Thermomechanical and dynamic bond-exchange behaviour in WS2 and MoS2 reinforced epoxy vitrimer nanocomposites
Abstract
Vitrimers are a new class of polymers that contain a covalent adaptable network where molecular bonds can be exchanged and reformed when heated above its Tv, providing them with stress relaxation, self-healing and reshaping abilities, which traditional thermosets lack. Herein, MoS2 and WS2 were used as nanofillers to fabricate nanocomposites based on a disulphide exchange based vitrimer. Their microstructure, Tg and thermomechanical properties were evaluated and compared to those observed for their epoxy-based analogues, and the vitrimer materials dynamic bonding exchange behaviour was investigated. The vitrimer exhibited a lower Tg than the neat epoxy due to its dynamic covalent network, with both matrices showing further Tg reductions upon filler addition as particles hindered cross-linking. Only the WS2/vitrimer nanocomposite displayed mechanical reinforcement, attributed to stronger vitrimer/WS2 interactions enabled by thicker, flatter WS2 platelets providing more accessible surfaces for H···S bonding. The stress-relaxation of the vitrimer was found to accelerate with increasing temperature due to a promotion of the chain mobility and thermal bond-exchange activation, with the addition of the fillers further promoting such acceleration at T > 75 °C and leading to slower stress relaxation at lower T, due to them increasing the vitrimer’s activation energy (Ea). The Tv of the polymer and the nanocomposites were determined and related to their Tg in order to establish their optimal processing windows. These results suggest that by strategically choosing a filler, the disulphide bond exchange process can be modified through tuning the filler-polymer interactions, hence changing Ea and Tv, and thus the range of temperatures for providing structural integrity or, alternatively, promoting their reprocessing, reshaping or recycling abilities can be tuned, opening the door to a wide range of new applications.