Physical ageing in poly(vinyl acetate) - 3. Structural relaxation and its effect on the stress relaxation modulus

The extent of physical ageing of a sample of poly(vinyl acetate) (PVAc) has been determined by measuring the shear stress relaxation at three temperatures below the glass transition. The data obtained after various ageing times could be superimposed by shifts along the log time axis using the characteristic mechanical relaxation time as shift factor. A comparison of these ageing time shift factors with enthalpy and volume data already established for the same sample indicates that the mechanical properties of PVAc reach equilibrium before either the volume or enthalpy. These results are compared with those of others and for the case where dynamic mechanical methods are used to obtain shift factors. The possibility of interrelating ageing measurements using different physical properties is discussed. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.