NEUTRON SPIN-ECHO INVESTIGATIONS ON THE SEGMENTAL DYNAMICS IN SEMIDILUTE POLYMER-SOLUTIONS UNDER THETA-SOLVENT AND GOOD SOLVENT CONDITIONS

Semidilute polymer solutions can be considered as transient networks of lifetime, tau(g) and of average meshsize xi. xi depends on the polymer concentration, c, and the solvent conditions. If the segmental diffusion of such systems is studied on timescales t << tau(g), using the neutron spin echo technique, different relaxation modes and continuous transitions between them are identified by varying the magnitude of the scattering vector, q. Under good solvent conditions, a crossover from single chain Zimm dynamics, as valid in dilute solutions on the entire intramolecular length scale, to the collective many chain diffusion is found at q xi(c) approximate to 1. The corresponding relaxation rates, Omega(q), vary from Omega(q) similar to q(3) to Omega(q) similar to q(2). In Theta-solvents, the many chain regime was not accessible. However, due to the existence of self-entanglements, which introduce an additional length scale xi(i)(c) < xi(c), the crossover from disentangled (Omega(q) similar to q(3)) to entangled single chain relaxation (Omega(q) similar to q) becomes visible.