Effect of Surface Interactions on Adhesion of Electrospun Meshes on

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Effect of Surface Interactions on Adhesion of Electrospun Meshes on Substrates Qiang Shi,*,† Qunfu Fan,†,§ Xiaodong Xu,§ Wei Ye,† Jianwen Hou,† Shing-Chung Wong,‡ and Jinghua Yin*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § Polymer Materials Research Center, Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China ‡ Department of Mechanical Engineering, University of Akron, Akron, Ohio 44325-3903, United States S Supporting Information *

ABSTRACT: Despite the importance of adhesion between electrospun meshes and substrates, the knowledge on adhesion mechanism and the method to improve the adhesion remain limited. Here, we precisely design the model system based on electrospun poly(ethylene oxide) (PEO) meshes and the substrate of styrene-b-(ethylene-co-butylene)-bstyrene elastomer (SEBS), and quantitatively measure the adhesion with a weight method. The surfaces of SEBS with different roughness are obtained by casting SEBS solution on the smooth and rough glass slides, respectively. Then, the surfaces of casted SEBS are respectively grafted with PEG oligomers and long PEG chains much larger than the entanglement molecular weight by surface-initiated atom transfer radical polymerization (SI-ATRP) of poly(ethylene glycol) methyl ether methacrylate (PEGMA). The detached surfaces of SEBS and electrospun fibers after adhesion measurements are analyzed by scanning electron microscopy (SEM). The adhesive force and adhesion energy are found to lie in the range from 68 to 220 mN and from 12 to 46 mJ/m2, respectively, which are slightly affected by surface roughness of substrate but mainly determined by surface interactions. Just as the chemical cross-linking induces the strong adhesion, the chain entanglements on the interface lead to the higher adhesion than those generated by hydrophilic− hydrophobic interactions and hydrophilic interactions. The long grafted chains and the enhanced temperature facilitate the chain entanglements, resulting in the strong adhesive force. This work sheds new light on the adhesion mechanism at molecular level, which may be helpful to improve the adhesion between the electrospun fibers and substrates in an environmentally friendly manner. electrospun meshes.11−17 However, this kind of adhesive force is relatively weak to stabilize electrospun meshes on the substrates for long-term applications.9,10,18 Although the adhesion can be improved by chemical cross-links, most effective cross-linking agents are toxic.10,19 Hence, the research on adhesion and further establishment of adhesion mechanism are necessary to enhance the adhesion in an effective and environmentally friendly way. However, investigation of the adhesion and adhesion mechanism remains challenging. Most techniques that have been used to quantitatively measure the adhesion at micro/nano contacts, such as surface forces apparatus (SFA),14 AFM,14,20 and indentation method,15 are not suitable for measuring the adhesion of meshes on large substrate. In addition, the obstacles in precisely designing

1. INTRODUCTION Electrospinning is a simple, straightforward, and versatile technique which allows creating micro/nanofibers from varied polymers and composite materials.1−7 The polymer micro/ nanofibers fabricated are continuous, entitling them with high axial strength and extreme flexibility. Therefore, the electrospun meshes assembled by electrospun fibers have excellent structural mechanical properties.1−3 Meanwhile, multiple functions can be incorporated into electrospun mats to broaden their application from textile, ultrafiltration, tissue engineering, and catalysis to sensors, solar cells, and other types of devices.3−7 The adhesion between electrospun meshes and substrates plays a key role in the stability and function of materials and devices fabricated by electrospinning.8−10 Polymer micro/ nanofibers are characterized by high surface area-to-volume ratio and highly active chains at surface, which render them adhesive to each other to improve the mechanical properties of © 2014 American Chemical Society

Received: June 24, 2014 Revised: November 2, 2014 Published: November 3, 2014 13549

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The length of grafted chains was estimated with a mass spectrometry (Bruker autoflex III smart beam MALDI-TOF/TOF, Germany), and the surface wettability of SEBS was evaluated by the sessile drop method with a pure water droplet (ca. 3 μL) using a contact angle goniometer (DSA, KRUSS GMBH, Germany). 2.3. Electrospinning of PEO onto Surface of SEBS. PEO was dissolved in a solvent mixture of water and ethanol in the ratio of 3:2 (w/w) to obtain the solution with the concentration of 1.5 wt %. The PEO nanofibers were electrospun onto the surface of virgin SEBS and PEG-grafted SEBS at a temperature of about 45 °C, humidity 43%, and solution feed rate ∼0.5 mL/h with applied voltage of 10−11 kV. The thickness of electrospun meshes was controlled within 2.2 μm. For simplicity, the SEBS coated with electrospun fibers was referred as “electrospun SEBS”. The morphology of electrospun SEBS was then characterized by a field-emission scanning electron microscopy (SEM, Sirion-100, FEI). Single electrospun fiber was collected by two parallel electrodes12 and transferred to silicon wafer for atomic force microscopy (AFM) measurement. 2.4. Cross-Linking of Electrospun Meshes on Surface of PEG-Modified SEBS. The cross-linking of electrospun meshes on the surface of SEBS-g2 was performed with glutaraldehyde.10 The electrospun SEBS films (1 cm × 1 cm) were put on a mesh screen and incubated in vapor phase of glutaraldehyde (5 wt % glutaraldehyde solution) for 24 h. The cross-linked electrospun SEBS was dried on a vacuum oven for at least 24 h to completely remove unreacted glutaraldehyde. 2.5. Measurement of Surface Roughness. The surface roughness of electrospun fiber, virgin SEBS, and grafted SEBS is determined by a SPI 3800/SPA 300HV AFM (Seiko Instruments Inc., Japan) with a heating stage. A 150 μm scanner and a commercially available SiN4 cantilever with a spring constant of 3 N/m were used. Dynamic force (tapping) mode is used to minimize the damage of the sample during scanning. Data were analyzed using the SPIWIN software, version 3.0. All images were processed using procedures for plan-fit and flatten. Root-mean-square roughness (rms) of the surface and average roughness (Ra) of the fiber surface were calculated with the methods used by Watanabe and co-workers.24 2.6. Adhesion Measurement. Adhesion measurements of electrospun fibers on SEBS were carried out based on the procedure developed by Dhinojwala et al.25 The setup for mesurement is shown in Figure 1. Electrospun SEBS (1 cm × 1 cm) was stabilized on a

model system are remained because of the difficulties in manipulating well-defined surfaces of the substrate.8 To overcome the above difficulties, we precisely design the model system based on electrospun PEO meshes and the substrate of SEBS and quantitatively measure the adhesion with a weight method. The surfaces of SEBS with different roughness and different length of grafted chains are obtained by casting SEBS solution on various substrates and subsequent SI-ATRP of PEGMA on the casted films. The SEBS surfaces are grafted with PEG oligomers and long PEG chains much larger than the entanglement molecular weight of PEG (4400),21,22 respectively. The detached surfaces of SEBS and electrospun fibers after peeling off are analyzed by SEM. The adhesive force and adhesion energy lie in the range from 68 to 220 mN and from 12 to 46 mJ/m2, respectively, which are slightly affected by surface roughness of substrate but mainly determined by surface interactions. Just as the chemical crosslinking induces the strong adhesion, the chain entanglements on the interface lead to the higher adhesion and adhesion energy than those generated by hydrophilic−hydrophobic interactions and hydrophilic interactions. The long grafted chains and the enhanced temperature facilitate the chain entanglements, resulting in the strong adhesion. This work sheds new light on the adhesion mechanism at molecular level, which may be helpful to improve the adhesion between the electrospun fibers and substrates in an environmentally friendly manner.

2. EXPERIMENTAL SECTION 2.1. Materials. SEBS copolymer with 29 wt % styrene (Kraton G 1652) was provided by Shell Chemicals. Poly(ethylene glycol) methyl ether methacrylate (PEGMA) monomer (Mn ∼ 475) and copper(I) bromide (CuBr, 98%) were obtained from Sigma-Aldrich. Copper(II) bromide (CuBr2, 99%) and 2,2′-bipyridine (Bpy, >99%) were purchased from Alfa Aesar. PEGMA was passed through a silica gel column to remove the inhibitor and stored under an argon atmosphere at −10 °C. Glutaraldehyde was obtained from Sigma-Aldrich. Acetone and xylene were reagent grade products. Other reagents were AR grade and used without further purification. 2.2. Manipulation of Surface Properties of SEBS. SEBS was dissolved in xylene to form 15% (w/w) solutions and casted onto a flat glass and rough glass to obtain SEBS films (0.2 mm thick) at room temperature. For clarity, the surfaces of SEBS contacting air, flat glass, and rough glass were referred as “SEBS-a”, “SEBS-f”, and “SEBS-r”, respectively. As SEBS-r was easily detected during sample preparation, the surface of SEBS-r was used for subsequent “grafting from” reactions. SI-ATRP of PEGMA onto SEBS films was performed according to the procedures in our previous works.23 Briefly, the immobilization of ATRP initiators on SEBS surface was carried with an aqueous-based method. The virgin SEBS films were subjected to oxygen plasma for 90 s and immersed in 90 mL of HBr/H2SO4 (5/1, v/v) mixture solution for 16 h at 60 °C. Then these treated films were washed drastically with deionized water to remove physically adsorbed HBr and dried overnight under vacuum at 25 °C. Finally, SI-ATRP of PEGMA was carried out at 40 °C in a Pyrex tube for 10 min and 2 h, respectively. The obtained hybrid films were subsequently rinsed by ethanol and then dried under vacuum overnight at 25 °C. Grafting density (GD, μg/cm2) was calculated as follows: GD = (Wm − W0)/S

Figure 1. Setup for adhesion measurements. substrate with a double-sided adhesive tape (Yongwei, China), followed by covering with a tape (Hongxiang, China) with size of 3 cm × 0.8 cm. A 5 g weight was put on the surface of tape for 30 s to ensure complete contact of tape with meshes. Then, a paper cup was connected at the end of tape, ensuring the vertical distance of tape is 2 cm. A support plate was put under the bottom of cup with the distance of 0.1 cm. Finally, the cup was continuously injected with water by a pump at a rate of 0.05 mL/s until the peel off occurred (0.1 cm). The adhesion was calculated based on the weight of injected water (N = Mg, where M is the mass of water and g is acceleration of gravity, g =

(1)

where W0 and Wm are the masses (μg) of the brominated and modified SEBS films, respectively; S is the area of film specimen (cm2). For clarity, the grafted SEBS obtained for 10 min (1.1 μg/cm2) and 2 h reactions (12.5 μg/cm2) were referenced as “SEBS-g1” and “SEBSg2”, respectively. 13550

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surface roughness between the casted films are obvious. For example, the surface roughness of SEBS-a, SEBS-f, and SEBS-r are 55, 62, and 70 nm, respectively. However, the surface roughness of SEBS-g1 and SEBS-g2 is similar, namely, 48 and 42 nm, respectively. The wettability of virgin and grafted SEBS is tested by the static water angle measurements (Figure S3), and the corresponding results are listed in Table 1. Virgin SEBS exhibits the hydrophobicity with a WCA larger than 90°. The WCA increases in the order of SEBS-a, SEBS-f, and SEBS-r, confirming the hydrophilicity is influenced by surface roughness.24 The surface of SEBS becomes hydrophilic ( 55 nm) while the grafted surface becomes relatively smooth, demonstrating the full coverage of grafted chains changes the surface topology.26,27 The surface roughness of all samples is listed in Table 1. The differences of Table 1. Surface Properties of Model SEBS surface roughness (nm) SEBS-a SEBS-f SEBS-r SEBS-g1 SEBS-g2

55 62 70 48 42

± ± ± ± ±

7 12 15 4 8

water contact angle (deg) 95 97 103 69 62

± ± ± ± ±

7 6 9 4 7 13551

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Figure 3. SEM images of electrospun meshes on the substrate of SEBS: (a) electrospun meshes on virgin SEBS; (b) electrospun meshes on grafted SEBS; (c) cross-linked electrospun meshes on grafted SEBS; (d) dry cross-linked electrospun meshes after incubation in distilled water for 1 h. All the scale bars are 2 μm.

within the range of adhesion energy between the casted poly(vinylidene fluoride−trifluoroethylene) copolymer film and mica sheet (∼20 mJ/m2),14 two polydimethylsiloxane rubber spheres (43.6 mJ/m2),32 and gecko foot setae and substrate (∼50 mJ/m2).33 The slight differences are observed for the adhesion and adhesion energy between electrospun fibers and virgin SEBS with the notable variation of surface roughness, indicating the surface roughness of substrate has slight effects on the adhesion between electrospun meshes and substrate. Compared with the adhesion on virgin SEBS, the adhesion on grafted SEBS increases substantially and nearly approaches the strong adhesion obtained by cross-linking, showing the enhanced adhesion is mainly caused by interactions between grafted layers and electrospun fibers.34 Because most effective crosslinking agents are toxic,10,19 enhancement of adhesion by interactions between grafted layers and electrospun fibers are highly desired. Therefore, the adhesion between electrospun fibers and grafted SEBS is further investigated. The electrospun SEBS is well sealed and preserved at 0, 20, and 40 °C for 24 h, respectively, before adhesion measurements. As the melt temperature of electrospun fibers is about 65 °C (Figure S4), these fibers do not melt at 40 °C and the surface roughness varies slightly with the increasing temperature (Figure S6). The temperature-dependent adhesion on grafted SEBS is shown in Figure 5. A small increment of adhesion on SEBS-g1 with the increasing temperature is observed. In contrast, the adhesion and adhesion energy on the surface of SEBS-g2 increase substantially with the enhanced temperature. The adhesive force at 40 °C (∼200 mN) is comparable to that after cross-linking (∼220 mN). Because the hydrophilicity and surface roughness are similar on the surface of SEBS-g1 and SEBS-g2, the different tendency of adhesion with temperature indicates the adhesion mechanisms on the

Figure 4. Adhesion and adhesion energy of electrospun meshes on SEBS.

The adhesion between electrospun meshes and susbstrates can be analyzed by JKR model,31 which shows that the peel-off force, F*, is determined by the number of contacting fibers, fiber diameter, and adhesion energy. The relationship is expressed as follows 3 F * = πRNW (2) 4 where R is the fiber diameter, N the contacting number of fibers, and W the adhesion energy. Based on the image of electrospun fibers in Figure 3 (about 1 fiber per 50 μm2), the number of electrospun fibers that contact substrate at initial peel-off area (1 cm × 0.1 cm) is assumed to be 105. With the data of average R (200 nm) and N (105) of electrospun fibers, the adhesion energy is obtained (shown in Figure 4). The adhesion energy lies in the range of 14.6−46.7 mJ/m2, which is 13552

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higher than hydrophilic−hydrophobic interactions in the model system. Some electrospun fibers remain on the surface of SEBSg2 and the electrospun fibers become distorted due to peel-off force (Figures 6c and 6c1). These morphologies are similar to the peel-off surfaces of fibers and SEBS-g2 with cross-linking (Figures 6d and 6d1), confirming the chain entanglements dominantly determine the adhesion between electrospun fibers on the surface of SEBS-g2. The entanglement of grafted PEG on SEBS-g2 with PEO chains in the fibers is further confirmed by a wide-angle X-ray diffraction (WAXD) measurement (Figure S7). WAXD pattern of SEBS-g2 after adhesion measurements supports the existence of electrospun PEO fibers remained on the peel-off surface. Thus, both the SEM and WAXD measurements provide the direct evidence for the chain entanglements between electrospun fibers and SEBS-g2. 3.4. Adhesion Mechanism. Based on the above investigation, four kinds of surface interactions in the model system are found to control the adhesion between electrospun fibers and SEBS, i.e., hydrophobic−hydrophilic interaction, hydrophilic interaction, chain entanglement, and cross-linking (Figure 7). Hydrophobic−hydrophilic interaction is weak and

Figure 5. Adhesion and adhesion energy of electrospun meshes on SEBS-g1 and SEBS-g2 at different temperatures.

surface of SEBS-g1 and SEBS-g2 are not the same in nature. The interactions between grafted layers and electrospun fibers are hydrophilic interaction, which mainly determine the adhesion between electrospun fibers and SEBS-g1. Considering the long grafted chains on SEBS-g2, the entanglement between PEO chains in electrospun fibers and grafted PEG chains readily occurs.35 As the chain entanglement is stronger than hydrophilic interactions, the chain entanglement mainly controls the adhesion between electrospun fibers and SEBS-g2. To prove the hydrophilic interactions and the chain entanglements between electrospun meshes and grafted SEBS, the detached surfaces after adhesion measurements are analyzed by SEM (Figure 6). The interfaces between electrospun meshes and virgin SEBS, as well as cross-linked surfaces, are investigated for comparison. All samples are treated at 40 °C for 24 h before adhesion measurements at 20 °C. The surfaces of virgin SEBS and corresponding electrospun fibers seem intact (Figures 6a and 6a1), indicating the hydrophilic−hydrophobic interaction is weak.36 For the grafted surfaces, the rough surface of SEBS-g1 and deformed surface of electrospun fibers can be detected (Figure 6b and 6b1), showing the higher adhesion on SEBS-g1 than that on virgin SEBS. Thus, it proves that the hydrophilic interactions are

Figure 7. Adhesion mechanism.

leads to the lowest adhesion. In contrast, cross-linking forms chemical bonds between electrospun fibers and the grafted

Figure 6. SEM images of peel-off interfaces of samples. Surfaces of (a) SEBS-r, (b) SEBS-g1, (c) SEBS-g2, and (d) SEBS-g2 with cross-linking; surfaces of electrospun fibers peeled off from (a1) SEBS-r, (b1) SEBS-g1, (c1) SEBS-g2, and (d1) SEBS-g2 after cross-linking. These samples are heated at 40 °C for 24 h before adhesion measurement. The scale bars for SEBS and electrospun fibers are 5 and 2 μm, respectively. 13553

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SEBS, resulting in the highest adhesion. The hydrophilic interactions and the chain entanglements cause the intermediate adhesion. For the SEBS grafted with short oligomers, the hydrophilic interactions between electrospun fibers and grafted SEBS is dominant. As the hydrophilic interactions are stronger than hydrophilic−hydrophobic interactions,36 the adhesion induced by the former (∼100 mN) is higher than that caused by the latter (∼50 mN). When the length of grafted chains is much larger than the entanglement molecular weight, chain entanglements become dominant to control the adhesion. Because of high mobility of grafted chains, chain entanglements at interfaces depend on the mobility of polymer chains in the fiber. It has been established that the mobility of polymer chains on the fibers is restricted by the confinement effect.16 As the temperature increases, the degree of molecular orientation decreases notably, thus enhancing the mobility of polymer chains in the surface of the fiber12 and resulting in substantial increase of chain entanglement with grafted chains.35 The high degree of entanglements renders the adhesive force comparable to the adhesion induced by cross-linking. Because most crosslinking agents are toxic, improving the adhesion by the introduction of chain entanglments as physical cross-links between electrospun fibers and the substrate is an effective and environmentally friendly method.37

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Q.S.). *E-mail [email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (Projects No. 51273199, 21274150, and 51103030).

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REFERENCES

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4. CONCLUSIONS We precisely designed the model system based on electrospun PEO meshes and the SEBS substrate and quantitatively measured the adhesion with a weight method. The surfaces of SEBS with different roughness and grafted chains with varied length were obtained by casting SEBS solution on various substrates and subsequent SI-ATRP of PEGMA on the casted films. The SEBS surfaces were grafted with PEG oligomers and long PEG chains much larger than the entanglement molecular weight of PEG, respectively. The surfaces of SEBS and electrospun fibers after adhesion measurements were analyzed by SEM. The adhesion and adhesion energy were found to lie in the range from 68 to 220 mN and from 12 to 46 mJ/m2, respectively, which were slightly affected by the surface roughness of substrate but mainly determined by the surface interactions. Just as the chemical cross-linking induces the strong adhesion, the chain entanglements on interface led to the higher adhesion and adhesion energy than those generated by hydrophilic−hydrophobic interactions and hydrophilic interactions. The long grafted chains and the enhanced temperature facilitated the chain entanglements, resulting in high adhesions. This work throws new lights on the adhesion mechanism at molecular level, which may be helpful to improve the adhesion between the electrospun fibers and substrates in an environmentally friendly manner.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of virgin SEBS and P(PEGMA)-grafted SEBS, determination of length of grafted P(PEGMA) chains, wettability of virgin SEBS and grafted SEBS, determination of melting, crystallization and glass transition temperature of electrospun PEO fibers, surface roughness of electrospun fibers, analysis of detached surfaces of SEBS after measurement with WAXD. This material is available free of charge via the Internet at http://pubs.acs.org. 13554

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