Tg-normalized Temperature Research Articles

Surface Mobility of Amorphous Indomethacin Containing Moisture and a Surfactant: A Concentration-Temperature Superposition Principle.

An amorphous material can have vastly higher mobility on the surface than in the bulk and, as a result, shows fast surface crystallization. Most amorphous materials contain multiple components, but the effect of composition on surface dynamics remains poorly understood. In this study, the surface mobility of amorphous indomethacin was measured using the method of surface-grating decay in the presence of moisture and the surfactant Tween 20. It is found that both components significantly enhance the surface mobility, and their effects are well described by the principle of concentration-temperature superposition (CTS); that is, the same surface dynamics is observed at the same Tg-normalized temperature T/Tg, where Tg is the composition-dependent glass transition temperature. For doped indomethacin showing CTS, the mechanism of surface evolution for a 1000 nm wavelength surface grating transitions from viscous flow at high temperatures to surface diffusion at low temperatures at 1.04 Tg. For the surfactant-doped system, the Tg used is the value for the surface layer that reflects the surface enrichment of the surfactant (measured by X-ray photoelectron spectroscopy). At a high surfactant concentration (>10% by weight), the surface-grating decay rate in the surface-diffusion regime is limited by the large, slow-diffusing surfactant molecules; in this case, CTS holds only for the viscous-flow regime. The CTS principle allows the prediction of the surface dynamics of multicomponent amorphous materials.

Molecular Weight Dependence of Fragility in Polystyrene

Segmental relaxation was measured for a series of polysytrenes (PS) of varying molecular weight. The normalized Arrhenius plots of segmental relaxation times (fragility curves) become less steep for shorter chain lengths. Since the shape of the relaxation function is invariant to molecular weight, such behavior is a deviation from the usual correlation between time and temperature dependencies. Although the dependence of relaxation times on Tg-normalized temperature is weaker (less fragile), lower molecular weight PS has a larger heat capacity change at Tg. This result is at odds with an interpretation of temperature dependencies in terms of the degrees of freedom available to the glass upon transition to the viscous liquid.