Optical properties of UV-C irradiated polyvinylidene chloride films

Abstract

Polyvinylidene chloride (PVDC) polymer films were exposed to ultra-violet light (UV–C light of wavelength 254 nm) in air for different exposure times varying from 1 h up to 6 h. These films were characterized by using ultraviolet–visible (UV–Vis) and Fourier transform infrared (FTIR) spectroscopic, scanning electron microscopy (SEM), as well as powder X-ray diffraction (XRD) techniques. XRD scans reveal that the degree of crystallinity (Xc) increased from 4.7% up to 28.6% on increasing the UV exposure time from 1 h up to 4 h, and a further increase in the exposure time causes a decrease in its value. Analysis of the SEM images revealed an increase in particle size (chemically modified regions) on increased the UV exposure time of PVDC films from 1 h up to 6 h, and the maximum size of these regions were observed for an exposure time of 4 h. The SEM images showed a damaged PVDC surface, the formation of cotton-like fibrous regions, as well as many cracks due to the photo-degradation of polymeric chains. FTIR scans reveal the formation of hydrogen bonded hydroxyl groups as well as carbonyl structures in the PVDC sample on exposure to UV-C radiation. For the study of modification in band structure of the UV irradiated PVDC films, UV–Visible spectroscopy was used. It was found that the optical band gap decreases from 4.70 eV down to 3.92 eV, as obtained from the plot of √εihν versus hν. The penetration depth (δ) decreased from 0.0074 cm down to 0.0035 cm at 4.88eV, the energy corresponding to incident UV-C radiation. The calculation of percentage decrease in the value of δ with respect to wavelength and with variation in UV-C exposure time reveals that UV-C irradiated PVDC films are good radiation shields against both UV-A and UV-B electromagnetic radiation. The study of refractive index with respect to wavelength reveals that UV irradiation of PVDC films is an effective method to modify the refractive index of PVDC, thereby making the material more suitable for use in different optical applications. The optical dielectric constant was studied systematically, which indicated that the energy storage properties of the PVDC films can be tuned and tailored by UV-C radiation. Details are discussed.