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Thermally Driven Photonic Actuator Based on Silica Opal Photonic Crystal with Liquid Crystal Elastomer Huihui Xing, Jun Li, Yang Shi, Jinbao Guo, and Jie Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01033 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 27, 2016

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ACS Applied Materials & Interfaces

Thermally Driven Photonic Actuator Based on Silica Opal Photonic Crystal with Liquid Crystal Elastomer

Huihui Xing†,a,b Jun Li†,a,b Yang Shi, a,b Jinbao Guo* a,b and Jie Wei* a,b a

College of Materials Science and Engineering, Beijing University of Chemical

Technology, Beijing 100029, P. R. China. b

Beijing Engineering Research Center for the Synthesis and Applications of

Waterborne Polymers, Beijing 100029, P. R. China.

†

These authors contributed equally to this study.

ACS Paragon Plus Environment


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ABSTRACT: We have developed a novel thermo-responsive photonic actuator based on three-dimensional SiO2 opal photonic crystals (PCs) together with liquid crystal elastomers (LCEs). In the process of fabrication of such photonic actuator, the LCE precursor is infiltrated into the SiO2 opal PC followed by UV light-induced photo-polymerization, thereby forming the SiO2 opal PC/LCE composite film with a bilayer structure. We find that this bilayer composite film simultaneously exhibits actuation behavior as well as the photonic band gap (PBG) response to external temperature variation. When the SiO2 opal PC/LCE composite film is heated, it exhibits a considerable bending deformation and its PBG shifts to shorter wavelength at the same time. What’s more, this actuation is quite fast, reversible, and highly repeatable. The thermo-responsive behavior of the SiO2 opal PC/LCE composite films mainly derives from the thermal-driven the change of nematic order of the LCE layer which leads to the asymmetric shrinkage/expansion of the bilayer structure. These results

will

be

of

interest

in

designing

optical

actuator

systems

for

environment-temperature detection.

KEYWORDS: Opal photonic crystal, liquid crystal elastomer, thermo-responsive photonic actuator, photonic band gap, bending deformation


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1

INTRODUCTION As a kind of artificial smart material, responsive photonic crystals (PCs), whose

optical properties can be tuned in response to external stimuli, have recently evoked a lot of interest due to their applications in stimuli-responsive sensing devices,1-11 color-changing paints and inks,12-14 and photonic active elements.15-17 As well known that, applying the external stimuli could lead to the changes of the effective refractive index,8, 18-19 the lattice parameter,3, 20-23 and/or the order degree of the PC structures,24 allowing for the reversible switching of the photonic band gap (PBG) of responsive PCs. Liquid crystal elastomers (LCEs) are unique LC polymers that combine the entropy elasticity of polymer elastomers with orientational order of LCs.25-27 A fascinating feature of LCEs is that they could reversibly change orientational order and the corresponding macroscopic shape in response to a certain external stimulus. In the case of thermo-responsive ones, a macroscopic deformation happens when temperature variation leads to a change in molecular orientation. Recently, LCEs-based inverse opal materials have been investigated by our and other groups.23-24, 28-29 By heating or irradiating with UV, a change of the LCE inverse opal film’s lattice constant was introduced, and the corresponding Bragg reflection band could

be

switched

continuously.

For

example,

we

recently

reported

a

thermal-switching patterned inverse opal materials based on LCEs, in which the dual structural colors of inverse opal film could be reversibly switched by temperature.23 However, to the best of our knowledge, there are no reports on coupling the optical



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properties of PCs with the deformation abilities of LCEs to show an optical output signal and an actuation behavior at the same time. In this study, we develop a novel thermo-responsive photonic actuator by filling the LCE monomers into the three-dimensional SiO2 PC template and following with UV irradiation, then finally detaching it from the glass substrates. Thermal switching of molecule orientation in LCEs allows for a simultaneous control of macroscopic shape and optical spectrum of the composite film. Upon the increasing temperature, the SiO2 opal PC/LCE composite film exhibits an obvious bending deformation and its PBG shifts to shorter wavelength at the same time. Cooling leads to the opposite effect. Furthermore, the relevant mechanisms of the thermo-responsive behavior are addressed in detail. This SiO2 opal PC/LCE composite film shows a great potential for a typical thermal stimuli-responsive actuator as well as an optical sensor.

2

EXPERIMENTAL SECTION

2.1 Materials All of the chemicals and materials including anhydrous ethanol (AR, J&K Scientific Ltd.), hydrogen fluoride(40%, J&K Scientific Ltd.), tetraethyl orthosilicate (98%, J&K Scientific Ltd.), ammonium hydroxide (25%, J&K Scientific Ltd.), photo-initiator Darocur 2959 (99%, J&K Scientific Ltd.), monoacrylate mesogenic monomer A6OCB (99%, Bayi Space LCD) were used as purchased. Nematic diacrylate monomer C6M was synthesized according to the previous study by Broer et al.30 The molecule structures of A6OCB, C6M and Darocur 2959 are shown in Figure


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1a. The structure characterizations of C6M and A6OCB were described in Figure S1 of the Supporting Information.

2.2 Fabrication of Opal PC/LCE Composite Films Monodispersed SiO2 spheres with different diameters were prepared according to Stőber synthesis method. Then vertical deposition method was employed to fabricate SiO2 opal PCs with close-packed face-centred cubic (FCC) structure, in which monodispersed SiO2 spheres were dispersed in anhydrous ethanol. The preparation of the composite film composed with SiO2 opal PC and LCE is shown schematically in Figure 1b. First, a LC mixture of A6OCB and C6M was prepared in a 9:1 molar ratio, then 0.1 mol% photoinitiator (Darocur 2959) was introduced for photopolymerization. Because A6OCB with high crystallizability is very narrow temperature range of nematic phase, C6M used as a cross-linker in the LC mixture was also employed to increase the temperature range of nematic phase.31 Then a glass cell was constructed based on SiO2 opal PC, where the upper glass substrate was coated with a rubbed polyvinyl alcohol orientation layer.23 The cell gap (20 µm or 6 µm) was controlled by the size of plastic spacers. The 6 µm glass cell was fabricated to estimate the effect of the LCE thickness on the bending behavior and spectral performance of SiO2 opal/ LCE composite film. After solvent evaporation, the LCE precursor was filled into the cell by capillary action at 80 °C to keep it in an isotropic phase. After that, the LC sample was cooled to 50 °C (nematic phase) and was cured for 10 min with UV illumination intensity of 3 mW/cm2. After UV photo-polymerization, the sample was ultrasonic treated in acetone for 20 min in order to dissolve the glue used to



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encapsulate the glass cell. Finally, the free-standing SiO2 opal PC/LCE composite film was obtained after being detached from the glass substrates.

Figure 1. (a): Molecule structures of LC monomers and photoinitiator. (b): Diagrammatic drawing for fabrication of SiO2 opal PC/LCE composite film.

2.3 Measurements Mesomorphic properties of the LCE precursor was studied by polarized optical microscope (POM) (Leica, DM2500P) with a heating stage (Linkam, THMS-600). Differential scanning calorimeter (DSC, Pyris Diamond) were used to examine the thermodynamic properties of the LCE with heating and cooling rate of 10°C/min


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under a dry nitrogen purge. Morphologies of SiO2 opal PC/LCE composite films were observed by scanning electron microscope (SEM, Hitachi S-4700). The reflection spectra were obtained by fiber spectrometer (Avantes, AvaSpec-2048). Note that all the reflectance measurements were manipulated at the condition that the angle between detecting beam and the centre of samples was 90°

3

RESULTS AND DISCUSSION

3.1 SiO2 Opal PC/LCE Composite Films As Figure 1b shows, the SiO2 opal PC/LCE composite film was prepared by filling LC mixture into the SiO2 opal PCs glass cells, and then clearing the glass substrates away after UV photo-polymerization. The LCE precursor was mainly composed of A6OCB and C6M with a 9:1 molar ratio, where 0.1 mol% Darocur 2959 was used as photoinitiator. The phase transitions and the thermodynamic properties of the LCE precursor were investigated by POM and DSC. As shown in Figures S2 and S3 of the Supporting Information, the LCE precursor exhibits nematic phase at the temperature range of 60-48 oC on the cooling process. The LCE precursor was loaded into the cell, in which the SiO2 opal template was sandwiched, by capillary action at 80 °C in isotropic phase. Then the sample was cooled down to 50 °C, and polymerized under UV irradiation at 50 °C to ensure the nematic phase being kept by the LCE. In addition, DSC measurement results show that the glass transition temperature (Tg) and the nematic–isotropic transition temperature (TNI) of the LCE film are around 43 °C and 123 °C, respectively (see Figure S4 of the Supporting Information). Here TNI is



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not very clear in DSC curve and it was further confirmed by POM measurement. The structure of the SiO2 opal PC/LCE composite films were observed by SEM. It is worth noting that we directly employed the resulting SiO2 opal PC/LCE composite films after UV photo-polymerization for SEM observation. Figure 2 shows the SEM images of SiO2 opal PC from the self-assembly of 214 nm SiO2 spheres and the bilayer structure of the SiO2 opal PC/LCE composite film. From Figure 2a and 2b, we observe that the SiO2 opal PC has a closely packed hexagonal structure. The whole thickness of the composite film is approximately 27 µm and opal PC/LCE composite layer is about 6.5 µm (Figure 2c), which are respectively controlled through the gap of LC cell and the SiO2 PC thickness. As presented in the cross-sectional SEM image at high magnification (Figure 2d), SiO2 spheres regularly embed in LCE at the SiO2 opal PC/LCE composite layer. We mention that this bilayer structure brings about the deformability of the composite film as mentioned later.

Figure 2. (a) SEM image of SiO2 opal PC from the self-assembly of SiO2 spheres with


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214 nm diameter. (b) SEM image of SiO2 opal PC at high magnification. (c) Cross-sectional SEM image of the SiO2 opal PC/LCE composite film. (d) Cross-sectional SEM image of the composite film at high magnification. SiO2 opal PCs used to fabricate the composite films possess brilliant blue structure color with a reflection peak at 475 nm, as shown in Figure 3. After the infiltration of the LCE precursor and UV photo-polymerization, the resulting composite film shows dark green structure color, and the reflection band shifts to 535 nm as the green line indicts. We think it results from the changing of the effective refractive index. For the normal incidence condition, the reflection band position (or PBG) could be proximately calculated according to the following formula:29, 32 2 2 λ = 2 2 / 3D  nsilica ⋅ f + nvoid ⋅ (1 − f ) 

1/2

where ߣ is the position of the Bragg peak, D is the SiO2 particles diameter, while nsilica is the refractive index of the SiO2, nvoild is the refractive index of the medium in the voids of the opal PCs and f is the volume fraction of SiO2. It can be seen that the changes in nvoild may result in the shift of reflection bands when the D, nsilica and f keep constants. As we know that the refractive indices of LCE is greater than that of air, when the LCE was infiltrated into the voids into the SiO2 opal PC, the spectrum could shift from 475 nm to 535 nm, and the corresponding reflection color changed from blue to dark green.



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Figure 3. Reflection bands of the pure SiO2 opal PC and opal PC/LCE composite films. Inset images are the corresponding real images.

3.2 Thermally Driven Photonic Actuation of SiO2 Opal PC/LCE Composite Films By integrating of the elastic network with mesogenic unit, LCEs obtain their actuation behavior. When the LCEs translate to the isotropic state, the change of polymer chains of LCEs induces a macroscopic shape change.26 Owing to this fantastic performance of LCEs, the shape and reflectance performance of SiO2 opal PC/LCE composite could be reversibly tuned during the heating and cooling process. Figure 4 shows the actuation and the corresponding reflection bands of the composite films with temperature. When the temperature increases from 20°C to 180°C, the macroscopic shape of the composite film exhibits a considerable bending deformation. Simultaneously, the reflection band of the composite film shifts to blue (from 535 nm to 519 nm). The tuning mechanisms of the composite film will be discussed later shown in Figure 5 and Figure S5. In the case of SiO2 opal PC/LCE composite film


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with a thinner LCE layer (around 6 µm), we found that the spectral shift become smaller compared to the thicker LCE layer (20.5 µm) case. This may be due to the deformation behavior of LCE was limited by the rigid SiO2 opal layer, thereby leading to a little change of lattice length in SiO2 opal layer (see Figure S6 of the Supporting Information).

Figure 4. (a) Photographs of the reversible actuation behavior and (b) reflection spectra reversible shifts of the SiO2 opal PC/LCE composite film induced by temperature variation. The bending deformation and the shift of the PBG are thermally reversible. After the temperature falling back (from 180°C to 20°C), the shape of the composite film



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returns to flat, and the PBG shifts to its original wavelength at the same time. Video in the Supporting Information shows that the tuning process is quite fast as the composite films curl and re-uncurl completely in seconds. It should also be stressed that the bending behaviour and spectrum shift of SiO2 opal PC/LCE composite film have similar trends when the SiO2 size was changed (see Figure S7 of the supporting Information) Figure 5 illustrates the mechanism for the thermo-switching of the SiO2 opal PC/LCE composite films. According to the replication experiment in Supporting Information (see Figure S5), we conform that SiO2 opal array induces homogenous orientation of LC molecules. Therefore, the LC molecule of LCE layer in this bilayer structure film has an approximate planar alignment. Here, we think that the bending deformation of the SiO2 opal/LCE composite could be attributed to the bilayer structure of a LCE layer and a SiO2 opal infiltrated with a small quantity of LCE. The temperature variation significantly influences the nematic order of LCE layer of the SiO2 opal PC/LCE composite films. When the composite films were heated to close to their TNI, the LC moieties transition to isotropic (Figure 5a), here a decrease in local nematic order results in a contraction in planar-aligned LCE layer.31 Because of almost no change of profile in the SiO2 opal layer, mismatches of deformation in these two layers lead to a strain gradient in the thickness direction of the composite film, which induces the bending deformation toward the pure LCE layer. Meanwhile, the SiO2 spheres arrange more closely in the thickness direction, and the d111 of the opal PC decreases during the bending deformation (Figure 5b). Therefore the


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reflection band of the bilayer film shifts towards shorter wavelength accompanied with a structure color’s blue-shift, which is due to that the PBG of the SiO2 opal PC/LCE composite film is positively related to the lattice constant.34-35 When the films are cooled down, the LC molecules revert to the nematic state. As a result, both the reflection bands and macroscopic shape of the bilayer films may go back to the initial state.

Figure 5. Schematic diagram illustrating (a) the actuation and (b) dynamic change of the lattice distance in the SiO2 opal PC/LCE composite films. The bending curvature and PBG peak as a function of temperature has been demonstrated as shown in Figure 6a, it could be found that an increase of the temperature leads to the increase of the curvature of the composites film and a blue-shift of the corresponding reflectance peak, and vice versa. We also note that the bending deformation undergoes a gradient change even if around the glass and clearing transition temperature during the heating and cooling processes, which we think may be due to the rigid C6M as a crosslinker in the LCE. The repeatability of



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the actuation and the PBG’ shift of the SiO2 opal PC/LCE composite film has also been investigated. Figure 6b gives six cycles of the SiO2 opal PC/LCE composite film with low and high temperature test, which reveals the stability of the composite film. In each cycle, the macroscopic shape of the composite film could change from the original uncurled status (with the maximum radius of curvature) to curl (with the minimum radius of curvature) and vice versa, demonstrating that stability of the composite film is good. What’s more, a dynamic thermo-tuning process of the SiO2 opal PC/LCE composite film was revealed in a movie (see Video of Supporting Information).

Figure 6. (a) The bending curvature and PBG peak as a function of temperature and (b) radius of curvature of the SiO2 opal PC/LCE composite film in the low-high temperature cycles. It should be noted the tunability of the SiO2 opal PC/LCE composite films’ PBG is closely related with the space between the SiO2 spheres in the opal PC. Here, self-assembling monodisperse SiO2 spheres form highly ordered and closely-stacked face-centered cubic opal structures by the flow-controlled vertical deposition method, leading to that the space between the SiO2 spheres in the opal PC is very tiny. In


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principle, increasing the distance between the SiO2 spheres by changing assembly method or further controlling the interactions between SiO2 spheres may be likely to provide more space for the SiO2 spheres to close to each other when bending deformation happens. Accordingly, the decrease of d111 will be more drastic as well as the blue-shift of the PBG.

4

CONCLUSIONS To summarize, we proposed a new thermo-driven photonic actuator based on SiO2

opal PC together with LCE. The SiO2 opal PC/LCE composite film shows actuation behavior as well as optical response to temperature variation. We demonstrate that the macroscopic shape of the SiO2 opal PC/LCE composite films could be fast and reversibly changed between flat and curl. Simultaneously, the PBG of the composite films occur reversible migration. The thermo-responsive behavior of this composite film is mainly due to thermal-driven the change of nematic order of the LCE layer that results in asymmetric shrinkage/expansion of the bilayer structure. These results will be of interest in developing smart materials for color-changing actuators with an environmental response function.



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ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org Characterization of LCE precursor and LCE, controlled the composites film details, and contrast experimental (PDF) Video of thermo-tuning procedure of the SiO2 opal PC/LCE composite film in heating and cooling process (AVI)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. Wei). *E-mail: [email protected] (J. B. Guo).

ACKNOWLEDGEMENTS

This research was supported by the National Natural Science foundation (Grant no. 51573012, 51373013 and 51173013), Beijing Young Talents Plan (YETP0489) and BUCT Fund for Disciplines Construction and Development (Project No. XK1509).


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