Factors affecting the nascent structure and morphology of polyethylene obtained by heterogeneous Ziegler—Natta catalysts: 2. Crystallinity and melting behaviour

Following studies of the factors affecting the structures and morphologies of nascent polyethylene crystals, density, heat of fusion, crystallinity and melting behaviour of the samples as-polymerized and also after nitric acid oxidation and annealing, were determined by density gradient column, X-ray diffraction and differential scanning calorimetry. The results show that the polymerization temperature is the most important factor controlling the crystal structure. The highest density, crystallinity and crystal quality were found for polymerization temperatures under 100°C, where the growth of the crystals was controlled by a successive polymerization and crystallization mechanism. For this range of polymerization temperatures, density and crystallinity decrease as the polymerization temperature and time increase. When the polymerization temperature is higher than 100°C, near the polyethylene solubility, the crystal grows without any polymerization control, according to a separate polymerization and crystallization mechanism. In this case, density and crystallinity increase with polymerization temperature and time, as with crystallization from melt and solution. The density and crystallinity values of all nascent polyethylene crystals are in the typical range of folded-chain drawn polyethylene or crystallized from the melt. Furthermore, no as-polymerized crystals exhibit a superheating effect suggesting a folded-chain macroconformation. From the melting depression for folded-chain crystals and extended-chain paraffin crystals with finite dimensions, and also from the molecular weight after nitric acid oxidation, the crystal thickness could be calculated. The values are within the range of folded-chain melt crystallized polyethylenes. The results indicate that the crystal thickness of the samples polymerized at below 100°C increases as the polymerization temperature decreases. For the samples polymerized at above 100°C, the crystal thickness increases with the polymerization temperature.