Fracture, Fatigue, and Friction of Polymers
in which entanglements greatly outnumber cross-links

(A) Scheme of a polymer chain with a large number of physical entanglements along its length between crosslinks (red dots). (B) Scheme of a stretched polymer transmitting tension to other entangled chains. (C) Scheme of chain relaxation after bond rupture, which partly relaxes both entangled and crosslinked chains. Arrows indicate direction of applied load. Optical images of (D) highly entangled and (E) regular hydrogels with marked differences in stiffness.

Hydrogels and elastomers have similar mechanical properties as living tissues. Hence, they are being widely developed for biomedical applications. However, to date, it has not been possible to simultaneously enhance both their stiffness and toughness, as these are orthogonal properties. Existing toughening mechanisms also lead to large hysteresis, which is undesirable for materials subjected to cyclic loads.

A research team from the Harvard MRSEC, led by Zhigang Suo, has designed a new class of polymers, in which physical entanglements between polymer chains greatly outnumber their chemical crosslinks. Under mechanical load, tensile stress experienced by a given chain is transmitted to surrounding entangled chains, which are prevented from disentangling due to sparsely distributed crosslinks that connect them. These engineered polymers exhibit high toughness, strength, and fatigue resistance. Upon swelling in water, the resulting hydrogels display low hysteresis, low friction, and high wear resistance.

Li, S.; Deng, B.; Grinthal, A.; Scheneider-Yamamura, A.; Kang, J.; Martens, R.; Zhang, C.; Li, J.; Yu, S.; Bertoldi, K.; Aizenberg, J.; "Liquid-induced topological transformations of cellular microstructures", Nature 2021 (in press)

Zhigang Suo (MechE)
2021-2022 Harvard MRSEC (DMR-2011754)