Versatile deformation of plastics with light

【Abstract】

A research group led by Dr. Toru Ube at Chuo University enabled 3D-selective deformation of “photomobile polymer materials” with the aid of two-photon absorption processes. Spatial selectivity of previous photomobile polymer materials has been limited in two dimensions. By applying two-photon absorption processes, the group succeeded in inducing deformation at any selective depth of a sample, which led to the versatile deformations and motions with an enhanced degree of freedom.

This work enhances the applicability of photomobile polymer materials and contributes to the development of small, lightweight and soft robots.

【Background】

Polymer materials, such as plastics and rubbers, are soft, lightweight and flexible, which are widely used in daily life. Recently, polymer materials that deform under external stimuli, such as heat, electricity and light, attract much attention. These materials are candidates of actuators for soft robots with human friendly nature. Especially, photomobile polymer materials can be used wirelessly and contribute to miniaturization of robots.

Crosslinked polymers containing azobenzenes have been investigated as typical photomobile polymer materials. Azobenzenes show isomerization (the change in molecular shape) upon irradiation with light. Rod-like trans-azobenzenes, which are thermodynamically stable, change to cis-azobenzenes with bent shape upon irradiation with UV light. Crosslinked polymers are composed of network structures, which are formed by chemical bonding of a large number of molecules. If azobenzenes are incorporated into polymer networks, the deformation of the networks can be induced by trans–cis isomerization. The original shape of the networks can be recovered through cis–trans isomerization, which is induced by irradiation with visible light.

The deformation behavior of photomobile polymer materials depends on the location of azobenzenes that show isomerization within the samples. As azobenzenes absorb UV light with high efficiency, the light is predominantly absorbed near the surface of the sample. Thus, trans–cis isomerization and contraction of polymer networks occur in the vicinity of the surface. As a result, samples show bending toward the light source (fig.1).

In the previous photomobile polymer materials, light absorption and deformation occur near the surface of the samples. If you can induce deformation at desired positions within the sample, various deformations with a higher degree of freedom can be made possible. Two-photon absorption is a photophysical process that occurs when the photon density is high, so molecules near the focal spot, even those located deeply within the sample, can be selectively excited. Therefore, application of two-photon absorption to photomobile polymer materials allows deformation of desired positions within the samples.

【Results】

In this work, photomobile polymer materials were prepared by incorporation of azotolane into crosslinked polymers. From spectroscopic measurements, azotolane was proved to show two-photon absorption with high efficiency under irradiation with fs laser pulses. Trans–cis isomerization was subsequently observed after two-photon absorption.

Photoinduced deformation behavior was investigated for crosslinked polymer films with azotolane moieties. Upon irradiation with fs laser pulses, a film deformed at the spot and bent toward the light source. The local deformation could be induced at a very small irradiation spot with a size of < 50 μm. In addition, the direction of bending could be controlled by changing the position of focal spots of the laser beam. When focusing near the back surface, the film bent away from the light source. On the other hand, the film bent toward the light source when focusing near the front surface. Furthermore, scanning the laser beam along the edge of the sample induced spatially continuous twisting, which gave a spiral shape of the whole sample (fig.2). Thus, two-photon absorption processes enabled deformation of desired position within samples, enhancing preciseness and variety of photoinduced motions.

【Future Outlook】

This work drastically enhances the preciseness and variety of photoactuation of polymer materials, which could be applied to various fields, such as robotics. Especially, two-photon absorption allows precise actuation even for very small materials, which would be advantageous in application to microrobots. Further enhancement of preciseness in actuation would be possible by controlling the position of irradiation spots with computers under monitoring the shape with cameras. Interdisciplinary research including robotics and information technology would lead to actual uses of photomobile polymer materials.