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Xiaoyan Yuan, Chunxue Zhang, Bin Duan, Cunhai Dong, Yunhui Zhao
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P.R. China

　Recently much work has focused on electrospinning that can prepare fibers with diameters ranging from 5 nm to 500 nm, one hundred to ten thousand times smaller than those prepared by traditional methods of solution or melt spinning. Because of the small size of the fibers, the electrospun fibrous membranes exhibit high specific surface area and porosity, and show potential applications in biomedical devices, nano-templates, molecular composites, and so on [1-3]. In our studies, ultrafine fibers of poly(vinyl alcohol) (PVA), poly(d,l-lactide) (PLA) and chitosan were prepared by electrospinning respectively from their solutions for biomedical purposes.

EXPERIMENTAL METHODS
　PVA with 1750±50 of polymerization degree and 98% of hydrolysis degree was supplied by Beijing Organic Chemical Engineering Factory, China. Amorphous PLA with an inherent viscosity of 1.43 dl/g was purchased from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. A sample of crab shell chitosan with 654 kDa of viscosity-average molecular weight and 90% of deacetylation degree was purchased from Qingdao Hisound Biological Engineering Co., Ltd., China. Aqueous PVA solutions, PLA solutions in N,N-dimethylformamide (DMF), and chitosan solutions in 2 wt% acetic acid were prepared for electrospinning. The ultrafine fibrous membranes of PVA, PLA and chitosan were prepared directly on the grounded aluminium collector. Further characterizations by scanning electron microscopy, differential scanning calorimeter, tensile test and etc. were performed.

RESULTS AND DISCUSSION
　The ultrafine PVA fibers with diameters of 100 nm∼250 nm were prepared by electrospinning from an aqueous PVA solution with 7 wt% concentration. The fibrous PVA membranes containing aspirin or bovine serum albumin were also prepared as model drug delivery vehicles. The tensile modulus and tensile strength of the electrospun PVA membranes were 236±27 MPa and 5.10±0.77 MPa respectively, indicating poor mechanical properties compared with the casting PVA film.

　The PLA solution with 22 wt% concentration in DMF was suitable to prepare ultrafine PLA fibers by electrospinning. Entrance of an organic salt, triethylbenzylammonium chlorate, led to a great increase in the conductivity of PLA solution, so that the average fiber diameter of the electrospun PLA fibers decreased dramatically from 500 nm to 200 nm. Addition of a surfactant, Span-80, did not improve the fiber morphology but formed beaded fiber web.
　The ultrafine chitosan fibers with diameters from 80 nm to 180 nm were generated by addition of half of poly(ethylene oxide) (PEO) in mass to a 4 wt% chitosan solution. Meanwhile, microfibers with visually thicker diameters could be formed as well. Results of X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and differential scanning calorimeter exhibited the larger electrospun microfibers were almost entirely made from PEO, while the electrospun ultrafine fibers mainly contained chitosan. The electrospun membranes of chitosan as well as PLA would be potentially applied for tissue regeneration.

References
[1] Matthews JA, Wnek GE, Simpson DG, et al. Biomacromolecules, 2002, 3: 232-238.
[2] Fang X, Reneker DH. J Macromol Sci-Phys, 1997, B36: 169-173.
[3] Kenawy ER, Bowlin GL, Mansfield K, et al. J Control Release, 2002, 81: 57-64.

÷畚睦叢矗篒nternational Symposium on Polymer Chemistry,June,2004