CC BY
8Research Article
DOI: 10.18083/LCAppl.2021.4.61
NEW FERROELECTRIC LIQUID CRYSTALLINE MATERIALS WITH PROPERTIES SUITABLE FOR SURFACE STABILIZED AND DEFORMED HELIX EFFECTS
Michal Czerwinski, Marzena Tykarska*, Przemyslaw Kula
Institute of Chemistry, Military University of Technology, 2 gen. S. Kaliskiego St., Warsaw, 00-908, Poland Corresponding author: marzena.tykarska@wat.edu.pl
Abstract. Liquid crystals (LC) are widely used in optical devices. New electro-optic effects are still being established. The requirements of such devices increases thus still new materials are elaborated for such purpose. In this review, two electro-optic effects are described, namely Surface Stabilized Ferroelectric LC (SSFLC) and Deformed Helix Ferroelectric LC (DHFLC) effects. New ferroelectric liquid crystal mixtures were formulated which can be used in devices operating on the basis of the mentioned electro-optic effects. Two ways of mixture preparation have been used: mixing of chiral components or mixing of achiral components with addition of a small amount of chiral dopant. The ferroelectric mixtures with very short (less than 200 nm) or very long (more than 1 ¡¡m) pitches are discussed. The mixtures meet almost all requirements of SSFLC and DHFLC effects and therefore, have important prospects for applications in optical sensors.
Key words: liquid crystals, ferroelectric phase, ferroelectric mixtures, SSFLC effect, DHF effect
For citation: Czerwinski M., Tykarska M., Kula P. New ferroelectric liquid crystalline materials with properties suitable for surface stabilized and deformed helix effects. Liq. Cryst. and their Appl, 2021, 21 (4), 6173. DOI: 10.18083/LCAppl.2021.4.61.
Научная статья УДК 544.022.5
НОВЫЕ СЕГНЕТОЭЛЕКТРИЧЕСКИЕ ЖИДКОКРИСТАЛЛИЧЕСКИЕ МАТЕРИАЛЫ СО СВОЙСТВАМИ, ПОДХОДЯЩИМИ ДЛЯ ЭФФЕКТОВ СТАБИЛИЗАЦИИ ПОВЕРХНОСТИ И ДЕФОРМАЦИИ СПИРАЛИ
Михал Червиньски, Маржена Тыкарска*, Пшемыслав Кульа
Химический институт Военного технологического университета, ул. ген. С. Калискего, 2, 00-908 Варшава, Польша *Автор для переписки: marzena.tykarska@wat.edu.pl
Аннотация. Жидкие кристаллы (ЖК) широко используются в оптических устройствах. Несмотря на это новые электрооптические эффекты все еще устанавливаются для таких материалов. Возрастающие требования к таким устройствам требуют разработки новых материалов для этих целей. В обзоре описываются два электрооптических эффекта, а именно эффект поверхностно-стабилизированного сегнетоэлектрического ЖК (SSFLC) и эффект сегнетоэлектрического ЖК с деформированной спиралью (DHFLC). Сформированы новые смеси сегнетоэлектрических жидких кристаллов, которые могут быть использованы в устройствах, работающих на упомянутых электрооптических эффектах. Эти смеси были приготовлены двумя разными способами, а именно путем
© Czerwinski M., Tykarska M., Kula P., 2021
смешивания хиральных компонентов и путем смешивания ахиральных компонентов с добавлением небольшого количества хиральной добавки. В статье обсуждаются сегнетоэлектрические смеси как с очень коротким (менее 200 нм), так и с очень длинным (более 1 мкм) шагом спирали. Подобные смеси удовлетворяют почти всем требованиям эффектов SSFLC и DHFLC и поэтому имеют большие перспективы применения.
Ключевые слова: жидкие кристаллы, сегнетоэлектрическая фаза, сегнетоэлектрические смеси, SSFLC-эффект, DHF-эффект
Для цитирования: Czerwinski M., Tykarska M., Kula P. New ferroelectric liquid crystalline materials with properties suitable for surface stabilized and deformed helix effects. Liq. Cryst. and their Appl, 2021, 21 (4), 6173. DOI: 10.18083/LCAppl.2021.4.61.
Introduction
Ferroelectric liquid crystals (FLCs) are materials that possess a spontaneous polarisation (Ps) in the absence of an external field and they can be reoriented on the application of electric field. After the ferroelectricity in liquid crystals was discovered in 1975 [1], many books and articles were written about synthesis, studies and application of new compounds having such properties [2-12]. Since then, many compounds have been synthesized and new materials on their basis were prepared and tested [13-21]. FLCs are characterized by fast electro-optical response, low driving voltage and memory effects, which make them suitable candidates for many advanced applications, such as displays, gratings or sensors [22-25]. Different electro-optical modes can be successfully used for these applications: (i) Surface Stabilized (SS) FLC [26, 27]; (ii) Deformed Helix (DH) FLC [28-30]; (iii) Electrically Suppressed Helix (ESH) of FLC [29]; (iv) modulation of light in the diffraction FLC grating [31]; (v) orientational electro-optical Kerr effect [32, 33] and others. This mini-review summarises results on recently prepared ferroelectric liquid crystalline materials and their multicomponent mixtures [34-39].
Electro-optical effects of FLCs
The main electro-optical switching modes of ferroelectric LCs are based on the field-induced director rotation on the "smectic cone" under the torque connected with the coupling of spontaneous polarization and electric field PsxE. It allows obtaining the switching between dark and bright states with much lower energy barrier than that for nematic liquid crystals, hence the electro-optical effects based on FLCs are characterized by considerably shorter switching times. Two effects (SSFLC and DHFLC) are the most popular and they will be discussed here.
DHFLC electro-optical effect
In the DHFLC effect, the helix axis is parallel to the substrate's plane of LC cell, and the helix is partially unwound when the electric field E is applied (which should be smaller than the so-called critical field EC causing complete helix unwinding). The electric field also changes the optical axis orientation of LC medium relatively to crossed polarizers (Fig. 1). The basic condition for occurrence of DHFLC effect is that the helical pitch p should be much smaller than the thickness d of the LC cell (p << d). DHFLC effect is very promising because of tunable, continuous and hysteresis-free optical phase shift at low voltages and short response times (shorter than in other electro-optical effects based on smectic LCs) either in transmission or reflection modes [30, 40, 41].
The parameters characterizing the DHFLC effect: transmission between crossed polarizers (Tn), switching time (t) and critical electric field (Ec) can be described by the following equations (1-3) [41]:
Tn =sin22[p± Aa(E)]sin '-e//
X
t =
Y<pP
K4n2
Er =
n
К
4 psp2'
(1)
(2) (3)
where P is the angle between the polarizer axis and helix axis of FLC phase; Aa(E) denotes the deviation of optical axis from normal to layer z due to electric field; d is the thickness of LC cell; Anej is the effective birefringence of LC; X is the wavelength; is the rotation viscosity; K is the elastic constant. From the equations (1-3) it can be concluded that the helical pitch p is the most important physicochemical parameter of LCs in DHFLC effect as it strongly affects t and Ec. The pitch should be as low as possible (at least below 200 nm) in order to lead to the
decrease in switching times and increase in the operating voltage range of the described effect. A high tilt angle of the director 6 (ideally it should be as close to 45° as possible) promoting high transmission Tn and a moderate spontaneous polarization Ps (ideally it should be within the range 120-180 nC/cm2) are the other important parameters.
Concerning the values of spontaneous polarization PS [42], it can be mentioned that too low values of Ps can result in a strong decrease of contrast, and too high values of Ps cause the shortening of electric field range, in which the DHFLC effect appears. Additionally, low values of rotational viscosity yq> help to achieve short switching times.
Fig. 1. Arrangement of molecules (blue bars), director n (dark arrows) and spontaneous polarization Ps (red arrows) in spontaneously created helical structure in FLCs (a). The change in the arrangement of molecules and the resulting change of optical axis vs. crossed polarizers in DHFLC effect with the model shapes of electro-optical response (b). The two stable synclinic F-states in SSFLC as well as the model shape of electro-optical hysteresis with marked characteristic electric field and transmission points (c). Molecules and directors in four consecutive smectic layers as well as tilt angle 6 are indicated
SSFLC electro-optical effect
In the SSFLC effect, the anchoring surface of LC cell should be strong enough to unwind the SmC* helical structure close to the surfaces. When the LC cell gap d is sufficiently small (d << p), the helix is suppressed by the surface action also in the volume. This allows the formation of two possible stable director orientations, simultaneously fulfilling the conditions that the molecules must be parallel to the LC cell surfaces. In the SSFLC effect, two stable states (called synclinic states - F) are energetically equivalent at E=0, but can be switched between each other by means of an electric field E that causes the molecules to rearrange in a way that the sense of spontaneous polarization vector of each layer is consistent with that of the electric field (Fig. 1).
The optical indicatrix axis is along to the director of both states. In the SSFLC effect, the switching to the synclinic state is of threshold type and it begins at the threshold electric field Eth and ends at the saturation electric field EMt. In order to obtain a dark state between crossed polarizers, one director of two possible synclinic states should be parallel to one polarizer in LC cell.
The parameters characterizing the SSFLC effect are the transmission (Tn) between crossed polarizers and the reorientation time (te) of molecules under electric field are expressed by the following equations (4, 5) [26]:
„ /ndAnpff\ Tn = sin (2a)sin i-j^) (4)
where a is the angle between the polarizer and director axes of FLC.
To assure the maximum contrast in SSFLC effect, the tilt angle 6 should be 22.5o. Moreover, the helical pitch p should be as long as possible to obtain a good surface stabilization of smectic layers of FLCs. According to the equation (5), the spontaneous polarization Ps should have moderate values to ensure short time te and reduce the electric current in LC cell as well as the driving electric field used for switching of molecules in LC cell. To obtain short-time switching between the dark and bright states, it is also very important to design FLCs with low rotational viscosity Yq, Besides its fast electro-optical response, the SSFLC effect is known possessing a hysteresis of electro-optical switching behaviour. In-plane switching of the
optical axis in LC cell is also observed. According to the equation (5) and Fig. 1, c, the SSFLC effect exhibits symmetric switching times because both the rise and fall times (Ton and Toff, correspondgly) are dependent on the electric field E.
The major obstacle for commercialization of SSFLC effect in potential applications is the insufficient contrast of LC cell placed between crossed polarizers due to light leakage in the dark state. This imperfection is mainly caused by the tendencies of smectic layers to fold into a chevron-like structure when transferring from an orthogonal phase (SmA*) to a tilted phase (e.g. SmC*). The appearance of such structure results in an inhomogeneous arrangement of the optical axis of LC medium. The main source of this effect is the decrease of smectic layer thickness d due to the tilt of molecules (dSmC* ~ dSmA* cos 9). On cooling through the SmA* -SmC* transition, the layers do not slip along the glass surface of LC cell and the smectic periodicity close to this surface corresponds to SmA* phase. The only way for the material to satisfy the new smaller layer thickness of SmC* phase in the bulk of LC cell is to fold in chevron-like structure. Characteristic zig-zag defects appear and cause mentioned light leakage in the dark state of LC cell placed between crossed polarizers in the SSFLC effect. The method to minimize the above mentioned disadvantages is the use of FLCs with the possible smallest layer shrinkage during the transition to a tilted phase. Moreover, the presence of a nematic phase in the phase sequence of LC material affects the improvement of molecule alignment in LC cell.
Preparation of mixtures
Considering such a complex research task of simultaneous tailoring of several parameters in DHFLC and SSFLC effects, it is clear that those requirements cannot be reached in a single smectogenic compound, but the only possible solution is to design and prepare specific multicomponent LC mixtures. There are at least two main approaches to design and obtain desirable FLC mixtures. The first one is based on doping an achiral base smectic C mixture with a chiral dopant or dopants, to produce chiral smectic C* mixture possessing the optimal physico-chemical and electro-optical properties. The advantage of this FLC mixtures concept is the ability of controlling the values of spontaneous polarization Ps and helical pitch p in a broad range. Additionally, this approach consists in
keeping the rotational viscosity of the final material as low as in the basic smectogenic mixture and providing the possibility to obtain phase sequence with nematic phase at higher temperature. However, the director tilt angle in mixtures developed by this method is rather low (6 < 35°). Such mixtures can also exhibit strong instability when the concentration of chiral dopant is high. The instability often occurs in LC mixtures consisting of compounds significantly differing in chemical structure. It can cause precipitation of some components, mainly of chiral dopant.
The second approach relies on the mixing of chiral LC compounds promoting chiral tilted phases. This method provides advanced LC materials with high stability and allows to obtain high director tilt angle 6 in SmC* phase.
All above information about the requirements and drawbacks of LC materials dedicated to the DHFLC and SSFLC effects have become motivation for the Military University of Technology research team to design and develop new generation of FLCs. In this paper, we demonstrate summary of these efforts.
FLCs prepared from achiral SmC basic mixture and chiral dopants
In order to obtain FLCs material according to the idea of doping achiral mixture with chiral dopant, the bicyclic pyrimidine compounds with alkyl or/and alkoxy terminal chain (based on a general structure 1, where: Ri and R2 are CnH2n+i- or CnH2n+iO-; n = 4 ^11) were chosen and an achiral base mixture (denoted here as W) was prepared [37].
1
The prepared eutectic base mixture W exhibits the following phase transitions at heating: Cr 7.9 °C SmC 68.8 °C SmA 82.8 °C N 87.0 °C Iso.
To induce ferroelectric properties in the tilted SmC phase, the base mixture W was doped with several low-melting, non-mesogenic, terphenyl-based dopants having two homo-chiral terminal chains (A, F, G (mixture), I, J). Thus, five FLC mixtures were obtained containing 5.0 wt. % of each dopant. The general structure 2 and acronyms of dopants along with their phase transition temperatures are presented below.
2
A: (S, S); n = 6; k = 0; Xi, X2 = H; Ri, R2 = CH3; Cr 81.1 °C Iso [43] F: (S, S); n = 4; k = 0; X1, X2 = H; R1, R2 = CH3; Cr 75.7 °C Iso [43] G: 84 wt. %.: (R, R); n = 6; k = 0; X1, X2 = H; R1, R2 = CH3 [by analogy to 43]; 16 wt. %.: (R, R); n = 6; k = 1; X1, X2 = H; R1, R2 = CH3; Cr 82.7 °C Iso [unpublished results] I: (S, S); n = 6; k = 0; X1, X2 = H; R1, R2 = CF3; Cr 42.9 °C Iso [44] J: (S, S); n = 6; k = 0; X1, X2 = F; R1, R2 = CH3; Cr 34.3 °C Iso [43]
All prepared chiral mixtures exhibit the ferroelectric SmC* phase in very broad temperature ranges and their melting points are below 10 °C (crystallisation temperatures are below 0 °C). Almost all mixtures exhibit the desired phase sequence: Cr -SmC*- SmA* - N* - Iso. The exception is the mixture containing terphenyl and quaterphenyl chiral dopants (mixture W_G), which does not form a chiral nematic phase. It may be the evidence that the presence of long and flat molecular core in the base mixture hinders the
molecular movements along the director and promotes layered ordering. Surprisingly, in the case of W_G mixture, the lack of nematic phase allows to obtain nearly perfect, almost defectless surface stabilized geometry of smectic layers in LC cell. Such almost perfect alignment was obtained for all prepared mixtures and the dark state without light leakage exhibits beneficial properties for application. This is an uncommon feature in comparison to FLC materials reported previously (i.e., see ref. [15]).
This fact can be explained by the properties of multicomponent base mixture W, which forms smectic layers with a small layer contraction and shows low viscosity values. Such properties that hinder formation and support relaxation of emerging defects, can be connected with the structural similarity of mixture W components. Moreover, the helical pitchp obtained for the mixtures with 5.0 wt. % of chiral dopants is higher than 1.5 ^.m (the thickness of the used electro-optical
Table 1. Electro-optical and physicochemical properties < T k 30 °C in SmC* phase. The table prepared on the basis
cell is lower) and it also contributes to a better ordering of mixture molecules in LC cell.
From the obtained results [37], many interesting conclusions regarding the influence of the chiral dopants structure on the properties of final multicomponent FLC mixtures can be highlighted. Some of the physicochemical and electro-optical parameters of prepared mixtures described in Ref. [37] are presented in Table 1.
the compositions prepared on the basis of mixture W at F the data from [37]
Properties W_A W_F W_G W_I W_J
0[°] 21.4 24.1 21.1 25.0 21.9
Smectic layer thickness [A] 27.8 27.7 28.6 28.2 28.3
The difference of the layer thickness between SmC* and SmA* phases [A] 1.8 1.9 1.7 1.9 1.7
te [ps] 80 71 88 88 104
Ps [nC/cm2] 9.5 21.4 14.0 28.4 16.0
Y<p [mPa s] 28 53 45 92 61
The smectic layer spacing for both mixtures W_A and W_Fis near equal. Compound F with shorter terminal chain in comparison with dopant A causes a greater increase of spontaneous polarisation Ps, the director tilt angle 6 and rotational viscosity yv and decrease the switching time Te in the final FLC mixture. Dopants I and J differ from each other in the location of fluorine atoms - in the terminal chain of dopant I and in the core of dopant J. The higher values of spontaneous polarisation Ps, optical tilt angle 6 and rotational viscosity yv are observed in the mixture with dopant I. It suggests that the presence of the heavy and electronegative fluorine atoms close to the chiral centre reflects in a hindrance of molecular rotation and, hence, results in a higher value of the spontaneous polarization Ps. Apparently, the location of the fluorine atom in chiral molecules, as studied in work [37], does not affect the layer spacing in the final FLC mixture. It can also be noted, that fluorine substitution in dopants causes an increase in the rotational viscosity yv of the resulted mixtures.
Among prepared materials, the mixture W_A exhibits a set of parameters that altogether make it the best candidate for applications in SSFLC mode (Table 1).
Here, the most uniform and defectless surface stabilized geometry, the director tilt angle 6 approximated to the required one, the lowest spontaneous polarisation Ps and short switching time te observed are of the highest importance.
Furthermore, we also designed and prepared a series of mixtures consisting of an excessive amount of chiral dopant in the base mixture W (up to 30.0 wt. %) [37]. At the chiral dopants concentration above 20.0 wt. %, the values of helical pitchp are very low (in some cases being lower than 150 nm at the room temperature). Such mixtures show high application potential in DHFLC effect.
FLCs prepared from chiral compounds
FLC mixtures consisted of "all chiral" compounds were formulated according to the procedure based on mixing of selected chiral compounds with ferroelectric and antiferroelectric properties. The proper composition allows the competition between the synclinic and anticlinic order, leading to appearance of frustrated ferroelectric phase, which provides the V-shape electro-optical switching [45]. This method allows to optimize the physicochemical and electro-optical properties of FLC materials beneficial in DHFLC effect.
In the first step of these studies, we selected compounds with the identical chiral terminal chain but different structure of rigid molecular core and achiral opposite chain (see compounds 3b-e and 4a-d in Table 2). Those compounds were added to the basic high tilted ferroelectric compound (see compound 3a in Table 2)
[34]. The aim of the preparation of such bicomponent systems was to establish the miscibility of phases, especially ferroelectric phase, and to determine the temperature dependence of the helical pitch p and the tilt angle of the director 6 in the mixtures.
Table 2. Compounds selected for miscibility investigation and their phase transition temperatures suitable for DHFLC effect
3
4
Compound acronym n m Xi X2 k l X3 X4 Phase transition temperatures determined on heating, °C
3a 6 - - - 1 0 H - Crn 80.7 CrI 98.9 SmC* 141.4 SmCa* 149.0 SmA* 184.0 Iso [46,47]
3b 6 - - - 0 1 H - Crn 94.3 CrI 97.9 SmC* 155.6 SmA* 184.6 Iso [46]
3c 6 - - - 1 0 F - Cr 89.7 SmC* 133.6 SmCa* 134.8 SmA* 154.7 Iso [46]
3d 6 - - - 0 0 H - Cr 60.0 SmA* 63.4 Iso [by analogy to 46]
3e 8 - - - 0 0 H - Cr 82.2 SmA* 90.7 Iso [48]
4a 1 5 H H - 0 H H Cr 43.4 Iso [49]
4b 3 6 H H - 0 H H Cr 36.2 Iso [by analogy to 50]
4c 3 6 F H - 0 H H Cr 32.8 Iso [by analogy to 50]
4d$ 3 0 H H - 0 H F Cr 14.9 Iso [49]
4e 3 5 H F - 1 H H Cr 28.1 SmCA*99.0 SmC* 101.0 Iso [50]
4f 3 7 F H - 1 H H Cr 37.4 SmCA* 103.1 SmC* 104.3 SmA* 109.1 Iso [50]
$ - compound without oxygen atom between olighomethylene group (CH2)m and rigid core
Results of studies [34] show that two-ring compounds, which do not form ferroelectric phase, destabilize it in the mixtures with compound 3a. Among them, compounds 3d and 3e, showing SmA* phase, cause the smallest destabilization. They have the longest fluorinated part of achiral terminal chain, similar to that of the basic compound 3a. Among compounds without liquid crystalline phase, the compound 4b having the longest achiral terminal chain without lateral substitution of fluorine atom gives the
smallest phase destabilization in mixtures with the base compound 3a. Systems containing compounds 4a, 4c and 4d show the phase coexistence area (SmC* and Iso) without stable SmC* phase above 0.3 molar fraction in the mixture with compound 3a at room temperature. Almost all investigated compounds cause small decrease of a helical pitch length in the ferroelectric phase of basic compound 3a except compounds with SmA* phase (compounds 3d and 3e). The tilt angle of mixtures with 0.2 molar fraction of the two-ring
compounds is still relatively high (above 30° at lower temperatures). Among biphenyl compounds, the compound 3d causes the lowest reduction of tilt angle in mixture with compound 3a. Based on the analysis of the above results [34], it can be concluded that compounds 3b, 3c, 3d, 3e and 4b seem to be the most promising components for creation of ferroelectric mixtures with compound 3a. They should be characterized by broad temperature range of SmC* phase, short helical pitch p and high director tilt angle 6.
In the next step of preparation of the FLC mixtures with the properties appropriate for DHFLC effect and consisting of compounds having ferro- and/or antiferroelectric phase, the multicomponent mixtures based on compounds presented in Table 2 were designed and characterized [35, 36]. The compositions of the developed mixtures and their phase transition temperatures are presented in Tables 3 and 4, respectively.
The first prepared mixture W-212B [35] was an eutectic mixture consisting of three-ring ferroelectric
Table 3. Compositions of the studied mixtures [35, 36]
compounds (3a-c) and two low melting two-ring compounds 3d and 3e, forming the SmA* phase. In order to decrease melting point as well as a helical pitch length of this mixture W-212B (see Table 4 and Fig. 2, a), three modified mixtures have been developed.
The second mixture W-212B2 additionally contains compound 4e possessing a wide-temperature range of antiferroelectric phase. This modification allows to decrease melting point and helical pitch (see Table 4 and Fig. 2, a) but at the same time, antiferroelectric phase is induced at lower temperatures in this mixture.
The third mixture W-212B3 was prepared on the base of two eutectic mixtures: the first developed one with acronym W-212B and the antiferroelectric one with acronym W-1000 [51, 52] (consisting of compounds 4e and 4f). Using the phase diagram of the system of these two mixtures, we chose the composition of the final W-212B3 mixture with the frustrated ferroelectric phase at a broad temperature range.
Component acronym Mixtures acronyms
W-212B W-212B2 W-212B3 W-212B3A W-212C W-212C2 W-212C3
Concentration (wt. %)
3a 11.88 7.97 7.72 8.22 8.35 7.52 7.52
3b 12.13 8.14 7.88 8.39 8.31 7.48 7.48
3c 24.29 16.30 15.79 16.81 18.54 16.69 16.69
3d 38.80 26.04 25.22 26.85 29.75 26.77 26.77
3e 12.90 8.66 8.39 8.93 - - -
4b - - - - 35.05 31.54 31.54
4e - 32.89 18.38 11.47 - 10.00 5.25
4f - - 16.63 10.37 - - 4.75
A - - 8.97 - - -
Mixture acronym Cr T, °C SmCA* T, °C SmC* T, °C SmA* T, °C Iso
W-212B 11.4 - - • 83.9 118.0
W-212B2 <-20.0 • -4.8 • 91.0 111.0
W-212B3 <-20.0 - - • 85.2 105.0
W-212B3A <-20.0 - - • 71.2 98.9
W-212C <-20.0 - - • 61.8 97.0
W-212C2 <-20.0 - - • 61.7 71.0
W-212C3 <-20.0 - - • 64.4 74.7
Table 4. Mesomorphic behaviour of the studied mixtures (on cooling) [35, 36]
The W-212B3 mixture exhibits the Cr - SmC* -SmA* - Iso phase sequence with low melting point and short helical pitch (see Table 4 and Fig. 2, a). Moreover, it is characterized by similar values of spontaneous polarization Ps (Fig. 2, c) and the director tilt angle 6 lower only by 3° than the mixture W-212B2 (Fig. 2, b). It is worthy to notice that W-212B3A differs from W-212B3 by an additional chiral component A and has the smallest value of the helical pitch length - around 150 nm - suitable for DHFLC effect (Fig. 2, a).
Unfortunately, the mixture W-212B3A has one disadvantage, i.e. too high values of spontaneous polarization Ps (higher than 200 nC/cm2, Fig. 2, c). According to equation (3), high Ps considerably decreases the value of Ec and, thereby, reduces the voltage range, over which the mixture W-212B3A could be driven in DHFLC effect. Therefore, we have modified the composition of the base mixture W-212B and the newly designed mixture W-212C [36] was formulated. Instead of compound 3e with the SmA* phase, the mixture W-212C contains a non-mesogenic compound 4b having much lower melting point and possessing strong tendency to decrease the helical pitch p [34]. Due to that, the SmA*-SmC* phase transition, as well as the melting point of W-212C mixture became much lower than those of W-212B mixture. Furthermore, and importantly, the helical pitch length of W-212C is lower than 180 nm (Fig. 2, a), thus, it was possible to avoid the use of compound A, which significantly increases spontaneous polarization Ps. We used again the procedure of blending compounds with ferroelectric and antiferroelectric compounds at proper concentrations. For this purpose, we chose compounds 4e and 4f to tune the features of W-212C. Two systems, namely W-212C + compound 4e as well as W-212C + mixture of compounds 4e and 4f (in the proportion corresponding to mixture W-1000), were prepared in order to find a material with a broad temperature range of frustrated ferroelectric phase and with the shortest possible helical pitch p. Based on the analysis of phase diagram and temperature dependence of helical pitch lengths p for both systems, we have chosen two mixtures consisting of 10.0 wt. % of compound 4e or W-1000 blended with the mixture W-212C, these
mixtures have been denoted with the acronyms W-212C2 and W-212C3, respectively. These mixtures are characterized by low melting points and relatively broad temperature ranges of the ferroelectric phase (over 70 °C). The values of their helical pitch lengths p are lower than 180 nm (see Fig. 2, a), similar as for W-212C mixture. As the values of helical pitch length p for W-212C2 mixture are the highest from all three discussed mixtures, the studies of temperature dependence of the director tilt angle 6, spontaneous polarization Ps, switching times t and rotational viscosity yv were performed for W-212C and W-212C3 mixtures only. The lowest values of spontaneous polarization Ps were detected for the base W-212C mixture, which contains the highest quantity of ferroelectric compounds and no components with antiferroelectric phase (see Table 3). On the other hand, antiferroelectric components of W-1000 mixture cause saturation of the tilt angle values 6 (about 40°) at a broader temperature range in W-212C3 mixture (Fig. 2, b). It is worthy to notice that both mixtures W-212C and W-212C3 exhibit lower values of spontaneous polarization Ps and comparable values of tilt angle 6 in comparison with the mixtures based on W-212B series (Fig. 2, b, c). Taking into account the basic information from Introduction section, the designed W-212C and W-212C3 mixtures fulfilled almost all material requirements for DHFLC effect, including not too high spontaneous polarization Ps. Thanks to this, the basic disadvantage of the previously prepared W-212B3A mixture was eliminated. On the other hand, W-212C and W-212C3 mixtures exhibit two times higher rotational viscosity y^ but also much lower melting point than the reported FLC materials for DHFLC effect [41]. So, it means that they can extend the switching times t, while allowing to work in DHFLC mode at lower temperatures. Furthermore, some mixtures of the designed binary systems (W-212B+ W-1000, W-212C+W-1000 and W-212C+compound 4e) described in Refs. [35, 36] possess antiferroelectric phase with a very short and simultaneously, temperature independent helical pitch p. That fact makes them potentially applicable and useful for DHAFLC effect [53].
Fig. 2. Temperature dependences of the helical pitch lengthp (a), the director tilt angle 6 (b) and spontaneous polarization Ps (c) of the mixtures belonging to W-212 series
Conclusions
The mixtures showing broad temperature range of ferroelectric phase have been prepared. Both methods of mixture preparation, namely the mixture formulation from chiral as well as from achiral components with addition of chiral dopant, have specific advantages. The former method is better suitable for preparation of the mixtures with classic ferroelectric phase having the director tilt angle around 22.5°. The latter method is good for the mixtures with orthoconic ferroelectric phase having the director tilt angle around 45°. The properties of presented mixtures satisfy the requirements of SSFLC as well as DHFLC electro-optic modes and by that have opened the possibility of using them for some optical sensors and other devices.
Acknowledgments: This work was supported by the European Union: [Grant No. COST Action IC1208]; the Czech-Polish bilateral project of MEYS: [Grant No. 8J20PL008]; the Ministry of Education and Science, the Republic of Poland: [Grant No. RMN/08-737/2015/WAT,
Grant No. RMN/08-964/2014/WAT, Grant No. PBS1/B3/9/2012]; the Ministry of National Defence, the Republic of Poland [Grant No. MUT 13-995, Grant No. PUM-09-429]; the National Science Centre of Poland [Grant No. UM0-2011/03/N/ST4/0136, Grant No. UMO-2015/19/D/ST5/02730].
References
1. Meyer R., Liebert L., Strzelecki L., Keller P. Ferroelectric liquid crystals. J. Phys. Lett, 1975, 36 (3), L69-L71. DOI: 10.1051/jphyslet:0197500360306900.
2. Lagerwall S.T. Ferroelectric and antiferroelectric liquid crystals. Weinheim: Wiley-VCH, 1999. 427 p. D0I:10.1002/9783527613588.
3. Takezoe H. Ferroelectric, antiferroelectric, and ferrielec-tric liquid crystals: applications. Encyclopedia of Materials: Science and Technology. Oxford: Elsevier, 2001, 3063-3074. DOI: 10.1016/B0-08-043152-6/00546-5.
4. Fukuda A., Ouchi Y., Arai H., Takano H., Ishikawa K., Takezoe H. Complexities in structures of ferroelectric liquid crystal cells. The chevron structure and twisted states. Liq. Cryst, 1989, 5 (4), 1055-1073.
DOI: 10.1080/02678298908026410.
5. Rieker T.P., Clark N.A. The layer and director structures of ferroelectric liquid crystals. Phase Transitions in Liquid Crystals / ed. by S. Martellucci and A.N. Chester. New York: Plenum Press, 1992, 287-341.
DOI: 10.1007/978-1-4684-9151-7_21.
6. Lagerwall S.T., Matuszczyk M., Matuszczyk T. Old and new ideas in ferroelectric liquid crystal technology. Proc. SPIE, 1998, 3318, 2-38.
DOI: 10.1117/12.299943.
7. Bubnov A., Novotna V., Hamplova V., Kaspar M., Glogarova M. Effect of multilactate chiral part of liquid crystalline molecule on mesomorphic behaviour. J. Mol. Struct, 2008, 892, 151-157.
DOI: 10.1016/j.molstruc.2008.05.016.
8. Novotna V., Hamplova V., Bubnov A., Kaspar M., Glogarova M., Kapernaum N., Bezner S., Giesselmann F. First photoresponsive liquid-crystalline materials with small layer shrinkage at the transition to the ferroelectric phase. J. Mater. Chem, 2009, 19, 3992-3997.
DOI: 10.1039/b821738f.
9. Tykarska M., D^browski R., Czerwinski M., Chelstow-ska A., Piecek W., Morawiak P. The influence of the chi-ral terphenylate on the tilt angle in pyrimidine ferroelectric mixtures. Phase Transitions, 2012, 85 (4), 364-370. DOI: 10.1080/01411594.2011.646274.
10. Ziobro D., D^browski R., Tykarska M., Drzewinski W., Filipowicz M., Rejmer W., Kusmierek K., Morawiak P., Piecek W. Synthesis and properties of new ferroelectric and antiferroelectric liquid crystals with a biphenylyl benzoate rigid core. Liq. Cryst., 2012, 39 (8), 10111032. DOI: 10.1080/02678292.2012.691560.
11. W^glowska D., D^browski R. Highly tilted ferroelectric liquid crystals with biphenylyl benzoate rigid core. Liq. Cryst., 2014, 41(8), 1116-1129.
DOI: 10.1080/02678292.2014.907454.
12. Kula P., Herman J., Perkowski P., Mrukiewicz M., Jaroszewicz L.R. On the influence of the chiral side linking bridge type upon the synclinic vs. anticlinic balance in the case of 2',3'-difluoroterphenyl derivatives. Liq. Cryst, 2013, 40 (2), 256-266.
DOI: 10.1080/02678292.2012.738311.
13. W^glowska D., Perkowski P., Piecek W., Mrukiewicz M., D^browski R. The effect of the octan-3-yloxy and the octan-2-yloxy chiral moieties on the mesomorphic properties of ferroelectric liquid crystals. RSC Adv., 2015, 5, 81003-81012. DOI: 10.1039/c5ra14903g.
14. Pramanik A., Das M.K., Das B., Hamplova V., Kaspar M., Bubnov A. Self-assembling properties of lactic acid derivative with several ester linkages in the molecular core. Phase Transitions, 2015, 88, 745-757.
DOI: 10.1080/01411594.2015.1025782.
15. Galadyk K., Piecek W., Czerwinski M., Morawiak P., Chrunik M., Raszewski Z. An effect of chiral dopants on mesomorphic and electro-optical properties of ferroelectric smectic mixture. Liq. Cryst., 2019, 46 (15), 21342148. DOI: 10.1080/02678292.2019.1613689.
16. Dardas D. Tuning the electro-optic and viscoelastic properties of ferroelectric liquid crystalline materials. Rheologica Acta, 2019, 58, 193-201.
DOI: 10.1007/s00397-019-01129-z.
17. Fitas J., Marzec M., Kurp K., Zurowska M., Tykarska M., Bubnov A. Electro-optic and dielectric properties of new binary ferroelectric and antiferroelectric liquid crystalline mixtures. Liq. Cryst., 2017, 44 (9), 1468-1476. DOI: 10.1080/02678292.2017.1285059.
18. Nataf G.F., Guennou M., Scalia G., Moya X., Wilkinson T.D., Lagerwall J.P.F. High-contrast imaging of 180° ferroelectric domains by optical microscopy using ferroelectric liquid crystals. Appl. Phys. Lett, 2020, 116 (21), 212901-1-5. DOI: 10.1063/5.0008845.
19. Zurowska M., Dziaduszek J., Szala M., Morawiak P., Bubnov A. Effect of lateral fluorine substitution far from the chiral center on mesomorphic behaviour of highly titled antiferroelectric (S) and (R) enantiomers. J. Mol. Liq, 2018, 267, 504-510.
DOI: 10.1016/j.molliq.2017.12.114.
20. Barman B., Das B., Das M.K., Hamplova V., Bubnov A. Effect of molecular structure on dielectric and electro-optic properties of chiral liquid crystals based on lactic acid derivatives. J. Mol. Liq., 2019, 283, 472-481. DOI: 10.1016/j.molliq.2019.03.071.
21. Herman J., Aptacy A., Dmochowska E., Perkowski P., Kula P. The effect of partially fluorinated chain length on the mesomorphic properties of chiral 2',3'-difluoroterphenylates. Liq. Cryst., 2020, 47 (14-15), 2332-2340. DOI: 10.1080/02678292.2020.1811410.
22. Iwasaki Y., Yoshino K., Uemoto T., Inuishi Y. Colour switching in ferroelectric smectic liquid crystal by electric field. Jpn. J. Appl. Phys, 1979, 18 (12), 2323-2324. DOI: 10.1143/JJAP.18.2323.
23. Pozhidaev E., Chigrinov V.G., Li X.H. Photoaligned ferroelectric liquid crystal passive matrix display with memorized gray scale. Jpn. J. Appl. Phys., 2006, 45, 875-882. DOI:10.1143/JJAP.45.875.
24. Fan F., Srivastava A.K., Chigrinov V.G., Kwok H.S. Switchable liquid crystal grating with sub millisecond response. Appl. Phys. Lett, 2012, 100 (11), 111105-1-4. DOI: 10.1063/1.3693601.
25. Brodzeli Z., Silvestri L., Michie A., Guo Q., Pozhidaev E.P., Chigrinov V.G., Ladouceur F. Reflective mode of deformed-helix ferroelectric liquid crystal cells for sensing applications. Liq. Cryst, 2013, 40, 14271435. DOI: 10.1080/02678292.2013.807942.
26. Clark N.A., Lagerwall S.T. Submicrosecond bistable electro-optic switching in liquid crystals. Appl. Phys. Lett, 1980, 36 (11), 899-901.
DOI: 10.1063/1.91359.
27. Clark N.A., Handschy M.A., Lagerwall S.T. Ferroelectric liquid crystal electro-optics using the surface stabilized structure. Mol. Cryst. Liq. Cryst, 1983, 94, 213233. DOI: 10.1080/00268948308084258.
28. Beresnev L.A., Chigrinov V.G., Dergachev D.I., Poshi-daev E.P., Fünfschilling J., Schadt M. Deformed helix ferroelectric liquid crystal display: A new electrooptic mode in ferroelectric chiral smectic C* liquid crystals. Liq. Cryst, 1989, 5, 1171-1177.
DOI: 10.1080/02678298908026421.
29. Chigrinov V.G., Pozhidaev E., Srivastava A., Qi G., Kwok H.S. Fast ferroelectric liquid crystal modes for field -sequential-color and 3D displays. SID Symp. Dig. Tech. Pap, 2012, 43 (1), 117-120.
DOI: 10.1002/j.2168-0159.2012.tb05725.x.
30. Guo Q., Brodzeli Z., Pozhidaev E.P., Fan F., Chigrinov V.G., Kwok H.S., Silvestri L., Ladouceur F. Fast electrooptical mode in photo-aligned reflective deformed helix ferroelectric liquid crystal cells. Opt. Lett., 2012, 37, 2343-2345.
DOI: 10.1364/0L.37.002343.
31. Srivastava A.K., Hu W., Chigrinov V.G., Kiselev A.D., Lu Y-Q. Fast switchable grating based on orthogonal photo alignments of ferroelectric liquid crystals. App. Phys. Lett, 2012, 101, 031112-1-4.
DOI: 10.1063/1.4737642.
32. Pozhidaev E.P., Kiselev A.D., Srivastava A.K., Chigrinov V.G., Kwok H.S., Minchenko M.V. Orientational Kerr effect and phase modulation of light in deformed-helix ferroelectric. Phys. Rev. E, 2013, 87 (5), 0525021-8. DOI: 10.1103/PhysRevE.87.052502.
33. Kiselev A.D., Kesaev V.V., Pozhidaev E.P. Orienta-tional Kerr effect in liquid crystal ferroelectrics and modulation of partially polarized light. J. Phys.: Conf. Ser, 2018, 1092, 012056-1-4.
DOI: 10.1088/1742-6596/1092/1/012056.
34. Kurp K., Czerwinski M., Tykarska M. Ferroelectric compounds with chiral (S)-1-methylheptyloxycarbonyl terminal chain - their miscibility and a helical pitch. Liq. Cryst, 2015, 42 (2), 248-254.
DOI: 10.1080/02678292.2014.982222.
35. Kurp K., Czerwinski M., Tykarska M., Bubnov A. Design of advanced multicomponent ferroelectric liquid crystalline mixtures with submicrometre helical pitch. Liq. Cryst, 2017, 44 (4), 748-756.
DOI: 10.1080/02678292.2016.1239774.
36. Kurp K., Czerwinski M., Tykarska M., Salamon P., Bub-nov A. Design of functional multicomponent liquid crystalline mixtures with nano-scale pitch fulfilling deformed helix ferroelectric mode demands. J. Mol. Liq, 2019, 290, 111329-1-10.
DOI: 10.1016/j.molliq.2019.111329.
37. Czerwinski M., Galadyk K., Morawiak P., Piecek W., Chrunik M., Kurp K., Kula P., Jaroszewicz L.R. Pyrimi-dine-based ferroelectric mixtures-The influence of oli-gophenyl based chiral doping system. J. Mol. Liq., 2020, 303, 112693-1-14.
DOI: 10.1016/j.molliq.2020.112693.
38. Wfglowska D., Perkowski P., Chrunik M., Czerwinski M. The effect of dopant chirality on the properties of self-assembling materials with a ferroelectric order. Phys. Chem. Chem. Phys., 2018, 20 (14), 9211-9220.
DOI: 10.1039/C8CP01004H.
39. Lalik S., Deptuch A., Fryn P., Jaworska-Gol^b T., W^glowska D., Marzec M. Physical properties of new binary ferroelectric mixture. J. Mol. Liq., 2019, 274, 540-549. DOI: 10.1016/j.molliq.2018.11.010.
40. Kotova S.P., Samagin S.A., Pozhidaev E.P., Kise-lev A.D. Light modulation in planar aligned short-pitch deformed-helix ferroelectric liquid crystals. Phys. Rev. E, 2015, 92 (6), 062502-1-13.
DOI: 10.1103/PhysRevE.92.062502.
41. Mikhailenko V., Krivoshey A., Pozhidaev E., Popova E., Fedoryako A., Gamzaeva S., Baibashov V., Srivastava A.K., Kwok H.S., Vashchenko V. The nano-scale pitch ferroelectric liquid crystal materials for modern display and photonic application employing highly effective chiral components: Trifluoromethylalkyl diesters of p-terphe-nyldicarboxylic acid. J. Mol. Liq, 2019, 281, 186-195. DOI: 10.1016/j.molliq.2019.02.047.
42. Malik P., Raina K.K., Bubnov A., Chaudhary A., Singh R. Electro-optic switching and dielectric spectroscopy studies of ferroelectric liquid crystals with low and high spontaneous polarization. Thin Solid Films, 2010, 519 (3), 1052-1055. DOI: 10.1016/j.tsf.2010.08.042.
43. Kula P, Herman J, Chojnowska O. Synthesis and properties of terphenyl- and quaterphenyl-based chiral diesters. Liq. Cryst, 2013, 40 (1), 83-90. DOI:10.1080/02678292.2012.733033.
44. Pozhidaev E.P., Torgova S.I., Molkin V.M., Minchenko M.V., Vashchenko V.V., Krivoshey A.I., Strigazzi A. New chiral dopant possessing high twisting power. Mol. Cryst. Liq. Cryst., 2009, 509 (1), 300-308.
DOI: 10.1080/15421400903054667.
45. Matsumoto T., Fukuda A., Johno M., Motoyama Y., Yui T., Seomun S.S, Yamashita M. A novel property caused by frustration between ferroelectricity and anti-ferroelectricity and its application to liquid crystal dis-plays-frustoelectricity and V-shaped switching. J. Mater. Chem, 1999, 9, 2051-2080.
DOI: 10.1039/A903273H.
46. Drzewinski W., Czuprynski K., D^browski R., Neubert M. New antiferroelectric compounds containing partially fluorinated terminal chains. Synthesis and mesomorphic properties. Mol. Cryst. Liq. Cryst, 1999, 328, 401-410. DOI: 10.1080/10587259908026083.
47. Mandal P.K., Jaishi B.R., Haase W., D^browski R., Tykarska M., Kula P. Optical microscopy, DSC and dielectric relaxation spectroscopy studies on a partially fluorinated ferroelectric liquid crystalline compound MHPO(13F)BC. Phase Transitions, 2006, 79 (3), 223235. DOI: 10.1080/01411590500500724.
48. Murashiro K., Kikuchi M., Inoue H., Fukushima M., Saito S. Dopant effect on threshold electric field of anti-ferroelectric liquid crystal switching. Liq. Cryst, 1993, 14 (2), 371-380.
DOI: 10.1080/02678299308027652.
49. Sokól E., Drzewinski W., Dziaduszek J., D^browski R., Bennis N., Otón J. The synthesis and properties of novel partially fluorinated ethers with high tilted anticlinic phase. Ferroelectrics, 2006, 343, 41-48.
DOI: 10.1080/00150190600962051.
50. Zurowska M., D^browski R., Dziaduszek J., Garbat K., Filipowicz M., Tykarska M., Rejmer W., Czuprynski K., Spadlo A., Bennis N., Oton J.M. Influence of alkoxy chain length and fluorosubstitution on mesogenic and spectral properties of high tilted antiferroelectric esters. J. Mater. Chem., 2011, 21 (7) 2144-2153.
DOI: 10.1039/C0JM02015J.
51. Piecek W., Perkowski P., Raszewski Z., Morawiak P., Zurowska M., D^browski R., Czuprynski K. Long pitch orthoconic antiferroelectric binary mixture for display applications. Mol. Cryst. Liq. Cryst., 2010, 525, 160172. DOI: 10.1080/15421401003796223.
52. Piecek W., Bubnov A., Perkowski P., Morawiak P., Ogrodnik K., Rejmer W., Zurowska M., Hamplová V., Kaspar M. An effect of structurally non-compatible additive on the properties of a long-pitch orthoconic anti-ferroelectric mixture. Phase Transitions, 2010, 83 (8), 551-563. DOI: 10.1080/01411594.2010.499496.
53. Pozhidaev E.P., Vashchenko V.V., Mikhailenko V. V., Krivoshey A.I., Barbashov V.A., Shi L., Srivastava A.K., Chigrinov V.G., Kwok H.S. Ultrashort helix pitch anti-ferroelectric liquid crystals based on chiral esters of ter-phenyldicarboxylic acid. J. Mater. Chem. C, 2016, 4, 10339-10346. DOI: 10.1039/C6TC04087J.
Contribution of the authors:
1Czerwinski Michal - development of the research concept and planning of the experiment, including the compositions of the developed mixtures; performing measurements of the parameters of the helicoid structure, director angle, spontaneous polarization, switching times and rotational viscosity of chiral phases of chiral mixtures; participation in collecting and analyzing the results; participation in the preparation of the manuscript
2Tykarska Marzena complicity in development of the research concept, including the compositions of the developed mixtures; participation in the preparation of the manuscript; carrying out the publication process 3Kula Przemyslaw - participation in the preparation of the manuscript.
Authors declare that the manuscript is a review of recent work and in such form was not published anywhere.
1https://orcid.org/0000-0002-9961-535X 2https://orcid.org/0000-0002-1409-8374 3https://orcid org/0000-0002-7862-7968
Поступила 2.11.2021, одобрена 22.11.2021, принята 25.11.2021 Received 2.11.2021, approved 22.11.2021, accepted 25.11.2021
