Abnormal pressure dependence of the phase boundaries in PEE-PDMS and PEP-PDMS binary homopolymer blends and diblock copolymers

Both polymer blends and diblock copolymers have been investigated with smal l-angle neutron scattering (SANS) in varying temperature and pressure field s. Four samples were studied: a polymer blend of near critical composition and the corresponding symmetric diblock copolymer of poly(ethylethylene) (P EE) and poly(dimethylsiloxane) (PDMS) and two further symmetric diblock cop olymers of poly(ethylenepropylene) (PEP) and PDMS differing in their molecu lar mass. From the SANS results the phase transition temperatures, the Flor y-Huggins interaction parameter, the Ginzburg number, and the sizes of the chain were determined. As the transition temperature and the Flory-Huggins parameter are independently determined from the SANS data, the Clausius-Cla peyron equation offers a cross-check of the theoretical background used for SANS analysis. The resulting parameters showed for all samples qualitative ly similar behavior. In particular, a quite unusual decrease of the phase b oundaries in low increasing pressure regimes was observed. Analysis based o n the Clausius-Clapeyron equation shows that the reason for this pressure-i nduced decrease of the phase boundary is a dominating increase of the entro pic Flory-Huggins parameter. The Ginzburg parameter was found constant with pressure. The size of the diblock copolymer chains changes-with temperatur e and pressure. Beyond the chain stretching observed near the ordering temp eratures (T-ODT) for both decreasing temperature and applied pressure, a di scontinuous decrease of the chain size was consistently found at the TODT T he chain size vs temperature was found to follow a scaling behavior with sl ightly different exponents in the disordered and the ordered regimes consis tent with former simulation calculations.