Fragility is a Key Parameter in Determining the Magnitude of Tg-Confinement Effects in Polymer Films
2013
Evans, Christopher M. | Deng, Hui | Jager, Wolter F. | Torkelson, John M.
Using fluorescence-based measurements, the effect of confinement on the glass transition temperature (Tg) was investigated in seven polymer systems of single-layer films supported on silicon substrates, all chosen such that they do not have substantial substrate interactions. Poly(vinyl chloride) exhibited the steepest reduction in Tg with decreasing film thickness. The largest magnitude of Tg reduction was observed in polysulfone where a 24-nm-thick film reported a Tg reduced by 53 °C relative to bulk. A system was also selected which exhibited no confinement effect at a thickness of 13–14 nm. Differential scanning calorimetry was used to determine dynamic fragilities in order to test the hypothesis that fragility is a key variable governing the magnitude of Tg-confinement effects. The systems in the present study show a one-to-one correlation between higher fragility and larger magnitudes of Tg reduction with decreasing film thickness. In particular, Tg/Tg,bᵤₗₖ exhibits a universal nature as a function of a scaled film thickness for all seven systems. Fragility is a key variable which reflects the local packing efficiency in a polymer glass and which determines the susceptibility of a glass former to perturbations, in this case induced by confinement.
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