Correlation between processing parameters and microstructure of electrospun poly(D,l-lactic acid) nanofibers

Using dimethyl formamide as the solvent, electrospinning of poly(D,l-lactic acid) (PDLLA, d-lactide content:10%) solutions with various concentrations was performed by means of a heating jacket for controlling the solution temperature range from 25 to 104 °C. In addition, an IR emitter was used to control the surrounding temperature at ∼110 °C. The effects of solution properties and processing variables on the morphologies of the cone/jet/fiber were investigated, and the internal structure of the electrospun fibers was characterized using polarized FTIR, WAXD and DSC. A sufficient entanglement density existing in a given solution was an important requirement for successfully obtaining uniform fibers without beads. The log-log plot of specific viscosity (η_sp) versus PDLLA volume fraction (φ{symbol}_v) provided us with a useful guideline to determine the entanglement concentration (c_e) for preparing fiber-shaped electrospun products. The φ{symbol}_v-dependence of η_sp varied from η_sp ∼ φ{symbol}_v ^1.1 for a dilute solution to η_sp ∼ φ{symbol}_v ^4.7 for a solution possessing entangled chains. From the incipient concentration of entanglements, the determined c_e was ∼10 wt%, which was in fair agreement with what was predicted theoretically by a simple relation of 2M_e/M_w, where M_e and M_w were the molecular weight between melt entanglements and the average molecular weight of PDLLA, respectively. To obtain uniform PDLLA fibers without beads, however, a minimum concentration of ∼1.9c_e was required for the entangled solutions possessing sufficient network strength to prohibit the capillary instability during jet whipping. The log-log plots of the jet diameter (d_j) and fiber diameter (d_f) versus zero shear viscosity (η_o) showed two scaling laws existing for the present solution, that is, d_j ∼ η_o ^0.07 and d_f ∼ η_o ^0.45. For a given solution, an intimate relation between d_j and d_f was derived to be d_f ∼ d_j ^0.61, regardless of the variations of processing variables applied. High-temperature electrospinning produced small diameter fibers because of the reduction of η_o, but the effect was gradually diminished for solution temperatures higher than 56 °C owing to the enhanced solvent evaporation. The as-spun nanofibers of this thermally slow-crystallizing PDLLA species were amorphous, and the Hermans orientation function calculated from the polarized FTIR results was ca. -0.063 regardless of the electrospinning conditions applied. This suggests that there was no preferential chain orientation developed in the nanofibers. In the heating in a DSC cell at a rate of 10 °C/min, however, rapid crystallization took place at 97 °C, followed by two well-separated melting endotherms centered at 121 and 148 °C, respectively. WAXD and FTIR results exhibited the exclusive presence of α-form crystals. These unique features were attributed to the occurrence of phase separation during electrospinning, which interrupted the chain orientation along the fiber during jet stretching, and yielded more trans-trans conformers with more extended chain structure to readily facilitate the cold crystallization during post-heating.