PREPARATION OF NANOCOMPOSIT

ES

CHINCHU KRISHNA

II MSc BPS

CBPST, KOCHI

A nanocomposite is as a multiphase solid material

where one of the phases has one, two or three

dimensions of less than 100 nanometers (nm),

OR

structures having nano-scale repeat distances

between the different phases that make up the

material.

NANOCOMPOSITES

Properties of Nanocomposites

Tiny particels with very high aspect ratio, and hence larger surface area.

Larger surface area enables better adhesion with the matrix/surface.

Improvement in the mechanical performance of the parent material.

Better transparency due to small size(>wavelength of light).

PREPARATION METHODS

In situ polymerisation

Melt mixing

Solution mixing

Precipitation

Sol-gel process

Electrospinning

These methods enable the final

product with the following

characteristics :

-Nano size particles

- Narrow particle size distribution

- High surface area- Homogenous

- Pure

-Improved properties

SOL – GEL PROCESS

Sol–gel synthesis is a very viable alternative

method to produce nanocrystalline elemental,

alloy, and composite powders in an efficient

and cost-effective manner.

Sol-gel process involve the formation of sol

,followed by a formation of gel.

Sol is a colloidal suspension of solid particles in a

liquid phase

Gel the interconnected network formed

between phases

The sol-gel process is a wet-chemical technique that

is widely used to deposit nanocomposite films.

In this process, sol (or solution) containing sources for

component materials, such as metal alkoxides and

metal chlorides precursors for metal oxides, metallic

nanoparticles for metals, tetraethoxysilane for silica

matrix, catalysers, stabilizers and other additives for

porosity generation, was prepared first.

The sol then undergoes hydrolysis and

polycondensation reactions to evolve gradually

towards the formation of a gel-like network .

The basic structure or morphology of the solid phase

can range anywhere from discrete colloidal particles

to continuous chain-like polymer networks.

The formation of the nanocomposite film from

the sol–gel precursor involves either dip coating

or spin coating on a substrate, decomposition

and pyrolysis of compounds, removal of water

and organics from the resulting network

followed by nucleation and growth of the

crystallites.

The thermal decomposition behaviour of the

gel precursor plays an important role in

crystallites size and in film porosity.

Sol-gel is an excellent technique for preparing

high purity multicomponent films. Various types

of nanocomposite films have been prepared by

the sol-gel process and used as active materials

for gas sensors.

For example

OHHClOHHCSiOHC 252452 )(

2SiO

OHHClOHHCSiOHC 252452 )(

With diameter20-50nm Nitration with NaOH

aerosol

Annealing with 600- 1000 C

Homogeneous mixture

Drying

Gel

Sol

Xerogel

Nitration with NH4OH

Drying in air

Glass of Nanoparticles

Drying with H in 1200 C and 1atm

The preparation of a silica glass begins with an appropriate

alkoxide which is mixed with water and a mutual solvent

to form a solution.

Hydrolysis leads to the formation of silanol groups (Si—OH).

These species are only intermediates.

Condensation reactions produce siloxane bonds (Si—O-Si).

The silica gel formed by this process leads to a rigid,

interconnected threedimensional network consisting of

submicrometer pores and polymeric chains.

During the drying process the solvent liquid is removed and

substantial shrinkage occurs.

The resulting material is known as a xerogel. When solvent

removal occurs under hypercritical (supercritical)conditions,

the network does not shrink and a highly porous, lowdensity

material known as an aerogel is producd.

Heat treatment of a xerogel at elevated temperature produce

viscous sintering

(shrinkage of the xerogel due to a small amount of viscous

flow) and effectively transforms the porous gel into a dense

glass.

During formation of gels, sample may adhere to wall and cause crack

Gel Drying

Gel drying period, can get kinetic data from weight loss

Similar to ordinary drying process, classified as (a) constant rate drying period; (b) reach a critical point (prone to cracking); (c) first falling rate period; (d) second falling rate period

To prevent cracking during drying, control drying rate (slow during certain period), some proposed to add “drying control chemical additive (DCCA) – objective: to lower capillary pressure, to lower solvent pressure; or to use supercritical evaporation method

A typical procedure for the preparation of PI/silica

hybrids via the sol-gel route,

The polyamic acid (PAA) (PI precursor) is formed by a polyaddition

reaction of a di anhydride [e.g., pyromellitic anhydride (PMDA), 3,3′,4,4′-

biphenyltetracarboxylic dianhydride(BPDA), or 2,2-bis(3,4-dicarboxy-

phenyl)-hexafluoropropane dianhydride (6FDA)] with a diamine [e.g.,

4,4′-oxydianiline (ODA), or p-phenylene diamine (PPA)] in a common

solvent [e.g., dimethylacetamide (DMAc), or N-methyl pyrolidone (NMP)].

The reaction and chemical structures of monomers are reported in fig.

Figure 1. Formation of polyamic acids, the precursor of

polyimides

This method consisted of the silica

precursor [e.g., tetraethoxysilane(TEOS)

or tetramethoxysilane (TMOS) is added to

the PAA solution, and the hydrolysis and

polycondensation is carried out using an

appropriate catalyst.

The PAA/silica solution is film cast by

drying the solvent, and then the film is

cured by successive heating treatments up

to 300°C.

The heating induces the imidization

reaction to convert PAA to PI and the

crosslinking of the siloxane

component to form a silica

network.

The prime requisite for obtaining

good quality in sol-gel process

Variation of PH

Temperature

Time

Concentration of Reagent

Concentration of Catalyzor

Phase transition Sol Gel

Drying

One of the most interesting advantages of

the use of sol-gel method is its compatibility

with polymers and polymerization processes,

which allows the formation of nanoparticles

in the presence of organic molecules.

Advantages of Sol-Gel Processes

Able to get uniform & small sized powder

Can get at low temperature high density glass, without high temperature re-crystallization

Can get new compositions of glass

New microstructure and composition

Easy to do coating for films

Can get objects or films with special porosity

For improved adhesion

Can get metal (inorganic) –organic composites

Can coat onto large area or complex shape objects

Can get fibers

ELECTROSPINNING

Electrospinning is a unique technique for

producing polymer nanofibers with nanofillers.

It is a promising method for producing an

extremely light weight coating, where

nanoscale polymer fibers with a large specific

area are produced on a substrate from a

polymer melt, solution, or dispersion.

The nanoparticle functionalities are

incorporated into the fibers, as with fillers in

polymers that reinforce or increase the

electrical conductivity of the fibers.

Every polymer solution and dispersion has a

unique fiber-forming limit, above which a

continuous fiber network is obtained.

The solvent evaporates from the polymer

dispersion jet as it travels to the substrate in

electrospinning, and a polymer nanofiber or -

drop nanocomposite deposits on the substrate.

A non-miscible third component can be used

to encapsulate the filler in emulsion drops

during travel. It must evaporate during the

travel from the electrode to the substrate.

The polymer solution and dispersion properties

influence the electrospun polymer/nanoparticle

fiber and network formation.

Bead-like polymeric structures may appear.

The fiber diameter, the network structure, and

the filler distribution along the fibers can be

controlled by modifying the dispersion

properties, such as

the molecular weight of the polymer,

the interactions between the component

concentration of the polymer and the filler in the

dispersion,

The processing parameters, such as

the voltage between electrodes

their distance

the feeding rate.

Filler particle agglomerates can disturb the flow

of a spinning dispersion and the formation of a

continuous jet.

Viscosity is one of the main factors influencing

composite formation in electrospinning

because it determines the formation of the

electrospun fiber and the network structure.

It is relatively easy to change viscosity by

1) changing the concentration of the filler and

polymer,

2) changing the feeding order of the

components during the mixing,

3) changing the pH of the solution,

4) tailoring the interactions between the

components by functionalizing the components

in the solution.

Electrospraying is a process similar to

electrospinning but is used when the viscosity of

the liquid is sufficiently low.

The electric charge draws the liquid from the

capillary nozzle in the form of a fine jet, which

eventually disperses into droplets

The droplets produced by electrospraying are

highly charged, usually close to one-half of the

Rayleigh limit, and can be smaller than 1 mm.

The size distribution of the droplets is usually

narrow,with low standard deviation.

Electrospraying can be used for the production

of small, nearly monodisperse particles when a

colloidal suspension of solid nanoparticles or a

solution of a material is sprayed .

In electrospraying the size of the droplets can be

controlled mainly by the liquid flow rate, and the

droplet charge by adjusting the voltage applied

to the nozzle.

The charged aerosol is selfdispersing, which

prevents the droplets from coagulation.

a) b)

Fig.ure 1. Schematic of the methods for the production of

nanocomposite mats: a) in a two-step process: electrospray deposition

after electrospinning; b) via simultaneous electrospinning and

electrospraying from two separate nozzles.

Experiment

Fibres were electrospun from a stainlesssteel capillary of 0.45 mm o.d. and 0.25 mm i.d. diameter, and 15 mm length, placed horizontally.

The fibres were collected on an aluminium drum of 60 mm diameter, covered with aluminium foil (10 mm thick), rotating at a rotational speed of about 3000 r.p.m.

The distance between the nozzle tip and the drum was about 120 mm.

In the simultaneous electrospraying process, a second capillary nozzle of 0.7 mm o.d. and 0.5 mm i.d. diameter was placed vertically above the drum.

This nozzle served as a source of nanoparticles which were deposited onto the fibres on the rotating drum. The distance between the nozzle tip and the drum was 50 mm.

In the two-step process: consecutive electrosprayingafter electrospinning, the electrospun nozzle was replaced with an electrospray nozzle.

The electrospinning process was carried out at a 1 ml/h flow rate of polymer solution for a time of 30 to 60 min.

The particle colloidal suspension was electrosprayed at a flow rate

of 0.5 ml/h for a time of 30 - 60 min.

The polymer solutions and particle suspensions were supplied to

the nozzles using two syringe pumps.

In these experiments the voltage and distances were adjusted for

each configuration and particle suspension separately in order to obtain a stable multijet mode.

Although the viscosity of the polymer solutions was not measured

due to the large volume of solution required for such

measurements.

The concentration of each polymer in a suitable solvent was chosen experimentally in order to obtain a stable electrospun jet

that produced even, bead and pore free fibres.

Before deposition, the particles were stirred for a time of 20 h in methanol with an addition of surfactant in order to stabilise the

suspension.

EXAMPLE :

Polyurethane/ montmorillonite (PU/MMT) nanocomposites were

electrospun and the effect of MMT on the morphology and physical

properties of the PU/MMT nanofiber mats were investigated for the

first time.

The average diameters of the PU/MMT nanofibers were ranged

from 150 to 410 nm.

The conductivities of the PU/MMT solutions were linearly increased

with increasing the content of MMT, which caused a decrease in the average diameters of the PU/MMT nanofibers.

The electrospun PU and PU/MMT nanofibers were not microphase

separated.

The exfoliated MMT layers were well distributed within the PU/MMT

nanofibers and oriented along the fiber axis.

When the PU/O-MMT nanofibers were annealed, the exfoliated

MMT layers hindered the microphase separation of the PU.

The electrospinning of PU/MMT nanocomposites resulted in PU nanofiber mats with improved Young's modulus and tensile strength.

a

)

b

)

Figure 3. SEM images of PVC electrospun fi bres with deposited MgO nanoparticles(particle concentration: 0.6 wt.%): a) Simultaneous electrospinning andelectrospraying process; b) Postspinning deposition.

a

)

b

)

Figure 4. SEM images of PVC electrospun fi bres with deposited Al2O3 nanoparticles (particle

concentration: 0.6 wt.%): a) Simultaneous electrospinning and electrospraying process; b) Postspinning

deposition.

CONCLUSION

The progress in nano composites is varied and covers many industries.

Nano Composites can be made with a variety of enhanced physical,

thermal and other unique properties.

They have properties that are superior to conventional micro scale

composites synthesized using simple and inexpensive techniques.

Materials are needed to meet a wide range of energy efficient applications

with light weight, high mechanical strength, unique color, electrical

properties and high reliability in extreme environments.

Applications could be diverse as biological implant materials, electronic

packages and automotive or aircraft components. Although some of the

properties will be common between the applications, others will be quite

different.

An electronic package polymer composite must be electrically insulating,

while an aircraft component may need to be electrically conductive to

dissipate charge from lighting strikes.