Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 186-197). Available also in a digital version from Dissertation Abstracts.

Th. doct.-ing.--Génie électrique--Toulouse--I.N.P., 1984. N°: 355.

No description available.

Virginia Polytechnic Institute and State University laboratories have produced a wide variety of polyimide films which have been modified by the incorporation of metal compounds. These polymer/metal composites have potential use as coatings in aerospace applications where enhanced electrical conductivity and thermal stability are desirable. Generally, these materials are produced by heating a polyamic acid solution which contains a soluble metal salt. While the electrical and thermal properties of some of these films have been studied in great detail, little is known about the factors which control the ultimate distribution of metal in the polymer matrix. In this work, the segregation of components in polyimide modified with the gold salt, HAuCl₄·H₂O, is described. Thermal treatment usually promoted three changes in the film: (1) conversion of an initially present polyamic acid to the thermally stable polyimide, (2) reduction of chloroauric acid to metallic gold and (3) redistribution of the metal into gold domains either in the bulk or at a surface of the film. The third event listed above has been termed metalization. Most of the gold modified polyimide I films exhibited bulk metalization as evidenced by the dispersion of very small gold particles through out the film. However, the gold aggregates which comprised the metalized surface layer possessed several different morphologies which indicated that a diffusion limited aggregation processes controlled the aggregate growth. The appearance of these aggregates was central to developing a model of phase separation in these metal modified films. / Ph. D.

Novel, polyand weight, randomly coupled, poly(imide siloxane) segmented copolymers were prepared and characterized. The copolymers were synthesized in two steps, the first involving the generation of soluble poly(amic acid) intermediates through reaction of various aromatic dianhydrides and aromatic diamines with a series of bis(aminopropyl)polydimethylsiloxane oligomers. These difunctional siloxane oligomers were prepared through the anionic equilibration of octamethylcyclotetrasiloxane with bis(3-aminopropyl)tetramethyldisiloxane in the presence of a siloxanolate catalyst. The aromatic monomers and siloxane oligomers were quantitatively reacted in a cosolvent system. The poly(amic acid) intermediates were then cyclodehydrated employing two different thermal treatments to afford the imidized homo- and copolymers. The first imidization process, conducted on solution cast poly(amic acid) films, employed stepwise heating cycles to 300°C. Quantitative thermal imidization was also achieved in solution at temperatures in the 140 to 170°C range. This novel method of imidization used a coamide solvent system. Kinetic studies employing FTIR indicated that the imidization process could be described by first order kinetics. An activation energy of 26 kcal/mole (109 kj/mole) was derived. The homopolymers and siloxane modified copolymers were characterized as a function of chemical composition and imidization method. Polymer solubility and processability greatly improved upon siloxane oligomer incorporation and the use of the solution imidization procedure. Regardless of the method of imidization, all homo- and copolymers possessed excellent thermal, mechanical, and adhesive properties. These properties were found to be a function of siloxane content and siloxane oligomer molecular weight. In all segmented copolymer systems, a two-phase microstructure developed at relatively low block molecular weights. X-ray photoelectron spectroscopy (XPS) results indicated that the surface of copolymer films was largely dominated by siloxane. Because of this, the siloxane modified copolymers advantageously displayed lower water uptake and much improved resistance to oxygen plasma degradation. XPS and SEM studies showed that the stabilization mechanism involved a siloxane to silicate transformation under an oxygen plasma environment. / Ph. D.

It is widely held in the field of adhesion science that the properties of the interfacial region, or interphase, between two bonded surfaces are of critical importance to the performance of an adhesive bond. This thesis describes a study in which a polyimide was modified by the addition of metal compounds in an effort to develop a graded interface between the adhesive and aluminum adherends. The results of mechanical adhesion testing and instrumental analysis of the failed surfaces indicated that the added compounds did in fact preferentially segregate toward the adhered surfaces, but that this segregation decreased the strength of bonds tested in peel. It was concluded that the collection of metal compounds at the metal surfaces did not occur in such a manner as to improve the integrity of adhesive bonds, but the possibility remains that an improved, graded interface might still be formed given a more appropriate adhesive/dopant system and improved specimen preparation and testing techniques. / Master of Science

An addition polyisoimide prepolymer was modified through the incorporation of metal particles. The response of this metal/polymer composite to mechanical vibrations and the passage of electric current was measured. Model aluminum conductor bar joints containing this material were assembled and exposed to elevated temperatures for extended periods of time while the electrical properties of the composites were monitored. In the most favorable systems, no thermal degradation of the electrical properties was observed. Dynamic mechanical behavior of the metal/polymer composites indicated good adhesion between particles and the matrix and also a broadening of the glass transition region as well as a post Tg dispersion in the temperature spectrum. The adhesive properties of these metal/polymer composites to aluminum were studied and found to be influenced by the loading level of the metal in the composite. Chemical reactions occurring during the cure of a neat resin sample of the polyisoimide prepolymer were monitored using infrared spectrometry and differential scanning calorimetry. Both the crosslinking and isomerization reactions were found to be apparently first order with the isomerization having a lower activation energy than the crosslinking. Linear, high molecular weight, thermoplastic polyimides and poly(imide-siloxane) homo- and copolymers prepared by bulk and solution thermal imidization were investigated as structural adhesives for titanium. The solution thermal imidization procedure was found to result in favorable adhesive characteristics while the presence of siloxane segments in the polymer backbone improved the resistance of stressed specimens to moisture. Aluminum-sec-butoxide used as a primer was also found to improve the moisture durability of bonds prepared with these materials. / 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.

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.

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.