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

Abstract

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) (dPS-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\u202fMPa. 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\u202fK/100\u202fMPa 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 χH and entropic χS coming from Δ e j j mediated by the copolymer bulk modulus. Pressure strengthens χH due to densification, whereas the augmented bulk modulus reduces χS. The diminution of χS is shown to be the source of baroplasticity.