Article pubs.acs.org/Langmuir
Reversible Photorheological Lyotropic Liquid Crystals Shuhua Peng,†,‡ Qipeng Guo,*,† Timothy C. Hughes,*,‡ and Patrick G. Hartley*,‡ †
Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 2000, Geelong, Victoria 3220, Australia CSIRO Materials Science and Engineering, Bayview Avenue, Clayton South, Victoria 3169, Australia
‡
S Supporting Information *
ABSTRACT: We describe novel lyotropic liquid-crystalline (LLC) materials based on photoresponsive amphiphiles that exhibit rapid photoswitchable rheological properties of unprecedented magnitude between solidlike and liquidlike states. This was achieved through the synthesis of a novel azobenzene-containing surfactant (azo-surfactant) that actuates the transition between different LLC forms depending on illumination conditions. Initially, the azo-surfactant/water mixtures formed highly ordered and viscous LLC phases at 20−55 wt % water content. Spectroscopic, microscopic, and rheological analysis confirmed that UV irradiation induced the trans to cis isomerization of the azo-surfactant, leading to the disruption of the ordered LLC phases and a dramatic, rapid decrease in the viscosity and modulus resulting in a 3 orders of magnitude change from a solid (20 000 Pa) to a liquid (50 Pa) at rate of 13 500 Pa/s. Subsequent exposure to visible light reverses the transition, returning the viscosity essentially to its initial state. Such large, rapid, and reversible changes in rheological properties within this LLC system may open a door to new applications for photorheological fluids.
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INTRODUCTION Smart materials whose rheological properties, such as the viscosity and modulus, can be triggered by external stimuli in a controlled manner are of great technological interest, with potential applications in brakes and valves, actuable armor, and related fields such as robotics and sensors.1,2 External stimuli that can modulate such material property changes include thermal,3 ultrasound,4 electric and magnetic fields,2 photoirradiation,5,6 and chemical (pH, ionic strength).7 Among these external stimuli, light has attracted much attention because it provides a broad range of tunable parameters (e.g., wavelength, intensity, and duration) to manipulate the rheological properties of photoresponsive systems.8,9 Moreover, light stimulation can also be accurately localized, enabling both temporal and spatial control.10 Several photosensitive units, such as azobenzene,11−13 stilbene,14,15 dithienylcyclopentene,16 naphthopyran,6,17 spiropyran,18,19 trans-2,4,4′-trihydroxychalcone,20 cinnamic acids,21,22 coumarins,23 acridizinium bromide,24 anthracenes,25 and merocyanine,19 show changes in viscosity on exposure to particular wavelengths of electromagnetic radiation. Light-induced molecular conformational changes can be used to switch the rheological behavior of materials. In particular, the trans−cis isomerization in azobenzene compounds has been used to switch rheological or dimensional properties under alternating UV and visible light irradiation.5,26−30 However, to date, reversible azobenzene photorheological fluids have been based on either simple micellar association/dissociation in solution,26,28 supramolecular hydrogels,22,31 or the rearrangement of order within polymer melts.5 © XXXX American Chemical Society
In these cases, the photoisomerization of azobenzene typically causes relatively small changes in the modulus and/or viscosity. For example, although remarkable levels of viscosity change have been achieved (up to a million fold), the resulting materials can switch only between low- and high-viscosity liquids having maximum viscosities on the order of 500 Pa·s.32 Moreover, the transition of such materials was relatively slow, requiring irradiation for about 50 min to induce the rheological changes, or in some cases were not reversible at all.22,33 The development of photorheological fluids with a greater dynamic range of rheological properties, in particular, involving rapid and reversible transitioning between a solid and a liquid, is required for these materials to be applied to real-world applications. Azobenzene-containing amphiphiles comprise a family of photosurfactants that can modulate interfacial and bulk properties under light stimulation.34 There are also recent reports of their application as photorheological fluids.28 Although the azobenzene moiety has been widely incorporated into polymers to induce orientation in neat thermotropic liquid crystal systems,35 to the best of our knowledge, no studies have been reported correlating the photorheological effects of azobenzene-containing materials in aqueous solution (i.e., lyotropic liquid-crystalline (LLC) phase behavior). At the same time, the rheological properties of different lyotropic Received: August 9, 2013 Revised: September 4, 2013
A
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Nikon Ds-Fi1 CCD camera equipped with DS-U2 controller (Nikon Australia Pty. Ltd., Melbourne, Australia).
liquid crystal mesophases are known to show distinct and significant differences depending on their phase state.36 For instance, a cubic-to-hexagonal phase transition has been shown to result in a 2-order drop in modulus over a 5 °C temperature change.36 Here we present a class of materials that exhibit rapid photoswitchable rheological properties based on novel photoresponsive amphiphiles with rationally designed LLC phase behavior for the first time. Reversible rheological changes can be achieved through photoinduced conformational changes of the photosensitive amphiphile. Significantly, the rapid and reversible photorheological responses observed for transitions between different phases within this binary amphiphile−water system are of unprecedented magnitude.
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RESULTS AND DISCUSSION To achieve photoresponsive LLC phases in water, we rationally designed several amphiphiles containing photoresponsive azobenzene (azo) groups based on known LLC-forming surfactants that typically possess bulkier C12−C18 alkyl chain lengths.39 The structures combined a photoresponsive azobenzene moiety within a hydrophobic alkyl block coupled to a hydrophilic oligoethylene glycol block. The amphiphilic property of the surfactants was controlled by the variation of the alkyl chain length. The ability of azo-surfactant to form LLCs in water was evaluated by penetration scans using polarized optical microscopy (POM). It was found that the surfactant containing a C9 alkyl chain length was the only one to display LLC phase behavior, the C6 alkyl chain derivative was soluble in water, and the C13 alkyl chain derivative was solid at room temperature and water could not penetrate it. Therefore, just one azo-surfactant with an appropriate amphilphile was chosen for further studies. The novel surfactant, shown in Figure 1 (synthesis and characterization
EXPERIMENTAL SECTION
Materials. Poly(ethylene glycol) methyl ether (PEG) with a number-average molecular weight of about 350 (Aldrich) was dried in a vacuum oven at 80 °C overnight before use. Sodium nitrite, 4butylaniline, phenol, ethyl 6-bromohexanoate, triethylamine, ethyl paminobenzoate, and 1-bromononane were purchased from SigmaAldrich and used without further purification. No purification was performed on the solvents except for the tetrahydrofuran (THF), which was taken from an Innovative Technologies, Inc., solventpurification system. Sodium hydrogen carbonate (NaHCO 3), potassium hydroxide (KOH), hydrogen chloride (HCl, 37%), potassium carbonate (K2CO3), neutral alumina, and potassium iodide (KI) were from Merck Co. and used as received. Thionyl chloride was purchased from Scharlau, and all of the organic solvents were from Merck. Small Angle X-ray Scattering (SAXS). SAXS experiments were performed at the Australian Synchrotron on the small/wide-angle Xray scattering beamline working at 12.0 keV with a q range of 0.2−9 nm−1. Silver behenate was used to calibrate the sample-to-detector distance. Data reduction (calibration and integration) of data collected using a 2D detector was achieved using AXcess, a custom-written SAXS analysis program written by Dr. Andrew Heron from Imperial College, London.37 The determination of the LLC phases was based on the characteristic diffraction pattern of the SAXS ratio of Bragg peaks. In particular, the relative peak positions are 1:31/2:41/2:71/2 for hexagonal phase and 1:2:3:4 for the lamellar phase.38 Photorheometer. In situ monitoring of the photoisomerization process for the liquid crystal system was conducted using an ARES photorheometer (TA Instruments, USA) connected to an EXFO Acticure 4000 UV light source (365 nm, 200 mW/cm2) and a quartzhalogen visible light source (>540 nm, 38 mW/cm2) via a liquid light guide. A Peltier temperature controller was also connected to the rheometer to maintain the temperature at 25 °C, thereby ensuring that both the rheological and the SAXS experiments were performed under the same conditions for the phase behavior study. The sample was loaded in the center of two parallel plates of 20 mm diameter. The gap between the two plates was set at 0.3 mm. The storage shear modulus (G′), loss shear modulus (G″), and viscosity (η*) were measured as a function of time at a constant frequency of 10 rad/s and a strain of 1.0%. UV−Vis Spectroscopy. Photoisomerization of the azo-surfactant was measured on a Cary 50-Bio UV−vis spectrophotometer (Varian) against a background of water in a quartz cuvette. We used a dilute sample (5.0 × 10−6 mol/L in water) to ensure an absorbance below 1. Meanwhile, to more clearly capture the rapid evolution during the photoisomerization of azo-surfactant, the UV light intensity was reduced from 200 to 3.8 mW/cm2 and the visible light intensity decreased from 38 to 12.1 mW/cm2 accordingly. UV−vis data was collected every 30 s. Polarized Optical Microscopy (POM). Liquid crystal textures were observed with a Nikon Eclipse 80i cross-polarized optical microscope equipped with a Linkam hot stage and controller (LTS 120 with PE94 controller, Linkam UK). Images were captured with a
Figure 1. Molecular structures of trans and cis isomers of the azosurfactant.
of azo-surfactant in Supporting Information), was obtained as a viscous orange liquid at room temperature. No ordered structure was observed for the neat azo-surfactant using POM observations and SAXS measurements. However, the addition of water resulted in anisotropic LLC phases in water at certain surfactant−water compositions, as determined using POM observations. As shown in Figure 2, typical streaked textures from lamellar phases for the azo-surfactant/water mixtures with water content ranging from 20 to 35 wt % were recorded in A−D respectively. When the water content was increased further to between 40 and 60 wt %, characteristic textures for hexagonal phases were observed in Figure 2E−H, respectively. Given that these hexagonal phases occur at higher water contents, we deduce that they are of the normal hexagonal (H1) variety. Isotropic phase regions were observed for the azo-surfactant/water mixture with less than 20 wt % and more than 60 wt % water contents, respectively (data not shown). UV irradiation of azo-surfactant in aqueous solution at room temperature results in the trans−cis isomerization of the azobenzene group as shown schematically in Figure 1. Spectroscopic measurements showed the expected decay of the maximum absorption band at around 349 nm and the generation of a new absorption at 442 nm (Figure 3A). Importantly for practical applications, this photoinduced isomerization is reversible, as the azo-surfactant in the cis B
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Figure 4. SAXS patterns for azo-surfactant/water binary system with 30 and 50 wt % water contents, respectively.
and water in response to alternating UV and visible light irradiation. As shown in Figure 5A for the mixture of azosurfactant and water (50 wt %), rheological measurements were conducted for 5 min prior to exposure to UV light in order to obtain baseline modulus and viscosity values. Initially, the sample was viscous with η ≈ 2000 Pa·s. Here, the closely packed and highly ordered structures of the initial normal hexagonal phase reinforce the system, resulting in high values of viscosity.5 Immediately following exposure to UV light, a dramatic decrease in viscosity was observed within a few seconds (Figure 5A). Specifically, the viscosity plummeted from about 2000 Pa·s to around 10 Pa·s, and a pronounced reduction was recorded for the storage modulus (G′), which dropped from around 20 000 to 50 Pa (Figure 6). The UV−vis spectral measurements shown in Figure 3A imply that this was due to the trans-to-cis isomerization of azo-surfactant under UV irradiation, resulting in a disruption of the ordered high viscosity and modulus hexagonal phase. This phase transition was confirmed by POM (Figure 5A). As shown in Figures 2A and 5A, before UV irradiation, the mixture of azo-surfactant and water (50 wt %) exhibited the characteristic birefringent texture of the hexagonal LLC phase. Within 2 min of UV irradiation, an anisotropic-to-isotropic phase transition was induced (Figure 5A), which was observed as the disappearance of birefringence in POM. After a period of approximately 9 min of UV exposure, a steady state of low viscosity was established and the UV light was turned off. At the same time, white light was turned on. The reversibility of the system was demonstrated by the rapid recovery of the rheological properties upon exposure of visible
Figure 2. POM images (100× magnification) for azo-surfactant/water mixtures with different water contents at room temperature: (A) 20, (B) 25, (C) 30, (D) 35, (E) 40, (F) 45, (G) 50, and (H) 55 wt %.
form relaxes to the trans form within a few minutes after exposure to visible light (Figure 3B). The POM observations of LLC behavior were confirmed using SAXS analysis. As is evident in Figure 4, representative SAXS patterns with relative peak positions of 1:2:3 and 1:31/2:41/2:71/2 indicating the formation of lamellar and hexagonal phases, respectively, were recorded for the mixture of azo-surfactant/water with 30 and 50 wt % water contents, respectively. It should be noted that the first-order peak recorded for the lamellar phase was of low intensity, which is not a common feature observed in SAXS of LLC systems. However, similar SAXS patterns for the lamellar phase can be found in self-assembled polymer systems.40,41 The lower intensity of the first-order peak has been attributed to the small electron density contrast between the hydrophobic domain and the hydrophilic domains. Quantitative analysis of the SAXS patterns yielded d spacings (that describes the distance between repeating units42) of 6.63 and 7.14 nm for lamellar and normal hexagonal phases, respectively. In situ photorheology was used to characterize the dynamic rheological properties of the LLCs comprising azo-surfactant
Figure 3. Changes in absorption of UV−vis spectra over time under (A) UV light and (B) visible light. C
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Figure 6. Degel point and gel point for the azo-surfactant/water binary system with 50 wt % water under alternating UV and visible light irradiation at 25 °C.
The solidlike to liquid and liquid to solidlike phase transitions (degel point and gel point) can be clearly observed when the storage modulus (G′) and loss modulus (G″) were plotted as a function of light irradiation time, as shown in Figure 6. The high rates of photorheological change of 13 500 and 220 Pa/s under UV and visible light irradiation, respectively, confirmed the rapid nature of the transformation. It is worth noting that there was a delayed/two-step increase in both G′ and G″ after turning on the visible light and the system returning to the initial high modulus state. This was not observed for the isotropic−lamellar phase transition in the 30 wt % water sample. This may be due to the more complex nanostructural rearrangements required to reconstruct the hexagonal phase. A similar two-step relaxation process (in the reverse direction) was observed for hexagonal−isotropic phase transitions as a function of temperature by Mezzenga et al.36 Similar reversible rheological changes were also demonstrated for the mixture of azo-surfactant with 30 wt % water. For this mixture that forms a lamellar phase (Figure 2C), the initial viscosity was much lower than that of the hexagonal phase with 50 wt % water. Compared to the hexagonal phase, the units in the lamellar phase are organized in sheets such that they can slide over each other, resulting in a lower viscosity.43 As shown in Figure 5B, this mixture was capable of reversible viscosity changes of between 45 and 4 Pa·s on demand by exposure to UV or visible light. In addition, POM results again indicated that these rheological changes were associated with switchable phase transitions between anisotropic (lamellar) and isotropic phases. To gain a deeper understanding of the reversible response of these photoresponsive liquid crystals, in situ synchrotron SAXS was used to follow nanostructure evolution upon alternating exposure to UV and visible light. SAXS data were acquired every 4 s at 25 °C, with the first data point collected in darkness. As shown in Figure 7A, a hexagonal phase with SAXS peaks in a ratio of 1:31/2:41/2:71/2 was initially observed for the mixture of azo-surfactant and water (50 wt %). Exposure of this mixture to UV light significantly changed the nanostructure of the system as the periodicity of SAXS peaks associated with a hexagonal phase rapidly faded away within a few seconds. The disruption of the hexagonal phase may be due to either molecular−geometrical changes or to the difference in hydrophobicity of the two isomers of azo-surfactant. Azobenzene based surfactants in the trans form usually have a greater hydrophobicity.34 Regardless, the trans-to-cis isomer-
Figure 5. (A) Real-time photorheology measured over alternating UV and visible light irradiation for the mixture of azo-surfactant/water (50 wt %) at 25 °C. (B) For the mixture of azo-surfactant/water (30 wt %) at 25 °C. POM images (100× magnification) were taken before UV irradiation, after UV irradiation, and after visible light irradiation.
light. Specifically, viscosity was restored close to that of the preUV initial state, and the sample regained a gel-like viscous consistency. At the same time, the anisotropic (hexagonal) phase reappeared under POM observation within 2 min. This indicated that the reversible rheological behavior was correlated with the anisotropic−isotropic phase transition. We note that the visible-light-induced increase in viscosity was somewhat slower than the UV-induced viscosity decrease. This phenomenon was consistently observed for this and other examples discussed later. It is likely that this difference results from the relatively low intensity of the visible light source used in the measurements and/or differences in absorption coefficients for the different wavelengths of light. D
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Time-resolved synchrotron SAXS patterns for the mixture of azo-surfactant and water (30 wt %) are also shown in Figure 7B. Under UV irradiation, the characteristic scattering peaks for the lamellar phase also quickly faded away as a result of the photoisomerization of the azo-surfactant. Upon removal of UV light and the switching on of visible light, the scattering peaks for the lamellar phase appeared again, indicating that the reversible nanostructural rearrangement observed by SAXS was in good agreement with the macroscopic rheological behavior (Figure 5B).
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CONCLUSIONS For the first time, a novel binary (amphiphile−water) lyotropic liquid crystal system was developed that can reversibly and rapidly switch between solidlike and liquidlike states upon exposure to UV and visible light. Moreover, this study has demonstrated how photorheological behavior can be correlated with LLC phase transitions through the development of a novel surfactant−water system based on an azobenzene-containing amphiphile. This photoresponsive amphiphile in its trans form assembles into highly ordered, high-modulus gel-like LLC mesophases at specific water contents. Upon irradiation by UV light, the trans form of the azo-surfactant rapidly photoisomerizes to its cis form. The difference in hydrophobicity and/or molecular geometrical properties of the two isomers leads to the disruption of the ordered LLC structure, resulting in low-viscosity fluids. Subsequent exposure to visible light reverses the transition, returning the viscosity essentially to its initial state. Different magnitudes of rheological change have been achieved by altering the water content of the system, thus resulting in different liquid-crystalline systems (lamellar versus hexagonal). In situ synchrotron SAXS measurements and POM results have revealed that this reversible viscoelastic response is accompanied by a rapid reversible rearrangement of the LLC nanostructure. This work provides new insights into relating the bulk rheological response with the light-stimulated isomerization of azobenzene on the molecular scale and nanostructural evolution within a lyotropic liquid-crystalline system.
Figure 7. Time-resolved synchrotron SAXS patterns for (A) the azosurfactant/water binary system with 50 wt % water upon UV light exposure and visible light at 25 °C and (B) for the azo-surfactant/ water binary system with 30 wt % water.
ization clearly results in the dissociation of the hexagonal nanostructure and an abrupt reduction in the rheological properties (Figure 5A). The UV-induced phase transition for the mixture of azo-surfactant and water (50 wt %) was complete within 2 min of exposure, as demonstrated by an unchanging SAXS pattern. This pattern exhibits two broad scattering peaks. Taken together with the low viscosity and lack of birefringence under crossed polarizers, this suggests the presence of disorganized micelles after UV irradiation. Similar phase transitions between low-viscosity micellar solutions and highly viscous LLC structures have been induced through temperature changes in surfactant−water binary systems.44−46 When the UV light was switched off and visible light was immediately turned on, the characteristic SAXS peaks for the hexagonal phase returned, demonstrating the reversibility of self-assembly in the system. In other words, the azo-surfactants in the cis form under UV light relaxed to their rodlike trans conformation upon visible light exposure. These nanoscale observations of structural transitions by changing light exposure were in good agreement with the photorheology results (Figure 5A), which shows that the sample became highly viscous and almost recovered its initial viscoelasticity after visible light was switched on. Moreover, multiple cycles of this switching response can be achieved. Quantitative analysis of the SAXS patterns yields a d spacing of 7.92 nm for the sample in the initial state and d spacings of 7.89, 7.91, and 7.84 nm after one, two, and three cycles, respectively, underscoring the nanostructural reversibility of the system.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis and characterization of azo-surfactant, experimental methods, and supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]; patrick.
[email protected]. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS SAXS/WAXS research was undertaken at the Australian Synchrotron, Victoria, Australia, and the authors thank Dr. Nigel Kirby for his assistance. S.P. gratefully acknowledges Deakin University and CSIRO for the provision of a collaborative Ph.D. scholarship and thanks Dr. Deng Hong (Zhejiang University, China and CSIRO, Australia) for help with chemical synthesis and purification. E
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dx.doi.org/10.1021/la4030469 | Langmuir XXXX, XXX, XXX−XXX
