Phase Behavior of a Block Copolymer in Solvents of Varying Selectivity

Abstract

The effect of the thermodynamic selectivity of a solvent on the self-assembly of a styrene−isoprene (SI) diblock copolymer with block molecular weights of 1.1 × 10 4 and 2.1 × 10 4 g/mol is investigated. We explore the phase behavior from the melt state to dilute solution in solvents that are of varying selectivities for the two blocks. Bis(2-ethylhexyl) phthalate (DOP) is a neutral good solvent for SI. Di- n -butyl phthalate (DBP) and diethyl phthalate (DEP) are good solvents for PS but marginal and poor, respectively, for PI. Tetradecane (C14) is utilized as a complementary solvent that is good for PI but poor for PS. Small-angle X-ray scattering (SAXS) and static birefringence are used to locate and identify order−order (OOT) and order−disorder transitions (ODT). Dynamic and static light scattering were used to characterize the dilute solution micellar behavior. The neat polymer forms the gyroid (G 1 ) morphology at low temperature, an OOT to hexagonal-packed cylinders (C 1 ) occurs at 185 °C, and the ODT is located at 238 °C. Dilution with DOP decreases the OOT temperature in agreement with the dilution approximation, i.e., χ OOT ∼ φ -1, but the ODT follows a stronger dependence of χ ODT ∼ φ -1.4 . The slightly selective solvent DBP stabilizes the ordered state relative to DOP. Rich lyotropic and thermotropic behavior is observed among regions of lamellae (L), inverted gyroid (G 2 ), inverted hexagonal-packed cylinders (C 2 ), and inverted body-centered-cubic spheres (S 2 bcc ). Solutions in DEP display similar morphological behavior, along with significantly increased ODT temperatures. Because of the asymmetric block copolymer composition, the phase behavior in the isoprene-selective solvent C14 is markedly different, as only G 1, C 1, and S 1 bcc phases are observed. The overall sequence of phases with dilution and/or heating is rationalized on the basis of diagonal trajectories across the phase map (temperature vs composition) for undiluted block copolymers: addition of a neutral solvent corresponds to increasing the temperature and thus a “vertical” trajectory, whereas the partitioning between microdomains of a selective solvent amounts to a “horizontal” trajectory to a renormalized block volume composition. However, a variety of novel features are observed in DEP: the formation of face-centered-cubic packed micelles, a reentrant thermotropic ODT, and a large window of L + C 2 coexistence. The dependence of the principal length scale, d *, on φ, T, and structure is also considered. The strongly temperature-dependent selectivity produces a crossover in the scaling of d * vs φ for the lamellar phase: the addition of a selective solvent increases d *, but as the solvent becomes neutral, d * decreases. This phenomenon is captured by self-consistent mean-field calculations.