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Gel-like aspect of a miscible polymer mixture studied by small-angle neutron scattering



By employing small-angle neutron scattering (SANS), we have investigated a miscible mixture of polystyrene (PS) and poly(vinyl methyl ether) (PVME) to answer the question of what a gel state is. The large difference in the glass-transition temperatures (Tg's) of PS and PVME uniquely enhances dynamic asymmetry by a temperature change. In an intermediate temperature range between the two Tg's, SANS reveals two interesting findings: (1) the anomalous suppression of small-angle scattering in a quiescent state and (2) an abnormal butterfly scattering pattern due to shear-induced phase separation. In this temperature region, we have further confirmed experimentally that the mixture behaves in a gel-like limit at which concentration fluctuations relax more quickly than rheological relaxation. With the gel-like limit and enhanced dynamic asymmetry, which is required for the gel state, our two SANS findings can be interpreted within the theoretical framework of dynamical coupling between stress and diffusion. For comparison, we have studied a swollen real chemical gel of poly(N-isopropyl acrylamide), in which covalent bonds crosslink chains heterogeneously. A combined method of SANS and neutron spin echo has been used to quantitatively determine the static inhomogeneity, which gives rise to a structure factor of a squared Lorentzian shape. The static inhomogeneity is closely related to the nonergodic feature of excess scattering when the gel is subjected to swelling or nondecaying abnormal butterfly scattering under deformation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3148–3164, 2004

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