Abstract:

Objective This research was an effort to prepare electrospun nanomembranes having good waterproofness, mechanical properties, moisture permeability, improved wearing comfort and required mechanical properties. It also expected to expand the applications of nanomembranes into areas such as outdoor sports, protection and filtration.

Method PU/PDMS nanofiber membranes (PU/PDMS NMs) were prepared by electrospinning by adding non-fluorinated hydrophobic polydimethylsiloxane (PDMS) into PU spinning solution. The electrostatic sprayed PU/PDMS NMs were then prepared by depositing PU/PDMS microspheres on the PU/PDMS NMs substrate, and the PU/PDMS NMs and electrostatic sprayed PU/PDMS NMs were modified by using a heat thermal induction process, respectively. The influences of heating temperature and heating time on the morphology, pore size distribution, waterproof, permeability and mechanical properties of nanofiber membranes were studied, and the influencing mechanism was analyzed.

Results The waterproofness and moisture permeability of the electrostatic sprayed PU/PDMS waterproof and moisture permeable membrane before heat treatment is better than that of the electro spun PU/PDMS waterproof and moisture permeable film (Tab.3), where the waterproofness and moisture permeability of the electrostatic sprayed PU/PDMS waterproof and moisture permeable membrane is greatly improved. Heat treatment of electrostatic sprayed PU/PDMS waterproof permeable membrane achieves the waterproofness and permeability property not as good as that of the heat treatment of electrostatic spun PU/PDMS, mainly because heat treatment of electrostatic sprayed PU/PDMS waterproof permeable membrane causes more adhesion, porosity reduction, making the waterproof and moisture permeability not as good as its electrostatic spun counterpart (Tab.3 and Tab.4). The pore size of the electrospun PU/PDMS NMs after heat treatment was greatly reduced (Tab.2), and the beading structure of the electrospun PU/PDMS NMs was transformed into a spider structure (Fig.3 and Fig.4), which not only greatly improved the waterproof property of the PU/PDMS NMs but also significantly improved its mechanical properties. When the heat treatment temperatures were 80 ℃ and 100 ℃, the temperatures were not enough to allow the nanofibers and microspheres in the membrane to be fully softened, so that the heat treatment time had to be increased to form a sufficient adhesion structure. However, when the heating temperature was 120 ℃, the excessive temperature caused the polymer molecules to become relaxed and disrupted the micro-nanostructure, resulting in a reduction in the waterproof and mechanical properties of the membrane (Tab.3 and Tab.5). When the heating temperature was 100 ℃ and the heating time was 90 min, the electrospun PU/PDMS NMs showed the excellent performance with a water contact angle of 144.7°, and also exhibited good moisture permeability, air permeability and mechanical properties, the moisture permeability was up to 5 666.7 g/(m²·d), air permeability was up to 9.91 mm/s, breaking strength was up to 17.89 MPa, and the elongation at break was 210.68%.

Conclusion The heat treatment transforms the bead structure of the electrospun PU/PDMS NMs into a spider web structure, which improves the performance of the waterproof and permeable membrane. However, after heat treatment, the spider web structure of the former PU/PDMS NMs disappears and the waterproof and permeable properties of the membrane decreases. On the other hand, the spider web structure of the membrane becomes stable after heat treatment. By increasing the heat treatment temperature and time period, the adhesive structure in the waterproof permeable membrane can be increased, the pore size can be reduced, and the surface roughness can be improved, so as to improve the mechanical and waterproof properties of the waterproof permeable membrane. It was found that excessive temperature would relax the polymer molecules, while prolonging the heating time reduces the mechanical properties of the membrane, so that the adhered nanofibers almost become smooth, leading to a decrease in waterproofing properties. Although the membrane prepared by this method has good waterproof, permeable and mechanical properties, it cannot meet the demand for waterproof property under harsh conditions and still has a certain gap compared to the waterproof permeable fabric containing fluorine, which calls for further improvement.

Key words: polydimethylsiloxane, polyurethane, nanomembrane, waterproofness and moisture permeability, electrospinning, electrostatic spraying