Bis(isomaleimide) and diphenol blends: Non-isothermal degradation kinetics and TG-FTIR studies

Abstract

Blends of 4,4′-bisisomaleimidodiphenyl methane (VS) with structurally different diphenols are made in 1:1 molar ratio and thermally polymerized. Thermogravimetric studies of the cured materials show that the thermal stability, the degradation pattern and the char yield are much dependent on the structure of the diphenol that is used for blending. The decreased thermal stability of materials from the blends is attributed to decreased cross links owing to the opening of the isomaleimide rings by diphenols during thermal polymerization. The materials with trimethylphenylindane and tetramethylspirobiindane units exhibit char residue of 21% at 800°C. This is due to the thermal stability of the indane and spirobiindane moieties present in the diphenol molecules. The apparent activation energy for thermal degradation (E_a-D) and pre-exponential factor (ln A) are derived. The Friedman, corrected Flynn–Wall–Ozawa, corrected Kissinger–Akahira–Sunose and advanced Vyazovkin methods are used to calculate the E_a-D values for various reaction extents (αs). The E_a-D values of poly(4,4′-bisisomaleimido-diphenyl methane) vary from 168 to 226 kJ mol⁻¹. A slight decrease in E_a-D is noted for the initial α levels and increases constantly up to α = 0.3–0.75 and then the E_a-D values decrease with increasing α values. The highest values of E_a-D and ln A are observed for the polymer derived from the blend of VS with tetramethylspirobiindane diphenol. The volatile products obtained during the thermal degradation of these polymers are analysed using thermogravimetry–Fourier transform infrared (TG-FTIR) spectroscopy. The TG-FTIR studies showed the compounds carbon monoxide, carbon dioxide and aromatic amines are the major degradation products from the polymerized blends.