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 temperatur e. The chosen polymer is an ethylene-vinylacetate copolymer (EVA) with diff erent amounts of vinylacetate (VA). Dielectric spectroscopy analysis is per formed directly on polymer-glass assemblies and the relaxation temperature corresponding to the maximum of the loss factor, tan delta, is examined. Th is temperature is related to the glass transition of the copolymer. The res ults show that at high thickness values, the glass-relaxation temperature c orresponds to the glass-transition temperature of the bulk copolymer. Howev er, when the polymer thickness decreases, the relaxation temperature increa ses greatly whatever is the VA content. These results evidence the formatio n of an interphase in the polymer layer, localised in the vicinity of the g lass surface, and whose properties are different from that of the bulk. The increase of the relaxation temperature when the polymer thickness decrease s is a consequence of the reduced mobility of the polymer chains in the int erphasial zone. (C) 1999 Elsevier Science S.A. All rights reserved.