Styrene and isoprene friction factors in styrene-isoprene matrices

Monomeric friction factors, zeta, for polystyrene (PS), polyisoprene (PI), and a polystyrene-polyisoprene (SL) diblock copolymer have been determined as a function of temperature in four poly(styrene-b-isoprene-b-styrene-b-is oprene) tetrablock copolymer matrices. The Rouse model has been used to cal culate the friction factors from tracer diffusion coefficients measured by forced Rayleigh scattering. Within the experimental temperature range the t etrablock copolymers are disordered, allowing for measurement of the diffus ion coefficient in matrices with average compositions determined by the tet rablock copolymers (23, 42, 60, and 80% styrene by volume). Remarkably, for a given matrix composition the styrene and isoprene friction factors are e ssentially equivalent. Furthermore, art a constant interval from the system glass transition temperature, T-g, all of the friction factors (obtained f rom homopolymer, diblock copolymer, and tetrablock copolymer dynamics) agre e to within an order of magnitude. This is in marked contrast to results fo r miscible polymer blends, where the individual components generally have d istinct composition dependences and magnitudes at constant T - T-g. The hom opolymer friction factors in the tetrablock matrices were systematically sl ightly higher than those of the diblock, which in turn were slightly higher than those of the homopolymers in their respective melts, when all compare d at constant T - T-g. This is attributed to the local spatial distribution of styrene and isoprene segments in the tetrablocks, which presents a nonu niform free energy surface to the tracer molecules. (C) 1998 John Wiley & S ons, Inc.