Mechanism of Polymer Glass Transition

利用中子反射率和X射线反射率研究了聚合物薄膜的退火效应对玻璃化转变的影响。发现先前报道的低于约20 nm的聚合物薄膜的负膨胀率是由由于缺乏退火而导致的未松弛结构引起的。这种结构松弛在玻璃化转变温度以上的一个或两个小时内发生。除了弛豫过程，我们还发现了一个非常缓慢的玻璃化转变温度以上的弛豫过程。在423 K时的弛豫时间为30 - 50小时。这种慢动作的候选者之一是de Gennes提出的“滑动运动”。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100和40 nm厚的薄膜的玻色子峰，并在很宽的Q值范围内研究了玻璃化聚丁二烯在玻璃化转变温度附近的运动减慢。在这个实验中，我们第一次发现弛豫时间的Q依赖性在结构因子S（Q）的第二个峰位置处显示出最大值。

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).