| The effect of the thermodynamic selectivity of a solvent on the self-assembly behavior of styrene-isoprene (SI) diblock copolymers 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-ethyl hexyl phthalate) (DOP) is a neutral good solvent for SI, while di-n-butyl phthalate (DBP), diethyl phthalate (DEP), and dimethyl phthalate (DMP), are increasingly PS-selective; n-tetradecane (C14) is utilized as a complementary PI-selective solvent. The model SI diblock copolymers were synthesized via anionic polymerization and are of the appropriate molecular weights and compositions to access all initial melt-state morphologies with differing degrees of segregation. Small-angle X-ray and neutron scattering, rheology, and static birefringence are used to locate and identify order-order transitions (OOT) and order-disorder transitions (ODT). Dynamic and static light scattering were used to characterize the dilute solution micellar behavior.; Dilution with DOP lowers the OOT and ODT temperatures due to the decrease in segregation between the S and I-blocks. For each polymer examined, the experimental results indicate the scaling of the interaction parameter at the ODT follows a stronger scaling than that of the predicted “dilution approximation”. In contrast, three different morphological OOTs followed exactly the dilution approximation. These results confirm an increased stability of the disordered state that has previously been ascribed to non-universal, non-random mixing.; The effect of increasing the solvent selectivity is to enlarge the region of ordered states. Rich lyotropic and thermotropic behavior is observed among regions of lamellae, and normal and inverted versions of the bicontinuous gyroid, hexagonal-packed cylinder, body-centered and face-centered cubic-packed sphere phases. The overall sequence of phases with dilution and/or heating is rationalized on the basis of diagonal trajectories across the phase map (temperature versus composition) for undiluted block copolymers. However, a variety of novel features emerge in the more selective solvents DEP and DMP: the formation of FCC-packed micelles, re-entrant thermotropic ODTs, and substantial windows of lamellar/hexagonal-packed cylinder coexistence. The temperature dependence of the principal length scale, d*, on &phis;, T and structure is also considered via experiment and theory. |
