STRUCTURE AND DYNAMICS OF DISORDERED TETRABLOCK COPOLYMERS - COMPOSITION AND TEMPERATURE-DEPENDENCE OF LOCAL FRICTION

The structure factor, viscosity, and diffusivity of four (styrene-b-is oprene-b-styrene-b-isoprene) tetrablock copolymers have been examined as functions of temperature (T). The copolymers have styrene compositi ons (f) of 23, 42, 60, and 80 vol % and total degrees of polymerizatio n ca. 120; polystyrene and polyisoprene homopolymers with similar degr ees of polymerization have been used for comparison. Small angle neutr on scattering (SANS) measurements in the disordered state are well-des cribed by the appropriate Leibler/RPA structure factors, and extrapola tion of the inverse peak intensities to lower T yields estimates of th e order-disorder transition temperatures, which are at or below -50 de grees C. Consequently, over the T range of interest (25-180 degrees C) and over length scales greater than the chain dimensions, the tetrabl ocks provide homogeneous matrices containing varying amounts of styren e and isoprene, in which the f and T dependence of segmental friction may be examined. The diffusivity (determined by pulsed-field-gradient NMR and forced Rayleigh scattering) and viscosity provide estimates of the effective monomeric friction factor zeta(eff)(f,T) via the Rouse model; the two dynamic properties yield equivalent values of zeta(eff) . The T dependence of zeta(eff) is well-described by the WLF function, with the f dependence almost entirely contained in the composition de pendence of the glass transition temperature (T-g). Thus, when compare d at constant T - T-g, zeta(eff)(f) is only slightly larger than zeta( PS)degrees or zeta(PI)degrees, in marked contrast to the results for m iscible blends such as PS/PVME and PS/PPO. Prediction of zeta(eff)(f,T ) on the basis of the homopolymer values alone, i.e., zeta(PS)degrees (T) and zeta(PI)degrees(T) is only successful when T-g(f) is incorpora ted explicitly. An approach using equation of state estimates oi;free volume is significantly less successful, implying that the most import ant determinant of local friction in the mixture is the effective T-g sensed by each chain; T-g(f) does not represent an iso-free volume sta te.