Abstract
The motion of artificial molecular machines has been amplified into the shape transformation of polymer materials that have been compared to muscles, where mechanically active molecules work together to produce a contraction. In spite of this progress, harnessing cooperative molecular motion remains a challenge in this field. Here, we show how the light-induced action of artificial molecular switches modifies not only the shape but also, simultaneously, the stiffness of soft materials. The heterogeneous design of these materials features inclusions of free liquid crystal in a liquid crystal polymer network. When the magnitude of the intrinsic interfacial tension is modified by the action of the switches, photo-stiffening is observed, in analogy with the mechanical response of activated muscle fibers, and in contrast to melting mechanisms reported so far. Mechanoadaptive materials that are capable of active tuning of rigidity will likely contribute to a bottom-up approach towards human-friendly and soft robotics.
Highlights
The motion of artificial molecular machines has been amplified into the shape transformation of polymer materials that have been compared to muscles, where mechanically active molecules work together to produce a contraction
Over a given threshold for the amount of interfacial area, illumination decreases the miscibility between the liquid crystal and the polymer network, because the polymer becomes less ordered and more polar and this decrease of miscibility leads to an increase in the interfacial contribution to mechanical properties, and eventually to photo-stiffening
When the resulting material is illuminated with ultraviolet light, activation of the cross-linking molecular switches modifies both the polarity and the morphology of the polymer network (Fig. 1), thereby diminishing the miscibility of the two components, and we show that this molecular event has a major consequences on the stiffness of the material
Summary
The motion of artificial molecular machines has been amplified into the shape transformation of polymer materials that have been compared to muscles, where mechanically active molecules work together to produce a contraction. Examples range from chiral liquid crystals[19,20] to liquid crystal elastomers[21] and other liquid crystal networks[22], and liquid crystal networks incorporating light-responsive molecules have been developed to mimic cilia movement[23], tendrils[12,24], opening of seedpods[25], flytraps[26], and continuous wave propagation[27] These macroscopic shape transformations, as complex as they are, have not been combined with a stimuli-induced enhancement of stiffness so far[28], which limits, e.g., the interactions that they can establish with unpredictable surroundings. Over a given threshold for the amount of interfacial area, illumination decreases the miscibility between the liquid crystal and the polymer network, because the polymer becomes less ordered and more polar and this decrease of miscibility leads to an increase in the interfacial contribution to mechanical properties, and eventually to photo-stiffening These mechanically adaptive materials can convert the work produced by molecular switches efficiently, by generating stresses in the MPa order. They display light-responsive enhancement of their nonlinear response to stress, which is a salient characteristic of myosin-activated muscle fibers
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