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 1.1 104 and 2.1 104 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 (G1) morphology at low temperature, an OOT to hexagonal-packed cylinders (C1) 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., łOOT -1, but the ODT follows a stronger dependence of łODT -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 (G2), inverted hexagonal-packed cylinders (C2), and inverted body-centered-cubic spheres (S2bcc). Solutions in DEP display simila