Optical Properties of Vanadium Oxide/Cellulose Triacetate Photochromic Films

The properties of polymer-based nanocomposites strongly depend on the fillers added and their dispersion on the matrix. Proposing a simple method that can control these variables is essential to obtain nanocomposites with enhanced properties. In this study, cellulose triacetate based nanocomposites modified with sol-gel synthesised vanadium oxide nanoparticles (V2O5) and poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO-b-PEO or EPE) triblock copolymer were obtained by two methods: solvent casting (SC, drying at ambient conditions) and solvent vapour annealing (SVA, drying under solvent vapour atmosphere). Nanocomposites were characterised by Fourier-transform infrared spectroscopy (FTIR) and UV-vis spectroscopy. Nanocomposites presented green colour and high transparency, improving the SVA method the surface finish of the films. Moreover, V2O5 nanoparticles provided switchable photochromic properties, changing the film colour from green to pale blue when exposed to UV radiation. Nanocomposites with EPE triblock copolymer presented a more noticeable colour change. As for the speed of the recovery process to the initial state, it increased with the addition of EPE and the sol-gel content. Thus, it was proved that the SVA preparation method was more appropriate that the SC, as well as corroborate that the EPE triblock copolymer and the sol-gel content affected the properties of developed CTA nanocomposites.


Introduction
Due to its renewable, sustainable and biodegradable nature [1], cellulose acetate (CA) is one of the most employed polymers even over polycaprolactone (PCL) and polylactide (PLA) [2]. Derived from cotton and wood [3], it presents odourless and non-toxic properties [4], as well as low cost and excellent biocompatibility [5], which make it ideal to cover a wide range of applications from textile, medical and pharmaceutical products to engineering materials, filtration membranes or cigarette filters [6,7], among others.
In order to achieve unique properties of polymeric matrix-based nanocomposites, different fillers such as nanoparticles are usually added [8][9][10]. Moreover, nanostructuration agents such as block copolymers are an interesting option to obtain polymeric blends, since their addition is strongly related with their ability to act as nanostructuration agents leading to nanostructured polymers [11].
The choice of the preparation conditions and method employed to obtain the nanocomposites is also crucial, since different factors such as the nanoparticles dispersion, which are influenced by the preparation conditions, determine the ultimate properties of the nanocomposites. In this way, the solvent evaporation rate and the type of solvent employed in solvent casting can affect the optical properties of cellulose triacetate (CTA) films due to the differences in the movement and molecular orientation of the polymeric chains in the solvent removal [12,13]. In the case of sorption films, the adsorbing capacity to remove dyes of chitosan-based nanocomposites can be enhanced dispersing by sonication previously synthesised Fe3O4 magnetic nanoparticles into the film forming solution [14] and applying ultrasound to TiO2 and ZnO nanoparticles [15]. Moreover, the solution method is more effective than melting blending method in the preparation of montmorillonite/thermoplastic starch films, since the mechanical properties improve due to the differences in the water absorption [16].
In this study, cellulose triacetate (CTA) based nanocomposites modified with sol-gel synthesised vanadium oxide nanoparticles (V2O5) and poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO-b-PEO or EPE) triblock copolymer were obtained by solvent casting (SC) and solvent vapour annealing (SVA) preparation methods. On the one hand, the V2O5 nanoparticles provided photochromic properties to the obtained nanocomposites. On the other hand, the SVA preparation method enhanced the surface finish of the nanocomposite films. The obtained nanocomposites were analysed by Fouriertransform infrared spectroscopy (FTIR) and UV-vis spectroscopy. Photochromic properties were also studied.

Preparation of Nanocomposites
Cellulose triacetate-based nanocomposites modified with V2O5 nanoparticles, without and with EPE triblock copolymer and denominated V2O5/CTA and V2O5-EPE/CTA, respectively, were obtained. Solutions of CTA (1 g) in acetone (25 mL) were prepared, adding EPE (0.1 g) in the case of V2O5-EPE/CTA nanocomposites, and stirred for 24 h. Additionally, isopropanol, VOIT and acetic acid were mixed (80:1:1 volume ratio) and stirred for 2 h. After that, sol-gel solution was added in the suitable proportion to CTA and EPE/CTA, obtaining solutions with 1, 3, 5, and 10 vol% of sol-gel (with respect to the initial 25 mL solution) and stirred for 1 h. Finally, nanocomposites were solvent casted employing two different drying conditions. On the one hand, nanocomposites were dried uncontrolled at ambient conditions (SC). On the other hand, films were solvent vapour annealed keeping them under acetone vapour atmosphere in an oven at 25 °C for 1 week (SVA).

Fourier-Transform Infrared Spectroscopy (FTIR)
Fourier transform infrared (FTIR) spectra were performed using a Nicolet Nexus FTIR spectrometer equipped with a Golden Gate (Specac) ATR sampling accessory. The background was recorded before every sample and the spectra were obtained in the range of 4000-700 cm −1 with 32 scans.

UV-Vis Spectroscopy
UV-3600 Shimadzu UV-VIS-NIR spectrophotometer was employed to obtain the transmittance spectra of the nanocomposites within the range of 200−800 nm. Transmittance values at 650 nm were taken as an indicative of nanocomposites transparency. As for the switchable photochromic properties, nanocomposites were firstly irradiated with 254 nm UV light during 5 min using UV lamp (XX-15S UV Bench Lamp, 15 W). The distance between the sample and the source was set in 12 cm. The irradiated samples were then monitored with the spectrophotometer to estimate the time needed to reach the initial state, denominated recovery time.

Appearance of Investigated Films
Both SC and SVA nanocomposites presented high flexibility and green colour due to the presence of vanadium oxide nanoparticles [17]. However, nanocomposites prepared by SVA method showed improved surface finish than those prepared by SC, without visible defects and smoother surface. This is due to the SVA method, which allowed a controlled solvent evaporation that affected the surface of the nanocomposites. Moreover, the addition of EPE triblock copolymer darkened the nanocomposites. These differences are reflected in Figure 1, where 5V2O5/CTA and 5V2O5-EPE/CTA nanocomposite films are shown as example. Consequently, SVA was presented as an alternative method to obtain micrometrethick nanocomposite films, being its implementation uncomplicated and providing great improvements in the surface finish. Thus, the employed SVA method would be suitable to scale-up the preparation of films from lab to industry in a simple way. Therefore, it was considered that the SVA method is more appropriate than the SC and in consequence a more comprehensive analysis of SVA nanocomposites was conducted.    CTA and EPE/CTA presented similar UV-vis curves, both with high transparency. Similarly, V2O5/CTA and V2O5-EPE/CTA nanocomposites displayed also high transparency. However, both series of nanocomposites lost transparency as long as sol-gel content increased. Moreover, V2O5-EPE/CTA nanocomposites displayed lower transmittance values than V2O5/CTA if compared nanocomposites with the same sol-gel content due to the incorporation of EPE triblock copolymer.

UV-Vis Spectra
As for the V2O5/CTA serie, 3V2O5/CTA, 5V2O5/CTA and 10V2O5/CTA nanocomposites presented transmittance values nearby to 0% until 400, 450 and 500 nm, respectively. This can indicate that these nanocomposites absorb UV radiation, which would protect them from the deterioration caused by it. Nevertheless, only 3V2O5/CTA and 5V2O5/CTA displayed this property with high transparency. On the other hand, in the case of nanocomposites with EPE triblock copolymer, 5V2O5-EPE/CTA and 10V2O5-EPE/CTA presented these UV-shielding properties, although they also showed a decrease in the transparency.
Additionally, photochromic properties of nanocomposites were studied, irradiating them with 254 nm UV light during 5 min. Figure 4 shows the colour change of SVA nanocomposites.
Nanocomposites changed their colour after UV irradiation from light green to pale blue in 1V2O5-EPE/CTA nanocomposite and from light green to darker green in the remaining ones. This blue colour indicated that vanadium presented an oxidation state of +4 [18], as a consequence of a reduction process from +5 to +4 [17,19]. As observed in Figure 4, this colour change was more noticeable in the case of V2O5-EPE/CTA nanocomposites, being almost indiscernible for V2O5/CTA nanocomposites. As reported by other researchers [20][21][22], this can be due to the plasticising effect of EPE triblock copolymer. The local environment around the photochromic molecule, which is influenced by free volume, fluidity and polarity, among others, can affect the photochromic response. In this case, the addition of EPE triblock copolymer produced a soft environment, encouraging the molecular mobility, which seemed to help the colouring process of the nanocomposites. After a few hours, nanocomposites presented an oxidation process due to the atmospheric oxygen, recovering the initial state and colour as vanadium oxidises from +4 to +5. This process is also influenced by the presence of EPE triblock copolymer and the sol-gel content. In order to perform a thorough analysis, the UV-vis spectra at different times of 1V2O5/CTA, 1V2O5-EPE/CTA and 3V2O5/CTA prepared by SVA were investigated ( Figure 5).
1V2O5/CTA nanocomposite displayed an 88% transmittance value at 650 nm. The irradiation process decreased it to 86%, needing 120 h (5 days) to reach the initial transmittance value. As for 1V2O5-EPE/CTA, it presented a transmittance value of ~78% at 650 nm, decreasing to 75% after the irradiation (0 h). The recovery process was carried out at a higher switching speed than in the case of 1V2O5/CTA nanocomposite, reaching a value of 77%, i.e., a recovery percentage of 83%, at 28 h. It can be said that the initial state was reached after approximately 2 days.
Before the irradiation, 3V2O5/CTA nanocomposite showed a transmittance value of 66% at 650 nm. It decreased to 64% after the irradiation, reaching the initial state after 48 h, a lower recovery time if compared with the 120 h of 1V2O5/CTA nanocomposite. Therefore, it can be said that the switching speed of the recovery process increased as long as sol-gel content increased.
With all the above mentioned, it can be concluded that the addition of EPE triblock copolymer provoked a more effective colouring of the nanocomposites and a higher switching speed of the recovery process. Moreover, it seems that the switching speed increased with the sol-gel content.

Conclusions
CTA-based nanocomposites were modified with EPE triblock copolymer and sol-gel synthesised V2O5 nanoparticles by SC and SVA preparation methods. Both SC and SVA nanocomposites displayed green colour and high flexibility. However, SVA nanocomposites presented smoother surfaces if compared with SC nanocomposites. Moreover, SVA nanocomposites presented high transparency, which decreased with the addition of EPE and the increase of sol-gel content. Additionally, CTA nanocomposites showed switchable photochromic properties when exposed to UV radiation, changing their colour from green to pale blue. This colour change was more noticeable in the nanocomposites with EPE triblock copolymer. In addition, the speed of the recovery process increased with the sol-gel content and the addition of the EPE triblock copolymer. For all the above mentioned, it is demonstrated that the preparation method affects the properties of the developed nanocomposites, with the SVA method being more suitable than the SC one.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not yet publicly available due to the performance of on-going studies of the scientific findings that have been reported.

Abbreviations
The following abbreviations are used in this manuscript: CTA cellulose triacetate EPE poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) SC solvent casting SVA solvent vapour annealing VOIT vanadium (V) oxytriisopropoxide