SAXS Investigation of the Effect of Temperature on the Multiscale Structure of a Macroporous Poly(<i>N</i>-isopropylacrylamide) Gel

Abstract

The temperature-induced structural modifications of poly(N-isopropylacrylamide) hydrogel (pNIPA) were investigated by small-angle X-ray scattering (SAXS) over a broad range of q values (3.5 × 10−2−12 nm−1) at temperatures ranging between 18 and 37 °C. The sample studied was elaborated by cryopolymerization yielding a macroporous gel (cryogel). The pNIPA gel forms the walls (the thickness at 23 °C is about 12 μm). The SAXS curves display an isoscattering (or isosbestic) point located at qiso = 3.633 nm−1 and disappearing above 30 °C. This feature has never been reported up to now. The SAXS curves obtained at each temperature are well fitted by a sum of four equations describing respectively the scattering resulting from the gel surface (power law), from the solidlike (Guinier equation) and liquidlike (Ornstein−Zernike equation) heterogeneities, and from the chain−chain correlation yielding a broad peak (pseudo-Voigt equation) in the high-q domain. The temperature dependence of the parameters obtained from the fit is analyzed and discussed.