The mechanical properties of covalent polymer networks often arise from the permanent end-linking or cross-linking of polymer strands, and molecular linkers that break more easily would likely produce materials that require less energy to tear. We report that cyclobutane-based mechanophore cross-linkers that break through force-triggered cycloreversion lead to networks that are up to nine times as tough as conventional analogs. The response is attributed to a combination of long, strong primary polymer strands and cross-linker scission forces that are approximately fivefold smaller than control cross-linkers at the same timescales. The enhanced toughness comes without the hysteresis associated with noncovalent cross-linking, and it is observed in two different acrylate elastomers, in fatigue as well as constant displacement rate tension, and in a gel as well as elastomers.

Slide-ring gels are polymer networks with crosslinks that can slide along the chains. In contrast to conventional unentangled networks with cross-links fixed along the chains, the slide-ring networks are strain-softening and distribute tension much more uniformly between their strands due to the so-called "pulley effect". The sliding of cross-links also reduces the elastic modulus in comparison with the modulus of conventional networks with the same number density of cross-links and elastic strands. We develop a single-chain model to account for the redistribution of monomers between network strands of a primary chain. This model takes into account both the pulley effect and fluctuations in the number of monomers per network strand. The pulley effect leads to modulus reduction and uniform tension redistribution between network strands, while fluctuations in the number of strand monomers dominate the strain-softening, the magnitude of which decreases upon network swelling and increases upon deswelling.

Extending polymer chains results in a positive chain tension, f ch, primarily due to conformational restrictions. At the level of individual bonds, however, tension, f b , is either negative or positive and depends on both chain tension and bulk pressure. Typically, the chain and bond tension are assumed to be directly related. In specific systems, however, this dependence may not be intuitive, whereby f ch increases while f b decreases; i.e., the entire chain is extended while bonds are compressed. Specifically, increasing the grafting density of a polymer brush results in chain extension along the direction perpendicular to the grafting surface while the underlying bonds are compressed. Similarly, upon compression of polymer networks, the extension of chains oriented in the “free” direction increases while their bonds are getting more compressed. We demonstrate this phenomenon in molecular dynamics simulations and explain it by the fact that the pressure contribution to f b is dominant over a wide range of network deformations and brush grafting densities.