Langmuir 1994,lO,4022-4026
4022
Microporous Polymeric Materials from Microemulsion Polymerization L. M. Gan, T. H. Chieng, C. H. Chew, and S. C. Ng* Department of Chemistry, National University of Singapore, Republic of Singapore Received April 6,1994. In Final Form: August 3,1994@ A new microemulsion system consisting of water, methyl methacrylate (MMA), cross-linking agent ethylene glycol dimethacrylate (EGDMA),and a combination of 2-hydroxyethylmethacrylate (HEMA)and sodium 11-(N-ethylacry1amido)undecanoate(Nall-EAAU) was investigated. Both surfactant Nall-EAAU and cosurfactant HEMA are highly polymerizable monomers. Transparent polymeric solids could rapidly be formed by photoinitiated polymerization of some of these microemulsion compositions. It seems that a minimum of 21 wt % of Nall-EAAU is required for producing a single-phase (transparent) polymer network. Only about 4 wt % organic compounds could be extracted out from the polymers which were formed from the systems containingmore than 15wt % MMA. Scanning transmission electron micrographs of the polymeric materials reveal the existence of microporous structures. Open-cell porous structures of transparent polymeric solids could be obtained from those precursor microemulsions with water content higher than 20 w t %, while closed-cell structures from those with less than 20 wt % water. The microporous structures of these polymeric solids are related to the microstructures of the precursor microemulsions.
Introduction There has been an increasing interest in the study of the formation of porous polymeric solids by polymerizing monomers containing microemulsions.1-6 Three types of thermodynamically stable microemulsions are known, namely oil-in-water (o/w), water-in-oil (w/o), and bicontinuous micro emulsion^.^^^ For a bicontinous microemulsion, both organic and aqueous phases coexist in interconnected domains with surfactant molecules mostly confined to the oil-water interface. Porous polymeric solids have been produced by polymerization of MMA or styrene in w/o microemulsions.lt2 Qutubuddin et al.3have reported the polymerization of styrene in bicontinuous microemulsions to produce porous solid materials with maximum porosity. Recently,Cheung et al.4-6 polymerized several microemulsion systems exhibiting both w/o and bicontinuous microstructures. Their microemulsion systems consist of either styrend sodium dodecyl sulfate (SDS)/pentanol or MWacrylic acid (M)/SDS with a cross-linking agent ethylene glycol dimethacrylate (EGDMA) or MMA/EGDWpotassium undecenoate (PUD). It seems possible to control the microcellular structures by the polymerization of microemulsions formulated with PUD, which is not a readily polymerizable anionic surfactant. This is because PUD is very prone to allylic chain transfer reaction that produces only o l i g o m e r ~ . ~In J ~ a recent report, Price"
* To whom correspondence should be addressed at the Department of Physics, National University of Singapore, Republic of Singapore.
Abstract published inAdvance ACSAbstracts, October 1,1994. (1)Stoffer, J. 0; Bone, T. J. Dispersion Sci. Technol. 1980,I , 393. (2)Menger, F. M.; Tsuno, T.; Hammond, G. S.J. Am. Chem. SOC. 1990,112,1263. (3)Qutubuddin,S.;Haque, E.;Benton,W. J.;Fendler, E. J. Inpolymer Association Structures: Microemulsion and Liquid Crystals;El-Nokaly, Magda, 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.;Sasthav, M.; Cheung, H. M. Langmuir 1992, I, 2686. (6)Palani Raj, W.R.; Sasthav, M.; Cheung, H. M. Langmuir 1992, 8. -, 1931. -- - - . (7)Scriven, L.E.Nature 1976,263,123. (8)Talmon, Y.; Prager, S. J. Chem. Phys. 1978,69, 2984. (9)Larrabee, C. E., Jr.; Sprague, E. D. J.Polym. Sci., Polym. Lett. Ed. 1979,17,749. @
0743-7463/94/2410-4022$04.50l0
used a readily polymerizable cationic surfactant in a ternary bicontinuous microemulsion to produce transparent solids with microstructures. Since a decade ago, we have attempted to prepare transparent polymeric solids by polymerizing w/o m i c r o e m ~ l s i o n s . ~ The ~ - ~ ~systems investigated were M W M w a t e r using both nonpolymerizable surfactant SDS and copolymerizable anionic surfactants of sodium acrylamidoundecanoate (NaAAU)or sodium acrylamidostearate (NaAAS). One of the major difficulties encountered in this type of polymerization was the occurrence of phase separation as indicated by the gradual appearance of turbidity during the polymerization. Polymeric solids thus formed were mostly opaque. Although transparent polymeric solids could also be obtained from these systems a t lower water contents (< 15 wt %), they did not seem to exhibit any microstructures. Currently we are investigating the preparation of transparent porous polymeric solids from new microemulsion systems which consist of MMA, 2-hydroxyethyl methacrylate (HEMA), a very reactive surfactant sodium 11-(N-ethylacry1amido)undecanoate(Nal1-EMU), and a cross-linker, EGDMA. Transparent polymeric solids with various microstructures are readily obtainable from polymerization of this new type of microemulsion system, the results of which are discussed in this paper.
Experimental Section Materials. MMA (BDH), EGDMA (Merck), and HEMA (Merck)were purified under reduced pressure. Dibenzylketone (DBK) of greater than 98%purity was used as received from TCI (Japan). Nall-EAAU was synthesized as previouslyreported.16 The deionized and doubly distilled water used has electrical conductivity of approximately 1.0 pS cm-1. Microemulsion Phase Diagram. The single-phase region of each microemulsion system was determined visually by titrating a specific amount of M U , HEMA, and Nall-EAAU with water in a screw-cappedtube at 30 "C. It was thoroughly (10)Paleos, C. M.; Stassinopoulou, C. I.; Malllaris,A. J.Phys. Chem. 1985,87,261. (11)Price, A. US Patent 6161217,1992. (12)Gan, L.M.; Chew, C. H. J.Dispersion Sci. Technol. 1985,4,291. (13)Gan, L. M.; Chew, C. H. J.Dispersion Sci. Technol. 1984,6,179. (14)Chew, C.H.:. Gan,, L. M. J.Polvm. Sci.: Polvm. Chem.Ed. 1985, 23,2226. cl6)Yeoh, K.W.; Chew, C. H.; Gan, L. M.; Koh, L. L. J.Macromol. Sa.-Chem. 1990,A27(1), 63.
0 1994 American Chemical Society
Langmuir, Vol.10, No. 11, 1994 4023
Microporous Polymeric Materials MMA
x (wt%)
20
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I \.
Water
I
I
I
20
I
I
I
50
I
(lOO-x)% N a l l - E M U +x%HEMA
Figure 1. Pseudoternary phase diagram of M W E W Nall-EAAU/water a t 30 "C. The single-phase microemulsion region is represented by the shaded area. The numbers indicate the compositions of which electrical conductivity and viscosity have been investigated as shown in Figure 2. Table 1. Compositions for Microemulsion Polymerization to Form Transparent Polymer Solids microemulsionn M1 M2 M3 M4
water
Nall-EAAU
HEMA
MMA
(wt %)
(wt %)
(wt%)
(wt%)
15 20 25 30
25.5 24.0 22.5 21.0
25.5 24.0 22.5 21.0
34 32 30 28
Equal weight ratio of Nall-EAAU and HEMA. Cross-linker EGDMA was 4 wt % on the basis of the total weight of monomers used in each system, while photoinitiator DBK was 0.3 wt % on the basis of the total weight of each microemulsion system. mixed using a vortex mixer. The clear-turbid boundaries were established from the systematic titrations. The transparent microemulsion regions obtained are shown by the shaded areas in Figure 1. Conductivity and Viscosity Measurements. The conductivity of each microemulsion was measured with an Omega CM-155 conductivity meter with a cell constant 1.029 cm-l. The viscosity of each microemulsion sample was measured using a Cannon (State College, PA) Calibrated Ubbelohde Dilution Viscometer (size 100). Both conductivity and viscosity measurements were carried out at 30 & 0.1 "C. MicroemulsionPolymerization. The amount of the photoinitiator (DBK) added to the system was 0.3% by weight, based on the total weight of each microemulsion sample. The polymerization was performed in nitrogen atmosphere in sealed ampules which were placed in a Rayonet Photochemical reactor chamber operated a t a wavelength of 235.7 nm for a duration of 2 h a t 35 f 0.1 "C. These polymerized samples were then used for morphology observation, leaching test, and thermogravimetric analysis. The rate of polymerization was investigated for sample M4 of composition shown in Table 1. A series of ampules containing microemulsion sample M4 were prepared and polymerized for different durations of time. At the end of each duration time, a sample was taken out and washed thoroughly in alarge quantity of methanol and hexane. The sample was then washed in hot water a t 50 f 0.1 "C for 48 h. This is to ensure that the unpolymerized surfactant, Nall-EAAU, and uncross-linked polymer, poly(Nal1-EMU), which is water-soluble, and any unreacted monomers would be extracted out from the solid polymer. Each sample was then dried in avacuum oven a t room temperature until a constant weight was attained. Polymer conversion in terms of cross-linked polymer was calculated on the basis of the total amount of monomers added to the precursor microemulsion. Morphology Observation. A JEOL JEM lOOCXII scanning transmission electron microscope (STEM)was used to study the polymer morphology. Samples of STEM studies were first cooled in liquid nitrogen and then fractured mechanically. The fractured
samples were vacuum-dried for 24 h a t room temperature before being coated with gold using a JEOL ion sputter JF'C-1100 coating machine. DryingRate ofwater Desorption. The dryingrate ofwater desorption from the polymerized samples were monitored using a Dupont Instruments TGA 2100 thermogravimetric analyzer. The polymer was dried in a stream of dry nitrogen gas isothermally a t 75 "C for 5 h. The drying rate curve for each polymer sample was constructed using the data of weight loss of the sample, which was recorded as a function of time by the TGAusingthe Dupont thermal analyst systemTGA5.1 program. On the basis of the shape of the drying rate curve obtained, a distinction can be made between open-cell porous structures and closed-cell porous structures in the polymer. Solid materials having the closed-cell structure will exhibit a drying rate curve which has an exponential falling rate period whereas the curve for the open-cell structure has a linear falling rate period.'B-l* Leachingof Polymer Solids. The final cross-linked polymer matrix was investigated by a leaching test. The vacuum-dried polymer samples were washed with methanol and hexane and then leached in hot water a t 50 f 0.1 "C for 48 h. The loss of weight was attributed to the unreacted monomers and possibly the homopolymer of Nall-EAAU, which is water soluble.
Results Phase Behavior of Microemulsions. In the absence of reactant HEMA, which acts as a cosurfactant for the system of H20/MMA/Nall-EAAU, the ternary microemulsion region emanates narrowly from the water-rich region and extends toward the Nall-EAAU region as shown in Figure 1. The maximum solubility of MMA was only about 20 wt % in this ternary system which required about 40 wt % Nall-EAAU. However, when HEMA was added to the ternary system, the single-phase region expanded enormously and extended toward both the oilrich and surfactant-rich apexes. The solubility of MMA in the system increased tremendously to about 85 wt % when HEMAreplaced 70 wt % of N a l l - E M U . The large microemulsion regions enclosed by the boundary lines as shown in Figure 1are generally known as isotropic Winsor IV microemulsion system^.'^^^^ In an attempt to probe into the microemulsion structure of the system containing equal amounts of Nall-EAAU and HEMA, electrical conductivity and viscosity of compositions indicated by the numbers of Figure 1 were measured. In Figure 2,the electrical conductivity and viscosity are plotted as a function of the water content in the microemulsion system. There was a small increase in the electrical conductivity (about 1.5 mS cm-') with increasing water content in the system when the water content was less than 20% by weight. This was followed by a rapid increase in the electrical conductivity up to a maximum of 14 mS cm-l when the water content in the microemulsion was increased to about 70 wt %. I t then decreased on further increase in the water content. A similar trend was also observed in variation of viscosity as a function of the water content in the microemulsion. The systems with the water contents ranging from about 20 to 50 wt % exhibited higher viscosities than those of other compositions. Polymerizationof MicroemulsionSystems. Transparent microemulsions of various compositions based on Figure 1,in principle, can be chosen for polymerization study. Table 1shows some ofthe compositions that would (16) Coulson, J. M.; Richardson, J. F. Chemical Engineering, 2nd ed.; Pergamon Press; New York, 1968; Vol 11, p 620. (17) McCabe, W. L.; Smith, J. C.; Harriot, P. Unit Operations of Chemical Engineering, 4th ed.; McGraw-Hill New York, 1985; p 716. (18)Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. J.Appl. Polym. Sci. 1993,47,499. (19)Winsor, P. A. Trans. Faraday SOC.1948,44, 376. (20)Clausse, M.; Heil,J.;Peyrelasse,J.;Boned,C. J.ColloidZnterface Sci. 1982, 87, 584.
Gun et al.
4024 Langmuir, Vol. 10,No.11, 1994 15
, 8
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40
60
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Water content in microemulsion (wt%)
Figure 2. Electrical conductivity and viscosity curves plotted as a fbnction of the water content across the single-phase
microemulsion region with compositions indicated by the numbers shown in Figure 1. An equal weight ratio of NallEAAU and HEMA was used.
30
0
SO
80
Time/min
Figure 8. Polymer conversion as a function of reaction time
for sample M4.
Table 2. Effect of Surfactant Concentration on the Stability of Microemulsions Before and After Polymerization
microemulsion systema
physical
MMA
HEMA
Nall-EMU
water
(wt%)
(wt%)
(wt 9%)
(wt%)
BP
AP
25 25 25 25 20 20 20
25 25 25 25
12.8 15 20 25 20 25
37.2 35 30 25 40 35 30
C C C C C C C
T T T1 C T1 C C
20 20 20
30
Cross-linkerEGDMAwas 4 w t % on the basis of the total weight of monomers used in each system, while DBK was 0.3 wt % on the basis of the total weight of each microemulsion system. BP = before polymerization, AP = a h r polymerization, C = clear, T = turbid, Tl = transluscent. 5
produce transparent polymer solids after polymerization. The effect of Nall-EAAU concentration on the stability of microemulsions before and after polymerization is shown in Table 2. Some of the microemulsion compositions produced only transluscent or turbid solid polymers after polymerization. The rates of polymerizations for the systems investigated (Table 1)were generally fast. For example, Figure 3 shows a typical polymerization curve for sample M4. The polymerization initially proceeded rather slowly up to about 10min. The system then became progressively more viscous and formed a transparent gel as accompanied by a sharp increase in the polymer conversion. The transparent polymer solid with no apparent phase separation was obtained within 1 h of polymerization. The gelation process of this kind of polymerizable microemulsion system has also studied recently by usz1 using Brillouin light scattering. Characterization of Transparent Solid Polymer. Figure 4 shows the weight loss of organic compounds on leaching of solid polymers as a function of MMA content in the precursor microemulsions which contained equal amounts of Nall-EAAU and HEMA. The cross-linker EGDMA added to the systems was 4 wt %, based on the combined weight of MMA, HEMA, and N a l l - E M U . The weight loss decreased with the increase of MMA content up to about 15 wt % in the precursor microemulsion. No (21)Ng, 5. C.; Gan, L. M.; Chew, C. H.; Chieng, T. H. Polymer 1994, 36, 2701.
o ! 0
7
14
21
28
35
MMA content (%) Figure 4. Weight loss on leaching of polymer matrix as a function of MMA content in the precursor microemulsion containing an equal amount of Nall-EAAU and HEMA. The cross-linker EGDMA was 4 wt % on the basis of the combined weight of MMA, HEMA, and Nall-EMU.
further decrease in weight loss was observed for the system containing more than 15w t % MMA. Thus the minimum weight loss for the system was about 4 wt %. This implies that almost all N a l l - E M U , HEMA, and MMA might have cross-linked with EGDMA or among themselves to form a stable polymer matrix. STEM micrographs of the polymerized solids from precursor microemulsions M3 and M4 containing 25%and 30% water are shown in Figures 5 and 6, respectively. Figures 5a and 6a show the morphology of the polymer surface facing the nitrogen atmosphere when photopolymerized in the glass ampules, while Figures 5b and 6b show the morphology of their fractured surface. Both samples revealed the existence of porous structures resembling that of a bicontinous structure. The dimensions of the pores (channels) were in the range of 20-50 nm. The porous structure of the sample containing 30% water seems to be more connected than that containing 25% water. The water desorption from the transparent solid polymer was studied by thermogravimetric analysis (TGA). The results from TGA are presented in Figures 7 and 8. Figure 7 illustrates the drying rate curve for the polymer obtained from a precursor microemulsion containing 15% water. The exponential falling rate period exhibited is typical for the closed-cell porous structure. On the other hand, the drying rate curves for polymers obtained from precursor microemulsions containing 25
Langmuir, Vol. 10, No. 11, 1994 4025
Microporous Polymeric Materials
Figure 5. STEM micrograph of the porous polymer sample M3: (a) surface facing the nitrogen atmosphere; (b) fractured surface.
and 30 wt % water are shown in Figure 8. These drying rate curves show a linear falling rate period which is characteristic of open-cell porous materials. 17918 It is to be noted that the linear falling rate period increased with the water content of the precursor microemulsions. Discussion Phase behavior studies on the MMA/HEMA/NallEAAU/water system indicate the existence of a singlephase region having microstructures resembling that of Winsor IV microemulsions. The enlarging single-phase region, as depicted in Figure 1, on increasing the ratio of H E W N a l l - E M U , shows clearly that HEMA can function as a cosurfactant. This is attributed to the amphiphilic nature of HEMA which possesses both the hydrophilic hydroxyl group and hydrophobic vinyl group. In these microemulsions, water domains (droplets or channels) would be stabilized by both polymerizable surfactant N a l l - E M U and cosurfactant HEMA, with MMA being the oil continuous phase. These three components consist of a common polymerizable vinyl group. It is anticipated that some microstructures of the microemulsions could still be observed after total polymerization of these reactive monomers. The variation of electrical conductivity as shown in Figure 2 is due to the continuous change of the microemulsion structure from water-in-oildroplets at low water content ( 70%). The transformation of these microstructures is well k n ~ ~ n . ~ ~ - ~ ~ With an increase in the water content in the oil-rich region of microemulsions, the increase in the viscosity (22) Clausse, M.; Zradba, A.; Nicholas-Morgantini,L. In Microemulsion Systems; Rosano, H. L.; Clausse, M., Eds.; Marcel Dekker Inc.: New York, 1987; p 387. (23) Chen, S. J.; Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1984, 88, 1631. (24) Georges, J.; Chen, J. W. Colloid Polym. Sci. 1986,264, 896.
Gun et al.
4026 Langmuir, Vol. 10, No. 11, 1994 700
420
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1
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R h
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a photoinitiator DBK. For instance, some transparent solid polymers could even be formed within 15 min. The polymers obtained from microemulsions containing less than 20 w t k water show closed-cell type microstructures, as evinced from Figure 7. These microstructures may be related to the water domains of their precursor microemulsions. The STEM micrographs of fractured solid polymers containing 26 and 30 wt % water reveal the existence of bicontinuous structures with the dimension of the pores on the order of 20-50 nm, which is comparableto the typical width (60-80nm) ofchannels of bicontinuous microemulsions as reported in the literature.a6-27The increment in the linear drying rate period with the increase of the water content of the precursor microemulsions is attributed to the formation of more interconnected microstructures in the polymer sample. It is substantiated by the STEM shown in Figure 6. In order to minimize the modification of microstructures of precursor microemulsions during the polymerization, it is imperative to have a fast rate of cross-polymerizations at a relatively low temperature. Since monomers NaE M U , HEMA, MMA, and EGDMA are all very reactive, they might have cross-polymerized in forming the crosslinked polymer matrix, as evidenced from the low leaching content of the polymer. This is highly possible because all four monomers possess the common polymerizable vinyl group. We have reported earlier that sodium acrylamidoalkanoates, which are similar to Nall-EAAU, can readily be copolymerized with vinyl monomer^.^'
W0 0
0.07
0.14
0.21
0.28
0.35
Free Moisture (mg/mg polymer)
Figure 8. Drying rate curves for porous polymer samples M3 and M4.
may be due to the continuous increase in size and number of w/o droplets in the system. Similarly, the substantial increase in the viscosity for the water-rich region of microemulsion may also be due to the increase in number and size of o/w droplets when the high water content was decreased. However, microemulsions in between the two extreme regions exhibit a nearly plateau region of the maximum viscosity. This is attributed to the existence of bicontinuous structures in which the connected water channels become larger in dimension, without a considerble increase in number, on increasing the water content, as reported6pa4for other microemulsions. Not all the transparent microemulsions would produce transparent solid polymers aRer polymerization. Some systems produced transluscent or turbid polymers. This is possibly due to the coalescence of some water domains that somewhat destabilize the system during the polymerization. However, most of the turbid polymers change to transparent polymers after being dried in an oven at 100 "Cfor about 30 min. This means that the turbidity of the polymers may possibly be due to the presence of the segregated water in the polymer matrix. A certain minimum amount of polymerizable surfactant Nall-EAAU (about 21 wt %) was required for producing transparent solid polymers, as shown in Tables 1and 2. It is believed that a certain weight ratio of Nall-EAAU to water is crucial for stabilizing oil-water interfaces during the polymerization up to the gel state. The formation of copolymer and/or terpolymer films of N a l l EAAUiHEMAiMMA around water droplets or water channels might minimize the collapse of water domains. The inclusion of 4 w t % cross-linker of EGDMA in the system further strengthens the interfacial films. In addition, this type of polymerization should proceed rapidly so as to minimize the time available for the reacting system to undergo possible rearrangements of microstructures of the precursor microemulsion. The fast polymerization rate was achieved in this study by using
Conclusions The results from the conductivity and viscosity measurements on the system of MMA/water/HEMA/NallEAAU/EGDMA indicate the existence of three different microstructures of microemulsions. The microstructure of the system is dependent on its composition. At water contents less than 20 wt %, the w/o droplet structure exists, whereas o/w droplets are formed at water contents greater than 70 wt %. At intermediate water contents, the bicontinuous microstructure prevails. All the organic components in the microemulsions could easily be copolymerized or/and cross-polymerized to form polymeric solids. A minimum amount of 21 w t % N a l l EAAU and a fast rate of polymerization are essential for the system to produce a transparent polymer matrix from a precursor microemulsion. The microstructures of the polymers are related to the water contents in the precursor microemulsions. By adjustment of the water content, transparent solid polymers with a closed-cell or an opencell structure can be produced readily. Moreover, the solid polymers are highly cross-linked when more than 15 w t % MMA and 4 wt % EGDMA are used. (26) Bodet, J.F.;Bellare, J. R.;Davis, H.T.;Sciven, L. E.;Miller, W. G. J . Phys. Chem. 1988,92, 1898. (26) Jahn,W.;Strey, R. J. Phys. Chem. 1988,92,2294. (27) Yeoh, K W.;Chew, C. H.; Gan, L. M.;Koh, L.L.; Ng, S.C. J. Macromol. ScL.-Chem. lSe0, A27(6), 711.
