Polyethylene Secondary relaxations

First is the crankshaft mechanism of Schatzki.16 It has been observed for many polymers containing linear (CH2) sequences with n = 4 or greater, that a secondary relaxation occurs at about -120 °C at 1 Hz. This seems to be true regardless of whether the CH2 sequences occur in the main chain or in the side groups. Thus both polyethylene and poly-n-butyl methacrylate exhibit this relaxation. The mechanism proposed by Schatzki is shown in Figure 5-14. 

Colombini and co-workers [42] used DMTA and DETA (Chapter 12) to explore the relaxation processes occurring in amorphous and semi-crystalline polyethylene naphthalene-2,6,-dicarboxylate. The two secondary relaxations P and P, the main a-relaxation and the p-relaxation processes were revealed by both mechanical and electro viscoelastic responses of the polymer. The DMTA results clearly identified the T(a) loss factor peak. 

Maxwell, A.S., Monnerie, L. and Ward, I.M. (1998) Secondary relaxation processes in polyethylene terephthalate-additive blends 2. Dynamic mechanical and dielectric investigations. Polymer, 39, 6851. 

During the course of these and related studies, notably those concerned with the temperature dependence of the mechanical anisotropy and the identification of relaxation processes in structural terms, it became apparent that the aggregate model was successful in low density polyethylene because it described effectively the influence of the very anisotropic x-relaxation process on the mechanical behaviour. Stachurski and Ward were even able to extend the aggregate model to deal with the anisotropy of dynamic loss factor. (See Chapter 9 for further discussion.) It was, however, more in the spirit of the original conception of the aggregate model that it would deal with mechanical anisotropy in glassy polymers, where morphology was of secondary importance. 

Figures 10 and 11 show the TSDC curves at 60 °C polarization temperature of the EVA before and after exposure at CDER and URAER sites respectively. There are differences in relative magnitudes of the low and high temperature relaxation peaks. These results suggest that prolonged exposure selectively affects the poly (vinyl acetate) rich phase, with much less impact on the polyethylene rich phase. This is due to the progress of EVA crosslinking reaction such as temperature increase by long-term exposure. We found also, that the aged EVA after exposure showed considerable decrease in current intensity for the high temperature polarization due to secondary melting peaks as it will be revealed by DSC technique.

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