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ABSTRACT
The mechanism of the strain-amplitude dependence (non-linearity) of viscoelastic properties of a PVC plastisol was examined. The range of amplitude was 0.02 to 10.24 and the range of frequency was 0.1 to 100 rad/s. The amplitude dependence of dynamic viscosity was very similar to its frequency dependence in that with the increase of amplitude or frequency it decreased first, reaching a minimum and then increased. In the preceding series of papers, the decrease of viscosity with the increase of frequency, i.e. pseudo-plasticity, was attributed to shear-induced phase separation into immobilized layer and mobile phase. In the present work, it is shown that the same mechanism is responsible for the decrease of viscosity with the increase of amplitude. The increase of viscosity with the increase of amplitude (after the minimum) is explained by the dilatation of the immobilized layer. Subsequently, the normal stress acting on the immobilized layer is calculated from the difference of the osmotic pressure between the mobile phase and immobilized layer. The shear deformation of the immobilized layer is estimated to be extremely small, although the deformation in the normal direction, i.e. the dilatation, is significant.
INTRODUCTION
In our previous paper of this series,1 strain dependence of viscoelastic behavior, (nonlinear behavior), was reported. The sample was a commercial PVC resin dispersed in DOP at 50% volume. The measurements were made in shear at room temperature in the frequency range of [omega] = 10^sup -1^- 10^sup -2^ rad/s. The strain amplitude, [gamma]^sub 0^, was varied from 0.02 to about 10. The general pattern showed that at a fixed frequency both storage modulus, G', and dynamic viscosity, [eta]', decreased with the increase of the strain amplitude, reaching a minimum at about [gamma]^sub 0^= 0.30. As the strain was increased further, both G' and [eta]' increased, going over a maximum in some case.
The objective of this work is to examine the mechanism of non-linearity of viscoelastic behavior with respect to initial decrease and then, increase of viscosity with the increase of the strain amplitude.
In our previous study of pseudo-plastic behavior of PVC plastisols,3 both frequency dependence of dynamic viscosity and shear rate dependence of steady state viscosity were explained with the same mechanism. When frequency or shear rate was increased, a...