Elucidating the electronic structure and optical properties of gamma-irradiated poly(methyl methacrylate): Experimental study and DFT approach

Abstract

In this work, the resulting effects on the lifetime properties of poly (methyl methacrylate) after exposure to different doses of gamma radiation were studied. Multiple techniques were used to analyze and characterize the resulting changes in properties, such Fourier transform infrared spectroscopy, surface roughness spectroscopy, and optical spectroscopy. There is a significant structural improvement in the material chains as a result of gamma ray stimulation, and the surface roughness of the samples increased after gamma ray exposure. Analysis of the optical properties measurements after gamma ray exposure revealed a shift in the absorption edge with increasing gamma ray doses, shifting toward longer wavelengths. A reduction in the band gap energy and a rise in the refractive index of the irradiated material were also observed with improved optical parameters. The band gap energy of the sample exposed to the highest dose reached 3.76 eV and 2.63 eV for the direct and indirect transitions, respectively. Also, with increasing gamma doses, both the tail bandwidth energy of the new states and the refractive index of the material increase from 0.565 eV to approximately 0.877 eV and from 1.3935 to 1.8555, respectively, at exposure to a dose of 185 kGy. The density functional theory (DFT) using the B3LYP method with a 6-311G (d,p) basis set was applied to explore the relationship between spectral and structural characteristics of PMMA molecules. This theoretical approach examined alterations in molecular structure and electronic transitions. Our vibrational analysis conducted in the ground state using DFT showed close alignment between experimental infrared spectra and calculated vibrational wave numbers. The molecular energy gap was defined through frontier molecular orbital energies (HOMO-LUMO), with observations indicating intermolecular charge transfer and chemical reactivity within the molecule. The molecular electrostatic potential (MEP) surface highlighted hydrogen bonding and the molecule's reactive nature, while Mulliken atomic charge analysis revealed electron density shifts within the structure.