Solvent quality, phase coexistence, and dynamics in ultrahigh molecular weight diblock copolymer solutions

The static S(q) and dynamic S(q,t) structure factor in disordered and ordered solutions of ultrahigh molecular mass (3.6 × 10⁶ g/mol) symmetric poly(styrene)-block-poly(isoprene) (SI) diblock copolymer were measured by photon correlation spectroscopy for wave vectors q on both side of the maximum S(q*) as a function of concentration and temperature in two solvents. At ambient temperature, the SI concentration CODT at the disordered-to-ordered transition decreases with increasing solvent selectivity from toluene (4.2 wt %) to decaline (2.7 wt %). In the disordered region, deviation from solvent impartiality enhances and slows down the short (q > q*) length order parameter fluctuations and hence leads to deviation of S(q > q*) from its theoretical mean-field form. From ultrasmall X-ray and light scattering, the S(q) over a broad q range shows hexagonally arranged cylinders for the ordered solutions with decaline, suggesting a biased solvent partition in the two microphases. At C_ODT, a two-phase regime which is visually observed in both solvents due to light diffraction is comprised of two separated regions with different S(q) and S(q,t) only at the vicinity of q*. For a rigorous interpretation of these effects, the solvent selectivity along with the renormalization of the neat block copolymer composition should be considered in the framework of the blob theory.