Kathy C. Chuang

Thermal characterization of PMR polyimides

Polyimides are attractive for high temperature applications as the matrix resins, due to their excellent thermo-oxidative stability and mechanical properties. As composite materials for aerospace applications, they possess certain unique characteristics such as toughness, resistance to temperature and solvents as well as high tensile strength and modulus. A series of novel PMR polyimides based on substituted benzidines are examined and compared to the state-of-the-art PMR-15. The assigned glass transition temperatures and the mechanism for the thermal decomposition of four specific PMR polyimides are obtained, using thermal mechanical analysis and TGA/FTIR/MS techniques. Their thermal decomposition steps are proposed, identified and also compared to each other.

6F-Polyimides with Phenylethynyl Endcap for 315-370 °C Applications

6F-Polyimides with a phenylethynyl endcap (HFPE) were fabricated into carbon fiber composites using the standard polymerization of monomer reactant (PMR) approach via the ester/acid route. T650-35/HFPE carbon fiber composites were evaluated against the corresponding T650-35/PMR-II-50 composites with nadic endcap at 315-370 °C (600-700 °F) for their physical and mechanical properties as well as thermo-oxidative stability. In addition, concentrated HFPE monomer solutions were infused into stitched AS4 and T650-35 preforms and their mechanical properties were compared with those of the commercial BMI-5270 composites from cryogenic temperature to 343 °C (650 °F). The stitched HFPE composites out-performed BMI-5270 composites in terms of property retention at elevated temperature and microcrack resistance during thermal cycling from −54 to 288 °C (−65 to 550 °F). The stitched composites also showed more resistance toward blistering and delamination than conventional laminate composites during a rapid heating rate simulating launch and re-entry of reusable launch vehicles (RLV).

THERMAL DEGRADATION STUDY OF POLYMERIZATION OF MONOMERIC REACTANTS (PMR) POLYIMIDES

A novel PMR polyimides (TMBZ-15) based on substituted benzidines is examined and compared to state-of-the-art PMR-15. The mechanism for the thermal decomposition of two specific PMR polyimides is obtained using TG/FTIR/MS techniques. In order to verify the pathway of polyimide degradation, a pyrolysis/GC-MS technique was employed to evaluate the organic degradation products, particularly the larger components that are destroyed in traditional TG/MS. A proposed degradation mechanism involves two main stages of decomposition, each of which produce a variety of products. The first group includes aromatic hydrocarbons, aromatic amines and nitriles, which correspond to partial fragments of polymer chains. The second group consists largely of fluorene, naphthalene and phenanthrene, which are attributed to the isomerization, rearrangements and cyclizations of the aforementioned pyrolyzates at high temperature.

Blistering of Moisture Saturated Graphite/Polyimide Composites Due to Rapid Heating

Polyimide matrices extend the role of composite materials to applications in extreme temperature environments. However, composites can be susceptible to damage under extreme hygrothermal environments such as rapid heating of moisture saturated materials. Here, rapid is defined as reaching high temperature in less than the drying time at that temperature. A new method to predict initiation of steam-pressure induced damage for rapidly heated neat resin and graphite/polyimide composites is proposed. This method entails comparing the calculated, available steam pressure within the laminate to an experimentally determined critical pressure—temperature envelope. Through experiments performed in a thermal mechanical analyzer it is shown that the onset of steam-induced damage can be detected by measuring the expansion of moisture-saturated specimens subjected to a rapid temperature ramp. Optical microscopy of damaged samples shows that the process of initiation and evolution of damage in neat resin and laminates begins with void growth and coalescence in the polyimide resin matrix. Data from tests performed over a range of heating rates and initial moisture saturations are used to develop a critical pressure—temperature envelope. With this envelope we show the dependence of damage on initial moisture content and heating rate and propose an application of this envelope to failure prediction and design of laminated structures subjected to rapid heating.

Synthesis and characterization of polyimides with ether linkages

Thermoplastic and thermoset polyimides derived from 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) and 4,4′-bis(4-aminophenoxy)-2,2′-dimethylbiphenyl (BAPD) were prepared and characterized. Their physical and thermal properties as well as the polyelectrolyte effect exhibited by BTDA–BAPP polyamic acids in NMP solution were discussed.

Synthesis and Characterization of Hyperbranched Polyazomethine Ablators for Space Exploration Applications

A novel series of ablative composites containing a hyperbranched polyazomethine synthesized inside a carbon fiber preform (HyPAZA) were prepared, which have similar density to phenolic impregnated carbon ablators (∼0.3  g/cc). A novel method of synthesizing strong hyperbranched polyazomethine thermosets has been developed, enabling polyazomethines to be studied in ablators for the first time. Several formulations of HyPAZA perform better than the phenolic impregnated carbon ablator in terms of polymer char yield, composite mechanical strength, CO2 laser ablation tests at heat fluxes of 550 and 1100  W/cm2, and small-scale arcjet testing at a heat flux of 400  W/cm2. Char yields of hyperbranched polyazomethines were as high as 79% at 1000°C by thermogravimetric analysis. This is one of the highest char yields ever reported for a fully organic polymer. Some HyPAZA composites are over 10 times stronger than the carbon fiber preform, as determined by compression tests. Specimens were also tested in an arcjet ...