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Formation and Mechanism of Superhydrophobic/ hydrophobic Surfaces Made from Amphiphiles Through Droplet-mediated Evaporation-induced Self-assembly Fangyuan Dong, Mi Zhang, Waiwa Tang, and Yi Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b00011 • Publication Date (Web): 02 Apr 2015 Downloaded from http://pubs.acs.org on April 6, 2015

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The Journal of Physical Chemistry

Formation and Mechanism of Superhydrophobic/Hydrophobic Surfaces Made from Amphiphiles through Droplet-Mediated Evaporation-Induced Self-Assembly

Fangyuan Dong, 1 Mi Zhang, 1 Wai-Wa Tang, 1 Yi Wang*,1,2,3

1

Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic

University, Hong Hum, Kowloon, Hong Kong SAR 2

Food Safety and Technology Research Center, Hong Kong PolyU Shenzhen Research

Institute, Shenzhen, PR China 3

Shenzhen Key Laboratory of Food Biological Safety Control, Hong Kong PolyU

Shenzhen Research Institute, Shenzhen, PR China

[email protected], [email protected], [email protected], [email protected]

*To whom correspondence should be addressed. Tel: 852-34008673. Fax: 852-23649932. E-mail: [email protected].

1 ACS Paragon Plus Environment


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ABSTRACT Superhydrophobic/hydrophobic surfaces have attracted wide attention because of their broad applications in various regions, including coating, textile, packaging, electronic devices, and bioengineering. Many studies have been focused on the fabrication of superhydrophobic/hydrophobic surfaces using natural materials. In this paper, superhydrophobic/hydrophobic surfaces were formed by an amphiphilic natural protein, zein, using electrospinning. Water contact angle (WCA) and scanning electron microscopy (SEM) were used to characterize the hydrophobicity and surface morphology of the electrospun structures. The highest WCA of the zein electrospun surfaces could reach 155.5±1.4°. To further understand the mechnism of superhydrophobic surface formation from amphiphiles using electrospinning, a synthetic amphiphilic polymer was selected, and also, a method similar to electrospinning, spray drying, was tried. The electrospun amphiphilic polymer surface showed a high hydrophobicity with a WCA of 141.4 ± 0.7 ° .

WCA of the spray-dried zein surface could reach 125.3 ± 2.1 ° . The

secondary structures of the zein in the electrospun film and cast-dried film were studied using ATR-FTIR, showing that α-helix to β-sheet transformation happened during the solvent evaporation in the cast drying process but not in the electrospinning process. A formation mechanism was proposed based on the orientation of the amphiphiles during the solvent evaporation of different fabrication methods. The droplet-based or jet-based evaporation during electrospinning and spray drying led to the formation of the superhydrophobic/hydrophobic surface by the accumulation of the hydrophobic groups of the amphiphiles on the surface, while the surface-based evaporation during cast drying


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led to the formation of the hydrophilic surface by the accumulation of the hydrophilic groups of the amphiphiles on the surface.

Keywords:

zein,

electrospinning,

superhydrophobic,

evaporation


droplet-mediated

solvent


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INTRODUUCTION Hydrophobic surface is a surface exhibiting water contact angle (WCA) higher than

90°. It has wide application in various areas, such as coating, textile, packaging, electronic devices, and biomedical engineering.1-2 Superhydrophobic surface is a special kind of hydrophobic surface with a WCA higher than 150°, and it has a self-cleaning property.3 Increasing the surface roughness and reducing the surface energy are the two general ways to improve the hydrophobicity of a surface.4 Electrospinning is one of the effective ways to make superhydrophobic surfaces from synthetic polymers, such as polystyrene (PS)5-7 and polylactide (PLA).8 In electrospinning, a highly porous network consisting of nanofibers is usually formed by the deposition of charged liquid jets from a syringe pump under a high electrical field.9 Electrospinning could largely increase the surface hydrophobicity by increasing surface roughness and changing surface chemistry.6 The electrospun nanofibrous mats are usually special, with additional wrinkles, grooves, or pits on individual fibers which largely increase the surface roughness. The rearrangement of polymer molecules also happens during electrospinning. Cui et al. studied the effect of chemical groups on surface wettability during electrospinning. It has been noticed that an enriched hydrophobic methyl group could be detected by X-ray photoelectron spectroscopy on the electrospun poly (D, L-lactide) fibers and an enriched methylene group was detected on polyethylene glycol electrospun fibers as well.10 Zein is a major protein from corn. It is not water-absorbing and can be dissolved in 40~95% ethanol-water mixture.11 Zein is an amphiphilic protein extracted from corn endosperm. More than 50% of its amino acids constituents are hydrophobic.12 The


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molecular structure of zein in 70% methanol has been investigated by Argos et al. using CD spectra.13 They proposed a structural model of a ribbon-type structure of the size 13×1.2×3 nm3, in which 9-10 helical segments were aligned in an antiparallel style and the segments were linked together by glutamine-rich turns. Later, Matsushima et al. studied the structure of zein in 70% ethanol solution using SAXS and a rectangular prism model, similar to the one given by Argos et al., was proposed.14 In both arrangements, the top and bottom of the zein molecule, which are made by the glutamine-rich bridges, are hydrophilic, while the sides of the zein molecule, which are the zein α-helices, are hydrophobic. Because more than 50% of the amino acids in zein molecule are hydrophobic, zein has a great potential for hydrophobic surface formation.12,

15

There

have been many studies about the fabrication of zein nanofibers using electrospinning. Most of the studies were aiming to improve the mechanical properties of the zein electrospun films to make it more suitable for clinical use, especially in tissue engineering.16-17 However, the research on the improvements of the surface properties, including surface hydrophobicity, of the zein electrospun film (ZEF) was seldom done. The study on the formation of the electrospun nanofibers will not only make the control of their surface properties possible but also give a mechanism that can explain the relation between the surface hydrophobicity and the morphology. The electrospinning of zein is a droplet-mediated, or jet-mediated evaporationinduced self-assembly process. Different from the evaporation style in the cast drying of zein solution to make zein film, during electrospinning the zein solution is dried in the air in the shapes of droplets or jets. It was learned that the α-helix to β-sheet transformation of zein happened during the cast drying of zein solution.12 The α-helix to β-sheet



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transformation of zein could be promoted by the increased solvent polarity.12 For the αhelix to β-sheet transformation, the intrinsic hydrogen bond among the peptide backbones played an important role in the determination of the peptide propensity.18 As the α-helix was more constrained by the hydrogen bonds, the structure would transform to a more energy favorable β-sheet structure.19 The α-helix to β-sheet transformation of zein was focused during the mechanism study of zein electrospinning, because the different behaviors of the α-helix to β-sheet transformation during zein electrospinning may result in its superhydrophobic surface formation. The use of FTIR to determine protein secondary structures has been well applied and confirmed in scientific research.20-21 In FTIR spectra of zein, the peak at 1600 cm-1 to 1800 cm-1 is generally corresponding to the amide group, and it is also called the amide I band. In principle, the amide I band is mainly attributed to the C=O stretching and vibration.22 Different types of secondary structures are the results of different hydrogen bonding modes and molecular geometries, which show differences on the amide I band of the FTIR spectra.20-22 The secondary structures of zein have been studied by Forato et al. using FTIR in 2003.21 It was stated that the α-helix structure was corresponding to a symmetric signal at about 1656cm-1 and the β-sheet structure could be observed at a shoulder below the peak at about 1620 cm-1. In this study, we reported the formation the superhydrophobic/hydrophobic surface from zein, a natural amphiphile, using electrospinning. The formation mechanism of the superhydrophobic/hydrophobic surface from amphiphiles during droplet-mediated evaporation-induced self-assembly, for example electrospinning and spray drying, was proposed.


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EXPERIMENTAL Materials. Zein was purchased from Wako Pure Chemical Industries, Ltd. (Tokyo,

Japan). PEG (5,000)-b-PLA (100,000) (PEG-PLA) was obtained from Polysciences Asia Pacific, Inc. (Taipei, Taiwan). Ethanol (96% v/v) was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. (Guangzhou, China). Sample Solution Preparation. Zein solutions of three different concentrations (10, 15 and 30 wt%) were prepared by dissolving zein in 80% (v/v) aqueous ethanol, and followed by 10-minute sonication. PEG-PLA solution was prepared by dissolving PEGPLA in chloroform (5 wt%). Electrospinning. The prepared sample solutions were electrospun using a nanofiber electrospinning unit (Kato Tech Co., Ltd, Tokyo, Japan). The solution was filled in a 10 ml syringe, which was connected to a metal needle with an internal diameter of 0.9 mm. The solution was delivered to the needle via the syringe pump, and the flow rate was controlled at 2 ml/h. The samples (0.7 mm thick) were collected on a rotated collector covered with an aluminum foil. The collector was placed at a distance of 15 cm from the needle tip. The applied voltage was adjusted to 18 kV. All the electrospinning processing was carried out under the ambient temperature of 25 ℃ and 1 atm ambient pressure. The samples obtained were dried overnight before further analysis. Spray Drying. The zein solution with a concentration of 10 wt% was spray dried using a SD-04 spray drier (Lab Plant, England) equipped with a 0.5 mm nozzle atomizer. Zein solution was pumped to the spray drier at a flow rate of 10 ml/min. The inlet temperature was set at 105 ℃ and the outlet temperature is 68 ℃. The dry powders were collected and stored at 4 ℃ before analysis.



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Water Contact Angle (WCA).

WCA was used to measure the surface

hydrophobicity. A hydrophobic surface has a WCA larger than 90 ° and a superhydrophobic surface has a WCA larger than 150°. WCA was measured using a standard goniometer Kruss GmbH DSA 100 (Hamburg, Germany). The water droplets were introduced by a micro-syringe and the images were captured by a camera system. The WCAs were calculated according to the Young-Laplace equation by the software. A total of 3 random spots were selected and measured in each sample and the average of the three WCA values was obtained and recorded. The results were expressed as mean ± standard deviation. Scanning Electron Microscopy (SEM). The morphology of the ZEF and PEG-PLA electrospun film was examined using SEM. The samples were cut into small pieces in the size of 0.5×0.5 cm2 and mounted rigidly on a specimen holder. The samples were then coated with 15 nm Au layer using an Edwards S150B sputter coater to improve the electrical conductivity. SEM images were obtained using a JEOL JSM-6490 SEM (Tokyo, Japan). Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATRFTIR).

ATR-FTIR spectra of ZEF, zein cast films (ZCF), and spray dried zein powder

(SDZP) were recorded using Nicolet FTIR Spectrometer coupling the ATR accessory with 16 scans at 4 cm-1 resolution. Baseline correction was conducted by subtracting a straight line from 900 cm-1 to 2100 cm-1 using Omnic software. The same processing of spectra smoothing was applied for each spectrum before the calculation of the secondary derivatives to improve the signal-to-noise ratio. The Savitsky-Golay derivative routine


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was conducted for the secondary derivative calculations. The relative proportions of the zein secondary structures were obtained by comparing the obtained sample spectra (1600 cm-1 to 1800 cm-1) to the documented α-zein spectra.21

RESULTS AND DISCUSSION Superhydrophobic/Hydrophobic Surfaces Made from amphiphiles Using

Electrospinning. Zein solutions of three different concentrations (10, 15 and 30 wt%) were electrospun using a nanofiber electrospinning unit. WCA was used to characterize the surface hydrophobicity of the formed ZEFs and the results are shown in Figure 1. It showeds that the WCAs were 155.5±1.4°, 150.4±0.8°, and 135.4±2.0°, respectively. The results indicate that the measured surfaces were all highly hydrophobic. The ones made from 10 and 15 wt% zein solution, respectively, were superhydrophobic. Moreover, as shown in Figure 2, the WCAs of ZCFs made from 10, 15 and 30 wt% zein solutions were 73.7 ± 2.6 ° , 72.8 ± 1.9 ° , and 76.5 ± 1.1 ° , respectively. The results show that the hydrophobicity of the zein surface was largely increased by electrospinning. To have a better understanding of the surface of the ZEFs, the morphology of the surfaces was examined using SEM and the images are shown in Figure 3. The SEM images show that there were two kinds of zein structures in the images: collapsed beads (10 wt%) and smooth fibers (30 wt%). There was a transition state of the collapsed beads and smooth fibers, which was made from 15 wt% of zein concentration. It is believed that the collapsed beads formed when the zein concentration in the original solution was low while the smooth fibers formed when the zein concentration in the original solution was high.16 When the zein concentration was low, the viscosity of the zein solution was low, 9 ACS Paragon Plus Environment


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and the beads formed instead of fibers.23 At even lower concentration, the beads were collapsed after solvent evaporation. The collapsed beads had rough surfaces while the fibers had smooth surfaces. The roughness of the collapsed beads was higher than that of the smooth fibers. Therefore, at selected zein concentrations (10, 15 and 30 wt%), the WCAs of the surface decreased as the concentration was increased.

Figure 1. WCA images of ZEF made from a solution containing (a) 10, (b) 15, and (c) 30 wt% of zein in 80% ethanol, respectively.

Figure 2. WCA images of ZCF made from a solution containing (a) 10, (b) 15, and (c) 30 wt% of zein in 80% ethanol, respectively.


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Figure 3. SEM images of ZEF made from a solution containing (a) 10, (b) 15, and (b) 30 wt% of zein in 80% ethanol, respectively. Scale bar = 5 µm

It is considered that the superhydrophobic/hydrophobic surfaces can also be formed from other amphiphiles using electrospinning. A synthetic amphiphilic polymer, PEGPLA, was selected. The PEG (5,000)-b-PLA (100,000) is amphiphilic because the PEG group is hydrophobic while the PLA group is hydrophilic. The PEG-PLA was dissolved in chloroform and the concentration was 5 wt%. After electrospinning at 18 kV and 2 ml/h flow rate, which were exactly the same as the electrospinning set-up for 10 wt% of zein solution, the electrospun PEG-PLA film (EPPF) formed. The hydrophobicity of the EPPF was measured using WCA. The WCA image (Figure 4a) of the EPPF showed that the formed surface was highly hydrophobic with a WCA of 141.4±0.7°. A control sample was prepared by cast-drying of the PEG-PLA solution (5 wt%) (CDPP), and the WCA was measured. Compared to CDPP, which was hydrophilic and had a WCA of 88.3 ± 0.6° (Figure 4b), the hydrophobicity of EPPF was largely improved by electrospinning, where the WCA had a dramatic increase from 88.3 ± 0.6° to 141.4 ± 0.7°.



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Figure 4. WCA images of surface obtained from a solution containing 5 wt% of PEGPLA in chloroform by (a) electrospinning and (b) cast drying, respectively.

Hydrophobic Surfaces Made by Spray Drying.

After the surface

hydrophobicity comparisons between ZEF and ZCF, and between EPPF and CDPP, respectively, it is obvious that, although both samples were prepared by solvent evaporation induced self-assembly, the samples made from cast drying of amphiphiles were

hydrophilic

and

those

made

from

electrospinning

were

hydrophobic/superhydrophobic. It is considered that the styles of the solvent evaporation, droplet-based, as in electrospinning, or surface-based, as in cast drying, during the fabrication are the key to control the surface hydrophobicity. Another fabrication method, spray drying, similar to electrospinning

on

the

solvent

evaporation

style,

was

used

to

study

the

superhydrophobic/hydrophobic surface formation and to explain the large improvement on the surface hydrophobicity over cast drying. Spray drying is a widely used method for producing a dry powder. During spray drying, the solution is sprayed through a nozzle into a hot vapor stream and the liquid phase is vaporized quickly.24 The droplet-based 12 ACS Paragon Plus Environment

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solvent evaporation occurring in spray drying is very similar to that in electrospinning. To further study the surface formed by spray drying, 10 wt% of zein solution was spray dried and the dried powder was collected. WCA was used to characterize the surface hydrophobicity of the sample while SEM was used to examine the surface morphology of the sample (Figure 5). The WCA of the sample surface was 125.3 ± 2.1°, which indicated that it was hydrophobic. SEM showed that the formed zein particles were collapsed beads too. The SEM result indicated that the spray-dried particles were in very similar shape as the electrospun beads, which are shown in Figure 3a. During spray drying, the zein solution was first spread into small droplets by the atomizer. The droplets were then carried into the chamber by hot air, and the collapsed particles with wrinkled skins formed after droplet-mediated solvent evaporation.

Figure 5. (a) WCA and (b) SEM image of SDZP prepared from 10 wt% of zein in 80% ethanol. Scale bar = 10 µm.

The α-helix to β-sheet Transformation during Evaporation-induced SelfAssembly. To further study the formation of zein superhydrophobic surface, the ATR-



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FTIR spectra of the ZCF, ZEF, and SDZP were collected. Their ATR-FTIR spectra (18001600 cm-1) and the corresponding secondary derivatives were presented in Figure 6. As shown in Figure 6, all the samples had the similar peak with high intensity at 1650 cm-1, which is attributed to the α-helix structure in zein. In the secondary derivative of ZCF spectra (Figure 6a, red solid line), it clearly shows the presence of the β-sheet structure by the shoulder of the peak at 1618cm-1. While, for ZEFs and the SDZPs (Figure 6b, Figure 6c, and Figure 6d), no obvious peak at the shoulder of 1618cm-1 canbe observed and identified from their secondary derivative spectra, so there are no or few β-sheet structures in the ZEF or SDZP. The results indicate that no α-helix to β-sheet transformation occurred during electrospinning and spray drying.


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Figure 6. FTIR spectra (blue dotted line) at Amide I region and the corresponding secondary derivatives (red solid line) of (a) ZCF, (b) ZEF prepared from 10 wt% zein solution, (c) ZEF prepared from 30 wt% zein solution, and (d) SDZP, respectively.

Zein has high α-helix content in its original solution.25-26 The β-sheet conformation happened during the formation of ZCF could be explained by the theory of evaporationinduced self-assembly (EISA). In the case of EISA of the zein solution, the faster evaporation rate of ethanol over water made the solvent more and more hydrophilic, which drove the conformational transitions of zein from α-helix to β-sheet.12 Thus, a strong contribution of β-sheet could be observed in the spectra of ZCF. The differences among the secondary structures of ZCF, ZEF, and SDZP could also be explained by the differences of the styles of the solvent evaporation. The evaporation style of the ZCF formation was surface-mediated, while the evaporation style of the ZEF and SDZP formations was droplet-mediated. The α-helix to β-sheet transformation involved a process of hydrogen bond rearrangement, which needed the solvent as a medium for it to take place. Because of the loss of solvent medium in a short time during the droplet-mediated evaporation in electrospinning and spray drying, there was no occurrence of the α-helix to β-sheet transformation. So, in the ZEFs and SDZPs, the structure was dominated by α-helix and no obvious β-sheet structure exists. Formation Mechanism of Superhydrophobic/Hydrophobic Surfaces. Based on the experimental results and related analysis, we propose a formation mechanism of superhydrophobic/hydrophobic surfaces made by amphiphiles through droplet-mediated evaporation-induced self-assembly, such as electrospinning and spray drying.



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For both the amphiphilic molecules discussed above, zein and PEG-PLA, electrospinning improved their surface hydrophobicity over cast drying. The key difference between the two methods, cast drying and electrospinning, was the style of solvent evaporation. Cast drying is a surface-mediated solvent evaporation process, while electrospinning is a droplet-based solvent evaporation process. The droplet-mediated solvent evaporation can also be recognized by the formation of the beads with collapsed skins and the ribbon-like fibers, as shown in Figure 3. The formation of collapsed beads and fibers could be explained by the premature aggregation and solidification of zein during the droplet-mediated or jet-mediated evaporation of the solvents. With low concentration solutions, the droplet-mediated or jet-mediated solvent evaporation resulted in a pressure difference between the inside and the outside of the zein aggregates, and, therefore, collapsed structures were formed.16 It is believed that the droplet-mediated solvent evaporation contributed significantly to the orientation of zein molecules, which resulted in the formation of superhydrophobic/hydrophobic surfaces. Scheme 1 illustrated the mechanism of zein superhydrophobic/hydrophobic surface formation. During both electrospinning and spray drying, solution was ejected as liquid droplets, and the solvent was evaporated from the droplet surface in a fast speed. In an extremely brief period before the solvent completely evaporated, a radial concentration gradient of zein was generated within each droplet. The maximum of the zein concentration was at the air-liquid interface, which was moving inside towards the center of the droplet during the solvent evaporation.27-28 Therefore, zein solidified from outside to inside “layer by layer”. At the same time, the relatively nonpolar side (air side) forced the hydrophobic compartment of zein molecules


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face outside. The radial concentration gradient and the presence of the liquid-air interface29 led to the orientation of the zein molecules as well as the growth of the dried zein, the direction of which was radially inward.30 As a result, the surfaces of the ZEF and SDZP were superhydrophobic or hydrophobic. Moreover, with low concentration solution, the solvent evaporation inside the droplet caused the atmospheric pressure exerted onto the prematurely solidified zein particles, and therefore collapsed structures formed. As indicated by the increasing WCA value, the collapsed surface increased the roughness of surface. As a comparison to the droplet-mediated solvent evaporation of electrospinning and spray drying, the surface-mediated evaporation of cast drying had a different mechanism. During surface-mediated evaporation, the solvent evaporated from the surface of the solution and the solvent molecules moved to the solution surface. A concentration gradient was formed, as there were more solvent molecules on the surface and less solvent molecules at the bottom of the solution. The air-liquid interface would be kept on the surface of the solution. The hydrophilicity of solvent was increasing since ethanol evaporates faster than water. Thus, the hydrophilic sides of zein molecules, which contained glutamine, turns would face outside to the solvent to achieve solvent-solute stability.12 In this way, the ZCF formed by cast drying had a hydrophilic surface.



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Scheme 1. Schematic illustration of zein superhydrophobic/hydrophobic surface formation during electrospinning and spray drying.

CONCLUSIONS ZEFs were fabricated using electrospining, a droplet-mediated evaporation-induced

self-assembly process, and their surface hydrophobicity was studied. The surfaces of the ZEFs made from different concentrations of zein solutions were highly hydrophobic, with WCA of 155.5 ± 1.4°, 150.4 ± 0.8°, and 135.4 ± 2.0°, respectively. Fibers were formed when the zein concentration was high (30 wt%), while beads were formed when the zein concentration was low (10 wt%). There was also a transition state of the fibers and bead when the zein concentration was 15 wt%. Hydrophobic surfaces could also be 18 ACS Paragon Plus Environment

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successfully formed using the synthesized amphiphilic molecule PEG-PLA. Spray drying of zein was also tried and hydrophobic surfaces of zein have also been successfully formed.

Based

on

all

of

these

studies,

a

formation

mechanism

of

superhydrophobic/hydrophobic surface of amphiphiles through droplet-mediated evaporation-induced self-assembly process was proposed. It is believed that the formation of superhydrophobic/hydrophobic surfaces from amphiphilic molecules using electrospinning and spray drying was mainly attributed to the droplet-mediated solvent evaporation style. The droplet-mediated solvent evaporation forced the hydrophobic sides of zein molecules to face outside, and made the surface hydrophobic. The chemical analysis by ATR-FTIR showed that the droplet-mediated solvent evaporation resulted in no obvious transformation of α-helix to β-sheet of zein, while the transformation usually occurred during the surface-mediated evaporation-induced zein self-assembly, such as cast drying.

ACKNOWLEDGMENTS

This work is ﬁnancially supported by the National Natural Science Foundation of China (project number: 51303153) and the Hong Kong Polytechnic University (project number: 1-ZVA9, 5-ZDAJ, G-UC07, and G-YK99). We appreciate the help from the Material Research Center of the Hong Kong Polytechnic University.

REFERENCES



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(1) Fahmy, T. M.; Samstein, R. M.; Harness, C. C.; Mark Saltzman, W. Surface Modification of Biodegradable Polyesters with Fatty Acid Conjugates for Improved Drug Targeting. Biomaterials 2005, 26, 5727-5736. (2) Vilčnik, A.; Jerman, I.; Šurca Vuk, A.; Koželj, M.; Orel, B.; Tomšič, B.; Simončič, B.; Kovač, J. Structural Properties and Antibacterial Effects of Hydrophobic and Oleophobic Sol−Gel Coatings for Cotton Fabrics. Langmuir 2009, 25, 5869-5880. (3) Fürstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Wetting and Self-Cleaning Properties of Artificial Superhydrophobic Surfaces. Langmuir 2005, 21, 956-961. (4) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Bioinspired Surfaces with Special Wettability. Accounts of Chemical Research 2005, 38, 644-652. (5) Zheng, J.; He, A.; Li, J.; Xu, J.; Han, C. C. Studies on the Controlled Morphology and Wettability of Polystyrene Surfaces by Electrospinning or Electrospraying. Polymer 2006, 47, 7095-7102. (6) Kang, M.; Jung, R.; Kim, H. S.; Jin, H. J. Preparation of Superhydrophobic Polystyrene Membranes by Electrospinning. Colloid Surface A 2008, 313, 411-414. (7) Menini, R.; Farzaneh, M. Production of Superhydrophobic Polymer Fibers with Embedded Particles Using the Electrospinning Technique. Polym Int 2008, 57, 77-84. (8) Buruaga, L.; Gonzalez, A.; Irusta, L.; Iruin, J. J. Production of Hydrophobic Surfaces in Biodegradable and Biocompatible Polymers Using Polymer Solution Electrospinning. J Appl Polym Sci 2011, 120, 1520-1524. (9) Sill, T. J.; von Recum, H. A. Electro Spinning: Applications in Drug Delivery and Tissue Engineering. Biomaterials 2008, 29, 1989-2006.


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(10) Cui, W. G.; Li, X. H.; Zhou, S. B.; Weng, J. Degradation Patterns and Surface Wettability of Electrospun Fibrous Mats. Polym Degrad Stabil 2008, 93, 731-738. (11) Wang, Y.; Padua, G. W. Formation of Zein Microphases in Ethanol-Water. Langmuir 2010, 26, 12897-12901. (12) Wang, Y.; Padua, G. W. Nanoscale Characterization of Zein Self-Assembly. Langmuir 2012, 28, 2429-2435. (13) Argos, P.; Pedersen, K.; Marks, M. D.; Larkins, B. A. A Structural Model for Maize Zein Proteins. J Biol Chem 1982, 257, 9984-9990. (14) Matsushima, N.; Danno, G.-i.; Takezawa, H.; Izumi, Y. Three-Dimensional Structure of Maize Α-Zein Proteins Studied by Small-Angle X-Ray Scattering. Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology 1997, 1339, 14-22. (15) Mark, H. F.; Bikales, N. M., Encyclopedia of Polymer Science and Technology : Plastics, Resins, Rubbers, Fibers : Supplement; Interscience Publishers: New York, 1976, p v. (16) Torres-Giner, S.; Gimenez, E.; Lagarona, J. M. Characterization of the Morphology and Thermal Properties of Zein Prolamine Nanostructures Obtained by Electrospinning. Food Hydrocolloid 2008, 22, 601-614. (17) Torres-Giner, S.; Martinez-Abad, A.; Gimeno-Alcaniz, J. V.; Ocio, M. J.; Lagaron, J. M. Controlled Delivery of Gentamicin Antibiotic from Bioactive Electrospun Polylactide-Based Ultrathin Fibers. Adv Eng Mater 2012, 14, B112-B122. (18) Cerpa, R.; Cohen, F. E.; Kuntz, I. D. Conformational Switching in Designed Peptides: The Helix/Sheet Transition. Folding and Design 1996, 1, 91-101.



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(27) Jayanthi, G. V.; Zhang, S. C.; Messing, G. L. Modeling of Solid Particle Formation During Solution Aerosol Thermolysis - the Evaporation Stage. Aerosol Sci Tech 1993, 19, 478-490. (28) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. Evaporation-Induced SelfAssembly: Nanostructures Made Easy. Advanced Materials 1999, 11, 579-585. (29) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Free-Standing and Oriented Mesoporous Silica Films Grown at the Air-Water Interface. Nature 1996, 381, 589-592. (30) Yang, H.; Coombs, N.; Ozin, G. A. Morphogenesis of Shapes and Surface Patterns in Mesoporous Silica. Nature 1997, 386, 692-695.

Table of Contents (TOC) Image



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Figure 1. WCA images of ZEF made from a solution containing (a) 10, (b) 15, and (c) 30 wt% of zein in 80% ethanol, respectively. 1393x347mm (56 x 56 DPI)

ACS Paragon Plus Environment


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Figure 2. WCA images of ZCF made from a solution containing (a) 10, (b) 15, and (c) 30 wt% of zein in 80% ethanol, respectively. 1393x347mm (56 x 56 DPI)


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Figure 3. SEM images of ZEF made from a solution containing (a) 10, (b) 15, and (b) 30 wt% of zein in 80% ethanol, respectively. Scale bar = 5 µm 1393x347mm (56 x 56 DPI)



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Figure 4. WCA images of surface obtained from a solution containing 5 wt% of PEG-PLA in chloroform by (a) electrospinning and (b) cast drying, respectively. 1418x526mm (55 x 55 DPI)


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Figure 5. (a) WCA and (b) SEM image of SDZP prepared from 10 wt% of zein in 80% ethanol. Scale bar = 10 µm. 1418x526mm (55 x 55 DPI)



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Figure 6. FTIR spectra (blue dotted line) at Amide I region and the corresponding secondary derivatives (red solid line) of (a) ZCF, (b) ZEF prepared from 10 wt% zein solution, (c) ZEF prepared from 30 wt% zein solution, and (d) SDZP, respectively. 508x381mm (96 x 96 DPI)


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Scheme 1. Schematic illustration of zein superhydrophobic/hydrophobic surface formation during electrospinning and spray drying. 666x500mm (96 x 96 DPI)



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Table of Contents (TOC) Image 550x550mm (96 x 96 DPI)


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Hydrophobic

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