Benzothiazole-based poly(phenoxy-imine)s containing ethoxy and methoxy units: synthesis, thermal and optical properties

Schiff bases (S1 and S2) were synthesized from the condensation reaction of 2-amino-6-ethoxybenzothiazole with 3-hydroxy-4-methoxybenzaldehyde and 3-hydroxy-4-ethoxybenzaldehyde. P-S1-A and P-S2-A poly(phenoxy-imine)s, and P-S1-O and P-S2-O poly(phenoxy-imine)s were synthesized through oxidative polycondensation (P) of S1 and S2 by NaOCl oxidant in alkaline (A) solution (10% KOH aquation solution) and by H₂O₂ (35% aquation solution) in THF organic (O) medium, respectively. The structures of the Schiff bases and poly(phenoxy-imine)s were characterized by FTIR, NMR, UV–Vis and TG/DTA investigations. The limit oxygen index (LOI) and heat-resistance index temperature (T_HRI) values were determined from thermogravimetric analysis data of compounds. The LOI values of poly(phenoxy-imine)s were between 21.16 and 33.87. The molecular weight distributions and glass transition temperatures of poly(phenoxy-imine)s were determined from gel permeation chromatography (GPC) and DSC, respectively. The M_n values of P-S1-O, P-S1-A, P-S2-O and P-S2-A were found to be 5800, 15500, 5600 and 9400 Da, respectively. Moreover, the polymerization degree (DP) values of P-S1-O, P-S1-A, P-S2-O and P-S2-A were calculated to be 18, 16, 47 and 28, respectively. The glass transition temperature (T_g) of P-S1-O, P-S1-A, S2, P-S2-O and P-S2-A were calculated from DSC curves at 115, 111, 102 and 128 °C, respectively. The surface images of poly(phenoxy-imine)s were determined with SEM analysis. The optical bandgap values of Schiff bases and poly(phenoxy-imine)s were calculated from UV–Vis measurements. The optical bandgap value of P-S2-A was calculated to be 1.95 eV.

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