Complex cure kinetics of self-healing copolyester vitrimers via isothermal thermogravimetric analysis

Abstract

Thermogravimetric Analysis (TGA) is a valuable tool for studying chemical reactions that release volatile compounds. Constant-temperature TGA can be particularly useful for polymer cure reactions with complex, time-evolving phenomenology, by ensuring time and temperature effects remain separable. In this work, a compound isothermal TGA methodology is developed to study the cure process of high-performance vitrimers known as Aromatic Thermosetting coPolyesters (ATSP). By fitting observed mass loss to a nondimensional phenomenological model with generalized terms and introducing additional nonphysical terms to account both known and suspected non-cure behaviors, changes in cure kinetics were evaluated across a wide range of temperatures (210°C–380 °C). The activation of the added nonphysical terms was used to differentiate between expected cure behavior (240°C–340 °C), and cures involving precursor decomposition (>340 °C). An unexpected cure-like reaction below cure temperature (<240 °C) was hypothesized to correspond to self-healing bond exchange, which is well-understood to cause changes in molecular weight in thermoplastic polyesters. This was directly validated via evolved gas analysis, and activation energies were calculated for the cure-dominant region of the reaction. These activation energies were found to be similar across different polymer formulations. When combined with significant observed mass loss below the expected cure temperature and findings in prior work, this indicates that the apparent cure process may be driven by thermodynamically-favorable bond exchange reactions.