Preparation And Morphology Control Study Of Electrospun Poly(Lactic-acid) Fibers

First,the influence of mix-solvents(chloroform/ethanol) on the electrospinng was studied.The poly(lactic-acid)(PLLA) was dissolved in mix-solvents with different ratios of 100/0,95/5,85/15,75/25,65/35,45/55(v/v).The volume composition of ethanol in mix-solvents should not be more than 55%;otherwise,homogenous polymer solution cannot be obtained,since ethanol is the nonsolvent of PLLA. Polymer solutions were prepared by dissolving PLLA in mix-solvents at concentrations of 4wt%,6wt%,8wt%,and were electrospun with different processing parameters.The results showed that,the ratios of nonsolvent/solvent had a great effect on the spinning ability of PLLA:for PLLA solution with concentration of 4wt%,if the solvent was pure chloroform,many droplets were ejected from the spinneret,and no fibers were formed.However,when mix-solvents(chloroform/ethanol) were used, continuous fibers were obtained,and at the same time,with increasing the volume of ethanol in mix-solvents,the spinning ability was getting better.After the volume ratio of ethanol in mix-solvents up to 55%,the beads of fibers disappeared,and the diameters of fibers were uniform.Considering the end-use application of PLLA electrospun fibers in the tissue engineering and mechanical areas,the property studies of electrospun fiber mats are necessary.The effects of the ratio of mix-solvents and the concentration of polymer solutions on the diameters and distributions,the thermal properties,the crystal ability, surface wetting properties,mechanical properties of fibers were investigated by SEM, TGA,XRD,contact angle and mechanical measurements,and these properties of membrane were also studied by comparison.The results showed that,with the increasing of ethanol in mix-solvents,the diameter of fibers decreased gradually,the diameter distribution became narrow;the thermal properties of membrane and electrospun fiber mat were different,the volume composition of ethanol in mix-solvents had a little effects on the thermal properties of fiber mats;the crystallinity for membrane was 44.8%,for electrospun fiber mat was 20.8%,the volume of ethanol and the concentration of solution had a little effect on the crystallinity;the contact angle for membrane was 79.2°,for electrospun fibers was more than 120°,and it increased with the increasing of fiber diameter.The intensity for membrane was 21.33Mpa,for electrospun fiber mat was 1.2Mpa,the diameter and the format of collected fibers had some effects on the intensity of fiber mats.In order to find how the mix-solvents affect the spinning ability of PLLA,the solutions properties of PLLA in different mix-solvents were studied.These included the intrinsic viscosity of PLLA in different mix-solvents,the conductivities,the surface tensions and shear viscosities of PLLA solutions at different concentrations of 4wt%,6wt%and 8wt%.The results showed that,with the increasing of ethanol in mix-solvents,the intrinsic viscosity was getting higher at the early stage.After the volume composition of ethanol in mix-solvents was higher than 15%,the intrinsic viscosity of PLLA in mix-solvents was decreasing gradually.With the increasing of ethanol in mix-solvents,the conductivity increased gradually,but the max value was only 1.2μs/cm;the surface tension decreased firstly,and increased latterly;the shear viscosity decreased gradually.The analysis of results showed that,the reason of solution spinnability getting better was that,the addition of ethanol enhanced the entanglement of macromolecules,and at the same time increased the conductivity of polymer solution.The changes of surface tension and viscosity had little effect on the modified spinnability.In the former part of this study,the effect of the ethanol volume in mix-solvents was discussed.In order to enlarge the application of PLLA electrospun fibers, controlling the surface and the interior structure of fibers is necessary,so a facile method for fabricating porous fibers was presented.Experimental results demonstrate that,porous fibers can be easily achieved if the solvent is more volatile than the nonsolvent in the spinning solution,and the applied voltage is lower enough. Increasing the ratio of nonsolvent/solvent close to 40/60 is necessary if high porosity electrospun fibers are desired.The processing temperature should be lower enough to avoid the coalescence phenomenon.The large surface area and rough surface structure make these porous biodegradable polymer fibers to be ideal candidates for drug delivery and tissue engineering.