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Abstract
Static and dynamic light scattering measurements have provided good information concerning the temperature-dependent associative behavior of a triblock copolymer polystyrene-b-poly(ethylene/butylene)-b-polystyrene dissolved in solvent mixtures of n-octane and 4-methyl-2-pentanone. Both are selective solvents, and when the solvent composition is varied, the solvent mixture shifts from a selective solvent of the copolymer end block to a selective solvent of the middle block. For solvent mixtures with a high 4-methyl-2-pentanone percentage (selective solvent for polystyrene blocks), two kinds of particles have been detected in solution. The smallest ones had a hydrodynamic radius close to 14 nm and were considered as free copolymer chains because they were only detected at temperatures higher than the critical micelle temperature (CMT). Below this temperature, a second kind of particle was detected with a hydrodynamic radius close to 37 nm, and those particles were considered as ordinary micelles with a core formed by poly(ethylene/butylene) blocks. For solvent mixtures with a high n-octane percentage (selective solvent for poly(ethylene/butylene) blocks) three kind of particles were detected. The smallest ones had a hydrodynamic radius close to 14 nm and corresponded to free copolymer chains. These particles were only detected at temperatures higher than CMTs. Below the critical micelle temperatures a second kind of particle of Rh ≈ 47 nm was detected, and those particles were considered flowerlike micelles with a core formed by polystyrene blocks. A third kind was detected at any temperature. The hydrodynamic radii of these particles were around 200 nm. Those were considered to be aggregates with a loose structure different from that of micelles. n-Octane/4-methyl-2-pentanone mixtures with similar solvent content behaved as a nonselective solvent, and no micelles were detected at 25 °C. Finally, the self-association process is enthalpy-driven, yielding closed association structures in the form of flowerlike micelles in solvent mixtures with a high n-octane content and ordinary micelles in solvent mixtures with a high 4-methyl-2-pentanone content.
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