Synergistic effects of oxidative environments and mechanical stress on in vitro stability of polyetherurethanes and polycarbonateurethanes.

The in vitro structural stability of polyetherurethanes (PEUs) and polycarbonateurethanes (PCUs and PCUUs) was examined under strong oxidative conditions (0.5N HNO3, pH 0.3; and NaClO, 4% Cl2 available, pH approximately 13) and in the presence of a constant strain state. Solvent-cast dog-bone shaped specimens were strained at 100% uniaxial elongation over extension devices and completely immersed in the oxidative solutions at 50 degrees C for 15 days. Unstrained polyurethane (PU) samples were treated in the same way for comparison. The modification of the PU molecular structure was determined by DSC, GPC, ATR-FTIR, static contact angle, and surface roughness analyses. The incubation in nitric acid and sodium hypochlorite brought about a greater degradation of samples tested under the applied strain with the exception of PEU treated with nitric acid. PEU was the most affected material, showing bulk deterioration in NaClO and significant modifications in nitric acid, with the appearance of new IR bands, which were assigned to oxidation products. A higher phase separation between soft and hard domains occurred in PCUs upon incubation in nitric acid, the treatment with NaClO gave rise to new bands in the IR spectra, denoting the presence of oxidation products at the surface. The surface roughness greatly increased in strained PCUs with SEM evidence of deep cracks and holes or ragged and stretched fractures perpendicular to the direction of stress. PCUU underwent complex chemical modifications with a marked decrease of N-H and urea IR absorptions and showed a lower degradation than PEU and PCUs under mechanical constraint. From these results, sodium hypochlorite appears to be able to create an ESC-like degradation for PUs that are resistant to other aggressive chemical environments.