Pressure effects on entropically driven phase transitions in block copolymers

The binary polymer system of polystyrene and poly(n-pentyl methacrylate) was recently found to exhibit closed-loop type phase behavior. This is the first known example of a weakly interacting system exhibiting such a phase diagram. At atmospheric pressure the block copolymer displays both a lower disorder-to-order and upper-order-to-disorder transition, representing the lower and upper bounds of the closed-loop phase diagram. The application of hydrostatic pressure served to shrink the closed-loop, yielding pressure coefficients of the lower disorder-to-order and upper order-to-disorder transitions of 725°C/kbar and -725°C/kbar respectively. These pressure coefficients were consistent with those calculated from the Clausius-Clapeyron equation, using the experimentally determined DeltaHdisorder and DeltaV disorder for each transition. The chieff determined from small angle neutron scattering (SANS) was found to decrease, pass through a minimum, increase to a maximum, and then decrease with increasing temperature. Swelling the system with carbon dioxide served to promote an expansion of the closed-loop. This was due to the entropic nature of both transitions, with differential dilation of the copolymer domains resulting in dissimilar compressibilities of the blocks. In addition to influencing block copolymer phase behavior, carbon dioxide can have a profound impact on resulting morphological structure. A 42/58 PS-b-PnPMA diblock copolymer was found to exhibit lamellar morphology at ambient pressures. With the application of 2500 psi carbon dioxide the morphology shifted to hexagonally-packed cylinders due to preferential absorption into the PnPMA block. Furthermore, the influence of carbon dioxide sorption on the morphology of the PS/poly(n-alkyl methacrylate) block copolymer series was studied both in thin films and in the bulk.