Synthesis, Characterisation and Mesophase Transition of Hexasubstituted Cyclotriphosphazene Molecules with Schiff Base and Azo Linking Units and Determination of Their Fire Retardant Properties

Abstract

Hexakis(oxy-4-benzaldehyde)cyclotriphosphazene, 1 was successfully synthesised by nucleophilic substitution reaction between hexachlorocyclotriphosphazene, HCCP and 4-hydroxybenzaldehyde. Azotization reaction of p-nitroaniline with phenol formed 4-(4-nitrophenylazo)phenol, 2 which was alkykated with heptyl, nonyl, decyl, dodecyl and tetradecylbromide to give a series of nitro compounds, 3a-e. Reduction of 3a-e and 2 formed the subsequent amine compounds 4a-f, 4-(4-alkyloxyphenylazo)phenylamine. Another similar reaction of protected aniline with a series of substituted aniline formed a series of compounds 4g-i. A series of hexasubstituted cyclotriphosphazene molecules containing Schiff base and azo linking units, 5a-i were synthesised from the reaction between intermediates 1 and 4a-i. Further reduction of compound 5i afford compound 5j with amino terminal end. All the synthesised intermediates and compounds were characterised using Fourier Transform Infrared spectroscopy (FT-IR), 1H and 13C Nuclear Magnetic Resonance spectroscopy (NMR) and CHN elemental analysis. The liquid crystal properties of intermediates and final compounds were determined using Polarised Optical Microscope (POM) and their phase transitions confirmed using Differential Scanning Calorimetry (DSC). Only intermediates 3a-e showed mesophase of smectic A and compounds 5a-e with alkoxy chain were mesogenic with smectic A and nematic phases. In addition, compound 5h exhibited nematic phase only. However, all the other intermediates and compounds were found to be non-mesogenic. Furthermore, the fire retardant of final compounds were determined using Limiting Oxygen Index (LOI) testing. The LOI value of pure polyester resin was increased from 22.53 to 24.71% when incorporated with 1 wt% of HCCP. Moreover, all the final compounds showed a positive in LOI value the highest LOI value was belonged to compound 5i with 27.90%.