On the effects of modifiers on the viscoelastic behavior of composites

Abstract Some of the details concerning the reasons for the difference in behavior between untoughened and toughened composites are presented. This work demonstrates the influence of localized plastic deformation on the viscoelastic behavior of polymer composites. As an example, the presence of a thermoplastic modifier and its potential influence on the microscale deformation is presented by using two micromechanical models to determine the conditions under which the observed behavior may occur. Results suggest that the effect of thermoplastic tougheners is to increase the amount of time‐dependent plastic deformation in loadings that produce sufficiently large local stress concentrations. The tougheners, however, do not seem to change the functional form of the time‐dependent response, rather they change the magnitude of this response. The two models discussed are used to qualitatively characterize these phenomena by relating model parameters to constituent properties. Differences in observed behavior with respect to plastic deformation in creep tests vs. tests such as dynamic mechanical analysis are attributed to the small deformations in the latter test types, which do create insufficient localized zones of plastic deformation.