Mechanism of Polymer Glass Transition

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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).