Fragility is a Key Parameter in Determining the Magnitude of T_g-Confinement Effects in Polymer Films

Abstract

Using fluorescence-based measurements, the effect of confinement on the glass transition temperature (T g) 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 T g with decreasing film thickness. The largest magnitude of T g reduction was observed in polysulfone where a 24-nm-thick film reported a T g 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 T g-confinement effects. The systems in the present study show a one-to-one correlation between higher fragility and larger magnitudes of T g reduction with decreasing film thickness. In particular, T g/T g,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.

A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films

“…(14) can describe our T g changes in Fig. 13; similar behavior of T g shifts has been reported in previous experimental studies 7,13,84,85 and simulations on thin films. 25 In our polymer film model, a becomes negative for ε 0.9.…”

Segmental dynamics and the correlation length in nanoconfined PMMA

“…These multilayer films were prepared by melt co-extrusion, using a forced assembly technique developed by Baer and coworkers [13e15]. Unlike conventional thin films [16,17], the macroscopic size of these multilayer films makes possible modulated scanning calorimetry (MDSC) to determine the complex heat capacity, supplementing the dielectric measurements. The ability to measure both dynamic and thermodynamic properties allows two independent determinations of the dynamic correlation length.…”

A coarse-grained molecular dynamics study of segmental structure and mobility in capped crosslinked copolymer films

“…31,22 Dynamic fragility, a measure of the glass transition abruptness of glass-forming materials, has been employed to explain why certain polymers display different confinement effects than others. For example, recent results of Evans et al, 32 obtained by differential scanning calorimetry experiments with single-layered polymer films supported on silicon substrates (systems with no substantial polymer-substrate interactions), showed a one-to-one correlation between higher fragility and the amplitude of the T g shift upon changing the film thickness.…”