Structural relaxation of polystyrene in nanolayer confinement

Abstract

The physical aging of polystyrene (PS) confined in a multilayered film arrangement was explored using differential scanning calorimetry (DSC). The multilayered films were produced via multilayer coextrusion and consisted of alternating layers of PS and polycarbonate (PC), with PS layer thicknesses ranging from 50 nm to 500 nm. A 125 μm bulk control film of pure PS was also extruded and studied for comparison. The glass transition temperatures (Tg) of the PS in multilayered films did not appear to be systematically dependent on layer thickness, and Tg values in all PS/PC films were similar to the bulk value of 104 °C. Two approaches were used to investigate the structural relaxation of PS in the layered films. In the first method, PS layers were aged isothermally at 80 °C after annealing above the Tg of PS (135 °C for 15 min) to reset the thermal history and provide a well-defined starting point for aging experiments. Recovered enthalpy data for aged films (calculated from DSC thermograms) showed that the aging rate in the PS layers decreased with decreasing layer thickness. Calculated aging rates were also compared with the fraction of interphase material (which increases significantly with decreasing layer thickness), and the decrease in aging rate for films with thinner layers was found to correlate with an increase in interphase fraction. The elevated Tg of the interphase material (compared to pure PS) was suggested as a possible reason for reduced aging rates in the thin PS layers. In the second method, PS layers were cooled from above their Tg at different rates under confinement by PC layers. After this cooling step was performed, subsequent heating thermograms revealed that the enthalpy recovered upon reheating through the Tg of PS was similar for bulk and nanolayered films.