SHEAR-INDUCED HOMOGENIZATION OF SEMIDILUTE SOLUTION OF POLYMER MIXTURE .2. COMPOSITION DEPENDENCE

Shear-induced homogenization of semidilute solutions of polybutadiene (PB) and polystyrene (PS) with dioctylphthalate as a solvent was studied as a function of temperature T, polymer compositions at a fixed total polymer concentration of 3.3 wt%. The drop of homogenization temperature DELTAT(c)(S) as measured in the increasing and decreasing cycles of shear rate S (s-1) was found to be given by DELTAT(c)(S)/T(c)(0) = (1.3 +/-0.1) x 10(-3) S0.5 +/- 0.02 for all the mixtures with PS/PB compositions 30/70, 40/60, 50/50, 60/40, and 70/30 where DELTAT(c)(S) = T(c)(0) - T(c)(S), T(c)(S) is the homogenization temperature at a given S, and T(c)(0) is the cloud point at S = 0. A remarkable hysteresis effect was discovered on the shear-induced homogenization for off-critical mixtures with 20/80 and 80/20 compositions: the critical shear rates S(c,i) and S(c,d) for the homogenizations observed, respectively, in the increasing and decreasing cycles of S satisfy S(c,i) > S(c,d). The difference deltaS(c) = S(c,i) - S(c,d) is greater at a lower DELTA T(0) = T(c)(0) - T for a given mixture and at more biased compositions for a given DELTAT(0). DeltaS(c) is essentially zero for the 50/50 mixture of PS/PB at all DELTAT(0) covered in this experiment. The hysteresis on S(c) also gives the hysteresis on T(c)(S) or DELTAT(c)(S), viz., DELTAT(c)(S)/T(c)(0) = P x 10(-3) S0.5 +/- 0.1 for the data obtained in the decreasing cycles of S, and DELTAT(c),(S)/T(c)(0) = Q x 10(-4) S1.0 +/- 0.1 for the data obtained in the increasing cycles of S for 20/80 and 80/20 mixtures, where P and Q are the constants of order unity. The hysteresis effect is none or small for the mixtures with compositions between 30/70 and 70/30.