Concentration
The polymer concentration in the solution has great effects on the structure of electrospun nanofibers owing to the resulting significant variation in the viscosity, which have been widely studied by researchers. Bead-on-string fibers are usually formed from solutions with low polymer concentration, which could be ascribed to the low viscosity. With increasing solution concentration, the spherical beads change to spindlelike beads and then form uniform nanofibers, and the fiber diameter increases accordingly. Similarly, changes in solution concentration result in different structures of nanonets during the electronetting process. Ding et al. first fabricated PAA nanonet membranes from solutions with various concentrations and then investigated the structures of the nanonets systematically. The nanonet membranes obtained from 6 wt% PAA solution had the largest pore size and nanowire diameter deviation, while the pore size of the nanonets formed from 8 wt% solution decreased, and their shapes became uniform. Moreover, with further increasing PAA concentration, the resultant membranes showed the smallest average pore size compared with the two other membranes. Then, Zhang et al. found that PA-6 nanofibers obtained from 5 and 7.5 wt% solutions showed microsized beads along the fiber axis, due to the low solution viscosity. As shown in Fig. 8.9AeD, when the PA-6 concentration was higher than 10 wt%, the beads disappeared and a handful of nanonets appeared in the membranes, as indicated in the dashed circle. In particular, the coverage rate of nanonets in PA-6 membranes obtained from a 12.5 wt% solution increased sharply, which could be ascribed to the increased conductivity and reduced surface tension of the solution (Fig. 8.9).
