In this study, poly(vinyl chloride) (PVC) is considered one of the most versatile plastics. It is the second largest manufactured resin by volume worldwide. Results: To study the optical properties of modified PVC containing 1,3,4-thiadiazole and phthalyl groups in the repeating units, thin films of the prepared polymers (P I, P II, P III, P IV, P V, and PVC) were casted from tetrahydrofuran solvent with a thickness of 30 μm. The optical properties were studied in the range of 200 to 600 nm,

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... the optical data were analyzed and interpreted in terms of the theory of phonon-assisted direct electronic transitions; the measured E g of pure and modified PVC increases in the following order: P II; PIV; PV; PI; PIII; and PV → C: Also, the refractive index was measured. Conclusions: The results showed that for all compositions, the refractive index decreases with increasing wavelength.