Light-induced mechanical response in crosslinked liquid-crystalline polymers with photoswitchable glass transition temperatures

Abstract

Energy conversion of light into mechanical work is of fundamental interest in applications. In particular, diligent molecular design on nanoscale, in order to achieve efficient photomechanical effects on macroscopic scale, has become one of the most interesting study topics. Here, by incorporating a “photomelting” azobenzene monomer crosslinked into liquid crystalline (LC) networks, we generate photoresponsive polymer films that exhibit reversible photoswitchable glass transition temperatures (Tg) at room temperature (~20 °C) and photomechanical actuations under the stimulus of UV/visible light. The trans-to-cis isomerization of azo chromophores results in a change in Tg of the crosslinked LC polymers. The Tg of the polymer network is higher than room temperature in the trans-form and lower than room temperature in the cis-form. We demonstrate the photoswitchable Tg contribute to the photomechanical bending and a new mechanism for photomechanical bending that attributes the process to an inhomogeneous change in Tg of the film is proposed.

Azobenzene‐Containing Block Copolymers as Light‐Induced Reworkable Adhesives: Effects of Molecular Weight, Composition, and Block Copolymer Architectures on the Adhesive Properties

“…Subsequent examination of the effect of the block copolymer architecture on the adhesive property revealed that the terminal PAz blocks contribute to enhance the adhesion strength and lower the residual adhesion strength upon UV irradiation. This is because these terminal PAz blocks functioned as physical crosslinks in the solid state and also imparted polymer chain mobility when softened by UV irradiation . Consequently, the 4‐arm (AB) 4 star‐block copolymers with high molecular weight ( M w ) of 136 kg mol −1 exhibited the strongest adhesion of 3.15 and 2.67 MPa for the thermoset and photoset specimens, respectively, while maintaining a relatively low residual adhesion strength of 0.45 MPa.…”

Fabrication of Photoactuators: Macroscopic Photomechanical Responses of Metal–Organic Frameworks to Irradiation by UV Light

“…Generally,inorganic and crystalline materials are brittle in comparison with soft materials [33] and thus in isolation they are unlikely to fulfill au seful technological role in which predictable macroscopic responses to irradiation are required. We describe herein am acroscopic photoactuation device in which the photoactive MOF is combined with polyvinyl alcohol (PVA) to form acomposite membrane.Itis known that the orientation and distribution of photoresponsive moieties in composite membranes can play acritical role in modulating the photomechanical actuation.…”

Semicrystalline poly(vinyl ether)s with high and phototunable glass transition temperature: application for thermally stable and reworkable adhesives

“…The intensity, wavelength, and polarization of light facilitate precise control of the reactivity, selectivity, geometry, and other properties . Diverse applications for photoinduced solid–liquid phase transition materials have been proposed, for example, photopatterning, photoactuators, transfer printing, self‐healing materials, remote shutters, energy storage materials, and phototunable adhesives …”

“…There are only a few polymers reported, which exhibit a polymer glass–liquid phase transition: azobenzene‐based poly(meth)acrylates and a cross‐linked liquid‐crystal (LC) polymer with an azobenzene‐based cross‐linker . The mechanism of the phase transition was based on glass‐transition temperatures ( T g 's) that were above room temperature for the trans ‐form and below it for the cis ‐form . As polymer structure–phase transition behavior relationships are of fundamental interest, the effect of the length of methylene spacers, connecting the polymerizable group and the side‐chain azobenzene moiety, was investigated .…”