The Bridge Between Responsive Moieties And Reversible Deformation: Liquid Crystal Polymer Network

Sophisticated movement in living organisms,such as the elongation of muscle and the beating of heart,is essentially cooperative action of biological molecules.To emulate living organisms,chemists have attempted to harness molecular motion in a concerted manner and transform it into sophisticated movement at macroscopic level.The employment of responsive and dynamic molecules has demonstrated the implementation of functionality and adaptability in artificial polymers.The rapid development of bio-inspired polymers challenges the current functionalities of polymer composites.The aim of this dissertation is to integrate responsive and dynamic moieties into liquid crystal polymer network capable of reversible and sophisticated deformation.To optimize polymer network and implement functionalities of intelligent polymers,aggregation-induced emission molecule,near infrared absorbing dye,photo-responsive azobenzene,and electrofluorochromic molecule are respectively incorporated into liquid crystal polymer network in this dissertation.The combination of aggregation-induced emission molecules and liquid crystal polymers that is recognized as classical two-way shape memory polymers is featured with both reversible fluorescence and shape morphing properties.The first part of this dissertation presents the synthesis of aggregation-induced emission moiety(TPE-10C)based on tetraphenylethene(TPE)crore and vinyl terminal group,which can be chemically incorporated into liquid crystal polymer network via acyclic diene metathesis polymerization/crosslinking.Due to the existence of aggregation-induced emission moieties,the fluorescence in functionalized liquid crystal polymer behaves similarly with fluorescent small molecules,which depends on swelling capabilities in various organic solvents.In organic solvents with strong swelling capability(such as chloroform and tetrahydrofuran),the fluorescence intensity of the polymer declines significantly.The fluorescence intensity can not only be controlled by altering the solvent,but also adjusted by varying the temperature.The elevated temperature active intramolecular motion of TPE-10 C,resulting in a decrease of fluorescence intensity;meanwhile,macroscopic deformation is also observed due to the phase transition from ordered liquid crystal to disordered isotropic phase.Therefore,temperature variation can be employed to alter macroscopic behavior,such as shape morphing and fluorescent behavior.Such temperature-and solvent-dependent fluorescent polymers act as potential building blocks in responsive optical elements.Benefiting from its extraordinary bio-capability and efficient penetration in living organisms,NIR light with long wavelength is one of the ideal candidates for tether-free remote triggers.Currently,NIR absorbing agents including mental nanoparticles,carbon-based materials,organic dye,and conjugated polymers are physically doped into polymers to construct NIR responsive polymers.However,the poor combability between NIR regents and organic polymer matrix sacrifices the mechanical property or photo-responsive speed of NIR regent/polymer composite.To tackle the aforementioned challenge,the second part of this dissertation is focusing on the synthesis of soluble and polymerizable NIR absorbing moiety,capable of chemically incorporating into liquid crystal network,which optimizes the both mechanical property and photo-responsive speed of NIR-responsive composites.The NIR absorption and polymerizable property are endowed in NIR absorbing dye due to the simultaneous presence of the conjugated thiophene-croconaine core and alkenyl terminal group.Based on these properties,functional liquid crystal polymer with high content of NIR absorbing dye is fabricated via acyclic diene metathesis polymerization/crosslinking.The functional liquid crystal polymer can transform the absorbed NIR light into heat because of photothermal effect,resulting in disordered mesogens and contraction along the longitudinal direction.Additionally,the functionalized liquid crystal polymer still maintains super strong mechanical properties in the isotropic phase,which has potential applications in photo-responsive actuators.The incorporation of photo-responsive moieties into polymer is of great importance for light responsivity,and typical photo-responsive moieties which can be employed in both wet and dry conditions are azobenzene derivatives.Although the cooperative bending motion in liquid crystal fibers based on azobenzene have been demonstrated for directional transportation,the moderate photo-response under ambient condition hinders its further application.To achieve efficient photo-response in fibers under ambient condition,an alternative strategy of optimizing the skeletal structure of liquid crystal polymer network and altering azobenzene location in polymer network is adopted in the third part of this dissertation.Azobenzene can be either incorporated into liquid crystal oligomer during the first step(Michael addition reaction),or introduced into liquid crystal polymer network during the second step(photo-polymerization/crosslinking).Due to the asymmetric contraction ratio along the fiber thickness,all fibers show a bending motion toward light under the illumination of UV light at room temperature,particularly for the fiber containing azobenzene in oligomer which responds to light more efficiently.The photo-induced bending behavior of liquid crystal fibers in wet condition is similar to the one in dry condition since the predominantly photo-chemical effect.In addition,the as-prepared liquid crystal fibers are also capable of light tracking and light guiding,which serve as a potential candidate for optical devices.The cooperative bending motion of cilia or fiber in the outer surface of living organisms is of crucial importance for nutrition capture,signal delivery and other life activities.Liquid crystal fibers in previous chapter have demonstrated the light-fueled bending toward UV light,hence,tunable bending direction in fibers is implemented in the fourth part of this dissertation.Through simplifying the composition of previous fibers,two smectic C fibers with same chemical composition and opposite bending direction are fabricated.When azobenzene is incorporated as crosslinker small molecule into polymer network,the fiber presents a bending behavior toward UV light because of the photo-contraction in the exposed surface.However,fiber bends away from UV light when the azobenzene is incorporated into main-chain liquid crystal oligomer,which is due to the photo-expansion in the exposed surface during smectic C to smectic A phase transition.Such fibers as light-fueled actuators have great potential application in directional as well as bi-directional transportation.Liquid crystal polymer network is endowed with electrochromism and electrofluorochromism via incorporating redox-active molecule.Responsive fluorescent materials featured with controllable photoluminescence have gained a lot of interest in the fields of anti-counterfeiting coatings,sensors,fluorescent imaging,optics,and electronic devices.Among all kinds of responsive fluorescent materials,electrofluorochromic materials based on redox reactions are capable of reversible and stable fluorescence regulation behavior.The fifth part of this dissertation is to synthesize liquid crystal monomer and crosslinker with redox active core.The smart polymer is endowed with reversible deformation,electrochromic and electrofluorochromic properties through the combination with liquid crystal polymers.