Highly compressible glass-like supramolecular polymer networks

Abstract

Supramolecular polymer networks are non-covalently crosslinked soft materials that exhibit unique mechanical features such as self-healing, high toughness and stretchability. Previous studies have focused on optimising such properties using fast-dissociative crosslinks (i.e. for aqueous system, k d > 10 s -1 ). Herein, we describe non-covalent crosslinkers with slow, tuneable dissociation kinetics (k d < 1 s -1 ) that enable high compressibility to supramolecular polymer networks. The resultant glass-like supramolecular networks have compressive strengths up to 100 MPa with no fracture, even when compressed at 93% strain over 12 cycles of compression and relaxation. Notably, these networks show a fast, room-temperature self-recovery (< 120 s), which may be useful for the design of high-performance soft materials. Retarding the dissociation kinetics of non-covalent crosslinks through structural control enables access of such glass-like supramolecular materials, holding significant promise in applications including soft robotics, tissue engineering and wearable bioelectronics.Supramolecular polymer networks (SPNs) are a class of soft materials composed of linear polymers transiently crosslinked through non-covalent interactions. 1, 2 On account of the dynamic nature of these crosslinks, they can serve as sacrificial bonds to dissipate applied energy, thus imparting SPNs with remarkable material properties including high toughness, 3 enhanced damping capacity, 4 extreme stretchability, 5-7 rapid self-healing 8-10 , and reversible mouldability. 11 These superior material properties have lead to the use of SPNs as repairable electrodes, 12, 13 artificial skins, 14,15 and drug-delivery devices 16,17 . Although promising strides have been made, the material requirements for some demanding applications have not yet been met. A major limitation of SPNs is achieving extreme compressibility with ultra-high compressive strength and complete self-recovery on short time scales.Comparing covalently to non-covalently crosslinked polymers, the dissociation kinetics for dynamic networks plays a critical role in the material design and mechanical properties of the SPNs. 1 Craig and co-workers revealed that it is in fact crosslink dynamics, rather than equilibrium thermodynamics, that are paramount in determining the material properties (e.g. viscoelasticity) of SPNs. 18,19 They reported that slower dissociation kinetics resulted in more intact crosslinks within a transient network under an applied force, leading to a higher complex modulus. Holten-Anderson et al. further demonstrated control over hierarchical polymer mechanics through tuning the relative ratio of two kinetically-distinct metal-ligand crosslinks, which allowed for decoupling of the material mechanics from crosslink structure. 20 These pioneering reports established the basis for understanding the relationship between crosslink kinetics and SPN material properties.

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