Nonlinear rheological behavior of telechelic ionomers with the distribution of ionic stickers at the ends

This study examines the nonlinear rheology properties of telechelic ionomers with connecting ions at the chain ends. Our previous studies revealed that these telechelic ionomers relaxed in a hierarchical way depending on the average number of connecting ionic groups per chain end m. The terminal relaxation time increased with m because the dissociation was slower for the chain end with more connecting ions. In this study, the stress growth and damping functions are examined for these telechelic ionomer samples and a reference conventional telechelic sample having strictly one ion per chain end, both based on poly(isobutyl acrylate). During both the steady shear and elongation measurements, the stress overshoots are observed, and the corresponding strains at the stress overshoots, γpeak and εH,peak, are insensitive to the Weissenberg number Wi for Wi≫1. This feature suggests that γpeak or εH,peak are critical strains for the induction of dissociation, which is rationalized by the phenomenological analysis of the yielding stress based on the von Mises criterion. The approximate time-strain separability was surprisingly observed for the telechelic ionomer networks with a moderate m, which may be due to the fact that, although the high strain leads to a rupture of strand, the network structure has been maintained and its terminal relaxation is governed by the ionic dissociation (at least in the strain range of our measurements). These arguments are supported by the proper normalization of damping functions obtained for different samples using γ/γpeak as the horizontal axis.