Research On Photoplastic Elastomers Based On Hexaarylbiimidazoles And Its Optomechanical Properties

Smart polymer materials can respond to external stimuli(temperature,light,humidity,etc.)and convert photochemical reactions at the molecular level into the change of macroscopic properties.The photochromic molecules are introduced into the polymer networks to prepare the smart polymer elastomers with photo-controlled deformation function.Compared with the traditional thermoplastic elastomers,this kind of new materials that allows for photoprocessing is named photoplastic elastomer.As a photochromic molecular switch with fast photochemical response characteristics,hexaarylbiimidazole(HABI)can be integrated into polymers and used for the manufacture of smart polymer materials.However,for the synthesis of HABI-based smart polymers,the reaction efficiency of traditional synthetic methods is low,and there are many applications in optomechanical properties need to be further developed.In this article,the functional HABI molecular is introduced into polymers with different network structures by adjusting the polymerization methods,and a series of intelligent polyurethane elastomers and hydrogel materials integrating photochromic,photo-induced self-healing and photo-controlled deformation functions have been prepared through simple and efficient reactions.It mainly includes the following four aspects:(1)The dihydroxy functionalized HABI molecules is introduced into linear polyurethane systems through copolymerization,a novel kind of photo-responsive linear polyurethane elastomer(HABI-PU)was designed and synthesized.The self-healing efficiency of the solvent-free HABI-PU elastomer materials has been greatly improved.When exposed to405 nm irradiation with the power density of 325 m W/cm~2,the scratches on the HABI-PU film take only 30 s to heal.Under the dual actions of light and external tension,the reversible dissociation of the C-N bond of the HABI units caused the scission of long polymer chains into short chains,and the polymer chains on the microscopic level are dislocated and rearranged,the HABI-PU elastomers are further endowed with the properties of photo-induced macroscopic deformation and fracture.(2)Typical cross-linked polymer networks have rubbery elasticity and do not exhibit thermoplasticity such as linear polymers,and do not allows for thermal processing.Linear polymers generally cannot achieve reversible deformation.Four hydroxyl-decorated HABI(2-Cl-4-diol-HABI)and glycerol were used as reversible dynamic and permanent static cross-linking agents respectively,the novel kind of dual-cross-linking photoplastic polyurethane(PPU)elastomers was prepared from the copolymerization.Upon 405 nm irradiation,the mechanical strength of the PPU materials decreases remarkably due to the reversible cleavage of dynamic HABI cross-linking points,PPU elastomers exhibit plastic deformation behaviors,and allowing for optical processing.Meanwhile,the PPU elastomers is endowed with the capability of reversible deformation because of the existence of the double cross-linking networks.The concept of"photoplasticity"was proposed.Acting like a photo-controlled springs,the photoplastic elastomers exhibit reversible elongation and contraction under the dual actions of light and external tension.When light is applied from one side of the PPU materials under the external tension,the PPU exhibits negative phototaxis effect,and the light is applied from the same side after removing the tension,the positive phototaxis effect of PPU elastomers is observed.The photo-driven open and closing of the biomimetic gripper constructed with this PPU materials can be realized.(3)Existing self-healing materials generally have the shortcoming of dependence on high intensity UV light.The photoplastic polyurethane(PPU)elastomers synthesized in the work of Chapter 3 is used for the self-healing experiments under white light.Compared with UV light,white light is easier to be available,has no potential harm to organisms,and is a mild trigger condition.Within 12 hours,the PPU elastomers can be completely healed under the irradiation of white light with a power density of only 4 m W/cm~2.After healing,PPU elastomers can still perform reversible photoplastic deformation repeatedly without significant attenuation of mechanical strength.Furthermore,the molar ratio of the dynamic2-Cl-4-diol-HABI cross-linking agent to the static glycerol cross-linking agent,the molecular weight of the poly(tetramethylene ether glycol),and the temperature will all affect the self-healing efficiency of the PPU materials,which can be further improved by optimizing the structure of the materials.(4)The driving speed of isotropic hydrogels based on photochemically reaction is generally slow,the transformation morphologies is limited,or could only be operated in specific acidic conditions.A new class of visible light-driven underwater biomimetic actuators are synthesized by the introduction of photo-responsive dynamic HABI crosslinking points and static pentaerythritol crosslinking points.The dual-covalently cross-linked network effectively promotes the mechanical stability and repeatability of PU gel materials.Further photo-induced secondary swelling occurs on the phototropic face due to the rapid cleavage of dynamic HABI cross-linking points in the polymer gels under 405 nm irradiation.Impressively,diversified three-dimensional(3D)deformation,such as photo-controlled curling,twisting and folding,photomechanical conversion and underwater biomimetic actuators can be spatiotemporally programmed and achieved within tens of seconds,which solves the limitation of the complicated pre-programming process of traditional hydrogel actuators during the synthesis procedure.Hexaarylbiimidazole molecule switchs is introduced into polymer materials to construct the multifunctional photo-driven actuators,which providing a new idea for the development of high-performance intelligent polymer materials.