STRUCTURE, CRYSTALLIZATION, AND INFRARED-SPECTRA OF AMORPHOUS PERFLUORO-NORMAL-ALKANE FILMS PREPARED BY VAPOR CONDENSATION

Amorphous films of n-C8F18, n-C14F30, n-C16F34, and n-C20F42 were prepared by vapor deposition under vacuum onto the surface of a CsI crystal at 8 K and studied by infrared spectroscopy. The amorphous films thus prepared consist of conformationally disordered perfluoro-n-alkane chains that contain both trans and gauche bonds. Annealing the as-deposited films at successively higher temperatures resulted in increased conformational ordering. The highest annealing temperatures attained for C8, C14, C16, and C20 were 180, 150, 210, and 250 K, respectively. Relative to the other chains, however, C8 was transformed very little. At the highest annealing temperature, the C16 and C20 films were highly ordered but retained a small but significant fraction of conformational disorder that appeared to be similar to the disorder observed for these compounds at room temperature. The C14 film underwent a well-defined crystallization at a temperature between 135 and 150 K, producing essentially complete conformational ordering. The gauche-trans energy difference Eg was estimated from infrared band intensities for C14 and C16, resulting in a value of 1.1 ± 0.1 kcal/mol. The concentration of conformationally ordered chains in a film of C20 was determined as a function of annealing temperature. The onset of ordering was found to occur near 65 K and to be one-half completed near 130 K. These temperatures are somewhat higher than those found for n-C20H42 in our earlier study. An activation energy for conformational ordering in amorphous C20 was also estimated and found to be significantly larger than that for the corresponding n-alkane. These results were obtained from an analysis of the infrared bands observed in the 1150-700-cm-1 region, which is associated with the ν3 dispersion curve. The ν3 infrared bands identified with ordered chains have been used to extend the earlier analyses of the ν3 dispersion curve. For amorphous films of the perfluoro-n-alkanes, the 1150-700-cm-1 region is extremely complex, but it displays a band pattern that resembles the pattern predicted for a model system of conformationally disordered chains. © 1990, American Chemical Society. All rights reserved.