Transition Phenomena in Vitrimers

Vitrimers, a special type of dynamic polymer network, are able to transform temperature-dependently from a cross-linked thermoset phase with static shear stiffness to a flowable, yet still cross-linked phase. This peculiar transformation raises a number of fundamental questions, such as if this isostructural transformation is accompanied by a phase transition or not, the existence of a proper order parameter and thus, what susceptibility to probe to investigate the vitrimer transition properly. Here, we present and discuss the challenges accompanying these questions from a physics-driven point of view with a perspective on the current state of research in the field of vitrimer transition phenomena. Then, in the framework of an assessment of classical pathways to evaluate classical phase transition in polymers, two methods (temperature-modulated optical refractometry and Brillouin scattering) are introduced that provide access to promising susceptibilities to probe the vitrimer transition, the dynamic thermal volume expansion β as well as the elastic constants E and G. Finally, experimental results for two model vitrimers are presented and our current perspective on the vitrimer transition phenomenon is discussed.