Effect of Hydrostatic Pressure on Closed-Loop Phase Behavior of Block Copolymers

The effect of hydrostatic pressure ([Formula presented]) on closed-loop phase behavior of deuterated polystyrene-block-poly([Formula presented]-pentyl methacrylate) copolymers [dPS-PnPMA] was investigated by using small-angle neutron scattering and birefringence. For [Formula presented], dPS-PnPMA exhibited a lower disorder-to-order transition temperature ([Formula presented]) at [Formula presented], and then an upper order-to-disorder transition temperature ([Formula presented]) at [Formula presented]. With increasing pressure both [Formula presented] and [Formula presented] were markedly changed, where [Formula presented] was [Formula presented] and [Formula presented] was [Formula presented]. These are consistent with predictions by the Clausius-Clapeyron equation using measured values of the volume and enthalpy changes of both transitions. The large pressure coefficients imply that the closed-loop phase behavior observed for PS-PnPMA is an entropic-driven phase transition.