The transparency of polymers in the ultraviolet region of the spectrum

Abstract

THE absorption spectra of organic compounds show that there are very few substances that are transparent over the whole visible region with a limit of transparency at a wave length shorter than 3000 A [1-4]. At the same time there is a need for polymeric adhesives that are transparent in the UV-region down to 2350 A. They are required for glueing optical components for ultraviolet and luminescent microscopy. The transparency of existing adhesives begins at about 3000 A. We have analysed the published information on the absorption spectra of a large number of organic compounds. I t was found that transparency limits from 2100 /~, to 2250 A, with complete transparency in the visible region, are possessed by members of the aliphatie seires C,~H~+ 2 where n is not greater than 5, alcohols of the series CnH2n+IOH or CnH2~(OH)2 where n is not greater than 2, aldehydes of the series CnH2,~+ICOH where n is 1 and corresponding compounds with unconjugated double bonds. We class these compounds arbitrarily as members of the first group. To the same group we have assigned four and five membered cyclic compounds of the type CnH~n+2. In these the hydrogen atoms can be subst i tuted by OH groups. I t will be seen subsequently that other groups of compounds not mentioned here can be transparent in the same region of the spectrum. In the second group we class compounds with transparency limits from 2300 A to 2500 A; this group includes members of the aliphatic series CnH2,,+ 2 where n is not greater than 6, alcohols (see above) with n up to 3, aldehydes with n up to 2, esters of the series C~H2n+IOCOC m H~,~+lWith n + m up to 4 and the halogensubst i tuted series C~H2n+ 1 hal with n up to 3. The same group includes corresponding compounds with uneonjugated double bonds and six-membered cyclic compounds of the type C~H,2,,+.2 in which the hydrogen atoms can be substi tuted by OH groups. On the basis of this information an adhesive consisting of a solution of polyvinylacetate, of molecular weight 20,000, in cyclohexanol was made up. Vinyl acetate CHaOCOCH=CH ~ and eyclohexanol CeHllOH belong to the second of the above groups and should give an adhesive with a transparency limit between 2300 and 2500 A.