FTIR reflectance experiments have been performed on thin ethylene-vinyl acetate (EVA) copolymer layers deposited on aluminum mirrors in order to determine the orientation of polymer functional groups at the interface. This was accomplished by using various reflection angles under p-polarization states of the incident IR wave. Film thickness is estimated by ellipsometric experiments. Kramers-Kronig analysis is first applied to the external infrared reflection spectrum from a single copolymer surface measured near the normal incidence angle. Absorption spectra, k(nu), are then deduced and used to calculate specular reflectance intensities of the functional groups of interest, as a function of reflection angle, polarization state, and film thickness. The calculated values are compared to the experimental ones. An original layer model was developed and allows us to determine the persistence length of the molecular orientation of EVA carbonyl groups at the interface. This approach is based on the fact that molecular orientation persists only over a given distance from the geometrical interface. This distance is called the "persistence length of molecular orientation". We then suppose that the nanofilm adsorbed is stratified and consists of an oriented layer (in the near interface region) plus an isotropic one. It seems that only carbonyl groups involved in specific electron donor-electron acceptor interactions at the interface are subjected to specific orientation.