Optical and Structural Properties of Composites Based on Poly(urethane) and TiO₂ Nanowires

This article's objective is the synthesis of new composites based on thermoplastic polyurethane (TPU) and TiO₂ nanowires (NWs) as free-standing films, highlighting their structural and optical properties. The free-standing TPU-TiO₂ NW films were prepared by a wet chemical method accompanied by a thermal treatment at 100 °C for 1 h, followed by air-drying for 2 h. X-ray diffraction (XRD) studies indicated that the starting commercial TiO₂ NW sample contains TiO₂ tetragonal anatase (A), cubic Ti_0.91O (C), and orthorhombic Ti₂O₃ (OR), as well as monoclinic H₂Ti₃O₇ (M). In the presence of TPU, an increase in the ratio between the intensities of the diffraction peaks at 43.4° and 48° belonging to the C and A phases of titanium dioxide, respectively, is reported. The increase in the intensity of the peak at 43.4° is explained to be a consequence of the interaction of TiO₂ NWs with PTU, which occurs when the formation of suboxides takes place. The variation in the ratio of the absorbance of the IR bands peaked at 765-771 cm^-1 and 3304-3315 cm^-1 from 4.68 to 4.21 and 3.83 for TPU and the TPU-TiO₂ NW composites, respectively, with TiO₂ NW concentration equal to 2 wt.% and 17 wt.%, indicated a decrease in the higher-order aggregates of TPU with a simultaneous increase in the hydrogen bonds established between the amide groups of TPU and the oxygen atoms of TiO₂ NWs. The decrease in the ratio of the intensity of the Raman lines peaked at 658 cm^-1 and 635 cm^-1, which were assigned to the vibrational modes E_g in TiO₂ A and E_g in H₂Ti₃O₇ (I_TiO2-A/I_H2Ti3O7), respectively, from 3.45 in TiO₂ NWs to 0.94-0.96 in the TPU-TiO₂ NW composites, which indicates that the adsorption of TPU onto TiO₂ NWs involves an exchange reaction of TPU in the presence of TiO₂ NWs, followed by the formation of new hydrogen bonds between the -NH- of the amide group and the oxygen atoms of Ti_xO_2x-mn, Ti₂O₃, and Ti_0.91O. Photoluminescence (PL) studies highlighted a gradual decrease in the intensity of the TPU emission band, which is situated in the spectral range 380-650 nm, in the presence of TiO₂ NW. After increasing the TiO₂ NW concentration in the TPU-TiO₂ NW composite mass from 0 wt.% to 2 wt.% and 17 wt.%, respectively, a change in the binding angle of the TPU onto the TiO₂ NW surface from 12.6° to 32° and 45.9°, respectively, took place.