Crystallization of polymers. Part II : Fractionated polyethylene quenched from the liquid state

Abstract

We have measured by low angle X-ray scattering (SAXS) the long period of fractionated polyethylene (P.E.) quenched from the melt. The P.E. samples were annealed at different temperatures Ti above the melting temperature and for a time greater than the relaxation time defined in Part I. As for polydispersed material, the long period is given by the relationship L(Ti) = L(To) (1 + α(T i - To)) : α is equal to β/2, β being the temperature coefficient of the ideal chain in the liquid state. Three types of behaviour are distinguished according to the value of the molecular weight M of the fractionated samples : M < M1 L ∼ M (1) M1 < M < M2 L∼ M1/2 (2) M > M2 L ∼ M1/22 (3) M1 is the critical molecular weight, above which the chain has a Gaussian behaviour in the liquid state. M1 is of the order of the molecular weight M e of a portion of chain between two entanglements. The scaling law (2) is new, the long period varies like the end to end distance of the coil in the melt. Our results indicate the necessity of introducing a critical value M2 for the molecular weight, above which the long period is constant. From WAXS and SAXS measurements the crystalline core lamella thickness lc is found to be constant, of the order of 170 Å, the distance between two consecutive entanglements along a chain. The experimental scaling laws giving L and lc give information about the process of crystallization of fractionated P.E

Crystallization of quenched polyethylene. Part III : Mixtures of fractions

“…Therefore, these relations imply that in quenched fractions the long period is a function of the melt temperature, just before crystallization. This effect has been analysed in Parts I [1] and II [2]. (1).…”

“…In quenched polydispersed commercial materials, in general the long period is believed to be independent of the molecular weight and its distribution. In fact, this is not true, small differences in the long period are observed in materials presenting small differences in the molecular weight distribution [5] and important differences when the molecular weight distributions are very different [2]. Kinetic By mixing fractions, the molecular weights MN and M, and therefore the polydispersity I = MW/MN were varied quasi-continuously.…”

“…Introduction. -We have reported recently [ 1,2,3] the correlations existing between the solid state and the melt of polyolefines. In quenched polyethylene fractions of molecular weight M M*, the long period L varies as the end-to-end distance ro of the coils in the liquid state, and is of the same order of magnitude.…”

“…The observation of similar variations of the long period and the weight average end-to-end distance permits an explanation of the dependence of the crystallinity on the molecular weight and the polydispersity. [2]. It is well known that for slow cooling rate L is not independent of the cooling rate [4,55].…”

“…Above the critical molecular weight M*, the long period does not vary. The laws of variations of L with molecular weight [2] [2] :…”