Design and thermal imidization of new 1,3-thiazine-based polyimides and copolyimides for high-performance corrosion inhibition

A new polyimide, poly(1,3-thiazine imide) (PTzI), together with two co-polyimides (CoPTz-DsI and CoPTz-HaI), were synthesized from the monomer named 4,4'-(1,4-phenylene)bis(6-(benzene-2-yl)-6 H-1,3-thiazine-2-amine) (PTA), which has a di-1,3-thiazine heterocyclic structure. The structure of PTA was confirmed with FT-IR, along with NMR spectra and mass spectroscopy. A thermal imidization process was used to prepare PTzI from the corresponding polyamic acid (PAA), which was obtained by polycondensation of PTA with 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA). Similarly, the copolymers were synthesized via thermal imidization of PAAs derived from the polycondensation of PTA with 4,4'-diaminodiphenyl sulfone (DDS) or 1,6-hexane diamine (HAD), in addition to BTDA. The structure and imidization of each polymer were verified at various curing temperatures using Fourier transform infrared FT-IR spectroscopy. X-ray diffraction verified their amorphous nature, while scanning electron microscopy (SEM) morphological investigation indicated different levels of homogeneity and compactness. The rigid aromatic backbones and imide linkages provide exceptional thermal stability and chemical resistance, demonstrating their high performance. Thermogravimetric study (TGA and derivative TGA) showed that the materials had outstanding thermal stability, with high degradation initiation and initial decomposition temperatures (Td_5%) in the range of 294-418 °C, and char yields of 52-63% at 800 °C. In acidic environments, the synthesized polymers have shown exceptional corrosion prevention performance for mild steel MS, according to electrochemical evaluations. Due to its structural composition rich in 1,3-thiazine structure, PTzI exhibited the highest inhibition efficiency (99.4%). Additionally, CoPTz-HaI and CoPTz-DsI demonstrated excellent efficiencies of 95.2% and 96.4%, respectively. The inhibitory performance was shown to be improved as the PTA concentration increased, suggesting that it may be used to create more sophisticated coatings that resist corrosion.