Crystallization of quenched polyethylene. Part III : Mixtures of fractions

Abstract

Abstract. 2014 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. support 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 semicrystalline state when the molecular weight is increased.

Microstructural aspects of failure in semicrystalline polymers

“…This is shown for linear polyethylenes in Fig. 1 1 . A similar decrease in level of crystallinity with molecular weight is invariably found on continuously cooling also (70,(72)(73)(74)(75)(76)(77). It is found also that the crystallite thickness increases with molecular weight (72).…”

Etude du mélange PVDF-PTMA

“…8) A0/0 -0,7 (PVDF) et As/0 -0,4 (PTMA) du même ordre de grandeur (0,6) que celles des matériaux monodispersés de polyéthylène cristallisés par trempe. D'après les lois de corrélation liquide-solide dans les polymères [25,26], la plus grande valeur de la longue période du PVDF, indique que les chaînes de ce matériau sont plus grandes que celles du PTMA. La comparaison entre les valeurs du paramètre A0/0 conduit à la même conclusion (pour des parafines 039403B8/03B8 ~ 0,1 à 0,3).…”

“…Il est connu maintenant que les longues périodes L dans l'état semi-cristallin sont le reflet des dimensions des pelotes juste avant cristallisation[25,26] et il faut aussi rappeler que lors d'une trempe, le rayon de gyration R des chaînes ne change pas. Le nombre de chaînes reliant deux lamelles cristallines contigues dépend alors de la distribution moléculaire; l'effet d'un plastifiant ou d'un diluant qui ségrège entre les lamelles cristallines et qui augmente la distance entre les lamelles est donc de diminuer ce nombre de chaînes de connexion.…”

Relationship between the relaxation of polyethylene coils in the melt and the molecular weight distribution. 1. Crystallization measurements

“…It has been shown recently1,2 that in polyethylene quenched from the melt the long period of the lamellar structure is given by the weight average of the end-to-end distance according to the relationship L = rw* = µ , ,•1/2 + VjM*1/2 (1) where w¡ and u>j are the weight fractions of species having molecular weight M¡ and M¡.…”