Fractionation of linear polyethylene during bulk crystallization under high pressure

Abstract

Two linear polyethylene fractions (Mη, 11,260 and 100,000) and mixtures of these fractions have been isothermally crystallized from the melt under pressures up to 3000 atm. Characterization of individually crystallized fractions with transmission electron microscopy indicates that pressure can be used to produce a crystallite whose thickness is a measure of the chain length within it. Although the high molecular weight fraction yields spherulites containing individually varying lamellae thicknesses, the maximum thickness of each lamella is a measure of the chain length within it. Both electron micrographs and differential thermal analysis results show that crystallization of homogeneous mixtures of the high and low molecular weight fractions under high pressure results in a distinct fractionation and segregation according to molecular weight.

Solid Solution Formation and Fractionation in Quasi-Binary Systems of Polyethylene Fractions

“…They arrived at the important conclusion that fractionation acts upon each molecule, and it is not impeded by the complex process of high-polymer crystallization under ambient conditions. Apart from studies by Kardos et al 10 at elevated pressure and investigations on mixtures of low molecular weight n-paraffins,n~14 most of the research on fractionation in heterodisperse systems has been carried out with polymer samples with rather broad molecular weight distributions. In these systems fractionation and solid-solution formation occur simultaneously, thus obscuring differences in the properties and structure of the solid solutions and the separate solid phases.…”

Compressibility of polyethylene under pressure up to 5000 kg/cm2

“…Several authors have evaluated the melting temperature of polyethylene under pressure by dilatometry or thermal analysis (8-10, 15, 16). Though the disparity has also been observed, in this case, the melting temperature of the extended chain crystal of polyethylene is nearly represented by Tm = T~o + 0.02P [4] where Tmo is the melting temperature at ordinary pressure. It is convenient to adopt this T,, as melting temperature of this sample.…”

Promising aspects of the structural modification of polymers and polymer composites with the aid of high pressures

“…In mixtures of fractions, fractionation and segregation take place during crystallisation, which results in the formation of a eutecticlike mixture of crystallites comprising fully straightened molecules with a low M and crystallites made up of molecules with a high M having both straightened and folded shapes. 39 The principal differences in the morphology of PE specimens, obtained at atmospheric and high pressures, are as follows. In the pressure-modified PE, the lamellar thickening occurs only up to a certain limiting extent, the twisting of the lamellae is not observed, and spherulitic formations with a fanlike structure are present.…”