Preparation of breathable and superhydrophobic polyurethane electrospun webs with silica nanoparticles

Abstract

Polyurethane (PU) is a unique polymeric material with excellent chemical and physical properties and is widely used in textile materials. There has been a need for superhydrophobic PU for wider applications, such as coating materials. In this research, SiO₂ nanoparticle (SNP) incorporated PU webs with superhydrophobic and breathable properties were prepared by one-step sol-gel electrospinning and post-treated with a non-fluorinated water repellent chemical, n-dodecyltrimethoxysilane (DTMS). SNPs were observed to be distributed evenly all over the fiber surfaces when 1–6 wt% SNP and tetraethoxysilane (TEOS)/acetic acid solution were added. TEOS was hydrolyzed to form larger nanoparticles while developing cross-linking with aromatic groups of the PU matrix. Interestingly, the addition of 20 nm SNPs was thought to act as nucleating seeds for enhanced hydrolysis of TEOS within the PU matrix. The hierarchical surface roughness consists of different sized SNPs and polymer beads, which resulted in superhydrophobicity with water contact angles as high as 157° and shedding angles as low as 5°. Laminating PU/SNP/DTMS webs onto polyester fabrics maintained the air permeability and water vapor transmission rate, which proves the potential of the developed PU/SNP/DTMS webs for practical applications as textile laminate materials with simple processing.

Keywords

silica nanoparticles sol-gel electrospinning superhydrophobicity multiscale roughness polyurethane

Polyurethane (PU) is a thermoplastic elastomer composed of hard segments (diisocyanate) and soft segments (polyether).¹ PU is a unique polymeric material with excellent chemical and physical properties, such as resistance to abrasion, flexibility, excellent stretch, high adhesiveness, low cost, and processibility.² Due to these characteristics, PU is widely used in textile materials for coating, artificial leather, and garment fabrics, despite of some drawbacks such as poor hydrophobicity that still exist.^3,4

Considering these desirable properties for textile materials, a number of studies have been conducted to utilize PU as a breathable coating in many forms, such as the electrospun web^5–7 or film.⁸ Chung et al.⁵ developed a shape memory PU electrospun web for intelligent coating, which can control its transport properties as temperature changes. Meng et al.⁸ synthesized waterborne PU and applied it as a textile coating material. Ahn et al.⁶ reported that electrospun web-laminated textiles possessed much better water vapor transmission rates (WVTRs) than film (polytetrafluoroethylene (PTFE)) laminated textile did. Lee and Obendorf⁷ showed that by applying PU electrospun webs onto commercial nonwovens, protective textiles with high transport properties could be obtained. Based upon previous research, electrospinning can be suggested as one of the most promising methods to develop breathable PU materials with multi-functionality.

Meanwhile, development of superhydrophobic surfaces has received considerable attention due to their various applications, such as self-cleaning and water repellency. Drops of water on superhydrophobic surfaces exhibit high contact angles (CAs) (>150°) and roll off at slight inclination (<10°).^9–12 Researches have been conducted to develop superhydrophobic PU by introducing inorganic nanoparticles on the PU surfaces^13–16 or fluorinating PU surfaces.^17,18 It is well known that micro-nano roughness is the prerequisite for superhydrophobic surfaces,^9–12 thus inorganic nanoparticles have been widely used to impart nano-roughness on the electrospun fibers. However, inorganic nanoparticles are known to be easily aggregated¹⁶ and have little affinity with the polymer matrix.¹⁹ In seeking to overcome these drawbacks of nanoparticle incorporation, Guo et al.¹³ synthesized PU copolymers having specific end groups that can be hydrolyzed into SiO₂ particles. Several researchers suggested using tetraethoxysilane (TEOS)^14,19 or 3-aminopropylethoxysilane (APTES)¹⁵ as a cross-linker between the inorganic particle and polymer matrix. However, these methods require complicated reaction steps. Recently, Pirzada et al.²⁰ reported on the one-step sol-gel electrospinning of poly(vinyl alcohol)-silica composite nanofibers. Their research was meaningful because of simple approach, but few studies have been done to apply one-step sol-gel electrospinning for developing superhydrophobic and breathable textile materials.

In this study, non-fluorinated PU/SiO₂ nanoparticle (SNP) composite webs with superhydrophobic properties were prepared by the one-step electrospinning process. TEOS/acetic acid precursor solution was added to the PU matrix to prepare PU/SNP composite webs. The addition of hydrophobic silica nanoparticles was believed to enhance hydrolysis of TEOS to form well-dispersed multiscale silica nanoparticles on the electrospun fiber surfaces. Different size of SNPs and polymer beads within electrospun fibers resulted in multiscale roughness, which is critical for hydrophobic surface design. N-dodecyltrimethoxysilane (DTMS) was deposited so as to reduce the surface energy of the webs. The developed webs were laminated onto polyester fabrics to examine their practical applications in the textile area. Surface morphology, surface chemical composition, superhydrophobicity, and breathability of PU/SNP/DTMS webs were investigated.

Experimental details

Materials

All chemicals were used without further purification. PU (pellethane 2103-80AE), a polyester type with average M_w of 80,000, melting temperature (T_m) of 182–210℃, glass transition temperature (T_g) of –40℃, and density at 25℃ of 1.13 g/mL was purchased from Lubrizol (USA). Hydrophobic silica nanoparticles (SNPs) (Aerosil® R 972) were provided by Evonik Industries (USA). Dimethylformamide (DMF) (>98%) and tetrahydrofuran (THF) (>98%) were used as solvent and TEOS (>98%) was used as the crosslinking agent. These chemicals were purchased from Daejung Chemicals (Korea). DTMS (>98%) was used to lower the surface energy of the web by vapor deposition and was purchased from Sigma-Aldrich (USA).

Preparation of PU webs

PU solutions of 10, 11, 12, 13, and 14 wt% were prepared by using a DMF/THF mixture with a volume ratio of 4:1.²¹ PU pellets were poured into the prepared solvent and mixed uniformly with a magnetic stirring bar for 2 h. PU pellets were dissolved evenly so that a colorless and transparent solution was obtained.

The electrospinning process was conducted at 25 ± 2℃ and 40 ± 5% relative humidity (RH). The solution was placed into a 10 mL syringe capped with a 25 G blunt end needle. An aluminum foil was bundled around a rotating collector with a rotating rate of 100 rpm and placed 14 cm away from the tip of the needle. The voltage was kept at 14 kV, and the solution feed rate was 1 mL/h.

Preparation of PU/SNP webs

The PU/SNP webs were prepared by three steps. Firstly, a sol-gel reaction was carried out with the composition of TEOS/acetic acid (1:2/ w:w) to form the primary silica nanoparticles. Secondly, 1–6 wt% relative to the PU concentration of Aerosil® R 972, which is a type of commercial hydrophobic silica nanoparticles, was added to the first mixture. The silica gelation mixture was added to 8.2 wt% PU solution and mixed by a magnetic stirrer for 24 h and sonicated for another 1 h. Then the prepared solutions were electrospun and dried to form the PU/SNP webs.

The electrospinning process was conducted at 25 ± 2℃ and 40 ± 5% RH. The collector was placed 18 cm away from the tip of the needle. The voltage was kept at 14 kV, and the solution flow rate was 0.2 mL/h.

Preparation of PU/SNP/DTMS webs

A total of 200 µl of DTMS was placed in ceramic dish at the bottom of a Teflon container in which PU/SNP webs were placed, and the Teflon container was placed in a vacuum oven at 150℃ for 3 h for vaporization of DTMS and the following vapor deposition onto the webs.^22,23 –OH groups of DTMS could be physically attached to PU fiber surfaces.

Preparation of PU/SNP web laminated fabrics

Polyester fabrics were purchased from Youngpoong Filltex Co. Ltd (Korea). The fabrics were fixed on the grounded collector of the electrospinning machine. PU solution containing 5 wt% of SNP (PU/SNP5), which were prepared according to the method described in the prior section, was electrospun onto the polyester fabric. DTMS was deposited onto the PU/SNP5 laminated polyester fabrics. Overall reactions regarding the development of superhydrophobic PU electrospun web with silica nanoparticles is summarized in Scheme 1.

Characterization

Fiber morphology was observed by a field emission scanning electron microscope (FE-SEM, JEOL, JSM-7600 F, USA) with acceleration voltage of 120 kV. The samples for SEM observation were sputter coated with gold. Fiber diameters of the electrospun web were measured with the image processing software, Image J (Image J, National Institutes of Health, USA). A total of 60–80 measurements upon SEM images were averaged and presented as the average fiber diameter.

Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Thermo Scientific, USA) was used to identify the interactions of TEOS and SNPs within the polymer matrix. The collected electrospun webs were analyzed in the absorbance mode in the range of 600–4000 cm^–1. Surface chemical composition and bonding of developed webs was investigated by energy dispersive spectroscopy (EDS, Aztec, Oxford Instruments, UK) and X-ray photoelectron spectroscopy (XPS, AXIS-HSI, Kratos Analytical, UK).

The static CA was determined by using the CA measurement device (Theta Lite, Attention, KSV Instrument, Finland). A 4 µl droplet of distilled water was placed on five different positions on the sample surface and the angles of drops on the specimens were determined at 23 ± 5℃. The static CA values for the sample reported were the average of five measurements. The shedding angle (SA) of samples was measured according to the method of Zimmermann et al.²⁴ After releasing a drop of water (12 ± 0.5 µl) at a height of 1 cm, the minimum angle of inclination at which the drop completely rolls off the surface was determined. The SA values for the sample reported were the average of five measurements.

A 1100-AEHXL capillary flow porometer (Porous Media Inc., USA) was used to measure air permeability of the developed web laminated polyester fabrics. The measurement was carried out according to the ASTM D737-04 standard method. Specimens were prepared to fit within a 1.4 mm orifice and 17.742 cm² test area and were put under 762 mm mercury pressure at 21℃, 65% RH condition. Water vapor transmission rates (WVTRs) of webs and laminated polyester fabrics were measured according to ASTM E96. The specially designed cup was filled with calcium chloride and the specimen was fixed onto its opening. The cup mouth defined the area of specimen exposed to the environment. Then the assembly was hanged up-side-down in a thermo-hygrostat at 40℃, 90% RH. After 1 h, the evaporation of water through the specimen was monitored by the weight change of the cup and the WVTR was calculated by the following equation:

WVTR = \frac{G}{tA} \times 24

(1)

where G is the weight change [g], t is the measurement time [h], A is the test area (cup mouth area) [m²] and WVTR is the water vapor transmission rates [g/m²*24 h].

Results and discussion

Development of PU/SNP webs

The DMF/THF mixture was used as a solvent for PU, which is composed of the hard segments and the soft segments. THF serves as an excellent solvent for the soft segments of PU, and DMF does for the hard segments. PU solutions with different concentrations of 10, 11, 12, 13, and 14 wt% were prepared (Figure 1). The average fiber diameter was increased as PU concentration increased; our result was in line with Huang et al.’s²⁵ investigation of the relationship between concentration of nylon -4,6 and the electrospun fiber diameter. This is because at lower polymer concentration, the polymer jet can be elongated more before being solidified by solvent vaporization. Adhesions between fibers appeared in PU-14 webs because the polymer concentration was too high to eject the solution continually, which might result in incomplete solvent vaporization. Residual solvents were reported to cause physical interdiffusion among individual electrospun fibers and make them bonded together.^26–28 PU-12 webs with fiber diameter ranging from 600 to 700 nm were chosen as the optimal condition because of their uniformity. Researchers have reported several different hypotheses on the relationship between polymer concentration and fiber diameter uniformity. Among them, Lin et al.²⁹ reported the phenomenon similar to our experimental results. They assumed that enhanced fiber uniformity is due to the more stable jet, which has suppressed jet bending instability at high solution concentration.

Figure 1.

The morphology of polyurethane (PU) webs electrospun from different PU concentrations: (a) 10 wt%; (b) 11 wt%; (c) 12 wt%; (d) 13 wt%; (e) 14 wt%.

After adding silica nanoparticles, electrospinning conditions including PU concentrations were adjusted again based on the uniformity and fiber diameter achieved from 12 wt% PU electrospun web. Polymer solutions with different PU concentrations with the same SNP amounts were electrospun and it was found that only 1.2 wt% increase in PU concentration caused around an 80 nm increase in average fiber diameter. In this research, we were seeking to understand the influence of SNPs that appeared on the electrospun fiber on the hydrophobicity of developed webs. To this end, it was attempted to obtain fiber diameters of both PU and PU/SNP webs that were as similar as possible. Various concentrations of SNPs were added with TEOS/acetic acid solution to the PU solution to decide optimal concentration of SNPs in developing PU/SNP composite webs. The SNP contents were varied in a range from 1 to 6 wt% relative to the PU concentration. The FE-SEM images of webs obtained from various concentration of PU/SNP solution are shown in Figure 2.

Figure 2.

The morphology of polyurethane (PU)/SiO₂ nanoparticle (SNP) webs electrospun from 8.2 wt% PU and tetraethoxysilane/acetic acid with different SNP concentrations: (a) 1 wt%; (b) 2 wt%; (c) 3 wt%; (d) 4 wt%; (e) 5 wt%; (f) 6 wt% of SNP.

As the SNP contents increased, PU/SNP webs exhibited the growth of fiber diameter. This is because as hydrophobic SNPs were added more, viscosity was likely to increase.^30–32 According to Fong et al.,³³ polymer bead formation during electrospinning is a complicated phenomenon that can be affected by viscosity, net charge density, surface tension of the solution, etc. Their study found that as solution viscosity increased, the polymer beads on fibers had a spindle-like shape, which can be observed in Figure 2. In this research, polymer bead structures on electrospun fibers were beneficial because they could impart a hierarchical structure along with SNP, which is crucial in designing superhydrophobic surfaces.^12,22

It was also thought that an excessive amount of SNPs addition would lead to aggregation of SNPs on the fiber surface. While PU/SNP6 showed some aggregation of SNPs, the PU/SNP5 web showed good distribution of SNPs on the surface of the fibers without aggregation. Overall, SNPs appeared on the PU/SNP web surface, and the surface roughness of the fiber was greatly increased. Based on these results, PU/SNP5 was chosen as an optimal condition for developing PU/SNP webs.

Meanwhile, the addition of TEOS/acetic acid played a crucial role in fiber morphology. As shown in Figure 3, without adding TEOS/acetic acid into the electrospinning solution, beads were seriously formed on the fibers but the addition of TEOS/acetic acid solution helped fibers have a more uniform diameter and SNP dispersed well on the web surfaces. It was suspected that TEOS was hydrolyzed to form silica particles inside of PU, and also formed cross-linking (Si-C) between PU molecules and SNPs (Scheme 1). Additional SNPs were thought to appear on the surface of fiber. Moreover, the addition of acetic acid is believed to increase net charge density causing less bead formation and more uniform fiber formation.^33–35

Figure 3.

The morphology of electrospun webs prepared from 5 wt% SiO₂ nanoparticles added to 8.2 wt% polyurethane solution (a, b) with and (c, d) without tetraethoxysilane/acetic acid solution.

FTIR spectra of PU and PU/SNP5 webs are shown in Figure 4. Peaks in the 1700–1670 cm^–1 region were observed from both PU and PU/SNP5, due to vibration of the C = O bond in the carbamate group of PU. It was noticed that additional signals in the spectrum of PU/SNP5 at 728 cm⁻¹ assigned to Si–C and 930 cm⁻¹ assigned to Si–H appeared. This is because the SNPs produced by the hydrolysis of TEOS have reacted with the Ar–H on the backbone of the PU. That is, the H atom was transferred and bonded with the Si atom to form new Si–H or Si–C bonds.¹⁶ Moreover, in FTIR spectra of PU/SNP5, the bands around 800 and 1100 cm^–1 correspond to the stretching and angular vibrations of the Si–O–Si bond also proved hydrolysis of TEOS within fibers and distribution of SNPs on fiber surfaces.^36,37 There were no discernible FTIR spectra changes after DTMS treatment. This is because peaks assigned for PU and reacted TEOS are overlapped with those of DTMS. So as to verify the efficacy of DTMS deposition, surface elements were detected via EDS and XPS techniques.

Figure 4.

Fourier transform infrared spectra of (a) polyurethane (PU), (b) PU/n-dodecyltrimethoxysilane (DTMS), (c) PU/SNP5 with tetraethoxysilane (TEOS)/acetic acid, and (d) PU/SNP5/DTMS with TEOS/acetic acid webs.

The surface energy of PU/SNP webs was lowered by treating DTMS onto their surfaces. The morphology of the webs was not changed after DTMS treatment, as shown in Figure 5. Surface chemical composition of the PU, PU/DTMS, PU/SNP5, and PU/SNP5/DTMS were verified by EDS (Figure 6) and XPS (Table 1). Both EDS and XPS measurements showed similar results on quantitative surface elements analysis. PU webs did not contain any contaminants shown by the sole presence of C, N, and O atoms. PU/DTMS had Si composition on the surface, which indicated DTMS was well deposited on the PU web surfaces. High Si content of PU/SNP5 revealed that the SNPs were placed on the fiber surface. PU/SNP5/DTMS had the highest Si content_, showing that DTMS and SNP5 were well deposited on PU surfaces.

Figure 5.

Field emission scanning electron microscope images of (a) untreated PU-12, (b) untreated polyurethane (PU)/SNP5 with tetraethoxysilane (TEOS)/acetic acid, (c) n-dodecyltrimethoxysilane (DTMS) treated PU-12, and (d) DTMS treated PU/SNP5 with TEOS/acetic acid webs.

Figure 6.

Energy dispersive spectra of the (a) polyurethane (PU), (b) PU/n-dodecyltrimethoxysilane (DTMS), (c) PU/SNP5 with tetraethoxysilane (TEOS)/acetic acid, and (d) PU/SNP5/DTMS with TEOS/acetic acid webs.

Table 1.

Surface elements atomic concentration (%) results obtained by X-ray photoelectron spectroscopy analysis of polyurethane (PU), PU/n-dodecyltrimethoxysilane (DTMS), PU/SNP5 with tetraethoxysilane (TEOS)/acetic acid, and PU/SNP5/DTMS with TEOS/acetic acid

Sample	Surface elements atomic concentration (%)
Sample	C1s	N1s	O1s	Si2p
PU	70.53	1.99	27.48	0
PU/DTMS	77.55	0.39	14.69	7.38
PU/SNP5	61.14	2.14	30.08	6.63
PU/SNP5/ DTMS	68.09	1.43	22.51	7.96

For functional group identification on each specimen, further analysis of C1s and Si2p peaks were conducted (Figure 7). Much stronger C-C peaks assigned to 284.8 eV were noticed than C-O peaks assigned to 286 eV or C = O peaks assigned to 289 eV^38,39 on DTMS treated samples, which could be attributed to CH₃- or CH₂- functionalities of DTMS covering up C-O and C = O bonds of the PU matrix. Si2p peaks of PU/SNP5 consist of Si-O _x assigned to 103.2 eV and Si-C assigned to 100.5 eV³⁸ peaks, which are in line with FTIR analysis. While TEOS was hydrolyzed to form silica nanoparticles, some of them developed cross-linking with PU molecules. Interestingly, Si2p peaks of PU/SNP5/DTMS showed a very strong O-Si-C peak at 101.8 eV,³⁸ which is a distinctive functional group of DTMS, in addition to a weaker Si-O _x peak at 103.2 eV, proving DTMS deposition on electrospun web surfaces as well.

Figure 7.

X-ray photoelectron spectroscopy spectra of (a) polyurethane (PU), (b) PU/n-dodecyltrimethoxysilane (DTMS), (c) PU/SNP5 with tetraethoxysilane (TEOS)/acetic acid, and (d) PU/SNP5/DTMS with TEOS/acetic acid.

Superhydrophobicity of PU/SNP webs

Table 2 shows that after introducing SNP on the PU fiber surfaces and DTMS treatment, the static CA of PU/SNP5/DTMS webs increased up to 157°. This could be attributed to the increased surface roughness and lowered surface energy. Since the mean fiber diameters of PU, PU/DTMS, PU/SNP5, and PU/SNP5/DTMS webs were 648 ± 97, 678 ± 72, 752 ± 149, and 729 ± 152 nm, the effect of fiber diameter could be negligible. Lowering surface energy by DTMS deposition affected the static CA, as it can be inferred from the CA change of approximately 6–7° between PU and PU/DTMS and between PU/SNP5 and PU/SNP5/DTMS. However, introducing a hierarchical structure onto the fiber with the combination of polymer bead formation and SNP distribution seemed to be more significant in improving hydrophobicity, which can be noticed from about 20° of CA change between PU and PU/SNP5. In this study, surface roughness had a greater effect than surface energy on the increase of CA of rough surfaces, as Park et al.¹⁹ pointed out.

Table 2.

Static contact angles and shedding angles of the PU-12, PU-12/n-dodecyltrimethoxysilane (DTMS), polyurethane (PU)/SNP5 with tetraethoxysilane (TEOS)/acetic acid, and PU/SNP5/DTMS with TEOS/acetic acid web (N/A indicates that shedding did not occur in this specimen)

	PU	PU/DTMS	PU/SNP5	PU/SNP5/DTMS
Static contact angle [°]	131.1 ± 5.0	138.5 ± 5.7	151.3 ± 5.9	157.4 ± 5.9
Shedding angle [°]	N/A	N/A	32.6 ± 1.7	5.1 ± 0.9

SAs of PU, PU/DTMS, PU/SNP5, and PU/SNP5/DTMS are also shown in Table 2. When 12 µL of water droplet was placed on the PU and PU/DTMS webs, the droplet was adhered to the surface of the web. Since carbamate groups in hard segments and the ether groups in soft segments of PU provide comparable hydrophilicity, PU has a high surface energy which is larger than 40 mN/m.⁴⁰ The static CA of PU/SNP5 was as high as 151.3°, and its SA was relatively high at 32.6°. However, the SA of PU/SNP5/DTMS was measured as 5°. Due to its hierarchical structure and lower surface energy, SA was decreased dramatically. It is also possible to explain this phenomenon in that SA is better at discriminating the level of hydrophobicity of rough surfaces than static CA is.²² Likewise, superhydrophobic PU/SNP5/DTMS webs were successfully developed by introducing TEOS/acetic acid and SNPs in electrospinning solution without fluorination.

Practical application of PU/SNP webs

So as to examine the practical application of developed superhydrophobic PU/SNP webs, they were laminated onto commercial polyester fabrics. Their air permeability and WVTRs were measured because those properties were directly connected to comfort property of textiles.⁴¹

Air permeability of polyester fabric (P), PU/SNP5 laminated polyester fabric (P/PU/SNP5), and PU/SNP5/DTMS laminated polyester fabric (P/PU/SNP5/DTMS) were measured according to the ASTM D737-04 standard method (Table 3). After laminating PU/SNP5 or PU/SNP5/DTMS, air permeabilities of polyester fabrics were not much affected. WVTRs were measured according to ASTM E96. The amount of absorbed vapor was weighed and WVTRs were calculated according to Equation (1), as can be seen in Table 3 as well.

Scheme 1.

Reaction schematics of the process: (1) sol-gel reaction of tetraethoxysilane (TEOS) and acetic acid to form silica nanoparticles; (2) interaction between silica nanoparticles and polyurethane (PU) molecules: dotted line indicates hydrogen bonding, solid line indicates Si-C bond caused by hydrolysis reaction of TEOS within the PU matrix, secondary interactions such as Van der Waals interaction or dipole-dipole interaction are not depicted; (3) physical n-dodecyltrimethoxysilane (DTMS) deposition onto the developed nanofibers.

Table 3.

Air permeability and water vapor transmission rates of polyester fabric (P), polyurethane (PU)/SNP5 laminated polyester fabric (P/PU/SNP5), and PU/SNP5/n-dodecyltrimethoxysilane (DTMS) laminated polyester fabric (P/PU/SNP5/DTMS). PU/SNP5 and PU/SNP5/DTMS were electrospun with tetraethoxysilane/acetic acid solution

	P	P/PU/SNP5	P/PU/SNP5/DTMS
Air permeability [cm³/s/cm²]	213.8 15.0	211.6 ± 7.5	205.2 ± 5.0
Water vapor transmission rates [g/m²•24 h]	6211.0 ± 222.2	6162.6 ± 333.3	6082.3 ± 83.3

According to the test results of the air permeability and WVTRs of the P, P/PU/SNP5, and P/PU/SNP5/DTMS, it was found that the air permeability and WVTRs were slightly decreased after DTMS treatment, possibly because DTMS molecules blocked some pores and rendered the treated surface hydrophobic. Air permeability was reported to be directly related to textile porosity,^28,42 but water vapor transmission involves different factors such as porosity, surface hydrophobicity, etc.^43–45 Water vapor transmission through textiles has been discussed thoroughly for a long time. Most researchers agree that water vapor travels through air spaces of textiles and within fibers.^43–45 Many researchers have tried to explain this complicated phenomenon mathematically. Among those, Wehner et al.⁴³ presented a simple yet reasonable model as Equation (2):

ε \frac{\partial C}{\partial t} + (1 - ε) \frac{\partial C_{f}}{\partial t} = \frac{D ε}{τ} \frac{\partial 2 C}{\partial x 2}

(2)

where ε is the fabric porosity, t is time, C is the moisture concentration in the air, C_f is the moisture concentration in the fibers, D is the diffusion coefficient of water in air, x is the distance, and τ is the effective tortuosity of the fabric. The left-hand side of Equation (2) indicates moisture accumulation in the air and fibers, and the right-hand side of the equation indicates water vapor transmission through the air inside textiles. At the equilibrium state, the amount of water vapor absorbed by air and fibers within textiles should be the same as water vapor transmitted through textiles. Since highly hydrophobic textile materials (polyester and developed electrospun webs) were examined in this research, the amount of water vapor transferred or absorbed by fiber itself would be very minimal. Thus, water vapor traveling through air spaces of textiles would majorly impact water vapor transmission of the specimen. As shown in Table 2, the trend of air permeability and WVTR value were very similar for the investigated specimens, and this correlation was in accordance with Gibson’s⁴⁴ study.

As the decrease of air permeability and WVTRs due to SNPs or DTMS deposition was minimal, the newly developed P/PU/SNP5 and P/PU/SNP5/DTMS webs were thought to have satisfying breathability to be applied as clothing materials.

Conclusion

In this study, non-fluorinated superhydrophobic PU/SNP composite webs were prepared by the one-step sol-gel electrospinning process. Hydrolysis of TEOS/acetic acid precursor within the PU matrix led to the formation of silica nanoparticles within PU electrospun fibers, which were cross-linked by Si-C bonds and secondary interactions such as hydrogen bonding. The addition of 20 nm of hydrophobic silica nanoparticles was thought to enhance hydrolysis of TEOS to form multiscale silica nanoparticles appearing on the electrospun fiber surfaces. DTMS deposition helped reduce the surface energy of the PU/SNP composite, and thus it was possible to develop superhydrophobic PU/SNP/DTMS webs via this simple process.

In addition to the water CA and SA to identify the superhydrophobicity of the developed webs, the potential of the developed webs to be applied as a textile laminate material was examined through air permeability and WVTR measurements. A maximum CA of 157° and minimum SA of 5° were obtained at the condition of 5 wt% SNP concentration and DTMS treatment. When PU/SNP5 and PU/SNP5/DTMS webs were laminated onto polyester fabrics, air permeability remained higher than 200 cm³/s/cm² and WVTRs of them were higher than 6000 g/m²*24 h, implicating the potential as clothing material. Durability of webs, however, should be evaluated further in the form of abrasion, washing durability, and tensile-strength tests to solidify their practical applications in the textile area.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2015R1A2A2A03002760) and ‘BK21 Plus’ project.

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