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Langmuir 1996, 12, 3320-3325
Microporous and Embedded Polymeric Composites of Vinyltoluene from Microemulsion Polymerization J. Santhanalakshmi* and K. Anandhi Department of Physical Chemistry, University of Madras, A. C. College Campus, Guindy, Madras 600 025, India Received June 26, 1995. In Final Form: December 13, 1995X Microporous homopolymers of vinyltoluene (VT) were prepared by the polymerization of water in oil (w/o) and bicontinuous microemulsions (µEs) using sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) as surfactants and butylcellosolve (BuCe), and butylcarbitol (BuCa) as cosurfactants, respectively. The morphology and porosity are found to vary from w/o to bicontinuous µEs. Phase separations were observed during polymerization in certain formulations. The phase regions of the pseudophase diagrams were determined by conductivity and viscosity measurements. Hydrophilic comonomers like acrylamide, acrylic acid, acrylonitrile, and (dimethylamino)ethyl methacrylate were added to the aqueous phase of µEs to prepare the embedded graft copolymer of polyvinyltoluene containing hydrophilic polymer beads. The polymers were characterized using SEM, DSC, FTIR, and viscosity average moleuclar weights (M h v). Investigation of the morphology of the polymers by SEM and TGA revealed the existence of a microporous structure dependent to a certain extent on the precursor microemulsions.
Introduction Microporous polymers find applications in ultrafiltrations, for decorative purposes, as novel adsorbents, as flame retardants, as polymer composites, etc.1-3 These have been synthesized via polymerization of vinyl monomers with suitable initiators in water/oil microemulsions (µEs).4-10 Micropore sizes are controlled to lie within the droplet dimensions of µEs by choosing the appropriate surfactant (S) and cosurfactant (CS) couple during microemulsification. A combination of monomers (hydrophobic as the oil and hydrophilic in the aqueous phase) is also utilized to produce microporous copolymers.11-13 The micropore fillings of the porous polymers with hydrophilic homopolymers lead to their grafting as beads on the hydrophobic polymer backbone. These materials mimic specific substrate-bound enzymes and can be employed for immobilization of enzymes, controlled hydrophilic drug release, microencapsulation of drugs, etc.1,14,15 The existing synthetic methods consist of the X
Abstract published in Advance ACS Abstracts, June 1, 1996.
(1) Rosff, M. Controlled Release of Drugs: Polymers and Aggregate Systems; VCH Verlagsgosellschaft: Weinheim, 1989. (2) Gillberg-Laforce, G. E. Chim. Actual. 1975, 1552, 43. (3) Gillberg, G. In Emulsions and Emulsions Technology: Part III; Lissant, K. J., Ed.; Surfactant Science Series No. 6; Marcel Dekker: New York, 1984. (4) Stoffer, J. O.; Bone, T. J. Dispersion Sci. Technol. 1980, 1, 393. (5) Gan, L. M.; Chew, C. H.; Friberg, S. J. Macromol. Sci. Chem. 1983, A19, 739. (6) Haque, E.; Qutubuddin, S. J. Polym. Sci., Polym. Lett. Ed. 1988, 26, 429. (7) Qutubuddin, S.; Haque, E.; Benton, W. J.; Fendler, E. J. In Polymer Association Structures. Microemulsions and Liquid Crystals; El-Nokaly, Magda, A., Eds.; American Chemical Society: Washington, DC, 1989; Chapter 5. (8) Anderson, D. M.; Strom, P. In Polymer Association Structures. Microemulsions and Liquid Crystals; El-Nokaly, Magda, A., Eds.; American Chemical Society: Washington, DC, 1989; p 204. (9) Menger, F. M.; Tsuno, T.; Hammond, G. S. J. Am. Chem. Soc. 1990, 112, 1263. (10) Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 1378. (11) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Langmuir 1992, 7, 2586. (12) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Langmuir 1992, 8, 1931. (13) Gan, L. M.; Chieng, T. H.; Chew, C. H.; Ng, S. C. Polymer 1995, 36, 1941. (14) Gupta, A.; Nagarajan, R.; Kilara, A. Biotechnol. Prog. 1991, 7, 348. (15) Chen, D.; Kennedy, J. P.; Allen, A. J.; Korey, M. M.; Fly, D. M. J. Biomed. Mater. Res. 1989, 23, 1327.
S0743-7463(95)00512-9 CCC: $12.00
sequential additions of living prepolymers with relatively low amounts of hydrophilic-hydrophobic comonomers aided by micellar copolymerizations.16,17 However a homogeneous dispersal of microbeads of hydrophilic polymers into a block of hydrophobic polymer can be tailored via polymerizing water in oil µEs containing a hydrophilic monomer in the water (polar) phase and a suitable surfactant-cosurfactant mixture.18 Conventional free radical initiators (azobis(isobutyronitrile) (AIBN) or potassium peroxodisulfate (K2S2O8)) can be used to initiate polymerization. Styrene and methyl methacrylate (MMA) water/oil µEs have been used earlier to prepare polymer blocks with microporous structure.4,5,9 Qutubuddin et al.7 have utilized bicontinuous µEs to produce porous polymers of maximum porosity. Cheung et al.10-12 have reported polymerization of MMA and styrene with acrylic acid (AA) and ethylene glycol dimethacrylate using sodium dodecyl sulfate (SDS) or potassium 10-undecenoate as the surfactant. The relation between the microstructure of the polymer and the precursor µE was investigated. Recently, Price19 reported the polymerization of a ternary bicontinuous µE with a polymerizable cationic surfactant to produce a transparent microporous polymer. Gan et al.13,20-22 polymerized several vinyl systems to produce transparent polymeric solids. The systems investigated were MMA/AA/water using both SDS and sodium acrylamidoundecenoate or acrylamidostearate as surfactant and MMA/2-hydroxyethyl methacrylate/11-(N-ethylacrylamido)undecenoate and ethylene glycol dimethylacrylate as the cross-linker. Transparent polymeric solids with various microstructures were obtained. Vinyltoluene has been used earlier as a potential monomer for the production of functional polymers, prepolymers, anchored polymers, etc. So far it has not been utilized to synthesize porous polymers. The objective (16) Kaszar, G.; Puskar, J. E.; Chen, C. C.; Kennedy, J. P. Polym. Bull. 1988, 20, 413. (17) Briggs, S.; Hill, A.; Selb, J.; Candau, F. J. Phys. Chem. 1992, 96, 1505. (18) Qutubuddin, S.; Lin, C.; Tajuddin, Y. Polymer 1994, 35, 4606. (19) Price, A. US Patent 5151217, 1992. (20) Gan, L. M.; Chew, C. H. J. Dispersion Sci. Technol. 1984, 5, 179. (21) Chew, C. H.; Gan, L. M. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2225. (22) Gan, L. M.; Chew, C. H.; Friberg, S. J. Dispersion Sci. Technol. 1983, 4, 291.
© 1996 American Chemical Society
Polymeric Composites of Vinyltoluene
of this study is to investigate the possibility of utilizing vinyltoluene for the production of porous and embedded polymers in microemulsion media. In this present work, the microemulsion (µE) systems containing water, vinyltoluene as the oil, and SDS/butylcellosolve (BuCe) or cetyltrimethylammonium bromide (CTAB)/butylcarbitol (BuCa) as the surfactant-cosurfactant mixture were availed for preparing porous and embedded polyvinyltoluene. The phase behavior was analyzed by conductivity and viscosity studies. Acrylamide (AM), acrylonitrile (AN), vinylpyridine (VP), acrylic acid (AA), and (dimethylamino)ethyl methacrylate (DMAEM) were dissolved in water and utilized as comonomers to prepare embedded polymer composites of polyvinyltoluene (PVT). The cross sections of porous polymer composite were characterized with the aid of scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and viscosity average molecular weights (M h v).
Langmuir, Vol. 12, No. 13, 1996 3321
A
B
Experimental Section Materials. Vinyltoluene (VT), vinylpyridine (VP), (dimethylamino)ethyl methacrylate (DMAEM), acrylonitrile (AN), and acrylic acid (AA) were from Fluka and were distilled under reduced pressure to remove the inhibitor and stored at -5 °C until use. Acrylamide from BDH was recrystallized twice from distilled water. All the alcohols, 1-pentanol (PeOH), butylcarbitol [BuCa, 2-(2-butoxyethoxy)ethanol], and butylcellosolve [BuCe, 2-butoxyethanol], were distilled under reduced pressure. Cetyltrimethylammonium bromide (CTAB) from BDH and sodium dodecyl sulfate (SDS) from Sd.Fine, India, were recrystallized from an ethanol-acetone mixture (3:1 by volume) and ethanol, respectively. Potassium persulfate (KPS) from LOBA, India, and 2,2-azobis(isobutyronitrile) (AIBN) from Eastman were recrystallized from distilled water and ethanol, respectively. Water was doubly distilled. Microemulsion Preparation. The microemulsions (µEs) were prepared by titration of a mixture of surfactant, alcohol, and vinyltoluene with water with constant stirring until transparency or slight translucency was obtained. Clear and turbid phases and boundaries were detemrined from visual tests based on systematic titrations. Conductivity and Viscosity Measurements. Conductivities (σ) of µEs were measured with an Elico CM82T conductivity bridge (Elico, India) and with a conductivity cell of cell constant 0.5 cm-1 in a double-walled vessel through which thermostated water was circulated. The viscosities of all the µEs were measured using a Ubbelohde dilution viscometer precalibrated in the viscosity range 1-100 centipoise and thermostated in a water bath. All the measurements were carried out at 30 ( 1 °C. Microemulsion Polymerization. KPS or AIBN of 0.5% weight of the monomer was used for initiating polymerization. After initiator additions, the solutions were N2 purged and polymerized at 60 °C in a thermostat. After polymerization, the adhered surfactant was removed through washings with water and methanol. Conversion of polymer was determined for different durations of time by precipitating the reaction mixture in a large excess of methanol. Copolymers were prepared using the µEs comprising a hydrophilic monomer in the water phase. In the case of PVT-PAM, the copolymers were washed with DMSO and excess methanol while PVT-PAA copolymers were washed with hot water and toluene. PVT-PDAEM was washed with hot methanol to remove any adhered surfactant while PVTPAN was washed with hot water. The absence of surfactant in all the polymers was established by measuring the conductivity of the final washings. Polymer Molecular Weight Determination. The molecular weight of polyvinyltoluene was determined by measuring the intrinsic viscosity of the polymer solutions in toluene using an Ubbelohde viscometer. The Mark-Houwink constant values
Figure 1. Pseudoternary phase diagram for the systems composed of (A) water (w)/VT/SDS/BuCe with a weight ratio of SDS to BuCe ) 0.43 and a weight ratio of VT to BuCe ) 5.0 and (B) water (w)/VT/CTAB/BuCa with a weight ratio of CTAB to BuCa ) 0.75 and a weight ratio of VT to BuCa ) 8.7 at 30 °C. Regions: O, O/W microemulsion; W, W/O microemulsion; B, bicontinuous microemulsion; D, two-phase regions. Solid line represents the W/O µE region when pentanol is used at the same compositions as that of BuCa and BuCe. (K and R) were taken as 8.86 × 10-3 mL/(g K) and 0.74, respectively.23 Morphology Observation. The polymer morphology was examined using a JEOL 5300 scanning electron microscope operating at a 20 kV voltage. The sample was fragmentized and dried at 45 °C under vacuum for 12 h. A sample piece was gold coated using gold foil in a JEOL JFC 1100E ion sputtering device and secured to the sample mount using carbon paint before the microscopic examination. Glass Transition Temperature Determination. The thermal properties of the polymers were analyzed using a differential scanning calorimeter (Delta series, DSC-7). The glass transition temperature (Tg) was measured at a scan rate of 10 °C/min in the temperature range 40-400 °C. Pore Continuity Determination. The continuity in the pore structure of the polymers was determined by examining the drying rate curve obtained using a Mettler TA3000 thermogravimetric analyzer. The porous polymers were dried isothermally at 70 °C in a stream of dry nitrogen gas for 5 h. The drying rate curve was constructed by measuring the weight loss of the polymer as a function of time. FTIR Studies. Polymer samples were pelletized with KBr and scanned using a Shimadzu Fourier transform infrared spectrometer. IR spectra were recorded after applying background corrections.
Results and Discussion Phase Behavior of Microemulsions. In the pseudoternary phase diagrams (Figure 1), water in oil µEs are represented by region W, oil in water µEs by region O, and bicontinuous µEs by region B. Hydrophilic cosur(23) Brandrup, J., Immergut, E. H., Eds. In Polymer Handbook; Wiley: New York, 1975; Chapter 4.
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centipoise)
Table 1. Phase Composition (% by wt), Viscosity Average Molecular Weight (M h v), and Tg (°C) of Polyvinyltoluene Prepared by Microemulsion Polymerization at 60 °C system water/oil
bicontinuous
oil/water
water/oil
Figure 2. Conductivity and viscosity as a function of water content for the SDS/BuCe/VT microemulsion at 30 °C: SDS ) 10 wt %, VT/BuCe (wt/wt) ) 70/14. bicontinuous
factants like butylcellosolve (C4H9OCH2CH2OH, BuCe) and butylcarbitol (C4H9OCH2CH2OCH2CH2OH, BuCa) when present are found to solubilize the maximum amount of water, resulting in µEs containing SDS/CTAB as surfactants at constant proportions of vinyltoluene compared to 1-pentanol (Figure 1). BuCe and BuCa were successfully used in earlier studies as cosurfactants for the µE polymerization of styrene.5,24 In water in oil regions (region W of Figure 1), the solubility of water increased with vinyltoluene content in CTAB µEs, while it decreased with vinyltoluene content in SDS µEs. The amount of surfactant needed to obtain the maximum solubility is lower in SDS than in CTAB µEs. The amount of cosurfactant required depends on the type of the surfactant. At fixed weight ratios of surfactant to cosurfactant (0.43 in SDS and 0.75 CTAB) and vinyltoluene to cosurfactant (5.0 in SDS µE and 8.7 in CTAB µE) an increase in either water or surfactants with fixed composition of vinyltoluene results in a transition from water in oil to bicontinuous to oil in water µEs. When hydrophilic monomers are dissolved in the aqueous phase, the amount of cosurfactant necessary to obtain isotropic single phase microemulsions is lowered. There is a sharp increase in conductivity on dilution with water in the lower water content region (Figure 2). This can be explained by the existence of a µE structure consisting of droplets of water dispersed in oil.25-27 Further dilution with water results in the water droplets becoming connected and a bicontinuous µE structure with a network of conducting channels being formed. This results in a sharp increase in conductivity around 3035% by weight of water. A further increase in water content above 70-75% by weight decreases the conductivity. This is due to the breakdown of the conducting network to form an oil in water µE structure.28 These results are similar to the observations made in earlier studies on the conductivity of microemulsions.27,29,30 The viscosity curve indicates sections of sharp increase followed by a decrease (Figure 2). The sharp increase in (24) Gan, L. M.; Chew, C. H.; Lye, I. Makromol. Chem. 1992, 192, 1249. (25) Warr, G.; Sen, R.; Evans, D. F. J. Phys. Chem. 1988, 92, 718. (26) Evans, D. F.; Mitchell, D. J.; Ninham, D. W. J. Phys. Chem. 1986, 90, 2817. (27) Kim, M. W.; Huang, J. S. Phys. Rev. A 1986, 34, 719. (28) Santhanalakshmi, J.; Anandhi, K. J. Colloid Interface Sci., in press. (29) Chen, S. J.; Evans, D. F.; Ninham, D. W. J. Phys. Chem. 1984, 88, 1431. (30) Loic, A.; Cotton, J. P.; Ober, R.; Taupin, C. In Microemulsion Systems; Rosano, H. I., Clausse, M., Eds.; Marcel Dekker Inc.: New York, 1987; p 225.
oil/water
compositiona water: 10% vinyltoluene: 70% SDS: 6% butylcellosolve: 14% water: 40% vinyltoluene: 35% SDS: 7% butylcellosolve: 18% water: 78% vinyltoluene: 10% SDS: 5% butylcellosolve: 7% water: 16% vinyltoluene: 70% CTAB: 6% butylcarbitol: 8% water: 43% vinyltoluene: 40% CTAB: 6% butylcarbitol: 11% water: 80% vinyltoluene: 11% CTAB: 5% butylcarbitol: 4%
Tg/°C
10-6 M hv
107
5.05-6.90
104
1.02-2.65
111
0.98-3.00
108
5.0-7.0
104
0.95-2.5
111
1.0-3.02
a The compositions were utilized to monitor polymer conversion with time at 60 °C. AIBN or KPS of 0.05 wt % of monomer was used as an initiator.
the viscosity of the water in oil µEs on addition of water indicates the appearance of connected water channels in the system. The reasoning is consistent with earlier studies on the viscosity behavior of µEs.31,32 The sharp increasing trend in the viscosity-water content curve above 35% water (Figure 2) indicates a continuous increase in the number of connected water channels of the µEs on the addition of water to form a bicontinuous structure. The viscosity of the microemulsion decreases at water contents above 75% as the continuous aqueous phase is progressively diluted with water. This is due to the decrease in the restricted shear flow of the bicontinuous structure which changes to oil in water at higher water contents. A comparison of the viscosity behavior with that of conductivity predicts three distinct structural regions of µEs, discontinuous water in oil at low water contents, bicontinuous in water and oil at intermediate water contents, and discontinuous oil in water at high water contents. These different phases of the µEs can be utilized to produce morphologically different polymers. Polymerization in Microemulsions. Table 1 shows some of the compositions utilized for polymerization. Polymerization of vinyltoluene was performed using the water soluble initiator potassium persulfate (K2S2O8) and the oil soluble initiator AIBN, both about 0.05 wt % of monomer. Unlike the case for oil/water µEs33,34 polymerization rates are slower in water/oil µEs. Friberg et al.5 had also earlier reported such lower rates in styrene water/ oil µEs. The compositions listed in Table 1 were chosen for the polymer conversion measurements at 60 °C. The polymer percentage conversion with time plots are shown in Figure 3 for water/oil µEs initiated by KPS of 0.05 wt % of monomer. Except slight variations in the initial rates (31) Bennert, K. E.; Hastfield, J. C.; Davis, H. T.; Macosbo, C. W.; Scriven, L. E. In Microemulsions; Robb, I. D., Ed.; Plenum Press: New York, 1982; p 65. (32) Georges, K.; Chen, J. W. Colloid Polym. Sci. 1986, 264, 896. (33) Santhanalakshmi, J.; Anandhi, K. J. Surf. Sci. Technol. 1995, 9, 37. (34) Santhanalakshmi, J.; Anandhi, K. J. Appl. Polym. Sci., in press.
Polymeric Composites of Vinyltoluene
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Figure 3. Percentage conversion versus time plots for polymerization of water in oil microemulsions containing (A) 16% water, 70% vinyltoluene, 6% CTAB, and8% BuCa and (B) 10% water, 70% vinyltoluene, 6% SDS, and 14% butylcellosolve at 60 °C [KPS ) 0.05 wt % of VT].
Figure 5. (A) SEM photograph of a porous PVT polymer sample from a bicontinuous microemulsion of composition SDS (7%), BuCe (18%), VT (35%), and water (40%). (B) Magnified SEM photo of the marked channels in part A.
Figure 4. SEM photograph of a porous polymer sample from a water in oil microemulsion of composition CTAB (6%), BuCa (8%), VT (70%), and water (16%).
the final conversions obtained are similar for both KPSand AIBN-initiated systems. Maximum conversions are obtained before phase separation. No gelation is observed. The overall rates do not vary appreciably with different types of surfactant and cosurfactant. The rate of polymerization in SDS µEs is 1.5% conversion per hour, and in CTAB µEs it is 1.7% conversion per hour. The maximum conversions reached before the phase separations are lower in the bicontinuous than in water/oil µEs. When hydrophilic monomers are dissolved in the aqueous phase of w/o µEs, phase separations are considerably lowered and rates are increased. Morpholoy of Polymers. Oil in water microemulsion polymerization of vinyltoluene yielded stable microlattices. The size characterizations of polyvinyltoluene (PVT) obtained from oil/water µEs are reported elsewhere.34 Porous polymeric materials are obtained by polymerizing microemulsions with water contents below 75% by weight. The SEM micrographs of polymers obtained from precursor µEs containing 10% and 40% water are shown in Figures 4-6. The micrographs reveal the existence of a microstructure related to the water content. Porous PVT polymers obtained from water/oil µEs show a closed cell (discrete) microstructure (Figure 4). The bicontinuous µEs produced polymers with a microstructure existing as interwined sheets which extend three dimensionally. Even though distinct void spheres are not seen, interwined vacant channels in lieu of water space may be detected,
indicating the presence of an open cell structure (Figure 5). Such open and closed cell morphologies from the W/O and bicontinuous µEs have been detected and reported by Palaniraj et al.11,2,35 The results go hand in hand with the present case. The sizes of the pores obtained from SEM results are between 2 and 3 µm. This is greater than the size of the bicontinuous µEs (600-800 Å) reported in the literature.36 Palani Raj et al.12 have attributed this change to the existence of phase separation effects during polymerization and structural deformation effects during drying of the polymer. Similar observations have been made in earlier studies.7,8,11 The marked squares in Figure 5A indicate the vacant channels that can be visualized at higher magnification (Figure 5B). In the case of CTAB µEs, the pores are more elongated and slightly cylindrical in nature. Changes in microstructure during polymerization due to phase separation and collapse of some pores during polymer drying can be the cause for the slight difference in the observations. Replacing the pure water components by hydrophilic monomer dissolved water in water/oil vinyltoluene µEs, the polymers contain no vacant micropores but exist as pore-filled copolymers of the hydrophilic homopolymer beads although distinct hydrophilic and hydrophobic parts could not be detected (Figure 6A). From the SEM micrograph with more particles as shown in Figure 6B as well as with the magnified SEM photo of a single particle, spheroidal beads with nearly monodispersity in size may be visualized. The radius of the beads may be used to (35) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1995, 36, 2637. (36) Bodet, J. F.; Bellare, J.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J. Phys. Chem. 1988, 92, 1898.
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Figure 7. Drying rate curve of closed-cell porous polymer obtained from a microemulsion containing 10% water, 70% vinyltoluene, 6% SDS, and 14% BuCe.
Figure 8. Drying rate curve of open-cell porous polymer obtained from a microemulsion containing 40% water, 35% vinyltoluene, 7% SDS, and 18% BuCe. Figure 6. (A) SEM photograph of a single embedded polymer sample from a SDS-BuCe-vinyltoluene water/oil microemulsion containing acrylamide in the aqueous phase. (B) SEM photo indicating a larger number of embedded polymer beads.
know the number density of the beads formed from the unit volume of water in oil microemulsions. Similarly the number density of the micropore may be known from the micropore radii (spherical) of the pure water/oil microemulsion without the hydrophilic monomer. For this purpose the equation that relates droplet radii to the number density of the microemulsion droplets may be used; i.e., φd ) (4/3)πRd3Nd, where φd is the volume fraction of the droplet, Rd is the radius of the droplet, and Nd is the number density of the droplets. On comparing the microporous homopolyvinyltoluene with pore-filled (embedded) PVT, the number density of the embedded beads is found to exceed the number density of the vacant pores of microporous homopolymer counterparts. In fact the polymer prepared from water/oil µEs resembles the hydrophilic polymer spherical beads embedded homogeneously into the PVT polymer block. The artifact of the µE polymerization is such that, instead of surfacial grafting, homogeneous grafting with spherical or nonspherical polymer beads (Figure 6B) is possible. When hydrophilic comonomers dissolved in water are microemulsified into vinyltoluene bicontinuous µEs using ionic surfactants, the hydrophilic polymer beads and sheets are found to be embedded in the bicontinuous PVT. At higher magnification, a film-like structure slightly resembling nonporous polyvinyltoluene solid is obtained. In polymers prepared from bicontinuous µEs, the collapse of the pores leading to a sheet-like morphology is more evident than in water/oil µEs due to greater loss of water and alcohol by evaporation during sample drying and sample preparation for SEM studies. Such a behavior has been reported by Qutubuddin18 on polymerizing styrene and acrylamide with ionic surfactants in micro-
emulsions. Microphotographs of the various cross sections show the embedding to be homogeneous in the entire block. The morphology of porous polymeric solids obtained by the polymerization of w/o and bicontinuous µEs can be confirmed by thermogravimetric analysis (Figures 7 and 8). Solid materials containing water in interconnected pores exhibit a drying rate curve with a linear falling rate period whereas materials with closed cell structure have an exponentially decreasing falling rate.13,37-39 The exponential (Figure 7) and linear (Figure 8) drying decay rates of the polymers obtained from water/oil and bicontinuous vinyltoluene µEs indicate the existence of closed and open cell structures in them. This confirms that the polymerization in µE media results in the original structure of the precursor µEs being preserved to a considerable extent. Typical DSC curves of the dried homo- and some copolymers of porous PVT blocks are given in Figure 9. In Tables 1 and 2, the Tg values of the reprecipitated polymers are given. A single Tg was obtained for all the polymers, indicating random copolymerization. The Tg values of polymer samples depend on crystallinity, compactness, porosity, additives, high or low molecular weights, etc.40-42 The polymer segments on the surfaces of the pores suffer lower compactness than in the bulk. The values Tg of porous homopolymers are found to be lower than the nonporous ones obtained thermally. This might be due to an increase in molecular weight and cross (37) Coulson, J. M.; Richardson, J. F. Chemical Engineering, 2nd ed.; Pergamon Press: New York, 1968; Vol. II, p 620. (38) McCabe, W. L.; Smith, J. C.; Harriot, P. Unit Operations of Chemical Engineering, 4th ed.; McGraw-Hill: New York, 1985; p 716. (39) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. J. Appl. Polym. Sci. 1993, 47, 499. (40) Stevens, M. P. Polymer ChemistrysAn Introduction; Oxford University Press: New York, 1990; Chapter 3. (41) Van Krevelen, D. W.; Hoftyzer, P. J. Properties of Polymer: Their Estimation and Correlation with Chemical Structure; Elsevier Publishing Company: Amsterdam, 1976; Chapter 6. (42) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: London, 1941.
Polymeric Composites of Vinyltoluene
Langmuir, Vol. 12, No. 13, 1996 3325
Figure 10. FTIR spectra of polyvinyltoluene (A) and copoly(vinyltoluene-acrylamide) (B) prepared by microemulsion polymerization. Asterisks indicate peaks characteristic of acrylamide.
Figure 9. DSC curves for porous polymers prepared by microemulsion polymerization. Table 2. Tg (°C) Values of Embedded Copolymers of Vinyltoluene from DSC in N2 Tg/°C Samplea
SDS
CTAB
PAM-VT PAA-VT PDMAEM-VT PAN-VT PVP-VT
Water/oil 118 110 115 98 106
120 110 117 95 108
PAM-VT PAA-VT PVP-VT PDMAEM-VT
Bicontinuous 112 108 102 110
115 105 100 110
a Compositions utilized for polymerization at 60 °C are the same as in Table 1. The aqueous phase contained hydrophilic initiator, KPS (0.05% by weight of the total weight of monomer), and the respective hydrophilic monomer (5% by weight of vinyltoluene).
linking. In embedded copolymer blocks the Tg’s are greater than those for nonporous and microporous homopolymers. This is due to the presence of hydrogen bonds and also to the compact pore volume fillings which reduce the mobility of the polymeric chains. FTIR spectra of nonporous and microporous PVT resemble each other but with a slight difference: sharp peaks are seen for the microporous polymers. This is due to the increased surface area exposures. The inclusion of hydrophilic polymers is inferred from the FTIR spectra. An illustration of PVT-PAM copolymer spectra (Figure 10) possessed characteristic PAM peaks (indicated in Figure 10 with asterisks) in addition to the PVT peaks.
The lack of the characteristic peaks of adhered surfactants, if any, also confirms their absence in the polymer solids. The absence of CdC peaks confirms complete polymerization. The viscosity average molecular weights of the solid homopolymers are given in Table 1. The molecular weights of the polymers are different depending upon the initial microstructure of the microemulsions. The molecular weight of the nonporous polyvinyltoluene is less than that of the porous polymer. This might be due to the compartmentalized environment of the oil droplet in the oil/water µE in which a normalized addition of monomers to the growing polymer occurs owing to droplet collisions. Similar dependencies of the molecular weight of the polymer on the morphology have been reported earlier. Conclusions The results of viscosity and conductivity of vinyltoluene/ water/SDS/butylcellosolve and vinyltoluene/water/CTAB/ butylcarbitol µE systems indicate the existence of distinct water/oil, oil/water, and bicontinuous structures depending upon their composition. The study reveals a unique method of forming porous polymers with microstructure being related to the precursor microemulsions. Micropore fillings or modification of hydrophobic polymer networks with hydrophilic polymers through graftings may be effectively brought out by polymerizing hydrophilic monomers in water containing water/oil and bicontinuous microemulsions. Due to the homogeneous nature of microemulsions, a uniform embedding is achieved. Such materials are expected to play a vital role in catalysis, separation science, controlled release of drugs, production of technology important polymers like conducting polymers, polymer composites, etc. The morphology, porosity, and thermal properties are affected by the type of surfactant and the initial µE structure. Acknowledgment. The Authors thank UGC, India, for their financial support of this work. They also acknowledge the Department of Metallurgy and Department of Catalysis, IIT, Madras, for SEM and DSC results. LA9505125
