Polymer Structure Development in Lyotropic Liquid Crystalline Solutions

Aug 14, 2014 - Dow Corning Corporation, 2200 West Salzburg Road, Midland, ... behavior were characterized to understand the connection between polymer...
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Polymer Structure Development in Lyotropic Liquid Crystalline Solutions Michael A. DePierro†,‡ and C. Allan Guymon*,‡ †

Dow Corning Corporation, 2200 West Salzburg Road, Midland, Michigan 48686, United States Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, Iowa 52242, United States

‡

ABSTRACT: Polymerization in lyotropic liquid crystalline (LLC) media is a promising method enabling synthesis of nanostructured organic polymers. To understand polymer structure development in LLC systems, this study investigates the polymerization of acrylate monomers of differing polarity and size within the hexagonal and lamellar phase of dodecyltrimethylammonium bromide and water. LLC structure, polymer morphology, and polymerization behavior were characterized to understand the connection between polymerization environment and formation of polymer structure. While the order of the template phase highly influences final polymer order, variations in monomer chemistry can lead to significantly different polymer structures even from the same liquid crystalline phase. More rapid polymerization, which leads to higher conversion and higher cross-link density of the relatively low molecular weight monomers, yields highly anisotropic polymer structure exhibiting alignment of 800 nm channels extending for lengths greater than 150 μm. Less ordered polymer structure results with analogous monomers of higher molecular weight. While polymer structure appears to result from a phase separation process in these systems, the structure directing influence of the liquid crystalline media is exhibited in several systems including the formation of highly oriented cylindrical structures with diameter of 200 nm from polymerization of poly(ethylene glycol) dimethacrylate in the hexagonal phase. Significant control of polymer structure has been demonstrated using LLC templates through proper selection of monomer chemistry, concentration, and LLC template structure.

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INTRODUCTION

While concentrated surfactant solutions form fundamentally interesting nanostructures spontaneously, they lack rigidity and are thermally unstable. Therefore, while lipidic LLC selfassemblies have been investigated for drug delivery systems,6−8 LLC phases by themselves are not commonly used in materials applications. The nanoscale morphology of these ordered phases, however, can be utilized as structure directing templates for polymerization to yield robust cross-linked materials. Two primary methods have been used to generate nanostructured polymers in LLC media: polymerization of liquid crystalline monomers and polymerization of nonmesogenic monomers within ordered surfactant assemblies.9 A significant body of research has demonstrated direct preservation of liquid crystal morphology in robust cross-linked networks utilizing mesogenic monomers.10−14 Several applications of polymerizable surfactants have been investigated, with promising results reported in catalysis,4,15 nanocomposite materials,16 and size selective barriers.17−21 The “template” approach to polymer synthesis, utilizing nonmesogenic monomers in LLC phases formed with low molecular weight surfactant, has also been investigated. This method offers many distinct advantages for the synthesis of nanostructured polymers such as facile control of template morphology through utilization of numerous well-characterized

Polymerization in self-organizing surfactant media is one of the most promising methods for the control of inorganic and organic polymer nanostructure. Nanoparticles, hollow spheres, membranes, and porous bulk polymers have all been obtained through polymerization within ordered surfactant solutions. One particularly interesting method allowing significant flexibility and control in the synthesis of polymers with periodic nanostructure involves photopolymerization within lyotropic liquid crystalline (LLC) phases.1 This method of synthesis has greatly impacted the field of nanotechnology by enabling formation of mesoporous silica monoliths and other nanostructured inorganic materials.2,3 Polymerization of organic monomers in LLCs could also be advantageous to generate nanostructured materials in applications requiring greater ductility, processability, and more versatile chemistry.4,5 Lyotropic liquid crystals form at relatively high concentrations of surfactant in a typically aqueous solvent. A variety of complex morphologies are exhibited in these systems depending on the nature of the surfactant and its concentration. As the surfactant concentration is increased in water, many systems exhibit several transitions in phase from isotropic arrangements of spherical micelles at low concentrations to cubic arrangements of spherical and elongated micelles. At higher concentrations of surfactant, cylindrical micelles pack in hexagonal arrays and lamellar structures of bilayer aggregates are often observed as well. © 2014 American Chemical Society

Received: April 18, 2014 Revised: July 21, 2014 Published: August 14, 2014 5728

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surfactant systems.9 In certain biomedical applications, such as drug delivery and tissue engineering, this method could allow control of nanostructure in the limited number of polymers approved for living systems.22−24 While surfactant must be removed for many of these applications, the control of structure enabled by LLC templating provides access to properties not available in traditional polymer systems. For example, hydrophobic polydimethylsiloxane oligomer (PDMS) has been compatibilized and incorporated into a poly(N-isopropylacrylamide) (PNiPAAm) network using LLC surfactant, increasing mechanical stability without altering thermoresponsive properties.25 Perhaps the greatest advantage of the polymerization of nonmesogenic monomers is low cost, as inexpensive, commercially available monomers that do not require special synthesis, can be used. Significantly different polymerization behavior has been observed in ordered LLC phases compared to isotropic bulk polymerization media. The rate of polymerization of several mono and diacrylate monomers is highly dependent on the order of the LLC phase.5 Even if bulk monomer concentration is held constant in each phase, the localized or effective concentration varies substantially depending on monomer ordering and segregation. The impact of LLC order is evidenced by sharp changes in polymerization rate coinciding with LLC phase transitions.26,27 The effect of order is particularly pronounced in the polymerization of acrylamide with the rate increasing by more than 10 times between the isotropic and cubic phase.28,29 The polar and nonpolar domains of lyotropic liquid crystals confine reacting monomer in welldefined 2−10 nm compartments, and the polymerization behavior is therefore highly dependent on monomer polarity. Investigations of the polymerization kinetics in LLC systems have revealed distinctly different polymerization behavior for oil- and water-soluble monomers due to segregation of monomer in distinct regions of the LLC phase.5 Significantly different thermal stability of the LLC phase after polymerization is observed with oil- and water-soluble polymers.30 The enhanced thermal stability observed with polar monomers has been attributed to polymerization in the continuous phase resulting in encapsulation and close interaction of the surfactant aggregates. The initiation mechanism is also affected by LLC morphology.31,32 Initiation efficiency varies significantly depending on initiator size, polarity, and LLC topology. Initial research has met considerable challenges in the direct templating of LLC phases with organic polymers, yet polymers with interesting anisotropic morphologies have been obtained.9,33 Several water-soluble monomers, including hydroxyethyl methacrylate, acrylic acid, and acrylamide yield highly ordered micrometer-scale polymer morphologies when polymerized in the ordered phases of nonionic and ionic surfactants.34,35 While direct templating of the LLC is not realized in these systems, anisotropic transport of monomer and polymer in the liquid crystalline reaction environment has been reported to lead to generation of the ordered polymer structures. Formation of polymers with nanometer scale features have been shown with other surfactant/monomer systems. Polymerization of styrene in the inverse hexagonal phase of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) results in the formation of extended polymer strands with diameter less than 100 nm.36 Similar morphology results from the sol−gel synthesis of silica within the inverse hexagonal phases of AOT.37 Conductive polyanaline nanowires have also been obtained through polymerization in the same phase.38

Many materials applications could be enhanced by the nanometer scale and organized structure of these polymers. Significant preliminary research of LLC systems has investigated the evolution of LLC order during polymerization. While retention of order of the template mesophase has been reported after polymerization in many systems, analysis of polymerization products has relied on small-angle X-ray scattering analysis alone, and polymer structures were not directly studied in many cases.39,40 The preservation of the LLC phase may be a strong indication of a highly ordered polymer structure, although the exact relationship between retention of LLC phase order and polymer morphology is still not understood. Previous research indicates that polymerization kinetics may play a vital role in the type of structure obtained in liquid crystalline solutions.26,27,29,34,36 The confinement of high molecular weight polymer to LLC geometries is entropically unfavorable, and rapid polymerization could enable the trapping of polymer structure in a nonequilibrium state. Monomer structure and interaction of the monomer with surfactant have also been suggested as vitally important parameters controlling polymer structure and corresponding physical properties.30,41 Just as monomer polarity largely influences the polymerization kinetics in LLC media, it is reasonable to believe that partitioning of monomer in LLC media will also significantly impact ultimate polymer structure. Recent work combining polymerizable and nonreactive surfactant in LLC-templating of nonmesogenic monomers to enhance thermodynamic stability of the system during polymerization has led to greater periodicity in the morphology of the final polymer.42 Before novel nanostructured polymers with beneficial physical properties can be controllably synthesized using LLC templates, greater knowledge is required of the factors controlling polymer structure, including monomer chemical structure, polymerization kinetics, cross-link density, and LLC phase order. To understand how monomer selection influences polymer structure, several acrylate monomers of varying hydrophobicity and molecular weight will be polymerized in the hexagonal phase of a cationic surfactant and water. Polymerization will also be examined with varying concentrations of monomer within the hexagonal phase of a common cationic surfactant. The evolution of the LLC phase will be examined using small-angle X-ray scattering and polarized light microscopy to determine the degree of order of the surfactant template throughout the polymerization and to understand the interaction of monomer with the LLC system. Polymer structure will be characterized after removal of surfactant and water. While this study will primarily investigate polymerization within the hexagonal phase, for comparison, polymerization will also be conducted in the lamellar phase to gain better understanding of the relationship between LLC template order and final polymer structure. This study will thus provide significant information regarding the polymer structural development occurring in lyotropic liquid crystalline systems. A greater understanding of the possible structures as well as insight into the control of polymer nanostructure will result.

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EXPERIMENTAL SECTION

Materials. The monomers examined in this study were hexanediol diacrylate (HDDA, Polysciences) and several poly(ethylene glycol) diacrylate monomers including those with ethylene oxide molecular weight of 258, 575, and 700 g/mol (PEG-258, 575, and 700-DA, Aldrich). Poly(ethylene glycol)-400-dimethacrylate (PEGDMA, Poly5729

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Figure 1. Chemical structures of the monomers, photoinitiator, and surfactant used in this study. Shown are (a) hexanediol diacrylate (HDDA), (b) poly(ethylene glycol)-400-dimethacrylate n = 9 (PEGDMA), (c) poly(ethylene glycol)-258,575,700-diacrylate n = 6, 13, and 16 respectively (PEGDA), (d) 2 hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (HEPK), and (e) dodecyltrimethylammonium bromide (DTAB). sciences) was also investigated in this study. Polymerizations were initiated with the photoinitiator, 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (HEPK, Ciba Specialty Chemicals). Liquid crystalline samples were synthesized using different concentrations of the cationic surfactant, dodecyltrimethylammonium bromide (DTAB, Aldrich) and deionized water. The monomers, surfactants, and initiator were all used as received. Samples were mixed, centrifuged, and sonicated repeatedly until homogeneous gels were obtained. Figure 1 shows the chemical structures of the materials used in this study. Procedures. Lyotropic liquid crystal morphology and phase boundaries were characterized with a polarized light microscope (Nikon, Eclipse E600W Pol) equipped with a hot stage (Instec, Boulder, CO) by examining the optical texture of each sample. For corroboration of microscopy data, phases identities were determined with small-angle X-ray scattering by measuring ratios in d-spacing. These measurements were conducted utilizing a Nonius FR590 X-ray apparatus with a standard copper target Röntgen tube as the radiation source with a Cu Kα line of 1.54 Å, a collimation system of the Kratky type, and a PSD 50 M position sensitive linear detector (Hecus M. Braun, Graz). By comparing SAXS profiles and PLM images from before and after polymerization, the degree of LLC structure retained upon polymerization was compared. Morphology of the freeze-fractured polymer gels was examined using scanning electron microscopy (SEM, Hitachi S-4000). Prior to analysis, surfactant was removed by gradually exchanging water for ethanol. The ethanol was subsequently replaced by supercritical CO2 using a BIO-RAD E3000 critical-point drying apparatus. The criticalpoint method employed has been shown to minimize collapse of the gel structure during drying.43,44 The dried and freeze-fractured samples were sputter-coated using a Au/Pd target with a coating thickness of approximately 2.5 nm. Polymerization rate data was acquired with a PerkinElmer differential scanning calorimeter. A medium pressure UV arc lamp (Ace Glass) was used to initiate polymerization. A band-pass filter was used to limit the initiating light to around 315 nm. A light intensity of 2 mW/cm2 was achieved by adjusting the height of the lamp above the sample. Error caused by water evaporation was minimized by covering the approximately 5 mg samples with thin transparent films of FEP (Dupont fluorinated copolymer). Samples were purged with nitrogen for 5 min prior to polymerization to prevent oxygen inhibition. Isothermal reaction conditions were maintained during polymerization using a refrigerated circulating chiller. The normalized polymerization rate was determined as a function of time from the heat flow (Q) according to eq 1 Rp [M ]0

=

Q × MW ΔH × M

error for these kinetic experiments was calculated by dividing the standard deviation of the maximum polymerization rate from five identical experiments by the average. The relative standard error for the vast majority of samples was less than 5%.

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RESULTS AND DISCUSSION Polymers templated from lyotropic liquid crystalline media could provide useful nanostructures and properties for a number of applications ranging from catalytic media, biological membranes, tissue scaffolds, and ultrafiltration.45,46 Recent investigations have demonstrated formation of highly anisotropic polymers in a variety of LLC phases.9,33 Although liquid crystalline morphology is not often directly templated in the final polymer, the type of structure obtained varies substantially depending on the order of the LLC template phase. As liquid crystalline order can be varied over a wide range quite easily, this method could enable generation of nanostructured polymers with tunable morphology and properties. While recent studies have shown significant promise in the generation of nanostructured polymer in LLC media, still, relatively little is known of the nature of the polymer structure or how this structure can be controlled.36 Several factors that could largely affect polymer structure development in LLC solvents include the polymerization kinetics, stability of the LLC phase, monomer polarity, monomer molecular weight, and the interaction of monomer and surfactant.22 By understanding the influence of these factors on polymer structure, novel polymers with anisotropic morphology and nanometer features could be developed using widely available and inexpensive monomers. Since polymer structure and properties are closely related, enhanced control of polymer morphology could lead to enhancements in several applications as well as the development of novel polymer applications. While polymerization often results in the complete disruption of the liquid crystalline template phase, the order of the template has been retained throughout the course of polymerization in several systems.5,35,39,40 It is naturally expected that polymerization in a more stable liquid crystalline phase should yield a more highly ordered polymer structure. While systems involving monovinyl monomers have been investigated in a parallel study,47 this work focuses on the liquid crystalline order during the polymerization of several divinyl monomers. From preliminary studies involving extensive comparison of PLM images and SAXS profiles before and after polymerization, it is clear that liquid crystalline order can be preserved after polymerization in LLC phases that are formed with a variety of cationic and nonionic surfactants and with a wide variety of acrylate monomers and concentrations of monomers. The stability of the phase, however, greatly varies depending on the chemical structure of the surfactant. Of the

(1)

where [M]0 is the initial monomer concentration, MW is the monomer molecular weight, ΔH is the enthalpy of polymerization of the monomer, and M is the sample mass. Maximum rates were taken from the peak in the rate profiles obtained and double bond conversion was calculated by integrating the heat flow profiles. For these studies the theoretical value of 20.6 kcal/mol was used as the heat evolved for reacted acrylate double bonds. The relative standard 5730

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templated, HDDA-based materials, compared to nanostructured hexyl acrylate based polymers prepared with similar monomer concentration, network formation likely facilitates the apparently higher degree of structure retention with HDDA as a cross-linked structure would have a greater stabilizing effect on the LLC phase. Additionally, the polymerization of HDDA occurs more than two times faster than hexyl acrylate in the hexagonal phase. Such rapid polymerization could enable formation of a polymer network structure at an earlier stage in the demixing process and minimize disturbance of the LLC phase.33 The slight shift of SAXS peak position to higher scattering angle in the presence of HDDA compared to the neat LLC system indicates a decrease in the size of the periodic LLC structures. For nonpolar materials such as HDDA, this shift suggests that the monomer aggregates at the surfactant/ water interface.5 Even though the SAXS scattering profile of the polymerized sample directly overlaps with the unpolymerized sample, the increased scattering intensity after polymerization is indicative of polymer/liquid crystal phase separation. A higher degree of order and hence, higher scattering intensity could result when polymer demixes from the LLC phase. Regardless, the persistence of the high degree of order throughout the polymerization could have important implications in the order of the final polymer. The structure of the polymer obtained from polymerization of 10 wt % HDDA in the hexagonal phase of the 50 wt % DTAB/water system was examined directly using SEM. The SEM image as shown in Figure 3 reveals a highly ordered

surfactants studied, the LLC phases of the cationic surfactant, DTAB, were found to be most stable forming with a wide variety of monomers in concentrations of 10 to 30 wt % in samples ranging from 45 to 65 wt % DTAB. The LLC phases formed with other nonionic surfactants such as polyoxyethylene (2) cetyl ether (Brij 56) were more sensitive to the chemical structure of the monomer and were generally less easily retained upon polymerization. The DTAB system is therefore ideal for this study as a wide variety of monomers can be polymerized in the same phase, which can be maintained throughout the polymerization. By comparing the morphology of polymers synthesized using monomers of different chemical structure in the hexagonal phase, this approach allows comparison of the effects of monomer polarity, monomer concentration, cross-link density, and polymerization kinetics on final polymer structure. As polymer structure formation is governed by the morphology of the LLC template phase, it is critical to understand not only the structure of the phase before polymerization but also the alterations that may occur during polymerization. In most of the systems that have been studied, the hexagonal phase appears to be retained throughout the polymerization, but the degree of change of the lattice spacing may still provide significant insight into the polymerization process. Consider for instance, the polymerization of the relatively hydrophobic difunctional monomer, HDDA, in the LLC phases of DTAB and water. As shown in Figure 2, the

Figure 2. SAXS profile of 10 wt % HDDA in the hexagonal phase of 50 wt % DTAB and water shown before (gray ●) and after (□) polymerization. The scattering profile of a sample containing only water and surfactant in the same ratio is shown for comparison (▲).

SAXS profile before polymerization exhibits diffraction peaks at 37.2 and 21.5 Å in the characteristic 1:31/2 ratio of the hexagonal phase. Interestingly, polymerization does not disrupt the hexagonal structure, and the same d-spacing is retained after polymerization. In a similar study involving polymerizations conducted with the monofunctional analogue, hexyl acrylate, the hexagonal phase is also present after polymerization but a significant decrease in d-spacing occurs.47 The SAXS profile of the hexyl acrylate system after polymerization overlaps with that of a sample containing only surfactant and water in the same ratio. This change demonstrates that the polymer is not interacting with the surfactant thus indicating complete phase separation of polymer and monomer.35 With HDDA (Figure 2) no shift in dspacing occurs, but the SAXS profile of both the unpolymerized and polymerized samples overlap the profile of the LLC phase, making detection of phase separation difficult. In LLC-

Figure 3. SEM image of 10 wt % HDDA polymerized in the hexagonal phase of 50 wt % DTAB and water, analyzed after removal of surfactant.

polymer structure with well-aligned, interconnected layers. This anisotropic polymer structure exhibits size scale 3 orders of magnitude larger than the LLC template phase, demonstrating that LLC morphology is not directly imprinted in the final polymer. However, the influence of the LLC template is clearly visible from the appearance of definitive grain boundaries in the polymer structure. Similar lamellar-type structures have been reported in poly(acrylamide) hydrogels cured in the LLC 5731

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phases of several nonionic surfactants.34,35 These interesting polymer morphologies have been attributed to anisotropic diffusion of monomer and polymer during polymer/liquid crystal demixing. It is clear from these results that retention of LLC order during the polymerization, does not translate to direct templating of LLC nanostructure in the final polymer. However, the preservation of original liquid crystalline order throughout the polymerization process is likely a critical factor allowing the formation of such highly aligned polymer morphology. For example, when higher concentrations of HDDA are polymerized in the hexagonal phase of DTAB and water, the phase is largely disrupted and randomly ordered polymer structure is obtained. Several poly(ethylene glycol) diacrylate monomers with different molecular weight of the ethylene glycol spacer unit have also been studied to simultaneously investigate the influence of monomer polarity and cross-link density on polymer structure development in the hexagonal phase. As shown in Figure 4 the X-ray scattering peaks before and after

monomer. Unlike hexyl acrylate,47 which clearly undergoes gross phase separation from the LLC system, PEG-258-DA continues to interact with the LLC aggregate structure after polymerization in such a way that the original phase morphology of the unpolymerized sample is retained upon polymerization. This is evidenced by the overlapping profiles of the primary and secondary diffraction peaks before and after polymerization. To obtain more direct information regarding polymer morphology, these samples were characterized with SEM after extraction of the surfactant and water. Figure 5 shows the SEM image of the sample taken at two magnifications to demonstrate both the long-range order and the nature of the polymer surface. A very high degree of order is observed in this polymer. The structure is homogeneous throughout the sample and parallel alignment of polymer layers extends for lengths greater than 200 μm. However, viewing was limited to the freeze-fracture surface plane and alignment likely continues over a much greater length. The alignment of cylindrical rods occurs indefinitely in the hexagonal phase, and this structure is therefore indicative of the role of the LLC in assisting polymer development.48 While oriented lamellar type structures have been observed in other monomer/LLC systems in our lab (HDDA and acrylamide) PEG-258-DA exhibits periodicity on a much smaller size scale. Primary polymer layers are separated by less than 800 nm. Each layer appears composed of additional layers as can be seen with higher magnification. These secondary layers, measuring less than 100 nm, consist of sphere-like polymer agglomerates. Densely packed polymer globules separated by pores on the order of 20−50 nm are exhibited in other domains not exhibiting such layer structure. The random defects in polymer structure such as the relatively large gaps could be due to the dehydration and freezing occurring during sample preparation or from the defects in the LLC template structure itself.12 The sample preparation could also lead to collapse of structure and the aggregation of multiple layers to form thicker primary layers. Prior to dehydration and freezing, the gel structure likely exhibits greater uniformity and smaller size scale of interlayer spacing. From the examination of the polymer structures resulting from PEG-258-DA and HDDA, pore size appears relatively independent of any difference in cross-link density for this LLC system. These observations are consistent with Anderson and

Figure 4. SAXS profiles of 20 wt % PEG-258-DA before (gray ●) and after (□) polymerization in hexagonal phase of 50 wt % DTAB and water as well as that of a sample containing only surfactant and water in the same relative ratio (▲).

polymerization of PEG-258-DA correspond to a hexagonal morphology. Decreased lattice spacing results when this monomer is added to the LLC sample, indicating swelling of the hydrophilic domains, as expected with such a polar

Figure 5. Freeze-fracture SEM images taken at two magnifications of 20 wt % PEG-258-DA polymerized in the hexagonal phase of 50 wt % DTAB and water, after removal of surfactant. 5732

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pattern of the neat LLC system of surfactant and water and no added monomer is shown. This more polar monomer shifts the d-spacing to lower values after polymerization than observed with PEG-258-DA, demonstrating that the monomer segregates to an even more significant degree in the polar domains of the LLC phase. After polymerization, the primary and secondary scattering peaks retain the d-spacing ratios of the hexagonal phase although a much greater shift in lattice spacing occurs upon polymerization of the higher molecular weight molecule when compared to PEG-258-DA. The post polymerization lattice spacing with PEG-700-DA and PEG-575-DA is intermediate to that of the respective unpolymerized and neat surfactant/water samples. The dspacing of the LLC phase before and after polymerization of PEG-575-DA is nearly identical to that of PEG-700-DA and is therefore not shown. Complete phase separation may be expected to yield lattice spacing identical to that of the neat LLC. However, such intermediate d-spacing could still result after phase separation due to competition for water between the LLC and polymer or from the continued interaction of the polymer with surfactant. The latter case would expectedly yield polymer with highly anisotropic polymer morphology. Polymer structure resulting with the higher molecular weight poly(ethylene glycol) monomers was also characterized using SEM. Figure 7 shows the SEM images of the polymer formed when 20% PEG-700-DA is photocured in the hexagonal phase of 50% DTAB and water. The images were acquired at the same magnification as the PEG-258-DA shown in Figure 5 to facilitate comparison of structure. The polymer structure resulting with the higher molecular weight monomers is much less ordered than that from PEG-258-DA. Polymer layers form more randomly and are not well-aligned. Layer spacing is approximately 2.5 μm, three times greater than that of the PEG-258-DA samples. The surface of the polymer has a smooth glassy appearance, distinctly contrasting with that of PEG-258-DA. Close inspection of the layer surface at higher magnification reveals a much more densely packed and less porous material. The effect of liquid crystalline order on polymerization rate can provide insight into the dynamics controlling structural development in these systems. Photodifferential scanning calorimetry was used to investigate the polymerization kinetics of these hydrophilic monomers both in neat isotropic phase

Strom’s results indicating independence of pore size and crosslink density in the polymerization of acrylamide in a bicontinuous cubic phase.49 O’Brien et al. have shown that the LLC phases of polymerizable lipids interrupt crosslinking.50 The different pore size resulting from HDDA and PEG-258-DA can better be explained by localization of the monomer in regions of different polarity as well as different affinity for surfactant. From the shift in the main peak observed in SAXS data, it is clear that that HDDA segregates preferentially within nonpolar domains while PEG-258-DA localizes in more polar domains of the surfactant aggregates. Strong interactions of PEG-258-DA with the polar region of the surfactant and the stabilization of the LLC from polymerization in the surrounding continuous phase appears to yield a structure more closely related to that of the template phase. Further insight into the role of monomer polarity was obtained through similar studies of the phase retention and polymer structure formation resulting with poly(ethylene glycol) diacrylate monomers with higher ethylene oxide content and lower cross-link density. The LLC structure before and after polymerization of PEG-575-DA and PEG-700-DA in the hexagonal phase of 50% DTAB/water was characterized with SAXS as shown in Figure 6. Additionally, the scattering

Figure 6. SAXS profile of 20 wt % PEG-700-DA in the hexagonal phase of 50 wt % DTAB and water shown before (gray ●) and after (□) polymerization. The scattering profile of a sample containing only water and surfactant in the same ratio is shown for comparison (▲).

Figure 7. SEM images taken at two magnifications of 20 wt % PEG-700-DA polymerized in the hexagonal phase of 50 wt % DTAB and water. 5733

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the higher molecular weight monomer in the organized liquid crystalline system. As seen in the neat phase polymerization, PEG-258-DA polymerizes 30% faster than PEG-700-DA when both are polymerized in the ordered hexagonal phase. The different rate of polymerization and cross-link density could be related to the significantly different polymer structure resulting from these monomers varying only in their ethylene oxide content. The organized polymer alignment observed with PEG-258-DA may be due to more rapid polymerization kinetics and higher crosslink density. Previous studies of acrylamide polymerization in LLC phases show significantly different structure depending on the speed of polymerization.29 Lester et al. also observed correlations between the rate of polymerization and the degree of LLC order retained during the polymerization of a cationic amphiphile.51 A smoother, more dense structure appears to result with PEG-700-DA from the diffusion of monomer to distinct domains in which growth of the polymer layers occurs. The higher cross-link density resulting with PEG-258-DA, yields relatively rapid polymerization and the formation of polymer structure in a nonequilibrium state at a relatively early stage in the phase separation process. LLC morphology is more effectively “templated” with this monomer, and a porous morphology with smaller scale features results. While monomer chemistry significantly impacts polymer morphology in these systems, the concentration of monomer within an LLC phase may also largely affect the structure obtained with a given monomer. Relatively low concentrations of monomer have typically been used in these studies because LLC phases often become disrupted at higher concentrations. The higher mechanical strength gained with higher monomer concentration, however, could justify the decreased order. In addition to obtaining a more robust material, investigation of the effects of monomer concentration on polymer structure could provide a greater fundamental understanding of how polymer structure forms in LLC media. To determine the influence of monomer concentration on the LLC phase evolution during polymerization PEG-400-DMA was polymerized in the hexagonal phase of DTAB/water with monomer concentrations from 10 to 40 wt %. The X-ray scattering profiles are shown for the 10 and 30 wt % percent samples in Figure 9, parts a and b, respectively. With low concentrations of PEG-400-DMA a well-defined hexagonal phase is observed both before and after polymerization and the lattice spacing increases only slightly from 39.7 to 40.26 Å during the polymerization. As seen with PEG-258-DA the diffraction peak of the polymerized sample overlaps with that of the unpolymerized material and both are observed at lower dspacing than the neat surfactant/water sample. Preservation of the lattice spacing in the LLC phase after polymerization again provides evidence that the water-soluble polymer interacts favorably with and stabilizes the surfactant aggregate structure. When 30 wt % PEG-400-DMA is added to the same LLC phase, a shift to even lower lattice spacing occurs, although a well-defined hexagonal phase still persists. Polymerization of the 30 wt % monomer sample causes a relatively large 2.7 Å shift to higher d-spacing, when compared to samples with lower monomer concentration, and the d-spacing in the polymerized sample is similar to that of the neat surfactant and water phase. Polymerization of a sample with 40% PEG-400-DMA yields similar behavior althoughthe X-ray scattering peaks are broader

and in various liquid crystalline phases of DTAB and water. Preliminary examination of the neat polymerization reaction reveals the fastest polymerization occurring with PEG-258-DA and the slowest rate with PEG-700-DA, with a 30% lower rate. PEG-575-DA exhibits an intermediate polymerization rate to these monomers. The general trend of decreasing rate of polymerization with increasing monomer molecular weight can be attributed mainly to the lower concentration of double bonds in systems with higher ethylene glycol content. Polymerization rate is shown in Figure 8 as a function of time for 20 wt % PEG-258-DA and PEG-700-DA in the micellar,

Figure 8. Polymerization rate vs time of PEG-258-DA (a) and PEG700-DA (b). Shown are polymerizations conducted in 30 wt % micellar (●), 50 wt % hexagonal (□), 60 wt % hexagonal (▲), and 70 wt % lamellar (◇) in DTAB/water.

hexagonal, and lamellar phases that form at DTAB concentrations ranging from 30 to 70 wt %. The rate of polymerization of both monomers is highest in the micellar phase and decreases with the transition to the more highly ordered phases forming at higher concentrations of surfactant. While both monomers exhibit a decrease in rate between micellar and hexagonal phases, the increased order has a greater impact on the polymerization of PEG-700-DA, decreasing by more than half between the micellar and lamellar phase. The polymerization rate decreases only 30% with PEG-258-DA over the same range of LLC phases. At surfactant concentrations greater than 50 wt % little variation is observed in the polymerization rate with PEG-700-DA. The degree of double bond conversion decreases from 90 to 50% for PEG-700-DA while PEG-575DA, in contrast, exhibits slight increase in conversion between the micellar and lamellar phase. The greater reduction in the polymerization rate and decreased double bond conversion with PEG-700-DA likely stems from more limited mobility of 5734

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20% PEG-400-DMA in the hexagonal phase of DTAB and water while Figure 10b depicts the polymer structure formation resulting with 30 wt % PEG-400-DMA. The 10 wt % samples lack the mechanical strength necessary for sample preparation and therefore could not be included in the comparison. Polymerization in the LLC results in a phase directed layered polymer structure formed with 20 wt % PEG400-DMA and exhbiting size scales 3 orders of magnitude larger than the LLC template structure. This material exhibits relatively large interlayer spacing (2−5 μm) compared to the previously described materials. Contrasting polymer morphology is observed when 30 wt % PEG-400-DMA is polymerized in the hexagonal phase. This material exhibits closely packed cylinders of diameter ranging from 150 to 250 nm. Even though the diameter of these structures is more than 50 times greater than that expected from direct templating of the hexagonal phase, the cylindrical nature of these structures gives strong indication of a template assisted mechanism. This is a surprising result in consideration of the SAXS data, which shows relatively large alteration in the LLC phase of this sample during polymerization. The extensive swelling of the polar domains with the water-soluble monomer appears to be a favorable condition enabling the formation of such a highly ordered nanostructured polymer. These results present an interesting paradox in that SAXS scattering has often been utilized to determine the degree to which LLC phases are templated during polymerization. Several studies have reported direct templating of LLC morphology based on SAXS data alone.39,40 This study, however, provides contradicting results by demonstrating the formation of nanoscale rod-like polymer structure in a system in which the LLC phase is significantly disrupted during polymerization. As shown in several studies, SAXS analysis can provide significant information regarding the polymerization dynamics, but cannot by itself reveal details about the polymer structure formed with nonmesogenic monomers in LLC media.35,52 One possible explanation for the enhanced templating of the LLC phase at higher concentration of PEG-400-DMA is a higher polymerization rate. Although phase separation of polymer and liquid crystal occur as demonstrated by the SAXS results, nucleation and initial formation of rod-like structure may be directed by the hexagonal phase before phase separation occurs as a result of fast polymerization. After phase

Figure 9. SAXS profile of (a) 10 wt % and (b) 30 wt % PEG-400DMA in the hexagonal phase of 50 wt % DTAB and water shown before (gray ●) and after (□) polymerization. The scattering profile of a sample containing only water and surfactant in the same ratio is shown for comparison (▲).

and less intense, indicating a less ordered LLC structure even before polymerization. Even though a hexagonal phase is still present, these SAXS results indicate significantly greater alteration in the dimensions of the LLC phase with higher concentration of monomer. Further understanding of the effects of monomer concentration has been obtained by comparing the morphology of samples polymerized with different monomer concentrations. Figure 10a shows the SEM images of the polymer formed with

Figure 10. SEM images of 20 wt % (a) and 30 wt % PEG-400-DMA (b) polymerized in the hexagonal phase of 50 wt % DTAB and water. 5735

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Figure 11. SEM images of (a) 20 wt % and (b) 30 wt % PEG-575-DA polymerized in the hexagonal phase of 50 wt % DTAB and water.

separation monomer may continue to swell and become incorporated in the polymer cylinders, leading to the expanded cylinder diameter. Faster polymerization kinetics resulting from higher monomer concentration similarly leads to more ordered structure when poly(acrylamide) is templated from nonionic surfactant mesophases.34 While kinetic effects could explain the variation in structure resulting with the 20 and 30 wt % samples, other monomers exhibiting faster polymerization kinetics do not result in this type of rod-like polymer nanostructure. For example, PEG-575DA polymerizes five times more rapidly than PEG-400-DMA in the hexagonal phase but yields a less ordered structure with layer spacing on the order of micrometers. Figure 11 shows the polymer structure resulting from polymerization of 20 and 30 wt % PEG-575-DA in the hexagonal phase of DTAB and water. Both concentrations of monomer yield a similarly ordered layered polymer structure, although a more dense morphology and smaller pore size results at the higher concentration. Despite the faster polymerization rate of PEG-575-DA, the polymer structure does not appear to be templated from the hexagonal phase and rod-like structure is not formed at the concentrations examined in this study. Different interaction of each monomer with surfactant and varying monomer mobility could account for the distinct polymer structures resulting with PEG-400-DMA and PEG-575-DA. The SAXS profiles of PEG575-DA interestingly exhibits a similar trend to that observed with PEG-400-DMA with apparent retention of the LLC phase with 10 wt % monomer and larger alteration of the lattice spacing occurring with 30 wt % samples. The larger polymer layer thickness, resulting in the more monomer-rich samples, leads to a lower polymer surface area to volume ratio causing a lower degree of polymer/surfactant interaction. This study has demonstrated the variability in polymer structure that can result within the same LLC template even with slight variations in monomer structure. In many instances polymers with lamellar morphology have been obtained using a hexagonal LLC phase template. Even though the polymer structure is not directly templated from the LLC phase, highly anisotropic polymer morphologies that are directed by LLC structure have been obtained. To determine further the role of the LLC phase in the structural evolution, polymerization has also been conducted in the lamellar phase formed at higher concentrations of DTAB in water. The SEM image of the polymer formed using 20% PEG-575-DA in the lamellar phase of 70% DTAB/water is shown in Figure 12. The polymer

Figure 12. SEM images of 20 wt % PEG-575-DA polymerized in the lamellar phase of 70 wt % DTAB and water.

templated from the lamellar phase exhibits parallel alignment of well-defined polymer layers. The interlayer spacing is higher (3−5 μm) in this sample and layers exhibit a greater degree of parallel alignment than observed from polymerization of the same monomer in the hexagonal phase. The layers are much smoother and consistent in thickness measuring less than 500 nm. As with all of the polymers studied, the size scale is much higher than that of the template phase. The contrasting high degree of order and homogeneity of the polymer generated in lamellar media relative to that from the hexagonal phase demonstrate the important role of LLC topology in polymer formation. The differing degree of order observed in samples templated from hexagonal and lamellar phases may be explained by the degree and type of order inherent in the template phase itself. Defects are known to occur in the LLC phases of DTAB. In fact some studies have incorporated double tailed analogues of DTAB to obtain ideal mixing in a hexagonal phase.53 A decreased termination rate parameter observed in the lamellar phase of the DTAB/water system provides evidence of more restricted diffusion in this phase.5,32 Significant variation in polymer structure can thus be obtained by adjusting surfactant concentration to alter the structure of the LLC phase. As polymer structures and properties are better understood using a 5736

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wider variety of LLC template structures, greater control of polymer nanostructure could be realized. By tuning polymer nanostructure and properties using LLC templates, many novel polymer applications could develop based on long established polymer chemistries.

REFERENCES

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CONCLUSIONS Polymerization in lyotropic liquid crystalline media enables controllable synthesis of highly ordered polymers. The size scale and alignment of polymer features varies widely depending on monomer chemistry and concentration even for polymerizations conducted using identical LLC template phases. The degree of LLC order retained during polymerization appears directly related to the order of the final polymer. While polymer structure is not directly imprinted from the LLC phase in the systems investigated in this study, polymers with anisotropic nanostructures are obtained with the degree of order highly dependent on LLC alignment and monomer chemical structure. Uniform orientation of polymer layers with 500 nm spacing results from polymerization of PEG-258-DA in the hexagonal phase while more random polymer structure with larger channels results with PEG-700-DA. The more organized structure can be attributed to the faster polymerization rate and higher cross-link density with the relatively mobile, lower molecular weight monomer. The influence of the liquid crystalline order on final polymer structure is further demonstrated by the polymerization of PEG-400-DMA in the hexagonal phase of DTAB/water. A bulk polymer morphology exhibiting parallel cylinders measuring approximately 200 nm in diameter is obtained upon photopolymerization of this system. Significant control of pore size can be achieved through simple variation in monomer concentration with a more dense material with smaller pore size forming at higher concentrations. The importance of LLC order in the polymer structure formation is further demonstrated by the significantly different polymer structure obtained from polymerization of PEG-575DA in lamellar and hexagonal template phases. The ability to tailor polymer structure and features from the nanometer to micrometer scale using widely available and inexpensive monomers makes polymerization within LLC solvents an attractive method for synthesis of polymers with potential applications in controlled transport, biological membranes, and tissue engineering.

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AUTHOR INFORMATION

Corresponding Author

*(C.A.G.) Telephone: +1 319 335 5015. Fax: +1 319 335 1415. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Science Foundation for financial support of this project through a grant (CBET-0933450) and through partial support of this research by the University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288). The authors would like to also thank Dr. Celine Baguenard for useful discussions. 5737

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