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 mole cular 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 no nsolvent for the S blocks and a theta solvent (UCST) for the BMA block s. 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 [eta] of P(S-b-BMA) and PBMA in 2-POH and of their variat ion with temperature demonstrates that [eta](P(S-b-BMA)) reflects the presence of micelles rather than unimers. The theological behavior of moderately concentrated solutions can be understood in terms of an ove rlap concentration c() defined as [eta](P(S-b-BMA))(-1). Below c(*) t he viscosity eta changes with temperature as usual, and the liquids ar e:Newtonian up to rates gamma of several thousand inverse seconds. Abo ve c() the viscosity increases up to 1 order of magnitude as T is rai sed; shear thinning is observed within the entire gamma range under in vestigation. All experimental findings can be consistently explained i n terms of micelles in which the S blocks are effectively hidden in th e cores and the PBMA blocks form the outer shell. The inverse temperat ure dependence of eta stems from the expansion of the PBMA layers resu lting from the improvement of thermodynamic quality of the solvent wit h rising distance from the miscibility gap of the system; the higher t he temperature becomes, the more the coronae of the micelles penetrate each other and the more their relative movement is hindered.