FULLY ATOMISTIC MODELS OF THE SURFACE OF AMORPHOUS POLYETHYLENE

Fully atomistic models of very thin films of amorphous polyethylene ar e studied. The density profile of the thin films shows that the surfac e layer is thicker than the result obtained previously for thin films of poly(1,4-cis-butadiene), using a similar method. This difference in thickness of the surface layer is attributed to the fact that polyeth ylene is a stiffer chain than poly(1,4-cis-butadiene), as reflected by the difference in their characteristic ratios. The C-C bonds are rand omly oriented in the interior of the film, but in the outer portion of the surface layer they have a slight preference for an orientation in the plane of the film. The contribution of the internal energy to the surface energy of the film is estimated to be 20 erg cm(-2), which is lower than the experimental value of 26 erg cm(-1) reported in the li terature for the surface energy. The roughness of the surface varies f rom 0 to 6 Angstrom as the radius of the spherical probe used in the m easurement becomes smaller. (C) 1997 Elsevier Science Ltd.