SELF-DIFFUSION AND TRACER DIFFUSION IN STYRENE 2-VINYLPYRIDINE BLOCK-COPOLYMER MELTS

Forced Rayleigh scattering (FRS) has been used to probe self-diffusion and tracer diffusion in unentangled, symmetric diblock copolymer melt s. Two poly(styrene-b-2-vinylpyridine) (PS-PVP) diblocks, with M(w) = 1.1 x 10(4) and 2.2 x 10(4), were synthesized anionically and end-labe led with a photochromic o-nitrostilbene derivative, and two PS homopol ymers, with M(w) = 1.4 x 10(4) and 2.5 x 10(4), were labeled with the same dye, randomly along the chain. Rheological measurements demonstra ted that the lower M(w) diblock was in the disordered state and the hi gher M(w) diblock was in the ordered (lamellar) state, over the measur ement temperature range (110-210-degrees-C). The ordered samples were quenched, in the sense that no attempt was made to induce a preferred orientation to the lamellae. The self-diffusion coefficients of the tw o copolymers superpose, when scaled by the ratio of molecular weights, as expected for disordered Rouse chains, thus indicating that the lam ellar order has little effect on the overall mobility. Similarly, the copolymer and homopolymer self-diffusion coefficients are nearly ident ical, for equal total M(w). This result is interpreted as reflecting u nhindered motion of the copolymer chains in the lamellar planes. Trace r diffusion measurements for the homopolymers and the lower M(w) copol ymer in the ordered copolymer matrix also showed little difference fro m the corresponding self-diffusion coefficients but were qualitatively consistent with perturbation theory. In all cases, the FRS signals we re well-described by single exponential decays, possibly suggesting th at diffusion through the lamellae is not greatly retarded relative to diffusion in the lamellar planes and/or that the orientational correla tion length of the lamellae is small.