2492
Ind. Eng. Chem. Res. 1994,33, 2492-2503
Structure and Thermodynamics in Associating Model Ionomer Solutionst Eleni Karayiannit and Stuart L. Cooper' Department of Chemical Engineering, University of Wisconsin-Madison, 1415 Johnson Drive, Madison, Wisconsin 53706
This paper presents an overview of our current research on structureproperty relationships of ionomer solutions. Viscometric measurements of ionomers in nonpolar solvents show a strong intermolecular association even at low concentrations. Small-angle neutron scattering (SANS) and quasi-elastic light scattering (QELS)experiments were performed on model telechelic ionomer solutions to investigate this association phenomenon. The results show that with increasing polymer concentration both the extent of association and the size of the associating particles increase. During association the single chain dimensions increase until a plateau value that extends into the bulk is reached at higher concentrations. The Flory-Huggins interaction parameter shows strong concentration dependence, increasing with decreasing concentration. Due t o association the diffusion of the particles in solution decreases which results in reduced mobility. Combination of SANS and QELS shows that, a t all association levels studied, interchain association leads to formation of highly extended clusters. Introduction Incorporation of ionic groups in an otherwise nonpolar polymeric backbone has long been known to result in unique behavior and material properties that are dramatically different from the nonionic parent polymer. This phenomenon is more widely known for polyelectrolytes that contain one ionic group per monomer unit, but has also been demonstrated quite dramatically for polymer systems that contain only a few ionic groups per polymer chain. In the latter category of materials belong the ionomers that are usually defined as polymers that contain a small fraction of ionic groups-usually less than 10 mol %-covalently bound to a polymeric backbone. The ionic groups, usually carboxylate or sulfonate groups, are accompanied by counterions, usually a metal cation. The presence of ionic groups has a strong effect in any physical state of the material. In the bulk, improved physical properties such as marked increases in modulus, adhesive strength, tear resistance, glass transition temperature, abrasion resistance, and impact strength can be achieved (Eisenberg and King, 1975). In solution and in the melt dramatic enhancement of the solution and melt viscosity, display of shear thickening behavior, and polyelectrolyte behavior can be obtained (Lundberg, 1987a). Because of the importance of this unique behavior in commercial applications, extensive investigation of the solid state of ionomershas been carried out. It is now generally accepted that the unusual behavior of ionomers originates from ionic group interactions that lead to aggregation of ions into microdomains, which act as physical cross-links in the material (Eisenberg, 1970). However, the mechanism by which ionic aggregation forms is still a subject of controversy (Lantman et al., 1989a). Compared to the numerous studies that exist in bulk, relatively few studies have focused on the solution behavior t We are pleased to dedicate this paper to Professor Arthur B. Metzner on the occasion of his retirement from full-time teaching at the University of Delaware and in recognition of his many contributions to chemical engineering. * Author to whom correspondence should be sent. Current address: College of Engineering, University of Delaware, 135 Dupont Hall, Newark, DE 19716. E-mail address: Stuart. Cooper@ mvs.udel.edu. E-mail address: Karayiannia che3la.che.wisc.edu.
*
0888-5885/94/2633-2492$04.50/0
of ionomers. The main reason for this has been the limited solubility of ionomers in low-polarity solvents (Otocka et al., 1969). However, when solubility is not an issue, selection of the appropriate solvent environment can lead to a control over the final solution properties. As the ionic groups are usually incorporated in a nonpolar polymeric backbone, in the bulk they find themselves surrounded by a nonpolar environment which promotes aggregation into ionic microdomains. In solution, however, the environment surrounding the ionic groups can be easily modified, leading to diverse properties. Depending on the polarity of the solvent, the level of the ionic group interactions may be quite different (Lundberg and Makowski, 1980). In nonpolar solvents ionomers have been shown to exhibit intermolecular association due to strong dipolar interactions between ion pairs of different chains (Tant and Wilkes, 1988). This behavior is similar to that in the bulk where ionic aggregation has also been observed. The formation of ionic association in solution leads to a strong enhancement of solution viscosity over that of the nonionic parent polymer and unique solution properties that have many potential applications such as fluid additives and viscosity modifiers for oil drilling muds (Lundberg, 1987b). Ionic association greatly affects the rheological behavior of semidilute solutions of these materials which have been observed to display time dependencies (Peiffer et al., 19871, shear thickening followed by shear thinning behavior (Lundberg and Duvdevani, 1989), as well as intersection of the storage and loss moduli in the frequency domain that is evidence of the network-like character of the material (Tant and Wilkes, 1988). Addition of even low levels of polar cosolvents, such as alcohols or amines, into the nonpolar ionomer solutions has marked effects on the solution properties as well as ionomer solubility. The presence of the polar cosolvent markedly reduces the solution viscosity and can even eliminate the ionic association. Depending on the system, both a thermal thinning and a thermal thickening behavior can be observed with increasing temperature, leading to a tremendous control in solution properties and the achievement of desired properties that could not be obtained in conventional systems (Lundberg, 1978,1982; Lundberg and Makowski, 1980). It is believed that this 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 10,1994 2493 unique behavior is due to the solvation or plasticization of the metal cation by the cosolvent which disperses the ionicpair interactions. The presence of ionic groups leads to an enhanced solubility of ionomers in high-polarity solvents even at low ionic contents and the display of a behavior that is quite different than that in low-polarity solvents. In polar solvents a dramatic increase of the reduced viscosity with decreasing concentration has been observed which is identical to the behavior exhibited by aqueous polyelectrolyte solutions (Rochas et al., 1979; Lundberg and Phillips, 1982;Peiffer and Lundberg, 1984). Although surprising, due to the large difference in the ionic content that exists between an ionomer and a polyelectrolyte, the identification of the polyelectrolyte behavior in high-polarity ionomer systems has also been supported by scattering experiments that show features which are characteristic of polyelectrolyte solutions (MacKnight et al., 1986; Lantman et al., 1988a). Typical polyelectrolyte behavior has even been shown for an ionomer with an average of one ionic group per chain (Hara et al., 1988). Due to the substantially lower ionic content of ionomers compared to polyelectrolytes, ionomers can be used in similar applications as polyelectrolyte solutions as well as serve as a system to obtain a better understanding of the still controversial questions regarding the structure of aqueous polyelectrolyte solutions. In this paper we shall focus on the investigation of the association phenomenon that has been observed in lowpolarity ionomer solutions. Apart from the potential applications (Lundberg, 1987b; Peiffer and Lundberg, 1985; Goncharova et al., 19851, the study of this behavior is of utmost importance in order to obtain a better understanding of the behavior in the bulk. Up to now the solution properties of ionomers have been mostly focused on random copolymer ionomer systems that have the ionic groups located in random positions along the polymeric backbone. The most extensively investigated system of this type is the copolymer of sulfonated polystyrene (SPS) due to its industrial and commercial importance (Makowski et al., 1975; Lundberg et al., 1977). Viscometric measurements of these systems have shown that the intrinsic viscosity of the ionomer solution is reduced compared to the nonionic parent polymer, a fact that was originally attributed to single-chaincontraction that forms due to intramolecular ion association. At higher concentrations the viscosity of the ionomer solution is significantly enhanced which is evidence of intermolecular association in solution and the formation of particles that contain many polymer chains which will be referred to hereafter as multimers (Lundberg and Phillips, 1982). However, both light and small-angleneutron scattering experiments have shown that ionic association persists even at very low concentrations (Lantman et al., 1987, 1988b). In addition, single-chain studies via the technique of SANS have shown that the single-chain dimensions are not affected by the association (Lantmanet al., 1988b;Gabrys et al., 1989). As the scattering experiments give information on the average dimensions of the single chains that exist in solution, assumption of the existence of clusters containing a few chains in the very dilute concentration regime led to the conclusion that while association still persists, the polymer chains that exist as single chains in solution are in the collapsed state (Pedley et al., 1990a,b). Pedley et al. (1990a) used an open association equilibrium model, according to which multiple equilibria between unimers and multimers of all sizes take place, to interpret the scattering data for a 1.39 mol 96 random SPS ionomer solution in xylene. The ionic group concentration has been
shown to change the interaction parameter of the system as reflected by the changes in the second virial coefficient (Lantman et al., 1987; Hara and Wu, 1988; Pedley et al., 1990a). Despite the number of studies that exist in the ionomer solution literature, the mechanism of the association phenomenon of ionomers in low-polarity solvents is still an open question. In this study we focus on a model halatotelechelic ionomer system that has the ionic groups positioned only at the two ends of the polymer chain. Because of the well-defined morphology of such a system, it is expected that a more direct correlation between the structure-property relations of the association behavior can be obtained. Extensive viscometric and solubility studies in telechelic ionomer solutions have shown that the association phenomenon is also observed in these systems. Telechelic ionomers exhibit a greater extent of aggregationin nonpolar solvents compared to similar ionic level random ionomers as evidenced by a sharper increase of their solution viscosity and gel formation at very low concentrations (Broze et al., 1981,1982a;Tant et al., 1985). However, apart from the viscometric measurements, structural studies in solution of model ionomers are lacking in the literature. Very recently Vanhoorne et al. (1994) presented small-angle X-ray scattering results on concentrated nonpolar solutions of halato-telechelicpolymers. This study showed that there is no structural discontinuity from the bulk to the solution state. Upon dilution the intermultiplet distance increaseswhile the average number of chains per multiplet remains constant. Further dilution results in partial multiplet dissociation while the intermultiplet distance remains constant. In order to understand the unusual physical and rheological properties of these materials, scattering techniques have been applied in the dilute concentration regime to explore the influence of ionic groups on the structure and thermodynamics of these solutions. Confinement in the dilute regime enables the detection of early stage association formation and therefore the monitoring of the mechanism that underlies the association phenomenon. To this end, small-angle neutron scattering (SANS) is a unique tool to probe the structure and characteristics of both the multimers and the single chains that form the multimers. By applying different labeling techniques, the contrast of the solution can be modified so as to probe both intrachain and interchain correlations. In this contribution we show the results of measurements of the extent of association, the determination of single-chain and multimer dimensions, and the effect of association on the polymer-solvent interaction parameter. These results are combined with those obtained from dynamic light scattering experiments in order to show the effect of association on the diffusion coefficient as well as the hydrodynamic behavior in solution. Experimental Section Materials. The materials used for this study are polystyrene telechelic ionomers in which the carboxylic anion end groups are neutralized with sodium. The materials were synthesized by Professor R. JBrame from the University of LiBge, Belgium, accordingto the synthetic procedures reported by Horrion et al. (1986)and Register et al. (1990). These materials have been synthesized via anionic polymerization to achieve a narrow molecular weight distribution. Due to the low molecular weight of the chains, the initiator used for the syntheses was naphthyllithium that produces a chain with exclusively polystyrene units. The functionality of the chains was
2494
Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994 (i)
CH,OOC-[
-CH,CH-In-COOCH,
6
(e)
Figure 1. Molecular structure of Na-neutralized carboxy-telechelic polystyrene ionomer (i) and corresponding methyl ester form (e). Table 1. Molecular Characterization of the Materials Studied material i-6 i-8 i-17
Mw 6000 8000 17500
ionic content, mol 76 3.7 2.6 1.1
better than 1.95 as was determined from potentiometric titration of the acid end groups in 90/10 (v/v) toluene/ methanol mixture using tetramethylammonium hydroxide. Along with the ionomer systems the corresponding methyl ester forms of the ionomers were also synthesized from the same batches of the telechelics so as to retain the same chain characteristics. The ester form is expected to lack any ionic association and is used as a reference system for exploring the influence of the ionic groups. The chemical structures of the halato-telechelic ionomer and the corresponding ester are shown in Figure 1. Table 1 shows a summary of the molecular weight and corresponding ionic level of the materials used in this study as well as the nomenclature that will be used in this paper for the specific system. The ionic content included in Table 1is determined on a mol percent basis of the ionic groups per repeat unit of the polymer chain. Viscometric Experiments. Viscosity measurements were performed using an AVS 300 Schott-Gerate viscometer measuring station for dilute sequences via an automatic dilution control unit, piston buret, and basic unit with computer output. For the experiments KPGUbbelohde capillary viscometers placed in a measuring stand were used whose size was selected according to the appropriate viscosity range. Temperature control of the measured solutions was achieved to 0.05 "C or better by immersing the measuring stand with the capillary viscometer in a transparent thermostat. The flow times of the solutions were in all cases larger than 200 s. The solutions were filtered with a 0.5 pm pore size filter from Millipore, Co., prior to measurements and left for 30-45 min for temperature equilibration in the thermostat bath. The viscosity values determined from this experiment are the kinematic viscosities of the solutions, and the specific viscosities that are therefore reported in this paper have been evaluated with respect to kinematic viscosity values. Small-Angle Neutron Scattering Experiments. The small-angle neutron scattering experiments (SANS) were performed at the Cold Neutron Research Facility (CNRF) at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. Because of the wide range of the multimer sizes expected, the experiments were carried out at the two 30 m SANS instruments at NIST that are connected to the NG-5 and NG-7 neutron guides of the neutron facility. The solutions were held in quartz cells tightly closed to prevent solvent evaporation. The cell thickness was determined to be 5 mm for the multimer scattering and 2 mm for the single-chain scattering in order to keep the transmission of the samples in the range of 0.5-0.8. The cells were placed in a multispecimen holder that could hold five to seven
different samples simultaneously and which was located inside the sample chamber. The temperature of the solutions was controlled electronically to f O . l "C. The time required for data collection depended on the sample. In most of the cases at least 4 X lo6 and up to 7 X 106 counts were collected to achieve satisfactory statistics in the scattering data. The scattering data from each solution were corrected for the scattering from the empty cell and the background (using a 6Li blocked beam), and the similarly corrected scattering of the solvent was subtracted. The subtracted data reflect the scattering from the polymer alone. The transmissions of the sample, empty cell, and solvent were measured, and their values were used to correct for the differences in transmission that exist between the sample and the empty cell. The data were also corrected for the detector response, which becomes nonlinear toward the edges, according to procedures developed at NIST. Finally, the data were converted to absolute intensity using appropriate standards that are maintained at NIST. The standards allow reduction of the two-dimensional scattering data to absolute crosssection (dz/dQ),b, versus scattering momentum vector (4) which facilitates further analysis. Dynamic Light Scattering Experiments. The dynamic light scattering experiments were performed using a Malvern 4700C system with a 2 W argon ion laser with etalon that provides monochromatic light at 488 nm. The sample cell used was a quartz cylindrical cell that is placed in the sample holder which contains a refractive matching fluid (decahydronaphthalene). The temperature of the sample was kept constant through external circulation of water around the sample holder. The time autocorrelation function was evaluated via a digital correlator that consists of 128 channels. For the ionomer solutions sample times from 0.1 to 100 ps were used. The total time of the experiment was 128 X T , where T is the selected sample time. T was assured to be sufficient for achieving the complete autocorrelation function by comparison of the measured baseline and the value of the autocorrelation function obtained from far point channels that measure at a longer time than the experimentally set time. The angular dependence of the diffusion coefficient was determined using a goniometer in the range of scattering angles from 30" to 120". Data Evaluation and Analysis Viscometric Measurements. Viscositymeasurements are one of the oldest techniques for the characterization of polymers in solution. One expects an associating polymer in solution to display an enhanced viscosity compared to a nonassociating parent polymer. From hydrodynamic analysis of both rigid and flexible polymer molecules, in the dilute concentration regime where [TIC
