Optical Properties of Vanadium Oxide/Cellulose Triacetate Photochromic Films

Cellulose acetate is one of the most versatile polymers due to its non-toxic nature and low price. Its blend with inorganic nanoparticles enables the design of new multifunctional materials, in which a good dispersion of the nanoparticles is essential to improve the properties of the matrix. In this work, micrometric films of cellulose triacetate based nanocomposites modified with both sol-gel synthesised V₂O₅ nanoparticles and poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer (PEO-b-PPO-b-PEO or EPE) were prepared and characterised by colour measurements, UV-vis spectroscopy, tensile tests, atomic force microscopy (AFM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR).

In some cases, the use of different preparation methods involves the modification of the final properties of the nanocomposites. Thus, nanocomposites were prepared by solvent casting (SC) and solvent vapour annealing (SVA) methods. In the case of SC, nanocomposites were dried at ambient conditions, whereas SVA nanocomposites were dried in an oven at constant temperature. The use of different preparation methods affected the final macroscale properties of the nanocomposites, including the surface finish.

The obtained nanocomposites displayed green colour due to V₂O₅ nanoparticles, presenting the serie with EPE a darker hue. Using the SVA method, nanocomposites with smoother surfaces and without any visual defects were obtained due to a controlled slow solvent evaporation in an acetone vapour atmosphere. In both preparation methods, nanocomposites presented high flexibility as long as high transparency. Furthermore, nanocomposites displayed photochromic properties when exposed them to UV radiation, changing their colour from green to pale blue. The speed to change the colour and recover the initial state depend on the preparation method, V₂O₅ nanoparticles content and the presence of EPE triblock copolymer. In conclusion, the developed nanocomposites presented interesting properties and good surface finish to be employed in different applications fields.