Preparation and fabrication of aromatic polyimides

Abstract

synopsisAromatic polyimides were prepared from pyromellitic dianhydride and a number of aromatic diamines. The effect of certain variables on the polymerization to, and the degradation of, the intermediate polypyromellitamic acids was studied, and a previously unrecorded reaction intermediate was isolated and identified. From these studies the conditions necessary to obtain high molecular weights were defined. Techniques were developed for the fabrication of satisfactory films and moldings. INTRODUCTIONPolyimides are one of a growing number of aromatic and heterocyclic polymers recently developed in the search for structural stability at elevated temperatures Depending on the environment and the exposure time, useful service lives may be obtained in the region of 300-500°C.1The fundamental reactions for synthesis of the imide ring have long been known. In the post-wax period these were applied to give aliphatic polymers via nylon-type condensation routes from aliphatic diamines with both aliphatic and aromatic tetracarboxylic acids and their esters. The polyimides formed were soluble and fusible and were suggested for use in injection-molding processes. These reaction routes proved to be unsatisfactory when attempts were made to prepare fully aromatic polyimides because of the precipitation of low molecular weight "brickdusts" at an early stage in the reaction. As a result of these failures, solution polymerization methods were developed by various ~o r k e r s .~-'~ Related reactions have also been described for the preparation of amide-imide copolymers. lo, l 4In the course of the preparation of polyimides of varying structure for thermal stability determinations, the polymerization reaction was studied in some detail and the formation of a prepolymer complex noted, which has not previously been reported. The conditions necessary for the formation of high molecular weight polymer were confirmed and techniques developed for producing satisfactory films and moldings. EXPERIMENTAL ReagentsPyromellitic Dianhydride (PMDA). This was purified via complex formation with anisoleg to give a pure white material melting at 286°C. This method was found to be preferable to either vacuum sublimation from phosphorus pentoxide or recrystallization from acetic anhydride.Two spot tests were used to detect small degrees of hydrolysis in PMDA. AROMATIC POLYIMIDES 611(1) A few crystals of PMDA were added to dry anisole in a flamed semimicro test tube and the solvent heated to boiling. The formation of turbidity in the hot yellow solution indicated that traces of acid were present.(2) A few crystals of PMDA were added to dry tetrahydrofuran in a flamed test tube and dissolved by shaking. Two drops of pure dry pyridine were then added. The immediate formation of a white precipitate indicated the presence of pyromellitic acid. The dipyridinium salt could be filtered off , dried, and estimated gravimetrically.Diamines. These were purified by recrystallization from benzene or from an alcohol-tetrahydrofuran mixture, followed by prolonged...

Characterization of a thermally imidized soluble polyimide film

“…However, we have noted (14) the previously reported embrittlement (3,15) of certain films at intermediate (150°-200°C) stages of cure.…”

“…Many early reports were rightfully concerned with studying both initial poly(amic acid) formation(1, 3) and imidization kinetics (4). The effect of temperature and residual water on poly(amic acid) solution stability was noted(1-3,5,6) and the potential for hydrolysis of both the poly(amic acid) and polyimide by water produced during imidization was speculated (3,6). Numerous studies have also noted the possibility of side reactions which may accompany the thermal imidization process (3,5,(7)(8)(9).…”

“…Dilute solutions are known to exhibit polyelectrolyte behavior (3,11,22,23). When this occurs, the hydrodynamic volume of the polymer increases, affecting viscosity, GPC, light scattering and other dilute solution property measurements.…”

The properties, morphology and structure of BPDA/ PPD/ BOA polyimide fibers

“…In fact, this material has been successfully used as polyimide film with trade name Upilex-S [15]. Unfortunately, polyimide fiber of this system has not been successfully used because of the inferior mechanical properties.…”

Two-Dimensional Raman Correlation Spectroscopy Study of the Pathway for the Thermal Imidization of Poly(amic acid)

“…Aromatic polyimides (PIs) are widely used in various applications [1][2][3][4] due to their outstanding thermal and mechanical properties. PIs are generally synthesized from poly(amic acid) (PAA) precursors by either stepwise thermal or chemical imidization.…”