Intermolecular interaction and properties of cross-linked materials from poly(ester-urethane) and nitrochitosan

Two series of semi-interpenetrating polymer network (semi-IPN) films, coded as PUNT and PUN, have been prepared from amorphous nitrochitosan (NCH) and semicrystalline poly(ester-urethane) (PU) cross-linked to the hydroxy groups of tris(hydroxymethyl)propane (TMP) and NCO groups of PU prepolymer, respectively, by a casting method. The effects of different cross-linking methods and NCH content on the crystallization and hydrogen bonding strengths of the films were studied by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with the curve fitting method and wide angle X-ray diffraction (WAXD). The miscibility and mechanical properties of the films were measured by dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), density measurement, thermogravimetric analysis (TGA), tensile testing and solvent swelling testing. The results revealed that the degree of crystalline for the semi-IPN films was lower than that of PU, because of the addition of amorphous NCH to destroy the original ordered structure in the PU film. Moreover, the crystal domain formation for the PUNT films cross-linked with TMP can be more easily interrupted than to other self-crosslinked PUN films without TMP, resulting in a new disordered hydrogen bonded band in ATR-FTIR spectra. The PUNT films exhibited better miscibility as well as higher density, thermal stability, tensile strength and water resistance than the PUN films, suggesting an effective interpenetration and dense network structure. The differences between the two series of the films can be attributed to a relatively dense cross-linking network and strong intermolecular hydrogen bonding occurred between NCH and PU in the PUNT films as a result of the reaction cross-linked with TMP. It is noted that with an increase of NCH from 5 to 30% w/w, the thermal stability, tensile strength and water resistance of the PUNT films increased.