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This study systematically investigates the morphological appearance of azo-chiral dye-doped cholesteric liquid crystal (DDCLC)/polymer coaxial microfibers obtained through the coaxial electrospinning technique and examines, for the first time, their photocontrollable reflection characteristics. Experimental results show that the quasi-continuous electrospun microfibers can be successfully fabricated at a high polymer concentration of 17.5 wt% and an optimum ratio of 2 for the feeding rates of sheath to core materials at 25 °C and a high humidity of 50% ± 2% in the spinning chamber. Furthermore, the optical controllability of the reflective features for the electrospun fibers is studied in detail by changing the concentration of the azo-chiral dopant in the core material, the UV irradiation intensity, and the core diameter of the fibers. Relevant mechanisms are addressed to explain the optical-control behaviors of the DDCLC coaxial fibers. Considering the results, optically controllable DDCLC coaxial microfibers present potential applications in UV microsensors and wearable smart textiles or swabs.
© 2016 Optical Society of America
1. Introduction
One-dimensional micro-to-nanofibers with a core-sheath structure have attracted considerable attention in recent years because of their novel properties and intriguing applications in numerous areas [1]. Many advanced techniques have been developed to fabricate one-dimensional micro- and nanostructures with well-controlled morphologies and chemical compositions. Among these methods, the novel coaxial electrospinning technique is ideal for generating core-sheath fibers. The advantages of this method include a high fiber production rate, simple setup, and ability to control fiber diameters through different operating parameters [2]. Coaxial electrospinning uses an electrical charge and two immiscible solutions to draw very fine core-sheath fibers (typically on the micro- or nanoscale). These electrospun fibers may be used in several applications, such as biological sensors [3], drug delivery [4], and smart textiles [5].
The cholesteric liquid crystal (CLC) is a nematic liquid crystal (NLC) with a periodic helix; thus, it is considered a one-dimensional photonic crystal. The helical structure of the CLC sample can selectively reflect the circularly polarized light with the same handedness as the CLC helix within a certain reflection band (known as Bragg reflection). CLC is suitable for use in certain applications such as color filters and reflective displays [6–8].
Combination of LC and the electrospinning technique remains a challenge in the LC field. A group led by Lagerwall first fabricated thin composite fibers with an NLC core through coaxial electrospinning. Their research showed that the director was aligned along the fiber, and the nematic phase range was substantially extended through a confinement effect within the thin fibers [9]. Moreover, Enz and Lagerwall successfully encapsulated CLC inside the fibers and obtained novel fibers with entirely new optical properties, such as iridescent colors caused by selective reflection within a narrow band of the visible wavelength spectrum [5]. The aforementioned investigation indicates that electrospun LC fibers are an advanced material with potential applications in optical devices. Research on electrospun fibers with CLC is interesting because the photonic bandgap (PBG) that can be continuously and dynamically tuned or controlled in response to weak external influences, such as temperature.
This study systematically demonstrates the morphological appearances of dye-doped cholesteric liquid crystal/polymer coaxially electrospun microfibers and first investigates their photocontrollable reflective features using reflection-mode polarized optical microscopy and transmission-mode optical microscopy (R-POM and T-OM, respectively). Experimental results show that different feeding rates of the sheath polymer solution and core material can induce three different morphological appearances for the formed microfibers, namely, beading, quasi-continuous, and escaped types. Among these types, quasi-continuous LC microfibers are the most practical for further application. Quasi-continuous microfibers can be successfully fabricated at an optimum polymer concentration of 17.5 wt%, sheath-to-core material feed rate ratio of 2, 25 °C, and high humidity of 50% ± 2% in the spinning chamber. The photoisomerizable azo-chiral dye doped in CLC (DDCLC) as a core material plays a key role in the photocontrol of the reflection features of the fabricated quasi-continuous microfibers. Optical control of the reflective features of the formed electrospun microfibers is investigated in detail by changing the concentration of the azo-chiral dopant in the core material, the intensity of UV irradiation, and the core diameter of the fibers. Related mechanisms are addressed to explain the optical-control characteristics of the DDCLC coaxial fibers. Given the optically tunable properties of the DDCLC and flexibility in the selection of the fiber fabrication parameters, a new class of functional composite microfibers can be produced simply by the coaxial electrospinning method with potential use in intelligent non-woven textiles or UV sensors.
2. Sample preparation and experimental setups
The materials used to fabricate the coaxial microfibers in this study mainly include MLC-6882 (NLC, from Merck; ne = 1.5816, no = 1.4838 at 589.3 nm and 20 °C), S811 (left-handed chiral dopant, from Fusol Material), ChAD-2-S (chiral azobenzene dye with left handedness, from Beam Co.), and polyvinylpyrrolidone (PVP, Mw = 1,300,000 g/mol, from Sigma-Aldrich). The helical twisting power (HTP) of S811 in MLC-6882 is about −11.1 μm−1. The components of the three sheath solutions, including mixtures A, B, and C, are presented in Table 1. The mixture for the core solution is DDCLC, including 72 wt% MLC-6882, 20 wt% S811, and 8.0 wt% ChAD-2-S. Mixtures A, B, and C for the sheath solutions include PVP concentrations of 12.5, 15.0, and 17.5 wt%, respectively; ethanol concentrations of 87.0, 84.5, and 82.0 wt%, respectively; and an identical NaCl concentration of 0.5 wt%. NaCl is added to the electrospinning system to improve the conductivity of the sheath solution and obtain smooth fibers [10]. Each polymer sheath solution is stirred for 6 h at 40 °C to obtain good homogeneity. The prepared polymer solution and DDCLC are taken up into 3 and 1 ml syringes, respectively, and these syringes are installed on two syringe pumps (KDS 100 from KD Scientific Inc. and YSP-101 from YMC) for coaxial electrospinning.
Table 1. Prescriptions in the mixtures A, B, and C of the sheath solutions.
Figure 1 displays the experimental setup for coaxial electrospinning, including two syringe pumps, two Teflon tubes, a coaxial nozzle (commercially available from Falco Co.), a high voltage DC power supply (AU-30P1, from Matsusada), and a collector. Two syringes with filled polymer solution and DDCLC are pre-fixed on the syringe pumps. The output ends of the two syringes with refilled polymer solution and DDCLC connect with the left ends of two Teflon tubes, and the other ends of the two Teflon tubes connect with the outer and inner capillaries of the coaxial nozzle (spinneret) from the side and top of the nozzle, respectively. The diameters of the outer and inner needles are 0.96 and 0.26 mm, respectively. The spinneret is fixed to the cradle, and aluminum foil is fixed to the bottom of the cradle as the collector of the electrospun fibers. Before starting the coaxial electrospinning process, the positive electrode of the high voltage DC power supply is connected to the spinneret and the negative electrode (ground) is connected to the aluminum foil. In this work, the distance between the nozzle and the grounded collector (aluminum foil) is 10 cm, and the operating voltage is maintained at 10 kV. The feeding rates for the DDCLC and the polymer solution vary from 0.1 ml/h to 0.8 ml/h with an interval rate of 0.1 ml/h and from 0.4 ml/h to 1.4 ml/h with an interval rate of 0.2 ml/h, respectively. The temperature and humidity in the electrospinning chamber are steady at 25 °C and 50% ± 2%, respectively. A glass slide is placed over the aluminum foil to collect the microfibers after electrospinning. The morphological appearance and reflection feature of the collected microfibers on the glass slide can be observed under R-POM with crossed polarizers or T-OM with an analyzer and then recorded with a CCD camera. One UV beam and one blue beam are used to perform the experiments for the photocontrol of the reflection feature of the electrospun microfibers.
3. Results and discussion
3.1 Morphological appearances of electrospun coaxial microfibers
Numerous factors can influence the morphology of electrospun LC microfibers, including the applied voltage, polymer concentration, feeding rate, and distance between the spinneret and collector. In the present study, the applied voltage and working distance are maintained at 10 kV and 10 cm, respectively. The feeding rates for both the core and sheath materials and the concentration of the polymer sheath solution are all adjustable. Experimental results (details are shown in subsequent sections) show that three different DDCLC fiber morphologies of fibers, namely, beading, quasi-continuous, and escaped types, can be fabricated by adjusting the polymer concentration and the feeding rates of the materials for the core and sheath. Figures 2(a)–2(c) show the morphological appearances of the beading, quasi-continuous, and escaped microfibers observed under the R-POM with crossed polarizers and the T-OM with an analyzer. The T-OM images presented in Fig. 2(a) show that the formed microfibers exhibit some degree of randomly distributed unevenness and discontinuities; here, the DDCLCs are encapsulated as oval-shaped beads in the fiber cores. The bright green dots in the R-POM image are attributed to the reflection of the DDCLC beads shown in the T-OM image. This beading morphology generally appears at low concentrations of polymer sheath solution and low ratios of the feeding rates of DDCLC core solution to sheath solution.
Fig. 2 POM images of three types of electrospun microfibers: (a) beading, (b) quasi-continuous, and (c) escaped types. Upper row: Images obtained through R-POM with crossed polarizers; Lower row: Images obtained through T-OM with an analyzer. Each scale bar in the POM images corresponds to 100 μm.
The quasi-continuous fiber type presented in Fig. 2(b) indicates that the DDCLC distribution in the core of the microfibers is quasi-continuous and quite uniform. This type of fiber can be obtained at moderate concentrations of the polymer sheath solution and moderate ratios of the feeding rates of core solution to sheath solution. The escaped type of fibers presented in Fig. 2(c) is typically found at higher ratios of the feeding rates of core solution to sheath solution, such that the excess DDCLC escapes from the polymer sheath.
3.2 Variations in the morphological appearance for the electrospun microfiber in feeding rates of sheath and core solutions at different concentrations of polymer solution
The optimum conditions to achieve coaxially electrospun quasi-continuous microfibers can be obtained by adjusting the PVP concentration and feeding rates of the polymer solution and DDCLC. The rate at which the polymer solution (DDCLC) is fed into the sheath (core) is maintained within the range of vsheath = 0.4–1.4 ml/h (vcore = 0.1–0.8 ml/h), with an interval of 0.2 ml/h (0.1 ml/h). Three different concentrations of PVP solution (i.e., 12.5, 15, and 17.5 wt%) are prepared for this experiment. Results of the variations in the morphological appearance of the formed coaxial fibers under different feeding rates of the sheath and core solutions are displayed in Figs. 3(a)‒3(c). As shown in Fig. 3(a), only escaped and beading types of microfibers can be obtained at 12.5 wt% PVP solution. The higher the vcore (vsheath) is, the higher the probability for obtaining escaped (beading) microfibers becomes. The beading (escaped) region in Fig. 3(a) broadens if the vsheath/vcore ratio increases (decreases). The microfibers with the beading type after the coaxial electrospinning usually appear at a relatively slow feeding rate of the DDCLC into the core. This fiber morphology can be enhanced, even into a quasi-continuous type, if the vcore slowly increases. However, the expected results are not realized. Figure 3(a) indicates that the changes in the feeding rates are unsuitable for the formation of quasi-continuous microfibers when the PVP solution concentration is as low as 12.5 wt%. Scalia et al. in 2013 first proposed that the formation of droplets in the LC microfibers can be attributed to the mismatch in the elongational viscosities between the inner and outer fluids during coaxial electrospinning [11]. Such droplets formed in the electrospun LC microfibers of the present work can be eliminated by adjusting the concentration and thus the viscosity of the outer PVP solution. Related results are discussed in the following paragraphs.
Fig. 3 Variations of the morphological appearance for the electrospun microfibres at conditions of different feeding rates (vsheath and vcore) and at (a) 12.5 wt%, (b) 15 wt%, and (c) 17.5 wt% PVP solution.
Figure 3(b) shows the variation in the morphological appearance of the formed coaxial fibers in feeding rates of sheath and core solutions at 15 wt% PVP solution. These experimental results are similar to those observed when the concentration of PVP solution is 12.5 wt%, that is, only beading and escaped LC microfibers can be formed. Compared with the abovementioned results obtained in 12.5 wt% PVP solution, the obtained microfibers in 15 wt% PVP solution are more ineffective in forming the escaped type even if the vsheath is as high as 1.0 ml/h. This result indicates that the increased polymer concentration can effectively suppress the escaped effect during the coaxial electrospinning process.
Figure 3(c) shows the variation in the morphological appearance of the formed coaxial fibers at different feeding rates if the PVP solution concentration increases to 17.5 wt%. Based on the results, quasi-continuous type microfibers can be successfully generated at certain ranges of the vsheath to vcore ratio. The quasi-continuous region shown in Fig. 3(c) broadens as both vsheath and vcore gradually increas. This result indicates that both higher feeding rates of the core and sheath solutions are beneficial for the formation of quasi-continuous LC microfibers in the coaxial electrospinning process. Although the vsheath to vcore ratio for the formation of quasi-continuous microfibers cannot be predicted precisely, this result is guaranteed to have a ratio of 2, provided that the vsheath and vcore values are higher than 0.4 ml/h and 0.2 ml/h, respectively.
In summary, 17.5 wt% is the ideal polymer concentration for generating practical quasi-continuous coaxial microfibers in the electrospinning process. Notably, the measured humidity and temperature in the electrospinning chamber during the electrospinning process are 50% ± 2% and 25 °C, respectively. Humidity is an important factor influencing the evaporation rate of the solvent in the electrospinning process [12]. The high humidity is the main reason why a high PVP concentration of 17.5 wt% is required for the formation of quasi-continuous fibers in this work, compared with the low PVP concentration (12.5 wt%) used in previous studies [5]. However, the precise ratio between the feeding rates of the polymer sheath and DDCLC core for obtaining quasi-continuous coaxial microfibers is difficult to determine. The optical-controlling experiment introduced later is performed based on the following effective examinations of polymer concentration and feeding rates to obtain quasi-continuous LC microfibers.
3.3 Discussions of morphological appearance of electrospun microfibers at a constant polymer concentration or constant ratio of vsheathto vcore
This section discusses the influence of the ratio of vsheath to vcore at a constant polymer concentration or of the polymer concentration at a constant ratio of vsheath to vcore on the morphological appearance of electrospun microfibers. Figures 4(a)–4(g) show the R-POM images of the electrospun microfibers obtained with a moderate vsheath = 1.0 ml/h and vcore = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 ml/h, respectively, at a constant polymer concentration of 17.5 wt%. The LC fiber morphology is characterized by successive appearances of beading, quasi-continuous, and escaped types if vcore increases from 0.1 ml/h to 0.7 ml/h. When vcore increases from 0.1 ml/h to 0.3 ml/h, the beads gradually transform into elongated segments, as shown in Figs. 4(a)–4(c), respectively. The length of the segments increases as the feeding rate of the core increases.
Fig. 4 (a)−(g) R-POM images of the electrospun microfibers obtained at vsheath = 1.0 ml/h and vcore = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 ml/h, respectively. (h) T-OM image with analyzer corresponding to (g). Scale bars in all images correspond to 100 μm.
Once the core feeding rate is increased from 0.3 ml/h to 0.4 ml/h, the segments gradually transit to quasi-continuous microfibers. At vcore = 0.4–0.6 ml/h, the fiber morphology remains in a quasi-continuous type as indicated in Figs. 4(d)–4(f). The colors (e.g., blue, green, and yellow) that are reflected from the microfibers can be observed under R-POM. This condition indicates that the DDCLCs are filled in the cores to form quasi-continuous microfibers and exhibit a radially helical structure. The discrete variation in the selective reflection from the DDCLC microfibers is caused by the confinement of the polymer sheath on the CLC [5]. The fiber morphology transits from the quasi-continuous to escaped type [Figs. 4(f)–4(g)] by continuously increasing the feeding rate of the DDCLC core from 0.6 ml/h to 0.7 ml/h. This condition can be attributed to DDCLC overflowing out of the microfibers when the core feeding speed is excessively fast. The escaped appearance of the fibers is particularly obvious observed under the T-OM with an analyzer, as displayed in Fig. 4(h).
Figures 5(a)–5(e) show the R-POM images of the electrospun microfibers at constant vsheath/vcore = 1.4 ml/h:0.7 ml/h, 1.2 ml/h:0.6 ml/h, 1.0 ml/h:0.5 ml/h, 0.8 ml/h:0.4 ml/h, and 0.6 ml/h:0.3 ml/h, respectively, when the PVP concentrations are 12.5, 15, and 17.5 wt%. The morphology of the electrospun microfibers successively transforms from beading to segment, and then into quasi-continuous type if the PVP concentration increases from 12.5 wt% to 17.5 wt% at any abovementioned constant ratio vsheath:vcore of 2:1, although both the vsheath or vcore values vary. When the elongation viscosities between the inner and outer fluids of the microfibers are matched, the quasi-continuous microfibers can be obtained as described in Section 3.2. The PVP concentration of 17.5 wt% used to fabricate the microfibers conforms to these conditions.
Fig. 5 R-POM images of the electrospun microfibers at (a)−(e) vsheath = 1.4, 1.2, 1.0, 0.8, and 0.6 and vcore = 0.7, 0.6, 0.5, 0.4, and 0.3 ml/h, respectively, if the PVP concentration is 12.5 wt%, 15 wt%, and 17.5 wt% (left, middle, and right rows). The scale bars in all images are 100 μm.
3.4 Photocontrollable reflective features of electrospun microfibers
3.4.1 Discussion of the LC structure in the electrospun microfibers
Enz et al. previously demonstrated four possible director configurations of LC structure inside the core of a quasi-continuous fiber: radially and axially helical structures and unwound structures along and normal to the fiber axis [5]. In the radially helical structure, the helical axis of the CLC is perpendicular to the fiber axis with a planar anchoring at the core-sheath interface. Such a configuration along any radial axis is similar to a one-dimensional planar texture. Given the confinement of the wall of the fiber, the helical pitch of the CLC in the fiber may be forced to expand or compress from its natural pitch. Therefore, the reflective coloration of the CLC fiber is discrete and depends on the fiber core diameter. If the core diameter is on the order of the natural CLC pitch length or larger, selective reflection occurs. The helical axis of the CLC may be along the fiber axis if anchoring at the core-sheath interface is weak. This type of CLC configuration in the fiber exhibits strong birefringence. Unwound LC structures along and normal to the fiber axis can be obtained in a strong anchoring condition in an extremely thin fiber or a strong forced alignment by the presence of externalities, such as a strong electric or magnetic field. In the four types of LC structure in the electrospun coaxial fibers, only the first type (radially helical structure) has the property of selective reflection laterally exhibited in the core of the fibers. This study fabricates electrospun coaxial fibers with DDCLC core, which has a natural pitch smaller than the core diameter and possesses selective reflection in the visible region. The results imply that the LC structure in the core of the fibers is a radially helical structure.
3.4.2 Optical control of reflection characteristics of electrospun microfibers with different concentrations of azo-chiral dopant under the irradiation of one weak UV beam
Given that the PBG for the electrospun microfibers is difficult to measure directly, a planar DDCLC cell is used to measure the reflection spectrum that imitates that of the DDCLC core in the electrospun microfibers. Figure 6 shows the variation in the reflection spectrum under UV irradiation based on a 4 μm-thick planar DDCLC cell with anti-parallel alignment. The original reflection band of the DDCLC (black curve in the figure below) can red-shift to a longer wavelength region after UV irradiation. The central wavelength of the DDCLC continuously red-shifts from 550 nm to 628 nm after UV irradiation with an intensity of 867 µW/cm2 for 5 min. After continuous UV irradiation, the rod-like trans isomers of the azo-chiral dopant in the DDCLC can continuously transform into bended cis isomers through trans-cis photoisomerization. Considering the variations in the conformations and the discrepancy between the intermolecular interactions of the LC molecules and two isomers, the HTP values of the trans and cis azo-chiral isomers in identical NLC are different. In general, the HTP value of the trans isomer is larger than that of the cis one, which leads to the elongation of the pitch after UV irradiation and the red-shift of the PBG. When the intensity of UV irradiation is strong, the PBG feature of the DDCLC varies in a different way. As shown in Fig. 6(b), the PBG of the DDCLC red shifts while its reflectance decreases after UV irradiation with an intensity of 2.75 mW/cm2 for 25 s. The decreasing reflectance implies that the planar texture of the DDCLC gradually collapses after strong UV irradiation.
Fig. 6 Variation of the reflection spectrum of a 4-μm-thick DDCLC planar sample when the sample is irradiated by one UV beam with (a) weak intensity of 867 µW/cm2 for 5 min and (b) strong intensity of.2.75 mW/cm2 for 25 s.
Direct measurement of the reflection spectrum of a single electrospun DDCLC fiber is difficult to achieve because the electrospun DDCLC coaxial microfibers are very thin (≤ a few micrometers) and cylindrical. Moreover, the reflection spectrum measured at a specific region that is full of randomly distributed DDCLC coaxial microfibers is poorly characterized by strong scattering. Such scattering is attributed to both the mismatch between the refractive indices of the DDCLC microfibers and the air and the curved shape of the microfibers. Hence, direct measure of the data for the PBG change of the DDCLC coaxial microfibers under light irradiation cannot be accomplished. Instead, changes in the reflective features of the microfibers can be qualitatively observed and recorded using R-POM with a CCD camera.
Figure 7(a) shows the R-POM images of the DDCLC coaxial fibers with 8 wt% azo-chiral dopant under the weak UV-irradiation (intensity: 845 μW/cm2) at tuv = 0, 3, 6, and 9 min (images from left to right). The DDCLC coaxial fibers shown in Fig. 7(a) are collected for around 10 s. The initially reflective color of the fibers is not uniform (from blue to green region), similar to those images shown in Figs. 4(d)‒4(g). This result is formed likely because different electrospun fibers cannot be produced with completely identical core diameter at the same time and the reflective color of the CLC core is significantly dependent on the core diameter of the microfibers [5]. Under weak UV-irradiation, the reflective colors of the blue and green microfibers with 8 wt% azo-chiral dopant gradually turn into green and red, respectively, at tuv = 9 min. The optically controlled reflective features of DDCLC coaxial fibers with higher concentrations of azo-chiral dopant (12 wt% and 16 wt%) under weak UV irradiation for 5 min are also demonstrated and displayed in Figs. 7(b) and 7(c), respectively. In addition to the faster red-shift of the reflective color of the fibers, some fibers with 12 wt% and 16 wt% azo-chiral dopant obviously darken under weak UV irradiation for 5 and 3 min, respectively. Weak UV irradiation-induced darkening of fibers with 16 wt% azo-chiral dopant is fairly extensive after 5 min, as displayed in the right-most image of Fig. 7(c). UV irradiation-induced red-shifting of the reflective color of the fibers can be attributed to pitch elongation of the CLC in the fiber core, the results of which are consistent with those based on the DDCLC planar sample [Fig. 6(a)]. The darkness of the reflection of DDCLC coaxial fibers with higher concentrations of azo-chiral dopant (12 wt% and 16 wt%) indicates the collapse of the radially helical structure in the CLC core, similar to that of DDCLC coaxial fibers containing 8 wt% azo-chiral dopant with strong UV irradiation. The other possible reason for the darkness of the reflection of DDCLC coaxial fibers is that the reflection of the fibers shifts into infrared (IR)-region after the UV irradiation. The relevant mechanism of the influence of the concentration of the azo-chiral dopant on the optical-control of the reflection features of DDCLC coaxial fibers will be explained in details later.
Fig. 7 R-POM images of the electrospun DDCLC fibers containing (a) 8 wt% of azo-chiral dopant with weak UV-irradiation (845 μW/cm2) at tuv = 0, 3, 6, and 9 min (images from left to right), (b) 12 wt% of azo-chiral dopant with weak UV-irradiation (845 μW/cm2) at tuv = 0, 1, 3, and 5 min (images from left to right:), and (c) 16 wt% of azo-chiral dopant with weak UV-irradiation (845 μW/cm2) at tuv = 0, 1, 3, and 5 min (images from left to right).
3.4.3 Optical control of the reflection characteristics of electrospun microfibers under successive irradiation of one weak/strong UV beam and one blue beam
Thin fibers with core diameters of roughly 1.25 μm and an initial blue color are selected for optical control experiments of the associated reflection features under the successive irradiation of one weak/strong UV beam and one blue beam. The relevant results and discussion are presented as follows. Figure 8(a) [Fig. 8(b)] shows the reflective appearances of DDCLC electrospun microfibers with a core diameter of ~1.25 μm under the R-POM with crossed polarizers before and after weak (strong) UV irradiation with 845 μW/cm2 (2.75 mW/cm2) for 10 min (45 s), respectively. Under weak irradiation, the microfibers slowly change their color from blue to green at all fiber regions within 10 min (see Visualization 1). By contrast, the color change from blue to green of the microfibers becomes much faster and even darker at some regions of the microfibers under strong UV irradiation for 45 s (see Visualization 2). The dark appearance for these regions observed under the R-POM with crossed polarizers reflects the collapse of the PBG and, thus, the radially helical structure of the core DDCLC.
Fig. 8 R-POM images for the reflective appearances of the DDCLC microfibers with core diameter of roughly 1.25 μm at different stages. (a) Before and after the weak UV-irradiation with 845 μW/cm2 for 10 min, after the blue-beam-irradiation with 5.53 mW/cm2 for 15 min following the UV-irradiation, and in dark for 30 min following the blue-beam-irradiation (see Visualization 1). (b) Before and after the strong UV-irradiation with 2.75 mW/cm2 for 45 s, after the blue-beam-irradiation with 5.53 mW/cm2 for 15 min following the UV-irradiation, and in dark for 2 h following the blue-beam-irradiation (see Visualization 2). Scale bars in all images correspond to 20 μm.
Since the photosensitive azo-chiral dopant in the DDCLC core of the microfibers plays a key role in optical tuning of the reflective appearance of the DDCLC microfibers under weak and strong UV -irradiation, the relevant mechanism is described based on the azo-chiral-associated model described as follows. In general, azo-chiral molecule exists as either rod-like trans isomers or kinked cis isomers. The absorption spectra of the azo material in these two states relatively differ. Figure 9 shows the evolution of the measured absorption spectrum of 1.0 wt% azo-chiral dopant dissolved in alcohol when the dopant is irradiated by a weak UV beam with 513 μW/cm2 at tuv from 0 s to 180 s. The black curve in the absorption spectra indicates that the azo-chiral dopant initially presents two absorption peaks at the UV (approximately 365 nm) and visible (approximately 442 nm) regions in the dark (tuv = 0 s). These two peaks are associated with π-π* and n-π* transitions of the azo-chiral molecule. As tuv increases from 0 s to 180 s, the concentration of cis isomers slowly increases to a steady state through weak UV beam-irradiation induced trans-cis isomerization of the azo-chiral dopant. As a result, the two absorption peaks slowly decrease and increase at 365 and 442 nm, respectively. The change rate of the absorption curve can increase if the Iuv (intensity of UV irradiation) increases. The following equation shows the relation between the increased concentration rate of the cis isomers and optical irradiation intensity:
where ∂Nc/∂t is the increased concentration rate of the cis isomer, q is the quantum efficiency of trans-cis photoisomerization, σ is the absorption cross-section, Nt (Nc) is the concentration of trans (cis) isomer, Iuv is the power density of incident light expressed in photons/cm2∙s, and τc is the lifetime of the cis isomer [13, 14]. Equation (1) shows that a higher UV irradiation intensity on the azo-chiral dopants corresponds to an increase in cis-isomer concentration rate. The twisting strength of the helical structure of the DDCLC microtube is given by the sum of the HTP values of the left-handed chiral dopant S811 and left-handed azo-chiral dopant. When the azo-chiral molecules transform at a low concentration rate from a rod-like trans state to a bent cis state under weak UV illumination [Iuv is relatively low in Eq. (1)], the twisting strength of the CLC slowly decays. This result is caused by the slight disturbance of the local order of LCs per unit time that may be attributed to the low concentration rate of the increased kinked cis isomers. Thus, the CLC pitch in the microfibers per unit time becomes slightly elongated, and the gradual red-shift of the DDCLC fiber coloration is observed with increasing tuv under low UV irradiation [Fig. 8(a)]. By contrast, numerous chiral trans isomers in the microfibers can rapidly transform to the kinked cis state at a high concentration rate under strong UV irradiation [Iuv is relatively high in Eq. (1)]. The production of a large amount of kinked cis isomers within a short period of time can considerably disturb the helical structure, resulting in severe distortion of the corresponding PBG in the core. Consequently, the coloration reflection can disappear and darken under the R-POM [Fig. 8(b)]. Equation (1) also shows that a higher concentration of the azo-chiral dopant (e.g., 12 wt%) may increase the cis-isomer concentration rate. This effect can result in faster red-shifting of the PBG of the coaxial fibers [Fig. 7(b)]. However, if the concentration of the azo-chiral trans isomer increases significantly (e.g., 16 wt%), the concentration rate of the cis isomer must greatly increase under UV irradiation, according to Eq. (1). This effect can also cause excess cis isomers to seriously perturb the helical structure, resulting in the disappearance of the PBG of the fibers [Fig. 7(c)].Fig. 9 Evolution of the absorption spectrum for 1 wt% ChAD-2-S dissolved in ethanol after successive UV-irradiation with intensity of 513 μW/cm2 for 3 min.
Figure 8(a) [Fig. 8(b)] shows the reflective appearances of the DDCLC coaxial microfibers after blue-beam-irradiation with 5.53 mW/cm2 for 15 min following weak (strong) UV irradiation and fibers in the dark for 30 min (2 h) following blue-beam-irradiation [see Visualization 1 (Visualization 2)]. Blue-beam-irradiation for 15 min following termination of weak or strong UV irradiation can induce most chiral cis isomers to transform back to the trans form through blue-beam-induced cis-trans back isomerization within 15 min. Nevertheless, although most chiral cis isomers can convert back to their trans form, strong confinement in the core of the thin microfibers can impede the LC structure from recovering back to its initial state after blue-beam-irradiation. On account of the distortion of the LC structure after strong UV irradiation, a longer recovery time after blue-beam-irradiation is necessary, as shown in Fig. 8(b). In comparison with the recovery time obtained after blue-beam-irradiation, the time required by the DDCLC structure in the microfibers to revert back to their initial state in the dark thermally (with no light irradiation) exceeds 24 h. Blue-beam-irradiation may accelerate cis-trans back isomerization and promote the recovery of the LC structure in the microfibers.
To realize the influence of the diameter of the fiber core on the UV-controlling reflection features of the fibers, we perform similar optical-control experiments under weak/strong UV irradiation using thicker electrospun microfibers. Figures 10(a), 10(b), and 10(c) show the reflective appearances of DDCLC coaxial microfibers with core diameters of around 3.7 μm as observed under the R-POM with crossed polarizers before UV irradiation, after weak UV irradiation (845 μW/cm2), and after strong UV irradiation (2.75 mW/cm2), respectively. The reflection color of the thicker fibers red-shifts from the blue region to the green region under weak UV irradiation for around 17 min and darkens under strong UV irradiation for around 100 s. These results are similar to those based on thin fibers but the optically induced tuning (switching) time from blue to green (dark) increases because of the thick fibers. This result is similar to that based on similar azo-chiral doped CLC planar cells investigated by White et al. [15]. According to the Beer-Lambert law, attenuation caused by the azo-chiral dopant increases as the cell depth increases. Thereafter, the time required by thicker fibers to optically induce color tuning/switching increases.
Fig. 10 R-POM images of the reflective appearances of the DDCLC microfibers with a core diameter of roughly 3.7 μm (a) before UV-irradiation, (b) after the weak UV-irradiation of 845 μW/cm2 for 17 min, and (c) after the strong UV-irradiation of 2.75 mW/cm2 for 100 s. Scale bars in all images correspond to 20 μm.
Figures 11(a) and 11(b) show that the temperatures of the irradiated spots are 22.2 °C and 22.4 °C, respectively, before and after the strong UV-irradiation on the DDCLC fiber sample for 100 s. The small variation in temperature is reasonable because the intensity (2.75 mW/cm2) of the strong UV-irradiation is not high. Because the measured clearing point of the DDCLC is at around 39 °C which is far higher than the after-irradiation temperature of the irradiated spot, we can confirm that the strong UV-irradiation induced concomitant heat less impacts the above-mentioned optical control of the DDCLC fibers.
Fig. 11 The thermal images of the DDCLC fiber sample (a) before and (b) after the strong UV-irradiation (2.75 mW/cm2) for 100 s. The thermal images are taken with a thermal image viewer (Ti10, from Fluke).
4. Conclusion
This study demonstrates systematically the morphological appearances for the DDCLC/PVP coaxial electrospun fibres and studies for the first time their photo-controllable reflection property. Experimental results reveal the quantitative impacts of the feeding rates of the DDCLC and polymer solution and the polymer solution concentration on the morphological appearance of the formed microfibers. Quasi-continuous coaxial microfibers can be successfully fabricated at a higher PVP solution concentration of 17.5 wt% because of the high humidity (50% ± 2%) in the spinning chamber. Moreover, optical control of the electrospun fibers are examined in detail under varying concentrations of the azo-chiral dopant, intensities of UV irradiation, and core diameters of the fibers. Relevant mechanisms are referred to accurately interpret the optical-controllable behaviors of the DDCLC coaxial fibers. Given the photocontrollable characteristics of the electrospun CLC microfibers, the proposed fiber mats may be potentially used as wearable smart textiles or swabs and environmental UV sensors.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology of Taiwan (Contract number: MOST 103-2112-M-006-012-MY3) and the Advanced Optoelectronic Technology Center, National Cheng Kung University, under the Top University Project from the Ministry of Education, for financially supporting this research.
References and links
1. J. T. McCann, D. Li, and Y. Xia, “Electrospinning of nanofibers with core-sheath, hollow, or porous structures,” J. Mater. Chem. 15(7), 735–738 (2005). [CrossRef]
2. Y. Wu, Q. An, J. Yin, T. Hua, H. Xie, G. Li, and H. Tang, “Liquid crystal fibers produced by using electrospinning technique,” Colloid Polym. Sci. 286(7), 897–905 (2008). [CrossRef]
3. A. Greiner and J. H. Wendorff, “Electrospinning: a fascinating method for the preparation of ultrathin fibers,” Angew. Chem. Int. Ed. Engl. 46(30), 5670–5703 (2007). [CrossRef] [PubMed]
4. X. Lu, C. Wang, and Y. Wei, “One-dimensional composite nanomaterials: synthesis by electrospinning and their applications,” Small 5(21), 2349–2370 (2009). [CrossRef] [PubMed]
5. E. Enz and J. Lagerwall, “Electrospun microfibres with temperature sensitive iridescence from encapsulated cholesteric liquid crystal,” J. Mater. Chem. 20(33), 6866–6872 (2010). [CrossRef]
6. T. J. White, R. L. Bricker, L. V. Natarajan, V. P. Tondiglia, C. Bailey, L. Green, Q. Li, and T. J. Bunning, “Electromechanical and light tunable cholesteric liquid crystals,” Opt. Commun. 283(15), 3434–3436 (2010). [CrossRef]
7. S.-Y. Lu and L.-C. Chien, “A polymer-stabilized single-layer color cholesteric liquid crystal display with anisotropic reflection,” Appl. Phys. Lett. 91(13), 131119 (2007). [CrossRef]
8. K. H. Kim, D. H. Song, Z. G. Shen, B. W. Park, K. H. Park, J. H. Lee, and T. H. Yoon, “Fast switching of long-pitch cholesteric liquid crystal device,” Opt. Express 19(11), 10174–10179 (2011). [CrossRef] [PubMed]
9. J. P. Lagerwall, J. T. McCann, E. Formo, G. Scalia, and Y. Xia, “Coaxial electrospinning of microfibres with liquid crystal in the core,” Chem. Commun. (Camb.) 42(42), 5420–5422 (2008). [CrossRef] [PubMed]
10. J. P. F. Lagerwall, Liquid Crystals Beyond Displays: Chemistry, Physics, and Applications (John Wiley & Sons, Inc., 2012), Chapter 7.
11. G. Scalia, E. Enz, O. Calò, D. K. Kim, M. Hwang, J. H. Lee, and J. P. F. Lagerwall, “Morphology and core continuity of liquid-crystal-functionalized, coaxially electrospun fiber mats tuned via the polymer sheath solution,” Macromol. Mater. Eng. 298(5), 583–589 (2013). [CrossRef]
12. P. Lu and Y. Xia, “Maneuvering the internal porosity and surface morphology of electrospun polystyrene yarns by controlling the solvent and relative humidity,” Langmuir 29(23), 7070–7078 (2013). [CrossRef] [PubMed]
13. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, T. J. White, and T. J. Bunning, “Optically switchable, rapidly relaxing cholesteric liquid crystal reflectors,” Opt. Express 18(9), 9651–9657 (2010). [CrossRef] [PubMed]
14. J.-D. Lin, M.-H. Hsieh, G.-J. Wei, T.-S. Mo, S.-Y. Huang, and C.-R. Lee, “Optically tunable/switchable omnidirectionally spherical microlaser based on a dye-doped cholesteric liquid crystal microdroplet with an azo-chiral dopant,” Opt. Express 21(13), 15765–15776 (2013). [CrossRef] [PubMed]
15. T. J. White, R. L. Bricker, L. V. Natarajan, N. V. Tabiryan, L. Green, Q. Li, and T. J. Bunning, “Phototunable azobenzene cholesteric liquid crystals with 2000 nm range,” Adv. Funct. Mater. 19(21), 3484–3488 (2009). [CrossRef]
