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Modification of Polymer Properties through Ion Incorporation C. Geraldine Bazuln and Ad1 Ebnbarg’ osparhnsnr Of ‘2WlWy, W UnhmsHy, Mnireal, CbLxsx?, Carla& KIA 2K6
This artide desdbesa g w p of k ” t a i n l n g polymers calkd homers. the focus being on ethylene-. styrene-, and ethyl acrylate-based materiais. Ion aggregation in these polymers is discussed, extensive experimental evidence indicating two types of aggregates: multiplets and clusters. Attempts to elucidate tl-m sbuchne of ionomers thrwgh models and theories are summarized. Concerning the properties of bulk ionomers.
melt viscosities. glass-transition temperatures. and rubbery moduli are shown to be raised drastically by ion introduction. In particular. the effects on the modulus-temperature. stress relaxation. and dynamic mechanical characteristics, on crystallinity, and on melt rheology are examined. Selective plastC cization is also discussed. Where possible. structure-property relations are explained, particulariy as concerns the degree of clustering and the sizes and strengths of clusters. Molecular parameters influencing ion aggregation and ionomer properties include the dielectric constant of the backbone, position and type of ionic group, counterion type, and degree of neutralization.
I. Introduction The past few years have witnessed an almost explosive growth of research into the properties of ion-containing polymers (Holliday, 1975; Eisenberg and King, 1977). Some areas,notably the polyeledrolytes, have already been well explored. Other areas have only recently been receiving extensive attention, in particular the solid-state or bulk properties. Still other areas-elective plasticization and nonaqueous solution properties, for example-are in their infancy. The primary reason for the great interest in them materials lies in the major changes which can be achieved in the properties of polymers by introducing ions. Some of these changes are still unexplained. Among the dramatic effecta that have been observed are increases in the moduli, in the glass transition temperatures, and in the viscosities. A few examples are cited to illustrate these effects. The 10-smodulus of styrene copolymerized with ea. 9 mol % sodium methacrylate is ca. lo8 N/m2 at 180 OC. Pure polystyrene of a comparable molecular weight is an oil a t the same temperature. Furthermore, although the glass transition (T,) of the copolymer is raised by only 30 “C, the material behaves like a phase-separated block copolymer that contains ca.60-70% of a “hard phase having a TI of ca. 200 O C (Eisenberg and Navratil, 1973). Magnesium methacrylate incorporated into polyethylene raises the Tgby ea. 10 OC/mol% ionic comonomer (Otocka and Kwei, 1968). More dramatically, the T, of the polyphosphate system rises from -10 “C for the nonionic polyacid [(HPO,),] to +520 “C for the calcium polyphosphate (Eisenberget al., 1966). In still another system, when ca. 13 mol % of LiCIO, is introduced into low molecular weight poly(propy1ene oxide), the viscosity rises by a factor of 10‘ (Eisenberg et al., 1980b). Other effects are equally intriguing. The incorporation of 4.1 mol % of methacrylic acid into polyethylene (only some of which needs to be ionized) renders the polymer practically transparent, even though the degree of crystallinity is not significantly lowered (Longworth, 1975,p 0196-4321/81/1220-0271$01.25/0
C G Basuin is a Ph.D. gradunte student a t McCill Uniuersity, Montreal, Canada. She receiued her A.B. (Philosophy) in 1973 a t Caluin College, Grand Rapids, Mich., and her BSc. (Mathematics, Physics, and Chemistry) in 1977 a t McCill Uniuersity. Ms. ... Bazuin is currently doing research in rheological properties of ionomer solutions. She is the holder of Natural Sciences and Engineering Research Council Canada and Prouince of Quebec scholarships. Adi Eisenbeg is a Professor of
.a 4
Chemistry at McCill Uniuersity, Montreal, Canada. He received his BSc. in 1957 at Worcester Polytechnic Institute, and his M.A. in 1959 and Ph.D. in 1960 a t Princeton Uniuersity. He spent a postdoctoral year at Princeton and a one year NATO Fellowship a t Basel, Switzerland. Dr. Eisenberg serued as Assistant Professor of Chemistry a t the Uniuersity of California, Los Angeles, from 1962 to 1967, Visiting Professor at Kyoto Uniuersity, Japan, 1973-1974, and at the Weizmann Institute, Israel, 1974. His research interests include coulombic interactions in polymers and structure-property relatiom in organic and inorganic macromolecules. Dr. Eisenberg is the author of about 100 papers and eo-author or editor of three books in the field.
70). Of potential industrial importance is the fact that thermoplastic elastomers can be formed through the introduction of ions into rubbery systems, for instance by the incorporation of zinc sulfonate groups into ethylenepropylene rubbers (EPDM),(Makowski et al., 1980). In another system, selective plasticization of either the ionic regions or of the neutral hydrocarbon backbone can be effected. This is exemplified by the differing plasticizing behavior of glycerol and dioctyl phthalate on sulfonated polystyrene salts, where the dual plasticization can yield a material similar to plasticized poly(viny1 chloride) (Lundberg et al., 1980). An unusual dilute solution property of ion-containing polymers is illustrated by sodium sulfonated polystyrene. When this substance (containing less than 5 mol % sulfonate groups) is dissolved in a solution of xylene and a small amount of hexanol, the viscosity can be observed to increase significantly with temperature, the exact behavior depending on the amount of sulfonation and on the amount of polar solvent present (Lundberg, 1978; Lundberg and Makowski, 1980). This indicates a potential for using ion-containing polymers as novel viscosity control additives such that viscosity can be made to increase with temperature. 8 1981 A n “
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Diffusion coefficients in polymers can also be affected markedly by the introduction of ions. For example, the copolymerization of ca. 9 mol % of pendant carboxylate ions into styrene decreases the diffusion coefficient of water in the material by ca.two orders of magnitude (Brock”, 1981). In contrast, the incorporation of sulfonate groups at the end of short side chains into a polytetrafluoroethylene-based material yields a system (Nafion) in which the diffusion coefficient for water is so drastically enhanced that it is close to the self-diffusion coefficient (Yeo and Eisenberg, 1977). The objective of this article is to introduce the reader to the major phenomena observed in polymers into which a relatively small mole percentage of ionic co-units has been incorporated, the focus being on their bulk properties. It can thus be considered as a brief introduction to the field of solid-state copolymers modified through the presence of coulombic interactions. The specific aim will be to describe both how polymer properties change as a result of the presence of ionic groups and the molecular phenomena which underlie these changes, insofar as they are understood. I t is evident that the materials comprising the field of ion-containing polymers range all the way from naturally occurring biopolymers through synthetic organic polymers to inorganic glasses and ceramics. They can be characterized as having from a low to a high degree of ionic and/or covalent cross-linking, and they can possess a low to a high degree of ionic character (Eisenberg and King, 1977, p 5). The materials that will primarily be dealt with in this article are synthetic organic polymers having an ion content of up to 10-15 mol % ; hence they are insoluble in water in most cases. Moreover, they possess weak ionic-network character (usually no covalent-network character), and are therefore thermoplastic at relatively low temperatures. The members of this group of ioncontaining polymers have come to be classified as ionomers, a term first introduced by Rees and Vaughan (1965a,b) in connection with the Surlyns which are neutralized polyethylene-based materials copolymerized with methacrylic acid. Specifically, an ionomer can be defined as an ionized copolymer whose major component is a nonionic backbone (which may or may not be crystallizable) and whose minor component is an ionizable or ionic comonomer. The latter component may either have been copolymerized with the major component or have been introduced by modifying the nonionic polymer through appropriate chemical techniques to yield a partly ionic material. The ionic component, usually in the form of pendant acid groups, is then partially or completely neutralized to form the polymer salts, the neutralization being effected by the addition of small ions, or counterions, such as sodium, cesium, barium, zinc, or ammonium. It should be noted here that polyelectrolytes, which contain a high percentage of salt groups (and for which a large body of literature already exists), are specifically excluded from the definition of ionomers. To date, the most extensive ionomer studies have involved polyethylene and polystyrene backbones. Other backbones utilized include polybutadiene, poly(ethy1 acrylate), polytetrafluoroethylene, polyisoprene, and, most recently, polypentenamers and ethylene-propylene copolymers. Carboxylic and sulfonic acid moieties have been the most common acid groups employed-the former usually as the pendant portion of the comonomer, methacrylic or acrylic acid. Both the carboxylic and sulfonic acid moieties have also been introduced by appropriate chemical means as substituents on, for instance, benzene
rings of polystyrene in postpolymerization reactions. Addition across double bonds has been used in preparing ionic derivatives of polypentenamers; these include thioglycolate and phosphonate as well as sulfonate and carboxylate derivatives (MacKnight and Earnest, 1980). Ionic content has been introduced into polymers, too, through quaternization of amine groups. Most of these techniques are well known and extensively described in the literature. It is noteworthy that the field of ionomers, in principle, covers a considerably wider area than the field of nonionic polymers in that any nonionic polymer can be modified in numerous ways through ion incorporation. Such modification can be accomplished by varying one or more of such parameters as the type of ionic comonomer, the ion content, the ion placement, the degree of neutralization, the counterion type, etc. It must be stressed here that, due to the extensive literature that has already accumulated in the field, this article can by no means be thought of as an exhaustive treatment. Neither can it contain a catalog of applications; rather, the primary purpose is to illustrate and explain selective structure-property relations and property-modification possibilities in the hope that these will suggest new applications. A more intensive study of the field can be obtained from the existing literature, namely, two books (Holliday, 1975; Eisenberg and King, 1977),several review articles (Eisenberg, 1967; Otocka, 1971; MacKnight and Earnest, 1980),and a number of symposia (Polym. Prepr., 1968; Bikales, 1973; Eisenberg, 1974; Rembaum and SBlBgny, 1975). Hence, this article is designed, in part, to supplement that literature. Topics related to ion-containing polymers that will not be included in this article are the biological macromolecules, water-soluble nonionic systems and ion-exchange resins. Furthermore, crystalline systems will not be considered in very much detail, and solution properties will not be discussed at all except insofar as they are related to plasticization effects. With regard to the contents of the article, it will first describe attempts to elucidate the structure of bulk ionomers as revealed by various kinds of experimental evidence and as advanced by models and theories. Discussion will then center on the property-modification effects of ion incorporation: in particular, the effects on glass transitions, on mechanical properties, and on melt rheology. These effects will be related to underlying structural explanations wherever such explanations are suggested by the evidence. Finally, plasticization behavior in ionomers will be reported on. Before concluding the Introduction, it is worthwhile to establish a consistent nomenclature, as proposed by Eisenberg and King (1977, p 9) that will succinctly identify the particular ionomer under discussion. The ionizable copolymer (or terpolymer) will be denoted by: (i) its composition, with the mole fraction(s) of the minor component@) specified; (ii) the counterion as well as the fraction of the total ionizable material neutralized (where applicable); and (iii) the plasticizer (where applicable), expressed as the weight fraction of the total plasticized weight. Scheme I illustrates such a designation. Symbols employed for common monomeric components are as follows: S, styrene; B, butadiene; E, ethylene; EA, ethyl acrylate; (4-VP),4-vinylpyridine; MAA, methacrylic acid; AA, acrylic acid. When an ionomeric system in general is referred to (rather than the specific makeup of a material), it will usually be denoted by its two components, as in PS-MAA, where the first and major component is preceded by a P indicating “poly”; when a salt form of the ionomeric system is referred to, it will be denoted by the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 273 Scheme I -0.034M
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4-0.75Na-0.08DOP sample designation -plasticizer (dioctylphthalate) weight fraction plasticizer counterion fractional degree of neutralization (if number is omitted, assume 1.0) minor component (methacrylic acid) mole fraction of ’
l i
two components and the counterion, as in PS-MAA-Na. Finally, to avoid possible confusion, it is to be noted that when the term “ionic” is used to describe a material, it means that the material contains ions or ionic units and not that it was synthesized by ionic methods. 11. Structure A. Introduction. As mentioned above, the incorporation of ions into polymers can cause a drastic modification of their bulk properties. From a structural point of view, this effect must be attributed to aggregation of the ions: the medium, which is the nonionic hydrocarbon portion of the polymer, is one of a low dielectric constant which means that ion pairs form and that they can coalesce into even larger aggregates. This is in marked contrast to what is observed for dilute aqueous solutions of polyelectrolytes where the ions exist as solvated single entities. Why two overall neutral ion pairs are able to attract each other at all can be understood from the point of view that a contact ion-pair is a dipole, and therefore dipole-dipole interactions are present. When two of these ion-pair dipoles interact to form an ion quartet, we have a quadrupolar system, the break-up of which into two ion pairs requires ca. 100-130 kJ/mol (Potts et al., 1968; Otocka et al., 1969). In ionomers, the approach of two ion pairs to each other results in a four-centered aggregate; this aggregate must then behave essentially like a cross-link. Clearly, larger aggregates resulting in crosslinks of still higher functionality are similarly possible. On the other hand, due to the fact that each ion pair is associated with a chain segment and that this chain segment is large compared to the size of the ion pair itself, space limitations must prevent this aggregation from proceeding indefinitely, unless unusual geometries such as rods or sheets are postulated. Experimental evidence indicates that aggregation is in fact present in ionomers and that it is the cause of the unique characteristics of these polymers. This evidence, as well as the nature of the aggregates that it suggests, will be discussed in the next section. Following that, a description will be given of a number of models and theories that have been proposed to illuminate the specific structures thought to result from this phenomenon of aggregation. It should be noted, however, that the field is in an early stage in that, despite many attempts, neither the detailed structure of these ionic aggregates nor their precise size have as yet been elucidated. B. Direct Experimental Evidence for Ion Aggregation. 1. Small Angle X-ray and Neutron Scattering. Small angle X-ray (SAXS) and neutron (SANS) scattering provide the most straightforward evidence for aggregation in ionomers. Copolymers of styrene (Eisenberg and Navratil, 1974; Roche, 1979, Ch. 3), ethylene (Wilson
0
10
20
30
angle 28
Figure 1. X-ray diffraction patterns oE (a) branched polyethylene; (b) acid copolymer, E-O.06MAA; (c) ionomer, E-O.06MAA-O.SONa (Wilson et al., 1968).
et al., 1968; Longworth and Vaughan, 1968; Delf and MacKnight, 1969; Roe, 1972; Marx et al., 1973; Kao et al., 1974; MacKnight et al., 1974; Roche, 1979, Ch. 2), and butadiene (Marx et al., 1973; Pineri et al., 1974; Moudden et al., 1977) with acrylic or methacrylic acid, as well as polypentenamers with phosphonate side groups (Roche, 1979, Ch. 4), have all been studied by the SAXS method in their acid and salt forms. It was found that, above a certain ion content, a low angle SAXS peak is present for the salts containing cations of a high electron density that is not present for the acid forms. Figure 1illustrates this phenomenon for PE-MAA. The existence of the peak appears to be independent of any crystallinity which may be present, of backbone nature, and of the type of cation, although the magnitude and location of the peak do depend on these factors (MacKnight et al., 1974). In addition, this peak is relatively insensitive to temperature, persisting beyond 300 “C for PE-MAA salts (Wilson et ai., 1968). Low levels of humidity were found to enhance the PE-MAA salt peak, but saturation of the sample with water destroyed it (Read et al., 1969; Marx et al., 1973; MacKnight et al., 1974). Disappearance of the peak due to water saturation has also been found for PS-MAA ionomers and polypentenamer-phosphonic acid ionomers (Roche, 1979). When the angular position of the low angle peak is related to structural spacings, it yields values between 2 and 3.5 nm for PE-MAA salts (Longworth and Vaughan, 1968; Delf and MacKnight, 1969; Marx et al., 1973; MacKnight et al., 1974). Other values have been reported for other ionomers-such as 7 nm for low molecular weight carboxy-terminated butadienes (Pineri et al., 1974), 8 nm for a telechelic PB-MAA ionomer (Moudden et al., 1977),5-6 nm for PS-MAA-Cs containing more than 6 mol % ionic comonomer and 1.8-2.0 nm for the same ionomer with less than 6 mol 5% ion content (Eisenberg and Navratil, 1974). In interpreting the peak, many authors have attributed it to interaggregate interference, the distances between the
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aggregates being Bragg spacings (Binsbergen and Kroon, 1973; Marx et al., 1973; Pineri et al., 1974). There is considerable controversy, however, concerning the specific features thought to be reflected in the interference patterns. Other authors have attributed the peak to intraaggregate interference (MacKnight et al., 1974); and, in fact, it will be seen that the most recent information favors the latter interpretation (Roche et al., 1980a; MacKnight and Earnest, 1980). In any case, it is clear that, whatever the detailed explanation of the peak, its presence must reflect the existence of scattering centers in the salt which are not present in the acid. These are generally thought to be microphase-separated regions containing a high concentration of ionic material. A recent SAXS study (Roche, 1979, Ch. 2; Roche et al., 1980a) has contributed significantly to the current debate concerning the model that best describes the physical structure of ionomers. The work essentially consists of a deformation analysis of E-O.OfjlMAA-0.94Csin the range of 0-300% elongation, the ionomer having been quenched to avoid polyethylene crystallization. The results show that the scattering pattern at elongations of ca. 60% and above is strongly azimuthally dependent. Furthermore, upon elongation, the scattering peak at I)= 0" shows an intensity decrease and a shift to lower angles up to ca. 45% elongation above which the peak disappears, and at I)= 90" the peak shows an intensity increase with little change in position. The I)'s here are the azimuthal angles parallel and perpendicular, respectively, to the stretching directions. When the Bragg spacings for various elongations at the two azimuthal angles are compared with those predicted by affine deformation, it is clear that the sample undergoes nonaffine deformation. In other words, the microscopic structure does not change according t o the macroscopic dimensions of the sample. The conclusion was drawn that these data are best described by a three-phase rather than a two-phase model-that is, by the existence of two types of ionic material in addition to the polymeric matrix. The exact models that were evaluated against the results will be discussed in section 1I.C. Small angle neutron scattering (SANS) studies have been introduced recently as a useful complement to SAXS studies (Schelten and Hendricks, 1978; Higgins and Stein, 1978). Where X-ray scattering is associated with varying electron densities, neutron scattering is associated with the type of nuclei causing the scattering. A SANS study of E-0.061MAA-0.85Cs (Roche et al., 1980b) shows a lowangle peak and a strong zero-order scattering component corresponding to that seen in SAXS of the same copolymer; however, there is a significant difference in the angular dependence of scattering displayed by these two techniques. Upon model correlation, this result can be explained, once again, in terms of a three-phase rather than a two-phase model, the former model having been shown to predict such a difference between SAX8 and SANS. Recent SAXS and/or SANS studies have also been done to a limited extent on both dry and water-saturated sulfonated polypentenamer cesium salts (Roche et al., 1980b), on hydrogenated and unhydrogenated cesium salts of polypentenamers containing phosphonate units (Roche, 1979, Ch. 41, on sodium (Pineri et al., 1980) and cesium (Roche, 1979, Ch. 3; Roche, et al., 1980b) salts of PS-MAA, on sodium salts of sulfonated PS (Earnest et al., 1980), on NAFION (Pineri et al., 1980), and on a butadiene-styrene-Cvinylpyridine (B,S,4-VP) terpolymer cross-linked by coordination complexes of Fe(III), specifically B0.15S-0.05 (4-W)-Fe(III) (Meyer and Pineri, 1978), all of which definitely support the phase-separated morphology.
For water-saturated samples, the technique of isotopic replacement can be used in Conjunction with SANS, since saturation is possible with either H20 or D20. Information about the chemical composition of the phases can then be obtained from an analysis of the intensity ratio ZDzo/ZHzo. Results of such an analysis of DzO- and H20-saturated samples of both sulfonated polypentenamer and PE-MAA cesium salts are consistent with the presence in a nonionic matrix of a separate phase containing the water molecules and the ionic units (Roche et al., 1980b). A particularly useful application of SANS and SAXS lies in analyzing the zero-order scattering region where intraparticle interference terms are dominant, thus providing a way to obtain radii of gyration using the so-called Guinier approximation (Guinier and Fornet, 1955)- Meyer and Pineri (1978), for example, in applying this analysis to the Fe(II1)-complexed B,S,4-Vp terpolymer, have inferred a broad distribution of particle sizes, where most radii are less than 3 nm and a very few greater than 10 nm. They also analyzed the tail of the scattering curve (ie., at large scattering angles where the interference terms are predominantly of an interparticle nature), using the theory of Porod (1951), and interpreted the results for B,S,CVP (namely, positive deviations from Porod's law) as being due to the presence of isolated ionic complexes or of very small ionic groups such as dimers and trimers in the nonionic phase. It should be noted, in conjunction with the above analysis of particle size and particle size distribution by SAXS, that there are fundamental difficulties that have not been overcome. For, in general, a particular form of the distribution function used must be assumed, and this may or may not reflect the existing size distribution. There are also ambiguities concerning the effects of particle size distributions and of particle shape distributions and uncertainties (Roche, 1979, p 43). 2. Electron Microscopy. Although electron microscopy, which potentially affords the simplest method of characterizing phase structure, has been applied extensively to the study of ionomers, the results of various investigators have frequently been at variance with one another. The photomicrographs do provide evidence that there is something present in the salt copolymers that is not present in the acid or parent polymers. However, different investigators have interpreted photomicrographs obtained for similar materials as being due to the presence of drastically different aggregates. The first comprehensive study, using this technique, was done by the Dupont group on PE-MAA (Longworth, 1975). This study shows that photomicrographs of the acid samples are very similar to those of low-density PE, both materials exhibiting lamellar and spherulitic structure, whereas neutralized copolymers show no evidence of spherulitic structure and instead give rise to a random grainy appearance. It has been found, subsequently, that the occurrence of this granular structure depends on the detailed method of preparation of the samples-probably indicating that these structures, insofar as they are reflected in the photomicrographs, depend on sample history (MacKnight, 1970). Furthermore, most of the samples studied have been cast from solution, a technique which, based on microscopic studies of amorphous polymers, does not give samples representative of bulk materials (Roche, 1979, p 14). Lately, it has been pointed out (Thomas, 1980) that even the results of electron microscopy must be interpreted with extreme caution. In the most recent use of electron microscopy, Meyer and Pineri (1978) found that the photomicrograph of the
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Fe(II1) complexes B,S,4-VP terpolymer showed many heterogeneously distributed iron-rich regions. The diameters of these regions were observed to vary from 100 to 5 nm, the majority being less than 10 nm. The conclusions drawn from the results of the various techniques used to study the terpolymer are described in the next section. 3. Mossbauer Spectroscopy, Magnetization Measurements, and Electron Spin Resonance. All three techniques have been applied, to date, by Pineri and coworkers only. For the terpolymer mentioned above, Mossbauer and magnetic susceptibility measurements have been coupled to supplement the information obtained by electron microscopy and small angle scattering. The results indicate that the iron exists in three distinct environments or levels of aggregation, the relative proportions depending on concentration. These were identified as follows: (i) 20-40% exist as dimers with antiferromagnetic coupling; (ii) less than 20% are represented by “quasiisolated complexes having a weak ferromagnetic coupling that could either be very small aggregates or be located in the vicinity of much larger aggregates; and (iii) 4040% are contained in large amorphous aggregates having a speromagnetic structure (i.e., an ordered spin structure with a statistical distribution of spin directions), where 90% of these aggregates are less than 3 nm in radius, which is consistent with the SAXS/SANS measurements. The mean distance between them was evaluated by SANS to be ca. 7.5 nm and by magnetization measurements to be ca. 6 nm-a good agreement in view of the uncertainty involved. Althougli it is not clear to what extent these conclusions concerning the size distribution of aggregates in the terpolymer are transferable to ionomers, they are certainly suggestive of what might be expected in the latter also. The single reported study utilizing electron spin resonance (Pineri et al., 1974) was done on B-O.09MAA copolymers neutralized with Cu2+ions. It suggests that ions in that system are present in aggregates that are at least as large as dimers. 4. Mechanical Properties. Many studies of the mechanical properties of ionomer systems have been conducted, and they will be discussed in greater detail later. For most of these systems, Nafion being a notable exception, the mechanical properties of the acid form resemble those of the parent polymer and differ significantly from those of the salt forms. This suggests that the morphology of the salt forms is quite distinct from that of the acid form and the parent polymer. In particular, all the neutralized materials investigated behave like systems that are temporarily cross-linked in that either a short rubbery plateau (Otocka and Eirich, 1968) or an additional inflection point (Eisenberg and Navratil, 1973) is present in modulus-time or modulus-temperature curves that is not present in those of the parent materials. Furthermore, time-temperature superposition invariably fails for salts above a certain ion content, both in stress relaxation studies and in melt rheology studies. In the absence of chemical degradation, this indicates the presence of one or more additional relaxation mechanisms having different temperature dependences from the primary one-a feature which has been attributed to microphase separation. This is found, for instance, in block copolymers of styrene and butadiene. Dynamic mechanical behavior, too, has its most convincing interpretation in terms of microphase separation: for, where there is one loss tangent peak for the acid or parent polymer, twq such peaks, both of relatively high intensity, appear upon neutralization of the acid. As will further be discussed later, this phenomenon is thought to reflect two
glass transition temperatures, one being the usual Tgof the matrix which contains a small concentration of ionic groups, the other being the Tgof the phase-separated regions containing a high percentage of ionic material. In other words, it seems that below a certain ion content small tight ionic aggregates, which manifest themselves as cross-links, dominate the rheological behavior, whereas above that critical concentration large ionic aggregation is apparent in the mechanical properties in the form of phase-separation, or reinforcing filler, effects. 5. Dielectric Properties. Dielectric studies reveal basic features very similar to dynamic mechanical studies except that more quantitative information can be obtained from the former. In the former, the sum of the areas under two loss tangent vs. reciprocal temperature peaks, which, again, are typically seen in the glass transition region, correlates very well with the total ion concentration (the correlation coefficient being 0.975) in the styrene ionomers (Hodge and Eisenberg, 1978). The most significant trends revealed by these studies are the following. The area under the low temperature peak initially increases and then levels off with increase in ion concentration (at ca. 5 mol % ion content for styrene ionomers). In contrast, the area under the high temperature peak increases more slowly initially and then, beyond ca. 5 mol % of ions for styrene ionomers, accelerates. The positions of both peaks move to higher temperatures with increasing ion content. The interpretation of these data is essentially the same as that of the mechanical properties data; that is, the low temperature peak originates with a small concentration of polar groups in the matrix and the high temperature peak originates from large aggregates having a much higher glass transition temperature. The new feature indicated by the dielectric studies is that the two species can coexist. It is their relative concentrations, as reflected by the area under the appropriate peak, that varies with ion content, thereby showing the differing effects above and below the “critical” ion concentration. 6. Raman Spectroscopy. Three very recent Raman studies (Neppel et al., 1979a,b;Neppel et al., 1980) provide strong corroboration of the views of ion aggregation that were obtained from the dielectric investigations. For both styrene sodium methacrylate and sodium p-carboxystyrene and for ethyl acrylate ionomers, two weak bands appear in the Raman spectra at ca. 250 and ca. 170 cm-’, respectively, that are not observed in the corresponding acid copolymers. The band intensities and intensity variations of the bands correlate remarkably well with the dielectric results (a correlation coefficient of 0.999 is obtained for the 170-cm-l band and the high temperature dielectric loss peak), both the intensities and areas under the loss peaks being directly proportional to ion content. On the basis of mass and interaction energy considerations, it was again suggested that two different polar species are present in these salts. The 250-cm-’ band corresponds to the small isolated polar groups that are interspersed in the polymer matrix and that possess a strong interaction energy but low mass. In accordance with the terminology of preceeding studies, these groups will now be identified as multiplets. The 170-cm-’ band corresponds to the large aggregates which possess a large mass and a weaker interaction energy. These have previously been termed clusters. It should be noted here that these types of aggregates had, in fact, been postulated earlier on theoretical grounds (Eisenberg, 1970). The Raman studies provide additional information, particularly concerning saturation limits of multiplets and clusters in different materials. The styrene systems (as
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4 5 6 7 0 9 10 M O L % I O N S IN COPOLYMERS
Figure 2. Relative intensities of the bands at 254 and 166 cm-’ vs. mol % sodium methacrylate for PS-MAA-Na where 0 denotes multiplet intensity, 0 denotes cluster intensity, and denotes the sum of the two intensities. The relative intensities are a direct measure of the ion contents. (Reprinted with permission from Neppel et al. (1979b). Copyright 1979 John Wiley and Sons, Inc.)
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40 60 MOL % IONS IN COPOLYMERS
Figure 3. Plots of the relative Raman intensities of the multiplet and cluster bands in the ethyl acrylate-sodium acrylate copolymers at four different temperatures vs. mol % sodium acrylate. Since their sum is essentially the same at all four temperatures, only that at 22 “C is shown (Neppel et al., 1979a).
the dielectric data also demonstrate) show a multiplet solubility limit at ca. 1 mol ‘70 ions in copolymers, whereas the clusters in these systems have no solubility limit in the region studied (Figure 2). The EA system, on the other hand, shows a multiplet solubility limit at ca. 10 mol 70 ions in copolymers and a cluster solubility limit at ca. 40 mol % ions in copolymers at 22 “C (Figure 3). The one-to-one correspondence between the mol % ions in copolymers and the sum of the mol % ions in clusters and in multiplets, apparent in Figure 2, indicates that all of the ions in the styrene systems are accounted for up to ca. 10 mol % total ion content. The EA system, which was investigated over the entire ion content range, and hence also the polyelectrolyte region, illustrates a new phenomenon. That is, since the sum of the ions in multiplets and in clusters levels off at ca. 35 mol % for all concentrations above that mol % total ion content (Figure 3), all of the ions in excess of ca. 35 mol % must be present as still other species. Two additional features revealed by the Raman measurements are noteworthy. First, it is significant that the multiplet saturation limits of the styrene systems and of the EA system are reached, respectively, at ca. 1 and ca. 5 mol % ions in multiplets. This points out that the multiplet solubility limit is a function of the host polymer; in fact, it is probably a direct consequence of the dielectric constant t of the polymer backbone [poly(ethyl) acrylate:
Figure 4. Schematic distribution of ion aggregates in ionomers.
= -4.0; polystyrene: E = -2.5 (Brandrup and Immergut, 1966)]. Secondly, as can be observed in Figure 3 for the EA system, the multiplet band intensity increases whereas the cluster band intensity decreases with increasing temperature; that is, when the temperature is raised, the number of multiplets increases and the number and/or average size of clusters decreases. In view of all the experimental evidence presented above concerning ion aggregation in ionomers, the broad picture that emerges can be depicted qualitatively and schematically as shown in Figure 4. C. General Models. Although it is now virtually indisputable that multiplets and clusters exist in some form in ionomers, the exact structure of the ion-rich regions is currently the subject of much discussion. Several models have been proposed to explain the experimental results obtained to date, particularly the scattering data which provide many quantitative details. In one early model (Longworth and Vaughan, 1968),the low-angle X-ray scattering peak is ascribed to a kind of intraaggregate interference caused by structural repeat distances of ca. 2 nm within ionic domains having a minimum diameter of 10 nm. In another model (Marx et d., 1973),this peak is attributed to interference between small aggregates (of ca. 0.5 nm in radius and containing 7 or 8 ion pairs) arranged on a paracrystalline lattice with a spacing of 3-4 nm and extending over the entire sample. MacKnight et al. (1974), however, suggested that the scattering maximum arises from internal particle structure, or intraparticle interference, rather than from interparticle interference. Accordingly, based on RDF analysis [which is concerned with the radial distribution function (RDF) of electron density through the Fourier transform of the angular dependence of the scattered intensity] as well as analysis of SAXS using the Porod and Guinier theories, they proposed a three-phase “shell-core” model as follows. The ionic aggregate in the dry state is pictured as consisting of an ionic core, or cluster, 0.3-1.3 nm in radius, that is shielded from the matrix ions by a shell of hydrocarbon chains. Since the surrounding matrix ions are attracted to the cluster by electrostatic forces, but cannot approach the inner core more closely than the outside of the hydrocarbon shell, a preferred distance is established between the cluster and matrix ions. It is to this shell-core distance, which is of the order of 2 nm, that the low angle scattering peak is attributed. (The shell-core structures are considered to be widely separated and randomly arranged within the sample, thus giving rise to intraaggregate SAXS interference only; i.e., the model is a dilute scattering model.) It is thought that, when the sample is saturated with water, the water molecules congregate in the neighborhood of the inner cluster, raising the local dielectric constant. This then causes destruction of the preferred distance, but leaves the cluster intact, thereby 6
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 2, 1981 277
accounting for the disappearance of the low-angle peak upon water saturation. The deformation studies described earlier (Roche, 1979, Ch. 2; Roche et al., 1980a) had been conducted with the object of better evaluating the various models proposed. The infinite paracrystalline lattice model was found not to fit the data obtained, particularly in that it gives no source of the strong zero-order scattering of the undeformed sample observed experimentally. In addition, the peak shifts, or changes in Bragg spacings, upon deformation of this model closely follow the affine predictions, contrary to what is indicated by the experimental data. A finite lattice (of ca. 10-15 lattice points) does give a satisfactory fit to the data of the undeformed sample; however, it is questionable if lattice statistics apply to such a small lattice. I t was noted that models featuring short-range or local structure with overall dimensions less than 10-15 nm afford a much better approach in that they are able to account for the necessary zero-order scattering component and in that they will show local strains in the region of the ionic structures that are not exhibited in the strain patterns of the sample as a whole. The latter condition is indicated by calculations, but is not satisfied by the paracrystalline lattice model. In the version of the spherical shell-core model tested, both the core and the shell constitute regions of differing excess electron densities, while the region between the core and shell and the matrix outside the shell have lower and equal electron densities. Upon deformation, the shell and core structures are considered to deform elliptically in the direction of stretching, their dimensions in the perpendicular direction remaining constant. This model correctly predicts the observed SAXS data (section II.B.l) for the undeformed sample as well as the decrease in peak intensity and position with stretching at the Oo azimuthal angle. The comparative rates of intensity and position decreases are not in agreement with experiment, however. More seriously, instead of corroborating the observed increase in intensity with stretching at the 90' angle, the shell-core model predicts a decrease. A second local structure model that was examined consists of two-dimensional lamellae (where analysis of the model was confined to two dimensions largely for mathematical simplicity). This model was suggested by the known occurrence of layered structures in salts of lipids (MacKnight, 1970). Again characterized by three phases, this one consists of three layers of ion-rich regions of the same thickness, with the central layer having an electron density different from that of the outer layers (a two-layer version that was initially examined does not yield a sufficient scattering maximum). Using the assumption that sample deformation causes orientation of the lamellae along the stretching direction through a rotational mechanism, this model was observed to predict the deformation changes in peak intensities and positions with stretching at both the Oo and 90° azimuthal angles correctly. There is again a discrepancy, however, in the rate of intensity decrease relative to the rate of peak shift at 0' between model and experiment, just as was true for the spherical shell-core model. It is possible that this discrepancy might be avoided by use of a three-dimensional model or by incorporating stretching in addition to rotation of the lamellae with deformation. It was concluded that, of the models examined, the lamellar model has perhaps the best potential for explaining the scattering data. At the same time, it was noted that other local structure models which might incorporate a combination of rotation and stretching of ellipsoids or incomplete ellipsoids can also presumably
fit the data, but at present it is not possible to discriminate unambiguously among these models. Moreover, the real case is undoubtedly more complex than the mechanisms and structures that have been evaluated to date. D. General Theories. The first theoretical approach to phase separation of ions in organic polymers was that of Eisenberg (1970), who considered ion aggregation to occur in two steps. First, based on analysis of the energy needed to separate contact ion pairs, it was shown that in organic media of low dielectric constant ions exist most probably as pairs or higher multiplets-up to a maximum of eight ion pairs for a spherical multiplet, which would then have a radius of 0 . 3 4 5 nm. These multiplets, coated completely with nonionic material, can further aggregate to form the larger clusters which are assumed to be stable below a characteristic temperature T,. The factors controlling cluster formation are the entropy increase due to chain elasticity and the free energy decrease caused by coulombic interactions as the multiplets approach one another. By equating the elastic and electrostatic forces at T,, an equation can be derived which relates the number of ion pairs in the cluster, and hence intercluster distances, to the spacing of ionic groups along the chain, to the density, the dielectric constant, the root-mean-square end-to-end distance of the polymer chain, the size of the ions, the average multiplet size above T,, and a constant which depends on the cluster geometry. Consideration of several different cluster geometries for E-0.045MAA-Na gives calculated values for the intercluster distance ranging from 4.4 to 9.5 nm. This compares reasonably well with estimates that have been determined from low-angle scattering data based on the assumption that the peak maximum corresponds to a Bragg spacing. Thus, despite the highly approximate nature of the theory-due to the neglect of factors such as crystallinity and to the oversimplification of other factors such as assuming a unique T, rather than a critical temperature range, assuming that the contact ion pair is the fundamental structural unit, and including all ionic groups into the clusters-it demonstrates the feasibility of the approach taken even if it cannot be regarded as predicting true cluster geometries or distribution of ions between clusters and multiplets. It particularly demonstrates the strong tendencies in ionomers for phase separation and shows that the coulombic energetics involved in aggregation are on a competitive level with chain entropic forces opposing aggregation. The theory also suggests that a minimum concentration exists, depending in part on the dielectric constant of the parent polymer, below which clustering is not to be expected. For, as ion content is decreased, the distance between multiplets should increase to the point where the attractive forces between them become smaller than the elastic forces keeping them apart. A mathematically highly complex theory of clustering has been developed by Ponomarev and Ionova (1974). These authors attempted to evaluate more accurately the minimum free energy configuration through the use of the nonequilibrium thermodynamic distribution function approach of Bogolyubov (Kuhn and Grun, 1942). They also considered more exactly the effect of shielding by the nonionic segments. The resulting equations are, unfortunately, not experimentally tractable, and it is impossible to draw conclusions from them regarding how the various molecular parameters influence the clustering phenomenon. Nevertheless, it is to be hoped that this theory will provide the basis for future theoretical developments whose predictions can be experimentally tested.
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Very recently, Forsman (1981) began to analyze the ion aggregation problem from the point of view of coil dimensions and developed an equation which relates this factor to the various chain parameters such as the number of units per aggregate, polymer concentration, molecular weight of repeat unit, fraction of repeat units substituted, and a parameter that depends on the geometry of the clusters. Forsman's analysis predicts that ion aggregation should be accompanied by coil expansion. This has indeed been observed by Earnest et al. (1980) from SANS experiments on sulfonated polystyrene ionomers where a small amount of perdeuterated ionomer was embedded in a protonated ionomer matrix. E. Models and Theories for Hydrated Nafions. Nafion is a recently developed polymer which has found extensive applications as an electrochemical separator because of its high selectivity with regard to ion transport. It is based on a backbone of perfluoroethylene and has a sidechain with the structure
Table I. Glass Transitions of Various Polyphosphatesa Pauling radius charge, material Tg,"C r, A . q/ab HPO -10 0.00 LiPO, t335 0.60 1 0.50 NaPO, t280 0.95 1 0.42 2 0.84 t520 0.99 Ca(P03)2 1.13 2 0.79 Sr(PO,)* +485 1.35 2 0.73 Ba(PO),), +470 2 0.93 +520 0.74 Zn(P03)2 2 0.84 t450 0.97 Cd(PO,),
,
a Reprinted with permission from Eisenberg et al. (1966). Copyright 1966 John Wiley and Sons, Inc. cation radius + oxygen anion radius.
a=
-OCF,CFOCF,CF,SO,H
I
CF3
Like other ionomers, the salt form shows irrefutable evidence of clustering. A novel feature of Nafion salts, however, is the dramatic increase in permeability to water. Several researchers have focused their attention on this material, especially in its hydrated form, in developing and tesing models and theories. These are being discussed separately here in view of the novelty and importance of the NAFIONs. Based on Mossbauer and other studies that indicate the presence of two types of iron atoms in Nafion-Fe samples, Pineri and co-workers (Duplessix et al., 1979) proposed a three-phase model for these salts where the ions exist both in clusters and in the matrix, the matrix containing ions in the form of dispersed multiplets as well as crystallites. Upon hydration, two things occur: water diffuses into the clusters thereby increasing their volume sometimes to the point of coalescence, and it hydrates the matrix ions. Mauritz et al. (1980) extended the Eisenberg theory, and developed a model to describe ionomeric structure using Monte Carlo simulation. It is based largely on first principles and incorporates four empirical parameters (three packing constants corresponding to various types of hydration shells, and a molecular elastic modulus force constant which reflects the stiffness of the membrane material). This approach was applied to Nafion membranes using experimental data on water uptake and densities. The model is pictorially described by a collection of intermeshed hoses along which are attached balloons at regular intervals. In the dry state the balloons are uninflated; with water saturation, the balloons become inflated until a certain equilibrium point is reached. Clusters form due to the ion dipoles (pictured as strong bar magnets) resulting from hydrated ion pairs. The strength of the theoretical development just described lies in the fact that meaningful numbers have been proposed and that it opens the way for testing predictions of the theory experimentally. Still another model for ion clustering in water-saturated Nafions has been advanced by Gierke (1977). It is a cluster-network model characterized by approximately spherical clusters (of ca. 2 nm radius, spaced ca. 5 nm apart) connected by short narrow channels (ca. 1 nm in length and width). These clusters and channels consist of the phase-separated ions and absorbed water. The polymeric charges are probably imbedded in the water very near the water/fluorocarbon backbone interface to mini-
-501
o
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,
a2
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I
0.6 q/a
, OB
,
,
1.0
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Figure 5. TBvs. q / a for various ion-containing systems where p is the cation charge, and a is the distance between centers of charge in angstroms (Eisenberg and King, 1977).
mize both the hydrophobic interaction of water with the backbone and the electrostatic repulsion of proximate sulfonate groups. This model was shown to provide a reasonable mechanism for the hydroxyl ion rejection in that once the hydroxyl ion enters the cluster, inside of which it is largely shielded from the repulsive forces, it does not easily migrate from one cluster to the next since a fairly high electrostatic barrier must be overcome in the narrow channels. 111. Physical Properties A. Glass Transitions. Since studies of the effects of ion incorporation on glass transition temperatures (Tis) have already been described in published reviews, they will be mentioned only very briefly here. It had been observed first that polyphosphates (Eisenberg et al., 1966) show a very large increase in Tgwith ion incorporation, as is evident from Table I. This increase in Tgdepends on the nature of the cation, with the effect being greater as the ionic forces are greater. The data were analyzed in terms of the anion-cation interaction on the assumption that this is the factor which limits segmental mobility and hence causes the Tgeffect. This led to the relationship, Tg0: q / a ,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981
279
t 200-
8
--I.....'. 203 '" , -, - 4I " 80
Kx)
1xI
140
160
180
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5
,
,220
240
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Figure 6. TBof EA-AA copolymers aa a function of cq/a, where c is the metal carboxylate content: 0,Na; 0 , Cs; 0 , Li; 0,K; @, Ca; 0,Ba (Reprinted with permission from Matsuura and Eisenberg (1976). Copyright 1976 John Wiley and Sons, Inc.)
where q is the cation charge and a is the distance of closest approach between the centers of charge of the anion and cation. As shown in Figure 5, Tgplotted against q / a does, in fact, yield a straight line for polyphosphates as well as for other completely ionized homopolymers over certain ranges, all having essentially the same slope. The partly neutralized HP03-NaP03 system also fits a linear Tgvs. q / a plot; however, the partially neutralized acrylate system does not, probably due to the presence of highly structured, rigid carboxylic acid dimers which decrease the segmental mobility of the polymer appreciably. For organic ionomers of low ion content, the initial rise of T with ion content is in most cases linear, the values of d f g / d c ranging from 2 to 10 OC/mol %. Furthermore, the largest effects are observed for host polymers that have a relatively low T and that contain ions of large q/a. With respect to the effect of ionic unit type on dTg/dc, it has been observed that for four polypentenamer derivatives studied, dTg/dc increases in the order thioglycolate N carboxylate < sulfonate < phosphonate up to 10 mol % ionic group content. The behavior of the phosphonate derivative can be attributed to the bulky phosphonate unit with its two cations which can be expected to elevate the Tg more efficiently than would the smaller carboxylate groups in the thioglycolate and carboxylate derivatives. On the other hand, dynamic mechanical data indicate little evidence for phase separation in phosphonate salts in the concentration region investigated, whereas strong evidence exists in the case of thioglycolate and sulfonate salts. Hence, the greater effectiveness of the phosphonate group in raising Tgcompared to the thioglycolate and sulfonate units could also be attributed to the greater concentration of homogeneously dispersed salt groups in the hydrocarbon phase of the phosphonate derivatives (MacKnight and Earnest, 1980). When ionic ethyl acrylate copolymers were investigated with respect to ion concentration and type of ion (Matsuura and Eisenberg, 19761, it was observed that for every ion studied a plot of Tgas a function of metal acrylate content, c, for the dry ionomers gives unusual sigmoidal curves. When Tg is plotted against c q / a , all of these curves coalesce into one sigmoidal curve, as shown in Figure 6. Significantly, the curved portion coincides with the onset of failure of time-temperature superposition in viscoelastic studies of the ethyl acrylate-sodium acrylate system (ca. 12 mol 5% ionic content). Since the latter phenomenon presumably reflects ion clustering, it appears
Figure 7. Young's modulus (10-s) vs. temperature for samples of different NaMA content: curve 1, PS; 2, O.GNa(h); 3, 1.9Na(h); 4, 2.5Na(h); 5,3.7Na(l); 6, 3.8Na(h); 7,4.6Na(h); 8, 5.5Na(l); 9,6.2Na(h); 10, 7.9Na(l); 11, 7.7Na(m); 12, 9.7Na(h); where h, m, and 1 indicate high, medium, or low molecular weight samples (Eisenberg and Navratil, 1973).
again that in the region below the sigmoid the dominating influence is exerted by multiplets, whereas clusters dominate the behavior above the sigmoid. Styrene sodium methacrylate copolymers likewise show the beginning of a sigmoid in a graph of Tgas a function of ion content, the upswing again occurring in the region where time-temperature superposition fails (ca. 6 mol % MAA). Two conclusions can be drawn from these observations. First, it appears likely that the onset of cluster-dominated rheological behavior is a function of q / a as well as of ion concentration. Second, it seems clear that the sigmoidal nature of the Tg vs. q / a plot is encountered only in materials in which sufficient clustering occurs. One further observation has recently been made, namely, that plotting Tgas a function of mol % ions in clusters for PS-MAA-Na, as determined by Raman spectroscopy (Neppel et al., 1979b), linearizes the plot of Tk vs. ion content. This suggests that it is the clusters which exert the strongest effect on the matrix glass transition temperatures, at least of the styrene ionomers. B. Mechanical Properties. 1. Modulus-Temperature. The trends that are typically observed for ionomers in 10-s modulus curves as a function of temperature and increasing ion content up to ca. 10 mol % are illustrated with PS-MAA-Na in Figure 7. It is clear that an increasing ion content raises the modulus and enhances the rubberlike plateaus. The molecular weight effect, manifesting itself below the "rubbery" modulus only, is indicated by the 3.7 (low molecular weight) and 3.8 (high molecular weight) mol % ion content samples. Curves obtained for the PS-MAA-Na (Eisenberg and Navratil, 1973) and PEA-MAA-Na (Eisenberget al., 198Oa) ionomer systems are compared in Figure 8. It is apparent that the ion concentration in ethyl acrylate salts must be about twice as high as in the styrene salts in order to achieve the same effect. This is believed to be due to the lower cluster content in ethyl acrylate compared to styrene salts at the same total ion content. This, in turn, is most probably a result of the fact that the dielectric constant of the acrylates is higher than that of the styrenes, so that at comparable ion concentrations the percentages of ions in clusters are much smaller in the acrylates than in the styrenes. In the phosphates, which have a still higher dielectric constant, no clustering is observed at any ion content. It has also been shown (Navratil and Eisenberg, 1974; Eisenberg et al., 1980a) that different counterions result in changes not only in Tgbut also in positions and shapes of modulus-temperature curves, as illustrated for ethyl
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2 , 1981
280
9 h
. E
U
m 0 c
*
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-z 8
2
6
10
14
18
v
LOG t ( S E C )
Y
Figure 10. Stress-relaxation master curves for styrene ionomers (4.6 and 7.7 mol % NaMA) and ethyl acrylate ionomers (4.4, 7.6, 12.0, and 16.3 mol % NaA) (Eisenberg and Navratil, 1973; Navratil and Eisenberg, 1974; Eisenberg et al., 1980a).
PP 0 -1
I
7
I
I
I
,
I
\
,
,
loo
50
0
1-Tg(0)
,
, "C
Figure 8. Plots of 10-smodulus vs. T - Tgfor the styrene ionomers (dotted lines) and the ethyl acrylate ionomen (solid lines) for various ion contents. (Reprinted with permission from Eisenberg (1979). Copyright 1979 Plenum Press).
0
I
1
I
40
80
120
I 160
T ("C) Figure 9. Plots of 10-s modulus vs. temperature for several ethyl acrylate ionomers with different counterions. Symbols indicate concentration and type of ion (Eisenberg et al., 1980a).
acrylates in Figure 9. It is especially noteworthy that the curve for a sample containing 16.3% Na is similar to those for samples containing 12.0% Li and 24.7% Cs. A comparision of the 24.7% ionomers, on the other hand, shows that the Na curve is very much higher than the Cs curve, while the 12% ionomer curves show that the Li curve is higher than the Na curve. This is exactly what would be expected on the basis of the q / a effect. In other words, the larger the inter-ion distance, the less stable the cross-link, and therefore the less well-developed the rubbery moduli. 2. Stress Relaxation. Stress relaxation studies of the styrene- and ethyl acrylate-based ionomers in the Tgregion offer another direct comparison of these two systems. These studies also illustrate the stress relaxation behavior that seems to be typical of ionomers. In particular, it was
found that the time-temperature superposition typical of nonionic and noncrystalline homopolymers breaks down when salt content is greater than ca. 6 mol % for the styrene sodium methacrylate copolymers and ca. 12-16 mol 5% for the ethyl acrylate-sodium methacrylate system (Figure 10 illustrates the breakdown for the 7.7 mol % styrene ionomer and 16.3 mol 5% ethyl acrylate ionomer, where the master curves have been constructed in such a way as to maximize overlap of the short-time regions). One observes again that there is a factor of 2 difference in ion content between the two systems for achieving the same effect. This also suggests that failure of time-temperature superposition is a diagnostic for the presence of clusters at a sufficiently high concentration. In other words, once the cluster concentration has reached a certain critical level, which depends on the system (i.e., on the backbone, on the ionic group, and on the counterion), one should expect to find such a superposition breakdown for all systems where clustering occurs. This phenomenon is apparently due to the onset of a second relaxation mechanism associated with the presence of the clusters. Interestingly, it appears that time-temperature superposition can be reestablished above a certain temperature for low molecular weight ionomers of greater than the critical ion content. In particular, this has been observed for S-0.079MAA-Na of 0.7 X lo5 molecular weight above ca. Tg+60 "C. It is possible that above that temperature, clusters no longer exist; however, it is more likely that cluster lifetimes have become too short to contribute appreciably to the relaxation (Eisenberg and King, 1977, p 152). This reestablishment of time-temperature superposition has not yet been observed in other materials, except for a Nafion-K salt of an undetermined molecular weight (Yeo and Eisenberg, 1977), presumably because measurements have not gone to sufficiently high temperatures. That it was observed in the Nafion-K sample was taken as an indication that its molecular weight was low. Further comparison of the styrene and ethyl acrylate ionomers indicates that in both cases the distribution of relaxation times (calculated only for ionomers obeying time-temperature superposition) is broadened considerably by increasing ion incorporation. However, the rate of this broadening differs by a factor of 2 in the two materials, being slower for the EA ionomers. There is likewise a broadening of the transition and flow regions with increasing ion content. Undoubtedly, this can be attributed
(an)
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 281
. S-MA*-No
0 10
5
mole
k
Figure 11. “Rubbery” inflection points vs. total ion content for styrene ionomers with sodium p-styrenecarboxylate (a),sodium p-styrenesulfonate (O), and sodium methacrylate (A),(Brockman, 1981).
to the finite ionic interactions that effectively act to increase the molecular weight and, at high enough ion concentrations, give the materials properties of a filled system. Finally, sufficient ion introduction causes the appearance of two inflection points in the master curves (Figure lo), analogous to what is seen in the modulus-temperature curves. The lower inflection point corresponds to the normal entanglement spacing seen in the parent polymer and lies at a relatively constant modulus of ca. 105.6N/m2. The upper inflection point occurs at ca. lo6to 108s relative to the Tgfor both systems, the exact position depending on the ion content; its height, which may reach values as high as lo8 N/m2, seems to be a linear function of ion content, the slope depending on the system. The difference in this slope between the ethyl acrylate and styrene ionomers is once again approximately a factor of 2 being smaller for the EA system. The fact that the modulus of the upper inflection depends on ion content provides further convincing evidence of temporary cross-linking or of microphase separation. In a very recent study (Brockman, 1981), the aim of which was to determine the effects of type and position of ionic pendant group, a comparison was made of the styrene sodium methacrylate copolymer and two styrene ionomers containing, respectively, carboxylate and sulfonate groups in the para position of the benzene ring. The gross features of the sodium salts of all three systems are similar in terms of what has just been described. Especially noteworthy is the breakdown of time-temperature superposition at ca. 6 mol 7% ions. This reinforces the argument that it is primarily the dielectric constant of the polymer matrix which determines the degree of clustering and not the type or position of the pendant ion. Semilogarithmic plots of the rubbery inflection point of the three ionomer systems as a function of ion content reveal a new feature. As is appqrent from Figure 11,these plots display a linear relation where the slope for the styrene-sodium methacrylate system is much higher than the coincident slopes for the two styrene-ring-substituted systems. This reflects the observation that at comparable ion contents, the styrene-sodium methacrylate system yields a rubbery plateau at a very much higher modulus value than either of the ring-substituted systems. That, in turn, suggests that there is more phase-separated material in the former system than in the latter two systems; that is, that the total volume of the clusters in the former system is larger than that in the latter systems. Since the
50
100
150
200
250
T, “C
Figure 12. Mechanical loss tangent vs. temperature for polystyrene and sulfonated polystyrene of different ion concentrations. For the sake of clarity the experimental points are shown for only one curve (Rigdahl and Eisenberg, 1981).
total ion content in clusters is the same for all three materials, judging from the point where time-temperature superposition breaks down, it appears that the clusters in the chain-carboxylated system are larger than those in the ring-substituted systems (the validity of this deduction could be confirmed most directly, perhaps, by SAXS/ SANS measurements if the difference in size of the clusters is large enough to be detected by those techniques). The implication, then, is that the size or effectiveness of the cluster is a function of the position of the ion, but not of its type. This feature is in marked contrast to the strengths of the interaction in carboxylate and sulfonate ionomers, as will be seen in the next section. The increase in the cluster size in the styrene-methacrylate ionomers relative to the ring-substituted styrene ionomers is further reinforced by water uptake experiments. At equilibrium and at lower ion contents (below ca. 3 mol % ), all three systems take up approximately one water molecule per ion pair. Since, at those low ion concentrations, most of the ionic groups are present in the form of multiplets, it appears that water saturation of the material occurs at one water molecule per ion pair. In contrast, the PS-hUA-Na ionomers of greater than 7 mol % ion content absorb many more water molecules per ion pair than do the ring-substituted systems of comparable ion content. All three systems at the higher ion contents show an increase with ion content in the number of water molecules absorbed per ion pair, but the rate is considerably greater for the chain-carboxylated material than for the other two. Since most of the water in the high ion content systems must reside in the clustered regions because of the high local dielectric constant of the clusters, it seems reasonable to conclude that the ring-substituted materials must have smaller clusters than the chaincarboxylated material-just as was suggested above by the positions of the rubbery moduli in the modulus-temperature curves. 3. Dynamic Mechanical. Dynamic mechgnical studies largely corroborate the views gleaned from stress relaxation. Loss tangent curves as a function of temperature for various ion concentrations illustrate the type of behavior displayed by dynamic mechanical measurements. Examples for the styrene-p-sulfonic acid copolymers, along with the curve for pure polystyrene, are shown in Figure 12 for three ion concentrations. The geperal features are the same for all the other styrene ionomers, notably in the presence of two peaks. The lower temperature peak re-
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4 1
Y
150
,,.I’
S-PSC-No S-MAA
100
s-PSS
0
5
IO
No No
15
mole
Figure 13. Dynamic mechanical (ca. 1 Hz)peak positions vs. ion content for three styrene ionomers (symbols same as in Figure 10) (Brockman, 1981).
flects the Tgof the matrix containing the dispersed multiplets and moves to higher temperatures with increasing ion content. This is accompanied by a decrease in peak intensity. The high temperature peak, which presumably reflects the T of the clusters, also moves to higher temperatures wita increasing ion content, but it increases in intensity. As indicated by Figure 13, the position of the low temperature peak as a function of ion content is identical for all three styrene ionomers investigated to date, whereas that of the high temperature peak depends very much on whether the carboxylate or the sulfonate ionomers are involved. For the carboxylated systems, the high temperature peak position is considerably lower than that for the sulfonated system, the difference at an ion content of ca. 5 mol % being ca. $0 “C. This suggests that the strength of the interaction within the cluster is greater in the sulfonated system than in either of the carboxylated systems, and that it is independent of the position of the ion. As was pointed out in the last section, this feature contrasts markedly with that regarding the size of the clusters, the size being influenced by the position of the ion rather than by its type. This size feature, in dynamic mechanical properties, appears to be reflected in the rate of decrease of plots of the dynamic modulus relative to temperature at the matrix glass transition temperature [ (dG ’/dT) ] as a function of ion content. For, the results of plotting?$G’/dTIq, vs. c for the three styrene systems parallel what is indicated in Figure 10; that is, the values for the two ring-substituted systems fall more or less on one line, although with a decreasing slope, and the values for the chain-carboxylated material fall on another line having a steeper slope. However, it is not clear at present just how these data should be interpreted. The loss tangent peak that has been identified with the ionic Tgin the polyethylene-based salts shows the same behavior as that described above. That is, even in the presence of crystallinity, this peak increases in both magnitude and temperature with increasing degree of neutralization. Furthermore, divalent ions effect a greater increase in magnitude and temperature of the peak than do monovalent ions. 4. Miscellaneous. Mechanical properties of ionomers other than those described above have also been investigated to some extent. As can be gathered from a perusal of the book by Eisenberg and King (1977), where most of these studies are documented, the results generally seem to support, or at least not to contradict, the trends in mechanical properties and the underlying cluster-multiplet explanations for those properties, as they have been de-
scribed in this article. From studies of metal oxide-cured, carboxyl-containing elastomers, for example, it is evident that these rubber-based ionomers possess an unusual combination of properties-high initial modulus, high elongation at break, and low permanent set. The low permanent set can be explained by the presence of a small concentration of highly stable cross-links, namely multiplets. The high elongation at break can be attributed to the relaxation by ionic bond interchange of the strained network elements which would initiate rupture in permanently cross-linked systems. Finally, the presence of clusters which act as a reinforcing filler (but which can fall apart under lower strain values than multiplets) can account for the high initial modulus. Crystalline polymers also exhibit similar trends upon ion introduction, although they are modified, and sometimes even masked, by the presence of crystallinity. This is understandable with the realization that the effects of ion aggregation and of crystallinity are, in some respects, similar-as is evidenced by the similarity in shapes of stress relaxation curves of noncrystalline ionomers and of partially crystalline nonionic materials. The relatively slow dip in modulus with time of both types of materials is particularly indicative, as is the observation that the differences in properties between the ethylene ionomers and the parent polymer are not as drastic as those encountered in materials that are devoid of crystallinity. Secondly, ion aggregation competes to some extent with the degree of crystallinity, thereby resulting in a slight reduction of crystalline content, as well as major changes in morphology, with increasing ion content. Those factors have the result that the effect of ion introduction on the physical properties that are due to presence of crystallinity are diminished to some extent, and that the parallel effects due to ion aggregation are enhanced. This phenomenon appears to be evident, for instance, in the thermomechanical behavior of PE-MAA ionomers. Here, the low-temperature modulus is seen to decrease somewhat with increasing ion content, in marked contrast to the behavior in noncrystalline ionomers where the modulus increases with ionic content. The fact that quenching the samples accentuates, and annealing deemphasizes, the differences in the lowtemperature moduli indicates that it is the degree of crystallinity that is primarily responsible for the modulus level in the low temperature range. On the other hand, in a higher temperature range, the modulus increases with ion content, indicating that in this region the crystallites are no longer of primary importance and it is the ions which contribute appreciably to the overall modulus (Eisenberg and King, 1977, p 18). Nafion, which possesses a small degree of crystallinity, also generally resembles other organic ionomers in its mechanical properties, notably in the effect of ion clustering on these properties; however, it displays some unusual features as well (Ye0 and Eisenberg, 1977). First, dry Ndion in its acid form yields a stress relaxation curve having an unusual shape with an extremely broad transition region. Except for the fact that time-temperature superposition is valid and that the Young’s modulus in the glass transition region is considerably lower, this curve resembles those for ion-clustered styrene ionomers more than those for unclustered systems. This suggests that ion aggregation occurs even in Nafion acids, in contrast to other ionomers in their acid form. This is undoubtedly due to the fact that the former are very strong acids. Nafion salts yield curves similar to that of the acid, again with unusually low Young’s moduli in the Tgregion; however, time-temperature superposition for these does
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 283
break down. For Nafion-K, superposition is reestablished above 180 "C. Dynamic mechanical studies show another feature not yet encountered in other ionomers, namely, that, at least in the presence of water, the glass transition of the ionic regions seems to occur at a lower temperature than that of the matrix. In all other ionomers studied, the reverse is true. The effects of partial neutralization of ionomers, specifically PS-MAA, have also been examined (Navratil and Eisenberg, 1974). In stress-relaxation master curves of S-0.046MAA neutralized to varying degrees with sodium, the disappearance of the upper inflection point below about 50% degree of neutralization suggests that the ions cease to act as cross-links below that point for this series of ionomers. Furthermore, calculations based on the theory of rubber elasticity indicate that the apparent cross-link density (which determines the height of the rubberlike plateau) is significantly less than would be expected on the basis of degree of neutralization. For example, the apparent cross-link density of the 60% neutralized material is only 25% of that of the fully neutralized material. The average lifetime of the remaining cross-links, moreover, appears to be reduced, as manifested by the poorly developed inflection points for the lower degrees of neutralization. It is possible that the unneutralized carboxyl groups participate in the multiplets and clusters and thereby weaken them, perhaps through an exchange mechanism involving the hydrogen and the counterions associated with the carboxyl groups. This concept is supported by the observation that the stress relaxation behavior of the partially neutralized copolymers is somewhat different from that of fully neutralized copolymers of comparable total salt content (lower carboxyl content), the difference being greater the lesser the degree of neutralization. C. Melt Rheology. Melt rheological studies of ionomers carried out to date have not been extensive. Those that have been performed were devoted primarily to only two types of materials, namely ethylene and styrene ionomers, although some recent attention has been given to EPDM. Very early investigations of the ethylene ionomers have shown that the melt strength of this polymer can be improved considerably by incorporating ions. For instance, a molten ethylene ionomer film can be drawn down by vacuum over a sharp metal nail without tearing or puncturing (Longworth, 1975). It has also been observed generally that dramatic increases in melt viscosities are displayed upon the introduction of ions into polymers. The extent of the increase seems to depend primarily on ionic content or degree of neutralization and, to a lesser extent, on the counterion. A marked sensitivity to shear rate is shown as well. In an attempt to isolate the specific effects of ionic content, Shohamy and Eisenberg (1976) investigated the melt rheological properties of the acid, the sodium salt, and the methyl ester derivatives of low to medium molecular weight PS-MAA samples. It was found that, in the range studied, time-temperature superposition is applicable to modulus vs. frequency curves for all three materials up to 7.7 mol % ion content. More interestingly, by selecting appropriate reference temperatures, the master curves of the dynamic modulus as a function of frequency for all the derivatives of the same molecular weight can be made to superimpose. This superposition phenomenon suggests that the chain dynamics in all three systems are identical in the range studied. On the other hand, the differences in the reference temperatures (AT = 8 "C for the ester-acid and 38 "C for ester-salt at 1.9 mol % ion content) appear
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Figure 14. Melt viscosities at 220 "C of lightly sulfonated and lightly carboxylated polystyrene sodium salts (Lundberg and Makowski, 1980).
to be directly related to differences in the interchain forces, namely to the hydrogen bonding strength (11kcal/mol) in the acid and to ionic multiplet formation (23 kcal/mol) in the salt. In addition, it is noteworthy that AT as a function of ion content gives a sigmoidal plot for the ester-salt pairs, in contrast to the linear relation observed for ester-acid AT'S. At low ion contents (ca. 1.9-3.7 mol % ), the ester-salt AT values seem relatively insensitive to ion content, probably reflecting a constancy in multiplet structure in this range. In contrast, above ca. 5 mol 90of ions, AT rises appreciably with ion content, reaching a value of ca. 130 "C at 7.7 mol 9%. This is clearly not a glass transition effect since the matrix Tgrises by about only 30 "C between 0 and 7.7 mol % ion content. However, it may well be related to the onset of the cluster T which, for the 7.7 mol % material, occurs at ca. 170 "c', or 70" higher than the Tgeofthe parent material. The melt viscosities of sulfonated and carboxylated polystyrenes are compared as a function of mole percent functionality in Figure 14. The data indicate that there are stronger ionic associations in the sulfonated than in the carboxylated ionomers. This may be merely another reflection of the difference in cluster T i s between the two materials, as was described earlier. Consistent with previous studies of ethylene ionomer melts (Sakamoto et al., 19701, Earnest and MacKnight (1978) found that, above the crystalline region, PE-MAA salts do not obey time-temperature superposition even for quite low ion contents (e.g. E-0.035MAA-0.70Na). Furthermore, ester-acid-salt master curves of PE-MAA can be constructed, just as was possible for PS-MAA. However, the frequency shifts that are necessary are different from those in the PS-MAA case, that between ester and acid being relatively small and that between ester and salt being enormous. The ester-acid AT (33 and 54 "C for E-0.035MAA-0.70Na and E-0.061MAA-0.78Na, respectively) corresponds to the Tgdifference between ester and acid, but this is not true for the ester-salt AT (165 and 202 "C, respectively, for the same 3.5 and 6.1 mol % ionomers) (MacKnight and Earnest, 1980). It is noteworthy that whereas the absolute values of the A T s for the styrene and ethylene systems mentioned are very different, the corresponding ratios of AT for the ester-salt to AT for the ester-acid are remarkably similar-the ratios being 4.8 for the styrene system and 5.0 and 3.7, respectively, for the 3.5 and 6.1 mol % ethylene systems. These ratios appear to corroborate the hypothesis that the differences between
284
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 2, 1981
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