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The pressure-and temperature-induced micro-and macrophase separations of hydrophobically solvated block copolymer aqueous solutions, consisting of poly[2-(2-ethoxy)ethoxyethyl vinyl ether]-block-poly-(2-methoxyethyl vinyl ether) (pEOEOVE-b-pMOVE), have been investigated with dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The following facts were disclosed. (1) For an increase in temperature under atmospheric pressure, a series of scattering peaks appeared in SANS intensity curves at about 40 °C, indicating microphase separation with pEOEOVE domains in a bcc packing in the matrix of pMOVE and water. This was followed by a precipitation of the polymer at about 65 °C. (2) A pressure-temperature (P-T) phase diagram for microphase separation was determined by DLS, which was a convex-upward function of P with a symmetric axis of P 0 ≈ 150 MPa. (3) At ambient temperature, an increase of the correlation length was observed by increasing P. This was confirmed to be due to macrophase separation by pressurizing. (4) On the other hand, the solution underwent a reentrant microphase dissolution-separation transition at 45 °C with increasing P. The SANS peaks disappeared and the intensity decreased at 100 MPa. Then, by further increasing P, the SANS intensity increased accompanying a scattering maximum. In view of the symmetry of the phase diagram, it is conjectured that this reentrant microphase separation at high pressure is different from one at low pressure. These findings lead to a conclusion that the selectivity of hydrophobic solvation is suppressed at high pressures.

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