Coarse-grained molecular dynamics simulations of macrocycle crystallization in polyrotaxane hydrogels

Self-healing and improved toughness are highly sought-after properties for synthetic hydrogels. Covalently crosslinked polyrotaxanes (PR), composed of α-cyclodextrin (αCD) and polyethylene glycol (PEG), exhibit both functionalities due to the propensity for αCD to form inter-chain hydrogen-bonded domains. These physically linked domains increase material toughness through the realization of a higher crosslinking density, and their ability to break and reform under stress imparts self-healing characteristics. To better understand the role of macrocycle crystallization in the performance of PR hydrogels, we perform simulations using a generic coarse-grained model. This model allows us to quantitatively demonstrate the crystallization of macrocycles under both quiescent conditions and tensile deformation of the hydrogel. We find that exposing the system to extensional stress can affect the crystal fraction, redistribute the macrocycle composition of the crystalline domains, induce anisotropic crystalline orientation, and in turn, lead to robust material properties. We expect our results to aid in the rational design of high-performance PR hydrogels.