Structure and dynamics of disordered tetrablock copolymers: composition and temperature dependence of local friction

The structure factor, viscosity, and diffusivity of four (styrene-b-isoprene-b-styrene-b-isoprene) tetrablock copolymers have been examined as functions of temperature (T). The copolymers have styrene compositions (f) of 23, 42, 60, and 80 vol % and total degrees of polymerization ca. 120; polystyrene and polyisoprene homopolymers with similar degrees of polymerization have been used for comparison. Small angle neutron scattering (SANS) measurements in the disordered state are well-described by the appropriate Leibler/RPA structure factors, and extrapolation of the inverse peak intensities to lower T yields estimates of the order-disorder transition temperatures, which are at or below -50 °C. Consequently, over the T range of interest (25-180 °C) and over length scales greater than the chain dimensions, the tetrablocks provide homogeneous matrices containing varying amounts of styrene 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 ζ_eff(f,T) via the Rouse model; the two dynamic properties yield equivalent values of ζ_eff. The T dependence of ζ_eff is well-described by the WLF function, with the f dependence almost entirely contained in the composition dependence of the glass transition temperature (T_g). Thus, when compared at constant T - T_g, ζ_eff(f) is only slightly larger than ζ_PS° or ζ_PI°, in marked contrast to the results for miscible blends such as PS/PVME and PS/PPO. Prediction of ζ_eff(f,T) on the basis of the homopolymer values alone, i.e., ζ_PS° (T) and ζ_PI°(T), is only successful when T_g(f) is incorporated explicitly. An approach using equation of state estimates of free volume is significantly less successful, implying that the most important 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 state.