Small-Angle Scattering Study of Mesoscopic Structures in Charged Gel and Their Evolution on Dehydration Masaaki Sugiyama* Department of Physics, Kyushu UniVersity, Fukuoka 812-8581, Japan Masahiko Annaka † Department of Materials Technology, Chiba UniVersity, Chiba 263-8522, Japan Kazuhiro Hara Institute of EnVironmental Systems, Kyushu UniVersity, Fukuoka 812-8581, Japan Martin E. Vigild Danish Polymer Centre, Department of Chemical Engineering, Technical UniVersity of Denmark, 2800 Lyngby, Denmark George D. Wignall Condensed Matter Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393 ReceiVed: December 20, 2002; In Final Form: April 18, 2003 Mesoscopic structures, with length scales ∼10 2 Å, were investigated by small-angle X-ray and neutron scattering (SAXS and SANS) in several N-isopropylacrylamide-sodium acrylate (NIPA-SA) copolymeric hydrogels with varying [NIPA]/[SA] ratios and water contents. The SAXS experiments reveal that, depending upon the [NIPA]/[SA] ratio, the dehydrated NIPA-SA gel shows two mesoscopic structures: one consists of randomly distributed SA-rich islands in NIPA matrix, while the other is a microphase-separated structure, composed of NIPA-rich and SA-rich domains. In addition, the SANS experiments reveal the mesoscopic structural features during the dehydration process. As the concentration of the network polymers increases, NIPA-rich and water- rich domains segregate in the gel. Then, an electrostatic interaction between the segregated domains induces a microphase-separated structure in the limit of the dehydrated NIPA-SA gel. I. Introduction Nanostructures in multicomponent systems have been a longstanding interest for researchers in the fields of both fundamental science and technological applications. Some such systems undergo phase separation with decreasing compatibility among the constituents, and the consequent morphology is determined by a thermodynamic balance of the entropy of mixing and the surface energy. 1 For example, some homopoly- mer solutions phase separate on macroscopic (>10 3 Å) length scales, as a result of a coil-globule transition. 2 With the decrease in compatibility between polymer and solvent in the globule phase, the polymer minimizes its interfacial area with respect to the solvent and forms a coil, despite the fact that the induced macrophase separation decreases the entropy of mixing. When the system is composed of heteropolymers, as in the case of surfactants, a microphase separation occurs so as to increase the entropy of mixing. Surfactant systems exhibit a variety of morphologies in thermodynamic balance from lamellae to multicontinuous microemulsions. 3 A gel is a system composed of a cross-linked polymer network and solvent and is therefore a multicomponent system. When the constituents are highly compatible with each other, the polymer network spreads out into the solvent to maximize the entropy of mixing. Conversely, the network shrinks to minimize the surface energy when the constituents are incom- patible. For example, with increasing temperature, a homopoly- mer hydrogel of N-isopropylacrylamide (NIPA) exhibits a continuous volume reduction at 36 °C as the hydrophilic NIPA group turns hydrophobic. This phenomenon is well-known as the Volume phase transition, 4,5 and the mesoscopic structural change in the NIPA gel in this transition (which results from enhancement of the static and dynamic fluctuations) has been observed via small-angle neutron scattering (SANS). 6 When the polymer network is composed of monomers with different solvent compatibility, the behavior of the gel is more complicated. For example, in hydrogels composed of N- isopropylacrylamide and acrylic acid (NIPA-AAc) segments, the volume changes of the NIPA and AAc groups act in opposite directions as the temperature is increased. The NIPA groups turn from hydrophilic to hydrophobic, and at higher tempera- tures, this generates a contractive force within the polymer network, while the AAc groups induce an expansive force due to a positive osmotic pressure, which comes from the thermal motion of counterions in the Donnan potential. 5 Therefore, compared with the pure NIPA gel, the NIPA-AAc gel has an * To whom correspondence should be addressed. Electronic address: sugi8scp@mbox.nc.kyushu-u.ac.jp. † Present address: Department of Chemistry, Kyushu University, Fukuoka 812-8581, Japan. 6300 J. Phys. Chem. B 2003, 107, 6300-6308 10.1021/jp0277816 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/10/2003