Photopolymerization kinetics of monolayer and bilayer Langmuir-Blodgett films of acetylenic acid salts

The photopolymerization of thin (monolayer and bilayer) Langmuir-Blodgett films of the lead salt of 2-docosynoic acid (CH₃(CH₂)₁₈C≡CCOOH, DCA), with a triple bond near the carboxylic group, and the lead salt of 23-tetracosynoic acid (HC≡C(CH₂)₂₁COOH, TCA), with a triple bond far from the carboxyl group, has been investigated by IR spectroscopy. The principal distinctions between the polymerization kinetics of the DCA salt and that of the TCA salt are observed for bilayers. It is hypothesized that the perfection of the molecular packing in the bilayers is governed by the interlayer interaction of carboxyl groups, which exerts a stronger effect on the mutual orientation of the triple bonds in the DCA salt films as compared to the TCA salt films. A model is suggested for describing the kinetics of the two-dimensional photopolymerization of monoacetylenic compounds. A comparison between simulated and experimental data for the monolayer films demonstrates that the observed saturation of conversion (α) as a function of the UV exposure time (t) at the α ≈ 0.5?0.6 level can be attributed to the fact that the intermolecular distance lengthens with local film densification during polymerization. The effects of the substrate and the orientation of molecules in the layer on α (t) is reported.