Diblock copolymers consisting of polystyrene (S) attached to a polybutadiene (B) block (which is either hydrogenous or perdeuterated) have been synthesized and blended in such a way that the microphase-separated S and B domains have equal scattering-length densities, thus eliminating the component of small-angle neutron scattering (SANS) due to the domain structure. Two samples were studied: one with small spherical polybutadiene microdomains whose size was in essential agreement with calculations assuming equilibrium, and a second one of larger molecular weight in which the sphere size, while larger, was considerably smaller than predicted from equilibrium theory. The SANS spectra of these samples were analysed to give the radii of gyration Rg and molecular weights Mw of the labelled polybutadiene blocks from plots of I-1 versus Q2 and least-square fits to the single-chain scattering function proposed by Debye. Results for the first sample agreed with the molecular weight obtained from chromatography and u.v. absorption and with the Rg found in bulk polybutadiene of similar Mw. The SANS estimates of both Mw and Rg for the second sample were anomallously large; these deviations may be due to (a) non-Gaussian conformations of the polybutadiene chains imposed by the nonequilibrium state of the microdomain, or (b) clustering of the deuterated polybutadiene chains within the microdomain due to small isotopic differences in chemical potential, enhanced by the larger Mw. Observations on other systems suggest that the second effect is the dominant one.
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ACKNOWLEDGEMENTS We are grateful to Professors S. N. Jahshan (SUNY), G. C. Summerfield (Michigan) and J. T. Koberstein (Princeton) for helpful discussions, and for communicating their calculation of the small-angle scattering from blends prior to publication. We thank the referees for helpful suggestions. This research was supported by the National Science Foundation, Division of Materials Research--Polymers Program under grant number DMR-8001674. SANS experiments were performed at the National Center for Small-Angle Scattering Research (NCSASR), which is funded by NSF grant number DMR-7724458 through interagency agreement number 40-637-77 with DOE.
- Small-angle scattering
- chain conformation
- deuterated polymer chains
- heterogeneous block copolymers