Fundamental insights into the covalent adaptable networks of phosphate esters: Effects of condensation and of the dynamic exchange of phosphate diesters with hydroxyl groups

Abstract

Vitrimers are one of the most widely studied and implemented classes of sustainable engineering plastics today. Among the vitrimers, the transesterification of carboxylate esters is dominant, although in recent years the transesterification of phosphate esters has gained attention due to the favorable exchange conditions between phosphate esters and hydroxyls and the autocatalytic effect of the phosphate group. Mono-, di- and tri-ester phosphates are all used in vitrimers and offer various advantages as exchangeable groups or internal transesterification catalysts. Here we have systematically studied the dynamic exchange in methacrylate photopolymer networks crosslinked with phosphate diesters and shown that, in addition to exchange, there is a significant condensation reaction between diester phosphates and free hydroxyl that accompanies the exchange process. When heated to temperatures above 120 °C, phosphate diesters are converted in situ to triesters, resulting in up to a 50-fold increase in elastic modulus, a 40 °C increase in glass transition temperature and a 50-fold increase in mechanical strength. The kinetics of the condensation was studied by differential scanning calorimetry (DSC) and the evolution of the properties by dynamic mechanical analysis (DMA). The dynamic properties were evaluated in stress relaxation experiments and were found to be complex. The relaxation curves were fitted to a two-element Maxwell model and showed that the dynamic behavior and mechanical properties evolve during condensation. This research highlights new aspects of phosphate ester exchange that need to be considered in the design of dynamic networks, and would also serve as a guide in the selection of phosphate group type for dynamic materials applications.