Chiral Photonic Crystalline Microcapsules with Strict Monodispersity

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Applications of Polymer, Composite, and Coating Materials

Chiral Photonic Crystalline Microcapsules with Strict Monodispersity, Ultra-high Thermal Stability and Reversible Response Pengcheng Lin, Qi Yan, Zhan Wei, Ying Chen, Shuqin Chen, Huiyuan Wang, Zhuoran Huang, Xuezhen Wang, and Zhengdong Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02561 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Applied Materials & Interfaces

Chiral Photonic Crystalline Microcapsules with Strict Monodispersity, Ultra-high Thermal Stability and Reversible Response Pengcheng Lin,*[1] Qi Yan,[1] Zhan Wei,[1] Ying Chen,*[1] Shuqin Chen,[1] Huiyuan Wang,[1] Zhuoran Huang,[1] Xuezhen Wang,[2] and Zhengdong Cheng*[2] 1

Guangdong Provincial Key Laboratory on Functional Soft Condensed Matter, School of

Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China 2

Artie McFerrin Department of Chemical Engineering, Texas A &M University, College

Station, Texas, 77843-3122, USA. *Corresponding Author: E-mail: [email protected], [email protected], [email protected] KEYWORDS: chiral photonic crystals, microcapsules, monodispersity, high thermal stability, reversible response ABSTRACT Tunable photonic crystals (TPCs) reflecting selected wavelengths of visible light and responding to external stimuli are widely applied to fabricate smart optical devices. Chiral nematic liquid crystals (CNLCs) possessing response to temperature, electric field and magnetic field are considered as one-dimensional TPCs. The encapsulation of CNLCs provides responsive photonic devices with stand-alone macroscopic structure and excellent processability. However, when CNLCs as cores are wrapped by polymeric shells to form core-shell structured microcapsules, the polydispersity of microcapsule size, the irregular spatial geometry and the low thermal stability inevitably result in a deterioration of the optical performance and limited application at high temperatures. Herein, a combination of microfluidic emulsification and interfacial polymerization is employed to fabricate polymer wrapped photonic crystalline microcapsules (PWPCMs). The sizes and reflected colors of PWPCMs can be simultaneously controlled by adjusting the flow rates in the microfluidic chips. PWPCMs possess strictly mono-dispersed sizes with coefficients of variation less than 1%. The free-standing PWPCMs have high thermal stability. The deformation temperature of PWPCMs is as high as 210 °C. The colored PWPCMs also exhibit a reversible thermochromic property between the chiral nematic phase and the isotropic phase. The highly stable and tunable PWPCMs provide new opportunities for a wide range of photonic

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applications, including smart optical window, tunable microlasers, responsive microsensors, and various photonic devices. 1. INTRODUCTION Since the milestone research by Eli Yablonovitch and Sajeev John in 1987,1,2 photonic crystals (PCs) have made tremendous contributions to the rapid development of functional optical materials such as PC fibers, PC sensors, PC waveguides, PC lasers and PC cavities as a result of their fascinating ability to control and manipulate light flow.3-7 In recent years, the main research interest has been focused on creating tunable photonic crystals (TPCs) whose properties could be dynamically modulated by external stimuli, such as chemicals, temperature, electric field, magnetic field or mechanical stress.8-12 Among these TPCs, chiral nematic liquid crystals (CNLCs) or cholesteric liquid crystals possessing variable helical layered arrangement and tunable photonic bandgap are considered as one-dimensional PCs, and when the photonic bandgap coincides with the wavelength of visible light, CNLCs are able to prevent light propagation within a specific wavelength range and reflect a brilliant color, and the reflection wavelength can be determined based on the Bragg’s law.13,14 In the process of utilizing the structure color to realize colorful display, such as host-guest effect display based on dye-doped liquid crystals, polymer dispersed liquid crystal display or CNLC display, CNLCs as optical layers are commonly hermetically packaged in a vacuum glass cell to limit the fluidity at high temperatures and to avoid the structure destruction by active chemicals,15-20 in the meantime, the optically active layers are essentially in the form of a flat thin film, the thickness of which is non-automated controlled by different size of paved spacers. The frangibility of glass package and the angle dependence of reflected color by flat geometry structure hinder the flexible application, particularly the wide viewing angle display of CNLCs. Three-dimensional CNLC microcapsules with center symmetrical architecture and flexible shell are assumed to perfectly solve the above challenges. Previous study concerning fabricating CNLC microcapsules with a shell-core structure is mainly conducted by solute co-diffusion method, emulsification polymerization or phase separation method,21-24 however, there are several disadvantages when applying the above methods, firstly, the encapsulation efficiency of CNLCs in bulk polymer is not large enough and is difficult to be improved due to the random phase separation of a polymer matrix from continuous liquid phase into separated solid shell, secondly, the heterogeneous mixture consisting of CNLCs and polymerizable precursors hardly can be evolved into shell-core 2


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structured microcapsules with uniform size in the process of phase separation or emulsification polymerization, lastly, the direct contact between CNLCs and polymerizable precursors leads to the physical extrusion and chemical adhesion of neighboring microcapsule shells, making the shell morphology exhibit irregular sphere or ellipsoid. Recently, microfluidics as a controllable and high-throughput technology has been applied in the fields of materials synthesis and micro-scale assembly.25-29 Application of microfluidics could solve the above problems caused by solute co-diffusion method, emulsification polymerization or phase separation method. CNLC microcapsules have been fabricated by double emulsification and the UV curing.30 However, application of single emulsification and interfacial polymerization has not been reported previously to fabricate CNLC microcapsules. Herein, we establish a facile and continuous strategy to assemble colored TPC microcapsules with symmetrical architecture, flexible shell and strict monodispersity by using microfluidic chips as template setup and interfacial polymerization for encapsuling mechanism. TPC used in this study is CNLC mixture composed of nematic liquid crystals (NLCs) and chiral dopant S811, a kind of isocyanate, 2, 4-tolylene diisocyanat (TDI) acting as an interfacial polymerization monomer, is dissolved in CNLCs and join the chiral matrix of CNLCs by high compatibility and guest-host effect to form homogeneous inner PC solution, sodium dodecyl sulfate (SDS) aqueous solution acts as an outter aqueous phase. With a single-channel, double-channel or triple-channel capillary devices, monodispersed O/W single-emulsion CNLC droplets are generated, upon flowing into a collecting aqueous phase containing another interfacial polymerization monomer tetraethylenepentamine (TEPA), the interfacial polymerization of TDI and TEPA occurs at the interface between inner PC solution and collecting aqueous phase to form a thin and transparent polymer shell. The colors of CNLC microcapsules can be controlled by changing the CNLC components in the injection channels and the flow rates of injected PC solutions in the double-channel or triple-channel. The size of the microcapsules can be precisely controlled by adjusting the flow rates of the hydrophobic PC solution and aqueous phase. 2. EXPERIMENTAL SECTION 2.1 Materials. The CNLC mixtures were composed of a host nematic LC (JK-1001) (Yongsheng Huatsing Liquid Crystal Co., Ltd, China) and a chiral dopant (S811) (Merck, China), where the dopant concentrations were set to be 20 wt%, 23 wt%, and 25 wt% for the red, green, and 3



blue-colored CNLCs, respectively. The red, green, and blue-colored PC solutions were prepared by dissolving 2 wt% TDI into the red, green, and blue-colored CNLCs at their clearing point, followed by incubating at 25 °C for 24 h, the stable PC solutions were used as the dispersed phase to fabricate the colored polymer wrapped photonic crystalline microcapsules (PWPCMs) in the microfluidic chip. The continuous phase was 5 wt% a SDS (Sigma-ldrich) aqueous solution. 5 wt% SDS and 2 wt% TEPA (BASF) were dissolved in the ultra-pure water to form the collecting phase. 2.2 Device preparation. For the assembly of single-channel of capillary microfluidic device, two tapered cylindrical capillaries were integrated in a square capillary. The left cylindrical capillary was tapered by puller (P-1000, Sutter Instrument) and then carefully sanded to have 80-µm-orifice or 120-µm-orifice. 2-[methoxy(polyethyleneoxy)propyl] trimethoxy silane (Sigma-Aldrich) was applied to selectively render inner surface of the capillary hydrophilic, the outer wall of the capillary was modified with trimethoxy(octadecyl)silane (Sigma-Aldrich) to render it hydrophobic. The right cylindrical capillary was tapered and sanded to have 240-µm-orifice. The whole surface of the capillary was treated with 2-[methoxy(polyethyleneoxy)propyl] trimethoxy silane to render it hydrophilic. These two cylindrical capillaries were coaxially assembled in a square capillary, the left and right cylindrical capillaries kept a tip to tip distance of 200 µm. For the assembly of double-channel capillary microfluidic device, two identical cylindrical capillaries with a orifice of 60 µm which followed the same modification procedure as the left capillary in the single-channel chip were side-by-side inserted into the square capillary from the left side, a hydrophilic cylindrical capillaries with a orifice of 240 µm was inserted into the square capillary from the right side, the left and right cylindrical capillaries kept a tip to tip distance of 200 µm. For the assembly of triple-channel capillary microfluidic device, three identical cylindrical capillaries with a orifice of 40 µm which followed the same modification procedure as the left capillary in the single-channel chip were side-by-side inserted into the square capillary from the left side, a hydrophilic cylindrical capillaries with a orifice of 240 µm was inserted into the square capillary from the right side, the left and right cylindrical capillaries kept a tip to tip distance of 200 µm. For these three microfluidic devices, the interstices between the left capillary and square capillary remained open, while the interstices between the right capillary and square capillary was sealed by epoxy resin. 4


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2.3 Preparation of PWPCMs. For the preparation of PWPCMs based on the single-channel of capillary microfluidic device, the dispersed phase of red, green or blue PC solution was injected into the left capillary. For the preparation of PWPCMs based on double-channel of capillary microfluidic

Figure 1. (a) A single-channel microfluidic capillary chip for encapsulation of PCs to form SWPCDs, single-emulsion SWPCDs are generated at the junction of the dispersed phase and the continuous phase. (b) The production process of PWPCMs by melting interfacial polymerization and room temperature ripening. (c, d, e) Optical microscopy images of red PWPCMs (Vd=2 µLmin−1, Vc=40 µLmin−1), green PWPCMs (Vd=2 µLmin−1, Vc=80 µLmin−1) and blue PWPCMs (Vd=2 µLmin−1, Vc=160 µLmin−1). (f, g, h) Orthogonal polarized optical microscopy images of red PWPCMs (Vd=2 µLmin−1, Vc=40 µLmin−1), green PWPCMs (Vd=2 µLmin−1, Vc=80 µLmin−1) and blue PWPCMs (Vd=2 µLmin−1, Vc=160 µLmin−1) in the transmission mode at 25 °C. device, the dispersed phases of red and blue PC solutions were independently injected into the left two capillaries. For the preparation of PWPCMs based on the triple-channel of capillary 5



microfluidic device, the dispersed phases of red, green and blue PC solutions were independently injected into the left three capillaries. The continuous phases in the singlechannel, double-channel and triple-channel were injected into the interstices between the left capillary and square capillary. The flow rates of the dispersed phase and continuous phase were independently controlled by syringe pumps (Harvard Apparatus).The collecting phase was set outside the outlet of microfluidic device to collect the surfactant wrapped photonic crystalline droplets (SWPCDs). Then the collected SWPCDs were heated to 80 °C to form surfactant wrapped isotropic photonic crystalline droplets (SWIPCDs) and to initiate the interfacial polymerization of the TDI and TEPA to result polymer wrapped isotropic photonic crystalline microcapsules (PWIPCMs). Finally, the PWIPCMs gradually formed PWPCMs by spontaneous self-assembly when decreasing the temperature from 80 °C to room temperature. 2.4 Characterization. The observation of polarized optical textures and reflected optical images of PWPCMs was conducted by using polarized optical microscope (POM) (Olympus, BX51). The Bragg selective reflection experiment of PWPCMs was carried out with UV-Vis-NIR spectrophotometer (PerkinElmer, Lambda 950). The rheological behaviors of CNLCs at different temperatures were analyzed by rotary rheometer (Malvern, Kinexus Pro). The scanning electron microscopy (SEM) of PWPCMs was performed using a JEOL JSM-6700F field emission scanning electron microscope. X-ray diffraction (XRD) data of PC and PWPCMs were obtained with a powder diffractometer (Empyrean, PANalytical) using unfiltered Cu Kα radiation (λ=1.5406 Å) at 45 kV and 40 mA. The weight loss curve of polyuria (PU) was determined by a NETZSCH TG 209C instrument at a heating rate of 10 °C min−1 in nitrogen. The optical images of PWPCMs in vials were obtained by a digital camera (Canon, 6D).The temperatures in the whole study were controlled by a Linkam heating and cooling stage at a rate of 5 °C min−1. 3. RESULTS AND DISCUSSION The universally applicable methodology of incubating stable and mono-dispersed PWPCMs is mainly conducted by the following three steps as shown in Figure 1, which are microfluidic-based fabrication of SWPCDs, melting interfacial polymerization of SWIPCDs to form PWIPCMs and room ripening of PWIPCMs to form PWPCMs. Firstly, to produce O/W single-emulsion colored droplets with precise controllability and strictly mono-dispersed size, we have designed capillary microfluidic setup, two tapered cylindrical capillaries are 6


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Figure 2. (a, b, c) Sets of photographs of red, green and blue PWPCMs produced by singlechannel microfluidic capillary chip, Vd is fixed at 2 µLmin−1, Vc varies from 40 µLmin−1 to 360 µLmin−1. (d, e, f) Size distribution of red, green and blue PWPCMs prepared at different flow rates. (g, h, i) Flow rate dependence of the average size of strictly mono-dispersed red, green and blue PWPCMs. 7



assembled in a square capillary, where the inner surface of left cylindrical capillary is rendered to be hydrophobic and the inner surface of the right one is rendered to be hydrophilic. The dispersed phases in this work are red PC solution (a host NLC containing 20 wt% chiral dopant S811 and 2 wt%TDI), green PC solution (a host NLC containing 23 wt% chiral dopant S811 and 2 wt%TDI), and blue PC solution (a host NLC containing 25 wt% chiral dopant S811 and 2 wt%TDI), respectively. The continuous phase is a SDS aqueous solution, the collecting aqueous phase out of the microfluidic chip is a mixture of deionized water, 5 wt% SDS and 5 wt% TEPA. The PC solution is injected through the left hydrophobic capillary, whereas the continuous aqueous phase is simultaneously injected through the interstice between the cylindrical capillary and square capillary. The PC solution is divided into SWPCDs through single-step emulsification of surfactant aqueous phase at high flow rates. Secondly, the newborn individual SWPCDs flowed into the collecting aqueous phase to obtain a large number of droplet piles and followed by maintaining the collecting phase at 80 °C for 6 hours to form PWIPCMs, during this process, PC solution changes from anisotropic state with high viscosity into isotropic state with low viscosity to increase the free diffusion of TDI towards the interface between SWPCDs and collecting phase and promote the interfacial polymerization of TDI and TEPA, the resulted polyurethane (PU) layer can effectively prevent the fusion of adjacent microcapsules and the overflow of inner core materials, on the contray, the SWPCDs without PU layer can easily fuse together when collected in water due to the unstable wrap of surfactant layer (see Figure S1). Finally, the PWPCMs are obtained by keeping solidating the microcapsule shells at room temperature for 24h, the mesogens in isotropic solution revive the periodic PC structure by spontaneous selfassembly. The resulted red, green and blue PWPCMs fabricated by single-channel microfluidic chip are shown in Figure 1c-h, where the volumetric flow rates of the dispersed phase (Vd) and continuous phases(Vc) for the preparation of red, green and blue PWPCMs are set to be (2 µLmin−1, 40 µLmin−1), (2 µLmin−1, 80 µLmin−1) and (2 µLmin−1, 160 µLmin−1) respectively, the optical images in Figure 1c-e taken by stereo microscope indicate that the PWPCM sizes are strictly mono-dispersed, PWPCMs present a brilliant Bragg reflection in Figure 1f-h due to the periodical helical assembly of rodlike molecules inside the PWPCMs and ultrahigh light transmittance of PU layers. In the process of forming microcapsules, a key point is to utilize microfluidic chips as standard templates to continuously and controllably fabricate mono-dispersed emulsions. 8


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However, large viscosity of dispersed phase frequently leads to the formation of long jet or short jet emulsion due to the slow shear dynamics on the O/W interface, and the resulted emulsions inevitably exhibit wide size distribution. As a result of the dense molecular stacking in PC solutions, which consists of highly viscous NLC fluid, chiral dopant S811 powder and a small amount of TDI, the viscosities of red, green and blue PC solution at 25 °C are as high as 42.17 mPs, 48.60 mPs and 59.97 mPs (see Figure S2). When the preparation of PC droplets are conducted at 25 °C, the high viscosity of PC solution is a disadvantage in the process of forming droplets with small size polydispersity. In order to ensure the resulted PWPCMs possess as small as possible size polydispersity, we utilize single-emulsion microfluidic chips to simplify the droplet formation and avoid the uneven sizes, meanwhile, the Vc is set to be far larger than Vd (Vc/Vd≥20) to enhance the interface dynamics and induce the templated SWPCDs. In order to figure out the flow rate range for template preparation of mono-dispersed single-emulsion SWPCDs, the influence of flow rates on the microcapsule sizes is investigated as summarized in Figure 2. Mono-dispersed microcapsules are generated within a relatively wide Vc range (from 40 µLmin−1 to 360 µLmin−1) when Vd is fixed at 2 µLmin−1, the diameters of mono-dispersed single-emulsion microcapsules can be controlled by adjusting the flow rates, and the flow rate dependence of microcapsule sizes is generally obey a power exponential function. The influence of viscosity of dispersed phase is analyzed in Figure 2h and 2i when maintaining the same chip structure and the same flow rate parameters, the average sizes of blue PWPCMs are obviously larger than those of green PWPCMs, the reason for this result is that the large viscosity of dispersed phase induces small shear force to form individual emulsion droplets from bulk dispersed phase. The sizes of PWPCMs also can be tuned by changing the chip size, as shown in Figure 2g and 2h, the orifice sizes of chips for generating red and green PWPCMs are 120 µm and 80 µm, respectively. Even the viscosity of green PC solution is larger than that of red PC solution, the average sizes of green PWPCMs are smaller than those of red PWPCMs when maintaining the same flow rate parameter. The size distribution of PWPCMs at fixed Vc and Vc is extremely narrow, and the coefficients of variation (CVs) of PWPCMs in Figure 1f-h based on the following equation are calculated as small as 0.142%, 0.173% and 0.134%,  n (di − d )  ∑  d i =1 n   2

CV =

1

1 2

(1)

× 100%

9



where di, d and n are diameter of single PWPCMs, average diameter of PWPCMs fabricated at fixed flow rates and number of statistic PWPCMs. It is worth noting that CVs of PWPCMs in this work are all below 1% (see Figure S3). The reason for this sufficiently small size variation of PWPCMs can be ascribed to the simplified microfluidic setup only with single emulsification31 and the distinctive immiscibility between the dispersed phase (organic CNLCs) and the continuous phase (5wt% SDS aqueous solution).

Figure 3. (a) Scanning electron microscopy (SEM) image of the purified red PWPCMs with a radius of 125 µm. (b) X-ray diffraction (XRD) patterns of red, green and blue PC film and free-standing red, green and blue PWPCMs on a glass substrate. (c) Thermogravimetric analysis thermograph and corresponding derivatives thermal analysis of PU prepared by polymerization of aqueous solution of TEPA and tetrahydrofuran solution of TDI. (d, e, f) Orthogonal polarized optical microscopy images of red PWPCMs in the transmission mode from 25 °C to 200 °C and back to 25 °C. (g, h, i) Orthogonal polarized optical microscopy images of red PWPCMs in the transmission mode from 25 °C to 210 °C and back to 25 °C. The PWPCMs in the collecting solution are purified by deionized water, and then deposited on the flattened wafers for SEM characterization, result shows that the as-prepared 10


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PWPCMs possess free-standing feature on a hard substrate without collapse, and remains the complete spherical structure out of the liquid environment (see Figure 3a). The PWPCMs also exhibit ultra-smooth surface due to the in-situ polymerization around the surfactant layer. As the PC cores enclosed inside the polymer shell are composed of ordered rod-like moleculars, the red, green and blue PC solutions before being wrapped exhibit intensive diffraction peaks located around 20°, which is attributed to the orientational order of layered arrangement of PC domains. When PC bulk solutions are splited into emulsion droplets and followed by melting interfacial polymerization and room temperature ripening, the red, green and blue PWPCMs still show diffraction peaks around 20°, indicating that PCs inside the microcapsules maintain the chiral layered arrangement. Based on the first order Bragg diffraction as following, (2)

2dsinθ = nλ

Where n is equal to 1, λ is the wavelength of incident X ray 0.154 nm. The interlayer spacing d of PCs can be figured out as 2.25 Å, which proves the dense stacking of PCs in threedimensional spherical confinement. The thermal stability of free-standing PWPCMs endowed by the PU layer can be identified by observing their deformation at high temperatures. Results show that the red PWPCMs reach an isotropic state and present dark state under crossed polarized light when the temperatures (200 °C) are much higher than the clearing point (73 °C) of red PC solution. When the temperature is cooled from 200 °C back to 30 °C, the shape of red PWPCMs are basically the same with that of the original one (see Figure 3d-3f). However, when the temperature is heated to 210 °C and then cooled back to 30 °C, the red PWPCMs occur obvious shape deformation (see Figure 3g-3i) as a result of the fact that the PU layer begin to decompose around 210 °C (see Figure 3c). CNLCs self-assembling into single-helix structure can be regarded as one-dimensional PCs. Selective reflection of circularly polarized light according to Bragg’s law (see equation 3 and 4) is the most important optical property of PCs, where λ is the central reflection wavelength of reflection spectra, n̅ is the average refractive index of the CNLC medium, P is defined as the distance over which the director of CNLCs rotates by a full 360°. HTP is the helical twist power of chiral dopant.32 (3)

λ = nP

P=

1 HTP*c

(4)

The PWPCMs dispersed in the collecting phase are washed 5 times to completely remove the surfactants and unreacted TEPA and followed by vacuum drying on a quartz glass at 25 °C 11



for 24h to obtain dried PWPCMs. Red, green and blue PWPCMs all exhibit sharp and symmetrical reflection peaks in the visible area, it should be that the melting interfacial polymerization and room temperature ripening do not affect the chemical structure of PCs, and PC domains keep the chiral arrangement inside the regular spherical shell at 25 °C. λ of

Figure 4. (a, b, c) Bragg reflectance spectra of the red, green and blue PWPCMs with different sizes at 25 °C. (d) The correspondence between reflection peak positions and the average sizes of red, green and blue PWPCMs. red, green and blue PWPCMs in Figure 4a-c are around 680 nm, 560 nm and 440 nm, respectively, as n̅ of red, green and blue PWPCMs are measured to be around 1.685, based on equation 3 and 4, where c is the concentration of chiral dopant in PC solution (20% for red PWPCMs, 23% for green PWPCMs and 25% for blue PWPCMs as mentioned above), the HTP in red, green and blue PWPCMs can be figured out as -11.70 µm-1, -13.08 µm-1 and 15.32 µm-1, respectively (Minus sign means the left handness). However, the peak positions of reflectance spectra for red, green or blue PWPCMs have a small deviation when the PWPCM sizes change within a large range. This phenomenon can be ascribed into the fact that the path of incident light between neighboring PWPCMs is different due to the difference 12


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of microcapsules sizes, the slight fluctuation of reflectance in Figure 4a-c is mainly caused by the different amount of tested PWPCM s in the optical detection path.

Figure 5. (a) Double-channel microfluidic capillary device for simultaneous injection of two distinct PC solutions to generate colored PWPCMs, where the channels above and below contain the red PC solution with a S811 concentration of 20 wt% and the blue PC solution with a S811 concentration of 25 wt%. (b) Sets of photographs of colored PWPCMs produced by double-channel microfluidic capillary chip suspended in the middle of pure water, VR+VB is maintained at 2 µLmin−1, Vc is fixed at 300 µLmin−1. The pitch and reflection color of the PWPCMs at room temperature can be controlled by changing the concentration of the chiral dopant, a higher concentration leads to a shorter chiral pitch and a reflection color at smaller wavelength. Therefore, the colors of the PWPCMs can be controlled by simultaneously injecting distinct PC solutions with different concentration of chiral dopant to form an integrated core. To accomplish such, we designed and assembled a double-channel microfluidic device of which the injection capillary had two parallel channels, which called double-channel chip as shown in Figure 5a. The blue PC solution containing 25 wt% chiral dopant and and 2 wt% TDI, and the red PC solution containing 20 wt% chiral dopant and 2 wt% TDI are independently injected into the two channels, and are then merged to form single emulsion droplets at the tip of the injection capillaries. The as-prepared SWPCDs are then experienced the melting interfacial polymerization and room temperature ripening to incubate PWPCMs with various colors. The chiral dopant concentration and the reflected colors of PWPCMs are determined by the flow 13



rates of the red and blue PC solutions. In the two-channel chip, the flow rates of blue and red PC solutions are set to be (2 µLmin−1, 0 µLmin−1), (1.7 µLmin−1, 0.3 µLmin−1), (1.3 µL min−1

Figure 6. (a) Reflectance spectra of the colored PWPCMs incubated at different flow rate of (VR, VB) on glass substrate at 25 °C. (b) Flow rate dependence of the average size of strictly mono-dispersed colored PWPCMs incubated by double-channel microfluidic capillary chip and the correspondence between reflection peak positions of colored PWPCMs and the flow rate formulations of (VR, VB), the insert shows the reflected images of PWPCMs. , 0.7 µLmin−1), (1 µLmin−1, 1 µLmin−1), (0.7 µLmin−1, 1.3 µLmin−1), (0.3 µLmin−1, 1.7 µLmin−1) and (0 µLmin−1, 2 µLmin−1), while the flow rate of continuous phase is maintained at 300 µLmin−1. The corresponding optical images of PWPCMs are shown in Figure 5b (suspended in the middle of pure water), red and blue PWPCMs as shown in the last and first optical microscopy images in Figure 5b can be prepared by modulating only one of the two channels. Fully operating two channels, the reflection color is adjusted between the blue and red according to the flow rates of red and blue PC solutions. The reflectance spectra of these seven distinctive PWPCMs are shown in Figure 6a. The peak position (λ) of Bragg selective reflection can vary from 680 nm to 425 nm with the increase of VB from 0 µLmin−1 to 2 µLmin−1, the PWPCMs with different colors are shown in Figure 6b, the reason for this blueshift of peak positions is that the pitch of PCs is inversely proportional to dopant concentration, larger VB leads to higher chiral dopant concentration in the red/blue PC mixture. As the flow rates of total dispersed phase (VR+VB) and continuous phase are kept at 2 µLmin−1 and 300 µLmin−1, the sizes of PWPCMs produced at different VR/VB by twochannel chip have no significant differences as shown in Figure 6b, the slight size increase from 78 µm to 84 µm is caused by the increased concentration of blue PC solution with higher viscosity in the PC mixture. 14


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Figure 7. (a) Triple-channel microfluidic capillary device for simultaneous injection of three distinct PC solutions to generate colored PWPCMs, where the channels above, middle and below contain the red PC solution with a S811 concentration of 20 wt%, the green PC solution with a S811 concentration of 23 wt% and the blue PC solution with a S811 concentration of 25 wt%. (b) Sets of photographs of colored PWPCMs precipitated in the bottom of pure water produced by triple-channel microfluidic capillary chip, VR+VG+VB is maintained at 2 µLmin−1, Vc is fixed at 300 µLmin−1. The chromatic PWPCMs in the visible region also can be produced by mixing threeprimary PCs, based on this principle, a capillary microfluidic device, named as triple-channel chip, whose injection inlet has three parallel channels is designed and assembled as Figure 7a. Through the triple-channel chip, the red PC solution containing 20 wt% chiral dopant and 2 wt% TDI, green PC solution containing 23 wt% chiral dopant and 2 wt% TDI, and the blue PC solution containing 25 wt% chiral dopant and 2 wt% TDI are independently injected into the capillaries, and the controlled preparation procedure of PWPCMs by a triple-channel chip is similar to that by a double-channel chip. The flow rates of total dispersed phase (VR+VG+VB) is 2 µLmin−1, the formulations of VR/VG/VB are set to be (2 µLmin−1, 0 µLmin−1, 0 µLmin−1), (1.0 µLmin−1, 1.0 µLmin−1, 0.0 µLmin−1), (0.0 µLmin−1, 2.0 µLmin−1, 0.0 µLmin−1), (0.67 µLmin−1, 0.67 µLmin−1, 0.67 µLmin−1), (0.67 µLmin−1, 0.33 µLmin−1, 1.0 µLmin−1), (0.0 µLmin−1, 1.0 µLmin−1, 1.0 µLmin−1) and (0.0 µLmin−1, 0.0 µLmin−1, 2.0 µLmin−1), and Vc is kept at 300 µLmin−1. The corresponding optical images of PWPCMs are shown in Figure 7b (precipitated in the pure water), red, green and blue PWPCMs can be prepared by modulating only one of the three channels, as shown in the last, fifth and first 15



optical microscopy images in Figure 7b. Fully operating three channels, the reflection color is adjusted between the blue and red according to the formulations of VR/VG/VB. The reflectance spectra of these distinctive PWPCMs are shown in Figure 8a. The peak positions (λ) of Bragg selective reflection are 682 nm, 612 nm, 553 nm, 549 nm, 521nm, 478nm and 425 nm, the PWPCMs with different colors are shown in Figure 8b. The PWPCM sizes have slight increase from 86 µm to 92 µm with the increase of S811 concentration in the PC mixture (Figure 8b).

Figure 8. (a) Reflectance spectra of the colored PWPCMs incubated at different flow rate of (VR, VG, VB) on glass substrate at 25 °C. (b) Flow rate dependence of the average size of strictly mono-dispersed colored PWPCMs incubated by triple-channel microfluidic capillary chip and the correspondence between reflection peak positions of colored PWPCMs and the flow rate formulations of (VR, VG, VB), the insert shows the reflected images of PWPCMs. CNLCs in PWPCMs are temperature-sensitive PCs due to the reconfiguration of CNLCs at different temperatures, and the optical properties of PCs strongly depend on the selfassembled structure, thereby providing the temperature responsive property. PWPCMs can serve as colorimetric micro temperature sensors of which the reflection color indicates the temperature of their surroundings. The thin PU layers are conducive to the heat conduction and diffusion, and making the inner PC cores response to surrounding temperatures. Meanwhile, the controllable size of PWPCMs ranging from 80 µm to 145 µm enables the temperature detection in some tiny region and precision instrument, whose temperatures are inappropriate to be tested by conventional large thermometer or temperature sensors. In order to analyze the thermochromic property of PWPCMs, the microcapsules are drop-cast onto a quartz glass to research their color evolution during the heating and cooling process. The photonic crystalline microcapsule has a high uniformity within the observed area and small 16


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Figure 9. (a-f) Sets of reflected optical microscopy images of red PWPCMs with a size of 103 µm, green PWPCMs with a size of 108 µm and blue PWPCMs with a size of 105 µm on black substrates with various temperatures ranging from 30°C to their clearing points (a, c, d) and from clearing points to 30°C (b, d, f). (g)The schematics of the phase transition of the red, green and blue PWPCMs.

Figure 10. (a) Chromaticity diagrams of reflected images of red, green and blue PWPCMs with temperatures increasing from 30 °C to their clearing points. (b) Chromaticity diagrams of reflected images of red, green and blue PWPCMs with temperatures decreasing from their clearing points to 30 °C. positional variations of the color coordinates with the maximum difference of 5.25% (see Figure S7). As shown in Figure 9, red PWPCMs reflect brilliant red in a wide temperature 17



range from 30 °C to 60 °C in the heating process, the corresponding color coordinates are located around (0.58, 0.44) (see Figure 10a), the detailed information about R, G, B values and the conversion process from R, G, B values to color coordinates are listed in Table S1, when the temperature reaches 70°C, the mesogens in PWPCMs are in a transition state, most part of PWPCMs turns to be dark, indicating that the chiral orientation structure of PCs collapses to a great extent. PC core changes from a chiral nematic phase to an optically isotropic phase, and red completely disappears when increasing the temperature to 73°C. In the cooling process, the red PWPCM changes from isotropic phase to ordered phase at 73°C, when further decreasing the temperature from 70 °C to 30°C, PWPCM restores red, and the corresponding color coordinates are located around (0.57, 0.40) (see Figure 10b), the phase transition process is illustrated in Figure 9g. The green PWPCM reflects brilliant green in a wide temperature ranging from 30°C to 55°C in the heating process, and the corresponding color coordinates are located around (0.36, 0.50) (see Figure 10a), when the temperature reaches 60°C, the mesogens in PWPCMs are in a transition state most part of PWPCM turns dark. PC core is in an optically isotropic phase, and green completely disappears when increasing the temperature to 62°C. In the cooling process, the green PWPCM occurs the transition from isotropic phase to ordered phase at 62°C, when decreasing the temperature to 30°C, PWPCM restores green, and the corresponding color coordinates are located around (0.34, 0.45) (see Figure 10b). The blue PWPCM reflects brilliant blue in a wide temperature range from 30°C to 48°C in the heating process, and the corresponding color coordinates are located around (0.18, 0.07) (see Figure 10a). When the temperature reaches 53°C, the mesogens in PWPCM are in a transition state, the chiral orientation structure of PCs collapses to a great extent. PC core is in an optically isotropic phase, and blue completely disappears when increasing the temperature to 55°C. The phase transition (from chiral nematic phase to isotropic phase) temperature presents an obvious decrease from 73°C to 55°C for the red, green and blue PWPCMs, the reason for this phenomenon is that the photonic crystals inside the PWPCMs are composed of nematic liquid crystal and chiral dopant without liquid crystal characteristics. As blue PWPCMs contain the most chiral dopant, the blue PWPCMs possess the lowest phase transition temperature. In the cooling process, blue PWPCM occurs the transition from isotropic phase to ordered phase at 55°C, when decreasing the temperature to 30°C, PWPCM restores blue, and the corresponding color coordinates are located around (0.18, 0.07) (see Figure 10b). It is worth noting that the reflected images of red, green and 18


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blue PWPCMs show a reversible thermochromic feature as the corresponding chromaticity coordinates and reflectance spectra at a certain temperature in the heating and cooling process are basically the same (see Table S1-S6, Figure S4-S6), which makes the PWPCMs possess potential application in the field of tunable microlasers and stand-alone microsensors. 4. CONCLUSIONS In this work, we report the microfluidic production of TPC microcapsules consisting of CNLC core wrapped by a thin PU layer. With single-channel, double-channel and triplechannel glass capillary microfluidic chips, strictly mono-dispersed O/W single-emulsion droplets, consisting of innermost CNLC and outer surfactant layer, are fabricated in the wide flow rate range, and are then subjected to melting interfacial polymerization and room temperature ripening, thereby creating a PU layer outside the PWPCMs. The sizes of PWPCMs can be precisely controlled by adjusting the flow rates of injected fluids, the core component and reflection colors of PWPCMs can be accurately tuned in real time by changing the flow rates of PC solutions in double-channel microfluidic chip and triplechannel microfluidic chip. The PWPCMs possess excellent size mono-dispersity, the CVs are controlled below 1%. The free-standing PWPCMs have high thermal stability, the deformation temperature of PWPCMs can be as high as 210 °C. The colored PWPCMs exhibit a reversible thermochromic property, which can be applied in the field of tunable microlasers, responsive microsensors, optical display and anti-counterfeiting materials. Moreover, the highly stable PWPCMs can be used as temperature warning device to monitor the working temperature of micro photonic devices. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The fusion process of SWPCDs. The stability of PWPCMs. The temperature dependence of rotational viscosity of PC solutions. The CVs of red PWPCMs, green PWPCMs and blue PWPCMs prepared at different flow rates. The reflectance spectra of PWPCMs in heating process and in the cooling process. The RGB values, tristimulus values, and chromaticity coordinates of red, green and blue PWPCMs in the process of increasing and decreasing the temperatures. ACKNOWLEDGMENTS

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This work was supported by the National Natural Science Foundation of China (No.51736005), the Guangzhou Science Technology and Innovation Commission (No. 201807010108), Guangzhou Science Technology and Innovation Commission (grant number 2016201604030063),

Foshan

Municipal

Science

and

Technology

Bureau

project

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Table of Contents

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Chiral Photonic Crystalline Microcapsules with Strict Monodispersity

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