The Impact of Internal Stress and Annealing on the Oxidation Stability of Polyurethane

It is widely known that internal stress is one of the driving forces to ignite cracks in biomedical polyurethane when implanted in the body for a long period. However, it is hard to quantitatively characterize internal stress and evaluate its effect on the polyurethane stability. In this study, small angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA) were employed to qualitatively and quantitatively assess the internal stress of polyurethane. The results show that, the higher the degree of orientation within the polyurethane, the higher is the internal stress. Subsequently, in vitro accelerated oxidation studies were conducted on specimens with various orientations and different annealing conditions. Fourier transform infrared (FTIR) spectroscopy was utilized to measure the oxidative degradation of ether bonds-(C-O-C). It is demonstrated that the ether bonds of the specimens with a well-ordered hard segment structure exhibit better oxidative stability. The lower the internal stresses, the less crack propagation appears. Annealing removes internal stresses, so that physical cracking is reduced during oxidative degradation. The crack density decreased by 89.3% and 88.4% after annealing at 70 °C and 80 °C, respectively. However, annealing also affects the microphase structure of polyurethane. Excessively high annealing temperature facilitates the mixing of polyurethane phase structure, while excessively low annealing temperature is not sufficient to induce the formation of ordered hard segmented phase regions. Therefore, both result in significant oxidative degradation of the ether bonds. This work establishes a foundation and insights for a deeper understanding of the failure of long-term implanted medical devices and for guiding product processing.