Kimberly B. Shepard

Fragility of an Isochorically Confined Polymer Glass.

We report the effect of isochoric confinement on the dynamic fragility of a polymeric glass-former, that is, polystyrene (PS). Utilizing silica-capped PS nanospheres as a model system, the fictive temperature (Tf) and the isochoric heat capacity (Cv) are measured as a function of diameter via differential scanning calorimetry. By examining Tf as a function of cooling rate for each sample, the isochoric fragility (mv) is obtained, which decreases significantly as the diameter of the nanospheres is reduced from 260 to 129 nm. Hence, the temperature dependence of structural relaxation near the glass transition is weakened with isochoric confinement.

Origins of nanostructure in amorphous polymer coatings via matrix assisted pulsed laser evaporation

We investigate the nanostructure of sub-monolayer and monolayer amorphous polymer films deposited via Matrix Assisted Pulsed Laser Evaporation (MAPLE). The structure is quantified by analyzing the size distribution of polymer nanoglobules as a function of deposition parameters: time and polymer concentration. Two deposition regimes are observed in the early stages of MAPLE deposition, with a transition at a critical time. The observed distribution of nanoglobule sizes that is present after the critical time agrees well with prior molecular dynamics simulations of the MAPLE process. We discuss the mechanism of nanostructured coatings within the framework of the Zhigilei model.