We study the surface states of an excess electron at polyethylene/vacuum interfaces using an accurate reaction-field method, specifically designed to take into account the long range interaction of the excess electron and the dielectric surface. The method is shown to validate the energy levels recently reported with a simple perturbation theory scheme, while providing a better description of the wave function at the vacuum. The use of a single particle pseudopotential allows the simulation of large interface samples, showing distinct differences between the electron surface states at amorphous and crystalline interfaces due to their different atomic density.