The influence of sorption of small molecules on the yield stress of glassypolymers

Uniaxial stress-strain measurements have been performed on two glassy polyc arbonates and one polyimide in an atmosphere of CO2, H2O-vapour or acetone- vapour. The yield stress has been determined as a function of the concentra tion of sorbent molecules. In all observed cases the polymer is softened du e to the sorption. The depression of the yield stress at a given concentrat ion is dependent an the size of the sorbent molecules and of interchain vol ume sites within the polymer. For H2O and CO2 a unique relation is found be tween the softening and the volume dilatation accompanying the sorption. Th is dilatation has been associated with an internal isotropic stress via Hoo ke's law. That enables us to explain the softening as a consequence of the well known dependence of the yield stress on the isotropic stress component . (C) 1999 Elsevier Science Ltd. All rights reserved.