SANS study of hydrophobic effects on pressure-induced micro- and macrophase separations of block copolymers

Hydrophobic effects on pressure-induced microphase separation of block copolymers were investigated by using small-angle neutron scattering (SANS). A block copolymer, pEOEOVE-b-pMOVE aqueous solution, which shows LCSTs (at 40 and 60 deg. C, for pEOEOVE and pMOVE homopolymers, respectively) was used. A phase diagram was obtained by tracing light-transmittance at 632.8nm by increasing temperature (T) at various pressures (P), which was a convex-upward function. The change in the slope of the phase boundary curve could be explained by volume change associated with mixing as well as 'iceberg' formation. SANS revealed that a microphase separation took place by pressurizing or increasing temperature. At 28 deg. C, the correlation length was observed to show continuous increase with divergence at 350MPa. This result showed that the block copolymers at 28 deg. C underwent macrophase separation and the transition was a second-order transition. At 45 deg. C, under atmospheric pressure, the SANS curve suggested formation of a BCC lattice structure with micelles composed of a central core of pEOEOVE. By pressurizing, the SANS peak related to the microphase separation disappeared, meaning dissolution of block copolymers. However, by further pressurizing, another peak re-appeared at 250MPa. This peak indicates the microphase separation of block copolymers is different from those observed under low pressures