A PROTON NMR INVESTIGATION OF CROSS-LINKS AND ENTANGLEMENTS IN POLYETHYLENE NETWORKS AND STAR POLYMERS

This paper investigates the ability of proton transverse NMR relaxatio n to study the structure and dynamics of polyethylene melts. Commercia l linear samples of similar M-n and various M-w have been examined, to gether with samples crosslinked by electron beam irradiation. Apart fr om the linear sample of lowest M-w, all the free induction decays (FID s) are very similar and this is attributed to the presence of free cha in ends. The magnitude of this problem ir illustrated by generating th eoretical FIDs from networks of different crosslink density and reptat ing linear chains of different entanglement spacings, with and without free chain ends. A new result for the FID using the reptation spectru m is presented and used to fit data from the linear polyethylenes. Thi s yields an ''NMR average'' molar mass, similar to the accepted value and provides information on a sample's polydispersity. Three-arm polye thylene 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 system s. (C) 1997 Elsevier Science Limited.