Effect of Cooling Rate and Molecular Weight on the Nonisothermal Crystallization of Polyethylene

Abstract

The effects of cooling rate and molecular weight on the nonisothermal crystallization behavior of polyethylene were investigated. In systems with slower cooling rates, chains exhibit greater mobility and have sufficient time to transition from cis to trans conformations. The average length of the trans chain is also larger, and there are fewer entanglements, which facilitate the crystallization. The coupling effect between segmental conformation transition and local segmental orientation and their relationship with nucleation was examined, revealing that nucleation occurs predominantly in regions with a high concentration of conformationally ordered chain segments. As the cooling rate increases, the proportion of conformationally ordered segments decreases. Additionally, in systems with higher molecular weight, there are more entanglement points, leading to reduced segment mobility, which hinders conformational transitions and ordered arrangements. This structure results in increased conformational entropy and reduced nucleation capability. At the early stages of crystallization, the nucleation mechanism of molecular chains is primarily characterized by intramolecular chain folding. Systems with higher molecular weights contain more chain-folded atoms.