Cholesteric liquid crystals (CLCs) spontaneously organizing into a periodic helical structure with a twist axis perpendicular to the local director can reflect selected wavelengths of circularly polarized light. The structural color can be easily controlled by manipulating the helical structure, temperature, electric field, and magnetic field. Despite the unique structure and superior performances, free-standing CLCs with confined fluidity, stimuli response, and high structure stability at high temperatures still remain a challenge. Herein, we report a simple and controllable preparation of a novel type of free-standing 3D confined polymer stabilized cholesteric liquid crystal particles (PSCLCPs) based on the microfluidic emulsification, interfacial polymerization, and UV curing. The size of PSCLCPs can be precisely controlled by adjusting the flow rates of the injected fluids in a microfluidic chip. The fluidity of CLCs is effectively restricted within the physical confinement of the polymer layer, the PSCLCPs present reversible thermal response between the cholesteric phase and the isotropic phase. The CLC domains in microcapsules possess superior microstructure stability at high temperatures (near 220 °C). The stand-alone PSCLCPs with confined fluidity, stimuli response and high structure stability at high temperatures will provide ever better performances in their tremendous applications in the field of smart photonic and electro-optical devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The application of polymer networks to stabilize and functionalize liquid crystals (LCs) in the fields of optical, electronic, mechanical and sensing materials has recently become widespread [1–4]. Composites consisting of polymer networks and liquid crystals have provided a new field of liquid crystal science and technology, there are mainly two forms of polymer/LC composites, polymer dispersed liquid crystals (PDLCs) and polymer stabilized liquid crystals (PSLCs). Different from PDLC that is isolated liquid crystal droplets dispersed in a polymer matrix [5, 6], in the process of fabricating of PSLCs, small amounts of acrylate monomers and photoinitiators are dissolved in LCs, the monomers are then in situ cross-linked in the LC matrix, leading to a polymer network randomly dispersed in the LC matrix [7, 8]. Such polymer networks formed in liquid crystalline solvent play a role in controlling the aggregation structure of LC domains, stabilizing the orientation of liquid crystal directors or freezing the ordered structure of LCs. The PSLC is exploited to make light shutter [9, 10], bistable and multi-responsive electro-optic materials [11–13], and haze-free transparent optical devices in the field of display [14, 15]. The PSLCs also can be used to fabricate fast switching and high contrast electro-optic devices .
Among the LC family, cholesteric liquid crystals (CLCs), as a one dimensional photonic crystal possess the most interesting periodic helical structures which give rise to selective Bragg reflection of circularly polarized light at the stop band, cannot propagate but is full reflected [17–20], the reflective wavelength λ is directly determined by λ = nP, where P is the helical pitch and n is the average refractive index, and the P is adjustable according to type and concentration of chiral dopants with a helical twisting power . According to the different refractive light wavelength, CLCs reflect the red (R), green (G), and blue (B) bands. Furthermore, the helical pitch is strongly sensitive to temperature and electric field [22–24], which lead to thermochromic properties and electro-optical response properties. Due to their unique characteristics, CLCs possess high application values in the fields of CLC lasers [25, 26], reflection-mode displays  and thermometers . However, the fluidity severely limits the easy processing of the CLCs, thereby restricting their high-level applications. Although polymer network has been used to locally confine the fluidity of CLCs in the polymer stabilized cholesteric liquid crystal (PSCLC) system, CLC molecules only can anchor on the polymer chains, the fluidity cannot be completely restricted . One way to overcome the limitation is to confine the CLCs in microspheres with a three-dimensional rigid network, thereby yielding CLC microspheres with high mechanical stability [30–32]. Another limitation of CLCs is their low thermal stability of the chiral structure at high temperatures due to the phase transition from a chiral phase to an isotropic phase of organic building blocks at relatively low temperatures, especially for the small molecule CLCs. Based on the previous research, the free-standing CLCs with confined fluidity and enhanced thermal stability at high temperatures still remain a challenge.
In recent years, microfluidics as an effective methodology has been widely used to fabricate optical materials and optical devices [33–37]. CLC microcapsules possessing photonic band gaps have been fabricated by double emulsification in microfluidic chips [38–42]. The preparation conditions in the micro-channels typically need precise control to form the core-shell structure microcapsules, however, the thermal stability of these CLC microcapsules exhibits no improvement. Herein, we report a simple and controllable synthesis of a novel type of free-standing 3D confined polymer stabilized cholesteric liquid crystal particles (PSCLCPs) based on the microfluidic single emulsification, interfacial polymerization and UV curing. With a capillary microfluidic device, a mixture of CLCs and photocurable monomers is emulsified into monodisperse oil-in-water (O/W) droplets, and then enclosed by a polyurethane (PU) layer through interfacial polymerization, finally, the photocurable monomers inside the PU layer are then irradiated by UV light to form polymer network dispersed in the CLC matrix and free-standing PSCLCPs. The size of PSCLCPs can be precisely controlled by adjusting the flow rates of the injected fluids, and the coefficient of variation (CV) of size is controlled below 1.5%. The fluidity of CLCs is effectively restricted within the physical confinement of PU layer, the PSCLCPs present reversible thermal response between the cholesteric phase and the isotropic phase as a result of the relatively low concentration of polymer component in the PSCLCPs. The dispersed polymer network and 3D confinement have synergistic effect on the thermal stability improvement of CLCs, the additional entrapping and solidification of CLCs within the PU shell can further enhance the thermal stability of the marginal parts of the PSCLCPs.
2. Experimental section
The CLC is composed of a host nematic liquid crystal (BHR-59001, Ba Yi Space LCD Tech. China) and a left-handed chiral dopant (S-811, Ba Yi Space LCD Tech. China) to induce a cholesteric phase, where the dopant concentration is selected to be 21.2 wt%, 25.3 wt%, and 28.6 wt% to control the photonic band position for the red, green, and blue-colored CLCs, respectively. The hydrophobic phase (dispersed phase) in the microfluidic chip is a mixture of CLC, 10 wt% (or 15 wt%) photocurable precursors, and 2 wt% 2,4-tolylene diisocyanate (TDI, Sigma-Aldrich). The photocurable precursors are composed of 1, 6-hexanediol diacrylate (HDDA, Sigma-Aldrich), trimethylolpropane triacrylate (TMPTA, Sigma-Aldrich), epoxy acrylate (EA, Sigma-Aldrich) and 3 wt% photoinitiator irgacure 1173 (BASF), the weight ratio of HDDA, TMPTA and EA is 1:1:1. In order to promote the miscibility among all the components, the CLC mixtures are stirred at their clearing points for 10 min and then cooled to room temperature. The hydrophilic phase (continuous phase) is an aqueous solution containing 5 wt% of sodium dodecyl sulfate (SDS, Sigma-Aldrich) to stabilize the interface between the dispersed phase and the continuous phase. An aqueous solution containing 5 wt% SDS and 5 wt% tetraethylenepentamine (TEPA, Sigma-Aldrich) is prepared as the collecting phase.
2.2. Preparation of polymer stabilized cholesteric liquid crystal films (PSCLCFs)
PSCLCFs are fabricated as reference samples when analyzing the structure stability of PSCLCPs. Firstly, the mixtures consisting of CLCs and photocurable precursors are injected into indium tin oxide (ITO) cells with a gap of 5 μm, after sealed by epoxy glue, the mixtures are exposed in UV light (365 nm, 20 mWcm−2) for 15 min to prepared PSCLCFs. Based on the type of CLCs and the concentration of photocurable precursors, six PSCLCFs are prepared, RPSCLCF 10%, RPSCLCF 15%, GPSCLCF 10%, GPSCLCF 15%, BPSCLCF 10% and BPSCLCF 15%.
2.3. Preparation of CLC-photocurable precursor droplets and PSCLCPs
The microfluidic device is made up of two tapered cylindrical capillaries inserted in a square capillary, as shown in Fig. 1a. The left tapered cylindrical capillary has an 80-μm-orifice, the right tapered cylindrical capillary has a 200-μm-orifice, and the outer square capillary has a size of 600 μm. All capillaries are purified with ethyl alcohol and dried in the vacuum to remove residual solvents. The dispersed phase is injected into the left tapered cylindrical capillary and the continuous phase is injected into the interstice between the left tapered cylindrical capillary and square capillary. The flow rates of the dispersed phase and continuous phase are independently controlled by two injection pumps (TYD01, Lead Fluid), and the flow rate of dispersed phase is kept at 2 µLmin−1 and the flow rate of continuous phase is adjusted from 40 to 200 µLmin−1. Thereby droplets generation in the two tapered cylindrical capillaries is observed by an optical microscope (Olympus, BDS400) equipped with a digital camera (SONY, TP310) as shown in Fig. 1b. Then the newborn CLC-photocurable precursor droplets are collected in the collecting phase and incubated for 1 h at their clearing points to improve the formation of PU shell. The polymerization of photocurable precursors in the microcapsules are performed using an UV lamp (365 nm) with an intensity of 20 mW cm−2. The UV curing is lasted for 15 min to promote the complete polymerization of the acrylates to form the polymers network in the PSCLCPs. After the UV curing, the PSCLCPs are transferred into deionized water and change from an isotropic phase to a cholesteric phase as shown in Fig. 1c.
2.4 Instruments and characterization
The optical images of PSCLCPs are obtained by a digital camera (Canon, 6D). The scanning electron microscopy (SEM) is performed to observe the micromorphology of PSCLCPs by using a JEOL JSM-6700F field emission scanning electron microscope. The fourier transform infrared spectroscopy (FTIR) is used to investigate the polymerization degree of unsaturated C = C in the photocurable precursors. The Braag selective reflection experiment of PSCLCPs is carried out with UV-Vis-NIR spectrophotometer (PerkinElmer, Lambda 950). The affinity between PU layer and PSCLC is analysed by contact angle instrument (dataphysics, OCA25). The polarized images of CLC films and PSCLCPs are observed under a polarized optical microscope (POM) (Olympus, BK-POL) with crossed polarizers and a digital CCD camera (SONY, TP310). The temperatures in this work are changed at a rate of 5 °C min−1.
3. Results and discussion
The CLCs used in this study are prepared by mixing the nematic liquid crystal with a few percent of chiral dopant S-811, whose concentration determines the pitch P, therefore, the reflected colors of CLCs can be controlled by changing the concentration of chiral dopant. As shown in Fig. 1, we report a microfluidic-based system for the controlled generation of photonic droplets containing CLCs, TDI and photocurable precursor, whose interfaces are stabilized by SDS. After the photonic droplets are collected in the collecting phase, TDI and TEPA carry out a fast interfacial polymerization to generate a very thin PU layer outside the droplets due to their high reactivity, then the polymer network can be formed by in situ UV polymerization of photocurable precursors to anchor the inside CLCs, it is worth noting that the photoinitiative polymerization is conducted at the clearing point of CLCs to ensure the uniform dispersion of photocurable precursors and the resulted polymer network in CLC matrix. After the UV curing, the temperature is decreased to 25 °C, the mesogens change from isotropic state to liquid crystalline state. With glass-capillary microfluidic devices, the O/W emulsion droplets are continuously generated at the tip of two tapered cylindrical capillaries. Compared with conventional emulsification methods, the microfluidic approach presents the great advantage of producing microcapsules with regular spherical and highly mono-dispersed sizes. The optical microscope images in Figs. 2(a)-2(f) show that red PSCLCPs (RPSCLCPs), green PSCLCs (GPSCLCPs) and blue PSCLCs (BPSCLCPs) present narrow size distribution and center symmetrical structure, proving that microfluidic chip is an excellent template to incubate PSCLCPs. Results in Figs. 2(g)-2(l) show that the maximum size difference of PSCLCPs is 15 μm, and the minimum size difference is as low as 4 μm in Figs. 2(g)-2(l). In order to precisely quantify the monodispersity of those as prepared PSCLCPs, a coefficient of variation (CV) is introduced as Eq. (1).Figs. 2(a)-2(f) are calculated to be 0.63%, 1.44%, 1.17%, 0.77%, 0.92% and 0.78%, respectively.
The production of the PSCLCPs can be precisely controlled by adjusting the flow rates according to the fluid shear effect. To study the influence of flow rates on the production of PSCLCPs, the flow rate of the dispersed phase of the CLC-photocurable precursor is fixed at 2 μLmin−1, while the continuous phase of aqueous solution containing 5 wt% of SDS is supposed to be adjustabe in a wide range to steadily form emulsion droplets in dripping mode. This is because, compared with long jetting mode, droplets generated in the dripping mode have larger mono-dispersity. The CLC/TDI/photocurable precursor is highly viscous, which frequently leads to the formation of a long jet when the flow rate is sufficiently low due to the slow dynamics of the interface. To enhance the dynamics of the interface and find an appropriate range of flow rates for producing mono-dispersed emulsion droplets, the flow rates of the continuous phase are set to be far higher than that of the dispersed phase. Besides the control of the mono-dispersity in the fabrication process, the size of the PSCLCPs can be dynamically controlled by adjusting the flow rates of the continuous phase according to Eq. (2) .Figs. 3(a)-3(c), there is an exponential relationship between the microcapsule sizes and the flow rates.
By adjusting the flow rates of the continuous phase, we can prepare PSCLCPs within a wide size range from 160 μm to 280 μm. The decreased size of the PSCLCPs is caused by the following mechanism, the continuous phase exerts a drag force on a droplet hanging on the tip of the injection capillary and its flow rate determines the frequency of the droplets generated in dripping modes, at higher flow rate of the continuous phase, a larger drag force is exerted, which leads to more frequent breakup and a smaller size of the droplets. At the same flow rate profile, we note that the sizes of PSCLCPs with higher concentration of photocurable precursors are larger than those with lower concentration of photocurable precursor. This is because the viscosity of the photocurable precursor is much larger than those of the red CLCs, green CLCs and blue CLCs, therefore, adding more photocurable precursors can increase the viscosity of the dispersed phase and the increased viscosity of the dispersed phase will lead to slow dynamics of the interface and larger size of PSCLCPs. The SEM in Fig. 3(d) shows that the PSCLCPs possess regular morphology, smooth shell and self-supporting feature on a silicon substrate. The FTIR spectrum presents that the absorption at 3009 cm−1 and 3077 cm−1 corresponding to the characteristic absorption of HC = CH bonds in the photocurable precursors disappears after the UV curing, which indicates the photocurable precursors have completely evolved into polymer network in the PSCLCPs (see Fig. 3(e)). The reflection spectra in Fig. 3(f) show that the resulted RPSCLCPs, GPSCLCPs and BPSCLCPs possess obvious reflection peaks at 402 nm, 524 nm and 608 nm, respectively, proving that the mesogens in PSCLCPs still keep their helical arrangement and reflect bright colors. We deduce that the reason for this orientation is the complete phase separation between the dispersed polymer network and CLCs, the self-assembly ability of CLCs and the stabilization effect of polymer anchoring on the molecular orientation. The color of the cross-linked PU shell is entirely different from the chromatic colors of the inside CLCs. Therefore, the thickness (T) of the PU shell is estimated to be about 4.20 μm based on the color difference (see the inserted T in Fig. 3(f)). Results show that PSCLCPs possess excellent stability in the air as well as in the water containing SDS as their structure colors can maintain for one month (see Fig. 3(g)). PSCLCPs possess poor stability in ethanol as their structure colors will gradually disappear when dispersed in ethanol as a result of the solubility of LCs in ethanol. PSCLCPs also possess poor stability in acidic or alkaline environment as a result of the degradation ability of H+ or OH- towards the PU layer, thus leading to the leakage of LCs (see Fig. 3(h)).
The UV photochemical reaction provides inner stabilization element for PSCLCPs, the encapsulation by microfluidic emulsification and interfacial polymerization provides the outer stabilization element for PSCLCPs. As reference samples, the phase transition process of CLCs and PSCLCFs are analyzed in Fig. 4, the clearing points of RCLC, GCLC and BCLC are 49 °C, 46 °C and 43 °C, the clearing points of the RPSCLCF 10%, GPSCLCF 10%, BPSCLCF 10%, RPSCLCF 15%, GPSCLCF 15% and BPSCLCF 15% are 64 °C, 62 °C, 52 °C, 70 °C, 65 °C and 59 °C. Results in Figs. 5(a)-5(b) show that RPSCLCPs 10% and RPSCLCPs 15% all reflect bright red at 35 °C, which indicates that mesogens inside the microcapsules can self-assemble into helical layered arrangement. POM images in Figs. 7(a)-7(b) and Figs. 9(a)-9(b) show that GPSCLCPs 10% and GPSCLCPs 15% reflect bright green at 35 °C, BPSCLCPs 10% and BPSCLCPs 15% reflect bright blue, indicating that mesogens inside the microcapsules also can self-assemble into helical layered arrangement. When increasing the temperature from 35 °C, the central part of the RPSCLCP begin to change from bright state to dark state, indicating an isotropic orientation of LC moleculars, the clearing points of the central parts of the RPSCLCPs 10%, RPSCLCPs 15%, GPSCLCPs 10%, GPSCLCPs 15%, BPSCLCPs 10% and BPSCLCPs 15% are 76 °C, 80 °C, 67 °C, 72 °C, 55 °C and 62 °C, respectively. While the marginal parts of the PSCLCPs still reflect red, green and blue above the clearing points of the central PSCLCPs, when the temperature reach 220 °C, all the marginal parts of PSCLCPs turn into an isotropic state and become dark. The temperature dependence of photonic band gaps of PSCLCPs in the heating process is listed in Fig. 6(a), Fig. 6(c), Fig. 8(a), Fig. 8(c), Fig. 10(a) and Fig. 10(c), the Bragg reflection gradually disappears when the temperatures increase from 35 °C to 220 °C.
In the process of decreasing the temperature from 220 °C, the marginal parts of the RPSCLCPs firstly change from isotropic state to cholesteric state reflecting red color, then when the temperature is below 60 °C, all the parts of the RPSCLCPs evolve into a cholesteric, thus forming a reversible thermal response process. The thermal-induced phase transition process of RPSCLCP 10% and RPSCLCPs 15% is basically the same (see Figs. 5(a)-5(b)). We note that the GPSCLCPs and BPSCLCPs present the similar phase transition process with RPSCLCPs. The clearing points of PSCLCPs (both the central and the marginal part) are significantly higher than those of PSCLCFs and CLCs as shown in Fig. 11, the reason for this phenomenon is that both the dispersed polymer network and 3D confinement have synergistic effect on the thermal stability improvement of CLCs (see Fig. 12). It's worth noting that the clearing points of the marginal parts of the PSCLCPs are obviously higher than those of the central parts. This phenomenon can be ascribed by the following mechanism, besides the polymer network stabilization and 3D confinement, during the interfacial polymerization of the PU shell, a part of the CLC may be entrapped and solidified with the PU shell. Therefore, it is possible to maintain the helical structure without becoming an isotropic state even at the clearing point temperature of CLC. In general, the heat-resistant temperature of thermotropic PU is about 200°C. Therefore, it is understood that the PU shell is swelled at a temperature of 220°C or more, and the alignment of the liquid crystal molecules stabilized in the PU shell is not maintained and becomes an isotropic state at a temperature of 220°C. The temperature dependence of photonic band gaps of PSCLCPs in the cooling process is listed in in Fig. 6(b), Fig. 6(d), Fig. 8(b), Fig. 8(d), Fig. 10(b) and Fig. 10(d), the Bragg reflection gradually restores when the temperatures decrease from 220 °C to 35 °C.
In summary, a novel and effective approach for fabricating free-standing 3D confined PSCLCPs based on the microfluidic emulsification, interfacial polymerization and UV curing is firstly realized. The size of PSCLCPs can be precisely controlled within the range from 160 μm to 280 μm by adjusting the flow rates of the injected fluids, and the coefficient of variation of PSCLCP size is controlled below 1.5%. The fluidity of CLCs is effectively restricted within the physical confinement of PU layer, the PSCLCPs present reversible thermal response between the cholesteric phase and the isotropic phase. The dispersed polymer network and 3D confinement have synergistic effect on the thermal stability improvement of CLCs, the additional entrapping and solidification of CLCs within the PU shell can further enhance the thermal stability of the marginal parts of the PSCLCPs. The stand-alone PSCLCPs with confined fluidity, stimuli response and high structure stability at high temperatures will provide ever better performances in their tremendous applications in the field of thermometers, anti-forgery materials, tunable LC lasers, LC display, and may open up new fields of application for smart photonic and electro-optical devices.
The Guangzhou Science Technology and Innovation Commission (No. 201807010108); Foshan Municipal Science and Technology Bureau (015IT100162); Innovative project of College Students (201711845027, 201711845154, and xj201711845085).
The authors gratefully acknowledge Qi Yan and Zhan Wei for helpful discussions and thank Zhengdong Cheng for his contribution to the construction of the early-stage experimental setup.
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