Preparation And Stimuli-responsiveness Of Azobenzene-based Polymer Film

Stimuli-responsive polymer film is a category of smart polymer materials which is sensitive to external stimuli.The chemical or physical structure of the polymer film can undergo reversible or irreversible changes after receiving the stimulus signals of environmental changes(such as heat,light,p H,electricity,moisture),so that the function or behavior of the system can be transformed to adapt to the changes of the external environment.Thus,this kind of polymer film material can be widely applied in intelligent sensors,flexible robots,separation,catalysis and controlled drug release.Therefore,it is a practical and challenging task to design and prepare stimuli-responsive polymer film materials with stable mechanical properties,high sensitivity,controllable response behavior and multiple functions.Azobenzene compounds are the most typical and commonly used photosensitive molecules.The photo-responsive mechanism is that the azobenzene molecule will undergo photoisomerization reaction upon UV irradiation,transforming from stable trans-isomer to metastable cis-isomer,and the cis-structure will revert to trans structure upon visible light irradiation or heating.Therefore,these photosensitive compounds are frequently introduced into the polymer structure to construct photo-responsive polymer materials with good stability and responsive performance.Based on the photoresponsive properties of azobenzene compounds,this paper discusses the preparation methods of two kinds of azobenzene-based stimuli-responsive polymer films,and explores their responsive performance and mechanism.Firstly,a multiple responsive polymer film HA@BAHA was prepared by free radical copolymerization of hydrophilic monomer and azobenzene crosslinking agent.The azobenzene groups has the properties of reversible photo-induced cis-trans isomerization,reversible acidinduced azo-hydrazone tautomerism and the photothermal effect,synergistic with the hygroscopic of hydrophilic groups,endowed the film with multiple responsiveness including acidochromism,reversible elastic-plastic transformation,shape memory,shape memory and in-situ swelling.Secondly,a photo responsive non-crosslinked liquid crystal polymer film HPDPHA@HMCS was prepared by free radical copolymerization of cholesteric liquid crystalline monomer with azobenzene liquid crystalline monomer,and the polymer chain and side-chain liquid crystal mesogens of the polymer were highly oriented by mechanical stretching.The orientation of liquid crystal elements in the stretched films was perpendicular to the direction of stretching,and the free volume expansion of the oriented azobenzene mesogens leaded to a bending away from the light source upon UV irradiation.In addition,the oriented noncrosslinked films possessed high responsive performance,and the photo-induced bending behavior of the film was reversible and repeatable,which could realize a variety of reversible photodynamic behaviors in both air and solution environments.