The glass transition in confined and heterogeneous systems
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The glass transition (Tg) is an interesting but challenging problem. Although this phenomenon has been studied for over half a century, Tg is still not well understood especially at the molecular level. One important feature which seems to be missing in the current understanding is its inherent heterogeneous dynamics. The aim of this work is to study the Tg behavior under various conditions such as for material freeze-dried from dilute solutions, confined in nanopores, and blended with other components, and to examine the relation between the dynamic heterogeneity and the observed Tg behavior. Freeze-dried materials from dilute solutions show different Tg behavior from the bulk; however, the origin of the difference remains unclear. In this work, the residual solvent effect on the calorimetric Tg of freeze-dried polystyrene is investigated. A linear correlation is found between the Tg depression and the residual solvent concentration, in agreement with data in the literature, indicating that the Tg depression observed for polymers freeze dried from dilute solution is due to residual solvent. Confinement at the nanoscale is also found to affect the glass transition behavior. Two hydrogen-bonded liquids, glycerol and propylene glychol, confined in silanized and unsilanized nanopores are studied to elucidate the confinement effects on Tg. Upon confinement, these two materials show similar behavior except that an additional Tg is observed for propylene glycol. We find that the confinement effects strongly depend on the competition between size effects and surface effects. Recently, a self-concentration model was proposed to predict the segmental dynamics of misicible polymer blends. To test this model, in this work, the dynamic properties of athermal blends of poly(á-methyl styrene) with its oligomer is examined. The effective Tgs of the components determined from the calorimetric transition can be described by the self-concentration model. However, the self-concentration value obtained is much lower than the theoretical prediction, indicating weak chain connectivity effects in the athermal mixture. Moreover, compared to the pure materials, the blends exhibit considerably broadened transitions and depressed enthalpy overshoots, presumably resulting from their broader relaxation time distribution. In addition to studies dealing with confined or heterogeneous systems, calorimetric measurements of Tg and the limiting fictive temperature, Tf', as a function of cooling and heating rates are performed for a polystyrene to examine the relationship between the Tg and Tf' and to examine the conversion dependence of the apparent activation energy.