Crystallization Behavior Of Electrospun Poly (L-Lactic) Nanofibers

Electrospinning is widely accepted as a simple and versatile technique to produce nanofibers due to its decisive advantage in preparation of continuous and uniform nanofibers. The technique has been used in tissue engineering, protective material and environmental engineering. However, formation of PLLA crystals has been greatly restrained during the electrospinning because of the retardation of high elongational rates and rapid solidification process. Low crystallinity limited their applications. More and more attentions have been taken to proclaim the crystallization behavior of electrospun nanofibers. Up to now, there is not a systematic study about internal structure of polymer nanofibers, especially the transformation of comformational order during the crystallizing process.In this work, semi-crystalline PLLA was prepared through electrospinning to form ultrafine nanofibers. Normal analytical approachs such as XRD, DSC and FTIR were used to show crystal structure in PLLA nanofibers. One new method --in situ FTIR was introduced to trace the change of conformational order of PLLA chains when heating process at the first time.Experimental dates have shown that the chains in PLLA nanofibers are non-crystalline, but highly oriented, which leads to crystallization happened at lower temperature. DSC analysis detects the evident decrease of some thermodynamic quantity (Tg, Tc) for PLLA electrospun nanofibers. Complementally, some processing parameters including drawing rate of rotating drum, relative molecular mass of PLLA and solution concentration are introduced to clarify the internal structure of PLLA nanofibers. Highly drawing rate was found to be one of the dominant factors to induce orientation of PLLA chains. Experiment data shows that crystallinity of PLLA nanofibers has improved through decreasing of relative molecular mass and solution concentration. It shows different crystallizing mechanics between PLLA solution cast film and the electrospun nanofibers. In the case of solution cast film, the conformational ordering begins with -CH3 interchain interactions during the crystallization. Conversely, C-O-C and -CH3 interchain interactions in electrospun nanofibers, which may be explain in terms of the highly orientation of PLLA chains. Complementary, effects of CNTs to induce crystallization of PLLA nanofibers were investigated preliminary. Addition of CNTs could improve the crystallization properties as much as the chain orientation. However, exceeding nanoparticles will hider the movement of polymer chains.