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Organic–Inorganic Hybrid Materials that Rapidly Swell in Non-Polar Liquids: Nanoscale Morphology and Swelling Mechanism Colleen M. Burkett, Laura A. Underwood, Rebecca S. Volzer, Jessi A. Baughman, and Paul L. Edmiston* Department of Chemistry, College of Wooster, 943 College Mall, Wooster, Ohio 44691 ReceiVed June 15, 2007. ReVised Manuscript ReceiVed NoVember 26, 2007
A novel type of sol–gel derived zerogels that instantaneously swell greater than three times their dried volume in nonpolar solvents were investigated. Hybrid organic–inorganic materials that swell were only produced from a narrow set of precursors that possess an organic bridging group that contains an aromatic segment that is flexibly linked to the alkoxysilane polymerizable ends. Careful control over the processing conditions (catalyst, solvent, aging time) was necessary to yield animated zerogels. Various materials were studied by electron microscopy, infrared spectroscopy, nitrogen adsorption, and fluorescence spectroscopy using a covalently linked pyrene reporter. Collectively, the data support a model where swelling is derived from a morphology of interconnected organosilicate nanoparticles that are cross-linked to a particular extent during the gel state. Upon drying to the zerogel form, tensile forces generated by capillary-induced collapse of the polymeric matrix are released when interparticle interactions holding the dried material in the shrunken state are disrupted by a suitable solvent system. Swelling produced forces in excess of 50 N/g and is completely reversible. The molecular-scale organization of the nanoparticle structure seems critical for this swelling behavior. Further experiments indicate that the organosilicate materials can also swell in response to gas-phase organic molecules in a concentration dependent manner. These hybrid materials show promise for use in remediation technologies and chemical sensing.
Introduction The sol–gel process is a convenient method to prepare hybrid inorganic–organic materials with unique properties both in terms of chemical composition and physical microstructure. Recently, we reported a type of hard optically transparent sol–gel that rapidly swells ∼4–5 times its dried volume in nonpolar solvents.1 These highly animated zerogels were prepared by polymerization of a bridged organosilane precursor bis(trimethoxysilyethyl)benzene. Polycondensation of a wide variety of bridged precursors has been reported and subsequently used to create nanostructured materials.2 Control of the sol–gel processing conditions (i.e., solvent, catalyst, aging temperature) can be used to tailor the texture of the resulting sol–gel derived solids which is advantageous when designing new materials.3 In this case, we have been able to create highly animated materials by * Author to whom correspondence should be addressed: (fax) 330-263-2386, (phone) 330-263-2113, (e-mail )
[email protected].
(1) Burkett, C. M.; Edmiston, P. L. J. Non-Cryst. Solids 2005, 351, 3174. (2) (a) Shea, K. J.; Loy, D. A.; Webster, O. W. Chem. Mater. 1989, 1, 572. (b) Loy, D. A.; Shea, K. J Chem. ReV. 1995, 95, 1431. (c) Loy, D. A.; Jamison, G. M.; Baugher, B. M.; Myers, S. A.; Assink, R. A.; Shea, K. J. Chem. Mater. 1996, 8, 656. (d) Shea, K. J.; Loy, D. A. Chem. Mater. 2001, 13, 3306. (3) (a) Small, J. H.; Shea, K. J.; Loy, D. A. J. Non-Cryst. Solids 1993, 160, 234. (b) Cerveau, G.; Corriu, R. J. P.; Lepeytre, C. J. Organomet. Chem. 1997, 584, 99. (c) Boury, B.; Corriu, R. J. P. AdV. Mater. 2000, 12, 989. (d) Cerveau, G.; Corriu, R. J. P.; Framery, E.; Ghosh, S.; Mutin, H. P. J. Mater. Chem. 2002, 12, 3021. (e) Boury, B.; Corriu, R. J. P. AdV. Mater. 2000, 12, 989. (f) Shea, K. J.; Loy, D. A. MRS Bull. 2001, 368. (g) Cerveau, G.; Corriu, R. J. P.; Lepeytre, C.; Mutin, P. H. J. Mater. Chem. 1998, 8, 2707.
carefully controlling chemical composition and morphology of the polymeric organosilicate. Silica-based sol–gel derived solids are typically characterized as inelastic and do not swell when fully dried, with only rare exceptions to this rule being reported.4–7 Typically, swelling involves some type of physical or chemical change, for example, a change in temperature6 or pH.7 The amount of swelling in these instances is modest, 400%). Only nonpolar liquids are taken up by the sol–gel polymer, even when placed in biphasic mixtures of water and organic solvents. This allows for a degree of chemical selectivity and sets the material apart from biomaterials8,9 or hydrogels10 that absorb water or other polar liquids. Swelling of the hybrid material is completely reversible after removing the solvent by evaporation or drying. No loss in the swelling behavior occurs even after repeated use. Swelling occurs instantaneously upon addition of liquid, so rapidly that large glass-like monoliths (>5 mm) can fracture during the rapid incursion of solvent. In addition (4) Burt, M. C.; Dave, B. C. J. Am. Chem. Soc. 2006, 128, 11750. (5) Rao, M. S.; Gray, J.; Dave, B. C. J. Sol-Gel Sci. Technol. 2003, 26, 553. (6) Rao, M. S.; Dave, B. C. AdV. Mater. 2001, 13, 274. (7) Rao, M. S.; Dave, B. C. AdV. Mater. 2002, 14, 443. (8) Khalid, M. N.; Agnely, F.; Yagoubi, N.; Grossiord, J. L.; Couarraze, G. Eur. J. Pharm. Sci. 2002, 15, 425. (9) Elvira, C.; Mano, J. F.; San Román, J.; Reis, R. L. Biomaterials 2002, 23, 1955. (10) Peppas, N. A.; Mikos, A. G. Preparation Methods and Structure of Hydrogels. In Hydrogels in Medicine and Pharmacy; Peppas, N. A., Ed.; CRC Press: Boca Raton, FL, 1986; Vol. 1, pp 1–27.
10.1021/cm0716001 CCC: $40.75 2008 American Chemical Society Published on Web 01/08/2008
Rapidly Swelling Organic–Inorganic Hybrid Materials
to adsorbing large amounts of nonpolar substances from the neat liquid phase, adsorption of nonpolar organic molecules is also possible from the gas phase or from aqueous solution. Because of these unique properties there are a number of potential applications of such a material including encapsulating molecules for controlled drug release,11 chemical remediation of hydrophobic species from aqueous systems, and chemical sensing.12 Highly swellable organosilicas have been only produced under a narrow set of processing conditions (precursor, solvent, catalyst, aging time). In previous work, sol–gels were prepared from bis(trimethoxysilyethyl)benzene and polymerized in tetrahydrofuran (THF) using tetrabutylammonium fluoride (TBAF) as a catalyst. Prior to drying, the wet gels are rinsed to remove unreacted silane, water, and catalyst. Rinsing is followed by derivatization of the residual silanol groups using a chlorosilane reagent. The identity of the derivatizing reagent has no effect on the swelling behavior;1 however, derivatization is required to produce a material that swells. In this report, we significantly expanded the number of processing variables to more fully understand the prerequisites needed to produce swellable organosilicates. In addition, the materials were further characterized to understand the physical processes that lead to the swelling behavior. Experimental Section Materials. Bis(trimethoxysilyethyl)benzene (1); 1,4-bis(trimethoxysilylmethyl)benzene (2); 1,4-bis(triethoxysilyl)benzene (3); tris(3-trimethoxysilylpropyl)isocyanurate (4); 1,6-bis(trimethoxysilyl)hexane (5); phenyltrimethoxysilane (PTMS); methyltrimethoxysilane (MTMS); tetramethoxysilane (TMOS); and mercaptopropyltrimethoxysilane (MPTMS) were obtained from Gelest. All other solvents and reagents were obtained from Sigma-Aldrich except for N-(1-pyrenemaleimide) which was obtained from Invitrogen. All reagents were used without further purification. Sol–Gel Preparation. Materials were prepared as described previously.1 Briefly, 1.80 × 10-2 moles of precursor and solvent to yield a final volume of 35 mL was initially mixed. One hundred microliters of 1.0 M TBAF, 375 µL of concentrated HCl, 90 µL of 1.0 M NaOH, 100 µL of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), or a specified amount of n-butylamine was added as a polymerization catalyst. Prior to addition, catalyst was mixed with a stoichiometric amount of water (0.5 mol H2O/mol alkoxysilane group). After gelation, the materials were aged for a prescribed time (6 days unless otherwise specified) in a closed container at 25 °C, crushed into pieces ∼3–5 mm in size, and sequentially extracted with ethanol twice and acetonitrile twice over a 4 day period. The sol–gels were then incubated in 20 mL of a 5% v/v solution of cyanopropyldimethylchlorosilane in acetonitrile for 48 h at room temperature. The gels were then rinsed three times with acetonitrile over 3 days and dried for 2 h at 60 °C in a standard laboratory oven. The material was then ground in a bead mill to generate a coarse powder. Characterization. Scanning electron microscopy (SEM) was performed using a Hitachi S-4700 instrument. Solid samples were dried in a vacuum oven at 60 °C for 12 h and coated with platinum prior to measurement. Multiple areas were examined to ensure the (11) Langer, R.; Peppas, N. A. AIChE J. 2004, 40, 2990. (12) Lavine, B. K.; Kaval, N.; Westover, D. J.; Oxebford, L. Anal. Lett. 2006, 39, 1773.
Chem. Mater., Vol. 20, No. 4, 2008 1313 micrographs depicted were a representative of the sample. Swollen sol–gels composed of 1 (THF solvent, TBAF catalysis) were prepared for microscopy by critical point drying from ethanol using a Tousimis AUTOSAMDRI-814 apparatus or by swelling the sol–gel in a 5% w/v solution of poly(2,2,3,3,4,4,4-heptafluorobutylmethacrylate) in acetonitrile followed by drying in at 60 °C for 24 h. Surface area and pore volume were measured with a BeckmanCoulter 3100 surface area analyzer using the BET method.13 Pore size distributions were measured by N2 desorption using the BJH method.14 Fourier transform infrared (FT-IR) spectra were obtained by preparing KBr pellets of the dried sol–gels. Spectra were obtained using a Perkin-Elmer 2000 FT-IR at a resolution of 4 cm-1. Polarized light microscopy was done using an Olympus 1 × 70 inverted microscope with crossed polarizers. Images were acquired with a Diagnostic Instruments Color Mosaic CCD using Spot image capture software. Sol–gel solutions were prepared as described above and immediately placed in cells constructed of glass microscopy slides derivatized with (tridacefluoro-1,1,2,2,-tetrahydroctyl)dimethylchlorosilane. A 50 µm Teflon gasket was placed between the slides to generate a sealed system of fixed thickness containing the sol–gel solution. Swelling was measured by adding solvent dropwise to 100 mg of zerogel until the swelling behavior had visibly ceased while the exterior of the material continued to appear fully dry. The amount of solvent was determined by the difference in weight. The amount of swelling was also confirmed by optical microscopy. Swelling force was measured as described in the text using a 3.3 cm diameter glass cylinder with a course glass frit sealed to one end. The piston was loaded with a fixed weight, and the volume increase was recorded after the system had come to equilibrium (abicyclo[4.3.0]non-5-ene. d All sol--gels were prepared with 1 in THF using TBAF catalysis. For sol-gels that were not aged, one type was immediately rinsed to remove water and catalyst (rinsed) while another type was immediately treated with excess cyanopropyl-dimethylchlorosilane (silane treated). All aging was performed at 25 °C unless otherwise indicated. e 1X sol-gel composition is defined as 0.51 M 1 in THF prepared using TBAF catalysis. A stoichiometric amount of water was used for all types, except when 8.5 ( 0.5. The rate of swelling was slightly enhanced by grinding the material to a coarse powder, presumably because there is a higher surface area to volume ratio resulting in a higher rate of solvent transport into the zerogels. Larger monolithic
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groups. A typical formulation that yielded a swellable sol–gel (1, THF, TBAF) exhibited ∼10% loss in volume during aging leading to cracking of the wet gel and expulsion of solvent. The importance of syneresis was demonstrated by the observation that a newly formed wet gel treated with a large excess of cyanopropyldimethylchlorosilane which derivatized silanol groups and thus prevented syneresis yielded a final material that could not swell.
Figure 1. Optical micrograph (20 × magnification) of an organosilicate composed of 1 dry (top) and swollen in acetonitrile (bottom).
material also swelled but will typically begin to fragment as the outer portions of the polymer increase in size more rapidly leading to fracturing. In all cases, if a material fit the criteria as swelling, then swelling was completely reversible after drying. Processing conditions were varied using 1 as the precursor (Table 1). Overall, a narrow window of conditions exists that leads to swelling behavior. For instance, the use of acetonitrile in place of THF as a solvent resulted in the complete loss of swelling behavior. In general, a high concentration of base catalyst is needed to achieve swelling. TBAF is particularly effective as a catalyst to yield animated materials. This is presumably due to fluoride ion being an effective nucleophile catalyst shown to generate morphologies similar to those obtained by base catalysis.18,19 Additionally, a fairly defined concentration of organosilane precursor must be used to yield zerogels that swell. One of the most important factors that contributes to conditions that yield swelling is syneresis during the aging in the wet gel state. Syneresis is due to further cross-linking of the polymer via hydrolysis and condensation of the alkoxy and silanol (18) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, 1990; Chapter 9. (19) Tilgner, I. C.; Fischer, P.; Bohnen, F. M.; Rehage, H.; Maier, W. F. Microporous Mater. 1995, 5, 77.
Zerogel materials prepared by TBAF catalysis in various solvents were studied by SEM to determine if morphological differences were responsible for the changes in swelling behavior (Figure 2). Regardless of precursor type, all of the organosilicates were found to be composed of approximately 10–20 nm sized particles clustered into small groups (Figure 2; see Supporting Information for a complete set of images for a wide variety of zerogel types). This familiar morphology is consistent with previous studies of sol–gel derived hybrid materials prepared by base or fluoride ion catalysis.20–22 Variation in the microstructure of the particle clusters was observed which was particularly affected by solvent used during the initial gel formation step. Polymerization in acetonitrile yielded the more mesoporous material. Increased particle aggregation led to a concomitant increase in pore volume/size and decrease in surface area as measured by N2 adsorption/desorption (Table 3). Interestingly, swelling behavior was lost when the nanoscale assembly of particles was either highly cross-linked and microporous as observed in materials polymerized in ethanol or too mesoporous as demonstrated by polymerization in acetonitrile. THF, acetone, and similar solvent systems apparently yield an optimal level of particle interconnectivity for swelling behavior. This further demonstrates the type of control that is possible in sol–gel processing conditions where morphology is dependent on the rate and potentially the mechanism of chemical reactions occurring during polymerization. To understand how the zerogel materials swell, the morphology of the swollen state was examined by SEM after the following treatments: (1) critical point drying of the material swollen in ethanol and (2) swelling the material in a solution of relatively low molecular weight polymer followed by drying, thereby leaving the polymer entrapped in the matrix (Figure 3). These methods had to be employed because the material swollen with solvent reversibly returns to the collapsed state when dried. On the basis of changes in morphology, individual nanoparticles appeared to be tethered together in clusters, but the individual particles are flexible within the arrangement. Microscopy of swollen materials revealed that the individual clusters of particles expanded upon exposure to solvent yielding larger spheroidlike structures. The nanoparticles themselves do not swell. This was especially apparent in the critical point dried sol–gels (Figure 3D) since some of the organosilicate may be in the completely expanded state which would not be the (20) Kurumada, K.; Nakabayashi, H.; Murataki, T.; Tanigaki, M. Colloids Surf., A 1998, 139, 163. (21) Cerveau, G.; Corriu, R. J. P.; Fischmeister-Lepeytre, C. J. Mater. Chem. 1999, 9, 1149. (22) Reale, E.; Levya, A.; Corma, A.; Martinez, C.; Garcia, H.; Rey, F. J. Mater. Chem. 2005, 15, 1742.
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Figure 2. SEM micrographs of zerogel compositions prepared by TBAF catalysis. A. 1 prepared in THF. B. 1 prepared in acetonitrile. C. 5 prepared in THF. D. 1 prepared in ethanol. All images are the same scale. Of these formulations, only the material depicted in image A swells. Table 3. Surface Area and Pore Volume of Various Sol–Gelsa
precursorb 1 2 3 MTMS 1 (0.25X) 1 1 1 1 1 (no aging)
solvent THF THF THF THF THF THF CH3CN acetone ethanol THF
pore BET surface volume pore size 2 catalyst area (m /g) (mL/g) < 6 nm swell TBAF TBAF TBAF TBAF TBAF DBN TBAF TBAF TBAF TBAF
736 694 853 374 415 543 460 543
