Effect of temperature on molecular weight measurements in polyethylene

Abstract

AbstractLight‐scattering evaluations of molecular weight and gyration radii have been carried out over a range of temperatures for linear and branched polyethylenes in α‐chloronaphthalene solutions. In a number of cases the molecular size parameters have been found to depend on the solution temperature. When observed, the temperature dependence can change molecular weight values by as much as 50%. The temperature dependence tends to be more pronounced at higher molecular weights, in linear than in branched polyethylenes, and is particularly noticeable at solution temperatures below that of the normal crystalline melting temperature of the polymer. It is suggested that the temperature dependence of molecular weight parameters may be due to a gradual dissociation in solution of polymer aggregates arising from persisting interchain entanglements. A relationship was established between intrinsic viscosity and molecular weight for selected fractionated polyethylenes by using molecular weight data obtained under conditions unfavorable to the persistence of polymer aggregates. A number of published viscosity–molecular weight functions for polyethylenes was compared with the calibrated relationship. Some of the published equations gave results in good agreement with the present function, but a number of others resulted in gross overestimates of molecular weight. Polymer aggregation problems may have been encountered in the derivation of some of these viscosity–molecular weight functions. The comparison indicates the danger of indiscriminate choice of viscosity–molecular weight functions for use in rapid estimates of polymer molecular weight.