Polymer Dynamics by Dielectric Spectroscopy and Neutron Scattering — a Comparison

Polymeric systems show a very rich variety of dynamical processes which manifest themselves in different frequency ranges at a given temperature. Between vibrations taking place at time scales faster than the picosecond range and rep-tation at very long times, a number of dynamical processes can been detected in such systems. Some of them can be specific for the particular microstructure of the polymer, like, for instance, rotations of methyl groups. However, it is well established that two relaxation processes are present in all glass-forming polymers (see, e.g., [1, 2]): the primary or structural α-relaxation and the secondary or β-relaxation, also known as the Johari-Goldstein process [3]. The two relaxations coalesce in what we will call α/β-process in a temperature range 10%–20% above the glass transition temperature T_g, The α-relaxation is commonly believed to be related to segmental relaxations of the main chain. The temperature dependence of its relaxation time shows a dramatic increase around T_g, leading to the glassy state at lower temperatures. The β-relaxation is active above as well as below T_g, and occurs independently of the existence of side groups in the polymer. This relaxation has traditionally been attributed to local relaxation of flexible parts, e.g., side groups, and, in main chain polymers, to twisting or crankshaft motion in the main chain [1]. On the other hand, the a-relaxation relates to the structural relaxation of the material and is necessarily of intermolecular nature [4]. However, the molecular nature of the secondary relaxation and its relationship with the primary relaxation are still poorly understood.