ORDERED STRUCTURE OF BLOCK POLYMER HOMOPOLYMER MIXTURES .4. VESICLE FORMATION AND MACROPHASE SEPARATION

Self-assembled structures in solvent-cast films of binary mixtures of poly(styrene-block-isoprene) (SI) and homo-polystyrenes (HS) were studied as a function of r(s), the ratio of molecular weights of HS and polystyrene block (PS) in SI and of W(HS), the weight fraction of HS. For r(s)<1 (a criterion for "wet brush"), HS tends to be solubilized into the microdomain of PS and to swell the PS chains, causing a change in microdomain morphology with long-range order. Spherical microdomains composed of polyisoprene block (PI) dispersed randomly in the matrix of HS and PS ("spherical micelles") are formed at the limit of high W(HS) (W(HS)-->1) . For r(s)greater-than-or-similar-to 1 (a criterion for "dry brush"), HS tends either to be segregated from SI, forming a macrophase-separated morphology (at r(s) much greater than 1), or to be solubilized into the PS microdomains without swelling PS or without a significant change of a degree of swelling with W(HS), if any, (at r(s)congruent-to 1). In the latter case, HS does not significantly affect the lamellar domains of PI for the particular SI studied here, which by itself gives alternating lamellar microdomains of PS and PI, but increases the mean distance between them. As W(HS)-->1, the long-range spatial order of the PI lamellae is lost, while keeping their thicknesses constant and uniform, resulting in morphologies of lamellar, cylindrical, and spherical vesicles. The vesicular morphology was analyzed by small-angle X-ray scattering and transmission electron microscopy. At r(s) much greater than 1 (still in the criterion of dry brush), SI and HS undergo macrophase separation, forming SI domains composed of alternating lamellae in the matrix of HS.