Mechanism of Adhesion between Polymer Fibers at Nanoscale

Article pubs.acs.org/Langmuir

Mechanism of Adhesion between Polymer Fibers at Nanoscale Contacts Qiang Shi,*,† Shing-Chung Wong,‡ Wei Ye,†,§ Jianwen, Hou,†,§ Jie Zhao,†,§ and Jinghua Yin*,‡ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China ‡ Department of Mechanical Engineering, The University of Akron, Akron, Ohio 44325-3903, United States § Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022, Changchun, China ABSTRACT: Adhesive force exists between polymer nano/microfibers. An elaborate experiment was performed to investigate the adhesion between polymer nano/ microfibers using a nanoforce tensile tester. Electrospun polycaprolactone (PCL) fibers with diameters ranging from 0.4−2.2 μm were studied. The response of surface property of electrospun fiber to the environmental conditions was tracked by FTIR and atomic force microscopy (AFM) measurements. The effect of temperature on molecular orientation was examined by wide angle X-ray diffraction (WAXD). The adhesive force was found to increase with temperature and pull-off speed but insensitive to the change of relative humidity, and the abrupt increase of adhesion energy with temperature accompanied by a reduced molecular orientation in the amorphous part of fiber was observed. Results show that adhesion is mainly driven by van der Waals interactions between interdiffusion chain segments across the interface.

1. INTRODUCTION The adhesion between micro/nanofibers plays an important role in fibrous structures mimicking biological systems.1−3 These fibers fall in a range from a few hundred nanometers to a few micrometers. The adhesion between these fibers influences the mechanical properties and function of the adhesives. In addition, nonwoven materials comprise continuous polymer fibers with diameters ranging from a few micrometers to tens of nanometers, which can be applied to filtering membrane, fuel cells, protective clothing, catalyst support, drug delivery device, tissue scaffolds,4,5 and nanocheese cutter.6 The adhesion between the fibers is especially important in determining the macroscopic mechanical properties of such mats and controlling the application of the nonwovens. Investigation of adhesive behaviors in fibers plays a pivotal role in future designs for biomimicking devices.6−8 Adhesion between nanometer-scale contacts is different from that at the macroscopic scale due to an increased surface areato-volume ratio.9 As the physical dimensions of a material structure are reduced, one observed outcome is an enhanced surface/ boundary effects8,10 whereby the surface/boundary properties dominate the bulk. Recently, some investigators evaluated the influence of size effect on adhesion.7,11−13 Some noted the influence of material stiffness and size, as well as surface properties. In most circumstances, adhesion could be examined by the rather well-established Johnson−Kendal−Robert (JKR)14 or Derjaguin− Muller−Toporov (DMT) contact mechanics models.15 © 2012 American Chemical Society

The use of electrospun polymer fibers as nanoscale adhesives is attractive because of its versatility in processing. Aligned fibers can be collected using a rotating disk collector16 or in between two parallel metallic strips.17 Polymer micro/nanofibers are characterized by highly oriented polymer crystals and amorphous region in comparison to that of the bulk, and geometry of crossed fibers affects the adhesion substantially.17,18 In addition, polymer micro/nanofibers are sensitive to the change of environmental conditions. In the previous work, we studied the adhesion between polymer micro/ nanofibers in a cross-cylinder geometry using a nanoforce tensile tester.7 We found that, at room temperature (∼25 °C) and low relative humidity (≤20%), the adhesion force lied in the range of 10−6 N, and the adhesion energy was 190 ± 7 mJ/m2. The adhesion depended on fiber radius following a linear relationship and the van der Waals interactions were primarily operative between the micro/nanofibers. However, the effects of environment conditions and molecular orientation on adhesion were outside the scope of our previous work. In this article, we investigate the adhesion between polymer micro/nanofibers as a function of environmental and loading conditions. They include relative humidity, pull-off speed, and temperature. The temperature-dependent adhesion is analyzed Received: November 23, 2011 Revised: February 17, 2012 Published: February 21, 2012 4663

dx.doi.org/10.1021/la204633c | Langmuir 2012, 28, 4663−4671

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Figure 1. Schematic of the dry adhesion test. (a) Single freestanding fiber was taped to the two prongs of a cardboard, (b) polarized light microscopy was used to obtain digital images of the fibers and measure their diameters, (c) cardboard sheet was cut into U-shape and two fibers with the same diameter were used for adhesion test, (d) U-shape cardboards were mounted on the nanoforce tensile tester, and vertical compressive load was applied to deform the two fibers into V- and inverted V-shapes, and the force and displacement were recorded at the same time, (e) two freestanding fibers were arranged in the cross-cylinder geometry. reagent grade were obtained from Acros and BMD Chemicals Inc., respectively. 2.2. Samples Preparation. Polycaprolactone (PCL) fibers are fabricated by electrospinning techniques.4,5 Electrospun fibers are prepared using two concentrations of PCL solutions, viz., 12 and 14 wt % PCL. PCL is dissolved in a solvent mixture of DMF and DCM in the ratio of 1:3 (w/w). The PCL micro/nanofibers are electrospun at a temperature of about 22 °C, humidity 43%, solution feed rate ∼0.5 mL/h, and applied voltage 10−11 kV. Two parallel electrodes are used to collect the electrospun fibers. The specimens are prepared by collecting as-spun fibers on a primary collector that consists of two parallel electrodes, followed by transferring single fibers onto a secondary collector.17 As shown in Figure 1, a single freestanding fiber is taped to the two prongs of a cardboard mount that are 15 mm apart

in terms of adhesion energy, surface roughness, and molecular orientation in the fibers. Then, the mechanism of adhesion at nanoscale contacts is proposed. This study not only provides the pressingly needed data for an understanding of the adhesion between polymer micro/nanofibers but potential for future applications of electrospun polymer fibers as adhesives.

2. EXPERIMENTAL WORK 2.1. Materials. The biodegradable polymer, polycaprolactone (PCL) (Mn = 80,000), was purchased from Sigma-Aldrich (CAS = 24980−41−4) and dried under vacuum at 40 °C for 36−48 h before use. N,N-dimethylformamide (DMF) and dichloromethane (DCM) at 4664


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(part a of Figure 1). Polarized light microscopy (Leica DMLB) is used to obtain digital images of the fibers and their diameters are measured at six different locations using NIH Image 1.63 (part b of Figure 1).7 The cardboard is cut and two fibers with the same diameter are selected for adhesion test (part c of Figure 1). Directional microfiber bundle is obtained from a rotational disk collector at a tangential velocity from 20 to 35 m/s. The alignment of microfibers is measured relative to one another and characterized with a standard deviation of 2.4°. The fiber bundle is used for FTIR and WAXD measurements. 2.3. Adhesion Test. Two cardboard holders are mounted onto the moveable crosshead and fixed load-cell respectively of an MTS Nano Bionix system, also known as the NANO UTM, (load and displacement resolutions of 50 nN and 1 nm, respectively (part d of Figure 1). The two freestanding fibers are arranged orthogonal to each other at the midpoint, in a cross-cylinder configuration (part e of Figure 1). The fibers are then pressed into contact with an approach distance of 2.5 mm and held for 1200 s. According to the load curves in the part a of Figure 2, the

investigated with attenuated total reflection-infrared spectroscopy (BRUKER Vertex 70, Crystal, 45°) at a resolution of 4 cm−1 for 32 scans. Micro/nanofiber bundles are placed in a home-built environmental chamber equipped with a hygrometer for 3600 s. The partial pressure of water vapor (relative humidity, RH) in the chamber is controlled by varying the ratio of water-saturated argon feed stream and dry argon stream. The calculated humidity from the flow rates and the measured humidity agree within 3% of each other. Then the fiber bundles are taken out from the chamber and measured with ATRFTIR at RH of ∼20% and temperature of ∼25 °C. 2.5. WAXD. WAXD is conducted using a Rigaku 18 kV rotating anode X-ray generator attached to the R-AXIS-IV plate system. The X-ray beam is directed perpendicular to the fiber bundle axis. The temperature rises from 25 to 70 °C at the heating rate of 5 °C/min and the data are collected at 180 s per scan. The orientation factor of the crystallites and the order parameter of amorphous phase of fibers are determined with the same method used by Zussman and co-workers.19 2.6. AFM. The morphology of electrospun fiber is imaged by a SPI 3800/SPA 300HV AFM (Seiko Instruments Inc., Japan) with a heating stage. A 150 μm scanner and a commercially available SiN4cantilever with a spring constant of 3 N/m are used. Dynamic force (tapping) mode is used to minimize the damage of the sample during scanning. Temperature-controlled AFM imaging is started after the sample is kept at selected temperature for 1800 s. Data are analyzed using the SPIWIN software, version 3.0. All images are processed using procedures for plan-fit and flatten. Average roughness (Ra) and rootmean-square roughness (rms) of the fiber surface is calculated with the methods used by Watanabe and co-workers.20

3. RESULTS AND DISCUSSION 3.1. Load Deflection Curves. Figure 2 shows a typical load deflection as a function of (a) time, (b) displacement, and (c) positive displacement at an enlarged scale. The fiber diameter is 2.2 μm, temperature 25 °C, and a relative humidity 20%. The fiber is pulled off at a speed of 0.1 mm/s. Part a of Figure 2 reveals the process of adhesion experiment: (I) two fibers come into contact with each other in 20 s, (II) two fibers keep in contact for 1200 s, (III) the two fibers are separated by an applied force in 80 s. The corresponding load-deflection curve is shown in part b of Figure 2. During the pressing step (I), the force changes with displacement and reaches the negative maximum; at the contacting step (II), the magnitude of force decreases possibly due to the stress relaxation, cardboard, and tensile tester drifts; during the pull-off (III), the force initially drops to nearly zero when displacement is negative, then continues positive as pull-off kicks in. A precipitous load drop is observed at displacement of ∼1.8 mm. The enlarged load− displacement curve at pull-off step is shown in part c of Figure 2. The load continues to increase with displacement followed by an abrupt load drop suggesting that the fibers spontaneously separate from each other. The load−displacement curve is similar with the typical adhesive force-displacement data in the indentation experiment.21 It should be noted that when two fibers are separated, the pull-off force does not drop to zero, but remains positive. This may be attributed to a slight crosshead drift of the tensile tester and cardboard drift. The pull-off force F* is then determined by the difference between peak load and residue value after two fibers are separated, and the value of pull-off force is indicated by the line with two arrows. The drift effect proves to have slight impact on the measurement of adhesive force because the data of adhesive force are corrected to accommodate system instability. 3.2. Effect of Relative Humidity on Adhesion. Figure 3 shows the adhesive force between two fibers as a function of relative humidity (RH), two pair of fibers with fiber diameter of

Figure 2. Representative force-time curve (a) force-displacement curve (b) and enlarged load-displacement curve at pull-off step (c) of samples in adhesion test. The fibers with the diameter of 2.2 μm were first pressed into an adhesion contact with an approach distance of 2 mm for 20 min with temperature of 25 °C and relative humidity of 20%. Then the upper fiber was pulled apart at the speed of 0.1 mm/s. maximum load is 36 μN and contact radius 240 nm.7 The maximum contact pressure from JKR is thus estimated to be 200 MPa, which is of the same order of tensile modulus of fiber (250 MPa). Similar to coldwelding for metals, the compression facilitated polymer micro/nanofibers to interdiffuse with one another.7 It also ensures the signal of adhesion be detected by the instrument. Though slightly stronger adhesion is observed with increasing fiber−fiber contact time up to 3600 s, an optimal 1200 s contact time is set for the current study. The two micro/nanofibers are then pulled off at the speed of 0.1 mm/s if not otherwise specified. The whole process is simultaneously recorded and monitored in situ by a video camera. The Nano Bionix is custom fit with an environmental chamber to control relative humidity (RH) and temperature. The RH is controlled by varying the ratio of water-saturated argon feed stream and dry argon stream with the accuracy of 3% and temperature by the thermocouple feedback system with the accuracy of 1% within a range from 20 to 50 °C. Measurements are repeated at least 5 times for a single pair of fibers to ensure reproducibility. 2.4. FTIR. To check whether water absorption on the surface of electrospun fibers during adhesion test, micro/nanofiber bundles are 4665


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Because of the hydrophobicity of PCL electrospun fibers,25 the static water contact angle of which is about 125°, water will neither be absorbed by the fiber to change material properties26 nor form layers on the fiber surfaces. Thus, the adhesion should be determined by the surface interaction of two electrospun PCL fibers. 3.3. Effect of Pull-off Speed on Adhesion. Figure 5 shows the adhesion between two electrospun fibers as a function

Figure 3. Adhesive force as a function of relative humidity (RH). Two pairs of fibers with fiber diameter of 1.6 and 2.2 μm were tested at 25 °C. No obvious effect of RH on the adhesion was observed.

1.6 and 2.2 μm are tested at 25 °C, respectively. No obvious effect of RH on the adhesion is observed. Generally, the humidity influences the adhesion in two ways: (i) by standard capillary action or (ii) by a change of the effective short-range interaction due to adsorbed monolayers of water.22 Moisturefree adhesion will suggest no water is adsorbed on PCL fiber surface during the testing procedure. This conclusion is confirmed by the results from the ATR-FTIR spectra of electrospun PCL mesh which have been placed under the environmental chamber with varied RH for 3600 s. Figure 4

Figure 5. Adhesive force as a function of pull-off speed. The fiber diameter was 0.8 μm and the tests were performed at 25 °C. The adhesion increased with the increase of the pull-off speed.

of pull-off speed. The diameter of fiber is 0.8 μm and the adhesion tests are performed at 25 °C. The adhesion increases with the increase of the pull-off speed. This tendency can be attributed to the nonequilibrium state of fiber surface which is fabricated during electrospinning.16 Adhesive interface exists in several possible metastable states separated by energy barriers. When a force is applied to separate two surfaces of fibers, the separation needs to overcome several energy barriers.9 Two fibers may be pulled apart from each other through a point-by-point detaching process at low pull-off speed or through an entire contact area peeling process at high pull-off speed. As the pointby-point separation requires a much lower force than that for pulling off the entire contact area, the adhesion is shown to increase with the increase of pull-off speed. The dependence of adhesion on pull-off speed is also connected with viscoelastic behavior of polymers. The effect of pull-off speed on viscoelasticity of polymer can be indicated by the Deborah number, De, which is defined as the ratio between the time scale of the relaxation of polymer and that of the observation method.27

Figure 4. ATR-FTIR spectra of PCL electrospun fibers treated at different relative humidities. a, 10% RH; b, 20% RH; c, 40% RH; d, 60% RH; e, 80% RH; f, 100% RH. Water layers were not present on the fiber surfaces.

De = τ

shows the ATR-FTIR spectra of electrospun PCL mats treated at different RH (from 10% to 100% RH). The inserted figure shows the details of absorption band in the 3100−3700 cm−1 region. The strong carbonyl band at 1727 cm−1, ether band at 1170 cm−1, asymmetric CH2 stretching band at 2950 cm−1, and broad OH stretching band at 3448 cm−1 are typical of PCL structure.23 If PCL fiber absorbs water, there should be observable bands at 1640 cm−1 due to H−O−H bending vibration and at 3000− 3800 cm−1 region due to the O−H stretching vibrations.24 These bands are not observed in the spectra of PCL microfibers even treated at the saturated vapor pressure of water (100% RH).

Ḋ D

(1)

where τ is the relaxation time of polymer, Ḋ pull-off speed, and D the separation distance. The relaxation time of PCL at 25 °C is about 0.05 s,28 Ḋ in the range of 0.1 to 1.0 mm/s, and D = 2.5 mm. The Deborah number is smaller than 1 indicating the material behaves more like viscous fluid. As the viscous resistance in the contact zone increases with increasing pull-off speed, the pull-off force is observed to increase with increasing pull-off speed. The dependence of adhesion on pull-off indicates that the state of and interactions between surface chains affect the adhesion substantially.9,29 3.4. Effect of Temperature on Adhesion. Figure 6 shows the adhesive force as a function of fiber diameter in the 4666


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b* ≈ 0.2 nm,30 we get L/R ≈ 4.4φ. This shows that soft polymer nanofibers with the length more than 3 μm will easily stick to each other if they come into contact. In contrast, for R = 0.1 μm thick keratin fibers (typical of Gecko’s spatula) with E ≈ 4 GPa, they will condense into a compact structure only if the fibers are longer than 30 μm. The comparison supports the statement that biological adhesive systems need to generate relative soft effective elastic modulus to make contact everywhere at the interface30 and shows the advantage of electrospun fibers in fabricating biomimicking devices.7 Table 1 reveals that adhesion energy increases with temperature from 190 ± 7 mJ/m2 at 25 °C to 206 ± 3 mJ/m2 at 30 °C, and then an abrupt increase occurs at 35 °C (247 ± 5 mJ/m2) and continues to 296 ± 6 mJ/m2 at 40 °C. This abrupt increase is correlated with transition of surface properties and/or internal structure of fiber with increasing temperatures. The surface of electrospun fiber contains surface irregularities, when two fiber surfaces come into contact, surface roughness is reflected by discrete contact points. Because the sum of the area of all the contact points constitutes the real area of contact, surface roughness contributes to a reduced contact area.31 Characterization of surface roughness is therefore important for understanding the temperature-dependent adhesion. The surface roughness of fiber is characterized by AFM equipped with a hot stage. The fiber is heated in situ to make sure that the same area on the fiber is detected for each measurement. AFM images of electrospun fiber at different temperature are shown in Figure 7. The single fiber is clearly detected and the diameter of fiber roughly estimated at 700 nm by AFM. AFM images show that the shape of electrospun fiber does not change with increasing temperature even to 45 °C (the melting point of PCL ∼60 °C). The surface roughness (rms) of fiber is determined by scanning along the fiber axis with a line profile,7 and the images of line profile are shown at the bottom of corresponding images of fiber. The data of surface roughness are listed in Table 1. The surface roughness changes slightly from 6.5 nm at 25 °C to 6.0 nm at 40 °C indicating that the variation of real area of contact between electrospun fibers with temperature is negligible. Thus, the increased adhesion energy is not caused by the increase of real contact area. The tensile modulus of electrospun fiber with the fiber diameter about 0.7 μm is measured with the tensile tester at a temperature range from 25 to 40 °C. The tensile moduli are listed in Table 1. The tensile modulus varies slightly form 262 MPa at 25 °C to 242 MPa at 40 °C indicating the change of bulk modulus is not the main reason for the increase of adhesion energy. The internal structure of electrospun fiber is characterized by WAXD to investigate the change of the domain at subsurface or bulk phase with temperature, which may affect the adhesion energy of polymer fibers.32 The variation of WAXD patterns of electrospun PCL fibers with temperature is shown in Figure 8. PCL has an orthorhombic crystalline structure with (110) and (200) planes appearing at a 2θ = 21.7° and 2θ = 24.09°, respectively. Although crystalline structure remains at 55 °C, the intensity of I110 and I200 peaks and amorphous part change with temperature indicating the degree of molecular orientation changes within the temperature range.33 On the basis of the WAXD patterns of electrospun fibers, the orientation of crystallite and order parameter of amorphous region at different temperatures are determined and listed in Table 1. The orientation of crystallite phase remains almost

Figure 6. Adhesive force as a function of diameter of fibers at different temperatures. Data were fitted to a linear relationship according to the JKR theory.

temperature range from 25 to 45 °C. The adhesive force is found to increase with diameter and temperature. The adhesion between electrospun fibers can be analyzed by JKR model, which shows that the pull-off force, F*, to detach the two fibers, is determined by the fiber diameter and adhesion energy.7,14 The relationship is expressed as follows,

F* =

3 πWd 4

(2)

where d is the fiber diameter, and W the adhesion energy. The linear curve fit in Figure 6 shows a slope of 1 at each temperature indicating the adhesion between two microfibers is well predicted by JKR model in the range from 25 to 45 °C.7 The adhesion energies can be deduced from the vertical intercept of log (3πW/4) and these results are listed in Table 1. Table 1. Adhesion Energy, Young’s Modulus, Surface Roughness, Molecular Orientation in Crystals, and Amorphous Part of Electrospun Fibers at Different Temperatures temp (°C) 25 30 35 40

adhesion energy (mJ/m2) 190 206 247 296

±7 ±3 ±5 ±6

Young’s modulus (MPa) 262 258 246 242

± ± ± ±

7 5 6 5

surface roughness (nm) 6.7 6.5 6.2 6.0

orientation degree of crystals (%) 88.4 88.3 88.3 88.1

± ± ± ±

0.3 0.1 0.2 0.4

orientation degree of amorphous part (%) 28.1 28.0 25.0 23.2

± 0.2 ± 0.1 ± 0.3 ± 0.2

Table 1 shows that the adhesion energy between two electrospun fibers at 25 °C is 190 ± 7 mJ/m2, which is much higher than that between two PCL films,7 indicating the electrospun fibers at a nanocontact easily stick together. The susceptibility of adhesion between two fibers can be predicted by the Persson’s model.30 ⎛ ER3/2 ⎞1/2 L ⎟⎟ φ ≈ ⎜⎜ R ⎝ 3Wb*1/2 ⎠

(3)

where L and R are the length and radius of fiber, respectively. E and W are the elastic modulus and surface energy, respectively. b* is of the order of the distance necessary to break an interfacial bond (interaction), and φ an angle, typically φ ∼1. For R = 0.7 μm fibers with E = 262 MPa,17 W = 190 mJ/m2, 4667


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Figure 7. AFM images of electrospun fibers at different temperatures. a, 25 °C; b, 30 °C; c, 35 °C; d, 40 °C. The fiber surface roughness was determined by a line profile. A slight change in the surface roughness was observed.

constant from 25 to 40 °C. In contrast, the order parameter of amorphous region decreases with temperature, especially from 30 to 40 °C, where a decrease of molecular orientation from 28% to 25% is observed. These data indicate the state of polymer chains in amorphous part of fiber is sensitive to the temperature and reduces molecular orientation, which gives rise to enhanced chain mobility on the fiber surface.29 As shown in Table 1, the abrupt increase of adhesion energy with temperature accompanies a decrease of molecular orientation in the amorphous part of fiber. It is, therefore, reasonable to assume that an abrupt increase of adhesion energy with temperature is a result of the decease of molecular orientation in the amorphous part of fiber with temperature. 3.5. Mechanism of Adhesion. To support the hypothesis that an abrupt increase of adhesion energy with temperature is attributed to the decrease of the degree of molecular orientation in the amorphous part of fiber, a comparison between measured adhesion energy and the calculated value based on

Figure 8. Temperature-dependent WAXD pattern of PCL electrospun fibers. 4668


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where K is a constant, ΔE the activation energy, R a gas constant, and T the absolute temperature. As the surface property and internal structure of electrospun fibers remain almost the same at 25 and 30 °C, as shown in Table 1, the measured adhesion energies at 25 and 30 °C are used in eq 5 to predict the adhesion energy at higher temperatures. Figure 9 shows the comparison between measured and calculated data of adhesion energies. The difference between measured and calculated data is observed at 35 °C and becomes significant at 40 °C. The deviation mainly comes from the negligence of the contribution from the temperature-dependent molecular orientation to the mobility of polymer chains. Therefore, the comparison supports the hypothesis that an abrupt increase of adhesion energy is a result of the decrease of degree of molecular orientation. In addition, the comparison provides direct evidence that adhesion is derived from the van der Waals interactions between polymer segments on the surface of fibers. During the electrospinning process, flexible polymer chains are elongated by a high voltage and tend to follow the direction of the inflexible oriented domains. Within the electrospun fiber, the polymer chains are extended along the axial direction.4,5 As shown in Figure 10, the mobility of polymer chains is restricted by the confinement effect on the fibers, which decreases the probability of interdiffusion and interaction between polymer chains at the interfaces. As the temperature increases, the degree of molecular orientation decreases notably, thus enhances the mobility of polymer chains in the surface, resulting in substantial increase of adhesion energy. The capillary bridges between two electrospun fibers have been precluded in this evaluation. The direct interaction between two fibers is the origin of adhesion. Furthermore, the dominating force of adhesion has been shown to be weak intermolecular van der Waals forces.7 Thus, a conclusion can be made that the adhesion between two polymer micro/nanofibers largely depends on the rearrangement or restructuring of surface molecular structures to enhance the number and/or strength of interactions, viz. weak van der Waals interactions, across the interface. The enhanced mobility of polymer chains at surface due to the reduced size makes the interdiffusion and

the temperature-dependent mobility of polymer chains is performed. Adhesion energy, W, reflects the strength of the interfacial van der Waals interactions between two polymer fibers, and can be expressed as follows,30 W = n*f *b*

(4)

where b* is of the order of the distance necessary to break the interfacial interactions, f * the typical force to break van der Waals interactions, and n* the number of attachments per area. f * and b* are assumed to be constant since they change slightly in the temperature range examined in this experiment, and n* is determined by the attachment rate and contact time.34 Because attachment rate mainly depends on the mobility of chain segments at the contact zone29,30 and contact time is constant in this experiment, n* is proportional to the mobility of chain segments. The mobility of chain segments is shown to be in an Arrhenius relationship.35 Thus, the dependence of adhesion energy on temperature can be expressed as follows, W (T ) ∼ K exp( −ΔE /RT )

(5)

Figure 9. Comparison of adhesion energies between measured and theoretical data. (a) Measured data, (b) the data calculated based on the mobility of chain segments in the surface.

Figure 10. Schematic representation of variation of chain mobility with temperature. The degree of molecular orientation decreased with temperature, resulting in enhanced chain mobility at the interface. 4669


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interaction plausible.8,28 We herein propose that adhesion between polymer micro/nanofibers can be controlled by temperature and degree of molecular orientation, awaiting additional experimental evidence. The orientation and temperature effects, in turn, influence the mobility of polymer chains on the fiber surface. This article not only provides the preliminary data that suggests such an interaction between nanoscale contacts6,8 but also provides fruitful insights on the design of biomaterials such as electrospun polymer meshes for tissue engineering5 and bioinspired dry adhesives.2,3

(8) Buell, S.; Rutledge, G. C.; van Vliet, K. J. Predicting polymer nanofiber interactions via molecular simulations. ACS Appl. Mater. Interface 2010, 2, 1164−1172. (9) Kendall, K. Adhesion: molecules and mechanics. Science 1994, 263, 1720−1725. (10) Cuenot, S.; Frétigny, C.; Demoustier-Champagne., S.; Nysten, B. Surface tension effect on the mechanical properties of nanomaterials measured by atomic force microscopy. Phys. Rev. B 2004, 69, 165410. (11) Carrillo, J. Y.; Raphael, E.; Dobrynin, A. V. Adhesion of nanoparticles. Langmuir 2010, 26, 12973−12979. (12) Xing, M.; Zhong, W.; Xu, X.; Thomson, D. Adhesion force studies of nanofibers and nanoparticles. Langmuir 2010, 26, 11809− 11814. (13) Erath, J.; Schmidt, S.; Fery, A. Characterization of adhesion phenomena and contact of surfaces by soft colloidal probe AFM. Soft Matt. 2010, 6, 1432−1437. (14) Johnson, K. L.; Kendall, K.; Roverts, A. D. Surface energy and the contact of elastic solids. Proc. R. Soc. London A 1971, 324, 301− 303. (15) Derjaguin, B. V.; Muller, V. M.; Toporov, Y. P. Effect of contact deformations on the adhesion of particles. J. Colloid Interface Sci. 1975, 53, 314−326. (16) Arinstein, A.; Zussman, E. Electrospun polymer nanofibers: Mechanical and thermodynamic perspectives. J. Polym. Sci. Part B. Polym. Phys. 2011, 49, 691−707. (17) Wong, S.- C.; Baji, A.; Leng, S. Effect of fiber diameter on tensile properties of electrospun poly (-caprolactone). Polymer 2003, 49, 4713−4722. (18) Wu, X.; Dzenis, Y. A. Collapse analysis of nanofibres. Nanotechnology 2007, 18, 285702. (19) Arinstein, A.; Burman, M.; Gendelman, O.; Zussman, E. Effect of supramolecular structure on polymer nanofibre elasticity. Nat. Nanotech. 2007, 2, 59−62. (20) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Effects of the surface roughness on sliding angles of water droplets on superhydrophobic surfaces. Langmuir 2000, 16, 5754−5760. (21) Vajpayee, S.; Long, R.; Shen, L.; Jagota, A.; Hui, C. Effect of rate on adhesion and static friction of a film-terminated fibrillar interface. Langmuir 2009, 25, 2765−2771. (22) Huber, G.; Mantz, H.; Spolenak, R.; Mecke, K.; Jacobs, K.; Gorb, S. N.; Arzt, E. Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 16293−16296. (23) Lumelsky, Y.; Zoldan, J.; Levenberg, S.; Silverstein, M. S. Porous polycaprolactone- polystyrene semi-interpenetrating polymer networks synthesized within high internal phase emulsions. Macromolecules 2008, 41, 1469−1474. (24) Asay, D. B.; Kim, S. H. Evolution of the adsorbed water layer structure on silicon oxide at room temperature. J. Phys. Chem. B. 2005, 109, 16760−16763. (25) Han, D.; Steckl, A. J. Superhydrophobic and oleophobic fibers by coaxial electrospinning. Langmuir 2009, 25, 9454−9462. (26) Puthoff, J. B.; Prowse, M. S.; Wilkinson, M. Autumn, K. Changes in materials properties explain the effects of humidity on gecko adhesion. J. Exp. Biol. 2010, 213, 3699−3704. (27) Moura-Ramos, J. J.; Correia, N. T. The Deborah number, relaxation phenomena and thermally stimulated currents. Phys. Chem. Chem. Phys. 2001, 3, 5575−5578. (28) Lu, J.; Mirau, P. A.; Tonelli, A. E. Dynamics of isolated polycaprolactone chains in their inclusion complexes with cyclodextrins. Macromolecules 2001, 34, 3276−3284. (29) Maeda, N.; Chen, N.; Tirrell, M.; Israelachvili, J. N. Adhesion and friction mechanisms of polymer-on-polymer surfaces. Science 2002, 297, 379−382. (30) Persson, B. N. J. The effect of surface roughness on the adhesion of elastic plates with application to biological systems. J. Chem. Phys. 2003, 118, 7614−7621.

4. CONCLUSIONS Electrospun PCL fibers with diameters ranging from 0.4−2.2 μm were investigated. Adhesion between two polymer fibers were studied using a nanoforce tensile tester. We found that the adhesive force increased with temperature and pull-off speed. There was no observable difference arising from the change in relative humidity. Water was found not to adsorb onto the fiber surface. Surface roughness of fibers changed slightly with temperature. As a result, the abrupt increase of adhesion energy with temperature was attributed to a decrease in the degree of molecular orientation in the amorphous part of fiber. These results corroborated our previous findings that adhesion is derived from weak van der Waals interactions between the interdiffusion chain segments across the interface. Strength and number of van der Waals interactions depend on temperature and degree of molecular orientation. Adhesion between polymer fibers can thus be manipulated by adjusting the mobility and state of the polymer chains on a surface.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 431 85262161, fax: +86 431 85262109, e-mail: [email protected] (Q.S.); [email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Project No. 50803061, 50833005, and 50920105320) and National Science Foundation under a CAREER Award to SCW (NSF-CMMI 0746703). The authors thank the reviewers for their constructive comments.

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