High-pressure neutron scattering and random-phase approximation analysis of a molten Baroplastic diblock copolymer

Abstract Through high-pressure, small-angle neutron scattering (SANS) and a random-phase approximation (RPA) theory based on a molecular model, we investigate the origin of the pressure responsiveness of a phase-segregating deuterated polystyrene-block-poly (2-ethyl hexyl acrylate) ( d PS- b -PEHA) copolymer melt for use as a material for low-energy processing. The SANS intensities for the copolymer were measured over a temperature range at applied pressures between 0.1 and ∼83 MPa. The copolymer exhibited an order-disorder transition (ODT) and revealed baroplasticity, where its ODT was suppressed upon pressurization as Δ T O D T / Δ P ≈ -15.8 K/100 MPa to ease subsequent processing. Theory predicts that a disparity in the self interactions e j j ’s ( Δ e j j = e P S − e P E H A ) or a disparity in the compressibilities between blocks precisely yields the observed Δ T O D T / Δ P . The effective Flory-Huggins χ is argued to consist of the conventional enthalpic χ H and entropic χ S coming from Δ e j j mediated by the copolymer bulk modulus. Pressure strengthens χ H due to densification, whereas the augmented bulk modulus reduces χ S . The diminution of χ S is shown to be the source of baroplasticity.