The phase behavior of polystyrene−polyisoprene diblock copolymers diluted with solvents strongly selective for polyisoprene was investigated using variable-temperature small-angle X-ray scattering. Tetradecane was employed as a model selective solvent; its addition induced lyotropic order−order transitions, with as many as five ordered phases found in solutions of one diblock as tetradecane content was varied. The mesophases observed for tetradecane solutions of 10 different diblocks can be partially collapsed into a single two-dimensional map of segregation strength vs overall polystyrene content, as if the selective solvent simply alters the volume fraction of the soluble block. Among strongly selective solvents (tetradecane, tributylamine, squalane), measurable and even large differences in diblock−solvent phase behavior are revealed, in both the locations of lyotropic order−order transitions and the thermotropic order−disorder transition. The addition of squalane to polystyrene-rich diblocks actually elevates the order−disorder transition temperature relative to the diblock melt.

The dependence of block copolymer interdomain spacing d on polymer volume fraction φP in solution, d ∼ P , is examined through a series of styrene−isoprene diblock copolymers in four solvents, three of them strongly selective (nonsolvents for polystyrene). The exponent β is found to depend on solvent selectivity (both temperature and chemical identity of solvent; β increases with selectivity), on mesophase (β is largest when the insoluble blocks form lamellae, then cylinders, then spheres), and on diblock composition within a given mesophase (β is largest when the soluble blocks are most highly diluted). Values of β as large as unity are observed, indicating that selective solvents can swell d quite substantially over the value for the bulk diblock. In one example, a 2.5-fold increase in d is obtained when a cylinder-forming polystyrene-rich diblock is diluted into its mirror-image polystyrene-poor cylindrical mesophase.

We use steady-shear rheology to study the shear-disordering phenomenon in block copolymers in the body-centered-cubic (bcc) mesophase. Polystyrene-polyisoprene and polystyrene-poly-(ethylene-alt-propylene) diblock, triblock, and starblock systems of varying molecular weight and composition were studied in the melt and as concentrated solutions in a selective solvent. The bcc lattice exhibits a low-frequency modulus G bcc 0 which scales with the cube of the lattice spacing and additionally depends only on the proximity to the order-disorder transition (ODT). Steady shear destroys the bcc lattice at a critical shear stress, which is found to be directly proportional to the lattice modulus: τc ) 0.038G bcc 0 for all block copolymer melts and highly concentrated solutions investigated. The critical stress is thus uniquely determined by the lattice spacing and the proximity to the ODT. The rheological behavior conforms in a remarkably quantitative manner to the shear-melting processes observed in other soft crystalline systems, such as colloidal crystals.

At present, diblock copolymer phase behavior in the long-chain limit is considered to be governed by three factors: the volume fraction of one block, φ; the segregation strength of the diblock, χN; and the conformational asymmetry parameter, . This implies that the phase diagrams for polymers of different chemistry but similar molecular weight should evolve simply and smoothly as is varied. We present here partial phase diagrams (covering the cylinder-gyroid-lamella region) for two derivatives of the well-known styrene-isoprene diblocks: styrene-(ethylene-alt-propylene) and vinylcyclohexane-(ethylene-alt-propylene), all of similar molecular weights. Comparison of the S/I, VCH/EP, and S/EP phase diagrams reveals that the phase behavior does not vary simply with . In particular, the shape and extent of the gyroid region differ greatly between the three chemistries; in the S/EP system, this region disappears into a cusp at moderate segregation strengths (χN ≈ 24). Our findings indicate that factors other than φ, χN, and do influence the free energy of diblock copolymer mesophases and their regions of stability.

The steady-shear rheology of polystyrene-polyisoprene (S/I) block copolymer melts having a body-centered-cubic (bcc) morphology has been characterized as a function of applied shear stress. At temperatures below the order-disorder transition (ODT) and at low stresses (τ < 100 Pa), both diblocks and triblocks manifest finite but extremely large (η 0 ) 10 7 -10 8 Pa‚s) zero-shear viscosities. Small-angle X-ray scattering (SAXS) measurements indicate that at these low stresses the bcc lattice remains intact. This Newtonian region ends abruptly when a critical shear stress is reached (τc ≈ 200 Pa), the steady shear viscosity decreasing by almost 4 orders of magnitude with only a factor of 3 further increase in stress. The viscosity then enters a weakly shear-thinning regime very similar to the behavior of the same block copolymer above its ODT temperature. As shown by SAXS, there is no lattice order in this highstress regime, indicating that "shear-disordering" has occurred. If the disordered polymer is held fixed or subjected to a low stress (τ < 100 Pa), it gradually regains its original equilibrium state, following the same kinetics as after a thermal quench from above to below the ODT temperature.