Design and Thermal Profiling of Ethyl Carbazole-Based Poly(phenoxy-imine)s: Synthesis and Characterization

A series of Schiff base monomers were synthesized via the condensation of 3-amino-9-ethylcarbazole with various aldehydes, including 3-hydroxybenzaldehyde (3-HBA), 4-hydroxybenzaldehyde (4-HBA), 2-hydroxy-1-naphthaldehyde (2-HNA), and ortho-vanillin (OV). These monomers were subsequently polymerized through oxidative polycondensation using NaOCl as the oxidant to yield corresponding poly(phenoxy-imine)s, namely Poly(3ECIMP), Poly(4ECIMP), Poly(ECIMN), and Poly(ECIMMP). The chemical structures of the synthesized compounds were confirmed using FT-IR, UV-Vis, and both 1H and 13C NMR spectroscopy. Thermal characterization using TG-DTA and DSC demonstrated that the synthesized polymers possess excellent thermal stability, as evidenced by their glass transition temperatures between 145 and 161°C and char yields reaching up to 38.22% at 1000°C. Furthermore, limiting oxygen index (LOI) values exceeding 28% indicate that these materials exhibit self-extinguishing behavior. Photoluminescence studies conducted in DMF demonstrated intense fluorescence, particularly for Poly(ECIMN), which exhibited yellow-orange emission upon excitation at 502 nm due to the presence of naphthalene moieties that enhance π-electron delocalization. Optical and electrochemical band gap analyses indicated significantly reduced E _g values for the polymers compared to their monomers, with Poly(ECIMN) showing the lowest band gap of 2.39 eV. Cyclic voltammetry results aligned with optical measurements, confirming improved charge transfer characteristics in the conjugated polymer backbone. Surface morphology assessed by FE-SEM revealed porous structures, suggesting applicability in gas adsorption or catalytic systems. Size exclusion chromatography (SEC) confirmed the formation of high molecular weight polymers with narrow polydispersity indices. Collectively, the unique combination of thermal durability, photophysical responsiveness, and structural robustness highlights the potential of these ethyl carbazole-based poly(phenoxy-imine)s in applications spanning optoelectronics, thermal protection, and fluorescence-based sensing.