Crystal structure changes during isothermal crystallization, cooling and heating of linear polyethylene

The dynamic process of crystal structure change during isothermal crystallization, cooling and heating of linear polyethylene with different molecular weight and polydispersity was followed by wide-angle X-ray diffraction (WAXD) measurement. From the WAXD data, variations of unit cell parameters a and b and changes in crystallinity were estimated. During isothermal crystallization, both cell parameters were found to decrease with time, suggesting that the crystal structure was becoming more perfect. With an increase in molecular weight or crystallization temperature, the rate of crystal perfection and the attainable crystallinity were found to decrease. This behavior can be explained by the formation of thicker lamellae, which probably have a lower degree of defects and a reduced surface-to-volume ratio in the crystals. Upon cooling and heating, the cell parameters appeared to contract and expand accordingly. The thermal contraction and expansion of parameter a is considerably larger than that of b, which probably results from the weaker intra-chain interactions along the a-axis, which is perpendicular to the spherulite growth direction.