Influence of carbon dioxide swelling on the closed-loop phase behavior of block copolymers

The influence of carbon dioxide sorption on the closed-loop phase diagram of polystyrene-block-poly(n-pentyl methacrylate) copolymers, PS-b-PnPMA, was investigated. Differential dilation of the constituent blocks was used to induce microphase separation of the PS-b-PnPMA. With increasing carbon dioxide loading an appreciable expansion in the size of the closed loop was found. At a carbon dioxide activity of 0.1 the upper order-to-disorder transition temperature (UODT) was elevated by more than 20°C, whereas the lower disorder-to-order transition temperature (LDOT) was depressed by the same magnitude. This arises from the entropic nature of the closed-loop miscibility gap in this weakly interacting system, where microphase separation is driven by disparity in free volumes between dissimilar segments of the chain.