Effects of quenching temperature on threshold, driving voltage and morphology of reverse mode liquid crystal gel films

This study demonstrated that the electro-optical properties and morphology of reverse mode liquid crystal (LC) gel films are strongly influenced by the quenching temperature (TQ) during the manufacturing processes. Composite films were self-assembled using LC and gelator molecules by using different TQ values (70, 80, 90, 100, 110 and 120 °C) and a cold brine (−15 °C). As TQ increased, the morphology of the gel in the polarised light microscope images are changed from neatly arranged fibre-like patterns to random disordered networks. Furthermore, the threshold and driving voltages of the films decreased. For further investigating the gelation processes, Steady cooling rates of 3, 10 and 30 °C min−1 were adopted during the manufacturing process of the LC films. The corresponding morphologies of the films were carefully inspected and compared with those of the quenched films.


Introduction
Self-assembly is an interesting natural process, and various complex biological structures are formed. Studies on soft matter have investigated the use of self-assembly to design and construct suitable structures with unique functionalities [1,2]. Because liquid crystal (LC) molecules are highly ordered, the morphology of LC films can be designed and controlled [3,4] in nanomaterial systems. Furthermore, when colloidal particles are dispersed in a LC host, namely LC colloids, anisotropic ordered structures are formed according to the shapes of the colloidal particles. To better understand the configurations of LC colloids, several studies have adopted different intermolecular interactions between colloids and LC molecules. These interactions include van der Waals forces, ionic interactions, π-π interactions, and hydrogen bonds [4][5][6]. These studies have increased the understanding regarding self-assembling composites of LCs [7][8][9][10][11][12][13]. Furthermore, applications of LC physical gel materials have been proposed [14][15][16][17][18][19].
Nematic LCs are widely used thermotropic LC [20] materials, which combine the fluidity of a liquid and long-range orientational order of a solid. Tipical nematic LC molecules tend to be aligned along a local dominant direction spontaneously, which is referred to as the director. This characteristic leads to special physical properties including birefringence, anisotropic scattering, and anisotropic responses to external fields. Nematic LC-gel composite systems are known for Polymer Dispersed Liquid Crystal (PDLC) [21][22][23][24] and Polymer Stabilized Liquid Crystal (PSLC) [25][26][27][28][29]. In the normal mode LC composite films are opaque (scattering state) when the applied voltage is OFF state and transparent when the applied voltage is ON state (above the switching voltage). Conversely, reverse mode LC (R-LC)-gel composite films are transparent in the OFF state and scatter light in the ON state [30,31].
In general, the switching voltages of PDLC and PSLC are higher than those of traditional LC displays. It is believed that the polymer fibres tightly hold their surrounding LC molecules so that the switching voltage increases accordingly [32,33]. To reduce the energy consumption, low switching voltages of LC-gel cells are desired. In this study used a quenching process to create a LC-gel film with a low switching voltage. In materials science, quenching is a rapid cooling process that uses water or oil to cool hot material [34][35][36]. This process can Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. change the properties of the material. For example, quenching can increase hardness of metallic or plastic materials by reducing the grain size of the materials. To date, most reports on quenching have focused on the mechanical properties and microstructures of metals. In this study, the morphology of the LC films formed by different quenching temperatures (T Q s) are investigated, and compared to that of the processes of slow cooling.

Experimental methods
Our samples include a gelator G12 (12-Hydroxydodecanoic acid, Sigma-Aldrich) and a LC HTW (HTW106700-100, Fusol material Co., Ltd). A drying oven (Deng Yng, DG40) was used to control temperature during heating processes. The ordinary, extraordinary refractive index, and isotropic-nematic transition temperature (T iso-N ) of HTW at 20°C and 589 nm are 1.504, 1.745, and 103.3°C, respectively. The sol-gel transition temperature (T sol-gel ) of G12 is 85-88°C. The phase transitions of different binary mixtures of HTW and G12 were studied using differential scanning calorimetry (DSC).
In this study, HTW and G12 were added to a sample bottle and stirred for 3 h while heating to the clearing point of HTW. The mixture was filled to empty cells by capillary action. The cell gaps were approximately 15 μm with planar alignment in antiparallel directions. Figure 1 shows a schematic diagram illustrating the preparation processes of the R-LC-gel cells.
The cooling processes were divided into two parts: quenching and steady cooling. In the quenching case, the prepared cells were heated to quenching temperatures T Q (70, 80, 90, 100, 110, and 120°C) for 30 min in an oven and then quickly immersed in cold brine at −15°C. The cooling process of cell was photgraphed. Several of the cells were taken out to record the surface temperatures by a thermocouple. The remaining cells were kept in the cold brine for 30 min. After that, polarised optical microscope (POM) images and voltage-transmittance curves were obtained at room temperature.
In the steady cooling case, the prepared cells were placed in a temperature controller (Linkam, T95-PE) at 110°C for 30 min, and then were cooled at rates of 3, 10 and 30°C min −1 to 20°C and kept in there for 30 min The morphology of the cells were taken by POM at room temperature.
To measure the electro-optical properties, a He-Ne laser (632.8 nm) was used as the light source and followed by a polariser. Transmittance axis of the polariser was parallel to the rubbing direction of the R-LC-gel cells. Voltages of 1 kHz square-wave were applied to the cells by a function generator (Tektronix AFG3022). Scattered light from the cells within an angle of 3.8°were detected. The threshold voltage (V th ) and driving voltage (V d ) values of the R-LC-gel films were acquired when the transmittances reach 90% and 10% of the maximum values, respectively.  Figure 2(a) shows the DSC diagram of binary mixtures of G12 and HTW with different concentrations. The phase transition points of pure HTW and G12 closely align with the information provided by the manufacturer, at approximately 103°C and 87°C, respectively. As the concentration of G12 increases, the isotropic-sol to nematic-sol phase transition temperature (T i-s to n-s ) shows a shift towards lower temperatures (indicated by the blue arrows). The nematic-sol to nematic-gel phase transition temperature (T n-s to n-g ) exhibits a shift towards higher temperatures (red arrows). Figure 2(b) shows the phase transition curves of the R-LC-gel films observed using POM at a heating rate of 1°C min −1 . The insets show the corresponding POM images of the R-LC-gel films at various temperatures and G12 concentrations. As the concentrations of G12 increases, the T i-s to n-s (blue line) decreases and the T n-s to n-g (red line) increases. These results are consistent with the DSC analysis shown in figure 2(a).

Results
Figures 3(a)-(f) shows the images of the 1 wt% G12 LC gel cell immersed in the cold brine (−15°C) between crossed polarisers. The cell appeared black at 0.00 s indicating that the LC was in the isotropic state. During 0.00 s to 0.37 s, the colors of the cell changed from black to white and then to green. These changes began from the edge and spread to the centre, which were the same as the temperature decrease paths. The green color of the LC cell means that the LC material was in the nematic state. The white appearance of the cell indicated that numerous randomly aligned LC domains were formed during the quenching, and these domains scattered light. Over time, the LC molecules in the domains aligned themselves along the rubbing direction of the cell and the domain boundary conditions, causing the cell's color to change to green. Figure 3(g) shows that the measured surface temperatures of the LC gel cells. The surface temperature changes were recorded after the cells were immersed in a cold brine for less than 1 s and 10 seconds. The results show that the surface temperatures of all cells quickly passed through T i-s to n-s . Within 10 sec of immersion, the surface temperatures were below T n-s to n-g , and the decline rates were smaller for cells with lower T Q s. Figure 3 demonstrates that all the sample cells quickly transitioned from the isotropic state to the nematic state during the quenching process. Figure 4 illustrates the POM images of R-LC-gel films containing 1 wt% G12 with various T Q values (70, 80, 90, 100, 110, and 120°C) in the nematic gel phase. Figure 4(a) demonstrates the uniformity of the films with G12 networks on a large scale. However, the G12 network patterns observed at T Q = 70 and 120°C are different. The POM image of the film with T Q = 70°C exhibits a fine, closely parallel structure of G12. The LC domains appear thin and long. As the quenching temperature increases, the film with T Q = 110°C displays broken and sparse branches of G12. The LC domains appear wider and larger. The corresponding enlarged morphology is clearly depicted in figure 4(b). When T Q = 70°C, the G12 networks are insignificant and aligned with the LC molecules in the rubbing direction. When T Q = 120°C, the G12 networks are rough and randomly aligned. That is, if T Q is close to and below T i-s to n-s , which is 103°C, the G12 molecules are aligned with the LC molecules through all the cooling processes and the growth of gel branches is approximately parallel to the rubbing direction of the cell, and the LC molecules form elongated domains. When T Q is higher than T i-s to n-s , the G12 and LC molecules align randomly when the quenching processes start . This is because the temperature dropped so quickly that the G12 molecules grow irregularly before the LC molecules are well-aligned; therefore, the network becomes chaotic. While the temperature continued to decrease, the G12 and LC molecules become less disordered and align according to the chaotic network. Figure 5 show that the applied 1 V POM images of HTW+ G12 1 wt % films under crossed polarisers. The gel structures are more obvious, as shown in figure 5(a). Figure 5(b) depicts that the lighter blue is the network texture of gelator G12 (white arrows), and the darker blue is the texture of LC HTW (orange arrows). When T Q = 70 and 80°C, we observed that the network texture of G12 is relatively thick. The direction of G12 network texture distributed in the rubbing direction. When T Q = 90°C, it can be observed that the network structure of gelator was a horizontal network, and small gelator G12 (purple arrows) began to appear. When T Q = 100°C, the texture showed a G12 network with the non-rubbing direction. When T Q = 110°C, more small gelator G12 gradually appeared without specific directionality and formed a slender network distribution. In addition, when T Q = 120°C, the network texture similar that of T Q = 100-110°C.
The normalised voltage-transmittance curves of the R-LC-gel films at various quenching temperatures are shown in figure 6(a). As the voltage increased, the films changed from the transparent state to the scattering state. As T Q increased, V th and V d decreased ( figure 6(b)). When the T Q is higher than 90°C, V th are below 0.6 V, and V d are below 1 V. Figure 6(c) displays the POM images (T Q = 80°C HTW+G12 1% cell) under an applied voltage of 0, 0.4, 0.6, 0.8, 1.0 V. In the image of 0 V, the light blue region which orange arrow points has larger LC material (LC region) while the black translucent region which white arrow points has large amount of G12 (G12 region) and forms a network. It is not until an applied voltage of 0.6V that the color of the G12 region becoming bright white and differing in color from the LC regions. This is consistent with the V-T curve in figure 6(a). The V-T curve shows no change from 0 to 0.4 V, but the transmittance starts to decline once the applied voltage reaches around 0.6 V. As the applied voltage is further increased to 0.8 V and 1.0 V, the color surrounded the boundary between the G12 and LC regions becomes progressively darker. The photographs of figure 6(d) correspond to the cell images shown in figure 6(c). At low electric field, R-LC-gel films are transparent, allowing us to see through the cell and read the word 'LAB'. As the electric field gradually increases, the transmittance decreases and the cell changes to scattering state. The word 'LAB' becomes blocked by the R-LC-gel film. Figure 7 shows the POM images the cell of steadily cooled from 110°C to 20°C with different concentration of G12 and three different cooling rates (3, 10, and 30°C min −1 , from top to bottom). In the 0.5 wt% G12 film with the lowest cooling rate, grain-like gel(approximately 30-50 μm) dispersed in the film. As the concentrations of G12 increased, the gel grains elongated, and the grain density increased. As the cooling rate increased, the gel grains became smaller and thinner. Compared with quenching, slow cooling results in fewer nuclei, larger gel grains, a less dense network, and fewer branches.

Discussion
We have investigated the influence of the cooling process and gelator concentrations on the nucleation and final patterns of the gel networks in the LC composite films. According to the non-isothermal kinetic models of M A Rogers and A G Marangoni [37], the number of nuclei and fibrous morphology depend on the solvent used in the gel formation process. The most important parameter for nucleation is the interaction between the solvent and gelator molecules. Generally, the network structures are divided into two categories depending on the order of T sol-gel of the gelator and T iso-N of LC [38][39][40][41]. When the LC is in the nematic state and T sol-gel is higher than T iso-N , the gelator molecules are irregularly distributed between LC molecules. The structure exhibits a random and disordered network after gelation. If T iso-N is higher than T sol-gel , the gelator molecules are well-aligned with LC molecules, and an anisotropic gel network is obtained.
In this study, the materials belonged to the latter category. Nevertheless, we obtained different gel networks by using different cooling processes, and the corresponding electric-optic behaviors changed accordingly. Figure 8 illustrates the influence of temperature and cooling rates on the gelation of the mixture. When the beginning temperature T > T i-s to n-s , the LC and gelator molecules were randomly aligned. During the slow cooling period, the gelator molecules aggregated and began to form small gels according to the alignment of the surrounding LC molecules. Gradually, these small gels developed into large gels. At a higher cooling rate, the gelator molecules formed a larger number of small gels, which indicates that the gel size and shape depend on the cooling rates. If the cooling rate is extremely high (i.e., quenching), the gelator molecules form a random directional network. When the beginning temperature T < T i-s to n-s , the LC molecules are in the nematic phase, and the gel molecules are distributed accordingly. The final morphologies of the gel networks are nearly aligned in the rubbing direction of the LC cell on the cooling processes.
In the results quenching, the gel networks became slender and more chaotic as T Q increases. Figure 5 show that the T Q = 70 and 80°C films have relatively complete G12 network. The LC regions are clean and no small G12 grain. When T Q = 90°C, the main network became slender, and the small and broken G12 grains were dispersed in the LC regions. When T Q = 110°C, a high quantity of small G12 grains were distributed in the LC regions. The boundaries of the G12 network when T Q = 90 or 110°C were not as flat as those observed when T Q = 70 or 80°C ( figure 5(b)). Large numbers of G12 grains can disorder the LC alignment. Because the LC alignment is influenced by the gel boundaries, the directors of the LC inside the networks are less ordered. The LC molecules respond more easily to the electric field force, which results in lower values for V th and V d .

Conclusions
This study demonstrated the influence of cooling rates on the formation of gel in the LC-gel composite films. Slow cooling rate results in the formation of large fibres. As the cooling rate increases, the morphology of the film changes to smaller and finer fibre distribution. To decrease V th and V d and increase the scattering in the ON state, a large quantity of fine fibres are required in an LC film. Quenching enables the highest cooling rate to be achieved. In this study, T Q influences gel formation. The microstructures of the gel networks and the alignment of LC molecules were affected by T Q . V th and V d decrease as T Q increases. Comparing the quenching temperature of 80 and 110°C, the threshold and driving voltage decrease by 15% and 20%, respectively. Understanding the effect of T Q s on the morphology of self-assembled LC-gel films is crucial for improving their properties and performance.  The POM image and the DSC curve provide evidence that the iso-N phase transition point of HTW + G12 1%, using the quenching method, is crucial for determining the order and disorder of the G12 network. This method, which involves varying the quenching temperature, proposes a new, convenient, and fast approach to modifying the internal structure of LC physical gels. Furthermore, other LC physical gels of the same type, using heating and cooling processes, may yield new stable state processes as a result of this method.