Isothermal physical aging of thin PMMA films near the glass transition temperature

Isothermal physical aging and the glass transition temperature ( T g ) of PMMA thin films were investigated by means of differential scanning calorimetry (DSC). Freestanding thin films of different molecular weights ( M w = 120,000, 350,000, 996,000 g/mol) and film thicknesses (40–667 nm) were obtained by spin coating onto a silicon wafer substrate and then releasing the coated film using a water floating technique. The thin films were stacked in a DSC pan and isothermally aged for different aging times ( t a = 1 and 12 h) and aging temperatures ( T a = 105, 110, and 115 °C) below but near T g . Enthalpy relaxation (Δ H Relax ), resulting from the isothermal physical aging, initially increased with increasing Δ T ( T g − T a , driving force of aging), reached a maximum value, and then decreased with further increase in Δ T . Below ~100 nm film thickness, Δ H Relax of samples aged near their T g (i.e . , T a = 110 and 115 °C) decreased with decreasing film thickness, indicating the suppression of physical aging. Up to 9.9 °C depression in T g was observed for thinner films (~40 nm), when compared to the thicker films (~660 nm) in this study. The decrease in Δ H Relax with decreasing film thickness at a given T a appears to be associated with the reduction in T g .