Liquid-Crystal and Fire-Retardant Properties of New Hexasubstituted Cyclotriphosphazene Compounds with Two Schiff Base Linking Units

Abstract

A series of new hexasubstituted cyclotriphosphazene compounds (4a–j) consisting of two Schiff base linking units and different terminal substituents was successfully synthesized and characterized. The structures of these compounds were confirmed using Fourier Transform Infra-Red (FTIR), Nuclear Magnetic Resonance (NMR), and CHN elemental analysis. Polarized optical microscopy (POM) was used to determine their liquid-crystal behavior, which was then further confirmed using differential scanning calorimetry (DSC). Compounds 4a–i with heptyl, nonyl, decyl, dodecyl, tetradecyl, hydroxy, 4-carboxyphenyl, chloro, and nitro terminal ends, respectively, showed the liquid-crystal properties, whereas compound 4j with the amino group was found to be non-mesogenic. The attachment of an electron-donating group in 4j eventually give a non-mesogenic product. The study of the fire-retardant properties of these compounds was done using the limiting oxygen index (LOI). In this study, polyester resin (PE) was used as a matrix for moulding, and the LOI value of pure PE was 22.53%. The LOI value increased to 24.71% when PE was incorporated with 1 wt.% of hexachlorocyclotriphosphazene (HCCP), thus indicating that HCCP has a good fire-retardant properties. The result showed that all the compounds have good agreement in their LOI values. Compound 4i with a nitro terminal group gave the highest LOI value of 28.37%.

Synthesis and Determination of Thermotropic Liquid Crystalline Behavior of Cinnamaldehyde-Based Molecules with Two Schiff Base Linking Units

“…For other homologues, compounds 3 – 9 showed a slight difference in the chemical shift values of each compound due to the different chemical environment, electronegativity effect, and bond angles [ 31 ]. Only compounds 7 and 9 showed an additional signal for the methoxy and dodecyloxy chains in the 1 H and 13 C NMR spectra, respectively.…”

“…The same trend occurred in compounds 8 and 9 . In addition, longer terminal chains lower the melting and clearing temperatures [ 31 ]. The DSC thermogram of compound 8 is illustrated in Figure 12 as an example in this series, while the DSC thermograms for other mesogenic compounds ( 2 , 3 , 7 , and 9 ) are provided in the Supplementary Materials section.…”

“…The Schiff base is one of the most well-known linking units used in connecting the rigid core groups. Schiff base molecules provide a stepped core structure which can maintain molecular linearity to provide high stability and enable mesophase formation [ 29 , 31 ]. Liquid crystal molecules with Schiff base linking units, with two cinnamaldehyde cores attached, did not show any liquid crystal properties.…”

“…The increased dipole moment can enhance the stability of lattice and melting temperatures [ 34 ]. Moreover, the electronegativity of the halogen atom reduces the degree of molecular order, which enhances a higher clearing temperature [ 31 , 35 ]. As a result, compounds 2 and 3 showed nematic phase behavior at high temperatures.…”

“…As a result, the mesophase transition cannot be induced. Meanwhile, the lone pair of the hydroxy groups caused the cancellation of dipole moments of the molecules, which reduced the polarizability and molecular interactions needed for the formation of the mesophases [ 31 , 40 ]. The POM observation of compounds 2 – 9 is summarized in Table 6 .…”