Correlating polyethylene microstructure to stress cracking; correlations to post yield tensile tests

Abstract

From experimental size exclusion chromatography/infrared detection and differential scanning calorimetry data coupled with random walk statistics, the primary structure parameter (PSP3*) and the average number of crystals per chain (ANCC) were estimated and used to correlate microstructure effects of various high density polyethylenes to their measured natural draw ratio (NDR) and strain hardening modulus (SHM). Results suggest that while both the NDR and SHM reflect the extent of connectivity between crystalline lamella, the SHM reflects the strength of those bridging entanglements and tie molecules. Based on this view and supporting stress strain curves, different but related mechanisms for SHM were suggested for homopolymers versus copolymers. Moreover, a delineation between the effects of branch type was suggested. A “crystal tie mesh (CTM)” model that views the system to be a type of network formed by “crosslinking” multiple bridging and entangled tie chains through common crystalline lamella was proposed. In this semi-empirical model, rather than a rigorous development, basic scaling arguments using the PSP3* and ANCC parameters along with branch length factors (fB) were used. A linear correlation (R2 = 0.9615) was found between the SHM and the structural relationship, fB (PSP3*3/ANCC), for samples used in this study which included monomodal and bimodal resins made using various catalyst types.