Molecular Weight Dependence of Deuterium Exchange on Polyethylene: Direct Measurement and SANS Model

Abstract

Following a hydrogen–deuterium exchange reaction, size-exclusion chromatography with infrared detection (SEC-IR) was used to measure the distribution of deuterium with respect to molecular weight on a polyethylene resin. The SEC-IR method reveals significant heterogeneity in the distribution of deuterium across molecular weight, including a fraction of high molecular weight chains that are nearly or completely unlabeled. Small-angle neutron scattering (SANS) was performed on the labeled polymer and an isotopic blend; the random phase approximation model prediction for the scattering from an n-component blend was successfully used to model the measured SANS data. Additionally, a Monte Carlo algorithm was used to fit the measured SANS data to a deuterium distribution, yielding a measurement consistent with that obtained from SEC-IR. The methods are compared to literature approaches for describing nonideal deuterium labeling. These results demonstrate the value of including detailed information about the distribution of deuterium in a scattering model and provide methods for probing the structure and interactions of complex polyolefin resins that have been labeled via heterogeneous catalysis.