(361e) Blend Miscibility of Sulfonated Polystyrene Ionomer Systems

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.