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Abstract
An advisable perfluoroalkyl acrylates functionalized system is proposed and experimentally demonstrated to drastically enhance the electric-field responsiveness of blue phase liquid crystal (BPLC), which overcomes the common tradeoff between the driving voltage and response time dependent on the polymer concentration. On one hand, a quick electrical response can be readily obtained from a denser polymer network due to the participation of perfluoroalkyl acrylates in photo-crosslinking; on the other hand, the large rising trend of driving voltage with the growing polymer concentration can be alleviated due to the reduced anchoring energy between the BPLC and surrounding polymer attributed to the lower surface tension of perfluoroalkyl acrylate. In consequence, a faster decay time of 0.54 ms and almost hysteresis-free electro-optical (E-O) performance of the BPLC is achieved, with an efficient reduction by almost half in the driving voltage.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As a ubiquitous existence in nature, the optical microstructure with periodic superstructure has become a widespread topic during past decades due to its distinct structural color and unique optical properties [1–4]. More prominently, inspired by the self-adaptive manipulation of structural color, for instance the tunable reflection color of chameleon [5,6], artificial soft superstructures composed of liquid crystals (LCs) have drawn enormous interests for great advantages such as multifunctional properties and flexible response to various external stimuli [7,8]. Therefore, it is potential in a variety of applications like fast electro-optic components [9,10], optical field modulators [11], and tunable photonic devices [8,12].
Blue phase liquid crystal (BPLC) exhibits self-organized three-dimensional soft cubic optical microstructure formed by the stacking of double twisted cylinders (DTCs) [13,14]. In detail, with the decreasing temperature, the DTCs sequentially stack into a fog-like phase [15], simple cubic lattice, and body-centered cubic lattice, referring to different subphases as BPIII, BPII and BPI [13,14]. Despite the remarkable progress made in display [16,17], micro-lasers [11,18,19], and biosensing [7,20], BPLCs normally suffer from the narrow temperature range (about 0.5∼2 °C) which limits more developments. Tremendous explorations and efficient endeavors have been devoted to broaden the temperature range of BPLCs. By means of doping monomers [21], nanoparticles [22,23] as well as modifying the geometry of LC host [24–26], the BPLCs can successfully remain stable over large thermal variations, which originates from the reduction of free energy costs in defects during temperature change [21,27]. In some cases, the temperature range can be drastically expanded over 60 °C [21], thus providing the BPLC-based multifunctional devices with improved thermal compatibility. In particular, the stabilization of BPLC by polymer networks is considered as the preferable approach due to the ability to provide broader temperature range and excellent electro-optical (E-O) responsiveness [28–30]. However, constrained by the strong anchoring energy between LC molecules and polymer networks, driving BPLCs by electrical field is burdened with the applied voltage commonly greater than 100 V [31–33]. According to the previous works, the driving voltage is dependent on the unwinding ability of the DTCs, which is directly related to the dielectric and viscous properties. While the viscous property is positively correlated to the anchoring energy between the polymer and LC [34]. Although the relative driving voltage can be reduced by decreasing the polymer content, it usually deteriorates the E-O performance of BPLCs, as evidenced by slow response time and severe hysteresis over 5% [33,35]. In view of the tradeoff of polymer content, various approaches have been demonstrated to further reduce the relative driving voltage of BPLCs, such as doping ion-suppressor [36], nano-particles [37] and configuring a hydrogen-bond-contained component to replace partial common polymer [38,39]. However, the effects are all limited due to the restriction of strong anchoring energy of common polymer has not been fundamentally alleviated.
More remarkably, by modifying the polymer properties of BPLCs with fluorine-containing compounds, not only a wide temperature range of BPLCs but also considerable improvements in E-O performances can be obtained [40–43]. Fluorine materials can effectively reduce the anchoring energy between the polymer networks and BPLCs on account of small radius and large negative charges [44], thus relieving the obstacles of LC reorientation under applied voltage. In particular, fluorinated monomers, by virtue of higher fluorine content, have been introduced to be polymerized with the common polymer system and integrally incorporated into the BPLCs [42,43]. Perfluoroalkyl acrylates, characterized as a kind of fluorinated monomer, consist of aromatic methacrylate and flexible side chains with fluorocarbon (C-F) bonds. After polymerization, the side fluorine tightly winds the main chain and forms a shielding layer [45], resulting in a lower surface tension, which further reduces the anchoring energy between the BPLCs and the polymers [46–48]. Taking advantage of the relieved constraint of perfluoroalkyl acrylates to LC molecules, the perfluoroalkyl acrylates functionalized BPLCs have presented the enhanced E-O performance in a single respect such as lower driving voltage less than 100 V [42], faster responsiveness and free-hysteresis [43]. However, the comprehensive improvement of E-O performance in all respects remains challenging.
In this work, an advisable perfluoroalkyl acrylates functionalized polymer stabilized BPLC is proposed and experimentally demonstrated to achieve the excellent E-O performance under an undemanding electrical stimulation. The side-chain fluorinated dopant enables a lower anchoring energy of polymer, thus alleviating the constrain to the BPLCs under the electrical field. The systematic investigation suggests that at a condense polymer network with a 12 wt% total polymer concentration, the driving voltage can be decreased almost in half relative to the common polymer with the same concentration, while maintaining a sub-millisecond and hysteresis-free electrical response. Furthermore, the critical relationship between the concentration of perfluoroalkyl acrylates and stability of BPLCs is studied for the first time. We anticipate this work may provide a promising approach to enhance the electrical modulation of the BPLCs, thereby opening up new possibilities and removing current obstacles for applications such as high-refresh displays and low-power tunable photonic devices.
2. Experimental
2.1 Materials
The BPLC consists of the nematic LC host TEB300 (Slichem, China, Δn = 0.166 at 589 nm, Δɛ = 29.3 at 1 kHz, clearing point at 63 °C), and the common chiral dopant R5011 (HCCH, China). The common polymer was composed of RM257 (supported by Merck) and TMPTA (1,1,1-Trimethylolpropane, Aldrich, USA), with the weight ratio of 2:1. The perfluoroalkyl acrylates marked as n-Fs where n = 6, 13, 17 (2,2,3,4,4,4-hexafluorobutyl methacrylate, 6-Fs; 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl methacrylate, 13-Fs; 1H,1H,2H,2H-heptadecafluorodecyl acrylate, 17-Fs, all from Aldrich) were doped with the weight ratio of 3 wt% and 7 wt%, whose chemical structures are schematically illustrated in Fig. 1(a). In addition, a small amount of photo initiator (Irgacure 184 (I-184), provided by BASF) was doped to promote photo-crosslinking in the monomers. Table 1 lists the compositions of pre-polymerized samples including different contents of perfluoroalkyl acrylates. The mixtures were uniformly stirred, then injected into a 12 µm-thick cell with the inter-digital stripe-shaped electrodes on one substrate (width of the electrodes: 15 µm; the gap between adjacent electrodes: 15 µm).
Fig. 1. (a) Chemical structure of the perfluoroalkyl acrylates n-Fs, where n stands for the number of C-F bonds. (b) Schematic of the polymer stabilized BPLCs with fluorinated polymer dopants.
Table 1. Chemical components of the different BPLC samples.
2.2 Sample testing
Since the temperature range of BPLCs is varied among the samples with different doping species and concentration of perfluoroalkyl acrylate, a temperature hot stage (HCS410, Instec) with the cooling rate of 0.2 °C min−1 was settled to precisely guide the formation of BP structure, and a polarizing optical microscope (POM, Eclipse LV100POL, Nikon) equipped with a charge-coupled device (CCD, DS-U3, Nikon) and a fiber spectrometer (ULS2048, Avantes) was used to observe the phase transition behavior. Each sample in Table 1 was controlled to generate the BP-I arrangement (Fig. S1). The following photo-polymerization was implemented by UV exposure (365 nm, 1.5 mW cm−2) for about 30 minutes to form the polymer stabilization. A contact angle measuring device (SDC-80, SINDIN) and a scanning electron microscope (SEM, S-3400N, Hitachi) were used to measure the contact angle and morphology of the pure polymer film, where the film was immersed into acetone for 12 hours to squeeze the involved LCs out. The Kössel diagram was collected by a monochromatic laser source (455 nm) and a high numerical aperture (NA) oil-immersed objective (×100, NA = 1.25, Nikon). A 633-nm probing laser was used and a 1 kHz AC voltage was applied to the sample to study the E-O performance of BPLCs. The response time was measured by a photoelectric detector connected to an oscilloscope (detailed in Fig. S3). All the tests were performed at room temperature (25 °C).
3. Results and discussion
In our scheme, the doped perfluoroalkyl acrylate forms a polymer network with reduced anchoring energy, which improves the E-O performance of BPLCs. Figure 1(b) illustrates the framework involving both the common and fluorinated polymer as well as BPLCs. The common crosslinking polymer forms a tight network to stabilize the DTC stacking architecture of BPLCs [49]. While the fluorinated polymer dispersed around the interface between the common polymer and DTCs makes a dual contribution that: both consolidates the density of polymer network and reduces its strong anchoring force, thus effectively relieving the burdensome constrain to the BPLCs.
In correspondence, Fig. 2 shows the E-O performance of a variety of fluorinated BPLC systems with different fluorinated components and varying doping concentrations in Table 1. Note that the concentration of fluorinated dopant 6-Fs was kept as 3 wt% (Sample B1) since excessive dopants result in the instability of BPLCs after photo-polymerization. Different from the general rule that a higher polymer concentration often causes an increasing of relative driving voltage [28], as shown in Fig. 2(a), the relative driving voltages of non-fluorinated sample A1 (containing only 5 wt% common polymer) and 3 wt%-doped Sample B1, C1 and D1 (8 wt% total concentration including the common and fluorinated polymer) are almost equivalent as 55 V. Notably, compared to the non-fluorinated Sample A1, a series of decreasing hysteresis is observed from Sample B1 to D1 which involve the respective dopants as 6-Fs, 13-Fs and 17-Fs. Correspondingly, as shown in Fig. 2(b), the reduced rise time (τon) among the samples implies more simplicity to drive the BPLCs; and the decay time (τoff) of BPLCs is significantly reduced by 0.37 ms due to the increase of total polymer concentration from 5 wt% to 8 wt%.
Fig. 2. The E-O performance of the different BPLC samples. (a) Measured normalized voltage transmittance curves at 25 °C. The relative driving voltage as indicated by the dotted line (blue and orange), and the direction of the arrow represents the decreasing trend of the driving voltage. δ is the voltage difference at the normalized transmittance of 0.5 during the processes as increasing and decreasing the driving voltage; the hysteresis is evaluated by the ratio between δ and the corresponding driving voltage. Lines with same color stands for the BPLCs with the same total concentration of polymer among i-iv. (b) Measured response time under driving voltage, and data group with same background color stands for the BPLCs with the same total concentration of fluorinated dopants. (c) Light intensity curve of sample C2 during the time period as switching on and off the driving voltage. Inset: the POM images corresponding to the ‘on’ and ‘off’ states.
To investigate the influence by larger fluorinated concentration, the 13-Fs and 17-Fs contents are increased to the maximum as 7 wt% (more doping concentration will cause the instability of BPLCs as shown in Table 1). Similarly, the decent E-O performance with negligible hysteresis is also maintained in both cases. On the other hand, from the perspective of electrical response rate including the rise and decay time (τon and τoff), as shown in Fig. 2(b), the values of τoff in different groups present a declining gradient with the ascending doping concentration spanning zero (i.e., without perfluoroalkyl acrylate), 3 wt% and 7 wt%. While for a constant ratio of dopants (data group with same background color), τoff increases along the different fluorinated polymer as 6-Fs, 13-Fs and 17-Fs. According to the overall investigation among all the samples, sample C2 with 7 wt% 13-Fs was validated to enable the optimal E-O performance of BPLCs, where the electrical response holds an almost hysteresis-free property and fastest decay time as 0.54 ms. Compared to typical BPLCs, where growing polymer concentration to 12 wt% results in a significant rise on the relative driving voltage (greater than 50 V), the sample C2 only displays a negligible 7.5 V increasement [25,33]. It is worth noting that the responsiveness and hysteresis are better than current researches while maintaining a relatively low driving voltage.
Since the electrical response of BPLC is strongly dependent on both the molecular geometry and doping concentration of fluorinated polymer, the respective anchoring energy and polymer density among different fluorinated components (6-Fs, 13-Fs and 17-Fs) and doping ratio (3 wt%, 7 wt%) as aforementioned are investigated to explain their different influence on the relative driving voltage and response time of BPLCs. The Oseen-Frank theory has described the stability of BPLC with the free energy equation, and the hysteresis and responsiveness would be improved with more stable BPLC. According to the equation, the second term shows the anchoring energy between the DTCs and polymer-containing defect core, which is positively correlated to the surface tension (see S2 in Supplement 1 for details) [21,27]. Therefore, the perfluoroalkyl acrylates with low surface tension would increase the stability of BPLC through reducing the free energy of system. In view of which the property of contact angle that implies the surface tension is measured to evaluate the anchoring energy of each individual perfluoroalkyl-acrylate-doped BPLCs [50,51]. As shown in Fig. 3, the contact angle of LCs on fluorinated polymer surface gradually increases with the increasing fluorocarbon tail chain lengths among 6-Fs, 13-Fs and 17-Fs. On the other hand, the contact angle also becomes larger as the mass ratios of the 13-Fs and 17-Fs increase from 3 wt% to 7 wt%. Overall, both the longer fluorocarbon tail chain and rising ratio of fluorine atoms would induce a lower anchoring energy between the BPLCs and surrounding polymer, which correspondingly alleviates the rising trend of relative driving voltage with the growing total concentration of polymer and decreases the constrain of LC molecules. However, since the robustness of BPLC structure is highly relied on the polymer localized in the disclination core [21], an excess of fluorinated dopants such as over 7-wt%-doped 17-Fs (sample D2), whose contact angle reaches over 92°, will reduce the solubility between the polymer and the LC host, and consequently break down the stable DTCs structure of BPLCs owing to the insufficient anchoring force [50]. The same conclusion can be drawn from the dissipative particle dynamics (DPD) method that simulates the contact angle of polymer and LCs induced by the microscopic particle interaction force (see detailed information in S4 of Supplement 1, Fig. S4) [51–53].
Fig. 3. Measured contact angle at the interface between the LC droplet and polymer film. The image of contact angle is also shown above the corresponding column.
From the other perspective, the morphology of polymer film suggests that the differences in polymer density have a critical impact on the electrical response time [54]. In principle, a denser polymer network highlights the electrical response with a faster decay time because the increasing stability of BPLCs. As shown in Fig. 4-(i) to 4-iii, the respective SEM images with a 3-wt% doping of 6-Fs, 13-Fs and 17-Fs exactly imply a series of less compact polymer network, which confirms to the descending tendency among the measured response time as shown in Fig. 2(b). These phenomena are attributed to the decreasing molar concentration of fluorinated dopants converted from the constant doping mass fraction. Likewise, with the increasing doping concentration of the respective 13-Fs and 17-Fs from 3 wt% to 7 wt% (sample C2 and D2), the formed polymer network also becomes denser (for example the SEM image from 7 wt%-doped 13-Fs is exhibited in Fig. 4-iv), thus further enhancing the E-O performance with quicker response. It is noted that the SEM topographies of sample B1 and sample C2 are similar due to the close molar concentration between both systems. As the dopant concentration increases, the solubility of LCs in the polymer decreases with the growing molecular weight of the perfluoroalkyl acrylates during the photo-crosslinking process, until the phase separation occurs [55]. In consequence, the optimal E-O performance of BPLCs from sample C2 can be convincingly explained from the lower surface tension and dense polymer network of the fluorinated polymer system, which are comprehensively evidenced by the molecular geometry of the perfluoroalkyl acrylate, the contact angle, the content of the BPLC mixture, and the morphology of polymer network. Furthermore, a faster rise time τon of 0.15 ms can be obtained as an overdriving voltage of 75 V is applied to the sample C2 (Fig. S5).
4. Conclusions
In conclusion, we have proposed an advisable perfluoroalkyl acrylates functionalized BPLC with significantly enhanced E-O performance. From the mechanism, the doped fluorinated monomer has a two-fold influence on the electric- responsiveness of BPLCs. On one hand, the fluorinated monomers with polymerizable acrylate terminal group provide a denser polymer network, thereby realizing a sub-millisecond faster decay time; on the other hand, the lower surface tension of perfluoroalkyl acrylate drives down the anchoring energy between the BPLCs and surrounding polymer, which alleviates the large rising trend of relative driving voltage with the growing total polymer concentration. To be specific, for the BPLC system containing 5 wt% common polymers and 7 wt% 13-Fs fluorinated dopants, a faster decay time of 0.54 ms and almost hysteresis-free E-O performance is achieved, with an efficient reduction by almost half in the relative driving voltage. Furthermore, the measured contact angle and polymer morphology indicates the underlying mechanism that the growth of fluorocarbon chain and increasing polymer network density contribute to the enhance electrical response. In correspondence, the critical relationship between the concentration of fluorinated dopant and stability of BPLCs is demonstrated. Such a bi-functional perfluoroalkyl acrylates system with both the fast responsiveness and undemanding relative driving voltage holds great promise for various applications such as color sequential displays and low-power tunable photonic devices.
Funding
National Natural Science Foundation of China (51873060, 61822504, 62035008); Shanghai Municipal Education Commission (2021-01-07-00-02-E00107); Shanghai Education Development Foundation (21SG29).
Acknowledgements
The authors acknowledge the support from the National Science Foundation of China (grant nos. 61822504, 51873060, and 62035008), Innovation Program of Shanghai Municipal Education Commission, Scientific Committee of Shanghai (2021-01-07-00-02-E00107), and “Shuguang Program” of Shanghai Education Development Foundation and Shanghai Municipal Education Commission (21SG29).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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