Effect of dilution on a block copolymer in the complex phase window

The phase behavior of a styrene-isoprene (SI) diblock copolymer, with block molecular weights of 1.1 x 10(4) and 2.1 x 104 g/mol, respectively, is exa mined in the neutral solvent bis(2-ethylhexyl) phthalate (DOP) smd the styr ene-selective solvent di-n-butyl phthalate (DBP). DBP is a good solvent for PS, but is near a theta solvent for PI at approximately 90 degrees C. Smal l-angle X-ray scattering (SAXS), rheology, and static birefringence are use d to locate and identify order-order (OOT) and order-disorder transitions ( ODT); all three techniques gave consistent results. The neat polymer adopts the gyroid (G) phase at low temperatures, with an OOT to hexagonally-packe d cylinders (C) at 185 degrees C, and the ODT at 238 degrees C. Upon diluti on with the neutral solvent DOP, the C window is diminished, until for a po lymer concentration phi = 0.65, a direct G to disorder (D) ODT is observed. These results reflect increased stability of the disordered state, based o n the different concentration scalings of the interaction parameter, chi, a t the OOT and ODT. The OOT follows the dilution approximation, i.e., chi(OO T) similar to phi(-1), but the ODT is found to follow a stronger concentrat ion dependence, i.e., chi(ODT) similar to phi(-1.4), Similar to the scaling of phi(-1.6) found previously for lamellar SI diblocks in toluene and DOP. Addition of the selective solvent DBP produces dramatic changes in the pha se behavior relative to DOP and the melt state; these include transitions t o lamellar (L) and perforated layer (PL) structures. The observed phase seq uences can be understood in terms of trajectories across the SI melt phase map (temperature vs. composition): addition of a neutral solvent or increas ing temperature corresponds to a "vertical" trajectory, whereas adding a se lective solvent amounts to a "horizontal" trajectory. When the solvent sele ctivity depends on temperature, as it does for the SI/DBP system, increasin g temperature results in a diagonal trajectory. For both neutral and select ive solvents the domain spacing, d*, scales with phi and chi as anticipated by self-consistent mean-field theory. (C) 1998 John Wiley & Sons, Inc.