Pressure-Induced Phase Transitions of Hydrophobically Solvated Block-Copolymer Solutions

The structures of poly(2-(2-ethoxy)ethoxyethyl vinyl ether)-block-poly(2-methoxyethyl vinyl ether) in ${\mathrm{D}}_2\mathrm{O}$ have been investigated with small-angle neutron scattering (SANS) as a function of temperature $T$ and pressure $P$. At ambient pressure, the solution underwent a two-step transition at 40 and 65 °C, both of which were convex-upward functions of $P$ having a maximum around $P_0\approx 150\mathrm{MPa}$. The first transition was assigned to a microphase separation to form a bcc structure, and the second was to a macrophase separation. Pressurizing at 28 °C resulted in a macrophase separation with divergence at 350 MPa. At 45 °C, a reentrant microphase separation was observed by increasing $P$. Differences in the states of hydrophobic solvation in the low ($P<P_0$) and high pressure regions ($P>P_0$) are discussed based on the SANS structure factors.