Effect of the solvent type and polymerization conditions on the curing kinetics, thermal and viscoelastic performance of poly(amide-imide) resins

Abstract

Isothermal and non-isothermal curing kinetics of both N-methyl-2-pyrrolidone (NMP) and N-methylimidazole (MI) based poly(amide-imide) (PAI) resins were investigated by DSC analysis using tightly closed high-pressure crucibles. Several exothermal peaks on the non-isothermal DSC-traces were observed and attributed to the reactions of different functional groups of PAI-resin. Furthermore the final conversion (polymerization degree) of PAI was determined under isothermal conditions, simulating three programs with the post-curing temperatures set as 215, 240 and 270°C. For the MI-PAI based resin, the conversion values were found to be much higher compared to those for the NMP-PAI system. Compared to NMP-based PAI-resin, a shift of the main exothermal peaks to the lower temperatures was observed in the non-isothermal kinetic investigations when MI was used as a solvent. This was accompanied with a reduction of activation energy (Ea) values, as up to a factor of 3 determined by the Flynn-Wall-Ozawa approach for all the main formation reactions. It indicates a catalytic effect of MI on the PAI polymerization. In addition, conversion values were determined according to the Di Benedetto equation for both systems cured using open molds in the oven. Regardless the different post-curing temperatures, the conversion values were similar for all the samples. Thermal and viscoelastic properties as well as crosslink density (nc) were also investigated for these systems. It was found that the MI-based samples demonstrate lower nc values compared to the NMP-based ones at an almost two times higher storage modulus (E') at room temperature.

Keywords

• Thermosetting resins
• curing kinetics
• activation energy
• crosslink density
• storage modulus