聚合物薄膜玻璃转化温度的降低：分子动力学研究

本文报告了分子动力学（MD）模拟结果，针对两种完全光滑且排斥性壁面之间的受限聚合物熔体进行研究。该系统中，内部粒子与壁面通过势能 $U_{\rm wall}\myeq (\sigma/z)^9$ 进行相互作用，其中 $z \myeq |z_{\rm particle}-z_{\rm wall}|$，$\sigma$ 约为单聚合体直径。系统在 various 薄膜厚度（壁面间距）$D$ 从约 3 倍至约 14 倍于粗糙半径 $R_g$ 的范围内进行了约束。比较薄膜中均方位移与 bulk 中者，发现壁面存在导致动力学加速。%Consistent with this result is the observation that, within %the film, regions closer to the walls have higher mobility than those further %away towards the film center. This leads to a reduction of the critical temperature, $T_{\rm{c}}$, of mode-coupling theory with decreasing film thickness. Analyzing the same data by the Vogel-Fulcher-Tammann equation, we also estimate the VFT-temperature $T_{\rm{0}}(D)$. The ratio $T_{\mr{0}}(D)/T^{\mr{bulk}}_{\mr{0}}$ decreases for smaller $D$ similarly to $T_{\mr{c}}(D)/T^{\mr{bulk}}_{\mr{c}}$. These results are in qualitative agreement with that of the glass transition temperature observed in some experiments on supported polymer films.