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 (ΔV/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.