Neutron spin echo investigations on the segmental dynamics in semidilute polymer solutions under Θ- and good solvent conditions

Abstract

Semidilute polymer solutions can be considered as transient networks of lifetime, τ g, and of average meshsize, ξ. ξ depends on the polymer concentration, c, and the solvent conditions. If the segmental diffusion of such systems is studied on timescales t ⪡ τ 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 qξ( c) ≈ 1. The corresponding relaxation rates, Ω(q), vary from Ω(q) ∼ q 3 to Ω(q) q 2. In Θ-solvents, the many chain regime was not accessible. However, due to the existence of self-entanglements, which introduce an additional length scale ξ i ( c) < ξ( c), the crossover from disentangled (Ω(q) ∼ q 3) to entangled single chain relaxation (Ω(q) ∼ q) becomes visible.