The Effect of Synthesis Conditions and Chemical Structure of Thermoplastic Polyimides on Their Thermomechanical Properties and Short-Term Electrical Strength

Polyimides (PIs) are materials that are resistant to high temperatures and crucial for the manufacturing of films, fibers, coatings, and 3D-printed items. PIs are widely used as electrically insulating materials in electronics and electrical engineering. This study investigated how the chemical structure (i.e., choice of initial monomers), the synthesis conditions of the prepolymer (i.e., choice of amide solvent), and the conditions for forming polyimide films (i.e., final curing temperature) affect the thermophysical properties and short-term electrical strength of obtained polyimide films of different chemical structures. In this work, we varied the compositions of the dianhydrides used for synthesizing polyamic acids-pyromellitic acid (PMDA), tetracarboxylic acid diphenyl oxide (ODPA) and 1,3-bis(3',4-dicarboxyphenoxy)benzene acid (R)-with a constant diamine: 4,4'-oxydianiline (ODA). Additionally, we varied the amide solvents employed: <i>N</i>,<i>N</i>-dimethylacetamide (DMAc), <i>N</i>,<i>N</i>-dimethylformamide (DMF), and <i>N</i>-methyl-2-pyrrolidone (NMP). This study represents the first investigation into how the choice of solvent in the synthesis of thermoplastic polyimide prepolymers affects their short-term electrical strength. The molecular weights of the polyamic acids were determined using gel permeation chromatography (GPC). The deformation and strength characteristics of the investigated films were also assessed. The thermophysical properties of the polyimides were evaluated via dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). X-ray diffraction analysis and infrared spectroscopy (IR) were conducted on the examined film samples. The short-term electrical strength was also evaluated.