Mechanical Structure-Property Relationships for 2D Polymers Comprised of Nodes and Bridge Units

Abstract

2D polymers have emerged as an infinitely-tailorable material with remarkable, tunable response and density-normalized mechanical properties far exceeding structural materials such as steel, high-performance fibers or reinforced composites. It is critical that the vast material design space of 2D polymers be mapped in order to achieve optimal mechanical performance, since hundreds of permutations of one class of 2D polymers known as covalent organic frameworks have already been synthesized in the decade since the introduction of these materials. To this end, this work establishes a general structure-property relationship for elastic modulus and strength for a common 2D polymer motif consisting of nodes linked by linear bridge polymer chains to form a two-dimensional network. The length of the bridge chains are parametrically varied to study the impact of chain compliance on stiffness and strength. The density-normalized isotropic strength of the graphene/polyethylene hybrid material known as graphylene begins at 0.015 GPa/kg·m³ (50% higher than that of perfect crystalline Kevlar®) and the density-normalized isotropic stiffness is 0.143 GPa/kg·m³ (31% higher than Kevlar®) and decreases non-monotonically with increasing bridge chain length. The mechanical response is mapped and correlated to the inherent molecular structure of these general 2D polymer as a framework for designing 2D polymer molecules for mechanical applications from the ground up.