Solvent Selectivity Controls Micro- Versus Macrophase Separation in Multiblock Chains

Monte Carlo simulations in the grand canonical ensemble were used to obtain critical parameters and conditions leading to microphase separation for uncharged block copolymers with solvophilic and solvophobic segments. Solvent selectivity was systematically varied to distinguish between systems that undergo direct macrophase separation and ones that initially microphase separate in the dilute phase. Finite-size scaling was used to obtain the critical parameters. Interestingly, corrections to scaling increase significantly for systems that form aggregates. The threshold value of solvent selectivity for aggregation was determined for symmetric diblock chains of varying length. The results suggest that long diblock copolymers form micelles in the dilute phase prior to macrophase separation, even in marginally selective solvents. The dependence of critical temperature on solvent selectivity was also obtained for triblock, multiblock, and alternating chains. For highly selective solvents, strong structuring in both dilute and dense phases makes it harder to reach equilibrium.