Chem.Maer.200517.931-934 931 Spontaneous Formation of Characteristic Layered properties(high elongation and strength),high transparency (structural homogeneity),high swelling ratio,and excellen a(e Kazutoshi Haraguchiand Kaori Matsuda NCs with high contents of exfoliated clay were readily obtained by drying the NC gels.56 Here,we report the Material Chemistry Lab ration of poro apa characteristic lavered morphologieswith controlled poros ties. NC gels,as monoliths,were synthesized and analyzed by decades,polyme -clay nanocomposite the same pro reporte PIC-NC isopropyl acrylamide,10 g), catalyst (N.N.N N'-tetra ator epoxy resin.It was rep w rema rati )thermo-mechanical heat distortion 0.33.water content 88.3 wt %The solution wa ntrodu ced to a glass v rogen atmosphere an by the kept a amoun b s we to 0.5.or more.by changing the clay concentration under alkylammonium surfac tant In most cas es.P/C-NCs include the fixed monomer content in the initial reaction mixtur 1-5 wt %and less than 10 wt%.of clay. P/C NCs with -NO xhbaed high clongations near tommand or greater than 00% prepa ted that a r ss).fere in liau nit 0C)or in a fre (-20C).and freeze-dried for 24 h using a freeze-dryer in and which the press ure was always ept belc pressure (61 the orpor gels (NC gels)with unique organic (poly ani obtained without large volume shrinkages b the (clay)network structures+These NC gels were obtained b water in the freeze-drying process.In contrast,when NCgels e-radical polymerizatio dried con ntio ly,e.g.,by heating or e media In the clay sh the are linked by large numbers of long flexible polymer chains prepared from NC gels were named porous G-nanocor Each clay sheet acts as a sup multifunctional cross-inking G-NCs).whic polyme poro Th by using seanning electron mic scony (SEM)after coa freeze-dried samples with Pt to a thickness of 5 nm contribut Becaus struc For all NC gels prepared here,the porous G-NC disk logies as s should be addressed.E-mail:haraakicr.or.ip nhal (A) ting of fine-norous dense and coarse es. 2a porous layers,(B)a two-layer morphology consisting of 93.8.1185 lense and coa ers,and (C)a un 32493249%88 00 (3)Ok M.;Nam. (5)1 uchTakehisa,T Fan,S.Macromolecules 0035. IK- .Ade.Mater.2002.14.1120-1124 10.1021/cm Spontaneous Formation of Characteristic Layered Morphologies in Porous Nanocomposites Prepared from Nanocomposite Hydrogels Kazutoshi Haraguchi* and Kaori Matsuda Material Chemistry Laboratory, Kawamura Institute of Chemical Research, 631 Sakado, Sakura-shi, Chiba 285-0078, Japan ReceiVed October 31, 2004 ReVised Manuscript ReceiVed December 19, 2004 In the last two decades, polymer-clay nanocomposites (P/C-NCs) have been extensively investigated as advanced composite materials.1 Conventional P/C-NCs consist of exfoliated clay sheets and thermoplastic or thermosetting polymers, such as nylon 6, polypropylene, polyurethane, or epoxy resin. It was reported that P/C-NCs show remarkable improvements compared with virgin polymers in mechanical (e.g., modulus), thermo-mechanical (e.g., heat distortion temperature), surface (e.g., gas-barrier), and thermal (e.g., nonflammability) properties by the inclusion of small amounts of clay sheets.2 These P/C-NCs were generally prepared by using organophilic clay pre-modified by an alkylammonium surfactant. In most cases, P/C-NCs include 1-5 wt %, and less than 10 wt %, of clay. P/C-NCs with higher clay contents were not satisfactory in use because of difficulties in both preparing uniform P/C-NCs and their molding. Also, it was reported that a porous (polypropylene/ organophilic clay) P/C-NC could be prepared using super￾critical CO2 as porogen,3 and that the average pore size was reduced from 153 to 93 µm by incorporating 4 wt % clay. We recently developed a novel series of nanocomposite hydrogels (NC gels) with unique organic (polymer)/inorganic (clay) network structures.4 These NC gels were obtained by in situ free-radical polymerization of N-substituted acryl￾amide derivatives in the presence of exfoliated inorganic clay in aqueous media. In the network, neighboring clay sheets are linked by large numbers of long, flexible polymer chains. Each clay sheet acts as a super-multifunctional cross-linking agent. Here, the most probable interaction between polymer and clay, which is not covalent bonding, is a hydrogen bonding interaction, although an ionic interaction involving initiator (potassium peroxodisulfate) fragments might also contribute. Because of their unique network structure, NC gels simultaneously exhibit extraordinarily tough mechanical properties (high elongation and strength), high transparency (structural homogeneity), high swelling ratio, and excellent thermo-responsive characteristics (gel volume and transpar￾ency transitions).4-6 It was also found that transparent P/C￾NCs with high contents of exfoliated clay were readily obtained by drying the NC gels.5,6 Here, we report the preparation of porous nanocomposite materials composed of poly(N-isopropyl acrylamide) (PNIPA) and clay by freeze￾drying NC gels. The resulting porous nanocomposites exhibit characteristic layered morphologies with controlled porosi￾ties. NC gels, as monoliths, were synthesized and analyzed by the same procedures as reported previously.5 Standard uniform aqueous solutions containing clay (synthetic hec￾torite [Mg5.34Li0.66Si8O20(OH)4]Na0.66, 3.31 g), monomer (N￾isopropyl acrylamide, 10 g), catalyst (N,N,N′,N′-tetra￾methylethylenediamine, 80 µL), initiator (potassium peroxo￾disulfate, 0.1 g), and water (100 g) were first prepared at 1 °C. A standard solution was clay/monomer weight ratio ) 0.33, water content ) 88.3 wt %. The solution was introduced to a glass vessel under a nitrogen atmosphere and kept at 20 °C for 20 h for free-radical polymerization. The clay/polymer weight ratio can be varied over the range 0.05 to 0.5, or more, by changing the clay concentration under the fixed monomer content in the initial reaction mixture. All NC gels prepared were uniform and transparent, and exhibited high elongations near to or greater than 1000%. NC gels were cut into disks (5.5-mm diameter, 5-mm thickness), frozen in liquid nitrogen (-200 °C) or in a freezer (-20 °C), and freeze-dried for 24 h using a freeze-dryer in which the pressure was always kept below the triple point pressure (611 Pa), and mostly below 5 Pa. White porous materials with low bulk density (ca. 0.12 g cm-3 ) were obtained without large volume shrinkages by removing the water in the freeze-drying process. In contrast, when NC gels were dried conventionally, e.g., by heating or evaporation under vacuum, normal dense nanocomposites of PNIPA and clay were always obtained.5,6 Therefore, the porous materials prepared from NC gels were named porous G-nanocompos￾ites (hereinafter, abbreviated to porous G-NCs), which signifies porous nanocomposites produced from gel. The morphologies of cross sections (and surface) were observed by using scanning electron microscopy (SEM) after coating freeze-dried samples with Pt to a thickness of 5 nm. For all NC gels prepared here, the porous G-NC disks exhibited characteristic layered morphologies as shells of different porosity regardless of the clay/polymer ratio. The layered morphologies were of three types: (A) a three-layer morphology consisting of fine-porous, dense, and coarse￾porous layers; (B) a two-layer morphology consisting of dense and coarse-porous layers; and (C) a uniform morphol￾ogy consisting of only a fine-porous material. On the other * To whom correspondence should be addressed. E-mail: hara@kicr.or.jp. Fax: 81-43-498-2182. Tel: 81-43-498-0062. (1) (a) Giannelis, E. P. AdV. Mater. 1996, 8, 29-35. (b) Pinnavaia, T. J.; Beall, G. W. Polymer-Clay Nanocomposites; Wiley: Chichester, 2000. (2) (a) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1185-1189. (b) Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2493-2498. (c) Gilman, J. W. Appl. Clay Sci. 1999, 15, 31-49. (3) Okamoto, M.; Nam, P. H.; Maiti, P.; Kotaka, T.; Nakayama, T.; Takada, M.; Ohshima, M.; Usuki, A.; Hasegawa, N.; Okamoto, H. Nano Lett. 2001, 1, 503-505. (4) Haraguchi, K.; Takehisa, T. AdV. Mater. 2002, 14, 1120-1124. (5) Haraguchi, K.; Takehisa, T.; Fan, S. Macromolecules 2002, 35, 10162-10171. (6) Haraguchi, K.; Farnworth, R.; Ohbayashi, A.; Takehisa, T. Macro￾molecules 2003, 36, 5732-5741. Chem. Mater. 2005, 17, 931-934 931 10.1021/cm048093x CCC: $30.25 © 2005 American Chemical Society Published on Web 02/11/2005