Polymer Layered Silicate Nanocomposites

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Item 1Macromolecules

33, No.6, 21st March 2000, p.2171-83EFFECT OF THERMAL HISTORY ON THE RHEOLOGICALBEHAVIOR OF THERMOPLASTIC POLYURETHANESPil Joong Yoon; Chang Dae HanAkron,University

The effect of thermal history on the rheological behaviour of ester- andether-based commercial thermoplastic PUs (Estane 5701, 5707 and 5714from B.F.Goodrich) was investigated. It was found that the injectionmoulding temp. used for specimen preparation had a marked effect on thevariations of dynamic storage and loss moduli of specimens with timeobserved during isothermal annealing. Analysis of FTIR spectra indicatedthat variations in hydrogen bonding with time during isothermal annealingvery much resembled variations of dynamic storage modulus with timeduring isothermal annealing. Isochronal dynamic temp. sweep experimentsindicated that the thermoplastic PUs exhibited a hysteresis effect in theheating and cooling processes. It was concluded that the microphaseseparation transition or order-disorder transition in thermoplastic PUs couldnot be determined from the isochronal dynamic temp. sweep experiment.The plots of log dynamic storage modulus versus log loss modulus variedwith temp. over the entire range of temps. (110-190C) investigated. 57 refs.GOODRICH B.F.USA

Accession no.771897

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ISBN 1-85957-391-6

Polymer/Layered SilicateNanocomposites

Masami Okamoto

(Toyota Technological Institute)

Polymer/Layered Silicate Nanocomposites

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Contents1. Introduction .............................................................................................................................................. 3

2. Layered Silicates ...................................................................................................................................... 3

2.1 Structure and Properties .................................................................................................................. 3

2.2 Organophilic Modification ............................................................................................................. 5

3. Preparative Methods for PLS Nanocomposites .................................................................................... 5

3.1 Intercalation of Polymer or Pre-Polymer from Solution ................................................................ 6

3.2 In Situ Intercalative Polymerisation Method .................................................................................. 6

3.3 Melt Intercalation Method .............................................................................................................. 6

4. Structure and Characterisation of PLS Nanocomposites .................................................................... 6

4.1 Structure of PLS Nanocomposites .................................................................................................. 6

4.2 Characterisation of PLS Nanocomposites ...................................................................................... 7

5. Types of Polymers for the Preparation of Nanocomposites ................................................................ 7

5.1 Vinyl Polymer Systems ................................................................................................................... 75.1.1 PS/LS Nanocomposites ...................................................................................................... 95.1.2 PMMA/LS Nanocomposites .............................................................................................. 95.1.3 PVA/LS Nanocomposites ................................................................................................. 105.1.4 Block Copolymer/LS Nanocomposites ............................................................................ 10

5.2 Condensation Polymers and Rubbers ............................................................................................115.2.1 Nylon/LS Nanocomposites .............................................................................................. 125.2.2 PCL/LS Nanocomposites ................................................................................................. 145.2.3 PET/LS Nanocomposites ................................................................................................. 145.2.4 PBT/LS Nanocomposites ................................................................................................. 145.2.5 PC/LS Nanocomposites .................................................................................................... 155.2.6 PEO/LS Nanocomposites ................................................................................................. 155.2.7 LCP/LS Nanocomposites ................................................................................................. 165.2.8 PBO/LS Nanocomposites ................................................................................................. 165.2.9 EPR/LS Nanocomposites ................................................................................................. 165.2.10 PU/LS Nanocomposites ................................................................................................... 225.2.11 Polyimide/LS Nanocomposites ........................................................................................ 22

5.3 Polyolefins .................................................................................................................................... 225.3.1 PP/LS Nanocomposites .................................................................................................... 225.3.2 PE/LS Nanocomposites .................................................................................................... 24

5.4 Speciality Polymers ...................................................................................................................... 245.4.1 PANI/LS Nanocomposites ................................................................................................ 255.4.2 PNVC/LS Nanocomposites .............................................................................................. 25

5.5 Biodegradable Polymers ............................................................................................................... 255.5.1 PLA/LS Nanocomposites ................................................................................................. 255.5.2 PBS/Clay Nanocomposites .............................................................................................. 28


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6. Properties of PLS Nanocomposite Materials ...................................................................................... 29

6.1 Dynamic Mechanical Analysis (DMA) ........................................................................................ 296.2 Tensile Properties .......................................................................................................................... 32

6.3 Flexural Properties and Heat Distortion Temperature .................................................................. 33

6.4 Thermal Stability .......................................................................................................................... 36

6.5 Fire Retardant Properties .............................................................................................................. 36

6.6 Gas Barrier Properties ................................................................................................................... 37

6.7 Ionic Conductivity ........................................................................................................................ 38

6.8 Optical Transparency .................................................................................................................... 39

6.9 Biodegradability ............................................................................................................................ 39

6.10 Crystallisation ............................................................................................................................... 416.10.1 Spherulite Growth ............................................................................................................ 416.10.2 Effect of Intercalation on Enhancement of Dynamic Modulus ....................................... 416.10.3 Crystallisation Controlled by Silicate Surfaces ............................................................... 42

7. Melt Rheology ........................................................................................................................................ 42

7.1 Linear Viscoelastic Properties ...................................................................................................... 42

7.2 Elongational Flow and Strain-Induced Hardening ....................................................................... 45

8. Processing Operations ........................................................................................................................... 478.1 Foam Processing Using Supercritical CO2 ................................................................................................................................................ 47

8.2 Shear Flow Processing .................................................................................................................. 47

8.3 Electrospinning ............................................................................................................................. 47

8.4 Porous Ceramic Materials ............................................................................................................. 48

9. Multifunctional Polyhedral Oligomeric Silsesquioxane Nanocomposites ....................................... 48

10. Carbon Nanotube Polymer Composites .............................................................................................. 49

11. Outlook ................................................................................................................................................... 49

Additional References ................................................................................................................................... 49

Abbreviations and Acronyms ....................................................................................................................... 51

Abstracts from the Polymer Library Database .......................................................................................... 53

Subject Index ............................................................................................................................................... 153Company Index ............................................................................................................................................ 163

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1 Introduction

Polymer/layered silicate (PLS) nanocomposites havereceived a great deal of attention during the pastdecade. They often exhibit attractive improvementof material properties (393) when compared withpure polymer or conventional composites (bothmicro- and macro-composites). These improvementscan include, high moduli (a.1), increased strengthand heat resistance (a.2), decreased gas permeability(410) and flammability (a.3) and increasedbiodegradability of biodegradable polymers (a.4).On the other hand, these materials have also beenproved unique model systems to study the structureand dynamics of polymers in confined environments(175, 176, 394).

The main reason for these improved properties isinterfacial interaction between the polymer matrix andorganically modified layered silicate (OMLS) asopposed to conventional composites. Layered silicates(LSs) have layer thickness in the order of 1 nm andvery high aspect ratios (e.g., 10-1000). A few weightpercent of OMLS that is properly dispersedthroughout the matrix thus creates a much highersurface area for polymer-filler interfacial interactionsthan in conventional composites (130).

Although the intercalation chemistry of polymerswhen mixed with appropriately modified layeredsilicate and synthetic layered silicates has been knownfor a long time (a.5, a.6), the field of PLSnanocomposites has gained large momentum recently.Two major findings stimulated the revival of thesematerials. First, Usuki, Fukushima and theircolleagues of Toyota Central Research &Development Co. Inc. (TCRD) successfully prepared,for the first time, exfoliated Nylon 6/LS hybrid (NCH)via in situ polymerisation of -caprolactam, in whichalkylammonium-modified montmorillonite (MMT)

was thoroughly dispersed in advance (a.7, a.8). Theresulting composite with a loading of only 4.2 wt%clay possessed a doubled modulus, a 50%-enhancedstrength, and an increase in heat distortion temperature(HDT) of 80 C compared to the neat Nylon 6, asshown in Table 1. Second, Vaia and co-workers (408)found that it is possible to melt-mix polymers withlayered silicates without the use of organic solvents.Today, efforts are being conducted globally usingalmost all types of polymer matrices.

This review is intended to highlight the majordevelopments in this area during the last decade. Thedifferent techniques used to prepare PLSnanocomposites, their physicochemical characterisation,and the improved materials properties that thosematerials can display; the processing and probableapplications of PLS nanocomposites will be reportedin detail.

2 Layered Silicates

2.1 Structure and Properties

The commonly used layered silicates for thepreparation of PLS nanocomposites belong to the samegeneral family of 2:1 layered- or phyllosilicates. Theircrystal structure consists of layers made up of two silicatetrahedral layers fused to an edge-shared octahedralsheet of either aluminium or magnesium hydroxide.The layer thickness is around 1 nm and the lateraldimensions of these layers may vary from 30 nm toseveral microns and even larger depending on theparticular layered silicate. Stacking of the layers leadsto a regular van der Waals gap between the layers calledthe interlayer or gallery. Isomorphic substitution withinthe layers (for example, Al+3 replaced by Mg+2 or byFe+2, or Mg+2 replaced by Li+1) generates negative

)HCN(dirbyhyalc/6nolyNfoseitreporplamrehtdnalacinahceM1elbaTseitreporP 6nolyN etisopmoconaN taeN 6nolyN

)%tw(tnetnocyalC 2.4 0ytivargcificepS 51.1 41.1

)aPM(htgnertselisneT 701 96)aPG(suludomelisneT 1.2 1.1

m/Jk(tcapmI 2) 8.2 3.2(TDH )aPM8.1taC 741 56


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charges that are counterbalanced by alkali and alkalineearth cations situated inside the galleries. The type oflayered silicate is characterised by a moderate surfacecharge (known as cation exchange capacity (CEC), andgenerally expressed by mequiv/100 g). This charge isnot locally constant as it varies from layer to layer andmust rather be considered as an average value over thewhole crystal. MMT, hectorite, and saponite are themost commonly used layered silicates. LSs have twotypes of structure, i.e., tetrahedral-substituted andoctahedral substituted. In the case of tetrahedralsubstituted layered silicates the negative charge islocated on the surface of the silicate layers, and hence,the polymer matrices can react with tetrahedral-substituted silicate more readily compared to

octahedral-substituted. Details regarding the structureand chemistry of these layered silicates are providedin Figure 1 and Table 2, respectively.

There are two particular characteristics of layeredsilicates that we generally consider in PLSnanocomposites. The first is the ability of the silicateparticles to disperse into the individual layers. Thesecond characteristic is the ability to fine-tune theirsurface chemistry through ion exchange reactions withorganic and inorganic cations. These two characteristicsare, of course, interrelated since the degree of dispersionof layered silicate in a particular polymer matrixdepends on the interlayer cation.

Figure 1Structure of 2:1 phyllosilicates

(Adapted from (a.9), S. Shinha Ray. et al., Macromolecules, 2003, 36, 2355, with permission from the AmericanChemical Society)

setacilisollyhp1:2desuylnommocfosretemarapcitsiretcarahcdnaalumroflacimehC2elbaTsetacilisollyhP1:2 alumroflacimehC a )g001/viuqem(CEC )mn(htgnelelcitraP

etinolliromtnoM Mx lA( x-4 gM x iS) 8O 02 )HO( 4 011 051-001etirotceH Mx gM( x-6 iL x iS) 8O 02 )HO( 4 021 003-002etinopaS Mx gM 6 iS( x-8 lA x iS) 8O 02 )HO( 4 6.68 06-05

a.)3.1dna3.0neewteb(noitutitsbussuohpromosifoeerged=x;noitactnelavonom=M


5

2.2 Organophilic Modification

Any physical mixture of a polymer and layered silicate,however, does not form a nanocomposite. This situationis analogous to polymer blends, and in most casesseparation into discrete phases normally takes place. Inimmiscible systems, which typically correspond to themore conventionally filled polymers, the poor physicalinteraction between the organic and the inorganiccomponents leads to poor mechanical and thermalproperties. In contrast, the strong interactions between thepolymer and the layered silicate in PLS nanocompositesleads to the organic and inorganic phases being dispersedat the nanometer level. As a result nanocomposites exhibitunique properties not shared by their micro counterpartsor conventionally filled polymers.

Pristine layered silicates usually contain hydrated Na+or K+ ions (a.10). Obviously, in this pristine state layeredsilicates are only compatible with hydrophilic polymers,such as polyethylene oxide (PEO) (139, 175, 195, 239,384, 417), polyvinyl alcohol (PVA) (223, a.11) etc. Torender layered silicates compatible with other polymermatrices, one must convert the normally hydrophilicsilicate surface to organophilic, which makes theintercalation of many engineering polymers possible.Generally, this can be done by ion-exchange reactionswith cationic surfactants including primary, secondary,tertiary, and quaternary alkyl ammonium oralkylphosphonium cations. The role of alkylammoniumor alkylphosphonium cations in the organosilicates is tolower the surface energy of the inorganic host and toimprove the wetting characteristics with the polymermatrix, and results in a larger interlayer spacing. Onecan evaluate that about 100 alkylammonium saltmolecules are localised near the individual silicate layers(~8 10-15 m2) and active surface area (~800 m2/g).

Additionally, the alkylammonium or alkylphosphoniumcations could provide functional groups that can reactwith the polymer matrix or in some cases initiate thepolymerisation of monomers to improve the strength ofthe interface between the inorganic and the polymermatrix (386, a.12).

Traditional structural characterisation to determine theorientation and arrangement of the alkyl chain involvedprimarily the use of wide angle X-ray diffraction(WAXD). Depending on the packing density,temperature and alkyl chain length, the chains werethought to lie either parallel to the silicate layersforming mono or bilayers, or radiate away from thesilicate layers forming mono or bimoleculararrangements. Vaia and Giannelis (a.13) have shownthat alkyl chains can vary from liquid-like to solid-like,with the liquid-like structure dominating as theinterlayer density or chain length decreases (Figure 2),or as the temperature increases. They used Fouriertransform infrared spectroscopy (FTIR), because of therelatively small energy differences between the transand gauche conformers. The idealised modelsdescribed earlier assume all trans conformations. Inaddition, for the longer chain length surfactants, thesurfactants in the layered silicate can show thermaltransition akin to melting or liquid-crystalline to liquidlike transitions upon heating.

3 Preparative Methods for PLSNanocomposites

So far there have been many papers published devotedto developing PLS nanocomposites with different

Figure 2Alkyl chain aggregation models: (a) short chain lengths, the molecules are effectively isolated from each other, (b)medium lengths, quasi-discrete layers form with various degrees of inplane disorder and interdigitation between the

layers and (c) long lengths, interlayer order increases leading to a liquid-crystalline polymer environment. Opencircles represent the CH2 segments while cationic head groups are represented by filled circles.

(Reprinted from (a.3), R. A. Vaia et al., Chem. Mater., 1994, 6, 1017, with permission from the AmericanChemical Society)

(a) (b) (c)


6

combinations of OMLS, employing somewhat differenttechnologies appropriate to each polymer and matrixpolymers such as:

epoxy resin (143, 193, 201, 255, 261, 277, 283,313, 395),

polyurethane (PU) (118, 136, 171, 215, 225, 270,272, 326, 356),

polyetherimide (PEI) (258, 289),

polybenzoxazine (212, 315),

polypropylene (PP) (149, 169, 180, 181, 187, 197,198, 205, 208, 214, 227, 230, 262, 268, 271, 278,319, 372, 374),

polystyrene (PS) (155, 160, 167, 204, 222, 235,240, 248, 276, 300, 310, 323, 340),

polymethyl methacrylate (PMMA) (170, 240, 246,280, 286, 323, 330, 389),

poly(-caprolactone) (PCL) (124, 141, 152, 154,398) and

liquid crystalline polymers (LCP) (148, 285).

The technologies are broadly classified into three maincategories.

3.1 Intercalation of Polymer or Pre-Polymerfrom Solution

This is based on a solvent system in which polymer orpre-polymer is soluble and the silicate layers are swellable.The layered silicate is first swollen in a solvent, such aswater, chloroform or toluene. When the polymer andlayered silicate solutions are mixed, the polymer chainsintercalate and displace the solvent within the interlayerof the silicate. Upon solvent removal, the intercalatedstructure remains, resulting in PLS nanocomposites.

3.2 In Situ Intercalative Polymerisation Method

In this method, the OMLS is swollen within the liquidmonomer or a monomer solution so that the polymerformation can occur in between the intercalated sheets.Polymerisation can be initiated either by heat or radiation,by the diffusion of a suitable initiator, or by an organicinitiator or catalyst fixed through cation exchange insidethe interlayer before the swelling step by the monomer.

3.3 Melt Intercalation Method

This method involves annealing, statically or undershear, a mixture of the polymer and OMLS above thesoftening point of the polymer. This method has greatadvantages over either in situ intercalativepolymerisation or polymer solution intercalation.Firstly, this method is environmentally benign due tothe absence of organic solvents. Secondly, it iscompatible with current industrial processes, such asextrusion and injection moulding. The meltintercalation method allows the use of polymers whichwere previously not suitable for in situ polymerisationor the solution intercalation method. This solvent-freemethod is much preferred for practical industrialmaterial production because of its high efficiency andpossibility of avoiding environmental hazards.

Other possibilities are exfoliation-adsorption (a.14),and template synthesis (a.15).

4 Structure and Characterisation ofPLS Nanocomposites

4.1 Structure of PLS Nanocomposites

Layered silicates have layer thickness in the order of1 nm and very high aspect ratio (e.g., 10-1000), thuscreating a much higher surface area for polymer/fillerinteraction than in conventional composites. Dependingon the strength of interfacial interaction betweenpolymer matrix and layered silicate (modified or not),three different types of PLS nanocomposites arethermodynamically achievable (Figure 3) (a.9).

(1) Intercalated nanocomposites: in an intercalatednanocomposite, the insertion of polymer matrixinto the layered silicate structure occurs in acrystallographically regular fashion, regardless ofthe silicate layer (clay) to polymer ratio. Propertiesof the composites typically resemble those ofceramic materials.

(2) Flocculated nanocomposites: conceptually this isthe same as with intercalated nanocomposites,however, silicate layers are sometimes flocculateddue to hydroxylated edge-edge interaction of thesilicate layers. The length of the orientedcollections in the range of 300-800 nm is far largerthan the original silicate layer (mean diameter 150nm) (165, 286, a.9). Such flocculation presumably


7

is governed by an interfacial energy betweenpolymer matrix and organoclays and controlled byammonium cation-matrix polymer interaction. Thepolarity of the matrix polymer is of fundamentalimportance in controlling the nanoscale structure.

(3) Exfoliated nanocomposites: in exfoliatednanocomposites, the individual silicate layers areseparated in a continuous polymer matrix by anaverage distance that totally depends on the layeredsilicate loading. Usually, the clay content of anexfoliated nanocomposite is much lower than thatof an intercalated nanocomposites.

4.2 Characterisation of PLS Nanocomposites

The structure of the PLS nanocomposites has typicallybeen established using wide-angle X-ray diffraction(WAXD) analysis and transmission electron microscope(TEM) observations. Due to its ease of use andavailability WAXD is most commonly used to probethe PLS nanocomposite structure and sometimes to studythe kinetics of the polymer melt intercalation. Bymonitoring the position, shape and intensity of the basalreflections from the distributed silicate layers, thenanocomposite structure either intercalated or exfoliatedmay be identified. For example, in the case of exfoliatednanocomposites, the extensive layer separationassociated with the delamination of the original silicatelayers in the polymer matrix results in the eventualdisappearance of any coherent X-ray diffraction fromthe distributed silicate layers. On the other hand, forintercalated nanocomposites, the finite layer expansionassociated with the polymer intercalation results in theappearance of a new basal reflection corresponding tothe larger gallery height. Although, WAXD offers aconvenient method to determine the interlayer spacing

of the silicate layers in the original layered silicates andin the intercalated nanocomposites (within 1-4 nm),however, little can be said about the spatial distributionof the silicate layers or any structural inhomogeneitiesin the PLS nanocomposites. Additionally, some layeredsilicates initially do not exhibit well-defined basalreflection. Thus, peak broadening and intensity decreasesare very difficult to study systematically. Therefore,conclusions concerning the mechanism ofnanocomposites formation and their structure basedsolely on WAXD patterns are only tentative.

On the other hand, TEM allows a qualitativeunderstanding of the internal structure, spatialdistribution of the various phases, and defect structurethrough direct visualisation. However, special caremust be exercised to guarantee a representative cross-section of the sample. The WAXD patterns andcorresponding TEM images of three different types ofnanocomposites are presented in Figure 4.

5 Types of Polymers for thePreparation of Nanocomposites

5.1 Vinyl Polymer Systems

These include the vinyl addition polymers derived fromcommon monomers like:

methyl methacrylate (MMA) (170, 240, 246, 280,286, 314, 320, 323, 330, 389, 341, a.16-a.18),

methyl methacrylate copolymers (280, 286, 367),

other acrylates (322, 325, 392),

Figure 3Schematic illustration of three different types of thermodynamically achievable polymer/clay nanocomposites.(Adapted from (a.9), S. Sinha Ray et al., Macromolecules, 2003, 36, 2355, with permission from the American

Chemical Society)

Intercalated Intercalated-and-flocculated Exfoliated


8

acrylic acid (259, a.19),

acrylonitrile (AN) (132, 420, a.20, a.21),

styrene (S) (155, 160, 167, 204, 222, 235, 240, 248,276, 300, 310, 323, 327, 340, 371, 386, 390, 408,a.22, a.23),

4-vinylpyridine (224, a.24), and

acrylamide (344, 354).

In addition, more specialised polymers have also beenused like:

PVA (223, 299, 352, 375, a.11),

poly(N-vinyl pyrrolidone) (PVP) (365),

polyvinyl pyrrolidinone (55, 318, 353),

polyvinyl pyridine (224, 331),

polyethylene glycol (PEG) (a.25),

Figure 4 (a) WAXD patterns and (b) TEM images of three different types of nanocomposites

Intercalated

Intercalated-and-flocculated

Exfoliated

(a) (b)


9

ethylene-vinyl alcohol copolymer (PEVA) (126, a.26),

polyvinylidene fluoride (67),

poly (p-phenylenevinylene) (a.27),

polybutadiene (373),

styrene-acrylonitrile copolymer (SAN) (218,294), and

polystyrene-polyisoprene diblock copolymer(133, 317).

5.1.1 PS/LS Nanocomposites

Akelah and Moet (390) have used the in situintercalative polymerisation technique for thepreparation of PS/LS nanocomposites. They modifiedNa+-MMT and Ca+2-MMT with vinylbenzyltrimethylammonium cation by the ion exchange reaction andthese modified MMTs were used for the preparationof nanocomposites. They first disperse and swellmodified clays in various solvent and co-solventmixtures such as acetonitrile, acetonitrile/toluene andacetonitrile/THF by stirring for 1 hour under N2atmosphere. To the stirred solution, S and N,N-azobis(isobutyronitrile) (AIBN) were added, andpolymerisation of S was carried at 80 C for 5 hours.The resulting composites were isolated by precipitationof the colloidal suspension in methanol, filtered off anddried. In this way, intercalated PS/MMTnanocomposites were produced and the extent ofintercalation completely depends upon the nature ofthe solvent used. Although, the PS is well intercalated,a drawback of this procedure remains that themacromolecule produced is not a pure PS but rather acopolymer between S and vinylbenzyltrimethyl-ammonium cations.

For the preparation of PS based nanocomposites, Dohand Cho (357) have used more commonly used MMT.They compared the ability of several tetra-alkylammonium cations incorporated in Na+-MMTthrough the exchange reaction to promote theintercalation of PS through the free radicalpolymerisation of S initiated by AIBN at 50 C. Theyfound that the structural affinity between S monomerand the surfactant of modified MMT plays animportant role in the final structure and the propertiesof nanocomposites. This concept was nicely employedby Weimer and co-workers (a.23) for the preparationof PS/MMT nanocomposites. They modified Na+-

MMT by anchoring an ammonium cation bearing anitroxide moiety known for its ability to mediate thecontrolled/living free radical polymerisation of S inbulk. The absence of WAXD peaks in the low anglearea together with the TEM observations of silicatelayers randomly dispersed within the PS matrix attestfor the complete exfoliation of the layered silicate.

PS was also the first polymer used for the preparationof nanocomposite using the melt intercalation techniquewith alkylammonium cation modified MMT (408). Ina typical preparative method, PS was first mixed withhost organoclay powder, the mixture was pressed intoa pellet and then heated in a vacuum at 165 C. Thistemperature is well above the bulk glass transitiontemperature of PS ensuring the presence of a polymermelt. The WAXD patterns of the hybrid before heatingshow peaks characteristic of the pure organoclay andduring heating the organoclay peaks were progressivelyreduced while a set of new peaks corresponding to thePS/LS appeared. After 25 hours, the hybrid shows theWAXD patterns corresponding predominantly to theintercalated structure.

Syndiotactic polystyrene (s-PS)/organoclaynanocomposites have also been prepared by thesolution intercalation technique by mixing pure s-PSand organoclay with adsorbed cetyl pyridiniumchloride (207).

5.1.2 PMMA/LS Nanocomposites

Okamoto and co-workers (286, 323) used organicallymodified smectite clays for the preparation ofPMMA/LS and PS/LS nanocomposites. Organicallymodified smectite clays (SPN and STN) were preparedby replacing Na+-smectite with quaternary ammonium(QA), oligo (oxypropylene)-, diethylmethylammoniumcation (SPN) or methyltrioctylammonium cation (STN)by exchange reaction. In a typical synthesis, bothlipophilised smectite clays (SPN and STN) weredispersed in MMA and S via ultrasonication at 25 Cfor 7 hours to obtain suspensions. After that t-butylperoxy-2-ethylhexanate and/or 1,1-bis(t-butyl peroxy)cyclohexane as an initiator was added to the suspensionsand then free-radical polymerisation was carried out inthe dark at 80 C for 5 hours (for MMA) and at 100 Cfor 16 hours (for S) in a silicon oil bath. For comparisonthe workers also prepared PMMA and/or PS includingQA as the references under the same conditions andprocedure. WAXD analyses were performed directlyfrom the suspensions of MMA/SPN, MMA/STN andS/SPN, and corresponding nanocomposites.


10

From WAXD patterns of MMA/STN suspension (seeFigure 5a), the higher-order peaks of interlayer spacingcorresponding to d(002) and d(003) are clearly observedalong with the basal spacing d(001) peak, suggestingMMA intercalated into the STN gallery without theloss of layer structure. The correspondingnanocomposite, PMMA/STN exhibits rather broadBraggs peaks, indicating the formation of disorderedintercalated structure. In contrast, for MMA/SPNsuspension (see Figure 5b), the absence of any Braggdiffraction peaks indicates that the silicate layer has

been completely exfoliated or delaminated in thesuspension. An almost similar pattern was observed inthe corresponding PMMA/SPN nanocomposite butwith a small remnant shoulder as shown in Figure 5b.

Further studies (286) have demonstrated the effect ofthe nature of co-monomers on the structure of PMMAnanocomposites prepared via in situ free-radical co-polymerisation of MMA in the presence of lyophilisedsmectite clays (each contain 10 wt%). They used threedifferent types of co-monomers (each 1 mol%): N,N-dimethylaminopropyl acrylamide (PAA), N,N-dimethylaminoethyl acrylate (AEA) and acrylamide(AA) for the free-radical polymerisation of MMA.Figure 6 shows TEM pictures of the observedstructures. In the case of PMMA-PAA/SPN10 (seeFigure 6c), individual silicate layers connected throughthe edge are clearly observed in the PMMA-PAA matrixand large anisotropy of the dispersed clay is observed.In contrast, the PMMA-AA/SPN10 nanocomposite(see Figure 6d) exhibited less stacking of 4-5 silicatelayers with a thickness of the stacking layers of about5 nm as a fine dispersion in the PMMA-AA matrix.The coherent orders of the silicate layers in this systemare higher than that in other systems.

5.1.3 PVA/LS Nanocomposites

Strawhecker and Manias (299) have produced PVA/MMT nanocomposite films. PVA/MMT nanocompositefilms were cast from MMT/water suspension where PVAwas dissolved. Room temperature distilled water wasused to form a suspension of Na+-MMT. The suspensionwas first stirred for 1 hour and then sonicated for 30minutes. Low viscosity, fully hydrolysed atactic PVAwas then added to the stirring suspensions such that thetotal solid (silicate plus polymer) was 5 wt%. Themixtures were then heated to 90 C to dissolve the PVA,again sonicated for 30 minutes, and finally films werecast in a closed oven at 40 C for 2 days. The recoveredcast films were then characterised by both WAXD andTEM. Both the d-spacing and their distribution decreasesystematically with increasing MMT wt% in thenanocomposites.

5.1.4 Block Copolymer/LS Nanocomposites

Krishnamoorti (133, 317) prepared block copolymer-based layered silicate nanocomposites. Disorderedpolystyrene-polyisoprene block copolymer/LSnanocomposites were prepared by solution mixing ofappropriate quantities of finely ground dimethyl-

Figure 5WAXD patterns of various monomer/organoclay

suspensions and corresponding polymer/LSnanocomposites: (a) PMMA/STN, (b) PMMA/SPN

and (c) PS/SPN. The dashed lines indicate thelocation of the silicate (001) reflection of organoclayfrom suspensions and nanocomposites. The asterisk

indicates the position of (001) reflections fromsuspensions and nanocomposites. The arrows

indicate a small shoulder or a weak peak.(Reprinted from (323), M. Okamoto et al., Polymer,

2000, 41, 3887, with permission from ElsevierScience Ltd.)


11

dioctadecylammonium cation modified MMT (2C18-MMT) and an anionically synthesised monodispersepolystyrene-1,4-polyisoprene (7 mol% 3,4 and93 mol% 1,4) diblock copolymer (PSPI18) in tolueneat room temperature. The homogeneous solution wasdried extensively at room temperature and subsequentlyannealed at 100 C in a vacuum oven for ~12 hours toremove any remaining solvent and to facilitatecomplete polymer intercalation between the silicatelayers. The intercalation of PS into the silicate layersmay be due to the slight Lewis base character impartedby the phenyl ring in PS, leading to favourableinteractions with the 2C18-MMT layers. Further, theinterlayer gallery spacing for the PSPI18/2C18-MMTcomposites is independent of the silicate loading. Allthe hybrids exhibit clear regular layered structure,demonstrated by the presence of the d001 and higher-order diffraction peaks. This independence of galleryheight on the silicate loading is consistent with theresults obtained by Vaia and coworkers on model PS-based nanocomposite systems.

5.2 Condensation Polymers and Rubbers

Several technologically important polycondensateshave also been used in nanocomposite preparation withLS. These include:

Nylon 6 (85, 90, 91, 138, 145, 147, 162, 177,179, 202, 203, 210, 211, 213, 217, 228, 232, 233,237, 238, 256, 282, 288, 301, 305, 351, 358, 368,369, 378, 399, 400, 404, 412, 413, 414, 415, 416,a.28, a.29),

several other polyamides (65, 92, 129, 194, 287,309, 311, 355, 370, 401),

poly(-caprolactone) (PCL) (88, 102, 124, 141,152, 154, 378, 380, 398, 409),

polyethylene terephthalate (PET) (120, 199, 229,274, 348, 350, 381, 379, 391, a.30, a.31),

polybutylene terephthalate (PBT) (135),

polypropylene terephthalate (PPT) (a.32, a.33),

polycarbonate (PC) (284, 321, a.34, a.35),

PEO (128, 139, 142, 175, 195, 209, 221, 239,247, 252, 293, 304, 332, 339, 384, 402, 411, 417,a.36-a.38),

Figure 6TEM images of: (a) PMMA/SPN, (b) PMMA-AEA

(1 mol%)/SPN, (c) PMMA-PAA (1 mol%)/SPN, and(d) PMMA-AA (1 mol%)/SPN.

Each contains 10 wt% SPN.(Reprinted from (286), M. Okamoto et al., Polymer,

2001, 42, 1201, with permission from ElsevierScience Ltd.)


12

ethylene oxide copolymers (347),

polyethylene imine (a.39),

polydimethyl siloxane (PDMS) (241, 250, 316,329, 363, 396),

liquid crystalline polymer (LCP) (148, 285),

polybenzoxazole (PBO) (140),

butadiene copolymers (231) (345) (297) (296),

epoxidised natural rubber (216, 343),

epoxy polymer resins (EPR) (95, 96, 130, 137, 143,193, 201, 249, 255, 261, 277, 281, 283, 313, 364,366, 388, 395, 403, 405, 407, a.40-a.43),

phenolic resins (275, 298),

polyurethanes (PU) (118, 136, 171, 184, 215, 225,270, 272, 326, 356, a.44),

polyurethane urea (PUU) (171, 270),

polyimides (131, 185, 206, 242, 254, 263, 258, 266,273, 279, 289, 290, 335, 337, 346, 377, 410, 406),

polyamic acid (200, 260, 269, 335, 342),

polysulfone (123, 220) and

polyetherimide (289).

5.2.1 Nylon/LS Nanocomposites

In 1993, Usuki, Fukushima and their colleagues ofTCRD successfully prepared, for the first time,exfoliated Nylon 6/LS hybrid (NCH) via in situpolymerisation of -caprolactam (see Section 1). TCRDreported (a.8) the ability of ,-amino acids (COOH-(CH2)n-1-NH2+, with n = 2, 3, 4, 5, 6, 8, 11, 12, 18) andmodified Na+-MMT to be swollen by the-caprolactam monomer at 100 C and subsequentlyto initiate ring opening polymerisation to obtainNylon 6/MMT nanocomposites. For the intercalationof -caprolactam, they chose the ammonium cation of-amino acids because these acids catalyse ring-opening polymerisation of -caprolactam.

Liu and co-workers (351) first used this techniquefor the preparation of commercially availableNylon 6/C18-MMT nanocomposites by using a twin-

screw extruder. They prepared nanocomposites withMMT content from 1 to 18 wt%. WAXD patterns andTEM observations respectively indicated thatnanocomposites prepared with MMT content of lessthan 10 wt% leads to the exfoliated structure, but morethan 10 wt% MMT leads to the formation ofintercalated structure. After that VanderHart and co-workers (202, 203) prepared Nylon 6/LSnanocomposites using the melt intercalation method.

Fornes and co-workers (211) have reported thepreparation of Nylon 6/LS nanocomposites undermolten state using a twin-screw extruder. They usedthree different molecular grades of Nylon 6 for thepreparation of nanocomposites with bis(hydroxy-ethyl)(methyl)-rapeseed-quaternary ammonium((HE)2M1R1) modified MMT, and tried to find out anyeffect of matrix molecular weights on structure,properties, rheology, etc. Nanocomposites wereprepared using a Haake, co-rotating, intermeshing twin-screw extruder, which was operated at 240 C with ascrew speed of 280 rpm, and a feed rate of 980 g/h.They also examined the effect of OMLS structure onNylon 6 nanocomposite morphology and properties(85). In order to understand this, a series of organicamine salts were ion exchanged with Na+-MMT to formOMLSs varying in amine structure or exchange levelrelative to the MMT. Each OMLS was melt-mixed witha high molecular weight (HMW) Nylon 6 using a twin-screw extruder; some organoclays were also mixedwith low molecular weight (LMW) Nylon 6. Thestructure and corresponding nomenclature of variousamine compounds that were used for the modificationof Na+-MMT using the ion exchange method arepresented in Figure 7. They concluded that threedistinct surfactant effects were identified that lead togreater extents of exfoliation, higher stiffness, andincreased yield strengths for nanocomposites basedon the HMW Nylon 6:

(a) one long alkyl tail on the ammonium ion ratherthan two;

(b) methyl groups on the amine rather than2-hydroxyethyl groups, and

(c) an equivalent amount of amine surfactant on thelayered silicate as opposed to an excess amount.

Gilman and co-workers (105) reported the preparationof Nylon 6- and PS-based nanocomposites of MMTmodified with trialkylimidazolium cation in order toobtain high stability of OMLS at high processingtemperature. Figure 8 represents the various kinds ofimidazolium salts used for the modification of MMT.


13

CH3

N+

CH3

CH3T(C18)

Cl- CH3

N+

CH3

CH3HT(C18)

Cl-

CH2CH2OH

N+

CH2CH2OH

CH3R(C22)

Cl- CH3

N+

HT(C18)

CH3HT(C18)

Cl-

CH2CH2OH

N+

CH2CH2OH

CH3T(C18)

Cl-CH2CH2OH

N+

CH2CH2OH

CH3C*(C12)

Cl-

H

N+

CH3

CH3HT(C18)

(HSO4)- H

N+

HT(C18)

CH3HT(C18)

(HSO4)-

M3T1 M3(HT)1

(HE)2M1R1 M2(HT)2

(HE)2M1T1 (HE)2M1C*1

M2H1(HT)1 M1H1(HT)2Figure 7

(a) Molecular structure and nomenclature of amine salts used to organically modify Na+-MMT by ion exchange.Symbols M: Methyl, T: Tallow, HT: hydrogenated tallow, HE:2-hydroxy-ethyl, R: rapeseed, C: cocoalkyl, and H

hydrogen designate the substitutents on the nitrogen.(Reprinted from (85), T.D. Fornes et al., Polymer, 2002, 43, 5915, with permission from Elsevier Science Ltd.)

Figure 8Structures of various imidazolium salts used to treat Na+-MMT

(Reprinted from (105), J.W. Gilman et al., Chem. Mater., 2002, 14, 3776, with permission from theAmerican Chemical Society)

NN

R Me

Me

X- = Cl-, BF4-

Dimethyl alkyl imidazolium salts

propylbutyl

decylhexadecyl

R =

+


14

For the preparation of nanocomposites they used amini-extruder, which was operated at 10 C above themelting point of the polymer with a residence time of3-5 min and screw speed of 200-300 rpm.

5.2.2 PCL/LS Nanocomposites

Messersmith and Giannelis (409) modified MMT usingprotonated aminolauric acid and dispersed the modifiedMMT in liquid -caprolactone (CL) beforepolymerising at high temperature. The nanocompositeswere prepared by mixing up to 30 wt% of the modifiedMMT with dried and freshly distilled -caprolactonefor a couple of hours followed by ring openingpolymerisation under stirring at 170 C for 48 hours.The same authors (398) have also reported on the-caprolactone polymerisation inside a Cr+3-exchangedfluorohectorite at 100 C for 48 hours.

Pantoustier and co-workers (88, 102) used this in situintercalative polymerisation method. They used bothpristine MMT and -amino dodecanoic acid modifiedMMT for the comparison of prepared nanocompositesproperties. For nanocomposite synthesis, in apolymerisation tube, the desired amount of pristine MMTwas first dried under vacuum at 70 C for 3 hours. Agiven amount of -caprolactone was then added undernitrogen and the reaction medium was stirred at roomtemperature for 1 hour. A solution of initiator, dibutyltin dimethoxide or tin (II) 2-ethylhexanolate in drytoluene was added to the mixture in order to reach a[monomer]/[Sn] molar ratio equal to 300. Thepolymerisation of CL with pristine MMT gives PCL witha molar mass of 4800 g/mol and a narrow distribution.For comparison the authors also conducted the sameexperiment without MMT but there was nopolymerisation of CL. These results demonstrate theability of MMT to catalyse and to control CLpolymerisation, at least in terms of molecular weightdistribution, to a remarkably narrow range. For themechanism of polymerisation, the authors assume thatthe CL is activated through interaction with acidic siteson the clay surface and the polymerisation is likely tobe proceeding via the activated monomer mechanismby the cooperative function of Lewis acidic aluminiumand Bronstrated acidic silanol functionalities on theinitiator. On the other hand, the polymerisation of CLwith protonated -amino dodecanoic acid modifiedMMT, gives a molar mass of 7800 g/mol with a monomerconversion of 92% and again a narrow molecular weightdistribution. In other very recent publications (150, 151),the same group prepared PCL/MMT nanocompositesby using in situ ring opening polymerisation of CL usingdibutyl tin dimethoxide as an initiator/catalyst.

5.2.3 PET/LS Nanocomposites

There are many reports on the preparation andcharacterisation of PET/LS nanocomposites using thein situ polymerisation method. Unfortunately no reportsgive a detailed description of the preparative method.One report describes the preparation of a PETnanocomposite by in situ polymerisation of a dispersionof organoclay in water; however, characterisation ofthe resulting composite was not reported (350). Thisreport claims that water serves as a dispersing aid forthe intercalation of monomers into the galleries of theOMLS and discloses that a wide variety of smallmolecules can serve as dispersing aids in place of, orin combination with, water. Imai (120) reported thepreparation of higher-modulus PET/expandablefluorine mica nanocomposites with a novel reactivecompatibiliser. Details regarding the synthetic route arepresented in reference (120).

Davis and co-workers (a.31) first reported thepreparation of PET-based nanocomposite using themelt intercalation method. They used 1,2-dimethyl-3-N-alkyl imidazolium salt modified MMT(hexadecyl-MMT) for the nanocomposite preparationwith PET. PET/MMT nanocomposites werecompounded via melt blending in a co-rotating minitwin-screw extruder operated at 285 C. WAXDanalyses and TEM (see Figure 9) observationsrespectively established the formation of mixeddelaminated/intercalated structure in thenanocomposites.

5.2.4 PBT/LS Nanocomposites

As well as PET/LS systems, this method wassuccessfully applied by Chisholm and co-workers (135)for the preparation PBT/LS nanocomposites. They usedsulfonated PBT for the preparation of nanocomposites.Because of the ionic nature of the -SO3Na groups andthe expected insolubility of the -SO3Na groups in thepolyester matrix, it was thought that the presence ofthe SO3Na groups may provide a thermodynamicdriving force for the production of nanocompositesderived from MMT. But after preparation andcharacterisation of the nanocomposites it was foundthat the degree of intercalation was not stronglydependent on the amount of -SO3Na groups, however,the mechanical properties increased significantly withincreasing -SO3Na content. This behaviour indicatesthat with high -SO3Na content the number ofinteractions increases between the clay particles andthe matrix via strong specific interactions involvingthe -SO3Na groups.


15

5.2.5 PC/LS Nanocomposites

Huang (321) reported the synthesis of a partiallyexfoliated bisphenol A PC nanocomposite usingcarbonate cyclic oligomers and ditallow dimethyl-exchanged MMT. WAXD patterns indicate thatexfoliation of this organoclay occurs after mixing withthe cyclic oligomers in a Brabender mixer for 1 hour at180 C. Subsequent ring-opening polymerisation of thecyclic oligomers converts the matrix into the polymerwithout disruption of the nanocomposite structure. TEMrevealed that a little exfoliation is obtained.

Mitsunaga and Okamoto (a.34, a.35) have reported thepreparation of intercalated PC/LS nanocomposites,using the melt intercalation method in the presence ofcompatibiliser. The morphology of thesenanocomposites and degradation of the PC-matrix afternanocomposite preparation could be controlled byvarying the surfactants used for the modification ofthe clay and compatibilisers. The intercalated PC/LS

nanocomposites exhibited remarkable improvementsof mechanical properties when compared with PCwithout silicate layers. They also conducted foamprocessing of PC/LS nanocomposites by usingsupercritical CO2 at 10 MPa in a batch process.

5.2.6 PEO/LS Nanocomposites

In 1992, Aranda and Ruiz-Hitzky (417) first reportedthe preparation of PEO/MMT nanocomposites. Theyhave carried out a series of experiments to intercalatePEO (MW = 105 g/mol) into Na+-MMT using differentpolar solvents, e.g., mixtures (1:1) of water/methanoland methanol/acetonitrile. In this method the nature ofthe solvents is very crucial to facilitate the insertion ofpolymers between the silicate layers, the polarity ofthe medium being a determining factor for intercalation.The high polarity of water causes swelling of Na+-MMTprovoking the cracking of the films. Methanol is notsuitable as a solvent for high molecular weight PEO,

(a)

Figure 9TEM images of PET/LS nanocomposites: (a) high levels of dispersion and exfoliation, average tactoids of four

sheets per stack and (b) similar levels of dispersion and delamination(Reprinted from (a.31), C.H. Davis et al., J Polym. Sci. Part B Polym. Phys., 2002, 40, 2661, with permission

from John Wiley & Sons, Inc.)

(b)

500 nm 50 nm

50 nm500 nm


16

whereas water/methanol mixtures appear to be usefulfor intercalations, although cracking of the resultingmaterials is frequently observed. PEO intercalatedcompounds derived from homoionic M+n-MMT andM+n-hectorite, can satisfactorily be obtained usinganhydrous acetonitrile or methanol/acetonitrile mixtureas solvents. In addition, the lack of PEO replacementby organic compounds having high affinity toward theparent silicate, such as dimethyl sulfoxide and crownethers, indicates again the high stability of PEO-intercalated nanocomposites. On the other hand,treatment with salt solutions provokes the replacementof the interlayer cations without disturbance of the PEO.For example, Na+ ions in PEO/Na+-MMT are easilyreplaced by NH4+ or CH3(CH2)2NH3+ ions, aftertreatment (2 hours) at room temperature with aqueoussolution of their chloride, perchlorate and thiocyanatesalts (1N solutions), in a reversible process.

Various other authors (142, 195) have used the samemethod and the same solvent for the preparation ofPEO/LS nanocomposites. Vaia and co-workers (402)applied this method to intercalate PEO in Na+-MMTlayers. Intercalation of PEO in layered silicate wasaccomplished by heating the PEO with the Na+-MMTat 80 C. The WAXD patterns before any heating containpeaks characteristic of both Na+-MMT and crystallinePEO. After heating to 80 C, the intensity of the peakscorresponding to the unintercalated silicate andcrystalline PEO is progressively reduced while a set ofnew peaks corresponding to the PEO-intercalated MMTare observed signifying the completion of intercalation.Recently Shen and co-workers (139) reported thepreparation of PEO/OMLS nanocomposites using themelt intercalation technique. In order to find out the effectof thermal treatment on the amount of PEO andpolyethylene-polyethylene glycol diblock copolymer(PE-PEG) intercalated into the layers of Na+-MMT andon ionic conductivity of PEO/Na+-MMT, Liao and co-workers (209) have prepared PEO/Na+-MMT andPE-PEG di-block copolymer/Na+-MMT nano-composites using a melt intercalation technique. It wasfound that PEO can be intercalated into the layers ofNa+-MMT by simple mechanical blending and part ofthe PE in PE-PEG diblock copolymers was alsointercalated into the layers of Na+-MMT. The intercalatedamount increases with the thermal treatment time, whichultimately improves the ionic conductivity of thePEO/Na+-MMT nanocomposites.

5.2.7 LCP/LS Nanocomposites

Vaia (285) reported the reversible intercalation betweenOMLS and liquid crystalline polymers (LCP) in the

nematic state. Melt intercalation of a model main chainliquid crystalline copolymer based on 4,4-dihydroxy--methylstilbene and a 50:50 mole ratio mixture ofheptyl/nonyl alkyl dibromide was accomplished byannealing a powder mixture of the polymer and OMLSwithin the nematic region of the polymer. In anotherreport, Chang and co-workers (148) have preparednanocomposites of thermotropic liquid crystallinepolyester (TLCP) and Cloisite 25A (a commercialorganoclay) using a melt intercalation method abovethe melt transition temperature of the TLCP. Liquidcrystallinity of the nanocomposites was observed withOMLS content up to 6 wt%.

5.2.8 PBO/LS Nanocomposites

Zhu (204) used phosphonium salt for the modificationof clay and then tried to find out the differences betweenorgano ammonium and phosphonium salt treatmentsof clay fillers in nanocomposites in their effects onthermal stability. This technique was successfullyapplied by Hsu (140) in order to preparepolybenzoxazole (PBO)/LS nanocomposite from aPBO precursor, polyhydroxyamide (PHA) and anOMLS. The PBO precursor was made by the lowtemperature polycondensation reaction betweenisophthaloyl chloride (IC) and 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane with an inherentviscosity of 0.5 dl/g. For the preparation of PBO/LSnanocomposite, the OMLS was first dispersed indimethyacetamide in which PHA was dissolved. ThePHA/LS film was obtained from solution casting anddried at 80 C under vacuum. Finally PBO/LSnanocomposite was obtained by curing the film at350 C to form the benzoxazole ring.

5.2.9 EPR/LS Nanocomposites

The chemistry of ring opening polymerisation ofepoxides to form polyether nanocomposites wasfollowed by studies of both rubbery and glassy thermosetepoxy (EPR)/LS nanocomposites using different typesof amine curing agents. The mechanisms leading to themonolayer exfoliation of clay layers in thermoset epoxysystems have been greatly elucidated. In addition, thepolymer/LS interfacial properties have been shown toplay a dominant role in determining the performancebenefits derived from nanolayer exfoliation.

Giannelis and co-workers (405) first reported thepreparation of epoxy resin based nanocomposites ofOMLS. They have analysed the effect of different curing


17

agents and curing conditions on the formation ofnanocomposites based on the diglycidyl ether ofbisphenol A (DGEBA) and a MMT modified bybis (2-hydroxyethyl) methyl hydrogenated tallowalkylammonium cation. They found that this modifiedclay dispersed readily in DGEBA when sonicated for ashort time period, as determined by the increase inviscosity at relatively low shear rates and the transitionof the suspension from opaque to semitransparent. Theincrease in viscosity was attributed to the formation of aso-called house-of-cards structure in which edge-to-edge and edge-to-face interactions between dispersedlayers form a percolation structure. Wang and Pinnavaia(407) used a series of acidic cations such as H+, NH4+,and acidic onium ions of the type [H3N(CH2)n-1COOH]+,[H3N(CH2)nNH2]+, [H3N(CH2)nNH3]+2 (n = 6 and 12)for the modification of MMT and carried out thepolymerisation-delamination process over thetemperature range of 198-287 C. They found that theEPR-clay delamination temperature (PDT) wasdependent on the heating rate and nature of the cationused for the modification of the clay. In general, the PDTincreased with decreasing cation acidity and basalspacing of the clay.

The delamination of MMT in the polymerised epoxyresin was confirmed by X-ray powder diffraction (XRD),as shown by the powder patterns in parts (a) and (b) ofFigure 10 where [H3N(CH2)11COOH]+-MMT remainscrystalline over the temperature range 25-229 C. Onlythe very diffuse scattering characteristic of the

amorphous polyether appears in the XRD pattern of thecomposite. The absence of a 17 peak for[H3N(CH2)11COOH]+-MMT suggests that the clayparticles have been exfoliated and the 9.6 -thick claylayers dispersed at the molecular level. TEM providesunambiguous evidence for the delamination of the MMTin the polyether matrix. PDT values and thermodynamicdata for MMT-polyether nanocomposites formed frombifunctional onium ion MMT and onium ion NH4+, andH+ MMT are presented in Table 3 and Table 4.

A group from Australia (137) has reported themorphology, thermal relaxation and mechanicalproperties of PLS nanocomposites of high-functionalityepoxy resins. Three different types of resins were used:bifunctional DGEBA, trifunctional triglycidyl p-aminophenol (TGAP), tetrafunctional tetraglycidyldiaminodiphenylmethane (TGDDM) and all were cured withdiethyltoluene diamine (DETDA). The structure of theresins and curing agent are presented in Table 5. MMTmodified with octadecylammonium cation was usedfor the preparation of nanocomposites. The morphologyof the cured samples was investigated using WAXDand different microscopy techniques. Figure 11(a)shows the WAXD patterns of the MMT concentrationseries showing that the organoclay with an initial d-spacing of 2.3 nm is mainly exfoliated in the DGEBA-based system. On the other hand, high content (10 wt%)OMLSs show intercalated structure, while DGEBA-based systems, resins of higher functionality showdistinctive peaks even at low OMLS loading, indicating

Figure 10(a) XRD powder patterns for a freeze-dried [H3N(CH2)11COOH]+-MMT, (b) [H3N(CH2)11COOH]+-MMT freeze-dried

and then heated at 229 C, and (c) clay-polyether nanocomposite containing 5 wt% [H3N(CH2)11COOH]+-MMT(Reprinted from (407), M.S. Wang et al., Chem. Mater., 1995, 7, 468, with permission from the

American Chemical Society)


18

that these nanocomposites have a lower degree ofexfoliated structure. WAXD patterns are shown inFigure 11b for TGAP and Figure 11c for TGADDMbased nanocomposites of MMT. In the case of anynanocomposite systems, the peak observed at around2.5 nm correlates to the (002) plane and thereforerepresents only half the distance of the d-spacing.

Figure 12 comprises atomic force microscopy (AFM)phase contrast images of the DGEBA nanocompositecontaining 5 wt% layered silicate. Individual layerscannot be seen by AFM as they usually are by the TEM.A striated structure, however, can be seen withincreasing phase intervals at the top of the picture. Fromthe AFM images it is evident that silicate layers are

not homogeneously distributed in the matrix, somestacked layers are present.

Chen and co-workers (130) synthesised epoxy-MMTnanocomposite using a surface initiated method in orderto understand the interlayer expansion mechanism andthermal-mechanical properties of these nanocomposites.MMT modified with bis-2-hydroxyethyl methyl tallowammonium cation (C30B) was used as OMLS fornanocomposite synthesis. 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate was used as theepoxy monomer, and hexahydro-4-methylphthalicanhydride (HHMPA), ethylene glycol (EG), andbenzyldimethylamine (BDMA) were respectively usedas curing agent, initiator and catalyst during synthesis.

morfdemrofsetisopmoconanyalc/rehteyloprofatadcimanydomrehtdnaseulavTDP4elbaTHN,noimuino 4+ Hdna, + TMM

noitacreyalretnI gnicapslasablaitinI )( TDP a ( )CnoitcaerfotaeH

)g/J(mylopfotaeH b

)lom/Jk(H[ 3 HC(N 2) 11 HC 3]+ 9.51 2.0 891 1 055 3 912 2H[ 3 HC(N 2)5 HC 3]+ 9.41 1.0 782 1 455 6 022 3HN 4 +2 5.21 1.0 742 1 455 5 022 2

H+ 9.31 1.0 132 1 555 21 122 5a 02foetargnitaehatanoitanimaledyalc-noitaziremylopedixoperoferutarepmettesnoehtsiTDPehT nim/C

.)w/w(59:5fonoitisopmocremylop:yalcetisopmocadnab

.stnelaviuqeedixopeowtrofnoitasiremylopfotaeH.yteicoSlacimehCnaciremAehtmorfnoissimrephtiw,864,7,5991,.retaM.mehC,.lategnaW.S.M,)704(morfdetnirpeR

morfdemrofsetisopmoconanyalc/rehteyloprofatadcimanydomrehtdnaseulavTDP3elbaTTMM-noimuinolanoitcnufib a

noitacreyalretnI gnicapslasablaitinI )( TDP b ( )CnoitcaerfotaeH

)g/J(fotaeH

noitasiremylop c)lom/Jk(

H[ 3 HC(N 2) 11 ]HOOC + 0.71 1.0 922 1 275 61 822 6H[ 3 HC(N 2)5 ]HOOC + 3.31 0.0 842 1 565 60 522 2H[ 3 HC(N 2) 21 HN 3] +2 4.31 1.0 172 1 665 80 522 3H[ 3 HC(N 2)6 HN 3] +2 1.31 1.0 372 2 865 70 622 3H[ 3 HC(N 2) 21 HN 2]+ 5.31 0.0 182 2 365 70 422 3H[ 3 HC(N 2)6 HN 2]+ 2.31 1.0 782 2 755 30 222 2

a.)w/w(59:5sawnoitisopmocremylop:yalcehT

b 02foetargnitaehatanoitanimaledyalc-noitasiremylopedixoperoferutarepmettesnoehtsiTDP .nim/Cc

.stnelaviuqeedixopeowtrofnoitcaerfotaeH.yteicoSlacimehCnaciremAehtmorfnoissimrephtiw,864,7,5991,.retaM.mehC,.lategnaW.S.M,)704(morfdetnirpeR


19

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,2002,remyloP,.laterekceB.O,)731(morfdetnirpeR 34 .dtLecneicSreiveslEmorfnoissimrephtiw,5634,

CHCH2O C

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O

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The curing mechanism for an epoxy-anhydridesystem with an alcohol initiator is shown inFigure 13. Amine catalysts like BDMA were addedto the mixture to accelerate the reaction byfacilitating the ring opening of epoxy groups. Severalpublished papers indicate that intragallery oniumions can catalyse the epoxy curing reaction and thuslead to favourable conditions for obtaining exfoliatedPLS nanocomposites. Chen and co-workers (130)verified that the crosslinking reactions in thepresence of C30B were due to hydroxy initiation andnot due to catalytic reactions. The extent of reactionof a resin containing C30B was compared to theextent of reaction for a neat resin and resinscontaining either EG or BDMA.

Time-resolved high-temperature-XRD is used toprobe the expansion behaviour of the silicate layersduring curing of the PLS nanocomposites. InFigure 14 the changes in d-spacing are plottedagainst the isothermal cure time for various clayloadings and cure temperature. On the basis ofvarious characterisation methods, the authorsproposed an exfoliation mechanism for surface-

initiated epoxy nanocomposites consisting of threestages. In the first stage, the interlayer expansioninduced by intragallery polymerisation mustovercome any polymer chains that bridge the silicatelayers. The interlayer expansion cannot proceedbeyond the first stage if the number of bridging unitsbecomes too great. The second stage wascharacterised by a steady and linear increase ininterlayer spacing and accounts for the majority ofthe total expansion realised. In this stage, the silicatelayers could be monitored via isothermal differentialscanning calorimetry experiments. Also, for samplesthat exhibited a large increase in interlayer expansion,it was found that the activation energy associated withthe interlayer expansion was less than the activationenergy associated with the curing. The reverse wastrue for samples that showed no increase in interlayerspacing. In the third stage, the interlayer expansionslowed then stopped, and in some cases decreasedslightly. This was ascribed to the evolving modulusof the extragallery polymer such that the interlayerexpansion stopped when the modulus of theextragallery polymer became equal to or exceeded themodulus of the intragallery polymer.


20

Figure 11WAXD patterns of (a) DETDA cured DGEBA nanocomposites, (b) DETDA cured TGAP nanocomposites and

(c) DETDA cured TGDDM nanocomposites containing 0-10 wt% organoclay(Reprinted from (137), O. Becker et al., Polymer, 2002, 43, 4365, with permission from Elsevier Science Ltd.)

(a) (b)

(c)

Figure 12Phase contrast AFM images of DETDA cured DGEBA containing 5 wt% organoclay

(Reprinted from (137), O. Becker et al., Polymer, 2002, 43, 4365, with permission from Elsevier Science Ltd.)


21

O

O

O

+

HO

OH

OH

O

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O

OH

O

O

O

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HO

OH

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BDMA

Figure 13Schematic illustration of generalised curing reaction involving the epoxy monomer, HHMPA, EG, and BDMA(Reprinted from (130), J.S. Chen et al., Polymer, 2002, 43, 4895, with permission from Elsevier Science Ltd.)

Figure 14Changes in d001 as a function of the curing time and temperature: (a) 5, (b) 10, and (c) 15 wt% silicate loading.

The dashed lines denote the quantitative detection limit of the XRD setup(Reprinted from (130), J.S. Chen et al., Polymer, 2002, 43, 4895, with permission from Elsevier Science Ltd.)

(a) (b)

(c)


22

5.2.10 PU/LS Nanocomposites

Chen (a.44) has used a PCL-based nanocompositesynthesis technique for the preparation of novelsegmented PU/LS nanocomposites fromdiphenylmethane diisocyanat