Abstract
This study examined the effects of the aging temperature (Tage) on structural relaxation in ultrathin-supported polystyrene (PS) films with thickness ~0.2Rg. X-ray reflectivity and differential scanning calorimetry were used to evaluate the glass transition temperature (Tg) of ultrathin PS and bulk PS, respectively. The ultrathin films in their confined state showed anomalous thickness relaxation behaviors that could be classified into three types: (1) Tage≤60°C, a slow increase in thickness with time and a decrease in relaxation magnitude with temperature; (2) 70°C≤Tage≤110°C, a slow decrease in thickness with time and an increase in relaxation magnitude with temperature; (3) Tage>110°C, an increase in thickness with time due to dewetting of PS films deposited on SiO2. Such an interesting relaxation behavior can arise from the enhanced dynamics at the surface and reduced mobility at the interface inherent in ultrathin-supported films. Two distinct Tgs peculiar to ultrathin films were confirmed. The lower Tg has not been reported for supported PS films. The relaxation summarized above revealed a strong correlation with the Tg values. The de Gennes modified sliding motion model was applied to explain the correlation with relaxation dynamics.
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