Polyethylene, considered relaxations

Let us consider now the effect of flow retardation in extension under constant force 25). The above-mentioned experiments wer,e carried out with low-density polyethylene at 125 °C and with polyisobutylene 11-20 at 44 °C. It should be noted right away that at the above-specified temperatures these melts have approximately similar characteristics under shear strain maximum viscosity is t) 3x 105 Pa s and relaxation time is 0 102 s. Flow curves in the investigated region of shear strain velocities also did not differ significantly from one another. 

Time dependent dichroic measurements during the stress relaxation of low density polyethylene have been reported by Gotoh et alP and more recently by Fukui et alP and by Uemura and Stein. Fukui and coworkers determined both crystalline and amorphous orientation functions at a constant strain between 2-5 and 5%. At room temperature, values of —f and —f>, estimated from data on the 730 and 720 cm" bands respectively, increased with time towards steady values which were attained after about 10 s. These time dependencies were considered to largely determine the observed time dependence of the strain-optical coefficient. Values of f from the 1352 cm" band showed little time dependence, but the amorphous orientation was estimated to contribute the larger amount to the magnitude of the strain-optical coefficient. 

The initial published reports on high density polyethylene were dynamic mechanical studies, but before considering them it is necessary to compare the mechanical relaxations in isotropic material with those observed in unoriented low density polyethylene. From the schematic curve of tan S v. temperature f Fig. 7(b)] it can be seen that the p relaxation, which was ascribed to branch point mobility, is not present, and that the high temperature relaxation is frequently resolvable into a and a peaks. 

Therefore, if distorted region is assumed to be a sphere with similar dimensions as the lattice constants of polyethylene crystals, the relaxation time t can be estimated from the value of D determined from the data of the decay reaction. The rdaxation time is considered to be a time constant of the molecular motion causing a slight distortion associated with the diffusion of the free radical. Thus, the relaxation time of the molecular motion associated with the decay reaction can be estimated. In order to validate this procedure, the diffusion constant was estimated from the known relaxation time obtained in dynamic mechanical studies of polyethylene within the temperature region of the so-called relaxation process in a crystalline phase and... 

The molecular weight of a polymer influences its relaxation behaviour considerably. However, these effects can be modified considerably by the crystallinity of the material. Consequently it seems convenient to consider the influence of molecular weight under the separate headings of amorphous and crystalline polymers. Two representative examples will be discussed in some detail i.e. atactic polystyrene [25] which is a typical amorphous polymer and polyethylene oxide [33, 42 ] whose crystallinity varies with molecular weight, whilst brief illustrative references will be made to a number of other materials. 

The mechanism for the a-relaxation in oxidised and chlorinated polyethylenes has been considered in detail by many workers (see Hoffman et al., 1966 for several proposed mechanisms and a review of the earlier literature). In a recent paper Mansfield and Boyd (1978) have reviewed the eadier work of Frohlich, Tuijnman, Booij, WUliams and co-workers and of Boyd and co-workers on the theory of the rotation of a polymethylene chain within its own crystal. It is well-known that the molecular reorientation may occur as a rigid-rod for short chains, but for long chains the reorientation process involves a twisting of the chain — as indicated first by Frohlich. Hoffman and co-workers (1966) and Williams and co-workers (1967) extended the Frohlich model to include a double-parabola ener and a cosine energy function. It was found that at short chain lengtlu the chains would reorientate between two states... 

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