Mechanism of Polymer Glass Transition

用中子反射率和X射线反射率研究了聚合物薄膜在玻璃化转变过程中的退火效应。研究发现，以前报道的聚合物薄膜在约20 nm以下的负膨胀是由于没有进行热处理而导致的结构不松弛造成的。这种结构松弛发生在玻璃化转变温度以上一到两个小时内。除了驰豫过程外，我们还发现在玻璃化转变温度以上有一个非常缓慢的驰豫过程。在423K时，松弛时间估计为30-50h。这一慢动作的候选者之一是德·根恩提出的“滑动动作”。用非弹性中子散射直接研究了聚合物薄膜的动力学，首次观察到了100 nm和40 nm厚的玻色子峰。我们还在很宽的Q范围内研究了玻璃形成的聚丁二烯在玻璃化转变温度附近的运动减慢。在本实验中，我们首次发现弛豫时间的Q依赖关系在结构因子S(Q)的第二峰位置出现极大值。

We have studied annealing effects of polymer thin films on glass transition using neutron and X-ray reflectivity. It was found that the previously reported negative expansivity of polymer thin films below about 20 nm is caused by unrelaxed structure due to lack of annealing. This structural relaxation occurs within one or two hours above the glass transition temperature. In addition to the relaxation process, we also found a very slow relaxation process above the glass transition temperature. The relaxation time was estimated at 423K to be 30 - 50h. One of the candidates of this slow motion is "sliding motion" proposed by de Gennes. The dynamics of polymer thin fims were directly studied using inelastic neutron scattering and for the first time we observed boson peak of the films 100 and 40 nm thick.We also studied motional slowing down of glass-forming polybutadiene near the glass transition temperature in a very wide Q range. In this experiment for the first time we found that the Q-dependence of relaxation time does now show a maximum at the second peak position of structure factor S(Q).