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Breathable and Colorful Cellulose Acetate-Based Nanofibrous Membranes for Directional Moisture Transport Aijaz Ahmed Babar, Dongyang Miao, Nadir Ali, Jing Zhao, Xianfeng Wang, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07393 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Applied Materials & Interfaces

Breathable and Colorful Cellulose Acetate-Based Nanofibrous Membranes for Directional Moisture Transport Aijaz Ahmed Babar, †ǁ Dongyang Miao,‡ Nadir Ali,‡ǁ Jing Zhao,‡ Xianfeng Wang,†‡§* Jianyong Yu,§ Bin Ding,†‡ §* †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of

Materials Science and Engineering, Donghua University, Shanghai 201620, China ‡

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles,

Donghua University, Shanghai 201620, China §

Innovation Center for Textile Science and Technology, Donghua University, Shanghai

200051, China ǁ

Textile Engineering Department, Mehran University of Engineering & Technology, Jamshoro

76060, Pakistan

Keywords: Electrospinning, colored nanofibers, cellulose acetate, directional moisture transport, moisture management testing

*

Correspondence to:

Prof. Xianfeng Wang, Prof. Bin Ding Key Laboratory of Textile Science & Technology, Ministry of Education, College of textiles, Donghua University, Shanghai 2016, China. E-mail: [email protected], [email protected]

Notes: The authors declare no competing financial interest.

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ABSTRACT Textiles with excellent moisture transport characteristics play key role in regulating the comfort of the body, and use of color in textiles helps in developing aesthetically pleasing apparels. Herein, we report an aesthetically pleasing and breathable dual-layer cellulose acetate (CA)/dyed CA (DCA) nanofiber membrane with exceptional directional moisture transport performance. Outer layer was synthesized by subjecting CA nanofibers to hydrolysis and reactive dyeing processes, which converted moderately hydrophobic CA nanofibers into uniformly colored superhydrophilic CA nanofibers with an excellent wettability. Besides, excellent wettability and superhydrophilic nature DCA nanofibers also offered high color yield and dye fixation as well as considerable colorfastness performance against washing and light, thus were opted as outer layer. Whereas, pristine CA nanofibers were chosen as inner layer for their moderate hydrophobicity. The subsequent CA/DCA nanofiber membrane produced high wettability gradient which facilitated directional moisture transport from CA to DCA layers. The resultant dual-layer nanofiber membranes offered high color yield of 16.33 with ~82% dye fixation, excellent accumulative one-way transport index (AOTI, 919%), remarkable overall moisture management capacity (OMMC, 0.89) and reasonably high water vapor transport (WVT) rate (12.11 kg d-1 m-2), suggesting to be a potential substrate for fast sweat release applications.


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1. INTRODUCTION Growing need for the directional water (moisture) transport technology has driven scientists for developing new materials and techniques for various application from clean water collection from fog to designing functional textiles for comfortable apparels.1-5 Moisture transport behavior is of critical significance for regulating the comfort (primary need of the textiles) characteristics of textiles, more specifically apparels which are designed for sportswear or work-wear.6-7 Performance attitude of the wearer is directly influenced by moisture transport characteristics of the textile. Since moisture transport acts as one of the major performance regulating factors of the comfortable textile as it controls the body heat dissipation of the wearer. Therefore, inefficient moisture transport would result into heat stress and thus would impair the performance of the wearer.8-12 To address this problem, various approaches are being applied such as surface modified single layered microfiber based textiles and double-layered hydrophobic/hydrophilic micro and/or nanofiber based textiles.1319

Former ones have low moisture carrying and high moisture releasing characteristics owing

to their inherent hydrophobic nature, and are mainly produced from polyester or polypropylene at commercial scale.20-21 Whereas, later ones offer more effective moisture transport behavior as each layer can be designed and tailored individually. Since two layers are of different nature, i.e. inner layer is hydrophobic and outer layer hydrophilic, therefore, they generate high wettability gradient. This wettability gradient and unique characteristics of each layer generate pull-push phenomenon, where inherent high moisture releasing property of inner layer pushes the moisture and hydrophilic nature of outer layer pulls out the moisture.13-15 Generally, two or more substrates and additives of different nature (i.e. hydrophobic and hydrophilic) are required for successful development of considerably high wettability gradient. For instance, Dong et al. developed a dual layer nanofiber membranes comprising of polyvinyldene-polyacrylonitrile

(core-shell) fibers in combination with

cellulose acetate, synthesized membranes also demonstrated considerable moisture transport 3 ACS Paragon Plus Environment


performance.13 Additionally, use of surface modified polystyrene nanofibers in combination with polyacrylonitrile for developing a dual layer fibrous membranes for moisture transport have also been reported.14 Moreover, in addition to high comfort, aesthetic appearance of the textiles is also of immense importance. Amongst aesthetic characteristics regulating factors, color of the substrate is the most important factor that plays major role of value addition to the textiles. Additionally, color is not only the feature that firstly attracts targeted end-user towards substrate but use of color also serves certain technical purposes. For instance, using yellow color enhances the visibility of construction workers and traffic police to avoid accident on highways, and is also used by mountaineers,22 or use of especially designed colored patterns help in developing camouflage for military purposes.23 Therefore, functional textiles which can easily be colored and retain their functional characteristics after coloration are always desired. Nowadays, nanofibers are gaining massive attention owing to their exceptional physical characteristics. The high surface area to volume ratio, light weight, regulated porous structure, excellent flexibility and considerable mechanical strength make them superior compared the fibers in micro and macro scales.24-27 Currently, electrospinning is the one of the major fabrication techniques capable of producing continuous fibrous strands at nanoscale. The fabrication technique is not only simple but also versatile enough to generate nanofibers from a wide variety of polymers (natural as well as synthetic), ceramics, metals and mixtures.28-30 Additionally, free-standing nature of the electrospun nanofibers makes them readily applicable to a wide range of applications which include air and water filtration, energy conversion and storage, sensors, drug delivery, biomedical and protective textiles, and apparels.31-34 Nanofibers are well known for their functional characteristics,35-37 however, only few works have been reported about their aesthetic characteristics.38 In 2013, dyeing of cellulose acetate (CA) nanofibers with disperse dyes via pad-dry-bake process was first 4 ACS Paragon Plus Environment

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reported as an attempt to analyze the aesthetic properties of nanofibers. Resultant nanofibers showed reasonable dyeing results (color yield of ~11 at 50 g L-1 dye concentration).38 Later on, dyeing of cellulose nanofibers with reactive and vat dyes, and polyurethane nanofibers with disperse dyes has been asserted via various dyeing processes to improve the aesthetic appeal of the nanofibers.39-41 However, to be the best of our knowledge, no work has reported the functional characteristics of the membranes developed from dyed nanofibers. In the present work, we report for the first time, an scalable strategy for designing aesthetically appealing and breathable dual-layer CA/dyed CA (DCA) nanofiber membrane with directional moisture transport performance. Since CA is frequently used biodegradable textile substrate of easter family. Fibers synthesized from CA have hydrophobic nature in the pristine state, however, the CA nanofibers show excellent hydrophilicity once they are hydrolyzed in weak alkaline solution. Therefore, electrospun CA nanofibers were first hydrolyzed in presence of sodium hydroxide (NaOH) to improve its wettability. After hydrolysis, CA nanofibers revealed superhydrophilic behavior, excellent wettability, and outstanding dyeability when subjected to reactive dyeing. Hydrolyzed CA (HCA) nanofibers after dyeing with reactive dyes yet retained the inherent wettability and superhydrophilic behavior, therefore, were opted for outer layer. Since pristine CA nanofiber showed moderate hydrophobicity, thus, a layer of comprising pristine CA nanofibers was then deposited on the DCA nanofibers, and was used as inner layer. Combination of hydrophobic-superhydrophilic layers generated wettability gradient, and synthesized dual-layer nanofiber membrane showed excellent directional liquid moisture transport performance.

2. EXPERIMENTAL SECTION 2.1 Materials CA (39.8% acetyl content, average Mw = 30 kDa), acetone, dimethyl formamide (DMF), sodium hydroxide, sodium carbonate and sodium chloride were procured from Aladdin 5 ACS Paragon Plus Environment


Reagent Co., Ltd., China. Three commercial dyes CI Reactive Blue 19 (RB 19), CI Reactive Yellow 86 (RY 86) and CI Reactive Red 195 (RR 195) were supplied by Shengwei Chemical Co., Ltd, China. Heal-Force water purifying system was used for getting ultrapure water. All chemicals except dyes were of analytical grade and were used as received. 2.2 Preparation of dual layer CA/DCA nanofiber membrane A 60-gram solution of CA polymer (16 wt.%) dissolved in mixture of acetone/DMF (2/1, w/w) was electrospun using DXES-3 electrospinning machine (SOF Nanotechnology Co., Ltd., China). Plastic syringes (10 ml) with metallic needles were used for the electrospinning. Solution was fed at 0.5 ml/l under an applied voltage 25 kV. Fibers were collected on a rotating metallic drum at a distance 25cm. Regulated relative humidity (45±3%) and temperature (25±2°C) were maintained throughout the process. Afterwards, collected web of CA nanofibers was vacuum dried at 80°C for 2 h. Resultant membrane was then soaked in NaOH (0.05 M) solution for 30 h for hydrolysis, followed by careful washing in ultrapure water. Subsequent membrane after vacuum drying for 4 h at 80°C was dyed with various commercial reactive dyes to obtain colored nanofibers. Colored nanofibers were then washed in cold and warm water seperately. The process was carried out untill no dye bleeding was observed. Finally, a fresh layer of CA (19 wt.%) was deposited on dyed web, using same electrospinning setup, to develop a dual layer nanofiber membrane. 2.3 Characterization Scanning electron microscope (SEM, TESCAN VEGA 3, TESCAN Ltd., Czech Republic) and Fourier Transmission Infrared FT-IR (Nicolet 8700 FT-IR spectrometer, USA) were utilized for examining the surface morphology and chemical characteristics of the resultant membranes. Thermogravimetric analyzer (TGA, TA Instruments model Q600, USA), X-ray diffraction (XRD, TD-3500, Tongda, China), differential scanning calorimetry (DSC 4000, Perkin Elmer Thermal Analyses Instruments) were employed for thermal and structural characterizations of the membrane. Digital Goniometer (Kino SL200B, USA) was used for 6 ACS Paragon Plus Environment

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water contact angle (WCA) analysis. Spectrophotometer (SF650, Datacolor, USA) was employed for assessing K/S values to determine the color yields of the dyed nanofibers. Following equations were used for calculating the color yields (K/S values) and percentage of dye fixed on the nanofiber surface.42 - 43

K S=

(1− R)2 2R

(1)

where R = decimal fraction of the reflectance of the dyed nanofibers, K & S are absorption and scattering coefficients, respectively. K   after wash S Dye Fixation (%) =   K   before wash S

(2)

ISO 105-C10:2006 and ISO 105-BO2 standards were followed for evaluation of colorfastness of nanofibers against washing and light.41 Moisture management performance of the synthesized membranes was investigated following AATCC 195-2009 standard via moisture management tester (MMT, SDLATLAS, USA). Equation 3 & Equation 4 were used for determining the AOTI and OMMC values of the synthesized membranes via in built software of MMT.9, 44 AOTC =

∫ (U

b

) − ∫ (U t )

(3 )

T

Where, Ub is moisture content in the bottom layer, Ut is moisture content in top layer and T is total testing time.

OMMC = 0.25 BAR + 0.5 AOTC + 0.25 BSS

(4)

Where, BAR = bottom absorption rate (% s-1), AOTC = accumulative one-way transport index and BSS = bottom spreading speed. ASTM E96 up right cup standard was followed for observing WVT rate examination. Following equation was involved for determining the WVT rate values.45



WVT rate =

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W1 − W 2 × 24 S

(5)

Where W1-W2 is difference in mass of water and S is the area of samples.

3. RESULTS AND DISCUSSION Figure 1 demonstrates the synthesis process of the dual layer CA/DCA nanofiber membrane. First of all, CA nanofibers were electrospun via a multi-needle electrospinning setup. The fabricated fibers were then hydrolyzed in NaOH (0.05 M) solution for at least 30 h followed by an intensive washing in ultrapure water until neutral pH was achieved and then were dried at 80oC for 4 h under vacuum. The resultant HCA nanofibers showed excellent wettability on contrary to the CA nanofibers which were hydrophobic in nature. The HCA nanofibers were then dyed with three different commercial reactive dyes to analyze dyeability of the HCA nanofibers. After reactive dyeing, DCA nanofibers were the carefully washed to remove unfixed dye molecules from the surface of nanofibers and vacuum dried at 80oC for 4 h. DCA nanofibers displayed not only excellent dyeability but also retained superhydrophilic nature and excellent wettability of HCA nanofibers. Finally, a fresh layer of pristine CA nanofibers was deposited on the DCA nanofibers via same electrospinning setup to develop the duallayer CA/DCA nanofiber membranes. Water transport through porous fibrous materials is complex phenomenon and is triggered by various factors such as surface chemistry and roughness, porosity and pore size of the material. Generally, when a water droplet comes into contact with a fibrous material, the first parameter that determines rate of water transport is the surface chemistry (i.e. in this case WCA). Once the surface chemistry of the material allows some room for water transport then role of pore size come into action which activates the capillary force of material, this way, the water is repidly transported through the fibrous material. This rate of water transport through

fibrous

materials

can

be

estimated

via


Young’s

laplace

equation

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( Laplace pressure =

4γ ⋅ cos θ ), where ߛ is surface tension of the liquid, ߠ represents the Dpore

advancing water contact angle (WCA) on inner surface of capillary of the material, and Dpore is the pore size of the fibrous material (Figure 1b-c ). Positive laplace pressure determines rapid water transport through the fibrous materials whereas negative one indicates the water resistance offered by the fibrous materials. Value of the laplace pressure is directly dependent on surface chemistry and indirectly on pore size of the materials.

Figure 1 Figure 2a-c illustrates the morphology of pristine, hydrolyzed and dyed CA nanofibers. It is apparent from Figure 2a that electrospun CA nanofibers possessed randomly oriented threedimensional solid smooth rod like fibrous structure. Average fiber diameter of electrospun CA nanofibers 276±15 nm (Figure S1). After hydrolysis and reactive dyeing in presence of NaOH, fibers retained their morphology, however, minor increase in the average fiber diameter (325±15 nm) was observed (Figure 2b-c). After dyeing, no significant change in fiber morphology as well as in fiber diameter was seen for DCA nanofibers when compared to HCA nanofibers. The hydrolyzed as well as dyed fibers showed certain roughness on the fiber surface which may be ascribed to mass reduction of the polymer during the hydrolysis and subsequent dyeing processes. This roughness on the fiber surface may be attributed to mass loss during hydrolysis process. Moreover, when the synthesized membranes were subjected to TGA, significant weight loss at ~100oC for HCA and DCA nanofibers compared to pristine CA nanofibers was observed. This weight loss was credited to water molecules evaporation displaying the hydrophilic nature and excellent wettability of HCA and DCA (Figure 2d). Alike TGA results also endorsed similar chemical properties of the HCA and DCA nanofibers. The successful hydrolysis of CA nanofibers was further confirmed through fourier transmission infrared 9 ACS Paragon Plus Environment


attenuated total reflection (FTIR-ATR) spectroscopy analysis. Characteristic adsorption peaks of cellulose acetate attributed to the vibrations of the acetate group C-O at 1740, C-CH3 at 1370 and C-O-C at 1225 cm-1 were observed in the pristine CA (Figure 2d). Vibration peaks of acetate group nearly disappeared after hydrolysis, and a very obvious broad bend between 3500-3100 cm-1 showing the presence of hydrogen bonded –OH group was witnessed for both HCA and DCA nanofibers. Identical spectra of HCA and DCA nanofibers illustrates that DCA nanofibers owed similar chemical characteristics as did HCA nanofibers, and this behavior of DCA nanofibers could be ascribed to incidence of low dye quantity. These outcomes are also supported by previous studies.41, 43 The samples were then subjected to DSC analysis for further validation of the FTIR and TGA evaluations to confirm the successful hydrolysis of the CA nanofibers. Endorsing FTIR as well as TGA results, DSC investigation of HCA and DCA nanofibers showed nearly unchanged thermograms showing no fusion peaks or phase transitions up to 260°C, whereas, a deep melting peak at ~231°C in the thermogram of CA nanofibers demonstrated typical thermal behavior CA nanofibers as reported.46

X-ray diffraction patterns of pristine CA, HCA and DCA nanofibers are

demonstrated in Figure S2. Three major characteristic peaks of cellulose II structure were viewed at 2θ = 12.1°, 20.1° and 22.0° corresponding to (7.19 Å), (4.42 Å) and (4.06 Å) dspacing. More intensive and nearly identical peaks of HCA and DCA nanofibers compared to pristine CA nanofibers in X-ray diffraction patterns also confirmed the successful hydrolysis.41

Figure 2 Figure 3a-e depicts the dyeing performance and wettability features of dyed and pristine CA nanofibers. It was noticed that the DCA nanofibers exhibited excellent dyeability and displayed color yield of ~12.56-16.33 for all the samples (Figure 3a). The dyeing results are relatively better than reported works, and could be credited to the nanofiber membrane 10 ACS Paragon Plus Environment

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thickness.47 The outcomes are also validated by dye fixation results showing that majority of the dye molecules which came in contact with substrate were attached to the fiber. Thus, over 80% dye fixation was obtained for all dyed samples regardless of the dye type (Figure 3a). Considerable color fastness to washing and light results also endorsed the successful dyeing of the resultant nanofibers (Table S1). Samples dyed with RB 19 showed relatively better colorfastness to light owing to having anthraquinone chromogen. Additionally, the aesthetic appearance of the dyed fibers also confirmed the successful even dyeing of the nanofibers (Figure 3d). Figure 3b depicts the water contact angles (WCA) of the pristine, HCA and DCA nanofibers compared to CA flat film. It is apparent from the Figure 3c that CA flat film and CA nanofibers revealed no significant change in WCA over the period 100 s. CA nanofibers presented relatively higher WCA compared to CA flat film which was ascribed to the improved surface roughness owing to fibrous structure at nanoscale. After hydrolysis, when nanofibers were exposed to water droplet, the subsequent nanofibers readily absorbed water droplet within ~2 s. Similar behavior of DCA nanofibers indicated that they also well retained the characteristics of HCA nanofiber and there was no obvious change in the wettability of two membranes, these results also support the FTIR outcomes. Water uptake results also validated outstanding wettability of synthesized membranes showing the fibers could absorb nearly ~17 times more mass of water compared to the mass of the membrane (Figure 3c). Additionally, Figure 3d demonstrates visual appearance of dyed samples, it can be seen that all the nanofibers were not only show highly uniform dyeing but also retained their inherent flexibility. Dyed nanofibers also showed considerable mechanical performance (Figure S3). Moreover, digital images showing before and after dyeing wettability also confirmed the excellent wetting behavior of the hydrolyzed as well as dyed nanofibers (Figure 3e).

Figure 3 11 ACS Paragon Plus Environment


Figure 4a-b presents the surface morphology of top (pristine CA) and bottom (DCA) layers of the synthesized dual layer nanofiber membrane. It could be seen that both CA as well as DCA the layers showed randomly oriented fibrous structure. However, DCA layer nanofibers revealed higher roughness on the surface compared to CA fibers which showed smooth and cylinder like surface, whereas CA nanofibers had relatively bigger diameter (916±15 nm, Figure S4), which may be useful for better water movement by offering larger inter-fiber gaps. Furthermore, the wicking outlook of the two layers is depicted in Figure 4c. It was noticed that DCA nanofiber layer offered ~9 times higher wicking height than CA nanofibers under identical conditions, and this huge water uptake capacity of DCA was credited to the presence of excessive –OH groups in fiber structure.46 Thus, a high wettability gradient between two layers was developed owing to the difference in fiber diameter and wettability of the layers. Furthermore, when the composite membrane was exposed to WCA characterization, it was seen that water droplets could easily transfer from top (CA) to bottom (DCA) layer for all the samples (Figure 4d-i). However, rate of water droplet transport increased with reduction in top layer (Figure 4d-f) and development in bottom layer (Figure 4g-i) thickness, respectively. Digital images displaying the directional moisture transport also validated above findings (Figure 4j). Dual-layer nanofiber membranes fabricated from CA/DCA showed that droplet (aqueous blue ink) readily passed through top layer but did not spread much on top layer (CA nanofibers) owing to its hydrophobic nature. On contrary, water droplets were absorbed in bottom layer and spread throughout the bottom layer surface owing to the strong capillary action and presence of high number of –OH bonds. The difference in the water spreading on the two sides of the dual-layer CA/DCA visually confirmed the directional moisture transport capability of the fabricated membranes.

Figure 4


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Moreover, WCA outcomes and visual water transport behavior of the dual-layer CA/DCA nanofiber membranes are well in line with moisture management test (MMT) findings. Figure 5 depicts the quantitative moisture management performance of the synthesized membrane, red line corresponds to the water content in the top layer (pristine CA nanofiber layer) and blue line symbolizes the water content in the bottom layer (DCA nanofiber layer). During the MMT examination of dual-layer nanofibrous membranes, a total mass of 0.2 g of salted water was consistently dropped for 20 s, and water movement in the membrane was observed for 500 s. It was observed that, for all samples, water content in the bottom layer was higher than the water content in the top layer. Figure 5a-c exhibits water transport in the dual layer membrane with respect to top layer thickness, keeping the bottom layer thickness constant (60±15 µm). Thickness of the top layer was regulated by variation of electrospinning time (30–60 min). It was noticed that water dropped on top layer of membrane could readily reach to the bottom layer, and initially the water content in top as well bottom layers increased in parallel. This initial behavior of the membrane, in the first 20 s of examination could be credited to the water feeding. Since water feeding rate was higher than water transport rate from top to bottom layer in dual-layer membranes during that span, therefore, water content in the both layers increased. However, as soon as the water feeding stopped, apparent difference in the water content of the two layers developed rapidly (Figure 5a). Additionally, top layer still retained significant quantity of water after 500 s, which was attributed to high thickness of hydrophobic (top layer) that hindered the movement water movement. Additionally, when the thickness of top layer was reduced to analyze to effect of top layer thickness on the moisture management performance of the synthesized membrane, keeping the bottom layer thickness constant. A relatively sharp increase in the rate of water transport from top to bottom was observed which is evident from the enhanced difference in the water content of the two layers (Figure 5b-c). This improved rate of water transport was credited to the increased wettability gradient between layers by virtue of top layer mass 13 ACS Paragon Plus Environment


reduction which dropped the retarding (hydrophobic) force.45 Further reduction in the top layer thickness affected the uniformity of the layer, therefore, 30 min duration of electrospinning time for top layer was opted for further research. Furthermore, to evaluate the effect of bottom layer thickness on the moisture management capacity of the fabricated membrane, top layer thickness was kept constant. Increase in the bottom layer thickness (80±15 to 120±15 µm) enriched the mass of layer which enhanced the water uptake capacity of the bottom layer. This improvement in the water uptake capacity, indeed, also gave rise to the wettability gradient between two layers as the mass enrichment in the bottom layer enhanced its water uptake capacity. Thus, rate of water transport further enhanced, distinction between the water content of two layers expended and ultimately optimum thickness of bottom layer could pull out almost all water of the top layer (Figure 5 d-f). Since this thickness (120±15 µm) of bottom layer was enough to pull out almost all water of the top layer, therefore, it was believed that further development in the top layer thickness would only add to the cost of the product. Performance of the optimized sample was further verifies by finger print analysis via MMT software, findings are depicted in Figure S5, obtained results also confirmed outcomes of Figure 5d. Such excellent moisture management performance of the resultant membrane was credited to the improved water uptake capacity of bottom layer, lower retarding force offered by the top layer, and variation in the fiber diameter between two layers. Critical balance of these factors boosted the overall performance of the synthesized membrane.

Figure 5 Figure 6a depicts the accumulative one-way transport index (AOTI) and overall moisture management performance (OMMC) of the synthesized membranes with respect to top layer thickness. A linear increment in AOTI was noticed with the consistent decline of the top layer thickness. This trend of AOTI results was also supported by OMMC outcomes, and could be 14 ACS Paragon Plus Environment

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credited to decrement in the retarding (hydrophobic) force owing to reduction in the mass of the top layer of the membrane. Additionally, the results are also well in line with Figure 5a-c. Figure 6b illustrates the influence of the bottom layer thickness on AOTI and OMMC at fixed top layer mass. It was evaluated that AOTI outcomes as result of incline in the bottom layer thickness well supported the trend of findings conferred in Figure 5d-e. Furthermore, tendency of OMMC results also retained the AOTI outcomes trend and sustained the conclusions of Figure 5d-e. These findings could be credited to lowered hydrophobic force and enhanced wettability gradient which was generated by developing the critical balance between top and bottom layers’ thickness (i.e. providing maximum moisture absorption sites and optimum retarding force), as well as regulated fiber structure of the two layers. Optimum values of AOTI and OMMC determined for the optimized synthesized membrane were 919% and 0.89, respectively, which are better when compared to many reported findings.48-52 Breathability of the textiles is another highly demanded feature for comfortable textiles, and is calculated via water vapor transmission rate (WVTR) evaluation. Therefore, a comparative analysis of WVTR performance of the dual layer CA/DCA nanofiber membrane in the forward (CA/DCA) and reverse (DCA/CA) directions was carried out, results are depicted in Figure 6c. It was observed that the optimized nanofiber membranes offered relatively higher WVTR of 12.11 kg d-1 m-2 in the forward direction than in the reverse direction 9.85 kg d-1m-2. This variation of WVTR in the two directions of the same membrane was attributed to huge difference in fiber diameter and hydrophobic nature of CA nanofibers which provided larger inter-fiber gaps, thus, supplement the water vapor transport in the forward direction (Figure S6). The results are also comparable with reported works.53-58 On the other hand, superhydrophilic nature and relatively compact membrane structure of DCA nanofibers owing their fairly smaller fiber diameter allowed relatively narrow passage for water vapors, thus, water vapors faced relatively more retardation in reverse direction compared to the forward one. Figure 6d compares the current work outcomes with previously 15 ACS Paragon Plus Environment


repoted works. It could be seen that the current work not only offered excellent AOTI but also shown relatively higher OMMC results than most of reported works.48-52

Figure 6 Figure 6e-f demonstrate the possible mechanism of directional moisture transport of the fabricated membranes. Initially, when the water droplet comes into contact with top layer (pristine CA nanofibers, Figure 6e), two opposite forces immediately come into existence i.e. hydrostatic pressure (FHP) pushing water towards the hydrophilic layer, and hydrophobic force (FHF) which opposes the water penetration through the pristine CA nanofibers. Additionally, wettability gradient (FWG) present between two layers and relatively higher inter-fiber gaps of the top layer further push the water droplet towards bottom layer. When the water droplet reaches the surface of bottom layer, superhydrophilic nature and capillary force (FCF) of bottom layer rapid capture the water droplets and ensure to restrict its movement in the opposite direction by quick spreading of water droplet in the bottom layer. On contrary, when the the droplet is exposed first to bottom layer (DCA nanofibers, Figure 6f), FHP pushes the water to penetrate through the membrane but hydrophilic nature and presence of effective FCF ensure the quick dissipation of water droplet in the layer, reduce the FHP per unit area, thus, restrict the water droplet from reaching surface of external surface (i.e. surface next to skin) of the top layer. Moreover, the FHF offered by top layer and presence of FWG between two layers also hinders the water moment from reaching external surface of the top layer. Thus, bottom layer withholds water within itself and does not let water droplet to reach external surface of the top layer.

4. CONCLUSION In summary, an aesthetically appealing dual-layer nanofiber membrane was successfully synthesized with the combination of CA and hydrolyzed and dyed CA nanofibers. One 16 ACS Paragon Plus Environment

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(bottom) of two layers of the dual-layer nanofiber membrane developed from same polymer was hydrolyzed and dyed with reactive dyes. Successful hydrolysis and dyeing of the one layer did not only make it superhydrophilic and developed excellent water up capability but also made it uniformly colored to fulfill aesthetic demands. Dyed layer also offered high color yield (~16.33) and dye fixation (~82%) as well as considerable colorfastness performance against washing and light. Physical combination of relatively open and high diameter CA nanofibers (top layer) with compactly packed DCA nanofibers (bottom layer) with relatively rough surface produced an excellent wettability gradient in the resultant dual-layer nanofiber membrane. This exceptional wettability gradient facilitated the directional liquid moisture transport from CA to DCA layer. Directional moisture transport performance was further confirmed via MMT characterization, and the resultant dual-layer nanofiber membrane exhibited excellent AOTI (919%), remarkable OMMC (0.89) and reasonably high WVT rate of 12.11 kg d-1m-2. This outstanding directional moisture transport performance and uniformly colored appearance of the resultant nanofiber membrane make it potential to be used in the fields of sports- and workwear for improved sweat release. SUPPORTING INFORMATION Table containing colorfastness results of dyed nanofibers, fiber diameter distribution frequency and XRD of CA, HCA and DCA nanofibers, and finger print of moisture management properties of optimized sample. This material is available free of charge via the internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS This work is supported by the Fundamental Research Funds for the Central Universities (No. 2232016A3-03), the Shanghai Rising-Star Program (No. 16QA1400200), the Shanghai Committee of Science and Technology (No. 15JC1400500), the Program for Professor of 17 ACS Paragon Plus Environment


Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2016019), the National Key R&D Program of China (No. 2016YFB0303200), and the National Natural Science Foundation of China (Nos. 51503028 and 51673037).

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FIGURE CAPTIONS

Figure 1. (a) Schematic illustration of fabrication process, (b–c) Probable water movement mechanism through hydrophobic CA and superhydrophilic DCA nanofibers based on Young’s Laplace equation. Figure 2. (a-c) Surface morphology of CA, HCA and DCA nanofibers. (d-f) TGA, FTIR and DSC analysis of CA, HCA and DCA nanofibers. Figure 3. (a) Dyeing performance of DCA nanofibers. (b) Water contact angle and (c) water uptake capacity of CA, HCA and DCA nanofibers. (d) Digital images demonstration of dyed nanofibers showing highly uniform dyeing and their inherent flexible nature after dyeing. (e) Digital demonstration of before and after dyeing wettability of synthesized nanofibers. Figure 4. Morphology and wettability of composite membrane. (a-c) SEM images and wicking characteristics of top and bottom layers of composite layer. WCA composite membranes with respect to top layer spinning time (d-f) and bottom (gi) layer thickness. (j) Digital images showing wettability difference of two layers in composite membrane. Figure 5 MMT analysis of nanofiber composite membrane with respect to top layer electrospinning time (a-c) 60, 45 and 30 min, respectively, and bottom layer thickness (d–f) 80±10, 100±10 and 120±10 µm, respectively. Figure 6. (a-b) AOTI & OMMC with respect to top layer spinning time and bottom layer thickness. (C) WVT performance in forward and reverse direction of composite membrane, (d) comparative analysis with reported works, and (e-f) Schematic illustration water transport mechanism in forward and reverse directions. (FH = hydrophobic force, FWG = wettability gradient, FHP = hydrostatic pressure, FCF = capillary force). 25 ACS Paragon Plus Environment


Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



“Table of Contents" Graphic


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