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Crystalline Iodine Complexes of Amorphous Poly(vinyl acetate) as Studied by X‑ray Diffraction, Vibrational Spectroscopy, and Computer Simulation

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journal contribution
posted on 21.05.2020, 15:43 by Mengfan Wang, Tomohiko Takahama, Kohji Tashiro
One of the typical amorphous polymers, atactic poly­(vinyl acetate) (PVAc) has been found to form the two types of crystalline iodine complexes (α and β forms) with iodine ions when the sample is immersed into the iodine aqueous solutions of low and high concentrations (0.001–0.01 and 0.04–3 mol/L), respectively. By analyzing the experimental data of resonance Raman spectra and X-ray 00L diffractions, the iodine species were identified as I5 ions in the β form, and the mixture of I3 and I5 ions was in the α form. The quantitative analysis of the two-dimensional X-ray diffraction data combined with the density functional theory (DFT) calculation gave the two representative models for the iodine complex: in one model (interchain iodine model), the helical PVAc chains of irregular configuration are arrayed side by side, and the iodine columns are sandwiched between these chains. Another model (intrachain iodine complex model) consists of the doubly stranded helices of PVAc chains, inside which the I5 (and I3) ions are confined along the chain axis. Both of these two models reproduced the observed X-ray diffraction data well, suggesting the hybridized model of these two candidates as the more realistic complex model. The details of the packing mode of PVAc chains and iodine columns in the complex are different between the α and β forms. The occupancy of iodine ions is also different between the α and β forms. It is emphasized that the amorphous PVAc chains can form the crystalline complexes with iodine ions although the PVAc chains themselves are totally irregular in configuration. The stabilization mechanism of the iodine complexes of the structurally irregular PVAc chains was discussed on the basis of DFT calculation.