Ind. Eng. Chem. Res. 1992, 31,1414-1417
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Kubota, B. Decomposition of higher oxides of chromium under various pressures of oxygen. J. Am. Ceram. SOC. 1961, 44,
Tombs, N. C.; Croft, W. J.; Carter, J. R.; Fitzgerald, J. F. A new polymorph of CrOOH. Inorg. Chem. 1964,3, 1791-1795.
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Oppegard, A. L. U.S.Patent 2,885,365, 1959. Swoboda, T. J.; Arthur, P.; Cox, N. L.; Ingraham,J. N.; Oppegard, A. L.; Sadler, M. S. Synthesis and properties of ferromagnetic chrome oxide. J . Appl. Phys. 1961,32, 347s-3755.
Received for review July 19, 1991 Accepted January 22, 1992
RESEARCH NOTES Microcellular Materials via Polymerization in Supercritical Fluids A process is demonstrated whereby microcellular polymer foams can be obtained by polymerization directly in a near-critical diluent. Critical point drying can then be effected in the reactor vessel in a relatively efficient manner. T h e key to the process is the choice of diluent and matching it to the polymer system to be gelled and dried. Propane and Freon-22 were studied as diluents with the polymer system of poly(methy1 methacrylate-co-ethyleneglycol-dimethacrylate). Freon-22 proved to be the superior choice. Morphology and density of the resulting materials were comparable to microcellular foams prepared by a more conventional carbon dioxide wash and dry approach.
Introduction A relatively little-known class of materials called aerogels has recently been the focus of several studies by specialists in physics and chemical engineering. The distinctive feature of these materials is that they combine extremely small pore sizes with remarkably low density, leading to some unique properties. For example, silica aerogel can have a density of 0.05 g/cm3 and BET surface areas of lo00 m2/g while maintaining pore sizes small enough that the bulk material is completely transparent; the pores are too small to refract visible light. Many applications can be envisioned for such materials, including thermally insulating windows, catalyst supports, membranes, and timed release supports (Aubert and Sylwester, 1991). But of course, there are some technical obstacles. Most notably, the silica aerogels are extremely fragile. This has led to attempts to develop organic aerogels (Pekala and Stone, 1988), also known as microcellular foams (Aubert and Sylwester, 1991; Elliott et al., 1991). Organic chemistry offers a broad range of possible molecular designs and polymerization mechanisms which provide alternatives to the silica aerogel process. By combining an understanding of the fundamental processes occurring during the formation of aerogels with the ability to design and synthesize monomers tailored to a specific function, we hope to arrive at a systematic approach for developing robust materials with unique properties. The key to making a microcellular foam is to focus on the mode of phase separation. The phase separation in conventional foaming occurs when the bubble forms and inflates in a manner that is difficult to control. Forming a microcellular foam requires gaining much greater control over the phase separation process (Young, 1987). The method by which the lowest densities and smallest pore sizes have been obtained is reaction induced phase separation with critical point drying (Henning and Svensson, 1981; Pekala and Stone, 1988). Critical point drying is the process of exchanging a normal liquid diluent with a near-critical liquid diluent under preasure and then raising the temperature above the critical temperature while maintaining a pressure greater than the critical pressure. 0888-5885/92/2631-1414$03.00/0
The pressure may then be reduced without ever having experienced a liquid-vapor phase transition which would exert destructive capillary forces on the microcellular material. Critical point drying was first applied to foam drying by Kistler (1932) in hie preparation of aerogels, and the work of Pekala and Stone has adapted Kistler’s approach to an organic resorcinal-formaldehyde foam. Recent work by Elliott et al. (1991) demonstrated the effects of diluent, cross linking, and critical point drying on formation of organic aerogels from methacrylates. As an extension of the work by Elliott et al. (1991), we have recently been pursuing direct polymerization in a near-critical solvent. In this way, the polymerization, washing, and drying is converted into a polymerization and drying process in a single vessel. Previous attempts to apply a similar process to resorcinol-formaldehyde aerogels resulted in substantial changes in the polymer product (Hair et al., 1988). The strong solvent power of supercritical fluids (SCFs) toward polymers has been studied by several researchers (cf. Scholsky et al. (1986)). However, high concentrations of polymer were treated in only a few of these references, and the role of cross linking was evidently not given direct consideration. Polymerization in supercritical fluids has been reviewed by Scholsky (1990), but the polymers obtained have generally been variants on the original high pressure polyethylene process or the polymers have been of relatively low molecular weight (4000 or less) (Saraf and Kiran, 1990; Kumar and Suter, 1987). As for cross linking during polymerization, this has apparently led to precipitation of the polymer from the SCF phase (Hartman and Denzinger, 1987). The system which we have selected is copolymerization of methyl methacrylate (MMA) with ethylene glycol-dimethacrylate (EGDMA) diluted to roughly 50 vol % with either propane or Freon-22 as the near-critical diluent. We selected this system because poly(methy1 methacrylate) (PMMA) has been a popular polymer for many years and much is known about its polymerization process in conventional settings and the mechanism of the reaction is relatively simple and insensitive to small changes in conditions and concentrations. Furthermore, this is the same 0 1992 American Chemical Society
Ind. Eng. Chem. Rea., Vol. 31, No.5,1992 1416 system studied by Elliott et al. (1991) except for the choice of diluent. Elliott et al. performed their polymerizations at low pressure in several diluents ranging in solvent strength, and superdical drying was effected by w a s h i i in liquid C02 at 14 "C for 4 8 7 2 h, then heating under pressure to 45 "C, and finally dropping the pressure. The current study merely replaced the diluenta with compounds having critical temperatures near 97 OC. S i c e this polymerization system and drying procedure was so similar to the hditional critical point drying method, the findings of the current study focus clearly on the implications of using a near-critical solvent as the diluent. Experimental Section The methacrylate polymerizations which we pursue are free-radical polymerizations. A feature of this system is the substanti cross linking via the addition of ethylene glycol-dimethacrylate (EGDMA). Elliott et al. (1991) found that the density of the final product decreased monotonically as the percentage EGDMA increased to 40 vol %. EGDMA is difunctional, and it was selected for our system because its reactivity ratio in relation to MMA is near unity (Flory, 1943). Materials. The monomers methyl methacrylate (MMA) and ethylene glycol-diimethacrylate (EGDMA) were purchased from Aldrich Chemical Co. The monomers were first washed with 10% (w/w) sodium hydroxide solution to remove the free radical inhibitor. Then the inhibitor-free monomers were washed with distilled and deionized water to remove the sodium hydroxide. The residual water in the monomers was removed by adding a small amount of magnesium sulfate (Fisher Scientific). After 12-15 h, the monomers were filtered out. The pure and dry monomers were then used in the experiments. Freon-22 and propane were the two diluents that were used in the experiments. Freon-22 manufactured by Du Pont was obtained locally. Natural grade propane w s purchased from Lmde Specialty Gases. The diluents were used as received. tert-Butyl peroxypivalate (TBPP) was used as the initiator in all the experiments. This was obtained as 75% in mineral spirits from Pfaltz and Bauer Inc. Experimental Procedure. The monomer to diluent volume ratio was fixed at L1,but the ratio of MMA to EGDMA was varied. The initiator concentrationwas 0.1% of the monomer weight in all the experiments. All the polymerizations were conducted at 70 "C and loo0 psig. This is helow the critical temperature and above the critical pressure of either solvent. The polymerization time was set at five times the half-life period of the initiator. The half-life of TBPP at 70 OC is 1h and 40 min. The time was set to ensure that the reaction went to high conversions. Consequently, the polymerization was allowed to proceed for about 8 h before the conditions were changed for the drying step. The pure and dry monomers with the initiator were loaded into the pressure vessel fmt, and the diluent was added later. The experimental setup is shown in Figure 1. After the monomers were loaded, the pressure vessel (High Pressure Equipment Company, Erie, PA; Model HP 38223; 340.22 atm) was cooled to a temperature below the boiling point of the solvent at atmospheric pressure using dry ice. The boiling points of Freon-22 and propane are -40.8 and -42.1 O C , respectively. A measured amount of the solvent which was previously collected as a liquid in a beaker was then added to the monomer mixture. A plug was installed into the end cap of the pressure veasel and immediately tightened to seal that end of the apparatus. The vessel was then placed in an oven and connected to the hand pump (High Pressure Equip.
Q
v Figure 1. Experimental apparatus for near-critical polymerhtion.
men6 Model 67-65;340.22 atm) through a quick connect. The reactants were then pressured to loo0 psig quickly, and this was held constant as the temperature increased and remained steady at 70 O C . The polymerization was allowed to proceed for 8 h or more before it was heated for drying. For critical point drying, the temperature of the system was raised to 100 "C at the constant pressure of loo0 psig. This temperature is above the critical temperature of either solvent and below the glass transition temperature of PMMA which is 105 "C. Although the cross-linked material had different thermal properties from PMMA, the glass transition temperature was regarded as safe with respect to undesirable side effects. PMMA is known to depolymerizeat temperatures near 200 OC. The conditions were maintained at 100 "C and loo0 psig for about 6 h. The pressure was then g r a d d y reduced to atmospheric pressure at a temperature of 100 "C, by backing out the piston of the high pressure generator. The polymer was then removed after the apparatus was cooled to room temperature. Characterization. The polymer was characterized in terms of its density and morphology. For the density, the polymer was weighed and ita dimensions were measured. The apparent density was then calculated. The polymer morphology was characterized using scanning electron microacopy. The scanning electron microscope was a Jeol
USM-5. Results and Discussion The results of the experimemts are summarized in Table I and Figures 2-4. As we had hoped, our findings show a stmng correlationwith the findings of Elliott et al. (1991). Elliott et al. showed that the use of a nonpolar diluent with weak solvent power (*heptane) yielded a larger underlying structure in the polymer morphology, while a polar strong solvent (methyl isobutyl ketone) resulted in a smaller underlying structure. Figures 2a, 3a, and 4a compare the morphology of polymers prepared in propane to the morphology of equivalent polymers prepared in Freon-22 given in Figures 2b, 3b, and 4b. In each case the size of the primary particles which comprise the polymer prepared
1416 Ind. Eng. Chem.
Res., Vol. 31, No.5,1992
a
b
Figure 2. Scanning electron micrographs for 20% EGDMA + 80% MMA samples prepared with (a) propane as diluent and (b) freon as diluent.
Fmre 3. Scanning electron micrograph for 30% EGDMA + 70% MMA samples prepared with (a) propane as diluent and (b) freon aa diluent.
Table I. Results for Densities of Foams P ~ ~ e r by ed Polymerization in Near-Critical Propane (P) or Freon (F) with Direct Critical Point Drying sample % EGDMA diluent denaity/(g/cma) nsin 1 i.. n F 20 F 0.646
Recent light scattering studies have provided some insight into the manner in which the microcellular morphology is formed (Elliott and Cheung, 1991). These results show a rapid growth of polymeric colloids to a fairly monodisperse population of small primary particles which do not grow above a certain size. The number of these particles rapidly multiplies as the polymerization proceeds, but the particles maintain the same size. Eventually, the particles become so populous that they percolate and the solution gels. But there is no indication of flocculation or space-filling. A fmding which is munter to our previous reaults is that increasing cross liking does not always lead to a lower density. As shown in Table I, increased moss linking leads to lower density in the freon systems up to 40% EGDMA but higher densities result a t 60 and 80% EGDMA. A similar trend is evident in the propane systems, but the minimum density appears a t only 10% EGDMA. A poe sible explanation is that higher cma linkingleads to earlier phase separation and ‘squeezing” of diluent out of the polymer phase. The resulting primary particles would then be more prone to flocculation rather than interpenetration, and the density would increase. Similar to our previous results, the density reduction with paraffin was not an adequate characterization of the success of the process. This is because the SEMs showed that the morphology of material prepared with paraffms was basically equivalent to the morphology of material
30 40 60 80
90 10 11 12 13 14
10 20 30 40 60
F F F F F P P P P P
0.508
0.408 0.470 0.596 -1 0.378
0.100 0.753 1.017 1.200
in the more polar freon is smaller than the structure of the polymer prepared in nonpolar propane. The size of these primary particles controls the size of the open cells in a slightly indirect manner. Smaller primary particles permit smaller voids while the same density is maintained. It should be noted, however, that the smaller primary particle size does not directly imply smaller voids. If the primary particles were to flocculate into regions of high particle density, then either the void size or apparent density would increase. Therefore, small primary particles are a necessary hut not a sufficient condition in forming a microcellular foam. It is also necessary that these particles not form space-ffig structures.
Ind. Eng. Chem. Res., Vol. 31, No. 5,1992 1417 pursuing the supercritical drying in the w e v w l . While there may be some limitations owing to lower solvent density or chain transfer to solvent, these do not prevent us from obtaining foams. The primary limitations are more pertiient to choice of a diluent with a strong enough solvent power to stabilize the polymer matrix but with a low enough critical temperature to permit critical point drying without damage to the polymer matrix. We find that Freon-22 is sufficiently polar to act as a substitute for toluene in terms of its solvent power, while maintaining the low critical temperature necessary for direct drying. The establishment of a direct polymerization and drying methodology substantially improves the practicability of this approach. Registry No. Propane, 7498-6; Freon-22,75-45-6: (MMA)(EGDMA) (copolymer), 25777-71-3.
I
Literature Cited Auben, J. H.;Sylwestar, A. P. Microcellular Foams: What for? CHEMTECH IWI,21.234-238. Elliott, J. R, Jr.; Cheung, H. M. Light Scnttering Study of Polymer Network Formation in a Supercritical Diluent. Presented at the Fall Natianal Meeting of the AIChE. L a Angeles, CA, 1991; paper 20%.
Elliott, J. R, Jr.; Akhaury, R,Srinivasan, G.Microcellular Methacrylates: Effect of Supercritical Drying on Pore Size and Densitv. Palvm. Commun. 1991.31 (1). 11. FIoG P. J.-Prineiples of Polymer Chemistry; Cornell University Press: Ithaca, NY,1943; p 391. Hair,L. M.; Pekala, R. W.;Stone,R E.; Chen, C.; Buckley, S. R. Low-Denaity Resorcinol-FormaldehydeAerogels for Direct-Drive Laser Inertial Coflmement Fusion Targets. J. Vac. Sei. Technol. 1988,A6 (4),2559. Hartman, H.; Denzinger, W.German Patent 360982fial.1987. Hennine. S.:Svensson. L. Production of Silica Aeroeel. Phvs. Scr. 1981,-23,697. Kistler, S.5.Coherent Expanded Aerogels. J. Phys. Chem. 1932,36,
52.
3
Figure 4. Seaming electron micrographs for 4090 EGDMA + 60% MMA samples prepared with (a1 propane as diluent and (b) freon as diluent.
prepared by conventional methods. Only in the case of the polymerizations in freon were the cell sizes small enough to justify the supercritical drying process. As in the previous study, an advantage of the supercritical fluid process is that one can imagine adjusting conditions and concentrations in order to tailor the morphology to a specific application. The benefit of the current study is that the advantages of the SCF process can now be realized with little practical penalty in terms of changing vessels and exchanging diluent. Conclusions We have demonshated a proteas to obtain microcellular foams by polymerizing directly in a near-critical fluid and
Kumar, S.K.; Suter, U.W.Precipitation Polymerization of Styrene in Supercritical Ethane. Polym. Prepr. 1987,28,286. Pekala, R W.;Stone, R. E. Low Density Resorcinol-Formaldehyde Foams. Polym. Prepr. 1988,29,204. Saraf, V. P.; Kiran, E. Free Radical Polymerization of Styrene in Supercritical Fluids. Polym. Prepr. 1990,31,687. Scholsky, K. M. Supercritid Polymerization Reactions. Polym. Prepr. 1990,31,686. Scholsky, K. M.; OConnor, K. M.; Weisn, C. S.;Krukonis, V. J. Supercritical Fluid Processing of Synthetic Polymers. Polym. Prepr. 1986,27,140. Young, A. T. Polymer Solvent Phase Separation a8 a Route to Low Density, Microcellular Plantic Foams. J . Cell. Plast. 1987,23,55.
Gokul Srinivasan, J. Richard Elliott, Jr.' Department of Chemical Engineering The University of Akron Akron, Ohio 44325-3906 Received for review June 3, 1991 Revised manuscript received January 21,1992 Accepted February 14, 1992
