Pure & AppL Chem., Vol. 57, No. 1 1 , pp. 1667—1678, 1985.Printed in Great Britain.© 1985 IUPAC

Interfacial effects in composites

John A. Manson

Materials Research Center, and Departments of Chemistry and Metallurgical andMaterial Engineering, Lehigh University, Bethlehem, PA 18015

Abstract — It is well known that the behavior of diverse multicomponent systems

such as fibrous composites, particle—filled polymers, polymer blends, andpigmented coatings depends not only on the composition, morphology, andproperties of the constituents, but also on the nature of the interfacial region.

This paper reviews typical examples, with emphasis on interpretation ofinteractions between constituents considered in terms of Lewis acids or bases.

INTRODUCTION

With a simple well—bonded composite system, the value of a given property P is determinedby a summation of the constituent properties, usually weighted by the respective volumefractions. In this limiting case, so—called "perfect" adhesion is assumed (with a sharpdiscontinuity in properties at a negligibly thick interface). The value of P may be givenby one of many equations, of which the following upper and lower—bound expressions are thesimplest (1—12):

P =PAVA + PBVB (upper bound) (1)

AB= (lower bound) (2)AB BA

where the subscripts A and B refer to the two constituents. While a particular equationmay be appropriate for a given system, it should be noted that such continuum—type modelshold in principle for all multicomponent systems, including not only particle andfiber—reinforced polymers, but also such diverse composites as pigmented coatings,laminates, polymer blends, and even semicrystalline polymers. Also, models of this kindcan characterize many kinds of propertIes, from mechanical to transport behavior.Nevertheless, such models are often excessively simplistic when applied to real systems;in practice, one must consider real interfaces differently.

This paper reviews briefly implications of the interfacial region with respect tomechanical and transport behavior. A new approach is also described in which interfacialinteraction is interpreted in terms of the Lewis—acid or Lewis—base character of theconstituents.

The interface

In some cases, typically for elastic behavior, equations 1 and 2 (or variations thereof)may hold quite well. While good adhesion may be effected chemically, satisfactorymechanical coupling of stresses and displacements may be obtained due to shrinkagestresses, though in the latter case, the stress field will depend on temperature.However, it is often useful to consider that the surface of an actual inclusion containasperities, that the state of composition of the matrix may be different at the interfacethan in bulk, and that various imperfections and stress gradients or singularities mayexist in the interfacial region (1,4,6,11—18). Hence, the concept of an "interphase" hasbeen introduced (19) to reflect the presence of an interfacial layer with its own set ofproperties, and its own interface (sharp or diffuse) with the bulk polymer (20). To besure, even the concept of the interphase is itself simplistic, for the layer may itselfexhibit a gradient of composition or morphology (12,21). (Although an "interphase" shouldtherefore not be taken to imply the existence of a classically defined homogeneous,

discretely bounded phase, the term is commonly used for the sake of simplicity.) However,consideration of properties of a binary composite in terms of the two constituents plus aninterphase constitutes a step forward in our appreciation of reality.

1667

1668 J.A.MANSON

Interphases can be developed either deliberately or adventitiously. Indeed couplingagents such as silanes or titanates have long been used to improve mechanical properties(12,22) of composites, and the surfaces of fillers are commonly treated to modifyadhesion. Attention has also been given to the use of a polymeric interlayer between areinforcing fiber or a filler particle and the matrix. For example, a relatively softinterlayer has been claimed to give a useful balance between stiffness and toughness ingeneral composite systems (15,23—28); a controlled crystalline morphology at the interfacemay also yield improved properties (21,29). Adventitious phenomena include adsorption,chemical reaction (such as copolymerization of the matrix with the coupling agent, or theformation of an interpenetrating polymer network) and interdiffusion (between two polymerphases, or between a coupling agent and the matrix) (1,2,6,9,12).

In view of the diverse and often coupled phenomena involved, it is not surprising that theeffects of interfacial adhesion are complex (1,6,12,18). With well—bonded high—modulusinclusions, the modulus, and sometimes the tensile strength and fracture energy (1,30), isincreased; the specific properly balance achieved may depend on the modulus of theinterphase, as in the case of a deliberately developed interphase (23—28). With respectto large—scale deformation, it has been shown that a silane interphase can change thebalance between shearing and crazing (31). Swelling and permeability is uually decreasedand the relaxation spectrum is often shifted to higher temperatures or longer times (32).Thus, the Tg. of such a system is often, though not always, higher than in the pure matrix(1,18); in tact, the interphase may contain both mobile and immobile components (33).(For reviews on relaxation and transport behavior, see refs. 1 and 18.) Of course, withrubbery inclusions, good adhesion is needed to enable the rubbery phase to generatebeneficial delocalized crazing and shear (33a).

In any case, thermodynamic and kinetic factors must be considered. Even thoughthermodynamics may favor a particular equilibrium state, and hence a particular set ofproperties, the nature of the interphase will depend on the history of the system, thatis, on the conditions of film formation or solidification from the melt (12,18,34,35).For example, the T of a polystyrene—silica composite was lower than that of the matrixwhen first but became higher on annealing (34). Presumably the originalinterphase was not at equilibrium, and was relatively mobile, while subsequent annealingpermitted equilibration to a less mobile conformation. In another example, the blending ofa matrix with a filler that had been densely grafted with a matrix—compatible polymer ledto poor strength (36), evidently because crowding at the filler surface Inhibitedinterdiffusion with the matrix by creating an unfavorable entropy of mixing.

Unfortunately, it is often implied that "good" adhesion is generally to be desired. Asindicated, this may be so for some properties, e.g., tensile strength and modulus in thecase of fibrous composites, and permeability generally. However, the role of adhesion in

other properties and systems is more complex (1,6,30). For example, in many compositesmaximum fracture energy, impact strength or fatigue resistance [with fibers parallel to acrack (37)] may actually require poor or intermediate adhesion (1,4,5,37). Also, withzinc—filled, corrosion—resisting epoxy coatings, poor interfacial adhesion is required inorder to permit the formation of corrosion—inhibiting zinc compounds at the zinc surfaceand facile diffusion to the steel substrate (38).

As mentioned above, regardless of the inherent interfacial adhesion, good interpenetrationbetween the matrix and interphase [sometimes involving formation of a trueinterpenetrating polymer network (39)] is generally necessary to avoid weak mechanicalcoupling between the interphase and the matrix (12,22,20,20a,36). If such weak couplingleads to a brittle interphase—matrix interface, crosslinking or use of ahigher—molecular—weight matrix may be advantageous (20a). Good interpenetration will alsobe favored by inherently good miscibility (see below).

The behavior of composites under especially severe types of loading such as fatigue (40),stress corrosion cracking (41) and simple exposure to an aggressive medium such as water(22) is of particular interest. Indeed, coupling agents such as silanes have long beenknown to improve the resistance of composites to aqueous media. Nevertheless, even whenfibers are treated to enhance adhesion, debonding under stress—cracking (41) or fatigueconditions (42—47) is a major micromechanism of failure; with fatigue loading, waterdeteriorates performance still further. Thus, fatigue crack propagation resistance inpoly(ethylene terephthalate) reinforced with glass fibers disposed in various orientationsis increased by fiber treatment to improve adhesion (42); also, overall fatigue life inrandom—glass—fiber—reinforced polypropylene is increased by the use of an appropriatesilane coupling agent (46). Nevertheless, with polypropylene, aligned short glass fibershaving enhanced adhesion improved resistance to fatigue when the fibers wereperpendicular to the growing crack, but had a deleterious effect when the fibers wereparallel (37). In the latter case, the enhanced adhesion was shown to decrease the sizeof the damage zone, and hence the extent of energy dissipation.

Interfacial effects in composites 1669

Some models for thermal and mechanical behavior

In order to model the transport and mechanical behavior or real composite systems in whichan interphase exists, several approaches have been made. For example, a firstapproximation has been proposed (8,9,11) in which the interphase is assumed to be a

homogeneous and isotropic phase interposed between the two principal phases andwell—bonded to each of them. The filled polymer is as an aggregation of many sphericalvolume elements containing a filler particle and concentric shells consisting of aninterphase and matrix, volume fractions being constant. It was possible to estimate thethermal expansion coefficient as follows:

% it vj(Kj/Kc) j [1_a+a(Kj/Kc)i'(3)

where: N = the number of shells (in this case, 3); v.is the volume fraction of the jthphase; a = (1-I-v )/3(1—v), v being the Poisson's ratio3of the composite; and K. and K arethe bulk nodulic of the jth component and the composite, respectively. Using 4i suitaCblecomputer program the tensile modulus and volume fraction of the interphase werecalculated. This model was reported to successfully represent the thermonechanicalbehavior of resins containing particles of iron, and to permit estimation of the amount of

interphase formed (8,9).

In another approach, the van der Poel method (48) for predicting the complex shear andbulk modulus of particulate composites was extended recently to take into account thepresence of an interphase (49—51). The bulk modulus K was found in terms of bulk andshear moduli for the matrix, interface, and filler. Although no analytical solution wasfound for the shear modulus G, numerical solutions were found in terms of constituentmoduli, composition, and the corresponding values of Poisson's ratio. Complex moduli were

estimated by the correspondence principle. Good agreement with prediction was reportedfor a glass—bead—filled styrene—acrylonitrile copolymer in which the beads had been coatedwith a layer of an ethylene—vinyl acetate copolymer. Also, the dynamic mechanicalspectrum of the interphase (bound polymer) formed in a polyethylene/silica system wascharacterized, and the dynamic behavior of the composite was successfully predicted interms of the constituent properties. (Interestingly, the thickness of the interphase andits volume fraction were calculated to be 4 nn and 0.24, respectively.)

To model composites containing unidirectional fibers, the 3—phase concentric shells ofFig. la (8,9) can be replaced by concentric cylinders in which the fiber, matrix, andinterphase bear tensile, tensile, and shear stresses, respectively (12). A still morerealistic model was developed (52) in which the interphase was considered as a collectionof an infinite number of cylinders, each having its own modulus; thus, the modulus E(r)was taken to decrease continuously with the distance r from the fiber:

2k 2k2E(r) = Ef(rf/r) 1 + E

[1_(rf/r ] (4)

where f and rn refer to the filler and matrix respectively, and k1 and kY), which can bemeasured experimentally (53,54), are parameters characterizing the quali'ty of adhesion.When k1/k2 = 1, perfect adhesion would exist and E(r) would decrease continuously from Efto E ; when k /k < 1, E(r) would exhibit a minimum at or close to r = r , and then rise

m 21 . 0to E . Values or the volume fraction of the interphase can also be calculated (11). Themodu'us of the composite is given by the following rule of mixtures:

k -1

Ec = Em + (Ef_Em) uf + uf [Ef/(1_k1) (ufKl_l_1) — Em/(l_k2) (uf2 —in (5)

where u = rf2/r 2 is the filler volume fraction. Other models that have been proposed totake into accoun phase mixing in polymer blends (55,56) nay be adaptable to deal with the

interphase.

1670 J.AMANSON

The Nielsen model for permeability

The simplest expression relating the permeability coefficient of a composite, P , to the

volume fraction of a spherical filler v, Vf and the permeability coefficientcof thematrix, P, is (57):

r E'cI'Pp = v/(1 + vf/2) (6)

where P is the relative permeability, and the term (1 + v /2) corresponds to thetortuosfty T of the diffusion path. Deviations from sphericty can be allowed for bytaking •r to be [1 + (LVf/W)1 where L/W is the length—to—width ratio of the filler (58).While such equations constitute limiting cases at low vç, they hold reasonably well for

many composite systems, including semi—crystalline potymers (1), they assume goodinterfacial adhesion and do not take account of poor adhesion, phase mixing or thepresence of an interface. When the latter phenomena are involved, more general equationsare necessary (59,60,61; for reviews, see references 1,2 and 61). For the purpose of thispaper, the Nielsen equation (58) is particularly useful because it explicitly treats theproperties of an interphase;

P v. v+vP -lr n n T + T (7)

Pvf+ Pi(_vf)

where: v and v are the volume fractions of liquid permeant dissolved in theinterphase and pomer, respectively; P is the permeability coefficient of theinterphase; 1* j the tortuosity of the lnterphase ( T in many cases); and n is ageometrical factor ranging between zero and unity for thin plates perpendicular andparallel, respectively, to the membrane. (It is assumed that v is small with respect tovp.) Although Equation 7 is an approximation, it is said to hold well for many systems(58). If, as in the example discussed below, poor adhesion exists, P will becomeappreciable with respect to P and the value of n may approach zero. Equation 8 can alsobe modified to deal with aggregated filler particles (58).

Directcharacterization of interfacial interaction

In addition to the mechanical and transport measurements discussed above, many otherexperimental methods have been used to characterize the implications of interfacialinteractions as well as composition, especially in terms of miscibility. Useful methodsare based on such properties as density, viscosity, viscoeleastic relaxation,thermooptical behavior, morphology, scattering or diffraction behavior, solution behavior,inverse gas chromatography, and the various types of spectroscopy (for a review, seereference 63). With insoluble inclusions such as inorganic fillers, classical techniqueshave, of course, long been used to characterize wetting and adsorption (18), and directinformation about the interphase has been deduced from methods based on, for example,dilatometry (64), dynamic mechanical response (65), and nuclear magnetic resonance (66).Recent developments using improved instrumentation have made it possible to convenientlyobtain direct thermodynamic evidence about the interface, as well as about miscibility; insome cases, conformational data can be obtained as well (18,67,68). Such methods includestandard and Fourier transform infrared spectroscopy (FTIR) (12,12a,12b,67—72),microcalorimetry (72—75), and inverse gas chromatography (IGC) (76—81). The identity,nature, and concentration of surface atoms or functional groups is also of interest. Forthis purpose, both traditional techniques such as titration with acids or bases (82), andcontemporary instrumental techniques such as FTIR (12), x—ray photon spectroscopy (27),and IGC (76—78), are useful. For example, such measurements can be used to determine what

functional groups in a given coupling agent (applied in a given way) are actuallypresented to the matrix, and what kind of sites are available for adsorption on a bare

inorganic surface. (Sometimes the findings are unexpected.) Clearly, a combination offundamental thermodynamic information (e.g., enthalpies of interaction and polymer—polymeror polymer—solvent interaction parameters) with evidence for the number, nature, andidentity of adsorptive or reactive sites at an interface constitutes a powerful tool for

investigation.

At the same time, interpretation requires an adequate rationale. In this respect, anincreasing number of research groups, including our own, is finding the concept of (donor—acceptor interactions of which H—bonding interactions constitute a sub—set) to be veryuseful in both rationalizing behavior in many multicomponent systems, and in selectingcombinations of constituents (72,73,83—86). The approach is quite general and soundlybased on thermodynamics, and avoids theoretical and practical problems associated with,

for example, the use of solubility parameters (see below). In many cases, this concepthas already proved applicable to questions of both adsorption (obviously a sine qua nonfor adhesion) and miscibility. A discussion of principles follows, as well as specific

examples.

Interfacial effects in composites 1671

INTERFACIAL INTERACTION: A NEW LOOK AT AN OLD CONCEPT

Whether one is dealing with the miscibility of polymer blends, with polymer solutions, orwith interphase formation and interpenetration with the matrix in filled or reinforcedpolymers, it is useful to consider the fundamental basis for the interaction of phases.In the most general case (63, 72,73,83—95), the interaction of two phases involves somecombination of dispersion or London forces, "polar" or dipole—dipole forces, and specificinteractions such as proton transfer in acid—base reactions or hydrogen bonding. Althoughmany investigators of interfacial phenomena combine the dipole—dipole and specificinteractions into one "polar" contribution, it is now known (83—86, 87) that the specificinteractions usually dominate in both solution and the solid state. It is also useful totreat hydrogen bonding as a Lewis acid—base interaction and to consider all specificinteractions as involving a donor—acceptor relationship, whether or not charge transferactually occurs.

For many years the concept of the solubility parameter has found much use in predictingheats of mixing of two components that interact primarily through dispersion forces. Thescientific adage "like dissolves likes" has also been a popular, though often uncertain,guide to practice. In spite of the facts that the solubility parameter has long beenknown not to take account of specific interactions (63,83—86), and that specificinteractions are much more common than has been generally believed, the solubilityparameter is still commonly used in cases in which it is basically inapplicable. Althoughattempts have been made to include a polar or hydrogen—bonding component, these involve afundamentally incorrect evaluation, as shown below, and computer—forced correlation ofheat of mixing with work of adhesion are necessary. Such fitting, however, can lead toincorrect conclusions, e.g., finite polar terms for nonpolar polymers, and falsepredictions of miscibility or immiscibility (89,90).

The concept described here is based on extensive research by Drago et al. (87, 87a), whocorrelated enthalpies of interaction of more than 30 acids and bases in neutral solvents.

aDTo make the correlations, Drago expressed the acid—base interaction H with twoconstants for each base and acid (EB and CB, and EA and CA, respectively)'

_Hmab = CACB + EAEB (8)

The two constants for both the acid and the base reflect the idea that the strength ofinteraction of a pair of groups depends not only on the donor/acceptor characteristics butalso on the polarizability (E refers to the former, and C to the latter). The ratio C/Eis a measure of the relative ease of deforming outer electron orbitals by an electricfield; the greater C/E the "softer" the acid or base. Thus, iodine and Bronsted acidsbehave as soft and hard acids, respectively, while sulfur and oxygen behave as soft andhard bases, respectively. It is believed that maximum interaction occurs between softacids and softbbases, or hard acids and hard bases. Drago et al. were able to predictvalues of tH5 for many systems involving hydrogen bonds and both Bronsted and Lewisacids and bases with good accuracy. In addition, they found a most interesting result:good correlations could be made assuming the existence of only dispersion and acid—baseinteractions (the contribution of dipole—dipole interactions being usually very small or

negligible).

Turning to interfacial phenomena, Fowkes proposed long ago (91,92> that the work ofadhesion WA can be expressed as follows:

WA WA + WB" + WAh + . . . (9)

where the superscripts d, , and h refer to dispersion forces, dipole interactions, andhydrogen bonds, respectively. (In principle, any other kinds of interactions could beadded to Equation 10.) By analogy, the surface tension y would be given by

(10)

Several interfacial properties were successfully calculated using the geometric—mean

expression

2/dd (ri)An analogous geometric mean expression for WA" is also correct:

=2/y11'y21) (12)

1672 J.A.MANSON

However, it is quite incorrect to use a similar geometric mean to calculate WAY'; 1h iszero for hydrogen—bond acceptors suchhas ethers, or aromatics that cannot form hydrogenbonds with themselves, even though WA witjh a hydrogen donor may be large. Also, somehydrogen onors, e.g., chloroform, have y = 0, but large values of W . Note that

because y = 0 in such cases, one cannot calculate a solubility parameter contribution tothe enthalpy of mixing due to hydrogen bonding.

Fowkes then proposed that the heat of mixing, 1H, should be given by

1H PAVM VM4l42(tSl_62)_Xp(CACB + EAEB) MJM (13)

where t5 and 62 are the solubility parameters, VM is the volume of mixing, 4 and 2 arethe volume fractions, X is the mole fraction of acid—base pairs per mole of components,and AU is the chang of internal energy of mixing associated with dipole—dipoleinteracions. Similarly, the work of adhesion becomes:

d d moles acid—base pairs PWA = 2A 1B — f(CACB

+ EAEB) xunit area + A (14)

where W refers to dipole—dipole interaction. The fact that interfacial interactionsappear o be doninated by dispersion forces and specific acid—base or hydrogen bondingbehavior not only simplifies the picture considerably but also clarifies the significanceof the vague term "polar". Moreover, C and E constants can be determined for bothsubstrate and matrix materials (85).

Determination of C and E values

Values of _AHmab can be predicted for many acid—base pairs using Equation (8) andtabulated values of C and E (87,87a). Although the C and E values provided are for smallmolecules, the latter can often serve as approximate models for polymers, e.g., chloroform

for poly(vinyl chlide) and ethyl acetate for poly(vinyl acetate) (73). Directmeasurement of —AH can be effected calorimetrically or spectroscopically. The latteris suitable for naPy polymers because groups such as hydroxyl and carbonyl are ubiquitousand shifts in their spectral frequency are readily measured. b Thus, for example, thecarbonyl stretching frequency is decreased by acid—base (LVa ) and dispersion forceinteraction (Avd); the latter is proportional to the dispersion contribution to surface

tension (1d). For poly(methyl methacrylate) (PMMA)

ab(ü) = 1757 — 0.70 d(15)

can al:o be calibrated in terms Of AHab (measured calorimetrically); for Pt1NA

&v (C0) = (1.0 cm .kJ •nol ) AH (16)

Values of C and E , or C and E , are determinable for acidic or basic components,A A ab B Brespectively, from i}I with a complementary component whose C and E values are known.Procedures are described elsewhere (72).

EB' = _AHa/EA_CB? (CA/EA) (17)

For PMMA, CB and EB are about 1 and 0.7 (kcal.mol)½, respectively. This procedure shouldbe useful for other polymer systems as well.

In a similar manner, C and E values for filler surfaces can be estimated from heats ofadsorption or from spectral shifts. Thus, silica 1(an acidic filler) has been found toexhibit CA and EA values of 1.1 and.4 (kcal'mol )½, respectively (93) and a—iron oxidevalues of 1.1 an 0.5—1.0 (kcal.mol )½ (75). Using these constants, one can then predictthe heat of adsorption for any base of known C and EB; with CB and EB constants for abasic filler, a similar prediction can be made Por any acid of known CA and EA.

IMPLICATIONS AND APPLICATIONS

Miscibility

Until recently, it was thought that in general pairs of polymers tend to be immiscible (or"incompatible") because of the combination of a low (though favorable) entropy of mixingwith an often zero or negative charge (94). Whereas many pairs are in fact immiscible on

Interfacial effects in composites 1673

a segmented scale, it is now clear that many more systems are wholly or partially misciblethan may have been thought earlier. Thus the role of specific interactions leading tonegative (favorable) enthalpies of mixing is invoked more and more often to explainmiscibility (63,94). Such interpretations are also consistent with the increasing numberof systems found to exhibit rather than lower critical solution temperatures. Theformer are predicted by newer theories that admit specific interactions, while the latterare predicted by older theories, which also lead to the traditional solubility parameter,whose limitations have been discussed above.

As mentioned above, hydrogen bonding appears to play an important role in polymer—polymermiscibility. For example, poly(vinyl chloride) tends to be miscible with polymerscontaining carbonyl groups. The former acts as a hydrogen donor to the electron—richcarbonyl; non—H—bonded interactions may exist as well (63,69—71,94,95). Such interactionsprovide the basis for the use of polycaprolactone, for example, as a miscible polymericplasticizer. Whereas many miscibility studies have used infrared spectral shifts forcorrelation, calorimetric measurement have been useful also, e.g., in the prediction ofmiscibility in hydrogen—bonding polymers from the behavior of low—M analogues (96). Arecent example of a miscibility study (72) involved 1:1 blends of poly(methylmethacrylate) (PMMA) (a basic polymer) with the following acidic polymers: poly(vinylfluoride) (PVF), poly(vinylidene fluoride) (PVF2), postc1lorimated PVC (Cl—PVC), andpoly(vinyl butyral) (PVB). Calculations of values of AH based on carbonyl spectralshifts (after correction for the dispersion contribution) were in the order PVB >

PVF2>

PVF > Cl—PVC.

The implications of the Drago—Fowkes approach with respect to miscibility are quite clear.Once C and E values are determined for a polymer, it will be possible to predict the heatof mixing with any other polymer whose solubility parameters C and E values are known.

Solution and swelling can also be predicted better using the donor—acceptor concept incombination with the solubility parameter; the advantages of such an approach have beenrecently discussed and demonstrated (89,97). Polystyrene (a basic polymer) provides aninteresting example. Of 14 good solvents described in the literature, 9 are acidic,including 2 of the lowest, and three of the highest, ô. Whereas the range of for

nonacidic (essentially neutral) solvents was 8.6 — 10.0, the range for acidic solvents was

significantly greater, 0.4 — 10.7. Thus acid—base interaction broadened the range ofvalues corresponding to good solvation with an acidic polymer, poly(vinyl butyral) (6 =9.2 — 9.5), in which internal, intermolecular hydrogen bonding exists. Solubility wasfound to depend on acid—base interaction with strongly basic solvents (AH << 0) such as

cyclohexanone (6 = 10.4) and pyridine (10.6). HoweKer, the polymer was insoluble in themore strongly basic amines such as tributylamine (6 = 7.8). Evidently, in agrement witha traditional rule—of—thumb, the strong basicity cannot overcome the fact that 6 is about2 units below the value for the polymer. The need to consider 6 as well as specificinteraction was also seen in studies of the solbility of postc1lorinated PVC (6 " 8.65)in esters. With equally basic esters (same 611 3.4 kcal'mol ), maximum solubility wasfound when 6 was in the range 8.6 — 8.7, lower than the value for a nonbasic solvent likedichloromethane (6 = 9.9). It was suggested that the acid—base interaction resulted inthe orientation of hydrocarbon groups away from the polymer so that the complexed polymerinteracted with additional solvents more like a hydrocarbon. In any case, when acid—baseinteractions were present, the solubility and intrinsic viscosity showed a negativetemperature coefficient (consistent with the existence of upper critical solution

temperatures).

Adsorption and adhesion

The phenomenon of surface—bound polymer has long been known in composites such asreinforced elastomers (1,64). While actual chemical bonding may occur in a reinforcedelastomer, even relatively weak interactions such as those involved in hydrogen bonding

may suffice if the surface area of the filler, and hence number of active sites, is very

large. In any case, both typical carbon blacks and silica contain significant numbers ofacidic sites. Indeed, the fraction of bound rubber tends to increase with increasingunsaturation in the rubber (increasing concentration of electron—rich sites) (1). Inacrylonitrile—containing elastomers reinforced with silica (acidic), the bound rubberfraction was found to increase with an increase in the acrylonitrile content (i.e., withthe concentration of basic comonomer units) (98). Hysteresis also varied withacrylomitrile content, as would be expected if deformation in values the movement ofoppositely charged (or partially charged) units past each other. While in these cases the

possibility of acid—base interactions was not explicitly considered, such interactionsappear to constitute a reasonable explanation for at least some of the phenomena.

About a decade ago, explicit consideration of acid—base interactions and their role indispersion of pigments, adsorption, and adhesion began to occur. In 1966, it was proposedthat the adsorption of a basic polymer on an acidic filler involved protonation of theadsorbate, which could then desorb, leaving the polymer and filler with a positive and

1674 J.A.MANSON

negative charge, respectively (99). The consequent development of a negative zetapotential has since been confirmed (90,100), as has the development of a positive zetapotential when an acidic polymer is dispersed in a basic medium (90). Independently, andlater, it was shown that empirical observations of pigment—binder compatibility incoatings (as measured by gloss) could be correlated with the Lewis acidity or basicity ofthe pigment, binder, and solvent (101). Thus stability in colloidal dispersions requiresthe development of significant electrostatic interactions.

The concept of acid—base interaction in determining dispersion stability can be veryhelpful in understanding and solving industrial problems (72). For example, PMMA andchlorinated PVC were used as basic and acidic polymer probes, respectively, to investigate

reasons why three pigments (uncharacterized with respect to surface chemistry) gave gooddispersion with a complex polymeric binder solution, while a fourth, which had otherwisedesirable properties, posed serious problems. As shown in Table 1, significantdifferences in adsorption were noted (102). The first three pigments adsorbed PMMA wellfrom an essentially neutral solvent, while the fourth did not. Thus, the first threepigments behaved as Lewis acids, and the fourth as a Lewis base;

TABLE 1. Adsorption of acidic and basic polymers in pigments (102)

Pigment Adsorption of PNMAa Adsorption of

A 30 ( 100) 0.1 (0.3)B 97 (98) 1.7 (2)C 35 (80) 8.5 (20)D 6 (9) 61 (91)

aFirst number, mg/g; number in parenthesis, % adsorbed

the polymeric binder solution evidently behaved essentially as a Lewis base. [Of course,it should not be expected that a filler surface contain acidic or basic sites.]

The effect of acid—base interaction on dispersibility is not confined to solutions. Forexample, blends of barium titanate (basic) with polycarbonate (basic) were found topossess undesirably low electrical resistivity (103); this would be expected with poordispersion, which favors agglomeration and the formation of chains. However, when thetitanate was treated with aluminum sulfate to make the surface acidic, much more uniformdispersion was obtained. More recently (76), in a study of the mixing of PVC with calciumcarbonate (plain and treated to endure a predominantly acidic or basic surface) thecritical mixing time necessary for the torque to drop to a constant level was much shorterwith the basic filler (Table 2). With polyethylene (essentially neutral), the times werein fact somewhat increased by surface treatment of the filler. These results, i.e., thetendency of an essentially electrostatically neutral phase to be incompatible with a polarone, may be consistent with the established practice of coating fillers with a hydrophobiclayer for use with polyolefins.

TABLE 2. Critical mixing times for polymer with calcium carbonate (76)

Polymer £(polymer)a Filler Surface 2(filler)a Crit. mixing time, s

PVC 0.5 acidicno treatmentbasicmore basic

0.61.32.02.8

255903515

Polyethylene 1.1 acidicno treatmentbasicmore basic

0.61.32.02.8

14575105118

is a measurement of relative acidity from inverse gaschromatography; çZ < 1 for an acid and > 1 for a base.

Interfacialeffects in composites 1675

At the same time, it has been reported that the maximum torque developed during the meltmixing of an ethylene—vinyl acetate copolymer with various silicas varied directly withthe concentration of acidic silanol groups available for interaction with the basicacetate groups (104).

Confirmation of the competitive role of polymer—polymer, filler-.polymer, filler—solvent,and polymer—solvent interactions (90) was provided by a study of the adsorption of anacidic and basic polymer (postchlorinated PVC and PrIMA, respectively), from solutions inacidic, basic, and neutral solutions (90). Significant adsorption occurred only when thefiller and polymer constituted an acid—base pair, and when the solvent was either neutralor a much weaker acid or base than the polymer. Such selectivity of adsorption hasprovided the basis for the use of PNMA and postchlorinated PVC as model probes tosupplement titration and dye techniques in the characterization of the relative acidityand basicity of several fillers, including zinc particles and wollastonite (calciummetaeilicate) (105,106). Of course, while acidic or basic sites may dominate on thesurface of a pigment or filler, the surface may be amphoteric (101,105).

All these findings are consistent with earlier observation of the tendency ofcarbonyl—containing polymers to adsorb on acidic surfaces (for reviews, see 18,74, and75).

Recent research in our laboratory

During an early study, explanation of the transport and mechanical behavior of someglass—bead—filled epoxies requires postulation of an interphase (107,108); others alsoexamined effects of inclusion on viscoelasticity (109; 1, ch.12). When awareness of theresearch by Fowkes et al. developed (90), it seemed reasonable to extend the adsorptionstudies by investigating the implication of adsorption with respect to mechanicalproperties and morphology (110) of films chlorinated PVC and polycarbonate filled withbarium titanate, calcium carbonate or silica, and cast from tetrahydrofuran (TIIF) (basic)4or methylene chloride (acidic).

In general, the best films were obtainable from pigment—binder pairs that constituted anacid—base couple cast from methylene chloride, and that exhibited adsorption (confirmed bymicroscopy). Two exceptions were noted: silica and chlorinated PVC, and calciumcarbonate and polycarbonate, were compatible when cast from THF and methylene chloride,respectively; the solvent of the opposite electrostatic character appeared to act as acoupling agent. Strengths and moduli tended to exceed predicted bounds with appropriateacid—base coupling, and the tendency to retain solvent was clearly related to thepropensity for acid—base interaction with the matrix. Solvent complications were avoidedby the milling of ethylene—vinyl acetate (EVA) copolymers (0 to 32 wt Z acetate) withsilicas containing various concentrations of silanol groups, as determined by titration(104). Adsorption was favored with a given acetate content when the silanol concentrationwas high, and adsorption was correlated with tensile strength and modulus. The role ofdispensibility has already been mentioned (72,102). Adsorption studies have also beenuseful in correlating interfacial effects with tnechanical and transport behavior(105,106). Zinc powders adsorbed PNMA but not chlorinated PVC, and hence acidic sitespredominated; adhesion to polystyrene (basic) was much stronger than to the chlorinatedPVC or an epoxy (acidic). Wollastonite adsorbed both PMMA and chlorinated PVC, the latterto a lesser extent. Tensile and compressive fracture energy varied with the extent ofadsorption (Fig. 1). Although the data for Fig. 1 are not corrected to account forparticle size differences, a similar trend is seen after such correction.

ie I •i2,. COMPRESSION TEST SPLITTING TENSILE TEST 'I

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ADSORPTION (PMMA/CCI4), r (mg/rn2) ADSORPTION (PMMA/CCI4), I'. (mg/rn2)

Fig. 1. Fracture Energy (stress—strain curves forwollastonite as a function of adsorption of PNNA).

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