Structure development during isothermal crystallisation of high-density polyethylene: Synchrotron small-angle X-ray scattering study

Abstract

Isothermal melt crystallisation in high-density polyethylene (HDPE) was studied using the time-resolved SAXS method with synchrotron radiation over a wide range of crystallisation temperatures. The SAXS profile was analysed by an interface distribution function, g1(r), which is a superposition of three contributions associated with the size distributions of crystalline (LC) and amorphous (LA) layers and a distribution of long period (LP). The morphological parameters extracted from the g1(r) functions show that the lamellar thickness increases with time, obeying a logarithmic time dependence. The time evolution of LC observed for the sample crystallised at 122°C leads to the conclusion that crystallisation proceeds according to the mechanism of thickening growth. For samples crystallised at lower temperatures (116°C and 118°C), the lamellar thickening mechanism has been observed. The rate of lamellar thickening in these cases is much lower than that at 122°C. At 40°C, thickening of the crystalline layer does not occur. The interface distribution functions were deconvoluted, and the relative standard deviation σC/LC obtained in this way is an additional parameter that is varied during crystallisation and can be used for analysis of this process. Time-dependent changes in the σC/LC at large supercooling (TC=40°C) indicates that LC presents a broad distribution in which the relative standard deviation increases with time. At lower supercooling (TC=122°C), LC shows a much sharper distribution. In this case, the relative standard deviation decreases with time.