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 degrees C. In addition, an IR emitter was used to control the surrounding temperature at similar to 110 degrees 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 (eta(sp)) versus PDLLA volume fraction (phi(v)) provided us with a useful guideline to determine the entanglement concentration (c(e)) for preparing fiber-shaped electrospun products. The phi(v)-dependence of eta(sp) varied from eta(sp) similar to phi(1.1)(v) for a dilute solution to eta(sp) similar to phi(4.7)(v) for a solution possessing entangled chains. From the incipient concentration of entanglements, the determined c(e) was similar to 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 similar to 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 (eta(o)) showed two scaling laws existing for the present solution, that is, d(j) similar to eta(0.07)(o) and d(f) similar to eta(0.45)(o) For a given solution, an intimate relation between d(j) and d(f) was derived to be d(f) similar to d(j)(0.61), regardless of the variations of processing variables applied. High-temperature electrospinning produced small diameter fibers because of the reduction of eta(o), but the effect was gradually diminished for solution temperatures higher than 56 degrees 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 degrees C/min, however, rapid crystallization took place at 97 degrees C, followed by two well-separated melting endotherms centered at 121 and 148 degrees C, respectively. WARD and FTIR results exhibited the exclusive presence of alpha-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. (C) 2009 Elsevier Ltd. All rights reserved.