Crosslinked poly(hydroxyurethane) films from biobased carbonates: Structure-properties relationships and the influence of moisture in the mechanical properties

In this work, crosslinked poly(hydroxyurethane) (PHU) films were prepared by the reaction of biobased cyclic carbonates (BisCC), with different chain lengths, with hyperbranched poly(ethyleneimine) (PEI) under amenable conditions and in the absence of catalysts. Fourier transform infrared (FTIR) spectroscopy revealed that the carbonate groups were completely consumed in the crosslinking reaction and the gel content of the films was about 99 %. It was shown that the thermal and mechanical properties of the PHU films depended on the length of the carbon chain of the BisCC. In the presence of moist environment, the PHU films absorbed moisture (6–8 %) which is ascribed to the presence of hydroxyl groups. This resulted in a deterioration of the mechanical properties, which were not completely recovered after drying. To counteract this effect, a simple procedure was used in which the hydroxyl groups on the surface were modified by stearate groups. Although FTIR and water contact angle measurements demonstrated that the films of PHU became more hydrophobic after modification, the water absorption capacity was similar to that of the unmodified PHU films. This suggests that the modification process may cause some degradation in the bulk of the material that opens spaces for water molecules penetration. In summary, this work has shed light for the first time on the effects of moisture on the mechanical properties of pure PHU films and has shown that surface modifications are not sufficient to prevent moisture absorption.