Crystallization of quenched polyethylene. Part III : Mixtures of fractions

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Abstract
The crystallization of mixtures of P.E. fractions quenched from the melt has been studied by SAXS, WAXS and DTA. The laws of variation of the long period with the weight and number average molecular weights are given. It is shown that the long period, L, of the semi-crystalline state is given by the weight average of the end-to-end distance rw of the coils in the liquid state before quenching. For mixtures containing high molecular weight M > M* = 105, the long chains behave like a series of segments of chains of molecular weight M* which crystallize independently. The crystallinity measured by WAXS and DTA varies with the molecular weight and its distribution as the linear crystallinity lc/L ~ lc/rw where lc is the crystalline lamella thickness and is independent of the molecular weight. The correlations emphasized in this paper between the solid and melt states of P.E. sup- port the contention that the structure of lamellar semi-crystalline P.E. is not controlled by the phenomenon of secondary nucleation, but by the dimensions and the rigidity of the coils just before crystallization. The results reported in this paper show that the process of crystallization of short and long chains are different ; that would explain the differences observed in studies of the kinetics of crystallization, and in the morphology of the semi-