Study of ethylene copolymers films by dielectric spectroscopy: influence of the polymer thickness on the glass-relaxation temperature

The aim of this work is to investigate the influence of the thickness of a polymer layer deposited on a glass cover on the glass-relaxation temperature. The chosen polymer is an ethylene-vinylacetate copolymer (EVA) with different amounts of vinylacetate (VA). Dielectric spectroscopy analysis is performed directly on polymer-glass assemblies and the relaxation temperature corresponding to the maximum of the loss factor, tan delta, is examined. This temperature is related to the glass transition of the copolymer. The results show that at high thickness values, the glass-relaxation temperature corresponds to the glass-transition temperature of the bulk copolymer. However, when the polymer thickness decreases, the relaxation temperature increases greatly whatever is the VA content. These results evidence the formation of an interphase in the polymer layer, localised in the vicinity of the glass surface, and whose properties are different from that of the bulk. The increase of the relaxation temperature when the polymer thickness decreases is a consequence of the reduced mobility of the polymer chains in the interphasial zone. (C) 1999 Elsevier Science S.A. All rights reserved.