Photomotility of polymers

Abstract

Light is distinguished as a contactless energy source for microscale devices as it can be directed from remote distances, rapidly turned on or off, spatially modulated across length scales, polarized, or varied in intensity. Motivated in part by these nascent properties of light, transducing photonic stimuli into macroscopic deformation of materials systems has been examined in the last half-century. Here we report photoinduced motion (photomotility) in monolithic polymer films prepared from azobenzene-functionalized liquid crystalline polymer networks (azo-LCNs). Leveraging the twisted-nematic orientation, irradiation with broad spectrum ultraviolet-visible light (320-500 nm) transforms the films from flat sheets to spiral ribbons, which subsequently translate large distances with continuous irradiation on an arbitrary surface. The motion results from a complex interplay of photochemistry and mechanics. We demonstrate directional control, as well as climbing.

Figure 1. Photomotility of a polymeric strip.

(a) Light induced motion (photomotility) of a thin strip composed of an azo-LCNs in the twisted nematic geometry aligned with the nematic director offset +15° (top) and −75° (bottom) to the principle axes of the strip. On irradiation with 200 mW cm⁻² of 320–500 nm light, the 15 μm thick strip forms a spiral ribbon and to continuous irradiation moves to the right. (b) The relative displacement taken from frame by frame analysis is recorded as a function of time. (c) A histogram of normalized frequency versus relative displacement demonstrates the variability in the motion. (d) Schematic of the spiral ribbon force balance illustrating the mechanism of the photomotility.

Figure 2. Directional control of photomotility.

(a) Representative images illustrating the influence of the orientation of the nematic director at the surfaces. Red arrows indicate motile angle of the polymeric coils with respect to the primary axis of coiled film (b) The geometry of the helix (Red filled circle, left axis) and the directionality of the locomotion (Blue filled square, right axis) are summarized as a function of offset angle of the top surface of the azo-LCNs to the primary axis of the samples. (c) Velocity of the photomotility of the azo-LCNs is plotted against the helix diameter (D) of the spiral ribbons. The azo-LCN samples were 15 μm thick. The error bars shown in (b) and (c) are the standard deviation of each parameter calculated from data measured for 15 seconds monitored at an interval of 0.5 seconds.

Figure 3. Contribution of light intensity.

(a) Representative images of azo-LCN strips subjected to light intensities ranging from 100 to 500 mW cm⁻². (b) Displacement of the strips (15° to −75° TN azo-LCNs) when subjected to 100, 200 and 300 mW cm⁻² of light intensity. The relative displacement of the photomotile azo-LCN strips is recorded with 30 ms time interval for light intensity of 100 mW cm⁻² (c) and 300 mW cm⁻² (e). A histogram of normalized frequency versus displacement depicts the regularity of the motion at 100 mW cm⁻² (d) and 300 mW cm⁻² (f). The azo-LCN samples were 15 μm thick.

Figure 4. Photochemical and photothermal material responses.

(a) UV-vis spectra of azo-LCN films with light (320–500 nm) irradiation at 200 mW cm⁻² intensity. (b) Summary of ultraviolet–vis absorbance (at 365 nm) of azo-LCN films subjected to 30 mW cm⁻² (Empty circle) and 200 mW cm⁻² (Red filled circle) light intensities. (c) Thermal image and temperature evolution for azo-LCN films on exposure to the light intensities of 30 mW cm⁻² (Purple color diamond), 100 mW cm⁻² (Black filled circle), 200 mW cm⁻² (Red filled square), 300 mW cm⁻² (Blue color traingle) and 500 mW cm⁻² (Green color inverted traingle).

Figure 5. Defying gravity.

(a) An illustration of climbing setup with a 15° incline. (b) Photomotility of the azo-LCN film on an incline is demonstrated at 40 s time interval. Red line is presented as a reference. (c) The work accomplished by the film is deconvoluted into potential (Empty circle) and kinetic (Red filled circle) contributions. The azo-LCN samples were 15 μm thick.