Molecular structure of crosslinked polyethylene as revealed by 13C nuclear magnetic resonance and infra-red spectroscopy and gel permeation chromatography

Abstract

The molecular structure of low-density, high-pressure polyethylene crosslinked with peroxide or according to the silane process has been studied by gel permeation chromatography (g.p.c.), infra-red (i.r.) spectroscopy, 13C nuclear magnetic resonance (n.m.r.) spectroscopy, solvent extraction and rubber elastic modulus measurements. It is shown that the degree of chain branching in the soluble part of the crosslinked samples (SOL) is the same as that in the polymer prior to crosslinking (BASIC). G.p.c. analysis provides evidence that the SOL samples are low-molecular-weight material compared with the corresponding BASIC samples and that they have bimodal molecular weight distributions. Model calculations assuming a 100% efficient crosslinking agent predict a dominance of chemically totally unmodified molecules in the SOL constituting the low-molecular-weight peak of the bimodal distribution. The model calculations furthermore predict that the next most important member of the SOL is the combination of two molecules linked together by one crosslink. There is an almost perfect fit between the calculated molecular weight distribution of these molecules and the experimentally found high-molecular-weight peak of the SOL. The discrepancies observed between measured and calculated contents and molecular weight distributions of SOL can be explained on the basis of an inefficient crosslinking agent due to possible side reactions and intramolecular crosslinking. The crosslink density determined by elastic modulus measurements using the theory of rubber elasticity compares fairly well with calculated values based on a 100% efficient crosslinking agent.