Enthalpy recovery of ultrathin polystyrene film using Flash DSC

Abstract

Enthalpy recovery for a single polystyrene ultrathin film of 20 nm thickness is studied using Flash DSC over an extended time and temperature range. Results are compared to a bulk sample of the same polystyrene using a similar experimental protocol and analysis procedure in an effort to determine the effects of nanoconfinement. Examined is the cooling rate dependence of the glass transition temperature (Tg) of unaged films which informs the initial fictive temperature (Tfo) and thus the jump size (Tfo - Ta) for a given aging temperature (Ta). Isothermal enthalpy recovery is investigated as a function of both Ta for various cooling rates and as a function of jump size at constant Ta. The apparent activation energies at Tg and along the glassy line are determined and compared, as is the enthalpy recovery aging rate. Although the apparent activation energy along the glass line is the same within experimental error as the bulk, the aging rate is found to be slightly faster in the ultrathin film. Increasing the cooling rate prior to aging increases the aging rate. The results are discussed in the context of the two competing factors which influence the aging rate, namely, the driving force and the molecular mobility. The driving force for aging is dictated by the jump size or cooling rate, i.e., the value of Tfo - Ta. The mobility, on the other hand, is dictated by the relaxation time at the aging temperature, which increases during aging from the value on the initial glass line to that at equilibrium. The initial mobility in the glassy state is dictated by the jump size, being related to Tfo - Ta and the temperature dependence of the relaxation time along the glass line, whereas the mobility at equilibrium is dictated by Ta, Tg, and the temperature dependence and breadth of the equilibrium relaxation time.