Linear Viscoelasticity of Model Vitrimers: Decoding the Interplay between Bond Exchange and Segmental Dynamics

The viscoelastic behavior of vitrimers is closely linked to the nature of the dynamic bonds, the functional group density and distribution, and the topology of the network. The aim of this study is to investigate the influence of segmental motion versus exchange dynamics on the viscoelastic response of vitrimers. To this end, we synthesized two unentangled model polyacrylate-based vitrimers with varying cross-linking densities, using a low-Tg poly(n-butyl acrylate) (PnBA) precursor and a bis-dioxaborolane cross-linker. Small-amplitude oscillatory shear (SAOS) experiments were conducted across a wide range of temperatures. In addition to the high frequency Rouse relaxation, the relaxation modulus of these vitrimers displays two relaxation processes, which show different dependencies on temperature. Consequently, thermo-rheological complexity is obtained in the viscoelastic data. To understand the origin of these two relaxation processes, we modified the time marching algorithm (TMA) tube-based model to account for the exchange dynamics of the reversible bonds, in order to separate the influence of the segmental dynamics from that of bond exchange dynamics. This analysis allows us to attribute the slower relaxation process to the relaxation of the molecular segments which are unable to move without activating the bond exchange mechanism, and the intermediate relaxation process to the relaxation of molecular segments containing dynamic bonds poorly trapped in the network. The temperature dependence of these relaxation processes was quantified and rationalized, combining the Arrhenius-like temperature dependence of the exchange dynamics and the WLF-like temperature dependence of the segmental dynamics. The influence of the molar mass of the precursor and of the cross-linker density is also discussed and considered in our model. This study brings new insights on how to understand and control the viscoelastic properties of vitrimers.