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The mesophase ranges, phase transition temperatures and corresponding enthalpy changes of monomers M1-M6 are presented in Table 4.8. Although monomers

M1-M6 have similar molecular structure except terminal sixth position substituent on

the benzothiazole ring, different LC phases were exhibited during heating and cooling. In order to investigate the terminal substituent effect on mesophase formation, X = H, CH3, OCH3, OC2H5, F and Cl groups situated at the sixth position on the benzothiazole ring were chosen for this study. It can be clearly seen from figures (Figure 4.14 to Figure 4.25) and Table 4.8 that the terminal methacrylate group and the sixth position substituents on benzothiazole ring have significant role on the mesophase formation. Comparison among the six monomers reveals that M1, M5 and M6 showed only smectic mesophase whereas monomers M2, M3 and M4 exhibited both nematic and smectic phases. Monomers with an electron donating substituent (CH3, OCH3 and

OC2H5) exhibited both nematic and smectic mesophases; however, monomer without terminal substituent (M1) revealed only SmA phase.

Table 4.8: Mesophase lengths, phase transition temperatures and enthalpy changes for monomers M1- M6 upon heating and cooling scans.

Phase transitions, (°C) (enthalpy changes, J g-1) Mesophase range (oC) Second Heating First Cooling Nematic Smectic

M1 Cr 107.3 (82.2) I Cr1 63.3 (-8.1) Cr2 65.5 (-26.8) SmA 90.1 (-12.6) I - 24.6 M2 Cr 116.5 (78.9) I Cr 89.2 (-69.6) SmA 102.6 (-4.8) N 112.3 (-1.0) I 9.7 13.4 M3 Cr1 91.8 (44.7) Cr2 94.3(1.3) SmC 105.2 (0.2) N 106.9 (0.2) I Cr1 51.0 (-33.0) Cr2 56.2 ( -1.9) SmC 77.0 (-0.6) N 106.1 (-0.5) I 29.1 20.8 M4 Cr 97.3 (69.9) N 111.2 (0.4) I Cr 54.8 (-53.4) SmA 76.2 (-0.2) N 110.5 (-0.6) I 34.3 21.4 M5 Cr 120.2 (83.2) SmA 129.8 (8.3) I Cr 93.9 (-54.0) SmA 125.6 (-9.6) I - 31.7 M6 Cr 113.3 (77.8) SmA 138.2 (8.5) I Cr 57.3 (-8.7) SmA 134.1 (-9.1) I - 76.8

Transition temperatures (°C) and enthalpies (in parentheses, J g-1) were measured by DSC. Cr = Crystalline phase; SmA = Smectic A phase; SmC = Smectic C phase; N = Nematic phase; I = Isotropic liquid.

The terminal methacrylate group attached with alkyl spacer could play an important role to the formation of smectic phase in monomer M1. This statement can be explained by comparing the mesophase behaviour of M1- M4 with recently reported structurally similar compounds [118, 122] whereby only nematic LC phase of same mesogenic unit containing molecules was reported. This result may be attributed to the fact that the terminal polar methacrylate group enhanced the polarisability anisotropy and could favour the lateral attraction between molecules to generate a strong smectic phase. On the other hand, monomers M2, M3 and M4 exhibited nematic phase along with smectic phase. Replacement of hydrogen atom by the methyl, methoxy and ethoxy groups at the sixth position on the benzothiazole ring may play vital role to the formation of the nematic phase because the relatively short terminal chains in conjugation with a core of high longitudinal polarisability facilitates the generation of nematic phase [3]. Polarization or electron distribution in electron-deficient benzothiazole moiety may also be affected by the electron-donating substituents which could facilitate the formation of nematic phase. The mesophase temperature ranges were significantly influenced (Table 4.8) by the size of the sixth position substituent on benzothiazole ring. Monomer with ethoxy substituent exhibited greater mesophase stability than those of compounds with methoxy and methyl substituents. Like M1, monomers M5 and M6 having terminal electron withdrawing F and Cl atom respectively exhibited only SmA phase. However, monomer M6 containing a larger or more polarized group (X = Cl) which enhanced the SmA phase stability than monomers with a smaller polarized substituent (X = F) and without polarized group (X = H). The clearing temperature of monomer M6 (138.3oC) was higher than those of compounds M5 (129.2oC) and M1 (107.3oC), i.e., 138.3oC >129.2oC >107.3oC. On the other hand, monomer M6 (76.8oC) exhibited greater mesophase stability than those of monomers M5 (31.7oC) and M1 (24.6oC), i.e., 76.8oC >31.7oC > 24.6oC on cooling. The higher clearing temperature and greater mesophase

stability of monomer M6 may be due to the strong polarizing power of Cl atom. Therefore, it can obviously be concluded that the terminal sixth position substituent on the benzothiazole moiety has profound influence on the formation of mesophase as well as mesophase stability of the synthesized LC monomers.

4.4.3. Mesomorphic behaviours of LC monomers M7-M10

Figure 4.28 shows DSC thermograms of synthesized monomers M7-M10. The isotropization temperatures of monomers M7-M10 determined from DSC were almost identical with the onset thermal decomposition temperatures of azo-benzothiazole segment as observed from TGA analysis. As a result, no distinct DSC peaks were identified for monomers M7-M10 during cooling scan due to the partial decomposition of the studied monomers. Wei et al. [208] also reported similar observation for their studied compounds. Thus, only first heating data of monomers M7-M10 are considered for further discussion.

Figure 4.28: DSC traces of monomers M7-M10 on first heating scan at 10oC/min

Figure 4.29: POM images of compounds M7- M10: (a) & (b) M7 displays schlieren texture of nematic phase at 196.8oC and smectic C phase at 140.5oC; (c) M8 shows nematic phase at 225.5oC; (d) M9 exhibits nematic phase at 229.4oC (e) M10 reveals nematic (threaded) phase at 226.5oC (magnification: 50×)

It can be seen from Figure 4.28 that monomer M7 exhibits three thermal transitions: (i) a crystal to smectic at 104.8oC, (ii) smectic to nematic at 143.3oC and

(iii) nematic to isotropic at 197.2oC. On the other hand, monomer M8 shows two thermal transitions: (i) a crystal to nematic at 106.6oC and (ii) nematic to isotropic at 229.6oC. Similarly, monomer M9 exhibits a crystal to nematic transition at 121.0oC and (ii) nematic to isotropic transition at 236.7oC. Likewise M8 and M9, monomer M10 shows two thermal transitions: (i) crystal to nematic at 127.9oC and (ii) nematic to isotropic at 232.5oC.

The observed optical photomicrographs of M7-M10 are shown in Figure 4.29. The optical photomicrographs of monomer M7 were taken during cooling scan whereas POM images of M8-M10 were recorded during heating scan. Monomer M7 exhibited schlieren texture of nematic phase at 196.8oC upon cooling from isotropic liquid and further cooling schlieren texture of smectic C phase appeared at 140.5oC. The identification of SmC phase was made on the basis of the characteristic grey schlieren texture (Figure 4.29b) which appeared during nematic to smectic C transition [209]. During heating, monomer M8 melted around 107.0oC and upon further heating schlieren texture of nematic phase with four-fold brushes (Figure 4.29c) started appearing and the image was taken at 225.5oC. Monomers M9 and M10 also exhibited schlieren texture of nematic phases upon heating scan and the POM images of M9 and

M10 were recorded at 229.4oC and 226.5oC respectively.

4.4.4. The effect of terminal substituent and mesogen length on mesomorphic