Surface glass transition temperature

Numerous experiments and computer simulations have shown that dynamics of polymers within nanoscale distances of the free surface exhibit Arrhenius T-dependence, unlike the super-Arrhenius (namely the VTF or WLF) T-dependence typically found in glasses. This warrants the T_g in the nanometer surface region, T_g^surf, to be lower than the bulk T_g. To probe T_g^surf, we employ time-of-flight secondary ion mass spectroscopy (ToF-SIMS), which has a surface sensitivity of less than 1 nm, to monitor the surface dynamics. From our previous ƞ_eff measurements, we know that dynamics in the surface mobile region can be modified when the substrate surface is sufficiently close by ⁸ and vice versa. ⁵ An interesting question, among many other, to address is whether T_g^surf will show a similar sensitivity when the substrate surface is in the proximity. If it does, we would like to examine if chain connectivity may play a role especially when the polymer molecular weight, M_w, is increased past the entanglement limit. According to de Genne’s sliding chain model ⁶ and our high-M_w effective viscosity measurement result, ⁵ T_g^surf of polymer films with larger M_w’s should display the onset of h-dependence at larger h’s. It would also be interesting to examine how the glass transition dynamics in the nanometer thin surface region may change upon the onset of h-dependence.