| Polymeric composites are complex systems that are inherently heterogeneous, anisotropic, and viscoelastic. As they are considered for high-temperature applications, it is essential to evaluate their long term performance. This work addressed the thermo-oxidative stability of polymeric composites through small and large scale weight-loss experiments with special emphasis on developing an integrated approach to scale up and explain composite weight-loss behavior based on the fundamental characteristics of composite materials. Weight loss and composite performance are affected by phenomena that cannot be quantified from weight-loss experiments. These changes were investigated through dynamic mechanical measurements and a viscoelastic model that was developed for a reacting polymer system. High performance bismaleimide, polyimide, and epoxy composites and unreinforced resins were used as model systems. Thermogravimetric analysis was used to determine activation energies for degradation, and to develop a time-temperature superposition based concept (Equivalent Property Time, EPT) to facilitate extrapolation of weight-loss data outside the experimental time and temperature scale. An anisotropic degradation methodology, based on the shrinking core model, was used to scale data between different sample sizes, lay-ups, and geometries. Weight loss was expressed as an anisotropic surface flux, and described in terms of an effective diffusion coefficient and a time exponent. The effective diffusion coefficients were modified to account for heterogeneity and anisotropy in composite degradation. Specifically, the effective diffusion coefficients parallel and perpendicular to the fibers were expressed in terms of resin and interphase contributions, providing a connection between the unreinforced resin and composite degradation. Integrated with the EPT concept, this methodology provided an effective scale-up tool for investigating composite stability over different time and experimental scales. Finally, changes in polymer structure such as crosslinking affect performance and may also influence weight loss by affecting diffusion and degradation reactions. These changes were addressed through a non-isothermal viscoelastic model that connected conversion, glass transition temperature, and dynamic mechanical data. |
