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

The effect of hydrostatic pressure ($P$) on closed-loop phase behavior of deuterated polystyrene-block-poly($n$-pentyl methacrylate) copolymers [dPS-PnPMA] was investigated by using small-angle neutron scattering and birefringence. For $P<20.7\mathrm{b}\mathrm{a}\mathrm{r}$, dPS-PnPMA exhibited a lower disorder-to-order transition temperature ($T_{\mathrm{L}\mathrm{D}\mathrm{O}\mathrm{T}}$) at $175°\mathrm{C}$, and then an upper order-to-disorder transition temperature ($T_{\mathrm{U}\mathrm{O}\mathrm{D}\mathrm{T}}$) at $255°\mathrm{C}$. With increasing pressure both $T_{\mathrm{L}\mathrm{D}\mathrm{O}\mathrm{T}}$ and $T_{\mathrm{U}\mathrm{O}\mathrm{D}\mathrm{T}}$ were markedly changed, where $d{T}_{\mathrm{L}\mathrm{D}\mathrm{O}\mathrm{T}}/dP$ was $725°\mathrm{C}/\mathrm{k}\mathrm{b}\mathrm{a}\mathrm{r}$ and $d{T}_{\mathrm{U}\mathrm{O}\mathrm{D}\mathrm{T}}/dP$ was $-725°\mathrm{C}/\mathrm{k}\mathrm{b}\mathrm{a}\mathrm{r}$. 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.