Adaptable Crosslinks in Polymeric Materials: Resolving the Intersection of Thermoplastics and Thermosets

Abstract

The classical division of polymeric materials into thermoplastics and thermosets based on covalent network structure often implies that these categories are distinct and irreconcilable. Yet, the past two decades have seen extensive development of materials that bridge this gap through incorporation of dynamic crosslinks, enabling them to behave as both robust networks and moldable plastics. Although their potential utility is significant, the growth of covalent adaptable networks (CANs) has obscured the line between “thermoplastic” and “thermoset” and erected a conceptual barrier to the growing number of new researchers entering this discipline. This Perspective aims to both outline the fundamental theory of CANs and provide a critical assessment of their current status. We emphasize throughout that the unique properties of CANs emerge from the network chemistry, and particularly highlight the role that the crosslink exchange mechanism (i.e., dissociative exchange or associative exchange) plays in the resultant material properties under processing conditions. Predominant focus will be on thermally induced dynamic behavior, as the majority of presently employed exchange chemistries rely on thermal stimulus, and it is simple to apply to bulk materials. Lastly, this Perspective aims to identify current issues and address possible solutions for better fundamental understanding within this field.

Dynamic covalent polymer networks via combined nitroxide exchange reaction and nitroxide mediated polymerization

“…However, modulating and characterizing the crosslinking density and reducing defects, such as dangling chain ends and loops, remain a challenge to be mastered by polymer scientists. [6][7][8][9][10][11] Traditionally, the topology of a polymer network is static and permanent after synthesis. Consequently, the polymer community showed growing interest in designing "smart" materials that can respond to changes in the environmental conditions, or be modified by post-synthetic functionalization.…”

Covalent Adaptable Networks with Tunable Exchange Rates Based on Reversible Thiol–yne Cross‐Linking

“…Covalent adaptable networks (CANs) are polymer materials that combine the strength of covalently cross-linked materials,s uch as high chemical resistance and dimensional stability,w ith the reprocessability of thermoplastic polymers. [1][2][3][4] These responsive polymer networks have the ability to reversibly rearrange their network topology through the action of dynamic exchanges.Upon application of an external stimulus (such as heat, light), the dynamic chemistry is activated to allow bond exchanges on amolecular level, which enables network plasticity and allows reshaping,reprocessing, stress relaxation, self-healing, and imprinting.…”

Tuning polymer properties of non-covalent crosslinked PDMS by varying supramolecular interaction strength

“…More recently, focus has shifted to the reuse and reshaping of materials by developing re-organisable polymer networks using dynamic covalent bonds. [1][2][3][4][5][6] Apart from dynamic covalent bonds, dynamic non-covalent bonds have also been applied in polymeric networks. [7][8][9] Here, a linear polymer backbone is typically post-modified to incorporate supramolecular moieties along the backbone as grafts.…”