Investigations of Polyethylene Single-Crystal Suspensions by Light Scattering

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Abstract
The light intensity scattered by polyethylene single crystals in dilute suspensions was investigated as a function of temperature and time in order to obtain information on crystallization and crystal dissolution as a function of crystallization temperature and time, of molecular weight and polydispersity. Whole polymer and three fractions were studied. The crystallization is relatively fast and complete with a fractionated sample of Mv = 85 000 but extremely slow with Mv = 9600 and 300 000 and also with the unfractionated polymer. As a consequence, fractionation occurs during crystallization. The uncrystallized fraction remaining in solution even after prolonged isothermal crystallization crystallizes during subsequent cooling, partly yielding markedly less stable crystals which dissolve many degrees below the main dissolution temperature. During annealing the scattered intensity first drastically drops, indicating a high degree of crystal dissolving. With fractionated polyethylene (Mv = 85 000) it rises again nearly linearly with the log of annealing time. The increase is extremely small or nearly missing with unfractionated samples. With the latter the fractionation during prolonged annealing is significant. The scattering intensity of all samples shows a change of temperature coefficient around 70°C as a consequence of a change in slope of the temperature dependence of both refractive indices of polyethylene crystal lamellas.

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