Article pubs.acs.org/Macromolecules
Photochemical Mechanism and Photothermal Considerations in the Mechanical Response of Monodomain, Azobenzene-Functionalized Liquid Crystal Polymer Networks Kyung Min Lee‡ and Timothy J. White*,† †
Materials and Manufacturing Directorate, Air Force Research Laboratory, 3005 Hobson Way Ste. 1, Wright-Patterson Air Force Base, Ohio 45433, United States ‡ Employed by Azimuth Corporation, 4134 Linden Avenue, Dayton, Ohio 45432, United States ABSTRACT: The potential for wireless transduction of input light energy into mechanical outputs has led to a reinvigorated pursuit of photomechanical effects in polymeric materials and composites. We report here on factors influencing the photochemical mechanism (and thus the mechanical output) in monodomain azobenzene-functionalized liquid crystal polymer networks. Through systematic examination of a representative material with both mechanics and spectroscopic characterization the prevalence of the trans−cis and trans−cis−trans mechanisms is elucidated. Furthermore, the role of light intensity in generating heat (photothermal effects) is also reported.
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INTRODUCTION Azobenzene is a versatile photochromic material. Upon irradiation with ultraviolet (UV) light, azobenzene (“classical” azobenzene, e.g., unfunctionalized with electron-withdrawing/ contributing groups) efficiently photoisomerizes from the trans to the cis conformation which is accompanied by a bathochromic shift in absorbance (typically centered near 450 nm), a 3 D increase in polarity, and a change in the molecular axis from 9 to 5.5 Å.1,2 Azobenzene has been exploited for decades in analytical chemistry, in photochromic pigments, as a means to photosensitize liquid crystal systems,3 and as a patternable optical material system.4,5 This work builds upon recent efforts examining photomechanical effects in azobenzene-functionalized polymeric materials.6−8 In the past decade, photomechanical effects in azobenzene-functionalized polymers have primarily examined azobenzene-functionalized liquid crystal elastomers9 and liquid crystal polymer networks.10,11 We recently have reported on similar photomechanical effects observed in amorphous, azobenzene-functionalized polyimides.12,13 Regardless of the material system employed, the resulting photomechanical response is a manifestation of the interplay between material properties including polymer network morphology (liquid crystalline, amorphous, or semicrystalline), thermomechanical properties (glass transition temperature and modulus), absorption coefficient (regulated by azobenzene concentration and liquid crystalline orientation, if any), and sample geometry (cantilever or film, thickness). In appropriate conditions, most notably when incident light is strongly attenuated within the sample thickness (regulated by azobenzene concentration, sample thickness, and light intensity), the photomechanical response (photogenerated strain) can be visualized as bending in the cantilever geometry. Theoretical and computational © 2012 American Chemical Society
efforts have elucidated the connection of these variables to the expected mechanical outputs.14−20 The photomechanical effects realized in azobenzenefunctionalized polymers are a process in which input light energy (photons) is converted into mechanical work by photochemical eventstypically the photoisomerization of azobenzene. Evident in the work of Ikeda, 10,21−23 Broer,11,24−26 and Yu27−29 linearly polarized UV irradiation of these materials results in large magnitude, unidirectional bending of cantilevers. We have taken a distinctive approach by employing “blue-green” light.30−38 Light of these wavelengths has been extensively utilized in the formation of volume and surface relief gratings in glassy, azobenzene polymers and confirmed to induce a photochemical mechanism referred to as trans−cis−trans reorientation (also referred to as the Weigert effect).4,5 Prior examinations have reported large magnitude and bidirectional bending of cantilevers upon irradiation with linearly polarized blue-green light.12,13,30−37 A recent report confirms the original hypothesis38 that trans−cis−trans reorientation of azobenzene chromophores is the enabling photochemical mechanism to realize forward and reverse bending in glassy, polydomain azo-LCN.30 The trans− cis−trans reorientation mechanism occurs when azobenzene materials are irradiated with light with wavelength(s) nearly equivalently absorbed by both the trans and cis isomeric forms of azobenzene (in nearly all cases, 440−514 nm referred to here as blue-green wavelengths). As such, the covalently attached azobenzene units in the polymeric material cycle between conformational forms upon absorption of the incident photons. Received: June 29, 2012 Revised: August 1, 2012 Published: August 17, 2012 7163
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Figure 1. (a) Chemical structures of diacrylate monomers used to form the monodomain azobenzene-functionalized liquid crystal polymer network (MD-20CL) examined here. The alignment of the film is evident in the polarized optical micrograph. Photomechanical response of MD-20CL (15 μm thickness) upon irradiation with (b) 80 mW/cm2 442 nm light, (c) 2 mW/cm2 442 nm light, and (d, e) 40 mW/cm2 375 nm light. The alignment of the linear polarization (E) to the long axis of the cantilever (x) is annotated above each image. In all cases the nematic director of the MD-20CL was aligned approximately parallel to the cantilever long axis (x).
azobenzene from the long axis (x) to the short axis (y). Since light is attenuated within the sample, the strain is nonuniform through the thickness (strain gradient). As such, the contractile strain along the long axis (x) causes the cantilever to bend toward the direction of the incident light. When the incident light is linearly polarized such that it is parallel to the short axis (y) of the cantilever, the azobenzene chromophores reorient from y to the long axis (x) of the cantilever which is an expansive strain along x, observed as bending of the cantilever in the direction away from the incident light source. The mechanical response of monodomain azo-LCN materials is comparably convoluted. In previous examinations of both low34 and high37 azobenzene concentration azo-LCN materials, polarization-controlled unidirectional bending has been reported. So-called bidirectional bending has to date been observed only when the nematic director of the monodomain is aligned such that it is along the short axis of the cantilever.34 For a variety of reasons, including actuation speed, magnitude of photomechanical response, and potential to generate engineered, spatially varying photomechanical responses, anisotropic materials such as monodomain azo-LCN may be the most promising material systems for applications in actuation as well as topography. With this as motivation, the goal of this work is to further elucidate the convoluted mechanical response of monodomain azo-LCN materials by examining the photochemical and photothermal contributions to the observed mechanical response. Toward this end, we employ polarized UV−vis spectroscopy to map the dichroic
Two properties of the azobenzene chromophore allow reorientation. First, both the trans and cis forms of azobenzene are dichroic. Thus, linearly polarized light in which the electric field vector is parallel to the long axis of the trans or cis conformational forms of azobenzene are considerably more likely to be absorbed. Second, the azo bond is known to exhibit exceptional rotational freedom. As such, if the incident bluegreen irradiation is linearly polarized as the azobenzene molecules cycle between trans and cis isomeric forms, transazobenzene is statistically reoriented normal to the polarization vector of the incident photons. It is important to note that accompanying the reorientation of trans-azobenzene is a wavelength-dependent photostationary state concentration of cis-azobenzene. While the ability to reorient is surprising considering the glassy nature of these materials, trans−cis− trans reorientation has been reported in materials as much as 325 °C below the glass transition temperature of the material.39 We recently reported on optically fixable shape memory in both azo-LCN and azo-PI materials as upon removal of the incident light stimulus the driving force behind the local chain mobility ceases, and the material retains the optically or mechanically generated strain.12,32 The ability to regulate azobenzene reorientation by adjusting the alignment of the linear polarization to the sample has been exploited to allow for tailorable photomechanical effects.12,13,30−32,34,37,38,40 In polydomain azo-LCN or amorphous azo-PI’s, irradiation with linearly polarized blue-green light polarized parallel to the long axis of the cantilever reorients 7164
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Figure 2. UV/vis absorption spectra of MD-20CL (8 μm thickness) (a) before irradiation and after irradiation with linearly polarized (b) 60 mW/ cm2 375 nm or (c) 100 mW/cm2 442 nm light aligned such that the electric field vector was parallel to the nematic director of the monodomain. For each panel, the absorbance spectra were measured when the nematic director of the monodomain is (i) parallel and (ii) orthogonal to the polarization of the probe light of the UV−vis spectrometer. Absorption Measurements. Absorption spectra for these materials were captured on a Cary 5000 UV−vis−NIR spectrometer. 8 μm thick samples were used for the UV/vis absorption measurements. The Cary 5000 is a two-beam spectrometer. Because of the thickness and the strong absorption of the azo-LCN materials, an OD 3 filter was placed in the path of the reference beam to enable measurement of the azo-LCN films.30 A control experiment reported in the Supporting Information of ref 30 confirmed this technique can accurately measure absorption values to as much as OD 5. The polarization-dependent absorbance of the materials was monitored before and after irradiation with 442 nm light. Additionally, the polarization-dependent absorption of the material was also characterized after long-term storage in the dark. Temperature Measurements. The surface temperature of monodomain azo-LCN cantilevers exposed to various laser intensities was monitored by using an infrared camera (Lockheed Martin Santa Barbara, ImagIR). The surface temperature was extracted from the images along the length of the cantilever as a function of pixel number. Temperature calibration from 20 to 100 °C was accomplished by varying temperature of a carbon substrate on a Peltier heating device.
absorbance of monodomain azo-LCN before, during, and after irradiation with blue-green light. To emphasize and elucidate differences in the photochemical mechanism, the mechanical and photochemical response of these materials to blue-green light is contrasted to UV light.
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MATERIALS AND METHODS
Synthesis of Monodomain Azobenzene Liquid Crystalline Networks. Monodomain liquid crystal polymer networks containing azobenzene (azo-LCN) were synthesized by copolymerizing RM257 (Merck) and 4,4′-bis[6-(acryoloxy)hexyloxy]azobenzene (2azo) (BEAM Co.). The polymerization was initiated by 1 wt % of the metallo-organic photoinitiator Irgacure 784 (I-784) (Ciba). The chemical structures of these molecules are shown in Figure 1a. Mixtures of RM257, 2azo, and photoinitiator were melted and mixed at 100 °C and drawn by capillary action into 8 or 15 μm thick Elvamide-coated rubbed glass cells. In this examination of monodomain azo-LCN, the Elvamide layer was rubbed with a soft cloth, and the rubbed substrates were aligned antiparallel. Before photopolymerization, the sample was cooled to 75 °C and subjected to 60 mW/cm2 of 532 nm laser light for 15 min to form a monodomain azoLCN referred to here as MD-20CL. After polymerization, the MD20CL film was removed from the glass substrates and washed with methanol to dissolve Elvamide alignment material from the surface of the films. Characterization Methods. The alignment of the azo-LCNs was confirmed by polarized optical microscopy (POM, Nikon) (Figure 1a). Because of the anisotropy evident in the micrographs, we refer to the orientation of these materials as monodomain in the convention of the peer literature. Thermomechanical properties of the azo-LCN films were determined by dynamic mechanical analysis (RSA III, TA Instruments, strain of 0.5%, frequency 1 Hz, heating rate of 2.5 °C/ min) in tensile mode with gauge length of 6 mm by 1 mm by 15 μm (width, length, and thickness, respectively). The glass transition temperatures (Tg) are reported from the maximum of the tan δ curve. Bending Cantilevers. Photodirected bending was triggered by irradiation with linearly polarized 442 nm line of a 130 mW helium− cadmium (HeCd, Kimmon) laser. Note that 442 nm light is absorbed by both the trans and cis isomers of azobenzene. The HeCd laser beam was expanded and collimated with a spherical lens, uniformly exposing the entirety of the cantilever with intensities ranging from 2 to 500 mW/cm2 (although nearly all experiments reported here employed 80 mW/cm2). The orientation of the polarization of the laser to the sample geometry was controlled with a Fresnel rhomb (Newport). The optical setup has been illustrated and described in detail elsewhere.30 The bending of azo-LCN cantilevers was monitored with a charge coupled device (CCD). The bending angle was analyzed with a program that determined the tip displacement angle between the mounting point of the cantilever and the outside edge of the tip of cantilever
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RESULTS AND DISCUSSION The photomechanical response of a glassy, monodomain azobenzene liquid crystal polymer network (azo-LCN, also referred to as MD-20CL) to both blue-green and UV irradiation is summarized in Figure 1. In all experiments reported here, the nematic director of the monodomain azoLCN was aligned parallel to the long axis of the cantilever. As apparent in Figure 1b, irradiation with 80 mW/cm2 of 442 nm light induces polarization-controlled (0°: linear polarization of the irradiation is parallel to the long axis of the cantilever/ parallel to the nematic director; 90°: linear polarization of the irradiation is orthogonal to the long axis of the cantilever/ parallel to the nematic director) unidirectional bending, similar to that reported in other monodomain azo-LCN material systems.34,37 Reducing the laser intensity to 2 mW/cm2 reduces the magnitude of the photomechanical response (e.g., bending) (Figure 1c). As shown in Figure 1d,e, irradiating cantilevers composed of the monodomain azo-LCN with linearly polarized UV light generate significant strain gradients that cause the cantilever to bend to a large angle. However, unlike 442 nm irradiation, the magnitude of the bending angle does not exhibit any discernible difference on the alignment of the linear polarization of the UV light to the cantilever. The photomechanical responses summarized in Figure 1 are a manifestation of input light energy being converted into local strain by photoisomerization of azobenzene (which undergoes either trans−cis or trans−cis−trans mechanisms) that sum into a macroscopic response visualized as bending of a cantilever. As 7165
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Figure 3. Polarized UV−vis (a, c, e) absorption spectra of MD-20CL (8 μm thickness) before (a) and during (c, e) irradiation to linearly polarized 442 nm light aligned such that the electric field vector (E) was orthogonal (c) and parallel (e) to the nematic director (N) of the monodomain. The absorbance value at 365 nm is summarized in (b, d, f) as a function of angle of orientation of the polarization of the probe light to the nematic director of the monodomain during and immediately after irradiation of MD-20CL with a linearly polarized 442 nm: (b) before exposure to light, (d) during/immediately after exposure to E⊥N, and (f) during/immediately after exposure to E||N.
discussed, UV light causes trans to cis isomerization while bluegreen light can induce trans−cis−trans reorientation. To further elucidate the photochemical mechanism (trans−cis or trans−cis−trans) underlying the mechanical response of these materials, we employed polarized absorption spectroscopy to study the dichroic absorption of the monodomain azo-LCN before, during, and after irradiation to linearly polarized light aligned parallel to the nematic director of the azo-LCN. Evident in Figure 2a, before irradiation the anisotropy of the monodomain azo-LCN is apparent in the dichroic absorbance as the cross-linked azobenzene mesogenic units preferentially align along the nematic director. The initial dichroic ratio of the
monodomain azo-LCN is 1.68, and the order parameter of the azobenzene chromophore (not to be confused with the order parameter of the liquid crystal polymer network) is 0.185. These values were calculated from the equations previously reported by Ikeda et al. and others.2 Upon irradiation with 60 mW/cm2 375 nm irradiation for 3 min, the trans isomer is completely converted into the cis form (Figure 2b). The polarized absorption spectrum of cis-isomer peak centered at 450 nm for the monodomain azo-LCN material upon irradiation with UV light is also dichroic (dichroic ratio of 1.70, order parameter of 0.189). Importantly, the dichroism of the trans-isomer peak is retained when the sample is exposed to 7166
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Figure 4. Polar plots of maximum absorbance value at 365 nm as a function of the alignment of the polarization of the probe light to the nematic director of the monodomain azo-LCN (8 μm thickness). (a) Before (○) and during irradiation to linearly polarized 442 nm light aligned such that the electric field vector was orthogonal (△) and parallel (▽) to the nematic director of the monodomain. (b) Immediately (▽) and 3 days (□) after irradiation to linearly polarized 442 nm light aligned such that the electric field vector was parallel to the nematic director of the monodomain.
442 nm light and slightly increased to 1.70 (order parameter of 0.189) despite the buildup of a photostationary state cis-isomer concentration. Also apparent in Figure 2c is a small shoulder at 450 nm associated with the photostationary state concentration of cis-azobenzene. While informative in distinguishing the photochemical mechanisms associated with blue-green (trans−cis−trans) and UV (trans−cis) light irradiation, the experiments reported in Figure 2 do not elucidate the extent of reorientation in the monodomain azo-LCN material. To further isolate the extent of reorientation in the anisotropic monodomain azo-LCN material, we examined the polarized absorption spectra of the monodomain azo-LCN after irradiation with linearly polarized blue-green light polarized either parallel or perpendicular to the nematic director of the material. The results of the polarized absorption experiment are summarized in Figure 3, which contains plots of the raw absorption spectra (Figure 3a,c,e) as well as plots of the absorbance value at 365 nm (Figure 3b,d,e) as a function of the angle of the polarization of the UV−vis probe light to the nematic director of the sample. The absorption of the monodomain azo-LCN material before irradiation as a function of the probe light polarization is plotted in Figure 3a (spectra) and Figure 3b (peak absorbance summary). As in Figure 2a, the material is strongly dichroic with maximum absorbance observed when the angle of the polarizer is set to 0° or 180° and minimum absorbance when the angle of the polarizer is set to 90° or 270°. The sample was subsequently irradiated with blue-green light wherein the alignment of the linear polarization was perpendicular to the nematic director of the monodomain azo-LCN. The absorption of the monodomain azo-LCN material is plotted in Figure 3c (spectra) and Figure 3d (peak absorbance summary). Notably, the maximum and minimum absorbance values are also found at 0°/180° and 90°/270°, respectively. Finally, the absorption of the monodomain azo-LCN material after exposure to linearly polarized 442 nm irradiation polarized parallel to the nematic director is plotted in Figure 3e (spectra) and Figure 3f (peak absorbance summary). Notably, the maximum and minimum absorbance as a function of polarization shifts approximately 5.5° to approximately 355°/175° and 85°/265°, respectively.
These results are directly contrasted in polar plots shown in Figure 4. Evident in Figure 4a, irradiation with 442 nm light polarized orthogonal to the nematic director uniformly reduces the overall absorbance by 7%. This indicates the photostationary state concentration of the cis isomer is 7−9%. Any trans−cis−trans reorientation occurring in this irradiation condition should rotate trans isomers to align along the nematic director of the monodomain azo-LCN which would increase the dichroism. However, the magnitude of the absorbance changes from the buildup of the 7−9% increase in cis isomer concentration masks the increase in dichroism. Notably, rotating the polarization so that it is parallel to the nematic director of the monodomain azo-LCN induces a 5.5° shift in the alignment of the dichroism. The difficulty in perfectly aligning the orientation of the nematic director exactly along the x axis and the imperfect distribution of the alignment/orientation of azobenzene mesogens in the sample (evident in an orientation parameter of 0.46 measured by X-ray diffraction) seem to bias the reorientation in a single direction. The data plotted in Figures 3 and 4 are representative of a number of experiments, some of which showed identical magnitude reorientation in the reverse direction (e.g., −5.5° shift). Regardless, the results presented in Figures 3 and 4 indicate the directional orientation of azobenzene is rotating. As previously reported,32 the optically generated strain in polydomain azo-LCN was accompanied by a retention in dichroism for at least 30 days. To measure the retention of the photoinduced reorientation of azobenzene chromophores in the azo-LCN material, the sample was stored in the dark for 3 days, after which it was subjected it to the same polarized absorption experiment. Notably, the directionality of the dichroism (e.g., 5.5° shift in dichroism) is retained while the absorbance at all polarization alignments is increased. The increase in absorbance at all polarizations is likely due to cis− trans isomerization in the dark. Importantly though, the reorientation of the trans isomer is retained. The dichroic ratio of the MD-20CL film after 3 days in the dark increased to 1.74 due to the increase in the trans-isomer concentration over time. 7167
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Figure 5. Dark relaxation of the absorbance of MD-20CL (∼8−10 μm thickness) at 365 nm after (a) irradiation with 80 mW/cm2 at 442 nm and (b) irradiation with 40 mW/cm2 375 nm. (△) E⊥N and (▽) E||N. The initial absorption at 365 nm of the material before irradiation is indicated by the dashed lines.
The increase in absorbance apparent in Figure 4b is likely due to cis−trans isomerization in the dark. To measure this process, we monitored the absorbance of the monodomain azoLCN material after irradiation to both 442 and 375 nm light polarized either parallel or orthogonal to the nematic director. Evident in Figure 5a, the absorbance value at 365 nm increases over time reaching a plateau at ∼30 h. The time scale of the cis−trans isomerization to irradiation to 442 nm of either polarization condition is effectively identical, as expected. Comparatively, irradiation of the samples with UV light induces a large decrease in absorbance. However, the time required to restore to the original absorbance value is of a similar range 30−50 h. Comparing the values of the absorbance at 365 nm after cis−trans isomerization (e.g., after 60 h) reveals a subtle but important distinction. The absorbance values after UV irradiation of the parallel and orthogonal polarization states trend to the same limit while the absorbance values after 442 nm irradiation of the parallel and orthogonal polarization alignments trend to different limits. Contrasting these limits to the original absorbance values clarifies the directionality and extent of reorientation of azobenzene upon 442 nm irradiation. The absorption measurements reported in Figures 2−5 confirm a number of items. First, irradiation of monodomain azo-LCN samples with 442 nm light generates both a small concentration of cis isomer (calculated 7−9%) as well as a measurable reorientation of the azobenzene units within the polymer network that is only observed upon irradiation when the polarization is parallel to the nematic director of the monodomain. Second, the extent of trans−cis−trans reorientation in the monodomain azo-LCN is limited as for an 8 μm thick film, the alignment of the dichroism shifts 5.5°. As evident in Figure 1b,c, it is clear that alignment of the linear polarization of the 442 nm light to the cantilever geometry (as well as the nematic director of the monodomain azo-LCN) can regulate the magnitude of the bend. To correlate whether the reduced bending angle observed in Figure 1b,c is due to reorientation, the shift in the alignment of the dichroism was measured in samples subjected to intermediate polarization conditions (e.g., the linear polarization of the 442 nm light was aligned 23°, 45°, and 68° to the nematic director of the monodomain azo-LCN) (Figure 6). Exposing MD-20CL to these intermediate polarization conditions reduces the magnitude of the shift in the orientation of the dichroism from the 5.5° observed when the linearly polarized 442 nm
Figure 6. Comparison of the dichroic shift (○) and bending angle (△) induced by irradiating MD-20CL (8 μm thickness) with 80 mW/ cm2 at 442 nm in which the electric field vector of the linear polarization to the nematic director was varied from 0 to 90°.
irradiation is parallel to the nematic director (0° in Figure 6) to 5.3°, 4.65°, and 3°, respectively. To aid in the correlation of these numbers to the photomechanical response, the magnitudes of the bending angles reported in Figure 1b are plotted as a function of the 442 nm polarization, confirming the interrelation between the degree of reorientation of the dichroism and the bending angle. The divergence of the relationship between the dichroic shift and the bending angle at the 90° polarization condition is explainable by the dominance of trans−cis isomerization in this condition that is accompanied by no statistically measurable reorientation. Irradiation of the monodomain azo-LCN material with 442 nm light causes both trans−cis isomerization and trans−cis− trans reorientation. Previously, we have illustrated the ability to optically fix shapes in polydomain azo-LCN (optically fixable shape memory). The extent of the retention of the optically generated strain generated by linearly polarized 442 nm oriented both parallel and orthogonal to the long axis of the cantilever (e.g., the nematic director of the monodomain) is highlighted in Figure 7. Contrary to our prior examination of polydomain azo-LCN materials, the extent of the retention of the optically generated strain is polarization dependent. Upon irradiation with 442 nm light linearly polarized such that the electric field vector is parallel to the nematic director of MD20CL, we observed large magnitude bending and a fairly large 7168
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Figure 7. Photomechanical response of a 15 μm thick cantilever composed from MD-20CL upon irradiation with 80 mW/cm2 of 442 nm light (a) polarized parallel to the long axis of the cantilever (i: before irradiation; ii: after 5 min of continuous irradiation; iii: 3 days after removal of irradiation) and (b) perpendicular to the long axis of the cantilever (iv: before irradiation; v: after 1 h of continuous irradiation; vi: 3 days after removal of irradiation).
retention of the curvature over the course of 3 days. However, irradiation of the MD-20CL sample with 442 nm polarized such that the electric field vector was orthogonal to the nematic director does not induce any observable shape memory and, in fact, may result in the cantilever slightly reversing direction upon relaxation. Through the photomechanical and spectroscopy data reported here we can piece together the correlation of the mechanics to the photochemistry. The monodomain film is anisotropic, and as such, the azobenzene molecules are highly aligned before irradiation. To irradiation polarized orthogonal to the nematic director of the monodomain azo-LCN, the film forms a photostationary state concentration of cis isomer in addition to reorientation of a small percentage of azobenzene molecules align along this direction. The formation of cis isomer will result in a contraction along the long axis of the cantilever. The reorientation of a small percentage of azobenzene chromophore to the long axis of the cantilever should result in an expansive strain along the long axis of the cantilever. Clearly, from the absorption plots in Figures 3−5 the formation of cis isomers is the dominant mechanism in this polarization condition. If the light is turned off, cis−trans isomerization occurs over 1−2 days in the dark, resulting in a film with a slight increase in dichroism due to the reorientation of azobenzene chromophores toward the nematic director of the monodomain azo-LCN. Rotating the polarization such that it is now parallel to the nematic director of the monodomain results in a measured 5.5° shift in the orientation of the dichroism. This is indicative of the reorientation mechanism. It should be noted that this number is determined from a transmission experiment, which effectively averages the reorientation through the film thickness. Because of the large absorption coefficient of these materials, light is absorbed nonuniformly through the film. Thus, the magnitude of the reorientation is likely larger than 5.5° on the front surface of the film. If the light is turned off, cis−trans isomerization occurs over 1−2 days in the dark. However, the monodomain azoLCN retains the shifted dichroism for at least 3 days and likely longer. The dark relaxation of the cis isomer to the trans isomer is the primary cause of the slight reduction in curvature evident in Figure 7a-iii. Subjecting the film in any of these conditions to
heat treatment exceeding the glass transition temperature restores the material to the equilibrium, as-prepared condition. Evident in Figure 1, the intensity of the 442 nm light can influence the magnitude of the photomechanical response. Prior examinations31,37,40 of oscillatory behavior in monodomain azo-LCN materials have used markedly higher intensity irradiation than that reported here. To ascertain the photothermal contribution to this work as well as prior efforts, Figure 8 plots the measured front surface temperature as a function of
Figure 8. Maximum measured surface temperature of a cantilever composed from MD-20CL when subjected to irradiation from 2 to 400 mW/cm2 with polarization oriented parallel (□), 45° (○), and perpendicular (△) to the nematic director.
intensity of the 442 nm laser polarized parallel, 45°, and orthogonal to the nematic director of the monodomain azoLCN. Temperatures were measured with a thermal camera as specified in the Materials and Methods section. As apparent in Figure 8, increasing intensity can significantly increase the temperature of the front surface. Above ∼100 mW/cm2 the temperature rise begins to reach levels that could influence the mechanical response of the material. Because of the dichroic absorbance of azobenzene, the temperature increase depends on the incident polarization orientation due to the alignment of 7169
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Macromolecules
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azobenzene chromophores in the monodomain azo-LCN. As emphasized in the plot, the contribution of absorptive heating with intensity can be ascribed into two regimes: photochemical (100 mW/cm2). In samples containing larger concentrations of azobenzene chromophores the crossover intensity wherein photothermal effects merit consideration will shift to lower intensities.
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CONCLUSION In summary, we have examined the photochemical mechanism in a representative glassy, monodomain azobenzene-functionalized liquid crystal polymer network (LCN). By systematic analysis using both spectroscopic and mechanics experiments, it is concluded that both the trans−cis and trans−cis−trans mechanisms are prevalent in dictating the photomechanical output in these materials. The prevailing photochemical mechanism is shown to depend on the alignment of the incident polarization to the nematic director of the monodomain azo-LCN. When the electric field vector of the 442 nm light is parallel to the nematic director, trans−cis−trans reorientation dominates the response. However, when the electric field vector of the 442 nm light is orthogonal to the nematic director, the trans−cis isomerization dominates. The contribution of photothermal effects (e.g., absorptive heating) is measured with thermal imaging. Heating of less than 3 °C is observed up to 100 mW/cm2.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel 937-255-9551. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the Air Force Office of Scientific Research and the Materials and Manufacturing Directorate of the Air Force Research Laboratory. We thank Leo Gonzalez for allowing us to use the infrared camera and Brandon Lynch for assistance in collecting portions of the absorption data.
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dx.doi.org/10.1021/ma301337e | Macromolecules 2012, 45, 7163−7170
