Mechanoresponsive polymers hold great technological potential in drug delivery, 'smart' optical systems and microelectromechanical systems. Unfortunately, limitations include (i) hysteresis and fatigue - e.g. when the response involves large-scale molecular rearrangements, as is the case with order-to-disorder transitions, critical phase transitions, or melting and glass transitions, or (ii) the need for high-energy photons that are damaging and/or too readily attenuated - e.g. when the response is a cis-trans isomerization of stilbene or azobenzene moieties triggered by ultraviolet light.
Recently, a new class of polymers that contains the unit of dibenzocyclooctadiene (DBCOD), a flexible cyclooctane group connecting two rigid phenyl rings, can generate an anomalous giant mechanical contraction, -2300 ppm/K.1 More impressively, this giant contraction is completely reversible in response to low-energy heating or NIR photons. The unique advantages of the material include:
Low energy: It requires less than one-third of the energy needed for the photoisomerization of azobenzene and thus enables remote NIR actuation for biological applications;
Completely reversible: No signs of fatigue or failure have been observed after extended cycling. This is in stark contrast to conventional systems that employ melting, glass transitions, or order-to-disorder transitions (e.g. liquid crystals);
High output: Due to the deep penetration of low-energy NIR stimulation, the entire depth of a sample can be stimulated, thus has the potential to deliver considerably higher conversion efficiency and greater mechanical energy output.
Mechanical characterization, calorimetry, spectroscopic analysis and density-functional theory calculations all point to a conformational change of the DBCOD moiety, from the thermodynamic global energy minimum (twist-boat) to a local minimum (chair), as the origin of this abnormally large thermal strain.
Incorporation of few-walled carbon nanotubes (FWCNTs) into the films increases the mechanical stiffness twofold, and dramatically increases the NIR-induced contraction strain up to a factor of nearly 24 at the optimal FWCNT concentration of ~ 3 wt%.2
The DBCOD-containing polymers offer a new pathway to create macromolecular switches and motors, and to design novel systems for thermal energy storage and conversion as well as zero thermal expansion polymers.
 X. Shen, C. Viney, E.R. Johnson, C.C. Wang and J.Q. Lu. Nature Chemistry 5, 1035-1041 (2013).
 X. Shen, C. Viney, C.C. Wang and J.Q. Lu. Adv. Funct. Mater. 24, 77-85 (2014).