Thermorheologically complex behavior of multi-phase viscoelastic materials

The dynamic correspondence principle of viscoelasticity is used to study the nature of time-temperature behavior of multi-phase composites by means of finite element computation. The composite considered contains viscoelastic inclusions embedded in a viscoelastic matrix. Each phase of the composite is considered to be thermorheologically simple, but the resulting mechanical properties of the composite are thermorheologically complex. The deviation of the composite moduli from thermorheologically simple behavior of the matrix material is shown to occur at frequencies and temperatures where the glass-to-rubber transition of the included phases are reached. Properties of a styrene-butadiene-styrene (SBS) block copolymer are investigated based on the individual phase properties of polystyrene and polybutadiene. To achieve congruence of the results with experimental data, it is necessary to consider a transition phase of properties "intermediate" to those of polystyrene and polybutadiene. Using accurate physical information on the individual phase properties and on the interphase region, it is possible to predict properties of multiphase composites. Although detailed a priori knowledge of such an interphase is usually lacking, it is shown that the computational procedure presented here together with an extended range of test frequencies will aid in estimating the properties of the phase in question.