Studies of relaxation phenomena in polymers. III. Superimposed transition mechanisms in polyisobutylene and poly(vinyl acetate)

Abstract

AbstractBased on the general properties of the function log J2 = gα{log J1}, discussed in earlier papers,1,2 the existence of two superimposed dispersion mechanisms in the glass–rubber transition of polyisobutylene (PIB), and poly(vinyl acetate) (PVAc) is discussed. A simple weighting model is used for separation of the two mechanisms. For the interpretation of the β‐mechanism, in PIB, the following facts are taken into account: (a) The loss maximum of the β‐mechanism appears in a zone where only very reduced segment movements occur. (b) The methyl side groups in PIB require special conformations of the main chains before they can rotate. (c) The apparent activation energy for the β‐mechanism is approx. 20 kcal/mole. Thus, for PIB the following interpretations for the α‐ and β‐mechanisms are proposed: (1) The α‐mechanism is due to simple segment movements. (2) The β‐mechanism appearing in the “Tg region” is due to coupled movements between the main chain and methyl side groups. The ester groups of poly(vinyl acetate) need a lot more space for their rotations than the carboxy groups of poly(methyl acrylate) and can be expected to give considerable steric hindrance. It is therefore reasonable to suppose that a transition mechanism similar to the β‐mechanism of polymethacrylates will appear in the “freezing” region of poly(vinyl acetate). The analysis of the β‐mechanism is made using the same separation method as for polyisobutylene.