A proton NMR investigation of crosslinks and entanglements in polyethylene networks and star polymers

Abstract

This paper investigates the ability of proton transverse NMR relaxation to study the structure and dynamics of polyethylene melts. Commercial linear samples of similar M_n and various M_w have been examined, together with samples crosslinked by electron beam irradiation. Apart from the linear sample of lowest M_w, all the free induction decays (FIDs) are very similar and this is attributed to the presence of free chain ends. The magnitude of this problem is illustrated by generating theoretical FIDs from networks of different crosslink density and reptating linear chains of different entanglement spacings, with and without free chain ends. A new result for the FID using the reptation spectrum is presented and used to fit data from the linear polyethylenes. This yields an “NMR average” molar mass, similar to the accepted value and provides information on a sample's polydispersity. Three-arm polyethylene star polymers, of low polydispersity and various arm lengths, have also been studied. The large variation in the chain dynamics from the core to the free end of the star arm is not reflected in the FIDs, which can all be well-fitted by double exponentials. This highlights the importance of specific isotopic labelling in studying such systems.