Polymer dynamics in bimodal polyethylene melts: A study with neutron spin echo spectroscopy and pulsed field gradient nuclear magnetic resonance

We have investigated the dynamics of polymers in bimodal polyethylene (PE) melts in the transition region from Rouse- to reptationlike behavior by var ying the mass fraction Phi(t) of long tracer chains (N approximate to 3N(e) or 4N(e)) in a short-chain matrix (N approximate to N-e=entanglement segme nt number) over the full concentration range. At short times (ns) the dynam ic structure factor for single-chain relaxation was investigated by neutron -spin-echo (NSE) spectroscopy. To obtain information about the long-time (m s) dynamics the tracer diffusion coefficient (D-NMR) was measured by pulsed -field-gradient (PFG)-NMR. We discuss our NSE data within a mode analysis w hich includes the relaxation rates W-p of the independent normal modes of t he internal chain dynamics and the center-of-mass diffusion coefficient D-N SE as model parameters. Only modes exceeding the Phi(t)-dependent length of a single entanglement strand N-e(Phi(t)) are found to be strongly hindered by topological constraints. The D-NSE are Phi(t)-independent and systemati cally faster than the strong concentration-dependent D-NMR, suggesting an e ffective time-dependent diffusion coefficient. The Hess model, which we hav e generalized for polydisperse melts, provides a time-dependent diffusion c oefficient. Taking chain-end effects into account we get an excellent descr iption of the NSE data. The mobility of the chain ends is much higher than the mobility of the inner segments resulting in an entanglement segment num ber which increases with decreasing tracer concentration. The concentration dependence of N-e(Phi(t)), as obtained from the mode analysis and the Hess model, is in agreement with our calculation within a self-consistent modif ication of the model by Kavassalis and Noolandi for entanglement formation. (C) 1999 American Institute of Physics. [S0021-9606(99)50520-5].