Poly(styrene-ran-styrene sulfonate) (P(S-SS_x)) is a commercially important polymer that has been widely studied, but recent experimental and theoretical findings suggest that the phase behavior in P(S-SS_x) systems and the morphology of neutralized  P(S-SS_x) ionomers require renewed research efforts.  The purpose of this study is to examine the miscibility of P(S-SS_x) ionomers with homopolymer polystyrene (PS) and P(S-SS_x) acid copolymers. 

Our previous study probed the miscibility of PS and P(S-SS_x) acid copolymers and found a narrow window of miscibility.[1]  Specifically, the PS:P(S-SS_x) blend systems becomes completely immiscible at an unexpectedly low sulfonation levels, x = 2.6 mol%.  Intermediate levels of sulfonation (0.7, 1.0 and 1.2%) are partially miscible and exhibit an upper critical solution temperature (UCST).  We use a deuterated homopolymer, a bilayer sample geometry, and the ion beam technique of forward recoil spectrometry (FRES) to determine the coexistence compositions and thereby to construct the phase diagrams. 

Here, we present a study of blend miscibility of PS and P(S-SS_x)-M ionomers, specifically P(S-SS_0.007) neutralized with sodium (Na⁺), barium (Ba⁺⁺), and zinc (Zn⁺⁺) cations.  The PS:P(S-SS_0.007)-M systems have higher UCST than the PS:P(S-SS_0.007) system, indicating that the neutralization of the acid copolymer reduces the blend miscibility.  The UCST is higher when P(S-SS_0.007) is neutralized with divalent cations Ba⁺⁺ and Zn⁺⁺ than with a monovalent cation, Na⁺.  In addition, as the level of neutralization increases from 25% to 125%, the miscibility in the PS:P(S-SS_0.007)-Zn⁺⁺ blends decreases; this was not observed in the PS:P(S-SS_0.007)-Na⁺ blends. 

Our blend miscibility studies were complimented with linear viscoelastic measurements of the various components.  The longest relaxation times for PS is slight shorter than for P(S-SS_0.019) acid copolymer, while the longest relaxation time is much longer (~ 2 decades) for P(S-SS_0.019)-M neutralized with either Na⁺ or Zn⁺⁺.  The origin of the increased relaxation times is due to the transient physical crosslinks created by the self assembly of ionic functional groups.  The specific interactions that produce these crosslinks, also impede blend miscibility, so we make correlations between the relaxations times and the phase diagrams.

We are now extending our efforts to P(S-SS_x) ionomers with higher sulfonation levels and using of Rutherford backscattering (RBS) is monitor the concentration profile of the high atomic number cations, Ba⁺⁺ or Zn⁺⁺.  These miscibility studies will be complimented with both rheology and morphology studies.  [1] N.C. Zhou, C. Xu, W.R. Burghardt, R.J. Composto, K.I. Winey, Macromolecules, 2006, 39, 2373-2379.