Synthesis, characterization, thermodynamics and thermal degradation kinetics of imine-linked polymers

A series of new imine-linked polymers reveal variations of aliphatic and aromatic linkers were synthesized via Schiff base co-condensation polymerization and fully characterized. The polymers are classified as 1,4- and 1,6-based polymers according to their preparation methodology, which depicted by the polymerization of thiophene-based aryl aldehydes with1,4-butanediamine and 1,6-hexanediamine, respectively. The interpretation of their thermal stability from the thermal gravimetric analysis and the first derivative (TGA-DTG) curves have indicated that the variation in the contents of the aryl-aldehydes and the length of the linker are significantly affect the thermal stability behavior. For example, the polymer prepared via the connection of ter-thiophene-aldehyde with 1,4-butanediamine (1,4-TIP) has shown higher thermal stability, higher char residue content, higher limited oxygen index (LOI = 32) than the polymer prepared by bi-thiophene-phenyl-aldehyde (1,4-F2IP) in the same manner. Furthermore, 1,4-TIP has shown the highest LOI value over all polymers and thus, can be considered as a flame-retardant polymer. However, an inverse relation has been observed upon the inclusion of 1,6-hexanediamine with the same aryl-aldehydes. Further insight into the TGA curves was performed by applying well-known fitting models: Coats-Redfren (CR), Arrhenius, Briodo, and Horowitz-Metzeger for the determination and comparison of the activation energy (Ea) values upon degradation process. The CR-method was further used for the determination of the kinetic triplet parameters (Ea), reaction order (n) and pre-exponential factor (A) and thermodynamic parameters include entropy, the heat of enthalpy, and Gibbs free energy of the polymers. All these methods support the fact that the degradation mechanism follows first-order kinetics, thermodynamically favored and the higher thermal stability polymer requires higher (Ea) for thermal degradation.