Chapter 32
Photopolymerization Kinetics of Nanostructured Polymers Templated by Lyotropic Liquid Crystals Photoinitiated Polymerization Downloaded from pubs.acs.org by YORK UNIV on 12/04/18. For personal use only.
C. Allan Guymon and Christopher L. Lester Department of Polymer Science, University of Southern Mississippi, 2609 West 4 Street, Hattiesburg,MS39406-0076 th
Nanostructured materials have been the focus of much attention due to their applicability in nanocomposites, separations media, drug delivery devices, and many other applications requiring a nanometer size scale. Recently, using lyotropic liquid crystalline (LLC) phases to template their unique nanostructure onto organic polymers has been proposed. This work details the photopolymerization of acrylamide in various phases of L L C systems. The photopolymerization kinetics are correlated to monomer organization for different phases at a variety of concentrations and temperatures. The photopolymerization kinetics of acrylamide in the L L C phases depend strongly on the L L C morphology. The polymerization rates are significantly faster in compositions of surfactant resulting in a hexagonal geometry as the acrylamide monomer is preferentially oriented. These results indicate that acrylamide is strongly associated with the L L C interface and the surfactant. Photopolymerization of these templated systems results in structure retention of the parent L L C phase.
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© 2003 American Chemical Society
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Introduction In recent years the synthesis of nanostructured materials has become the focus of much research. Lyotropic liquid crystal (LLC) phases possess a wide range of controllable nanometer morphologies that are formed with mixtures of amphiphile and water. When amphiphiles are solvated in water at sufficient concentrations, a variety of ordered phases may form. At concentrations above the critical micelle concentration, micelles will form. At higher concentrations of amphiphile, these micelles may pack into discontinuous cubic arrays. At successively higher concentrations of amphiphiles, hexagonally packed rod-like aggregates and bilayer lamellar phases may form as shown in Figure I. These phases, however, are not robust and therefore not useful from a materials standpoint. If the nanostructure of the LLC phases could be templated onto the polymer, a wide variety of applications could benefit including ultrafiltration membranes, catalytic media, and drug delivery devices. Using LLC phases as templates for the synthesis of nanostructured materials has been attempted, but with mixed results. Typically, the polymer morphologies reported are a result of phase separation during polymerization as it is thermodynamically unfavorable for polymer chains to adopt such constrained conformations. In some cases the successfid templating of L L C phase structure onto polymers has been reported, but these studies do little to detail what mechanisms allow phase retention versus phase separation. Some studies have alluded to kinetic trapping of the otherwise thermodynamically unfavorable state as a possible mechanism to control structure. Polymerization kinetics, therefore, are important to consider when attempting to retain the original L L C phase structure of templated systems. Conversely, it is also 1
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Figure 1. Schematic ofselected lyotropic liquid crystalline phases. Shown in order of increasing amphiphile concentration are the a) micellar, b) hexagonal c) lamellar, d) inverse hexagonal and e) inverse micellar phases.
380 important to understand how the nanostructure of the L L C template impacts the polymerization mechanism and how the monomers are organized prior to polymerization. Recently, photopolymerization has been utilized as a means to develop controlled L L C nanostructures. Photopolymerization is not only extremely rapid, which may allow kinetic trapping, but also allows facile examination of the polymerization kinetics. Previous studies have shown that the final properties of analogous thermotropic systems are greatly dependent on the polymerization kinetics for a variety of monomers/LC systems. ' Other results indicate that the photopolymerization kinetics and structure retention of monomelic L L C amphiphiles are influenced by the type and degree of L L C order. " Additionally, the polymerization kinetics of a variety of monomers templated in L L C phases are quite dependent on the original L L C morphology. These studies illustrate the importance of understanding the polymerization kinetics of L L C systems and the relationship of the polymerization to structure retention. It is therefore extremely important to correlate the photopolymerization kinetics to the orginal L L C morphology as well as to the degree of structure retention. The goal of this work is to further elucidate the details of templated polymerization in L L C phases to allow the development of nanostructured polymers that can be reliably and controllably synthesized. Specifically, the polymerization kinetics of an acrylamide will be correlated to the organization of the monomer in an L L C template. The local mobility and environment of the monomer will be examined and the resulting polymeric materials will be characterized to determine phase retention. The results of this study will aid in optimizing templated L L C materials for applications requiring controlled polymeric nanostructures. 4
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Experimental Materials The monomer examined in this study was acrylamide, while the L L C template was formed by the nonionic surfactants Brij 56 and Brij 58 with deionized water. Figure 2 depicts the chemical structures of the monomer and surfactant. The photoinitiator used was Irgacure® 2959.
381 Methods Small angle x-ray scattering (SAXS) was used to characterize the phase behavior using a Siemens XPD 700P WAXD/SAXS with a CuKa line of 1.54 Â. A polarized light microscope (Nikon) equipped with a hot stage was utilized to corroborate data collected with SAXS by looking for characteristic textures of the various mesophases. Reaction profiles were monitored in real time with a differential scanning calorimeter (Perkin-Elmer DSC-7) modified with a medium pressure U V arc lamp and quartz windows. Samples were covered with a thin film of FEP (DuPont fluorinated copolymer) to prevent evaporation of water. The samples were heated to 60 °C at 20 °C/min after which they were held isothermally for one minute to allow the samples to flow and level to ensure good thermal contact. The samples were then allowed to cool at 10 °C/min to the desired polymerization temperature. The polymerizations were initiated with monochromatic 365 nm light at an intensity of 4.5 mW/cm . The heat of polymerization was utilized to directly calculate the rate of polymerization. For these studies the theoretical values of 18.5 kcal/mol was used as the heat evolved per acrylamide double bond reacted. The kinetic parameters, k and kt, were determined from after effect experiments as described elsewhere. NMR spectra were acquired using a Bruker MSL-400 NMR spectrometer operating at a frequency of 28.9 MHz for N . A standard 10mm probe was used. Spectra were obtained using the DEPTH sequence to suppress N background due to the probe. The 90° pulse width was 22 μβ, the probe dead time was 10 μβ, and the acquisition time was 131 ms. The recycle delay was 600 ms, and no proton decoupling scheme was implemented. Samples were typically acquired without spinning. 2
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Results and Discussion Previous work indicates that the polymerization kinetics is of utmost importance in structure retention of the original L L C phase. Interestingly, the
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Figure 2. Chemical structures of a) acrylamide, b) Brij 58, and c) Brij 56.
382 polymerization rate of acrylamide in various L L C phases of Brij 56 exhibits considerable differences depending on the morphology of the system. In Figure 3 the polymerization rate is plotted as a function of double bond conversion for 25% acrylamide with increasing concentration of surfactant at 35°C. By increasing the surfactant over this range, discontinuous cubic, hexagonal, and inverse micellar phases are observed. Also shown is a profile of an isotropic solution polymerization of the same concentration of acrylamide in water at the same conditions. An extraordinarily slow photopolymerization rate is observed in the isotropic sample. The rate is so slow, in fact, that it is difficult to obtain consistent results due to the small exotherm. On the other hand, with addition of an appropriate amount of surfactant to reach the discontinuous cubic phase the polymerization rate is more than six times faster than that observed in the isotropic system. Further increases are observed in hexagonal samples with 50% and 60% surfactant that yield a polymerization rate more than ten times that of the isotropic phase and also considerably faster than that of the cubic phase. Interestingly, with further increases in surfactant concentration to 70% an inverse micellar phase is formed that exhibits a polymerization rate less than the other ordered L L C phases with lower surfactant concentrations. This further substantiates that changes in polymerization kinetics are strongly dictated by the L L C morphology and not inherently by the surfactant concentration.
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Double Bond Conversion Figure 3. Polymerization rate versus double bond conversion for 25 wt% acrylamide in the LLC phases ofBrij 56/water with increasing Brij 56 concentration. Shown are 0% (Isotropic- *), 40% (Cubic-V), 50% (Hexagonal-*), 60% (Hexagonal- 0), and 70% (Inverse Micellar, A) surfactant.
383 To understand the kinetic mechanism driving these striking increases in rate, it is valuable to determine the apparent rate parameters of propagation (k ) and termination (kt). Figure 4 gives kp and k for the isotropic acrylamide solution as well as a solution of acrylamide in the micellar phase of Brij 58, a closely related surfactant to Brij 56. Rate increases in this system are virtually identical to those described for the Brij 56 system. The paramaters k and k for both the isotropic and micellar solutions remain fairly constant as the reaction proceeds. Additionally, kp appears to be approximately the same for both systems over the range of conversions studied. On the other hand, a large change is observed in kt. The values decrease almost an order of magnitude when comparing the isotropic to the micellar system. Such a decrease is indicative of decreased mobility of the growing polymer chains which leads to an increase in the radical concentration and thus a higher rate. Similar decreases in termination have been observed for polymerizations in thermotropic LCs, as well as the polymerization of reactive lyotropic L C . ' p
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384 From these results, it is logical to conclude that the order inherent in these templated lyotropic systems leads to dramatic changes in polymer evolution and kinetics. A valuable technique to provide direct information about the reaction environment and local ordering is N NMR. N is a quadrupolar nuclei that yields a splitting pattern indicative of the degree of order in an anisotropic environment. To determine the ordering of the amide nitrogen in the acrylamide as a function of global lyotropic order, spectra were obtained in the various L L C phases of the Brij 56 system. In Figure 5 the N spectra from the acrylamide before polymerization are plotted for three different concentrations of Brij 56. These three concentrations correspond to the hexagonal, cubic, and isotropic phases. The isotropic solution of acrylamide and water exhibits a relatively sharp peak with no splitting pattern indicating no anisotropy and therefore no organization of the monomer. The cubic L L C with 40% surfactant yields a broader peak with shoulders at the base of the peak at approximately 4 and 8 kHz. This broadening of the peak indicates a less mobile environment, which could account for the increase in polymerization rate through inducing an early onset of the gel effect that lowers the termination rate. Also, the presence of the shoulders on the base of the peak at this composition indicates some preferential orientation of the acrylamide. When increasing the 1 4
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Figure 5. Ν NMR spectra of 25% acrylamide in the various LLC phases of Brij 56/water with increasing concentration of surfactant in water. Shown are spectra with 0% (Isotropic-V), 40% (Cubic- *), and 50% (Hexagonal-O) surfactant.
385 concentration further to 50% surfactant, a hexagonal L L C phase is present. As stated previously this composition yielded a tenfold increase in the polymerization rate. The N spectra of this sample exhibits two splitting patterns with relatively broad peaks. The broadening certainly could be due to a decrease in mobility of the monomer which may account for some of the increase in polymerization rate. The splitting patterns are also an indication that the L L C phase is inducing ordering on the acrylamide and that it possesses a preferred orientation within the L L C system. This behavior shows that the acrylamide is strongly associated with the surfactant and the increase in rate may be due both to decreased termination, resulting from less mobility, and an increased local concentration of double bonds. The orientation of monomer may also enhance the polymerization rate by causing polymerization to occur topologically along the L L C phase interface. Although, the phase and ordering of lyotropic LCs is typically modulated by composition, they are also inherently thermotropic as well in that they exhibit changes in phases with increase in temperature. In fact, the polymerization of the Brij 56/aciylamide/water system exhibits a distinct dependence on temperature. Figure 6 shows the polymerization rate as a function of conversion for polymerizations of 25% acrylamide in 40% Brij 56 at 25, 30 and 50°C. Interestingly, a relatively slow polymerization rate is 1 4
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Double Bond Conversion Figure 6. Polymerization rate as a function of conversion for 25 wt% acrylamide in 40% Brij/35% water with increasing temperature. Shown are polymerizations at 25°C (Isotropic/Crystalline- W), 35°C (Cubic- 0), and 45°C (Isotropic- ·).
386 observed for the system at 25°C. At this temperature a biphasic system is observed using polarized microscopy with crystallites and isotropic micellar regimes. When the temperature is increased to 30°C, which corresponds to a completely homogeneous hexagonal phase, the polymerization rate almost doubles. This large increase cannot be accounted for simply by the increase in temperature alone. This significant increase in polymerization rate is due to the increased organization of the monomer in the L L C phase that may enhance the propagation rate and potentially lower the termination rate due to the high degree of order imposed on the propagating chains. Additionally, the sample at 25°C possesses crystalline domains that would not readily polymerize and therefore decrease overall rate. Further evidence of the dependence of the polymerization rate on the L L C phase morphology is given when the polymerization takes place at 50°C. At this temperature the polymerization rate actually decreases substantially. One might expect that the polymerization rate should increase with increasing temperature. Again, the ordering of the system yields a logical answer. With an increase in temperature to about 40°C, the sample changes to a micellar system. The decrease in order from the hexagonal to micellar phase appears to be directly responsible for the decrease in rate. While the kinetic results and ordering of the system before polymerization are important to develop controlled nanostructured L L C systems, it is also critical to look at structure retention after polymerization. Figure 7 shows the small angle X-ray scattering (SAXS) profile of both polymerized and unpolymerized samples with 25% acrylamide in 40% Brij 58 surfactant. The unpolymerized sample shows the strong primary reflection as well as the secondary reflections of the bicontinous cubic phase. After polymerization very little change is seen. Both primary and secondary reflections are evident and a large degree of structure retention is observed. Such behavior is in great contrast to thermally polymerized samples in which very little if any structure retention is observed. By retaining these L L C nanostructures, the physical properties of the polymer can also be dramatically altered. These results also show the importance of understanding polymerization kinetics and the potential of photopolymerization for nanostructure development and retention in templated lyotropic L C systems. 14
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Conclusions This work presents the photopolymerization kinetics of acrylamide correlated with monomer organization in various phases of an L L C system. The photopolymerization kinetics of acrylamide depend strongly on the L L C morphology. The polymerization rates are enhanced in the more ordered L L C phases as a result of the preferential orientation of the acrylamide monomer.
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Figure 7. SAXS profiles of 25% acrylamide in the bicontinous cubic phase formed with 40% Brij 58 and water before (*) and after (V) polymerization.
The effect is observed both with changing temperature and changing composition. These results indicate that aciylamide is strongly associated with the L L C interface and coaggregates with the surfactant. L L C order strongly influences photopolymerization behavior and consequently the polymer structure. Photopolymerization under appropriate conditions also allows successful templating of the L L C nanostructure onto the aciylamide polymer.
Acknowledgements The authors thank the University of Southern Mississippi, the Petroleum Research Fund, and the National Science Foundation (CTS-0093911) for financial support of this project. Also Dr. Bill Jarrett is gratefully acknowledged for obtaining the N spectra. 1 4
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