Size and Interfacial Effects on the Glass Transition and Associated Dynamics in Nanoscopically-Confined Polymer Glasses

Abstract

Polymers confined to the nanometer length scale are important in applications ranging from active layers in photovoltaic cells to vehicles for drug delivery. In this thesis, the 3-dimensional nanoparticle geometry is used to explore confinement-induced effects on the glass transition temperature (Tg) and associated properties, e.g., the dynamic fragility and the characteristic length of cooperative segmental motion, in amorphous polymers. For bare polystyrene (PS) and poly(4-methylstyrene) (P4MS) nanoparticles, the Tg, when measured in an aqueous environment from calorimetry and in the dried state from capacitive dilatometry, is observed to decrease systematically with decreasing diameter. The significant reduction in Tg with decreasing diameter is explained in terms of an enhanced mobile layer at the polymer free surface, which acts to reduce the local Tg. This viewpoint is supported by the observation that when the nanoparticles are capped with a rigid silica shell, the Tg remains invariant as a function of diameter, due to removal of the free surface. Aside from the Tg, the temperature dependence of structural relaxation, i.e., the dynamic fragility, is also examined. Using variable cooling rate differential scanning calorimetry, the isochoric fragility (mv) of silica-capped PS and P4MS nanoparticles is measured, which is observed to decrease significantly with confinement. These results suggest the existence of reduced dynamics caused by chain immobilization at the polymer/inorganic interface leading to the observed isochoric fragility-confinement effects. Furthermore, in the case of P4MS, the ratio of the isochoric fragility to the isobaric fragility, i.e., mv/mp, is found to be constant with diameter. Hence, the relative effects of thermal activation and volume contribution on dynamics near the glass transition remain unchanged under confinement. Lastly, the effect of isochoric confinement on the characteristic length of the glass transition for PS and P4MS is examined. Utilizing silica-capped PS and P4MS nanoparticles as model systems, characteristic lengths are determined from the thermal fluctuation model and calorimetric data. With decreasing nanoparticle diameter, the characteristic length decreases, which suggests a reduction in the number of segmental units required for cooperative motion at the glass transition temperature under confinement. Hence, a direct correlation is observed between the characteristic length and the isochoric fragility in confined polymers.