ, one can get the value of Effective Young’s modulus E*. For most soft

materials, the value of Poisson’s ratio 𝜈 is 0.499. Thus we can calculate Young’s modulus

of PDMS using equation 6 and equation 7

2.4 Results and discussion

2.4.1 Compression test

Table 1 and Table 2 summarize the results obtained for the modulus of PDMS experiment.

The modulus of PDMS is dependent on the ratio of mixing between the base and cross-

linker. For smaller ratios, the modulus of PDMS is high.

Table 1. Summary of compression study

Ratio Modulus

(MPa)

Error

(MPa)

7:1 3.09 0.1

10:1 2.4 0.2

12:1 1.88 0.1

15:1 1.2 0.07

20:1 0.89 0.04

25:1 0.39 0.04

21

2.4.2 Hertz Indentation

Table 1. Summary of Hertz Indentation study

Ratio Modulus

(MPa)

Error

(MPa)

7:1 2.01 0.05

10:1 1.77 0.06

12:1 1.35 0.07

15:1 1.05 0.1

20:1 0.59 0.05

25:1 0.41 0.003

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30

Mo

du

lus

(MP

a)

PDMS base/cross-linker ratio

Compression test

Figure 9 Plot of Modulus values using compression testing for various values of

PDMS base/curing agent ratios. The modulus value is high for smaller mixing

ration and decreases for higher base/curing agent ratios

22

2.4 Conclusions

Young’s modulus of PDMS samples was studied based on two types of macroscopic tests

viz Compression test and Hertz Indentation. Both the tests show that Young’s modulus of

PDMS is related to the degree of cross-linking, thus smaller mixing ratios of base/agent

show higher modulus values. The relationship between the PDMS network elastic modulus

and its base/cross-linker ratio are summarized in Figure 11. In order to compare our test

results with the work done by other groups, we have also plotted findings of a reference

article20

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30

Mod

ulu

s (M

pa)

PDMS base/cross-linker ratio

Hertz Indentation

Figure 10 Plot of Modulus values using Hertz Indentation for various values of

PDMS base/curing agent ratios. The modulus value is high for smaller mixing

ration and decreases for higher base/curing agent ratios

23

For smaller ratios of the base to cross-linker, the modulus values were found to be higher.

At such ratios, the degree of polymerization is higher, giving us a harder sample. For larger

ratios of the base to cross-linker, we get softer PDMS samples.

Figure 11. Summary of Modulus Experiments and reference values from the

literature18. It can be seen that the modulus values after using the Hertz correction

factor did not change drastically.

Both macroscopic tests showed similar trends, as described above. The modulus value was

observed to be dependent on the method used. This is consistent with the research done by

other groups.

3.59

2.91

2.61

1.21

0.98

0.56

1.871.68

0.86

0.6

0.36

3.09

2.4

1.88

1.2

0.89

0.39

2.01

1.77

1.35

1.05

0.590.41

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25 30 35

Mod

ulu

s M

Pa

)

PDMS base to cross-linker ratio

Compression Test

Hertz Indentation

reference

Hertz Correction Factor

24

CHAPTER 3. FRICTION AND WEAR

Friction is responsible for consuming about one-third of the world’s energy resources21. It

is the principal cause of wear and energy loss, and humans have been attempting to reduce

this energy loss for ages. Tribology is the science of lubrication, friction, and wear that

deals with a diverse array of natural and man-made systems of interacting bodies in relative

motion.

A process that involves friction always goes hand-in-hand with energy transformation.

Thus it is important to develop a thermodynamic framework for studying its characteristics.

The energy dissipation in friction processes is an irreversible phenomenon; thus, the

concept of the thermodynamic entropy production becomes an ideal tool for probing into

its complex behavior22.

When two bodies are pressed together and set in motion, it is always accompanied by the

dissipation or release of energy. The interfacial friction is the main factor that controls the

behavior of energy generation within the contact of a sliding system. The frictional energy

generated between contacting bodies is mainly governed by the velocity and combination

of the applied load. Other contacting factors, such as the relative velocity, material

properties, and size, also influence the dissipation and distribution of the frictional energy.

The friction and heat dissipation are, therefore, intimately related23.

26

scale. This roughness feature, known as asperities are present not just at macro-scale but

also down to nano-scale dimensions. As a result, a true solid to solid contact exists only at

a limited number of points, accounting for only a fraction of the apparent or nominal

contact area27. As higher force is applied, the true contact area rises due to the deformation

of the surface asperities. In order to quantify this linearity between the applied load, static

friction, and true contact area, Amonton-Coulomb’s law is used28. As mentioned in the

previous example, in order to move an object kept on a sloped surface, the static friction

force must be overcome by applying greater force. The maximum possible friction force

(Fmax) between two surfaces before sliding begins is defined as:

Fmax=µsFN [8]

It can be seen from the equation that Fmax is the product of the coefficient of static friction

(µs) and the normal force (FN):

When there is no sliding, the friction force can have any value from zero up to Fmax. If the

applied force is smaller than the force required to initiate sliding one surface over the other,

this force is opposed by a frictional force of equal magnitude and opposite direction. A

force greater than Fmax will cause sliding to occur. As soon as sliding begins, the static

friction is no longer applicable, and this type of friction between the two surfaces is called

kinetic friction.

A classic example of static friction, contrary to popular belief, is the force that prevents a

car wheel from sliding as it rolls on the ground. Although the wheel is in motion, the contact

area of the tire in contact with the ground is stationary relative to the ground.

https://en.wikipedia.org/wiki/Asperity_(materials_science)

https://en.wikipedia.org/wiki/Viscosity

28

All real fluids (except superfluids) offer some resistance to shearing and therefore are

viscous. For teaching and explanatory purposes, it is helpful to use the concept of an

inviscid fluid or an ideal fluid that offers no resistance to shearing, and so is not viscous.

3.1.4 Rolling friction

The force resisting the motion when a circular or spherical body (such as a ball, tire,

or wheel) is called Rolling resistance, sometimes called Rolling Friction or Rolling Drag.

It is mainly caused by non-elastic effects; that is, not all the energy needed for deformation

(or movement) of the wheel, roadbed, etc. is recovered when the pressure is removed. Two

forms of this are hysteresis losses (see below), and permanent (plastic) deformation of the

object or the surface (e.g., soil). Another cause of rolling resistance lies in

the slippage between the wheel and the surface, which dissipates energy. Note that only

the last of these effects involves friction; therefore, the name "rolling friction" is, to an

extent, a misnomer.

In analogy with sliding friction, rolling resistance is often expressed as a coefficient times

the normal force. This coefficient of rolling resistance is generally much smaller than the

coefficient of sliding friction.

Any coasting wheeled vehicle will gradually slow down due to rolling resistance, including

that of the bearings, but a train car with steel wheels running on steel rails will roll farther

than a bus of the same mass with rubber tires running on tarmac. Factors that contribute to

rolling resistance are the (amount of) deformation of the wheels, the deformation of the

roadbed surface, and movement below the surface. Additional contributing factors

include wheel diameter, speed, load on wheel, surface adhesion, sliding, and relative

micro-sliding between the surfaces of contact. The losses due to hysteresis also depend

https://en.wikipedia.org/wiki/Superfluid

https://en.wikipedia.org/wiki/Ideal_fluid

https://en.wikipedia.org/wiki/Motion_(physics)

https://en.wikipedia.org/wiki/Ball

https://en.wikipedia.org/wiki/Tire

https://en.wikipedia.org/wiki/Wheel

https://en.wikipedia.org/wiki/Plasticity_(physics)

https://en.wikipedia.org/wiki/Hysteresis

https://en.wikipedia.org/wiki/Rolling_resistance#Primary_cause

https://en.wikipedia.org/wiki/Plastic_deformation

https://en.wikipedia.org/wiki/Frictional_contact_mechanics

https://en.wikipedia.org/wiki/Friction

https://en.wikipedia.org/wiki/Sliding_friction

https://en.wikipedia.org/wiki/Vehicle

https://en.wikipedia.org/wiki/Train

https://en.wikipedia.org/wiki/Rail_tracks

https://en.wikipedia.org/wiki/Bus

https://en.wikipedia.org/wiki/Rolling_resistance#Dependence_on_diameter

https://en.wikipedia.org/wiki/Rolling_resistance#Dependence_on_speed

https://en.wikipedia.org/wiki/Rolling_resistance#Dependence_on_wheel_load

https://en.wikipedia.org/wiki/Hysteresis

29

strongly on the material properties of the wheel or tire and the surface. For example,

a rubber tire will have a higher rolling resistance on a paved road than a steel railroad

wheel on a steel rail. Also, sand on the ground will give more rolling resistance

than concrete.

Figure 13. Hard wheel rolling on and deforming a soft surface, resulting in the

reaction force R from the surface having a component that opposes the motion. (W is

some vertical load on the axle, F is some towing force applied to the axle, r is the wheel

radius, and both frictions with the ground and friction at the axle are assumed to be

negligible and so are not shown. The wheel is rolling to the left at a constant speed.)

Note that R is the resultant force from non-uniform pressure at the wheel-roadbed

contact surface. This pressure is greater towards the front of the wheel due to

hysteresis.

https://en.wikipedia.org/wiki/Tire

https://en.wikipedia.org/wiki/Steel

https://en.wikipedia.org/wiki/Wheelset_(rail_transport)

https://en.wikipedia.org/wiki/Wheelset_(rail_transport)

https://en.wikipedia.org/wiki/Sand

https://en.wikipedia.org/wiki/Concrete

30

3.2 Lubrication and Wear

Dr. H. Peter Jost coined the term “tribology” in the mid 1960s, and the term was accepted

as the study of friction, lubrication and wear, and their application. Even though the effects

of friction were studied by Leonardo da Vinci in the 1400s, it took nearly 600 more years

to quantify and study friction as a branch of science. There was a dramatic increase in the

reported failures of plant and machinery in the early 1960s. Most of these were due to wear

in moving parts of the machine and caused serious financial losses. Many industrial

operations required continuous operations of the machinery, and such breakdowns were

costlier than ever.

To overcome such failures, specialists involved in wear, lubrication, and friction fields

conducted several studies and reported on the impact of friction, lubrication, and wear on

machine efficiency, cost, and productivity. As a result, tribology became a mainstream

field of science, and many universities offer it as part of their mechanical engineering

department curriculum.

The primary focus of tribology was to improve the efficiency of industrial operations and

extending the lifecycle of industrial machinery. Today, those principles and design benefits

have evolved in their own branch of study and are making a major impact in a variety of

modern applications. Here are some examples.

3.2.1 Biotribology

The term “biotribology” was first reported by Dowson in 1970 as “those aspects of

tribology concerned with biological systems”32. This type of tribological system involves

an extensive range of synthetic materials and natural tissues like skin, cartilage, blood

vessels, heart, tendons and ligaments.

31

These materials involve complex interactions between biological components. In order to

study these systems, biotribologists incorporate concepts of lubrication, friction, and wear,

of these biological surfaces in various applications, such as the wear of screws and plates

in bone fracture repair, the design of joints and prosthetic devices, wear of replacement

heart valves, wear of denture and restorative materials, and even the tribology of contact

lenses.

3.2.2 Green Tribology

The concept of green tribology is relatively new can be defined as “The science and

technology of the tribological aspects of ecological balance and of environmental and

biological impacts.”33 A simple definition of green tribology is saving materials, energy,

improving the environment and the quality of life. The area of green tribology directly

affects the economy as it helps reduce the waste and extend the life of industrial

equipments. The specific field of green or environment-friendly tribology emphasizes the

aspects of interacting surfaces in relative motion, which are of importance for energy or

environmental sustainability or which have impact upon today’s environment.

3.2.3 Nanotribology

This branch of tribology studies and characterizes friction, adhesion and surface interaction

at the nano scale. As it is done at molecular level, atomic interactions and quantum effects

are not negligible and have to be taken into consideration. The systems such as MEMS

(Microelectrochemical Systems) and NEMS systems (nanoelectromechanical systems)

have been on the rise since 1990s. They can be simply described as miniature machines

that has both mechanical as well as electronic components. Such systems include disk

drives, inkjet printers, biodetectors, molecular sieves etc. In order to change surface

32

topography at such small scale, the surface interaction at nano and micro-scale has to be

studied and nanotribology allows us to do so.

33

CHAPTER 4 CARBON AND ITS FORMS

Elemental Carbon exists in two natural allotropes, diamond, and graphite. These two

allotropes consist of extended networks of sp3- and sp2 -hybridized carbon atoms,

respectively34. Both forms show unique physical properties such as hardness, thermal

conductivity, lubrication behavior, or electrical conductivity35. Theoretically, there are

many ways to construct carbon allotropes by altering the periodic binding motif in

networks consisting of sp3-, sp2- and sp-hybridized carbon atoms36. As a consequence of

the expected remarkable physical properties of these elusive carbon allotropes, it has been

appealing to develop concepts for their preparation on a macro-scale. Diamond and

graphite used to be the only known allotropes of carbon for a long time. This situation

changed in 1985, with the advent of fullerenes, which were observed for the first time by

Kroto et al37. This serendipitous discovery marked the beginning of an era of synthetic

carbon allotropes. Now, as we celebrate buckminsterfullerene’s 25th birthday, it is also the

time to reflect on a growing family of synthetic carbon allotropes, which includes the

synthesis of Carbon Nanotubes in 199138 and the rediscovery of graphene in 200439.

Keeping in mind the numerous possible carbon modifications and the number of scientists

investigating this challenge, these revelations have certainly not come to an end.

4.1 Nanotubes:

Carbon nanotubes are one of the allotropes of carbon which are intermediate to spherical

fullerenes and flat graphene sheets. There are two main types of carbon nanotubes, viz

Single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT), that can have

high structural perfection. SWNTs consist of a single graphite sheet seamlessly wrapped

34

into a cylindrical tube40. MWNTs comprise of an array of such nanotubes that are

concentrically nested like rings of a tree trunk41.

Figure 14. Single Walled and Multi Walled Carbon Nanotubes42. Single walled CNT

have 1 Graphene cylinder whereas Multi Walled CNT have multiple graphene

cylinders

Despite structural similarity to a single sheet of graphite, which is a semiconductor with

zero band gap, SWNTs can either be metallic or semiconducting, depending on the sheet

direction about which the graphite sheet is rolled to form a nanotube cylinder43. The

nanotube axis direction relative to graphene is denoted by a pair of integers (n, m).

Depending on the orientation of carbon bonds around the nanotube diameter, the nanotube

is either of the armchair (n = m), zigzag (n = 0 or m = 0), or chiral (any other n and m)

variety. All armchair SWNTs are conductors like metals. Those with n - m = 3k, (where k

is a nonzero integer) have a tiny band gap and are semiconductors.

Small-diameter SWNTs are exceptionally strong, meaning that they have a high Young's

modulus and high tensile strength. Literature reports of these mechanical parameters can

35

be confusing, because some authors use the total occupied cross-sectional area and others

use the much smaller van der Waals area for defining Young's modulus and tensile strength.

With the total area per nanotube in a nanotube bundle for normalizing the applied force to

obtain the applied stress, the calculated Young's modulus for an individual (10, 10)

nanotube is ~0.64 TPa44, which is consistent with measurements. Because small-diameter

nanotube ropes have been extended elastically by ~5.8% before breaking, the SWNT

strength calculated from the product of this strain and modulus is ~37 GPa, which is close

to the maximum strength of silicon carbide nanorods (~53 GPa45). This modulus of ~0.64

TPa is about the same as that of silicon carbide nanofibers (~0.66 TPa) but lower than that

of highly oriented pyrolytic graphite (~1.06 TPa)44. More impressive and important for

applications needing light structural materials, the density-normalized modulus and

strength of this typical SWNT are, respectively, ~2.4 and ~1.7 times that of silicon carbide

nanorods and ~19 and ~56 times that of steel wire. The challenge is to achieve these

properties of individual SWNTs in nanotube assemblies found in sheets and continuous

fibers.

4.2 Carbon Nano onions

Carbon Nano-Onions or CNOs are multi-shell fullerenes consisting of quasi-spherical

nested graphitic layers with a size ranging from 2 to 50 nm, depending upon the method of

synthesis; the innermost shell is composed of a C60 fullerene and the adjacent shells are

approximately spaced by 3.4 Å, slightly different to the distance between two [2 2 0] planes

(3.334 Å) in bulk graphite46. Firstly reported by Iijima in 198047, CNOs aroused great

interest in 1992 thanks to Ugarte’s experiment48. Under an intense electron beam, he

36

observed the in situ transformation of amorphous carbon into spherical particles, with a

diameter of about 45 nm, due to the highly energetic irradiation process, which induced the

graphitization and the curling of the amorphous carbon49.

Although various methods have been published for the synthesis of CNOs, the main

synthetic methods have been thermal annealing of ultradispersed nanodiamonds50 (NDs),

arc-discharge between two electrodes submerged in water51, pyrolysis, ion implantation52,

chemical vapor deposition53, or electron-beam irradiation54. Depending on the preparation

method of the onion-like structures, different types of nanostructures can be achieved with

varying sizes (‘‘big” and ‘‘small”), shapes (spherical and polyhedral), and types of cores

(dense and hollow

Figure 15. HRTEM image of Carbon Nano-onions (left) and schematic of Carbon

nano-onions formed by 3 concentric layers (C60, C240, C540) 55

4.3 Fullerenes

A fullerene is a carbon allotrope which consists of carbon atoms connected by single or

double bonds to form a closed mesh. Fullerenes have rings of five or seven carbon atoms

37

fused together to form spheres, ellipsoid, tube and many other shapes. The most abundant

and stable form of fullerenes is Buckminster fullerene (C60) with 60 carbon atoms arranged

in a spherical structure56. It contains 12 pentagons and 20 hexagons and its shape resembles

that of a soccer ball, which contains57. Fullerenes fulfill the EULER’s theorem which states

that in order to form a polyhedron from building a closed structure from pentagons and

hexagons; it has to contain exact 12 pentagons. Following this rule, the smallest stable

fullerene is C60, which has no two pentagons side by side, making it the most stable

structure.

Figure 16. Varieties of fullerenes found in natural shungite with different numbers of

carbon atoms: C24, C28, C32, C36, C50, C60, C7058

C60 molecule also known as Buckminsterfullerene is highly symmetrical59. There are 120

symmetrical operations, like rotation around the axis and reflection in a plane, which map

the molecule onto itself. This makes C60 the most symmetrical molecule. There are two

38

types of bond lengths in the fullerene: C5-C5 single bonds in the pentagons and C5-

C6 double bonds in the hexagons; the first is 1.45±0.015 Å and the other one is 1.40±0.015

Å60. Each carbon atom forms bond to three other adjacent atoms with sp hybridization. The

set of orbitals is arranged at 120-degrees angles and is centred in the xy-plane61. Hence

these delocalized pi electrons stabilize the spheroid structure by resonance.

A C60 molecule is about 7 Å in diameter62. C60 molecules condense to form a solid of

weakly bound molecules. This crystalline state is called fullerites. This solid is cubic,

weakly bound with a lattice constant a=14.71 Å and electrically insulating. It occurs as

yellow powder, which turns pink when dissolved in toluene. On exposure to strong UV

light, the Buckyballs polymerize, forming bonds between adjacent balls. In the

polymerized state, C60 no longer dissolves in toluene. NMR studies of C60 benzene solvates

show free rotation at room temperature. At about -13°, the balls spin freely in their

crystalline positions. At lower temperature, their movements begin to limit to certain

orientations. Eventually, below -183°, the balls become completely struck. Chemically the

molecule is quite stable; breaking the balls requires temperature of over 1000°. By heating

fullerenes up to 1500° in absence of air, they transform to graphite.

4.4 Nanofibres

Carbon nanofibre are a unique form of vapour-grown carbon fibre that bridge the gap

between the larger, conventional PAN or pitch-based carbon fibres and the smaller single-

wall or multi-wall carbon nanotubes63. The nanofibres have transport and mechanical

properties that approach the theoretical values of single crystal graphite, similar to the

nanotubes, but can be produced in high volume at low cost. Investigations performed over

39

last thirty years mainly devoted to the understanding the growth mechanism and property

development from various similar gas phase techniques. Figure 15 shows the typical SEM

image of as grown carbon nanofibres.

Figure 17. SEM image of Carbon Nano-Fibres64

Vapour phase produced carbon nanofibres are similar to fullerene nanotubes in the

nanoscale domain of initial formation and highly graphitic structure of the initial filament.

A variation makes carbon nanofibres of the vapour-phase catalytic method in which a

carbon containing feedstock is pyrolysed in the presence of small metal catalyst particles65.

The nanofibre growth normally proceeds in a two-stage process of lengthening followed

by thickening. In the first stage, carbon from the hot atmosphere is absorbed into the

catalyst particle and then precipitates out on one side to form a highly graphitic strand with

a diameter roughly equal to that of the catalyst particle. After lengthening, chemical vapour

deposition of carbon covers the catalyst and builds up the diameter of the nanofibre.

40

4.5 Carbon spheres

Spherically shaped carbon materials have been given many names. These include carbon

balls, carbon nanospheres, carbon microbeads, carbon blacks, onions, mesoporous

microbeads, etc. While the properties/sizes/morphology of the spheres can certainly vary

there appears to be no consistent nomenclature for naming these shaped materials;

consequently in this review we will use the generic term carbon spheres (CSs) to cover all

carbons that have a spherical or near spherical shape. This will not include the fullerene

family of carbons. The spherical shape is not peculiar to carbon; indeed numerous studies

of spherical silica have indicated the wide range of synthetic strategies and approaches that

can be used to make spherical materials. A key target when making spherical materials is

to ensure that they are monodisperse and that the size (diameter) and the chemical

properties of the external rim can be controlled. Many CSs have been shown to be made of

layers of carbon that typically form broken concentric layers emanating from the core. CSs

are attracted to each other by van der Waals forces and this leads to agglomerated

collections of CSs. When CSs have diameters of less than 1000 nm they also tend to

accrete, i.e. bond together to form necklace or bead like structures. Thus, although CSs are

normally discussed as though they are discrete, they often form connected solid extended

chains. These chains are formed early in the synthesis process and are not necessarily due

to post synthesis treatments. Mechanisms that have been proposed to explain their

formation are described in this review. Spheres can also be built up from carbon layers that

are perpendicular to the sphere rim.

41

Figure 18. Carbon Spheres66

The study of CSs can be categorized in four different ways. Firstly, spheres can be

described as solid, core–shell or hollow. Secondly, they can be classified according to their

‘nanometric texture’, i.e. in terms of whether the spheres are made of concentric, radial or

random layers . Thirdly, it has also been proposed that spheres can be categorized in terms

of their size, in particular their diameter. In this method three categories are recognized (i)

well graphitized spheres (2–20 nm), (ii) less graphitized spheres (50–1000 nm) and carbon

beads (>1000 nm). Fourthly, it is possible to classify spheres in terms of strategies used in

their synthesis. It appears that the procedures used to make spherical carbon materials such

as chemical vapor deposition (CVD), arcdischarge, laser ablation, autoclave processes, etc.

can produce quite different materials.

42

CHAPTER 5 STIMULI RESPONSIVE POLYMERS

The functions of living cells are regulated by macromolecules that respond to changes in

local environment and these biopolymers form the basis around which all major natural

processes are controlled. Many synthetic polymers that exhibit environmentally responsive

behavior can thus be considered as biomimetic and their development is central to

emerging ‘smart’ applications in biology and medicine. Of especial interest are synthetic

or modified biological materials that can undergo conformational or phase changes in

response to variations in temperature and/or pH. Polymers of this type are being developed

for uses in fields as diverse as bulk engineering and microscale medicine, while specific

examples range from microfluidic devices, pulsatile drug release systems, bioadhesion

mediators and motors/actuators. Responsive polymers are also a major focus in emerging

nanoscale technologies. In all these cases the key parameter defining the responsive or

‘smart’ behavior of the polymers is a non-linear response to an external signal. Although

there are many responsive elements that can be incorporated in synthetic materials or

engineered/modified biopolymers, much of the research to date has involved pH,

temperature or light as the stimulus. As in nature, the bulk response of the polymer is

usually due to multiple co-operative interactions such as progressive ionization or loss of

H-bonding, that, although individually small, ultimately evoke a large structural change in

the material when summed over the whole polymer. This behavior intrinsically lends itself

to biomedical applications and in this review the aim is to highlight selected yet diverse

recent research showing the potential for bringing these classes of materials into

therapeutic use.

43

5.1 Temperature Responsiveness

Figure 17 illustrates several examples of temperature-responsive monomeric blocks which,

upon polymerization maintain stimuli-responsiveness. A well known polymer with the

LCST behavior is poly(N-isopropylacrylamide) (PNIPAAm) which exhibits coil-to-

globule phase transition at 32 ◦C. Poly(N-vinylcaprolactone) (PVCL), poly(N-(dl)-(1-

hydroxymethyl) propylmethacrylamide) (p(dl)-HMPMA), and poly(N,N -

diethylacrylamide) (PDEAAm) are also temperature-responsive, and their LCSTs are

about 32, 37, 33 ◦C, respectively. Thus, molecular designs of a polymer backbone allow

one to control temperature at which a given system is responsive. It is well established that

the LCST phase transition is a nanometer scale event, where the particle or aggregate

dimensions change. However, for individual polymer chains, the coil-to globule transitions

can be thermodynamically controlled by adjusting polymer compositions, as determined

by Atomic Force Microscopy (AFM).

Figure 19. Temperature Responsive Polymers67

When copolymerized with hydrophilic or hydrophobic comonomers, LCST transitions

may shift to higher or lower temperatures, respectively. Block copolymers of poly(ethylene

44

oxide)–poly(propylene oxide) (PEO–PPO) also exhibit thermal responses in solutions, but

it is believed that the driving forces for these transitions originate from amphiphilic balance

5.2 pH responsiveness

Figure 18 illustrates examples of pH-responsive polymers. pH-responsive polymer

solutions represent another group, in which chemical structures of pH-responsive

compounds have ionizable functional groups capable of donating or accepting protons

upon environmental pH changes. In this case, electrostatic repulsions between generated

charges cause alternations of the hydrophobic volume along a polymer chain, which is

capable of extending or collapsing. Polyacids, such as poly(acrylic acid) (PAAc), and

poly(methacrylic acid) (PMAAc) with pKa values in the range of 5 will release protons

and swell under basic pH values.

Figure 20. pH Responsive Polymers65

In contrast, pH-responsive polybases accept protons and extend under acidic pH

conditions, where amino and amine functional groups in poly(N,N -dimethyl aminoethyl

methacrylate) (PDMAEMA) and poly(vinyl pyridine) (PVP), respectively, are responsible

for these transitions. One of the common trends in designing stimuli-responsive polymers

45

is to copolymerize monomers with different stimuli-responsiveness in order to achieve

multiple-responsive behavior.

5.3 Electromagnetic responsiveness

Incorporation of photo-chromic molecules provides opportunity to develop polymers

capable of responding to electromagnetic radiation. Figure 19 illustrates most common

photo-sensitive molecules, which are classified into the following categories: cis–trans

isomers (A), ionization monomers (B), and dimerization monomers (C). As shown in

Figure 19A, photo-responsive azobenzene is a molecule that exhibits trans-to-cis

photoisomerization with sufficiently low energy (2–3 kcal/mol) to induce photo-chromic

transitions. The rearrangement mechanism for lecuo and spiropyran derivatives shown in

Figure 19B is based on ionization upon exposure to electro-magnetic irradiation. When

exposed to UV light, dissociated ion pairs are generated, which can be further neutralized

when heated in the dark. As illustrated in Figure 19B, photo-induced polymer chains of

lecuo and spiropyran derivatives shrink and expand, which is attributed to the reversible

exchange of the electrostatic repulsion between ionic states. This process typically requires

less than 5 kcal/mol. Figure 19C illustrates another photoreactive molecule cinnamate

which is able to dimerize upon UV irradiation with the energy barrier of about 7 kcal/mol.

These molecular entities can be utilized as photo-reversible covalently crosslinkers in

polymers, thus offering potential applications as switching segments in shape memory

systems and other devices. Liquid crystalline materials are stimuli-responsive polymers

that have been known for a few decades. These are molecules with permanent dipole

moments embedded in polymer matrices, which due to optical and geometrical anisotropies

are able to respond to electromagnetic field by aligning their mean optic axis parallel to the

46

external field, which results in orientation changes. As shown in Figure 19A, liquid

crystalline molecules are freely dispersed between the two electrodes with no electric field.

When an electric field is applied, the molecules align along the electric field axis and the

driving force for the alignment results from the electrostatic interactions. Figure 19B

summarizes selected examples of chemical entities capable of responsiveness, and their

common feature is the permanent dipole moment, in this case generated by electron-

withdrawing groups nitrile (CN) and trifluoromethyl (CF3) groups.

Figure 21. Electromagnetic Responsive Polymers65

47

CHAPTER 6. TUNABLE FRICTION THROUGH STIMULI

RESPONSIVE HYBRID CARBON MICROSPHERES

Friction can be greatly reduced by introducing a rolling element between two sliding

surfaces68. Braun et al.69 showed that friction can be lowered 102−103 times compared to

sliding friction. Despite the superiority of rolling friction over sliding70, few attempts have

been made to study the rolling friction mechanism on the micro- and nanoscale. Past studies

have explored the use of fullerenes (buckyballs or C60) as an additive to synthetic oil71. The

fullerene molecules were expected to impart a rolling mechanism to reduce friction. The

small sizes of the fullerenes were found to hamper the predicted “rolling mechanism”, and

they were proven to be detrimental to the surfaces in contact by getting trapped in the

surface asperities. St. Dennis et al.72 showed that an aqueous suspension of carbon

microspheres (CM), which are larger in size compared to fullerenes, can be used to reduce

friction. These uniform CM (∼400 nm) can withstand high pressure (8 GPa) and have

diameters greater than the inherent surface roughness of a wide range of polished materials.

The CM work as small ball bearings and employ a rolling mechanism to reduce friction.

Furthermore, the surfaces in contact showed high durability and no visible surface wear.

In a follow-up study, Cheng et al.73 explored the influence of the size and concentration of

the CM on rolling friction. The coefficient of friction was found to increase with increasing

particle size, and for a given size, an optimal concentration of ∼4 mg/mL resulted in low

friction for prolonged shear cycles. Hybrid carbon/iron microspheres74 were also

demonstrated to be effective in the presence of a magnetic field that localized the particles

in the shear contact region. Several groups explored the use of nanometer-sized materials

other than carbon allotropes to reduce friction. Rod-shaped materials such as carbon

48

nanotubes75, ZnS76 were shown to reduce friction with no apparent wear. The properties of

particles at surfaces can be altered or enhanced by the grafting of polymers77. Stimuli

sensitive polymers are of particular interest as they impart new properties upon activation

of a trigger78. Poly(N-isopropylacrylamide) also known as PNIPAm is a temperature-

sensitive polymer widely used in drug delivery, biosensor design, and tissue engineering

for its stimuli-responsive property79. The polymer chains are well hydrated and form a

three-dimensional hydrogel network when present at temperatures below its lower critical

solution temperature (LCST) at 32 °C80. When heated above its LCST, the polymer chains

undergo a phase transition and lose their water molecules81. This dehydrates the chains and

shrinks the hydrogel structure82.

In this work, we show that PNIPAm-grafted CM show increased friction upon heating the

particles above the LCST. We also demonstrate that the low friction at room temperature

is concentration-dependent, and a relatively high particle concentration is needed to reduce

friction significantly. We hypothesize that upon increasing the temperature above LCST of

PNIPAm, the polymer chains dehydrate, rendering the PNIPAm-grafted CM surface to a

hydrophobic state. The latter causes the particles to aggregate and form clusters that

hamper the rolling ability of individual dispersed particles, resulting in high friction force

values as shown schematically in Figure 22.

49

Figure 22. Schematic illustration of the rolling mechanism of grafted CM

confined between two surfaces. Aggregation of the grafted CM at high

temperatures leads to increased friction.

6.1 Experimental Section

6.1.1 Carbon Microsphere (CM) Synthesis.

D-(+)-Glucose (Sigma Aldrich, Bio Reagent G7021) was dissolved in deionized (DI) water

(Millipore Direct-Q) to obtain a 0.5 M aqueous solution which served as the precursor to

synthesize the CM. The solution was stirred and sonicated to ensure complete dissolution

of the glucose. Once all the glucose was dissolved, the solution was transferred to a glass

vial and placed inside a stainless-steel pressure vessel. The latter was heated in an oven set

at 185 °C and heated for 4.5 h. The resulting black precipitate that formed in the solution

after the hydrothermal step was collected and sequentially washed with acetone and ethanol

with the help of a centrifuge. Samples were washed until the supernatant was clear. The

precipitate was then dried at 80 °C for 6 h in an oven under atmosphere. The dried

precipitate was then placed in a tube furnace and pyrolyzed at 800 °C for 10 h under an

inert argon atmosphere. All glassware and vials were cleaned by immersion in concentrated

sulfuric acid for 24 h followed by thorough rinsing in DI water.

6.1.2 Grafting of Carbon Microspheres with PNIPAm.

To functionalize the CM, an atomic transfer radical polymerization (ATRP) surface

“grafting from” process was used. For this purpose, first the CM surface was functionalized

with an initiator, 2-bromoisobutyryl bromide (BiBB), in a series of steps. In the first step,

3.0 g of CM were dispersed in 70% HNO3 (150 mL) and refluxed at 120 °C for 2 h to

50

introduce −COOH surface groups (denoted as CM-COOH). The CM-COOH particles were

then washed with water until a neutral pH was measured followed by drying in an oven. In

the second step, the CM-COOH particles were dispersed in thionyl chloride with the

concentration of 50 g/L and refluxed at 80 °C for 24 h. The liquid phase was evaporated

by a rotary evaporator. The solid was washed with tetrahydrofuran and dried at 100 °C for

2 h. The product is denoted as CM-COCl. After this step, 3.0 g CM-COCl was refluxed

with short chain dialcohol (ethylene glycol, 150 mL) at 120 °C for 48 h to introduce −OH

end groups, denoted as CM-OH. Lastly, esterification was carried out by reacting CM-OH

(2.0 g) with BiBB (16 g) to produce the initiator functional group for ATRP (CM-Br) (see

Figure 23).

51

Figure 23. Schematic illustration of the grafting process. (a, b) Macroinitiator

preparation, (c) polymerization of PNIPAm from the surface of the carbon

microsphere (CM).

52

PNIPAm was polymerized using the macro initiator (CM-Br, 2.0 g) in the presence of the

CuBr/PMDETA metal ligand system in 50 mL of DMF medium for 18 h. The reaction

conditions of CM-Br/NIPAm ratio was 1:6 (w/w), and NIPAM/CuBr/PMDETA ratio was

25:1:1 in moles. At the end of the reaction time, the mixture was transferred into excess

methanol and cleaned by centrifugation several times at 25000 rpm until the supernatant

became colorless. All the drying processes were performed under a vacuum using a freeze-

dryer (Christ Alpha 1-2 LD plus). Before drying under a vacuum, the sample was kept in

a deep freezer for 2 h. After each modification step, particles were separated and cleaned

using a centrifuge (Beckman Coulter Avanti J-25 I). The average sizes and corresponding

standard deviations of the bare and PNIPAm-grafted particles were measured by averaging

∼120 individual particles from SEM images (Figure 24 A, B).

53

Figure 24. SEM images of (A) bare carbon microspheres after pyrolysis, (B)

PNIPAm-grafted carbon microspheres. The scale bars are 500 and 300 nm for the

low and high magnification images, respectively

6.1.3 Friction Measurements.

A borosilicate lens (Anchor Optics, Barrington, NJ) with a radius of curvature of 7.75 mm

was used as the probe (or top shearing surface). A silicon wafer (University wafer, rms =

1.72 nm) was used as the bottom shearing surface. Both surfaces were sonicated in ethanol

and water, respectively, to remove any debris and/or organic contamination. The

borosilicate lens probe was also cleaned with an air plasma for 1 min. A universal material

54

tester (CETR nanotribometer, Campbell, CA) was used to measure friction forces between

the shearing surfaces with or without PNIPAm-grafted CM. An FL sensor (force range

5−50 g) was used with a spring attachment (spring constant = 520 N/m). The instrument

was programmed such that upon contact with the substrate, the borosilicate lens moves 10

mm to the right relative to the motionless bottom silicon surface at a speed of 0.5 mm/s

followed by moving back to its original position while maintaining a constant applied load.

The feedback mechanism in the instrument maintains the applied load while the force

sensor measures the resultant friction force (Fx) for a particular applied force (L). In a

typical experiment, a 40 μL drop of the sample (for e.g., aqueous PNIPAm-grafted CM)

was placed on the silicon wafer. The probe was then brought in contact with the drop, at

which point a wetting meniscus formed which ensured that the lubricant medium remained

present during the shearing motion. The system was allowed to equilibrate for 1 min prior

to the start of each experiment. The applied force (L) was varied from 5 g (49 mN) to 30 g

(294 mN), with increments of 5 g (49 mN), and the resultant friction force (Fx) was

measured for each of the predetermined applied loads. The reported friction force values

were obtained by averaging the friction forces in both directions, and each experiment was

run at least 5 times to calculate standard deviations. Various concentrations (1, 3, 5, and 10

mg/mL) of aqueous PNIPAm-grafted CM were prepared to test the effect of concentration

on the coefficient of friction (CoF). The resulting pH value of the aqueous solutions was

5.5. To perform the experiment at elevated temperatures, a hot plate was placed directly

underneath the silicon substrate. The hot plate was set at 45 °C, and the surface temperature

was measured at 40 °C using an infrared laser thermometer. Upon dispensing a drop of the

aqueous PNIPAm-grafted CM, the system was again allowed to equilibrate for 1 min and

55

the experiment was performed similarly as described earlier. The duration of each run was

approximately 4 min, during which no significant evaporation of the lubricant drop was

observed.

6.2 Results and Discussion

Figure 24 shows SEM images of the CM before and after grafting with PNIPAm. The CM

were produced using the method described by Wang et al.83 After the hydrothermal step,

the microspheres still have functional groups present84. However, the pyrolysis step

removes any functionality on the surface and renders them hydrophobic85. As seen in

Figure 24 A, the particles exist as singlets, which is essential so as to allow the particles to

roll as individual entities, thereby reducing friction.5 Figure 24 B shows SEM images of

PNIPAm-grafted CM. The grafting of PNIPAm onto the CM was achieved without

damaging, without causing aggregation or clustering of the individual particles. SEM

imaging was used to measure the particle sizes of the bare (240 ± 30 nm) and PNIPAm-

grafted CM (220 ± 43 nm). On the basis of these measurements, we cannot determine an

absolute thickness of the PNIPAm coating; however, we can estimate the thickness to be

on the order of 10s of nanometers or less. DLS measurements could not be performed

reliably or reproducibly on the bare carbon particles because of their aggregation behavior

as a result of being inherently hydrophobic.

56

Figure 25. DLS measurements of average particle size of PNIPAm-grafted CM

dispersed in deionized water within a temperature range from 25 to 50 °C. The

concentration of particles is 0.1 mg/ml. Each data point is an average of 6 individual

measurements along with their corresponding standard deviations

The average diameter of the PNIPAm-grafted CM obtained from DLS measurements did

not show a reliable change in particle size because of the relatively large standard deviation

of the data (see Figure 25) presumably because of multiple scattering and particle settling

effects. Nevertheless, the PNIPAm coating imparts hydrophilicity to the surface to the

particles68 and results in a ζ potential of −23 mV which allows the PNIPAm-grafted CM to

easily disperse in DI water without the aid of a surfactant. In addition, TGA and FTIR

measurements were also performed on the microspheres to further confirm the surface

modification (see Figures 26 and 27).

57

Figure 26. Thermogravimetry (TG) and derivative thermogravimetry (DTG) data.

Thermal gravimetric analysis (TGA) of CM and PNIPAm-grafted CM were carried

out by using a thermogravimetric analyzer (Seiko, SII EXSTAR6000) under dry air

at a flow rate of 150 ml/min and a heating rate of 15 °C/min. Approximately 4 mg of

the sample was placed in ceramic crucibles, and the weight loss was recorded over the

temperature range 50-900 °C

58

Figure 27. FTIR spectra of CM and PNIPAm-grafted CM

Figure 28 shows the friction forces as a function of applied load between a borosilicate lens

and a smooth silicon wafer in the presence of an aqueous solution containing PNIPAm

grafted CM at various concentrations.

59


spherical lens and a flat silicon wafer using an aqueous solution containing PNIPAm-

grafted CM at (A) 1 mg/mL, (B) 3 mg/mL, (C) 5 mg/mL, and (D) 10 mg/mL

concentrations. Data points marked by open circles (○) and filled circles (●)

correspond to measurements at 22 and 40 °C, respectively. Data points were fitted to

a straight line with the slope corresponding to the coefficient of friction (CoF). The

shear velocity is 0.5 mm/s over a distance of 10 mm. Error bars represent the standard

deviation of friction force obtained from at least 5 trials.

For each concentration, the friction experiment was performed at room temperature (i.e.,

22 °C) and repeated at a higher temperature (i.e., 40 °C). A similar trend was found in all

cases: the CoF is higher for high-temperature measurements compared to the room

temperature measurements. In the case of the aqueous lubricant having a CM concentration

of 1 mg/mL, a CoF of 0.37 was obtained at room temperature. The same experiment

60

performed at the higher temperature yielded a CoF of 0.47, which is even higher compared

to the CoF between the shearing surfaces in the absence of CM (i.e., pure water, CoF = 0.4;

see Figure S4). We can infer from these results that at a concentration of 1 mg/mL, the

PNIPAm-grafted CM aqueous lubricant only slightly reduces friction at room temperature

but is ineffective at the higher temperature. The fact that the CoF is even higher compared

to using pure DI water suggests that the particles add an additional contribution to the

friction force. Increasing the CM concentration in the aqueous lubricant to 3 mg/mL

improves the lubricity for both low and high temperature measurements. The CoF drops

even more as the concentration of the CM is increased to 5 mg/mL yielding a CoF of 0.04

at room temperature. Increasing the CM concentration beyond 5 mg/mL does not further

decrease the CoF at room temperature. The CoF values were obtained by fitting the data to

a modified version of Amontons’ equation 86

Fx= µ(L + L0 ) = µL + F0 [9]

where Lo is a constant “internal load” that is added to the external applied load L to account

for the new interactions generated as a result of having particles within the confined region

of the shearing surfaces. A positive value of Fo implies the presence of an internal attractive

force between the shearing surfaces while a negative value of Fo implies the presence of a

repulsive internal force between the shearing surfaces. In a control experiment (i.e., pure

DI water (Figure 29)), a fit of the data to equation 9 yielded a negative value for the y-axis

intercept suggesting the presence of a repulsive force-distance function between the

borosilicate lens and a silicon wafer in an aqueous environment.

61


spherical lens and a flat silicon wafer using DI water only at room temperature. Data

points were fitted to a straight line with the slope corresponding to the coefficient of

friction (CoF). The shear velocity is 0.5 mm/s over a distance of 10 mm. Error bars

represent the standard deviation of friction force obtaining from at least 5 trials.

To test the efficacy of grafted CM to reduce friction, another control experiment was

performed (Figure 30), in which bare CM (not grafted) were sheared under the following

conditions: a) In DI water b) In presence of 0.36 SDS (sodium dodecylsulfate)

A 10 mg/mL suspension of bare CM showed high friction (CoF=0.26) under aqueous

condition. Carbon Microspheres are inherently hydrophobic and tend to aggregate in DI

water to form clusters. This was evident by the immediate settling of particles after

sonication. As a result, the particles can not contribute to rolling and the friction force is

high. In order to disperse the particles under aqueous condition, a 0.36 M solution of

Sodium dodecylsulfate was made and bare Carbon Microspheres were dispersed in 10

62

mg/mL concentration. The CoF of bare CM in presence of SDS was 0.05. Due to the

presence of SDS, the particles were well dispersed and did not settle immidiently. This

allows the particle to roll effectively in the contact region and thus reducing friction. In our

project, grafting PNIPAm on Carbon Microspheres allows the particles to disperse readily

in water, without a need of a surfactant

Figure 30. Plot of Friction force Fx vs. Applied load L while shearing a spherical probe

on Silicon wafer in presence of: a)10 mg/mL of bare Carbon Microspheres (CM) in

DI water b)Bare CM in presence of 0.36M SDS

The addition of particles at low concentrations (i.e., 1 and 3 mg/mL) reduces the repulsive

interaction. Further increase in the particle concentration (i.e., 5 and 10 mg/mL) entirely

overcomes the repulsive interaction at low loads, and instead, an adhesive interaction is

measured: the y-intercept of fitted data to equation 9 in Figure 28 C,D is positive. These

results further indicate that a critical concentration of PNIPAm-grafted CM (in the range

of 3 to 5 mg/mL) is needed for the aqueous based lubricant to be effective. Below the

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300 350

Fric

tio

n F

orc

e F

x (m

N)

Load L (mN)

Bare CM no SDS

Bare CM with0.36M SDS

CoF=0.26

CoF=0.05

8

63

critical concentration, there are insufficient particles in the confined zone between the

shearing surfaces, and thus the particles cannot effectively contribute to lower friction

through a rolling mechanism. However, above the critical concentration, the PNIPAm-

grafted CM remain in the confined region and are able to lower friction. We hypothesize

that as the temperature of the system is increased above the LCST, the surface of the

PNIPAm-grafted CM becomes more hydrophobic, causing aggregation (see Figure 31).

64

Figure 31. Cryo-SEM images PNIMAm-coated CM. (A) Low temperature, and (B)

high temperature samples with a concentration of 1 mg/ml PNIMAm-coated CM

particles.

Figure 31 shows cryo-SEM images of PNIMAm-coated CM at low and high temperatures.

A 1 mg/ml PNIMAm-coated CM sample was prepared. Half of the sample volume was

65

left at room temperature while the other half was heated to 50°C. Since the aggregation

process is irreversible once the temperature of the sample is increased (unless the sample

is sonicated to re-disperse the particles), cryo-SEM captured the state (i.e., aggregates or

singlets) of the particles. We were unable to quantify the degree of aggregation. Although

aggregates were present at both low and high temperatures, the number and size of

aggregates were larger in the high-temperature sample. In doing so, clusters of particles are

formed, thereby losing their ability to effectively lower friction by rolling.

Figure 32 summarizes the CoF data for aqueous-based lubricants of 4 different PNIPAm

grafted CM concentrations.

Figure 32. Bar chart summarizing the CoF values for the PNIPAm-grafted CM-based

aqueous lubricants at various concentrations below and above the LCST

A closer look at the raw friction data provides greater insight into the lubrication

mechanism. During a shear cycle, the load and shear speed are maintained at predetermined

values while the probe shears in one direction followed by retracing the same path for the

66

second half of the cycle. At the end of the cycle, the applied load is increased to a new

value (denoted by the step increase in the load as shown in Figure 33, right axis) and the

next shear cycle begins. Figure 33A shows the friction force as a function of time using 1

mg/mL aqueous PNIPAm grafted CM as the lubricant from a typical experiment.

Figure 33. Plot of the friction force as a function of time using (A) 1 mg/mL and (B)

10 mg/mL concentrations of PNIPAm-grafted CM-based aqueous lubricant at room

temperature (gray data) and high temperature (black data). The stepwise increase in

67

the applied load as a function of time is superimposed on the graph. The shear

distance is 10 mm, and the shear velocity is 0.5 mm/s. The friction force data for the

room temperature experiments have been offset by 2 s to facilitate the visualization

of the stiction spikes

At this concentration, the lubricant is not effective, and the friction forces are high. The

variation in the friction force is large (i.e., friction spikes are present during sliding)

indicative of intermittent contact between the borosilicate probe and the substrate. At this

concentration, there are insufficient particles in the confined region to reduce friction and

the friction forces are dominated by the interaction of the borosilicate probe and the silicon

surface. However, by increasing the concentration of the PNIPAmgrafted CM aqueous

lubricant to 5 mg/mL, the friction force drops by a factor of 7 and smooth sliding is

observed at room temperature. At this concentration, particles remain in the confined

region and are able to reduce friction. Stiction spikes are present when the probe changes

shear directions which indicate that the probe has to overcome an initial larger friction

force before sliding. The stiction spikes are even larger when the experiment is run at high

temperatures. We speculate that the stiction spike originates from the adhesive interaction

of the PNIPAm-grafted CM with the hydrophilic shearing surfaces within the confined

region (i.e., surface-particle adhesive forces). At high temperatures, particle aggregates are

present in the confined region which lead to even higher stiction spikes because of the

surface-particle adhesive forces in addition to the particle−particle interactions. In previous

work, Zheng et al87 demonstrated that phospholipid liposome coated silk microspheres with

well-hydrated headgroups provided effective aqueous based lubrication. The surface

hydration of the PNIPAm-grafted CM at room temperature is also consistent with

68

providing smaller stiction spikes. At a concentration of 10 mg/mL, no significant decrease

in the friction force is measured at room temperature. However, the magnitude of the

stiction spike in the friction data at high temperatures is significantly reduced. We attribute

this reduction to the presence of a higher concentration of individual particles (i.e., singlets)

within the confined region. Presumably, at low temperatures, a single population of

individual, well-dispersed PNIPAm-grafted CM are present. Hydration forces provide the

repulsive force contribution to stabilize the particle dispersion. At a temperature above the

LCST, a second population of particle aggregates begin to form in addition to the individual

dispersed particles. The latter are still effective at reducing friction through a rolling

mechanism; however, the particle aggregates, if present in the confined region, are

detrimental to the rolling mechanism. We speculate that at higher concentrations of

PNIPAm-grafted CM, a larger population of individual dispersed particles exists which

can contribute to lowering friction through a rolling mechanism. Larger aggregates are also

expected; however, because of their size, they are excluded from the confined region. The

reversibility of the process as a function of temperature was also investigated. It was found

that once the temperature of the PNIPAm-grafted CM aqueous lubricant was increased, the

lower CoF was not recovered when the temperature was reduced back to room temperature.

However, if the solution was sonicated, the low CoF was recovered at room temperature.

We believe that the aggregation process is not reversible, and instead, the clusters of

particles remain trapped in the aggregated state unless energy is added to the system to

break the clusters. Colloidal systems are inherently kinetically stable88. When temperature

in the PNIPAm-grafted CM aqueous lubricant system is increased, the attractive

interactions (due to hydrophobic interactions) between the PNIPAm-grafted CM increase,

69

which in turn sufficiently lower the energy barrier to initiate and facilitate aggregation.

When the temperature is decreased, the aggregates are initially at their lowest (or primarily

minimum) energy state and therefore external energy (in the form of sonication) is required

to overcome the energy barrier so as to allow the particles to reside once again in the

secondary minimum.

A schematic of the proposed mechanism by which the PNIPAm-grafted CM aqueous

lubricant influences friction is shown in Figure 34 under the various conditions.

Figure 34. Schematic illustration of the proposed mechanism by which PNIPAm-

grafted CM affect the lubrication between two shearing surfaces at low and high

temperatures and under low and high concentrations of PNIPAm-grafted CM

dispersed in water. The upper surface moves in the x-direction relative to the bottom

surface at a velocity Vx.

At low concentrations, intermittent contact is made between the shearing surfaces because

of the lack of individual PNIPAm grafted CM within the confined region. The confined

PNIPAm-grafted CM experience high pressures and are potentially more susceptible to

damage the PNIPAm-grafted layer. When the temperature is increased above the LCST,

70

the particles form aggregates which do not contribute to lowering friction and instead

further impede the movement of the shearing surfaces (i.e., the probe has to plow away the

aggregates at the leading front) leading to an increase in the CoF. At higher concentrations,

a larger number of individual PNIPAm-grafted CM are present in the confined region

which effectively reduce friction by a rolling mechanism. The compliance of the softer

PNIPAm coating leads to an increased contact area between the shearing surfaces and the

confined PNIPAm-grafted CM which produces an internal adhesive force contribution and

also leads to the stiction spikes during the onset of motion. In the smooth sliding regime,

the rolling of the individual particles is not affected by neighboring particles. However, at

high temperatures, the friction force increases since there is a smaller population of

individual PNIPAm-grafted CM in the confined region and furthermore, the presence of

particle aggregates impedes the movement of the shearing surfaces.

6.3 Conclusions

In this study, we explored the use of PNIPAm-grafted carbon microspheres (CM) dispersed

in water as a stimulus responsive lubricant. It was found that a critical concentration

between 3 and 5 mg/mL PNIPAm-grafted CM was needed to achieve low friction

(coefficient of friction ∼ 0.04) at room temperature. Higher concentrations of PNIPAm-

grafted CM above the critical concentration did not further reduce friction at room

temperature. An increase in the temperature of the system above the lower critical solution

temperature (LCST) caused the aggregation of PNIPAm-grafted CM which led to an

increase in friction forces for all concentrations of PNIPAm grafted CM in water. The

process was not immediately reversible unless the lubricant was sonicated so as to

redisperse the aggregates. A mechanism to explain the lubrication properties of PNIPAm-

71

grafted CM was proposed which pointed toward the need of particle singlets at sufficiently

high concentrations within the confined region to achieve low friction through a rolling

mechanism.

6.4 Future Direction

In this research work, we successfully proved that by grafting PNIPAm on Carbon

Microspheres, friction could be changed upon application of temperature as a trigger. For

this particular research, the size of particles, as well as the polymer density, was kept

constant. For future projects, I propose using Carbon Microspheres of the same size and

studying the friction of these bare particles on Si wafer with varying surface roughness. A

standard Si wafer has a surface roughness of RMS 1-2 nm. The spherical particles can

effectively roll on a rough surface, provided their rolling is not hampered. If the particle is

too small compared to the roughness, it might not be able to effectively roll over the

obstacle; thus, we can expect the friction to be higher (see image below).

Varying levels of surface roughness can be created using a combination of lithography and

Reactive Ion Etching (see image below).

1

Low friction

Effective rolling

2

High friction

Particle stuck, no rolling

72

Once the relationship between the surface roughness and the frictional behavior is

established, PNIPAm of the same grafting density and length as used in this research, can

be grafted on these particles to test their rolling friction. PNIPAm at room temperature

shows a fully extended chain structure. Thus at high surface roughness values, the bare

particles that cannot roll effectively, can be expected to roll and overcome the obstacles

with the help of the polymer chains.

Ion etching

Solvent

washing to

remove

photoresist

Patterned

Silicon wafer

Grafted Carbon Microspheres rolling effectively on surface asperities

73

Appendix I

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300 350

Fric

tio

n F

orc

e Fx

(m

N)

Load L (mN)

Bare CM noSDS

0.36M SDS noCM

Bare CM with0.36M SDS

Plot of Friction force Fx vs. Applied load L while shearing a spherical probe on

Silicon wafer using a)bare Carbon Microspheres (CM) in absence of Sodium Dodecyl

sulfate (SDS) b)an aqueous 0.36M SDS solution as a boundary lubricant c) Bare CM

in presence of 0.36M SDS

74

BIOGRAPHY

Shreyas Oak was born on July 13, 1991 in Mumbai, India. He received his Bachelor of

Technology (B.tech) in Oils, Oleochemicals and Surfactants from Institute of Chemical

Technology (ICT), India in 2013. After completing his undergraduate education, Shreyas

came to the USA to pursue his PhD program in the Department of Chemical and

Biomolecular Engineering at Tulane University, New Orleans in June of 2013. He served

as a Research Assistant in Dr. Noshir Pesika’s lab where learned numerous techniques such

as Surface Modification, Lithography, Material Property Testing, Electron Microscopy etc.

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