Mesophase stability in binary mixtures of monotropic nematogens

Binary phase diagrams were constructed from laterally substituted methyl azo/ester derivatives, namely 4-(4″-substituted phenylazo)-3-methyl phenyl-4″-alkoxy benzoates (Ina–d). In this group of compound the unsubstituted and chloro-substituted derivatives possess the nematic phase monotropically, while the nitro and methyl analogues are enantiotropically nematogenic. The binary phase diagrams constructed were made once from the monotropic nematogens with each other, and another with the enantiotropic nematogens. In both the cases enantiotropic nematic phase was observed that covers a wide range of composition. The mesophase behaviour of all binary mixtures was investigated by differential scanning calorimetry (DSC) and polarised light microscopy (PLM). The nematic phase was exhibited in all binary mixtures. Independent of the alkoxy chain length, the entropy change, ΔSN–I of the N–I transition of pure components was found to vary irregularly with the anisotropy of polarisability (∆αX) of the polar substituent, X.


Introduction
Generally, liquid crystalline compounds are rod-like molecules with stable central linkages. [1] The ester carbonyl group is one of the most commonly used linking units since it is relatively thermally stable. [1][2][3] Azobenzene group represents very comfortable fragments to design and synthesise new structures, giving stable mesophases often with very interesting polymorphism. [4][5][6][7] Liquid crystalline materials, either low molar mass [8][9][10][11] or polymeric in nature, [12] containing an azo group in the mesomeric core are attractive with regard to studying their optical properties. Generally, it has been reported that the central linkages and the terminal groups play important roles in the formation, type, thermal stability and temperature range of the mesophase of the liquid crystalline compound. [13] Many series of dimeric liquid crystals have been reported, [8][9][10][11] all of these compounds may be termed symmetric, with identical mesogenic moieties, or non-symmetric dimers, with different mesogenic units. In both the cases, the specific interactions between the two mesogenic groups do lead to a significant variation in the mesophase behaviour of such materials. [8][9][10][11] Mesophase characteristics of mesogens may be greatly modified upon mixing of individual components. Earlier studies [14][15][16][17][18][19][20][21][22][23][24][25] have shown that the emergence of the mesophase upon mixing compounds where none, one or both components of the mixture are mesogens.
In our laboratory, various types of binary mixtures, dependent on the two components of the mixture, were investigated. For such mixed systems, all components were of the same core structure, namely, phenylazo phenyl benzoates (X-C 6 H 4 -N═N-C 6 H 4 -OCO-C 6 H 4 -OR′).
Binary mixtures were made once from any two homologues carrying the same terminal polar group X, [26,27] and another from any two terminally substituted analogues bearing different polar groups, X, but of the same alkoxy chain length. [26,28] A third type of binary system [29] was made from two analogously terminally substituted compounds but of different mesogenic core, one is an azo-ester and the other is the corresponding terminally substituted di-ester.
Going further in investigating mixtures of liquid crystalline components, two groups of positional isomers of the same skeletal structure, bearing the same terminal (polar and alkoxy) groups, but of lateral methyl substituent protruded with different angles with respect to the long axis of the molecule, were prepared, and their mesophase behaviour was investigated again in their pure and mixed states. [30] That is, binary mixtures were made from any two positional isomers, one from each group. In a more recent work, [31] the same core structure was used, but the difference between the two components of the binary mixtures lies in the polarity of the lateral substituent (CH 3 and Cl) attached to the same position of central benzene ring.
Generally, the physical properties of a nematogens are strongly influenced when a lateral substituent is appended to the mesogenic core. Lateral protrusion effectively widens the mesogenic core, thus increases intermolecular separation that leads to a reduction in the lateral association, [32] and hence the nematic phase stability is reduced.
In a previous work in our laboratory, [30] compounds In a-d were prepared and characterised for their mesophase behaviour. It was found that, independent of the length of the terminal alkoxy chain, compounds In b&c (X = H and Cl) possess a monotropic nematic phase, whereas, the electrondonating CH 3 group in In a , or the electron-withdrawing NO 2 substituent in In d induces the nematic phase enantiotropically. The goal of the present study is to extend investigation to the binary phase behaviour of mixtures once prepared from two monotropic mesogens and another from a monotropic nematogen with an enantiotropic one.

Experimental
Compounds, In a-d , were prepared by the method described in a previous work. [30] Calorimetric measurements were carried out using a differential scanning calorimeter, PL-DSC, of Polymer Laboratories, England. The instrument was calibrated for temperature, heat and heat flow according to the method recommended by Cammenga et al. [33] DSC measurements were carried out for small samples (2-3 mg) placed in sealed aluminium pans. All of the thermograms have been achieved at a heating rate of 10°C/min in inert atmosphere of nitrogen gas (10 ml/min). Binary mixtures were prepared by mixing accurately weighed samples of the appropriate amounts of the individual components, melting them together to give an intimate mixture and then cooling to room temperature while stirring.
Transition temperatures were measured by DSC and the type of mesophase identified for all mixtures prepared with a standard polarised light microscope, PLM (Wild, Germany) attached to a home-made hot stage. The transition temperatures obtained for all prepared blends as measured by both DSC and PLM agreed within 2-3°C.

Results and discussion
Transition temperatures as measured by DSC and PLM are given in Table 1. The data in this table, which match those reported previously, [30] revealed that all members of group In a-d are purely nematogenic, but the unsubstituted (In b ) and the chloro-substituted (In c ) homologues showed their nematic phase monotropically. In order to confirm the purity of prepared compounds, the elemental analysis data of In a-d , infrared (IR) and 1 H-NMR data of I14 a-d as representative compounds are collected in Tables 2-4, respectively. Generally, the mesophase stability is augmented by an increase in the polarity and/or polarisability of the mesogenic part of the molecule. Also it was reported [34] that the terminal substituent has relatively greater effect on the shape anisotropy of the unit to which it is attached and consequently the mesophase behaviour. The electron-donating (CH 3 ) and electron-withdrawing (NO 2 ) substituents (in In a or In d ) were found to enhance the nematic stability (T N-I ) over those of their unsubstituted analogues (In b ). This indicates that the polarity and/or polarisability play an important role in enhancement mesophase stability. On the other hand, the data in Table 1, when compared with those reported for their corresponding laterally neat analogues, [35] revealed that lateral methyl substitution on the central ring in position 3 is accompanied with an increase of the double bond character of the C=O group and, in addition to its special effect, consequently, furnishes poor mesomorphic compounds specially in the less polar terminal H-and Clsubstituents.
It is already known that materials that retain their liquid crystalline character over a wide range of temperatures are preferred for practical applications. One way to achieve this property is to use eutectic mixtures of materials exhibiting liquid crystallinity in their pure state, or at least when molecules in question resemble one another structurally. Under such condition, the mesophase-to-isotropic line is usually straight or even enhanced and the temperature range of the mesophase is greater for the eutectic mixture than either pure components.
Since the unsubstituted (In b ) and Cl-substituted (In c ) homologues are only monotropically  arrangement of their structurally similar enantiotropic nematogenic analogues (In a or In d ), the binary phase behaviour of the mixtures with other three types of components were investigated. In the first system (e.g., I8 b /I8 c and I16 b /I16 c ) the binary mixtures were composed of the two monotropically nematogenic analogues, the second system (e.g., I8 b /I10 b and I8 c /I10 c ) is a mixture of two homologues of each, and the third type (e.g., I8 b /I8 d and I10 c /I10 d ) is their mixture with a corresponding enantiotropic nematogen.

Lateral substitution and entropy change of transition, ΔS N-I
The entropy of the nematic-isotropic transition was calculated for the two series In a-d (Table 1) and IIn a-d . [31] The difference between the two groups of compounds lies in the orientation protrusion of the laterally substituted methyl group into the central benzene ring. In the first group (Group I), the methyl substituent, introduced in position 3 with respect to the ester group, makes an angle of 120°with the long axis of the molecule.
In the other group (Group II), the orientation angle is 60°since it is introduced in position 2.
The calculated entropies are represented graphically as a function of the anisotropy of polarisability (Δα X ) in Figure 1. As can be seen from the figure, the terminally unsubstituted isomers (In b and IIn b ) showed that lateral substitution with the ortho-methyl group is associated with lower values of ΔS N-I than those of their corresponding laterally meta-substituted analogues, except for those bearing longer alkoxy chain length (n = 16). The decrease observed in ΔS N-I is presumably, in part, a reflection of the increase in biaxiality of the mesogenic group resulted from the flexible terminal alkoxy chain being less strongly anchored at its end, thus resulting in a decrease in the conformational entropy. [35]   This comparison indicated that the introduction of a lateral substituent, whether it is in ortho or meta position with respect to the ester group, has led to an irregular ΔS N-I -Δα X dependency; in other words, ΔS N-I changes in an irregular fashion with Δα X upon increase of the alkoxy chain length (n). Such discrepancy may be attributed to the significant steric effect of the lateral substitution as well as their different electronic interaction with the central benzene ring; this consequently affects the polarisability of the molecule as a whole.
Upon terminal substitution with different polar groups, the behaviour became more complex, reflecting the variable extent of the strength of the mesomeric interaction within the mesogenic core resulting from the change in degree of conjugation. Hence, for the terminally 4-CH 3 substituted homologues in both the series In a and IIn a (Figure 1), their relative value of ΔS N-I vary according to the length of the flexible part of the molecule, which definitely affects the lateral intermolecular interaction to a variable degree. That is for n = 8-14, the order of the decrease in ΔS N-I is IIn a > In a , while for n = 16 is reversed, i.e., I16 a > II16 a . Terminal substitution by the 4-Cl group resulted in a big difference in ΔS N-I values between the homologues; IIn c in all cases are greater than In c . The low ΔS N-I value of the chloro derivatives is attributed to the repulsive force contribution that predominates and consequently reduces ΔS N-I . [36] On the other hand, when comparison is made between members of any group of derivatives bearing the same lateral substituent (2-CH 3 or 3-CH 3 ) and the same terminal alkoxy group, the entropy change ΔS N−I was found to decrease in an order that varies according to the nature of these substituents.
In summary, ΔS N-I decreases according to the terminal polar substituent in the order: Group I: Lateral methyl group in m-position    The low ΔS N-I of some nitro and chloro derivatives is in accordance with the concept of repulsion, [36] adjacent molecules being forced apart, making the nematic arrangements more random and less anisotropic. Thus, in the case of these nitro derivatives the repulsive contribution predominates and consequently reduces ΔS N-I .
In order to investigate the effect of the size of terminal substituent on the mesophase behaviour of prepared compounds, the dependence of the nematicisotropic transition temperature on the van der Waals radius of the terminal substituent, X, those for I8 a-d and I16 a-d , as representative examples, is shown in Figure 2. It was obvious that the transition temperatures are less linearly dependent on the shape of the terminal substituent, irrespective of the alkoxy chain length. The terminally unsubstituted and chloro-substituted analogues are associated with lower values of T N-I than those of their corresponding terminally methyl and nitro-substituted analogues. This suggests that the shape of the compact terminal substituent (X) has insignificant effect on the shape anisotropy of the unit to which it is attached and consequently the mesophase stability.

Binary mixtures of monotropic nematogens
3.2.1. Binary phase behaviour of the two monotropic nematogens bearing the same alkoxy chain, In b /In c In order to test the ability of the two monotropic nematogenic homologues In b and In c to form the nematic phase in their own, their binary mixtures, covering the whole composition range, were first prepared then thermally and optically investigated to construct their binary phase diagrams. Figure 3 shows two examples, where the binary mixtures are formed from the two corresponding lowest homologues I8 b /I8 c and the second from their highest homologues I16 b /I16 c . As can be seen from Figure 3, the addition of the unsubstituted analogue (In b ) to its 4-chloro analogue (In c ), both bearing the same alkoxy chain, resulted in a sharp decrease in its melting temperature, thereby allowing the appearance of the nematic phase enantiotropically covering a wide composition range that varies according to the length of the alkoxy chain.
Thus, in the mixture of the lower homologues, I8 b / I8 c , the nematic phase starts to appear, enantiotropically, upon addition of about 20 mol% of the unsubstituted, I8 b , to its chloro-substituted analogue, I8 c . This range of composition is some what increased in the mixtures of their higher homologues, I16 b /I16 c , to cover a range of 2-90 mol% of I16 b . Eutectic mixtures in both the systems (I8 b /I8 c and I16 b /I16 c ) showed to exhibit reasonable nematic temperature ranges, namely, 21.5°C and 27.5°C, respectively.  Examining these diagrams revealed that the addition of one homologue to the other is accompanied by the appearance of the enantiotropic nematic phase as a result of melting point depression. Again, the nematic composition range is widened as the difference (Δn) in the alkoxy chain length between the two components of the mixture is increased, namely 60 and 80 mol%. But, on the other hand, the nematic stability at the eutectic composition is not significantly affected by the difference Δn.

Mixtures of homologues of
In c . Two examples of the binary phase diagrams of mixtures of two homologues of the chloro analogues (I8 c /I10 c and I8 c / I12 c ) are represented graphically in Figure 5. As in the case of the mixtures of the terminally unsubstituted homologues (In c ), the mixtures of homologues of In c proved to exhibit nematic phase within a composition range dependent on Δn. That is, this composition range increases from ≈55 mol% (in the system I8 c /I10 c ) to 75 mol% (for I8 c /I12 c ) as Δn increases from 2 to 4 carbons. It is also evident from Figure 5 that the lower homologue (I8 c ) is more effective in promoting the nematic phase. The nematic phase starts to appear enantiotropically by the addition of less than 5 mol% of the other homologue (I10 c or I12 c ) to I8 c .

Binary phase behaviour of the monotropic components (In b or In c ) with their enantiotropic nematogenic (4-methyl and 4-nitro) substituted analogues
Since the 4-methyl (In a ) and 4-nitro (In d ) substituted homologues of group II are found to be enantiotropically nematogenic, they were chosen to be the second component in the binary mixtures with either of the two monotropic analogues (In b and In c ), in order to investigate the effect of the former components to enhance the nematic phase in the latter ones. constant except the 4-substituent, X. Two extreme chain lengths (n = 8 and 16) were chosen to investigate the effect of the alkoxy chain length. It is clear from Figure 6 that, in both the examples investigated, the nematic temperature range of the enantiotropic nematogen, In a , decreases in almost linear fashion, indicating the compatibility of the two molecules, so that the inclusion of In b molecules into the nematic arrangement of In a does not disturb the order significantly. Also observed from Figure 6 that the increase in n is accompanied by a reduction of the nematic composition range from ≈95 mol% (for n = 8) to ≈90 mol% (for n = 16 carbons). Another effect of the alkoxy chain length is that the eutectic composition changes from ≈68 mol% I8 b for the mixture of the lower homologues to ≈47 mol% I16 b for the higher homologues.

Mixtures of
In a with In c . The corresponding binary phase diagrams of In c with the strong nematogen In a are given for two representative examples in Figure 7. The resemblance between the diagrams in Figures 6 and 7 revealed that the unsubstituted analogues In b behave mesomorphically nearly similar to those of their corresponding 4-chloro analogues, In c . Again, the higher homologue is less nematogenic.  does not disturb significantly the nematic arrangement of either I8 d or I16 d . That is, the enantiotropic nematic phase of I8 d or I16 d is retained in the mixtures up to the addition of more than 90 mol% of the monotropic nematogens (I8 b or I16 b ). On the other hand, the eutectic compositions of the solid mixtures possess small contents (less than 20 mol%) of I8 b or I16 b .

Mixtures of
In c with In d . Figure 9 represents the binary phase diagrams of the monotropic nematogens (In c ) with the strong nematogens (In d ). As shown in Figure 9, similar behavior was observed for the binary mixtures of the (monotropic) 4-chloro with the (enantiotropic) 4-nitro-substituted analogues. Again, the lower homologue (I8 c ) proved to maintain the nematic arrangement of the nitro analogue (In d ) throughout nearly the whole composition range. Conversely, in these systems the eutectic composition possesses greater amounts (≈60 mol%) of the monotropic component (I8 c or I16 c ).

Summary and conclusions
When comparison was made between the entropy change associated with the nematic-to-isotropic transition, ΔS N-I , for two groups of compounds bearing different lateral substituent (3-CH 3 , In a-d and 2-CH 3 , IIn a-d ) keeping the alkoxy chain length constant, it was found that ΔS N-I decreases in an order dependent on the nature of the two (terminal and lateral) substituents.  Three types of phase diagrams were constructed for binary mixtures made from the laterally 3methyl substituted phenyl azo phenyl benzoates, namely, 4-(4ʹ-substituted phenyl azo)-3-methyl phenyl-4″-alkoxy benzoates. All compounds are nematogenic, but all the terminally unsubstituted and 4chloro-substituted homologues (for n = 8-16 carbons) where found to show their nematic phase monotropically. The other two homologous series bearing the 4-CH 3 and 4-NO 2 wing substituents are enantiotropically nematogen. The binary phase diagrams were constructed to possess at least one monotropic nematogenic, thus these three types were made from: (1) The monotropic nematogens In b and In c as the two components. (2) Two monotropic nematogenic homologues.
(3) A monotropic nematogens (In b or In c ) as the first component, while the second one is an enantiotropic nematogens (In a or In d ).
In all three cases, enantiotropic nematic phases were observed that cover a wide range of composition showing eutectic concentration dependent on the two terminal substituents (RO and X). Also, all the systems studied showed enhanced behavior in the N-I curve, which is attributed to the fact that all of these binary systems consist of two structurally similar components.

Supplemental data
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