Branching effects on the segmental dynamics of polyethylene melts

C-13 NMR spin-lattice relaxation time and nuclear Overhauser effect measure ments are reported for linear polyethylene, polyethylene with long branches , and linear polyethylene with infrequent short branches at 75 MHz over the temperature range of 400-535 K. A quantitative description of the segmenta l dynamics of the main-chain methylene units and the branch points is obtai ned for linear polyethylene with ethyl branches. At 400 K, the correlation time is 29 ps for the segmental dynamics of the branch point but 5 ps for t he segmental dynamics of the main-chain methylene units. The activation ene rgy for the segmental dynamics of the main-chain methylene units (4 kcal/mo l) is significantly less than the flow activation energy (E-eta = 7.2 kcal/ mol). In contrast, the activation energy for the segmental dynamics at the branch point (6.4 kcal/mol) is much closer to E-eta. This result supports o ur earlier hypothesis that the lack of side groups in polyethylene allows c onformational transitions to happen without substantial intermolecular coop eration. Thus, in contrast to many other polymers, conformational transitio ns are not the fundamental motion for flow in polyethylene. Branches as sma ll as a methyl group introduce intermolecular cooperation into conformation al transitions at the branch point, so conformational transitions at the br anch points have a stronger temperature dependence. No difference in local dynamics was found for methylene units in linear polyethylene and polyethyl ene with long branches. Therefore, local dynamics are not relevant for unde rstanding the factor-of-two difference in the flow activation energies of t hese two polymers. (C) 2000 John Wiley & Sons, Inc.