During the past few decades, cholesteric liquid crystals (CLCs) with intrinsic helical configuration of molecular directors have great perspectives towards a wide range of advanced photonic applications such as brightness-enhancement devices of liquid crystal (LC) displays, diffractive optical elements, smart windows, mirrorless lasers, and sensors[
However, two main limitations still exist in low molar mass CLCs and should be overcome in colorimetric methods, that is, the durability problem caused by fluidity, which makes them difficult to be prepared as portable systems, and the temperature-sensitivity problem, which may lead to a false-positive response. To address the above-mentioned issues, studies on recording and stabilizing the helical arrangement through the employment of mechanically robust polymer networks have attracted extensive attentions[
In this work, we report on the optical response of polymerized CLC (PCLC) networks templated by the “wash-out/refill” method in the presence of organic compounds. The dynamic coloration was facilitated by two key approaches to diffuse organic compounds into the polymerized cholesteric networks. The first one is based on the alternative injection of two mutually soluble fluids, that is, a nematic LC (NLC) E7 and an organic solvent benzyl alcohol (BA), into a microfluidic channel and refilling the cholesteric scaffold integrated within in turn, therefore enabling real-time and reversible tunability. The second one is to explore the diffusion and interaction between PCLC networks and representative volatile organic compounds (VOCs, using alcohol as a model compound) with low concentration. This work is expected to extend the study of PCLCs as a dynamically tunable optofluidic reflector, visually readable sensor, or compact anti-counterfeit label in response to organic compounds.
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2. Experiments and Methods
The LC/monomer premixture was composed of E7 (71.8% mass fraction), R5011 (2.2%), RMs (25%), and photoinitiator Irgacure 651 (1%). RMs were made up of RM257, RM82, RM006, RM021, and RM010, with a weight ratio of 3:2:2:2:1[
Figure 1.Microfluidic infiltration of PCLC networks. (a) Fabrication of PCLC networks enclosed with microchannel: (i) photoalignment with SD1 coated on glass substrates, (ii) self-assembly of CLCs into helical structures, (iii) UV-induced polymerization, (iv) cholesteric scaffolds after the wash-out procedure, (v) fabrication of microfluidic device, (vi) cholesteric scaffolds after the refill procedure. (b) Solubility test of E7 and BA in glass vial. (c) Stratification of E7 and BA in the serpentine channel of the microfluidic device.
Elastic polydimethylsiloxane (PDMS) is an excellent choice for preserving optical properties of PCLC networks in microfluidic operation. Photoresist SU-8 was used in the design and fabrication of microchannels by soft lithography. The microfluidic device consists of two injection ports, a serpentine channel for fluid mixing, and a rectangular region of for containing PCLCs [Fig. 1(a-v)]. Several meticulous manipulations were required in the integration of PCLC films into microfluidic channels. The PCLC film that adhered firmly to the substrate close to UV exposure was washed thoroughly in ethanol, reshaped carefully by a blade to fit in the rectangular region, and bonded immediately after oxygen plasma treatment.
3. Results and Discussion
In the microfluidic experiments, E7 () flowed in the microchannel at a controlled flow rate by using a syringe pump, infiltrating the cholesteric scaffold, which was washed and refilled with BA () beforehand [Fig. 1(a-vi)]. BA was chosen because of its miscibility with E7, higher boiling point, and weaker volatility compared with other commonly used organic solvents like acetone, alcohol, etc. However, the mixing time should be prolonged because of a higher viscosity coefficient. As shown in Fig. 1(b), although hand shaking will accelerate the dissolution process, E7 and BA are initially separate in two layers due to slow diffusion. According to the Einstein–Smoluchowski equation and the Stokes–Einstein equation, the mutual diffusion distance of E7 and BA molecules could be calculated as[
Figure 2 shows the reflectance spectra measured at different fabrication stages by using a fiber spectrometer (Ocean Optics USB4000). The blue-shift of the reflection band occurred after the polymerization because of the shrinkage of film thickness, and the resulting contraction of helical pitch of the PCLC network formed on one single substrate as the tethering force from the other substrate was absent[
Figure 2.Reflection spectra of the PCLC network during the “wash-out/refill”’ procedure. The inset shows the corresponding micrographs in reflection mode.
By injecting E7 and BA alternatively, can be varied between and reversibly, covering the chromaticity space from blue to orange, in less than 2 min for the flow velocity of 20 µL/min [Figs. 3(a) and 3(b)]. The reflection intensity is increased by injecting E7 with higher RI and . Moreover, as shown in Figs. 3(c) and 3(d), we compared the influence of flow velocities (20, 40, 60, and 80 µL/min) on the normalized wavelength shift factor (), which reflects the relative position of wavelength shift. Here, is defined as the difference between the varying reflection center wavelength and the minimum value, and is a constant value calculated from the difference between the maximum and the minimum value of the reflection center wavelength. Apparently, the time interval at which the reflective band shifts gradually decreases as the flow velocity increases.
Figure 3.Characterization of the dynamic coloration of PCLC networks by microfluidics. Reflection spectra of the PCLC network by injecting (a) E7 and (b) BA at a fixed flow velocity of 20 µL/min. The normalized wavelength of the reflection maximum recorded as a function of time at different fluid velocities by injecting (c) E7 and (d) BA and the corresponding Boltzmann fitting curves. The insets show the corresponding relationship between ηmax and the flow velocity. Illustration of the microstructure of PCLC network when injecting (e) E7 and (f) BA.
As shown in the insets of Figs. 3(c) and 3(d), the maximum change rate of , namely the maximum slope of the fitting curves in Figs. 3(c) and 3(d), was expressed as and also monitored at different flow velocities. E7 with a higher viscosity coefficient would have a larger friction resistance and smaller diffusion coefficient in the porous polymer network, leading to a lower . Conversely, for BA with a lower viscosity coefficient, the diffusion coefficient should be larger, and the shift of the reflection band to the short wavelength was accelerated. The results coincide well with the prediction by analyzing the mutual diffusion distance from Eq. (2). In addition, was connected with the flow velocity of the injected solution in both cases. The increase of flow velocity also leads to the concentration difference maintained at peak level, which facilitates the diffusion process at the interface between the PCLC film and the fluid. As depicted in Figs. 3(e) and 3(f), the changes of and dominate the variation of the reflection band[
Furthermore, to exhibit the dynamic coloration by the diffusion of VOCs, the as-prepared UV-PCLC films with retained helical skeleton in double-open-ended glass cells with a thickness of 20 µm were subject to the volatilization of alcohol in a home-made sealed chamber. In the chamber, 10 g of alcohol () was put () and volatilized at room temperature, allowing the diffusion of alcohol molecules into the glass cell from one open end and the infiltration of the PCLC film. As shown in Figs. 4(a)–4(d), when alcohol with lower RI was diffused into the film, the wavelength of the main reflection center blue-shifted from to . The variation of reflection spectra can be well explained by Fig. 4(e), which reveals that the diffusion of alcohol in the PCLC film might possibly experience four stages within 10 h.
Figure 4.Characterization of the dynamic coloration of PCLC networks by the diffusion of VOCs. (a)–(d) Four-stage reflection spectra of the PCLC network by the volatilization of alcohol. The corresponding insets show: the dependence of cell thickness on diffusion time, the evolution of reflection spectra of the polymer-poor sample fabricated by UV irradiation for 2 s, the blue-shift of the minor reflection band contributed by the polymer-poor layer, and the blue-shift of the major reflection band contributed by the polymer-rich layer, respectively. (e) Illustration of the microstructure of the PCLC network by the diffusion of alcohol vapor.
In Stage a, an unexpected slight red-shift of the main reflection band was observed within the first 30 min, ascribed to the elongation of due to the swelling of adhesive tapes used to assemble the glass cell. The inset of Fig. 4(a) illustrates the dependence of the thickness of an empty LC cell on the diffusion time of ethanol. The pitch increased and tended to be stable at a value of after 60 min.
The uneven distribution of the polymer network was caused by UV exposure. The polymer-rich layer could be formed close to the UV light, while the polymer-poor layer is on the other side[
In Stage c, as shown in Fig. 4(c), the peak at the major reflection band centered at 590 nm was relatively stable at the long wavelength edge due to the slight change of and in the polymer-rich layer. However, the sparse network facilitated the diffusion of ethanol molecules, with a further decrease of in the polymer-poor layer and the blue-shift of the minor reflection center from 445 nm to outside the detection range [Fig. 4(c)]. The diminishment of reflection intensity can be ascribed to the decrease of . Finally, although the lateral diffusion in the polymer-rich layer from the open end was not evident, when the concentration of alcohol in the polymer-poor layer reached a suitable value, the occurrence of vertical diffusion into the polymer-rich layer was allowed in the following Stage d. The inset in Fig. 4(d) shows the continuous blue-shift of the central wavelength from 590 nm to 440 nm, reflecting the significant decrease of in the polymer-rich layer.
Additionally, as shown in Fig. 5, the aforementioned approaches were adopted to infiltrate the cholesteric scaffolds in a uniform and a gradient way, respectively, to endow the 100th anniversary logo of Xiamen University (XMU) with vivid structural colors. A microprojection system based on digital micromirror device (DMD) was used to record the XMU logo[
Figure 5.Reflection micrographs of the 100th anniversary logo of XMU fabricated by the PCLC network. (a) Uniform coloration achieved by microfluidics. (b) Gradient coloration induced by the diffusion of alcohol vapor at 35 min, 300 min, and 600 min.
In summary, we disclosed the dynamic coloration of the PCLC network facilitated by two key approaches to refill organic compounds, that is, the alternative injection of mutually soluble organic fluids into a microfluidic channel and the diffusion of organic vapor. For the first approach, the relationship between flow velocity of fluid and the optical response was studied, enabling the reversible tuning of reflection color with a central wavelength located between and . For the second approach, the influence of the duration time in UV polymerization on optical characteristics was studied and interpreted. A VOC-diffusion-induced blue-shift of the reflection center was revealed from to . We anticipate that the class of polymer-templated LC composites with dynamic coloration by infiltrating the cholesteric scaffold with organic compounds can become one of the most promising candidates for dynamically tunable optofluidic reflectors, visually readable sensors, or compact anti-counterfeit labels in response to VOCs.
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