Ionomer solutions

The properties of ionomer solutions are sensitive to not only the degree of the ionic functionality and the polymer concentration, hut perhaps even to a greater extent, the ability of the solvent to ionize the ion-pairs (64). Thus, non-ionizing solvents, usually those with relatively low dielectric constant, favor association of the ionic groups even in dilute solutions. In contrast, ionomer solutions may exhibit polyelectrolyte behavior in polar solvents due to solvation of the ion-pair that leaves the hound ions unshielded. 

Polyelectrolyte Behavior. Figure 1 shows the characteristic Kc/R vs. c plot of lonomers in polar solvents the reciprocal reduced scattered intensity rises steeply from the intercept at zero polymer concentration, bends over, and becomes nearly horizontal at higher concentration. This type of behavior was reported for some salt-free polyelectrolytes in aqueous solution (23.26), although the reliability of these early measurements is rather poor because of very small scattered intensity from polyelectrolyte/aqueous solution systems. For example, the excess scattered intensity from salt-free sodium poly(methacrylate) in aqueous solution over that of water was only 10 to 100% (2i) - (0.1-1) x 10 °. in ionomer solutions,... 

HARA WU Light-Scattering Study of Ionomer Solutions... 

The decrease in Aj is attributed to two factors (1) poor solubility of solvent, since THF is not a good solvent for ionic groups (2) attraction between ion pairs in low dielectric constant medium. If the former is the only cause, we should obtain the same molecular weight irrespective of the ion content. However, as is shown in Figure 7, the molecular weight increases with ion content. Therefore, the decrease in definitely reflects the attractions between ion pairs in THF. The second virial coefficient of ionomer solutions is composed of two parts f32.331 i.e., — Ai +... 

Typical solution viscosities for SPS lonomers dissolved In THF are shown In Figure 1. As mentioned above, the reduced viscosity of the Ionomer solution at low concentrations Is less than that of unmodified polystyrene. It Is of Interest to focus on this low concentration limit and to determine the molecular basis for the lowered viscosity. 

The extent of aggregation observed by small-angle neutron scattering Is very dependent on concentration In agreement with light scattering and solution viscosity studies. These measurements on SPS solutions are expected to apply In general to Ionomer solutions In low polarity solvents with appropriate considerations for polymer architecture and counterion structure. 

Surprisingly few studies have focused on the effect of solvents or diluents on the structure and properties of ionomers. Solution results are scarce due to the limited solubility of ionomers in conventional solvents, because of the strong intermolecular associations of the ionic groups(6,7). 

In the 20-30°C temperature regime, certain solutions such as those in Sulfolane (Phillips Petroleum) begin to thicken. Therefore, these ionomer solutions are easier to prepare and more convenient to handle than the previously described precursor solutions. 

Lightly sulfonated polystyrene is soluble in mixed solvent systems, such as xylene containing low levels of alcohols, or in moderately polar solvents. In low polarity solvents the viscosity of such ionomer solutions can be substantially higher than polystyrene of comparable molecular weight due to ion pair association at concentrations >1% as shown in Table I. 

Typically conventional polymer isolation procedures do not permit product isolation within a specific concentration region however, under certain conditions the operation of freeze drying ionomer solutions would be expected to offer this option. Thus, freeze drying of S-PS at two different concentrations (0.3 and A.O weight percent polymer) from appropriate solvents should offer two different polymer species. This hypothesis, of course, assumes that once solutions of this polymer are frozen, the polymer conformations will be "locked in" at those concentrations, or the normal changes which might occur under other conditions of polymer isolation will be minimized. 

Solution behavior of ionomers can be divided into two types, primarily depending on the polarity of the solvent [46,47], One is polyelectrolyte behavior due to the dissociation of counterions in polar solvents (e.g., DMF), and another is association behavior due to the formation of ion pairs and even higher order aggregates in less polar solvents (e.g., THF). Table 2 shows the solvents frequently used for the study of ionomer solutions, as well as their dielectric constants. As the dielectric constant decreases, the degree of counterion binding and also ion pair formation changes (increases) gradually, and so does the solution behavior. In this chapter, only the polyelectrolyte behavior of ionomers in a polar solvent is described. Some brief... 

As is the case of polyelectrolyte aqueous solutions, the viscosity data from ionomer solutions apparently follow the Fuoss equation. A later study [50] has shown that the Fuoss equation is basically an empirical one and that the physical meaning of the constants A and B is not as clear as origi-... 

Rochas et al. [52] showed that the method of isoionic dilution can also be applied for an ionomer solution, as is the case for polyelectrolyte aqueous solutions. In this treatment, originally proposed by Pals and Hermans [53], it is assumed that the size of the polyion can be kept constant by keeping the total ionic concentration (CT) constant, where... 

Another characteristic viscosity behavior of polyelectrolyte solution, viz., the effect of added salts, has been reported for ionomer solutions [55]. The reduced viscosity of sulfonated PS (Li salt) in DMF increases markedly with decreasing polymer concentration in the absence of added salt, LiCl. However, as the concentration of LiCl increases, the reduced viscosity significantly decreases, then a maximum appears in the viscosity curve, and finally straight lines are obtained. The last behavior is characteristic of neutral polymer solutions. Table 3 summarizes ionomer nonaqueous solutions whose viscosity behavior has been reported. Those results have demonstrated that the viscosity behavior of random ionomers is basically similar to that of polyelectrolyte aqueous solutions. 

The characteristic scattering behavior of ionomer solutions shown in Figure 7 can be understood by using the following equation for the excess scattering from a solution of strongly interacting polymers [63] ... 

It is obvious from Figure 9 that the simple diffusion mode of PS spreads into two modes (a fast mode and a slow mode) by introducing a small amount of ionic groups into PS. Figure 10 shows the distribution of the decay rate at different concentrations. At least two distinct peaks are seen over a wide range of time scales for the ionomer solution, in contrast to the PS solution, which shows only a single peak. The average decay rate, (O,... 

The slow (diffusive) mode, whose diffusion coefficient is 1 -2 orders of magnitude lower than that of the fast mode, may be interpreted as arising from the presence of large-scale heterogeneities [87], The diffusion coefficient of the slow mode, Ds, is 5 x 10 s cm2/s for the ionomer solution (as seen in Figure 11). This corresponds to a hydrodynamic radius, Rh, of 475 A, 3.6 times that of the unmodified PS, 130 A (with a diffusion coefficient of 1.8 X 10 7 cm2/s). Here, the Stokes-Einstein relation,... 

Here the counterion binding of ionomer nonaqueous (polar) solutions is described. Figure 12 shows conductance data for a sulfonated PS ionomer in DMF. For comparison, conductance data for comparable small salts, sodium styrenesulfonate, which has a similar structure to the ionic repeat units of sulfonated PS ionomers, is also shown [29], A significant drop in conductance is clearly noted for the ionomer solution as compared with the simple salt. This is due to counterion binding, as discussed above for polyelectrolyte nonaqueous solutions. 

Rochas et al. [52] determined the osmotic coefficients, p, of the ionomer whose viscosity behavior was described before in DMF. The experimental value, cf)p(exp), was obtained by the measurements of osmotic pressure for the ionomer solution in the absence of added salts by using the following equation [93] < p(exp) = IIexp/nideal, where IIexp is the real osmotic pressure obtained by experiment and IIideal is the ideal osmotic pressure, in which all ionic species (macroions and counterions) are assumed to contribute to the osmotic pressure. Examples of < p(exp) values are 1.0 for 3.4 mol% ionomer and 0.80 for 7.3 mol% ionomer at 1 X 10 3 (g/cm3), and 0.98 and 0.48,... 

Hara M. Light scattering from ionomer solutions. In Brow W, Mortensen K, eds. Scattering in Polymeric and Colloidal Systems. London Gordon and Breach, 2000. 

Hara M, Wu J. Light scattering study of ionomer solutions. ACS Symp Ser 1989 395 446-458. 

---