The crucial role of external force in the estimation of the topology freezing transition temperature of vitrimers by elongational creep measurements

Abstract

Vitrimers are covalent adaptable networks that rely on associative bond exchange reactions. Along with the glass transition temperature, the service temperature of thermally activated vitrimers is strongly governed by the topology freezing transition temperature (Tv), at which the covalent bond exchange reactions become significantly fast. Despite the vast number of developed vitrimer systems, there is a serious lack in direct methods for the determination of the Tv. In the present work, we highlight a versatile and facile measurement routine, which enables a precise estimation of the Tv in epoxy-acid vitrimers undergoing thermo-activated transesterifications. In particular, elongational creep of the networks is measured as a function of temperature and the applied stress. The results show that the onset temperature of the plastic flow regime is clearly stress-dependent, possibly leading to an inaccuracy of Tv data in the existing literature obtained from so-called dilatometry measurements. We demonstrate that with increasing external force on the specimens, the onset of macroscopic flow is gradually shifted to lower temperatures and approaches the accurate Tv of the material at a particular stress level. This behaviour is found in different epoxy-acid vitrimers with varying Tg. The data is in very good accordance with (theoretical) Tv values, which are reported in literature and were received from stress relaxation data, confirming the validity and equivalence of the method presented.