epoxy resin based nanocomposites: 1. diglycidylether of bisphenol a (dgeba) with...

Polymer International Polym Int 53:2130–2137 (2004)DOI: 10.1002/pi.1638

Epoxy resin based nanocomposites:1. Diglycidylether of bisphenol A (DGEBA)with triethylenetetramine (TETA)Jane Brown, Ian Rhoney and Richard A Pethrick∗Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1XL,UK

Abstract: The preparation and properties of a series of nanocomposite materials obtained using differentorganically modified montmorillonite clays with a simple epoxy resin are reported. Dynamic mechanicalthermal analysis is used to assess the effect of the incorporation of the clay platelets into the matrix ofthe polymer. In this system, it is observed that with the well-dispersed clay system the low temperaturemodulus increases as would be predicted for a filled polymer system. The high temperature modulusincrease is consistent with the premise that the polymer is interacting directly with the clay platelets. Theglass transition temperature increases with the loading of the clay in the polymer resins. However, theextent to which enhancement of the physical properties of the composite occurs depends on the nature ofthe organic modifier. 2004 Society of Chemical Industry

Keywords: epoxy resin; nanocomposites; montmorillonite; organic modifiers; dispersion methods

INTRODUCTIONDespite the technological importance of epoxy resinsthe number of papers on nanocomposites obtainedfrom these materials is comparatively small.1–4 Ithas been found that the incorporation of as littleas 2–5 % of organically modified mica type silicate(OMTS) significantly reduces the flammability ofthe material without increasing the carbon monox-ide or smoke yields.5,6 Depending on the nature ofthe organic used, magadiite was found to be interca-lated or exfoliated in an epoxy resin matrix.7,8 Studiesof diglycidyl ether of bisphenol A (DGEBA) withpoly(etheramine) indicated that whether or not exfo-liation occurs depends on the organophile used todisperse the clay.9 Cures using dicyandiamide (DICY)and benzyldimethylamine (BDMA) produced onlyintercalated structures from melt mixed materials,whilst solution-mixed materials were fully exfoliated.1

The dynamic storage modulus of the nanocompositein both glass and rubbery plateau regions was increasedwith increasing OMTS content, but the glass transitiontemperatures (Tg) remained unchanged. Dependenceof the structure on the nature of the OMTS wasagain observed.2–4 In order to achieve the optimumenhancement of the physical properties, exfoliation ofthe platelets is the desired objective of the dispersionprocesses.10–13

The interlayer expansion in exfoliation of clayparticles should exceed the separation associated withthe organic modification and cause them to lose theirstructural registry.14 Among various clay minerals,layered smectic-type montmorillonite (MMT) clayhas been widely used to create nanocomposites.MMT, a hydrous alumina silicate mineral whoselamellae are constructed from octahedral aluminasheets sandwiched between tetrahedral silicate sheets,exhibits a net negative charge on the lamellar surfacewhen aluminium is substituted by magnesium oriron atoms.15 The anionic character of the surfacesites allows absorption of Na+ or Ca++. A numberof organically modified alkylammonium cations arenow available commercially to aid the dispersion ofindividual platelets into the reaction mixture.

In this paper, a study is reported of the preparation,characterization and properties of a series of epoxynanocomposites based on the cure reaction of DGEBAwith triethylenetetramine (TETA).

EXPERIMENTALMaterialsThe epoxy resin, diglycidyl ether of bisphenol A ofCiba Specialty Chemicals (Araldite resin, MY 750)and the hardener triethylenetetramine (TETA) of

∗ Correspondence to: Richard A Pethrick, Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building,295 Cathedral Street, Glasgow G1 1XL, UKContract/grant sponsor: EPSRCContract/grant sponsor: Southern ClayContract/grant sponsor: Du Pont(Received 11 July 2003; revised version received 16 October 2003; accepted 18 December 2003)Published online 12 October 2004

2004 Society of Chemical Industry. Polym Int 0959–8103/2004/$30.00 2130

Diglycidylether of bisphenol A with triethylenetetramine nanocomposites

Table 1. Characteristics of the montmorillonite clays

Code Organic modifieraModifier concentrationa

(meq/100 g)∗ Moisture (%)Weight loss

on ignition (%)

Cloisite 6A Dimethyl, dihydrogenated tallow,b

quaternary ammonium chloride140 2 47

Cloisite 25A Dimethyl, hydrogenated tallow,b

2-ethylhexyl, quaternary ammoniumchloride

95 2 34

Cloisite 30B Methyl, hydrogenated tallow,b

bis-2-hydroxylethyl, quaternaryammonium chloride

92 2 30

Cloisite Na+ Natural 4 7

a Data provided by Southern Clay for the materials used.b Hydrogenated tallow consisting of approximately 65 % C18, 30 % C16 and 5 % C14 straight chain hydrocarbon.

Aldrich Chemicals were used as supplied. The epoxyresin nanocomposites were prepared using a stoichio-metric ratio of 100:6.95 (w/w) epoxy resin:hardener.The organically modified montmorillonite clays weresupplied and characterized by Southern Clay and theirproperties are summarized in Table 1. The epoxy resinwas heated to 70 ◦C and degassed until no furtherfrothing was observed. The stoichiometric amount ofTETA was added and the mixture degassed under vac-uum for ca 5 min. The resin mixture was then pouredinto a pre-heated (50 ◦C), pre-waxed mould of dimen-sions 80 × 4 × 35 mm3. The sample was initially curedat 60 ◦C for ca 20 h and then post-cured at 100 ◦Cfor 24 h and finally post-cured at 160 ◦C. Samplescontaining the montmorillonite were prepared usingnanoclays that had been dried at 110 ◦C for 24 h. Themixture of resin and clay were sonicated to producea smooth, uniform dispersion, and held at 80 ◦C for30 min, with occasional stirring. The mixture was thendegassed before and after addition of the curing agent,poured into the pre-conditioned mould and curedunder the conditions described above. With most ofthe epoxy–nanoclay systems, as much as 25 % of thenanoclay could be added before the solution viscosityprevented flow. A set of samples was cured at 100 ◦Cand 160 ◦C to determine the optimum cure conditions.It was found that all the DMTA data were highest forthe samples cured at 160 ◦C, so this temperature wasadopted for all subsequent measurements.

Dynamic mechanical thermal analysis (DMTA)A Polymer Laboratories dynamic mechanical thermalanalyzer MKIII operating at a frequency of 1 Hz, astrain of ×4 and a scanning rate of 3 ◦C min−1 wereused to analyze samples cut from the cured materials.Measurements were performed from −100 to 200 ◦Cand the resultant changes in tan δ and E′ plotted.

Differential scanning calorimetry (DSC)A DuPont 910 differential scanning calorimetercoupled with a DuPont 990 programme/recorder wasused, under nitrogen, to measure the heat changesoccurring in each cured sample. The instrumentwas calibrated using an indium standard. DSC

measurements were therefore performed between 40and 300 ◦C at a rate of 10 ◦C min−1 and the resultantheat changes plotted.

X-ray diffraction (XRD) measurementsA Siemens X-ray diffractometer with a Cu Kα (λ =1.54 A) radiation source and a curved graphitecrystal monochromator was used to analyze thesamples. XRD experiments were performed directlyon the organically modified montmorillonite (OMTS)powders and on the epoxy/OMTS cured samples.

Water absorption measurementsSamples of the composite with dimensions 20 × 34 ×4 mm3 and weighing typically 3 g were immersed inindividual glass jars containing de-ionized water at50 ◦C. The samples were weighed periodically usinga Mettler analytical balance capable of measuring to±0.0001 g. Samples were removed from the water andwiped with tissue paper to remove excess water andleft for 2 min to equilibrate before being weighed andthen re-immersed. Samples were out of the water forapproximately 5 min during this process.

RESULTS AND DISCUSSIONPrevious studies1–7 have indicated that the organicmodifier has a significant influence on the physi-cal properties of the nanocomposite. In this study,composites created with the following four differentclays are compared: a non-organically modified dis-persible montmorillonite, Cloisite Na+; a dimethyldi(hydrogenated tallow) quaternary ammonium chlo-ride modification, Cloisite 6A; a methyl hydrogenatedtallow 2-ethylhexyl quaternary ammonium chloride,Cloisite 25A; and a methyl tallow bis(2-hydroxyethyl)quaternary ammonium chloride, Cloisite 30B. Themethod used in the preparation of these samplesinvolves sonication to aid dispersion of the clay.In a previous paper17 it was observed that a well-dispersed clay significantly enhanced the viscosity ofthe prepolymer. The fluids exhibited dramatic shearthinning that was attributed to the time constant asso-ciated with the dynamics of edge-to-face interactions

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J Brown, I Rhoney, RA Pethrick

between clay platelets. It is well known that the addi-tion of molecules capable of blocking the edge sitescan significantly reduce the viscosity of the dispersionleading to a lowering of the effective viscosity. Themontmorillonite clays used in this study all exhibitedenhancements of the viscosity even at high clay load-ing. The TETA is suppressing the ‘house of cards’structure attributed to the viscosity thickening effectby killing edge-to-face interactions.

Nature of the dispersion in the epoxy compositesThe initial d-spacing of the dried organoclay wasdetermined and in each nanocomposite system a peakcan be observed at 2θ = 19.5 ◦: this peak representsthe (110) reflection of the montmorillonite. This peakis independent of the distance between the silicateplatelets and its location can be used as a reference. Itshould be noted that the lowest angle which could bemeasured in this work was 2θ = 1 ◦ which correlateswith a d-spacing of 88 A. In addition, the large increasein intensity of the XRD signal at very low angle mayoverlap or interfere with peaks in this region and henceobscure signals associated with a d-spacing greaterthan 50 A.

X-ray analysis of some of the cured montmorilloniteepoxy resins exhibited a high degree of exfoliation, andFig 1 indicates that the clay platelets were satisfactorilydispersed by the sonication method. The degree ofdispersion achieved varied with the type of organictreatment. Figure 2 indicates that, for the lowestloading of Cloisite 6A, a high degree of exfoliationis achieved. For the higher loading, however, theobserved spacing is essentially that of an intercalatedmaterial. The dispersion of the Cloisite 6A is stabilizedby a quaternary salt which has two long-chainhydrocarbons and it is possible that, above a certainloading level, re-aggregation of the platelets occursand is associated with the observation of creation ofthe intercalated structures in the X-ray diffraction.Figure 3 indicates that, for the Cloisite 25A, oncemore it is only at the lowest level of loading that

50 40 30 20

C

B

A

Cloisite 30B

Cou

nts

d-spacing (Å)

Figure 1. XRD spectrum of Cloisite 30B powder and Cloisite30B/DGEBA/TETA cured mixture. (A) 1.0 % Cloisite 30B, (B) 10.0 %Cloisite 30B, (C) 20.0 % Cloisite 30B.

50 40 30 20

C

B

A

Cloisite 6A

Cou

nts

(a.u

.)

d-spacing (Å)

Figure 2. XRD spectrum of Cloisite 6A powder and Cloisite6A/DGEBA/TETA cured mixture. (A) 1.0 % Cloisite 6A, (B) 10.0 %Cloisite 6A, (C) 20.0 % Cloisite 6A.

50 40 30 20

C

B

A

Cloisite 25A

Cou

nts

(a.u

.)

d-spacing (Å)

Figure 3. XRD spectrum of Cloisite 25A powder and Cloisite25A/DGEBA/TETA cured mixture. (A) 1.0 % Cloisite 25A, (B) 10.0 %Cloisite 25A, (C) 20.0 % Cloisite 25A.

a high degree of exfoliation is observed and athigher levels intercalation is the preferred state ofthe clay. Cloisite 25A contains one long-chain but hasa hindered 2-ethylhexyl grouping which appears to aidstability in the intercalated state at higher clay loading.In contrast, Cloisite 30B (Fig 1) with the strongerinteractions with the clay surface associated with thehydroxyl groups and the single long hydrocarbon chainappears the most easily and effectively dispersed of allthe clays and shows a high degree of exfoliation, evenat 10 % loading. It is evident from the above datathat the organic modifier plays an important role inachieving the required degree of exfoliation of the clayplatelets. X-ray analysis of the Cloisite Na+ powderand dispersion in the resin (Fig 4) indicated that therewas a small change in the gallery spacing, but no clearindication of exfoliation.

Dynamic mechanical thermal analysisTypical traces for the DMTA of the unfilled and filledOMTS–epoxy composites are presented in Figs 5–8.The data obtained for the composites containingCloisite 30B cured at 100 and 160 ◦C is summarized

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50 40 30 20

Cloisite Na+

CB

ACou

nts

(a.u

.)

d-spacing (Å)

Figure 4. XRD spectrum of Cloisite Na+ powder and CloisiteNa+/DGEBA/TETA cured mixture. (A) 5.0 % Cloisite Na+, (B) 10.0 %Cloisite Na+, (C) 20.0 % Cloisite Na+.

50 100 150

108

109

No filler5.0% Cloisite 30B10.0% Cloisite 30B20.0% Cloisite 30B

E′ B

endi

ng/P

a

Temperature /°C(a)

50 100 150

0.6

0.5

0.4

0.3

0.2

0.1

0.0

No filler5.0% Cloisite 30B10.0% Cloisite 30B20.0% Cloisite 30B

Tan

δ

Temperature /°C(b)

Figure 5. Dynamic mechanical thermal analysis: (a) E′ bending forthe Cloisite 30B/DGEBA/TETA system; (b) tan δ for the Cloisite30B/DGEBA/TETA system.

in Table 2. Post-curing at higher temperatures of160 ◦C shifts all Tg values, E′ bending values andE′′ bending peak heights to significantly higher values.The samples were not fully cured at 100 ◦C, whereasafter curing at 160 ◦C the DSC indicated that there isno residual exotherm and a significant improvementin Tg values was observed. The samples of the epoxycomposite with different Cloisites were all curedat 160 ◦C.

Cloisite Na+ –epoxy compositeCloisite Na+ is the natural form of the clay with noorganic modifier. This unmodified clay is produced

50 100 150

108

109

No filler5.0% Cloisite 6A10.0% Cloisite 6A20.0% Cloisite 6A

E′ B

endi

ng /

Pa

Temperature /°C


50 100 1500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Tan

δ

Temperature /°C

(a)

(b)

Figure 6. Dynamic mechanical thermal analysis: (a) E′ bending forthe Cloisite 6A/DGEBA/TETA system; (b) tan δ for the Cloisite6A/DGEBA/TETA system.

by high pressure steam treatment of the pristineclay. Figure 4 indicates that there is some evidenceof intercalation of the clay with a slight increase in thed-spacing occurring with increase in the clay content.For the highest level of clay incorporation there isan apparent decrease in the d-spacing. However, thesamples produced exhibited enhancement of boththeir low (25 ◦C) and higher temperature (100 ◦C)bending modulus. The enhancement of the lowtemperature modulus is consistent with the effectof inorganic filler on the polymer matrix. Theintroduction of the nanoparticles increases the lossmodulus (Table 3). Results indicate a slight decreasein Tg on addition of Cloisite Na+, then as theconcentration increases the Tg increases slightly;however, looking at the associated errors it ispossible that these apparent changes are within theexperimental error of the measurements. The hightemperature modulus (Fig 8a) shows an indicationof increasing crosslink content with increasing claycontent. This appears to imply that exfoliation hasoccurred to some extent in this system, liberating anincreased number of interaction sites or crosslinkingspecies.

Cloisite 25A–epoxy compositeCloisite 25A is a dimethyl, hydrogenated tallow, 2-ethylhexyl, quaternary ammonium chloride modified

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50 100 150

108

109


E′ B

endi

ng /P

a

Temperature / °C(a)


Temperature / °C50 100 150

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Tan

δ

(b)

Figure 7. Dynamic mechanical thermal analysis: (a) E′ bending forthe Cloisite 25A/DGEBA/TETA system; (b) tan δ for the Cloisite25A/DGEBA/TETA system.

clay. The organic molecule has a single long hydrocar-bon chain and hence aids the easily dispersion of theclay platelets in the resin mixture. Both the low temper-ature and high temperature modulus increase as wouldbe expected for a loaded system. Results indicate nosignificant change in the Tg with increasing concentra-tion of Cloisite 25A. The X-ray data (Fig 3), indicatethat there is intercalation rather than exfoliation, whichis consistent with the small changes observed in thephysical properties. The high temperature modulus(Fig 7a), shows a progressive enhancement, consis-tent with an increasing polymer–filler interaction withincreasing clay content.

50 100 150

108

109

No filler5.0% Cloisite Na+

10.0% Cloisite Na+

20.0% Cloisite Na+

No filler5.0% Cloisite Na+

10.0% Cloisite Na+

20.0% Cloisite Na+

E′ B

endi

ng /

Pa

Temperature / °C

Temperature / °C50 100 150

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Tan

δ

(a)

(b)

Figure 8. Dynamic mechanical thermal analysis: (a) E′ bending forthe Cloisite Na+/DGEBA/TETA system; (b) tan δ for the CloisiteNa+/DGEBA/TETA system.

Cloisite 6A–epoxy compositeCloisite 6A is a dimethyl, dihydrogenated tallow,quaternary ammonium chloride modified clay. Thetwo long-chain hydrocarbons are able to interactwith neighbouring molecules on adjacent platelets andstabilize the intercalated form of the nanocomposite.Results indicate that the Tg decreases on addingCloisite 6A, suggesting that there are two possibleeffects. Addition of small proportion of clay initiallydecreases the Tg, then potentially increases it withincreasing clay concentration. The results again showan initial increase in E′ bending at high temperaturefrom the standard DGEBA value, then a slight

Table 2. Influence of cure temperature on the physical properties of epoxy Cloisite 30B composites

Cured at 100 ◦C Cured at 160 ◦C

Tan δ E ′ bending (Pa × 109) atE ′′ bending


Cloisite 30BPeak

heightaTg (◦C)(±3) 25 ◦C 100 ◦C

(Pa × 108)

peak heightaPeak

heightaTg (◦C)(±3) 25 ◦C 100 ◦C

(Pa × 108)

peak heighta

0 0.65 113 1.14 0.8 1.34 0.52 141 1.90 1.50 1.671 0.67 114 1.48 0.87 1.49 0.61 133 1.97 1.45 2.065 0.63 113 1.53 0.86 1.61 0.58 133 2.25 1.70 2.37

10 0.60 114 1.59 0.96 1.69 0.55 133 2.30 1.73 2.3215 0.57 115 1.63 1.11 1.78 0.51 133 2.47 1.86 2.4620 0.52 115 1.52 1.11 1.72 0.47 132 2.36 1.92 1.7525 0.55 114 1.58 1.18 1.71 0.49 136 2.56 1.99 2.49

a Errors in tan δ, E′ and E′′ were typically ±0.03.

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decrease with increasing concentration of Cloisite6A. The X-ray data (Fig 2) indicate that the clayplatelets are predominantly intercalated. Once morethis is a filler effect rather than evidence of anyspecific effects of exfoliation. The higher temperaturemodulus values (Fig 6a), show a relatively smallchange compared with some of the other organicmodifications and this is consistent with a relativelymodest change in the strength of the polymer–fillerinteractions.

Cloisite 30BCloisite 30B is methyl hydrogenated tallow bishydrox-yethyl ammonium chloride modified clay. The singlelong chain has the potential of stabilization of the dis-persion of the clay platlets and the two hydroxylethylgroups anchor the chain firmly to the clay surface. Thisorganic modification appears to be the most effectivein achieving exfoliation as is shown from the X-ray datain (Fig 1). Increasing the concentration of Cloisite 30B(Table 3), gradually increases the E′ and E′′ bendingvalues but has little effect on the Tg, apart from aslight decrease with respect to the standard DGEBA;however, there is a gradual decrease in the peak heightof the tan δ curve with increasing concentration. Con-sistent with the idea of an increased polymer–surfaceinteraction, the high temperature modulus (Fig 5a) isobserved to progressively increase with increase in claycontent.

Influence of various organic surfacemodifications on the physical properties of thecompositesAt 25 % loading Cloisite 30B exhibits a 35 % increasein modulus, whereas Cloisite Na+ shows a 37 % andCloisite 25A an increase of 22 % at 20 % loading.It was not possible to incorporate more than 20 %of Cloisite 25A because of the magnitude of theviscosity exhibited by the initial dispersion. Cloisite6A appeared to be the least compatible with the epoxyand showed no significant improvement in E′. Thehigh temperature modulus values are indicative of theinteraction between the polymer and the platelets andthese are particularly strong in Cloisite 30B and may beattributed to hydrogen bonding between the pendanthydroxyl groups created on opening the oxirane ringand the hydroxyl groups present on the clay. The claymay also play a catalytic role in the base-catalyzedhomopolymerization of the epoxy that would modifythe topography of the matrix formed. At high loadinglevels, the local stress exceeds the yield stress of thematrix resulting in the formation of localized shearbands.16 This continuity of stress serves to blunt anycrack or defect propagation, thereby enhancing theoverall toughness of the epoxy.

Tg values for the epoxy compositesThe Tg values of the samples containing Cloisite30B were ∼7 ◦C lower than for the standard epoxyand did not seem to improve with increasing clay

Table 3. Influence of increasing Cloisite concentration on the dynamic mechanical properties of epoxy composites

Cloisite Na+ Cloisite 30B



CloisitePeak

heightaTg (◦C)(±3) 25 ◦C 100 ◦C

(Pa × 108)

Peak heightaPeak

heightaTg (◦C)(±3) 25 ◦C 100 ◦C

Peak heighta

(Pa × 108)

0 0.52 141 1.90 1.50 1.67 0.52 141 1.90 1.50 1.671 0.63 134 1.99 1.53 2.23 0.61 133 1.97 1.45 2.065 0.57 135 2.03 1.60 2.21 0.58 133 2.25 1.70 2.37

10 0.48 138 2.30 1.80 2.28 0.55 133 2.30 1.73 2.3215 0.43 137 2.20 1.74 2.21 0.51 133 2.47 1.86 2.4620 0.39 139 2.36 1.83 2.27 0.47 132 2.36 1.92 1.7525 0.36 138 2.60 2.06 2.40 0.49 136 2.56 1.99 2.49

Cloisite Cloisite 25A Cloisite 6A



Peakheighta

Tg (◦C)(±3) 25 ◦C 100 ◦C

(Pa × 108)

Peak heightaPeak

heightaTg (◦C)(±3) 25 ◦C 100 ◦C

(Pa × 108)

Peak heighta

0 0.52 141 1.90 1.50 1.67 0.52 141 1.90 1.50 1.671 0.54 140 1.99 1.53 1.72 0.77 120 1.98 1.60 2.605 0.51 139 2.17 1.60 1.76 0.66 125 2.05 1.60 2.20

10 0.49 140 2.17 1.66 1.71 0.62 130 1.91 1.47 2.1715 0.46 139 2.28 1.75 1.81 0.60 130 2.13 1.55 2.2020 0.45 139 2.31 1.74 1.81 0.60 129 1.98 1.47 2.1025 — — — — — 0.57 128 2.00 1.44 2.07

a Errors in tan δ, E′ and E′′ were typically ±0.03.

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loading. In contrast, the Tg values of the samplescontaining Cloisite Na+ gradually increased withincreasing concentration. The Tg values of the Cloisite25A–epoxy system did not change significantly withincreasing concentration. Improvements in Tg withincreasing concentration of Cloisite 6A were observed.The majority of the epoxy–nanoclay systems studiedexhibited lower Tg values than those of the unfilledresin. It would appear that the clay changes thenature of the crosslinked network that is formedand can lead in certain cases to the creationof greater chain mobility. The gradual increasein Tg with increasing concentration, observed forCloisite Na+, can be explained in terms of freevolume. As the nanoclay concentration is increased,more polymer–clay interactions occur, creating aconstrained region at the particle surface and resultingin immobilization of parts of the resin matrix. Asmore chains pack onto the surface, densification ofthe polymer occurs which ultimately reduces thefree volume. These restrictions hinder the relaxationmobility of the polymer segments near the interface,leading to an increase in Tg.

In the case of the Cloisite 6A–epoxy system, it isdifficult to explain why the Tg has decreased whenthe modulus results suggest very little interactionoccurring between the epoxy and clay. It is possiblethat the release of the organic modification could resultin plasticization of the matrix and this would lead to alowering of the Tg.

The Cloisite 30B–epoxy samples exhibit Tg valuessignificantly lower than the standard epoxy. The factthat the Tg values are not increased with increasingconcentration may be attributed to a major increase inthe spacing between the clay layers upon exfoliation.As a result, there is more free volume available and lessrestriction on the movement of the polymer segments.This is further supported by Giannelis et al18 who statethat the ability of chains to undergo centre-of-masstransport during melt intercalation is evidence that theinteractions within the interlayer do not completelyrestrict segmental motions, otherwise large-scale chainmotion would not be possible. There is always thepossibility the effects observed can be attributed tothe organic modifier being released into the matrixand plasticizing the system which would off-set theeffects of the increased ‘crosslinking’ attributed to theinteraction of polymer and clay particles.

The Cloisite 25A–epoxy samples have Tg valueswhich match that of the standard epoxy and, thereforeit can be concluded that Cloisite 25A has virtually noeffect on the Tg of the material. This suggests thatthis particular clay does not interact with the epoxyto the same extent as the other fillers. Perhaps itsbulky structure is only filling the free volume withinthe polymer instead of physically interacting.

Water uptake analysisThe water absorption characteristics of the compositeswere assessed using the mass uptake as a function oftime. In order to evaluate the effects of the variousnanoclay fillers, at increasing loading levels, wateruptake experiments were performed and diffusioncoefficients calculated using a normalized expressionfor Mt/M∞ which takes into account the initial weightof the sample, thus:

Mt − Mo

M∞ − Mo= 4√

π

(Dt

l2

)1/2

where Mt is the mass of the sample at time t, Mo is theinitial mass of the sample at t = 0 and M∞ is the massat equilibrium. The normalized data against the squareroot of time (t1/2) divided by the thickness (l) hasthe diffusion coefficient, D, as the initial gradient.The equilibrium amounts of water and the initialdiffusion coefficients are presented in Table 4. Thediffusion coefficients were calculated from the initialslope (0–500 s1/2 cm−1). The results obtained indicatethat the standard epoxy, containing no nanoclay filler,has the highest diffusion coefficient. The diffusionprocess describes the behaviour of the water as itenters the constrained epoxy resin matrix. As expected,diffusion is slow, reflecting the inhibition of diffusionas a consequence of the tortuous path created bythe platelets. The platelets will not necessarily beperpendicular to the diffusion direction and the secondstage coefficient will reflect both diffusion through thematrix and diffusion at an enhanced rate along theplatelet surfaces, which lie in the direction of thediffusion gradient.

The data indicates that improved barrier propertiesare being achieved with the higher levels of clayloading. Cloisite 30B, Cloisite 25A and Cloisite

Table 4. Diffusion coefficients for Cloisite-modified epoxy resins

Cloisite 30B Cloisite 25A Cloisite 6A Cloisite Na+Concentration (%) D × 109 (cm2 s−1) D × 109 (cm2 s−1) D × 109 (cm2 s−1) D × 109 (cm2 s−1)

0 11.5 11.5 11.5 11.51 7.25 7.60 8.79 6.465 7.73 7.18 9.37 6.85

10 6.04 7.76 11.13 5.6015 5.46 5.66 15.48 5.8620 6.93 8.45 7.81 6.6025 3.56 10.40 7.25

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Table 5. Water uptake at ‘equilibrium’ normalized for weight of

sample

Concentration(%)

Cloisite30B

Cloisite25A

Cloisite6A

CloisiteNa+

0 0.0163 0.0163 0.0163 0.01631 0.0128 0.0155 0.0178 0.01605 0.0151 0.0161 0.0184 0.0198

10 0.0149 0.0176 0.0230 0.025215 0.0155 0.0181 0.0267 0.030420 0.0155 0.034 0.0239 0.040925 0.0157 0.0259 0.0516

Na+ have smaller diffusion coefficients than thestandard epoxy. Samples created from Cloisite 6Ainitially exhibit a lower rate of diffusion than thestandard DGEBA/TETA systems, but the diffusionrate increases with increasing concentration of Cloisite6A. In addition the normalized water equilibriumvalues (Table 5) indicate that for Cloisite 30B there isno increase in the value with increasing concentration,whereas in the other systems the values increase withincreasing concentration, with the unmodified clayCloisite Na+ showing the maximum increase.

CONCLUSIONSThe use of sonication has enabled well-dispersedepoxy–Cloisite composites to be prepared. Thelow and high temperature modulus of the samplesindicated that the clay was acting as a filler and thatthe polymer interacted with the clay platelets. Thelack of a significant increase in the values of the Tg

is unexpected, but implies that densification of thematrix between the clay platelets is not occurringin a manner which might have been expected andin part could be attributed to plasticization effects.The organic treatment does influence the physicalproperties of the composite and in particular the extentto which water is absorbed into the composite.

ACKNOWLEDGEMENTSOne of us (IR) wishes to thank the EPSRC for theprovision of a post doctoral fellowship for the period ofthis study. The support of Southern Clay and DuPontis also gratefully acknowledged.

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