HIGH-PRESSURE EFFECTS ON THE ORDER-DISORDER TRANSITION IN BLOCK-COPOLYMER MELTS

We have measured the pressure dependence of the order-disorder transit ion temperature (T-ODT) in two styrene-isoprene diblock copolymers con taining 24 and 48 wt % styrene (M(w) = 20 500 and 16 500 g mol(-1), re spectively). Small-angle X-ray scattering measurements indicate that T -ODT rises linearly with increasing hydrostatic pressure in both syste ms at a rate of +20 degrees C kbar(-1) (approximately) over the range from 0 to 600 bar; deviations from linearity appear at higher pressure s. This increase is approximately twice the value that we have previou sly estimated from measurements of changes in polymer characteristics with high pressure.(1) We present dilatometric measurements across the ODT which show a volume change (Delta V/V) of order +5 x 10(-4) upon disordering. The observation of a change in specific volume at the ODT has implications for experimental and theoretical investigations of p olymer phase behavior, which we discuss. Finally, we demonstrate the e xistence of pressure-induced (''barotropic'') phase transitions in the se materials and compare their characteristics with those of the analo gous temperature-induced (''thermotropic'') transitions.