Glass Transition, Dynamics and Heterogeneity of Polymer Thin Films by Toshiji Kanaya

Author:Toshiji Kanaya
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

7 Concluding Remarks

We have reviewed recent studies on the dynamics of polymer thin films using neutron scattering at temperatures ranging from far below bulk T g to above bulk T g and found that heterogeneity is the essential notion for understanding the physical properties of polymer thin films from the perspective of dynamics. If we want to grasp the essence of polymer thin films, we have to aim at investigating the dynamics at a selective position in a thin film (not in entire films), that is to say the distribution of dynamics in a thin film. Such an experimental approach is quite new at present; however, some researchers have started investigations.

Kim et al. studied only the surface region of a polymer thin film by changing the incident angle of X-rays using XPCS [30]. Multilayer methods are also useful and Torkeloson and coworkers used fluorescence to study the aging dynamics (glassy dynamics) of multilayered labeled PMMA thin films [68]. Thanks to the sophistication of neutron source devices, isotope labeling of multilayered thin films might be a reliable method for studying the distribution of dynamics in thin films by INS directly. Is there an alternative method for evaluating heterogeneous dynamics without resorting to labeling or multilayered methods? Pratt et al. [69] studied the dynamics of polymer thin films at different positions in a polymer thin film using slow muons with different incident energies. The advantage of their method is that the dynamics at the desired position in a thin film can be probed just by changing the incident muon energy, which corresponds to changing the interpenetration depth in the thin film. At present, the energy resolution or the width of interpenetration depth is very broad; hence, analyses such as the distribution of relaxation time along the depth direction are not yet practical. However, the refinement of instrumentation would offer better opportunities to study the distribution of dynamics at a given position in polymer thin films.

If we can succeed in evaluating the distribution of dynamics near T g in thin films qualitatively, we have a chance to evaluate the distribution of CRR in thin films and can discuss the origin of anomalous dynamics in polymer thin films directly. For this purpose, new experimental methods and further theoretical approaches are indispensable.

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