Investigation Of Thermal Imidization Processes For Polyimide Precursors

Aromatic polyimides (PIs) are one of the most important classes of high-performance polymers. Due to their excellent electrical, thermal, and low and/or high-temperature mechanical properties, aromatic polyimides have found many applications as high-temperature insulators and dielectrics, coatings, adhesives, and so on. Perhaps the most highly developed synthetic route for these important materials is the classical two-step method, the first step being the synthesis of precursor, ie. poly(amic acid) (PA) from the reaction of diamine and dianhydride and the latter step being subsequent thermal or chemical imidization of the poly(amic acid). The conversion of a poly(amic acid) to the polyimide is most commonly accomplished by the thermal solid-phase imidization process, even this process is very easy to operate. However, the strong inter-molecular interaction, the rigid chain or cross-linking and so on, make many of these polymers insoluble and infusible even over decomposition temperature, rendering them impossible to process. Moreover, the water and bubble was produced during the high temperature imidization, which limit wider applications of the polyimides. Alternatively, the poly(amic acid) could be imidizated by solution imidization process at relatively mild imidization temperatures, which may avoid several possible side reactions that may occur at much higher temperatures, and improve solubility and/or processability of the polyimide. In order to prepare the high-performance polyimides with better solubility and/or processibility, the processes of the thermal solid-phase imidization and the solution imidization were investigated for polyimides synthesized from the long-chain monomers in this paper.Potentiometric titration technique was selected to determine the content of amic acid in polyimide precursors, which were synthesized by the two-step method. The basic potentiometric titration device was firstly improved, which can be controlled by PC and software programmed using VB. Two samples of PA were determined. The results showed that this technique was rapid and convenient within better repeatability, higher accuracy, and could be further used to investigate the imidization kinetic of polyamic acid to polyimide.By the improved potentiometric titration device and gel permeation chromatography (GPC), the kinetics of the thermal solid-phase imidization process of poly(amic-acid)s was investigated. The results showed that the number average molecular weight (Mn) of polymers were at the range of 1.2 to 1.4×104, and the molecular weight of the polymer was increased with the raise of the imidization time and temperature, and the molecular weight distribution (D) of the polymer presented an initial increase and then a decrease during the imidization process. The higher the imidization temperature, the faster the rate of imidization process was and the larger the imidization degree was. The solid-phase imidization process follows two first-order step kinetics models. The relevant parameters were obtained for the initial-stage and the later-stage. The activation energy values of the two stages were found to be 74.84 and 59.68 kJ·mol-1, respectively. The rate constant of the imidization process of the initial-stage is larger than that of the later-stage, the frequency factors and the activation entropies of the two stages were different. The poly(amic acid)s with different molecular weights end-capped with phthalic anhydride were synthesized, their viscosity were 6.4, 7.6, 9.3 mL/g, respectively. The results of the potentiometric titration technique showed that the imidization degree and rate increased with the decrease of the molecular weight. And during the process of imidization the end groups imidizated priorly, then the groups in the molecular chain imidizated.The three kinds of polyimides were synthesized using the nonconjugated monomers of p-BAPS, PMDA and ODPA, respectively. The solution imidization technique was applied and also investigated by the improved potentiometric titration device and GPC...