Chain Entanglements in Polyethylene Melts. Why Is It Studied Again?

The aim of this study is to revisit the characterization of entanglement density in polyethylene melts by studying a series of well-defined, high molecular weight polyethylene materials via transverse NMR relaxometry in the melt state at 423 K. The comparison of the relaxometry data with high temperature SEC-MALLS characterization allows the measurement and correlation of the fraction of chain-end fragments by two independent methods. As compared with rheological methods that measure volume average characteristics, the H-1 NMR method described here offers advantages for studying the entanglement molecular weight (M-e) and chain dynamics in entangled polyethylene melts due to the selectivity of dynamics to entangled chain fragments and disentangled chain-end blocks. The calculated M-e value for infinitely long chains equals 1760 +/- 80 g/mol. This value is in the range of previously reported M-e for polyethylene; however, it exceeds commonly accepted in rheology M-e of 1250 g/mol. The difference can be explained (1) by the effect of chain branching and molecular weight distribution, if samples are not well characterized, and (2) by complex chain dynamics in polymer melts that require several assumptions in rubber-elasticity theory used for calculation of M-e from the plateau modulus.