Progress in Polymer Science 35(2010)357–401
Contents lists available at ScienceDirect
Progress in Polymer
Science
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /p p o l y s c
i
Carbon nanotube–polymer composites:Chemistry,processing,mechanical and electrical properties
Zdenko Spitalsky a ,1,Dimitrios Tasis b ,∗,Konstantinos Papagelis b ,Costas Galiotis a ,b
a FORTH/ICE-HT,Stadiou Str.,26504Rion Patras,Greece
b
Department of Materials Science,University of Patras,26504Rio Patras,Greece
a r t i c l e i n f o Article history:
Received 8January 2009
Received in revised form 9September 2009Accepted 15September 2009
Available online 25September 2009
a b s t r a c t
reaction between pvp and amino
Carbon nanotubes have long been recognized as the stiffest and strongest man-made mate-rial known to date.In addition,their high electrical conductivity has roused interest in the area of electrical appliances and communication related applications.However,due to their miniscule size,the excellent properties of these nanostructures can only be exploited if they are homogeneously embedded into light-weight matrices as those offered by a whole series
Abbreviations:ABS,acrylonitrile–butadiene–styrene copolymer;ACMA,acrylonitrile–methyl acrylate copolymer;AIBN,2,2 -azobisisobutyronitrile;ATRP,atom transfer radical polymerization;CNT,carbon nanotube;CPP,chlorinated polypropylene;DWCNT,double-walled carbon nanotube;EDS,energy-dispersive X-ray spectroscopy;EMMA,ethyl–methyl methacrylate copolymer;EPDM,ethylene–propylene–diene rubber;EVA,ethylene–vinyl acetate copolymer;EVOH,ethylene–vinyl alcohol copolymer;HDPE,high-density polyethylene;HMW,high molecular weight;LDPE,low den-sity polyethylene;LMW,low molecular weight;MA,maleic anhydride;MBMA,methyl–butyl methacrylate copolymer;MDPE,medium density polyethylene;MEMA,methyl–ethyl methacrylate copolymer;MPTS,m
ethacryloxypropyltrimethoxysilane;MWCNT,multi-walled carbon nanotube;NMP,nitroxide-mediated polymerization;NMR,nuclear magnetic resonance;P3HT,poly(3-hexylthiophene);P3OT,poly(3-octylthiophene);PA,poly-acetylene;PAA,poly(acrylic acid);PABS,poly(m-aminobenzene sulfonic acid);PAM,polyacrylamide;PAMAM,poly(amidoamine);PAN,poly-acrylonitrile;PANI,polyaniline;Parmax,poly(benzoyl-1,4-phenylene)-co -(1,3-phenylene);PBA,polybutyl acrylate;PtBA,poly(tert-butyl acrylate);PBMA,poly(butyl methacrylate);PBO,poly(phenylenebenzobisoxazole);PBT,poly(butyl terephthalate);PC,polycarbonate;PCL,polycaprolac-tone;PDEAEMA,poly[2-(diethylamino)ethyl methacrylate];PDI,polydispersity index;PDMEMA,poly[2-(dimethylamino)ethyl methacrylate];PDMS,polydimethylsiloxane;PDPA,polydiphenylamime;PE,polyethylene;PEG,polyethyleneglycol;PEI,polyethyleneimine;PEMA,poly(ethyl methacry-late);PEO,polyethyleneoxide;PET,poly(ethyl terephthalate);PETI,phenylethynyl-terminated imide;PGMA,poly(glycerol monomethacrylate);PHEMA,poly(2-hydroxyethyl methacrylate);PHET,poly[3-(2-hydroxyethyl)-2,5-thienylene];PHPMA,poly[N-(2-hydroxypropyl)methacrylamide];PI,polyimide;PIMA,poly(imidazolium methacrylate);PLLA,poly(l -lactic acid);PLLA-g-AA,poly(l -lactic acid)grafted with poly(acrylic acid)chains;PMDMAS,poly[3-(N-(3-methacrylamidopropyl)-N,N-dimethyl)ammoniopropanatesulfonate];PMMA,poly(methyl methacrylate);PMMAHEMA,poly[(methyl methacrylate)-co -(2-hydroxyethyl methacrylate)];PmPV,poly(
m-phenylenevinylene-co -2,5-dioctoxy-p-phenylenevinylene);PNIPAAm,poly(N-isopropylacrylamide);PP,polypropylene;PPE,poly(p -phenylene ethynylene);PPEI-EI,poly(propionylethylenimine-co -ethylimine);PPS,poly(phenylene sulfide);PPY,polypyrrole;PS,polystyrene;PSS,poly(sodium 4-styrenesulfonate);PSV,poly(styrene-co -p-(4-(4 -vinylphenyl)-3-oxobutanol));PTH,polythiophene;PU,polyurethane;PVA,poly(vinyl alcohol);PVAc,poly(vinyl acetate);PVAc-VA,poly(vinyl acetate-co -vinyl alcohol);PVC,poly(vinyl chloride);PVDF,poly(vinylidene fluoride);PVK,poly(N-vinyl carbazole);PVKV,poly(N-vinyl carbazole-co -p-(4-(4 -vinylphenyl)-3-oxobutanol));PVP,polyvinylpyrrolidone;P2VP,poly(2-vinylpyridine);P4VP,poly(4-vinylpyridine);RAFT,reversible addition-fragmentation chain transfer polymerization;ROP,ring opening polymerization;SAN,styrene–acrylonitrile copolymer;SBA,styrene–butyl acrylate copolymer;SBBS,styrene–butadiene–butylene–styrene copolymer;SBR,styrene–butadiene rubber;SCMS,styrene-p -chloromethylstyrene copolymer;SE,silicone elas-tomer;SEC,size exclusion chromatography;SIBS,poly(styrene-b-isobutylene-b-styrene);SMA,styrene maleic anhydride copolymer;STM,scanning tunneling microscopy;SWCNT,single-walled carbon nanotube;TDI,toluene diisocyanate;TEMPO,2,2,6,6-tetramethylpiperidinyl-1-oxy;TGA,thermo-gravimetric analysis;THF,tetrahydrofuran;UHMWPE,ultra high molecular weight polyethylene;WBPU,waterborne polyurethane.∗Corresponding author.Tel.:+302610969811;fax:+302610969368.E-mail address: (D.Tasis).1
Present address:Polymer Institute,Slovak Academy of Sciences,Dubravska Cesta 4,84236Bratislava,Slovak Republic.0079-6700/$–see front matter ©2009Elsevier Ltd.All rights reserved.doi:10.1016/j.progpolymsci.2009.09.003
358Z.Spitalsky et al./Progress in Polymer Science35(2010)357–401
Keywords:
Carbon nanotubes Polymers Composites Processing Mechanical properties Electrical properties of engineering polymers.We review the present state of polymer nanocomposites research in which thefillers are carbon nanotubes.In order to enhance their chemical affinity to engineering polymer matrices,chemical modification of the graphitic sidewalls and tips is necessary.In this review,an extended account of the various chemical strategies for graft-ing polymers onto carbon nanotubes and the manufacturing of carbon nanotube/polymer nanocomposites is given.The mechanical and electrical properties to date of a whole range of nanocomposites of various carbon nanotube contents are also reviewed in an attempt to facilitate progress in this emerging area.
©2009Elsevier Ltd.All rights reserved.
Contents
1.Introduction (358)
2.Modification of carbon nanotubes with polymers (359)
2.1.Method“grafting to” (360)
2.1.1.Ester linkage between oxidized CNTs and polymers (360)
2.1.2.Amide linkage between oxidized CNTs and polymers (361)
2.1.3.Grafting by a radical mechanism (362)
2.1.4.Nucleophilic addition/coupling reactions (363)
2.1.5.Cycloaddition (363)
2.1.6.Condensation (364)
2.1.7.Sonochemical reaction (365)
2.2.Method“grafting from” (365)
2.2.1.Atom transfer radical polymerization(ATRP) (365)
2.2.2.Reversible addition-fragmentation chain transfer(RAFT) (367)
2.2.3.Ring opening polymerization(ROP) (367)
2.2.4.Free radical polymerization (368)
2.2.5.Cationic/anionic polymerization (369)
2.2.6.Condensation polymerization (369)
2.2.7.Reduction/oxidation polymerization (371)
2.2.8.Metallocene catalysis polymerization (371)
2.2.9.Electrochemical grafting (371)
2.2.10.Nitroxide-mediated radical polymerization (371)
2.3.Mixed mechanism (372)
2.4.Endohedralfilling (372)
3.Composite processing (372)
3.1.Solution processing of CNTs and polymer (373)
3.2.Bulk mixing (373)
3.3.Melt mixing (373)
3.4.In situ polymerization (374)
3.5.CNT-basedfibers andfilms (375)
3.5.1.Compositefibers (375)
3.5.2.Compositefilms (375)
4.Mechanical properties of carbon nanotube/polymer composites (376)
4.1.Literature data (376)
4.2.General conclusions-remarks (376)
5.Electrical properties of carbon nanotube/polymer composites (383)
5.1.Literature data (383)
5.2.General conclusions-remarks (383)
Acknowledgements (391)
References (391)
1.Introduction
The properties and applications of carbon nanotubes (CNTs)and related materials have been very active research fields over the last decade[1–3].CNTs possess highflexi-bility,low mass density,and large aspect ratio(typically >1000),whereas predicted and some experimental data indicate extremely high tensile moduli and strengths for these materials.Individual single-walled carbon nanotubes (SWCNTs)can be metallic or semiconducting.The latter can transport electrons over long lengths without significant interruption which makes them more conductive than cop-per[4,5].It is indeed this c
ombination of mechanical and electrical properties of individual nanotubes that makes them the ideal reinforcing agents in a number of applica-tions.Thefirst ever polymer nanocomposites using CNTs asfillers were reported in1994by Ajayan et al.[6].Since then,there have been many papers dedicated to process-ing and resulting mechanical and/or electrical properties of fabricated polymer nanocomposites.However,as-grown
Z.Spitalsky et al./Progress in Polymer Science35(2010)357–401359
CNTs are normally mixtures of various chiralities,diame-ters,and lengths,not to mention the presence of impurities and other defects.Furthermore,CNT aggregation has been found to dramatically hamper the mechanical properties of fabricated nanocomposites.Finally,due to their small size, CNTs are normally curled and twisted,and therefore indi-vidual CNTs embedded in a polymer only exhibit a fraction of their potential.Thus,the superb properties of CNTs can-not as yet be fully translated into high strength and stiffness finished products.
In view of the preceding,there has been an immense effort to establish the most suitable conditions for the transfer of either mechanical load or electrical charge to individual nanotubes in a polymer composite component.
A prerequisite for such an endeavour is the efficient dis-persion of individual nanotubes and the establishment of a strong chemical affinity(covalent or non-covalent)with the surrounding polymer matrix.Various methods of CNT chemical modification have been proved quite success-ful in introducing functional moieties which contribute to better nanotube dispersion,and eventually to efficient thermodynamic wetting of nanotubes with polymer matri-ces[7].Another area of intense research is the grafting of macromolecules onto the nanotube surface.Indeed,the addition of a whole polymer chain is expected to have greater influence on the nanotube properties and their affinity to polymer matrices as compared to the addition of low molecular weight functionalities.The modification of CNTs by polymers is separated into two main cate-gories,based on whether the bonding to the nanotube surface is covalent or not.The covalent modification itself involves either“grafting to”or“grafting from”strategies [8–10].
Apart from improving the chemical affinity of CNTs to polymer matrices,the various modification strategies also assist in effective processing to form CNT/polymer compo-nents with enhanced mechanical or electrical properties. As is well known,any aggregation of CNTs in polymer composites results in inferior properties,as it prevents effi-cient stress transfer to individual nanotubes[11].So far,the majority of the processing methods lead to materials that contain low volume fractions of CNTs t
hat,at least in abso-lute mechanical property values,cannot seriously compete with commercial polymer composites.For electrical appli-cations,on the other hand,the percolation threshold is so low that large quantities of CNTs are not required and cost-effective composites can be fabricated[12].Indeed,a large number of processing techniques have already been attempted,and useful conclusions may be drawn from a systematic review of the current situation.
In terms of tensile modulus,it has been established by numerous studies[13]that chemically modified nanotubes exhibit a significant increase in modulus as compared to the matrix resin.As mentioned earlier,this is mainly due to the fact that functionalization improves both disper-sion and stress transfer.As yet however the values of strength improvement are disappointing,being orders of magnitude lower than the tensile strengths of CNTs,which range from60to150GPa[14].However,in some cases certain improvements are observed as a function of CNT functionalization and,most importantly,CNT volume frac-tion.All these results are fully reviewed in the subsequent sections.
With reference to electrical properties[12,15,16],the present review compares the results obtained from a great number of un-reinforced and CNT reinforced polymers. The results are indeed quite revealing;in most cases, an enhancement of the electrical conductivity by several orders of magnitude is obtained by the addition of CNTs. Although a very broad range of both thermosetting and thermoplas
tic matrices have been employed and system-atic trends are difficult to discern,it is evident that only small quantities of single or multi-wall carbon nanotubes are required to achieve relatively high values of electri-cal conductivity.Needless to say,that this result alone can guarantee the future commercial viability of CNT materials, provided of course that cost-effective dispersion methods are employed.Finally,the replacement of carbon black,the most commonly industrially usedfiller material,with CNTs for the preparation of electrically conducting polymer com-posites is expected to have a great impact on a wide range of industrial applications.
Obviously,it is impossible to make a comprehensive overview of all aspects of this large subject in the frame-work of one article.Therefore,to keep our task manageable, we confine ourselves to discussing the most characteristic and important recent examples,where the homogeneous dispersion of CNTs within polymer matrices plays a crucial role in the fabrication of multifunctional composites.More detailed information is available in topical reviews devoted to particular issues.
2.Modification of carbon nanotubes with polymers
As mentioned above,the modification of CNTs by poly-mers may be divided into two categories,involving either non-covalent or covalent bonding between CNT and poly-mer.Non-covalent
CNT modification concerns the physical adsorption and/or wrapping of polymers to the surface of the CNTs.The graphitic sidewalls of CNTs provide the pos-sibility for␲-stacking interactions with conjugated poly-mers,as well as organic polymers containing heteroatoms with free electron pair.The advantage of non-covalent functionalization is that it does not destroy the conjugated system of the CNT sidewalls,and therefore it does not affect thefinal structural properties of the material.
The second modification is the covalent chemical bond-ing(grafting)of polymer chains to CNTs,where strong chemical bonds between nanotubes and polymers are established.There are two main methodologies for the grafting of CNTs depending on building of polymer chains. The“grafting to”approach involves a synthesis of a polymer with a specific molecular weight terminated with reac-tive groups or radical precursor.In a subsequent reaction, the polymer chain is attached to the surface of nanotubes by addition reactions.A disadvantage of this method is that the grafted polymer content is limited because of the relatively low reactivity and high steric hindrance of macromolecules.In comparison,the“grafting from”approach involves growing polymers from CNT surfaces via in situ polymerization of monomers initiated by chemical species immobilized on the CNT sidewalls and CNT edges.
360Z.Spitalsky et al./Progress in Polymer Science 35(2010)
357–401
Fig.1.Oxidation of CNTs and derivatization reaction with amines or alcohols.
This type of polymerization is an example of reactions called surface-initiated polymerization.The advantage of this method is that the high reactivity of monomers makes efficient,controllable,designable,and tailored grafting fea-sible.
2.1.Method “grafting to”
In general,the “grafting to”approach involves pre-formed polymer chains reacting with the surface of either pristine,oxidized or pre-functionalized CNTs.The main approaches exploited in this functionalization strategy are radical or carbanion additions as well as cycloaddition reactions to the CNT double bonds.Since the curvature of the carbon nanostructures imparts a significant strain upon the sp 2hybridized carbon atoms that make up their framework,the energy barrier required to convert these atoms to sp 3hybridization is lower than that of the flat graphene sheets,making them susceptible to various addi-tion reactions.Therefore,to exploit this chemistry,it is only necessary to produce a polymer-centered transient in the presence of CNT material.Alternatively,defect sites on the surface of oxidized CNTs,as open-ended nanostruc-tures with terminal carboxylic acid groups,allow covalent linkages of oligomer or polymer chains [7].The “grafting to”method onto CNT defect sites means that the ready-made polymers with reactive end groups can react with the
functional groups on the nanotube surfaces.In most cases,polymer chains terminated with amino or hydroxyl moi-eties are attached by amidation or esterification reactions with the nanotube surface-bound carboxylic acid groups (Fig.1).In the following paragraphs (Sections 2.1.1and 2.1.2),various examples of the CNT defect chemistry will be discussed.In the subsequent paragraphs,the chemical interactions between polymers with reactive end groups and the conjugated network of CNT sidewalls will be doc-umented in detail.
An advantage of the “grafting to”method is that pre-formed commercial polymers of controlled molecular weight and polydispersity can be used.The main limitation of the technique is that initial binding of polymer chains sterically hinders diffusion of additional macromolecules to the CNT surface,leading to a low grafting density.Also,only polymers containing reactive functional groups can be used.
2.1.1.Ester linkage between oxidized CNTs and polymers
The grafting of both oxidized SWCNTs and MWCNTs with a polystyrene (PS)copolymer was reported by Sun and co-workers [17],where a solution of poly(styrene-co -p-(4-(4 -vinylphenyl)-3-oxobutanol))(PSV)in tetrahydrofuran (THF)was mixed with acyl chloride-activated nanotubes.According to thermogravimetric analysis (TGA),the CNT contents in the PSV-functionalized SWCNT and MWCNT
Z.Spitalsky et al./Progress in Polymer Science35(2010)357–401361
samples are approximately12%and18%,respectively.Sim-ilarly,Zehua and co-workers[18]grafted a styrene–maleic anhydride copolymer(SMA)onto MWCNTs and the modified material was incorporated into poly(vinyl chlo-ride)(PVC)matrix.Mechanical testing showed significant enhancements of the elongation at break and the impact strength.Alternatively,the reaction of hydroxy-terminated PS with thionyl chloride treated MWCNTs was performed by Baskaran et al.[19],resulting in a hybrid containing 86wt%of CNTs.
Poly(vinyl alcohol)(PVA)was grafted by carbodiimide-activated esterification reactions of oxidized SWCNTs and MWCNTs[20].Riggs et al.[21]reported the graft-ing of poly(vinyl acetate-co-vinyl alcohol)(PVAc-VA)via ester linkages to acyl-activated SWCNTs for measure-ment of the optical properties of the prepared modified nanotubes.Other PVA-based copolymers used for func-tionalization of nanotubes was poly(ethylene-co-vinyl alcohol)(EVOH)copolymer under carbodiimide-activated esterification reaction conditions[22].Nuclear magnetic resonance(NMR)spectra showed that nanotube content in the SWCNT-EVOH is about14wt%,whereas thermogravi-metric analysis showed10wt%.
Silicone-functionalized CNT derivatives were prepared by opening terminal epoxy groups of functionali
zed poly-dimethylsiloxanes(PDMS)by the carboxylic groups of acid-treated MWCNTs[23].The prepared composite con-tained about85wt%of CNTs[24].The derivative was found to be homogeneous and viscous,almost tar-like,liquid at room temperature.It was dispersible at high concentra-tions(10mg/ml)in toluene.Itsfluid-like nature is suitable for applications in ink-jet printing technology.
The esterification reaction was used for grafting polyethylene glycol(PEG)chains to acyl chloride-activated SWCNTs[25].Such modified nanotubes were found to modulate neurite outgrowth,indicating a potential appli-cation in nerve regeneration[26].In the absence of solvent medium,a grafting reaction of hydroxy-terminated PEG with thionyl chloride treated MWCNTs was performed at temperatures above the melting point of the polymer[19]. However,grafting efficiency was low,as the TGA analysis showed the presence of93wt%of nanotubes.
Both oxidized SWCNT and MWCNT material were tar-geted for carbodiimide-activated esterifications with a derivatized polyimide endcapped with alkoxysilane groups [27,28]or having pendant hydroxyl groups[29].Based on the1H NMR signal integrations in reference to stan-dard,the CNT content in the sample was estimated to be about35wt%[29].A similar approach was used in the case of a poly(N-vinyl carbazole)copolymer contain-ing pendant hydroxyl groups(PVKV)which was grafted to oxidized SWCNTs under typical reaction conditions for the esterification via acyl-activation reaction[30].The CN
T content in the sample was estimated to be∼20wt%. Wang and Tseng prepared segmented polyurethanes car-rying carboxyl groups in the chain extender[31],with this polymer undergoing esterification reaction with acyl chloride-activated MWCNTs.According to TGA data,the CNT content was up to67wt%.It was found that longer acid treatment time in the CNT material resulted in a higher grafted amount of polymer chains.
MWCNTs were functionalized with poly(l-lactic acid) (PLLA)with molecular weights ranging from1000to15,000 by grafting PLLA chains to acyl chloride-functionalized tubes[32].The amount of PLLA bound to MWCNTs ranged from25to53wt%depending on the molecular weight of PLLA.The maximum amount of grafted chains was reached in the case of the PLLA polymer having molecular weight 3000.A different approach was used by Wu and Liao[33], where the acyl chloride-functionalized MWCNTs were con-verted to hydroxy-modified MWCNTs(MWCNT-OH)with hexanediol and chemically bonded to acrylic acid moieties grafted to PLLA(PLLA-g-AA)by a melt blending method.
Grafting reactions of hydroxyl terminated poly(me-thyl methacrylate)(PMMA-OH)and poly[(methyl me-thacrylate)-co-(2-hydroxyethyl methacrylate)](PMMA-HEMA)with acyl chloride-activated MWCNTs were carried out in different solvents at various temperatures[34].It was found that at higher temperatures and longer reac-tion times favored the grafting reaction.Increasing the concentration rati
o of hydroxyl groups to acid-chloride groups did not improve the grafting efficiency.Other examples of esterification reactions include the grafting of poly(bisphenol-A-co-epichlorohydrin)chains to oxi-dized MWCNTs[35]by a reactive blending process and the grafting of hyperbranched polyester based on2,2-bis(methylol)propionic acid to the surfaces of MWCNTs [36].
2.1.2.Amide linkage between oxidized CNTs and
polymers
Sun and co-workers[21,37–39]reported the grafting of poly(propionylethylenimine-co-ethylimine)(PPEI-EI)to acyl-activated tubes by direct heating or carbodiimide-assisted amidation.The polymer-bound nanotubes were found to be luminescent with quantum yields up to11%. The luminescence properties were found to be indepen-dent from the chemical grafting approach[21].Scanning tunneling microscopy(STM)images confirmed that the polymer interacts with the whole length of the tube and not just at the CNT ends[40].TGA measurements determined a ∼30wt%content of polymer for the“acylation-amidation”route and∼40wt%of polymer for the“heating”method [39].Hu et al.[41]grafted branched polyethyleneimine (PEI)to acyl chloride-modified CNTs and they used the CNT adduct as a substrate for neurite outgrowth and branching. Two independent techniques(UV–vis–NIR and TGA)indi-
cated that SWCNTs constitute about20%by weight of the graft copolymer SWCNT-PEI.Neurons grown on SWCNT-PEI showed more branched neurites than those grown on the as-prepared MWCNTs.
Ge et al.[42]grafted oxidized MWCNTs with a non-fluorinated polyetherimide.The reaction occurred in the solid state at high temperatures under inert atmosphere without the addition of a catalyst or forcefields.The authors speculated that the polymer was grafted onto MWCNTs via not only the amide but also the imide linkages.They grafted about30wt%of polymer.Similarly,grafting of amine-terminated polyimide by carbodiimide-activated reaction of SWCNTs and MWCNTs was reported by Qu et al.[43].
Sano et al.[44]grafted monoamine-terminated poly(ethylene oxide)(PEO)(M w∼5000)to acyl-activated

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。