Relaxation in Low-Crystallinity Polymers

In contrast to the a relaxation both the shape and location of the subglass process are insensitive to the degree of crystallization. Dielectric studies [17] yield the same conclusion. The process is therefore consistent with localized molecular motions, in contrast with the restrained long-range segmental motions involved in the glass-rubber a relaxation. 

Boyd [13] has analysed the d3mamic mechanical behaviour of PET in terms of a composite model of crystalline and amorphous phases. The relaxation strengths for the a and P processes, determined from the shear modulus results of Illers and Breuer, were shown to be related to the crystallinity, indicating that both relaxation processes related to the amorphous regions. For the P processes, the shear modulus for the amorphous phase lies between the upper (Voigt) and lower (Reuss) bounds in both the relaxed and unrelaxed states. The a process, however, appeared to fit best near the lower bound behaviour. 

180° flips and the low temperature side to motion of the carbonyl groups, which has a significantly lower activation energy and activation enthalpy (see Section 6.3.1). 

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