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 sing le crystal (ECSC) of polyethylene (PE) crystallized at various pressur es below or at the triple-point pressure (P-tri = 0.5 GPa) on the degr ee of supercooling (Delta T) and the pressure have been studied. The v alue of I increased with the decrease in Delta T [i.e. increase in the crystallization temperature (T-c)] at a fixed pressure, similar to th e 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 fr om 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 Delt a T dependence of I and the pressure dependence of I-max. It was propo sed that the value of I is determined by the cessation of lamellar thi ckening growth at the phase transition from metastable hexagonal to st able orthorhombic. The phase transition was also studied and it is sug gested to be a nucleation-controlled process of the primary nucleus.