1378
Langmuir 1991, 7, 1378-1382
Characterization and Polymerization of Middle-Phase Microemulsions in Styrene/Water Systems Mohan Sasthav and H. Michael Cheung' Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325-3906 Received July 13,1990. In Final Form: February 19, 1991 The products of photopolymerizing middle-phase microemulsions containing sodium dodecyl sulfate as the surfactant, styrene as the monomer, NaCl or NaSCN as electrolyte, and either 2-pentanol or a combinationof 5-hexene-1,2-dioland sodium styrenesulfonate as cosurfactant were characterized by using differentialscanningcalorimetry,gel permeationchromatography,and scanningelectronmicroscopy (SEMI. Phase separation during polymerization was evident for the 2-pentanol containing microemulsions. The use of 5-hexene-1,2-dioland sodium styrenesulfonateeliminated gross evidence of phase separation during polymerization although SEM examination indicated that phase separation still occurred at a length scale of around 10 pm. A higher rate of radical generation combined with chain transfer to the cosurfactant results in lower molecular weights for the microemulsion-derivedpolymer than for polymer formed from bulk polymerization. This was especially true when 2-pentanol was the cosurfactant. Polymer formed from microemulsions using the 5-hexene-1,2-dioland sodium styrenesulfonate combination as cosurfactant had a somewhat higher molecular weight. The presence of the surfactant raised the glass transition temperature of the microemulsion-derived polymer relative to the excess monomer phase polymer.
Introduction Transparent or translucent systems are formed spontaneously when oil and water are mixed with relatively large amounts of an ionic surfactant combined with a cosurfactant, e.g. a medium size alcohol. On the other hand nonionic surfactants and twin tail surfactants can individually generate microemulsions. They differ from ordinary emulsions not only in their lack of turbidity but, more essentially, in being thermodynamically stable. The middle-phase microemulsion may be bicontinuous in microstructure where both the organic and aqueous phases coexist in interconnecting domains with surfactant molecules located at the interface. Applications of microemulsions may be based on the low interfacial tensions, on the possibility of preparing nearly homogeneous mixtures of oil- and water-soluble substances, and also on the near uniform droplet size which occurs in microemulsions. The more common uses of microemulsions have been in tertiary oil recovery, cutting oils, dry cleaning fluids, and wax polishes. An application that has recently gained attention is polymerization in microemulsionsystems. Microemulsionsprovide an unique medium for polymerization of both hydrophilic and hydrophobic monomers. It may be possible to exploit this environment to produce unique polymers and copolymers and/or to produce microstructured polymeric solids. Most of the studies in the field of microemulsion polymerization have dealt with the formation of microlatex particles and primarily with oil-continuous microemulsions. Atik and Thomas' were the first to report an account of microemulsion polymerization that produced spherical latex particles ranging in size from 200 to 400A in diameter. An oil in water microemulsion consisting of cetyltrimethylammonium bromide, styrene, and hexanol in water was prepared and polymerized thermally (60 "C)and radiolytically (Cs y-ray source). Stoffer and Bone2 also reported the preparation and polymerization of monomers in W/O microemulsion systems. They showed that microemulsions were suitable as a medium for production of polymers in which molecular weight can be controlled and
* Authors to whom correspondence should be addressed.
(1) Atik, S. S.; Thomas, J. K. J.Am. Chem. SOC. 1981,103,4279-4280. (2) Stoffer, J. 0.; Bone, T. J. Dispersion Sci. Technol. 1980,1,37-54.
predicted by variations in initiator and chain transfer agents using known equation^.^ Molecular weight dependencies seemed to parallel solution, rather than emulsion, behavior. They studied the morphology of the resultant polymers and observed that the cosurfactant 1-pentanol acts as a chain transfer agent. They encountered phase separation problems resulting in a polymer with large pore structure. Turro and K U Ophotopoly~ merized styrene/water microemulsions by using dibenzyl ketone as initiator. The molecular weights of polymers produced were on the order of lo5, and the polydispersity indexes were in the range 1.6-2.2. Rabagliati, et aL6 reported the polymerization of styrene in a three-phase microemulsion-oil-water system. They concluded that polymerization in three-phase systems produces the same results, regardless of the location of the initiator. The system exhibited features of a solution polymerization process. When the initiator concentration was increased, the rate of polymerization increased, and the molecular weight decreased. No autoacceleration was detected. Qutubuddin andHaque6were the first to report an account of the preparation of solid porous materials by the polymerization of all three types of microemulsions. One of the major difficulties during the polymerization was the phase separation of the cosurfactant from the microemulsion. SEM micrographs indicated porous structures in the solid materials obtained by polymerization of microemulsions. Both Atik and Thomas' and Qutubuddin and Haques reported phase separation problems during the thermal polymerization of microemulsions. In this work, photopolymerization was examined as a possible' means of ameliorating the phase separation problem. Photopolymerization is an attractive method for producing polymer solids from middle-phase microemulsions. It alleviates concerns about attainment of thermodynamic (3) Stoffer, J. 0.;Bone, T. J. Dispersion Sci. Technol. 1980,1, 393412.
(4) Kuo, P. L.; Turro, N. J.; et alMacromolecules 1987,20,1216-1221. (5) Rabaglinti, F. M.; Falcon, A. C.; et al. J. Dispersion Sci. Technol. 1986, 7,245-258. (6) Qutubuddin, S.; Haque, E.; et al. Polymer Assoc. Structures; ACS
Symposium Series; El-Nokaly, M. A. Ed.; American Chemical Society: Washington, DC, 1989. 0 1991 American Chemical Society
Langmuir, Vol. 7, No.7, 1991 1379
Middle- Phase Microemulsions in StyrenelWater Systems
equilibrium prior to initiation of polymerization which may be partially responsible for phase separation, since it can be carried out at a low enough temperature that nonphotoinduced initiation is negligible. Though the systems studied, in this work all equilibrated within a few hours this feature of photopolymerization may be especially important for polymerization in slowly equilibrating systems, for example those using nonionic surfactants without cosurfactants. Also, higher polymerization rates are attainable with photopolymerization which may permit the polymerization to rapidly proceed beyond where phase separation would significantly alter the microstructure of the polymer. In this work we studied the phase behavior of styrene/ water/SDS microemulsionscontaining nearly equal volumes of styrene and water with varying surfactant, cosurfactant, and electrolyte concentrations. NaCl and 2-pentanol were used as the salt and cosurfactant for all of the phase behavior and much of the polymerization work. However we examined the effects on the polymeric products of using mixtures of sodium styrenesulfonate (SSS) and 5-hexene-1,2-diol as copolymerizable cosurfactants and NaSCN as the salt. The microemulsions were photopolymerized with either visible light from a 500-Wincandescent lamp or ultraviolet light from a 450-Wmercury vapor lamp. The resulting polymers were characterized by using GPC for molecular weight and polydispersity, SEM for morphology, and DSC for glass transition and melting temperature. Materials Styrene (99.9 % with 10-15 ppm 4-tert-butylcatecholinhibitor), 2-pentanol (98%), NaCl (99%), 5-hexene-1,a-diol (go%), and SDS (98%),allfromAldrichChemicalCo.,wereusedassupplied. Sodium styrenesulfonate (technical grade) was obtained from Du Pont. Water was doubly distilled. Samples for polymerization were prepared by using inhibitor-freestyrene. The initiator benzoin was used as supplied from Aldrich.
Experimental Section Samples were prepared and sealed in 10-mL disposable serological pipets (Fisher). The tip of the pipet was sealed with a blowtorch. The various components were then filled in the pipet with an Eppendorf digital pipet (100-1000 NL). A small plug of the sample, at the top of the pipet, was frozen by liquid nitrogen and then the open end of the pipet was sealed under vacuum with a blowtorch. The sealed samples were then placed in a water bath and equilibrated at the temperatures of interest for the phase behavior studies. Samples were placed in the bath over a period of 12 h to attain thermal equilibrium and then the volumes of the various phases were noted. The parameters that were varied in order to plot the phase diagram were as follows: styrene composition,50 and 40% (by volume); SDSconcentration, 0.5-9.0% (byweight);surfactant/cosurfactantratio,0.14.6;NaCl concentration, 1-5% (by weight); temperature, 30 and 40 OC. Samples for polymerization were prepared in culture tubes. Small holes were drilled on the screw cap and the inside of the cap was lined with a special butyl rubber gasket. After the cap was tightened onto the tube, two 6-in. syringes were pierced through the specially designed cap. One of them served as the inlet port for the purge gas, nitrogen, and the other syringe served as the outlet. Samples were nitrogen purged for 15-20 min. The purged sample was then placed in a photoreactor over a period of 15-24 h until a solid polymer was obtained. A 500-W incandescent lamp was used as the source of visible light while a 450-W mercury vapor lamp (Hanovia) was used for experiments with ultraviolet light. The results were the same regardless of the light source used; however the time required for ultraviolet photopolymerization was approximately half that required for visible photopolymerization. Both the virgin solid and the reprecipitated polymers were then subjected to analysis by using gel permeation chromatography (GPC), scanning electron microscopy (SEM), gas chro-
-0 0.7 2 0.6 0.8
*,
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-
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6.0
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Figure 1. Volumetric phase behavior of 50 % styrene containing microemulsion with sodium dodecyl sulfate to 2-pentanol ratio of 1:4 and 1%NaCl. Temperature was 30 OC. matography/mass spectrometry (GC/MS), and differential scanning calorimetry (DSC). Reprecipitation was accomplished by first dissolving the polymer in sufficient benzene to form a 5-7 w t % solution. Methanol was then added to precipitate the polystyrene. The precipitated solid was then slowly filtered and dried under vacuum. A JSM-U3 scanning electron microscope was used to study the structure of the polymer solids. The polymer obtained from the reactor was freeze shattered by immersing it in liquid nitrogen, followed by mechanical fracturing. Each sample was sputter coated with gold for 35 s before examination under the SEM. The thermal properties were measured with a Du Pont-910 DSC system. The temperature was ramped from room temperature to 550 OC for measuring the crystalline melting temperature and to 160 "C for glass transition temperature (T,)measurements. A ramp of 25 OC/min was used for most samples. Selected samples were also examined using a 10 OC/min ramp to verify that there were no ramp-related effects. The molecular weights (M,and M,,) and their distribution were determined with a Waters Associates Model 150C GPC. GPC studies were performed with both virgin as well as reprecipitated polymers. Twenty-five milligrams of the polymer was dissolved in 10 mL of tetrahydrofuran, filtered, and then injected into the GPC column.
Results The phase behavior of mixtures of water, styrene, SDS, 2-pentanol, and NaCl follows general patterns that originate from the interplay between the lower miscibility gap of the binary mixture oil-surfactant and the upper miscibility gap of the binary mixture water-surfactant. The trends were observed by plotting isothermal sections of the phase prism at a constant ratio of surfactant to cosurfactant and constant NaCl concentration. Figure 1 illustrates the phase behavior for an SDS:2pentanol ratio of 1:4 and 1 wt 7% NaC1. The three-phase body exists only within a well-defined interval. At very low SDS concentrations a turbid and coarse emulsion is formed. As the SDS concentration is gradually increased, a lower HsO-rich phase microemulsion (A) in equilibrium with an excess oil phase is formed. Further increase in the SDS level results in the formation of a middle amphiphile-rich phase (B)in equilibrium with an excess-oil and an excess-water phase. A t high concentrations of SDS (above 5.5%) an upper oil-rich microemulsion in equilibrium with an excess-water phase is formed. Thus a gradual increase in SDS concentration is accompanied by a transition from O/W through a three phase to a W/O microemulsion. The volume fraction of the middle phase is represented by the height between the curved boundaries at any SDS concentration. Volume fractions as high as 0.8 were
1380 Langmuir, VoZ. 7, No. 7, 1991
Sasthau and Cheung
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Figumt. Volumetricphasebehaviorof 50% styrenecontaining microemulaions, effects of electrolyte concentration. Microemulsions contained sodium dodecyl sulfate and 2-pentanol in 1:2 ratio. Temperature was 30 O C . Middle-phaseenvelopes are shown for 1% to 4% NaC1.
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Figure 4. Electron micrograph of middle phase microemulaion derived polymer. Microemulsion composition was 6% sodium dodecyl sulfate, 20 % 2-pentanol, 1% NaCl,M% styrene, and the balance water. Magnification was 6000X. Smooth microporous region shown. Sample is the same as shown in Figure 5.
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Figure3. Volumetricphase behaviorof 50% styrene containing microemulsions, effects of surfactant/cosurfactant ratio. Microemulsions contained 2% NaCl. Temperature was 30 O C . Middle-phaseenvelopes are shown for 25,1:4, and 3:20 sodium dodecyl sulfate to 2-pentanol ratios.
observed for some of the middlephase samples in this prospective study. The effect of the electrolyte on the phase behavior was also studied (Figure 2). At a constant SDS concentration and ratio of surfactant to cosurfactant an increase in NaCl concentration results in a transition from a lower phase to three-phase regime and consequently to an upper-phase regime. A small range of NaCl concentration of 1 to 3% seems to be an optimum composition to form sizable amounts of middle-phase microemulsions. The ratio of surfactant to cosurfactant (S/CS) has a significant bearing on the phase behavior of the microemulsion (Figure 3). For a particular concentration of NaCl (2%), an increase in S/CS shifts the three-phase region to the right. Thus by incorporation of a higher fraction of cosurfactant (lower S/CS) in the microemulsion, a lesser amount of SDS is required to form the microemulsion. This property can be exploited in preparing polymers from microemulsions where the cosurfactant is incorporated in the polymer microstructure. The effect of temperature on the phase behavior was studied at 30 and 40 O C . There was no significant difference in the phase behavior at the two temperatures. The anionic surfactnat, SDS, becomes more hydrophilic astemperature increases, whereas the opposite occurs with
the nonionic cosurfactant, 2-pentanol. Hence, an appropriate mixture of both kinds of surfactants becomes insensitive to temperature, owing to the trade-off between the opposite effects on each kind of surfactant. Scanning electron microscopy was used to examine the microstructure of the polymer solids produced by photopolymerization of the middle-phase microemulsions. Figures 4,5, and 6 are representative SEM micrographs for these samples. The middle phase polymers containing SDS and 2-pentanol were white and crumbly in appearance. The samples containing 2-pentanol had a cosurfactant-rich layer above the polymer, indicating that 2-pentanol was squeezed out of the microstructure. The "swiss cheese" type structure is a result of coalescence of a highly dispersed phase present in the original microemulsion. SEM examination showed that two types of structures were evident in these polymerized middle-phase microemulsions: (1)a somewhat microporous structure with pores in the 0.1-2 pm range, Figure 4; (2) a fibrous looking structure composed of needlelike polymer segments approximately 0.2 pm in diameter and a few micrometers in length, Figure 5. The smallest visible pores are in the range of 0.1 pm, which is much too large to be remnants of the original microemulsion structure. Polymers from middle-phase microemulsions containing sodium styrenesulfonate were more homogeneous, rubbery, yellowish, and opaque in appearance. These samples did not have a cosurfactant-rich layer above the polymer suggesting that sodium styrenesulfonate was incorporated in the polymer microstructure. SEM examination revealed a porous structure with pores in the 5-10 pm range, Figure 6. Table I lists the GPC results; the weight average molecular weight, M,,the number average molecular weight, M n , and their ratio, M w / M n , commonly called the polydispersity index, for the polymers obtained from polymerization of the middle phase and excess monomer phase for five representative SDS/styrene-based microemulsions are shown. The middle-phase polymers and the polymers formed by the excess organic phase (bulk phase) showed distinct differences in their molecular weights and their
Middle-Phase Microemulsions in StyrenelWater Systems
Langmuir, Vol. 7, No. 7, 1991 1381 Table I. Gel Permeation ChromatographyResults for Polymers Derived from the Excess Monomer Phase (Bulk Phase) and the Middlephase Microemuleiom* samcomposition Ple before no. polymerization system Mw Ma &/Ma 1 styrene. 40% excess 83500 26920 3.101 SDS, 7.0%
2
2-pentanol, 1.555 mL NaC1,2% benzoin, 0.1 g styrene, 40% SDS, 7.0% SSS, 5%; HD, 1.2 mL
3
NaCl, 12% benzoin, 0.25 g styrene, 40% SDS, 7.0% SSS, 0.2 g; 2-pent,
middle 83540 323M) P h excess 102400 54330 Phase
2.582
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72930 37800
1.93
78660 35500
2.216
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middle P h excess P h
57460 21320
2.695
80970 38920
2.081
middle 111800 61580 P h excess 69300 21370 Phase
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Figure S. Electron micrograph of middle phase microemulsion derived polymer. Microemulsion composition was 6% sodium dodecyl sulfate, 20% 2-pentanol, 1% NaCl, 50% styrene, and the balance water. Magnification was 6000X. Needle-type structure region shown. Sample is the same as shown in Figure
5
4. 6
7
NaCl, 2% benzoin, 0.15 g styrene, 40% SDS, 6.75% 2-pentanol, 1.35 mL NaCl, 3% benzoin, 0.2 g styrene, 40% SDS, 7.5% 2-pentanol, 1.5 mL NaSCN, 3% benzoin, 0.2 g styrene, 40% SDS, 6.75% 2-pentanol,l.35 mL NaCl, 3% benzoin, 0.3 g styrene, 40% SDS, 5.0% SSS, 0.1 g; 2-pent,
P
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Figure 6. Electron micrograph of middle phase microemulsion
derived polymer. Microemulsion composition was 7 % sodium dodecyl sulfate, 5% sodium styrenesulfonate, 12% 5-hexene1,2-diol, 50% styrene, 12% NaC1, and the balance water. Magnification was 1000X.
distributions. For example (sample 1 in Table I), the bulk polymer had a polydispersity index of 3.1 while the middlephase polymer from the same sample had a polydispersity of 2.58. It is known that the rate of bulk polymerization is greater than that of microemulsion polymerization. These results indicate that the restricted environment in the microemulsion promotes a more ordered addition of monomers to the growing polymer chains. This could also suggest that the polymer chains in the bulk polymer are atactic compared to the chains in the middle-phase polymer. Lower molecular weights of these polymers (80 O00) compared to polymers obtained in latexes (106)
NaCl, 3% benzoin, 0.15 g 8 styrene, 40% SDS, 7.0% 2-pentanol, 1.555 mL NaCl, 2% benzoin, 0.3 g 9 styrene, 40% SDS, 7.0% SSS, 6%; HD,1.1 mL NaCl, 13.5% benzoin, 0.25 g 10 styrene, 40% SDS, 6.75% 2-pentanol, 1.35 mL NaCl, 3% benzoin, 0.1 g 11 styrene, 40% SDS, 7.0% 2-pentanol, 1.555 mL NaCl, 2% benzoin, 0.2 g
middle P h excess P h
3.244
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1.961
middle 117600 76130 P h excess 145100 61490 P h
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74240 23540
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are probably due to a high rate of radical generation and chain transfer to the cosurfactant. The number average molecular weight (M,) of polymers containing 2-pentanol (sample 1)is lower than that of the polymers containing sodium styrenesulfonate and/or 5-hexene-1,2-diol as the cosurfactant (samples 2 and 3). This is probably due to the incorporation of the cosurfactants in the polymer chain and also due to the chain transfer effect of 2-pentanol. Table I1 presents the data for the thermal properties of the polymers (Tgand Tm).The glass transition temperature (Tg), which is a characteristic of the amorphouspart
1382 Langmuir, Vol. 7, No. 7,1991 Table 11. Differential Scanning Calorimetry Results for Polymers Derived from the Excess Monomer Phase (Bulk Phase) and the Middle-PhaseMicroemulsions. 88111composition before Tg, Tm 'no. le polymerization system OC OC 1 styrene, 40% excess 64.00 454.23 SDS,7.50% phase 2-pentanol, 1.5 mL NaSCN, 3% middle 86.54 443.74 benzoin, 0.2 g phase 2 styrene, 40% excess 451.44 SDS,5.0% phase 2-pentanol, 1.0 mL NaSCN, 4% middle 53.85 450.55 benzoin, 0.15 g phase 3 styrene, 40% excess 67.31 458.74 SDS, 6.0% phase SSS, 0.2 g; 2-pent, 1.0 mL NaCl, 3% middle 97.86 466.34 benzoin, 0.15 g phase 4 styrene, 40% excess 79.69 453.79 SDS,7.0% phase SSS,5%; HD,1.2 mL NaCl, 12% middle 62.50 464.25 benzoin, 0.25 g phase 5 styrene, 40% middle 60.58 463.48 SDS,7.0% phase SSS,5%; HD,1.2 mL NaC1, 12% benzoin, 0.25 g 6 styrene, 50% excess 73.08 454.62 SDS,6.0% phase SSS,4%; HD,1.2 mL NaC1,9% middle 93.27 464.05 phase benzoin, 0.1 g 7 styrene, 40% WIQ 64.90 453.36 microemulsion SDS,7.50% 2-pentanol, 1.667 mL NaCl, 3% benzoin, 0.2 g 8 styrene, 40% excess SDS, 7.0% phase 2-pentanol, 1.40 mL NaSCN, 3% middle 63.46 443.31 benzoin, 0.2 g phase 9 styrene, 40% excess 79.00 SDS,7.0% phase 2-pentanol, 1.556 mL NaCl, 2% middle 84.00 benzoin, 0.1 g phase 10 styrene, 40% middle 101.00 SDS,5.5% phase 2-pentanol, 1.10 mL NaSCN, 4% benzoin, 0.15 g 11 styrene, 40% middle 86.00 SDS,4.5% phase 2-pentanol, 0.90 mL NaSCN, 4% benzoin, 0.15 g 12 styrene, 40% w/o 77.33 microemulsion SDS,4.5% 2-pentanol, 1.8 mL NaC1,3% benzoin, 0.2 g 0 All samples were 10 mL total volume prior to polymerization.
of a polymer, occurs as a discontinuity in the temperature derivative of the energy, heat content, entropy, and volume. Among the variables that affect Tgare the chain microstructure (tacticity), molecular weight and distribution, cross-linking, and presence of low-molecular-weight com-
Sasthav and Cheung pounds such as surfactants. Any molecular parameter affecting chain mobility can be expected to influence TB' In this study DSC was used to determine T, and Tm. The polymers formed by the excess organic phase have a lower Tg compared to the middle-phase polymer, indicating that the chains in the middle-phase polymers are stiffer. There is a strong interaction between the ?r electrons of polystyrene and the surfactant molecules, in the presence of water. This results in increased stiffness and increased physical cross-linking. A higher T, (114 "C)for polymers obtained thermallp compared to the polymers, above, may be explained due to increased molecular weight and cross-linking (indicated by gelation). In order to find out how the surfactant was present in the polymer (i.e., physically trapped or whether there is some bonding), the virgin solid was dissolved in benzene and then precipitated from methanol. The Tgof the precipitated solid was found to be lower than that of the virgin solid. This suggests that most of the SDS was physically entrapped in the polymer matrix while a part of it was chemically bonded within the polymer. The melting point of the middle-phase polymer was also higher than that of the bulk polymer, suggesting that the crystalline portion of the polymer was more ordered in the middle-phase polymer.
Summary The bicontinuous nature of the middle-phase microemulsion provides an organized media, of unique characteristics, to perform polymerization reactions. The "swiss cheese" type structure observed in many of the solids was a result of the coalescence of a highly dispersed phase present in the original microemulsion. A high rate of free radical generation combined with chain transfer due to the cosurfactant resulted in low molecular weights. The higher molecular weights of polymers containing sodium styrenesulfonate and/or 5-hexene-1,2-diol compared to the ones with 2-pentanol suggest some incorporation of the cosurfactant in the polymer structure. The anionic microemulsion systems yield solids with a higher Tgthan the excess organic phase polymer due to interactions between the surfactant and polystyrene chains. The T, of the precipitated solid was higher than that of the excess phase polymer, indicating that there was a partial incorporation of the surfactant in the polymer structure. The higher T, of the middle-phase polymer indicated that a more ordered crystal was formed in the restrictive environment of a microemulsion. The microstructure inherent in microemulsions, particularly middle-phase microemulsions, makes them attractive as a polymerization medium. Not only might it be possible to exploit the unique environment of microemulsions to produce novel polymers, but the possibility exists of utilizing the microstructure of the microemulsion as a template for producing solid polymer with the same or similar microstructure. This combined with the ability to influence the interfacial characteristics of such microporous solids may make middle-phase microemulsions a useful route for the production of microfiltration media, separation membranes or their supports, and microstructured polymer blends. The results of our prospective study of SDS-stabilized styrenecontaining middlephase microemulsions are encouraging. Significant microstructure remains following photopolymerization, and the previously reported problems with phase separation can be suppressed by appropriate choice of cosurfactant.
