| Abstract
| - The structural and dynamic properties of astyrene−isoprene−styrene (SIS) triblockcopolymer, with block molecular weights of 104,105, and 104, respectively, and the matchingstyrene−isoprene (SI) diblock, with block molecular weights of 104and 5 × 104, respectively, are examined intheselective solvent di-n-butyl phthalate (DBP). DBP is agood solvent for PS, but a very poor solvent forPI; PI/DBP solutions exhibit a critical temperature near 80 °C.In dilute solution, the triblock copolymerforms micelles with a hydrodynamic radius of 260 Å, which melt at 49°C. In this work, solutions werepolymer concentrations near 20% are emphasized. Small-angle X-rayscattering (SAXS) indicates thatbelow 0 °C, the copolymers form elongated micelles, withisoprene-rich cores. Detailed analysis with anellipsoidal micelle model suggests core minor radii on the order of150−200 Å and overall average lengthsgreater than 1000 Å. As the solutions are heated fromca. 10 °C to ca. 40 °C, the dynamic shearmoduli,G‘ and G‘‘, increase by as much as 2 orders ofmagnitude. This is attributed to the swelling of themicelles,as the solvent begins to penetrate the micelle cores, leading tosubstantial steric interactions betweenmicelles. Upon further heating, the moduli drop sharply, until byabout 60 °C they correspond to valuesfor disordered, entangled solutions. The temperature at which themoduli first decrease signals amicrophase separation transition (MST), as the micelles disintegrate.The locations of the SAXS structurefactor peaks are also consistent with micelle overlap just prior to theMST. Oscillatory flow birefringencemeasurements indicate the same MST, but below the transition thebirefringence is very large, andindependent of frequency. This originates from form birefringence,due to the alignment of highlyanisotropic micelles. Small-angle neutron scattering under shearalso indicates alignment of anisotropicaggregates. In the region between the MST and ca. 60°C, the strong temperature dependences of themoduli, flow birefringence, and scattering intensities are consistentwith loose associations among thechains, reminiscent of the fluctuation regime in diblock copolymermelts.
|
