Synthesis and characterization of graft copolymers of melamine: Thermal stability, electrical conductivity, and optical properties

In this study, novel three Schiff bases of melamine were synthesized via condensation reaction of melamine with salicylaldehyde, 3-hydroxybenzaldehyde, and 4-hydroxybenzaldehyde namely N,N′,N′′,-tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (2-HPMTT), N,N′,N′′-tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (3-HPMTT), N,N′,N′′-tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (4-HPMTT), respectively. Then, oligo/polyphenol derivatives of these Schiff bases were obtained by grafting melamine onto oligosalicylaldehyde (OSA), oligo-3-hydroxybenzaldehyde, and oligo-4-hydroxybenzaldehyde that have generate names of poly-N,N′,N′′-tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-2-HPMTT), poly-N, N′,N′′-tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-3-HPMTT), and oligo-N,N′,N′′-tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (O-4-HPMTT), respectively. The structures of the synthesized compounds were confirmed by FT-IR, UV–vis, 1H NMR, and 13C NMR techniques. The characterization was made by TG-DTA, DSC, size exclusion chromatography (SEC), and solubility tests. Electrical conductivities of the synthesized materials were measured by four-point probe technique using a Keithley 2400 electrometer showing that the synthesized oligo/polyphenols have higher electrical conductivities than the monomeric Schiff bases. Also considerable increases in the conductivities were observed when they were doped with iodine as a doping agent. The order of increase rates of the conductivities were found as follows: 2-HPMTT > P-2-HPMTT > P-3-HPMTT > 3-HPMTT > 4-HPMTT > P-4-HPMTT. Additionally, the optical band gaps (Eg) were calculated by using the absorption spectra and found to be 2.78, 2.17, 3.58, 3.30, 4.03, and 2.82 eV for 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT, respectively.