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.

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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.

Results are presented for two unrelated studies. The primary topic involved the crystallization behavior and morphological features of two polyimides, namely LARCCPI (Langley Research .Center Crystalline folyimide) and LARC-TPI (Thermoplastic folyimide). The LARC-CPI study first considered feature~ affecting crystallization behavior including inherent viscosity, crystallization temperature, and melt temperature and time. Data were analyzed using the A vrami analysis. Morphological features were investigated using techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and small angle x-ray scattering (SAXS). Permanganic etching combined with SEM successfully revealed morphological detail, further supported by TEM data. These data support the Avrami analysis results. SAXS data reflected the influence of several crystalliz.ation variables on the long spacing. SAXS results also revealed the presence of a broad second scattering peak for semicrystalline samples appearing in the same position regardless of crystalliz.ation temperature or inherent viscosity. Molecular modelling predicted a low-energy helical conformation with a near-periodic repeat distance corresponding to the second SAXS peak. This conformation is offered as a tentative explanation for that peak. Finally, the effe.ct of nucleating agent on the recrystalliz.ability of a higher inherent viscosity LARC-CPI was considered. One of several nucleators appeared effective in enhancing recrystallization. The LARC-TPI study involved the development of a thermal cycle based upon crystalliz.ation kinetics results, allowing the successful recrystallization of LARC-TPI following exposure to elevated temperatures. This cycle was then used to modify an existing composite fabrication process to produce semicrystalline composites. Two carbon fiber types were utilized to make composites via both fabrication cycles. These composites were evaluated using several techniques. The second unrelated project examined very localized orientation behavior in two types of semi crystalline superstructure using FTIR microspectroscopy. Results were obtained utilizing linear dichroism for orientation within spherulitic and transcrystalline superstructure in poly(glycolide co-lactide) and isotactic polypropylene. An increase in orientation on an absolute basis as a function of position was successfully measured. Orientation function values were higher on an absolute basis for the transcrystalline superstructure. However, the sign of the isotactic polypropylene spherulite orientation function values was the opposite of what was anticipated. These results were considered in light of spherulite W AXS data also obtained. / Ph. D.

Earlier research relating to the use of polyimides modified with metal-ion complexes and metal particles indicate that enhanced conductivity and adhesive strength can be achieved. This research evaluated the employment of metal-modified polyimides to achieve optimum conductance at an aluminum joint. Condensation and addition polyimides were employed. The modification of the polyimides occurred in two ways. The first method involved homogeneous doping of the condensation polyimides with metal-ion complexes. The second modification method involved heterogeneous doping of condensation or addition polyimides with particles of a nickel-aluminum alloy. / Master of Science

LaRC-TPI, an aromatic thermoplastic polyimide, and Kapton®, a poly(pyromellitimide) were exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. Chemical changes which occurred in the surface as a result of the plasma treatments were investigated using x-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IR-RAS). Water contact angle analysis was utilized to characterize the changes in surface wettability, and the ablative effects of the plasmas were monitored using ellipsometry and high resolution scanning electron microscopy (HR-SEM). Both XPS and IR-RAS results revealed the formation of polar functional groups at the surface. Contact angle analysis showed enhanced water wettability of the plasma-treated surfaces. As monitored by ellipsometry, oxygen and argon plasmas were seen to be highly ablative, whereas an ammonia plasma was only moderately so. HR-SEM micrographs revealed texturized surfaces in the case of oxygen and argon plasmas, but not in the case of ammonia plasma. Oxygen and argon plasmas appear to react with the polyimides via a fragmentation/oxidation mechanism, forming a loosely attached layer composed of low molecular weight polymer chains. The effect of ammonia plasma is postulated to be imide ring-opening resulting in the formation of amide functional groups. The 180° peel test was utilized to determine the receptability of the plasma-treated polyimide surfaces toward bonding with other polymeric materials. Adhesives used were a pressure sensitive acrylate and poly(ether sulfone). The pressure sensitive adhesive, although not representing a realistic bonding situation, does represent a system which presents the least disturbance to the plasma-modified layer, allowing the physical nature of the plasma-treated surface to be probed. The peel test values of the pressure sensitive adhesive/plasma-treated polyimide systems fell below the level of the non-treated controls, regardless of the plasma treatment used. Peel surface analysis revealed the presence of polyimide on the pressure sensitive adhesive failure surface, indicating failure in the plane of a weak boundary layer created by plasma. The removal of the weak boundary layer by solvent treatment restored the peel values to the level of the controls. Bonding of Kapton® films with poly(ether sulfone) showed an opposite trend; peel strengths of the plasma-treated samples all showed improvements versus the non plasma-treated control. Plasma treatments of LaRC-TPI which had been deliberately contaminated with mold release and high density polyethylene illustrated showed that plasma treatments are not always detrimental to adhesion. It was shown that the physical as well as the chemical nature of a polymer surface is critical to the level of adhesion which can be achieved. / Master of Science

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.