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Abstract

This study aimed to examine the fabrication of bi-constituent nanofibrous membranes and investigation of their moisture management behavior in various environmental conditions. In doing so, polyurethane with a hydrophobic nature and superior mechanical behavior and poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS) with a hydrophilic nature were utilized. Different hybrid electrospun nanofibrous membranes were aligned based on different layer arrangements and composition ratios. Then, the impacts of the solid fraction of polymers, sequence of stacking, and environmental conditions on water vapor permeability, contact angle, and acidic water permeation were measured and discussed. Tracing the water vapor permeability behavior in samples was carried out through measuring the amount of permeation hourly and proposing some regression models. Bi-modal nanofibrous membranes were successfully fabricated using PAMPS and polyurethane with an average fiber diameter of 543.5 and 216.7 nm, respectively. As the volume fraction of PAMPS increased, the porosities of the samples remained unchanged, the number of pores increased, and the pore size decreased (the average pore diameter was 299.97 nm for the PAMPS sample and 492.35 nm for the polyurethane sample). Despite the better water vapor permeability of the polyurethane membranes than that of the PAMPS membranes, in the first 12 h of the water vapor permeability test, the trend was completely reverse. The results also revealed that in the relative humidity of 55%, the polyurethane layer had the highest water vapor permeability among all samples. The results of the acidic water permeation and contact angle tests showed that the hybrid electrospun nanofibrous membranes exhibit better wicking and wetting properties.

Keywords

Bi-modal nanofiber hybrid membrane moisture management protective clothing wound dressing

Introduction

Breathable and water/windproof fabrics are in high demand primarily due to their moisture vapor transfer and resistance to water and wind penetration. These functional materials can have applications in fields as diverse as sportswear, leisure and outdoor clothing, chemical/mechanical equipment, and, in particular, protective clothing (for example, for medical personnel, military personnel, and firefighters) [1 –5]. A thorough review of the related literature shows that in most conventional waterproof breathable fabrics such as densely woven fabrics and laminated and coated fabrics, there is an inverse relationship between comfort and protection level [6 –8]. On the other hand, water repellency and water vapor permeability (WVP) are two mutually exclusive functions. Little wonder, then, achieving high WVP for protective clothing is still a serious challenge for manufacturers of waterproof performance fabrics as they usually have to sacrifice breathability for barrier properties [1,7 –9]. Serious efforts have been made to address this limitation. New technologies have been introduced for the production of protective fabrics, resulting in the production of other types of breathable materials such as hydrophilic membranes [1,10], smart breathable fabrics [1,11], biomimetics fabric [1], as well as commercial products such as Gore-Tex [12] and Sympatex [12,13]. In recent years, many attempts have also been made to improve the protective properties of natural fabrics while retaining their comfort [14]. Thanks to new methodologies with various functionalizing agents, fabrics have become antimicrobial [15], self-cleaning [16], insect repellent [17], and resistant to ultraviolet radiation [18], features lacking in their older versions. Despite the ever-increasing advances in the technology of fabricating waterproof and breathable fabrics, there is still a lot of room for research and presentation of new technologies such as electrospinning in order to produce structures that combine higher barrier performance and comfort properties. Electrospinning, as an effective and versatile technique to produce porous nanofiber membranes, is widely used in fabricating water/windproof and breathable membranes [19 –21]. There are different electrospinning set-ups including solvent electrospinning [22], melt electrospinning [23,24], and also bubble electrospinning [25,26]. Bubble electrospinning has great potential in the mass production of nanofibers within 100 nm [27,28]. All these methods provide a web of extremely fine fibers with diameters both on the micro scale and nano scale, low weight, high flexibility, excellent structural properties, high porosity, and small pore size [1,3,7,8,29,30]. This prevents the penetration of water droplets, while its pore size which is larger than a water vapor molecule allows water vapor to transfer from the body to the environment [1].

Recently, a large body of research has investigated the potential of using an electrospun nanofiber web in water/windproof breathable materials as protective clothing. For example, transport properties of polyurethane electrospun nanofibrous membrane (PU ENM) have been investigated by Gibson et al. [3], and using ENMs as barriers to liquid penetration has been studied by Lee and Obendorf [31]. Gorji et al. [4] studied water/windproof electrospun nanofiber PU membranes. Hydrophobic-breathable membranes with water/windproof performance have also been investigated by Yang et al. [32] and the development of waterproof-breathable fabrics by direct electrospinning of PU on the polyester/nylon blended fabric as substrate has been performed by Kang et al. [33]. Some other works in this area have studied tunable porous structure fibrous membranes [34], multilayer fabrics based on a nanofiber mat in the middle layer to produce water/windproof fabrics [35], waterproof-breathable hydrophobic composite PAN@PDMS membranes [36], and superamphiphobic breathable membranes utilizing SiO₂ nanoparticles [37]. The use of hybrid nanofiber mats composed of two different polymeric nanofibers is a newly proposed practice for achieving desirable characteristics [38]. Amini et al. [39] showed how PU and nylon 66 hybrid electrospun nanofiber layers keep waterproof property intact while providing air permeability. Other reports demonstrated that the dual-mode hydrophilichydrophobic electrospun PU/(poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS)-GO) nanofiber membrane area is a good candidate for breathable membranes used in protective clothing [40,41].

Many kinds of electrospun breathable and water/windproof membranes have been prepared using various polymers including PU [33], polyacrylonitrile (PAN) [35], polypropylene [42], nylon [43], and polyvinylidene fluoride [44]. Depending on what kind of polymer is used in electrospinning, properties of the nanofiber web can vary. Hence, choosing the right polymer is crucial to attain a desired barrier and comfort performance of water/windproof breathable membranes. For example, PU has good mechanical properties despite its poor moisture absorption and transfer. On the other hand, PAMPS can absorb and transfer moisture, resulting in fast water vapor permeation and better cooling and comfort; however, its mechanical properties are poor [45,46]. Incorporating a hydrophilic polymer and a hydrophobic polymer into an electrospinning setup with two opposite nozzles can result in the production of bi-functional water/windproof breathable hybrid ENMs, with desirable features such as good mechanical strength, high water vapor transmission, and comfort [39,41]. Nevertheless, the moisture management behavior of PU/PAMPS ENMs still remains under-researched. One of the novelties and advantages of the present study is that it has taken into account the effect of layer arrangement (structural geometry), composition ratio (volume fraction of PAMPS), and different environmental conditions (relative humidity) on the moisture management behavior of PU/PAMPS hybrid layers. PU/PAMPS ENMs can be prepared via different polymer contents and layer arrangements which may affect the moisture management of hybrid ENMs. Therefore, different hybrid ENMs have been introduced following different arrangements of two components and different volume fractions of PAMPS in hybrid ENMs. The effects of the layer arrangement (in five levels) and the volume fraction of PAMPS (in three levels) on the WVP and wettability of membranes have been systemically addressed in this study. Regarding the uses of these membranes in different environmental conditions, the effect of relative humidity (in two levels) on WVP has also been investigated.

Materials and methods

Materials

N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were purchased from Merck (Germany). 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS, monomer) was obtained from Daejong Co. (South Korea). 2,2-Azobisisobutyronitrile (AIBN) was purchased from Sigma-Aldrich (St. Louis, MO). Commercial thermoplastic polyurethane (TPU) pellets were purchased from Bayer, Leverkusen, Germany.

Preparation of the polymers solution

PU solution with a concentration of 6% w/w was prepared by dissolving TPU pellets in a THF and DMF mixture (60:40, v/v) with stirring by a magnetic mixer for 7 h at room temperature. PAMPS was prepared by free radical polymerization. PAMPS was synthesized as follows (for further details, one can see Lin et al. [47]). To begin with, 17.031 g of AMPS and 40.623 g of DMF were placed into a three-necked round-bottomed flask (100 mL). Then, the three-necked round-bottomed flask was placed in a water bath. Next, the water bath was slowly heated up to 80℃ and kept in this condition for 10 h. Nitrogen atmosphere was used for preventing compounds from reacting with components of air. After reaching 80℃, 0.190 g of initiator AIBN was added to the solution. The mixture was stirred for 10 h at 80℃ in the water bath to form a 30% w/w PAMPS solution.

Electrospinning

In electrospinning set-up, first nozzle with a syringe contained 5 mL of the PU solution equipped with a 22 gauge (the internal and external diameters of the needle were 0.413 mm and 0.7 mm, respectively) blunt-tipped needle has been operated with a the feeding rate of the 0.4 mL/h and a voltage set at a constant value of 12 kV. Second nozzle with a syringe contained 5 mL of the PAMPS solution equipped with a 23 gauge (with internal and external diameters of 0.337 mm and 0.641 mm, respectively) blunt-tipped needle has been operated with a feeding rate of 0.1 mL/h and a voltage set at 16 kV. The optimum distance between the nozzle and collector was obtained to be 140 mm for both nozzles. The rotational speed and transverse speed of the drum were 100 r/min and 200 mm/min, respectively. Solution viscosity was measured at room temperature using a viscometer (LVDV II, Brookfield Co., USA). The electrospinning process was carried out at room temperature (22 ± 2℃) and room humidity (30 ± 5%). The electrospinning conditions are summarized in Table 1.

Table 1.

Electrospinning conditions for two opposite nozzles set-ups.

Factors	Polymer concentration (w/w%)	Solvent THF/DMF (v/v)	Solution viscosity (cps)	Voltage (kV)	Feeding rate (mL/h)	Tip to collector distance (cm)	Rotational speed of the collector (r/min)	Transverse speed of the collector (mm/min)
Nozzle 1 (PU)	6	60/40	400–520	12	0.4	14	100	200
Nozzle 2 (PAMPS)	30	0/100	2400–3000	16	0.1	14	100	200

DMF: N,N-dimethylformamide; PAMPS: poly(2-acrylamido-2-methylpropane sulfonic acid); PU: polyurethane; THF: tetrahydrofuran.

Layer arrangement and volume fraction of the components

Different hybrid ENMs were fabricated based on the different arrangements of the two layers and volume fraction of PAMPS. Thickness and electrospinning duration of all the samples were kept within 24 ± 2 µm and 6 h, respectively. Volume fractions of PAMPS on the hybrid electrospun mat were 33%, 50%, and 66%, and the arrangement of the two layers varied in five levels. These conditions and codes are summarized in Table 2.

Table 2.

Sample description.

Sample code^a	Number of layer	Layer arrangement^b	Electrospinning mode	Electrospinning duration (h)	Volume fraction of PAMPS
A0	1	A	One-nozzle	6	0%
B100	1	B	One-nozzle	6	100%
AB33	1	AB	Two-nozzle Simultaneous	4 + 2	33%
AB50	1	AB	Two-nozzle Simultaneous	3 + 3	50%
AB66	1	AB	Two-nozzle Simultaneous	2 + 4	66%
A/B33	2	A/B	One-nozzle Sequential	4/2	33%
A/B50	2	A/B	One-nozzle Sequential	3/3	50%
A/B66	2	A/B	One-nozzle Sequential	2/4	66%
B/A33	2	B/A	One-nozzle Sequential	2/4	33%
B/A50	2	B/A	One-nozzle Sequential	3/3	50%
B/A66	2	B/A	One-nozzle Sequential	4/2	66%
A/B/A33	3	A/B/A	One-nozzle Sequential	2/2/2	33%
A/B/A50	3	A/B/A	One-nozzle Sequential	1.5/3/1.5	50%
A/B/A66	3	A/B/A	One-nozzle Sequential	1/4/1	66%
B/A/B33	3	B/A/B	One-nozzle Sequential	1/4/1	33%
B/A/B50	3	B/A/B	One-nozzle Sequential	1.5/3/1.5	50%
B/A/B66	3	B/A/B	One-nozzle Sequential	2/2/2	66%

PAMPS: poly(2-acrylamido-2-methylpropane sulfonic acid); PU: polyurethane.

^aA: PU, B: PAMPS, the numbers after the letters indicate the volume fraction of PAMPS.

^bThe AB code indicates the simultaneous spinning of nozzles, and the sign “/” between the letters indicates the sequence of spinning.

Generally, electrospinning in hybrid samples can be broken down into two modes, namely simultaneous and sequential. In the simultaneous mode, which is denoted as the AB code in Table 2, both nozzles are utilized in real-time. Therefore, both polymers are simultaneously deposited on a collector. In the sequential mode denoted as the A/B code, only one nozzle is applied in real-time.

Characterization and measurements

Morphology of layers

Morphology of electrospun nanofiber layers was examined using a scanning electron microscope (SEM, VEGA\\TESCAN-XMU). The diameter of the nanofibers was determined for 50 randomly selected nanofibers using SEM images and Image J software (National Institute of Health, Bethesda, MD) and the average fiber diameter was reported. The average pore diameter was calculated by an image analysis technique using Image J software. To this end, the original SEM image of the sample was converted to grayscale with 256 levels based on a proper threshold level by a method developed and described by Ghasemi-Mobarakeh et al. [48]. The average pore size was calculated by Image J software. Finally, the equivalent pore diameter $(D_{Area}^{Eq})$ was calculated using equation (1) based on the same area

D_{Area}^{Eq} = \sqrt{\frac{4 A_{P}}{π}}

(1)

where A_P is the average area of pore calculated by Image J software [49].

Measuring geometrical properties of ENMs

The weight of the sample was measured using a microbalance (with 1 µg readability) in 10 trials. The thickness of the specimens was also measured using a micrometer (Dial Thickness Gauge, Mitutoyo, Japan) and the average of 10 measurements was reported as the web thickness [4,39]. The solid volume fraction (SVF) of ENMs was calculated using the following equation [31]

S = \frac{M / M ρ ρ}{A . T}

(2)

where S is the SVF of the ENMs, M is the mass of the sample in standard conditions (g), ρ is computed on the basis of polymers' composition and densities (g/cm³), A is the area of the specimen (cm²), and T is the thickness of the sample (cm). The density of the PU was obtained from the data sheet and the density of the PAMPS was calculated by the Archimedean principle [50].

Water vapor permeability

Breathability performance was evaluated by measuring the WVP of the electrospun samples using a tester M261 (Atlas, England) [4,40,41] based on the ASTM E 96-00 (standard test method for water vapor transmission of materials) [51]. The electrospun layer was sealed into the top of cups containing a predetermined amount of water. The WVP was calculated at relative humidity levels of 35% and 55% in four trials. A laboratory germinator was used to control temperature (27 ± 2℃) and relative humidity (35 ± 5%, 55 ± 5%). WVP test duration was four days (96 h). The total WVP was reported based on the weight loss of water according to equation (3)

WVP (g . m^{- 2} . {day}^{- 1}) = \frac{10^{4} (m_{2} - m_{1})}{4 S}

(3)

where WVP is water vapor permeability (g.m⁻².day⁻¹), m₂ − m₁ is the weight loss of water after four days (g), and S is the area of the sample subjected to the test (m²).

Contact angle

Wettability behavior of the ENMs was evaluated by measuring the static contact angle. The contact angle of the sample with water was measured on a drop shape analysis system OCA20 (Date-Physics Germany) at ambient temperature and 30% relative humidity. In doing so, 5 µL of distilled water were dropped carefully from a needle on a microsyringe onto the samples. Once the water droplet had been placed on the surface, images were captured at 1, 30 and 60 s during the test. Afterward, contact angles were calculated through analyzing the drop shape.

Acidic water permeation

This test had been reported previously by Amini et al. [39]. In their study, the waterproof property of ENMs in static state had been investigated using the acidic water permeation test. For modeling acidic rain, an aqueous solution of acetic acid (0.5%) was used as liquid. The sample was firmly clamped between two frames and acidic detector paper was placed underneath the sample. A mirror was also used to better trace changes in the detector color. A drop of the aqueous solution was placed on the sample using a syringe pump. After the color of the detector paper changed, the required time for permeation of the acidic water was recorded. In this test, 10 samples of each layer were tested as trial.

Results and discussions

Morphology of nanofibers

Figure 1 shows the SEM image and diameter distribution of PU and PAMPS electrospun nanofibers. The average diameters of PU and PAMPS electrospun nanofibers were 543.5 and 216.70 nm, respectively. The average diameter of the PAMPS nanofiber was about three times finer than that of the PU nanofiber. It seems that increasing the voltage and the needle gauge and also reducing the feeding rate have led to a reduction in the PAMPS nanofibers diameter [52 –54]. In addition to the above factors, the ionic properties of the PAMPS polymer can affect the diameter of the PAMPS nanofibers, where a high degree of sulfonic acid leads to an increase in charge density, and, finally, high elongation forces in the electrical field cause a reduction in the nanofiber diameter [40,55].

Figure 1.

SEM image (left) and the corresponding fiber diameter distribution (right): (a) A0 sample, (b) B100 sample.

Figure 2(a) shows the morphology of the electrospun PU and PAMPS hybrid layer which has been successfully produced by simultaneous electrospinning of the PU and PAMPS with two opposite nozzles. In this image, fine electrospun nanofibers with an average diameter of 236.36 nm were obtained from the PAMPS solution and a larger diameter of 659.76 nm was obtained from the PU solution. In contrast to the reference samples (Figure 1), the mean nanofiber diameter of the PU and PAMPS hybrid layer with two nozzles (two-nozzle simultaneous electrospinning mode) increased by 21% and 9%, respectively (Figure 2(f)). This phenomenon can be attributed to the nozzle electrical charging of electrospinning with two nozzles, i.e. the electrical charge of the opposite nozzle can cause electrical repulsion in the other working nozzle. Therefore, a decrease in electrical field due to electrical repulsion can lead to an increase in the nanofiber diameter. A further increase in the PU nanofiber can be attributed to the higher voltage of the opposite nozzle.

Figure 2.

(a) SEM image of the AB50 (50% volume fraction of PAMPS) sample, (b) PU nanofiber diameter distribution, (c) PAMPS nanofiber diameter distribution, (d) top view of the A/B50 hybrid layer, (e) side view of A/B50 hybrid layer, and (f) effect of the electrospinning mode on the nanofiber diameter.

Figure 2(d) and (e) shows the SEM image of the A/B50 sample with the PU on the inner face and the PAMPS on the outer face. As can be seen, the PAMPS nanofiber has been successfully deposited on the PU nanofiber layer. The analysis of variance (ANOVA) showed that in the one-nozzle sequential electrospinning mode, there is no significant difference among the mean nanofiber diameters of ENMs in various samples produced with different layer arrangements and volume fractions of two components (Table 3).

Table 3.

Significant difference among the mean nanofiber diameters for the PU and PAMPS nanofiber.

	PAMPS nanofiber diameter	PU nanofiber diameter
Sig.	.561	.358
Sample Size = 50.000.

PAMPS: poly(2-acrylamido-2-methylpropane sulfonic acid); PU: polyurethane.

Geometrical properties

The results for weight per unit area, thickness and SVF of the PU, PAMPS and hybrid layers are presented in Table 4. ANOVA at 95% confidence level showed that there was no significant difference between the mean value of the thickness and SVF of the samples (Table 5). The results showed that the thickness of the layer is approximately equal for all samples. Thus, it could be deduced that either the one-nozzle electrospinning or two-nozzle electrospinning with half the electrospinning time duration for a single nozzle, a layer with identical thickness will produce. The same result had also been reported in a study by Gorji et al. [4] and Amini et al. [39].

Table 4.

Weight per unit area, thickness, and SVF of the samples.

Sample code	Weight per unit area (g/cm²)	CV% of weight	Thickness (µm)	CV% of thickness	SVF
A0	66 × 10⁻⁵	5.48	24	10.39	0.2292
B100	76 × 10⁻⁵	4.73	26	7.35	0.2268
AB33	70 × 10⁻⁵	5.37	25	8.9	0.2271
AB50	73 × 10⁻⁵	5.61	26	13.6	0.2240
AB66	75 × 10⁻⁵	5.63	26	9.2	0.2275
A/B33	69 × 10⁻⁵	6.04	25	10.5	0.2254
A/B50	71 × 10⁻⁵	5.96	25	11.0	0.2245
A/B66	74 × 10⁻⁵	4.91	26	10.1	0.2277
B/A33	70 × 10⁻⁵	6.90	25	10.2	0.2236
B/A50	72 × 10⁻⁵	7.44	26	8.7	0.2211
B/A66	75 × 10⁻⁵	5.70	26	10.5	0.2319
A/B/A33	69 × 10⁻⁵	9.77	25	11.3	0.2228
A/B/A50	72 × 10⁻⁵	5.70	26	10.5	0.2232
A/B/A66	74 × 10⁻⁵	4.83	26	8.4	0.2223
B/A/B33	69 × 10⁻⁵	8.00	25	9.9	0.2209
B/A/B50	73 × 10⁻⁵	5.82	26	7.4	0.2246
B/A/B66	74 × 10⁻⁵	5.75	26	8.3	0.2240

CV%: percent coefficient of variation; SVF: solid volume fraction.

Table 5.

Significant difference among the thickness and SVF of the samples.

	Thickness	SVF
Sig.	.769	.999
Sample Size = 10.000.

SVF: solid volume fraction.

Pore characterization

The pore diameter and its distribution are among the most important characteristics of ENMs, with a significant effect on the breathability of the membrane [34,36,56]. The most common technique for pore diameter characterization is the bubble point method using a capillary flow porometer, which was applied in this method using a wetting liquid and high pressure. This method could not be used to measure the pore size diameter in this study due to the sensitivity of the PAMPS to most liquids as well as the structure of the PAMPS ENM, which is not strong enough to remain intact under such a high pressure. In this study, the pore diameter was measured using the image processing method. However, since this model is capable of evaluating initial surface layers, there are some restrictions in evaluating pore specifications of sequential samples. Therefore, only the pore specification of simultaneous and pure samples (A0, AB50 and B100) was considered in this study. Figure 3 shows typical SEM images of A0, AB50, and B100 samples in (a), (b), and (c), respectively; binary images of corresponding images ((d), (e) and (f)); pore visualizations ((g), (h) and (i)); and pore size distributions ((j), (k) and (l)). It can be seen that the pore size decreases as the volume fraction of PAMPS increases. The mean pore diameter was 492.35, 311.55, and 299.97 nm for A0, AB50, and B100 samples, respectively. In fact, the smaller diameter of the PAMPS nanofiber compared to that of the PU leads to a smaller pore diameter of the PAMPS ENM compared to that of the PU. As the volume fraction of the PAMPS increased, the porosities of the samples remained unchanged, while the number of pores increased and the pore size decreased (see Figure 4).

Figure 3.

Typical SEM images: (a) sample code A0, (b) sample code AB50, (c) sample code B100, binary corresponding images ((d), (e) and (f)), corresponding pore images ((g), (h) and (i)), corresponding pore size distribution ((j), (k) and (l)).

Figure 4.

Pore diameter, number of pores, and porosity of the A0, AB50, and B100 samples.

Moisture management behavior of the ENMs

Water vapor permeability

In this study, as mentioned earlier, the average pore diameter varied between 299.97 nm for the PAMPS sample and 492.35 nm for the PU sample. Regarding the average pore diameter, the water vapor transport mechanism of the nanofiber membrane in either the single layer or the hybrid layer occurred based on the convective flow model [57 –59]. The results of the overall WVP of the ENMs are presented in Figure 5(a). Obviously, the overall WVP of the PAMPS ENM is lower than that of the PU ENM. That explains why as the volume fraction of the PAMPS increases in the hybrid layer, the WVP difference between the hybrid layers and the PU sample increases, too. Figure 5(b) shows the WVP results of different samples during the initial hours (12 h) of the WVP test. The WVP of the hybrid ENMs containing 33%, 50%, and 66% PAMPS is illustrated in Figure 5(c), (d), and (e), respectively. According to these results (Figure 5(a) to (e)), the WVP behavior of the samples during the initial hours of the examination is quite the opposite of the WVP behavior of the samples at the end of the WVP test such that in the early hours of the WVP test, the hybrid samples and the PAMPS sample showed higher WVP compared to the PU sample. At the end of the WVP test, the hybrid samples and the PAMPS sample showed lower WVP than the PU sample. During initial hours (12 h) of measurements, the behavior of the WVP of the PAMPS was nonlinear and the WVP of this ENM seemed higher than that of the other samples. The high water vapor transfer through the PAMPS sample during the initial hours of the WVP test can be attributed to the hydrophilic nature of the PAMPS, resulting in more water molecule uptake, more convective flow transfer, and, subsequently, more WVP. This dual behavior of the PAMPS sample during the WVP test can be attributed to the structural change of the PAMPS nanofiber. Figure 5(f) shows the morphological change in the PAMPS ENM after the WVP test. As PAMPS is a water-soluble polymer, absorption of water molecules during water vapor transfer causes the dissolution of the electrospun PAMPS nanofibers. Eventually, PAMPS ENM changes to a barrier film like dense membranes. Also, the dense membrane has the same morphology as the PAMPS film layer and may prevent water vapor molecules from being transferred easily, but they can transfer slowly by the solution-diffusion mechanism [60,61].

Figure 5.

WVP of different samples (error bars are confidence interval at the level of 95%): (a) 96 h, (b) 12 h. WVP vs. time for different samples at relative humidity of 35%: (c) 33%, (d) 50%, and (e) 66% volume fraction of PAMPS. (f) SEM image of B100 membrane after the WVP test.

Determining the required time for film formation

To determine the required time for film formation of the sample, numerical differentiation methods and statistical methods were utilized. To this end, it was assumed that the nonlinear behavior could be modeled with two linear sections. Intersection of these lines was considered as the time of film formation. Therefore, to begin with, the slope of the amount of water vapor passed-time curve at each point was numerically calculated using the 9-point central differential method

\begin{matrix} f'_{(x)} = \frac{1}{840 h} (3 f_{x - 4 h} - 32 f_{x - 3 h} + 168 f_{x - 2 h} - 672 f_{x - h} + 672 f_{x + h} \\ - 168 f_{x + 2 h} + 32 f_{x + 3 h} - 3 f_{x + 4 h}) \end{matrix}

(4)

where

f_{x - 4 h}, f_{x - 3 h}, f_{x - 2 h}, f_{x - h}, f_{x + h}, f_{x + 3 h}, f_{x + 4 h}

are the values of WVP in the consecutive points (in this study h is 1 h).

It was followed by examining the difference between the slopes of the curve at each point at a significant level of 95% through variance analysis. The time of film formation and the equation of the WVP trend line before and after the film formation of different samples at 33% volume fraction of the PAMPS are shown in Table 6. According to the data, the structural change of the PAMPS nanofiber occurs in 4 h. Afterward, the WVP behavior of the PAMPS changes from a high WVP rate with a nonlinear behavior to a low WVP rate with a linear behavior. Because of the low volume fraction of the PAMPS at 33% volume fraction, the behavior of WVP in all hybrid structures was close to the behavior of WVP in the PU sample.

Table 6.

WVP trend line equation of samples at 33% volume fraction of PAMPS.

Sample	Required time for film formation (h)	Equation (m₁)	Equation (m₂)
A0	–	–	y = 55.01 x
A/B33	4	y = 63.76 x	y = 61.04 x
AB33	3	y = 79.72 x	y = 50.68 x
A/B/A33	3	y = 72.68 x	y = 45.08 x
B/A/B33	5	y = 73.08 x	y = 39.92 x
B/A33	4	y = 78.56 x	y = 35.95 x
B100	4	y = 153.44 x	y = 15.79 x

PAMPS: poly(2-acrylamido-2-methylpropane sulfonic acid); WVP: water vapor permeability.

According to Figure 5(a) and (c), sample A/B33 has the highest WVP among all samples. In this sample, the structure of the PAMPS nanofibers was destroyed in the first 4 h of the test (Figure 6(a) and (b)). Despite the presence of a polymeric film layer, WVP in this sample is higher than that in the PU sample. Figure 6(a) and (b) demonstrates the surface morphology of the A/B33 sample following the WVP test in two different magnifications. Obviously, there are many pores and cracks (indicated by arrows) in the polymeric film structure. It seems that the absorption of water vapor molecules by the PAMPS nanofibers caused swelling and shrinkage behavior in the PAMPS ENM, which in turn resulted in the formation of pores and cracks in the structure of the polymeric film. To better understand the behavior of water vapor transmission, Figure 6(g) and (h) schematically show water vapor transport through the A/B33 hybrid layer from the microclimate (the distance between skin and clothing) to the environment (Figure 6(g) shows it prior to film formation, and Figure 6(h) shows it following film formation). In these figures, the pale blue spheres represent a convective flow model, the violet spheres represent a solution-diffusion model, the dark blue spheres represent the absorption of water vapor molecules by the PAMPS nanofibers, and the pink arrows represent the absorbing and releasing moisture mechanism. Before and after film formation, in both structures, water vapor transfer through the membranes can occur according to both the convective flow and the solution-diffusion mechanisms; and before film formation, part of the water vapor molecules is absorbed by the PAMPS nanofibers. Before film formation, the convective flow is the dominant phenomenon, but after film formation, the solution-diffusion is the main phenomenon. Due to the slow process of water vapor transfer through the solution-diffusion model, the WVP decreases after film formation. During the electrospinning process, the solvent did not completely evaporate from the nanofibers, and the residual solvent led to the formation of bonding between the PU nanofibers and the PAMPS nanofibers deposited on the collector. This bonding caused adhesion between the two ENM layers. Here, the PU layer acts as the substrate and support for the PAMPS layer, where the adhesion force between the two layers can overcome the shrinkage force and prevent the formation of further pores. However, there are still plenty of pores and cracks in the polymeric film structure due to the low thickness of the polymeric film.

Figure 6.

SEM image of the A/B33 membrane after the WVP test: (a) magnification 1:3000, (b) magnification 1:500. SEM image of the hybrid layers after the WVP test: (c) AB33, (d) A/B50, (e) AB66, and (f) A/B66. Schematic representation of WVP through the A/B33 sample: (g) before and (h) after film formation.

The behavior of water vapor transport from the fibrous membrane can be explained by Fick’s first law of diffusion (equation (5)) [61]

WVP rate = - D \frac{c_{1} - c_{2}}{l}

(5)

where D is the diffusion coefficient, c₁ and c₂ represent water vapor concentration inside the cup (downstream) and outside the cup (upstream), respectively, and l is the thickness of the membrane. D is only affected by temperature and in a steady state, c₁ and c₂ are constant. Equation (5) can be rewritten as equation (6)

WVP rate = \frac{S}{l}

(6)

where s is a constant value. The presence of pores and cracks in the A/B33 sample reduces the overall thickness of the specimen. According to equation (6), the amount of water vapor transmission increases as the thickness of the sample decreases. On the other hand, the air flow around the polymeric film can improve the performance of water vapor transmission, creating a dynamic equilibrium in absorbing and releasing moisture between the polymeric film and the air outside. Figure 6(c) shows the SEM image of the AB33 sample after the WVP test. The uniform distribution of the PAMPS nanofibers through the entire structure and the presence of the PU nanofibers between the PAMPS nanofibers prevent the formation of an integrated polymeric film. As it can be seen, there are many pores in the structure, which have a positive effect on water vapor transmission. In the A/B/A sample, the presence of a PU nanofiber membrane at the top and bottom of the sample creates a suitable substrate and holder for the PAMPS nanofiber membrane. In this case, the adhesion force between the PAMPS membrane and the PU membrane can increase and prevent further shrinkage of the PAMPS ENM during film formation. This reduces the cracks and pores in the polymeric film structure, resulting in a decrease in the amount of WVP. On the other hand, due to the placement of a PAMPS polymeric film in the middle of the structure, it is not possible to create a dynamic equilibrium in absorbing and releasing moisture between the polymeric film and the outside air, ultimately reducing the amount of WVP. In the B/A/B sample, due to the presence of two film structures at the top and bottom of the sample, water vapor transmission is low. It seems that the location of the polymeric film layer in the sample structure has a great effect on water vapor transfer. Hence, by moving the polymeric film layer from the upper part of the structure (on the outside) in the A/B sample to the bottom of the sample (on the side of the cup) in the B/A sample, the WVP value drops significantly. It can be deduced that structural changes in the hybrid samples have a great impact on the characteristics of water vapor transmission. Figure 6(d) represents the morphological structure of the A/B50 layer following the WVP test. As it can be seen, unlike the A/B33 layer, there is no pore in the polymeric film structure and there is only a crack in the structure. Increasing the thickness of the PAMPS nanofiber membrane causes more resistance to shrinkage, but the thickness is not enough to prevent crack formation in the structure. As shown in Figure 5(a), in the volume fraction of 66%, the WVP of the AB66 sample is higher than that of the A/B66 sample and the other hybrid samples. Figure 6(e) and (f) shows the SEM image of the AB66 sample and the A/B66 sample after the WVP test, respectively. In the A/B66 sample, there are no pores and cracks in the polymeric film structure, which reduces the amount of water vapor transmission, but the AB66 sample has many pores in its structure. The higher water vapor transmission of the AB66 layer as compared to the A/B66 layer can be largely explained by the presence of these pores.

The effect of relative humidity on WVP

For examining the effect of environmental relative humidity on breathability performance, the water vapor permeation behavior of the samples at a relative humidity of 55% has been investigated. Figure 7 shows the WVP process of the samples at the 55% relative humidity level. In fact, the WVP of all the samples with a different structural geometry and volume fraction of the PAMPS significantly decreased as the humidity level increased from 35% to 55%. The results revealed that in the relative humidity of 55%, the PU layer had the highest WVP among all the samples. Unlike the humidity level of 35%, at a humidity level of 55%, for all three levels of the volume fraction of the PAMPS, among hybrid layers, the AB sample showed the maximum value of WVP. The higher rate of WVP in the AB sample as compared to the A/B sample can be attributed to the effect of the ambient humidity on the PAMPS nanofibers. Hence, in the A/B sample, the PAMPS nanofiber membrane transforms to a polymeric film faster under the influence of ambient humidity, causing a decrease in the amount of WVP. However, in the AB sample, the PU nanofibers act as a supporting layer and prevent the PAMPS nanofiber from film formation faster. The analysis of the WVP behavior in the other layers (at the humidity level of 55%) is the same as the humidity level of 35%.

Figure 7.

WVP vs. time for different samples at relative humidity of 55%: (a) 33%, (b) 50%, and (c) 66% volume fraction of the PAMPS.

In order to investigate the effect of the PAMPS structural change on the water vapor transmission rate, parameter α = m₁/m₂ has been introduced, which shows the ratio of the slope of a line prior to film formation (m₁) to the slope of a line following film formation (m₂). This ratio indicates the nonlinearity behavior of WVP. It can be observed (Figure 8) that both layer arrangement and volume fraction of the PAMPS factors can affect the WVP behavior. The amount of α at the relative humidity of 55% is greater than the amount of α at the relative humidity of 35%. At the relative humidity of 55%, due to a decrease in water vapor pressure, the absorbing and releasing moisture mechanism is less likely to happen; therefore, the rate of water vapor transmission after filming (m₂) decreases and, consequently, the amount of α increases.

Figure 8.

Value of α for different samples: (a) 35% and (b) 55% relative humidity.

In simultaneous electrospinning samples, the uniform distribution of the PU nanofiber between the PAMPS nanofibers in the AB hybrid layer prevented an increase in the water vapor transmission rate before film formation as well as the formation of an integrated polymeric film, which ultimately had the lowest change in the amount of α and the water vapor transmission rate among all the hybrid samples in both 35% and 55% relative humidity levels. Among sequential hybrid samples, the B/A sample and the A/B sample constituted the highest and lowest value of α, respectively. The location of the PAMPS layer in the hybrid sample structure had a great effect on the amount of α. In the A/B layer, due to moisture absorption and release mechanisms between the air and the polymeric film layer, the water vapor transmission rate from the polymeric film to the outside increased, which in turn increased m₂ and decreased the value of α.

The results show that different layer positions make it possible to choose the proper sample depending on different environmental conditions and the type and duration of usage. The results show that for a short period of time (12 h), the PAMPS and PU/PAMPS hybrid layers have high breathability properties. The PU/PAMPS hybrid layers are good candidates for disposable protective clothing with a short consumption period, for example in firefighting, disposable clothing, disposable medical supplies, and climbing outfits. At high levels of physical activity and when one breaks out into a sweat in cold and rainy weather, the best sample is the A/B/A hybrid layer with two PU layers placed on top and bottom of the sample. The PU layers act as a water/wind proof layer and the internal layer of the PAMPS accelerates the WVP performance, which has a higher comfort level and evaporation and cooling properties than the PU sample. Therefore, in cold/wet climates, the A/B/A hybrid layer may protect the wearer against hypothermia.

Contact angle

Wettability of a solid surface is one of the most important properties of a textile surface, which plays an important role in many applications and industrial processes. In order to investigate the wettability behavior of the hybrid ENMs, the contact angle (θ) was measured. The surface is considered hydrophilic when the water contact angle is smaller than 90° and hydrophobic if the water contact angle is large than 90° [62,63]. Figure 9(a) shows the result of the static contact angle measurement of the PU, PAMPS, and the hybrid samples at three levels of the volume fractions of the PAMPS for water droplets placed on the sample surface after 1, 30, and 60 s. To investigate the stability of the droplet on the ENM surface, the water contact angle difference between 1 and 30 s and 1 and 60 s was measured. Figure 9 presents the water contact angle and contact angle change of the samples obtained at different times. As shown in Figure 9(d), the PU and B/A samples have the highest water droplet stability among the hybrid samples, showing good waterproof properties. The PU ENM showed a good hydrophobic property with a contact angle of 132° and PAMPS ENM exhibited a high hydrophilic property with a water contact angle of 7°.

Figure 9.

(a) Water contact angle images of the ENMs. Contact angle of the samples obtained at different times (error bars are confidence interval at the level of 95%): (b) the one-nozzle sequential and (c) the two-nozzle simultaneous electrospinning mode. Contact angle change over time: (d) the one-nozzle sequential mode and (e) the two-nozzle simultaneous electrospinning mode.

The wettability properties (contact angle) of the PU, PAMPS, and hybrid layers can be studied based on geometric potential, such as the irregular distance between nanofibers, porosity, pore size, and porous structure [64,65]. The balance of attracting force, gravity and repelling force creates a water bridge (Figure 10(a)) between the two nanofibers [64]. The results of previous research [64,65] show that as the pore size decreases, so does the distance between the two nanofibers, resulting in a stronger repelling force and, consequently, an increase in the contact angle. Despite the smaller pore size of the PAMPS ENMs than that of the PU ENMs, the contact angle in the PAMPS ENMs is lower than that of the PU ENMs. It can be concluded that in this study the wettability characteristics of the samples (especially PAMPS layer) are most affected by the hydrophilic properties of the polymers (hydrophilic groups in the polymer chain). The boundary energy is known as geometrical potential [64]. The presence of hydrophilic groups in PAMPS nanofiber walls can create boundary energy in the walls, which this boundary energy (the interactions between hydrophilic groups and water) can be described as the geometric potential. The presence of hydrophilic groups in the PAMPS nanofiber walls can create boundary energy in the walls (red line in Figure 10)(c), which affects water bridge between the two nanofibers. It means that some water molecules will be attracted into the surface of PAMPS, which affects the water contact angle. The interaction between water and PAMPS nanofibers (hydrophilic groups in PAMPS) weakens the water bridge between the two nanofibers; as a result, the water contact angle becomes smaller.

Figure 10.

Geometrical potentials of PAMPS and PU nanofibers: (a) a water bridge, (b) PU, and (c) PAMPS [64].

As seen in Figure 9(c), the water contact angle of the AB sample decreases linearly as the volume fraction of the PAMPS increases from 33% to 66%. The results show that it can easily change the hydrophilic and hydrophobic properties of the AB sample as the volume fraction of the PAMPS changes.

At the 33% volume fraction, increasing the drop time on the A/B/A33 sample surface has no effect on the stability of the droplet. In this case, the surface of the PU layer in the A/B/A33 sample is almost waterproof. As the volume fraction of the sample increases to 50% and 66%, as a consequence of a reduction in the thickness of the PU layer, water drops on the surface of PU layer cannot be stable. Hence, as the contact time of the water droplet with the sample surface increases, the water droplet passes through the PU layer and enters the PAMPS layer. In this case, the PU layer loses its waterproof property. The thickness of the PU layer in the A/B/A33 sample is introduced as critical thickness, which is less than this thickness, the droplet on the surface of the sample is not stable, and also, the surface of the sample is not waterproof.

Acidic water permeation

In the contact angle test, droplet stability and wettability behavior of the samples, which are based on the adhesion properties between polymers and water, are not enough for determining hydrophilicity and waterproofness of the samples. Therefore, to evaluate the waterproofing properties of the samples and simulate acid rain, the wicking behavior of the samples was investigated by means of the acidic water permeation test. The behavior of water droplet penetration through a nanofibrous mat is influenced by porosity, adhesion interactions between the water, and polymer and capillary forces, which follow the Lucas–Washburn’s equation (equation (7))

h^{2} = \frac{r γ cos θ}{2 η} . t

(7)

where h is the distance of penetration at time t, γ is the surface tension of the liquid, r is the equivalent radius of capillary pores, θ is the contact angle, and η is the dynamic viscosity of the liquid [66 –68]. The results of acidic water permeation are shown in Figure 11. Among the hybrid samples, the AB sample constitutes the lowest penetration time. As the PAMPS nanofiber was added to the PU nanofiber, the hydrophilic properties improved and the contact angle decreased. According to equation (7), penetration time decreases. On the other hand, adding the PAMPS nanofibers into the PU nanofibers increased the specific surface area and led to an increase in the penetration of the liquid and, consequently, the penetration time decreased. The results presented in this section point to the importance of critical thickness. Since the thickness of the PU layer in the A/B/A50 and A/B/A66 layers is smaller than the critical thickness, the penetration time in these layers has decreased.

Figure 11.

Acidic water permeation of samples: (a) the one-nozzle sequential mode and (b) two-nozzle simultaneous electrospinning mode.

Conclusion

In this study, different PU/PAMPS hybrid ENMs with a hydrophilic-hydrophobic behavior were fabricated and characterized. The impacts of polymer volume fraction, layer arrangements, and environmental condition on the moisture management of these hybrid layers were investigated. The results showed that the WVP behavior of the PU nanofibers as a function of time was linear, while the PAMPS sample showed a nonlinear trend. The WVP of the PU samples could be improved up to 6.20% by utilizing 33% PAMPS with a sequential arrangement. The contact angle of the A/B33 ENM exceeded the 5.33° after 60 s of droplet contact, while this value for the PU ENM was found to be 131.08°. In other words, the wettability of the hybrid ENM increased up to 96%. From the viewpoint of layer arrangement, the location of the PAMPS layer in the hybrid sample structure has a great effect on the WVP because the moisture absorption and release mechanism occurring between the polymer film and outside air is a very important determining factor. At 33% and 50% volume fraction of the PAMPS, the highest value of WVP was observed for the A/B sample and the lowest one for the B/A sample. Considering the effect of the volume fraction of the PAMPS on the number of pores and cracks in the polymeric film structure, changes in volume fraction of PAMPS led to a change in WVP. The results showed that as the layer arrangement of the hybrid ENMs and the composition ratio of polymers in the hybrid ENMs changed, different PU/PAMPS hybrid ENMs with different levels of breathability-comfort and protection could be achieved for different environmental conditions. Therefore, it is possible to produce special PU/PAMPS hybrid ENMs based on our desired applications, different environmental conditions, type of usage, and the lifetime of the structure. Consequently, the PU/PAMPS hybrid ENMs can be potentially used in a range of fields as diverse as protective clothing, disposable medical supplies, and wound dressings.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Mukhopadhyay

Midha

. A review on designing the waterproof breathable fabrics part I: fundamental principles and designing aspects of breathable fabrics. J Ind Text 2008; 37: 225–262.

Mukhopadhyay

Midha

. A review on designing the waterproof breathable fabrics part II: construction and suitability of breathable fabrics for different uses. J Ind Text 2008; 38: 17–41.

Gibson

Schreuder-Gibson

Rivin

. Transport properties of porous membranes based on electrospun nanofibers. Colloids Surf A 2001; 187: 469–481.

Gorji

Jeddi

Gharehaghaji

. Fabrication and characterization of polyurethane electrospun nanofiber membranes for protective clothing applications. J Appl Polym Sci 2012; 125: 4135–4141.

Gugliuzza

Drioli

. A review on membrane engineering for innovation in wearable fabrics and protective textiles. J Membr Sci 2013; 446: 350–375.

Ahn

Park

Chung

. Waterproof and breathable properties of nanoweb applied clothing. Text Res J 2011; 81: 1438–1447.

Lee

Obendorf

. Transport properties of layered fabric systems based on electrospun nanofibers. Fibers Polym 2007; 8: 501–506.

Yoon

Lee

. Designing waterproof breathable materials based on electrospun nanofibers and assessing the performance characteristics. Fibers Polym 2011; 12: 57–64.

Bagherzadeh

Montazer

Latifi

, et al. Evaluation of comfort properties of polyester knitted spacer fabrics finished with water repellent and antimicrobial agents. Fibers Polym 2007; 8: 386–392.

10.

Painter

. Waterproof, breathable fabric laminates: a perspective from film to market place. J Coated Fabr 1996; 26: 107–130.

11.

Save

Jassal

Agrawal

. Smart breathable fabric. J Ind Text 2005; 34: 139–155.

12.

Shishoo

. Recent developments in materials for use in protective clothing. Int J Clothing Sci Technol 2002; 14: 201–215.

13.

Drinkmann

. Structure and processing of sympatex laminates. J Coated Fabr 1992; 21: 199–211.

14.

Emam

. Generic strategies for functionalization of cellulosic textiles with metal salts. Cellulose 2018; ▪: 1–17.

15.

Ahmed

El-Hawary

Emam

. Self-assembled AuNPs for ingrain pigmentation of silk fabrics with antibacterial potency. Int J Biol Macromol 2017; 105: 720–729.

16.

Emam

Abdelhamid

Abdelhameed

. Self-cleaned photoluminescent viscose fabric incorporated lanthanide-organic framework (Ln-MOF). Dyes Pigm 2018; 159: 491–498.

17.

Emam

Abdelhameed

. In-situ modification of natural fabrics by Cu-BTC MOF for effective release of insect repellent (N, N-diethyl-3-methylbenzamide). J Porous Mater 2017; 24: 1175–1185.

18.

Emam

Abdelhameed

. Anti-UV radiation textiles designed by embracing with nano-MIL (Ti, In)–metal organic framework. ACS Appl Mater Interfaces 2017; 9: 28034–28045.

19.

Sahay

Kumar

Sridhar

, et al. Electrospun composite nanofibers and their multifaceted applications. J Mater Chem 2012; 22: 12953–12971.

20.

Wang

Ding

Sun

, et al. Electro-spinning/netting: a strategy for the fabrication of three-dimensional polymer nano-fiber/nets. Prog Mater Sci 2013; 58: 1173–1243.

21.

Zhou

Gong

. Manufacturing technologies of polymeric nanofibres and nanofibre yarns. Polym Int 2008; 57: 837–845.

22.

Teo

Ramakrishna

. A review on electrospinning design and nanofibre assemblies. Nanotechnology 2006; 17: R89–R89.

23.

Hutmacher

Dalton

. Melt electrospinning. Chem Asian J 2011; 6: 44–56.

24.

Lyons

. Melt electrospinning of polymers: a review. Polym News 2005; 30: 170–178.

25.

Yang

, et al. Bubble-electrospinning for fabricating nanofibers. Polymer 2009; 50: 5846–5850.

26.

Liu Y, He J-H and Yu J-Y. Bubble-electrospinning: a novel method for making nanofibers. In: Journal of Physics: conference Series, 2008, Bristol: IOP Publishing.

27.

Liu

J-H

. Bubble electrospinning for mass production of nanofibers. Int J Nonlinear Sci Numer Simul 2007; 8: 393–396.

28.

Tian

Zhou

C-J

J-H

. Strength of bubble walls and the Hall–Petch effect in bubble-spinning. Text Res J 2019; 89: 1340–1344.

29.

Kim

Reneker

. Mechanical properties of composites using ultrafine electrospun fibers. Polym Compos 1999; 20: 124–131.

30.

Laurencin

Caterson

, et al. Electrospun nanofibrous structure: a novel scaffold for tissue engineering. J Biomed Mater Res 2002; 60: 613–621.

31.

Lee

Obendorf

. Barrier effectiveness and thermal comfort of protective clothing materials. J Text Inst 2007; 98: 87–98.

32.

Yang

, et al. Hydrophobic polyvinylidene fluoride fibrous membranes with simultaneously water/windproof and breathable performance. RSC Adv 2016; 6: 87820–87827.

33.

Kang

Park

Kim

, et al. Application of electrospun polyurethane web to breathable water-proof fabrics. Fibers Polym 2007; 8: 564–570.

34.

Yang

, et al. Hydrophobic fibrous membranes with tunable porous structure for equilibrium of breathable and waterproof performance. Adv Mater Interfaces 2016; 3: 1600516–1600516.

35.

Bagherzadeh

Latifi

Najar

, et al. Transport properties of multi-layer fabric based on electrospun nanofiber mats as a breathable barrier textile material. Text Res J 2012; 82: 70–76.

36.

Sheng

Zhang

, et al. Tailoring water-resistant and breathable performance of polyacrylonitrile nanofibrous membranes modified by polydimethylsiloxane. ACS Appl Mater Interfaces 2016; 8: 27218–27226.

37.

Wang

Raza

, et al. Synthesis of superamphiphobic breathable membranes utilizing SiO 2 nanoparticles decorated fluorinated polyurethane nanofibers. Nanoscale 2012; 4: 7549–7556.

38.

Tafreshi

Rahman

Jaganathan

, et al. Analytical expressions for predicting permeability of bimodal fibrous porous media. Chem Eng Sci 2009; 64: 1154–1159.

39.

Amini

Samiee

Gharehaghaji

, et al. Fabrication of polyurethane and nylon 66 hybrid electrospun nanofiber layer for waterproof clothing applications. Adv Polym Technol 2016; 35: 419–427.

40.

Gorji

Karimi

Nasheroahkam

. Electrospun PU/P (AMPS-GO) nanofibrous membrane with dual-mode hydrophobic–hydrophilic properties for protective clothing applications. J Ind Text 2018; 47: 1166–1184.

41.

Gorji

Sadeghian Maryan

. Breathable-windproof membrane via simultaneous electrospinning of PU and P (AMPS-GO) hybrid nanofiber: modeling and optimization with response surface methodology. J Ind Text 2018; 47: 1645–1663.

42.

Lee

Kay Obendorf

. Developing protective textile materials as barriers to liquid penetration using melt-electrospinning. J Appl Polym Sci 2006; 102: 3430–3437.

43.

Golmohammadi Rostami

Sorayani Bafqi

Bagherzadeh

, et al. Multi-layer electrospun nanofiber mats with chemical agent sensor function. J Ind Text 2015; 45: 467–480.

44.

Zhaohui

Zhenya

. A study on novel waterproof and moisture-permeable poly (vinylidene fluoride) micropore membrane-coated fabrics. J Appl Polym Sci 2001; 79: 801–807.

45.

Kim

Lim

Kim

, et al. Optimum parameters for production of nanofibres based on poly (2-acrylamido-2-methyl-1-propane sulfonic acid) by electro-spinning. Smart Mater Struct 2005; 14: N16–N16.

46.

Kim

Shin

Kim

. The effect of electric current on the processing of nanofibers formed from poly (2-acrylamido-2-methyl-1-propane sulfonic acid). Scripta Mater 2004; 51: 31–35.

47.

Lin

Tang

Cheng

. Electrospinning process of thermo-sensitive poly (N-isopropylacrylamide)/poly (2-acrylamido-2-methylpropanesulfonic acid) nanofibers. J Macromol Sci A 2012; 49: 980–985.

48.

Ghasemi-Mobarakeh

Semnani

Morshed

. A novel method for porosity measurement of various surface layers of nanofibers mat using image analysis for tissue engineering applications. J Appl Polym Sci 2007; 106: 2536–2542.

49.

She

Tung

Kong

. Calculation of effective pore diameters in porous filtration membranes with image analysis. Rob Comput Integr Manuf 2008; 24: 427–434.

50.

Resnick

Halliday

Walker

. Fundamentals of physics, Extended 5th edn. New York, NY: John Wiley and Sons, 1997.

51.

Intl A. Standard test method for water vapor transmission of materials. E 96-00. In: Annual book of ASTM standards. Philadelphia, PA: ASTM Intl, 2000, pp.307–314.

52.

Beachley

Wen

. Effect of electrospinning parameters on the nanofiber diameter and length. Mater Sci Eng C 2009; 29: 663–668.

53.

Deitzel

Kleinmeyer

Harris

, et al. The effect of processing variables on the morphology of electrospun nanofibers and textiles. Polymer 2001; 42: 261–272.

54.

Zong

Kim

Fang

, et al. Structure and process relationship of electrospun bioabsorbable nanofiber membranes. Polymer 2002; 43: 4403–4412.

55.

Kim

Lee

Kim

. Effect of ionic salts on the processing of poly (2-acrylamido-2-methyl-1-propane sulfonic acid) nanofibers. J Appl Polym Sci 2005; 96: 1388–1393.

56.

Shirazian

Ashrafizadeh

. 3D modeling and simulation of mass transfer in vapor transport through porous membranes. Chem Eng Technol 2013; 36: 177–185.

57.

Gibson

Rivin

Kendrick

. Convection/diffusion test method for porous textiles. Int J Clothing Sci Technol 2000; 12: 96–113.

58.

Kamada

Kamo

Motonaga

, et al. Gas permeation properties of conducting polymer/porous media composite membranes I. Polym J 1994; 26: 141–141.

59.

Zhong

Ding

Tang

. Analysis of fluid flow through fibrous structures. Text Res J 2002; 72: 751–755.

60.

Gorji

Karimi

Mashaiekhi

, et al. Superabsorbent, breathable graphene oxide-based nanocomposite hydrogel as a dense membrane for use in protective clothing. Poly Plast Technol Mater 2019; 58: 182–192.

61.

Wijmans

Baker

. The solution-diffusion model: a review. J Membr Sci 1995; 107: 1–21.

62.

X-M

Reinhoudt

Crego-Calama

. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chem Soc Rev 2007; 36: 1350–1368.

63.

Samuel

Zhao

Law

K-Y

. Study of wetting and adhesion interactions between water and various polymer and superhydrophobic surfaces. J Phys Chem C 2011; 115: 14852–14861.

64.

Liu

J-H

. Geometric potential: an explanation of nanofiber’s wettability. Therm Sci 2018; 22: 33–38.

65.

Song

. Permeability, thermal and wetting properties of aligned composite nanofiber membranes containing carbon nanotubes. Int J Hydrogen Energy 2017; 42: 19961–19966.

66.

Washburn

. The dynamics of capillary flow. Phys Rev 1921; 17: 273–273.

67.

Loeb

Schrader

. Modern approaches to wettability: theory and applications, Berlin: Springer Science & Business Media, 2013.

68.

Hwa Hong

Jin Kang

. Hydraulic permeabilities of PET and nylon 6 electrospun fiber webs. J Appl Polym Sci 2006; 100: 167–177.

Hybrid electrospun nanofibrous membranes: Influence of layer arrangement and composition ratio on moisture management behavior

Abstract

Keywords

Introduction

Materials and methods

Materials

Preparation of the polymers solution

Electrospinning

Layer arrangement and volume fraction of the components

Characterization and measurements

Morphology of layers

Measuring geometrical properties of ENMs

Water vapor permeability

Contact angle

Acidic water permeation

Results and discussions

Morphology of nanofibers

Geometrical properties

Pore characterization

Moisture management behavior of the ENMs

Water vapor permeability

Determining the required time for film formation

The effect of relative humidity on WVP

Contact angle

Acidic water permeation

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References