In recent years, the quest for the new materials having novel properties that can meet the day–by–day increasing technological demands has also revolutionized the research in the field of Liquid Crystals (LCs). This has given rise to a new approach called ‘Guest – Host’ approach where LC materials (the Host) are doped with some other materials (the Guest) like dyes, polymers, nanomaterials etc. to manipulate the inherent properties of these materials so that a new material can be synthesized that can fulfill the need as per the required specifications
Mixing of two or more liquid crystals makes it possible to prepare new materials as per the requirement because the intermolecular interaction between the two mesomorphic phases can greatly influence the optical anisotropy of both
Cholesteric Liquid Crystals (CLC) were the first liquid crystals discovered by Friedrich Reinitzer in 1888 and since then they have played a significant role in redefining the various technologies related to optics and nanomaterials
Generally, the CLCs are birefringent i.e. their refractive indices are different along and perpendicular to the optic axis
The microscopic order parameter gives the information regarding the molecular alignment about the director and is given as -
where < > denotes average behavior of
Though a lot of studies have been done in the past to characterize the order parameter of a large number of optically positive (ne > no) nematic and cholesteric materials by studying their optical anisotropy and barring a few studies on LC mixtures, very limited data is available on the optically negative CLCs and their mixture of homogeneous composition. Also, the objective of mixing the LCs in different concentrations to tune the transition temperature is rarely described in literature.
Here, we report the tuning of transition temperatures by studying the microscopic order parameter (S) from molecular polarizability and optical anisotropy of optically negative (ne < no) CLCs namely Cholesteryl Propionate, Cholesteryl Benzoate and their three homogeneous mixtures. For this purpose, Maier and Saupe’s anisotropic internal field model have been taken into account because this method provides direct and reasonably accurate value of the order parameter.
The materials were procured from Merck, Germany and were used as such without any further purification. The various steps involved in the calculation of order parameter using the optical anisotropy data are as under –
(a) Measurement of both ordinary (no) and extraordinary (ne) refractive indices.
(b) Measurement of density (
(c) Calculation of optical anisotropy (
(d) Determination of molecular polarizabilities (
(e) Calculation of order parameter (S) from molecular polarizabilities.
Abbe’s refractometer (range 1.3 – 1.7) was used to measure the refractive index (n) in the isotropic phase of samples. Since the CLC samples used in this study are anisotropic, the birefringence study was also carried out for measurement of ne and no till their values lies within the range of refractometer. This was done by attaching a polarizer to refractometer and blocking one of the rays and then rotating the polarizer by 900C from initial position. When the values of refractive indices were higher than the range of refractometer, a modified wedge method was used
For density measurements in cholesteric and isotropic phases, the capillary rise method was used by placing a sample filled glass capillary tube in a thermostat. The sample length in a capillary tube at different temperature was measured by a travelling microscope. After correction for expansion of glass tube, the density was calculated
The principal polarizabilities (αe, αo) were calculated using Maier and Saupe’s method and their values were used to calculate the values of order parameter (S)
where αe and αo are principal polarizabilities in mesogenic phase and V/s (T – T) graph at 0
In the present work, Maier and Saupe’s approach has been used to calculate the αe and αo values. This approach is applicable when the internal field is anisotropic. In this approach, a single internal field constant ‘a’ is assumed, whose value can be calculated from the relation-
where Viso and Vch are molar volumes in isotropic and cholesteric phases respectively.
The values of αe and αo are given by the relations -
where N denotes the number of molecules per c.c. and is written as-
NA,
The variation of principal polarizabilities
The temperature dependence of the order parameter (S) is calculated using Maier and Saupe’s anisotropic internal field model for pure liquid crystal samples and is shown in
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92 |
96 |
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175 |
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148 |
176 |
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104 |
114.5 |
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120 |
136 |
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133 |
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81 |
83.5 |
145 |
84.0 |
83 |
82.0 |
149 |
82.5 |
85 |
81.5 |
153 |
63.0 |
84 |
81.0 |
157 |
61.0 |
89 |
81.0 |
161 |
59.0 |
91 |
80.0 |
163 |
57.5 |
93 |
69.0 |
167 |
56.0 |
95 |
65.5 |
171 |
55.0 |
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100 |
82.0 |
113 |
81.0 |
127 |
81.0 |
101 |
81.0 |
115 |
81.0 |
129 |
81.0 |
103 |
81.0 |
117 |
80.0 |
131 |
80.0 |
105 |
84.0 |
119 |
79.5 |
133 |
66.0 |
107 |
62.5 |
121 |
67.5 |
135 |
63.0 |
109 |
61.0 |
123 |
66.0 |
137 |
63.0 |
111 |
60.5 |
125 |
62.0 |
139 |
63.0 |
113 |
57.5 |
127 |
59.0 |
141 |
62.0 |
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129 |
57.0 |
143 |
60.0 |
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131 |
57.0 |
145 |
60.0 |
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133 |
56.0 |
147 |
58.0 |
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149 |
56.5 |
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151 |
55.0 |
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153 |
53.0 |
From the graphs, it can be seen that the ratio of principal polarizabilities
From the graphs it can be seen that for pure samples as well as for their mixtures (
Earlier studies were performed by some researches on those samples for which the liquid crystalline state was existing in a normal temperature range
The present study shows that it is possible to tune the transition temperature of CLCs by mixing them in different concentrations as we have achieved transition temperature for solid to cholesteric phase, 1040C for Mixture 1, 1200C for Mixture 2 and 1330C for Mixture 3. Similarly, the transition temperatures are tuned from 114.50C for Mixture 1, 1360C for Mixture 2 and 1450C for Mixture 3 for cholesteric to isotropic phase transition. All these transition temperatures for mixtures lies within the range of transition temperatures of pure samples. Further, the abrupt change in the values of optical anisotropy, polarizability and order parameter at the transition temperature also confirms the different phases occurring in CLC samples. The fall in the degree of orderedness from a solid phase to isotropic liquid phase via mesogenic phase with rise in temperature is well indicated by the data. The close agreement of theoretical and experimental values for pure sample establishes the validity of the data obtained. The advantage of using Maier and Saupe’s approach to calculate the polarizability and order parameter is that this approach is more realistic as it considers the internal field as the anisotropic one, because liquid crystals are also a medium of anisotropic molecular distribution.
The novelty of this study is that it opens the door for preparing the new materials by mixing the two or more materials in different concentrations and altering their phase transition temperatures as per need. This has great significance in the field of material science while preparing new materials with new properties and novel performance.
One of the authors, Syed Salman Ahmad Warsi, is thankful to the Research and Development (R & D) wing of Integral University, Lucknow for providing the support and manuscript communication number (IU/R&D2022-MCN0001496) for the publication of this manuscript.