Shear-induced homogenization of semidilute solution of polymer mixture. II. 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 ΔTc(S) as measured in the increasing and decreasing cycles of shear rate S (s−1) was found to be given by ΔTc(S)/Tc(0)=(1.3±0.1)×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 ΔTc(S)=Tc(0)−Tc(S), Tc(S) is the homogenization temperature at a given S, and Tc(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 Sc,i and Sc,d for the homogenizations observed, respectively, in the increasing and decreasing cycles of S satisfy Sc,i≥Sc,d. The difference δSc≡Sc,i−Sc,d is greater at a lower ΔT(0)≡Tc(0)−T for a given mixture and at more biased compositions for a given ΔT(0). δSc is essentially zero for the 50/50 mixture of PS/PB at all ΔT(0) covered in this experiment. The hysteresis on Sc also gives the hysteresis on Tc(S) or ΔTc(S), viz., ΔTc(S)/Tc(0)=P×10−3 S0.5±0.1 for the data obtained in the decreasing cycles of S, and ΔTc,(S)/Tc(0)=Q×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.