J O U R N A L O F C H E M I S T R Y Materials Communication Polymer mesofibres Stacy A. Johnson,a Deepa Khushalani,b Neil Coombs,c Thomas E. Mallouk*a and GeoVrey A. Ozin*b aDepartment of Chemistry, T he Pennsylvania State University, University Park, Pennsylvania 16802, USA bMaterials Chemistry Research Group, L ash Miller Chemical L aboratories, University of T oronto, 80 St. George Street, T oronto, Ontario, Canada M5S 3H6 cImagetek: Analytical Imaging, 32Manning Avenue, T oronto, Ontario, Canada M6J 2K4 samples were then shaken in 48% hydrofluoric acid overnight and filtered to leave the organic replica materials. Ashing A novel process is described for synthesizing high aspect ratio, controlled diameter mesoscale poly(phenolformaldehyde) analysis of the products consistently gave compositions with fibres using the well defined channels of mesoporous silica as a <2% residual silicate.Characterization of the samples was mold and their extraction with the structure intact. accomplished using powder X-ray diVraction (PXRD), adsorption studies, elemental analysis, 13C CP MAS NMR, FTIR spectroscopy and HRTEM. PXRD analysis of the silica/polymer composite (before etching) displayed a pattern at low 2h values that is characteristic of the hexagonal form of mesoporous silica.The low Ever since the discovery of the MCM-41S family of mesoporous electron contrast of the carbon-based polymer compared to materials by Kresge et al.,1 a great deal of interest has been that of the silica host had little eVect on the intensity of the generated in the study of various aspects of these materials PXRD pattern of the composite relative to the empty host.At including varying pore sizes through a variety of techniques,2 a 2h value of ca. 25°, a broad peak was observed suggesting tuning the framework composition,3 and employing the the presence of disordered polymer in a glassy silica host internal channels for probing reactions in restricted matrix.Moreover, this peak was retained for the sample dimensions.4 obtained after etching, characterizing the presence of an Here we present a route to the formation of a polymer mold amorphous polymer. Elemental analysis on the silica/polymer of the channel structure of the hexagonal phase of MCM-41 composite sample gave 45% carbon content.For an ideal mesoporous silica. The technique involves creating a replica complete filling of the channels, the required carbon content by polymerizing formaldehyde and phenol inside the channels. needed to be 59%, and hence the channels can be assumed to This replica, through the novel idea of etching the host around be ca. 75% filled. This degree of mesopore filling is consistent it, is then able to be characterized ex situ by various physical with the results of 77 K N2 adsorption isotherms recorded for methods.High resolution transmission electron microscopy calcined mesoporous silica before and after polymer encapsul- (HRTEM) in particular allows the study of the structure and ation in the channels which show that the accessible pore morphology of the resultant polymer fibres after extraction. It volume of the mesoporous silica has diminished to about 85 should be noted that in the past there has also been some vol%. 13C CP MAS NMR spectra of the extracted polymer debate over the actual access of diVerent sized molecular displayed two broad signals at 40 and 130 ppm. These peaks adsorbates into the mesoporous silica. Channel restriction or can be assigned to the presence of the methylene and aromatic blockage was envisaged in certain cases.Also, it was debatable groups, respectively. Aside from some slight line narrowing in whether the reactions were occurring inside the mesoporous the aromatic region and small intensity changes in the methyl- structure or instead on the external surface of the particle.The ene region, the spectrum of the extracted polymer was more- technique presented here helps to clarify these issues and or-less the same as that of bulk poly(phenolformaldehyde). provides a way of directly visualizing the length of the channels Further evidence of the presence of the polymer was obtained within mesoporous silica materials. by FTIR spectroscopy. Characteristic bands for poly(phenol- Mesoporous silica was synthesized according to the literaformaldehyde) at ca. 3500 cm-1 were assigned to nOH stretch- ture procedure5 followed by calcination at 540 °C over 12 h. ing modes and several sharp bands at ca. 1500 cm-1 were The final surface area, pore size, and pore volume were attributed to CMO, CMC groups similar to those found for determined to be 969.2 m2 g-1, 38.7 A ° , and 1.40 ml g-1, the bulk poly(phenolformaldehyde). The combination of the respectively.In order to fill the pores with the phenolic resin, NMR and FTIR results indicated that the mesoscale size solid phenol was added to a flask containing the MCM-41 constraint aVorded by the channels did not cause extensive template and incubated at 65 °C under reduced pressure overdeviation in polymer connectivity and dynamics from that of night.The amount of phenol incorporated was determined the bulk. Nevertheless, NMR relaxation studies and the [1.57 g (g MCM-41)-1] by the pore volume of the dehydrated measurement of the elastic constants of the fibres by, for MCM-41 material. Excess solid paraformaldehyde [1.27 g (g example, AFM will be needed to explore whether there exists MCM-41)-1] was heated to 120 °C in order to liberate monany alteration in the dynamical and mechanical properties omeric formaldehyde, which was transferred as a vapour to between the mesofibre and bulk forms of the polymer.the phenol/MCM-41 composite. To initiate polymerization, The most interesting information was gained by HRTEM. anhydrous HCl vapor produced from a mixture of NaCl and It should be emphasized that because of the low electron H2SO4 was allowed to enter the reaction chamber.The contrast of the extracted sample, HRTEM studies of the resulting polymer/porous silica composite was cured in argon polymer necessitated the use of direct imaging on ultrathin at 125 °C for 3 h and then at 500 °C for 12 h in order to induce ‘holey’ carbon film (ca. 7 A° thick), and indirect imaging using cross-linking. A subsequent second loading of the monomer was performed to ensure complete filling of the channels. The heavy metal negative staining using a dilute solution of uranyl J. Mater. Chem., 1998, 8(1), 13–14 13Fig. 1 Transmission electron micrograph (using negative staining technique with uranyl acetate solution) of polymer mesofibres supported on a carbon film.Magnification bar=100 nm. Fig. 2 Transmission electron micrograph of control sample—polymer deposited on amorphous silica (Cab-O-Sil). Magnification bar= acetate on thin carbon-support film, Fig. 1. From the micro- 100 nm. graph, it is obvious that only fibre-like morphology is seen for the extracted poly(phenolformaldehyde).The width of the fibres, as determined through the micrographs, is found to be viously reported,4,6,7 no studies have been described on the extraction of these materials and their structural characteriz- ca. 20 A ° with an eVective resolution of 7 A ° (combination of uranyl acetate stain and other specimen parameters). These ation. Hence, the concept of synthesizing copies of the channels of mesoporous silica and extracting them provides access to a are comparable to the pore diameter of the host taking into consideration that only between 75 and 85% of the available diversity of mesofibres with a range of compositions and sizetunable properties.volume is occupied. Furthermore, depending on extraction, sonication and solvent protocols both single and bundles of polymer strands are observed.These bundles are discernible T.M. would like to acknowledge the National Institute of Health (GM 43844) and G.A.O. would like to acknowledge from single fibres by their size and texture. These display extensive regions of curvature thereby indicating appreciable the Natural Sciences and Engineering Research Council for support of this work.D.K. is grateful for a University of flexibility of the poly(phenolformaldehyde) mesofibres. The mesofibres were found to display extensive lengths which were Toronto Open Scholarship. comparable to the particle size of the mesoporous silica host, and the aspect ratios (ratio of the length to the diameter) of References the extracted polymer fibres were found to exceed 103.This suggests that for the mesoporous silicas, the channels run 1 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature (L ondon), 1992, 359, 710. practically the entire length of the particles (whose sizes are in 2 D. Khushalani, A. Kuperman, G. Ozin, K. Tanaka, J. Garces, the range 1–10 mm, depending on the synthesis conditions). M. Olken and N. Coombs, Adv.Mater., 1995, 7, 842.Control TEM experiments have been performed for bulk 3 W. Zhang, M. Froba, J.Wang, P. Tanev, J.Wong and T. Pinnavaia, forms of poly(phenolformaldehyde), as well as samples synthe- J. Am. Chem. Soc., 1996, 118, 9164. sized on the surface of non-porous Cab-O-Sil silica that had 4 T. Maschmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature (L ondon), 1995, 378, 159; J. Felipe Diaz and K. J. Balkus Jr., J. Mol. been pre-treated under identical conditions to those used for Catal. B, 1996, 2, 115. the mesoporous silica channel host material. The TEM images 5 D. Khushalani, A. Kuperman, N. Coombs and G. Ozin, Chem. of the polymer mesofibres show distinct morphologies, Fig. 1, Mater., 1996, 8, 2188. while only shapeless polymer particulates are extracted from 6 C.-G. Wu and T. Bein, Science, 1994, 264, 1757. Cab-O-Sil, Fig. 2, and only non-descript polymer agglomerates 7 T. Kyotani, T. Nagai, S. Inoue and A. Tomita, Chem. Mater., 1997, are obtained for the bulk form of the polymer. 9, 609. We wish to emphasize that while mesoporous silica encapsulated polymers, metals and semiconductors have been pre- Communication 7/06791G; Received 18th September, 1997 14 J. Mater. Chem., 1998, 8(1), 13–14