3.1 Physical Properties of Spinning Solutions and Properties of PU/RE Fiber

The electrospinning effect and VOC adsorption capacity of the PU/RE nanofibrous membranes were determined by the physical properties of the spinning solutions and the properties of the PU/RE composite nanofibers. The specific surface area was measured by the N₂ adsorption-desorption isotherms test via the Brunauer–Emmett–Teller (BET) method. It was found that the viscosity, conductivity and specific surface area increased by increasing the amount of RE powder in the spinning solution. However, the fiber diameter first increased from 489 nm to 524 nm and then decreased from 524 nm to 356 nm when the relative content of the RE powder reached 50 wt. %. The PU nanofiber containing 50 wt. % of the RE powder had the smallest fiber diameter, which was 356 nm.

3.2 Morphological Characteristics of the PU/RE Nanofibrous Membranes

Figure 2 shows the morphology and the fiber diameter distribution of the composite PU/RE nanofibrous membranes. From Fig. 2a–d, it was observed that the changes of the fiber morphology and fiber diameter distribution correlated very closely with the increasing amounts of RE powder in the PU fibers. The pristine PU fibers had smooth surfaces and an average fiber diameter of 489 nm. However, it was found that there were a lot of RE NPs embedded in the PU fibers, and the number of RE NPs increased with an increased concentration of RE powder (Fig. 2b–d). In addition, some particle agglomeration existed in Fig. 2d because higher concentrations of RE powder cannot be completely dispersed into the spinning solution. Fig. 2e–h show the average fiber diameters of the PU, PU/RE-10, PU/RE-30 and PU/RE-50 nanofibrous membranes, which were 489, 513, 524 and 356 nm, respectively. Each of the reported PU/RE nanofibers’ diameters represents an average of 50 random fibers. The fiber diameters increased slightly as the amount of RE powder increased from 0 to 30 wt. %, and then showed a marked decrease when the relative amount of RE powder was 50 wt. %. The PU/RE-50 possessed the smallest average diameter of 356 nm.

Figure 2. FE-SEM images and diameter distributions of (a) polyurethane (PU); (b) PU/RE-10; (c) PU/RE-30 and (d) PU/RE-50.

3.3 Absorption Characteristics of the Composite PU/RE Nanofibrous Membranes

The VOC absorption characteristics of the PU nanofibers containing different amounts of RE powder (0 wt. %, 10 wt. %, 30 wt. %, 50 wt. %) are shown in Figure 3. As shown in Fig. 3, it can be seen that the pure PU nanofibrous membrane had a weak VOC absorption capacity (about 10%), while the pure RE powder had a better VOC absorption capacity compared with the pure PU nanofibrous membrane. However, the RE powder will limit its usability. To integrate the merit and eliminate the defect of both PU and RE, the RE-incorporated PU fibers were electrospun before the VOC absorption experiment. The VOC absorption capacity of the composite PU/RE nanofibrous membranes increased as the amount of RE powder in the PU fibers increased. The PU nanofiber containing 50 wt. % RE powder had the highest VOC absorption capacity. In addition, the absorption capacity of the PU nanofiber containing 50 wt. % RE powder increased by 320%, 210%, 208% and 187% compared with that of the pure PU nanofibrous membrane during the absorption of styrene, xylene, toluene, benzene and chloroform, respectively. This may be due to the decreasing fiber diameter of the PU/RE composite fibers as the amount of RE powder increased, which subsequently increased the surface area of the composite fiber, producing a high physical absorption capacity. Furthermore, the higher BET surface area of the PU nanofiber containing 50 wt. % RE powder is 1.64 times greater than the pure PU nanofiber.

Figure 3. VOC absorption capacity of different nanofibrous mats.

4. Conclusions 

In summary, a series of PU nanofibers containing different amounts of RE powder (0 wt. %, 10 wt. %, 30 wt. %, 50 wt. %) with a high VOC absorption capacity can be successfully produced via electrospinning. At RE powder concentrations greater than 50 wt. %, however, the electrospinning nozzle will be seriously blocked. The PU/RE-50 fiber had the smallest fiber diameter, with an average fiber diameter of 356 nm. In addition, this fiber also had outstanding mechanical properties, with an average tensile strength of 8.28 MPa and a percentage strain that was above 700%. Most importantly, it had the highest VOC absorption capacity, more than three times that of the pure PU nanofiber. In addition, the composite PU/RE nanofibrous membranes most easily absorbed styrene, followed by xylene, toluene, benzene and chloroform.