Tailoring Optical Performance of Polyvinyl Alcohol/Crystal Violet Band-Pass Filters via Solvent Features

Optical filters are essential components for a variety of applicative fields, such as communications, chemical analysis and optical signal processing. This article describes the preparation and characterization of a new optical filter made of polyvinyl alcohol and incremental amounts of crystal violet. By using distinct solvents (H₂O, dimethyl sulfoxide (DMSO) and H₂O₂) to obtain the dyed polymer films, new insights were gained into the pathway that underlies the possibility of tailoring the material's optical performance. The effect of the dye content on the sample's main properties was inspected via UV-VIS spectroscopy analysis combined with colorimetry, refractometry and atomic force microscopy experiments. The results revealed that the colorimetric parameters are affected by the dye amount and are dramatically changed when the solvent used for film preparation is different. The rise in the refractive index upon polymer dyeing was due to the synergistic effect of the larger polarizability of the dye and the occurrence of hydrogen bonds among the system components. Spectral data evidenced that samples prepared in H₂O and DMSO preserve the absorption characteristics of the added dye, whereas H₂O₂ acts as an oxidizing agent and enhances transparency. Also, for the first two solvents, multiple absorption edges were noted as a result of dye incorporation, which was responsible for the occurrence of new exciton-like states, hence the band gap reduction. The films processed in H₂O were able to block radiations in the 506-633 nm range while allowing other wavelengths to pass with a transmittance above 90%. The samples attained in DMSO presented similar properties, with the difference that the domain of light attenuation was shifted towards higher wavelengths. Atomic force microscopy showed the dye's effect on the level of surface roughness uniformity and morphology isotropy. The dyed polymer foils in non-oxidizing agents have suitable features for use as band-pass filters.