| Abstract
| - The phase behavior of eight poly(styrene-b-isoprene) diblock copolymers in the neutralsolvent dioctyl phthalate (DOP) has been examined over the full concentration (φ) range and from roomtemperature up to 250 °C. The copolymer compositions (f) ranged from 0.15 to 0.76, and molecular weightswere chosen to make the melt order−disorder transition (ODT) accessible, or nearly so. Definitive phaseassignments were made on the basis of small-angle X-ray scattering (SAXS). Individual order−ordertransitions (OOTs) and ODTs were located by a combination of rheology, static birefringence, and SAXS;all three techniques gave equivalent results. The transition temperatures were converted to values ofthe effective interaction parameter, χ, using an established relation for this system. For all compositions,χODT scales as φ-α, with α varying from 1.3 to 1.6. These results are in conflict with the mean-field dilutionapproximation (α = 1.0), as was previously demonstrated for lamellae-forming samples. This result isattributed to the additional fluctuation stabilization of the disordered state in two-component solutions.Three of the samples displayed OOTs: cylinder to sphere (f = 0.15), gyroid to cylinder (f = 0.31), andlamellae to gyroid (f = 0.33). In each case, χOOT scaled as φ-1.0, in agreement with the dilutionapproximation. This result underscores the reliability of mean-field theory for computing relative freeenergies of the various ordered phases. The domain spacing, d, scales approximately as φ 0.33 and χ0.25,independent of morphology, in good agreement with both mean-field theory and previous experiments.
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