Phase Behavior and Dynamics of Binary and Multicomponent

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Phase Behavior and Dynamics of The Binary and Multicomponent Thioester Liquid Crystal Mixtures Arkadiusz Rudzki, S#awomir Zalewski, Beata Suchodolska, Jan Czerwiec, Miroslawa D. Ossowska-Chrusciel, and Janusz Chru#ciel J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b08370 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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The Journal of Physical Chemistry

Phase Behavior and Dynamics of the Binary and Multicomponent Thioester Liquid Crystal Mixtures Arkadiusz Rudzki1* Sławomir Zalewski1 Beata Suchodolska1, Jan Czerwiec2, Mirosława D. Ossowska-Chruściel1, and Janusz Chruściel1 1

The Faculty of Sciences, Siedlce University of Natural Sciences and Humanities, 3-go Maja 54,

08-110 Siedlce, Poland 2

The Faculty of Physics, Jagiellonian University, Ingardena 3, 30–060 Kraków, Poland

Email: [email protected] Tel. +48 25 643 11 08

ABSTRACT

ACS Paragon Plus Environment

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The results of studies conducted by means of the DSC, TLI, POM complementary methods and the electro-optical method as well as dielectric relaxation spectroscopy of two new liquid crystal mixtures are presented. The first mixture is an equimolecular binary mixture consisting of two ferroelectric chiraly liquid crystals from the homologous series, abbreviated as (S)-MHOBSn. The second mixture is multicomponent and consists of four mesogens from the homologous series of nOS5 as the base of the mixture abbreviated as 610712 and chiral MHOBS8. The binary MHOBS4+MHOBS7 mixture has the enantiotropic phase sequence

as follows: Cr,

SmG*, SmI*, SmC*, N*. The second mixture has the same phase sequence but a much wider temperature range a first of all with reference to ferroelectric SmC*. Switching time changes from 50 to 90 µs in the range of SmC* of the binary MHOBS4+MHOBS7 mixture, while in 610712+MHOBS8 the change is more than an order of magnitude greater and it ranges from 500 to 3500 µs. Numerical analysis of the dielectric spectra results points at the complex dynamics of the MHOBS4+MHOBS7 and 610712+MHOBS8 mixtures. The relaxation processes in the crystalline, SmC*, SmI* and SmG* sub-phases have been observed and described. The relaxation processes have been detected down to 12°C. There is a very low intensity Goldstone mode in SmC* and low frequency Non-cancellation mode NCM in the SmI* and SmG* phases.

INTRODUCTION


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The subjects of this study are two new ferroelectric liquid crystal mixtures consisting of calamitic chiral and achiral thioesters. The first mixture A is an equimolecular binary mixture composed of chiral thioesters from the homologous series of (S)-1-methylheptyl-4-(4alkyloxybiphenylthiocarboxy)-benzoates, referred to as (S) MHPSBOn (where n denotes the number of carbon atoms in the alkyl chain), in which the chiral branched chain is attached to the biphenyl system. The mixture has been studied by means of DSC calorimetry, dielectric spectroscopy and electro-optical methods.1-3 MHOBSn possesses a rich phase polymorphism, including the enantiotropic ferroelectric smectic C phase (SmC∗). The second mixture B is a multicomponent mixture of four achiral compounds from the homologous series 4-n-pentylphenyl-4’-n-alkoxythiobenzoates, nOS5 in short4-10, consisting of 6OS5, 7OS5, 10OS5 and 12OS5. The mixture has been defined as 610712. The compounds from the nOS5 homologous series are achiral thioesters, whose molecular core consists of two phenyl rings connected by the thioester bridge –COS-. The terminal groups in nOS5 are alkyl and alkoxy chains, where the alkyl chain has a stable length, which is the pentyl chain -C5H11, and the only variable element of the chemical structure of the nOS5 homologues is the alkoxy chain length.

The mesogens with shorter alkoxy chains, n=6 and 7, have the enantiotropic nematic

phase, and 7OS5 additionally has the monotropic SmC phase. The compounds with longer terminal chains, n=10 and 12, are characterized by rich phase polymorphism in a wide range of temperature. 10OS5 and 12OS5 have the following phases: N, SmA, SmC, SmJ and SmE. The MHOBS8 chiral mesogen from the MHOBSn series was added to the 610712 mixture, which has been assumed to be the base of the four-component system. The molecular model of the 7OS5 (a) and MHOBS8 (b) is presented in Fig. 1. An elongated shape of the 7OS5 is clearly visible in the model, whereas in the case of MHOBS8 the terminal chain, 1-methylheptyl C*H(CH3)C6H13,


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connected through O with the biphenyl system, is distinctly tilted from the para-molecule axis. This fact will probably have an impact on the arrangement of the molecules in the smectic phases layers, including the SmC* phase.

Figure 1. Molecular model of the 7OS5 (a) and MHOBS8 (b). Liquid crystal mixtures have been recently very often prepared, which is mainly due to their application possibilities. The use of such mixtures may lead to changes in the range of existence of condensed phases and changes in phase polymorphism, and it may also cause the alterations of physical properties and molecular dynamics. Liquid crystal mixtures constitute also a very important source of information in the scope of basic research. Recently a large number of scientific papers devoted to this problem have been published. They are dedicated to studies of mixtures of both calamitic and banana-shaped liquid crystals, containing achiral and chiral mesogens, and more lately also to studies of the liquid-crystal-nanomaterial systems, which are investigated with respected to their wide application spectrum.11-23 For many years also our team has been involved in studies of both chiral and achiral liquid crystal mixtures.24-31


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Both A and B mixtures are characterized by the existence of the ferroelectric SmC* phase in a wide range of temperatures. In one of the mixtures (mixture B) the SmC* phase occurs at room temperature, which is very significant from the application point of view. The aim of the present work is to confirm phase polymorphism of both mixtures: the binary mixture A and the multicomponent mixture B, using complementary methods (DSC, POM, TLI), electro-optical results and describe the details of dynamics in, first of all, the SmC* phase, and the SmI* and SmG* phases by dielectric spectroscope in a wide temperature range EXPERIMENTAL All ingredients were synthesized and both A and B mixtures have been preparated by us at the Institute of Chemistry of the Siedlce University of Natural Sciences and Humanities, Poland. The chemical structure components of binary and multicomponent mixtures were confirmed by elemental analysis, infrared spectroscopy (IR), 1H NMR, 13C NMR. The IR spectra were recorded on an FTIR Nicolet Magna 760 spectrometer in the range of 400-4000 cm-1 using 64 co-added scans at the resolution of 1 cm-1. The NMR spectra were obtained with a VARIAN Unity Plus spectrometer operating at 500 MHZ (CDCl3), with TMS as internal standard. The chemical purity was checked by thin layer chromatography (TLC) and further confirmed by elemental analysis using a Perkin-Elmer 2400 spectrometer. Transition temperatures were established from DSC scans, and cross-checked by polarized optical microscopy POM and transmitted light intensity TLI measurements, using a microscope with Bur-Brown OPT101 photodiode. Heating and cooling rates for all DSC and TLI measurements were the same (±2.0 K min-1) and for PMO ±1.0 K min-1. The temperature was controlled with a Linkam controller with an accuracy of ±0.1 K, coupled with a Linkam programmable heating stage THMSE 600. A


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detailed description of our TLI measurements can be found.32 The DSC scans were taken with a DSC 822e Mettler Toledo Star System differential scanning calorimetry. In order to thoroughly describe the phase polymorphism of both A and B mixtures, we used not only the DSC, POM and TLI complementary methods but also the results of electro-optical (EO) studies. The dynamics were investigated by means of dielectric relaxation spectroscopy carried out in low frequency. Basic information about the dielectric properties of the materials was obtained using a Turnkey Alpha-A High Performance Frequency Analyzer from Novocontrol Technologies in a frequency domain - ν from 0.1Hz to 10MHz. The sample was kept in the capacitor consisting of two gold electrodes, separated by 5 µm spacers (AWAT cells). The molecules were arranged perpendicularly (⊥) to the electric field. The surfaces of the electrodes were covered by a thin polymer layer placed by means of the Spin-Coating (SC) method and rubbing, which forced the planar arrangement of the molecules. All measurements were done in pure inert N2 gas atmosphere. Moreover, the thermal stability of both mixtures was also investigated using the calorimetric, DSC and thermogravimetric TG and DTG methods. Multiple cycles of heating and cooling at different rates in the range of temperatures from 273K to 373K proved the stability of the mass, while chemical and spectroscopic analyses demonstrated the chemical stability of both mixtures. These results are of high importance as they confirm the application possibilites of both mixtures in a wide range of temperatures. The subject matter of this paper are two new mixtures, of which one is composed of two chiral mesogens from the MHOBSn series, i.e. MHOBS4 and MHOBS7, and the other consists of four


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achiral thiobenzoates from the nOS5 series, i.e. 6OS5, 7OS5, 10OS5, 12OS5, here treated as the base, with different lengths of the terminal alkoxy chain, referred to as 610712, and the second chiral component, acronymed MHOBS8. Both the binary and multicomponent mixtures were composed in order to investigate how components with phases interacting with the field in a ferroelectric way can influence the mesogenity of the proposed systems of the mixtures. In the MHOBS4+MHOBS7 mixture both components have the SmC* phase, while in the 610712+MHOBS8 only MHOBS8 has a phase interacting with the field. Table 1 shows molecular structures of the components of the mixtures and their phase situations. The chiral compounds from the MHOBSn series are calamitic thioesters, whose molecular core consists of the biphenyl and phenyl ring, connected with each other by the thioester bridge. One of the terminal groups of stable length is the chiral 1-methylheptyloxyl chain, and the other substituent of variable length is the alkyl chain (Table 1). The nOS5 homologous series is composed of achiral thioesters, whose molecular core consists of two phenyl rings connected with each other by the thioester bridge. The terminal groups in nOS5 are alkyl and alkoxyl chains, where the alkyl chain has a stable length, the phenyl chain C5H11, and the only variable element in the chemical structure is the length of the alkoxyl chain.


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Table 1. Overview of component’s molecular structures and phase transitions for mixtures

mixtures

MHOBS4+MHOBS7 and 610712+MHOBS8.

Ingredient

Transition temperatures [°C]

MHOBS4_MHOBS7

MHOBS4 COS

H13C6*CH(CH3)O

C4H9

°C °C °C Iso 121 .6 → N * 90 .9 → SmC * 73 .4 → SmI * °C °C 71 .5 → SmG * 26 .7 → Cr

MHOBS7 °C °C °C Iso 123 .7 → N * 101 .7 → SmC * 78 .3 → SmI *

C7H15

COS

H13C6*CH(CH3)O

°C .5 ° C 70 .1 → SmG * 5 → Cr

6OS5

H13C6O

C5H11

COS

10OS5

H21C10O

C5H11

COS

.3°C .7°C .34°C Iso 84  → N 36  → SmC 7 → Cr

610712_MHOBS8

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7OS5

H15C7O

C5H11

COS

12OS5

H23C12O

C5H11

COS


H13C6*CH(CH3)O

COS

C8H17

°C °C 76 .0 → SmG * 51 .6 → Cr


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The equimolecular MHOBS4+MHOBS7 mixture consists of two compounds from the MHOBSn series, whose alkyl chains are of different length (n-4 and n-7) and are characterized by the identical phase sequence (Table 1). From the application point of view the most interesting phase is the SmC* ferroelectric phase. The range of the SmC* phase is 17.5 deg in MHOBS4 and 23.4 deg in MHOBS7, respectively (Table 1). The components of the five-component 610712+MHOBS8 mixture are as follows: the fourcomponent 610712 mixture and MHOBS8, mixed together with respect to the weight ratio of 0.0360:0.0502. The 610712 base, consisting of four homologues from the nOS5 series, has the nematic phase N and the smectic phase C, SmC, of a range of 29.4 deg. The chiral MHOBS8 mesogen is characterized by the range of the SmC* phase amounting to 23.6 deg (Table 1). In order to obtain the mixtures the components of the following compositions had to be prepared: -

mixture A: MHOBS4+MHOBS7 mixture, an equimolecular binary mixture consisting of

two chiral mesogens from the MHOBSn homologous series, with n=4 and n=7, i.e. MHOBS4 and MHOBS7; -

mixture B, 610712+MHOBS8, a multicomponent mixture consisting of four achiral

liquid crystals from the nOS5 homologous series, with n=6, 6OS5; n=7, 7OS5; n=10, 10OS5 and n=12, 12OS5 and one chiral mesogen from the MHOBSn homologous series, with n=8, i.e. MHOBS8. In order to compose mixture B, two transitional mixtures B1 and B2 consisting of achiral thioesters from the nOS5 homologous series had to be prepared. Phase polymorphism and the


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widest temperature range of phases were taken into account in the preparation of these two mixtures. Full ranges of phase diagrams of two mixtures: 6OS5+10OS5 and 7OS5+12OS5 were investigated and two transitional mixtures with the following compositions were selected for further studies: B1 : 6OS5 – 0.7 mole

B2 : 7OS5 – 0.82 mole

10OS5 – 0.3 mole

12OS5 – 0.18 mole

Next, the B1 and B2 mixtures were used to compose the B3 mixture with respect to the equilibrium rate of 1:1. The chiral MHOBS8 mesogen was added weightily to the achiral B3 mixture in the range from 10% to 90%. Based on the analysis of the phase diagram of the B3 + MHOBS8 mixture, a system containing 30% weightily of MHOBS8 with respect to the B3 mixture was selected for further studies. The comparison of both mixtures and their phase polymorphism are shown in Table 1. RESULTS AND DISCUSSION MIXTURE MHOBS4+MHOBS7 - MESOMORPHIC PROPERTIES DSC measurements were conducted in a few subsequent cycles of heating and cooling in order to confirm the repeatability of thermal effects with regard to both temperature and enthalpy of phase transitions, which indicates that this mixture is characterized by high thermal stability and lack of metastable phases. The phase situation of the mixture obtained during heating and cooling is presented in Fig. 2.


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Figure 2. DSC curve obtained during heating (thick line) and cooling (thin line) of mixture MHOBS4_MHOBS7 (±2oC·min-1). During heating and cooling e mixture the following phase polymorphism has been observed (Table 2) : Heating: Cr–13.74°C–SmG*–68.75°C–SmI*–74.13°C–SmC*–94.81°C–N*–122.5°C–Iso Cooling: Iso–122.4°C–N*–94.21°C–SmC*–74.04°C–SmI*–68.84°C–SmG*– -5.24°C –Cr The MHOBS4+MHOBS7 mixture has the following enantiotropic phases N*, SmC*, SmI*, which demonstrate similar temperature ranges of existence in both cycles, and the SmG* phase chracterized by a wider range of existence in the cooling cycle. The SmC* phase interacting with the field has the existence range of ca. 21 deg during both heating and cooling. When compared to the ranges of the SmC* phase in clean components (MHOBS4 – 17.5 deg, MHOBS7 – 23.4 deg), we can come to the conclusion that the range of the SmC* mesophase in the mixture has not increased and that there have been no significant shifts in the temperatures of the N* – SmC* phase transition in comparison to the temperatures of the same transition in clean components.


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Table 2. Phase polymorphism of MHOBS7+MHOBS8 mixture obtained by DSC and TLI methods.

Phase transition →SmG* →SmI* →SmC* →N* →Iso →N* →SmC* →SmI* →SmG* →Cr

measurement method DSC T (oC) ∆H (J·g-1) heating 16.7 17.7 13.25 67.4 69.1 0.35 74.3 74.6 4.71 92.0 95.2 2.77 124.3 122.7 2.63 Cooling 122.2 122.6 2.78 88.3 95.0 2.90 71.8 74.4 4.79 64.0 68.8 0.33 -6.9 -5.2 8.19

TLI T (oC)

Fig. 3 shows a phase thermogram obtained with the use of the TLI method. The characteristic changes in the intensity of light passing through the sample visible on the curves shown below correspond to the points of all phase transitions determined with the use of other methods, mainly with DSC. Minor differences in the values of the temperatures of the phase transitions in the TLI method may result from different arrangements of the samples and their different masses in both methods ((DSC – ca. 10 mg, TLI – ca. 1mg). Microscopic observations (POM) of the mixture corroborate its phase situation determined with the DSC and TLI methods. Figure 4 shows liquid-crystalline textures of the studied mixture in the nematic N* (A) and SmC* (B) phase.


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Figure 3. TLI curve obtained during heating (thick line) and cooling (thin line) of mixture MHOBS4_MHOBS7 (±2oC·min-1).

Figure 4. Textures for the N*(113.8°C - A) and SmC* phase (80.0°C - B) obtained during cooling. The photos of these textures were taken during cooling of the mixture at a rate of 1 deg·min-1. The texture of the cholesteric phase is characteristic, and similar textures have been observed in the N* phases of clean components. The texture of the SmC* phase visible in photo B has a fanshaped structure, similarly to clean components.


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MIXTURE MHOBS4+MHOBS7 - ELECTRO-OPTICAL PROPERTIES MHOBS4+MHOBS7 mixture is characterized by a quite wide range of the SmC* phase. Below the comparison of the results of electro-optical studies is shown. Within these studies the following electro-optical parameters were determined: spontaneous polarization, tilt angle of molecules, switching time, characteristics of optical switching. Spontaneous polarization measurements were performed by reversal current method - RCM. The experimental set-up consists of Agilent 3310A wave form generator, FLC electronics amplifier F20ADI, and digital scope Agilent DSO6102A. Figure 5 presents schematically Ps measurements in RCM. The sample was kept in the liquid crystal cell (LC Cell), this capacitor consisting of two indium-tin oxide-glass (ITO-glass) electrodes, covered with strongly rubbed polymer layers, separated by 5 mm spacers (Linkam–HG planar cells) and 0.25mm2 surface area interacting with electric field. Resistor was 100kΩ, capacity of empty cell was 53.2pF. All measurements were done in cooling, rapidity was 2°C·min-1. Figure 6 presents typical signal in channel 1 and material response in channel 2.

Figure 5. Scheme of experimental set-up for measurements of Spontaneous Polarization by reversal current method.


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Figure 6. Reversal current spectrum of ferroelectric SmC* sub-phase of MHOBS4+MHOBS7 in 11°C. Figures 7 and 8 present changes in the value of spontaneous polarization in the SmC* phase measured by means of the molecule reorientation method using the electric field of rectangular modulation of +/- 10V in two different frequencies, 10 and 50 Hz.

Figure 7. Changes in the value of spontaneous polarization as a function of temperature in SmC* phase for mixture MHOBS4+MHOBS7 (U=+/-10V, f=10Hz).


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Both the value of spontaneous polarization within the limits of 40 – 65 nC·cm-2 as well as its tendency to increase are analogous as in clean components.

Figure 8. Changes in the value of spontaneous polarization as a function of temperature in SmC * phase for mixture MHOBS4+MHOBS7 (U=+/-10V, f=50Hz). Figure 9 shows changes in the value of switching time of the molecules of the mixture in the SmC* phase as a function of temperature. The switching time amounts to ca. 50 µs in the temperature range of 91 - 83°C. It increases with increasing temperature and reaches a threshold value of even 90 µs. In comparison to the values of switching time in clean components (ca. 35 – 55 µs), the preparation of the mixture has resulted in the double prolongation of the switching time. Below (Fig. 10) the comparison of measurements of the tilt angle of molecules in the SmC* phase is shown. The values of the tilt angle of molecules are confined within the limits of 3 - 36 deg in each measurement. These values are characteristic of the smectic C* phase. The values of the tilt angle of molecules in clean components are similar, so no destabilization of this parameter has taken place.


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Figure 9. Changes in the value of switching time of molecules as a function of temperature in the phase of SmC * for mixture MHOBS4+MHOBS7.

Figure 10. Changes in the value of tilt angle of molecules as a function of temperature in the phase of SmC * for mixture MHOBS4+MHOBS7. Figure 11 presents measurement results of the properties of the sample optical response taken during cooling of the sample in four different temperatures of the SmC* phase existence. The SmC* phase in the mixture is of ferroelectric nature and is characterized by a W-shaped type of two-state switching, which is typical of ferroelectric liquid crystals. The properties of optical


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switching in the SmC* phase for the investigated mixture do not undergo any significant changes in different temperatures of the mesophase, which confirms the stability of the SmC* phase in the mixture.

Figure 11. Transmitted light curves vs. applied triangular-wave electric field of frequency 10Hz and amplitude ±15V/3,2µm for the mixture MHOBS4+MHOBS7 obtained during cooling at chosen temperatures of SmC* phase. MIXTURE 610712+MHOBS8 - MESOMORPHIC PROPERTIES The phase situation of the mixture obtained during heating and cooling is shown in Fig. 12. During heating and cooling of the mixture the following phase polymorphism has been observed (Table 3) : Heating: Cr -3.38°C - SmG*–7.83°C –SmI*–26.53°C –SmC*–52.10°C–N*–93.,04°C–Iso Cooling: Iso–94.04°C–N*–51.21°C–SmC*–6.87°C–SmI*–1.62°C–SmG*


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Figure 12. DSC curve obtained during heating (thick line) and cooling (thin line) of mixture 610712+MHOBS8 (±2oC·min-1). Table 3. Phase polymorphism of 610712+MHOBS8 mixture obtained by DSC and TLI methods.


measurement method DSC T (oC) ∆H (J·g-1) Heating -1.0 3.4 0.79 4.2 7.9 1.19 30.4 26.5 4.93 46.7 52.1 0.20 93.4 93.0 4.07 Cooling 91.0 92.6 4.01 45.2 51.9 0.26 3.2 6.9 2.54 0.7 1.6 0.96 -3.3 -a -

TLI T (oC)

The 610712+MHOBS8 mixture has enantiotropic phases N*, SmC*, SmI* i SmG*. The SmC* phase interacting with the field demonstrates the range of existence ca. 44 deg during cooling and its range encompasses room and lower temperatures, which makes it interesting from the application point of view. In comparison with the range of the SmC* phase in clean

a

not observed


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component MHOBS8 (ca. 24 deg) it can be concluded that the range of the SmC* mesophase in the mixture has increased. A considerable shift in the temperature of the N* – SmC* phase transition has taken place when compared with the temperatures of this shift in clean component MHOBS8. Fig. 13 presents a phase thermogram obtained with the use of the TLI method. The characteristic changes in the intensity of light passing through the sample visible on the curves shown below correspond to the points of all phase transitions determined with the use of other methods, mainly with DSC.

Figure 13. TLI curve obtained during heating (thick line) and cooling (thin line) of mixture 610712+MHOBS8 (±2oC·min-1). Microscopic observations (POM) of the mixture corroborate its phase situation determined with the DSC and TLI methods. Fig. 14 shows liquid-crystalline textures for the studied mixture in the nematic N* (A) and SmC* (B) phases.


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Figure 14. Textures for the N*(82.0°C - A) and SmC* phase (32.0°C - B) obtained during cooling. The photos of the textures have been taken during cooling of the mixture at a rate of 1 deg·min-1. The texture of the cholesteric phase is characteristic and similar textures can be observed in the N* phase for MHOBS8. The texture of the SmC* phase visible in photo B has a fine-grained structure and is similar to the texture of the SmC phase in the 610712 matrix. MIXTURE 610712+MHOBS8 - ELECTRO-OPTICAL PROPERTIES The 610712+MHOBS8 mixture is characterized by a quite wide range of the SmC* phase. Below the comparison of the results of electro-optical studies is shown. Within these studies the following electro-optical parameters were determined: spontaneous polarization, switching time, characteristics of optical switching. Figures 15 and 16 present changes in the value of spontaneous polarization in the SmC* phase measured by means of the molecule reorientation method using the electric field of rectangular modulation of +/- 60V in two different frequencies, 10 and 50 Hz.


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Figure 15. Changes in the value of spontaneous polarization as a function of temperature in SmC* phase for mixture 610712+MHOBS8 (U=+/-60V, f=50Hz).

Figure 16. Changes in the value of spontaneous polarization as a function of temperature in SmC* phase for mixture 610712+MHOBS8 (U=+/-60V, f=10Hz). The value of spontaneous polarization of the SmC* phase in the 610712+MHOBS8 mixture has undergone a significant reduction and amounts to 1 – 5 nC·cm-2, which is more than 10 times lower in comparison to clean MHOBS8 (40 – 50 nC·cm-2). Changes in the value of spontaneous polarization as a function of temperature for the mixture are of chaotic nature and are different


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from a relatively stable course of changes in the Ps value in MHOBS8. This may indicate that the molecular arrangement in the SmC* phase in the obtained mixture has gone considerable deterioration. This has been confirmed by measurements of switching time (Fig. 17), increasing as a function of temperature from 0.5 to 3.5 ms. The switching time of the molecules in the SmC* phase for the 610712+MHOBS8 mixture is much longer in comparison with τs in the SmC* phase of clean MHOBS8, which was within the limits of 35 – 55 µs.

Figure 17. Changes in the value of switching time of molecules as a function of temperature in the phase of SmC* for mixture 610712+MHOBS8. Figure 18 shows the comparison of properties of the optical sample response for selected temperatures in the SmC* phase. Their course is disturbed and changes with changing temperature, which may indicate that the molecular arrangement in the mixture is violated.


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Figure 18. Transmitted light curves vs. applied triangular-wave electric field of frequency 5Hz and amplitude ±10V/3.2µm for the mixture 610712+MHOBS8 obtained during cooling at chosen temperatures of SmC* phase. The properties of the optical response in the SmC* phase for clean MHOBS8 have a stable, Wshaped course, and the above characteristics obtained for the mixture of this compound with the 610712 base indicate a clearly disturbed course, which is hard to analyze and suggests considerable fluctuations in the molecular arrangement in the layers of the SmC* phase. DIELECTRIC MEASUREMENT - THEORETICAL BACKGRAOUND Impedance spectroscopy is an important tool used for investigating the dielectric relaxation processes in anisotropic materials. This is a powerful and most effective technique for analyzing the relationship between the micro-structure and properties. The basic information about the dielectric properties of materials was obtained using a Turnkey Alpha-A High Performance.


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Frequency Analyzer from Novocontrol Technologies in a frequency domain - ν from 0.1Hz to 10MHz, value of the probing voltage amplitude was 0.2V without bias field. This value is below the thermal voltage and was confirmed by the preliminary tests. The sample was kept in the capacitor consisting of two gold electrodes, separated by 5 µm spacers (AWAT cells). The molecules were arranged perpendicularly (⊥) to the electric field. On the surfaces of the electrodes a thin polymer layer was placed by means of the Spin-Coating (SC) method and rubbing, which forced the planar arrangement of the molecules. All measurements were done in pure inert N2 gas atmosphere. The equations present: (1) complex dielectric function, (2) generalized Cole-Cole equation (3) dielectric increment and (4) time relaxation. ߝୄ∗ ሺνሻ = ߝୄᇱ ሺνሻ − ݅ߝୄᇱᇱ ሺνሻ ߝୄ∗ ሺνሻ = ߝୄ ሺ∞ሻ +

∆ఌ఼

భషഀ

ν ଵା൬௜ ൰ νೃ

஻

+ νಿ + ݅ ఢ

(1) ఙ఼

ಾ బ ଶగν

∆ߝୄ = ߝୄ ሺ0ሻ − ߝୄ ሺ∞ሻ ଵ

߬ோ = ଶగν

ೃ

(2) (3)

(4)

where ߝୄ∗ ሺνሻ equation (5) is the perpendicular complex dielectric constant as a function of frequency, ߝୄᇱ ሺνሻ is the real component of permittivity representing the collective lossless permittivity given by the product of the free space permittivity ߳଴ and the relative real permittivity ߝ௥ᇱ . The ⊥ symbol in the lower index means that the electrostatic field was perpendicular to the gold electrode plates.


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ߝୄᇱ ሺνሻ = ߳଴ ߝ௥ᇱ

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(5)

ߝୄᇱᇱ ሺνሻ is the imaginary component of permittivity attributed to bound charge and dipole relaxation phenomena, which increases energy loss, ߝୄ ሺ0ሻ is the static (low frequency) permittivity, ߝሺ∞ሻ is the infinite (high) frequency permittivity, νோ is the characteristic relaxation frequency of the measured substance, ߙ is the distribute relaxation time parameter which takes a value between 0 and 1. When ߙ = 0, the Cole-Cole model reduces to the Debye model. ‫ ܤ‬is the parameter responsible for the polarity of the electrodes, ߪୄ is the parameter responsible for the ionic conductivity, the ܰ and ‫ ܯ‬parameters are exponents of the polynomial equation, ߳଴ is vacuum permittivity. Impedance analyzers can measure the ‫ܥ‬௣ capacity and the dielectric loss tangent ‫ ߜ݊ܽݐ‬are given by the equations (6) and (7): ௜ఙ

‫ܥ‬௣ = ‫ܥ‬଴ ቀߝୄᇱ + ଶగνఢ ቁ ‫= ߜ݊ܽݐ‬

బ

ᇲᇲ ሺνሻାఙ ଶగνఌ఼ ఼ ᇲ ሺνሻ ଶగνఌ఼

(6)

(7)

The ‫ܥ‬௉ apacity and the dielectric loss factor ‫ ߜ݊ܽݐ‬can be calculated to ߝୄᇱ ሺνሻ and ߝୄᇱᇱ ሺνሻ using equations (1-7). DIELECTRIC MEASUREMENT - RESULTS AND DISCUSSION Figure 19 presents dielectric measurement results: dispersion ߝ′ୄ ሺνሻ (upper portion) and absorption ߝ′′ୄ ሺνሻ (lower portion) of Goldstone mode (GM), which is one of the collective processes in the SmC* phase at 81°C of the MHOBS4+MHOBS7 mixture. This process is connected with the collective molecular motion around the cone by changing an azimuthal angle. Fig. 20 presents results in Cole-Cole representation at a complex plane in the same sub-phase


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and in the same temperature as in Fig. 19. The boxes (in Figs. 19 and 20) present fitted parameters of Eq. 2. All solid lines have been obtained by fitting Equation (2) to the experimental data.

Figure 19. Dielectric spectrum obtained for the SmC* phase. Dispersion and absorption curves (solid lines – red and blue, respectively) computed by fitting to the experimental points using Eq. (2). Dashed line (red) – computed dispersion curve of Goldstone mode. Dotted line (red) – electrode polarization contribution to electric permittivity. Dashed line (blue) – Goldstone mode absorption curve. Dotted (blue) – ionic conductivity contribution to dielectric absorption. The dielectric spectra were processed using ORIGIN 7.0 software from OriginLab Corporation. The following complex function was fit to the experimental points measured using eq. 2. Nonlinear least squares fitting (NLSF) methods were used. NLSF based on the LevenbergMarquardt - LM algorithm and is the most widely used algorithm in nonlinear least squares fitting


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Figure 20. Cole-Cole plot for the SmC* phase. Cole-Cole plot (blue solid line) for the Goldstone mode. Effects related to the polarity of the electrodes and ionic conductivity (red solid line). Fig. 21 presents dielectric increment and relaxation time vs. temperature. A characteristic flat line (red solid line) in the ferroelectric SmC* representing Goldstone mode can be observed, which is correct with Blinc and Žekš theoretical predictions based on the mean-field model.32 Figure 22 also presents activation energy of the process in the SmI* sub phase, which has been calculated using Arrhenius equation (8), where ν଴ denotes the pre-exponential factor (frequency factor) in [Hz], ݇஻ - Boltzmann constant, ܶ - absolute temperature in the Kelvin scale and ‫ܧ‬௔ is the activation energy in joules per mole of the substance [J·mol-1]. ா

νோ ሺܶሻ = ν଴ ݁‫ ݌ݔ‬ቀ− ௞ ೌ்ቁ ಳ

(8)


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Figure 21. Dielectric increment (left part) and relaxation time (right part) vs. temperature.

Figure 22. Arrhenius plots for the relaxation processes observed. Fig. 23 presents activation energy (calculated using modified Arrhenius equation of ionic conductivity (9). ா

ߪୄ ሺܶሻ = σ଴ ݁‫ ݌ݔ‬ቀ− ௞ ್்ቁ ಳ

(9)

where σ଴ is the pre-exponential factor (ion conductivity factor) in [S·m-1] and ‫ܧ‬௕ is the activation energy for ionic conductivity in joules per mole of the substance [J·mol-1].


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In the SmI* sub-phase the activation energy is more than two times greater than in SmC*. In the SmI* sub-phase, the molecules form a more compact structure than in SmC*, which means that a greater contribution of energy is necessary for ion transport between the molecular layers. Figure 24 presents dielectric measurement results: dispersion ߝ′ୄ ሺνሻ (upper portion) and absorption ߝ′′ୄ ሺνሻ (lower portion) of dielectric process in the SmI* phase at 24°C of 610712+MHOBS8 mixture.

Figure 23. Arrhenius plot for ionic conductivity.

Figure 24. Dielectric spectrum obtained for the SmI* phase. Dispersion and absorption curves (solid lines – red and blue, respectively) computed by fitting to the experimental points using Eq.


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(1). Dashed line (red) – computed dispersion curve. Dotted line (red) – electrode polarization contribution to electric permittivity. Dashed line (blue) – absorption curve. Dashed-dotted (blue) – ionic conductivity contribution to dielectric absorption. The temperature is a little bit shifted because the dielectric measurements have been performed at a near-zero rate than the calorimetric measurement. Figure 25 presents results in Cole-Cole representation at a complex plane in the same sub-phase and in the same temperature as in Fig. 24. The boxes (in Figs. 24 and 25) present the fitted parameters of Eq. 2. All solid lines have been obtained by fitting Equation (2) to the experimental data. Figure 26 presents the Arrhenius plot in the frequency representation and Fig. 27 - the Arrhenius plot in the conductivity representation.

Figure 25. Cole-Cole plot for the SmC* phase. Cole-Cole plot (blue solid line) for the dielectric process. Effects related to the polarity of the electrodes and ionic conductivity (red line).


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Figure 26. Arrhenius plots for the relaxation processes observed.

Figure 27. Arrhenius plot for ionic conductivity. In each phase there is one dielectric process as it is presented. The energy activation of this process in the SmC* sub-phase is lower than in than SmI* sub-phase. In frequency domain the energy activation quotient is more than 4, but in conductivity domain the energy activation quotient is near 2. Similar observations have been also conducted for other thermo-tropic liquid crystalline materials.33-37


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CONCLUDING REMARKS The MHOBS4+MHOBS7 mixture has the wide temperature range of a liquid crystal phases from +122.4oC to -5.24oC, during cooling. The switching time amounts to ca. 50 µs in the temperature range of 91 - 83°C (SmC*).The values of the tilt angle of molecules are confined within the limits of 3 - 36 deg. The SmC* phase in the mixture is of ferroelectric nature and is characterized by a W-shaped type of two-state switching. The 610712+MHOBS8 mixture has enantiotropic phases N*, SmC*, SmI* i SmG*. The SmC* phase interacting with the field demonstrates the range of existence ca. 44 deg during cooling and its range encompasses room and lower temperatures, which makes it interesting from the application point of view. The value of spontaneous polarization of the SmC* phase in the 610712+MHOBS8 mixture has undergone a significant reduction and amounts to 1 – 5 nC·cm-2. The switching time of the molecules in the SmC* phase for the 610712+MHOBS8 mixture is from 0.5 to 3.5 ms and much longer in comparison with µs in the SmC* phase of clean MHOBS8, which was within the limits of 35 – 55 µs. Dielectric spectroscopy measurements in a range up to GHz frequencies have given some crucial information on the complex dynamics of the MHOBS4+MHOBS7 and 610712+MHOBS8 mixtures. The relaxation processes (for both mixtures) have been observed and described in the crystalline: SmC*, SmI* and SmG* sub-phases. The relaxation processes have been detected down to 12°C. There is a very low intensity Goldstone mode in the SmC* and low frequency Non-cancelation mode NCM in the SmI* and SmG* phases. Moreover, the activation energy increases when ordering to increasing both in: frequency domain and ionic conductivity. The Cole−Cole function (Eq. 2) with two extra expressions in this polynomial


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Page 34 of 70

equation gives a good correlation between the theoretical and experimental points. MW process in each sub-phases was too small to fitted and presented them. The molecules have the ability to selectively pass ions, depending on the phase situation. The barrier permeability can be activated by temperature. Each of the sub phases has a different value of the energy required for the activation of the ions between the monolayers. Owing to its movement through the molecules, the molecule barrier could be also the movable conduction barrier. A precise description of this aspect will be the subject matter of the next publication.

REFERENCES (1) Ossowska-Chruściel, M.D.; Chruściel, J. Mesomorphic Properties of ( S)-MHPSBO nSeries Termochemica Acta 2010, 502, 51–59. (2) Zalewski, S.; Ossowska-Chruściel, M.D.; Chruściel, J. Mesomorphic Properties of Condensed Phases in MHOBSn, chapter in Neutron Scattering and Complementary Methods in Investigations of Condensed Phase, Soft Matter, University of Podlasie Publishing House, Monograph No. 60, 2005, Vol. 2, 232-245. (3) Mikułko, A.; Marzec, M.; Ossowska-Chruściel, M.D.; Chruściel, J.; Wróbel, S. ElectroOptical and Dielectric Studies in the Vicinity of the Smectic C*-Cholesteric Transition, chapter in Neutron Scattering and Complementary Methods in Investigations of Condensed Phase, Soft Matter, University of Podlasie Publishing House, Monograph No. 60, 2005, Vol. 2, 192-230. (4) Kim, YB; Seno, M. Thermiodynamic and Mesomorphic Properties of Some Phenylthiolbenzoate Derivatives. Mol. Cryst. Liq. Cryst. 1976, 36, 293-306.


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(5) Chruściel, J.; Richter, R.; Rachwalska, M. Study of the Phase Situation in 4-npentylphenyl-4′-n-heptyloxythiobenzoate (7S5). Mol. Cryst. Liq. Cryst. 1981, 75, 153-167. (6) Chruściel, J.; Wróbel, S.; Gestblom, B.; Haase, W. chapter in Modern Topics in Liquid Crystals, World Scientific, Singapore, New Jersey, London, Hong Kong, 1993. (7) Ossowska-Chruściel, M.D.; Karczmarzyk, Z.; Chruściel, J. The Polymorphism Of 4-NPentylphenyl-4″-N-Butyloxythio-Benzoate, (4OS5) In The Crystalline State. Mol. Cryst. Liq. Cryst. 2002, 382, 37-52. (8) Chruściel, J.; Pniewska, B.; Ossowska-Chruściel, M.D. The Crystal and Molecular Structure of 4-pentylphenyl-4′-pentioxythiobenzoate (5S5). Mol. Cryst. Liq. Cryst. 1995, 258, 32-331. (9) Karczmarzyk, Z.; Ossowska-Chruściel, M.D.; Chruściel, J. The Crystal and Molecular Structure of 4-n-pentylphenyl-4′-n-hexyloxythiobenzoate (6OS5). Mol. Cryst. Liq. Cryst. 2001, 357, 117-125. (10) Ossowska-Chruściel, M.D.; Chruściel, J.; Wróbel, S.; Makrenek, M.; Gestblom, B.; Haase, W. Molecular Dynamics in Liquid Crystalline Phases of Achiral Thioesters, chapter in Neutron Scattering and Complementary Methods in Investigations of Condensed Phase, Soft Matter, University of Podlasie Publishing House, Monograph No. 60, 2005, Vol. 2, 209-217. (11) Vieweg, N.; Shakfa, M.K.; Koch, M. BL037: A Nematic Mixture With High Terahertz Birefringence. Optics Communications 2011, 284, 1887-1889. (12) Zaki, A.A. Optical Measurements of Mixture Thermotropic Liquid Crystals. Optics and Lasers in Engineering 2010, 48, 538-542.


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(13) Nowinowski-Kruszelnicki, E.; Kędzierski, J.; Raszewski, Z.; Jaroszewicz, L.; Dąbrowski, R.; Kojdecki, M.; Piecek, W.; Perkowski, P.; Garbat, K.; Olifierczuk, M.; Sutkowski, M.; Ogrodnik, K.; Morawiak, P.; Miszczyk, E. High Birefringence Liquid Crystal Mixtures for Electro-optical Devices. Optica Applicata 2012, 42,165-180. (14) Gauza, S.; Wen, C.H.; Wu, B.; Wu, S.T.; Spadło, A.; Dąbrowski, R. High Figure‐of‐Merit Nematic Mixtures Based on Totally Unsaturated Isothiocyanate Liquid Crystals. Liquid Crystals 2006, 33, 705-710. (15) Kirchhoff, J. Investigations Into Complex Liquid Crystal Mixtures, The Florida State University, Electronic Theses, Treatises and Dissertations. Paper 2893, 2010. (16) Grasselli, J.G. Analysis of Liquid Crystal Mixtures. Anal. Chem. 1981, 53, 593A–602A. (17) Cinacchi, G.; Mederos, L.; Velasco,

E. Liquid-Crystal Phase Diagrams of Binary

Mixtures of Hard Spherocylinders. J. Chem. Phys. 2004, 121, 3854-3863. (18) Kapernaum, N.; Knecht, F.; Hartley, C.S; Roberts, J.C.; Lemieux, R.P.; Giesselmann, F. Formation of Smectic Phases in Binary Liquid Crystal Mixtures with a Huge Length Ratio. J. Org. Chem. 2012, 8, 1118–1125. (19) Kyu, T.; Chiu, H-W. Phase Equilibria of a Polymer–Smectic-Liquid-Crystal Mixture. Phys. Rev. E 1996, 53, 3618 –3706. (20) Boudah, S.; Sébih, S.; Guermouche, M. H.; Rogalski, M.; Bayle, J.P. Thermal Properties and Chromatographic Behavior of a Mixture of Two Liquid Crystals. Chromatographia 2003, 57, S307-S311.


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(21) Govindaiah, T.N.; Sreepad and Nagappa H.R. Formation of Lyotropic Micellar Nematic Phase in Binary Mixture of Liquid Crystals. Mol. Cryst. Liq. Cryst. 2015; 608, 166-173. (22) Katranchev, B.; Petrov, M.; Keskinova, E.; Naradikian, H.; Rafailov, P.M.; DettlaffWeglikowska U.; Spassov, T. Liquid Crystal Nanocomposites Produced by Mixtures of Hydrogen Bonded Achiral Liquid Crystals and Functionalized Carbon Nanotubes. Journal of Physics: Conference Series 2014, 558, 166-173. (23) Popa-Nita, V.; Barna, V.; Repnik R.; Kral, S. Mixtures Composed of Liquid Crystals and Nanoparticles, Chapter 7, Popa-Nita et al.; licensee InTech. 2013,145-165. (24) Czerwiec, J. M.; Chruściel, J.; Marzec, M.; Ossowska-Chruściel, M. D. Ferroelectric Mixtures of Chiral and Achiral Thioesters. Mol. Cryst. Liq. Cryst. 2011, 541, 276-283. (25) Suchodolska, B.; Rudzki, A.; Ossowska-Chruściel, M.D.; Zalewski, S.; Chruściel, J. ‘Guest–Host’ Effect in Liquid Crystal Mixtures. Phase Transitions. 2015, 88, 16-29. (26) Rudzki, A. Electro-Optical Properties of Nanodispersions Based on Ferroelectric Thiobenzoates. Phase Transitions. 2015, 88, 513-520. (27) Chruściel, J.; Wojciechowska, S.; Ossowska-Chruściel, M.D.; Rudzki, A.; Zalewski, S. Phase Behaviour of Novel Liquid Crystalline Mixtures Based on Thiobenzoates. Phase Transitions. 2007, 80, 615–629. (28) Rudzki, A.; Wojciechowska, S.; Ossowska-Chruściel, M.D. New Binary and Ternary Liquid Crystalline Mixtures Based on Thiobenzoates, chapter in Neutron Scattering and Complementary Methods in Investigations of Condensed Phase, Soft Matter, University of Podlasie Publishing House, Monograph No. 60, 2005, Vol. 2, 218-231.


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(29) Chruściel, J.; Wantusiak, B.; Ossowska-Chruściel, M.D. Equimolecular Mixture of Calamitic and Bent-Core Thiobenzoates. Acta Phys. Polon. A. 2012, 122, 375-380. (30) Zalewski, S.; Hutnik, J.; Jastrzębska, J. New Liquid Crystal Mixtures with Ferroelectric Mesogens, Proceedings of the IX National Conferenc of Neutron Scattering and Complementary Methods in the Investigstions of the Condensed Phases, Chlewiska, 2015, Section Liquid Crystals, Abstracts, 2015. (31) Ossowska-Chruściel, M.D.; Zalewski, S.; Rudzki, A.; Filiks, A.; Chruściel, J. The Phase Behaviour of 4- n -pentylphenyl-4′- n –heptyloxythiobenzoate. Phase Transitions. 2006, 59, 679–690. (32) Blinc, R.; Žekš, B. Dynamics of Helicoidal Ferroelectric Smectic-C̃ Liquid Crystals. Phys. Rev. A. 1978, 18, 740. (33) Juszynska-Gałązka, E.; Gałązka, M.; Massalska-. Arodź, M.; Bąk, A.; Chłędowska, K.; Tomczyk, W. Phase Behavior and Dynamics of the Liquid Crystal 4′-butyl-4-(2methylbutoxy)azoxybenzene (4ABO5*). J. Phys. Chem. B. 2014, 118, 14982-14989. (34) Wróbel, S.; Burakowski, Z.; Chruściel, J.; Czerwiec, J.; Marzec, M.; Ossowska-Chruściel M.D.; Wantusiak, B. Unusual Physical Properties in B Phase of Polar Bent-Core Thioester Compound. Phase Transitions. 2012, 85, 379–387. (35) Das, D.; Majumder T.P.; Ghosh, N.K. Influence of Ionic Conductivity on In-Phase and Anti-Phase Motions Of Antiferroelectric Liquid Crystals. Physica B. 2014, 436, 41–46.


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(36) Ray, T.; Kundu, S.; Pal Majumder, T.; Roy, S.K.; Dabrowski, R. Influence of Ionic Conductivity and Interfacial Charges on the Relaxation Dynamics of Smectic Phases of an Antiferroelectric Material. J. Mol. Liq. 2008, 139, 35-42. (37) Lahiri, T.; Pal Majumder, T. Theory of Ion-Chirality Relation in Ferroelectric Liquid Crystals. Eur. Phys. Lett. 2012, 98, 16008.


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TOC graphic


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a)

b)

Figure 1. Molecular model of the 7OS5 (a) and MHOBS8 (b).


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Figure 2. DSC curve obtained during heating (thick line) and cooling (thin line) of mixture MHOBS4_MHOBS7 (±2oC/min).


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Figure 3. TLI curve obtained during heating (thick line) and cooling (thin line) of mixture MHOBS4_MHOBS7 (±2oC/min).


43


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Figure 4. Textures for the N*(113.8°C - A) and SmC* phase (80.0°C - B) obtained during cooling.


44

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Figure 5. Scheme of experimental set-up for measurements of Spontaneous Polarization by reversal current method.


45


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Page 46 of 70

Figure 6. Reversal current spectrum of ferroelectric SmC* sub-phase of MHOBS4+MHOBS7 in 11°C.


46

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70 60

PS (nC·cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10Hz

50

PS=P0·(TC-T)β

40

P0=39.9±1.1

30

β=0.18±0.01

20

PS MHOBS4+MHOBS7

10

PS fitted

0 0

5

10

15

20

Tc-T (oC) Figure 7. Changes in the value of spontaneous polarization as a function of temperature in SmC* phase for mixture MHOBS4+MHOBS78 (U=+/-10V, f=10Hz).


47


70 50Hz

60

PS (nC·cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 70

50

PS=P0·(TC-T)β

40

P0=42.2±1.0 β=0.13±0.01

30 20

PS MHOBS4+MHOBS7

10

PS fitted

0 0

5

10

15

20

Tc-T (oC) Figure 8. Changes in the value of spontaneous polarization as a function of temperature in SmC* phase for mixture MHOBS4+MHOBS78 (U=+/-10V, f=50Hz).


48

Page 49 of 70

100

τ MHOBS4+MHOBS7 90 80

τ (µ µs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 0

5

10

15

20

Tc-T (oC) Figure 9. Changes in the value of switching time of molecules as a function of temperature in the phase of SmC * for mixture MHOBS4+MHOBS7.


49


40

35 ο θ( )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 70

30

25

θ MHOBS4+MHOBS7

20 0

5

10

15

20

25

Tc-T (oC) Figure 10. Changes in the value of tilt angle of molecules as a function of temperature in the phase of SmC * for mixture MHOBS4+MHOBS7.


50

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Figure 11. Transmitted light curves vs. applied triangular-wave electric field of frequency 10Hz and amplitude ±15V/3,2µm for the mixture MHOBS4+MHOBS7 obtained during cooling at chosen temperatures of SmC* phase.


51


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Page 52 of 70

Figure 12. DSC curve obtained during heating (thick line) and cooling (thin line) of mixture 610712+MHOBS8 (±2oC/min).


52

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13. TLI curve obtained during heating (thick line) and cooling (thin line) of mixture 610712+MHOBS8 (±2oC/min).


53


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Page 54 of 70

Figure 14. Textures for the N*(82.0°C - A) and SmC* phase (32.0°C - B) obtained during cooling.


54

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8 7

Ps 610712+MHOBS8

6 Ps (nC cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 4 3 2 1

10Hz

0 0

5

10 15 20 25 30 35 40 45 50 Tc-T (oC)



55


8 7

Ps 610712+MHOBS8

6 Ps (nC cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 70

5 4 3 2 1

50Hz

0 0

5

10 15 20 25 30 35 40 45 50 Tc-T (oC)



56

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4000 3500

τ 610712+MHOBS8

3000 2500

τ (µ µs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000 1500 1000 500 0 0

5

10

15

20

25

30

35

40

Tc-T (oC) Figure 17. Changes in the value of switching time of molecules as a function of temperature in the phase of SmC* for mixture 610712_MHOBS8.


57


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Page 58 of 70

Figure 18. Transmitted light curves vs. applied triangular-wave electric field of frequency 5Hz and amplitude ±10V/3.2µm for the mixture 610712+MHOBS8 obtained during cooling at chosen temperatures of SmC* phase.


58

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Figure 19. Dielectric spectrum obtained for the SmC* phase. Dispersion and absorption curves (solid lines – red and blue, respectively) computed by fitting to the experimental points using Eq. (2). Dashed line (red) – computed dispersion curve of Goldstone mode. Dotted line (red) – electrode polarization contribution to electric permittivity. Dashed line (blue) – Goldstone mode absorption curve. Dotted (blue) – ionic conductivity contribution to dielectric absorption.


59


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Page 60 of 70

Figure 20. Cole-Cole plot for the SmC* phase. Cole-Cole plot (blue solid line) for the Goldstone mode. Effects related to the polarity of the electrodes and ionic conductivity (red solid line).


60

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Figure 21. Dielectric increment (left part) and relaxation time (right part) vs. temperature.


61


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63


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Page 64 of 70

Figure 24. Dielectric spectrum obtained for the SmI* phase. Dispersion and absorption curves (solid lines – red and blue, respectively) computed by fitting to the experimental points using Eq. (1). Dashed line (red) – computed dispersion curve. Dotted line (red) – electrode polarization contribution to electric permittivity. Dashed line (blue) – absorption curve. Dashed-dotted (blue) – ionic conductivity contribution to dielectric absorption.


64

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Figure 25. Cole-Cole plot for the SmC* phase. Cole-Cole plot (blue solid line) for the dielectric process. Effects related to the polarity of the electrodes and ionic conductivity (red line).


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67


mixtures

Table 1. Overview of component’s molecular structures and phase transitions for mixtures MHOBS4+MHOBS7 and 610712+MHOBS8.

Ingredient

Transition temperatures [°C]

MHOBS4_MHOBS7

MHOBS4 COS

H13C6*CH(CH3)O

C4H9

°C °C °C Iso 121 .6 → N * 90 .9 → SmC * 73 .4 → SmI * °C °C 71 .5 → SmG * 26 .7 → Cr


C7H15

COS

H13C6*CH(CH3)O

°C .5 ° C 70 .1 → SmG * 5 → Cr

6OS5

H13C6O

C5H11

COS

10OS5

H21C10O

C5H11

COS

.3°C .7°C .34°C Iso 84  → N 36  → SmC 7 → Cr

610712_MHOBS8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 68 of 70

7OS5

H15C7O

C5H11

COS

12OS5

H23C12O

C5H11

COS


H13C6*CH(CH3)O

COS

C8H17

°C °C 76 .0 → SmG * 51 .6 → Cr


68

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Table 2. Phase polymorphism of MHOBS7+MHOBS8 mixture obtained by DSC and TLI methods. Phase transition →SmG* →SmI* →SmC* →N* →Iso →N* →SmC* →SmI* →SmG* →Cr

measurement method TLI DSC T (oC) T (oC) ∆H (J·g-1) heating 16.7 17.7 13.25 67.4 69.1 0.35 74.3 74.6 4.71 92.0 95.2 2.77 124.3 122.7 2.63 Cooling 122.2 122.6 2.78 88.3 95.0 2.90 71.8 74.4 4.79 64.0 68.8 0.33 -6.9 -5.2 8.19


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Page 70 of 70

Table 3. Phase polymorphism of 610712+MHOBS8 mixture obtained by DSC and TLI methods.


a

measurement method DSC T (oC) ∆H (J·g-1) Heating -1.0 3.4 0.79 4.2 7.9 1.19 30.4 26.5 4.93 46.7 52.1 0.20 93.4 93.0 4.07 Cooling 91.0 92.6 4.01 45.2 51.9 0.26 3.2 6.9 2.54 0.7 1.6 0.96 -3.3 -a -

TLI T (oC)

not observed


70

Phase Behavior and Dynamics of Binary and Multicomponent

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