Solution Properties of a Diblock Copolymer in a Selective Solvent of Marginal Quality. 1. Phase Diagram and Rheological Behavior

A diblock copolymer of styrene and butyl methacrylate with narrow molecular weight distribution [P(S-b-BMA)] containing 1200 monomeric units of S and 1250 of BMA was studied in 2-propanol [2-POH], which is a nonsolvent for the S blocks and a θ solvent (UCST) for the BMA blocks. The phase diagram of the present system is almost indistinguishable from that of 2-POH/PBMA if the molecular weight of the homopolymer is identical with that of the BMA block. The comparison of the intrinsic viscosities [η] of P(S-b-BMA) and PBMA in 2-POH and of their variation with temperature demonstrates that [η]_P(S-b-BMA) reflects the presence of micelles rather than unimers. The rheological behavior of moderately concentrated solutions can be understood in terms of an overlap concentration c* defined as [η]⁻¹P(S-b-BMA)· Below c* the viscosity η changes with temperature as usual, and the liquids are Newtonian up to shear rates γ of several thousand inverse seconds. Above c* the viscosity increases up to 1 order of magnitude as T is raised; shear thinning is observed within the entire γ range under investigation. All experimental findings can be consistently explained in terms of micelles in which the S blocks are effectively hidden in the cores and the PBMA blocks form the outer shell. The inverse temperature dependence of η stems from the expansion of the PBMA layers resulting from the improvement of thermodynamic quality of the solvent with rising distance from the miscibility gap of the system; the higher the temperature becomes, the more the coronae of the micelles penetrate each other and the more their relative movement is hindered.