Ellipsometric Characterization of Network Topology Transition in Vitrimers

Vitrimers are an emerging class of covalent adaptable networks, which can be reprocessed at elevated temperatures while preserving crosslinking density. In these systems, the onset temperature of bond exchange is often dubbed "topology freezing transition temperature (T_v)" and is characterized by a sharp reduction in material viscosity. Here, we provide a universal and external stress-free method to determine T_v in submicrometer (<μm) supported films, by measuring their thickness change as a function of temperature by ellipsometry. This study investigated a range of vitrimer systems, including catalyst-free, externally catalyzed, and internally catalyzed networks, to confirm the general applicability of our approach. We demonstrate the high sensitivity of ellipsometry in detecting changes in the apparent thermal expansion behaviors of vitrimer films, specifically linked to the onset of bond exchange in vitrimers, which is distinguished from most other methods that primarily capture macroscopic thermomechanical behaviors. Our results also suggest that the mechanism by which ellipsometry reveals the T_v in vitrimers is governed by their change in relaxation dynamics, which are fundamentally distinct from the thermodynamically driven glass transition observed in conventional polymers. We believe the ellipsometric method can not only streamline the characterization of T_v in vitrimers but also provide deeper insights into their dynamic exchange mechanisms by distinguishing between their microscopic and macroscopic properties.