Phase behavior of a block copolymer in solvents of varying selectivity

The effect of the thermodynamic selectivity of a solvent on the self-assemb ly of a styrene-isoprene (SI) diblock copolymer with block molecular weight s of 1.1 x 10(4) and 2.1 x 10(4) g/mol is investigated. We explore the phas e;behavior from the melt state to dilute:solution in solvents that are of v arying selectivities for the two blocks. Bis(2-ethylhexyl) phthalate (DOP) is a neutral good solvent for SI. Di-n-butyl phthalate (DBP) and diethyl ph thalate (DEP) are good solvents for PS but marginal and poor, respectively, for PI. Tetradecane (C14) is utilized as a complementary solvent that is g ood 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-d isorder 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-packe d cylinders (C-1) occurs at 185 OC, and the ODT is located at 238 degrees C . Dilution with DOP decreases the OOT temperature in agreement with the dil ution approximation, i.e., chi(OOT) similar to phi(-1), but the ODT follows a stronger dependence of chi(ODT) similar to phi(-1.4) The slightly select ive solvent DBP stabilizes the ordered state relative to DOP. Rich lyotropi c and thermotropic behavior is observed among regions of lamellae (L), inve rted gyroid (G(2)), inverted hexagonal-packed cylinders (C-2), and inverted body-centered-cubic spheres (S-2(bcc)). Solutions in DEP display similar m orphological 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 trajectori es across the phase map (temperature vs composition) for undiluted block co polymers: addition of a neutral solvent corresponds to increasing the tempe rature and thus a "vertical" trajectory, whereas the partitioning between m icrodomains of a selective solvent amounts to a "horizontal" trajectory to a renormalized block volume composition; However, a variety of novel featur es are observed in DEP: the formation of face-centered-cubic packed micelle s, a reentrant thermotropic ODT, and a large window of L + Ct coexistence. The dependence of the principal length scale, d*, on phi, T, and structure is also considered. The strongly temperature-dependent selectivity produces a crossover in the scaling of d* vs phi for the lamellar phase: the additi on of a selective solvent increases d*, but as the solvent becomes neutral, d* decreases. This phenomenon is captured by self-consistent mean-field ca lculations.