Mechanics of Non-Concatenated Ring Polymers – Effects of Topology Revealed by Molecular Simulations*

While many aspects of the statics and dynamics of non-concatenated ring polymers have been elucidated in the past several decades, the transformation of ring polymers into practically useful materials has remained less explored. Using molecular simulations with perfect control of polymer topology, we make elastomers and thermoplastics out of non-concatenated ring polymers and investigate their mechanical properties during large deformation and failure, which serve as an essential foundation of the proper function of polymeric materials. Advancing the knowledge of structure-property relationships in ring polymer mechanics is urgent given recent developments in the precise synthesis and characterization of ring polymers. The simulations reveal that the elastomers made of cross-linked ring polymers are significantly more stretchable than cross-linked linear polymers. Compared to linear polymers, the entanglements between ring polymers do not act as effective cross-links. As a result, the stretchability of cross-linked ring polymers is determined by the maximum extension of polymer strands between cross-links, rather than between trapped entanglements as in cross-linked linear polymers. The simulations also reveal that the thermoplastics made of ring polymers below the glass transition temperature fail through a mechanism similar to the crazing in their linear counterparts under tensile loading. The stable craze formation indicates the existence of an entanglement network in glassy ring polymers. Nevertheless, the entanglement network consists of only a fraction of the topological constraints that force ring polymers to be in self-similar loopy globular conformations. The structural features of the ring polymer craze and the drawing stress during the craze formation are related to the underlying entanglement network. Apart from the simulations, theories have been developed to delineate the mechanical behavior of both ring elastomers and ring thermoplastics. The simulations and accompanying theoretical studies demonstrate tuning polymer topology as a transformative pathway to design polymer mechanics, propelling the establishment of a new paradigm of topological polymer chemistry entering materials science.