Dicarboxylic acid-epoxy vitrimers: Influence of off-stoichiometric acid content on cure reactions and thermo-mechanical properties

The present study explores a broad range of stoichiometry, with [epoxy]/[acyl] ratio in excess to unity for a commercial diepoxide/sebacic acid vitrimer formulations with 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) used as catalyst. In particular, it investigates to what extent side reactions promoted by off-stoichiometry mixtures can achieve desirable thermomechanical properties (i.e. glass transition T, Young’s modulus, strain at break, and strength) for an optimised vitrimer that behaves like a stiff material at room temperature, keeping its capacity to flow at high temperature while remaining insoluble. Possible role of TBD as anionic initiator is tested on the homopolymerisation of the epoxy and compared to a known anionic initiator, 2-phenylimidazole (2-PI). Attenuated total reflection infrared (ATR-IR) spectroscopy reveals different reaction speeds, but identical scenario for either 2-PI or TBD. The acid + epoxy addition occurs first, then epoxy homopolymerisation takes place after di-carboxylic acid consumption; ester typically forms in less than 20 min at 125°C with TBD, while ether takes several hours. For all [epoxy]/[acyl] ratios ranging from 1:1 to 1:0.3, it is found that the integrity of the network remains while submitted to 1,2,4 trichlorobenzene (TCB) solvent. From 1:1 to 1:0.75 epoxy to acyl ratio, the material keeps full ability to flow and relax stresses under thermal stimulation, showing a 10 fold increase of viscosity and unchanged activation energy of about 100 kJ.mol-1. Beyond stoichiometry 1:0.6, a gradual transition from vitrimer to non exchangeable crosslinked materials is observed as these networks show only partial stress relaxation due to interpenetration with polyether network.