J O U R N A L O F C H E M I S T R Y Materials Communication Synthesis of IPN polymer hybrids of polystyrene gel and silica gel by an in-situ radical polymerization method Ryo Tamaki and Yoshiki Chujo* Department of Polymer Chemistry Graduate School of Engineering Kyoto University Yoshida Sakyo-ku Kyoto 606-01 Japan Homogeneous IPN polymer hybrids of polystyrene gel and silica gel were prepared by applying an in-situ polymerization method to styrene monomer and divinylbenzene. The monomers were mixed in the sol–gel reaction mixture of tetramethoxysilane and subjected to radical polymerization resulting in transparent glassy materials. The homogeneity was confirmed by nitrogen porosimetry methods. The organic part was found to be dispersed at the nanometer scale. The obtained IPN polymer hybrids are highly solvent resistant.In recent years a large variety of organic and inorganic polymer hybrids have been synthesized by utilizing the sol–gel technique Scheme 1 with alkoxysilanes.1,2 The sol–gel reaction comprises the hydrolysis and subsequent condensation reaction of alkoxysilanes. 3–7 When alkoxysilanes are used as precursors Si–OH sol–gel reaction of TMOS. The mixture was stirred at room temperature for 5 h and subsequently heated at 60 °C under groups are formed by hydrolysis of alkoxy groups and Si–O–Si linkages are obtained by condensation of the hydroxyl groups. nitrogen for 1 week with an aluminium foil cover having a few pinholes. Sol–gel reaction of TMOS and radical polymerization One of the interesting properties of the obtained silica gel is that it contains unreacted residual silanol groups even after of styrene and DVB were expected to proceed simultaneously entrapping the organic network in the silica gel.After removal gelation which allow us to utilize hydrogen bonding interactions between the silanol groups and amide groups of organic of the solvent glassy materials were obtained. The conversion of the monomer to polymer was confirmed polymers to obtain homogeneous organic–inorganic polymer hybrids. Poly(2-methyl-2-oxazoline) poly(N-vinyl-2-pyrroli- by thermogravimetric analysis (TGA) which shows the onset of polymer decomposition around 350 °C (Fig. 1).19 The weight done) and poly(N,N-dimethylacrylamide) have been incorporated homogeneously by adding them into a sol–gel reaction loss at around 100 °C corresponds to residual solvent.The polymer hybrids obtained were ground and subjected to CHCl3 mixture at an initial stage.8–18 On the other hand we have successfully prepared homo- extraction using a Soxhlet apparatus. The contents of organic polymer in the hybrids after CHCl3 extraction were confirmed geneous polymer hybrids of polystyrene which has no polar functional groups utilizing the in-situ radical polymerization by TGA as the weight loss at 900 °C. As shown in Fig. 1 the polymer contents decreased dramatically after the extraction of styrene monomer in the sol–gel reaction mixture.19 In contrast to the prepolymer incorporation method used for in the polymer hybrid prepared without DVB. The polymer contents before and after the extraction were 48% and 17% poly(2-methyl-2-oxazoline) and the other polymers described above hydrogen bonding interactions between silica gel and respectively (Table 1).The results indicate 64% of polystyrene was extracted from the polymer hybrid prepared without DVB. organic polymers are not so critical in this in-situ polymerization technique. It is rather confinement of the growing On the other hand the polymer contents hardly changed before and after extraction when 0.1 equiv. of DVB was used organic polymer by silica gel that aVects the homogeneity.20 Here we have applied this in-situ polymerization method for (Fig. 2). In this case the polymer contents before and after extraction were 53% and 50% respectively. The loss of organic the synthesis of polymer hybrids of polystyrene gel and silica gel with a so-called interpenetrating polymer network (IPN) polymer was only 6%.For clarity the polymer loss was plotted against DVB contents as illustrated in Fig. 3. The plot structure. The polymer hybrids were expected to be highly solvent resistant because of the presence of cross-linking points shows the considerable improvement of the resistivity to solvent extraction with increasing DVB content and the extrac- in the organic network. Polystyrene gel and silica gel polymer hybrids were prepared tion was almost prevented above 10 wt.% of DVB. The results indicate polystyrene formed a percolation network in silica gel by the in-situ polymerization method as shown in Scheme 1. Divinylbenzene (DVB) was used to introduce cross-linking with 10 wt.% of DVB resulting in strong resistance to dissolution in chloroform. points in the organic network.Styrene and DVB were mixed with 0.01 equiv. of 2,2¾-azobisisobutyronitrile (AIBN) and 1.0 g The homogeneity of IPN polymer hybrids could be estimated by optical observation. Since the refractive indices of organic of tetramethoxysilane (TMOS) in 10 ml of acetone. The weight ratio of DVB and styrene was varied from 0 to 0.2 to control and inorganic parts have diVerent values i.e. 1.46 and 1.60 at 20 °C for polystyrene and silica gel respectively the composites the density of cross-linking points while the total weight of the organic monomers was fixed at 1.0 g (Table 1). 0.24 ml of would become opaque when the domain of each component is larger than wavelength of visible radiation.2 As shown in 0.1 M HCl was added to the solution as a catalyst for the J. Mater. Chem.1998 8(5) 1113–1115 1113 Table 1 Synthesis of polystyrene IPN polymer hybridsa polym. cont.c run DVB/St homogeneityb before(%) after(%) weight lossc (%) 1 0 transparent 48.0 17.4 63.8 2 0.05 transparent 49.5 40.3 18.9 3 0.1 transparent 53.0 49.7 6.2 4 0.2 transparent 55.8 51.9 7.0 aEach hybrid was prepared with 0.01 equiv. of AIBN 1.0 g of TMOS 0.24 ml of 0.1 M HCl in 10 ml of acetone. The total amount of DVB and St was 1.0 g. The mixture was heated at 60 °C under nitrogen. bHomogeneity was evaluated optically. cThe polymer contents in the hybrids were calculated by TGA. dWeight loss of the organic part was calculated as follows weight loss={polym.cont.(before)-polym.cont. (after)}/polym.cont.(before)×100. Scheme 2 Table 1 the obtained polymer hybrids were all transparent indicating the homogeneous dispersion of the organic domain in the silica matrix.The homogeneity of the polymer hybrids was also evaluated quantitatively by nitrogen porosimetry of Fig. 1 TGA traces of polystyrene–silica gel IPN polymer hybrid porous silica obtained from the polymer hybrids. The organic (DVB/styrene=0) before (a) and after (b) CHCl3 extraction polymer was removed from the polymer hybrids by charring at 600 °C for 24 h resulting in porous silica with pores of comparable size to the organic domains in the polymer hybrids (Scheme 2).18 Therefore the dispersity of organic polymers in the hybrids could be evaluated by measuring the pores.18 The BET method was applied to the isotherm curves to calculate surface areas and pore volumes of porous silicas.21 As shown in Table 2 it was found that the porous silica obtained from these polymer hybrids had surface areas of more than 200 m2 g-1.If an aggregation of the organic segment occurred the porous silica would have much smaller values for surface area and pore volume.20 The pore size was calculated by the BJH method from the desorption isotherm curve from which the pore radius (Rp) was obtained assuming cylindrical pores.22 The results are illustrated in Fig. 4 and Table 2. Although the pore size is larger than that of porous silica obtained from a polymer hybrid of linear polystyrene and silica gel the pore size distribution plot for the porous silica obtained from the Fig. 2 TGA traces of polystyrene–silica gel IPN polymer hybrid polymer hybrids prepared with 10 and 20 wt.% DVB exhibited (DVB/styrene=0.1) before (a) and after (b) CHCl3 extraction peaks at 1.9 and 2.7 nm respectively.As the pore of the silica obtained from polymer hybrids corresponds to the domain size of the organic segment it seems reasonable to say that the polystyrene gel was dispersed at a nanometer level in these polymer hybrids. This is very interesting for the organic segment which does not possess hydrogen bond accepting groups and has poor solubility in the solvent. The homogeneity could be attributed to confinement of the polystyrene network Table 2 Pore volume and surface area of porous silicaa DVB/ pore volumeb/ surface areab/ pore radiusc/ run St ml g-1 m2 g-1 nm 1 0 63.6 277 1.8 2 0.1 64.7 282 1.9 3 0.2 67.9 295 2.7 aThe porous silicas were obtained by charring the polymer hybrids at 600 °C for 24 h.bCalculated by BET method. cCalculated by BJH Fig. 3 Weight loss by CHCl3 extraction for 1 week method from desorption curve. 1114 J. Mater. Chem. 1998 8(5) 1113–1115 3 H. Schmidt H. Scholze and A. Kaiser J. Non-Cryst. Solids 1984 63 1. 4 C. J. Brinker K. D. Keefer D. W. Schaefer R. A. Assink B. D. Kay and C. S. Ashley J. Non-Cryst. Solids 1984 63 45. 5 F. Orgaz and H. Rawson J. Non-Cryst. Solids 1986 82 57. 6 C. J. Brinker and G. W. Scherer J. Non-Cryst. Solids 1985 70 301. 7 C. J. Brinker and G. W. Scherer Sol–Gel Science Harcourt Brace & Co. Boston 1990. 8 Y. Chujo E. Ihara S. Kure and T. Saegusa Macromolecules 1993 26 5681. 9 M. Toki T. Y. Chow T. Ohnaka H. Samura and T. Saegusa Polym. Bull. 1992 29 653. 10 T. Saegusa and Y. Chujo J.Macromol. Sci. Chem. 1990 A27 1603.11 Y. Chujo and T. Saegusa Adv. Polym. Sci. 1992 100 11. 12 Y. Chujo J. T hermosetting Plast. Jpn. 1995 16 99. 13 Y. Chujo E. Ihara S. Kure N. Suzuki and T. Saegusa Makromol. Chem. Macromol. Symp. 1991 42/43 303. 14 Y. Chujo Organic/Inorganic Polymer Hybrids CRC Press Boca Raton New York London Tokyo 1996; vol. 6 p. 4793. Fig. 4 Pore size distribution plots of porous silica gels 15 Y. Chujo Polym. Mater. Sci. Eng. 1996 74 65. 16 T. Saegusa and Y. Chujo Makromol. Chem. Macromol. Symp. within the silica gel matrix. The formation of polystyrene gel 1991 51 1. and silica gel proceeded simultaneously resulting in an IPN 17 T. Saegusa and Y. Chujo Makromol. Chem. Macromol. Symp. 1992 64 1. structure in this in-situ polymerization method. It was thus 18 Y. Chujo H. Matsuki S. Kure T. Saegusa and T. Yazawa J. Chem. expected that the rigid structure of silica gel suppressed the Soc. Chem. Commun. 1994 635. mobility of polystyrene and then prevented aggregation of the 19 R. Tamaki K. Naka and Y. Chujo Polym. Bull. 1997 39 303. organic segment. 20 R. Tamaki K. Naka and Y. Chujo Polym. J. 1998 30 60. 21 S. Brunauer P. H. Emmett and E. Teller J. Am. Chem. Soc. 1938 60 309. References 22 E. P. Barrett L. G. Joyner and P. P. Halenda J. Am. Chem. Soc. 1951 73 373. 1 J.Wen and G. L. Wilkes Chem. Mater. 1996 8 1667. 2 B. M. Novak Adv. Mater. 1993 5 422. Communication 7/08915E; Received 11th December 1997 J. Mater. Chem. 1998 8(5) 1113–1115 1115