Electrospun Ultrafine Fiber Composites Containing Fumed Silica

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Electrospun Ultrafine Fiber Composites Containing Fumed Silica: From Solution Rheology to Materials with Tunable Wetting Martin K Dufficy, Mackenzie T Geiger, Christopher A. Bonino, and Saad A Khan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03545 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on November 1, 2015

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Electrospun Ultrafine Fiber Composites Containing Fumed Silica: From Solution Rheology to Materials with Tunable Wetting Martin K. Dufficy*, Mackenzie T. Geiger*, Christopher A. Bonino*† and Saad A. Khan 1* Department of Chemical and Biomolecular Engineering North Carolina State University, Raleigh, NC 27695-7905 Keywords: silica, particle distribution, surface modification, polyacrylonitrile, nanofiber Abstract Fumed silica (FS) particles with hydrophobic (R805) or hydrophilic (A150) surface functionalities are incorporated in polyacrylonitrile (PAN) fibers by electrospinning to produce mats with controlled wettability. Rheological measurements are conducted to elucidate the particle-polymer interactions and characterize the system while microscopic and analytic tools are used to examine FS location within both fibers and films to aid in fundamental understanding of the wetting behavior. Unlike traditional polymers, we find these systems to be gel-like, yet electrospinnable; the fumed silica networks breaking down into smaller aggregates during the electrospinning process and dispersing both within and on the surface of the fibers. Composite nanofiber mats containing R805 FS exhibit an apparent contact angle over 130o and remain hydrophobic over 30 minutes, while similar mats with A150 display surface-wetting with a static contact angle of ~30o. Wicking experiments reveal that the water absorption properties can be further manipulated, with R805 FS-impregnated mats taking up only 8% water relative to mat weight in 15 minutes. In contrast PAN fibers containing A150 FS absorbs 425% of water in the same period, even more than the pure PAN fiber (371%). The vastly different responses to water demonstrate the versatility of FS in surface modification, especially for sub-micron fibrous mats. We discuss in this regard the role of fumed silica in terms of their surface functionality, placement on nanofibers and induced surface roughness.

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co-first authors corresponding author: email:[email protected]; ph: 919-515-4519; fax: 919-515-3465 † current address: RTI International, Research Triangle Park, NC 27709

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Introduction The development of materials with tunable water repellency and absorbent tendencies is highly desirable in a wide range of applications, from moisture management in performance textiles [1][2] to peptide assembly for biomolecular sensors [3] and polymeric materials that inhibit biofouling [4][5]. Researchers employ many methods to achieve materials with such desired characteristics, the most common being topical approaches that includes coating and/or chemical grafting of functional moieties to the surface of the substrate of interest, be it films or fibers [6]. In this study we focus on fibers but take a different approach. We incorporate fillers of different surface groups into the material of interest prior to spinning them into fibers, thus presenting a facile, and one-step process. In this regard, we use polyacrylonitrile (PAN) as a model polymer and fumed silica (FS) as a filler comprising surface modifications to render the surface hydrophobic or hydrophilic.

These systems are electrospun to produce composite

nanofibers wherein PAN acts as ‘scaffolds’ for the FS particles. We chose PAN as a model matrix because the electrospinning of PAN has been extensively studied [7][8]. Our rationale for using electrospinning stems from the fact that it is a continuous, simple process but more importantly it enables formation of ultrafine composite fibers with various fillers [9][10]. Such small fiber diameters are needed to exploit the functionality of the fillers [10]. Finally, our motivation for using FS arises from its industrial relevance, and the versatility it offers in various applications from coatings to gel electrolytes and flame retardant materials [11][12][13][14][15][16]. The vast scope of uses for fumed silica can be attributed to its inherent structure and surface functionality. Individual fumed silica entities are a branched structure composed of fused SiO2 units (~12 nm diameter), the surface of which may be decorated with different moieties. While the latter allows for introducing


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functionalities, the irregular shape with its high surface may provide the desired properties at a much lower volume fraction compared to its spherical analog. Despite the promise of electrospun PAN/FS nanofibers, critical issues that dictate their success center around evaluating the electrospinning conditions, and whether bulk blending of hydrophobic or hydrophilic FS lends itself to producing the desired surface effect. These issues in turn entail understanding of the rheology of the system and examining the location of the FS in the nanofibrous membranes. Solution rheology, even as measured in shear mode and at low shear rates, has been related to electrospun fiber morphology. Studies [17][18][19][20] have shown that increases in solution viscosity and/or relaxation times can increase fiber diameters and affect bead defects. While these relationships have been well-established in polymer solutions, electrospinning literature has limited studies that correlate the rheological properties of polymer fillers, such as fumed silica, with fiber morphology. This becomes particularly relevant here as the complex structure of FS lends itself to the formation of networks and aggregates. Previous works have fabricated and characterized electrospun composites containing PAN, silica and other fillers [21][22][23][24]. However, these studies examined various applications such as energy and battery storage without heed to particle-polymer interaction and rheology. Lim et al [21], nevertheless, identified the importance of polymer-particle compatibility on spherical silica nanoparticle distribution within the electrospun fibers, but did not focus on understanding colloidal interactions. Regardless, the rheology of FS with its branched structure is significantly different and complex than spherical colloidal fillers used in their work. In this study, we investigate the effects of particle fillers on the morphology and wettability of PAN fibers. While several studies have been done on PAN/FS systems [21][22][23], none have focused on wettability and how it can be tuned using different FS fillers, adding novelty to our work. We use rheological methods to characterize differences in the PAN


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system with each type of FS (hydrophobic vs. hydrophilic), and compare rheological trends with the corresponding electrospun fiber morphology. Elemental analysis and microscopy on the composite PAN/FS fibers assess the role of FS surface modification in particle distribution within the electrospun fibers. The dispersion of FS particles was also investigated on/within composite PAN/FS films to decouple the effects of the FS surface energies and the electric field applied during the electrospinning process. Finally, we investigate wetting behavior of both films and fibers as they relate to water repellency, water absorption, and vertical wicking. Through the addition of functionalized FS, we demonstrate tailored wetting properties of PAN/FS fibers, with apparent contact angles ranging from 12-120° over 30 minutes and water uptake from 8-425 wt% after 15 minutes.

Experimental Materials. Polyacrylonitrile (PAN, MW = 150,000 g/mol, Scientific Polymer Products, Ontario, NY), polyethylene oxide (PEO, MW = 600,000 g/mol, Dow Chemical, Midland, MI), and anhydrous N, N-dimethylformamide (DMF, Sigma Aldrich, St. Louis, MO) were used as received. Fumed silica particles with different surface modifications, Aerosil A150 (silanol) and R805 (octyl) were obtained from Evonik Degussa Corporation (Piscataway, NJ).

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preparing solutions, fumed silica particles were dried in a vacuum oven for 2 days at 120ºC. Preparation of Polymer/FS solutions. Polymer solutions were prepared by adding PAN into DMF resulting in a 14 wt% solution and stirring with a magnetic stir bar overnight. A separate solution containing FS particles and DMF was mixed at 6000 RPM for 5 min using a high-shear mixer (L5M-A Laboratory Mixer, Silverson Machines, East Longmeadow, MA) and then added to the polymer solution. The combined solution was shear mixed for 10 min at 6000


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RPM. The concentration of the FS/DMF solution varied such that the combined solution contained 7.9 wt% PAN and 0.5, 1.3, or 2.2 wt% FS. Rheological measurements. Rheological experiments were performed on a Discovery Hybrid Rheometer-2 (TA Instruments, New Castle, DE) with a 40 cm, 2º cone and plate geometry. All samples were measured at 25ºC. A steady pre-shear stress (e.g., 200 Pa) was applied for 2 mins to each sample to erase any shear history followed by a 3 minute equilibrium period for the FS networks to restructure. Dynamic and steady-shear stress experiments were conducted on each sample. The stresses applied in the frequency sweeps were selected from the linear viscoelastic (LVE) regime in the stress sweeps. All rheological measurements were repeated on at least two different samples. Preparation of Polymer/FS Nanocomposites. Fibers were prepared by electrospinning in accordance with a setup described elsewhere [25][26]. In brief, a polymer solution was ejected from a 22 gauge needle using a syringe pump (model NE-1010, New Era Pump Systems, Inc., Wantagh, NY). An electric field was applied between the needle and grounded collector plate using a high voltage power supply (model AU-60P0.5, Matsusada Precision, Inc. Kusatsu-City Japan), which was adjusted until a Taylor cone produced a stable polymer jet. Each experiment was conducted using a flow rate of 1 mL h-1, 15 cm needle-to-plate separation distance, 35 – 55% relative humidity, and 21 – 25ºC. Spun-cast films were prepared using a spin coater (Laurell Technologies, North Wales, PA) with a spin speed of 2000 RPM and a 1000 RPM/s ramp rate. Solution cast composite films were prepared by film casting 40 µm of PAN/ FS solution onto 10 µm-thick Cu foil. Excess solvent was removed overnight in a vacuum oven at 60ºC. Characterization of Polymer/FS Nanocomposites. Composite fiber mats and films were analyzed with a field emission scanning electron microscope (FE-SEM, FEI XL300 and FEI


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Verios 470L) and a transmission electron microscope (TEM, FEI Tecnai G2 Twin). Cross sections fibers were prepared by freeze fracturing, in which fiber mats were submerged in liquid nitrogen and cut with a razor blade. All SEM samples were sputter-coated with 7 to 10 nm of gold prior to analysis. Fiber diameters were measured with ImageJ software (National Institutes of Health, USA) using 50 fibers per sample. Fibers were electrospun directly onto carbon-coated grids for TEM analysis. Surface analysis of the composite fibers was conducted on a SPECS Xray Photoelectron Spectroscopy (XPS) unit equipped with a dual Al/Mg anode monochromator source. Adventitious carbon (C1s, 285 eV) was used as a reference. A Shirley background was used for elemental transitions, and peaks were fit using a Gaussian-Lorentzian function, GL(30). Contact angle measurements were made using a goniometer (FTA1000, First Ten Angstroms, Inc., Portsmouth, VA) equipped with First Ten Angstroms software. Prior to making contact angle measurements, the samples in our study were dried in a vacuum oven at 80ºC for at least 12 h in order to reduce the effects of residual solvent. The temperature was kept below the glass transition of PAN to avoid deformation of the fibrous mat structure. Apparent contact angles were determined on films and electrospun fibers from the sessile profile of deionized water droplets (approx. 5 µL) 15 seconds after deposition. The water droplets were deposited on the sample by moving the stage until the droplet was in contact with the sample, and then slowly re-lowered. Each sample was evaluated in triplicate. Contact angles of hydrophobically-modified samples (films and fibers) over time were collected for a period of 30 mins. The deposition of water droplets on PAN and hydrophilically-modified electrospun mats were also recorded. Images were exported at the time of initial contact of the water droplet and at regular intervals to qualitatively compare wetting behavior of the hydrophilic nonwoven mats. A dynamic contact angle analyzer (DCA-315, Cahn) was used to measure the water uptake capabilities of the composite nonwoven mats. The nonwoven mats were each cut to 250


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mm by 350 mm portions and weighed pre/post analysis. The mats were immersed 1-mm into an infinite water reservoir. Weight measurements commenced once the zero-depth-of-immersion was reached and continued for 15 minutes. The vertical wicking height of the fiber mats was conducted in accordance with literature procedures [27][28]. Briefly, the mats were cut into 100 mm by 500 mm portions and weighed. The mats were hung over an infinite water reservoir and immersed 1 mm. Wicking height was measured with a ruler placed beside the immersed sample. We performed all wicking tests in triplicate to ensure reproducibility.

Results and Discussion Nanocomposites of PAN and FS with Different Surface Chemistries. We begin by examining the rheology of systems containing PAN, DMF, and either hydrophobically- (R805) or hydrophilically-modified (A150) fumed silica (Figures 1). The use of A150 and R805 FS allows for a comparison of fillers with similar surface areas (150 m2 g-1) and densities (~55 g L-1). Figure 1a shows the elastic (G’) and viscous (G’’) moduli of these systems as a function of frequency obtained using dynamic rheology. Unlike the PAN-only system (7.9 wt.% in DMF) that shows typical solution-like features with G’’ being larger than G’ and both moduli having strong frequency dependence, the PAN/FS (7.9:2.2 wt % in DMF) systems exhibit gel-like behavior [29] [30]. The PAN/R805 sample shows G’ dominating G’’ and being essentially flat, particularly at low frequency, reminiscent of a sample-spanning network [29]. The viscous moduli of both the filled systems are approximately equal over the observed frequency; however, the elastic modulus of the PAN/R805 sample is 5 times larger at low frequencies than that of the sample with A150, indicating the PAN/R805 sample forms a more interconnected network and a stronger gel [29]. Consistent results are also observed from steady shear experiments. Viscosity of both PAN/FS samples (Figure 1b) plotted as function of


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Figure 1. Solution rheology showing (a) frequency spectrum of the elastic, G’ (filled symbol) and viscous, G’’ (open symbol) modulus of 7.9 wt% PAN-only solution and those containing same amount of PAN together with 2.2 wt% R805 (hydrophobic) or A150 (hydrophilic) fumed silica (FS); (b) steady shear viscosity as a function of shear stress for the same samples. shear stress reveal the lack of a Newtonian plateau at low stresses indicative of the presence of microstructure. In fact, at low stresses, we observe a steep drop in viscosity with increasing stresses suggestive of sample yielding and presence of a ‘yield stress’ as one would expect from a gel network [13][30]. In this regard, the PAN/R805 system has a greater “yield stress” than the PAN/A150. Extrapolation of steady shear data towards the y-axis on the stress versus strain plot (Supplemental Figure S1) reveals yield stress of the PAN/R805 sample to be an order of magnitude higher than that of the PAN/A150 sample. At high stresses (Figure 1b), the viscosities


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of both FS-filled systems converge as a result of the breakdown of the microstructure. In contrast, the PAN solution exhibits a low viscosity with a large Newtonian plateau and slight shear thinning at high stresses. The rheology results taken together, the higher G’ and yield stress in the PAN/R805 system, indicate that the hydrophobic R805 FS creates stronger networks within the PAN solution compared to its hydrophilic analog, the A150 FS. To explain why R805 forms stronger networks than A150 in DMF, we look towards the interactions between the FS particles and their surroundings (solvent), which we have previously reported, using hydrophobic and hydrophilic FS in polyethylene glycol [29].

The hydrophobic R805 FS contains approximately 50%

nonpolar n-octyl surface chains, which sterically shields the remaining surface (silanol) groups on the FS [29]. These non-polar octyl surface layers attract other R805 FS particles resulting in the flocculation of R805 FS particles, in a process akin to a reverse of steric stabilization in which the ‘stickiness’ of surface chains arises from their poor solvency in the polar solvent matrix [29]. In contrast, flocculation of the silanol-decorated, hydrophilic A150 is dominated by weak van der Waals forces resulting in a weaker network. Figure 2 shows the dynamic and steady shear rheological behavior of systems containing 0.5, 1.3, and 2.2 wt% R805 FS in 7.9 wt% PAN/DMF. While the primary purpose of this experiment was to characterize these systems as part of using various loadings of R805 in PAN/DMF to create composite nanofibers with tunable wetting properties, the results also shed light on the colloidal interactions in the system. For the sample with low (0.5 wt%) R805 concentration (Figure 2a), we find the absence of any gel-like behavior with G’’ dominating G’ and both being frequency dependent. However, some microstructural interactions take place as evidenced from the absence of typical terminal slopes observed in polymer melts and solutions


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[30]. This result is consistent with steady shear viscosity data (Figure 2b), which shows no Newtonian plateau at low shear stresses indicating the presence of some aggregation.

Figure 2. Solution rheology of 7.9 wt% PAN with 2.2, 1.3, and 0.5 wt% R805 fumed silica ; (a) elastic (G’) and viscous (G’’) moduli as a function of frequency in which filled and open symbols represent G’ and G”, respectively; (b) steady shear viscosity as a function of shear stress for same samples. As the concentration of R805 is increased to 1.3 and 2.2 wt%, the percolation threshold is possibly crossed and we observe the presence of a sample-spanning gel network with G’ higher than G’’ and relatively frequency-independent (Figure 2a). The steady shear viscosity (Figure 2b) of these samples also reveals existence of yield stress at low stresses, as the viscosity rapidly decreases with increasing stress at a certain stress level. Extrapolation of steady shear data


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towards the y-axis on the stress versus strain plot (Supplemental Figure S2) provides a value for the yield stress of the 1.3 wt% R805 system approximately double that of the 2.2 wt% A150, further emphasizing that R805 networks are stronger than those formed by A150 in PAN/DMF. Electrospun Fibers of PAN/FS Electrospun fibers obtained from our systems reveal that the addition of fumed silica affects their morphology (Figure 3). Notably, an increase in (a) fiber diameter along with differences in the (b) bulk and (c) surface morphology are observed in PAN/FS fibers when compared to pure PAN fibers. PAN fibers without FS filler have uniform diameters (242±90 nm), without bead defects (Figures 3a, b). In comparison, the composite PAN fibers containing 2.2 wt% A150 (Figures 3c, d) and R805 FS (Figures 3e, f) have diameters ~150 nm larger and comprise a ~3-fold increase in variance of diameter size distributions when compared to pure PAN fibers pure polymer: 397±310 and 382±270 nm, respectively. A reduction in R805 FS content results in similar fiber diameters; composite fibers containing 0.5 (Figure 3g, h) and 1.3 wt% R805 have fiber diameters of 340±156 and 347±220 nm, respectively. The increase in fiber diameter may be attributed to the differences in the rheological behavior between the filled and unfilled systems; all PAN/FS samples have similar viscosities at high shear rates, approximately two times higher than that of pure PAN. The larger solution viscosities may lead to increased forces within the composite polymer jet resisting elongation during electrospinning and in turn producing larger fiber diameters. Such behavior has been widely reported in many polymer solutions (reviewed by Li and Xia [31]). Figure 3 also shows the presence of bead defects and a larger fiber size distribution in FS containing fibers. This may be explained in terms of the breakdown of the FS network and resulting non-uniform distribution of the FS during the electrospinning process. While the


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PAN/FS systems have yield stresses and a sample-spanning structure, the elongational forces at the formation of the electrospun jet (near the Taylor cone) are well beyond [32] the PAN/FS

Figure 3. Scanning electron micrographs of (a, b) PAN-only electrospun fibers, as well as composite PAN/FS fibers containing (c, d) 2.2 wt% A150, (e, f) 2.2 wt% R805, and (g, h) 0.5 wt% R805. All fibers were electrospun using a 7.9 wt% PAN solution. Silica appears whiter than the polymer fibers, due its high electron density.


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yield stresses leading to breakdown of the macro-scale network. However, SEM images of the fibers reveal that some aggregates remain intact. During the bending and whipping process of electrospinning, the non-uniform size distribution of remaining FS aggregates travel in the electrospun jet toward the collector plate. In this region, capillary instabilities in the partially solidified jet may lead to bead defects in the deposited fibers [33]. We speculate that the broad size distribution of FS aggregates within the jet lead to localized regions of disproportionate levels of moduli, which are more susceptible to the capillary instabilities. A report by Wu et al [34] on carbon nanotube (CNT)-filled electrospun nanofibers confirmed our finding; bead defects occurred when CNTs were not well dispersed with the polymer solutions, regardless of enhancements to the solution moduli. In addition to the fiber diameter and bulk morphology, FS also contributed to differences in the fiber surfaces when compared to pure PAN fibers. PAN fibers in the absence of FS have complex surface textures, which we ascribe to the ambient conditions during electrospinning (Figure 3b). The textured surface appearance on the fibers becomes more pronounced with PAN/FS fibers (Figure 3d, f). Due to the high volume fraction of FS fillers in the PAN solution, we speculate that the electric field and surface energy of the filled system have little impact on the positioning of FS particles. The surfaces of the composite fibers contain FS particles, as well as pits (the result of voids from weakly-bound FS or poor polymer surface coverage). Crosssectioning the composite fibers reveal FS within the fiber interior (Supplemental Figure S3) and display the same holes and pits as those observed on the fiber surface. TEM also confirmed the presence of FS aggregates below the fiber surface, which were not present in the PAN-only fibers (Supplemental Figure S3). Surface characterization of PAN/FS films and fibers


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We compared composite PAN/FS films properties to those of the corresponding fibers, to determine what role, if any, the fiber formation process has on FS distribution and/or surface migration. Figure 4a, d shows TEM images of spun cast films containing both types of FS. Aggregates of fumed silica particles, up to 2 µm in their largest dimension, are present within both films at loadings of 2.2 wt% FS. The existence of these FS aggregates is consistent with the origin of gel-like features observed in rheological measurements for these systems [35]. An array of FS structures form associations that span distances on the order of the rheometer gap size, creating a gel. Similar features are observed in films in which the large volume fraction of filler places the FS both inside the film cross-section (Figure 4c, f) and on the surface of the film (Figure 4b, e) irrespective of surface functionality of the FS; the surface energy of the FS particles plays a nominal role in the location throughout the film.

Figure 4. Transmission electron micrographs of (a) PAN/A150 FS and (d) PAN/R805 FS spuncast films; SEM images of the air-surface interface for solution-cast films containing (b) PAN/A150 and (e) PAN/R805 with the inset showing a high-resolution image of the film surface; cross sections of solution-cast films containing (c) PAN/A150 FS and (f) PAN/R805 FS.


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To probe further the role of FS surface functionality on its placement, both in fibers and films, we used elemental weight ratios of Si/N from XPS (Figure 5a) to characterize surface migration of the FS. It should be noted that the octyl groups present on the Si surface may hinder Si signal intensity (Figure 5b). The detection of Si and N in the system were solely attributed to fumed silica (SiO2) and PAN (C3H3N)n, respectively. As expected, composite FS/PAN films had

Figure 5: XPS data showing (a) the weight ratios of elemental Si to N for fibers and films and (b) the Si2p transition for the PAN/FS composites. All samples contained 7.9% PAN and 2.2 wt% FS.


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a similar Si/N ratio, which we attributed to network formation resultant of the high loadings of FS in the system. However, slight surface migration was seen on the PAN/FS films; the R805 films display a larger Si/N ratio than the A150 films, as the hydrophobic R805 FS migrates towards the hydrophobic air. Contrarily, more Si was measured on the surface of the hydrophilic A150 fibers than the R805 fibers, suggesting that the surface energy of the FS does not play a role in the migration and/or location of FS particles in the composite fibers. Elemental Si/N wt ratios of 0.45 and 0.35 for A150 and R805 fibers, respectively, were observed. Composite PAN/FS fibers had a much lower Si/N ratio than films possibly because the bending and whipping of the polymer jet forced the polymer to elongate, which increased inter-particle distances [36] and exposed more elemental nitrogen. It is possible that the greater yield stress of R805/PAN solutions leads to larger FS aggregates when compared to A150/PAN solutions, and exposes less surface Si surface area. We also examined if polymer-particle compatibility affected the FS particle distribution on and within the fibers by replacing PAN with a more hydrophilic polyethylene oxide (PEO). polymer. PEO was blended with fumed silica and DMF by the same procedure used with PAN solutions, and electrospun. Since the hydrophilic FS was not completely encapsulated within PEO fibers, and was distributed on the fiber surfaces similar to R805 (Supplemental Figure S4), we conclude that FS particle location was not determined completely by polymer-particle chemical compatibility. Other groups [37] have proposed that FS experiences charging in an electric field, which can lead to particle motion in the field direction. However, this is contrary to what we observe in our electrospun fibers (Figure 3). Composite PAN/FS fibers prepared by wet spinning in the absence of an electric field also reveal that FS is present within the fiber cross sections, as well as on the surface (Supplemental Figure S5), reiterating the point that the electric field does not play a role in surface migration of heavily filled PAN/FS systems.


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Tailored Wettability of Electrospun PAN Fibers with Fumed Silica The ability to incorporate FS with different functionality on the surface of composite fibers via electrospining presents a convenient, one-step strategy for modifying the water repellency of the resulting mat. Figure 6 compares the wetting behavior of filled systems to pure PAN fibers and film controls (Figure 6). PAN fiber mats in the absence of FS display hydrophilic behavior causing water droplets to quickly spread across the surface and absorb into

Figure 6. Water droplet profiles during absorption of (a) PAN/A150, and (b) PAN electrospun fiber mats; (c) apparent contact angles of films and electrospun fibers with and without filler incorporation.


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the fiber themselves. This result is consistent with wettability measurements on electrospun PAN mats previously reported in the literature [38]. Electrospun mats containing 2.2 wt% A150 rapidly absorbed water droplets as also seen in the unfilled PAN system. However, the A150containing fiber mats (Figure 6a) absorbed water droplets at a faster rate than that of the unmodified PAN mat (Figure 6b). The hydroxyl surface groups on the FS particles, which are accessible on the fiber surfaces, effectively influence the interactions between the mat and liquid. As hydrophobically-modified FS was introduced to the electrospinning solutions, the resulting mats began to show apparent contact angles exceeding 120° (Figure 6c), which may again be attributed to the accessibility of the octyl surface groups on R805 fumed silica. Interestingly, we observed no significant statistical difference in apparent contact angle between a 0.5 wt% and 2.2 wt% R805/PAN fiber mat. The high contact angles we observed with R805 fillers in PAN fibers may be influenced by a combination of (a) the roughened nanofibrous structure that was created via incorporation of nanoparticles, and (b) hydrophobic nature of the fillers. To decouple the effects of any induced surface roughness imparted by the fibrous mats, cast films were also investigated (Figure 6c). Prepared films containing A150, R805 and pure PAN were compared using static contact angle measurements. Unlike the A150 and pure PAN fiber mats, films of A150 and pure PAN produce contact angles of 28.1+2.3° and 52. 3+2.5°, respectively. A sessile profile is obtainable due to the lack of porosity in the casted films compared to the nanofiber mats. The R805-filled films show an increase in hydrophobicity as the 0.5 wt% R805 is increased to 1.3 wt% R805 with an increase in contact angle from 67.1+5.0° to 86.5+3.3°, respectively. The contact angle does not change significantly when the concentration of R805 is increased to 2.2 wt% (θc=84.3+2.4°) indicating that a maximum performance threshold may be present with respect to apparent contact angle.


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To further probe the repellent characteristics of the hydrophobically-modified nanofibers, contact angles were also recorded over time (Figure 7). Droplets remained on the top surface of a the 1.3, and 2.2 wt% R805/PAN mat with an average apparent contact angle (θc) of 131.7+6.8 º, and 133.3±8.3º, respectively, upon initial deposition. Both mats maintain their hydrophobic nature over duration of 30 min. However, the 0.5 wt% R805/PAN mat which initially displays hydrophobicity, registering a 124.2+9.8° apparent contact angle, shows a decrease in its apparent contact angle at an average rate of approx. 4°/min. This may be attributed to the porosity and hydrophilicity of the PAN fibers, in addition to the lower volume fraction of fumed silica present in the electrospun mat.

Figure 7. Apparent contact angle of 0.5, 1.3, and 2.2 wt% R805/PAN over time. The vastly different responses to water demonstrate the versatility of FS in surface treatments, especially in sub-micron fibrous mats. Regardless of the behavior of the pure PAN fibers, our results highlight the fact that incorporation of FS particles into the nonwoven mat of fibers can be exploited to modify the surface wettability. Additional refinements can be explored to expand the composite mat capabilities.

For example, strategies to generate highly


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hydrophobic properties (θc>120º) by manipulating fiber size and morphology [39], as well as fiber alignment [40] have previously been reported. To further investigate the wettability properties, we looked to characterize the wicking behavior in composite PAN/FS fibers. Vertical Wicking and Water Absorption Properties of Electrospun Fibers We measured wicking as a function of nanofiber mat weight and height using various concentrations of FS to alter sample wettability. Throughout all of our experiments, mats containing hydrophilically-modified FS were able to uptake more water than pure-PAN mats. The fiber mats containing A150 wicked approx. 33 mg water/mg mat while pure PAN fibers wicked around 29 mg water/mg mat (Figure 8a) in 15 minutes. The rate at which uptake occurred also increased when A150 was added to PAN fibers. We observed very little wicking behavior on the PAN/A150 samples after 10 minutes of immersion into the water reservoir (2% increase is mat weight) compared to the pure pan samples (10% increase in mat weight). Moreover, the incorporation of hydrophobically-modified FS into the mat decreased water uptake significantly. Composite fibers comprising 1.3 and 2.2 wt% R805 repelled water in the reservoir and pushed up against the scale, creating negative values of water uptake. The wicking measured is a combination of flow within (a) capillary spaces and (b) any corresponding diffusion into the nonwoven fiber mat as a result of capillary flow; the wicking and wetting of the mats occur in parallel. Water flows, initially with no saturation, from the smallest pores to the largest pores until the system reaches 100% saturation or a hydrodynamic equilibrium. The incorporation of FS into the 2.2 wt% A150/PAN mat effectively produced fibers with more pores/pits than pure PAN fibers, corresponding to a larger surface area and thus a greater amount of wetted fibers. The rapid uptake of water upon initial immersion of the PAN/A150 fibers may result in an increase in permeability [41], which drives faster capillary flow than PAN-only fibers. Both the pure PAN and PAN/A150 fiber systems exhibit contact


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Figure 8. Wicking behavior of the PAN/FS composites; (a) water uptake over time, normalized to the mass of the matt and (b) the corresponding water uptake after 900 seconds in relation to initial mat weight; (c) wicking height over time. All composite fiber mats were electrospun with 7.9 wt% PAN.


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angles R805) produced mats with apparent contact angle over 130o that maintained their hydrophobicity over 30 min, while lowering the R805 FS content to 0.5 wt% eventually led to a fully wetted fiber. In contrast, A150-containing mats absorbed water more than the umodified PAN mat. Additionally, wicking behavior was tuned corresponding to the surface modification and weight fraction of FS fillers; PAN/A150 took up 425% of water/mass of fiber mat compared to only 8% for the corresponding PAN/R805 mat.


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Acknowledgements This work was partially supported by National Science Foundation GOALI Program (#0555959) and a Department of Education GAANN Fellowship (CAB). The authors thank Prof. Jan Genzer and Dr. Kirill Efimenko for helpful discussions on surface energy. Supporting Information Stress-strain relationship for PAN solutions with and without FS; scanning and transmission electron micrographs of various systems that includes top surface and cross sectional images. This information is available free of charge via the Internet at http://pubs.acs.org

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