This dissertation describes kinetic and mechanistic studies of high performance polyimide formation, synthesis and characterization of fully cyclized, molecular weight and end group controlled polyimides, and investigations of high performance polymer blends based upon polyimides and polybenzimidazole. Imidization kinetics were successfully followed by the quantitative non-aqueous titration of the amic acid functional groups as a function of reaction conditions. The homogeneous solution imidization processes were described by auto-acid catalyzed second order kinetics. The effects of heteroatom bridging groups in the diamines and dianhydrides on reaction rates have been investigated and a possible reaction mechanism for the solution imidization processes has been proposed. Detailed mechanistic investigations of the thermal solution imidization of polyamic acids were performed. A small amount of hydrolysis and possibly some unimolecular decomposition of amide bonds in the polyamic acid during thermal solution imidization processes were observed via combination of NMR and intrinsic viscosity measurements. However, complete "recombination" of the degraded polymer chains and their further cycloimidization could be achieved under proper imidization conditions. Potential side reactions involving intermolecular imide formation reaction were also investigated using a well characterized polyimide and also a model imide. For polyimide systems containing benzophenone tetracarboxylic acid dianhydride (BTDA), direct evidence for network formation involving imine crosslinking, was observed by high field lH-NMR spectroscopy. The gel formation was a strong function of reaction conditions, occurring under extremely dry reaction conditions and being favored at moderate reaction temperatures. Various polyimide homo- and copolymers with controlled molecular weight and end groups were synthesized by the classic two step method and their thermal properties and solution viscosities were evaluated. Further, miscibility behavior of high performance polymer blends based upon polyimide (PI) and polybenzimidazole (PBI) was investigated. Several miscible PI/PBI blend material systems were identified, some of which showed a lower critical solution temperature (LCST), which was consistent with earlier observations. It was found that miscibility was a strong function of polarity and possible specific interactions with the polyimide components. Thus, miscibility was possible over a wide composition range with polyimides containing polar groups such as ketones, sulfones and ethers. However, immiscible blends were obtained when these polar polyimide components were replaced by non-polar groups such as the hexafluoroisopropylidene linkages. / Ph. D.

Polyimides are generally formed by combining a dianhydride and a diamine monomer in a polar aprotic solvent to form a poly(amic acid). The poly(amic acid) is then cyclodehydrated by either thermal or solution imidization at high temperatures to give the fully cyclized polyimides. This research focuses on the development of a low temperature solution polymerization route utilizing a transimidization method to make fully cyclized polyimide homopolymers and polyimide siloxane copolymers. Polyimide oligomers endcapped with 2-aminopyrimidine were reacted with aminopropyl terminated poly(dimethyl siloxane) to give perfectly alternating segmented polyimide siloxane copolymers. The polymerization was conducted under very mild conditions. At reaction temperatures of only 100 - 110°C in chlorobenzene, high molecular weight, fully imidized polyimide siloxane copolymers were obtained. The polyimide siloxane copolymers were cast into tough transparent films. Properties of the polyimide siloxane copolymers were found to be dependent on the molecular weight of the starting polyimide and poly(dimethyl siloxane) oligomers. The transimidization method was also applied to the synthesis of soluble, fully cyclized polyimide homopolymers at reaction temperatures as low as 60°C. Utilizing the transimidization route, fully cyclized polyimides were made at lower temperatures than can be made by conventional polyimide synthetic methods. The polymerization of the bis(N-pyrimidine phthalimide) derivative of 6F dianhydride and 4, 4' oxydianiline at ~65 °C in N-methyl pyrrolidone with acetic acid as a catalyst resulted in a high molecular weight, cyclized polyimide. Fully cyclized, processable, fluorine containing polyimides were developed with very high glass transition temperatures and good thermal stability. The 1, 1-bis(4- aminophenyl)-1-phenyl-2, 2, 2-trifluoroethane (3F diamine) based polyimides were soluble and amorphous, probably as a result of the non-coplanar structure. DSC and dynamic mechanical analysis showed a glass transition temperature exceeding 420°C for the PMDA-3F diamine based polyimide. These polyimides showed good thermooxidative stability at 600°F in air after 500 hours. / Ph. D.

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. / Ph. D.

Polyimides derived from 2,2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane (BDAF) and pyromellitic dianhydride (PMDA) displayed a glass transition temperature (Tg) at 306°C and a melting point (Tm) near 470°C as measured by differential scanning calorimetry (DSC) with a heating rate of 10°C/min. The degree of crystallinity increased with decreasing molecular weight. The incorporation of 20 mole percent of the comonomer, hexafluoroisopropylidene-2,2-bisphthalic acid anhydride (6FDA), gave a Tm of 440°C that appeared to be stable 20°C above the melt. A fluorinated diamine, 1,1-bis[4-(4-aminophenoxy) phenyl]-1-phenyl-2,2,2-trifluoroethane (SFEDAM), was synthesized in 80% yield from trifluoroacetophenone with a melting point of 169-170°C. The 3FEDAM-PMDA polyimide controlled to 30,000 g/mol with phthalic anhydride exhibited by DSC a Tg of 308°C and a Tm of 476°C. Similar to the BDAF based copolymer, the 3FEDAM-20%6FDA/80%PMDA-PA polyimide had a Tg of 286°C and a Tm of 440°C which appeared to show short term thermal stability above the Tm. Three 1,4-bis(4-aminophenoxy)benzene (TPEQ) homopolyimides were synthesized with 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), 3,3',4,4'-biphenylcarboxylic dianhydride (BPDA), and 4,4'-oxydiphthalic anhydride (ODPA). The Tg values were 255°C, 251°C, and 239°C for the TPEQ-BPDA-PA, TPEQ-BTDA-PA, and TPEQ-ODPA-PA polyimides, respectively. The TPEQ-ODPA-PA polyimide displayed the lowest Tm of 411°C. A series of TPEQ-ODPA polyimides of different molecular weights were synthesized using the Carother's equation and the monofunctional reagent, phthalic anhydride. The Tg values ranged from 214°C to 239°C for the 7.5K to the 30K polyimides. All the TPEQ-ODPA polyimides displayed an endotherm on the first heat of the DSC analysis. However, the 7.5K, 10K, and 15K polyimide also recrystallized upon heating. Polyimide powders were directly synthesized via preparation of the poly(amic acid) followed by cyclodehydration by solution imidization. One requirement is the polyimide must be crystalline so that as the polyimide is formed it will crystallize and precipitate from solution as small particles in the range of 2-16 pm. The particle size may be controlled by the concentration of the solution, temperature, and drying process. / Ph. D.