Scaling of interdomain spacing of diblock copolymers in a selective diluent

Interdomain spacing, d, of microphase-separated poly(styrene-b- dimethylsiloxane) in high-pressure carbon dioxide (CO₂) was found to follow a modified power law scaling, d/ξ ∼ φ_diblock ^-γ, where ξ is a swelling asymmetry factor and φ_diblock; is the volume fraction polymer. This interdomain spacing was measured in situ both isothermally and at constant φ_diblock using small-angle neutron scattering. The use of high-pressure CO₂ as the diluent enabled precise control of sorbed solvent volume fraction in highly concentrated systems at elevated temperatures. Data measured at constant φ_diblock are in agreement with theoretical predictions and experimental data for dilution by traditional liquid solvents and follow the scaling law d ∼ χ_AB ^-β. In the highly concentrated regime, the isothermal dependence on φ_diblock cannot be described by the simple power law scaling previously suggested for liquid diluents, d ∼ φ_polymer ^-γ. By accounting for solvent selectivity using ξ, however, d-spacing as a function of φ_diblock for a system exhibiting lamellar morphology collapses to a common scaling.