The Control Of Surface Morphology On Poly(Lacticacid) Composite Nanofibers And Its Effect On Properties Of Nanofibrous Mats

In this work, the poly(lacticacid)/poly(ε-caprolactone)(PLA/PCL) composite nanofibers with parallel line surface texture were successfully fabricated by electrospinning technique. The nanofibers had parallel-like textures surface which were different from the general fibers with smooth surface. In order to investigate the formation mechanism of surface texture structure on nanofibrous surface, we changed different influential factors in the electrospinning process and observed the change of the surface morphology. The results showed that the solution which sprayed from the nozzle translated into filament under high voltage electric field in electrospinning. The polymer-rich and solvent-rich regions were caused by temperature induced phase separation(TIPS) and vapor induced phase separation(VIPS). Then the solvent rich phase evolved into pores after the solvent rapidly evaporated, the formed voids were elongated into oriented lines by electric field force, and the nanofibers solidified to form line texture nanofibrous mats. The tests found that the morphology of nanofibers could be influenced by electrospinning processing parameters, such as solvent, temperature, humidity, voltage and receive distance. Due to PCL and PLA had already been proved to be thermodynamically immiscible, their molten blend mats form macrophase separation and the phase separation morphology in composite nanofibers could be conformed by atomic force microscope(AFM)and polarized optical microscope(POM). In the micrographs of AFM, the surface of nanofiber showed typical dispersion phase structure, in which the dark and bright parts corresponded to the discrete PCL phase and the continuous PLA phase, respectively. This result elucidated that a separated phase existed in the electrospun PLA/PCL nanofibers. The sample of PLA/PCL nanofibrous mat was first heated to 190 ℃ and then cooled to 80 ℃ rapidly, the nanofibers presented two phase morphologies observed by POM, in which the dark dispersed droplets corresponded to molten PCL phase, and the bright continuous phase corresponded to frozen PLA.The results are consistent with that of AFM, indicating that PLA and PCL existed separated phase in nanofibers.The macrophase separation between PLA and PCL may influence the mechanical properties of the nanofibrous mats, we added MWCNTs acting as compatibilizer to improve thecompatibility of the immiscible blend. The morphology of nanofibers could not be influenced by the addition of MWCNTs which could be observed by scanning electron microscopy(SEM), the average numbers of block dots in single PLA/PCL fibers with MWCNTs decreased which could be observed by POM. These results indicated that the presence of MWCNTs improved the compatibility of the immiscible PLA and PCL. Moreover, the images of AFM showed that the morphology of PCL phase changed from elongational structure to dispersed phase and the sizes of PCL domains reduced evidently which were consistent with that of POM, indicating that the compatibility of PLA and PCL were improved. In addition, amphiphilic MWCNTs acting as heterogeneous nucleation improved the crystallization degree of PLA in the blend, as demonstrated by differential scanning calorimetry(DSC) and X-ray diffraction(XRD). The mechanical tests showed that MWCNTs played a role in reinforcement improving tensile strength of the blend, and the decrease in elongation at break was due to fillers impeded the motion of matrix molecule chains. The results of proliferation revealed that the nanofibrous scaffolds with MWCNTs had no toxic effect on the proliferation of NIH3T3 mouse fibroblasts cells, making nanofibrous scaffolds with surface texture potential substrates for tissue engineering.The PLA-PCL composite material had good mechanical properties and distinctive degradation, however, it was short of cell recognition sites which was to disadvantage of the cellular growth and proliferation. The nature silk fibroin(SF) was added into PLA-PCL blend, the different mass ratios of polylactide-poly(ε-caprolactone)/silk fibroin(PLA-PCL/SF) composite nanofibrous mats were fabricated by electrospinning. The surface morphology of PLA-PCL/SF nanofibers changed from parallel-like textures to smooth with the increasing SF amounts. The content of SF was 10-50 %, the morphology of composite nanofibers treated by methyl alcohol presented high density of pores and the sizes of pores decreased with the addition of SF. When the contents of SF exceeded 50 %, the morphology of nanofibers gradually became smooth, the structure and properties of composite nanofibrous membranes were mainly influence by SF which can be characterized by Fourier transform infrared spectroscopy(FT-IR) and DSC. The porosity and adsorption rate of the nanofibrous mats decreased with the increasing of SF content in composites. The water contact angle test and mouse fibroblasts cells(NIH3T3) culture indicated that the hydrophilicity of materials were improved with the addition of SF whichsupported the cell adhesion and proliferation on composite nanofibrous mats.