ARTICLE pubs.acs.org/Langmuir
Enhanced Wetting Behavior at Electrospun Polyamide Nanofiber Surfaces Urszula Stachewicz† and Asa H. Barber‡,* †
Nanoforce Technology Ltd. and ‡Department of Materials, School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom ABSTRACT: Nanofibers of polyamide have been synthesized using electrospinning processes and their wetting properties determined directly from a nanoscale Wilhelmy balance approach. Individual electrospun polyamide nanofibers were attached to atomic force microscope (AFM) tips and immersed in a range of organic liquids with varying polar and dispersive surface tension components. AFM was used to measure nanofiber-liquid wetting forces and derive contact angles using Wilhelmy balance theory. Owens-Wendt plots were used to show a considerable increase in the polar component of the surface free energy of the polyamide nanofibers compared with bulk film of the same polymer. Chemical surface analysis of the polyamide nanofibers and films using X-ray photoelectron spectroscopy provided evidence for enhanced availability of polar oxygen groups at the electrospun nanofiber surface relative to the film. Our results therefore confirm chemical group orientation at the electrospun polyamide nanofiber surface that promotes availability of polar groups for enhanced wetting behavior.
’ INTRODUCTION Polymer nanofibers show promise as novel materials in a number of applications ranging from tissue scaffolds1 to filtration devices2 and fibrous reinforcements in synthetic and biological composite materials.3,4 In these applications, the surface properties of the nanofiber critically define the overall material behavior. Nanofibers have shown numerous advantages over conventional fibrous materials due to their large surface area-to-volume ratio and inherent improvement in mechanical properties as their diameter decreases.5-7 While the controlled processing of nanomaterials is a current manufacturing challenge, electrospinning has been particularly effective for the production of polymeric nanofibers in large quantities using a relatively simple experimental setup. Electrospinning applies a high electric field stress to a mobile polymer, either in a melt or solution, to form fibers with a controllable geometry.8,9 Previous work has shown that the electric field used to spin polymer nanofibers produces a polymeric structure different to that of the bulk with evidence of polymer chain alignment parallel to the fiber axis for structurally anisotropic nanofibers.10-12 However, nanofibers in applications such as filtration, tissue engineering and composites require understanding of fiber surface behavior to define function. For example, fibrous scaffolds in tissue engineering require optimization of the adhesion between the fibers and cells13,14 whereas the interfacial adhesion between nanofibers and binding polymer critically define the mechanical performance of the whole composite.3 The surface properties of electrospun nanofibers have been measured predominantly on macroscopic samples using wetting experiments where the organization of the fibrous network defines overall behavior. Surface properties of polyamide 6 (PA6) nanofiber mats decorated with silica nanoparticles coated with fluorosilane have been shown to exhibit hydrophobic behavior.15 Electrospun fibrous mats of blended polystyrene (PS) and PA6 fibers have been shown to be superhydrophobic r 2011 American Chemical Society
using a defined and controllable porosity.16 Further studies have modified the properties of nanofibers mats via chemical treatments by adding perfluorinated acridine to polyamide 6 (PA6) dissolved in formic acid for self-cleaning surfaces.17 Conversely, electrospun fibers with hydrophilic surfaces promote neurite extension in nanofibrous scaffolds for effective neural tissue engineering.18 PA6 is a material commonly used in numerous fibrous applications19-21 due to its excellent durability, abrasion resistance, low friction coefficient, and good chemical resistance. The origin of these physical properties is due to the structural organization of the polymer chains in PA6, which predominantly crystallize as either the R-form or γ-form.22 A limited number of studies have examined the molecular orientation of polymer chains in electrospun PA6 using Fourier transform infrared (FTIR) and Raman spectroscopy.23-25 Results indicated that molecular orientation was preferential along the fiber axis, especially when the electrospun fibers were produced with diameters below 250 nm.23 The orientation of the polymer chains in nanofibers is also possible due to thermal processing where recrystallization takes place.12,26,27 The alignment of polymer chains during electrospinning is expected to be because of the applied electric field stress creating whipping action of polymer jet or mechanical stretching of collected nanofibers onto a rotating drum. While evidence exists to show preferential polymer chain orientation using electrospinning, the influence of electrospinning processes on nanofiber surface structure receives little attention despite its importance in functional electrospun fibrous materials. The understanding of electrospun fiber surface properties is therefore critical to the development of optimized electrospun nanofibers applications. However, the surface properties of electrospun polymer nanofibers are often Received: November 23, 2010 Revised: January 21, 2011 Published: February 18, 2011 3024
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Langmuir assumed to be similar to the behavior of bulk polymers, despite the electrospinning process modifying structural organization of the polymer. Previous work has shown how molecular motion as well as the relaxation of polymer chains can cause changes in structure at the nanoscale due to different manufacturing processes, resulting in deviation in surface structure of the material relative to the bulk.28-30 Understanding the surface properties of electrospun polymer nanofibers, or indeed any nanofiber, is challenging as the inherent properties of the nanofiber cannot be separated from larger scale behavior. For example, the sessile drop method in static water contact angle measurement is used for the characterization of nanofibrous membranes31 but the effect of surface porosity using Cassie-Baxter and Wenzel theories need to be employed when considering these contact angles.32 Measurement of contact angle between liquids of differing surface tensions and an individual electrospun fiber is therefore the most direct method for understanding the inherent surface properties of the fiber. Previous work in the literature has examined highly crystalline nanomaterial surface properties of individual carbon nanotubes33,34 and inorganic nanowires35 but the potentially unique surface characteristics of electrospun polymer nanofibers, implied from previous work showing unique polymer structure in electrospun fibers when compared to the bulk,10 have not been examined. Measurement of the wetting angle on individual polymer electrospun nanofibers will thus define their inherent surface properties using established theories for describing wetting behavior of surfaces in terms of molecular contributions.36 The aim of this study is to examine how the surface properties of a typical semicrystalline polyamide (PA6), commonly used in a range of textile applications, is modified when electrospun as nanofibers. The effectiveness of individual nanofiber property measurements as shown in refs 33, 34, and 37 warrants the use of Wilhelmy balanced based wetting techniques. We partially immerse individual electrospun PA6 nanofibers in three different organic probe liquids using an atomic force microscope (AFM) and measuring the wetting forces acting on the nanofiber surface. The polymer nanofiber-liquid interactions are monitored by AFM and accurate measurements of the contact angle between probe liquids and the nanofiber surface calculated. Wetting data is used to produce Owens-Wendt plots for the calculation of the dispersive and polar components of the nanofiber surface and is compared to the surface properties of conventional PA6 films to examine the influence of electrospinning processes on polymer surface behavior. The surface properties of nanofibers and film are compared and verified with X-ray photoelectron spectroscopy (XPS) analysis.
’ EXPERIMENTAL SECTION Materials. Polyamide 6 (BASF, Ultramid B33 L, Germany) was dissolved in a mixture of acetic acid (g99.7%, Sigma Aldrich, U.S.A.) and formic acid (98%, Sigma Aldrich, U.S.A.) (50/50 mass ratio) to produce a resultant polymer concentration of 12 wt % in solution. Polyamide materials were kept at 22 °C and a relative humidity of 36% in the laboratory throughout this work to ensure a constant level of water absorption in all samples. Electrospinning of PA6 Nanofibers. The PA6 polymer solution was electrospun into nanofibers using a large scale multijet electrospinning setup (NanoSpider, Elmarco, Cze.). Briefly, the polymer solution is introduced into a reservoir containing a rotating wire cylinder. The rotating wires allow pickup of solution droplets at the wire surfaces
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Figure 1. SEM images showing (A) contact of the AFM tip containing glue to an individual electrospun PA6 nanofiber protruding from the spun fibrous mat and (B) isolation of the PA6 nanofiber attached to the AFM tip following FIB cutting. as they move through the polymer solution in the reservoir. A voltage of 60 kV was applied between the rotating cylinder and an aluminum sheet acting as a ground electrode 15 cm above the rotating cylinder. The action of the applied voltage causes charge build-up within the polymer solution droplets on the cylinder wires until repulsion of charge within these droplets causes ejection of the polymer solution toward the ground electrode. The ejected solution is stretched toward the top electrode under the action of the applied voltage to form electrospun fibers. Evaporation of the solvent as the fibers are stretched toward the ground electrode causes the deposition of solid PA6 nanofibers in the form of a mat on the aluminum ground electrode surface. Nanofiber Attachment to AFM Tip. Individual electrospun fibers were isolated from the deposited PA6 nanofiber mat and attached to an atomic force microscope (AFM) tip for subsequent wetting experiments. The attachment of a nanofiber to the AFM tip was carried out in the chamber of a scanning electron microscope (SEM, FEI Quanta 3D, U.S.A./E.U.) containing a custom built nanomanipulator (attoAFM II, attocube GmbH, Germany) A small section of the electrospun PA6 mat was placed on carbon tape and attached to a sample stage holder within the SEM chamber. A small droplet of vacuum compatible glue (Poxipol, Arg.) was also added to the sample stage holder. An AFM tip (Veeco, U.S.A., spring constant K = 0.02-0.04 N 3 m-1) was attached to the nanomanipulator system and the tip translated toward the glue while observing using the SEM. Contact of the AFM tip with the droplet of glue within the SEM chamber caused deposition of the glue at the apex of the AFM tip as shown in Figure 1. The AFM tip was immediately translated toward a suitable electrospun PA6 nanofiber protruding from the electrospun fibrous mat as shown in Figure 1A. The attachment of the AFM tip to the free end of an electrospun PA6 nanofiber was achieved within a time frame of 10 min. The glue solidified beyond this time frame to ensure that the nanofiber was securely attached to the end of the AFM tip. We noted that electrospun nanofibers under the electron beam for times ranging from 8 to 20 min in this work showed no variation in their surface behavior. To quantify the effects of the electron beam exposure on the PA6 structure, we can calculate the electron dosage from the SEM imaging conditions during the nanomanipulation using the electron beam accelerating voltage U = 5 kV and maximum beam current of I = 53.3 pA. The calculated dose rate DR for an electron accelerator is given using DR = KpI/R, where Kp is the stopping power of electrons in PA6, which depends on the energy of electrons and density of material being irradiated (25 MeVcm2/g for PA6).38 R is the irradiation field area and would be expected to be between 2 and 65 10-15 depending on the fiber diameter used in our experiments.38,39 The dose rate for our experiments was in the range of 7-35 10-7 Gy depending on the irradiation area, R, which is far below the threshold value of 104 kGy reported in the literature as modifying PA6 surfaces.40 We therefore conclude that for the electron beam conditions and manipulation times used in this work exposure of the electrospun PA6 nanofibers to the 3025
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Figure 2. Plots of the change in the force, measured from AFM cantilever deflection, as an individual electrospun PA6 nanofiber contacts three different probe liquids of glycerol, polyethylene glycol, and formamide. electron beam of the SEM during the manipulation process does not modify the nanofiber structure. Following solidification of the glue, a focused ion beam (FIB) system integrated within the SEM was used to cut the nanofiber and leave a fiber length of approximately 10 μm fixed to the AFM tip as shown in Figure 1B. The diameter of electrospun PA6 nanofibers attached to the AFM probe varied between 60 and 280 nm and a total of 3 nanofiber-AFM tips were produced.
Contact Angle Measurements on Individual Nanofibers. The wetting behavior of the individual electrospun PA6 nanofibers was carried out according to Wilhelmy balance experiments33-35 in air using a stand-alone AFM (NT-MDT NTegra, Russia) with the electrospun PA6 nanofiber-AFM tips replacing conventional AFM tips. The experiments were performed at 22 °C and a humidity of 36%. Probe liquids of polyethylene glycol, formamide, and glycerol were placed separately into a liquid cell situated below the nanofiber-AFM tip. The probe liquids were selected due to their low vapor pressure, consequently the liquid evaporation process is relatively slow and ensures a stable liquid surface is used. Contact between the liquid surface and the individual PA6 nanofiber attached to the AFM tip was achieved by first moving the liquid cell up toward the PA6 nanofiber in a standard AFM landing mode. Proximity of the liquid surface to the PA6 nanofiber was determined using the AFM in semicontact mode with the signal dropping by 15-20%. The AFM was then switched to contact mode and the liquid cell moved up more slowly using the z-piezo positioner of the AFM, situated underneath the liquid cell until contact of the PA6 nanofiber with the liquid surface was achieved, determined as an abrupt bending of the AFM cantilever down toward the liquid as the wetting of the nanofiber surface occurred as shown in Figure 2. Raising the liquid cell upward caused an increased fiber length to be submerged within the liquid but no further change in the cantilever deflection, indicating that the wetting behavior was consistent along the length of the nanofiber and no bending of the nanofiber at the liquid surface was achieved. The cantilever deflection was recorded during the partial immersion of the nanofiber length within each probe liquid and the nanofiber was
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removed by lowering the liquid cell using the AFM z-piezo positioner. Cantilever deflection was converted to force by first determining the cantilever spring constant K, calibrated using the thermal noise method.41 The displacement of the AFM cantilever, measured as the deflection signal x, was converted to force F acting on the nanofiber using F = Kx. Contact Angle Measurement on Polymer Films. For comparison, PA6 films were prepared by placing approximately 5 mL of polymer solution, as used for the electrospinning, onto a glass slide rotating in a spin coater (SCS G3 Spin Coater, U.S.A.) operating at 800 rpm for 1 min. After allowing the PA6 film to dry fully, small droplets of all three probe liquids, glycerol, formamide, and polyethylene glycol, were deposited on glass slides with PA6 film. Contact angles measurements of the probe liquids on the PA6 film were taken using a digital camera (Best Scientific, U.K.) with microvideo zoom lens. The contact angle was measured using MB ruler software. The error bars on the measured contact angles were determined based on the maximum and minimum measured values. X-ray Photoelectron Spectroscopy. The surface structures of electrospun PA6 nanofibers and film were analyzed with X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra-DLD system, U.K.). With this spectroscopy technique, we measured the elemental composition of a material surface (up to 10 nm) in an ultrahigh vacuum condition. The resulting energy spectra shows resonance peaks characteristic of the electronic structure for atoms at the analyzed surface. Electrospun PA nanofibers and corresponding films were mounted inside the XPS chamber using double sided carbon tape. XPS was performed over an area of 700 300 μm and elemental compositions of oxygen, corresponding to polar side groups in the polyamide chain, recorded. Nanofiber and film samples were tilted from normal (0°) to grazing (75°) angles in order to record the chemistry more at the surface as the tilting angle increased. The percentage of O(1s) surface composition for nanofibers and film is presented in Table 3. We note that the fiber assemblies are a relatively complex geometry presented to the incident XPS beam relative to the planar surfaces of the film samples. For example, grazing angles used to probe the surface of the electrospun nanofibers will examine surface structure at fiber edges parallel to the incident XPS beam and substructure at the fiber midpoint where the surface is presented perpendicular to the incident beam. Thus, increase in the O(1s) composition in nanofibers relative to the film represents a lower limit due to both surface and bulk being examined at these larger grazing angles for the nanofiber samples.
’ RESULTS AND DISCUSSION The wetting behavior of electrospun PA6 nanofibers was measured by contacting individual PA6 nanofibers, attached to AFM tips as shown in Figure 1, with three probe liquids of differing surface tension. The recorded AFM cantilever deflection with experimental progression time as an individual PA6 nanofiber contacted each of the three probe liquids is shown in Figure 2. The far left of the plot shows zero force as the PA6 nanofiber is above the probe liquid surface. Contact between the PA6 nanofiber and probe liquid surface is shown as a characteristic drop in the force corresponding to bending of the AFM cantilever toward the liquid surface due to wetting of the nanofiber surface by the liquid. The abrupt change in the measured force in Figure 2 is defined as the wetting force. The contact angle between the probe liquid and electrospun PA6 nanofiber surface is calculated based on the Wilhelmy balance method where the AFM cantilever deflection is equal but opposite to the wetting force acting on the nanofiber according to F ¼ γl cos θd 3026
ð1Þ
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Table 1. Surface Tension Data for the Three Probe Liquids, Stating the Total Surface Tension, γl, the Dispersive, γld, and Polar, γlp, Components from References 34 and 44a γl [mJm-2]
liquid
γld [mJm-2]
γlp [mJm-2]
θf [°]
θn [°]
polyethylene glycol
48.3
29.4
19.0
43.4 ( 4.2
48.7 ( 2.9
formamide glycerol
58.2 64.0
39.5 34.0
18.7 30.0
50.7 ( 3.5 61.3 ( 3.1
62.8 ( 3.3 58.5 ( 6.9
The corresponding contact angle measurements are given for each probe liquid on PA6 nanofibers, θn, calculated from eq 1 and observed optically on PA6 films, θf. a
Table 3. Percentage Content of Oxygen O(1s) at a PA6 Film and Electrospun Nanofiber Surface from XPS Measurements
Figure 3. Owens-Wendt plot for three probe liquids of differing surface tension wetting electrospun PA6 nanofiber and PA6 film surfaces.
Table 2. Surface Free Energy of Solid Surfaces, γs, Including Dispersive, γsd, and Polar, γsp, Contributions Determined from the Owens-Wendt Plot in Figure 3 γs [mJm-2]
γsd [mJm-2]
PA6 film
40.3 ( 1.7
34.9 ( 1.5
5.5 ( 0.2
PA6 nanofiber
47.0 ( 2.8
5.2 ( 0.3
41.8 ( 2.5
solid surfaces
γsp [mJm-2]
where d is the nanofiber diameter, γl is surface energy of the liquid, and θ is liquid-nanofiber wetting contact angle. The calculated contact angles between the three probe liquids and the PA6 nanofibers using eq 1 are presented in Table 1. In addition, the contact angles between the probe liquids and the bulk PA6 film are also shown in Table 1 for comparison. We note that all of the contact angles are less than 90°, indicating partial wetting behavior between the PA6 surfaces and three probe liquids. Table 1 indicates that the lower surface tension liquids form the largest contact angles on the nanofiber surfaces whereas the most polar probe liquid of glycerol shows increased wetting of nanofibers relative to the PA6 film as highlighted with a lower contact angle. Thus, our results indicate that the electrospun PA6 nanofiber surfaces show a different wetting behavior when compared to PA6 film surfaces of the same material. To explain the difference between the electrospun PA6 nanofiber and film surfaces, Owens-Wendt theory is employed to calculate the corresponding dispersive and polar components of the solid PA6 nanofiber and film surfaces investigated.36 Owens-Wendt theory determines the polar and dispersive contributions to a solid’s surface free energy using the known
XPS measurement
film PA6 O
nanofibers PA6 O
angle [deg]
(1s) [%]
(1s) [%]
0
9.84
10.48
15
9.42
10.03
30
9.2
9.94
45 60
8.93 8.36
9.85 9.70
75
7.87
9.12
polar and dispersive components of the probe liquids and their contact angles with the solid using eq 2 below pffiffiffiffiffiffip ! qffiffiffiffiffiffi pffiffiffiffiffiffi γs γl ð1 þ cos θÞ pffiffiffiffiffiffi ð2Þ ¼ γs p pffiffiffiffiffiffi þ γs d 2 γl d γl d where γl is the total surface tension of the probe liquid with dispersive (γld) and polar (γlp) components and γs is the surface free energy of a solid surface with dispersive (γsd) and polar (γsp) components. θ is the contact angle made between the probe liquid and solid surface. An Owens-Wendt plot is formed from eq 2 from a simple linear function of y = mx þ b where the axis terms of y = [(γl(1 þ cos θ)]/[2(γdl )1/2] and x = (γpl )1/2/(γdl )1/2 are known using the data in Table 1. Thus, a linear plot of these known axis terms for each probe liquid wetting the PA6 nanofiber and film surface can be used to determine the polar and dispersive components of the PA6 surfaces where m = (γps )1/2 and b = (γds )1/2 shown in Figure 3. The calculated polar and dispersive contributions and the summation of these contributions give the total solid surface free energy and is shown in Table 2. The total surface free energy of the PA6 film is calculated from the summation of the corresponding dispersive and polar components and was found to be 40 mJm-2, which is in agreement with PA6 literature values.42,43 In contrast, the electrospun PA6 nanofibers have a total surface free energy that is almost 20% higher than the bulk PA6 film equivalents. Interestingly, the surface free energy of the electrospun PA6 nanofibers is dominated by their polar contribution, which is in contrast to the PA6 film where most of the surface free energy contribution is from dispersive forces. To provide chemical information to justify the wetting behavior of the film and nanofiber surfaces, XPS is employed to examine the PA6 surfaces. Specifically, the amount of oxygen that will provide the greatest contribution to the polar component of the surface free energy is measured. XPS results in Table 3 show the oxygen O(1s) content for the PA6 nanofibers is higher compared to PA6 film, especially at the grazing angle values used to evaluate the immediate surface only. Thus, correlation exists between increased oxygen content at the electrospun nanofiber relative to the film and a corresponding 3027
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Langmuir enhancement of the surface free energy of the electrospun nanofibers due to an increased polar contribution. We therefore conclude that electrospinning PA6, shown in previous literature to alter the structural organization of polymer chains in the nanofibre9 causes an increase in the surface free energy of the resultant fiber surface. Currently, we can only speculate on the origin of this surface free energy increase but the Owens-Wendt plots showing enhanced polar contribution at the PA6 nanofiber surface suggest that a reorientation of the polar groups toward the nanofiber surface is occurring relative to the PA6 film. Generally, we would expect the electrospun fiber structure to organize so that the surface free energy is minimized with polar groups pointing away from the fiber surface. The amount of oxygen in the PA6 structures is shown in Table 3 and clearly demonstrates a drop in the oxygen content as the grazing angle increases, highlighting how the oxygen content drops as we sample more of the surface only and consistent with a surface free energy minimization process. However, the drop in the amount of oxygen present at the PA6 surface is more pronounced for the film relative to the electrospun nanofibers. Our results would therefore indicate that the organization of the polymer chains in the nanofibers occur so that orientation of polar groups away from the fiber surface is suppressed when compared to the PA6 film. As electrospinning is a rapid and dynamic process for nanofiber synthesis, we expect that the polymer chains do not have sufficient time to reorganize at the nanofiber surface. The formed solid nanofiber structures produced from electrospinning can therefore be considered as nonequilibrium when compared to the film form. Additionally, our results show no dependence of the contact angle on fiber diameter from 60 to 280 nm, indicating nanofiber wetting behavior is not a size effect but due to the surface structure formed from electrospinning processing.
’ CONCLUSIONS The surface free energy of electrospun PA6 nanofibers are measured directly using a Wilhelmy-based method using AFM. The surface free energy of the electrospun fibers are found to be almost 20% higher than corresponding solution processing of PA6 as a film. Owens-Wendt plots highlight how the surface free energy increase for electrospun nanofibers is predominantly from an increased polar component at the nanofiber surface. XPS analysis supports these results and indicates an increase in polar oxygen groups at the nanofiber surface relative to the film. Hence, the enhanced surface free energy of the PA6 nanofibers is potentially a result of this rapid electrospinning process and highlights how the electrospinning process produces polymer materials with unique physical properties when compared to more established production methods. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors would like to thank Dr. David Morgan (XPS Access Manager), Cardiff University, EPSRC Grant (EP/ F019823/1) for XPS analysis of our samples, Dr. Zofia Luklinska at Queen Mary University of London NanoVision Centre for assisting with microscopy facilities and Fei Hang for assistance with the nanofiber attachment process. We would like to thank
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EPSRC for partially supporting this work through Grant EP/ E039928.
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