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Langmuir 1998, 14, 762-767
Effects of Precursor Composition on Pore Morphology for Thermally Polymerized Acrylic Acid/Methyl Methacrylate-Based Microemulsions Edward Wayne Davis, Ramachandra Mukkamala, and H. Michael Cheung* Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325-3906 Received February 14, 1997. In Final Form: November 14, 1997 Thermal polymerization has not been extensively studied as a technique for the polymerization of bicontinuous microemulsions. Other more rapid techniques have been employed such as visible- and UV-initiated polymerization. However, the industrial preparation of porous solids would be facilitated by the use of the cheaper and simpler thermal polymerization. The effects of changing composition of the precursor on the resulting matrix morphology for thermally polymerized microemulsions were studied. The factors examined were the surfactant loading of the aqueous phase, the aqueous to organic phase ratio, the monomer ratio of the organic phase, and the cross-linker content of the organic phase. SEM, drying rate studies, and freezing point depression measurements were used to determine matrix morphology. The surfactant loading of the aqueous phase and the cross-linker content of the organic phase were found to have no significant effect on the morphology while the phase ratio and monomer ratio were found to have significant effects on the resulting polymer matrix morphology.
Introduction The preparation of porous polymeric solids by the polymerization of monomer containing microemulsions has been explored.1,2 The relationship between the microemulsion precursor structure and the polymer morphology has been demonstrated.3-6 Surfactant mixtures and polymerizable surfactants have been examined.4,7 Various monomer species have been examined.8-11 The microstructure of the obtained porous polymers has been examined by thermal analysis methods.12 The use of these systems in the preparation of porous polymer membranes has been explored.13 Previous studies have focused on visible light initiation of the polymerization reaction instead of thermal polymerization. The industrial preparation of porous solids would be facilitated by the use of the cheaper and simpler thermal polymerization. However, thermal polymerization has been shown to be slower than visible-initiated polymerization.14 Experience in our laboratory has shown * To whom correspondence should be addressed. (1) Stoffer, J. O.; Bone, T. J. J. Dispersion Sci. Technol. 1980, 1, 393. (2) Menger, F. M.; Tsuno, T.; Hammond, G. S. J. Am. Chem. Soc. 1990, 112, 1263. (3) Palani Raj, W. R.; Sasthav, M.; Cheung, M. Polymer, 1995, 36 (13), 2637. (4) Palani Raj, W. R.; Sasthav, M.; Cheung, M. J. Appl. Polym. Sci. 1993, 47, 499. (5) Haque, E.; Qutubuddin, S. J. Polym. Sci., Polym. Lett. Ed. 1990, 26, 429. (6) Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 1378. (7) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C.; Pey, K. L. Langmuir 1996, 12, 319. (8) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C. Polymer 1995, 36 (10), 1941. (9) Gan, L. M.; Chieng, T. H.; Chew, C. H.; Ng, S. C. Langmuir 1994, 10 (11), 4022. (10) Palani Raj, W. R.; Sasthav, M.; Cheung, M. Langmuir 1991, 7, 2586. (11) Palani Raj, W. R.; Sasthav, M.; Cheung, M. Langmuir 1992, 8, 1931. (12) Sasthav, M.; Palani Raj, W. R.; Cheung, M. J. Colloid Interface Sci. 1992, 152 (2), 376. (13) Palani Raj, W. R.; Sasthav, M.; Cheung, M. Polymer 1993, 34 (15), 3305. (14) Palani Raj, R. W. Ph.D. Dissertation. The University of Akron, Akron, 1994, p 140.
that slower polymerization rates increase the likelihood of the polymer morphology being affected by phase separation. The objective of this study was to determine the effects of factors associated with the microemulsion precursor composition on the resulting polymeric solid morphology for thermally initiated polymerization. The simple system containing the surfactant sodium dodecyl sulfate (SDS), water, the hydrophilic monomer acrylic acid (AA), the hydrophobic monomer methyl methacrylate (MMA), and the cross-linking agent ethylene glycol dimethacrylate (EGDMA) was examined. Four factors were studied: the surfactant loading of the aqueous phase, the aqueous to organic phase ratio, the monomer ratio of the organic phase, and the cross-linker content of the organic phase. Experimental Section Materials. The monomers, AA and MMA, and cross-linking agent, EGDMA, were obtained from Aldrich. The purity of the monomers was greater than 99%, and the cross-linking agent had a purity greater than 95%. The inhibitor was not removed from the monomers prior to sample preparation. The surfactant SDS was obtained from Sigma and had a purity of 99%. The water used in this study was deionized. The thermal initiator used in this study was azobis(isobutyronitrile) (AIBN) and was obtained from Aldrich. Polymerization Procedure. Thermally initiated free radical polymerization was used to prepare the porous monoliths from microemulsion precursors. The precursors were prepared by weighing the components of the microemulsion including the initiator into clean dry 16 mm × 100 mm test tubes. After the appropriate amount of each component was added to each test tube, the tubes were capped and vortex-mixed for 20-30 s. The caps were then removed, and the samples were purged with dry nitrogen for 15 min at a flow rate of 0.02 scfh. The test tubes were again capped and vortex-mixed for 20-30 s. The samples were then placed in a water bath thermostated to 55 ( 1 °C. After polymerization, the samples were cured at room temperature for a period of 24 h. The test tubes were wrapped in paper towels and then broken. The samples were removed and prepared for further examination. Scanning Electron Microscopy. The most direct method for examination of a microporous system is to photograph the system under high magnification to obtain a visual record of the
S0743-7463(97)00153-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/28/1998
Effects of Precursor Composition on Pore Morphology morphological characteristics of the system. The vacuum required for scanning electron microscopy necessitates that samples be well-dried. Sample drying has been shown to result in pore collapse. This morphology change is probably due to the forces applied to the pore walls from the water-air surface tension. Therefore, the micrographs cannot be assumed to be exact representations of the water-loaded system. However, qualitative comparisons of SEM results are valuable as an indication of changes in morphology of samples in the hydrated state. For this study an ISI SX-40 scanning electron microscope (SEM) was used to generate the images of the dried polymer samples. Samples were prepared by drying at room temperature for 1 week. The dried polymer samples were then freeze-fractured by immersion in liquid nitrogen.15 A fragment of the sample was mounted and sputter-coated with silver using a Polaron E5400 sputter coater. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was used to distinguish between open and closed cell morphologies. The differences in the shape of the drying rate curves between open and closed cell systems are used to differentiate between the two porous systems.16-20 The drying rates were measured according to the method of Sasthav et al.12 A DuPont Instruments TGA 2950 thermogravimetric analyzer was utilized for these measurements. A sample of known dimensions was cut from the polymerized sample immediately after removal from the test tube and placed in the sample pan of the instrument. The weight of the sample was monitored over 240 min as the system dried at 70 °C under a stream of dry nitrogen gas. After 240 min, the drying temperature was increased to 100 °C at a ramp rate of 1 °C/min, and drying continued for 30 min. The sample weight was monitored and recorded as a function of time throughout the experiment, and the sample dimensions were measured after drying. The drying rate was based on the average of the initial and final sample dimensions. The drying rate was plotted as a function of free moisture content of the sample to yield the dying rate curve. Pore Size Distribution (FPD). The distribution of pore sizes within the polymer monoliths was determined from the freezing point depression (FPD) of the water trapped in the pores. This freezing point depression is the result of capillary effects.21-25 Measurements of the depression of the freezing point of water within the pores were made using a Dupont Instruments 910 differential scanning calorimeter (DSC) system. A sample of approximately 10-20 mg was placed in an aluminum sample pan and hermetically sealed. The sample was then stored in dry ice overnight to ensure that all the water was frozen. The sample pan was then placed in the DSC instrument and cooled to -40 °C. The sample was heated to -20 °C at a ramp rate of 1 °C/min and held a -20 °C for 5 min. Then the sample was then heated to 4 °C at a ramp rate of 0.25 °C/min. The heat flow to the sample was recorded over the length of the experiment. By integrating the endotherm, the pore size distribution was determined. (15) Sawyer, L. C.; Grubb, D. T. Polymer Microscopy; Chapman and Hall: New York, 1987; p 146. (16) McCormick, P. Y. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley: New York, 1987; Vol. 5, p 179. (17) Foust, A. S.; Wenzel, L. A.; Clump, C. W.; Maus, L.; Anderson, B. L. Principles of Unit Operations, 2nd ed.; Wiley: New York, 1980; p 268. (18) Coulson, J. M.; Richardson, J. F.; Backhurst, J. R.; Harker, J. H. Chemical Engineering; Pergamon Press: New York, 1978; Vol. II, p 620. (19) McCabe, W. L.; Smith, J. C.; Harriott, P. Unit Operations of Chemical Engineering, 4th ed.; McGraw-Hill: New York, 1985; p 716. (20) Sherwood, T. K. Ind. Eng. Chem. 1929, 21, 12. (21) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1986; p 189. (22) Enstun, B. V.; Senturk, H. S.; Yurdakul, O. J. Colloid Interface Sci. 1978, 65 (3), 509. (23) Schneider, P.; Rathousky, J. Collect. Czech. Chem. Commun. 1989, 54, 2644. (24) Zander, P.; Arndt, K. F. Colloid Polym. Sci. 1990, 268, 806. (25) Homshaw, L. G. J. Colloid Interface Sci. 1981, 84 (1), 141.
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Figure 1. SEM micrograph of polymer sample obtained by polymerizing a microemulsion sample with the composition 28.8% AA, 19.2% MMA, 2% EGDMA, 41.25% water, and 8.75% SDS.
Results Four factors associated with the microemulsion system were studied for their effects on the resulting polymer matrix morphology. Two factors were associated with the composition of the organic phase, the monomer ratio, and the cross-linker content. One factor was associated with the aqueous composition and the surfactant loading of the aqueous phase. The other factor was the aqueous to organic phase ratio of the precursor system. The polymer matrixes obtained from thermal polymerization were examined using three methods to determine the morphology. SEM micrographs were taken to obtain a visual record of the pore morphology. Measurement of the drying rate curves gave an indication of how interconnected the pores were. Finally, a quantitative measurement of the pore size was obtained from freezing point depression measurements. Of the three methods SEM measurements are the most direct but also the most disruptive to the sample due to the required drying. Drying rate curves are indirect, and the results are qualitative and are most useful when used with other supporting information about the precursor microemulsion structure such as conductivity measurements. Freezing point depression measurements are the most indirect technique, due to dependence on the simple pore model used, but also the most quantitative. Surfactant Loading of the Aqueous Phase. The first factor examined was the loading of the surfactant SDS in the aqueous phase of the precursor system. The precursor system consisted of 50 wt % aqueous phase and 50 wt % organic phase. The organic phase consisted of 4 wt % of the cross-linker EGDMA and 96 wt % of a 60:40 weight ratio of the monomers AA and MMA. The surfactant loading was varied between 10 and 20 wt % SDS in deionized water. A SEM micrograph representative of samples containing varying surfactant loadings is shown in Figure 1. In general, the observed structure was similar regardless of the degree of surfactant loading. However, samples containing 10 wt % SDS in the aqueous phase exhibited some pores which were larger than those seen in other samples. While SEM micrographs give a visual representation of the structure of the polymer matrixes, there are problems associated with this method such as pore collapse on drying and limited three-dimensional relief. Other methods were employed to help determine the morphology
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Figure 2. Drying rate data as a function of the SDS content of the precursor. The SDS content is expressed as a percentage of the overall aqueous components by weight of the precursor, water, and SDS.
of these systems. One such method is examination of the drying rate characteristics of the polymer matrixes. This method can give qualitative information on the interconnectivity of the pores present in a sample. Open-celled systems display a two-stage drying process in which the drying rate decreases linearly with loss of the vaporizing fluid and then exponentially as remaining liquid evaporates through the polymer material. Drying rate curves for closed-celled systems only display an exponential decay in drying rate. It is possible to estimate the amount of water trapped in discrete pores in an open-celled system by noting the point on the drying rate curve at which the decay of the rate changes from linear to exponential. Figure 2 shows the changes in drying rate with changing surfactant concentration of the aqueous phase. All five drying rate curves exhibited a two-stage decay typical of open-celled systems. The change from linear to exponential decay occurred between 0.3 and 0.4 mg of water/ mg of dry polymer for all samples. This information indicates that changing the surfactant loading for this system produced little to no change in the continuity of the pores. The other method for examining the structure of polymer matrixes is the measurement of the freezing point depression of water trapped in the pores. This method allows a quantitative determination of the pore size and distribution. In addition, the peak melting temperature indicates in what size pores the majority of the water is contained. The variation in peak melting temperature obtained as a function of percent SDS in the aqueous phase is shown in Figure 3. The pore size distributions as a function of surfactant loading of the aqueous phase are shown in Figure 4. These results indicate that a change in the surfactant loading of the aqueous phase has little effect on the pore sizes. These results are in close agreement with those obtained by examination of the drying rate curves for these compositions. The range of pores for these samples is from about 50 to 550 Å in radius. The results from the 12.5 wt % SDS in the aqueous phase sample do not show pores below about 100 Å in radius, and the results from the 20 wt % SDS in the aqueous-phase sample showed pores in the 700 Å radius range. This representation of the data seems to suggest that the shapes of the distributions might be affected by the surfactant loading of the aqueous phase. However, the logarithmic scale compresses the data at higher values and expands it at lower values of the pore size. An alternate way to look at the data is to plot the cumulative
Davis et al.
Figure 3. Peak melting temperature as a function of SDS content of the precursor. The SDS content is expressed as a percentage of the overall aqueous components by weight of the precursor, water, and SDS.
Figure 4. Pore size distribution as a function of SDS content of the precursor. The SDS content is expressed as a percentage of the overall aqueous components by weight of the precursor, water, and SDS.
Figure 5. Cumulative pore volume as a function of SDS content of the precursor. The SDS content is expressed as a percentage of the overall aqueous components by weight of the precursor, water, and SDS.
pore volume as a function of pore diameter. This is shown in Figure 5. The figure’s linear scale limits the range of data which can be plotted. However, data for large pores are suspect because of the accuracy associated with the equipment and the small change in melting temperature with pore size for large pores. For these two reasons, data are only shown for pores less than 3000 Å in diameter. This representation of the data suggests that changing the surfactant loading does not alter the pore distribution.
Effects of Precursor Composition on Pore Morphology
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Figure 6. Drying rate data as a function of the overall aqueous fraction by weight of the precursor. Figure 8. SEM micrograph of polymer sample obtained by polymerizing a microemulsion sample with the composition 19.2% AA, 28.8% MMA, 2% EGDMA, and water.
Figure 7. Cumulative pore volume as a function of the overall aqueous fraction by weight of the precursor.
Aqueous to Organic Phase Ratio of Precursor. The second factor examined was the ratio of the aqueous to organic phases of the precursor system. The aqueous phase used was 15 wt % SDS in deionized water. The organic phase consisted of 4 wt % of the cross-linker EGDMA and 96 wt % of a 60:40 weight ratio of the monomers AA and MMA. The precursors examined ranged from 40 to 70 wt % aqueous phase with the remainder organic phase. Figure 6 shows the changes in drying rate with changing aqueous phase fraction. The curves associated with phase ratios of 30 wt % aqueous and 40 wt % aqueous precursor systems only exhibit the single-exponential decay rate typical of closed celled systems. The other curves display the two-stage decay typical of open-celled systems. This indicates that 30% and 40% aqueous fraction systems result in closed-celled systems while higher aqueous loading results in open-celled matrixes. The change from linear to exponential decay occurs between 0.3 and 0.4 mg of water/mg of dry polymer for the samples consisting of 50% aqueous phase and decreases to between 0.1 and 0.2 mg of water/mg of dry polymer for the samples consisting of 70% aqueous phase. This change was the largest noted for any factor studied and is consistent with the theory that matrix morphology is governed primarily by precursor phase ratios. The cumulative pore volume as a function of aqueous phase fraction of the precursor is shown in Figure 7, which indicates a strong dependence of pore size distribution on the fraction of precursor associated with the aqueous phase. Samples containing 40% and 50% aqueous phase fraction exhibit a narrower distribution than those of samples containing 60% and 70% aqueous fraction. The
Figure 9. SEM micrograph of polymer sample obtained by polymerizing a microemulsion sample with the composition 33.6% AA, 14.4% MMA, 2% EGDMA, 42.5% water, and 7.5% SDS.
range of pore radius for 40% and 50% aqueous phase fraction samples was 60-740 Å. For the 60% aqueous fraction it ranged from 170 to 1230 Å, and for the 70% aqueous phase fraction sample it ranged from 90 to 6630 Å. It is interesting to note that even samples with very large pores have some pores that were in the very small size range. The cumulative pore volume of these samples shows clearly the change of the system from small pores with a narrow distribution to larger pores with a wide distribution. Monomer Ratio of Precursor Organic Phase. The third factor examined was the ratio of the monomers AA and MMA in the organic phase of the precursor system. The precursor examined consisted of 50 wt % aqueous phase and 50 wt % organic phase. The aqueous phase used was 15 wt % SDS in deionized water. The organic phase consisted of 4 wt % of the cross-linker EGDMA and 96 wt % of a mixture of the two monomers. The mixture consisted of between 40 and 80 wt % AA with the remainder MMA. Representative SEM micrographs of samples with the varying monomer ratios in the organic phase are shown in Figures 8 and 9. The pore size increased with increasing MMA in the organic phase. Samples with greater than 60% AA in the organic phase did not seem to be porous. Figure 10 shows the changes in drying rate with changing monomer ratio. The sample with a monomer
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Figure 10. Drying rate data as a function of the ratio of the monomers AA/MMA by weight.
Figure 12. Pore size distribution as a function of the ratio of the monomers AA/MMA by weight.
Figure 11. Peak melting temperature as a function of the ratio of the monomers AA/MMA by weight.
Figure 13. Cumulative pore volume as a function of the ratio of the monomers AA/MMA by weight.
ratio of 40:60 wt % AA/MMA seemed to consist of only a linear portion. This result is consistent with the system being completely open-celled. The samples for monomer ratios 60:40 and 80:20 displayed the two-stage drying rate decay more typical of open-celled systems. The change from linear to exponential decay occurred between 0.3 and 0.4 mg of water/mg of dry polymer for the samples having a 60:40 AA/MMA monomer ratio and decreased to between 0.2 and 0.3 mg of water/mg of dry polymer for the sample having a 80:20 monomer ratio. These results indicate that changing the monomer ratio affected the extent of the continuity of the monoliths. However, none of the samples examined seemed to be completely closedcelled. The variation in peak melting temperature with changing monomer ratio of the organic phase is shown in Figure 11. The resulting pore size distributions are shown in Figure 12, and the cumulative pore volume is shown in Figure 13. The peak melting temperature data suggests a difference in the size of the pores contributing the most volume to the system. These pores seem to get larger as the fraction of MMA in the organic solution grows. This is in agreement with the drying rate data shown previously. The pore size distribution graphs show that as the fraction of AA in the organic phase is increased, the distribution narrows. The largest change occurs when the composition is changed from 40% to 50% AA. The range of pore sizes for the system with 40% acrylic acid in the organic phase is 110-6630 Å. The range for the other samples is 50-610 Å. The cumulative pore volume representation of the data shows clearly the increased pore size of the samples containing 40% AA in the organic phase.
Figure 14. Cumulative pore volume as a function of the crosslinker content of the precursor. Cross-linker content is expressed as a weight fraction of overall organic components: AA, MMA, and EGDMA.
Cross-linker Content of Precursor Organic Phase. The fourth factor examined was the amount of the crosslinker EGDMA present in the organic phase of the precursor system. The precursor examined consisted of 50 wt % aqueous phase and 50 wt % organic phase. The aqueous phase used was 15 wt % SDS in deionized water. The organic phase consisted of a 60:40 by weight mixture of AA and MMA and varying amounts of EGDMA. The amount of EGDMA present in the organic phase was varied from 0 to 8 wt %. The cumulative pore volume is shown in Figure 14. These data show that changing the cross-linker content of the organic phase of the precursor does not have a
Effects of Precursor Composition on Pore Morphology
significant effect on the range of pore sizes. The size range is from 50 to 740 Å in radius. Discussion The factor with the greatest effect on the matrix morphology was the ratio of the aqueous to organic phases of the precursor system. The pores which arise in the polymer matrix originate from the separation of the aqueous and organic phases in the microemulsion precursor. The more aqueous phase present, the larger the volume it occupies in the precursor and the larger the resulting matrix pores. It is this factor, aqueous-to-organic ratio, which has been primarily explored to date. One would expect that changing the surfactant loading would alter the size of the microemulsion system and thus the morphology of the polymer matrix. The lack of change to the matrix morphology with surfactant loading indicates that the surfactant is not the primary surface-active agent in the system. This is not surprising because previous studies have shown that at certain compositions this system will form porous polymer monoliths with no surfactant present. This phenomenon is observed because of the surface-active properties of the monomer AA. However, the complete lack of morphological change in the sample with changing surfactant loadings is suprising. The drying rate results for changing aqueous phase fraction indicate that the change from an open- to closed-
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celled system occurred between 40 and 50 wt % aqueous phase for these system. Sasthav found that the transition from open- to closed-celled system occurred at a much lower aqueous phase fraction.12 There are two possible explanations for this difference. The first is that here a surfactant loading of only 15% SDS in water was used, although the above results on changing this composition would seem to discount this theory. The second is that this study focused on thermal polymerization while that of Sastav examined more rapid polymerization methods. These more rapid polymerization techniques would limit the coalescence of the bicontinuous phases and therefore limit closed-cell formation. The ratio of the monomers in the organic phase also seemed to affect the pore sizes observed in the polymer matrix. Low AA content resulted in large pores being observed in the FPD measurements. One explanation for the large extent of water found in interconnected pores for the 40:60 AA/MMA monomer ratio samples is phase separation. These samples are located close to the onephase/two-phase boundary, and it seems likely that phase separation could occur on polymerization. The slower thermal polymerization technique used here would result in greater phase separation than more rapid polymerization methods. LA9701539
