Synthesis of Epoxy-Functional Microspheres by Cationic Suspension

Chem. Mater. 2004, 16, 5033-5041

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Synthesis of Epoxy-Functional Microspheres by Cationic Suspension Photopolymerization Benjamin Falk and James V. Crivello* Department of Chemistry and Chemical Biology, New York Sate Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180 Received April 15, 2004. Revised Manuscript Received August 11, 2004

This article describes the development of a novel rapid one-step cationic suspension photopolymerization method for the synthesis of epoxy-functional polymeric microspheres ranging in size from 50 nm to 100 µm. Multifunctional silicon-containing epoxy monomers and oligomers were photopolymerized to produce microspheres in both in aqueous and nonaqueous suspensions using a variety of cationic photoinitiators. Various solvents with controlled polarities were employed to produce macroporous particles. The effects of a variety of experimental and compositional variables on the particle diameter, size distribution, and epoxy content of the microspheres were examined. Among the parameters that were evaluated were the initiator composition and concentration, agitation method, UV irradiation time, viscosities of both the monomer and suspending medium, and type of porogen used. Following synthesis, the microspheres were characterized by SEM microscopy and by titration to determine the epoxy-functional group content. Microspheres that were subjected to pyrolysis at 700 °C retained their macrostructure despite undergoing a 60% weight loss.

Introduction There are a wide variety of current and potential applications for polymeric microspheres that bear specific reactive functional groups. Among these include the following: reinforcing additives to improve the mechanical properties of polymers; drug delivery carriers;1 new chromatographic materials;2,3 catalyst supports;4-6 metal ion complexing agents.7-10 Many of these applications require high functionality as well as discrete particle size ranges. Over the past decade, the desire for welldefined, functional polymer microspheres has led scientists to develop a wide variety of complex polymerization methods for their synthesis. The most common methods used include free radical suspension, seeded suspension, and both nonaqueous and aqueous dispersion polymerizations.11-13 Aerosol polymerizations have * To whom correspondence should be addressed. E-mail: crivej@ rpi.edu. (1) Peniche, C.; Fernandez, M.; Gallardo, A.; Lopez-Bravo, A.; San Roman, J. Macromol. Biosci. 2003, 3, 540-545. (2) Lai, J.-P.; Cao, X.-F.; Wang, X.-L.; He, X.-W. Anal. Bioanal. Chem. 2002, 372, 391-396. (3) Lai, J. P.; Lu, X. Y.; Lu, C. Y.; Ju, H. F.; He, X. W. Anal. Chim. Acta 2001, 442, 105-111. (4) Nicolaides, C. P.; Coville, N. J. J. Organomet. Chem. 1981, 222, 285-298. (5) Molinari, H.; Montanari, F.; Quici, S.; Tundo, P. J. Am. Chem. Soc. 1979, 101, 3920-3927. (6) Chauvin, Y.; Commereuc, D.; Dawans, F. Prog. Polym. Sci. 1977, 5, 95-226. (7) Alexandratos, S. D. Sep. Purif. Methods 1992, 21, 1-22. (8) Alexandratos, S. D.; Quillen, D. R. React. Polym. 1990, 13, 255265. (9) Alexandratos, S. D.; Crick, D. W.; Quillen, D. R. Ind. Eng. Chem. Res. 1991, 30, 772-778. (10) Alexandratos, S. D.; Kaiser, P. T. Solvent Extr. Ion Exch. 1989, 7, 909-923. (11) Downey, J. S.; Frank, R. S.; Li, W. H.; Stover, D. H. Macromolecules 1999, 32, 2838-2844. (12) Kiremitci, M.; Cukurova, H. Polymer 1992, 33, 3257-3261.

also been employed to produce microspheres using various carrier gases as suspending mediums.14,15 Thermally initiated free radical suspension polymerizations are most commonly used to synthesize microspheres. These polymerizations typically employ redox initiator systems, e.g., potassium persulfate or thermally activated initiators such as 2,2′-azobis(isobutyronitrile) (AIBN), and usually require reaction times on the order of hours to days for completion.16 Although many polymerization processes and monomer combinations have been examined for the free radical initiated synthesis of microspheres, there have been only a few reports of the use of photopolymerizations to prepare these materials. There are several potential advantages of using these latter reactions for the preparation of microspheres. Photoinitiated addition polymerizations can generally be carried out more rapidly than the corresponding thermal processes and are, therefore, potentially capable of producing microspheres on a much shorter time scale. In addition, using the high quantum yield photoinitiators available today, photopolymerizations can be efficiently conducted at a variety of temperatures, allowing better control of the polymerization conditions. Various groups have been investigating free radical photopolymerizable systems to produce microspheres. Saito et al.17 have employed seeded suspension photo(13) Gu, S.; Mogi, T.; Konno, M. J. Colloid Interface Sci. 1998, 207, 113-118. (14) Esen, C.; Schweiger, G. J. Colloid Interface Sci. 1996, 179, 276-280. (15) Ward, T. L.; Zhang, S. H.; Allen, A.; Davis, J. E. J. Colloid Interface Sci. 1987, 118, 343-355. (16) Arshady, R.; Ledwith, A. React. Polym., Ion Exch., Sorbents 1983, 1, 159-174. (17) Saito, R.; Ni, X.; Ichimura, A.; Ishizu, K. J. Appl. Polym. Sci. 1998, 69, 211-216.

10.1021/cm040057a CCC: $27.50 © 2004 American Chemical Society Published on Web 10/09/2004

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polymerizations to produce monodispersed microsphere particles with diameters less than 1 µm. Capek et al.18 carried out the photopolymerization of 2-methacryloyloxyethyl trimethylammonium chloride in microemulsions to produce ∼100 nm nanospheres. A high loading of surfactant, up to equal parts surfactant and monomer, is typically required. In addition, a specific surfactant must be developed for each monomer system used. Far less surfactant is used in suspension polymerizations, and as a result, the particle size and size distribution tend to increase.19 Photopolymerizations carried out in nonaqueous suspensions have also been employed for microsphere preparation. These polymerizations typically involve hydrophilic monomers and a hydrophobic suspending medium such as n-hexane or chloroform. Muxxalupo et al.20 carried out the photopolymerization of R,β-poly(N-2-hydroxyethyl)-DL-aspartamide that was partially functionalized with glycidyl methacrylate. Both Carver et al.21 and Fouassier et al.22 examined the radical photopolymerization of acrylamide in bis(2-ethylhexyl)sulfosuccinate reverse micelles. While the majority of photopolymerization techniques for microsphere synthesis involve free radical polymerization, there has been only one report of the use of a cationic photopolymerization to produce microspheres. In a recent article from this laboratory23 we described the use of a cationic aerosol photopolymerization to prepare microspheres with diameters of 10-50 µm. There are several incentives that make cationic photopolymerizations especially attractive for the formation of microspheres. Whereas most free radical photopolymerizations must be conducted under an inert atmosphere for the polymerization to proceed, the corresponding cationic processes are not inhibited by oxygen. Cationic photopolymerizations can also be applied to a broader range of monomers than their free radical counterparts. Thus, not only vinyl monomers but also wide range of heterocyclic monomers can be employed as potential substrates for the synthesis of microspheres. Of particular interest is the photopolymerization of multifunctional epoxide monomers and oligomers for the preparation of microspheres. This is because the resultant epoxy-functional microspheres can be readily modified using a wide variety of both acid24-26 and base27,28 catalyzed addition reactions. In this article we describe the synthesis and characterization of epoxy-functional microspheres using cat(18) De Buruaga, A. S.; Capek, I.; De La Cal, J. C.; Asua, J. M. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 737-748. (19) Mellott, M. B.; Searcy, K.; Pishko, M. V. Biomaterials 2001, 22, 929-941. (20) Muzzalupo, R.; Iemma, F.; Picci, N.; Pitarresi, G.; Cavallaro, G.; Giammona, G. Colloid Polym. Sci. 2001, 279, 688-695. (21) Carver, M. T.; Dreyer, U.; Knoesel, R.; Candau, F. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 2161-2177. (22) Fouassier, J. P.; Lougnot, D. J.; Zuchowicz, I. Eur. Polym. J. 1986, 22, 933-938. (23) Vorderbruggen, M. A.; Crivello, J. V.; Wu, K.; Breneman, C. M. Chem. Mater. 1996, 8, 1106-1111. (24) Winstein, S.; Ingraham, L. L. J. Am. Chem. Soc. 1952, 74, 1160-1164. (25) Berti, G.; Bottari, F.; Ferrarini, P. L.; Macchia, B. J. Org. Chem. 1965, 30, 4091-4096. (26) Rueppel, M. L.; Rapoport, H.; Moore, C. G. J. Am. Chem. Soc. 1972, 94, 3877-3883. (27) Colclough, T.; Cunneen, J. I.; Moore, C. G. Tetrahedron 1961, 15, 187-192. (28) Burness, D. M.; Bayer, H. O. J. Org. Chem. 1963, 28, 22832288.

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ionic ring-opening suspension photopolymerizations. These photopolymerization reactions were carried out in both aqueous and nonaqueous media. The resulting microspheres were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR), and the epoxy content was determined by standard titration methods. Experimental Section Materials. All organic starting materials and solvents employed in this investigation were reagent quality and were used as purchased from the Aldrich Chemical Co. (Milwaukee, WI) unless otherwise noted. Mineral oil (Aldrich) employed in this work had a viscosity of 34.5 cP at 40 °C and a density of 0.845-0.905 g/mL. Various viscosity silicone oils (SF96 fluids) were purchased from the General Electric Co. Silicone Products Department (Waterford, NY). Epoxysilicone monomers and oligomers PC1000 and PC2003 were gifts from the Polyset Co., Mechanicville, NY. The photoinitiators, (4-npentadecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC15),29 (4-n-octadecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC18),29 (4-n-octyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC8),29 and S-octadecyl-S-butyl-Sphenacylsulfonium hexafluoroantimonate (C4C18-DPS),30 were prepared as described earlier. Decahydronaphalene (mixture of cis and trans isomers) was purified first by washing with concentrated sulfuric acid followed by vacuum distillation. Photopolymerization Apparatus. Ultraviolet (UV) radiation used in these photopolymerizations was supplied by a Hanovia 679A36 (Hanovia Ltd., Slough, England) mediumpressure mercury arc lamp equipped with a 200 W power source. A control-cure radiometer obtained from UV Process Supply (Chicago, IL) was used to measure the light intensity. The light intensity used for all the photopolymerizations was 0.90 mW/cm2. A Vibra-Cell (VCX 750, Vibracell Sonics, Newton, CT) ultrasonicator equipped with a 13 mm solid probe was employed. A mechanical stirrer (RW 20 DZM.n) equipped with an integrated digital speed display was purchased from IKA, Inc. (Wilmington, NC), and fitted with a 5 cm 3-blade propeller stirrer purchased from Fisher Scientific (Burr Ridge, IL). A 6 cm (diameter) model DB2B Cowles stirrer obtained from Indco, Inc. (New Albany, IN), was also employed for high shear agitation of the aqueous suspension polymerizations. Synthesis of Microspheres. Shown in Figure 1 is a schematic diagram of the apparatus employed in this work for the photochemical synthesis of epoxy-functional polymeric microspheres in both aqueous and nonaqueous media. The apparatus consists of a cylindrical (7.6 cm diameter × 12.7 cm) quartz reaction vessel equipped with a high-intensity mechanical stirrer and an ultrasonicator wand. A mercury arc lamp is positioned directly below the reaction vessel behind a manual shutter, and the entire apparatus is enclosed in a metal irradiation chamber. Irradiation of the contents of the reaction vessel was accomplished by opening the external manual shutter. Unless otherwise indicated, the amount of photoinitiator used was 1.0 mol % based on the total number of epoxy-functional groups present in the monomer(s) used in the photopolymerization. Monomer Suspensions in Water. The following is a typical experimental procedure employed for photopolymerizations carried out in water. An aqueous suspending medium (200 mL) composed of dionized water and 85% hydrolyzed poly(vinyl alcohol) (0.05 g/L) was added to the quartz reaction vessel. The vessel was then placed into the UV chamber and stirred using a three-paddle mechanical stirrer at 400 rpm. Separately, 10 g of the monomer, 1.0 mol %/mol of epoxy groups of the photoinitiator, and 2.5 g of a porogen or nitromethane for the monomer were mixed together using a Vortex agitator until (29) Crivello, J. V.; Lee, J. L. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 3951-3568. (30) Crivello, J. V.; Kong, S. Macromolecules 2000, 33, 825-832.

Epoxy-Functional Microspheres

Figure 1. Schematic diagram of the photochemical apparatus for the synthesis of microspheres. homogeneous. The ultrasonicator wand was placed into the reaction vessel so that the tip was 2 cm above the bottom. The ultrasonicator was switched on at 70% power, and the monomer solution was slowly added to the vessel. Agitation of the solution was continued for 20 min, and then the ultrasonicator was switched off while continuing mechanical agitation. The shutter was opened, and the sample was irradiated for 5 min. Finally, the shutter was closed and 0.25 mL of triethylamine was added to terminate the polymerization reaction. A translucent suspension mixture was obtained and transferred to 50 mL centrifuge tubes and spun at 10 000 rpm for 10 min in a Beckman model J2-21 centrifuge (BeckmanCoulter Corp., Fullerton, CA). The clear supernatant layer was decanted from the settled polymer particles, and 20 mL of dionized water was added and the particles redispersed using a Vortex stirrer. Following centrifugation, the process was repeated using first acetone and then methyl ethyl ketone to wash the microspheres. After the final centrifugation, the product was transferred to a beaker and allowed to air-dry. The microspheres were further dried overnight at 25 °C under vacuum. Monomer Suspensions in Oil. The photoinitiator (1.0 mol %/mol of epoxy groups) was dissolved in 10 g of the monomer with the aid of a Vortex mixer, and if used, 25 wt % of a solvent or porogen was also added. Mineral oil, silicone oil, or ntetradecane (200 mL) used as the suspending medium was added to the quartz reaction vessel. A 6 cm diameter Cowles stirrer was employed to mix the suspension. The monomer solution was added to the reaction vessel, and the suspension was stirred at 1500 rpm for 20 min; then while still under agitation, the reaction vessel was exposed to UV irradiation (5000 mJ/(cm2‚min)) for 5 min. Triethylamine (0.5 mL) was added to terminate the polymerization reaction. The reaction mixture was transferred into four 50 mL centrifuge tubes and spun in a Beckman model J2-21 centrifuge (Beckman-Coulter Corp., Fullerton, CA) at 10 000 rpm for 10 min during which time the polymeric microspheres settled. After the suspending medium was decanted off, the solid microspheres were redispersed in 30 mL of n-hexane to remove residual suspending medium and then centrifuged once again. This process was repeated until the suspending medium was removed, and then the material was washed and centrifuged once with 30 mL of acetone to remove unpolymerized monomer. After the final centrifugation, the supernatant was decanted and the microspheres were transferred to a beaker and allowed to dry at room temperature overnight in air and then further dried at room temperature under high vacuum for an additional 24 h. Epoxy-functional microspheres were obtained in 93% yield.

Chem. Mater., Vol. 16, No. 24, 2004 5035 Microsphere Characterization. Particle size and distribution of the microspheres prepared during this investigation were determined using scanning electron microscopy (SEM) employing a JEOL JSM-840 microscope and by field emission scanning electron microscopy (FESEM) with a JEOL JSM6330F (JEOL Corp., Peabody, MA) microscope. Samples were vacuum sputter coated with gold for 30 s at 25 mA under argon using a Technics Corp. Hummer V platinum plate coater (Alexandria, VA). A minimum of 50 measurements of particle diameters of randomly selected microspheres was taken from each SEM image. The data were averaged, and the percent deviation was determined. Various groups have used this technique for determining particle size.31 The epoxy content of the microspheres was determined by titration using a modification of the standard HCl-dioxane method.32 The following three solutions were prepared prior to the titration. A 0.2 N solution of hydrochloric acid in dioxane was prepared by pipetting 1.6 mL of concentrated (36%) hydrochloric acid into 100 mL of dioxane. A cresol red indicator solution was prepared by adding 0.1 g of cresol red sodium salt to a 100 mL solution of 50 vol % ethanol and water. A 0.1 M sodium hydroxide solution was prepared by dissolving 4.0 g of sodium hydroxide in 1 L of methanol. The titration was performed as follows. A 1.0 g portion of the polymeric microspheres was added to a 250 mL Erlenmeyer flask. Then, 25 mL of the HCl-dioxane solution was transferred using a pipet into the flask. The suspension was allowed to stand for 15 min and swirled occasionally. Ethanol (25 mL) and 5 mL of the indicator solution were added to the flask. The red colored solution was tritrated with the standard sodium hydroxide solution until the suspension turned bright yellow. A blank was run in the absence of the microspheres to determine the normality of the HCl-dioxane solution. As a control, a separate titration was performed on the microspheres in which HCl solution was omitted. It was confirmed that the microspheres isolated from the reaction mixtures as described above were neutral. This method has an estimated sensitivity of (5 mequiv/100 g. Calculations:

R-epoxy equiv/100 g )

(B - S)N 10M

Here B ) volume of sodium hydroxide solution, mL, used in titrating the blank, S ) volume of sodium hydroxide solution, mL, used in titrating the microspheres, N ) normality of sodium hydroxide solution, and M ) mass of sample, g. FT-IR spectra were run on a Midac M Series FT-IR spectrometer MIDAC Corp., Costa Mesa, CA) using the KBr pellet technique. Pyrolysis of Microspheres. Pyrolysis and weight loss measurements on microspheres were carried out using a Perkin-Elmer Series 7 thermal analyzer (Perkin-Elmer Corp., Stamford, CT) equipped with a TGA-7 module. TGA runs were conducted in air at a heating rate of 20 °C/min. Pyrolysis samples prepared for elemental analysis and SEM were held at 700 °C for an additional 10 min. Sodium Azide Modification of Epoxy-Functional Microspheres. Epoxy-functional microspheres (2.0 g) were added to a 250 mL three-necked flask equipped with a condenser and a magnetic stirrer, and 1.9 g (0.03 mol) of sodium azide, 25 mL of acetone, and 25 mL of deionized distilled water were added. The reaction mixture was placed in an oil bath and brought to reflux and held at this temperature for 23 h. Following filtration, the spheres were washed first with water and then acetone and finally dried overnight under high vacuum at room temperature. (31) Noda, I.; Kamoto, T.; Yamada, M. Chem. Mater. 2000, 12, 1708-1714. (32) Weiss, F. T. Determination of Organic Compounds: Methods and Procedures; Wiley-Interscience: New York, 1970; p 207.

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Chem. Mater., Vol. 16, No. 24, 2004 Scheme 1

Results and Discussion General Considerations. As stated previously in the Introduction, the intent of this work was to investigate the possibility of preparing microspheres bearing reactive epoxy-functional groups using photoinitiated cationic suspension polymerizations. Such an approach would permit the synthesis of functional microspheres without contamination by large quantities of dispersants or suspending agents. In addition, particle size and distribution can be readily controlled by manipulation of such parameters as the intensity of stirring and the viscosities of both the monomer and suspending medium. The general concept is presented in Scheme 1. To accomplish this goal, we wished to proceed initially by dispersing a multifunctional epoxy monomer in a liquid suspending medium and then initiating the cationic ring-opening epoxide polymerization using UV irradiation and a monomer-soluble onium salt photoinitiator. There are several constraints placed on both the monomer and the suspending medium that must be considered at the outset if the stated goals are to be successfully met. First, both the monomer and the suspending medium should be nonvolatile, have no absorption in the UV spectral range, and be mutually incompatible. Solid monomers and high-viscosity oligomers can be used provided a solvent that is insoluble in the suspending media is employed. Since the particle size in a suspension polymerization is determined by the difference shear characteristics generated by stirring at the interface between the liquid monomer and the suspending medium, it would be desirable to be able to control the viscosities of both of these two components. The photoinitiator should optimally be soluble only in the monomer and completely insoluble in the suspending medium. There are several additional constraints placed on the system by the cationic nature of the polymerization reaction. For example, cationic epoxide ring-opening polymerizations are inhibited by bases and undergo highly efficient chain transfer reactions with water and alcohols. Initially, we sought to avoid both monomers and suspending agents that were problematic with respect to both of these considerations. Optimally, the monomer should be as highly functional as possible to maximize the number of epoxy groups remaining at the surface of the microsphere. It has been observed33 that the greater the functionality of a monomer, the lower the conversion that is obtained in a photopolymerization. This is due to the rapid immobilization of functional groups within the bulk and on the surface of the cross-linked matrix that is formed in such systems that renders these groups inaccessible for further polymerization. Last, we desired monomers with high reactivities for these suspension photopolymerizations (33) Moore, J. E. In UV Curing: Science and Technology; Technology Marketing Corp.: Stamford, CT, 1978.

Falk and Crivello

that would convert liquid monomer droplets to solid microspheres as rapidly as possible. This would minimize the coalescence and aggregation of partially polymerized microspheres during the suspension photopolymerization reaction. In a previous publication from this laboratory, we reported34 that among the most reactive cationically polymerizable epoxy monomers were those bearing the highly strained epoxycyclohexane ring and containing no other nucleophilic functional groups such as ethers, esters, amides, urethanes, or sulfides. One class of monomers that fulfills the above-stated requirements are epoxy-functional siloxanes. It has been reported that monomers such as 1,3-bis(2-(3,4-epoxycyclohexyl)ethyl)-1,1,3,3-tetramethyldisiloxane (PC1000) exhibit outstanding reactivity in photoinitiated cationic polymerizations using onium salts as photoinitiators.35 PC1000 is easily prepared as shown in eq 2 by the direct double rhodium-catalyzed hydrosilation of 1,1,3,3-tetramethyldisiloxane with 4-vinyl-1,2-epoxycyclohexane. Of particular interest to this research are a new class of epoxycyclohexyl functional oligomers that can be prepared as shown in eq 3 by the sol-gel reaction using 3,4-epoxycyclohexylethyltrimethoxysilane.36 The struc-

ture shown is idealized. The 1H and 13C NMR of this monomer/oligomer show that it contains linear, cyclic, and cage siloxane units in the backbone. PC2003 is a commercial product based on this chemistry that has an average of nine epoxycyclohexyl groups/molecule. (34) Crivello, J. V.; Linzer, V. Polimery (Warsaw) 1998, 43, 661672. (35) Crivello, J. V.; Lee, J. L. Polym. Mater. Sci. Eng. 1989, 60, 217221. (36) Crivello, J. V.; Mao, Z. Chem. Mater. 1997, 9, 1562-1569.

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Figure 2. Epoxy-functional polymeric microspheres produced by photopolymerizing a 1:3:1 mixture of PC2003-PC1000nitromethane in a mineral oil suspension using 1.0 mol % IOC8.

This monomer/oligomer was employed extensively in the investigation reported in this article. During the course of this research, we have used as cationic photoinitiators diaryliodonium and dialkylphenacylsulfonium salts (onium salts) with structures depicted as follows:

In all cases, these onium salts bear long-chain alkyl or alkoxy groups that impart lipophilic character permitting these photoinitiators to be readily soluble in the above monomers yet remain insoluble in the suspending medium. Monomer in Oil Suspension. Mineral oil, n-tetradecane, and silicone oil are ideal suspending media for photoinitiated cationic ring-opening epoxide polymerizations. They are high-boiling, inert liquids that exhibit no appreciable UV absorption in the range from 190 to 400 nm. Moreover, these suspending agents are unreactive in the cationic epoxide ring-opening polymerization chemistry that we wished to employ. Various mineral and silicone oils with different viscosities can be obtained or can be modified by admixture with n-tetradecane to attain a desired viscosity. Epoxysilicone oligomer PC2003 and the onium salt photoinitiators depicted above are insoluble in these hydrophobic nonpolar liquids. PC2003 has a viscosity of 27620 cP, as determined by a Brinkmann cone and plate viscometer (Brinkmann Instruments, Westbury, NY). The high viscosity of this oligomer makes it difficult to suspend and disperse into small droplets. For this reason, PC1000 with a viscosity of 60 cP was added to produce mixtures that can be readily dispersed in mineral oil. By suspending this monomer mixture with an appropriate photoinitiator in mineral or silicone oil, functional polymeric microspheres can be obtained using UV light to initiate polymerization. Figure 2 shows a typical SEM micrograph of microspheres obtained using this technique employing the diaryliodonium salt, IOC8, as the

Figure 3. Study of the effect of the viscosity of PC2003nitromethane solutions on the size of the resulting microspheres using 2 mol % IOC8. Viscocity measurements were taken at 30 °C using a Cannon Ubbelohde viscometer.

photoinitiator and mineral oil as a suspending medium. No surfactants or emulsifiers were added. The resulting product consisted of a large particle size distribution of solid, smooth surface microspheres with diameters from 30 µm to 30 nm. Since, in the above experiment, no surfactants or emulsifiers and a high shear mechanical stirrer were employed, the viscosities of the monomer solution and suspending medium play a major role in determining the final particle size. As mentioned previously, the viscosity of PC2003 can be adjusted through the addition of an appropriate solvent. Nitromethane is ideal for this purpose since it has a very low viscosity, is insoluble in mineral oil, and also does not interfere with the cationic ring-opening polymerization. Furthermore, it has a low UV absorption coefficient in the spectral region of interest (250-400 nm).37 Shown in Figure 3 is a plot of the average particle diameter of the microspheres obtained plotted as a function of the amount of nitromethane added to reduce the viscosity of PC2003. As the concentration of nitromethane is increased (monomer/oligomer viscosity decreases), the average particle size decreases. When the nitromethane concentration was above 25%, accurate measurements of the viscosity of the monomer solutions could not be obtained due to limitations of the viscometer. The viscosity of the suspending medium plays an important role in stabilizing the monomer droplets sizes before and during photopolymerization. As mixing takes place, monomer droplets are continuously formed by the shearing action of the stirrer or ultrasonicator while, at the same time, others collide and coalesce to form larger droplets. As the viscosity of the suspending medium increases, the number of collisions decreases and the average particle size decreases. Table 1 shows the effect of viscosity on the diameters of the microspheres polymerized using various silicone (poly(dimethylsiloxane)) oils as suspending media. PC2003 and PC1000 were copolymerized using a diaryliodonium salt photoinitiator. A high shear Cowles stirrer was employed to mix the system. As the viscosity of the suspending medium was decreased, the average particle size increased. When silicone oil with a viscosity of 5 (37) Pretsch, E.; Seibl, J.; Simon, W.; Clerc, T. Tables of Spectral Data for Structure Determination of Organic Compounds; SpringerVerlag: Berlin, 1983.

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Table 1. Dependence of Microsphere Diameter on Suspending MediumViscositya viscocity (cP)b 1000 500 100 50 5

particle diameter (µm)

variation (%)

18 27 115 129 no microspheres formed

35 28 29 31

a Nonaqueous suspension photopolymerization of 75 wt % PC2003 and 25 wt % PC1000 with 1.0 mol % IOC8 and 15 min UV irradiation at 2300 mJ/cm2 min. b Silicone oil, stirring speed 1000 rpm.

Figure 4. Effect of the size of the monomer (4:1 mixture of PC2003 and nitromethane) charge on the morphology of the resulting microspheres (1.0 mol % IOC15 as photoinitiator): (a) 25 g‚L-1; (b) 50 g‚L-1; (c) 100 g‚L-1; (d) 250 g‚L-1.

cP was used, the suspension was not stable and a large amount of the liquid monomer droplets coalesced and polymerized onto the surface of the quartz reaction vessel. The ability of the suspending medium to stabilize the monomer droplets also depends on the size of monomer charge used in the photopolymerization. To examine this effect, varying amounts of PC2003 were photopolymerized using the lipophilic diaryliodonium photoinitiator IOC15. Shown in Figure 4 are the results obtained as the monomer concentration in mineral oil was increased from 25 to 250 g‚L-1. The particle size and distribution were similar for the two samples obtained utilizing low concentrations of monomer. As the concentration was increased to 125 g‚L-1, the resulting microspheres show increasing evidence of agglomeration during photopolymerization resulting in irregular fused masses of microspheres. This is due to an increase in the rate of collisions of coexisting monomer droplets and partially or fully polymerized microspheres. As the monomer concentration increases, the number of collisions of the liquid monomer droplets also increase. The overall results are the average particle size increases and there is an increased likelihood of the formation of irregular shaped fused particles. Monomer in Water Suspension. Conventional cationic vinyl and heterocyclic ring-opening polymerizations are well-known to be sensitive to water and other hydroxyl-containing agents. Similar effects has been observed with the corresponding cationic photopolymerizations. During such epoxide ring-opening polymeriza-

Figure 5. Effect of ultrasonication time on the diameter and yield of the microspheres in the aqueous suspension photolymerization of PC2003 with 25 wt % decahydronaphthalene as the porogen and 1.0 mol % IOC15 as the photoinitiator.

tions, water and other hydroxyl-containing agents undergo both chain transfer reactions as well as simple addition reactions to generate glycols. The presence of large amounts of water cause a marked inhibition of polymerization. Surprisingly, we have found that it is possible to conduct the synthesis of epoxy-functional microspheres by cationic photopolymerization under aqueous suspension conditions using PC2003 as the substrate. The results can be rationalized by noting that PC2003 is highly hydrophobic and that the amount of water dissolved in a monomer droplet is likely to be small. In addition, since PC2003 is also highly functional (∼9 epoxy units/molecule), efficient cross-linking can readily occur within a monomer droplet even in the presence of small amounts of water. However, since the surface of the monomer droplet is in contact with water, chain transfer and addition reactions as well as simple inhibition predominate over polymerization at this interface. As the diameter of the monomer droplet decreases, the ratio of surface area to volume dramatically increases. For this reason, the above-mentioned side reactions occurring at the surface become increasingly significant as the droplet size decreases. If the average diameters of the droplets fall bellow approximately 1 µm, the side reactions dominate over propagation in the bulk of the droplets and solid particles do not form. As a result, unlike the monomer in oil suspensions, the microsphere size distributions are narrower when the suspension photopolymerizations are conducted in water. In addition, only relatively large particles (5-35 µm) are formed under aqueous suspension photopolymerization conditions. To examine this effect further, the particle size was altered by varying the sonication time, shown in Figure 5. As the mixing time is increased, the average diameter of the isolated polymer particles decreases from 30 µm to a limit of 5 µm and the yield of microspheres also decreases from 96% to 19%. Suspension cationic photopolymerizations conducted in an aqueous environment require a stabilizer to obtain spherical microspheres. Poly(vinyl alcohol) (PVA, 85% hydrolyzed poly(vinyl acetate)) was chosen because of its known ability to stabilize oil in water suspensions by reducing the surface tension of the droplets.38 PVA is not soluble in either PC2003 or PC1000. When this stabilizer is absent, most of the suspended monomer adheres to and polymerizes on the sides of the reaction (38) Mendizabal, E.; Castellanos-Ortega, J. R.; Puig, J. E. Colloids Surf., A 1992, 63, 209-217.

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Table 2. Effect of Photoinitiator on Microsphere Size, Yield, and Epoxy Contenta

photoiniator IOC15 SbF6 IOC18 SbF6 C4C18 DPS SbF6 C1C18 DPS PF6

particle diameter (µm) 11 9 14

variation (%)

yield (%)

epoxy content (mequiv/100 g)

67 52 58 47 59 56 no particles formed

185 193 84

a Aqueous suspension photopolymerization of 75% PC2003, 25% decahydronaphthylene, and 2.0 mol % photoinitiator, at 5000 mJ/ cm2 min for 5 min.

vessel and individual spherical particles are not formed but rather large aggregates of cross-linked material are obtained. Table 2 shows a study of the affect of the PVA concentration on the particle size, size distribution, and epoxy content of the resulting microspheres. A minimum concentration of 0.05 g/L of PVA is required to stabilize the system sufficiently to avoid immediate fouling of the reaction vessel. A further increase in the concentration of PVA does not appear to have an appreciable affect on the system. Effect of Porogens. Pore-forming agents called porogens are often added to free radical microsphere-forming polymerizations to increase the overall surface area by producing macropores (pores greater than 100 nm) within the bulk of the polymer particles. It was of interest to attempt to discover whether it might be possible to identify porogens that could be used for this purpose in the present cationic suspension photopolymerizations. Accordingly, the mechanism by which porogens function to produce pores must be considered. One possible mechanism is that a porogen that is initially soluble in the monomer may undergo phase separation during polymerization to form microdomains within the bulk of the polymer particle. Upon removal of the porogen after the cross-linking polymerization, voids are produced throughout the microspheres. If this mechanism is operative, the behavior of a given solvent as a porogen should exhibit a dependency on its solubility parameter. Further, it was expected that Θ solvents would be the most effective porogens. Shown in Figure 6 are the SEM micrographs of microspheres obtained using four different potential porogens. As the polarity of the porogen is decreased from chloroform (19.0 MPa1/2) to cyclohexane (16.6 MPa1/2), the pore structure changes in the resulting microspheres. Chloroform does not generate macropores and nanopores with diameters less than 50 nm cannot be imaged by SEM. When decahydronaphalene (18.0 MPa1/2) or bicyclohexyl (17.4 MPa1/2) is used, there is clearly evidence of the presence of macropores. Cyclohexane gives nonspherical crosslinked particles on photopolymerization. The loss in the control of the macrostructure of the particles in such a nonpolar porogen is not understood at this time. The effect of the solvent polarity on porosity in polymer microspheres by the free radical copolymerization of styrene and divinylbenzene has been extensively examined.39,40 Similar results were obtained. Whereas Θ solvents produced macropores, good solvents produced nanopores. Figure 7 shows fractured and intact epoxy (39) Cheng, C. M.; Vanderhoff, J. W.; El-Aasser, M. S. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 245-256. (40) Guyot, A.; Bartholin, M. Prog. Polym. Sci. 1982, 8, 277-332.

Figure 6. Effect of porogen type on the aqueous suspension photopolymerization of PC2003 (25 wt % porogen with 1 mol % IOC15 as photoinitiator): (a) cyclohexane; (b) bicyclohexyl; (c) decahydronaphthalene; (d) chloroform.

Figure 7. Left: A high-magnification image of a porous microsphere produced by aqueous suspension polymerization of PC2003 using 25% decahydronaphthalene as a porogen. Right: A SEM image of a fractured microsphere. Table 3. Influence of PVA Concentration on Microsphere Formationa PVA concn (g/L) 0.20 0.05 0.02 0

particle diameter (µm) 4 4 8

variation (%)

yield (%)

epoxy content (mequiv/100 g)

84 21 62 17 75 29 no particles formed

252 253 193

a Aqueous suspension photopolymerization of 10 g of PC2003, 2.5 g of decahydronaphthylene, and 1.0 mol % IOC15 under UV irradiation for 5 min at 5500 mJ/cm2 min.

microspheres produced using decahydronaphthylene as a porogen. The presence of large and small pores is clearly evident in the micrographs. Upon closer inspection, the pores appear to vary widely in size, to be randomly distributed throughout the particle, and to be partially interconnected. Effect of Photoinitator Type. Diaryliodonium salts IOC15 and IOC18 and S,S-dialkyl-S-phenacylsulfonium salt C4C18-DPS were employed as photoinitiators for the polymerization of aqueous suspensions PC2003 with decahydronaphthylene as a porogen, and the results are depicted in Table 3. The yields, particle sizes, and size distributions are similar for microspheres produced using all three photoinitiators. When the anions of these photoinitiators are changed from SbF6- to the more nucleophilic PF6- anion, microsphere formation was not observed.

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Figure 8. Effect of irradiation time on the size (b) and epoxy content (9) of microspheres prepared by suspension photopolymerization of PC2003 in mineral oil with 25 wt % nitromethane and 1.0 mol % IOC15 as photoinitator.

As can be also noted in Table 3, there is a marked difference between the epoxy contents of microbeads produced by diaryliodonium salt photoinitiators and that derived when the S,S-dialkyl-S-phenacylsulfonium salt photoinitiator is used. Microspheres with considerably lower epoxide contents were produced using C4C18DPS. A possible explanation for this observation is as follows. Since the microspheres are highly cross-linked, they do not swell to an appreciable extent in common solvents. Therefore, the epoxy groups available for titration or further chemical modification are located primarily at the surfaces of the microspheres. This implies that the initiation and polymer-forming crosslinking reactions occur mainly in the bulk of the microsphere. The epoxy functional groups at the surface do not polymerize efficiently because they are isolated from the active species due to steric effects and because polymerization is inhibited at the aqueous interface of the monomer droplet. In addition, the photoinitiator is not appreciably soluble in the suspending medium and is concentrated in the bulk of the monomer droplet. Porous microspheres usually have higher amounts of available epoxy groups than nonporous microspheres simply because they possess higher surface areas. However, if the photoinitiator is soluble in the porogen, polymerization can occur at the interface formed by phase separation between the porogen droplet and the polymerizing monomer. This will be overall detrimental to the objective of obtaining highly functional epoxy functional microspheres. This is the case with C4C18DPS, which is readily soluble in decahydronaphthalene while the iodonium salt photoinitiators IOC15 and IOC18 are insoluble. It follows, therefore, that to obtain the highest epoxy functional microspheres, the photoinitiators should be insoluble in the suspending medium as well as in the porogen. Influence of Irradiation Time. The length of the UV irradiation time has a marked effect on the epoxy content of the resulting microspheres. Figure 8 shows the results obtained when the polymerization of an aqueous suspension of PC2003 with 1.0 mol % IOC15 is carried out using different UV irradiation times. As the irradiation time is increased, microspheres with a larger average particle size and a correspondingly smaller epoxy contents are obtained. The increase in particle size is attributed to coalescence between polymerized particles with monomer droplets followed by further photopolymerization. The decrease in the epoxy content is due to a corresponding decrease in the surface area that occurs as a result of the increased particle

Figure 9. Comparison of the FT-IR spectra (KBr disk) of asprepared and sodium azide modified microspheres prepared by suspension photopolymerization of PC2003 with 25 wt % nitromethane in mineral oil.

diameter. The yield of microspheres also increases with irradiation time. A 5-min irradiation time produces microspheres in 22% yield while a 10-min irradiation time gives a 60% yield. An additional increase in the irradiation time does not further improve the yield. After 10 min of UV irradiation at 0.90 mW/cm2, all the diaryliodonium salt photoinitiator is decomposed, and extending the irradiation time does not produce additional active initiating species. Chemical Derivatization of Microspheres. Fourier transform infrared spectroscopy (FT-IR) was used to confirm the presence of epoxy groups in the microspheres. Two IR spectra of microspheres prepared by suspension photopolymerization of PC2003 with 25 wt % nitromethane in mineral oil are shown in Figure 9. The first spectrum was obtained from microspheres as isolated from the reaction vessel. The band at 800 cm-1 is indicative of the presence of cycloaliphatic epoxy groups.41 The microspheres were further derivitized by a ring-opening reaction with aqueous sodium azide as shown in eq 4 to produce microspheres bearing both azide and hydroxy functional groups.42

Evidence for this reaction is found in the second FTIR spectrum shown in Figure 9. After the sodium azide addition, the band at 800 cm-1 decreases while the strong, sharp absorption band at 2095 cm-1 due to the azide group appears. Pyrolysis. Since the monomers and oligomers used in this work to prepare polymeric microspheres contain a high percentage of silicon, it was of interest to determine whether these materials could be used as ceramic precursor templates for the production of silica microspheres. Accordingly, porous microspheres prepared by photopolymerization of PC2003 using IOC8 were pyrolyzed by heating from 40 to 700 °C at a rate of 20 °C/min in air. The thermogravimetric analysis (41) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press, Inc.: San Diego, CA, 1991. (42) Christoffers, J.; Schulze, Y.; Pickardt, J. Tetrahedron 2001, 57, 1765-1769.

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curred, the detailed macro- and microstructures of the microspheres are well preserved. We conclude that this procedure can be used to synthesize porous silica microspheres that may have a variety of interesting applications. Conclusions

Figure 10. TGA of epoxy-functional microspheres prepared by aqueous suspension utilizing PC2003 as the monomer, 25 wt % decahydronaphthalene, and 1.0 mol % IOC15 as the photoinitator (heating rate 20 °C/min in air; temperature held at 700 °C for 10 min).

Figure 11. Left: Microspheres obtained by the aqueous suspension technique using PC2003 as the monomer with 1.0 mol % IOC15 as the photoinitiator and 25 wt % decahydronaphthalene as the porogen. Right: Microspheres after pyrolysis at 700 °C in air for 10 min.

curve (TGA) given in Figure 10 shows the onset of a major weight loss at approximately 350 °C with an overall weight loss of 60% at 700 °C. This weight loss corresponds to the loss of virtually all the carbon residues present in the polymer. Elemental analysis confirms this with the carbon content before pyrolysis of 49.54% falling to 3.89% after pyrolysis. Figure 11 shows the SEM images of the microspheres before and after pyrolysis. Although a large weight loss has oc-

Suspension photopolymerizations of multifunctional epoxysilicone monomers using onium salt photoinitiators can be carried out in both aqueous and nonaqueous media to prepare epoxy-functional microspheres using a rapid, simple one-step process. Particle diameters obtained in the nonaqueous systems range from