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Morphology of Thermally Polymerized Microporous Polymer Materials Prepared from Methyl Methacrylate and 2-Hydroxyethyl Methacrylate Microemulsions Nathan Schmuhl, Edward Davis, and H. Michael Cheung* The University of Akron, Department of Chemical Engineering, Akron, Ohio 44325-3906 Received February 13, 1997. In Final Form: November 14, 1997 The system composed of water, sodium dodecyl sulfate, methyl methacrylate, and 2-hydroxyethyl methacrylate, with and without ethylene glycol dimethacrylate as a cross-linking agent was used to investigate the effects of changing from visible-light to thermal polymerization. Thermal polymerization yielded porous solids within the range 20-80 wt % aqueous phase content for systems with and without cross-linker. Microstructures similar to photoinitiated polymers were observed in all ranges analyzed. Closed cell microstructures were found to exist for polymers with aqueous fractions less than 50 wt % for polymers with and without cross-linker. Polymers with aqueous fractions greater than or equal to 50 wt % were found to exhibit an open cell microstructure. These microstructures for the higher aqueous fractions differed from the previously observed morphology in other microemulsion systems. These polymers were found to be porous with polymer droplets interconnected to produce a solid mass. Also, a flakelike appearance was found in certain compositions. Both of these occurrences were also recorded for a previous photoinitiated study. The level of cross-linker was found to have a negligible effect on the maximum pore sizes observed, although cross-linker did cause a general shifting of polymers toward closed cell microstructure.
Introduction Microemulsions are transparent, isotropic liquids exhibiting significant microstructure and consisting of water and oil subphases. They are thermodynamically stable, this stability often being facilitated by a surface-active agent (surfactant) or a combination of surfactant and cosurfactant. There are three types of microemulsions. These are water in oil (w/o), in which microdroplets containing primarily water are dispersed in a continuous oil phase, oil in water, in which microdroplets containing primarily oil are dispersed in a continuous aqueous phase (o/w), and then a bicontinuous microemulsion, in which continuous oil and water subphases coexist. The formation of porous polymeric solids through the polymerization of, mainly bicontinuous, microemulsions is a field of previous and current research activity.1-5 Several microemulsion systems have been used to form these porous solids. Two of the systems relating to our research involve the methyl methacrylate/acrylic acid/aqueous sodium dodecyl sulfate (MMA/AA/SDS) cross-linked with ethylene glycol dimethacrylate (EGDMA) studied by Cheung et al.2,6-8 and more recently the system consisting of MMA/2-hydroxyethyl methacrylate (HEMA)/SDS reported by Ng et al.4,9 * To whom correspondence should be directed either via regular mail or e-mail (
[email protected]). (1) Haque, E.; Qutubuddin, S. J. Polym. Sci., Part C: Polym. Lett. 1988, 26, 429. (2) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 2586. (3) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Langmuir 1992, 8, 1931. (4) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C. Polymer 1995, 12, 1941. (5) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 1378. (6) Sasthav, M.; Palani Raj, W. R.; Cheung, H. M. J. Colloid Interface Sci. 1992, 152, 376. (7) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1993, 34, 3305. (8) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. J. Appl. Polym. Sci. 1993, 47, 499.
In some potential applications, it would be more convenient to thermally polymerize these systems than to use the more commonly used photopolymerization method. One concern with thermal polymerization is whether it can be carried out at reasonable temperatures and within a short enough time period (comparable to photopolymerization) to exhibit microstructural characteristics similar to those obtained via photopolymerization. In this work we studied the effects of changing to thermal polymerization from photopolymerization in the microemulsion system consisting of the methyl methacrylate and 2-hydroxyethyl methacrylate system developed by Ng et al.4-9 Microemulsion macroscopic phase behavior, polymer microstructure, and pore size distribution were studied. Experimental Section 2-Hydroxyethyl methacrylate (HEMA) was obtained from Fluka with a purity of 95%, and methyl methacrylate (MMA) was obtained from Aldrich and had a purity of 99+%. The crosslinking agent ethylene glycol dimethacrylate (EGDMA) was also from Aldrich and was 98% pure. Sodium dodecyl sulfate (SDS) was the surfactant and was from Sigma with a purity of 99%. The thermal initiator for polymerization was azobis(isobutyrontrile) (AIBN) from Alfa Aesar and had a purity of 99%. The water used for all microemulsions was deionized. Two phase diagrams were prepared utilizing MMA and HEMA as comonomers. The first was the system MMA/HEMA/H2O. The microemulsion samples were prepared by titration of the given percentages of MMA, HEMA, and water into test tubes so that 2 g of total components was present. The test tubes were then capped and vortex-mixed to ensure complete mixing and allowed to equilibrate in a temperature-controlled environment at 24 °C for 24 h. The samples were allowed to set for a period of 24 h and then were visually inspected, and the number and appearance of the macroscopic phases were recorded. The second system was MMA/HEMA/aqueous SDS. These samples were identical to the MMA/HEMA/H2O samples described above except (9) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C.; Pey, K. L. Langmuir 1996, 12, 319.
S0743-7463(97)00152-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/28/1998
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Figure 1. Ternary-phase diagram for the system methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), and water (H2O) at 24 °C and at 1 atm. Compositions are based on weight. Domains are as follows: A, two-phase region; B, monophasic region; C, unstable three-phase region. that, in place of deionized H2O, a stock solution of 20% SDS by weight in H2O was used in their preparation. Polymerization samples in this study were produced using azobis(isobutyrontrile) (AIBN) as the initiator. The amount of AIBN used was 0.020 g for 10 g of microemulsion. Once prepared, the microemulsions were purged with dry nitrogen gas for 15 min at 1 atm. Thermal polymerization of the microemulsions was carried out in a model 1265-02 Cole-Parmer refrigerated circulator at 55 °C for 2 h. After polymerization, samples prepared for microscopic analysis were allowed to postcure for 24 h within the test tubes in which they were reacted. The tubes were then cracked and the samples removed and allowed to set in the open air for a period of not less than 1 week for complete drying. Prior to analysis, the polymerized samples were coated with silver with a uniform thickness of approximately 240 Å. The coating was performed using a model ISI-5400 Polaron SEM coating system, supplied by Polaron Equipment Limited. The samples were then examined with a ISI SX-40 scanning electron microscope (SEM) to study the pore morphology. Pore sizes and pore size distributions for the polymerized microemulsions were determined through freezing point depression calculations6 with data collected with a DuPont Instruments910 DSC system in conjunction with a DuPont Thermal Analyst 2100 system. Polymerized samples were opened 24 h after polymerization and immediately cut into small specimens ranging in weight from roughly 10 to 15 mg. Two samples per microemulsion composition tested were made for verification of the accuracy of the data collected. The specimens were then sealed in hermetic sample pans supplied by TI instruments and then chilled in dry ice for a period of at least 24 h to ensure complete freezing of the water within the pores. During testing, individual samples were placed in the DSC cell with an empty sample pan. The cell was then cooled to -30 °C and immediately heated 1 °C/min to -20 °C. The sample was then held isothermal at -20 °C for 5 min and then heated at a ramp of 0.25 °C/min to 5 °C.
Results The resulting pseudoternary-phase diagrams for the phase behavior studies of the MMA/HEMA/H2O and MMA/ HEMA/SDS systems are shown in Figures 1 and 2, respectively. The surfactant-free system depicted in Figure 1 displays three distinct phase regions. The compositions existing in regions A and C are two and three phases, respectively. The compositions in region B are macroscopically monophasic and transparent. Consistent with the behavior of similar systems studied by our group and others, region B is assumed to be made up of three
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Figure 2. Ternary-phase diagram for the system methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), and water (H2O) at 24 °C and at 1 atm. Compositions are based on weight, and the SDS/H2O solution was 20% SDS. Domains are as follows: A, two-phase region; B, monophasic region; C, unstable three-phase region.
subregions with microemulsions consisting of water in oil, bicontinuous, and oil in water microstructures. The boundaries between these regions were not determined in this study. A shrinking of the three-phase region shown in Figure 1 was noted over a period of several weeks. Over this period, several samples that were three phase initially changed to two phases. It is likely that this region might disappear entirely if given enough time to come to a thermodynamic equilibrium, which apparently cannot be reached in 24 h in all cases. This behavior was observed in a similar system studied by Cheung et al.2 In that study it was noted that a three-phase region similar to the one in this system was kinetically stable but thermodynamically unstable and would change over to a twophase region upon sonication.2 Figure 2 exhibits the two distinct regions found for the phase study with the 20% SDS in H2O solution replacing pure water in the system mentioned above. This phase behavior study is similar to the one appearing in the work of Ng et al.4 although this study lacks the 4% EGDMA included by Ng et al. The phases in this diagram were either two phase, as shown in region A, or one phase, as in region B. This coincides closely with what was found by Ng et al.4 As in the surfactant-free system, region B is presumably divided into the w/o, bicontinuous and o/w subregions, as was found in the previously mentioned work. Two sets of microemulsions were polymerized for morphological study through SEM analysis. Both sets of polymerizations ranged in composition from 20% to 80% aqueous content with the aqueous fraction consisting of 20% SDS by weight. The first set of microemulsions were polymerized without any cross-linker (EGDMA) added to the organic fraction which consisted of 75% HEMA and 25% MMA by weight. All of the microemulsions formed solid, self-supporting structures within the range polymerized. However, as the ratio of aqueous to organic content of the microemulsions was increased, the rigidity of the resultant polymers did decrease. SEM analysis of the polymerized samples exposed a porous microstructure within the polymers starting from an aqueous content of 20% all the way up to the 80% ceiling. The pores at the lower aqueous concentrations (20%-40%) seemed to be isolated and nonnetworked (see Figure 3),while growing
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Figure 3. SEM micrograph of a porous polymer sample obtained from polymerization of a precursor microemulsion consisting of a 30% SDS/water solution, with the balance being 3:1 HEMA/MMA. No EGDMA is included in this sample.
Figure 5. SEM micrograph of a polymer sample obtained from polymerization of a precursor microemulsion consisting of a 30% SDS/water solution, with the balance being 3:1 HEMA/ MMA with EGDMA as 4% of the total monomer weight.
Figure 4. SEM micrograph of a porous polymer sample exhibiting interconnected polymer droplets. The precursor microemulsion consisted of a 70% SDS/water solution, with the balance being 3:1 HEMA/MMA. No EGDMA is included in this sample.
Figure 6. SEM micrograph of a porous polymer sample exhibiting interconnected polymer droplets. The precursor microemulsion consisted of a 70% SDS/water solution, with the balance being 3:1 HEMA/MMA with EGDMA as 4% of the total monomer weight.
in size and becoming interconnected with larger water fractions (see Figure 4). At the 20% aqueous concentration, pores were observable ranging from 1.25 to 2.5 µm. At concentrations of 50% aqueous and above the pores appeared to be interconnected. The microstructure of these polymers was similar to that found by Ng et al. Instead of the normally expected solid walls with pores running throughout; small, connected, solid droplets were seen. However, the flaking phenomena noted by Ng et al. were not seen in samples prepared without EGDMA. The second set of polymerizations varied from the first by the composition of the organic phase. The ratio of HEMA to MMA was kept at 3:1, as it was in the previous microemulsions, but EGDMA for cross-linking was added in the amount corresponding to 4% of the comonomer mixture present. Just as for the non-cross-linked samples, all the compositions formed solid self-supporting polymers and rigidity steadily decreased as aqueous content was increased. Similar photopolymerized samples in the Ng et al.4 work lost rigidity also; however, they ceased being self-supporting at the 70% aqueous mark instead of the 80% mark, as in this study. Unlike their non-cross-linked counterparts, these samples did not begin to display pores until the aqueous content was increased to 30% (Figure
5). Even with that amount, the pores seen with the SEM were in the range of 5 µm in size, which is smaller than that seen with the non-cross-linked 20% aqueous polymerized microemulsion. Significant pore structure was not seen until 50% or higher aqueous content, at which point the samples took on the appearance of networked channels of pores surrounded by the previously described droplet microstructure within the polymer (Figure 6). The endothermic heating effects measured and recorded by the DuPont systems mentioned previously in the Experimental Section were used to calculate the pore sizes and resulting pore size distributions (PSD) of the porous polymer materials produced. The theory and calculations for freezing point depression from ice melting in capillarylike pores have been discussed previously.6 Precursor microemulsions with concentrations identical to those used for the SEM samples were polymerized. The cumulative pore volumes are presented in Figure 7 for samples formed from precursor microemulsions formulated without the cross-linking agent, EGDMA, and in Figure 8 for samples obtained by polymerizing precursor microemulsions formulated with EGDMA present as 4% of the monomer. In both figures the HEMA/MMA ratio is 3:1 and the aqueous phase composition was varied from 30% to 80%. The PSD
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Figure 7. Pore size distributions for polymers formed with no cross-linker and aqueous contents varying from 30% to 80%. The microemulsion aqueous phases contained 20% SDS by weight. The ratio of monomers was HEMA/MMA ) 3:1.
Figure 8. Pore size distributions for polymers formed with 4% EGDMA cross-linker and aqueous contents varying from 30% to 80%. The microemulsion aqueous phases contained 20% SDS by weight. The ratio of monomers was HEMA/MMA ) 3:1.
appears to depend primarily on the aqueous fraction and only slightly on the presence (or absence) of cross-linker. For cross-linked samples with 60% and above aqueous fraction and for non-crosslinked samples with 50% and above, neither aqueous fraction nor cross-linker presence seemed to play a role in pore size. For samples with water contents below this, both aqueous fraction size and crosslinker presence seemed to affect the PSD. A significant number of pores smaller than 500 Å were detected in the 30%-40% aqueous content samples containing EGDMA, with the same being true for 30%-50% aqueous content samples in the non-cross-linked system. Neither the crosslinked nor the non-cross-linked 20% aqueous content samples yielded analyzable results from the FPD analysis, since no shift in the slope of the thermal input was seen. Discussion Comparing the two-phase behavior diagrams of this study (Figures 1 and 2), the effect of the addition of the surfactant (SDS) to the mixture is evident. The fact that a significant portion of the HEMA/MMA/H2O diagram is monophasic is an indication of the ability of HEMA to act as a cosurfactant. This ability is an indication of the surface-active character of the HEMA molecule, which is weak however when compared to SDS. We have seen similar behavior using acrylic acid as a comonomer with methyl methacrylate.
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By a direct comparison of the measured sizes of the pores at various aqueous fractions from the SEM micrographs and the DSC FPD calculations, it is apparent that some type of morphological change takes place during the drying of these porous polymers, as previously discussed by Cheung et al. At the 30% aqueous mark for polymers containing no EGDMA cross-linker (Figure 3), pores averaging about 5-6 µm (5000-6000 Å) in diameter were observable from the SEM micrographs. However, in the DSC measurements, the pores measured between 70 and 400 Å in diameter. This enlargement in pore size after drying has been previously explained by the collapsing of capillary structures, resulting in larger pores after drying. It has been observed through FPD analysis that, after an initial increase in the size of the pores, a barrier is reached where, regardless of any increase in aqueous fraction, pore size is not increased. This analysis is verified by visual inspection of SEM micrographs of the porous polymers that seem to confirm that a maximum pore size is reached within the polymers. This occurrence has yet to have been explained fully although it has been speculated that the terminal pore size in this system may be dictated by ionic repulsion of HEMA with itself. Also in the micrographs, it was observed that as the aqueous fraction reached higher and higher fractions, the polymer walls changed structure. This occurrence can be seen by comparing Figures 3 and 4 to Figures 5 and 6. The polymers changed from solid with Swiss-cheese-like holes to interconnected droplets which formed a very porous structure. The size of the droplets also decreased with increasing aqueous fraction, as also noted by Ng et al. The presence of the spherical polymers is possibly the result of an initial oil in water microstructure with polymerization also occurring between the already polymerized droplets upon collision. These demarcations would not agree with the elementary boundaries previously determined by Ng et al., however, and would seem to agree with their synopsis that their is significant disruption of the original microstructure of the microemulsion during polymerization. In this case thermal polymerization provides the same disruption as the original photopolymerization. The effect that the cross-linker EGDMA had on the PSD of the resultant polymers was surprisingly minimal. The exceptions are 30, 40, and 50% aqueous fractions. These samples were also the only compositions to really vary noticeably from any of the other pore size distributions, and even then the effect of cross-linker is not clear. With the 30 and 50% aqueous fraction polymers, the presence of EGDMA shifted the pore size distribution of the polymers to the right toward larger size pores. However, it had the opposite effect on the 40% sample. In the SEM micrographs, it can be seen that the presence of EGDMA inhibits the presence of smaller pores. This is exhibited by comparison of Figures 3 and 5. This inhibition agrees with the shifting of the PSD for the 30% and 50% fractions to the right in Figure 8. Cross-linker has been successfully added in the past to help preserve the microstructure of the initial microemulsion. Although the initial microstructure of this system was not determined, if the EGDMA was successful in preventing the breakdown of the microstructure with this system, then the SEM micrographs indicate that the initial microemulsions with a bicontinuous microstructure near the water in oil boundary have a tendency to shift toward the w/o region upon polymerization.
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Conclusions The results of the phase behavior studies indicated that this HEMA/MMA system forms a large monophasic region. This region is large enough to make this system useful in examining the effects of changing the method of polymerization upon microemulsion systems. This study reports that polymers in this system are porous with increasingly large pores up to a limit at which further increases in aqueous content have no effect on the pore size of the pores. This effect was found in FPD testing and visually verified in the SEM micrographs. Addition of EGDMA was also found to have less of an effect on pore size as measured by DSC than expected, and the effect that was
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found was unclear in direction. Differences in microstructure caused from EGDMA addition were more apparent in SEM micrographs taken of the polymers. In general, the polymers formed in this study were very similar to polymers with identical components which were photopolymerized. No significant change in microstructure seemed to result in the switch from photopolymerization to thermal polymerization. Therefore, thermal polymerization, at least in this system, is successful at preserving many of the features of the initial microstructure of the microemulsion. LA970152G
