Phase Behavior of PS-(PS-b-P2VP)3 Miktoarm Star Copolymer

Abstract

We synthesized polystyrene-[polystyrene-b-poly(2-vinylpyridine)]3 miktoarm star copolymer [PSL-(PSS-b-P2VP)3], where PSL and PSS are long and short PS chains, respectively, by the combination of anionic polymerization, atom transfer radical polymerization (ATRP), and click reaction. We changed the volume fraction of the PS block (fPS) and the chain asymmetry of the PS chain τ = fPS,L/(fPS,L + fPS,S). Phase behavior of PS-(PS-b-P2VP)3 was investigated by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Inverted gyroids consisting of PS chains were formed at fPS = 0.64 and τ = 0.65, while asymmetric lamellae were observed at fPS = 0.81 and τ = 0.79. Because a long PS chain (PSL) and three short PSS-b-P2VP copolymer chains are linked at a single junction point, a huge configurational entropy penalty was expected, resulting in distorting the original phase boundaries observed for conventional AB diblock copolymer. PSS-b-P2VP chains are mainly located at the interface between PS and P2VP microdomains, whereas PSL chains fill the regions far from the interface, which causes a radial distribution to form interfacial curvature. Interestingly, the phase behavior was greatly affected by τ at a fixed fPS. For instance, at a fixed fPS (0.64), an inverted gyroid structure was formed at τ = 0.65, while a lamellar structure was observed at τ = 0.46. With the decrease in τ (or the difference of molecular weight between PSL and PSS becomes smaller), the interfacial curvature is not expected because all PSS-b-P2VP chains have no need to be arranged in the same direction. The experimental results are consistent with the predictions based on self-consistent field theory (SCFT).