On the ultraviolet absorption spectrum of polyvinyl alcohol

Abstract

AbstractA study of the ultraviolet absorption spectrum of polyvinyl alcohol (PVA) was made. Spectral examination of the solvolysis products of vinyl acetate and 1‐methoxyvinyl ester copolymers shows unequivocally that the absorption bands in PVA at 225, 280, and 330 mμ are related to carbonyl‐containing structures. The high intensities of these bands in the solvolyzed copolymers is additional evidence that the absorbing groups cannot be unconjugated carbonyl and are probably of the type \documentclass{article}\pagestyle{empty}\begin{document}$ - (\rm{C} \hbox{=\hskip-1.5pt=} \rm{C})_{n} \hbox{---} \mathop C\limits^{\mathop {||}\limits^O }\hbox{---}.$\end{document} Ultraviolet dichroism measurements tend to confirm this view. The ultraviolet absorbing bands in PVA arise predominantly from two sources: the presence of acetaldehyde, and dissolved air in the vinyl acetate monomer during polymerization. In the case of the latter, the formation of the vinyl acetate—oxygen copolymer and its subsequent decomposition to aldehydic‐type products introduce, into the polymer, chain structures similar to those introduced by acetaldehyde. By the careful exclusion of acetaldehyde and air during vinyl acetate polymerization and by a judicious choice of initiator, PVAs having minimal ultraviolet absorptions can be prepared. Treatment of PVA at room temperature with dilute aqueous alkali results in a marked diminution in the intensities of all three absorption bands. Hot dilute acid, while essentially removing the 330 mμ band, produces no consistent change in the relative intensities of the 225 and 280 mμ bands.