SOLVENT DISTRIBUTION IN WEAKLY-ORDERED BLOCK-COPOLYMER SOLUTIONS

The distribution of solvent in block copolymer solutions near their or der-disorder transitions is examined experimentally, by small-angle ne utron scattering (SANS), and theoretically, by the self-consistent mea n-field (SCMF) approach. Three lamellar-forming poly(styrene-b-isopren e) diblocks were employed, in toluene, a neutral good solvent, and in cyclohexane, a selective solvent. For a given copolymer concentration, two solutions were prepared, one in protonated and one in perdeuterat ed solvent, and the scattering profiles compared. For a neutral solven t, one expects a small partitioning of the solvent to the interface be tween microdomains, to screen unfavorable styrene-isoprene contacts. S uch partitioning should be manifest as a difference between the h- and d-solvents in the intensities of the second (and higher order, even) harmonic peaks (i.e., I(2q), where the lamellar spacing, L, is 2 pi/q ). This difference is observed experimentally, and is in quantitative agreement with SCMF predictions using Literature values for the three interaction parameters. Interestingly, the predicted relative scatter ing intensity between solutions in h- and d-toluene varies by several orders of magnitude over a small range in copolymer composition or ove r a small range of relative interaction parameters of the solvent for the two blocks, suggesting that SANS could be used in this way as a ve ry sensitive measure of copolymer composition and/or solvent selectivi ty. For a selective solvent, one expects partitioning of the solvent b etween microdomains and a concomitant change in the intensities of the primary (and higher order, odd harmonic) peaks between h- and d-solve nts. This effect is seen clearly in cyclohexane, with the partitioning of the solvent into the isoprene domains increasing with decreasing t emperature. However, the SCMF calculations can only match the behavior in both solvents by employing polyisoprene-cyclohexane and polystyren e-cyclohexane interaction parameters significantly different from thos e reported in the literature.