Photo-activated bimorph composites of Kapton and liquid-crystalline polymer towards biomimetic circadian rhythms of Albizia julibrissin leaves

Circadian rhythm is a built-in bioclock widely existing in living organisms, not only in animals but also in plants. Particularly, circadian rhythm is of great importance for the growth of plants. To mimic the circadian rhythm behavior of Albizia julibrissin leaves, we designed photo-activated bimorph composites with several kinds of photoresponsive liquid-crystalline polymers and commercially-available polyimide (Kapton). Compared with conventional photo-actuators, the fabricated bimorph composite possesses good mechanical properties, a large displacement angle and a fast photoresponsive rate at room temperature. Upon irradiation with actinic light, unique photomechanical behaviors were observed, in which the bimorph composites always bent towards the Kapton layer side independent of the incident direction of UV light, as a result of the photoinduced volume expansion of the liquid-crystalline polymer layer. To further explore the photomechanical properties, the F (photoinduced driving force)-I (light intensity) and (displacement angle)-I (light intensity) relationships of the photo-activated bimorph composites were theoretically proposed based on a classical double beam model. Taking advantage of their sensitivity to light intensities, artificial Albizia julibrissin leaves exhibiting circadian rhythms upon UV irradiation with time varying light intensities (simulating the sunlight change from sunrise to sunset) were successfully fabricated, which may extend the versatility of biomimetic research studies.