Covalent adaptable networks: smart, reconfigurable and responsive network systems

Abstract

Covalently crosslinked materials, classically referred to as thermosets, represent a broad class of elastic materials that readily retain their shape and molecular architecture through covalent bonds that are ubiquitous throughout the network structure. These materials, in particular in their swollen gel state, have been widely used as stimuli responsive materials with their ability to change volume in response to changes in temperature, pH, or other solvent conditions and have also been used in shape memory applications. However, the existence of a permanent, unalterable shape and structure dictated by the covalently crosslinked structure has dramatically limited their abilities in this and many other areas.These materials are not generally reconfigurable, recyclable, reprocessable, and have limited ability to alter permanently their stress state, topography, topology, or structure. Recently, a new paradigm has been explored in crosslinked polymers -that of covalent adaptable networks (CANs) in which covalently crosslinked networks are formed such that triggerable, reversible chemical structures persist throughout the network. These reversible covalent bonds can be triggered through molecular triggers, light or other incident radiation, or temperature changes. Upon application of this stimulus, rather than causing a temporary shape change, the CAN structure responds by permanently adjusting its structure through either reversible addition/condensation or through reversible bond exchange mechanisms, either of which allow the material to essentially reequilibrate to its new state and condition. Here, we provide a tutorial review on these materials and their responsiveness to applied stimuli. In particular, we review the broad classification of these materials, the nature of the chemical bonds that enable the adaptable structure, how the properties of these materials depend on the reversible structure, and how the application of a stimulus causes these materials to alter their shape, topography, and properties. Key learning pointsCritical differences between thermosets, thermoplastics, and covalent adaptable networks. Bond-breaking, bond-forming versus bond-forming, bond-breaking network strand rearrangement sequence. Behavior and mechanical properties derived from reversible addition/condensation and exchange crosslinking. Design and synthesis of covalent networks with reversible structures. Triggers enabling activation of covalent adaptable network.

Comprehensive study of the thermo-reversibility of Diels–Alder based PCL polymer networks

“…3 several research groups with the purpose to generate strong links but which can be easily cleaved at will under the action of a stimulus, leading to the breaking up of the network [15][16][17] .…”

One-Pot Double Modification of Polymers Based on Thiolactone Chemistry

“…This inherent reversibility means that it can be (re)formed or broken on the users' demand, generally under relatively mild conditions. Hence, it has been included in the dynamic covalent chemistry (DCC) toolbox [157][158][159][160][161] and it is essential in some covalent adaptable networks (CANs) [162][163][164] with self-healing capabilities [165][166][167][168]. However, undesired disulfide formation during thiol-click reactions and/or longterm storage of thiol-containing substrates is of particular concern and was one of the initial reasons for starting our thiolactone research (vide supra).…”

Reconfigurable surface patterns on covalent adaptive network polymers using nanoimprint lithography

“…[11]). This network reconfiguration process has been shown to relieve internal stresses, and enables permanent shape changes in these types of polymer networks [11]. To date, work has focused on quantifying and controlling stress relaxation in uniformly deformed bulk samples [1e3,8].…”

“…The chemical reaction mechanisms enabling these CANs include DielseAlder, transesterification, and reversible additionfragmentation chain transfer (RAFT) reactions, among others (for a complete review, see Ref. [11]). This network reconfiguration process has been shown to relieve internal stresses, and enables permanent shape changes in these types of polymer networks [11].…”