Structure and dynamics of concentrated solutions of asymmetric block copolymers in slightly selective solvents

The structural and dynamic properties of a styrene-isoprene-styrene (SIS) triblock copolymer, with block molecular weights of 10⁴, 10⁵, and 10⁴, respectively, and the matching styrene-isoprene (SI) diblock, with block molecular weights of 10⁴ and 5 × 10⁴, respectively, are examined in the selective solvent di-n-butyl phthalate (DBP). DBP is a good solvent for PS, but a very poor solvent for PI; PI/DBP solutions exhibit a critical temperature near 80°C. In dilute solution, the triblock copolymer forms micelles with a hydrodynamic radius of 260 Å, which melt at 49°C. In this work, solutions were polymer concentrations near 20% are emphasized. Small-angle X-ray scattering (SAXS) indicates that below 0°C, the copolymers form elongated micelles, with isoprene-rich cores. Detailed analysis with an ellipsoidal micelle model suggests core minor radii on the order of 150-200 Å and overall average lengths greater than 1000 Å. As the solutions are heated from ca. 10°C to ca. 40°C, the dynamic shear moduli, G′ and G″, increase by as much as 2 orders of magnitude. This is attributed to the swelling of the micelles, as the solvent begins to penetrate the micelle cores, leading to substantial steric interactions between micelles. Upon further heating, the moduli drop sharply, until by about 60°C they correspond to values for disordered, entangled solutions. The temperature at which the moduli first decrease signals a microphase separation transition (MST), as the micelles disintegrate. The locations of the SAXS structure factor peaks are also consistent with micelle overlap just prior to the MST. Oscillatory flow birefringence measurements indicate the same MST, but below the transition the birefringence is very large, and independent of frequency. This originates from form birefringence, due to the alignment of highly anisotropic micelles. Small-angle neutron scattering under shear also indicates alignment of anisotropic aggregates. In the region between the MST and ca. 60°C, the strong temperature dependences of the moduli, flow birefringence, and scattering intensities are consistent with loose associations among the chains, reminiscent of the fluctuation regime in diblock copolymer melts.