Speaker
Description
The dynamics of polymer glasses have been shown to be enhanced within a few nanometers of an interface. This enhanced surface mobility has technological implications for processing and applications of glasses. An open question is how the chemical structure of a polymer affects the dynamics in the near surface region, but this is extremely challenging to study. Previous studies using bulky fluorescent probes found a mobile surface layer in polystyrene (PS) but not in poly($\alpha$-methylstyrene) (P$\alpha$MS). We have used $\beta$NMR to study the depth dependence of phenyl ring dynamics in PS ($T_g$ = 370~K) and P$\alpha$MS ($T_g$ = 448~K) glasses at 317 and 261~K. The larger $T_g$ of P$\alpha$MS indicates the polymer chains are stiffer than PS. The $\beta$NMR results indicate there is a mobile surface layer in P$\alpha$MS contrary to previous reports and that this layer is much thicker than that of PS. The characteristic length scale for the return to bulk dynamics at 317~K is 26(3)~nm in P$\alpha$MS versus 4.0(2)~nm in PS. The stiffness of the polymer chains plays a role in the transmission of fast surface dynamics into the bulk. The phenyl ring motion appears to be decoupled from the backbone motion.
| iainmckenzie@triumf.ca | |
| Funding Agency | NSERC |
