STRUCTURE AND DYNAMICS OF CONCENTRATED-SOLUTIONS OF ASYMMETRIC BLOCK-COPOLYMERS IN SLIGHTLY SELECTIVE SOLVENTS

The structural and dynamic properties of a styrene-isoprene-styrene (S IS) triblock copolymer, with block molecular weights of 10(4), 10(5), and 10(4), respectively, and the matching styrene-isoprene (SI) dibloc k, with block molecular weights of 10(4) and 5 x 10(4), 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 solut ions exhibit a critical temperature near 80 degrees C. In dilute solut ion, the triblock copolymer forms micelles with a hydrodynamic radius of 260 Angstrom, which melt at 49 degrees C. In this work, solutions w ere polymer concentrations near 20% are emphasized. Small-angle X-ray scattering (SAXS) indicates that below 0 degrees C, the copolymers for m elongated micelles, with isoprene-rich cores. Detailed analysis with an ellipsoidal micelle model suggests core minor radii on the order o f 150-200 Angstrom and over all average lengths greater than 1000 Angs trom. As the solutions are heated from ca. 10 degrees C to ca. 40 degr ees 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 micell es, as the solvent begins to penetrate the micelle cores, leading to s ubstantial steric interactions between micelles. Upon further heating, the moduli drop sharply, until by about 60 degrees C they correspond to values for disordered, entangled solutions. The temperature at whic h the moduli first decrease signals a microphase separation transition (MST), as the micelles disintegrate. The locations of the SAXS struct ure factor peaks are also consistent with micelle overlap just prior t o the MST. Oscillatory flow birefringence measurements indicate the sa me 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 aggrega tes. In the region between the MST and ca. 60 degrees C, the strong te mperature dependences of the moduli, flow birefringence, and scatterin g intensities are consistent with loose associations among the chains, reminiscent of the fluctuation regime in diblock copolymer melts.