DEPENDENCE OF THE LAMELLAR THICKNESS OF AN EXTENDED-CHAIN SINGLE-CRYSTAL OF POLYETHYLENE ON THE DEGREE OF SUPERCOOLING AND THE PRESSURE

The dependence of the lamellar thickness (I) of an extended-chain single crystal (ECSC) of polyethylene (PE) crystallized at various pressures below or at the triple-point pressure (P-tri = 0.5 GPa) on the degree of supercooling (Delta T) and the pressure have been studied. The value of I increased with the decrease in Delta T [i.e. increase in the crystallization temperature (T-c)] at a fixed pressure, similar to the well known Delta T dependence of I for a folded-chain single crystal (FCC). The observed maximum value of I, obtained at the lowest Delta T(I-max) increased with increasing pressure and the crystal changed from FCC to an extended-chain crystal (ECC) at ca. 0.25 GPa. Application of the chain-sliding diffusion theory, previously proposed by one of the authors (M.H.) was found to explain well observed significant Delta T dependence of I and the pressure dependence of I-max. It was proposed that the value of I is determined by the cessation of lamellar thickening growth at the phase transition from metastable hexagonal to stable orthorhombic. The phase transition was also studied and it is suggested to be a nucleation-controlled process of the primary nucleus.