The phase structure of high-pressure-crystallized polyethylene

Abstract

The phase structure of three linear polyethylene (PE) samples, crystallized from the melt at high pressure, has been studied by electron microscopy and high-resolution solid-state 13C n.m.r. spectroscopy. In general, three phases are required to account for the n.m.r. data: the lamellar crystalline phase, the crystalline-amorphous interphase and the amorphous phase. All three are present in high-molecular-weight samples but there is no amorphous phase for samples with lower molecular weights and large lamellar thicknesses. The amorphous phase appears when the ratio of the number-averaged extended molecular chain length ( X n) to the number-averaged crystalline stem length ( L n) exceeds two. High-pressure-crystallized materials differ from those crystallized at atmospheric pressure in that the mass fraction of the amorphous phase does not exceed 0.05; the thickness of the crystalline-amorphous interphase reaches 8.0 nm for material with the highest molecular weight, a value which is considerably larger than those reported for samples crystallized at atmospheric pressure or which have been estimated theoretically. Extraordinarily long 13C spin-lattice relaxation times have been found: a figure of T 1C = 7000 s, higher than any previously reported, for the highest-molecular-weight sample is still less than would be expected from the large lamellar thickness. In consequence, this relaxation is attributed to molecular motion in the vicinity of the crystal defects; this is in addition to 13C spin diffusion to the non-crystalline region, occurring with a shorter T 1C. The discrepancy between the observed and calculated values of T 1C increases as the molecular weight falls in those samples for which the crystal stem lengths exceed the extended molecular lengths. For these, the unexpectedly shorter T 1C is attributed to defects such as methyl end-groups within the crystalline regions.