In this paper, an understanding of the physical relationships between the material structure and the infrared optical properties of polar ethylene homo and copolymers for solar applications is described. The infrared optical properties are relevant for the heat transport of transparent insulation structures based on these materials. To establish structure-property-correlations molecular structure parameters such as comonomer content and the concentration of carbon-oxygen single bond and carbon-hydroxyl groups, respectively, were determined. For 50 mu m thick films a good correlation between the concentration of the functional carbon-hydroxyl and the carbon-oxygen group and the infrared optical thickness as well as the hemispherical emittance was found. The carbon-hydroxyl group appears to be slightly more effective than the carbon-oxygen single bond. (c) 2005 Elsevier B.V. All rights reserved.