Tracer Diffusion of Polystyrene in Lightly Sulfonated Polystyrene

The tracer diffusion coefficient $D^{\ast }$ of deuterated polystyrene (d-PS) (M$_{w}$ = 65,900 g/mol) in lightly sulfonated polystyrene (P(S-SS$_{x}))$ (M$_{w}$ = 65,000 g/mol) as a function of sulfonation mole fraction (x) was measured by forward recoil spectrometry (FRES). For x $<$ 0.7{\%}, d-PS undergoes Fickian diffusion; however, as x increases beyond 0.7{\%}, partial miscibility and eventually immiscibility occurs. For x $<$ 1{\%}, $D^{\ast }$ exponentially decreases with sulfonation, according to $D^{\ast }$ = $D_{o}$ exp(-0.14 N$_{s})$, where N$_{s}$ is the number of sulfuric acid groups per chain. This slowing-down is attributed to an increase in the monomeric friction coefficient which increases with sulfonation. The diffusion mechanism includes both reptation and constraint release. The monomeric friction coefficient for d-PS in P(S-SS$_{x})$ is compared with the coefficient for P(S-SS$_{x})$ measured by rheology.