SOLVENT EFFECT ON THE CURING OF POLYIMIDE RESINS

The effect of the solvent 1-methyl-2-pyrrolidinone (NMP) on the curing of polyimide resins synthesized from pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA) has been investigated. Three polyimide precursors, i.e., the polyamic acid (PAA), with controlled amount of NMP were prepared. The study was aimed first to independently investigate the decomplexation process, which involved the evolution of hydrogen-bonded NMP from PAA, without interference from imidization. This was accomplished by TGA at varying heating rates using different solvent content in PAA. The observed one-stage decomplexation process suggested that the complex formation of NMP and PAA was not the same as the model compound studied by others. An average value of 150 kJ/mol for the activation energy of the decomplexation process was obtained. The study then sought to identify the effect of the decomplexation on the imidization kinetics by employing DSC at several drying temperatures and also varying heating rates. This allowed one to control the extent of plasticization that occurred to facilitate the imidization process. Our DSC data showed that over-drying PAA resulted in prolonged imidization due mainly to the lack of plasticization by decomplexed NMP. The estimated enthalpy of imidization and that of decomplexation were 114 KJ/mol and 53 kJ/mol NMP, respectively. Finally, the imidization kinetics was independently investigated using FTIR, without the interference from decomplexation process. The results indicated that there were four stages during the entire imidization process. Up to a temperature of 150-degrees-C, less than 20% of amide groups had reacted to give imide groups and the reaction was slow. Most of the imidization took place between 150 and 180-degrees-C with conversion as high as 90%. The imidization process was completed after the temperature was further raised to 250-degrees-C. Above 250-degrees-C, the reverse reaction became more significant (due probably to configurational and packing preference) and resulted in a lowering of final conversion back to 80%.