The Microscopic Basis of the Glass Transition in Polymers from Neutron Scattering Studies

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Abstract
Recent neutron scattering experiments on the microscopic dynamics of polymers below and above the glass transition temperature Tg are reviewed. The results presented cover different dynamic processes appearing in glasses: local motions, vibrations, and different relaxation processes such as α- and β-relaxation. For the α-relaxation, which occurs above Tg, it is possible to extend the time-temperature superposition principle, which is valid for polymers on a macroscopic scale, to the microscopic time scale. However, this principle is not applicable for temperatures approaching Tg. Below Tg, an inelastic excitation at a frequency of some hundred gigahertz (on the order of several wave numbers), the "boson peak," survives from a quasi-elastic overdamped scattering law at high temperatures. The connection between this boson peak and the fast dynamic process appearing near Tg is discussed.