Yasuhiko Ohta

Effect of strain amplitude on viscoelastic properties of concentrated solutions of styrene-butadiene radial block copolymers

The dynamic viscoelastic properties of 30 wt% solutions of styrene (S)‐butadiene (B) radial (star) block copolymers were measured to investigate the effect of deformation history. The block structure for SBH1 examined was (S‐B‐)m and that for BSH1 (B‐S‐)n, where m and n are about 5. The solvents used were chlorinated diphenyl (KC4) and dibutylphthalate (DBP); the former is a good solvent for both polystyrene (PS) block and polybutadiene (PB) block, and the latter is a good solvent for PS block and a poor solvent for PB block. Both storage modulus G′ and loss modulus G″ vary little with maximum strain amplitude (γ0) for the solutions in KC4, or for those in DBP at 80°C. On the other hand, G′, and G″ depend significantly on γ0 for solutions in DBP at temperatures lower than 60°C, implying that the former systems are homogeneous solutions and the latter contain microdomain structure. G′ and G″ of DBP solutions of SBH1 are independent of γ0 for γ0⩽0.03,G′ decreases and G″ increases in the range 0.04

Rheology and Phase Transition in 30% Solutions of Styrene‐Butadiene Radial Block Copolymers

The viscoelastic properties of 30 wt% solutions of styrene(S)‐butadiene(B) radial (star) block copolymers in dibutyl phthalate (DBP) have been measured to investigate the effect of temperature. Block structure used was (S—B—)m and (B—S—)n, where m and n are about 5. The solvent, dibutyl phthalate (DBP), is a good solvent for the PS block and a poor solvent for the PB block. The temperature dependences of the storage modulus G′ and loss modulus G″ for the solutions exhibit a rheological transition at Tc=66∼68°C. On raising the temperature, G′ decreases remarkably at the temperatures and correspondingly G″ jumps up almost discontinuously. On lowering the temperature, G′ and G″ show the apparent hysteresis behavior caused by slow kinetics of phase separation and values of G′ and G″ return to the original levels. When the temperature is lower than the transition temperature Tc, the measured G′ is higher than G″, and when T>Tc, G″>G′. These results suggest that the rheological transition at Tc is associated wi...