Abstract:

Objective Poly L-lactic acid (PLLA) nanofiber membrane with porous fibers has excellent adsorption and filtration properties, and it is widely used in biomedicine, flexible sensors, filtration materials and other related fields. However, the preparation of the PLLA nanofibers with controllable pore structure and high porosity is still a challenge.

Method With PLLA as raw material, chloroform and acetone as solvents, and bupleurum as additives, the multipored PLLA nanofibers were successfully prepared by phase separation method based on electrospinning technology. The microstructure, molecular structure and porosity of the PLLA nanofibers were characterized by thermal field scanning electron microscopy, fourier transform infrared and automatic surface and porosity analyzer.

Results The surface of virgin PLLA fiber was relatively flat, with no obvious pores. When the mass fraction of bupleurum was set to 1%, 2%, and 3%, a large number of pore structures appear on the fiber surface in PLLA fiber membrane, indicating that the addition of bupleurum additive greatly improves the fiber porosity (Fig. 2). When the mass fraction of radix bupleurum was at 2%, the porous structure in the fiber became more obvious, and the porosity is 70.98%. Bupleurum did not participate in the change of the chemical bond of PLLA in the blending electrospinning process, and the structure of PLLA macromolecule did not change significantly, which did not affect the functional groups. The additional functional groups of bupleurum were not introduced into the PLLA fiber membrane, but affected the speed of phase separation, which was conducive to the formation of pore structure. Under the condition of keeping the mass fraction of bupleurum mass fraction at 2%, the pore structure of fiber membrane prepared with different solvent mass ratios was obvious(Fig. 5). As the chloroform/acetone mass ratio was changed from 5∶1, 6∶1, 7∶1 to 8∶1, the fiber porosity membrane gradually increases(Tab. 1). When the chloroform/acetone mass ratio was altered from 8∶1, 9∶1 to 10∶1, the fiber membrane porosity gradually decreases. This indicated that the solvent mass ratio is an important factor affecting the fiber porosity, and when the solvent mass ratio is 8∶1, the fiber membrane porosity reaches the maximum (82.09%). The porosity of fiber membrane was increased from 6% to 9%, it is due to the increase in concentration of the mixed solution, which increases the viscosity of the solution, causing the originally collapsed pores on the nanofibers to grow into normal pores during the spinning process(Tab. 2). The concentration of PLLA in the spinning solution was found an important factor affecting the fiber pore structure, because the main structure of the fiber is composed of PLLA, and the change of the concentration of PLLA will directly affect on the degree of entanglement of the polymer molecular chain, determining the viscosity of the solution and ultimately whether the spinning can proceed normally.

Conclusion It is confirmed that adding bupleurum to NSS system can improve the porosity of fiber membrane. When the mass fraction of bupleurum is set at 2%, the mass ratio of chloroform to acetone is 8∶1, and the concentration of PLLA is 9%, the porosity of the fiber reaches the maximum value of 82.09%. When the porosity of the fiber membrane greatly increases, PLLA nanofiber membrane can be used in fields such as oily wastewater treatment and medical masks.

Key words: poly-l-lactic acid, nanofiber membrane, electrospinning, bupleurum, porous fiber, non-solvent solution system, pore-forming mechanism