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POLYMERS

STRUCTURE AND MECHANICAL BEHAVIOUR

2

What is a polymer?

Poly mermany repeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

repeat

unit

repeat

unit

repeat

unit

3

Ancient Polymer History

• Originally natural polymers were used

– Wood – Rubber

– Cotton – Wool

– Leather – Silk

• Oldest known uses

– Rubber balls used by Incas

– Noah used pitch (a natural polymer) for the ark

4

Polymer Composition

Most polymers are hydrocarbons– i.e. made up of H and C

• Saturated hydrocarbons– Each carbon bonded to four other atoms

CnH2n+2

C C

H

H HH

HH

5

Unsaturated Hydrocarbons

• Double & triple bonds relatively reactive – can form new bonds– Double bond – ethylene or ethene - CnH2n

• 4-bonds, but only 3 atoms bound to C’s

– Triple bond – acetylene or ethyne - CnH2n-2

C C

H

H

H

H

C C HH

6

Chemistry of PolymersAdapted from Fig.

14.1, Callister 7e.

Note: polyethylene is just a long HC

- paraffin is short polyethylene

7

Bulk or Commodity Polymers

10

MOLECULAR WEIGHT

iiw

iin

MwM

MxM

Mw is more sensitive to higher

molecular weights

Lower M higher M


xi: number fraction of chains

wi:weight fraction of chains

Mi:Mean molecular weight of size range i.

The melting or softening temperature increases

with increasing molecular weight (for up to about

100,000 g/mol)

At room temperature, polymers with very short

chains (having molecular weights on the order of

100 g/mol) exist as liquids

or gases.

Other properties that depend on molecular weight

include elastic modulus and strength

Example: A polymer has three species of molecular

weights: 3 × 106, 4 × 106, and 5 × 106. Compute its

number-averaged molecular weight Mn and weight-

averaged molecular weight Mw .

iin MxM

iiw MwM

Solution:

)10x5(3

1)10x4(

3

1)10x3(

3

1 666

610x4

)10x5(12

5)10x4(

12

4)10x3(

12

3 666

610x17.4

13

• Covalent chain configurations and strength:

Direction of increasing strength


Molecular Structures

B ranched Cross-Linked NetworkLinear

secondarybonding

14

Polymer CrystallinityEx: polyethylene unit cell

• Crystals must contain the polymer chains in some way

– Chain folded structure

10 nm

Adapted from Fig.

14.10, Callister 7e.

Adapted from Fig.

14.12, Callister 7e.

15

Polymer CrystallinityPolymers rarely 100% crystalline

• Too difficult to get all those chains aligned

• % Crystallinity: % of material

that is crystalline.-- TS and E often increase

with % crystallinity.

-- Annealing causes

crystalline regions

to grow. % crystallinity

increases.


(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,

and J. Wulff, The Structure and Properties of

Materials, Vol. III, Mechanical Behavior, John Wiley

and Sons, Inc., 1965.)

crystalline region

amorphousregion

Effect of CrystallinityCyrstallinity

elongation, toughness

stiffness, strength, thermal stability, chemical resistance

Density, crystallinity and tensile strength in polyethylene

Increasing the crystallinityThe strength of semi-crystalline polymers is larger than that of amorphous polymers. If

the crystallinity of a polymer can be increased, the mechanical properties are improved accordingly.

The crystallinity of a polymer can be changed by the manufacturing process and by the

structure of the chain molecules. Upon cooling from the melt, crystalline regions can only form if

there is sufficient time to arrange the molecules in the energetically favourable more densely

packed crystal structure. Crystallinity is thus a function of the cooling speed. If this speed is too

high, the polymer is purely amorphous. This is analogous to the production of glasses or to

precipitation and transformation processes (see section 6.4.4).

The crystallinity can also be increased by orienting the chain molecules under

mechanical loads. By applying a tensile load, the fibres are drawn and straightened, forming

crystalline fibres bundles.

The size of the side chains and thus the mobility of the chain molecules also influences

the crystallinity. The more immobile the molecules are, the more difficult it is to arrange them in a

closely packed and regular manner.

Polymers with stiff chain molecules usually have a lower crystallinity than those made of

mobile chains. Exceptions from this rule do occur, however: Due to its very stiff and straight chain

molecules, PTFE can reach crystallinity values of up to 90%. This is only possible if the cooling

speed is very low; in technical applications, the crystallinity is therefore usually less.

The molecules can also be oriented during cooling from the melt by shearing the melt

with high speeds because this will also align the molecules. This has to be kept in mind when

designing polymer components manufactured by injection moulding to avoid a strong fluctuation of

the crystallinity in the final component.

From: Mechanical Behaviour of Engineering Materials: Metals Ceramics Polymers and Composites

18

Melting vs. Glass Transition Temp.What factors affect Tm and Tg?

• Both Tm and Tg increase with increasing chain stiffness

• Chain stiffness increased by

1. Bulky sidegroups

2. Polar groups or sidegroups

3. Double bonds or aromatic chain groups

• Regularity – effects Tm only

Adapted from Fig. 15.18,

Callister 7e.

• Thermoplastics:

-- little crosslinking

-- ductile

-- soften w/heating

-- polyethylene

polypropylene

polycarbonate

polystyrene

• Thermosets:

-- large crosslinking

(10 to 50% of mers)

-- hard and brittle

-- do NOT soften w/heating

-- vulcanized rubber, epoxies,

polyester resin, phenolic resin

Adapted from Fig. 15.19, Callister 7e. (Fig.

15.19 is from F.W. Billmeyer, Jr., Textbook of

Polymer Science, 3rd ed., John Wiley and

Sons, Inc., 1984.)

Thermoplastics vs. Thermosets

Callister, Fig. 16.9

T

Molecular weight

Tg

Tmmobile liquid

viscousliquid

rubber

tough plastic

partially crystalline solid

crystalline solid

20

Mechanical Properties• i.e. stress-strain behavior of polymers

brittle polymer

plastic

elastomer

U of polymer ca. 10% that of metals

Strains – deformations > 1000% possible

(for metals, maximum strain 100% or less)

elastic modulus

– less than metal


Callister 7e.

23

Tensile Response: Brittle & Plastic

brittle failure

plastic failure

(MPa)

e

x

x

crystalline

regions

slide

fibrillar

structure

near

failure

crystalline

regions align

onset of

necking

Initial

Near Failure

semi-

crystalline

case

aligned,

cross-

linked

case

networked

case

amorphous

regions

elongate

unload/reload

Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from

Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, Prentice-

Hall, Inc., 1974, pp. 500-501.)

24

Predeformation by Drawing• Drawing…(ex: monofilament fishline)

-- stretches the polymer prior to use

-- aligns chains in the stretching direction

• Results of drawing:

-- increases the elastic modulus (E) in the

stretching direction

-- increases the tensile strength (TS) in the

stretching direction

-- decreases ductility (%EL)

• Annealing after drawing...

-- decreases alignment

-- reverses effects of drawing.• Compare to cold working in metals!

Adapted from Fig. 15.13, Callister

7e. (Fig. 15.13 is from J.M.

Schultz, Polymer Materials

Science, Prentice-Hall, Inc.,

1974, pp. 500-501.)

25

• Compare to responses of other polymers:

-- brittle response (aligned, crosslinked & networked polymer)

-- plastic response (semi-crystalline polymers)

Stress-strain curves

adapted from Fig. 15.1,

Callister 7e. Inset

figures along elastomer

curve (green) adapted

from Fig. 15.15, Callister

7e. (Fig. 15.15 is from

Z.D. Jastrzebski, The

Nature and Properties of

Engineering Materials,

3rd ed., John Wiley and

Sons, 1987.)

Tensile Response: Elastomer Case(MPa)

e

initial: amorphous chains are kinked, cross-linked.

x

final: chainsare straight,

stillcross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failurex

x

26

• Decreasing T...

-- increases E

-- increases TS

-- decreases %EL

• Increasing

strain rate...

-- same effects

as decreasing T.

Adapted from Fig. 15.3, Callister 7e. (Fig. 15.3 is from T.S. Carswell and

J.K. Nason, 'Effect of Environmental Conditions on the Mechanical

Properties of Organic Plastics", Symposium on Plastics, American Society

for Testing and Materials, Philadelphia, PA, 1944.)

T and Strain Rate: Thermoplastics

20

4 0

6 0

8 0

00 0.1 0.2 0.3

4°C

20°C

40°C

60°Cto 1.3

(MPa)

e

Data for the

semicrystalline

polymer: PMMA

(Plexiglas)

Schematic of the different

types of stress–strain curves

in a polymer.

Effect of strain rate and

temperature on stress–strain

curves.

a) Even though they have similar structures, glass fibers

are much stronger than the window glass. Why?

b) Polymer fibers are also much stronger than bulk

polymers. Why?

2014 Final:

Answers:

Answers:

On the other hand, random polymer chains with partial crystallinity

decreases the strength of bulk polymers.

VISCOELASTIC DEFORMATIONLoad versus time, where load is

applied instantaneously at time taand released at tr .

the strain-versus time response

for totally elastic material

the strain-versus time response

for viscoelastic material

str

ess

strain

str

ess

strain

• Stress relaxation test:

-- strain to eo and hold.

-- observe decrease in

stress with time.

o

r

ttE

e

)()(

• Relaxation modulus: • Sample Tg(C) values:

PE (low density)

PE (high density)

PVC

PS

PC

- 110

- 90

+ 87

+100

+150

Selected values from

Table 15.2, Callister

7e.

Time Dependent Deformation

time

strain

tensile test

eo

(t)

• Data: Large drop in Erfor T > Tg.

(amorphous

polystyrene)

Adapted from Fig.

15.7, Callister 7e.

(Fig. 15.7 is from

A.V. Tobolsky,

Properties and

Structures of

Polymers, John

Wiley and Sons, Inc.,

1960.)

103

101

10-1

10-3

105

60 100 140 180

rigid solid (small relax)

transition region

T(°C)Tg

Er (10s)

in MPa

viscous liquid (large relax)

Mechanical Models

Elastic

(spring)

Pure viscous

(dashpot or damping)

str

ain

time

str

ain

time

Mechanical Models (Viscoelastic flow

(Maxwell) (Voigt)

str

ain

time

str

ain

time

Mechanical Models

Four element viscoelastic model

A specimen made of a certain material is subjected to a tensile stress

at time 0 and the stress is removed at time t0. The strain behavior of the

specimen is given below as a function of time. What type of material

would that be? Explain the graph.

2011 Final:

Answer:

40

Polymer Fracture

fibrillar

bridges

microvoids crack

alligned chains


Callister 7e.

Crazing Griffith cracks in metals

– spherulites plastically deform to fibrillar structure

– microvoids and fibrillar bridges form

Strengthening Mechanismsa) Composites

b) Alignment of polymer chains

c) Increase crystallinity

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