POLYMER MODIFICATION WITH CARBON

NANOTUBES

MENON LAKSHMI SURESH

M.SC BPS

INTRODUCTION Incorporation of carbon nanotubes (CNTs) into a polymer

matrix is a very attractive way to combine the mechanical and electrical properties of individual nanotubes with the advantages of plastics.

Carbon nanotubes are the third allotropic form of carbon and were synthesized for the first time by Iijima in 1991 .

Two types of CNTs are distinguished

1) Single-walled CNTs (SWCNTs) consist of a single graphenesheet wrapped into cylindrical tubes with diameters ranging from 0.7 to 2nm and have lengths of micrometers.

2) Multi-walled CNTs (MWCNTs) consist of sets of concentric SWCNTs having larger diameters . The unique properties of individual CNTs make them the ideal reinforcing agents in a number of applications.

Single- walled Multi-walled

But the low compatibility of CNTs set a strong limitation to disperse them in a polymer matrix.

Indeed, carbon nanotubes form clusters as very long bundles due

to the high surface energy and the stabilization by numerous of π−π electron interactions among the tubes.

Non covalent methods for preparing polymer/CNTs Nano

composites have been explored to achieve good dispersion and

load transfer .

The non-covalent approaches to prepare polymer/CNTs

composites via processes such as solution mixing , melt mixing,

surfactant modification, polymer wrapping , polymer absorption and in situ polymerization are simple and convenient but

interaction between the two components remains weak.

Relatively uniform dispersion of CNTs can be achieved in polar polymers such as nylon, polycarbonate and polyimide

because of the strong interaction between the polar moiety

of the polymer chains and the surface of the CNTs .

Moreover, it was found that MWNTs disperse well in PS and

form a network-like structure due to π-stacking interactions

with aromatic groups of the PS chains .

However, it is difficult to disperse CNTs within a non polar

polymer matrix such as polyolefins.

To gain the advantages of CNTs at its best, one needs: (i)

high interfacial area between nanotubes and polymer; &, (ii) strong interfacial interaction.

The mechanical properties of polyethylene (PE) reinforced by

carbon nanotubes do not improve significantly because the

weak polymer-CNT interfacial adhesion prevents efficient stress

transfer from the polymer matrix to CNT .

A strategy for enhancing the compatibility between nanotubes

and polyolefins consists in functionalising the sidewalls of CNT to

introduce reactive moieties and to disrupt the rope structure.

Functional moieties are attached to open ends and sidewalls to improve the solubility of nanotubes while the covalent polymer

grafting approaches, including ‘grafting to’ and ‘grafting from’

that create chemical linkages between polymer and CNTs, can

significantly improve dispersion and change their rheological

behaviour.

Methods to process polymer/carbon nanotubes composites

Several processing methods available for fabricating

CNT/polymer composites based on either thermoplastic or

thermosetting matrices mainly include

1)Solution mixing

2)melt blending

3 ) in situ polymerisation.

Schematic representation of different steps of polymer/CNTs composite

processing: (a) solution mixing ; (b) melt mixing;

(c) in situ polymerisation

CNTs are considered ideal materials for reinforcing fibres due to their exceptional mechanical properties.

Therefore, nanotube−polymer composites have potential applications in aerospace science, where lightweight robust materials are needed.

It is widely recognised that the fabrication of high performance nanotube−polymer composites depends on the efficient load transfer from the host matrix to the tubes.

The load transfer requires homogeneous dispersion of the filler and strong interfacial bonding between the two components. A dispersion of CNT bundles is called “macrodispersion” whereas a dispersion of individual nonbundled CNT is called a nanodispersion

Surface modifications of carbon nanotubes

with polymers

To address these issues, several strategies for the synthesis of such composites have been developed.

Currently, these strategies involve physical mixing in solution, in situ polymerisation of monomers in the presence of nanotubes, surfactant-assisted processing of composites, and chemical functionalisation of the incorporated tubes.

As mentioned earlier, in many applications it is necessary to tailor the chemical nature of the nanotube’s walls in order to take advantage of their unique properties.

For this purpose, two main approaches for the surface modification of CNTs are adopted

i.e. covalent and noncovalent, depending on whether or not covalent bonding between the CNTs and the functional groups and/or modifier molecules is involved in the modification surface process. Figure depicts a typical representation of such surface modifications.

COVALENT ATTACHMENT OF POLYMERS

Functionalisation of carbon nanotubes with polymers is a key issue to improve the interfacial interaction between CNTs and the polymer matrix when processing polymer/CNT nanocomposites.

The covalent reaction of CNT with polymers is important because the long polymer chains help to dissolve the tubes into a wide range of solvents even at a low degree of functionalisation.

There are two main methodologies for the covalent attachment of polymeric substances to the surface of nanotubes, which are defined as “grafting to” and ‘grafting from’ methods .

A disadvantage of this method is that the grafted polymer contents are limited because of high steric hindrance of macromolecules.

NONCOVALENT ATTACHMENT OF

POLYMERS The noncovalent attachment, controlled by thermodynamic

criteria , which for some polymer chains is called wrapping, can alter the nature of the nanotube’s surface and make it more compatible with the polymer matrix.

Non-covalent surface modifications are based mainly on weak interactions, such as van der Waals, π−π and hydrophobic interactions, between CNTs and modifier molecules.

Non-covalent surface modifications are advantageous in that they conserve sp2-conjugated structures and preserve the electronic performance of CNTs.

The disadvantage of noncovalent attachment is that the forces between the wrapping molecule and the nanotube might be weak, thus as a filler in a composite the efficiency of the load transfer might be low.

Different routes for nanotubes’ functionalisation: a) sidewall covalent

functionalisation ; b) defect-group covalent functionalisation ; c)

noncovalent polymer wrapping ; d) noncovalent pi-stacking

Carbon nanotubes nanocompositesbased on Polyolefins

Polyethylene (PE) is one of the most widely used commercial polymer due to the excellent combination of low coefficient of friction, chemical stability and excellent moisture barrier properties.

The combination of a soft polymer matrix such as PE with nanosized rigid filler particles may provide new nanocompositematerials with largely improved modulus and strength.

To improve the stiffness and rigidity of PE, CNTs can be used to make CNT/PE composites.

The mechanical properties of polyethylene (PE) reinforced by carbon nanotubes do not improve significantly because the weak polymer-CNT interfacial adhesion prevents efficient stress transfer from the polymer matrix to CNT .

.

The lack of functional groups and polarity of PE backbone

results in incompatibility between PE and other materials such

as glass fibres, clays, metals, pigments, fillers, and most

polymers.

A strategy for enhancing the compatibility between

nanotubes and polyolefins consists in functionalising the

sidewalls of CNT with polymers either by a ‘grafting to’ or a ‘grafting from’ approach.

As discussed before, the “grafting from” approach involves the

growth of polymers from CNT surfaces via in situ polymerisation of olefins initiated from chemical species immobilised on the

CNT

As an example, Ziegler-Natta or metallocene catalysts for ethylene

polymerisation can be immobilised on nanotubes to grow PE

chains from their surface.

However, covalent linkages or strong interactions between PE

chains and nanotubes cannot be created during polymerisation.

The “grafting to” technique involves the use of addition reactions

between the polymer with reactive groups and the CNT surface.

However, the synthesis of end-functionalized polyethylene (PE),

which is necessary in the “grafting to” approach, is difficult.

Another promising route for a chemical modification of MWCNTs

by PE is to use free radical initiators such as peroxides.

The general mechanism of free radical grafting of vinyl

compound from hydrocarbon chains detailed by Russell,

Chung and Moad seems to express a widespread view.

The grafting reaction starts with hydrogen abstraction by

alkoxyl radicals generated from thermal decomposition of the peroxide.

Then, the active species generated onto the hydrocarbon

backbone react with unsaturated bonds located on the MWCNTs surface.

This chemical modification is thus conceivable during reactive

extrusion because the radicals’ lifetimes (in the range of few

milliseconds) are compatible with typical residence time in an

extruder (around one minute).

Carbon Nanotube Polymer

Composites While there are limitless applications for these materials, we

are interested in radiation shielding and radiation resistant materials for use in the space industry.

Initially, focused on optically transparent single wall nanotoube (SWNT) polymer composites .

Three different in situ polymerization/sonication methods, heat, light and gamma radiation, were used to produce poly (methyl methacrylate) (PMMA) nanotube composites.

When these composites are dissolved in methylene chloride and immediately cast into films, they exhibit a high degree of transparency (fig. 1).

All of the composites in fig. 1 contain 0.26 wt% carbon

nanotubes.

The dark sample on the bottom right was made by melt

blending 0.26% carbon nanotubes with the PMMA in a

Banbury mixer. This illustrates the dramatic effect of dispersion quality on transparency.

The dispersion in a typical sonicated sample is depicted in

the SEM image shown in fig. 2.

The dispersion in a typical sonicated sample is depicted in the SEM image

shown in fig. 2.

Epoxy/CNT Nanocomposites

To improve dispersion of CNT in an epoxy matrix, Surfactants, for example

polyoxyethylene-8-lauryl, have been used to disperse CNT before their introduction

into a polymer matrix

Carbon Nanotube Polymer

Composites: A Review of Recent

Developments

Nanotube Composite

Materials

• Engineering MWNT composite materials

• Lighter, stronger, tougher materials

• Lighter automobiles with improved safety

• Composite armor for aircraft, ships and tanks

• Conductive polymers and coatings

• Antistatic or EMI shielding coatings

• Improved process economics for coatings, paints

• Thermally conductive polymers

• Waste heat management or heat piping

• Multifunctional materials

High Strength Fibers

To achieve a high strength nanotube fiber:

High strength nanotubes (> 100 GPa)

Good stress transfer from matrix to nanotube

Or, nanotube to nanotube bonding

High loadings of nanotubes

Alignment of nanotubes (< 5° off-axis)

Perfect fibers

Each defect is a separate failure site

Two Approaches for Surface

Modification of MWNTS

Non-covalent attachment of molecules

van der Waals forces: polymer chain wrapping

Alters the MWNT surface to be compatible with the bulk polymer

Advantage: perfect structure of MWNT is unaltered

mechanical properties will not be reduced.

Disadvantage: forces between wrapping molecule / MWNT maybe weak

the efficiency of the load transfer might be low.

Covalent bonding of functional groups to walls and caps

Advantage: May improve the efficiency of load transfer

Specific to a given system – crosslinking possibilities

Disadvantage: might introduce defects on the walls of the MWNT

These defects will lower the strength of the reinforcing component.

Polymer Wrapping

Polycarbonate wrapping of MWNT (Ruoff group)

Ding, W., et al., Direct observation of polymer sheathing in carbon

nanotube-polycarbonate composites. Nano Letters, 2003. 3(11): p.

1593-1597.

In Situ Polymerization of PAN

Acrylate-functionalized MWNT which have been carboxilated

Free-radical polymerization of acrylonitrile in which MWNTs are dispersed

Hope to covalentely incorporate MWNTs functionalized with acrylic groups

Strong Matrix Fiber

Interaction

SEM images of fracture surfaces indicate excellent interaction with PAN matrix, note ‘balling up’ of polymer bound to the MWNT surface. This is a result of elastic recoil of this polymer sheath as the fiber is fractured and these mispMWNTs are pulled out.

20 wt% MWNT/Carbon Fiber

PP/SWNT Fibers

SWNT were dispersed into polypropylene

via solution processing with dispersion via ultrasonic energy

melt spinning into filaments

40% increase in tensile strength at 1wt.% SWNT addition, to 1.03

GPa.

At higher loadings (1.5 and 2 wt%), fiber spinning became more

difficult

reductions in tensile properties

“NTs may act as crystallite seeds”

changes in fiber morphology, spinning behavior

attributable to polymer crystal structure.

SWNT/Polymer Fibers

PMMA

PP

PAN

Fabricated fibers with 1 to 10 wt% NT

Increases in modulus (100%+)

Increases in toughness

Increase in compressive strength

Decrease in elongation to break

Decreasing tensile strength

PBO/SWNT Fibers high purity SWNT (99% purity)

PBO poly(phenylene benzobisoxazole)

10 wt% SWNT

20% increase in tensile modulus

60 % increase in tensile strength (~3.5 GPa)

PBO is already a high strength fiber

40% increase in elongation to break

.

Conclusion The field of CNT polymers composite is currently undergoing

rapid developments.

Over the last few years it has been demonstrated that polymers can serve as efficient tools for engineering the

interfacial behaviour of CNT without damaging the unique

properties of individual tube.

Polymers were shown to be efficient tools for dispersing

separating, assembling & organizing CNT in different media.

It is evident that harnessing the unique physical properties

CNT require development of throughout understanding of complex polymer-CNT systems.

Conclusions Nanotubes are > 150 GPa in strength.

Strain-to-break of 10 to 20%

Should allow 100 GPa composites

Challenges still exist

Stress transfer / straining the tubes

Controlling the interface

Eliminating defects at high alignment

Work is progressing among many groups

REFRENCES Dispersion and functionalization of carbon nanotubes for polymer-based

nanocomposites, P.-C. Ma et al. / Composites: Part A 41 (2010) 1345–1367

“Transparent PMMA/SWNT Composites with Increased Dielectric Constants”, L. Clayton, T. Gerasimov, M. Meyyappan and J. P. Harmon, Advanced Functional Materials, Vol. 15, No. 1, 101, (2005). 4

Kumar, S., et al., Fibers from polypropylene/nano carbon fiber composites.Polymer, 2002. 43: p. 1701-1703.

Kumar, S., et al., Synthesis, Structure, and Properties of PBO/SWNT Composites.Macromolecules, 2002. 35: p. 9039-9043.

Sreekumar, T.V., et al., Polyacrylonitrile Single-Walled Carbon Nanotube Composite Fibers. Advanced Materials, 2004. 16(1): p. 58-61

Kearns, J.C. and R.L. Shambaugh, Polypropylene Fibers Reinforced with Carbon Nanotubes. Journal of Applied Polymer Science, 2002. 86: p. 2079-2084

THANK YOU !!!!!