Crystallite Size Analysis of Pressure-Crystallized Polyethylene by Nitric Acid Etching and Gel-Permeation Chromatography. II. Folded-Chain Crystals and Ordinary Extended-Chain Crystals

Abstract

Statistical analysis of average lamellar thickness and its size distribution in the molecular chain direction has been made on a variety of folded-chain crystals and ordinary extended-chain crystals of linear polyethylene which were crystallized under high pressures of 106 MPa and 305 MPa. This was done by fuming nitric acid degradation followed by gel-permeation chromatography (GPC) measurement of molecular weight distribution of the degraded samples. The observed GPC curves were corrected for instrumental spreading using the Chang–Huang’s method of correction. Several new experimental facts became clear. First, the existence of folds was confirmed in the ordinary extended-chain crystals formed at 305 MPa. Second, the average lamellar thickness of the defect-free crystalline core of folded-chain crystals was often found to be approximately one tenth the average thickness of ordinary extended-chain crystals in the same sample. Third, in ordinary extended-chain crystals with average lamellar thicknesses of several hundred nanometers, it was observed that the average crystal thickness decreased markedly with increasing degradation time, compared with the folded-chain crystals and that the chain-thinning effect tended to be become greater with an increase in average crystal thickness. This suggests strongly that ordinary extended-chain crystals have surface structures easily accessible to nitric acid etching. Since the crystal thickness of ordinary extended-chain crystals is comparable to the number-average molecular weight of the original material, crystalline lamellae will be terminated predominantly in the form of extended-chains. It may be reasonable to assume extended-chain crystals with gradually disordered, long surface layers from about 20 to 50 nanometers as a possible model for ordinary extended-chain crystals.