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B7.01 - Low-Energy Driven Polymer Actuators Based on Sub-Molecular Conformational Change 
December 2, 2014   1:45pm - 2:15pm

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.


[1] X. Shen, C. Viney, E.R. Johnson, C.C. Wang and J.Q. Lu. Nature Chemistry 5, 1035-1041 (2013).
[2] X. Shen, C. Viney, C.C. Wang and J.Q. Lu. Adv. Funct. Mater. 24, 77-85 (2014).

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