On the Phase behavior of polystyrene-$b$-poly(methyl methacrylate) diblock copolymer

The phase behavior of polystyrene-$b$-poly(methyl methacrylate) (PS-$b$-PMMA) copolymers of various molecular weights has been studied by using small-angle X-ray scattering (SAXS) and depolarized light scattering (DLS). The empirical Flory \textit{$\chi $}, determined from scattering intensity profiles for a fully disordered PS-$b$-PMMA copolymer, was shown to behave differently depending on temperature range. \textit{$\chi $} was described mostly by enthalpic contribution at higher temperatures, but a dominant entropic contribution appeared in \textit{$\chi $ }at lower temperatures. The order-disorder transition (ODT) temperatures for the series of copolymers with the controlled molecular weights were directly measured through SAXS and DLS. The resultant ODTs were then compared with a compressible random-phase approximation theory to determine cross interactions between block components. It was found that effective \textit{$\chi $} from theory is also mostly described by enthalpic contribution, which yields a moderate change in ODT upon the increase of copolymer molecular weight. In addition, we discussed the pressure response of the copolymer using \textit{$\chi $} from theory.