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Effects of Monomer Organization on the Photopolymerization Kinetics of Acrylamide in Lyotropic Liquid Crystalline Phases Christopher L. Lester,† Shannon M. Smith,† William L. Jarrett,† and C. Allan Guymon*,‡ Department of Polymer Science, University of Southern Mississippi, Hattiesburg, Mississippi 39406-0076 and Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, Iowa 52242 Received February 24, 2003. In Final Form: July 14, 2003 The synthesis of nanostructured polymer gels via polymerization of hydrophilic monomers in lyotropic liquid crystal (LLC) phases has recently been of great interest. This work describes the effects of monomer organization on the photopolymerization kinetics of acrylamide in various LLC phases of surfactant/water systems. Resulting polymer morphology is reported as well. The polymerization rate of acrylamide is significantly enhanced when performed in the presence of various LLC phases. An unprecedented 10-fold increase in polymerization rate is observed in bicontinuous cubic and hexagonal phases as compared to polymerizations in water. This significant increase in polymerization rate is a result of diffusional limitations on the propagating polymer as well as segregation and ordering phenomena induced by the LLC phase. 13C T relaxation times and 14N NMR spectra show that the monomer strongly associates with the interface 1 of the surfactant/water assemblies and assumes the orientational order of the LLC phases to some degree. When formed using photopolymerization, the resulting polymer morphology appears to be a direct template of the parent LLC phase.
Introduction Polyacrylamide is a hydrophilic polymer that forms the basis for a variety of applications and has thus been the focus of considerable research. When cross-linked, gels are formed that swell to a considerable degree in water. These hydrogels have been widely used for electrophoretic separation of proteins and other biomacromolecules on the basis of molecular size. Polyacrylamide gels separate a wide variety of biomacromolecules adequately, but some limitations do exist. Polyacrylamide hydrogels are typically polymerized free-radically with thermal or redox initiators, and the morphology of the gels thus formed are typically random structures with fairly wide pore size distributions. Therefore, the size range of biomacromolecules that can be separated and the separation resolution is significantly limited. These random polymer morphologies also exhibit relatively weak mechanical properties and can be easily damaged.1 It has been recently proposed that controllably templating the nanostructure of lyotropic liquid crystal (LLC) phases onto polymers may eliminate such limitations in separatory devices.2-6 LLC phases exhibit some degree of orientational and positional order as found in crystalline solids while still * To whom correspondence should be addressed. E-mail:
[email protected]; phone: 319-335-5015. † University of Southern Mississippi. ‡ University of Iowa. (1) Allen, R. C.; Maurer, H. R. Electrophoresis and Isoelectric Focusing in Polyacrylamide Gel; Allen, R. C., Maurer, H. R., Eds.; Walter de Gruyter: Berlin, 1974. (2) Anderson, D. M.; Stro¨m, P. Polymer Association Structures; ElNokaly, M. A., Ed.; ACS Symposium Series 384; American Chemical Society: Washington, DC, 1989; p 204. (3) Holtzscherer, C.; Wittmann, J. C.; Guillon, D.; Candau, F. Polymer 1990, 31, 1978. (4) Antonietti, M.; Hentze, H. P. Colloid Polym. Sci. 1996, 274, 696702. (5) Go¨ltner, C. G.; Antonietti, M. Adv. Mater. 1997, 9, 431. (6) Antonietti, M.; Caruso, R. A.; Go¨ltner, C. G.; Weissenberger, M. C. Macromolecules 1999, 32, 1383.
having the relatively low viscosity and rheological properties typical of liquids. With relatively high concentrations of amphiphilic molecules in water, a variety of LLC phases can form including hexagonal, lamellar, and cubic geometries. Inverse cubic and hexagonal phases can also be observed in some systems.7,8 All of these LLC phases display nanoscale morphologies making them ideal candidates for enhanced separations and other applications. Unfortunately, LLCs lack material robustness and are unsuitable for separatory applications. Templating these LLC structures onto polymeric materials may aid considerably for the generation of nanostructured materials not only for enhanced separation but also for nanocomposites and biomimetic systems. Previously, the templated polymerization of acrylamide and other water-soluble monomers has been attempted in the LLC phases of a variety of ionic and nonionic surfactants. For example, the polymerization of acrylamide in the LLC phases of a ternary surfactant/water/oil mixture has been reported. The polymerizations performed in bicontinuous cubic and lamellar mesophases both resulted in the formation of homogeneous dispersions of water-swollen polyacrylamide particles in oil.3 Additionally, Antonietti and co-workers have examined the polymerization of acrylamides and other hydrophilic monomers in the mesophases of various LLC systems. The polymer morphology obtained is a function of phase separation rather than direct templating.4,6 The polymerization of styrene in the LLC phases of a cationic surfactant has also been shown to yield phase-separated systems in which no LLC order is evident in the polymeric material.9 Contrasting results have been presented elsewhere in the literature in which the polymerization of (7) Gray, G. W.; Winsor, P. A. Liquid Crystals & Plastic Crystals, 1st ed.; Gray, G. W., Winsor, P. A., Eds.; John Wiley & Sons: New York, 1974; Vol. 1, p 314. (8) Kelker, H.; Hatz, R. Handbook of Liquid Crystals, 1st ed.; Verlag Chemie: Weinheim, Germany, 1980.
10.1021/la0300784 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/26/2003
Effects of Monomer Organization
acrylamide and other monomers in bicontinuous cubic phases yields nanostructured polymer gels that appear to be directly templated from the parent LLC phase.10,11 These conflicting results raise many questions regarding the influence of the organized LLC phases on the polymerization process and monomer organization. Conversely, the polymerization kinetics may play a role in the retention of the original LLC phase if the polymerization occurs on a faster time scale than does phase separation. A more complete understanding of the polymerization kinetics may allow more reliable synthesis of polymers with LLC morphologies to facilitate development of materials useful in a variety of applications. The impact of monomer organization on polymerization kinetics and the resulting polymer morphology has proven significant in the polymerization of a variety of monomers in analogous thermotropic systems. The polymerization kinetics in thermotropic liquid crystals are highly dependent on the phase in which they are polymerized and also influence the final polymer architecture and properties.12-14 The role of LLC morphology in the polymerization of monomeric amphiphiles has also been examined.15,16 The polymerization rate of a fluorinated polymerizable amphiphile is substantially faster in LLC phases with higher degrees of order. The enhanced rates are a result of decreases in the termination rate. These decreases result from diffusional limitations on the propagating polymers induced by the highly ordered LLC phases. The fastest polymerizing samples display the greatest degree of structure retention upon polymerization.15 Similar behavior is observed in the polymerization of a cationic polymerizable surfactant. In this case, the polymerization is much faster in the lamellar LLC than in hexagonal and cubic morphologies. The differences in polymerization kinetics are due to significant differences in interfacial curvature that again result in decreases in termination. The LLC phases are apparently retained upon polymerization. Interestingly, the least amount of structural change occurs in the fastest polymerizing lamellar sample.16 The templated polymerization kinetics of nonmesogenic monomers exhibit a similar dependence on the LLC morphology. Monomers with different polarity segregate in different regions of the LLC phase. Nonpolar monomers segregate in the hydrophobic domains and the hydrophilic monomers are associated more with the aqueous domains or the interfacial region of the LLC phase. Correspondingly, these systems exhibit much different polymerization behavior. Hydrophobic monomers display faster polymerization rates in the micellar aggregates as a result of increased local concentration. Hydrophilic monomers, on the other hand, polymerize faster in the lamellar phase as a result of the increased degree of LLC order that yields a depression in the termination rate. The original nanostrucuture in these highly cross-linked systems is retained to a large extent.17 The polymerization kinetics during the formation of polyacrylamide gels may depend heavily on the different (9) Jung, M.; German, A. L.; Fischer, H. R. Colloid Polym. Sci. 2001, 279, 105. (10) Anderson, D. M.; Stroem, P. Physica A 1991, 176, 151. (11) Anderson, D.; Strom, P. Langmuir 1992, 8, 691. (12) Guymon, C. A.; Hoggan, N. A.; Rieker, T. P.; Walba, D. M.; Bowman, C. N. Science 1997, 275, 57. (13) Guymon, C. A.; Bowman, C. N. Macromolecules 1997, 30, 5271. (14) Guymon, C. A.; Dougan, L. A.; Martens, P. J.; Clark, N. A.; Walba, D. M.; Bowman, C. N. Chem. Mater. 1998, 10, 2378. (15) Lester, C. L.; Guymon, C. A. Macromolecules 2000, 33, 5448. (16) Lester, C. L.; Guymon, C. A. Polymer 2002, 43, 3707. (17) Lester, C. L.; Colson, C.; Guymon, C. A. Macromolecules 2001, 34, 4430.
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Figure 1. Chemical structure of monomers and surfactant used in this study. Shown are (a) acrylamide, (b) N,N-methylene bisacrylamide, (c) Brij 58, and (d) Brij 56.
templates provided by the LLC phases. Conversely, the overall polymer morphology may be highly influenced by the polymerization kinetics. The goal of this work is to investigate the polymerization kinetics of acrylamide in various phases of nonionic surfactant/water LLC systems and understand the mechanism driving changes in polymerization behavior to enhance the ability to controllably synthesize templated nanostructured gels. For these systems, the phase behavior will be determined as a function of composition of surfactant, monomer, and water. The orientational order of the monomer will also be determined within the various phases to determine if order is imposed on the monomer. The results detailing the phase behavior and orientational order of the monomer will be correlated with polymerization kinetics that are studied in real time in various LLC phases. The resulting polymer structure will be characterized as well. This systematic study of the polymerization mechanism of acrylamide in the highly ordered LLC phases will yield insight into the role of organized media on the polymerization mechanism and aid in optimizing conditions for the retention of the original lyotropic structure. The polymerization kinetics will be correlated to the polymer morphologies obtained. The understanding of the effects that these ordered nanostructures have on the polymerization kinetics of acrylamides will not only facilitate the generation of enhanced separations media but could also yield nanoporous materials for other applications including drug delivery systems, tissue scaffolding, and media for membrane-mediated chemistries. Experimental Section Materials. The monomers used in this study are acrylamide and N,N-methylene bisacrylamide (Acros). The surfactants utilized include nonionic Brij 58 and Brij 56 (Aldrich). The chemical structures of the monomers and surfactants are displayed in Figure 1. Systems were photoinitiated with Irgacure 2959 (Ciba Specialty Chemicals Corp., Tarrytown, NY). The monomers, surfactants, and initiator were all used as received. Each sample contained 25 wt % of an acrylamide mixture (93.0 wt % acrylamide, 2.0 wt % N, N-bisacrylamide, and 5.0 wt % initiator) with appropriate amounts of deionized water and surfactant. Solutions were mixed thoroughly and centrifuged repeatedly to ensure homogeneity. The samples were flushed with nitrogen and refrigerated until use. Procedure. Small-angle X-ray scattering (SAXS Siemens XPD 700P WAXD/SAXS with a CuKR line of 1.54 Å) was used to understand the phase behavior of the LLC mixtures and the morphology of the polymeric gels. Bragg’s law was used to determine the d spacing of the lyotropic mesophase. A polarized light microscope (Nikon Optiphot-2 pol) equipped with a hot stage (Instec, Boulder, CO) was used to corroborate data collected from SAXS by looking for characteristic textures of the various mesophases. Polymerization rate profiles were generated by following the polymerization in real time with a modified Perkin-Elmer DSC-7 with a medium-pressure UV arc lamp and UV transparent quartz windows in the DSC head cover. Samples of approximately 5.0 mg were used and covered with a thin film of FEP (DuPont fluorinated copolymer) to prevent evaporation of water. The
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samples were heated to 60 °C and cooled to the desired polymerization temperature at 10 °C/min to provide adequate thermal contact and films of uniform thickness. The DSC sample cell was purged with nitrogen to prevent oxygen inhibition during the polymerization. The polymerizations were performed at the desired temperature and initiated with monochromatic 365-nm light with an intensity of 4.0 mW/cm2. The heat of polymerization from the resulting exotherm was used to calculate the rate of reaction as well as the double-bond conversion as described elsewhere.13 The polymerization of each sample was monitored at least three times. For each figure, the most representative profile was chosen. Maximum rates for each composition typically did not vary more than 5%. The polymerization rate as given is normalized by the initial monomer concentration, allowing direct comparison of different monomer concentrations. For these studies, the theoretical value of 18.5 kcal/mol was used as the heat evolved per acrylamide double bond reacted.18 NMR spectra were acquired using a Bruker MSL-400 NMR spectrometer operating at a frequency of 28.9 MHz for 14N. A standard 10-mm probe was used. Spectra were obtained using the DEPTH sequence to suppress 14N background induced by the probe.19 The 90° pulse width was 22 µs, the probe dead time was 10 µs, and the acquisition time was 131 ms. The recycle delay was 600 ms, and no proton decoupling scheme was implemented. Spectra were typically acquired without spinning. Variable temperature studies were performed using the Bruker BUT 1000 temperature controller. The temperatures listed are within (2 °C.
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Figure 2. Polymerization rate of 25 wt % acrylamide at 25 °C with increasing Brij 58 concentration in water as a function of double-bond conversion. Shown are 0% isotropic (b), 20% micellar (3), 40% bicontinuous cubic (9), and 70% inverse micellar ()) Brij 58.
Nanostructured polyacrylamide gels could be beneficial for many applications such as biological separations and nanocomposite synthesis. Templating the unique geometries of LLC phases onto organic polymers is one route to such materials, but this approach has proven problematic as the resulting polymer morphology is typically a function of phase separation and not a direct template of the LLC geometry. As stated previously, the goals of this work are to develop a better understanding of the photopolymerization kinetics of acrylamide in various LLC phases, to correlate these kinetics to the acrylamide organization, and to determine if the original LLC phase is retained upon polymerization. To accomplish these goals, the nonionic Brij 58 and 56 surfactants were selected as they display a wide range of stable LLC phases over a broad composition range.4,6 With 25 wt % acrylamide and concentrations between 40 and 60 wt % Brij 58, the bicontinuous cubic phase is observed. Solutions at these concentrations are isotropic when viewed between crossed polarizers and exhibit triply periodic SAXS profiles characteristic of the bicontinuous cubic phase. Below 40% Brij 58, the solution is optically isotropic consisting presumably of micellar aggregates. An inverse micellar phase exists at concentrations above 70 wt % surfactant. The morphology of the LLC phase may have significant implications on polymerization kinetics. To understand the influence of the morphology on polymerization kinetics of the Brij 58/water/acrylamide system, polymerization rate profiles were obtained using photodifferential scanning calorimetry. The polymerization rate behavior of this system exhibits tremendous differences on the basis of the LLC phase. The polymerization rate profiles of a constant concentration of acrylamide (25 wt %) with increasing Brij 58 is displayed in Figure 2 as a function of double-bond conversion. Also shown in Figure 2 is the photopolymerization rate profile of an isotropic solution
of acrylamide in water at the same bulk concentration. The isotropic solution of acrylamide in water exhibits an extremely slow photopolymerization rate, taking nearly an hour to reach high conversion. With the surfactant concentration increased to form a micellar solution, the maximum photopolymerization rate is increased by a factor of 5 when compared to the isotropic solution. In the bicontinuous cubic LLC phase, the polymerization rate is enhanced even further. In fact, the maximum photopolymerization rate in the bicontinuous cubic LLC phase is fully 10 times higher than that of the isotropic sample and over twice that of the micellar solution. Subsequent increases in surfactant concentration while remaining in the bicontinuous cubic LLC phase result in small decreases in the polymerization rate.21 The small changes in rate could be attributed to subtle changes in the structure of the LLC phase as well as the dynamics of the system. However, when the wt % of Brij 58 is increased to form the inverse micellar phase, the polymerization rate drops dramatically from that of the bicontinuous cubic morphology. Interestingly, the polymerization rate of the inverse micellar phase is of similar magnitude to that of the normal micellar phase. The similar rates and conversions observed in these two different types of micellar solutions strongly indicate that the difference in polymerization kinetics results from the LLC morphology and not simply changes in surfactant concentration. To understand how other LLC phases impact the polymerization kinetics, the photopolymerization behavior of 25 wt % acrylamide was also studied in the Brij 56/ water system. This system displays discontinuous cubic, hexagonal, micellar, and inverse micellar phases. Significant polymerization rate enhancements are observed in this LLC system as well. In Figure 3, the polymerization rate profiles of 25 wt % acrylamide are depicted with increasing concentration of surfactant. The profile of an isotropic solution of acrylamide and water at the same concentration and temperature is also included for comparison. As shown previously, an extraordinarily slow photopolymerization rate is observed in the isotropic sample. In contrast, the polymerization rate in the discontinuous cubic liquid crystalline phase increases by a factor of 6. The hexagonal samples with 50% and 60% surfactant yield a polymerization rate 10 times that of the isotropic phase and considerably faster than that of
(18) Brandrup, J.; Immergut, E. H. Polymer Handbook, 2nd ed.; Brandrup, J., Immergut, E. H., Eds.; John Wiley and Sons Inc.: New York, 1975. (19) Cory, D. G.; Ritchey, W. M. J. Magn. Reson. 1988, 80, 128.
(20) Lester, C. L.; Smith, S. M.; Guymon, C. A. Macromolecules 2001, 34, 8587. (21) Odian, G. Principles of Polymerization, 3rd ed.; John Wiley & Sons: New York, 1991.
Results and Discussion
Effects of Monomer Organization
Figure 3. Polymerization rate with respect to time of 25% acrylamide at 35 °C with increasing Brij 56 concentration in water. Shown are polymerizations with 0% isotropic (b), 40% discontinuous cubic (3), 50% hexagonal (9), 60% hexagonal ()), and 70% inverse micellar (2) Brij 58.
the cubic phase. Interestingly, with further increases in surfactant concentration to reach an inverse micellar phase, the polymerization rate is significantly lower than the ordered LLC phases with lower surfactant concentrations. As in the Brij 58 system, these results indicate that the substantial changes in photopolymerization kinetics are heavily influenced by the type of LLC phase and not from changes in surfactant concentration. The increases in polymerization rate are induced in the ordered LLC phase by two different driving forces. One is an increase in the apparent rate parameter, kp, that results from segregation of the acrylamide monomer in the LLC phases. The other driving force comes from increased diffusional limitations on the propagating polymer chain because of the highly ordered LLC phase indicated by significant decreases in the observed termination kinetic parameter, kt,20 and in increases in polymer molecular weight of the acrylamide formed in ordered LLC phases. The fact that these two phenomena occur simultaneously in the Brij 58/acrylamide system implies that the monomer may be organized in some fashion within the LLC phase. The photoinitiator may also play a role, especially if similar segregation or diffusional limitations are present. On the other hand, these effects should be much less significant for the initiator as the initiator fragments are much smaller than are growing polymer chains and should therefore experience much lower diffusional limitations to reaction. Additionally, segregation of the initiator is expected to some degree, but to induce a 10-fold increase in rate, the local initiator concentration would need to be 100 times that in the bulk, a scenario which is highly unlikely. To corroborate the kinetic evidence of diffusional limitations and segregation phenomena, 13C T1 spin lattice relaxation times were measured to determine if mobility of the acrylamide monomer is being influenced. In Figure 4 relative 13C T1 (T1rel) relaxation times are shown for the carbonyl, methylene, and methine carbons of a constant concentration of acrylamide with increasing concentration of Brij 58. T1rel is obtained by dividing the relaxation time of acrylamide carbons found at different concentrations of surfactant by the relaxation time in the isotropic solution. This normalization of the relaxation times is important for comparison because different carbons have inherently different relaxation times. In the isotropic state, T1rel is at a maximum. When the surfactant concentration is increased to 10 and 20% (micellar), T1rel for all of the carbons decreases linearly. With a very concentrated micellar solution at 30% surfactant, T1rel decreases even
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Figure 4. Relative 13C T1 spin lattice relaxation times of the carbonyl (b), methylene (9), and methine (3) carbons of 25% acrylamide at 25 °C with increasing concentration of Brij 58. Relative standard error for each measurement is less than 7%.
Figure 5. Polymerization rate profiles of acrylamide in the bicontinuous cubic LLC phase (40% Brij 58) at 25 °C. Shown are profiles with 15% (b), 20% (3), and 30% (9) acrylamide.
further. In the bicontinuous cubic LLC phase at 40% surfactant, T1rel for all of the carbons of acrylamide is fully 5 times less than that of the isotropic solution. In solution NMR, short T1’s are indicative of hindered mobility at the site being studied. The shorter T1rel of the carbons of acrylamide show that the monomer is not simply solvated in the aqueous domains of the LLC but is interacting strongly with the surfactant. The acrylamide monomers appear to be aggregating at the surfactant water interface, which would account for the increase for kp through the consequent increase in the local concentration of double bonds. Additionally, if the monomer is experiencing a more hindered environment, it follows that the propagating polymer would as well. This decrease in mobility could account for the observed depression of the termination rate parameter. As water concentration decreases with increasing concentration of surfactant, it is possible even with the same bulk concentration of monomer in the system that the monomer could simply be more concentrated in the LLC phases, thereby inducing the observed rate changes. To determine if the polymerization kinetics result from different LLC morphology or from simple local concentration effects in the LLC phases, polymerization rate profiles were obtained with increasing concentration of monomer in 40% Brij 58. The bicontinuous cubic phase is exhibited for all monomer concentrations examined. In Figure 5, rate profiles in the cubic LLC phase of 40% Brij 58/water are displayed with increasing acrylamide concentration. The polymerization rate as reported is normalized by the initial monomer concentration allowing direct comparison of kinetics with different concentrations of monomer.
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Figure 6. Relative 13C T1 spin lattice relaxation times of the carbonyl carbon of acrylamide at 25 °C in 40% Brij 58 (bicontinuous cubic) with increasing acrylamide concentration. Relative standard error for each measurement is less than 7%.
Interestingly, on the basis of these results, concentration of monomer does not appear to be the only driving force for the differences in reaction behavior. For example, at 15 wt % acrylamide the polymerization rate is substantially faster than the isotropic polymerization rate discussed previously in Figure 2 even with the lower bulk concentration. While the rate does appear to increase slightly with increasing monomer concentration, the change is quite small and dramatically less than that observed in the LLC phases. Similar results are also observed with increasing concentration of monomer in an isotropic solvent (see Supporting Information). To ascertain how the mobility of the monomers change with increasing concentration, 13C T1 relaxation times were obtained for the samples with increasing acrylamide concentration in the 40% Brij 58/water system. In Figure 6, the T1rel values for the carbonyl carbon of acrylamide are shown for the various compositions of acrylamide. With increasing monomer concentration T1rel varies slightly, but not to a significant extent. Therefore, the diffusional limitations imposed on the monomers and correspondingly onto the propagating polymer chains appear to be fairly independent of monomer concentration. This behavior corresponds well with the similar polymerization kinetics observed in Figure 5. From this evidence, it is apparent that the enhancement of the photopolymerization rate in the LLC phases is not simply a function of monomer segregation but also must result from the LLC phase acting as a template for the polymerization. While decreased mobility and segregation phenomenon are obviously important in the polymerization kinetics, they alone may not completely account for the observed rate enhancements. The acrylamide itself could also be ordered by the LLC phases. Nitrogen is a naturally abundant quadrupolar nuclei that exhibits splitting patterns in 14N NMR spectra if in an anisotropic environment. To establish whether the acrylamide is itself ordered in an anisotropic LLC phase, 14N NMR spectra of the amide nitrogen were obtained in the Brij 56/water system. The Brij 56/water system was utilized for its anisotropic hexagonal phase. In Figure 7, the 14N NMR spectra of the amide nitrogen from 25% acrylamide with increasing concentration of Brij 56 water are displayed. The isotropic solution of acrylamide and water exhibits a relatively sharp peak with no quadrupolar splitting, indicating no anisotropy and therefore no monomer organization. The discontinuous cubic LLC sample with 40% surfactant yields a broader peak with shoulders at the base of the peak at approximately 0.4 and 0.8 kHz. This broadening
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Figure 7. 14N NMR spectra of the amide nitrogen of acrylamide in various concentrations of Brij 56/water at 35 °C.
Figure 8. 14N NMR spectra of the amide nitrogen of acrylamide with increasing temperature in the discontinuous cubic LLC phase of Brij 56/water (40% Brij 56).
of the peak indicates a less mobile environment, which could contribute to the increase in polymerization rate. The presence of the shoulders at the base of the peak indicates that some preferential orientation of the acrylamide may be occurring. When surfactant concentration is increased to 50%, a hexagonal LLC phase is present. As stated previously this composition yields a nearly 10fold increase in the polymerization rate. The 14N spectrum of this sample exhibits two splitting patterns with relatively broad peaks. Again, as in the discontinuous cubic phase, the broadening is probably due to a decrease in species mobility. In addition, the presence of 14N quadrupolar splitting is direct evidence of induced ordering of the acrylamide within the LLC phase. These results show that the acrylamide is strongly associated with the surfactant and the increase in rate is due both to decreased termination, resulting from less mobility, and to increased local concentration of double bonds. This orientation may also enhance the polymerization rate by causing the polymerization to occur topologically along the LLC phase interface. The organization of the Brij 56/acrylamide/water system exhibits interesting temperature dependence as well. 14N NMR is also a valuable technique for studying the organization of acrylamide in this LLC system at various temperatures. In Figure 8, the 14N spectra of the amide nitrogen of 25% acrylamide in the discontinuous cubic LLC phase with 40% surfactant is shown with increasing temperature. At 25 °C, the peak is quite broad and does not yield a splitting pattern that would indicate acrylamide organization. This mixture is extremely turbid at room temperature and, when viewed between crossed polarizers, exhibits isolated crystalline domains that correspond to acrylamide crystals not completely solvated. Upon in-
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Figure 9. Polymerization rate versus time for 25 wt % acrylamide with increasing temperature in the discontinuous cubic LLC phase (40% Brij 56). Shown are polymerizations at 25 °C (b), 35 °C (3), and 45 °C (9).
Figure 10. SAXS profiles at 25 °C of 25% acrylamide in the bicontinuous cubic phase of 40% Brij 58/water unpolymerized (b), photopolymerized at 25 °C (3), and thermally polymerized at 60 °C (9).
creasing the temperature to 35 °C, the peak remains broad at the base but sharpens considerably toward the top indicating that the acrylamide is being solubilized into the LLC phase. Also, at this temperature the sample becomes completely transparent and when viewed with polarized microscopy the crystalline domains disappear. The peak corresponding to 35 °C has shoulders at approximately 0.4 kHz and 0.08 kHz indicating that the monomer does in fact exhibit some degree of preferential orientation at this temperature. Upon increasing the temperature to 45 °C, a sharp isotropic line is observed that corresponds to completely solvated acrylamide that is isotropic in orientation. These results indicate that the acrylamide monomer displays significant differences in organization with increases in temperature that could correspondingly impact the polymerization kinetics. The different monomer organization observed in the 40% Brij 56/water/acrylamide system as a function of temperature may also have significant implications on the polymerization behavior. The polymerization kinetics of 25% acrylamide in the 40% Brij 56/water system at different temperatures show very interesting results as seen in Figure 9. A relatively slow polymerization rate is observed in the inhomogeneous sample at 25 °C. At this temperature, crystalline domains are observed that would not readily polymerize, therefore the overall rate decreases. When the temperature is increased to 35 °C, the polymerization rate doubles. This doubling of the polymerization rate cannot be accounted for by the increase in temperature alone. This significant increase in polymerization rate is instead due to the increased organization of the monomer in the LLC phase. This organization would enhance the segregation of monomer and potentially lower the termination rate because of the high degree of order imposed on the propagating chains. Further evidence that the LLC phase morphology alters the polymerization rate is shown at 45 °C. At this temperature, the polymerization rate decreases to values similar to that seen at 25 °C. Ordinarily free-radical polymerization rates increase with increasing temperature. However, as indicated by the 14N NMR results, the monomer has no order induced by the LLC phase at this temperature. Thus, the mechanisms that lead to rate enhancement at 35 °C are not in effect when monomer organization is no longer prevalent. Typically, polymerizations of monomers in LLC media have been initiated either thermally or via a redox mechanism, both of which require elevated temperatures to initiate polymerization.21 The type and degree of LLC order depends significantly on the temperature. In fact, at elevated temperatures the degree of LLC order de-
creases and eventually results in the complete loss of LLC order.7 By using initiators that require higher temperatures, the lower thermodynamic stability of the LLC phase allows greater diffusion of the individual components making phase-separation processes more likely. Therefore, using thermal and redox initiation may not be ideal methods for polymerization in LLC media. On the other hand, the initiation of photopolymerization is independent of temperature, thereby allowing polymerization at temperatures for which the proper balance of mobility and ordering exists. Additionally, photopolymerization is very rapid which may aid in kinetically trapping the LLC nanostructure, as it may be entropically unfavorable to form ordered high molecular weight polymers in such confined media. To determine if the LLC order is being truly templated onto the polyacrylamide gel, SAXS measurements were performed on gels synthesized using photoinitiation at 25 °C and thermal initiation at 60 °C. In Figure 10, SAXS profiles are depicted of the bicontinuous sample with 40% Brij 58/water and 25% acrylamide before polymerization and after polymerization using both initiation methods. All profiles are measured at 25 °C. A triply periodic SAXS pattern is observed for the unpolymerized sample indicative of the bicontinuous cubic phase with the primary reflection corresponding to 61 Å. Upon photopolymerization, this triply periodic pattern is retained with the same d spacing for the primary reflection. This indicates that the polymeric gel formed possesses a nanostructure that is truly templated from the original LLC. When using the thermal initiator, AIBN, the triply periodic pattern disappears even though the observation temperature is identical to that of the other samples. The primary reflection is still evident and is maintained at the same scattering angle, but its intensity decreases dramatically. This behavior indicates a loss of the original bicontinuous structure. The existence of a peak in the SAXS range implies that the sample still retains some degree of LLC order although obviously much less than that in either the original LLC system or the photopolymerized sample. To determine the LLC nature of the thermally polymerized sample, polarized light microscopy was used to observe any change in LLC phase. Interestingly, the thermally polymerized sample actually changes from the isotropic texture of the cubic phase before polymerization to a mottled birefringent texture indicating a phase change to a lamellar morphology. The thermally polymerized sample also becomes turbid and almost opaque, whereas the photopolymerized sample remains translucent.
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Conclusions This work presents the photopolymerization kinetics of acrylamide monomers in the LLC phases of the Brij 58/ water and Brij 56/water systems. The acrylamide monomers photopolymerize very slowly in isotropic solutions. When polymerized in micellar solutions, the polymerization rate for acrylamide is dramatically enhanced. The increase is even more pronounced in the LLC phases for which a 10-fold increase in polymerization rate is observed. These unprecedented differences in polymerization rate are partially due to monomer segregation, which yields a higher local concentration. Additionally, diffusional limitations of the propagating polymer are induced by the ordering of the LLC phase, driving further increases in polymerization rate. These two phenomena are a result of the acrylamide monomers segregating in the interfacial domain of the LLC phases and adopting LLC orientation. This segregation and association increases the local concentration of double bonds and decreases diffusion of
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the propagating polymers, thereby increasing polymerization rate. The observation of both of these kinetic phenomenon has tremendous implications in the developing polymer structure and molecular weight. Additionally, the photopolymerization of these LLC mixtures affords gels with morphologies that are a true template of the original nanostructure, whereas the original structure is not retained with thermal initiation. Acknowledgment. The authors would like to thank the Petroleum Research Fund and the National Science Foundation through a PECASE grant (CTS-0093911) for financial support. Supporting Information Available: Polymerization rate of increasing concentrations of acrylamide in formamide. This material is available free of charge via the Internet at http://pubs.acs.org. LA0300784
