The influence of the length of the degradable segment on the functional properties and hydrolytic stability of multi-component polyurethane elastomeric films

The hydrolytic degradation of aliphatic polyurethane (PU) films made from polycarbonate-based macrodiol (MD), diisocyanate-1,6-hexane, butane-1,4-diol (BD) and d,l-lactide-based oligomeric diol (DLL) was studied. The influence of the length of DLL was tested in phosphate-buffered saline (PBS) for periods of up to 12 months. One macrodiol (molecular weight ∼2000 Da), three DLL oligomers (∼400, 660 and 850 Da) and three MD-to-BD-to-DLL molar ratios were chosen for the PU synthesis. The isocyanate-to-total hydroxyl-group ratio was kept constant at 1.05. The functional properties of raw polyurethane films and samples immersed for 1, 3, 6, 9 and 12 months in a model physiological environment (37 °C, pH = 7.4) were studied from the segmental to the macroscopic level. Tensile testing and water uptake experiments, as well as differential scanning calorimetry (DSC), scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction (XRD) analyses, were used for the characterization of the raw and PBS-treated films. The study shows that the length of the DLL chain is much more important for functional PU properties than the mass content of DLL in the PU film. The incorporation of the shortest DLL into the PU backbone leads to a degradable PU material with outstanding tensile properties when not subjected to the hydrolytic treatment. However, the incorporation of oligomers with longer DLL chains results in PU materials with substantially deteriorated tensile characteristics due to more pronounced phase separation compared to systems without DLL or with the shortest DLL. The degradability of the PU films can be controlled to a relatively broad extent by altering DLL content and length. The investigation of functional properties of new PU materials during the hydrolytic process under physiology-mimicking conditions is important for potential medical/package coating/film applications.