Combustion characteristics and thermal degradation kinetics of microporous triazine-based organic polymers: the role of organic linkers

Abstract

This work aims to investigate the combustion characteristics, kinetics triplets, and thermodynamic parameters of microporous triazine-based organic polymers. The polymers were prepared by the incorporation of aliphatic and aromatic diamines (e.g., 1,4-hexane diamine (Hex) and 1,4-phenylenediamine (Bz)) with triazine core (Tr) via polycondensation polymerization. Both polymers Tr–Hex-diamine and Tr–Bz-diamine are microporous with a surface area of 212 and 524 m²/g, respectively. The successful synthesis was confirmed from FTIR and solid-state ¹³C CP-MAS. The combustion index (SN), kinetic triplets, apparent activation energy (Ea), pre-exponential factor (A), and thermodynamic parameters were estimated from the thermal degradation profiles of the polymers (TGA) at different heating rates. At the maximum heating rate (20 °C/min) the SN of Tr–Bz-diamine is 2.08, while it reached 4.2 for Tr–Hex-diamine, which indicates the high rate of combustion of the aliphatic hexyl chains. The other kinetic and thermodynamic parameters were determined by applying model-free isoconversional methods including Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (OFW), and Kissinger. From KAS, the average Ea for Tr–Bz-diamine and Tr–Hex-diamine are 163.4 and 147.8 kJ/mol, while 169.2 and 151.7 kJ/mol from OFW calculations. These values are higher in the case of the Kissinger method. The degradation mechanism and the rate of decomposition were determined from the Coats–Redfern method and by applying the master plot methods. Comparing the Ea values of the CR method with the integral method shows the possibility of the chemical reaction F3 mechanism beside multiple parallel reactions as shown by the master plot. The pre-exponential factor (A) along with the thermodynamic parameters (e.g., heat of enthalpy, entropy, and Gibbs free energy) were also determined and found to be within the same range of all methods.