1-s2.0-S0014305706004010-Factors and processes influencing the reinforcing effect of layered silicates in polymer nanocomposites main

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Macromolecular Nanotechnology

Factors and processes influencing the reinforcing effectof layered silicates in polymer nanocomposites

Laszlo Szazdi, Andras Pozsgay, Bela Pukanszky *

Department of Plastics and Rubber Technology, Budapest University of Technology and Economics, P.O. Box 91, H-1521 Budapest, Hungary

Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences,

P.O. Box 17, H-1525 Budapest, Hungary

Received 7 June 2006; received in revised form 26 October 2006; accepted 5 November 2006Available online 28 December 2006

Abstract

The analysis of the tensile yield stress of a large number polymer/layered silicate composites showed widely differingmechanical properties. The composition dependence of yield stress can be described and evaluated quantitatively by a sim-ple model developed earlier for particulate filled polymers. The comparison of data produced in our laboratory or takenfrom the literature indicated that several processes may take place during the preparation of the composites and a consid-erable number of factors influence composite properties. Quite a few of these are often neglected and percentage increase inmodulus, strength or other properties is reported in published papers instead. The most important of such effects are

changing matrix properties when a functionalized polymer is used to promote adhesion (PE, PP), modification of crystal-line structure due to nucleation (PA, PP), plasticization or lubrication (PVC), decreased interaction (PA, PVC, PET, rub-bers) or chemical reactions (PVC, PP, PET). Using a few simple assumptions, most of which are supported by previousexperience, the extent of exfoliation can be estimated quantitatively in nanocomposites. The analysis of the tensile yieldstress of more than 80 composites with various matrices indicated that the extent of exfoliation is very low in most com-posites; it reaches maximum 10% in the best case, which corresponds to about 10 silicate layers per stack. Although theapproach has limitations and several factors were neglected during analysis, this result is in agreement with observationsindicating that complete exfoliation rarely can be reached in thermoplastic/clay composites. In order to achieve larger rein-forcement, silicates must be exfoliated more perfectly in the future. 2006 Elsevier Ltd. All rights reserved.

Keywords: Nanocomposites; Layered silicates; Tensile yield stress; Modeling; Reinforcement; Extent of exfoliation

1. Introduction

One of the most important potential advantagesof layered silicate polymer nanocomposites is strongreinforcement at low filler content[19]. This expec-tation is based on the idea that an extremely largecontact surface combined with high aspect ratiois created by the exfoliation and homogeneous

0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.eurpolymj.2006.11.005

* Corresponding author. Address: Department of Plastics andRubber Technology, Budapest University of Technology andEconomics, P.O. Box 91, H-1521 Budapest, Hungary. Fax: +36 1463 3474.

E-mail address: [email protected](B. Pukanszky).

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dispersion of individual silicate platelets in the poly-mer matrix[1017]. Besides complete, or at least alarge extent of exfoliation, further conditions ofreinforcement are good adhesion between thematrix and the silicate, as well as the orientation

of the plates in the direction of the load [15,16,18].Although, often all of these conditions are assumedto be fulfilled, very few evidence is available whichproves the validity of the assumption. Strong rein-forcement is mentioned in many papers, but onlya few experimental results support this statement[5,19,20]. Usually an increase in stiffness is under-stood as reinforcement, which increases upon theincorporation of the silicate indeed, but strengthdoes not change or decreases in most cases. How-ever, modulus increases almost always when inor-

ganic fillers are added to a polymer, thus largerstiffness is not a sufficient proof for reinforcement.Consequently we consider the simultaneous impro-vement of stiffness and strength as an indication ofefficient reinforcement.

Even less attempt is made to estimate reinforce-ment quantitatively [12,15,21]. Improvement inproperties is often given in percentages, which doesnot allow the proper comparison of reinforcement,since the effect depends very much on componentproperties and also on processing conditions. Someauthors claim that the disappearance of clay reflec-

tion from the wide angle X-ray diffraction (XRD)pattern indicates complete exfoliation [14,2224],but ample evidence exists which proves otherwiseand shows that the extent of exfoliation cannot bedetermined by XRD [2527]. Attempts were alsomade to estimate the extent of exfoliation by rheo-logical measurements [28] or by the determinationof gas permeability and model calculations [2931]. TEM micrographs were also analyzed and theauthors determined the number of layers in stacksappearing on the micrographs [21,32]. Unfortu-nately these methods did not yield a specific valuefor the extent of exfoliation, which could be relatedto the mechanical properties of the composites. Sev-eral groups tried to estimate the extent of exfoliationby the modeling of the stiffness of the composites[15,21,33,34]. Micromechanical models or existingtheories are compared to measured values and con-clusions about the structure of the composites aredrawn from the results. Usually, the calculationsgive the aspect ratio of the silicate which increaseswith increasing extent of exfoliation. Although theseattempts are rather successful and supply valuable

information about exfoliation, the number of indi-

vidual layers dispersed in the matrix compared toall available platelets is not known yet.

We selected a different approach for the estima-tion of the extent of exfoliation. We assume thatreinforcement and composite properties are deter-

mined by the same factors as in traditional compos-ites, i.e. matrix properties, contact surface, and thestrength of interaction. These factors are allincluded into the simple model developed earlierby us to describe the composition dependence oftensile yield stress and tensile strength of particulatefilled polymers[35,36]. The evaluation of the com-position dependence of the tensile properties of alarge number of PP nanocomposites yielded inter-esting results about the extent of reinforcementand exfoliation [37]. The goal of this paper is to

check the validity of the model for matrices otherthan PP. The mechanical properties of variousnanocomposites (PE, PA, PET, PVC, etc.) are mod-eled and the extent of reinforcement is determinedquantitatively. We call attention to factors whichare usually neglected, but strongly influence rein-forcement and the performance of layered silicatenanocomposites. The validity of the model as wellas its limitations are discussed in the final sectionof the paper.

2. The approach

The approach and the model were described in aprevious publication[37], thus we refrain from theirdetailed discussion and only give some backgroundinformation which facilitates the understanding ofthe paper. The model assumes that an interphaseforms spontaneously in composites and yield stresschanges proportionally to its actual value as a func-tion of composition. The composition dependenceof tensile yield stress can be described by the follow-ing equation[35,36,38]:

ry ry01 u

1 2:5uexpBu 1

where ryand ry0are the yield stress of the compos-ite and the matrix, respectively, u the volume frac-tion of the filler in the composites and Bis relatedto the load carried by the dispersed component,i.e. it depends on interaction [35,36,3941]. Theterm (1 u)/(1 + 2.5u) expresses the effectiveload-bearing cross-section of the matrix. At zerointeraction all the load is carried by the polymer

and the load-bearing cross-section decreases with

346 L. Szazdi et al. / European Polymer Journal 43 (2007) 345359


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increasing filler content. The same correlation canbe used to describe the composition dependence oftensile strength, if ultimate elongation is small, usu-ally less than 100%[36].

The value of parameter Bdepends on all factors

influencing the load-bearing capacity of the filler,i.e. on the strength of interaction and on the sizeof the contact surface. The effect of these factorson Bis expressed as

B 1 Afqfl lnryi

ry02

where Af is the specific surface area of the filler(contact surface), qf is its density, while l and ryiare the thickness and the yield stress of the inter-phase. The latter two parameters were shown todepend on the strength of matrix/filler interaction

[42,43]. The approach used in this paper is basedon Eq.(2). The contact surface between the matrixand the silicate increases with increasing extent ofexfoliation, thus the value of parameter B shouldincrease proportionally and indicate reinforcement.The load carried by the second component dependsalso on the properties of the matrix; the extent ofreinforcement is larger in a softer than in a stifferpolymer[4446]. This factor must be also taken intoaccount when composites prepared with differentmatrices are compared with each other.

If the model is valid, we should obtain linear cor-relation when the natural logarithm of reduced yieldstress is plotted against filler content, i.e.

ln ryred lnry1 2:5u

1 u lnry0 Bu 3

Linearity is a necessary, but not sufficient conditionto prove the validity of the model. However, thestudy of a large number of various compositesproved than in the absence of structural effects, i.e.orientation of anisometric particles, aggregation,phase inversion in blends, changing matrix proper-ties, etc., plotting reduced yield stress against thevolume fraction of the dispersed phase resulted inlinear correlation[35,36,4749]. The validity of themodel was proved also by the agreement of experi-mental results with its predictions, i.e. increase ofBwith decreasing particle size (specific surface area ofthe filler)[35,43]or with increasing strength of inter-action (surface modification) [43,50], the thicknessof the interphase increases with increased adhesion[42,43], etc. It is easier to compare composites withdifferent matrices, if we normalize reduced strength

by the matrix value

lnryrel lnry

ry0

1 2:5u

1 u Bu 4

in spite of the fact that the extent of reinforcement,i.e. parameter Bdepends on matrix yield stress (see

Eq. (2)). In the representation of Eq. (4) the yieldstress of all composites should fall on a straight linewith zero interception and with slopes proportionalto the extent of reinforcement (B).

3. Experimental

The reinforcing effect of layered silicates is com-pared in a large number, in more than 80 compos-ites. Some of them were prepared by us, while theproperties of others were taken from the literature.The composites contain a wide variety of compo-

nents and they were prepared under very differentconditions. Basic information about the compositesevaluated can be found in Table 1, in which wegive also the references containing all experimentaldetails; accordingly we refrain from the further dis-cussion of these details here.

Practically all composites were characterized bystandard techniques used for the study of layeredsilicate nanocomposites. Their structure was investi-gated by X-ray diffraction (XRD), scanning (SEM)and transmission (TEM) electron microscopy. In

some cases additional methods were also used likerheology, differential scanning calorimetry (DSC),the measurement of thermal stability or transpar-ency. In this paper we focus our attention onmechanical properties and on the reinforcing effectof the silicate. As a consequence, we evaluate andcompare the composition dependence of tensileyield stress whenever possible. In some cases onlyfailure characteristics, i.e. tensile strength data wereavailable; we evaluated also these when the defor-mation of the composite was small, less than 100%.

4. Results

As mentioned earlier, the investigation of about40 PP composites proved the validity of theapproach[37]. The study showed that a wide rangeof reinforcement was achieved depending on compo-sition and processing conditions. As a consequence,we refrain from the detailed discussion of the princi-ple and focus our attention on factors which arelargely neglected in the study of layered silicatenanocomposites. Accordingly, we discuss the effect

of matrix properties, nucleation, plasticization and

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lubrication, as well as possible chemical reactionson the properties of nanocomposites. We compare

about 80 composites in a final section and discuss

the practical consequences of the results togetherwith the potentials as well as limitations of the

approach.

Table 1Mechanical properties of polymer/clay composites published in the literature and the reinforcing effect (parameterB) calculated from them

Matrix Filler ry0(MPa) ry0c(MPa) B R2 Ref.

Type Characterc Type Treatmenta

1 LLDPE 30% MMt M2H1C18 7.5 7.9 19.3 1.00 [13]b,c

2 LDPE 0% Cloi20A M2(C18)2 11.8 15.1 4.3 0.70 [12]c

3 LDPE Var Cloi20A M2(C18)2 11.8 14.8 13.7 0.99 [12]c

4 LDPE Var CloiExp M3C18 11.8 13.8 11.7 0.97 [12]c

5 HDPE 0% MMt M2(C18)2 27.0 26.9 1.8 0.87 [15]c

6 PA Wet N948 M2(C18)2 46.9 48.3 1.8 0.74 [86]7 PA Wet N748 x-amino-acid 46.9 50.0 1.7 0.68 [86]8 PA Wet NaMMT 46.9 46.8 2.9 0.69 [86]9 PA Dry N948 M2(C18)2 75.7 76.2 4.2 0.95 [72]

10 PA Dry N748 x-amino-acid 75.7 66.6 2.8 0.79 [72]b

11 PA Dry NaMMT 75.7 81.3 4.2 0.99 [72]12 PA Extrus. NmerI30 M2H1C18 72.0 81.4 3.3 1.00 [11]

b

13 PA HMW CloiExp (HE)2M1C22 69.7 72.2 12.6 1.00 [17]14 PA HMW CloiExp (HE)2M1C12 69.7 73.7 13.0 1.00 [17]15 PA HMW Cloi30B (HE)2M1C18 69.7 78.3 9.5 1.00 [17]16 PA HMW CloiExp M3C18 69.7 81.8 7.7 0.75 [17]17 PA HMW CloiExp M3C18 69.7 76.3 10.5 0.98 [17]18 PA HMW CloiExp M2H1C18 69.7 79.0 8.9 0.83 [17]19 PA HMW CloiExp M1H1(C18)2 69.7 82.6 5.2 0.63 [17]20 PA HMW Cloi20A M2(C18)2 69.7 77.9 8.4 1.00 [17]21 PA HMW Cloi15A M2(C18)2 69.7 77.7 5.5 1.00 [17]22 PA HMW CloiExp (HE)2M1C22 69.7 75.2 13.0 0.99 [87]23 PA MMW CloiExp (HE)2M1C22 70.2 76.7 11.5 0.99 [87]24 PA LMW CloiExp (HE)2M1C22 69.2 73.6 9.2 0.99 [87]25 PA Extrus. NmerI30 M2H1C18 49.5 54.4 3.8 0.99 [64]26 PA ? ? ? 43.9 49.9 22.9 0.98 [81]d

27 PA LMW MMT M2H1C18 71.1 85.3 4.4 0.63 [88]b

28 PA MMW MMT M2H1C18 71.6 87.2 5.0 0.66 [88]b

29 PVC 190 C NaMMT 53.2 52.5 0.8 0.83 [73]30 PVC 170 C N948 M2(C18)2 55.6 68.6 19.3 0.89 [73]31 PVC 180 C N948 M2(C18)2 53.9 58.9 6.8 0.88 [73]32 PVC 190 C N948 M2(C18)2 53.2 55.6 0.4 0.03 [73]33 PVC DOP NaMMT 51.3 61.9 2.5 0.51 [89]b

34 PVC DOP MMT M3C18 51.3 62.1 0.8 0.37 [89]b

35 PVC DOP MMT M2(C18)2 51.3 65.5 3.2 0.01 [89]b

36 PVC In situ MMT M2(C12)2 63.7 73.1 6.9 0.99 [90]b

37 PVC 35%DOP NaMMT 13.0 10.3 4.5 0.83 [91]38 PVC 35%DOP ann NaMMT 13.5 14.1 2.7 0.71 [91]39 PVC 5%DOP MMT (HE)2M1C18 26.0 45.2 2.5 0.97 [92]

b

40 PVC 35%DOP MMT (HE)2M1C18 12.0 10.1 3.9 0.99 [92]b

41 PVC 5%DOP ann MMT (HE)2M1C18 44.0 44.3 2.7 0.98 [92]b

42 PVC 35%DOP ann MMT (HE)2M1C18 13.0 15.3 2.4 0.99 [92]b

43 PET Recyc Cloi25 M2C18C6 60.0 60.0 3.4 1.00 [77]44 PET Recyc Cloi25 M2C18C6 60.0 60.1 3.3 0.99 [77]45 PET MMT M2H1C16 40.3 40.7 10.2 0.99 [78]46 PET PEG MMT M2H1C16 40.3 41.7 6.0 0.79 [78]

a Letters indicate the composition of the surfactant, i.e. M: methyl, HE: 2-hydroxy-ethyl, Cx: alkyl chain, H: hydrogen.b Tensile strength.c Character indicates MAPE content in wt% in PE composites, Var means varying MAPE content, ann means annealed samples.d Details of component properties were missing in the paper. Density of clay was assumed to be 2.4 g/cm 3.



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4.1. Matrix properties, blending (PE, PP)

Neither NaMMT nor OMMT can be efficientlydispersed in polyethylene and polypropylene,because of the lack of thermodynamic driving force

[15,16,22,51]. The attractive forces among the layersare much stronger in both cases than the interac-tion between the polymer matrix and the silicate.Exfoliation and homogenization is assisted by theaddition of a functionalized polymer, the mostfrequently maleinated PE or PP [1216,23,24]. Thefunctional groups of MAPE or MAPP may reactwith the surfactant, usually a primary ammoniumsalt, remove it from the surface making possiblethe adsorption of either the functionalized polymeror the matrix polymer itself[52]. Hydrolyzed func-

tional groups may interact also with non-ionicgroups on the silica surface as well.Functionalized polymers are used in various

quantities, but their amount can be considerable,exceeding 20% in some cases[14,53,54]. As a conse-quence not a homo- or copolymer, but a blend servesas matrix for the reinforcing silicate. The structure ofthis blend can be homogeneous, but phase separa-tion and the formation of a heterogeneous structurehave been also mentioned occasionally [15,53,55].The properties of functionalized polymers usuallydiffer from those of the matrix, thus the blend will

have different properties as well. Nevertheless, thechange in properties and reinforcement is usuallyrelated to the matrix polymer and the presence ofMAPE or MAPP is completely neglected.

The tensile yield stress of several PE/silicate com-posites is presented inFig. 1as a function of silicatecontent. Composition and other information aboutthe materials can be found in Table 1. The captionofFig. 1and all other figures indicates in bracketsthe identification number of each composite andalso the source of the data used in the evaluation.Yield stress increases in each case when functional-ized polymer was used indicating considerable rein-forcement. However, the extrapolation of thecomposition dependence of yield stress to zero sili-cate content results in a value, which is differentfrom the yield stress of the matrix polymer. The dis-crepancy may be explained by the orientation of thesilicate platelets[56,57]or by the different propertiesof the blend and the matrix polymer. The possiblenucleation effect of the silicate is moderate andcan be neglected in polyethylene[12,15,16].

The yield stresses of the same composites are

plotted in the form of Eq. (3)inFig. 2. We obtain

straight lines for all four sets of composites in thisrepresentation indicating that the model can beapplied for the evaluation of the reinforcing effectof silicates in these composites. The deviation ofthe intersection of the lines from the value of thematrix polymer is obvious again. Similar correla-tions and deviation of intersection from the matrixvalue were observed in the case of PP compositesas well. The comparison of reinforcing effect and

performance is difficult because in Fig. 2 both the

0.00 0.01 0.02 0.035

10

15

20

Tensileyieldstress(MPa)

Volume fraction of silicate

Fig. 1. Effect of silicate content on the tensile yield stress of PE/clay composites. Symbols: (h) Kato[13](1), (d) Hotta[12](3),(s) Hotta[12](4), (n) Hotta[12](2).

0.00 0.01 0.02 0.032.0

2.2

2.4

2.6

2.8

3.0

3.2

ln(reducedyieldstress)


Fig. 2. Linear representation of reduced yield stress for thecomposites shown inFig. 1. Symbols are the same as in Fig. 1.


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slopes and the intersections are different in eachcase.

Comparison of the composites and the estima-tion of the reinforcing effect of the silicates are mucheasier if relative yield stresses are plotted as a func-

tion of filler content in the form of Eq. (4). Theresults obtained for all PE composites found inthe literature by us are presented in Fig. 3 in thisform. Very large, one order of magnitude differencescan be observed in reinforcement among the com-posites as indicated by parameter B, which is pro-portional with the load carried by the dispersedsilicate particles. The two smallest values belong tocomposites containing only PE and OMMT withouta functionalized polymer. The results clearly provethat the introduction of MAPE changes structure

and improves the load-bearing capacity of the sili-cate i.e. increases the extent of reinforcement. TheB values obtained exceed those measured in shortfiber reinforced composites[35,36,48,38]. The inter-action between the polymer and the silicate must bealso excellent otherwise such large Bvalues and sig-nificant reinforcement could not have been achievedat all.

Because of the different components used in thesecomposites, the estimation of reinforcement is stilldifficult. Changing matrix properties influence alsothe value of B, as indicated by Eq. (2). According

to the equation, a linear correlation should exist

betweenBand the natural logarithm of matrix yieldstress. The existence of the correlation and thus thevalidity of the approach were proved in CaCO3composites prepared with various matrices asshown by the broken line plotted in Fig. 4. The

extent of reinforcement is indicated by the deviationfrom this line in the vertical direction. The figureshows that Bvalues for composites not containinga functionalized polymer lie on the line, i.e. these sil-icates behave like regular fillers, e.g. CaCO3. On theother hand, reinforcement is considerable in theother three cases; the deviation from the line islarge. The dispersion of silicates seems to be muchbetter and the reinforcement larger in PE than inPP composites since the largest B values did notexceed 8 or 9 in the latter case[37]. From the anal-ysis of the mechanical properties of PE compositeswe may conclude that blending changes matrixproperties, indeed, and this must be taken intoaccount when the effect of the silicate on compositeproperties is discussed. Silicates reinforce PE con-siderably when a functionalized polymer is alsoadded to the system.

4.2. Crystalline structure, nucleation (PP, PA)

Mineral fillers generally, and also layered silicateswere shown to nucleate polymers. A considerable

number of papers have been published already on

0.00 0.01 0.02 0.03 0.04 0.05

0.0

0.1

0.2

0.3

0.4

B = 1.8

B = 4.3

B = 11.7

B = 13.7B = 19.3

ln(rel

ativeyieldstress)


Fig. 3. Relative tensile yield stress of various PE/MMT com-posites. Widely differing reinforcing effect depending on compo-sition and processing conditions. Symbols: (h) Kato[13](1), (d)Hotta[12](3), (s) Hotta[12](4), (n) Hotta[12](2), () Osman

[15](5).

1 2 3 4 5

0

5

10

15

20

25

ParameterB

ln(matrix yield stress, 0c

)

Fig. 4. Comparison of the reinforcing effect of layered silicates inthe PE nanocomposites presented inFig. 3. The plot accounts forchanging matrix yield stress. Reinforcement is given by thedistance of the broken reference line. Symbols: (n) PE compos-ites (d) PP/CaCO3 reference line.



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the crystallization of PP in the presence of layeredsilicates [5860]. Similarly, polyamide was shownto crystallize in the c form, instead of the usual amodification in the presence of silicates [6165].On the other hand, various crystal modifications

have different properties, e.g. b polypropylene haslarger impact and fracture resistance than the usualaform[66]. Similarly to blending, changes in crystalmodification must also influence the reinforcingeffect of silicates in layered silicate nanocomposites.

The preparation and use of PA nanocompositesby the Toyota group initiated large interest in thesematerials [6770]. Considerable reinforcement wasachieved in in situ polymerized composites at verylow filler contents. A large number of compositeswere prepared and studied afterwards, but limited

attention was paid to the effect of changing crystalmodification on mechanical properties [59,65]. Werefrain from the presentation of primary resultshere, they can be found in the corresponding publi-cations (seeTable 1). Reduced yield stress of threecomposites prepared by three different groups ispresented in Fig. 5. Good linear correlations areobtained in both cases when we have a sufficientnumber of points for evaluation, what confirmsthe validity of our approach. The slope of the linesis different indicating dissimilar reinforcing effect ofthe silicate in the three composites. It is interesting

to note, however, that the intersection of the linesand the yield stress of the matrix differ from eachother in each case. A probable reason for the devi-

ation is the change in the crystal modification ofthe matrix polymer. The modification of crystallinestructure by the presence of the silicate is confirmedbyFig. 6, in which we present the WAXS traces of aneat PA used as matrix for layered silicate compos-

ites and that of three composites prepared in thepresence of 2 vol% of silicates with different organ-ophilization. Irrespectively of the treatment, the cmodification dominates in the composites, whilemainly the a form is present in the neat polymer.Since an adhesion promoter is not added in thiscase, the deviation between the measured and calcu-lated matrix yield stress cannot be caused by ablending effect and when an x-amino acid is usedfor organophilization even the surfactant cannotinfluence properties.

The relative yield stress of selected PA compositesis plotted inFig. 7as a function of silicate content.Straight lines are obtained in all cases again, thedeviation from the lines is negligible. The slopes ofthe lines differ considerably indicating widely differ-ing reinforcing effects. Rather surprisingly very smallBvalues of about 4 or 5 are obtained in some caseseven when organophilic MMTs were used, in spiteof the general belief that silicates exfoliate in a largeextent in PA and properties improve tremendously[10,24]. We must call attention here to the fact thatmost of the silicates were treated with aliphatic

amines and not with an x-amino acid [71]. Whilethe latter creates covalent bond and strong adhe-sion to the matrix, the former decreases interfacial

0.00 0.02 0.04 0.06 0.08

3.7

3.9

4.1

4.3

4.5

4.7

ln(reduced

yieldstress)


Fig. 5. Reduced tensile yield stress of selected PA nanocompos-ites with different measured and calculated matrix properties.Symbols: (h) Liu[64](25), (d) Shelly[81](26), (n) Fornes [87]

(23).

15 20 25 30

b)

a)

c)

d)

Intensity(a.u.)

Angle of reflection (2)

Fig. 6. Formation of the c modification of PA in PA/claynanocomposites. (a) PA, (b) PA/NaMMT, (c) PA/N948, (d) PA/

N784 (x-amino acid). Silicate content 2 vol%.


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interaction, results in poorer stress transfer as well asin weaker reinforcement[72]. Without exception allcomposites containing a silicate with aliphatic aminetreatment have smallBvalues, i.e. limited reinforce-ment. We must also mention here that the authors ofthe composite with the largest Bvalue of 22.9 do notsupply any information about the composition of

their material. We may conclude from these resultsthat changing matrix properties due to nucleationmay modify properties considerably and interfacialinteractions play also a crucial role in the determina-tion of composite properties.

4.3. Plasticization, lubrication (PVC)

Layered silicate nanocomposites usually containseveral components. The possible effect of some ofthese components on composite properties is com-pletely ignored or only one aspect is taken intoconsideration. We mentioned the role of the func-tionalized polymer in PE and PP composites. InPVC the surfactant used for the organophilizationof the silicate may play an important role. The basicamine initiates the degradation of the polymer [73],but it may act also as a plasticizer or lubricant.

The composition dependence of the tensile yieldstress of three PVC/layered silicate composites ispresented inFig. 8. Yield stress changes completelydifferently in the three cases. Very often plasticizedPVC is used as matrix or some plasticizer is also

added, since degradation is more severe in unplasti-

cized PVC. Amide waxes are used as lubricants inPVC thus we may assume that the surfactantsapplied for the organophilization of the silicate areat least partially soluble in it. The surfactant mayinteract also with the other components of thePVC formulation (plasticizer, stabilizer, lubricant,pro cessing aid, etc.). The different composition de-

pendence of tensile yield stress presented in Fig. 8for the three sets of composites is possibly the resultof dissimilar interactions, which depends on theformulation of the PVC compound.

These differences are further amplified byFig. 9,where the relative yield stress of some composites isplotted in the linear form. Negative slopes are alsoobtained, which are difficult to explain with themodel. In the case of zero interaction and no stresstransfer, the straight line should run horizontally.Negative slopes can be obtained only if the matrixpolymer changes properties proportionally to theamount of the silicate added. We must also callattention here to the fact that even in the case of apositive slope the extent of reinforcement is moder-ate,Bvalues do not exceed 7 compared to the muchlarger values obtained in PE and PA composites (seeTable 1).

One may object that in the case of a negativeslope dispersion is poor and this results in inferiorproperties. However, when the polymer adheres tothe surface and an interphase form, which is usuallythe case, the introduction of inorganic fillers into

polymers always results in a positive slope, even if

0.00 0.02 0.04 0.060.0

0.2

0.4

0.6

B = 3.8

B = 5.5

B = 9.2

B = 11.5

B = 22.9

ln(relativeyieldstress)


Fig. 7. Relative tensile yield stress of several PA/OMMTcomposites plotted as a function of silicate content. Symbols:(h) Liu [64] (25), (,) Racz [72] (11), (s) Fornes [17] (21), ()Fornes[87](24), (n) Fornes[87](23), (d) Shelly[81](26).

0.00 0.02 0.04 0.06 0.08

20

30

40

50

60

70

80

90

Tensileyieldstress(M

Pa)


Fig. 8. Composition dependence of the tensile yield stress ofPVC/OMMT composites. Widely differing effect of the clay onproperties. Symbols: (n) Pozsgay[73](31), (s) Wan[88](35), ()Gong[90](36).



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aggregation occurs. Moreover, the PVC compositescontaining OMMT are transparent compared tothose prepared with neat sodium montmorilloniteindicating exfoliation and good dispersion. Thetransparency of the composite containing NaMMTdecreases drastically with increasing silicate content,while it changes much moderately in the presence ofOMMT (Fig. 10). Decreasing transparency indi-

cates only partial exfoliation in the latter case, butexfoliation does take place in this composite. Thestrong decrease of yield stress is definitely the resultof changing matrix properties. The lubricating effectof the exfoliated silicate is also shown by thedecrease in torque values recorded during thehomogenization of the composite in the internalmixer (Fig. 11). Torque is proportional to viscosity,which decreases drastically already at very lowsilicate contents. These results clearly prove thatcompetitive interactions take place in PVC nano-composites containing an organophilic silicate,which may change the properties of the matrix poly-mer and the reinforcing effect of the silicate con-siderably.

4.4. Chemical reactions (PET, PP)

The possibility of chemical reactions was men-tioned already in previous sections. Amines usedas surfactants may initiate the degradation of PVC[73]and we proved earlier that maleinated polymersreact chemically with the surfactant in PP nanocom-posites [52]. Naturally chemical reactions modifythe structure and characteristics of the matrix dras-tically leading to considerable changes in interactionand composite properties. We present two examplesto demonstrate the possible effect of chemical

reactions.

0 2 4 6 8 10 120

10

20

30

40

50

60

70

Trans

parency(%)

MMT content (vol%)

Fig. 10. Light transmittance of PVC/MMT composites. Effect oforganophilization on clay dispersion. Symbols: (h) PVC/NaMMT (29), (d) PVC/OMMT (30), (n) PVC/OMMT (31),

(.) PVC/OMMT (32)[73].

0 2 4 6 8 10 12

12

13

14

15

16

17

Torque(Nm)

MMT content (vol%)

Fig. 11. Effect of organophilization on the viscosity of PVC/claycomposites. Lubrication effect of exfoliated silicate. Symbols: (h)PVC/NaMMT (29), (d) PVC/OMMT (30)[73].

0.00 0.04 0.08 0.12 0.16

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8


B = -19.3

B = -3.2

B = 0.8

B = 4.5

B = 6.9


Fig. 9. Relative tensile yield stress of PVC/OMMT compositesplotted according to Eq.(4). Results were obtained by us or takenfrom the literature. Symbols: (d) Pozsgay[73](30), (s) Wan[89](35), (h) Pozsgay[73] (29), (,) Wang [91] (37), () Gong [90](36).


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Liu and Wu[74]prepared PP nanocomposites byswelling OMMT with an acrylate, which containedalso a peroxide. Their composites had a Bvalue of15.6, while the next largest value was below 9 in aset of 40 PP composites [37]. Composition or pro-

cessing conditions did not justify this large B. Theonly reasonable explanation is that the decomposi-tion of the peroxide created radicals in the PPmatrix, which reacted with the acrylate leading tosignificantly modified matrix properties and interac-tion. We do not have any proof for this assumptionsince the authors did not investigate or even men-tion the possibility of chemical reactions. However,our tentative explanation is supported by experiencerelated to the effect of peroxides and acrylates on PP[75,76].

The possibility of reactions cannot be excluded inPET nanocomposites either. The tensile yield stressof several PET composites is plotted in Fig. 12.Pegoretti et al.[77]used recycled PET and OMMTto prepare their composites. Yield stress does notchange with composition in this case. On the otherhand, Yuan et al.[78]swelled their silicate also withpoly(ethylene glycol), PEG, before adding it to thepolymer. When relative yield stress is plotted in thelinear form negative slopes are obtained in two casesindicating changes in matrix properties (Fig. 13). Wemay safely assume that glycolysis or transesterifica-

tion takes place during the processing of the compos-ites resulting in a matrix with smaller molecular

weight and modified properties. The possibility andeffect of chemical reactions were not investigated inthis case either. Further examples can be mentionedwhere chemical reactions change the structure andproperties of nanocomposites, like the deintercala-

tion of silicates in rubber composites vulcanized withsulphur[79,80].

5. Discussion

Although we do not have unassailable proof forthe occurrence of chemical reactions in the abovementioned cases, we may safely state that the prep-aration and formation of layered silicate nanocom-posites is much more complex than often claimed inthe literature. A considerable number of processes

may take place as shown above, which can all influ-ence the properties of the composites and result inconsiderable reinforcement or on the opposite, ina decrease of stiffness and strength. The effect ofthe components and the results achieved dependon composition and also on processing conditions.Only the thorough characterization of compositestructure and the consideration of all possibleprocesses and factors can supply a plausible expla-nation for unexpected phenomena, behavior, orproperties.

In order to obtain a full picture about the rein-

forcing effect of silicates in various matrices, the Bvalue of all composites, including those having PP

0.00 0.01 0.02 0.03 0.0420

30

40

50

60

70

Tensileyieldstress(MPa)


Fig. 12. Composition dependence of the tensile yield stress ofPET/OMMT composites. Symbols: (n) Pegoretti [77] (43), (,)

Pegoretti[77](44), (h) Yuan[78](45), (s) Yuan[78](46).

0.00 0.01 0.02 0.03 0.04

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

B = -6.0

B = -10.2

B = 3.4



Fig. 13. Natural logarithm of the relative tensile yield stress ofPET/OMMT composites plotted as a function of silicate content.

Symbols are the same as inFig. 12.



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as matrix, is plotted against the natural logarithm ofcalculated matrix yield stress (Fig. 14). As men-tioned before, this representation compensates fordifferent matrix properties and makes possible thecomparison of various composites. The figure shows

that reinforcement covers a very wide range frompractically zero to a relatively high value. In com-posites falling on or situated near the broken linethe silicate does not reinforce the polymer. More-over, negative values were also measured in somecases explained by changes in matrix properties.The B values of these composites are not plottedin the figure.

Although Bvalues cover a very wide range, wemust emphasize that even the largest values indicatemoderate reinforcement. Apart from a few values, Bhardly exceeds 10 for most of the composites. Thisvalue is in the range of short fiber reinforced com-posites [35,36,48,38] and much below the expectedimprovement in strength. The largest values repre-sent considerable reinforcement, but they are verydifficult to explain. The highest value was obtainedin PA composites by Shelley [81], but we do notknow anything about the characteristics of the com-ponents in this case. In PP the largest Bvalue wasachieved by Liu et al. [74]by swelling OMMT withan acrylate and a peroxide. The large reinforcementachieved in PE by Kato et al. [13] cannot be

explained very easily either, the components, com-

position and preparation of the composite do notdiffer considerably from those of the other compos-

ites with much smaller Bvalues.The determination ofBvalues offers the possibil-ity to estimate also the extent of exfoliation in thesecomposites. We assume that NaMMT does notexfoliate at all, while the specific surface area ofcompletely exfoliated silicate is known to be about750 m2/g [4,18,82,83]. These two cases representthe boundaries for zero and maximum reinforce-ment. Bdepends linearly on specific surface area ifall other factors including interaction are the same(see Eq.(2)). Using PP/CaCO3composites as refer-ence we obtain the results listed in Table 2. This

shows that B values of about 200 should beobtained if exfoliation were complete down to indi-vidual silicate layers. The largest Bvalue calculatedfor the PA composite of Shelley[81]corresponds toa specific surface area of about 90 m2/g. This indi-cates the formation of stacks containing approxi-mately 10 silicate layers in the average. Thisnumber is larger than the value of 1.37 determinedby Fornes and Paul in PA [21] or the 3 plateletsper stacks found by Hotta and Paul in PE [12].On the other hand, it is very close to the predictionsof Manias et al. [32] who estimated the degree ofexfoliation as 2040%, or to the results of Tidjaniet al. [84], who observed stacks with 18 plateletsby TEM. Our result agrees well also with the expe-rience that complete exfoliation is very difficult toachieve and nanocomposites always contain differ-ent structural formations including individual sili-cate platelets, intercalated stacks, but sometimeseven large particles [85]. The complex structure ofa PA nanocomposite is demonstrated by Fig. 15showing most of the features mentioned above.

We must call attention here to the limitations

of the model and the calculations. Although B

1 2 3 4 5

0

5

10

15

20

25

30

Liu [74]

Kato [13]

Shelly [81]

ParameterB

ln(matrix yield stress,0c

)

Fig. 14. Estimation of the reinforcing effect of layered silicates inpolymer composites. Reinforcement is given by the distance of agiven point from the broken line. Symbols: (s) PA, (h) PP, (n)PE, () PVC, (,) others, (d) CaCO3 reference line (dashed) forthe effect of changing matrix properties.

Table 2Estimation of the extent of exfoliation from parameter Bdetermined in polymer composites

Filler ParameterB

Specificsurface area(m2/g)

Extent ofexfoliation(%)

Ref.

CaCO3 1.5 3.3 0 [40]MMT 1.8 26.0 0 [93]MMT 195a 750 100 [4,82,83]OMMT 22.9 91.0b 11.7 [81]

a Calculated from published specific surface area assumingcomplete exfoliation.b Calculated from the largest Bvalue published.


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measures reinforcement properly, its value is influ-enced by several factors. Bcan be used for the esti-mation of the extent of exfoliation only in the caseof good adhesion and in the absence of platelet ori-entation in a particular direction. Although this lat-ter condition might be fulfilled in many cases, since

published TEM micrographs usually show randomorientation of the platelets, the adhesion betweenthe matrix and the polymer may be very differentfrom one composite to the other. We may assumegood adhesion in PE, PP and in some of the PA com-posites, because of the use of functonalized polymersor e-caprolactam as a coupling agent, respectively.In these casesBis related to the degree of exfoliation.Interaction is weak in PA composites containing sil-icates treated with aliphatic amines as shown by thesmall Bvalues obtained in such cases and also byprevious results[72]. Interaction and other factors,like the occurrence of chemical reactions, determineproperties in PVC and PET composites. In suchcases, a negative slope may be obtained, which indi-cates the formation of a soft interphase or the con-tinuous change of matrix properties. Naturally, themodel cannot be used for the estimation of reinforce-ment and exfoliation in such cases, but the negativevalue ofBitself calls the attention to the existence ofspecial processes or effects.

Finally, we must comment on the use of tensileyield stress and strength data for the estimation of

reinforcement versus modulus, which is preferred

generally. As mentioned earlier, the basic conditionof efficient reinforcement is strong adhesion betweenthe matrix and the reinforcing component. Goodadhesion and reinforcement are usually indicatedby large stiffness and strength at the same time. If

strength or yield stress is small, reinforcement can-not be achieved, because of weak interaction orpoor interphase properties. On the other hand,modulus always increases in the presence of hardinclusions and specific surface area has only a mod-erate effect on it. Modulus is not very sensitive tointeractions or structure either. As a consequencewe are convinced that the evaluation of strengthor yield stress gives more accurate informationabout the reinforcing effect of silicates in nanocom-posites than modulus.

6. Conclusions

The analysis of the tensile yield stress of a largenumber polymer/layered silicate composites showedwidely differing mechanical properties. The composi-tion dependence of yield stress can be described andevaluated quantitatively by a simple model devel-oped earlier for particulate filled polymers. Compar-ison of data produced in our laboratory or takenfrom the literature indicated that several processesmay take place during the preparation of the com-

posites and a considerable number of factors influ-ence composite properties. Quite a few of these areoften neglected and percentage increase in modulus,strength or other properties is reported in publishedpapers instead. The most important of such effectsare changing matrix properties when a functional-ized polymer is used to promote adhesion (PE, PP),modification of crystalline structure due to nucle-ation (PA, PP), plasticization or lubrication (PVC),decreased interaction (PA, PVC, PET, rubbers) orchemical reactions (PVC, PP, PET). Using a few sim-ple assumptions, most of which are supported by pre-vious experience, the extent of exfoliation can beestimated quantitatively in nanocomposites. Theanalysis of the tensile yield stress of more than 80composites with various matrices indicated that theextent of exfoliation is very low in most composites;it reaches maximum 10% in the best case, which cor-responds to about 10 silicate layers per stack.Although the approach has limitations and severalfactors were neglected during analysis, this result isin agreement with observations indicating thatcomplete exfoliation rarely can be reached in ther-

moplastic/clay composites. In order to achieve larger

Fig. 15. TEM micrograph showing the structure of a PAnanocomposite containing 2 vol% OMMT organophilized withx-amino acid.



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reinforcement, silicates must be exfoliated moreperfectly in the future.

Acknowledgements

The authors are indebted to Ibolya Csapo, LajosRacz, Karoly Renner, Bela Pukanszky Jr., TundeFrater, Agnes Abranyi, Istvan Sajo and JanosMoczo for their help in sample preparation andin the characterization of various nanocomposites.Hyoung Jin Choi and Min Soo Yang are acknowl-edged for the preparation of TEM micrographs.We appreciate the help of Sud Chemie GmbH andZoltek Rt. in the donation of raw materials. The re-search on heterogeneous polymer systems waspartly financed by the National Scientific ResearchFund of Hungary (OTKA Grant No. T043517).

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