Small-angle X-ray scattering from styrene-butadiene-styrene block copolymers

Abstract

The effects of varying the 1, 2-addition (vinyl) content in polybutadiene(PB) and of varying casting solvent on the formation of randomly oriented domain structures of styrene-butadiene-styrene (SBS) block copolymers have been investigated by small-angle x-ray scattering (SAXS) and electron microscopy (EM). The degree of segregation of two phases was estimated by ⟨n ²⟩_S /⟨n ² ⟩F, where ⟨n ²⟩_S and ⟨n ²⟩_F are the mean square of the electron density fluctuations obtained by the absolute SAXS measurements on the given system and that estimated by flotation method on the assumption of an ideal two-phase system, respectively. It was found that if the block copolymer contains small amounts of the vinyl content (ca. 10%) and of polystyrene (PS) (less than 25 vol %), the PS phases form nearly spherical domains and the degrees of segregation (0.87-1.04) of the PS and PB phases are relatively high for the samples cast from benzene (BZ) and cyclohexane (CH) solutions. In these cases the radii of PS domains and distances between those domains obtained from the correlation function δ(r) and the radial distribution function are in fairly good agreement with those estimated from EM. If the same SBS is cast from methyl ethyl ketone (MEK) solution, the granular PB phases are embedded in the PS matrix and the degree (0.25) of segregation is lower than those for the BZ-and CH-cast films. δ(r) shows a rather broad maximum at larger r. This result is consistent with the EM observation that arrangement and size of the PB domains are irregular. For samples containing large amounts (ca. 60%) of vinyl content, the degrees (0.19–0.42) of segregation are also low for the BZ-and CH-cast films as well as for the MEK-cast film.