Synthesis and characterization of soluble, high temperature aromatic polyimides

Abstract

High molecular weight, soluble polyimides were synthesized by a non-traditional synthetic route utilizing solution imidization techniques and diester-diacid derivatives of various commercially available dianhydrides. "One pot" syntheses were conducted using a solvent system of N-methylpyrrolidinone and σ-dichlorobenzene at temperatures of 170°C to 180°C and times of 24 hours or less. The resulting polyimides were soluble in amide solvents at concentrations of 15 to 20 percent (w/v) at 25°C, were fully cyclodehydrated as determined by non-aqueous potentiometric titrations, possessed molecular weight distributions very close to the theoretical value of 2.0 and displayed glass transition temperatures consistent with accepted values for the same materials synthesized via conventional methods. Model studies indicated that polymerization proceeds via intermediate conversion of the esteracid functional groups to anhydride groups.

This method was also successfully employed in the synthesis of controlled molecular weight ethynyl-functionalized thermosetting imides. High T_g's, low end group concentrations and the relatively low cure temperature of the ethynyl end group restricted sample fabrication to thin, solution-cast films; nevertheless, several of these systems were evaluated for high temperature stability and were identified as potential candidates for 700°F (371°C) applications.

In addition, a novel polyimide synthesis utilizing diamine dihydrochlorides as substitutes for unstable diamines was also investigated, and a series of novel polyimides based on diaminoresorcinol and commercial dianhydrides was synthesized. Diaminoresorcinol dihydrochloride and dianhydride were heated in an NMP/dichlorobenzene mixture; at sufficiently high temperatures the insoluble dihydrochloride dissociates, liberating hydrogen chloride gas and the soluble free diamine, which rapidly dissolves and reacts with dianhydride before decomposition occurs. The poly(hydroxy-imide)s possess T_g's in excess of 250°C, are soluble in amide solvents and, as might be expected, are extremely hygroscopic.