J. Sep. Sci. 2004, 27, 874 – 886 www.jss-journal.de i 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Kanamori, Yonezawa, Nakanishi, Hirao, Jinnai Kazuyoshi Kanamori 1 Hideyuki Yonezawa 1, * Kazuki Nakanishi 1 Kazuyuki Hirao 1 Hiroshi Jinnai 2 1 Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyotodaigakukatsura, Nishikyo- ku, Kyoto 615-8510, Japan 2 Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Structural formation of hybrid siloxane-based polymer monolith in confined spaces Structural deformation of phase-separated methylsiloxane gel under the influence of a surface has been studied. Competitive wetting of siloxane gel phase on a surface during phase formation is found to significantly affect the final morphology in a confin- ed space. When the spinodal wavelength is sufficiently shorter than the size of the available space, a uniform bicontinuous structure forms in confined geometry. How- ever, gel skeletons in the vicinity of a surface are elongated with decreasing size of the space, and finally when the size of the space becomes shorter than the spinodal wavelength, all the gel phase wets on a surface, showing a “wetting transition”. Homogeneous bicontinuous methylsiloxane gels were successfully prepared, avoid- ing such structural deformation, in a long cylindrical fused silica capillary and used for capillary HPLC. The capillary gels exhibited excellent separation efficiency of nitro- benzenes and it was found that the surface character can be altered by incorporating surfactants, which will enable more advanced and extended control of surface char- acter, depending on the analytes. Key Words: Siloxane monolith; Sol-gel; Phase separation; Confined geometry; Miniaturized HPLC; Received: April 7, 2004; revised: May 3, 2004; accepted: May 4, 2004 DOI 10.1002/jssc.200401816 1 Introduction Porous silica gels with bicontinuous structure can be pre- pared via the sol-gel technique with accompanying phase separation, especially spinodal decomposition [1]. This kind of silica gel possesses well-defined micrometer- range continuous pores and its porosity reaches typically as high as 80%. After tailoring the micrometer-range structure, mesopores with diameters of 5 – 10 nm can be formed by heat treatment with a basic aqueous solu- tion [2]. This process is known as Ostwald ripening [3] in which convex structures of larger curvature dissolve and reprecipitate onto concave structures with larger curva- ture, transforming the entire silica gel into one with hier- archical double pores. The double pore structure is favor- able on application of the material as separation media for high performance liquid chromatography (HPLC) because the technique requires high macro-porosity to assure a high flow rate and the mesopores which contribute to the separation of a mixture of molecules [4, 5]. Recently, great efforts have been made to apply bicontin- uous silica gel in miniaturized HPLC [6] such as lab-on-a- chip or micro-HPLC because a porous bicontinuous struc- ture can be prepared in a variety of small confined spaces as long as a starting solution can be introduced into them. Also, porous bicontinuous organic-inorganic hybrid gels have been intensively studied to obtain a variety of sur- face characteristics and/or functions [7, 8]. For example, such hybrid materials can be tailored from three-functional methyltrimethoxysilane (MTMS) which can provide a hydrophobic surface [9] because the methyl groups are mainly directed out from the gel surfaces [10]. However, the “flexible” MTMS-derived network tends to deform under the influence of surfaces, which makes it difficult to prepare ideal structures in small confined spaces [11, 12]. Thus, investigations on the surface effects are highly desirable. In polymeric multi-component systems which undergo phase separation, the surfaces induce a wetting effect during phase separation [13 – 16]. Especially in bicontinu- ous spinodal decomposition, more wettable component(s) flows onto the contacting surfaces through bicontinuous pathways in a hydrodynamic manner [15]. This hydrody- namic flow leads to a significant deformation of siloxane Correspondence: Kazuki Nakanishi, Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyotodaigakukatsura, Nishikyo-ku, Kyoto 615-8510, Japan. Phone: +81 75 383 2411. Fax: +81 75 383 2412. E-mail: kazuki@bisco1.kuic.kyoto-u.ac.jp. Abbreviations: LSCM, Laser Scanning Confocal Microscopy; MTMS, Methyltrimethoxysilane; FA, Formamide; MeOH, Metha- nol; PB, Polybutadiene; dPB, Deuterated polybutadiene; MF sys- tem, the system containing MTMS and FA and 1 M nitric acid; MM system, the system containing MTMS and MeOH and 1 M nitric acid. 111111111 *Present address: Nitto Denko Corporation, Shimohozumi 1-1-2, Ibaraki-shi, Osaka 567-0041, Japan. 874