Application of density functional theory to the investigation of polymer degradation: Example of cross-linked ethylene-vinyl acetate-vinyl alcohol (EVA-OH) terpolymer de-acetylation

We report on the application of Density Functional Theory (DFT) methodologies to investigate the degradation of polymer materials and provide new insights into the effects of degradation on molecular geometry. The temperature- and radiation-assisted degradation of a cross-linked ethylene-vinyl acetate-vinyl alcohol (EVA-OH) elastomer was studied both experimentally and theoretically, in order to correlate observable parameters with theoretically calculated electronic properties. Experiments showed 'yellowing' of the material, outgassing of acetic acid, and attenuated IR deformation modes upon exposure to increased levels of gamma radiation or increased temperatures, consistent with the de-acetylation model in which acetic acid is abstracted from the vinyl acetate group via a molecular rearrangement involving the displacement of a hydrogen atom from the ethylene backbone toward the acetyl group, and the propagation of the mechanism to adjacent vinyl acetate groups, forming polyenes in the polymer backbone. DFT modeling predicted the molecular structures of the cross-linked EVA-OH polymer for various degrees of de-acetylation, with corresponding IR and UV-visible absorption spectra. Theoretical attenuated IR deformation modes matched experimental observations, and the theoretical absorption spectrum of polyenes with 5 double bonds matched the optical absorption data, shedding light onto the final chemical structure of the polymer fragment. In addition, DFT unveiled precise and local effects of de-acetylation on the geometry of both the remaining polymer chain and cross-linker, which could only be detected as methylene deformation mode reflectance changes by IR spectroscopy.