Grains and Grain Boundaries in Lamellar Styrene - Butadiene Block Copolymers

Due to the condition of the original material, there are unavoidable flaws in this reproduction. We have made every effort possible to provide you with the best copy available. If you are dissatisfied with this product and find it unusable, please contact Document Services as soon as possible. ABSTRACT Simultaneous determination of the lamellar morphological length scale and the grain size of several low molecular weight heterogeneous styrene-butadiene block copolymers was accomplished through the use of ultra small angle x-ray scattering measurements. NIST's X23A3 Ultra SAXS beamline in the Brookhaven National Laboratory provided a scattering vector, q, from 0.0004 to 0.1 A. Most of the block copolymer specimens display a clearly resolvable peak in the Ultra SAXS region, and grain size was determined using the spherical form factor. Determination of Porod's law constant and the value of the scattering invariant provided a verification of the scattering mechanism by solving for the contrast factor and the volume fraction of the grain boundaries in these specimens. Grain size in a given polymer was a function of annealing temperature and time. For the case of a block copolymer swollen with varying amounts of cumene, both the lamellar repeat distance, d, and the grain size, D, increased with the cube root of the volume fraction of solvent over the concentration range examined. Transmission Electron Microscopy validated Ultra SAXS grain size measurements for one of the block copolymer's solvent casting and annealed series. Grain size can be altered in commercial styrene-butadiene block copolymers through the use of evaporation solvent and temperature. The styrene rich polymers can be altered from about 0.3 to 3.5 gm and from about 3.5 to 6.5 pm for the butadiene rich polymers. The solvents caused the same relative grain size for all polymers studied, from smallest to largest: chloroform, toluene, methylene chloride, tetrahydrofuran, ethyl acetate, cumene, and methyl ethyl ketone. The mechanical properties of the commercial block copolymers were examined as a function of grain structure. In contrast to polycrystalline materials, the yield strength increases with increasing grain size for the two styrene rich block copolymers. The Ultra SAXS results suggest that the grain boundaries contain an enrichment of styrene which increases as the grains grow larger. Edge-view SAXS patterns, modulus measurements, and comparison with the mechanical properties of a highly oriented, grain-free specimen all indicate that the observed trends in mechanical yield are not dominated by variations of lamellar orientation …