Reversal of recovery direction during creep recovery of polymer blends

Abstract

At small stresses or strains, polymeric liquids show a linear viscoelastic behavior. This can be demonstrated in creep followed by creep recovery. If the creep period 0<t<t1 is long enough, the elastic portion Je(t) of the shear creep compliance is equal to the recovered creep compliance Jr(t’), the latter follows from the recovery of the shear strain for t≳t1 with the new time scale t’=t−t1. Both functions are monotonously increasing. Such a behavior, at least approximately, is shown in this paper for melts of polystyrene (PS) and poly(methylmethacrylate) (PMMA). However, when blends of these polymers are made with the PS dispersed in the continuous phase of PMMA, the relation Je(t)=Jr(t’) is no longer valid. Unexpectedly, the recovered creep compliance shows a maximum at which the recovery direction is reversed. The height of the maxima increases with the PS concentration. We attribute this surprising phenomenon to the action of the interfacial tension and the different retardation times of the PS and PMMA melts. When the PMMA is dispersed in the continuous phase of PS, such a reversal of the recovery direction is not observed.