Structure and Dynamics of Disordered Tetrablock Copolymers:  Composition and Temperature Dependence of Local Friction

Abstract

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 ζeff(f,T) via the Rouse model; the two dynamic properties yield equivalent values of ζeff. The T dependence of ζeff 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, ζeff(f) is only slightly larger than ζPS° or ζPI°, in marked contrast to the results for miscible blends such as PS/PVME and PS/PPO. Prediction of ζeff(f,T) on the basis of the homopolymer values alone, i.e., ζPS° (T) and ζPI°(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.

Styrene and isoprene friction factors in styrene-isoprene matrices

“…Four styrene-isoprene-styrene-isoprene (SISI) tetrablock copolymers of varying composition were synthesized via sequential living anionic polymerization as previously reported, 5 and used as matrices for the tracer diffusion measurements. Each tracer molecule was labeled with a photochromic dye in order to measure the diffusion with forced Rayleigh scattering (FRS).…”

“…The hydroxyl group permitted subsequent end-labeling through a base-catalyzed condensation reaction with ONS-COOH. 5 The living anionic polymerization of a styreneisoprene diblock copolymer, SI(5-6.5), was terminated with p-dichloroxylene, for end-labeling with the ONS-COO Ϫ Cs ϩ . 7 Fractionation of SI(5-6.5) removed the portion of coupled chains.…”

“…copolymers as a means to achieve this end. 5 By keeping the block lengths reasonably short, the microphase separation transition is suppressed, but the total chain length is still sufficient to minimize chain end effects. Such a copolymer should provide a homogenous matrix with a selectable composition for use, for example, as a matrix for tracer diffusion measurements.…”

“…This approach was demonstrated with four tetrablock copolymers of styrene and isoprene (SISI), prepared by sequential anionic polymerization; the chain lengths were roughly constant (N Ϸ 120), while the styrene contents were 23, 42, 60, and 80% by volume. 5 In this article we describe measurements of the tracer diffusion of a polystyrene homopolymer, a polyisoprene homopolymer, and a styrene-isoprene diblock copolymer as a function of temperature, in these SISI tetrablock matrices. Although the homopolymers were not misicible with all four tetrablocks, the diblock tracer was, and consequently, the individual styrene and isoprene friction factors could be extracted across the entire composition range.…”

“…5 Furthermore, the experimental scattering profiles were well described by the appropriate mean-field (RPA) structure factor. 5 The peaks were located near 0.08 Å Ϫ1 , corresponding to a length scale of 80 Å, and the chain radii of gyration were all ca. 40 Å.…”