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Dec 11, 2018 - Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada. § ... resulted in c...
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Dynamically evolving surface patterns through light-triggered wrinkling erasure Jaana Vapaavuori, Taylor Charlene Stimpson, and Jose M. Moran-Mirabal Langmuir, Just Accepted Manuscript • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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Langmuir

Dynamically evolving surface patterns through light-triggered wrinkling erasure Jaana Vapaavuori,*,a Taylor Stimpson,b and Jose M. Moran-Mirabalc a. Départment de Chimie, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, QC, H3C 3J7E, Canada. email: [email protected], b. Department of Chemical Engineering, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4M1, Canada. c. Department of Chemistry and Chemical Biology, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4M1, Canada. For many applications, it is an imperative that changes in polymer surface topography, especially periodic patterns, can be triggered on command by a well-defined remote signal. In this contribution, we report a light-induced cascade of changes in wrinkling wavelengths on thin polymer layers supported by an elastomeric substrate under tensile stress. Through the applied supramolecular design, the effect of varying the ratio of light active and light passive components can be easily assessed, and it is shown that both the cascade type as well as the rate of the progress of the dynamic light-induced changes can be tuned by this ratio, as well as by the light intensity. Furthermore, for the reported phenomena to occur, nominally only every 20th polymer repeat unit needs to be occupied by a chromophore, which makes the conversion of the sub-nm photoisomerization reaction into 10 µm - scale changes of periodic surface patterns extremely efficient.

INTRODUCTION Responsive surfaces that can be dynamically altered upon exposure to non-contact stimuli are of interest for a plethora of applications, including, but not limited to, adjustable surface wetting, adhesion, optical properties, mimicking of biological membranes, and as substrates for microfluidics and directed self-assembly.1–3 A simple way to produce surface undulations spanning the submicron to millimeter length scales is to expose a double layer of materials with sufficiently mismatched mechanical properties to tensile or compressive forces.4–6 If the critical buckling threshold is exceeded, a sinusoidal buckling pattern emerges, whose dimensions depend on the elastic modulus of the materials, the thickness of the thinner layer, and the magnitude of the disturbing force.7 In addition to being promising for practical applications, measuring the periodicity of the formed wrinkles can be used as a way to assess mechanical properties of, and the stimuli-related changes within, the thin film layer.1 Hence, this is the method of choice for mechanical measurements of films that are too thin or too brittle for the preparation of selfstanding films for dynamic mechanical analysis, and is especially beneficial for studying light-response of polymer materials, since the films of the size used in practical applications, such as coatings, can be studied in-situ under illumination. In azobenzene-containing materials, electromagnetic energy can be converted into mechanical energy via photo-induced isomerization. In practice, depending on the substitution on the azobenzene, both quasi-bistable switching and quasicontinuous cycling between the two geometrical trans and cis

isomers, are possible.8 In azopolymers, the photoisomerization can drive material motion at least over three different length scales.9 In addition, light-powered photoisomerization modifies the mechanical properties of the azopolymers. For example, the seminal atomic force microscopy work of Karageorgiev et al.10 demonstrated that both the elastic modulus and viscosity decreases in azo-containing methacrylate-based polymers upon irradiation. However, these changes were more moderate than would be expected if heating the material above its glass transition temperature (Tg). They speculated that illumination-induced mechanical changes could be related to a localized softening near azobenzenecontaining segments, as compared to non-localized overall softening upon heating. Nanoidentation studies further demonstrated the light-induced reduction of stiffness and hardness of both covalent and supramolecular azopolymers.11,12 Recently, FTIR results lent support to the interpretation that the photoinduced plasticization in amorphous azopolymers occurs at a submolecular level.13 Since the dimensions of buckling patterns are related to the mechanical properties of the polymer films, light-induced mechanical property changes should also manifest themselves in the concomitant modulation of these patterns. Indeed, Takeshima et al. have shown that by varying the azobenzene comonomer content in liquid crystalline copolymers, different levels of wrinkling reduction could be produced. The exposure of an azo homopolymer (100% azo content) to 365-nm light resulted in complete wrinkle erasure, whereas the copolymer containing 25% azo comonomer led to an 8% decrease in the wrinkling wavelength.14 It is known, that in these types of polymers, the modifications in mechanical properties result from a long-lived cis-state and photo-stationary state

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consisting of mainly cis isomers.15 Analogously, it was recently shown that a liquid crystalline azopolymer can undergo an approximately 60C drop in Tg, when side-chain azobenzenes are converted into the cis form.16 Similar to the above-mentioned liquid crystalline azopolymers, light-induced modifications in wrinkling have also been observed in amorphous polymers bearing such azobenzenes as pendant groups, for which single wavelength illumination can drive efficient trans-cis-trans cycling of chromophores. Zong et al. observed that inducing photoisomerization by light (the wavelength of which could be varied between 410–550 nm) of amorphous poly(disperse orange 3) resulted in the erasure of the wrinkle patterns without noticeable changes in the buckling wavelength.17 They attributed the wrinkle erasure to the light-induced stressrelease in the films, being thus a fundamentally different mechanism than the photoinduced decrease in Young’s modulus observed by Takeshima et al.14 Additionally, a simple illumination method for preventing wrinkle formation upon stretching of a covalent azopolymer as well as for creating hierarchically wrinkled surfaces through selective erasure were recently demonstrated.18 In this manuscript we show, for the first time, that the photoinduced modulations in wrinkling patterns are not limited to polymers bearing covalently grafted azobenzene sidechains, but can also be realized in simple supramolecular polymer-azobenzene complexes between poly(4vinylpyridine) (P4VP) and 4-hydroxy-4’dimethylaminoazobenzene (OH-azo-DMA). This material design strategy allows easy variation of the photoresponsive azobenzene content,19 and although the phenol-pyridine hydrogen bonding is an order of magnitude weaker interaction than covalent bonding, under certain conditions the mechanical energy from the azobenzenes can be efficiently transferred to the polymer chain.20 This specific model system was chosen based on previous work, where we have found that for this system no azobenzene aggregation or liquid-crystal formation occurs even at nominally equimolar concentration. This is significant, as it enables the direct comparison of samples with varying azobenzene content. These supramolecular azopolymers can also be utilized for photoinduced surface patterning under a light interference pattern – a phenomenon known as surface-relief grating (SRG) formation.21 Recently, we demonstrated that in these materials, SRGs can be erased by applying uniform illumination, indicating a photoinduced increase in mobility of the polymer chains under homogeneous illumination.22 To support the claim that very small quantities of hydrogenbonded azobenzenes are enough to cause large-scale lightinduced material responses, here we demonstrate that an azobenzene content of 5 mol% (10 wt%) is enough to induce light-mediated mechanical changes in the polymer to erase the wrinkles. Furthermore, it is shown that the stress relaxation in the illuminated area leads to a transient increase in compressive stress, which initiates a cascade of surface buckling patterns proceeding through frequency doubling and tripling of the original pattern. By ceasing the illumination before the erasure is complete, diverse patterns along the

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kinetic pathway of the erasure can be frozen in the materials, thus implying the on-off nature of the photomobile state of these polymers. Together, these findings show that light as a low-power remote stimulus can modify the dimensions of surface patterns at large surface areas in scales of tens of micrometers, which is of interest, among other things, for substrates for stimuli-controllable cell culture, dynamic templated self-assembly and microfabrication.

EXPERIMENTAL SECTION Sample preparation Poly(4-vinylpyridine), P4VP (50 kDa MW, Scientific Polymer Products, Inc., Ontario, NY), and the 4-hydroxy-4’dimethylaminoazobenzene, OH-azo-DMA (Tokyo Chemical Industries, Inc., Portland, OR), were first weighed to give the desired molar ratio mixture, ranging from 1 to 100 % of OHazo-DMA in P4VP. Then these samples were dissolved in a 1:5 mixture of methanol and chloroform (Caledon Laboratories Ltd., Georgetown, ON, Canada) to give solutions of dry content of 2 wt% of P4VP. For stretchable substrates, polydimethylsiloxane (PDMS, Sylgard-184, Sigma-Aldrich, Oakville, ON, Canada), was mixed in 10:1 elastomer to hardener ratio, spread into petri dishes, degassed for 30 minutes and then cured at 60 °C for 2 hours. The thickness of the ready-made PDMS substrates were measured by micrometer and found to be 1.3 ± 0.2 mm. Prior to spin-coating, these substrates were cleaned with air plasma for 90 seconds in a PDC001 Expanded Plasma Cleaner (Harrick Plasma, Ithaca, NY) at 30W power and partial pressure of 600 mTorr. To prepare films of two different thicknesses, the solutions were spin-coated at 1000 and 4000 rpm for 60 seconds. For measuring the UV-Visible spectra and the thickness of the films, additional films were deposited on clean glass substrates using the same parameters. Sample characterization The UV-Vis spectra were recorded with Cary 100 Spectrometer (Agilent, Santa Clara, CA) from films spincoated on clean glass substrates, and by using a clean glass substrate as a reference. For measuring the thickness, a DEKTAK XT stylus profilometer (Bruker, Billerica, MA) was used. The films were scratched by a needle, and the thickness was measured by recording the profile of this scratch. For capturing the videos on the stretching and the buckling erasure, a custom-built polymer stretcher and a Nikon Eclipse LV100N POL upright microscope (Nikon Instruments, Mississauga, ON, Canada) equipped with 4x/0.1NA AND 10x/0.25NA CFI P Achromat objectives. Images were acquired using either a Retiga 2000R cooled CCD camera (QImaging, Surrey, BC, Canada) or an Infinity 1 color CMOS camera (Lumenera, Ottawa, ON, Canada) and recorded with NIS-Elements AR software (Nikon Instruments). To illuminate the stretched polymer films, a Lumencor Spectra X light source (Lumencor, Beaverton, OR) coupled to the optical microscope and filtered through FITC excitation filter was applied. For measuring the 488 nm light intensity at the sample plane, optical power meter (PM16-120, Thorlabs) was employed.

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Langmuir The analysis of the periodicity of the wrinkled induced on the polymer films was done through a previously reported procedure.6 Briefly, color optical images were converted into greyscale and the brightness and intensity adjusted to cover the full 8-bit range. Then, the 2D-fast Fourier transform (FFT) power spectrum was obtained using a custom script written in Matlab. The normalized intensity vs. wavelength plots were obtained and the highest intensity peak selected. Since some samples had multiple local maxima contained within the peak, corresponding to variations in periodicity in the sample, a Gaussian profile was fitted using Matlab to capture the most representative wavelength for the sample. The center of the fitted curve was chosen as the characteristic wavelength and compared against sample spatial measurements from the original image. Three independent measurements of the periodicity were performed for each polymer composition, and the reported numbers correspond to the average and standard deviation for the n = 3 replicates.

RESULTS AND DISCUSSION Figure 1 shows the chemical structure of the polymerazobenzene complex employed, as well as the UV-visible spectrum recorded from a sample of P4VP(OH-azo-DMA)0.05 spin-coated on a glass substrate. To study the effect of the photoactive azo content in the material, a sample series of increasing azo content of 0.01, 0.05, 0.25 and 1.00 molar fractions was prepared. Previously, it has been shown by IR spectroscopy that hydrogen bonds form between the 4vinylpyridine units and OH-azo-DMA, and that irradiation by 450-nm light causes a minor shift of a steady state towards free pyridine.20,23 However, this shift is small and no macroscale phase separation is observed in the materials even after prolonged irradiation.20 Additionally, no changes in the shape of the UV-vis absorption spectra were observed upon varying the azobenzene content, indicating that azobenzeneazobenzene aggregation was not present even at nominally equimolar samples.

Figure 1. Absorption spectrum of a spin-coated thin film for x=0.05 complex, and the chemical structure of the employed hydrogen-bonded polymer-azobenzene complex in the inset. Profilometry measurements on samples spin-coated on glass showed that the film thickness of the azopolymer after spincoating 4000 rpm was 230 ± 40 nm. Since the same spincoating procedure was applied on plasma-cleaned PDMS

(having comparable surface chemistry to clean glass substrates), we expect this to be a realistic estimation for the azopolymer film thickness on PDMS. The PDMS substrates, having a thickness of 1.3 ± 0.2 mm, were thus several orders of magnitude thicker than the thin azo-containing films. When clamped to a mechanical stretcher, cracking of the azopolymer film was observed due to irregular strain applied to the sample. This indicates that the azopolymer film is not as pliable as the PDMS substrate it rests on, and thus upon large deformations stress release from the overall film is accomplished by cracking. When stretched to 125% of the original length of samples, sinusoidal buckling patterns having periodicities ranging from 10 to 15 μm were created. This buckling could be reversed by simply relaxing the tension resulting in the film regaining its original flat topography. Since the buckling periodicity is affected both by film thickness and the plane-strain modulus of the polymerazobenzene complex, the small variations of the buckling periodicity as a function of azobenzene content can be traced back both to small differences in the film thickness and to earlier results, which showed that the Tg of polymerazobenzene complexes would decrease as a function of increasing azo content due to an increased plasticization effect.22 The effect of tensile stress on the dimensions and structure of azopolymer films was studied by spin-coating thin films with pre-defined square shapes onto PDMS using 2  2 mm deposition masks made out of adhesive tape. Since the tape mask thickness was significantly higher than the spin-coated azopolymer film thickness, the edges of the azopolymer islands show some defects and non-uniform thickness. When the PDMS under the azopolymer (x=0.25) film was stretched uniaxially to 125% elongation, the elongation of the patterned polymer island along the stretching direction was 122.5%, while the dimensions in the perpendicular direction were compressed to 90.8% of the original. As shown in Figure 2a (phases i-ii), due to the mismatch in mechanical properties of the azopolymer and the PDMS, the thin azopolymer film buckles upon stretching the PDMS, as a response to the compressive stress generated by the changes of the supporting PDMS substrate dimensions. When the buckled film island was exposed to 488-nm light, trans-cis-trans isomerization cycles in the azopolymers lead to a partial stress release in the film, resulting in a slight change in its dimensions. As indicated by Videos S1 and S2 in the supporting information and the images in Figure 2a (iii), the film dimensions increased along the stretching direction and decreased in the perpendicular direction (in the case of original 2 x 2 mm films, the increase was to 123.9% and the decrease to 90.3%). This initial stress relaxation (at illuminations of 17 and 14 mW/cm2 for 4x and 10x objectives, respectively) occurred within 2 s from starting the exposure. Interestingly, this timescale is on the same order of magnitude as that reported for the photo-induced increase in volume of azopolymers, as measured by spectroscopic ellipsometry.24 It is plausible that, even though the light-induced expansion is very small, the increase in free volume is enough to increase the relaxation rate of the polymer chains in the thin film.

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Simultaneous to this stress relaxation, a significantly longer (one order of magnitude) cascade of dynamic photoinduced wrinkling erasure is initiated. It is noteworthy that, after the erasure, the original square shape of the polymer film is effectively changed into a rectangle. Furthermore, if the tensile stress was removed, the square shape of the initial azopolymer film was recovered, but this time the buckling appeared in a

perpendicular direction due to the PDMS applying a compressive force in the direction of the uniaxial stress release. The appearance of buckles in removing the stress after buckling erasure further highlights the strong adhesion between the azopolymer film and the PDMS, since no film delamination was observed.

Figure 2. a) Schematic and images of the changes in the dimensions and structure of a square azopolymer film upon stretching (iii), illumination and wrinkle erasure (iiiii), stretch release of the PDMS substrate (iiiiv), and second wrinkle erasure (v). Images of the light-induced erasure of the buckling patterns formed at the edge of the island after b) stretching and c) release of the PDMS substrate. When applying the illumination to a smaller region of the stretched film, selected wrinkled areas could be optically erased. The erasure began with similar stress relaxation of the azopolymer as in the island test described above. Following a relatively fast change in the dimensions of the illuminated area, the buckling periodicity started to evolve, with a rate and evolution pattern proportional to the applied illumination intensity, as depicted in Figure 3 for the P4VP(OH-azoDMA)0.25 sample. Low intensity light (0.1 mW/cm2) led to every wrinkle summit splitting into two (λ/2), whereas the

higher intensity light (1.0 mW/cm2) led to summits splitting into three (λ/3), effectively evolving into a frequency tripling of the original pattern. If illumination was continued, in both cases, the wrinkles were gradually erased and a flat film emerged. We attribute this development of hierarchical wrinkling to changes in boundary conditions resulting from the increase in compressive stress due to the change in the dimensions of the illuminated area.

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Langmuir Indeed, the formation of a pitchfork-type wrinklon upon splitting of a wrinkle into two is geometrically similar to the wrinklons described upon merging of two wrinkles of λ into one of 2λ in case of wrinkled curtains or graphene sheets that are suspended only on one edge.25 Similar wrinkling pattern evolution through wrinklons has also been reported in polymer films floating on water,26 as well as in thin metal films on oil

droplets27 or on polymers, whose mechanical properties have been gradually changed through solvent swelling28. The observed wrinkle evolution at low illumination intensities shed light into the stresses present during wrinkle erasure, where the initial expansion of the illuminated area leads to increased wrinkle frequencies, which are ultimately smoothed over due to the softening of the polymer film over a longer timescale.

Figure 3. Low intensity (upper row) and high intensity (lower row) illumination of a circular area on buckled double layers of P4VP(OH-azo-DMA)0.25 and PDMS. The illumination was turned on at t = 5s. Insets show zoomed-in regions. All images taken at the same magnification. Varying the azobenzene content in the polymer-azobenzene complexes revealed a threshold of x = 0.05 below which the pattern evolution either didn’t take place or was too slow to be observed in the measured time scale (60 s). As shown in Figure 4, above x = 0.05, the rate of the process increased with increasing azobenzene content, but the effect of the intensity on the pattern evolution was the same. In general, the lower azobenzene content decreased the rate of the pattern evolution, allowing the distinction of initial stress relaxation and the appearance of λ/3 wrinkles through an intermediate step of formation of the λ/2 wrinkles, whereas at higher azobenzene content, the wrinkling evolution was faster, making it more difficult to observe the intermediate steps before the formation

of λ/3 wrinkles. Hence, the supramolecular strategy described here provides a simple solution for tuning the timescale of the dynamics of photoresponsive surfaces. It is also noteworthy that any stage of the pattern evolution can be frozen in by turning off the illumination powering the pattern evolution on the surfaces. From images 2-4 it is clear that the photoinduced changes in surface patterns occur without the original cracks of the films filling in, regardless of the illumination intensity and the azobenzene content. This indicates that the photoinduced changes in the mechanical properties of the azopolymer films, and thus the “photomobile” state of the polymer, are inherently different from isotropic flow of the polymer that would result from heating it above its Tg.

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Figure 4. Early-stage pattern evolution upon 488-nm illumination (1.0 mW/cm2) on wrinkled samples with azopolymers of different azobenzene contents. All images taken at the same magnification. The fact that the wrinkle erasure was possible in P4VP(OHazo-DMA)0.05 is significant, since, thus far, wrinkling erasure has only been demonstrated in polymers for which every side chain carries a covalently grafted photoactive azobenzene unit. This means that nominally occupying only one in every twenty P4VP repeating units with azobenzene is enough to create a material that is photoresponsive on a macroscopic scale, i.e. capable of photoinduced stress release and wrinkle erasure. Since azobenzene photoisomerization has been shown to modify the mechanical properties of the polymer materials,10 it is of interest to compare this lower limit to the lower limit of x = 0.01 that was observed to be sufficient for inscription of all-optical surface relief gratings (SRGs).29 In this work, analogously to the wrinkle erasure, the process speed could be increased by increasing the azobenzene content in the supramolecular polymer. In both cases, extremely low azobenzene content bound to a polymer with relatively weak hydrogen bonds is sufficient to control the mechanical properties of the host polymers by illumination. Moreover, a lower MW P4VP was employed in the SRG study,29 thus making that system intrinsically more mobile and probably contributing to a lower azo content threshold. Likewise, it was recently shown that by employing uniform illumination with 488-nm light on a similar material system with x = 0.33, the SRGs of 200 nm deep could be completely erased and the rate of erasure correlated with the MW of the

P4VP backbone.22 Therefore, varying the MW of the polymers used for wrinkle erasure constitutes an important task for future studies. In summary, it is plausible that both the SRG formation, photoinduced SRG erasure, and the photoinduced buckling pattern evolution are enabled by the increased mobility in the polymer system due to the photoisomerization of the azobenzenes, and that this mobility can be controlled by varying the azobenzene content and the P4VP MW, at least when below the entanglement limit. Since the photoinduced buckling pattern erasure (as well as all-optical SRG inscription and erasure) in this material occurs at a temperature significantly below its Tg, an intriguing question is the nature of the molecular-level mechanism behind the photomobile state of these azopolymer complexes. Taking into account that our polarization-modulation FTIR spectroscopy results have shown that the local rotational motion of pyridine rings can be initiated by azobenzene photoorientation20 and that the β relaxation in the structurally similar polystyrene (PS) polymer have been assigned to phenyl group motion coupled to backbone libration or oscillation,30,31 one possible molecular-level explanation for the nature of the photomobile state is that photoisomerization enables or accelerates local sub-Tg motions, such as the β transition process, in P4VP. In other words, it is possible that the photoinduced changes in these amorphous materials occur at a more localized scale than typical changes in polymer chain

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Langmuir motions that give rise to the glass transition. Thus, there is no significant decrease in the Tg value. We propose that the mechanism leading to mechanical changes in these materials is inherently different to that of liquid-crystalline materials, where order in a larger range can be directly disturbed by azobenzene photoisomerization. Even though more research is needed to form a solid understanding of this mechanism, it is remarkable that very low illumination conditions and optimized material design allow converting the sub-nm-scale azobenzene photoisomerization into changes of the polymer wrinkling patterns having macroscopic dimensions.

CONCLUSIONS Overall, we have shown that supramolecular azopolymers can be used for photomorphing surface patterns with at least an order of magnitude larger dimensions than previously reported. This was achieved through the all-optical erasure of surface wrinkles that appear when stretching a thin azopolymer film on elastomeric substrate. The photoisomerization-induced mechanical changes in the azopolymer allow modifying the physical dimensions of a predefined square pattern into a rectangle. Additionally, the photomobile state of the material enables a cascade of wrinkling pattern changes proceeding through frequency doubling and tripling modes that can be frozen in by simply switching off the illumination. The supramolecular strategy allows easy tuning of the photoactive azobenzene content in the polymer matrix, which then directly tunes the rate of the pattern evolution at constant illumination intensity. Furthermore, this strategy allowed us to demonstrate that the light-induced changes in surface topography can be induced even in a sample in which nominally only one in every twenty polymer repeat units is occupied by a light-active molecule. We believe that the current phenomenon will also be generalizable to other azobenzene derivatives, thereby allowing changing the wavelength of the switching light and thus furthering the application prospects of these dynamic surfaces.

Prof. Kari Dalnoki-Varess, Prof. C. Geraldine Bazuin, Anna Nissen, and Lasha Shaw-Korchynski are heartfully thanked for discussions.

ABBREVIATIONS P4VP, Poly(4-vinylpyridine); dimethylaminoazobenzene

Supporting Information. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Videos demonstrating wrinkling erasure.

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AUTHOR INFORMATION Corresponding Author * e-mail: [email protected]

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Author Contributions All authors have given approval to the final version of the manuscript.

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Funding Sources JV was supported through the Banting Postdoctoral Fellow program. JMM is the recipient of an Early Researcher award from the Ontario Ministry of Research and Innovation and holds the Tier 2 Canada Research Chair in Micro- and Nanostructured Materials. Funding from the Natural Sciences and Engineering Research Council of Canada through a Discovery Grant to JMM is gratefully acknowledged.

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ACKNOWLEDGMENT

4-hydroxy-4’-

REFERENCES

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ASSOCIATED CONTENT

OH-azo-DMA,

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