The Directional Assembly Of Bacterial Cellulose Nanofibers For Optical Retardation Materials

Bacterial cellulose（BC）is a kind of cellulose biosynthesized by bacteria.Compared with plant-based cellulose,it does not contain lignin,hemicellulose,fructose and other components.Due to its higher purity,higher crystallinity,higher elastic modulus,excellent biocompatibility and biodegradability,BC has gained remarkable attention and been widely studied.However,BC synthesized by conventional methods has a disordered and isotropic essentially three-dimensional network structure,which greatly limits its certain practical applications in many fields,such as mechanics,optics,thermal science,electrochemistry and so on.Therefore,it is of great significance to realize the macroscopically order by inducing the alignment of BC nanofibers.So far,many classic methods for orienting BC have been proposed,such as addictive induced alignment（sodium carboxymethyl or lipid compounds usually）,rotation shear induced alignment,spinning with wet drawing,electrical field alignment,magnetic field alignment,directional freezing and template alignment derived with the development of nanotechnology.Although all aforesaid methods have their own advantages and disadvantages,we cannot obtain a high-orientation BC in one-step easily,which is still a challenge.In addition,due to pseudo-nematic structure of oriented BC,similar to traditional nematic materials,it may have the ability of retardation.Based on the above,focusing on the high-orientation BC aerogels and the retardation of transparent high anisotropic BC films,the following works have been done in this work:（1）In the second chapter,before static culture of BC,a certain amount of cellulose nanocrystal（CNC）suspension was added to the culture medium.The BC biosynthesized assisted with CNC was freeze-dried into an aerogel,and its uniaxial orientation structure was proved by scanning election microscopy（SEM）.The degree of orientation is related to the amount of CNC.and the result shows that it increases first and then decreases with the content of CNC increasing.When the amount of CNC is 0.14 wt%,it reaches as high as 0.9682.Moreover,we found the orientation tends to get worse when the charge of added CNC decreases,and its functional group is not decisive.In a word,an in-situ CNC-assisted method is successfully developed for fabricating the oriented BC aerogels with controllable orientation in one step.（2）In the third chapter,on the basis of previous work,we draw along the orientation first,and then fill the voids of the aerogel with epoxy resin.Thus,the transparent high anisotropic BC film was prepared successfully due to the similar refractive index.Due to its pseudo-nematic structure,the transparent BC film has the ability of retardation,and it can be expressed by birefringence index（Δn）of the material,which depends on its degree of orientation.When the amount of CNC is 0.14 wt%,it shows the best orientation and largest birefringence index(2.1×10^-3).When the thickness of sample is appropriate（50μm）,it shows half-wave retardation at near 440nm（340-520 nm）,which makes the chirality flip.Compared with most conventional retardation devices made by liquid crystal materials,BC is cheaper and more environmentally friendly.Therefore,the successful attempt to explore the retardation capability of BC in this work is of great significance.