Utilizing an optimized azobenzene precursor and polydimethylsiloxane-soft-template-based secondary replication, we achieved light-driven switching of superhydrophobic adhesion of water droplets on a micro-arrayed liquid crystal polymer film, which is cross-linked in nematic phase and guarantees a satisfying stability for practical application. Regulated by alternating irradiation of UV-Vis light (365 nm/530 nm), the micro-arrayed film showed an ideal quick (< 1 min) and reversible switch of superhydrophobic adhesion. This kind of non-contact real-time controllable switching of microdroplet’s adhesion might inspire and facilitate the designs and applications in novel microfluidic devices.
Two acrylate monomers and two diacrylate crosslinkers containing a long spacer were synthesized and crosslinked
liquid-crystalline polymer (CLCP) films with different concentrations of azobenzene were prepared by
photopolymerization of the mixtures of the monomers and the crosslinkers. All the monomers, crosslinkers and
polymerizable mixtures exhibited LC phases in heating or/and cooling processes. Due to the relatively long spacer, the
obtained CLCP films exhibited a glass transition around 30 °C, which allows the CLCP films to undergo
photodeformations at room temperature. In addition, when the concentration of azobenzene decreased, the films bent
towards the light source with a faster speed and a larger photoinduced mechanical force.
Crosslinked liquid-crystalline polymer film (CLCP) was prepared by thermal polymerization of the mixture of an
acrylate monomer and diacrylate crosslinker containing azotolane chromophores. Mesomorphic properties were studied
using a polarizing optical microscope and a polarized UV-Vis absorption spectrometer. Due to a longer conjugated
structure of the azotolane moieties in side chains, the CLCP film underwent photoinduced bending upon exposure to
short-wavelength visible light at 436 nm and the deformable film returned to its initial flat state completely by alternating
visible light to 577 nm. It was also observed that the bending process was accelerated by increasing the light intensity
and the temperature. And the maximum force generated in the film upon photoirradiation decreased with the increase of
the temperature and increased with the increment of the light intensity. In addition, a visible-light-driven microrobot was
prepared from CLCP and polyethylene bilayer films, which could successfully lift and move an object through its "head".
Various kinds of crosslinked liquid-crystalline polymers were synthesized with acrylate monomers and diacrylate cross-linkers
containing azobenzene moieties. The effect of spacer length on photoinduced bending behavior of the polymer
films was investigated. The films with long spacers exhibited low glass transition temperatures and underwent bending
and unbending behavior at room temperature. Mechanical force generated during the process of UV light irradiation was
also measured. It was found that the maximum force increased with the increment of the cross-linking density and the
intensity of UV light.
Photoinduced bending and unbending behavior of cross-linked polymer films, prepared by in-situ polymerization of liquid-crystalline (LC) monomers with azobenzene moieties, is discussed. Various modes of bending have been achieved with various alignments of the photoactive mesogens in the elastomers. Exposure of a monodomain film to UV light to cause <i>trans-cis </i>isomerization of the azobenzene moieties leads to the bending of the film toward irradiation direction of the actinic light. Irradiation of a polydomain film with linearly polarized light results in the bending along the polarization direction of the light, which enables bending of the film along any direction. The LC alignment in the films significantly affected the photoinduced bending behavior.
Photoinduced bending and unbending behavior of liquid-crystalline elastomers is discussed. Various modes of bending have been achieved with various alignments of the photoactive mesogens in the elastomers: oriented Liquid-crystalline elastomer films were found to undergo anisotropic bending and unbending behavior only along the rubbing direction, when exposed to alternate irradiation of unpolarized UV and visible light; in the case of polydomain Liquid-crystalline elastomer films, a single film could be bent repeatedly and precisely along any chosen direction by using linearly polarized light.