Concentration fluctuations in polymer gel investigated by neutron scattering: Static inhomogeneity in swollen gel

By using small-angle neutron scattering (SANS) and neutron spin echo (NSE), we have quantitatively investigated the static inhomogeneity in poly (N-isopropyl acrylamide) gel (PNIPA) in microscopic length scales of $0.015<q<0.16 A−1,$ where q is a wave number of scattered neutrons. NSE revealed that at lower $q(≅0.015 A−1),$ the concentration fluctuations in the PNIPA gel decays more slowly as compared to the PNIPA solution without crosslinks. According to our scenario that the slower decay found for the PNIPA gel is due to the static inhomogeneity coexisting in the swollen gel, small-angle scattering $S(q)$ obtained by SANS has been quantitatively decomposed into thermal and static scattering components, respectively, $Sth(q)$ and $Sst(q).$ It was further revealed that (i) the q-region where $Sst(q)$ becomes dominant is closely related to that for the abnormal butterfly scattering under stretching, and (ii) as the temperature increases toward the temperature for volume phase transition, $Sst(q)$ of a squared Lorentzian shape increases more drastically than $Sth(q)$ of a Lorentzian shape. These findings were quantitatively understood in the theoretical framework by Panyukov and Rabin [Macromolecules 29, 7960 (1996)] or by Onuki [J. Phys. II. France 2, 45 (1992)], taking into account stress-fluctuation coupling under coexistence of the inherent structural heterogeneity in the real gel. We further found that the static inhomogeneity showing $Sst(q)$ seems to relate to the necklacelike microstructure, appearing after a shallow quench into the collapsed phase.