Further investigation of the relationship between polymer structure and HDPE post yield properties

Abstract

Using typical characterization methods (hyphenated chromatography and calorimetry), the effects of molecular weight and short chain branching (SCB) on relative rates of crystallization and the resulting effects on the post yield tensile properties were quantified for a robust set of high-density polyethylene samples. A semi-empirical chain folding equation coupled with the new structure parameters PSP4 (tie molecule content) and the average number of lamella segments per chain (ALSC) were proposed. These parameters are based on a random polymer walk, including passes through the crystalline lamella, the number of times a chain folds in each lamella and the subsequent effects on the end to end distance of the polymer chain, <R>. Intuitively, the current approach is expected to yield enhanced correlations to mechanical performance since, in addition to maintaining the improvements in lamella size calculation, persistence length dependence on SCB content, and multi-ties per chain, it also accounts for polymer chain folding and lamella orientation. Results showed that homopolymers had a significantly higher level of chain folding compared to copolymers. Moreover, as the length of the SCB increased from ethyl to butyl, less folding was predicted thereby leaving more of the polymer chain to form tie molecules. The quantity PSP43/ALSC, exhibited a linear relationship with strain hardening modulus (SHM) with excellent correlation for a diverse set of unimodal and bimodal resins made using different catalyst systems, which included homopolymers, copolymers, and blends of different catalyst type and modality. Given the simplicity of calculation, use of actual polymer data, and the good predictability offered for SHM for a diverse set of resins, these primary structure parameters (PSP4 and ALSC) have high potential to contribute to the in-depth understanding of polymer structure-property relationships.