3. Results and Discussion
The Materials and Methods section contains a detailed description of the synthesis of new chiral additives. The quality of synthesized substances was checked with IR spectra (
Figure S1) and different NMR and mass spectral analyses. Several examples are provided in
Figures S2–S4. The results of the CHNS/O elemental analysis are presented in
Table S1. The first two dopants out of three are crystalline. This is proven by the crystalline textures of
CA1 and
CA2 (
Figure 2a), which undergo a transition to another crystal with a different texture. The dopants are melting to form isotropic liquids with the heat of fusion values given in
Table 1. The DSC curves of all three additives are given in
Figure 2b. The crystalline state of
CA1 and
CA2 is also confirmed by endothermic peaks (curves 1,2) that accompany the melting under heating. As for the dopant
CA3, it forms a glassy state and turns into a liquid at glass transition temperature
Tg, i.e., 17 °C (
Figure 2b, curve 3).
Table 1 summarizes the corresponding transition temperatures and melting enthalpies.
These compounds are optically active and, as shown in
Table 1, have different values for their specific optical rotation, namely ranging from +12 to +43 deg mL g
−1 dm
−1. The first two compounds contain an asymmetrical carbon atom that is responsible for the optical activity, and the other compound is the binol derivative
CA3, which has axial optical asymmetry that contributes to the optical activity (atropisomerism).
Figure S1 presents the IR spectra of additives, which reflect the structure of the additives. One can mention the p-substituted aromatic rings and carboxylic groups (marked in the spectra) and ether Ph-O-CH
2- groups that are presented as intensive peaks in 1250–1000 cm
−1 range. Intensive peaks at 2870–2960 cm
−1 and 1460 cm
−1 are characteristic for polymethylene chains in the
CA2 and
CA3 molecules, and the band splitting related to phenyl rings in the IR spectrum of
CA3 confirms the presence of naphthalene rings. A particularly important point is that the IR spectra show that
CA1 forms dimers due to hydrogen bonds between carboxylic groups (C=O at 1680 cm
−1), whereas
CA2 prefers the monomeric structure.
The introduction of optically active additives into liquid crystals assumes the induction of chiral phases, primarily chiral nematics (N*). In this case, we must find out a sufficiently good compatibility of components and elucidate the effect of the additive content on the transition temperature from the nematic to isotropic (N–I) phase. As an example,
Figure S5 demonstrates the DSC curves of LMN composites with
CA1,
CA2, and
CA3.
Figure 3 presents the change in
TNI with the increase in the CA component content for all systems under study. The comparison
TNI as a function of the mixture content (
Figure 3a) shows a continuous increase in the
TNI (curve 1) and its decrease (curve 2). One can see that the N–I phase transition temperature may either increase or decrease with the addition of
CA1 and
CA2. The increase in
TNI observed in mixtures with
CA1 indicate the possibility of the increase in order that is caused by the formation of molecularly rigid and elongated dimers of
CA1 in the nematic LC, as confirmed by the infrared spectra.
CA1 contributes to the increase of the transition enthalpy (from 3.4 up to 6.5 J g
−1) (
Figure 3b, curve 1). At the same time, one may see in LMN mixtures with
CA2 that there is a continued drop in
TNI without any visible destruction of the LC structure; however, the corresponding enthalpy decreases from 3.4 down to 2.2 J g
−1 (
Figure 3b, curve 2). It is a clear indication that the
CA2 additive destroys LMN, which is possibly due to the large size of its molecules in comparison with the nematic matrix molecules. These data are in a good agreement with the quantitative correlation established in [
19]. They show the doping of the liquid crystal with molecularly flexible acids to cause the lowering of
TNI. The increase in
TNI occurs if the rigid carboxylic acids form dimers are comparable in size with the nematic matrix molecules.
LMN mixtures with
CA3 behave in a quite different way. As shown in
Figure 3a, (curve 3) the
TNI of mixtures drops down to 5 wt.% of an additive and then does not change with the increase in
CA3 content. The enthalpy of the N–I transitions (
Figure 3b) changes in a similar way with the increase in
CA3 content. This result makes one consider the phase separation that is occurring in the system with greater than 5 wt.% of
CA3.
The analysis of the optical textures obtained in the crossed polarizers in cells without surface treatment shows that the layer of LMN has a homeotropic texture, with the long molecular axis being oriented preferably normal to the sample plane (
Figure 4a). The addition of
CA1 in amounts creating mixtures with a concentration below 10 wt.% does not change the homeotropic texture, even if the clearing temperature
TNI is higher (
Figure 3a, curve 1). However, the cooling of that sample to below the
TNI of the mixture (70 °C) results in the appearance of an oil stripe texture. (
Figure 4b). At a
CA1 content of below 25 wt.%, the solubility of the initially crystalline
CA1 is sufficiently high and is preserved at room temperature, indicating the weak influence of the chiral dopant on the nematic matrix and the preservation of the homeotropic texture. Roughly speaking, the anchoring energy of the nematic is higher than the twisting power of
CA1. Nevertheless, all samples with a
CA1 content of below 25 wt.% undergo a violation of the homeotropic orientation during the heating up to the vicinity of
TNI of LMN matrix. The higher the content of
CA1, the lower the temperature for the texture change. As for the mixture with 25 wt.% of
CA1, the homeotropic texture at room temperature is distorted and small light structures appear (
Figure S6), which are growing into the chiral texture (
Figure 4e). Generally, at
CA1 concentrations ranging between 15 and 25 wt.%, cooling below the
TNI of the mixtures allows one to observe the polydomain textures being formed by the chiral nematic structures and the chiral twisting being stronger than that of when it is under heating (
Figure S7).
Contrary to the LMN-
CA1 mixtures, the
TNI transition of mixtures with
CA2 decreases (
Figure 3b, curve 2) with the increase in
CA2 content. However, mixtures starting with 2 wt.% of
CA2 have a chiral twisted texture at room temperature (
Figure 5). The presence of
CA2 seems to change the anchoring of the matrix molecules, which transforms the homeotropic texture. This effect, in combination with the higher optical activity of
CA2 chiral molecules (
Table 1), allows for the selective reflection to appear in the visible spectral range.
As an example,
Figure 6a shows the curves related to the selective reflection of light of the LMN with 30 wt.% of
CA2 [
18]. When the content of the additive reaches 20 wt.%, the selective reflection happens at 850–950 nm (
Figure 6b). At 30 wt.%
CA2, the selective reflection is observed at 500–600 nm, whereas at 40 wt.%, it shifts down to 400–500 nm.
At the same time, the optical activity of
CA3 is already two times higher than that of
CA2 (
Table 1); that is why it induces chiral structure of the mixture at 0.5 wt.% at room temperature already (
Figure 7a). The chiral texture exists for all of the mixture contents studied (
Figure 7), but at 6 wt.%
CA3 and higher, one may observe the appearance of dark areas, which indicate the secretion of the excess of amorphous areas of
CA3 liquid. This means that the compatibility of the components becomes poor, and dark areas co-exist with the chiral texture. The maximum saturation of
CA3 by a nematic matrix seems to be about 5 wt.%. The concentration range where
CA3 shows full compatibility with LMN is quite narrow.
The ITO surfaces organize the nematic matrix with the formation of homeotropic orientation (
Figure 4a). However, the treatment of the surface with the planar surface orientant results in the formation of LMN planar texture.
Figure 8a shows the corresponding curves of the dielectric permittivity as a function of temperature. As for mixtures with
CA3, a small amount of the additive also induces the planar orientation without any prior treatment of ITO. The dielectric permittivity of the mixtures in the planar texture continuously increases with temperature leading up to
TNI, and then it becomes independent of temperature or slightly decreases. The latter is typical for low molecular liquid crystals in an isotropic phase.
As is observed from the optical textures, mixtures with
CA2 may form an initial planar orientation similar to those with
CA3. This is confirmed with the curve ε′/ε′
is(T) of 5 wt.% mixture with
CA2 (
Figure 8b). At a higher
CA2 content (about 20 wt.%), the dielectric permittivity behaves in a similar way while being heated. However, during the cooling of the sample from the isotropic phase, the value of the ε′/ε′
is begins to increase below
TNI. Notably, the LC optical texture practically does not change when the mixture is heated or cooled (
Figure S8). It seems that this effect may be related to the orienting influence of the field signal level on the LC composition with a positive dielectric anisotropy. Along with an increase in
CA2 content, which induces stronger chiral twisting, the anchoring effect becomes much weaker. This results in a partial molecular reorientation along the field direction during the cooling from the isotropic phase.
Contrary from the above the mixtures, the ones containing
CA1 do not reorient, and the homeotropic matrix orientation stays the same for the mixtures. That is why we have analyzed the dielectric constant dependence with temperature with the use of the surface orientant, which makes the initial liquid crystal mixture plane oriented for all concentrations of
CA1. Heating of the samples up to 10% results in the continuous increase in the relative dielectric permittivity ε′/ε′
is (
Figure 9).
The conditions of the experiment were as follows: the optical textures and dielectric measurements were simultaneously analyzed in cells with unidirectionally treated surfaces.
The analysis of the optical textures as a function of the dopant content allows us to hypothesize that, at concentrations below 25 wt.%, the planar unidirectional anchoring is preserved on both surfaces. The mixtures exhibit very similar twisted structures with changing temperatures and concentrations. At higher concentrations such as 25 wt.% and above, the set of various optical textures becomes broader. It may be likely that the planar anchoring is a degenerate one.
The initial homogeneous texture of the LMN matrix (
Figure 10a) does not really change when one inserts 10 wt.% of
CA1 (
Figure 10b). When approaching the
TNI of this mixture, the chiral texture appears, which exists in a very narrow temperature range (
Figure 10c). The increase in
CA1 content up to 20 wt.% also does not change the initial planar orientation (
Figure 10d). However, when approaching the
TNI of the initial nematic matrix (64 °C) by heating, the typical twisted texture appears with the axis located in the in-plane of the cell (
Figure 10e). One may suppose that at that moment, the interaction between the surface and mesogenic molecules is becoming weaker. At the same time, a small peak in the ε′/ε′
is(T) curve (
Figure 9) appears. With a further increase in
CA1 content up to 25 wt.%, the peak becomes more pronounced, and a second peak shows up in the vicinity of 72 °C. The corresponding optical textures are shown in
Figure 10f–j. The analysis of the textures lets us draw conclusions about the transformation of the N structure of the mixture to a more ordered one (
Figure 10f). Above 64 °C, the ordered texture is transformed to a chiral one, which also changes with the temperature (
Figure 10g,h). Note that in all cases, this system forms the planar orientation with the characteristic texture known as oil striping (
Figure 10h). At 72 °C, the peak on the ε′/ε′
is(T) curve indicates the next transformation of a chiral texture, which is accompanied by the relocation of the twisting axis into the plane of the cell (
Figure 10i,j). The cause of the change in the location of the axis from the normal to a parallel cell surface is not yet clear. One of the possible explanations may be the stronger twisting effect from heating (
Figure 10i). When approaching the
TNI of the 25 wt.% composition, the texture changes again to the planar one with characteristic oil stripes (
Figure 10j).
As for the mixture with 30 wt.% of
CA1, its curve has only one peak (
Figure 9) at about 70 °C. This case corresponds to the transformation of the smectic-like fan texture (
Figure 10k), which exists in a broad temperature range, into a different texture, which resembles the blue phase texture (
Figure 10l). The latter endures the transition to planar textures with oil stripes, preceding the transition to an isotropic phase at 85 °C (
Figure 10m).
The analysis of the optical textures as a function of the dopant content allows us to suppose that at concentrations below 25 wt.%, the planar unidirectional anchoring is preserved on both surfaces, and the mixtures exhibit very similar twisted structures when undergoing a change in temperature and concentration. At higher concentrations such as 25 wt.% and above, the set of various optical textures becomes broader. It may be likely that the planar anchoring is a degenerate one because of the increase in CA1 dopant content.
The texture transitions discussed above are in good agreement with the DSC data (
Figure 11). There are two transitions observed: during heating, the first transition corresponds to 72 °C with a transition enthalpy of 4 J g
−1, and the second one proceeds at 84.6 °C with a corresponding enthalpy of 5.5 J g
−1 (curve 1). These transitions are fully reversible, as shown in
Figure 11, curve 2.
Thus, by using the POM and DSC results, we observe the transition of the 30 wt.% system with a fan texture to the chiral phase, which in its turn is transformed in the isotropic phase. To analyze the transition observed, we have used a small-angle X-ray scattering analysis. The curves obtained at various temperatures are given in
Figure 12. One can observe an X-ray Bragg reflection with the corresponding d-spacing equal to 2.31 nm at room temperature. This d-spacing is roughly the size of paired
CA1 molecules, which are connected by a hydrogen bond. It is a supporting evidence of the presence of a layered mesophase.
The increase in temperature up to 50 °C leads to the temperature shift of the small-angle maximum to a d-spacing of 2.33 nm, which corresponds to the linear thermal expansion coefficient of β = 3.6 × 10
−4 K
−1 in the material, typical for LC. However, further heating leads to the drop in the diffraction intensity, and the maximum completely disappears at 72 °C. This leads one to conclude that the increase in
CA1 content leads the nematic phase at lower contents to be transformed in to a smectic one. While no second or third order reflections were detected, this pattern still can be attributed to the smectic ordering due to the narrow peak and the type of the texture (see
Figure 10k). Thus, the first transition is S-N*, and the second one—N*-I.