High-Pressure Effects on the Order−Disorder Transition in Block Copolymer Melts

We have measured the pressure dependence of the order−disorder transition temperature ( T ODT ) in two styrene−isoprene diblock copolymers containing 24 and 48 wt % styrene ( M w = 20 500 and 16 500 g mol -1, respectively). Small-angle X-ray scattering measurements indicate that T ODT rises linearly with increasing hydrostatic pressure in both systems at a rate of +20 °C kbar -1 (approximately) over the range from 0 to 600 bar; deviations from linearity appear at higher pressures. This increase is approximately twice the value that we have previously estimated from measurements of changes in polymer characteristics with high pressure. 1 We present dilatometric measurements across the ODT which show a volume change (Δ V / V ) of order +5 × 10 -4 upon disordering. The observation of a change in specific volume at the ODT has implications for experimental and theoretical investigations of polymer phase behavior, which we discuss. Finally, we demonstrate the existence of pressure-induced (“barotropic”) phase transitions in these materials and compare their characteristics with those of the analogous temperature-induced (“thermotropic”) transitions.