Controllable Core-shell Structure Of Chitosan Based Nanofiber By Emulsion Electrospinning

Chitosan(CS),the second largest amount behind cellulose in nature,is an environmentally friendly natural polymer material.Due to the existence of many active groups such as hydroxyl and amino groups on its molecular chain,CS has become a multifunctional polymer material,which has broad application prospects in many fields such as tissue engineering,drug controlled release,food packaging and environmental remediation.Electrospun CS nanofibers have high specific surface area,porosity,and continuous production,which achieve their functionality and are widely used.However,due to its poor solubility and unique polycationic properties,it is difficult to electrospin CS into fibers.In addition,CS has poor mechanical properties.The modification of CS by chemical method is a way to improve its spinnability.While during the modification process,the active group amount of CS would decrease.Preparation of CS-based blended fibers is another effective way for CS further use.Polycaprolactone(PCL)is a widely used biopolymer in drug sustained release and tissue engineering.It is an environmentally friendly synthetic polymer material.It has good mechanical properties,no toxic processability,biocompatibility,and eco-friendliness.However,PCL and CS have the poor compatibility,and there is also difficult to find the co-solvent for electrospinning.In this paper,PCL/CS composite nanofibers with controllable core-shell structure were prepared by emulsion electrospinning.The effects of solvent and polymer systems on the emulsion droplets size and stability of electrospun emulsion without the addition of any surfactants were studied.The structure and morphology of CS based composite fibrous membranes were analyzed.The main research contents include the following two parts:(1)CS and PCL,two kinds of environmentally friendly polymer materials,were used as electrospinning polymers,and low-toxic solvents formic acid(FA)and dichloromethane(DCM)were selected as solvent systems for electrospinning emulsion.The stability of emulsion with different ratios of solvent and polymer was observed and studied.The size of the emulsion droplets with different weight ratios of PCL/CS dissolving in FA/DCM solvent system with a volume ratio of 7/3(v/v)was observed with an optical microscope that the observation showed the emulsion droplets size gradually decreased with the decrease of CS content.The PCL/CS emulsion had the best stability in the solvent system with a FA/DCM volume ratio of 7/3(v/v).PCL/CS core-shell structured nanofibers were prepared by emulsion electrospinning.The microstructure of fiber was observed by FE-SEM and TEM.The results showed that with the increase of PCL content,the average diameter of fibers increased,the diameter distribution became wide,and the core-shell diameter ratio decreased.The EDS,XPS and water contact angle(WCA)tests were used to characterize the distribution of components in the fiber.PCL/CS fibers with different core-shell diameter ratios could be obtained by adjusting the solvent and PCL/CS content.Further,CS ultrafine nanofibers with an average diameter of 143 ± 49 nm were successfully prepared by removing the PCL shell from the PCL/CS core-shell fibers.(2)PCL was modified with a hydrophilic polymer polyethylene glycol(PEG)to synthesize block polymer(PCL-b-PEG).In order to further improve the uniformity of the emulsion droplet diameter,the solution blend of PCL-b-PEG and CS were electrospun.The affinity between the PEG segment and CS molecules could enhance the stability of the emulsion via improving the compatibility between the continuous phase and the dispersed phase.PCL-b-PEG/CS core-shell fibers were prepared by electrospinning technology.The effect of PEG segments with different molecular weights on the emulsion was observed by optical microscope.The results showed that comparing with PCL/CS emulsion,the emulsion droplet diameter of PCL-b-PEG/CS emulsion was smaller and the diameter distribution for the emulsion droplet was more uniform.The emulsion droplet size of long chain PEG(5K)segment was smaller than that of PEG(2K).The effects of different molecular weight PEG segments on the surface morphology and internal structure of the fibers were observed by FE-SEM and TEM.Compared with PCL/CS fiber,PCL-b-PEG(2K)/CS fiber had smooth surface,uniform diameter,and smaller core-shell diameter ratio.The surface morphology of PCL-b-PEG(5K)/CS fibers was greatly affected by the polymer concentration.When the content of PCL-b-PEG(5K)was low,a beaded structure appeared on the fiber,and the core-shell structure disappeard.When the content of PCL-b-PEG(5K)was high,the viscosity of electrospun emulsion was large and the fiber can not be obtained.The EDS and WCA tests were used to characterize the distribution of the components in the fiber,and it was proved that the shell component of the fiber was PCL-b-PEG and the core component was CS.The above research showed that CS based electrospun fibers with controllable core-shell structure could be prepared by emulsion electrospinning via changing the content and compatibility of core-shell components.