Langmuir 1995,11, 3316-3320
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Microporous Polymeric Materials from Polymerization of Zwitterionic Microemulsions L. M. Gan,*>tT. D. Li,t C. H. Chew,t and W. K. TeoS Department of Chemistry and Department of Chemical Engineering, National University of Singapore, Singapore
L. H. Gan Division of Chemistry, NIE, Nanyang Technological University, Singapore Received February 15, 1995. I n Final Form: May 25, 1995@ A new microemulsion system consisting of a polymerizable zwitterionic surfactant of (((acryloy1oxy)undecy1)dimethylammonio)acetate (AUDMAA), methyl methacrylate (MMA), water with or without a cross-linking agent of ethylene glycol dimethacrylate (EGDMA) was investigated. Transparent solid polymeric materials can be produced by photoinitiated polymerization of some of these microemulsion compositions. Microemulsions with higher concentrations of EGDMA (>4%)or MMA (>50%)produced only turbid polymeric materials. However, it required a minimum of about 25%AUDMAA for producing a transparent polymeric material. Most of the bicontinuous microemulsions investigated could be gelled within 10 min and transparent solid polymers were finally obtained. Electron micrographs of these transparent polymers reveal the existence of not only the open-cell type microstructures but also the bicontinuous nature. The widths ofthe bicontinuous structures were about 50-70 nm, their lengths were long and winding and they appear to be randomly distributed. The microporous structures of these transparent polymeric materials seem to be related to the microstructures of the precursor bicontinuousmicroemulsions. Perhaps, this is the first report to show the bicontinuous structures of microporous polymeric materials prepared by microemulsion polymerization.
Introduction Porous polymeric materials have been produced by polymerization of methyl methacrylate (MMA) or styrene in water-in-oil (wlo) microemulsions,1,2 bicontinuous (middle phase) micro emulsion^,^-^ or both types7ofthem. A middle phase microemulsion possesses bicontinuous microstructures comprising both oil and aqueous phase with interconnecting domains which are stabilized with surfactant molecules locating at the oil-water interface. Nonpolymerizable sodium dodecyl sulfate (SDS)was most often used for stabilizing those polymerized microemulsions. Solid polymeric materials thus formed were mostly opaque. Bicontinuous microemulsions yielded open-cell structures, whereas wlo microemulsions produced closedcell porous solid polymers. It is desirable to use a polymerizable surfactant that can be copolymerized with MMA or styrene to form a n integrated polymeric material. We have attempted to prepare transparent polymeric solids by polymerizing wlo microemulsions since 1983. The systemss-10investigated
* To whom correspondence should be addressed. +
Department of Chemistry.
e Department of Chemical Engineering.
Abstract published inAdvanceACSAbstracts, August 1,1995. (1)Stoffer, J. 0.;Bone, T. J . Dispersion Sci. Technol. 1980, 1, 393. (2) Menger, F. M.; Tsuno, T.; Hammond, G. S. J . Am. Chem. Soc. 1980, 112, 1263. (3) Qutubuddin, S.; Haque, E.; Benton, W. J.;Fendler, E. J. InPolymer Association Structures: Microemulsion and liquid Crystals;El-Nokaly, M. A,, Ed., ACS Symposium Series No. 384; American Chemical Society: Washington, DC, 1989; p 64. (4)Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 1378. (5) Palani Raj, W. R.; Sathav, M.; Cheung, H. M. Langmuir 1992, 7, 2586. (6) Palani Raj, W. R.; Sathav, M.; Cheung, H. M. Langmuir 1992,8, 1931. (7) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C. Polymer, in press. (8) Gan, L. M.; Chew, C. H. J . Dispersion Sci. Technol. 1983,4,291. (9) Gan, L. M.; Chew, C. H. J.Dispersion Sci. Technol. 1984,5,179. (10)Chew, C. H.; Gan, L. M. J . Polym. Sci.: Polym. Chem. Ed. 1986, 23, 2225. @
then were M W a c r y l i c acidwater and a copolymerizable anionic surfactant of sodium acrylamidoundecanoate or sodium acrylamidostearate. But only opaque polymeric solids were obtained from those systems with water contents greater than 15 wt %. Though transparent polymeric solids could also be produced from the systems at water contents lesser than 15 wt %, they did not reveal any obvious microstructures as examined by scanning electron microscopy. The less readily polymerizable anionic surfactant of potassium undecanoate6 (PUD) had also been incorporated in a bicontinuous microemulsion consisting of MMA, PUD, water and a cross-linking agent, ethylene glycol dimethacrylate (EGDMA). But PUD is very prone to allylic chain transfer reacti0ns.l' We have successfully prepared several types of transparent porous polymeric materials recently from the polymerization of microemulsions containing very reactive surfactants. These polymerizable surfactants are anionic sodium ll-(N-ethylacrylamido)undecanoate'2 ( N a l l EAAU), cationic ((acryloyloxyhmdecy1)trimethyla"onium bromide13 (AUTMAB), and zwitterionic (((acryloy1oxy)undecy1)dimethyla"onio)acetate (AUDMAA). The latest polymerization of this AUDMAA (CH2=CHCOO(CH2)11N+(Me)2(CH2C00-))in microemulsions containing MMA, water, and a cross-linker of EGDMA for producing transparent microporous materials i s discussed in this paper.
Experimental Section Materials. Both MMA and EGDMA were obtained from Merck and purified under reduced pressure. Photoinitiator 2,2dimethoxy-2-phenylacetophenone (DMPA)from Aldrich was used (11)Paleos, C. M.; Stassinopoulou, C. I.; Mallaris, A. J . Phys. Chem. 1983, 87, 251. (12) Gan, L. M.; Chieng, T. H.; Chew, C. H.; Ng, S. C. Langmuir 1994,10,4022.
(13)Li, T. D.; Chew, C. H.; Ng, S. C.; Gan, L. M.; Teo, W. K. J. Macromol. Sci., Pure Appl. Chem. 1996, A32 (5), 969.
0743-7463/95/2411-3316$09.00/00 1995 American Chemical Society
Microporous Polymeric Materials
Langmuir, Vol. 11, No. 9, 1995 3317
M MA
Jc
Water Content (wt%) 40 eo
0
5.0
-5
20
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100
I 36
4.0-
\
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.-c% , .c
3.0-
0
0
0
0
Water
2.0-
AUDMAA 1 .o
Figure 1. Ternary phase diagram of MMA/AUDMAA/water at 30 "C. The single-phasemicroemulsionregion is represented by the shaded area.
-
".", 0
as supplied. Water was purified by a Milli-Qwater system. The AUDMAA was synthesized according to our previous report.14 Microemulsion Phase Diagram. The single-phase region ofthe microemulsionsystem was determined visually by titrating a specific amount of MMA and AUDMAA with water in a screw capped tube at 30 "C. Each titration was thoroughly mixed using a Vortex mixer. The clear-turbid boundaries were established from the systematic titrations. The transparent microemulsion region is represented by the shaded area as shown in Figure 1. Conductivity and Surface Tension. Electrical conductivities of the microemulsion samples were measured using Omega CM-155 conductivity meter with a cell constant of 1.209cm-l at 30 "C and 1 atm. The surface tensions of the samples were determined using a torsion balance tension meter (WhiteElectric Instrument Co.) at 30 "C and 1 atm. Microemulsion Polymerization. The photoinitiated polymerization of the microemulsionsused 0.2 wt % of DMPA that was based on the combined weight of MMA and AUDMAA. Microemulsion samples were charged into ampules which were placed in an ice-water bath and purged with dry nitrogen gas at 1atm for 10min before they were sealed. The sealed ampules were sonicated for 1min in order to expel the possibletiny bubbles from the microemulsionphase. The polymerizationwas carried out in a Rayonet Photochemical Reactor for 1h at 35 "C. All samples changed from liquid to viscous gel and finally to transparent hard solids during polymerization. Morphology Observation. Both JEOL JEM lOOCXD transmission electron microscope (TEM)and HITACHI S-4100scanning electron microscope (SEM) were used to study the morphology of polymeric materials. During the early stage of polymerization (before gelation), one drop of samples taken out from the system at different time intervals was added to 2 mL of 0.2% photsphotungstic acid (PTA) solution and the mixture was thoroughly mixed. A drop of this mixture was then put on a copper grid coated with a thin layer of Formvar. As for the gelled samples, they were frozen in liquid nitrogen and were fractured mechanically. The fractured samples were vacuum dried for 24 h at room temperature and coated with gold using JEOL ion sputter JEC-1100 coating machine. Drying Rate of Water Desorption. The dryingrate ofwater desorption from each solid polymer sample was monitored using a Dupont Instrument TGA2100 thermogravimetricanalyzer. The polymer sample was dried in a stream of dry nitrogen gas isothermally at 70 "C for 5 h and then increased to 100 "C at a rate of 2 "C/min for 1 h. The weight loss of the sample was recorded as a function of time throughout the experiment and the sample dimensions were measured again after drying. The drying rate for the polymer sample based on the average of the initial and final surface areawas plotted against the free moisture content of the sample to yield the drying-rate curve. (14) Chew, C. H.; Li, T.D.;Gan, L. M.; Teo, W.K. J.Macromol. Sci., Pure Appl. Chem. 1995, A32 (21, 221.
-
, 20
.
, 40
.
I
80
.
I
80
.
I - "
100
Water Content (%) Figure 2. Change of electrical conductivity and surface tension of microemulsion as a function of the water content along line A in Figure 1.
Results Phase Behavior of Microemulsions. Figure 1 shows the phase behavior of the ternary system of M W AUD-20. The very large shaded region represents the wide range of compositions that can be used to form transparent microemulsions. In general, the viscosity of the microemulsions were higher at higher AUDMAA concentrations. Microemulsions along line A were characterized by electrical conductivity and surface tension measurements. The weight ratio of MMA to AUDMAA was fixed at 1:1.2along line A and only the water content was varied. When the water content exceeded 20%,its conductivity increased abruptly reaching a maximum of 4.3 slcm at about 50%water content as shown in Figure 2. On the other hand, the surface tensions were nearly constant within that region. Beyond which, the decreasing conductivities and the increasing surface tensions were observed on increasing the water content in microemulsions.
Polymerization Aspects. On the basis of the large microemulsion region of Figure 1,numerous compositions can be chosen for the polymerization study. However, many of those compositions will not produce transparent polymeric materials. On the other hand, some compositions may produce only soft transparent polymeric materials such as the composition at point P (30 w t % water, 38 wt % AUDMAA, and 32 w t % MMA). When a certain amount of cross-linker EGDMA was added to the microemulsion, a transparent rigid polymeric material was obtained. EGDMA not only increased in rigidities of the polymeric materials but also accelerated the gelation for the systems during polymerization as illustrated in Table 1. It was established that about 2 to 4 w t % EGDMA based on the total weight of MMA and AUDMAA was sufficient for producing transparent rigid polymers. At higher EGDMA concentrations, the phase separation of the systems would occur during polymerizations. Hence, subsequent polymerization for all microemulsions investigated used only 2 w t % EGDMA. Table 2 shows the physical appearance of some microemulsions before and after polymerization. The compositions of these microemulsions were based on the equal weight ratio of water to AUDMAA along line B of Figure 1. Transparent polymers could be obtained from the
Gan et al.
3318 Langmuir, Vol. 11, No.9, 1995 Table 1. Effect of EGDMA Concentration on Microemulsion" Polymerization and the Physical Appearance of Polymers sample no. A1
EGDMA (wt 96)
0.0 1.0 1.5 2.0 3.0 4.0
A2 A3 A4 A5 A6
gel time (min) 20.1 15.6 11.6 8.3 5.3 4.2
transparent polymer soft solid soft solid soft solid rigid solid rigid solid rigid solid
a Microemulsion composition: 30 w t % water, 32 wt %, MMA, and 38 w t % AUDMAA. Percentage of EGDMA and 0.2 wt % of DMPA were based on the combined weight of MMA and AUDMAA.
Table 2. Effect of MMA Concentration on the Appearance of Microemulsions before and after Polymerization
B1 B2 B3 B4 B5
20 30 40 50 60
40 35 30 25 20
40 35 30 25 20
C C C C C
C C C
tl t
a EGDMA and DMPA are 2 and 0.2 wt % based on the combined weight of MMA and AUMAA, respectively. BP, before polymerization;A I', after polymerization; c, clear;tl, translucent;t, turbid.
system with the MMA content increasing from about 20 to 50 w t %. At higher MMA concentrations, only turbid polymers were produced. Characterizationof TransparentSolidPolymers. Transparent solid polymers of samples B1 to B4 were dried under vacuum at 30 mmHg for 24 h. The weight losses were only slightly lower ( < 1%) than their respective water contents in precursor microemulsions. The dried samples were subsequently subjected to extraction by toluene, which would remove unreacted MMA and its possible homopolymer. The toluene-treated polymer samples were further extracted with hot water in order to remove unreacted AUDMAA and its possible homopolymers. After two successive extractions, the total weight losses for all samples were less than 1.5 w t % based on the total weight of both monomers. This implies that almost all MMA, AUDMAA, and EGDMA might have copolymerized and/ or cross-polymerized to form a stable polymer matrix. The formation of polymers in microemulsions during polymerization was also monitored by taking electron micrographs at different time intervals. Figure 3 shows the polymer formation in successive four stages for a microemulsion (point P of Figure 1)comprising 30%water, 32%MMA, 38% AUDMAA, and 2.0%EGDMA based on the total weight of MMA and AUDMAA. Numerous nanoparticles can be seen from the TEM of Figure 3a after 3 min of polymerization. At this early polymerization stage, the polymerized microemulsion was still very fluid. The sample became a semigel at 6 min polymerization and its morphology could be examined by SEM as shown in Figure 3b. Polymer particles were now in the form of interconnected clusters with interdispersed pores of sizes smaller than 250 nm. The sample was totally gelled after about 8 min of polymerization. The existence of the microporous structures resembling those of bicontinuous structures of microemulsion15J6can clearly be seen from Figure 3c. The dimensions of the channels were in the (15) Bodet, J. F.; Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Miller, W . G. J . Phys. Chem. 1988,92, 1898. (16)Jahn, W.; Strey, R. J . Phys. Chem. 1988,92,2294.
Figure 3. Electron micrographs for sample A-4 during polymerization: (a) TEM, 3 min; (b) SEM, 6 min (semigel); (c) SEM, 8 min (gel); (d) SEM, 1 h (solid). range of about 50-70 nm in width and 100-200 nm in length. These structures did not alter much even after
Langmuir, Vol. 11, No. 9, 1995 3319
Microporous Polymeric Materials
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1
0.0
0.1
0.2
0.3
0.4
0.5
Free Moisture (mg/mg polymer)
Figure 5. Drying rate curves for porous polymer samples of different water contents: 0,25% H2O; A, 40% H20.
(white strips) for a polymer sample with 35% water, 35% AUDMA, and 30% MMA. This microemulsion composition is identical to that at point M in Figure 1. It is interesting to note that the widths of these water channels and polymer islands were about 50 to 70 nm. Both of these infrastructures were long and winding and they appeared to be randomly distributed. Though SEM micrographs reveal the microstructures of the fractured polymer specimen, additional information pertaining to the continuity of the microporous structures of water channels is desirable. The water desorption from the polymer specimen was studied by thermogravimetric analysis (TGA). Polymer samples were prepared from the microemulsion compositions along line A of Figure 1 with the water content ranging from 20 to 50 wt %. All the drying rate curves show a linear falling rate period as shown in Figure 5. It is noted that the linear falling rate period increased with the water content of the precursor microemulsion. The linear falling rate of drying is the characteristic of open-cell porous materials. l8
Discussion
Figure 4. SEM micrographs of fractured porous polymer specimen with compositions varying along line A of Figure 1: (a) 20% H2O; (b) 25% H2O; (c) 35% H20.
polymerization for 1 h as shown in Figure 3d, and the final product was a transparent solid polymer. A series of bicontinuous microemulsions with a fixed weight ratio of MMA to AUDMAA at 1:1.2 were polymerized with increasing water contents along the composition-line A of Figure 1. Only three SEM micrographs of the transparent polymers obtained are shown in Figure 4. At 20% water content, the polymer already exhibited microporous structures (Figure 4a) and they became more porous at 25% water content (Figure4b) similar to those of parts c and d of Figure 3. More and longer channel-like or random sponge microstructure^^^ were formed at higher water contents up to 45%. Figure 4c shows numerous tiny water channels (dark strips) and polymer islands (17) Pleruschka, P.; Marcelja, S. Langmuir 1994, 10,345.
The ternary system of A U D W w a t e r / M M A exhibits a very large microemulsion region without the need of a cosurfactant. The terminal hydrophobic chain of AUDMAA consists of a polymerizable acryloyl group which is very compatible with MMA, while its hydrophilic head is a betaine type group which is very water soluble. These dual characteristics of AUDMAA are responsible for forming the exceptional wide ranges of compositions of the polymerizable microemulsions. The variation of electrical conductivity as shown in Figure 2 is due to the continuos changes of the microemulsion structure from water-in-oil droplets at low water content ( 60%). The transformation of these types of microstructures of microemulsions is well-known.19-21 It is thus believed t h a t all the compositions used in this polymerization study were bicontinuous microemulsions. (18)McCabe, W.L.; Smith, J. C.; Haniot, P. In Unit Operation of Chemical Engineering, 4th ed.; McGraw-Hill: New York, 1985; p 716. (19) Clausse, M.; Zradba, A.; Nicholas-Morgantini, L. In Microemulsion Systems; Rossano, H. L., Clausse, M., Eds.; Marcel Dekker, Inc.: New York, 1987; p 387. (20) Chen, S. J.; Evans, D. F.; Ninham, B. W.J. Phys. Chem. 1984, 88, 1631. (21)Geoges, J.; Chen, J. W.Colloid Polym. Sei. 1986,264, 896.
Gun et al.
3320 Langmuir, Vol. 11, No. 9, 1995
Not all the transparent microemulsions aRer polymerization would produce transparent solid polymers. Some systems with lower amounts ofAUDMAA ('25%) or high amounts of EGDMA (>4%) and MMA ('50%) produced only turbid polymers or caused the phase separation during polymerization. It is believed that a certain weight ratio of AUDMAA to water is required for stabilizing the oil-water interface of bicontinuous structures during polymerization up to the gel state. The fast formation of copolymer films of AUDMAA and MMA around water channels might minimize the collapse of water domains. The interfacial films could be strengthened by crosslinking with a few percent (14%)ofEGDMA cross-linker. This type of copolymerization and terpolymerization should be carried out as fast as possible in order to reduce the possible rearrangements of microstructures of a precursor microemulsion during the early stage of polymerization. In this study, the fast gelation time of about 5 min was achieved for some polymerizing microemulsion systems. In general, most of the microemulsions investigated could be gelled within 10 min and transparent solid polymers were finally formed. SEM micrographs reveal the randomly distributed bicontinuous structures of water channels and polymer islands for samples containing 20-50 wt % water. The widths of the bicontinuous structures were about 50-70 nm which are similar to those (60-80 nm) reported in literature15J6for bicontinuous microemulsions. This may imply that the bicontinuous structures of the precursor bicontinuous microemulsions might have been retained to a great extent duringpolymerization. The confirmation of these bicontinuous structures, before and after polymerization, by small angle X-ray scattering is under study. We believe that bicontinuous structures can be obtained from a fast polymerization of a bicontinuous microemul-
sion containing a polymerizable surfactant. In fact, similar bicontinuous structures have also been observed from SEM micrographs from our two previous investigat i o n ~ . ~Polymerizable ~J~ surfactants of Na 11-EMUl2and AUTMAB13 were used in the respective microemulsion systems. All three fast polymerization systems using either N a l l - E M U , AUTMAB, or AUDMAA produce transparent solid polymeric materials with open-cell type structures.
Conclusions
An extremely large region of ternary microemulsions can be obtained using a polymerizable zwitterionic AUDMAA, MMA, and water. The bicontinuous microemulsions exist in the water region ranging from about 20 to 50 wt %. These bicontinuous microemulsions can be polymerized to produce transparent solid polymeric materials with open-cell type microstructures. The microstructures also appear to be bicontinuous in nature and may resemble their precursor bicontinuous microemulsions. However, we are unable to confidently answer the question whether the general bicontinuous microstructures can be preserved or not during the polymerization. But we believe that using polymerizable surfactants in bicontinuous microemulsions and with a fast polymerization rate, transparent solid polymeric materials with bicontinuous microstructures can be often obtained. Acknowledgment. The authors are grateful to the National University of Singapore for financial support under Grant RP930630. LA9501187
