Thermal degradation kinetics of 6FDA/durene diamine/pPDA copolyimides

Aromatic copolyimides have been synthesised from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) with various mol-% of 2,3,5,6-tetramethyl-1,4-phenylene diamine (durene diamine (DDA)) and 1,4-phenylene diamine (pPDA) in N -methyl-2-pyrrolidone (NMP). Copolyimides were characterised by inherent viscosity, GPC, DSC, DMA, XRD, and TGA. Inherent viscosity and molecular weight increased with increase in DDA content in copolyimides. This was attributed to higher basicity of DDA than pPDA, which resulted in higher reactivity towards dianhydride. They were found to be soluble in all common solvents except CCl₄ and methanol. The glass transition temperature T _g of these amorphous copolymers increased with increase in DDA content; T _gs determined by DSC and DMA were comparable and T _g calculated using the Fox equation was compared with the experimental value. Thermal degradation of these copolyimides was found to follow second order kinetics in air. The activation energy E _a for the thermal degradation of these copolyimides in air was determined by both the Coats-Redfern and Chang methods and the result was comparable. Both Ea and ln A were found to decrease with increase in DDA content; CO₂ and trace amounts of CHF₃ were the decomposition products. The mechanism of thermal degradation is unique and is not affected by a change in the copolymer composition because all copolyimides display the kinetic compensation effect.