Molecular Dynamics Study on Glass Transitions in Atactic-Polypropylene Bulk and Freestanding Thin Films

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• 作者：Xiang Yu ; Rongliang Wu ; Xiaozhen Yang
• 刊名：Journal of Physical Chemistry B
• 出版年：2010
• 出版时间：April 22, 2010
• 年：2010
• 卷：114
• 期：15
• 页码：4955-4963
• 全文大小：294K
• 年卷期：v.114,no.15(April 22, 2010)
• ISSN：1520-5207

文摘

Molecular dynamics simulation has been adopted in investigation of different glass transition behaviors of the bulk and the freestanding thin films (about 8 nm) of atactic polypropylene (a-PP). For characterization of glass transition temperature (T_g) of above systems, both the specific volume and the local conformational transition rate of the systems were examined. The T_g characterization from the local conformational transition rate of polymer chains was recently developed. Our simulation results show that the films have lower T_g than the bulk, and the descent of T_g is in a range of 3010 K. These are consistent with experiments. a-PP chain consists of meso-dyad and racemic-dyad, the above results are obtained from both dyads. Individual contribution of the stereoregular dyads to the conformational transition rate was further studied. It was revealed that T_g obtained singly from the meso-dyads was almost the same with T_g individually from the racemic-dyads, although the racemic-dyads have much lower transition rate and higher transition barrier than the meso-dyads. In the present study, the reason for the thin films having lower T_g than the bulk is attributed to the novel behavior that the films have lower transition rate and higher barrier than the bulk when it is below T_g. Such behavior was discussed according to the “coupling rotation” of the dihedrals, which depends on the increase of free volume and the inhibition of kinetic energy.