| Vitrimers | |
Vitrimers, also known as Covalent Adaptable Networks (CAN), were first discovered and named by Ludwik Leibler [1, 2] in 2011. They are a third class of polymers [3, 4] which, like the thermosets, have covalent molecular networks. However, thermally activated bond-exchange reactions mean they can be formulated to crosslink at certain temperatures like thermosets, but at elevated temperatures vitrimers can be softened and reformed like thermoplastics. As flow is controlled by the chemical exchange reactions, the viscosity decrease has Arhennian characteristics similar to vitreous silica, making vitrimers the first reported strong organic glass formers [5]. Vitrimers have two transition temperatures: the classical glass transition temperature (Tg) and a topology freezing transition temperature (Tv). The latter is characterised by a change from a viscoelastic solid to a viscoelastic liquid. Tv recognises the increasing rate of the exchange reactions where vitrimers become processable but lose their resistance to creep [5].
Yue et al [6] have suggested that unrecyclable thermoset materials can be reprocessed by vitrimisation of the permenent networks into associative dynamic networks. The polymers can be reprocessed, and maybe even upcycled or healed, by swelling the thermoset network in solution with catalyst to enable a transesterification reaction exchange between the ester and the hydroxyl groups within the thermoset network. Potential catalysts include tin (II) 2‐ethylhexanoate (Sn(Oct)2), zinc acetylacetonate, triazobicyclodecene, triphenylphosphine, zinc acetate and zinc octoate. Solvents were dichloromethane (DCM) for polyurethane and dimethylformamide for epoxy.
Kloxin et al [7] discuss both thermoreversible and photoreversible (photochemically triggered) CAN with attributes including recyclability, healability, tunability, shape changes and low polymerisation stress.
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