Effects of surface treatment with coupling agents of PVDF-HFP fibers on the improvement of the adhesion characteristics on PDMS

Applied Surface Science 321 (2014) 378–386

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Effects of surface treatment with coupling agents of PVDF-HFP fibers on the improvement of the adhesion characteristics on PDMS O.M. Kwon a , S.J. See b , S.S. Kim b , H.Y. Hwang a,∗ a Department of Mechanical Design Engineering, Andong National University, 1375, Gyeongdong-ro, Andong-si, Gyeongsangbuk-do 760-749, Republic of Korea b Department of Organic Materials & Fiber Engineering, Chonbuk National University, 567, Baekje-daero, Daekje-daero, Deokjin-gu, Jeonju, Jeollabuk-do 567-756, Republic of Korea

a r t i c l e

i n f o

Article history: Received 6 August 2014 Accepted 7 October 2014 Available online 17 October 2014 Keywords: Adhesion PVDF-HFP PDMS Coupling agent Surface roughness

a b s t r a c t Surface treatment of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) fibers was conducted with coupling agents such as epoxy silane, amino silane, and titanate to improve the adhesion characteristics of PVDF-HFP fibers and polydimethylsiloxane (PDMS). The adhesion strength was largest when 4 wt% amino silane was used for surface treatment, showing a 250% improvement compared to the untreated case. Surface roughening and shrinking of the PVDF-HFP fibers were observed after surface treatment, but no chemical bonding occurred between the PVDF-HFP fibers and the coupling agents. It was thus concluded that the improvement of the adhesion characteristics of the PVDF-HFP fibers and PDMS was caused by the physical bonding between them due to the surface treatment with coupling agents. In addition, for the surface roughening mechanism, amino silane infiltration into the PVDF-HFP fibers during the surface treatment, followed by extraction during the drying process, was suggested. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The interfacial adhesion strength of each material is one of the critical parameters for the mechanical properties of the composite material, such as elastic modulus and strength, and is known to be greatly influenced by the types of reinforcements and matrices, the fabrication conditions, and the surface treatment [1–3]. Until now, many researches have been conducted to improve the adhesion strength using surface treatment with a coupling agent since coupling agents can provide a stable adhesion between incompatible surfaces [4–6]. Piezoelectric-sensor-embedded flexible devices have been developed recently for use as tactile sensors of humanoid robots. Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), a polymer-based piezoelectric material, and polydimethylsiloxane (PDMS), a silicon-based organic polymer, are widely used to provide flexibility to the tactile sensors. However, it is very hard to adhere PVDF-HFP and PDMS onto each other, because PVDF-HFP is a highly-non-reactive fluoropolymer and PDMS has hydrophobic characteristics. To improve the adhesion characteristics of PVDFHFP and PDMS, the surfaces of PVDF-HFP are generally modified via

∗ Corresponding author. Tel.: +82 54 820 6305; fax: +82 54 820 5167. E-mail address: [email protected] (H.Y. Hwang). http://dx.doi.org/10.1016/j.apsusc.2014.10.028 0169-4332/© 2014 Elsevier B.V. All rights reserved.

gas plasma treatment with argon, oxygen, nitrogen and ammonia, but plasma treatment is temporal and requires special equipment for generating plasma and controlling the gas flow. Therefore, if surface treatment with the generally used coupling agents on PVDF-HFP can be adopted to improve the adhesion characteristics of PVDF-HFP and PDMS, it can be very useful and cost-effective [7–10]. In this study, the adhesion characteristics of PVDF-HFP fibers and PDMS were investigated. To improve the adhesion strength of PVDF-HFP fibers and PDMS, surface treatment with several coupling agents was introduced. The effects of the coupling agents on the adhesion strength of the PVDF-HFP fibers and PDMS, the surface morphology, and the chemical composition and structures of PVDF-HFP fibers were analyzed via a microdroplet test, scanning electron microscopy (SEM), an atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). 2. Materials and methods 2.1. Materials PVDF-HFP fibers were prepared through the wet spinning method, using PVDF-HFP (Kynar flex2801, Arkema Inc., USA). Table 1 represents the fabrication conditions of the PVDF-HFP fibers. The fabricated PVDF-HFP fibers were 200–230 ␮m in

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Table 1 Fabrication conditions of PVDF-HFP fibers. Process

Wet spinning

Heat treatment

Annealing

Temperature (◦ C) Stretching (%)

80 900

80 350

120 (30 min)

diameter. PDMS (SYLGARD-184, Dow-Corning Co., USA) was used as matrix material for making a flexible structure. Table 2 shows the specifications of the PVDF-HFP fibers and PDMS. Three types of coupling agents — epoxy silane (3-glycidoxypropyl trimethoxysilane, KBM-403, Shin-Etus, Japan), amino silane (3-aminopropyltriethoxysilane, KBE-903, Sin-Etus, Japan), and titanate (neopentyl(dially)oxy tri(dioctyl)pyrophosphate titanate, Lica-38, Kenrich, USA) — were used for the surface treatment of the PVDF-HFP fibers. The chemical formulae of all the materials are shown in Fig. 1. 2.2. Surface treatments with coupling agents The surface treatment of the PVDF-HFP fibers was performed using three generally used coupling agents, as shown in Fig. 2 [11–14]. First, the solutions of the coupling agents were prepared by mixing 1–5 wt% epoxy and amino silane with distilled water, or 1–5 wt% titanate with isopropyl alcohol, for 30 min at 500 rpm. For the epoxy silane, acetic acid was added to adjust the acidity of the mixed solution to pH 5, and to stabilize it. Then the PVDF-HFP fibers were soaked in the coupling agent solutions for 30 min. Finally, the distilled water and isopropyl alcohol were evaporated in an oven at 80 ◦ C for 2 h. 2.3. Adhesion strength test (microdroplet test) In this study, microdroplet tests were conducted to evaluate the interfacial adhesion strength of the PVDF-HFP fibers and PDMS, because such test can be applied to fibers with small diameters, is less dependent on the fiber size and strength, and can assess various combinations of fibers and matrices [15–19]. Microdroplet specimens were prepared as shown in Fig. 3. Using a 200-␮mdiameter clean metal wire, liquid PDMS was carried and wetted onto the PVDF-HFP fibers, after which the fibers were cured in an oven at 80 ◦ C for 3 h. The embedded lengths and diameters of the cured microdroplets were measured using a high-resolution digital microscope. The embedded lengths of the microdroplets were kept at 700–1500 ␮m. Microdroplet tests were performed on a special micromaterial testing machine designed as a vertical type to prevent fiber bending, as shown in Fig. 4. A motor-driven stage (Am1-0803-3s, MMT, France) with a 10 N-capacity loadcell (BCL-1kgf, CAS, South Korea) and two microvises (XEG40, Misumi Co., Japan) with a knife edge were installed for tensile load application, and LVDT (DP-10, KOMEIN Co., South Korea) and a high-resolution digital microscope (AM4413T5 Dino-Lite Edge, Anmo Electronics Co., Taiwan) were used for measuring the displacement and for monitoring the microdroplet behavior.

Fig. 1. Chemical structures of (a) PVDF-HFP, (b) PDMS, (c) epoxy silane, (d) amino silane, and (e) titanate.

To make the loading around the microdroplet uniform, and to prevent stress concentration, a stainless-steel pin hole with a 250 ␮m hole diameter, 5 mm outer diameter, and 1 mm thickness was used, as shown in Fig. 4 [19]. One end of a single PVDF-HFP fiber with the microdroplet was inserted into a pin hole and was fixed onto the jig attached to the loadcell. Then the pin hole was positioned beneath the knife edges, and the microvises were controlled to make the PVDF-HFP fiber hang freely. Then the microdroplet tests were performed by moving the motor-driven stage at the speed of 0.25 mm/min. The failure modes of the specimens were classified as matrix failure, interfacial failure, or fiber fracture according to the failure shape monitored with the microscope, and the interfacial shear

Table 2 Specifications of PVDF-HFP fiber and PDMS. PVDF-HFP fiber

PDMS

Product information

Product Manufacturer Remark

Kynar flex2801 Arkema Inc. (USA) Wet-spinning

SYLGARD-184(A) and (B) Dow-corning (USA) Mixing 10:1 Curing 80 ◦ C, 3 h

Properties

Tensile modulus (MPa) Tensile strength (MPa)

240 60

1700 52

380

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Fig. 2. Procedure of surface treatments with (a) epoxy silane, (b) amino silane, and (c) titanate.

strength (IFSS) between the PVDF-HFP fiber and the matrices was calculated using the following equation [11]: d =

Fd , Df X

(1)

where  d , Fd , Df , and X are the interfacial shear strength, maximum load, fiber diameter, and embedded length, respectively. 2.4. Chemical analysis

Fig. 3. Photograph of the micro-droplet specimen.

As shown in Fig. 1, PVDF-HFP and PDMS are formed in stable structures based on F and Si, and have no molecules that can form chemical covalent bonds with the used coupling agents. Thus, chemical coupling between PVDF-HFP and the coupling agents, and between PDMS and the coupling agents, cannot occur. PVDF-HFP, however, include F and O atoms, which are strong electronegativity. Hydrogen bonds which connect atoms of electronegativity higher than hydrogen can be occurred with very low possibility [20,21]. Therefore, to determine the influence of the coupling agents on the chemical structures or bonds of PVDF-HFP fibers, chemical analyses were performed via XPS. The chemical composition of the PVDF-HFP fibers was determined using a Multilab-2000 X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., USA) with an Al K␣ X-ray source (1486.6 eV photons) at 14 kV and 17 mA. The chamber pressure was maintained below 1 × 10−9 mbar during the measurements. The component elements were analyzed based on the binding energies of each peak, using the AVANFAGE 4.73 program, from the measured signal. Only the amino-silane-treated PVDF-HFP fibers were considered, as in the surface morphology analysis. 2.5. Surface morphology analysis

Fig. 4. Experimental setup for the micro-droplet test.

To investigate the effects of the coupling agents on the surfaces of the PVDF-HFP fibers, the surface morphologies of the PVDFHFP fibers before and after the surface treatment were studied

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Fig. 5. (a) Measured maximum load and (b) interfacial shear strength of micro-droplet tests with respect to the embedded length of non-treated PVDF-HFP fibers.

using a scanning electron microscope (SEM) and an atomic force microscope (AFM). SEM was performed to analyze the surface configuration on a JSM-6300 scanning electron microscope (Jeol Ltd., Japan) at 20 kV accelerating voltage. For the measurement of the surface roughness, AFM images were obtained using Nano XpertII (EM4SYS Co., Ltd., South Korea) with a Si3 N4 tip, which had a 0.2 N/m spring constant in the contact mode, with 30 Hz scanning rate and 256 scanning line. Only the amino-silane-treated PVDF-HFP fibers were considered in this study because in the adhesion strength test, amino silane was shown to have led to the highest improvement. 3. Results 3.1. Adhesion strength Fig. 5 shows the maximum load and the interfacial shear strength obtained from the microdroplet tests with respect to the embedded lengths of the non-treated PVDF-HFP fibers. As shown

in Fig. 5(a), matrix failure occurred when the embedded length was shorter than 1030 ␮m. When the embedded length was longer than 1030 ␮m, interfacial debonding between the PVDF-HFP fiber and the microdroplet was observed, and the maximum load increased linearly as the embedded length increased. As the tensile strength and diameter of the PVDF-HFP fiber that was used in this study were 60 MPa and 200 ␮m, respectively, the maximum bearable load of the PVDF-HFP fiber was 1.5 N. Therefore, fiber fracture did not occur within the considered embedded length of the microdroplet. Fig. 5(b) shows the IFSS calculated by measuring the maximum load of the interfacial-debonding cases in Fig. 5(a). The obtained IFSS values were similar regardless of the embedded length. The average IFSS and standard deviation of the non-treated PVDF-HFP fibers and PDMS were 0.313 MPa and 0.027, respectively. According to the results obtained by Ahmadi, the average IFSS between the glass fibers and PDMS was 0.31 MPa [22]. Therefore, the IFSS of the non-treated PVDF-HFP fibers/PDMS droplet seemed reasonable. Fig. 6 shows the microdroplet test results of the surface-treated PVDF-HFP fibers with respect to the kinds and concentrations

Fig. 6. Micro-droplet test results of surface treated PVDF-HFP fibers with respect to the kind and concentration of the coupling agents: (a) epoxy silane, (b) amino silane, and (c) titanate.

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Table 3 Peak assignment of XPS (shown in Fig.7). Binding energy (eV)

Assigned peak

Reference

690.2 533.2 532.2 531.2 399.8 399.5 293.0 291.0 289.0 286.7 286.4 103.43 102.70 102.4

F1s Si O Si Si O Si OH NH2 C N CF3 CF2 CF CH2 C N Si O Si Si OH Si O

[24,33] [29] [26] [26] [23,28] [30,31] [32] [24,32] [33] [25,27] [24] [29] [27] [27]

of the coupling agents that were used. The IFSS values of the PVDF-HFP fibers surface-treated with epoxy silane were within the 0.73–0.84 MPa range and improved by about 180%. A slight difference was observed, however, with respect to the epoxy silane concentration, as shown in Fig. 6(a). When amino silane was used for the surface treatment, the IFSS remarkably increased to 1.05 MPa (a 250% increase compared to the non-treated case) as the amino silane concentration increased from 0 to 4 wt%, but it decreased when the amino silane concentration was 5 wt%, as shown in Fig. 6(b). The PVDF-HFP fibers surface-treated with titanate had an average IFSS of 0.33–0.39 MPa within the considered concentration, as shown in Fig. 6(c), but the surface treatment with titanate had little effect on the adhesion strength considering the measurement errors. Based on the microdroplet test results, it can be said that the surface treatment of the PVDF-HFP fibers with amino silane improved the adhesion strength of the PVDF-HFP fibers and PDMS the most. 3.2. Chemistry XPS was used to further explore the chemical characteristics of the non-treated PVDF-HFP fiber and the PVDF-HFP fiber surface-treated with amino silane. Fig. 10 shows the XPS results of the PVDF-HFP fibers before and after the surface treatment with 0–5 wt% amino silane. XPS peak assignments are presented in Table 3 [23–33]. As shown in the wide-scan survey spectra of the PVDF-HFP fibers (Fig. 7(a)), fluorine (F1s), oxygen (O1s), and carbon (C1s) atoms were observed for the non-treated PVDF-HFP fiber, but nitrogen (N1s) and silicon (Si2p) were added after the surface treatment. It can thus be said that the surface-treated PVDF-HFP fibers had amino silane because the chemical elements of amino silane were detected after the surface treatment. We analyzed peaks of each element by narrow scan survey for investigating the chemical bonding between PVDF-HFP and amino silane. The core level spectra of F1s as shown in Fig. 7(b) depicts that C–F (686.4 eV), CF2 (689.3 eV), and CF2 –CH2 (687.6 eV) bond structures of PVDF-HFP were detected before surface treatment, but there was no changes after surface treatment. Fig. 7(c) shows the core level spectra of O1s. A little O2 (532.8 eV) was observed before surface treatment, but Si O (531.2 eV) of amino silane, Si OH (532.2 eV) came from the silanols by the hydrolysis of the silane, and Si O Si (533.2 eV) came from oligomers by the partial condensation of the silanol were detected. In the case of N1s as shown in Fig. 7(d), no related element was detected before surface treatment, but C N (399.5 eV) and NH2 (399.8 eV) bond structures of amino silane were observed. The core level spectra of C1s as shown in Fig. 7(e) indicates that CH2 (286.7 eV), C F (289.0 eV), CF2 (291.0 eV), and CF3 (293.0 eV) bond structures of PVDF-HFP before surface treatment, but only

Fig. 7. XPS measurement results of PVDF-HFP fibers before and after the surface treatment with 0–5 wt% of amino silane: (a) wide-scan survey spectra, core level spectra of (b) F, (c) O, (d) N, (e) C, and (f) Si.

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Fig. 8. SEM images at ×300 magnification of the surface of PVDF-HFP fibers before and after surface treatment with amino silane.

C N (286.4 eV) bond structure of amino silane was added. Fig. 7(f) represents the core level spectra of Si2p. No element related Si was observed before surface treatment, but Si O (102.1 eV), Si OH (102.4 eV), and Si O Si (103.4 eV) bond structures of amino silane were detected. Since amino silane elements were detected but there was no hydrogen bond structure between the PVDF-HFP fibers and the amino silane after the silane surface treatment, it was concluded that the amino silane did not share its functional group with the PVDF-HFP fibers.

3.3. Surface morphology Fig. 8 shows the SEM images at ×300 magnification of the surfaces of the PVDF-HFP fibers before and after surface treatment with amino silane. The surface of the non-treated PVDF-HFP fiber was relatively smooth and glabrous, but the surface of the aminosilane-treated PVDF-HFP fiber became rougher and had caved-in parts. In particular, some amino silane remained on the PVDF-HFP fiber when the amino silane concentration was 5 wt%, as shown in

Fig. 9. AFM images of the surface of PVDF-HFP fibers before and after surface treatment with amino silane.

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Fig. 8. When the PVDF-HFP fiber was soaked in pure amino silane (100 wt%), the surface of the PVDF-HFP fiber became completely crumpled. For the quantitative analysis of surface roughness, AFM images were obtained, as shown in Fig. 9, and the surface roughness of each AFM image was measured using the Easy AFM program. Similar to the SEM images, the surfaces of the PVDF-HFP fibers became rougher as the amino silane concentration increased, but the PVDFHFP surface-treated with 5 wt% amino silane became smooth again due to the amino silane that remained on the surface. The arithmetical average roughness (Ra ) for the non-treated PVDF-HFP fiber was 215 nm, increasing to 622 nm for the 4 wt% case and then decreasing to 426 nm in the silane coated region but increasing to 755 nm in other region for the 5 wt% case. 4. Discussion It was determined through the microdroplet tests that the adhesion strength of the PVDF-HFP fibers and PDMS increased the most after the surface treatment with amino silane. The possible adhesion mechanisms of PVDF-HFP fibers and PDMS are physical and chemical bonding. The results of the chemical analyses via XPS, FTIR, and XRD showed that the surface treatment with amino silane did not trigger chemical coupling. This means that the improvement of the adhesion strength was not caused by chemical bonding. The SEM and AFM test results showed, however, that the surfaces of the PVDF-HFP fibers became rougher after the surface treatment. In particular, the surface roughness of the PVDF-HFP fibers surfacetreated with amino silane was very similar to the adhesion strength of the PVDF-HFP fibers and PDMS, as shown in Fig. 10. Therefore, it was concluded that the surface treatment with amino silane made the surface of the PVDF-HFP fibers rougher and increased the adhesion strength of the PVDF-HFP fibers and PDMS through physical bonding. For the case of surface treatment with 5 wt% amino silane, the adhesion strength decreased as the not-chemically-bonded but partially remaining amino silane on the fiber surface decreased the surface roughness and produced defects on the interface between the PVDF-HFP fibers and PDMS. There are several reports that the surface roughness affected the adhesion strength, and there was an optimal surface roughness according to stiffness, thickness and width of adherands [34–36]. Therefore, these reports can support our explanation for increasing the adhesion strength by the surface roughness changes due to the surface treatment. Fig. 11 shows the PVDF-HFP fibers with and without surface treatment just after being soaked in the amino silane solution,

Fig. 10. Comparison of the surface roughness calculated from 10 ␮m × 10 ␮m AFM images and IFSS with respect to the amino silane concentration.

amine ammonia water, or ethanol for 30 min. The color of the non-treated PVDF-HFP fiber was bright white, but it became transparent right after being soaked in the amino silane solution. This was caused by the infiltration of the amino silane solution into the PVDF-HFP fiber. The amino silane solution was made with amino silane and distilled water. The inorganic functional group ethoxy of amino silane fell off as the condensation reaction, and became ethanol. The PVDF-HFP fiber soaked in ethanol became transparent, but the color of the PVDF-HFP fiber soaked in amine ammonia water was not changed. Therefore, the infiltration into the PVDFHFP fibers was caused by the inorganic functional group ethoxy of amino silane rather than by the amine group, but the surfaces of the PVDF-HFP fibers surface-treated with ethanol did not change, as shown in Fig. 12. That is, surface treatment with ethanol alone does not affect the surface morphology of PVDF-HFP fibers. Fig. 13 shows why the surfaces of the PVDF-HFP fibers were changed by the surface treatment with amino silane. Amine infiltrated the amorphous layers of the PVDF-HFP fibers with ethanol in the amino silane solution during the soaking process of the surface treatment. As the molecular weight of amine is larger than that of ethanol, ethanol got out of the PVDF-HFP fiber faster than amine did during the drying process. Thus, some amine remained in the amorphous layer of the PVDF-HFP fiber [37]. This was well explained by the FTIR and XPS result: the PVDF-HFP fibers had the chemical elements of amino silane after the surface treatment. Therefore,

Fig. 11. Photographs of the PVDF-HFP fibers with and without the surface treatments just after soaked in the amino silane solution, ethanol, and amine ammonia water.

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Fig. 12. SEM images at ×300 magnification of the surface of PVDF-HFP fibers before and after surface treatment with the ethanol.

Fig. 13. Schematic diagram of the infiltration phenomena by the surface treatment with the amino silane.

Fig. 13 can explain why the surfaces of the PVDF-HFP fibers became rougher after the surface treatment with amino silane. According to our conclusion, main mechanism for the improvement of the adhesion strength between the PVDF-HFP fibers and PDMS is to increase the surface roughness of the PVDF-HFP fibers. This means that any ideas to make the PVDF-HFP fiber surface rougher with inexpensive way can be applied since the coupling agents are generally expensive materials for the industrial purpose.

the chemical composition and bonds of the PVDF-HFP fibers before and after the surface treatment. It can be concluded from the experiment results that the improvement of the adhesion characteristics of the PVDF-HFP fibers and PDMS through the surface treatment was caused by physical bonding. With regard to the supplementary observation of the color changes in the PVDF-HFP fibers after the surface treatment, the changes in the surface colors of the PVDF-HFP fibers surface-treated with amino silane was caused by the infiltration of amino silane.

5. Conclusions Acknowledgement In this study, the effect of surface treatment with coupling agents on the adhesion characteristics of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) fibers and polydimethylsiloxane (PDMS) was investigated. The results of the microdroplet tests showed that the surface treatment with amino silane was most effective, with the adhesion strength remarkably increasing from 0.31 to 1.05 MPa when 4 wt% amino silane was used. The surfaces of the PVDF-HFP fibers became rougher due to amino silane, but there was no difference in

This work was supported by Basic Defense Research Program of “Development of Sensor Fibers for the Smart Skin” from Agency of Defense Development of Korea (ADD-11-01-07-16), in part, Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023918) and BK21Plus Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.

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