Novel responsive materials based on reactive Liquid Crystals: from complex microparticles to thermal shielding

Fifty years after their initial theorisation, Liquid Crystalline Networks (LCNs) continue to attract great attention in the scientific community for their application as smart materials in numerous fields, from soft actuators to photonics. Indeed, LCNs possess characteristics such as anisotropic properties, long-range molecular order and responsiveness to external stimuli that are difficult to find in other soft materials. Monodomain LCNs, characterised by a macroscopically uniform nematic director, are well known for undergoing rapid and reversible deformation in response to stimuli including temperature variation, irradiation with light or the presence of magnetic fields. One of the most explored preparation techniques involves the photopolymerization of an aligned layer of reactive mesogens (e.g. acrylate based liquid crystals). Following this approach, photoresponsive side-chain polymers have been widely described as fast artificial muscles, while a detailed comparison of light-responsive LCNs with different architectures performances as actuators is not properly addressed. In this thesis, we focus on several synthetic strategies to develop LCN in different shapes, from polymeric films to micrometric particles, tailoring the need of different applications. In the first part of this study, we take in consideration the optimization of photoresponsive actuators as artificial muscles. Two synthetic approaches, involving respectively an Aza-Michael addition and a thiol-ene click reactions, are exploited to obtain photoresponsive LCNs with main-chain and mixed main-chain/side-chain architectures. The materials’ performances in terms of thermo- and photo-actuation were quantitatively characterized and compared with the ones of classical acrylate-based polymers. Then, we report a simple and scalable bulk-emulsification strategy to fabricate LCN microparticles spanning from single, double (Janus), and triple emulsion morphologies. Across morphologies, the particles exhibit reversible, large-amplitude deformations under heating, as well as chemoresponsivity through anisotropic swelling in organic solvents. Moreover, complex morphology enables new key features like multiple actuations modes and gravitational self-orientation. As a proof of concept, we exploit these responses to realize adaptive microlenses with thermally tuneable focal plane and magnification. In the third part of this work, we focus on polydomain Liquid Crystalline Elastomers (LCEs) with thermo- and mechanoresponsive optical properties. Application of a tensile stress on the polymeric films allows to transform these opaque materials into transparent monodomain ones, thus offering a possible solution for privacy mode interior windows or thermal shielding. Indeed, by appropriately choosing the initial stress applied on the LCE, we demonstrate a high reversibility of both shape deformation and transmittance.