Polymerization of two unsaturated fatty acid esters in Langmuir-Blodgett films as studied by IR spectroscopy

Abstract

The two isomers octadecyl fumaric acid and octadecyl maleic acid (C18H37OCOCHCHCOOH) have been synthesized, and Langmuir-Blodgett (LB) films of their cadmium salts have been prepared on silver, aluminum and thallium bromoiodide substrates. The orientation of the molecules relative to the substrate surface and the lateral packing of the chains in the unit cell were studied by means of transmission and grazing incidence reflection IR spectroscopy. Using the latter technique, polymerization of these films by both UV radiation and thermal heating was followed as a function of exposure time. By comparison with films polymerized on the water surface of a Langmuir trough, it was concluded that all three polymerization processes produce films with a similar molecular structure in the head group region, but with differences in orientation of the aliphatic chains. Interestingly, on polymerization on the water surface, the fumarate undergoes an expansion at constant surface pressure whereas the maleate contracts. A mixture of fumarate and maleate in the ratio of 1:3, however, forms a solution and preserves its area on polymerization, a critical feature required for the fabrication of crack-free LB films polymerized on solid substrates. © 1985.