Design and phase transition behavior of siloxane-based monomeric and dimeric liquid crystals bearing cholesteryl mesogenic groups
Introduction
Self-assembly in molecular science is one of the outstanding tools for fabricating functional nanomaterials. Highly ordered architectures in the nanostructured materials are fundamental factors dictating unique features in molecular biology, chemistry, polymer science, materials science, and engineering [1]. Such molecular self-assembly systems accelerate a significant advance in modern scientific technology, and are attracting a great deal of attention. Over the past few decades, the self-assembly of liquid crystals (LCs) responsive to external environments, has proven to be a very useful tool creating the well-ordered materials [2]. Depending on the chemical structure and molecular shape, a variety of LC phases can be formed, and furthermore their structures are changeable by external stimuli for example electric/magnetic field, light, temperature, and mechanical forces. For the construction of the nanostructured materials using the LCs, hybrid organic/inorganic molecules such as those based on siloxane moieties are attractive and feasible. The chemical immiscibility of the organic and inorganic parts leads to nanosegregation into highly ordered nanostructures [3].
There has been an increasing interest in LC dimers consisting of two mesogenic groups connected by a flexible spacer, due to their interesting phase behavior which significantly differs from that of the corresponding monomeric LCs. Since LC dimers were firstly reported by Vorländer in 1927 [4], symmetrical dimers composed of two structurally identical mesogenic groups linked through a flexible central spacer such as a polymethylene group have been widely investigated, especially during the past two decades [5]. The transition properties of the LC dimers are strongly dependent on the nature and length of the flexible spacer [6]. For example, an odd-even effect for the clearing and melting points has been observed by changing the length of the spacer between the two mesogenic groups. In addition, the design of twist bend nematic (Ntb) phases arising from the LC dimers is a very hot topic in the molecular science of the LCs recently [7]. From the scientific background mentioned above, the study of such unique phase properties in the LC dimers is still an important subject. Presently, a standard approach for understanding of the properties is to investigate stabilities of two identical mesogenic groups (symmetrical dimers) [8], unsymmetrical instabilities (unsymmetrical dimers) [9], innate mesogenic properties depending on its molecular structure [8], length and flexibilities of spacers [8], connection mode between mesogenic groups and spacers (terminally, vertically) [10], and conformations of overall molecule (linear, tilt, bent) [11].
The present study is focused on the siloxane-based monomeric and dimeric LCs bearing cholesteryl mesogenic groups, which could be categorized as hybrid organic/inorganic LC dimers, and their phase transition behavior was characterized by polarising optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The incorporation of the siloxane unit into the spacers for the dimeric LCs is an essential scientific topic for studying the molecular design of the LC materials with the unique properties deriving from its flexibility and nanosegregation. We investigated also how the siloxane unit in the spacer and tail influences on the thermal properties of the homologues having the normal alkyl chains and spacers. The synthetic scheme of the siloxane-based LCs is presented in Fig. 1.
Section snippets
Materials
The reagents, allyl bromide, methyl 4-hydroxybenzoate, cholesterol, 1,1,1,3,3-pentamethyldisiloxane, 1,1,1,3,3,5,5-heptamethyltrisiloxane, 1,1,3,3-tetramethyldisiloxane, 1,1,3,3,5,5-hexamethyltrisiloxane, 1,1,3,3,5,5,7,7-octamethyltetrasiloxane, and the organic solvents, 2-butanone, methanol, dichloromethane, toluene were purchased from Tokyo Chemical Industry Co., Ltd. 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N,N-dimethyl-4-aminopyridine (DMAP) were purchased from FUJIFILM Wako
Results and discussion
The phase transition behavior of the siloxane-based LCs obtained in this study is discussed in the following two sections: (1) the thermal behavior of the monomeric LCs (D-n, n = 2, 3) and dimeric ones (E-n, n = 2, 3, 4), and (2) the influence of the siloxane units on the thermal behavior of the corresponding LC materials having normal alkyl chains. The thermal properties of all the siloxane-based LCs summarized in Table S1 were studied by differential scanning calorimetry (DSC), polarising
Conclusion
Identical types of the siloxane-based monomeric and dimeric LCs bearing the cholesteryl mesogenic groups have been synthesized and characterized. The mesomorphic behavior was found to be greatly governed by the modification using the siloxane units in comparison of those of the corresponding LCs having the normal alkyl chains and spacers. Furthermore, the nature of the LC phases was associated with the nanosegregation of the constituent parts of the molecules consisting of the cholesteryl
Acknowledgment
This work was partially supported by MEXT-supported Program for the Strategic Research Foundation at Private Universities (2012–2016). We thank Mr. Yuta Kitajima (Ritsumeikan University) for technical assistance with the experiments and fruitful discussions.
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