Mechanostereochemical modulation of polymer mechanical properties.

Abstract

Stereochemistry profoundly affects the physical and mechanical properties of polymers, illustrated by the contrast between elastic natural rubber (<i>cis</i>-polyisoprene) and the stiffer, less extensible gutta-percha (<i>trans</i>-isomer). Traditional stereochemistry such as tacticity and <i>cis</i>/<i>trans</i> isomerism primarily governs polymer properties based on fixed structural or conformational factors. Herein, by incorporating the mechanostereochemistry concept into polymers, we demonstrate a stereochemistry paradigm wherein dynamic isomers dictate material properties, thereby defining the unprecedented transient-stereostructure-efficacy mechanism. Specifically, we engineer two mechanically interlocked networks based on [<i>c</i>2]daisy chains, where force-triggered intramolecular motion generates mechanostereoisomers with distinct geometric configurations: [<i>c</i>2]Daisy chain <b>1</b> in MIN-<b>1</b> contracts into a fisherman's knot, whereas [<i>c</i>2]daisy chain <b>2</b> in MIN-<b>2</b> extends into a loop. Due to reduced network elasticity from the loop structure, MIN-<b>2</b> exhibits a lower modulus in large-strain shear measurements and less pronounced strain hardening in tensile tests compared to MIN-<b>1</b>. Since these mechanostereoisomers are induced by force, material properties show strain-dependent character: both networks perform similarly under small or no strain, but diverge significantly at large strains. Our work expands the conceptual boundaries of polymer stereochemistry and provides insights for designing high-performance materials through stereochemical control.