Double bond system formation in the process of thermосatalytic dehydration of polyvinyl alcohol

Abstract

The formation of a conjugated bond system in the heat treatment of polyvinyl alcohol films with the addition of hydrochloric acid as an acid catalyst for thermal dehydration in the range of 80-150 °C has been studied on the base of electronic spectra. It is established that the fine structure in the absorption spectra in the 282-443 nm region is associated with electronic transitions in polyconjugated chains with the number of carbon atoms from 8 to 18, with the exception of the bands at 310 and 342 nm; only one peak at 361 nm of four lines in the wide band at 352-380 nm is associated with optical transitions in the polyconjugated chain containing 12 carbon atoms. It is shown that the long-wave boundary of the spectrum is due to electronic transitions in polyconjugated chains of the largest sizes; the maximum size of such chains can be determined by comparing the dependence of the Tauc band gap and the energy of the optical transitions as an inverse function of the number of carbon atoms in the conjugate chain. Since the typical sizes of such poly-conjugation chains are hundreds of carbon atoms (about 300 atoms in films heated at 100 °C), these chains can be considered as trans-polyacetylene nanoclusters incorporated into a polyvinyl alcohol matrix.