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Mechanical properties of polytetrafluoroethylene composites reinforced with graphene nanoplatelets by solid-state processing
Composites Part B 95 (2016) 317e323
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Mechanical properties of polytetrafluoroethylene composites reinforced with graphene nanoplatelets by solid-state processing Jiyeon Suh, Donghyun Bae* Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2016 Accepted 28 March 2016 Available online 4 April 2016
Polytetrafluoroethylene (PTFE)/graphene nanoplatelet (GnP) composites with varying GnP volume fractions are produced by solid-state milling and hot-pressing. This simple approach is able to be used for any polymer powder without any solvents. High-energy milling is performed to disperse GnPs into the PTFE in the solid-state condition. Morphology of PTFE/GnP powder and the fracture surface of PTFE/GnP composites are observed to investigate the GnP dispersion and the deformation behavior in the composites. The tensile test reveals improved mechanical properties of the PTFE/GnP composites. Yield stress of the PTFE/3 vol.% GnP composite increases by 60%, as compared to the neat PTFE. It is attributed to the interference with the movement of the PTFE chains by the randomly dispersed GnPs. A significant reinforcing effect appears on Young's modulus, which shows an increase of 223% at 3 vol.% GnP loading. © 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer-matrix composites (PMCs) B. Mechanical properties D. Electron microscopy E. Powder processing
1. Introduction Polymer composites have attracted interest for structural and functional applications owing to their low density, low cost, and easy formability. Polytetrafluoroethylene (PTFE) can be a suitable matrix for the multifunctional polymer composites, which shows low friction coefficient, high thermal stability, and high chemical resistance. PTFE and its composites are widely used in automotive and aerospace industries. However, due to poor mechanical properties, applications of PTFE are mostly limited to lubricants and seals using the low friction and chemical inertness [1,2]. It causes that the mechanical properties of PTFE composites have been barely studied so far. To enhance the mechanical properties of polymer composites, carbon nanomaterials not only of fiber type, but also of sheet type have been employed recently. In case of graphene, such enhancement is attributed to the high aspect ratio and extraordinary mechanical properties of monolayer graphene, e.g., tensile strength of 130 GPa and Young's modulus of 1 TPa [3]. For several decades, polymer composites have been synthesized by typical methods to incorporate reinforcements into the polymer matrix. The most common synthesis routes are categorized into solution mixing, melt mixing, and in-situ polymerization [4,5]. Recently, diverse approaches, which are modified from common
* Corresponding author. Tel.: þ82 2 2123 5831; fax: þ82 2 312 5375. E-mail address: [email protected] (D. Bae). http://dx.doi.org/10.1016/j.compositesb.2016.03.082 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
techniques or newly developed, have been used to disperse reinforcements and enhance the mechanical properties of polymer composites [6e9]. Tan et al. reported that Poly(butylene succinate) (PBS)/single-walled carbon nanotubes (SWNTs) composites were prepared by a new technique including hydrolysis, and the tensile strength and Young's modulus of the composite improved by 14% and 56%, respectively [7]. In this study, we produce polytetrafluoroethylene (PTFE) composites incorporating graphene nanoplatelets (GnPs) using solidstate milling and hot-pressing as a candidate of various approaches for the fabrication of polymer composites. PTFE powders and GnPs are mechanically milled for the dispersion of GnPs, and the obtained PTFE/GnP powders are hot-pressed. The effect of GnPs on mechanical properties and the microstructures of PTFE/GnP composites is investigated. 2. Experimental PTFE/GnP composites were produced by mechanical milling of PTFE powder and GnP powder followed by hot-pressing. GnPs with grade M, which have a particle diameter of 5 mm and a thickness of 8 nm, were purchased from XG science, Inc., Michigan, USA. PTFE powder with an average particle size of 250 mm was supplied by BOGO Chem. Co., Korea. PTFE powders and GnPs were prepared with varied GnP volume fractions of 1 vol.%, 2 vol.%, and 3 vol.%. The attrition milling was performed at 500 rpm for 12 h under ambient condition. Stainless steel balls, with a diameter and weight of 5 mm
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and 0.5 g, respectively, were used as milling media. The weight ratio between the milling media and the powders was 15:1. The temperature in a milling chamber was maintained at room temperature using a water-circulating system within the outside wall of the chamber. PTFE/GnP composite powders were compacted in a mold with 30 mm diameter and then hot-pressed under 10 MPa for 30 min at 400 C to fabricate the final PTFE/GnP composites. The schematic of the fabrication process is displayed in Fig. 1. Hot-press procedure was carried out at a temperature higher than the melting temperature of the PTFE in order to acquire the fluidity of a polymer matrix and to fully consolidate the composite. Morphology of PTFE/GnP powders and the fractured surface of PTFE/GnP composites were observed by scanning electron microscopy (SEM; JEOL-7001F, JEOL). To minimize damage on the specimen, low accelerating voltages of 1e5 kV were applied. The powders and the composites were coated with a thin platinum layer. To observe the composite surfaces, the composites are finepolished using SiC papers and buffer cloths. The structure of the PTFE/GnP composites was analyzed using X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and Raman spectroscopy. XRD patterns of neat PTFE, GnP, and the composites were obtained using a Rigaku CN 2301 diffractometer with Cu Ka source at 40 kV and 30 mA. ATR-FTIR spectroscopy was performed by Vertex 70 (Bruker). Raman spectroscopy (LabRam Aramis, Horriba Jobin Yvon) was carried out using an excitation wavelength of 532 nm. Mechanical properties of the composites were evaluated by a tensile test using an Instron-type machine with a constant crosshead speed of an initial strain rate of 102 s1 at room temperature. Dog-bone shaped tensile test specimens with a gauge length of 5.75 mm were prepared. Young's modulus values were obtained from an ultrasonic measuring system. 3. Results and discussion Fig. 2 shows the powder morphology of PTFE and GnP as starting materials and the ball-milled PTFE/GnP powders. The initial PTFE powder has a spherical shape and smooth surface (Fig. 2(a)). The powder size is widely distributed from 100 mm to 700 mm. Smaller PTFE powders adhere to larger PTFE powders or tangle each other. Many PTFE fibrils are observed which are connected to each other to form a network on the powder surface. In Fig. 2(b), pristine GnPs are severely agglomerated around 40 mm in size. Pristine GnPs are used as it is without any pre-treatment for reducing the layers of GnPs. Starting powders of PTFE and GnP are mechanically milled using an attrition mill at 500 rpm for 12 h to disperse GnPs into the PTFE. Ball-milling is performed under a solid-state condition with the PTFE powders and the GnPs, maintaining the room temperature. Vacuum condition is employed to prevent the adhesion of oxygen. After the high-energy milling, the
spherical PTFE powder and the GnP cannot maintain their initial shapes, as shown in Fig. 2(c)e(e). The PTFE powder easily deforms by collisions with powders and balls in the milling chamber, owing to the high strengths of GnP and the stainless steel ball. Also, striations appear on the surface of the deformed PTFE powder due to the shear slip arisen by the high speed rotation of the powders in the milling chamber. Morphological changes in the PTFE powders proceed similarly to the deformation of soft metal powders during milling. When metal powders are mechanically milled, they are repeatedly deformed, crushed, and cold-welded [10]. Deformation of the PTFE powder follows a procedure in which the powder is flattened, torn, and stacked to other powders. During the milling process, the GnPs are embedded and strongly attached on the PTFE powder surface. The GNPs on the PTFE powder are marked by white arrows in Fig. 2(f)e(h). Dispersion of GnPs occurs during the milling process where the GnPs are attached to the powder surface and the GNP-attached powder repeatedly undergoes the process of flattened deformation, tearing, and stacking to other GnP-attached powders. The GnPs are barely agglomerated and partially exfoliated. With increasing GnP volume fraction, the number of GnPs exhibited on the PTFE powder surface increases, indicating that the GnPs are well dispersed on the powder surface. The PTFE/GnP composite powders are produced into the bulk specimen of 30 mm in diameter by hot-pressing. The fine-polished surface of PTFE/3 vol.% GnP composite is shown in Fig. 3. The PTFE/ GnP powder shape and its boundary cannot be seen on the surface (Fig. 3(a)). The surface of the composite is very smooth, and pores are rarely observed. It confirms that the PTFE/3 vol.% GnP composite is fully consolidated. In Fig. 3(b), GnPs are exhibited on the surface (marked by the white arrows) as dispersed and embedded in the PTFE matrix. Plane and edge of the GnPs are shown on the surface of the composite. The GnPs are dispersed in the PTFE without any particular orientation since they are ball-milled with the PTFE powders, and the milled powders are randomly piled up in the mold for hot-pressing. XRD patterns of the starting materials and the PTFE/GnP composites with varying GnP volume fractions are given in Fig. 4. The neat PTFE shows strong intensity at 2q values of 18.12 , 31.64 , and 36.68 . These sharp peaks imply that the neat PTFE has a high crystallinity with closely packed chains, even after high-energy processing of ball-milling and hot-pressing. These peaks also maintain their sharpness in the XRD patterns of the PTFE/GnP composite. A small difference between the XRD patterns of the neat PTFE and the PTFE/GNP composites is observed in the shift of the main peaks of PTFE. Those three peaks are regularly changed to higher degrees by 0.08 , 0.1, and 0.12 for each PTFE/GnP composite with increasing GnP volume fraction. The peak shifts explain that the GnPs are dispersed with they embedded and entangled by fibrils of the PTFE powder during the milling. The GnPs are fixed in the PTFE domain after the hot-pressing, and the dispersed GnPs
Fig. 1. Fabrication process for PTFE/GnP composites.
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319
Fig. 2. SEM images of (a) PTFE powders, (b) GnPs, and ball-milled PTFE/GnP powders containing (c) 1 vol.% GnP, (d) 2 vol.% GnP, and (e) 3 vol.% GnP. (f)e(h) Display magnified images of (c)e(e), respectively.
induce the expansion of the domain composed of PTFE chains. The inset displays the magnified XRD patterns in the range of 23 e30 . The pristine GnPs clearly show a strong peak at 2q ¼ 26.5 , which is attributed to the (002) plane of GnPs. The d-spacing calculated from the peak at 2q ¼ 26.5 is 3.36 Å, which corresponds to the basal spacing of GnPs. For the PTFE/GnP composites, the peak appeared at 2q ¼ 26.6 , which corresponds to a d-spacing of 3.35 Å. The peak of GnPs in the composites is weakened and broadened compared to that of pristine GnPs. Crystallinity of the GnPs is decreased because the GnPs are partially exfoliated and folded during the milling process on collision with the PTFE powders and the balls. Characteristic peaks for the neat PTFE and the PTFE/GnP composites are detected in the ATR-FTIR spectra (Fig. 5). IR bands of the
neat PTFE are detected in the range from 600 to 3000 cm1. Conspicuous bands showing the maximum intensity appear at 1145 and 1200 cm1, which is assigned to CF2 symmetric stretching. Three bands from 721 to 777 cm1 are derived from CF2 scissoring vibration mode. CF deformation mode is detected in the range of 630e640 cm1. The bands between 630 and 640 cm1 are closely associated with the crystallinity of the PTFE [11]. These bands are directly related to fluorocarbons in the formula of PTFE. However, IR bands around 2900 cm1 belong to CH2 and CH3 stretching vibration modes. The PTFE/GnP composites exhibit similar ATR-FTIR spectra as compared to the neat PTFE. The peaks around 1470, 1600 and 2350 cm1 are newly detected in the composites, whose intensities increase with increasing GnP volume fraction. These bands are attributed to the in-plane CeH deformation, skeletal
Fig. 3. SEM images of (a) the surface of PTFE/3 vol.% GnP composite after hot-pressing and (b) the magnified image of (a).
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Fig. 4. XRD patterns of neat PTFE, GnPs, and PTFE/GnP composites.
Fig. 6. Raman spectra of neat PTFE, GnPs, and PTFE/GnP composites.
vibration of graphitic domain, and CH stretching vibration in aromatic sp2 carbon system, respectively [12]. There are not found any other functional groups in the spectra of the PTFE/GnP composites. To more investigate the specific structure of the GnP in the PTFE/ GnP composites, Raman spectroscopy is carried out (Fig. 6). Raman spectra for the neat PTFE, the GnPs, and the PTFE/GnP composites are obtained using an excitation wavelength of 532 nm at room temperature. For the neat PTFE, bands at 1216, 1299, and 1380 cm1 are clearly observed in 1000e3500 cm1 range. These lines are typical in Raman spectrum of the PTFE, which represent CF2 asymmetric stretching, CF2 wagging, and CeC stretching [13]. For the PTFE/GnP composites, the peak at 1380 cm1 weakly appears which overlaps with a peak of the pristine GnPs. The pristine GnPs exhibit strong bands at 1352, 1583, and 2700 cm1 corresponding to D band, G band, and 2D band, respectively. D band is called a disorder-induced band, which represents a breathing mode of sp2 carbon rings, and G band is a doubly degenerated E2g phonon mode, which appears due to in-plane CeC bond stretching vibration in sp2 hybridized carbon [14]. The 2D band at 2700 cm1 is the second order band of the D band. For the pristine GnPs, the intensity of the D band is weak and the intensity ratio of D band to G
band (ID/IG) is calculated to be 0.23. Meanwhile, an asymmetric shape of the 2D band indicates that the pristine GnPs consists of multilayered graphenes. The ID/IG is increased to 0.25, 0.52, and 0.57 for the composites containing 1 vol.%, 2 vol.%, and 3 vol.% GnPs, respectively as damage on the GnPs are slightly increases during the milling process. Whereas, the 2D band becomes more symmetric as the center of the band shifts to lower wave number with increasing GnP volume fraction. It indicates that the GnPs are partially exfoliated and damaged at the same time, upon being subjected to repeated a collision and sliding with balls and powders during the mechanical milling. The mechanical properties of the neat PTFE and the PTFE/GnP composites are evaluated by tensile stress and Young's modulus. The values of the yield stress, elongation at break, and Young's modulus are listed in Table 1. The stressestrain curves are displayed in Fig. 7(a) to examine the effects of the GnPs. It is clear that the addition of the GnPs leads to a great enhancement on the mechanical property of the PTFE/GnP composites. The yield stresses of the PTFE/GnP composites are improved compared to the neat PTFE. Furthermore, yield stress and Young's modulus of the composites gradually increase with increasing GnP volume fraction. Yield stress of the PTFE/3 vol.% GnP composite increases by 60%; i.e., the yield stress of the neat PTFE and the composite with 3 vol.% GnP content are 13.4 MPa and 21.5 MPa, respectively. It is assumed that the dispersed GnPs restrict the reorientation of the PTFE chains, causing an increase in the yield stress of the composites. In spite of the high reinforcing effect of the GnPs, the increasing tendency is attenuated with increasing GnP loading. When the GnP content increases from 2 vol.% to 3 vol.%, the yield stress is only slightly changed from 19.8 MPa to 21.5 MPa. Increased GnP loading results in partial accumulation and restacking and causes the slippage between the GnPs during the tensile test. It also affects the deformation behavior of the composites that the strain hardening and the elongation at break gradually decreases with an increase in the
Table 1 Mechanical properties of neat PTFE and PTFE/GnP composites.
Fig. 5. ATR-FTIR spectra of neat PTFE and PTFE/GnP composites.
Specimen
sy (MPa)
sUTS (MPa)
3b (%)
E (GPa)
Neat PTFE PTFE/1 vol.% GnP PTFE/2 vol.% GnP PTFE/3 vol.% GnP
13.4 18.3 19.8 21.5
19.6 27.2 20.0 21.5
250 300 208 96
0.81 1.81 2.36 2.62
± ± ± ±
0.01 0.03 0.04 0.02
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321
Fig. 7. (a) Stressestrain curves of PTFE/GnP composites and Young's moduli of (b) PTFE/GnP composites, (c) PTFE/GnP composites and HalpineTsai predictions, and (d) PTFE/GnP composites and polymer composites in other literatures [16e20].
GnP loading. The deformation of the polymer composites mainly depends upon the polymer matrix, such as the alignment of polymer chains during the tensile test. For the PTFE/GnP composites, the strain hardening and the elongation are weakened since the GnPs interfere with the movement of the PTFE chains and reduce the PTFE network. The Young's moduli of the PTFE/GnP composites are exhibited in Fig. 7(b). The Young's modulus increases with a similar trend as that of the yield stress. On the addition of 3 vol.% GnPs, Young's modulus increases by 223% as compared to the neat PTFE from 0.81 GPa to 2.62 GPa. It is caused by the high aspect ratio of GnPs and the interaction between the PTFE and the GnPs. The Young's moduli of the composites could be explained by the HalpineTsai equation. The HalpineTsai equation is modified for applying graphene as follows [15]:
Ec ¼ EP
3 1 þ hL xVG 5 1 þ 2hT VG þ 8 1 hL VG 8 1 hT VG
(1)
Ek ¼ EP
1 þ hL xVG 1 hL VG
(2)
hL ¼
EG Ep EG Ep
1 þx
(3)
EG Ep
1
EG Ep
þ2
hT ¼
x¼
2lG 3tG
(4)
(5)
where Ec and Ek stand for the Young's moduli of the composites containing randomly and unidirectionally oriented GnPs, respectively. EP and EG are the Young's moduli of the neat PTFE and the GnPs, respectively. The Young's modulus of the GnPs is replaced by that of the monolayer graphene, which is ~1 TPa [3]. VG is the GnP volume fraction in the PTFE composite. lG and tG represent the length and thickness of the GnP, respectively. The values of 5 mm and 8 nm are considered for the GnP as provided from the suppliers (XG science) for the values of lG and tG, respectively as the GnPs are used without any pre-treatment in this study. Fig. 7(c) shows the predicted and experimentally obtained Young's moduli of the PTFE/ GnP composites. The experimental values are slightly lower than the HalpineTsai model that considers randomly oriented GnPs. It is caused by the intrinsic property and the dispersion of the GnPs. Young's modulus of the GnPs would be lower than that of the monolayer graphene because the used GnPs are not exfoliated to monolayer graphene. Besides, HalpineTsai equation is based on a perfect adhesion between the matrix and the reinforcement. As described above, the GnPs slip each other when the GnPs with
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Fig. 8. Fractured surfaces of (a) neat PTFE and (b) PTFE/3 vol.% GnP. (c) and (d) Display magnified images of (a) and (b), respectively.
multilayers are dispersed in the matrix, interrupting the perfect adhesion with the PTFE. Nonetheless, the experimental values show a similar trend to the expected values, and it is concluded that the GnPs are randomly distributed in the composites. In Fig. 7(d), Young's moduli of composites with various polymer matrices and graphene structures are plotted as a function of graphene volume fraction [16e20]. Graphene oxide (GO) and functionalized graphene are commonly used to ameliorate the mechanical properties of the neat polymer. GO and functionalized graphene have advantages in the load transfer due to functional groups that can have covalent, non-covalent, and hydrogen bonding with the polymer matrix [21]. Besides, for semi-crystalline polymers, enhancement on the mechanical properties by graphene is facilitated as graphene act as a nucleating source to generate the crystalline interfaces in the semi-crystalline polymer [22]. Due to the reasons explained above, even though a direct comparison for the reinforcing efficiency is complicated, the addition of GnPs clearly shows significant enhancement in the Young's modulus of the PTFE/GnP composites, compared to other polymer composites reinforced with graphene. To scrutinize results of the tensile test, fractured surfaces of the neat PTFE and the PTFE/3 vol.% GnP composite after the tensile test are observed, as shown in Fig. 8. For the typical fracture surface of the PTFE, brittle fracture and ductile fracture coexist. The tensile fracture of the neat PTFE is initiated from the upper left corner of the specimen in Fig. 8(a). The initiation region is largely flat and shows broad hackle marks [23]. At a mirror zone on the corner, the crack slowly initiates, and the crack propagation abruptly accelerates from the boundary of the mirror zone, remaining the hackle region. The hackle region is produced by the shear yielding as the crack velocity rapidly increases [22]. The ductile fracture region commences around the mirror zone. In the ductile region, flat surface by the shear yielding partially appears among dimples (Fig. 8(c)). The region is covered with dimples of hundreds of nanometers in sizes. A few voids are present with submicron sizes. The PTFE/3 vol.% GnP composite shows a completely different fracture morphology. According to the overview of the fractured surfaces in Fig. 8(a) and (b), the fracture surface of the PTFE is
relatively smooth without any PTFE fibrils. In contrast, the fracture surface of the PTFE/3 vol.% GnP composite is rough and sharpened. The fracture morphology also reveals the ductile failure throughout the PTFE/3 vol.% GnP composite. The PTFE fibrils encompass the fracture surface of the PTFE/3 vol.% GnP composite, without any particular direction. The fibrils are tangled with each other, and the entanglement is shown as a fibril network. The GnPs do not exhibit strong chemical adhesion with the PTFE, but the GnPs are strongly entwined by the PTFE fibrils, as seen from Fig. 8(d). It alludes that the GnPs mechanically impede the movement of the PTFE chains that are elongated during the tensile test. It leads to a drastic decrease in the strain hardening because the strain hardening occurs when the polymer chain is reoriented along the tensile direction. Meanwhile, the behavior is attributed to the improvement in the mechanical properties of the composites with increasing GnP loading. 4. Conclusions In this study, we attempt to use the solid-state processing including mechanical milling and hot-pressing for the fabrication of PTFE/GnP composites. The high-energy ball milling results in random dispersion of the GnPs, with attached on and inserted in the PTFE powder, by the powder deformation and tearing without making use of any solvents. The mechanical properties of the hotpressed PTFE/GnP composites are enhanced compared to the neat PTFE. The yield stress of the composites is gradually improved with increasing GnP volume fraction, showing the increase of 60% at 3 vol.% GnP loading. In addition, a significant improvement of 223% is accomplished on Young's modulus for the PTFE/3 vol.% GnP composite. Further work requires to be continued in order to study key issue such as the strong adhesion in polymer composites. Acknowledgments This work is supported by Brain Korea 21 (BK 21) Program.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Mechanical properties of polytetrafluoroethylene composites reinforced with graphene nanoplatelets by solid-state processing Jiyeon Suh, Donghyun Bae* Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2016 Accepted 28 March 2016 Available online 4 April 2016
Polytetrafluoroethylene (PTFE)/graphene nanoplatelet (GnP) composites with varying GnP volume fractions are produced by solid-state milling and hot-pressing. This simple approach is able to be used for any polymer powder without any solvents. High-energy milling is performed to disperse GnPs into the PTFE in the solid-state condition. Morphology of PTFE/GnP powder and the fracture surface of PTFE/GnP composites are observed to investigate the GnP dispersion and the deformation behavior in the composites. The tensile test reveals improved mechanical properties of the PTFE/GnP composites. Yield stress of the PTFE/3 vol.% GnP composite increases by 60%, as compared to the neat PTFE. It is attributed to the interference with the movement of the PTFE chains by the randomly dispersed GnPs. A significant reinforcing effect appears on Young's modulus, which shows an increase of 223% at 3 vol.% GnP loading. © 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer-matrix composites (PMCs) B. Mechanical properties D. Electron microscopy E. Powder processing
1. Introduction Polymer composites have attracted interest for structural and functional applications owing to their low density, low cost, and easy formability. Polytetrafluoroethylene (PTFE) can be a suitable matrix for the multifunctional polymer composites, which shows low friction coefficient, high thermal stability, and high chemical resistance. PTFE and its composites are widely used in automotive and aerospace industries. However, due to poor mechanical properties, applications of PTFE are mostly limited to lubricants and seals using the low friction and chemical inertness [1,2]. It causes that the mechanical properties of PTFE composites have been barely studied so far. To enhance the mechanical properties of polymer composites, carbon nanomaterials not only of fiber type, but also of sheet type have been employed recently. In case of graphene, such enhancement is attributed to the high aspect ratio and extraordinary mechanical properties of monolayer graphene, e.g., tensile strength of 130 GPa and Young's modulus of 1 TPa [3]. For several decades, polymer composites have been synthesized by typical methods to incorporate reinforcements into the polymer matrix. The most common synthesis routes are categorized into solution mixing, melt mixing, and in-situ polymerization [4,5]. Recently, diverse approaches, which are modified from common
* Corresponding author. Tel.: þ82 2 2123 5831; fax: þ82 2 312 5375. E-mail address: [email protected] (D. Bae). http://dx.doi.org/10.1016/j.compositesb.2016.03.082 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
techniques or newly developed, have been used to disperse reinforcements and enhance the mechanical properties of polymer composites [6e9]. Tan et al. reported that Poly(butylene succinate) (PBS)/single-walled carbon nanotubes (SWNTs) composites were prepared by a new technique including hydrolysis, and the tensile strength and Young's modulus of the composite improved by 14% and 56%, respectively [7]. In this study, we produce polytetrafluoroethylene (PTFE) composites incorporating graphene nanoplatelets (GnPs) using solidstate milling and hot-pressing as a candidate of various approaches for the fabrication of polymer composites. PTFE powders and GnPs are mechanically milled for the dispersion of GnPs, and the obtained PTFE/GnP powders are hot-pressed. The effect of GnPs on mechanical properties and the microstructures of PTFE/GnP composites is investigated. 2. Experimental PTFE/GnP composites were produced by mechanical milling of PTFE powder and GnP powder followed by hot-pressing. GnPs with grade M, which have a particle diameter of 5 mm and a thickness of 8 nm, were purchased from XG science, Inc., Michigan, USA. PTFE powder with an average particle size of 250 mm was supplied by BOGO Chem. Co., Korea. PTFE powders and GnPs were prepared with varied GnP volume fractions of 1 vol.%, 2 vol.%, and 3 vol.%. The attrition milling was performed at 500 rpm for 12 h under ambient condition. Stainless steel balls, with a diameter and weight of 5 mm
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and 0.5 g, respectively, were used as milling media. The weight ratio between the milling media and the powders was 15:1. The temperature in a milling chamber was maintained at room temperature using a water-circulating system within the outside wall of the chamber. PTFE/GnP composite powders were compacted in a mold with 30 mm diameter and then hot-pressed under 10 MPa for 30 min at 400 C to fabricate the final PTFE/GnP composites. The schematic of the fabrication process is displayed in Fig. 1. Hot-press procedure was carried out at a temperature higher than the melting temperature of the PTFE in order to acquire the fluidity of a polymer matrix and to fully consolidate the composite. Morphology of PTFE/GnP powders and the fractured surface of PTFE/GnP composites were observed by scanning electron microscopy (SEM; JEOL-7001F, JEOL). To minimize damage on the specimen, low accelerating voltages of 1e5 kV were applied. The powders and the composites were coated with a thin platinum layer. To observe the composite surfaces, the composites are finepolished using SiC papers and buffer cloths. The structure of the PTFE/GnP composites was analyzed using X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and Raman spectroscopy. XRD patterns of neat PTFE, GnP, and the composites were obtained using a Rigaku CN 2301 diffractometer with Cu Ka source at 40 kV and 30 mA. ATR-FTIR spectroscopy was performed by Vertex 70 (Bruker). Raman spectroscopy (LabRam Aramis, Horriba Jobin Yvon) was carried out using an excitation wavelength of 532 nm. Mechanical properties of the composites were evaluated by a tensile test using an Instron-type machine with a constant crosshead speed of an initial strain rate of 102 s1 at room temperature. Dog-bone shaped tensile test specimens with a gauge length of 5.75 mm were prepared. Young's modulus values were obtained from an ultrasonic measuring system. 3. Results and discussion Fig. 2 shows the powder morphology of PTFE and GnP as starting materials and the ball-milled PTFE/GnP powders. The initial PTFE powder has a spherical shape and smooth surface (Fig. 2(a)). The powder size is widely distributed from 100 mm to 700 mm. Smaller PTFE powders adhere to larger PTFE powders or tangle each other. Many PTFE fibrils are observed which are connected to each other to form a network on the powder surface. In Fig. 2(b), pristine GnPs are severely agglomerated around 40 mm in size. Pristine GnPs are used as it is without any pre-treatment for reducing the layers of GnPs. Starting powders of PTFE and GnP are mechanically milled using an attrition mill at 500 rpm for 12 h to disperse GnPs into the PTFE. Ball-milling is performed under a solid-state condition with the PTFE powders and the GnPs, maintaining the room temperature. Vacuum condition is employed to prevent the adhesion of oxygen. After the high-energy milling, the
spherical PTFE powder and the GnP cannot maintain their initial shapes, as shown in Fig. 2(c)e(e). The PTFE powder easily deforms by collisions with powders and balls in the milling chamber, owing to the high strengths of GnP and the stainless steel ball. Also, striations appear on the surface of the deformed PTFE powder due to the shear slip arisen by the high speed rotation of the powders in the milling chamber. Morphological changes in the PTFE powders proceed similarly to the deformation of soft metal powders during milling. When metal powders are mechanically milled, they are repeatedly deformed, crushed, and cold-welded [10]. Deformation of the PTFE powder follows a procedure in which the powder is flattened, torn, and stacked to other powders. During the milling process, the GnPs are embedded and strongly attached on the PTFE powder surface. The GNPs on the PTFE powder are marked by white arrows in Fig. 2(f)e(h). Dispersion of GnPs occurs during the milling process where the GnPs are attached to the powder surface and the GNP-attached powder repeatedly undergoes the process of flattened deformation, tearing, and stacking to other GnP-attached powders. The GnPs are barely agglomerated and partially exfoliated. With increasing GnP volume fraction, the number of GnPs exhibited on the PTFE powder surface increases, indicating that the GnPs are well dispersed on the powder surface. The PTFE/GnP composite powders are produced into the bulk specimen of 30 mm in diameter by hot-pressing. The fine-polished surface of PTFE/3 vol.% GnP composite is shown in Fig. 3. The PTFE/ GnP powder shape and its boundary cannot be seen on the surface (Fig. 3(a)). The surface of the composite is very smooth, and pores are rarely observed. It confirms that the PTFE/3 vol.% GnP composite is fully consolidated. In Fig. 3(b), GnPs are exhibited on the surface (marked by the white arrows) as dispersed and embedded in the PTFE matrix. Plane and edge of the GnPs are shown on the surface of the composite. The GnPs are dispersed in the PTFE without any particular orientation since they are ball-milled with the PTFE powders, and the milled powders are randomly piled up in the mold for hot-pressing. XRD patterns of the starting materials and the PTFE/GnP composites with varying GnP volume fractions are given in Fig. 4. The neat PTFE shows strong intensity at 2q values of 18.12 , 31.64 , and 36.68 . These sharp peaks imply that the neat PTFE has a high crystallinity with closely packed chains, even after high-energy processing of ball-milling and hot-pressing. These peaks also maintain their sharpness in the XRD patterns of the PTFE/GnP composite. A small difference between the XRD patterns of the neat PTFE and the PTFE/GNP composites is observed in the shift of the main peaks of PTFE. Those three peaks are regularly changed to higher degrees by 0.08 , 0.1, and 0.12 for each PTFE/GnP composite with increasing GnP volume fraction. The peak shifts explain that the GnPs are dispersed with they embedded and entangled by fibrils of the PTFE powder during the milling. The GnPs are fixed in the PTFE domain after the hot-pressing, and the dispersed GnPs
Fig. 1. Fabrication process for PTFE/GnP composites.
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Fig. 2. SEM images of (a) PTFE powders, (b) GnPs, and ball-milled PTFE/GnP powders containing (c) 1 vol.% GnP, (d) 2 vol.% GnP, and (e) 3 vol.% GnP. (f)e(h) Display magnified images of (c)e(e), respectively.
induce the expansion of the domain composed of PTFE chains. The inset displays the magnified XRD patterns in the range of 23 e30 . The pristine GnPs clearly show a strong peak at 2q ¼ 26.5 , which is attributed to the (002) plane of GnPs. The d-spacing calculated from the peak at 2q ¼ 26.5 is 3.36 Å, which corresponds to the basal spacing of GnPs. For the PTFE/GnP composites, the peak appeared at 2q ¼ 26.6 , which corresponds to a d-spacing of 3.35 Å. The peak of GnPs in the composites is weakened and broadened compared to that of pristine GnPs. Crystallinity of the GnPs is decreased because the GnPs are partially exfoliated and folded during the milling process on collision with the PTFE powders and the balls. Characteristic peaks for the neat PTFE and the PTFE/GnP composites are detected in the ATR-FTIR spectra (Fig. 5). IR bands of the
neat PTFE are detected in the range from 600 to 3000 cm1. Conspicuous bands showing the maximum intensity appear at 1145 and 1200 cm1, which is assigned to CF2 symmetric stretching. Three bands from 721 to 777 cm1 are derived from CF2 scissoring vibration mode. CF deformation mode is detected in the range of 630e640 cm1. The bands between 630 and 640 cm1 are closely associated with the crystallinity of the PTFE [11]. These bands are directly related to fluorocarbons in the formula of PTFE. However, IR bands around 2900 cm1 belong to CH2 and CH3 stretching vibration modes. The PTFE/GnP composites exhibit similar ATR-FTIR spectra as compared to the neat PTFE. The peaks around 1470, 1600 and 2350 cm1 are newly detected in the composites, whose intensities increase with increasing GnP volume fraction. These bands are attributed to the in-plane CeH deformation, skeletal
Fig. 3. SEM images of (a) the surface of PTFE/3 vol.% GnP composite after hot-pressing and (b) the magnified image of (a).
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Fig. 4. XRD patterns of neat PTFE, GnPs, and PTFE/GnP composites.
Fig. 6. Raman spectra of neat PTFE, GnPs, and PTFE/GnP composites.
vibration of graphitic domain, and CH stretching vibration in aromatic sp2 carbon system, respectively [12]. There are not found any other functional groups in the spectra of the PTFE/GnP composites. To more investigate the specific structure of the GnP in the PTFE/ GnP composites, Raman spectroscopy is carried out (Fig. 6). Raman spectra for the neat PTFE, the GnPs, and the PTFE/GnP composites are obtained using an excitation wavelength of 532 nm at room temperature. For the neat PTFE, bands at 1216, 1299, and 1380 cm1 are clearly observed in 1000e3500 cm1 range. These lines are typical in Raman spectrum of the PTFE, which represent CF2 asymmetric stretching, CF2 wagging, and CeC stretching [13]. For the PTFE/GnP composites, the peak at 1380 cm1 weakly appears which overlaps with a peak of the pristine GnPs. The pristine GnPs exhibit strong bands at 1352, 1583, and 2700 cm1 corresponding to D band, G band, and 2D band, respectively. D band is called a disorder-induced band, which represents a breathing mode of sp2 carbon rings, and G band is a doubly degenerated E2g phonon mode, which appears due to in-plane CeC bond stretching vibration in sp2 hybridized carbon [14]. The 2D band at 2700 cm1 is the second order band of the D band. For the pristine GnPs, the intensity of the D band is weak and the intensity ratio of D band to G
band (ID/IG) is calculated to be 0.23. Meanwhile, an asymmetric shape of the 2D band indicates that the pristine GnPs consists of multilayered graphenes. The ID/IG is increased to 0.25, 0.52, and 0.57 for the composites containing 1 vol.%, 2 vol.%, and 3 vol.% GnPs, respectively as damage on the GnPs are slightly increases during the milling process. Whereas, the 2D band becomes more symmetric as the center of the band shifts to lower wave number with increasing GnP volume fraction. It indicates that the GnPs are partially exfoliated and damaged at the same time, upon being subjected to repeated a collision and sliding with balls and powders during the mechanical milling. The mechanical properties of the neat PTFE and the PTFE/GnP composites are evaluated by tensile stress and Young's modulus. The values of the yield stress, elongation at break, and Young's modulus are listed in Table 1. The stressestrain curves are displayed in Fig. 7(a) to examine the effects of the GnPs. It is clear that the addition of the GnPs leads to a great enhancement on the mechanical property of the PTFE/GnP composites. The yield stresses of the PTFE/GnP composites are improved compared to the neat PTFE. Furthermore, yield stress and Young's modulus of the composites gradually increase with increasing GnP volume fraction. Yield stress of the PTFE/3 vol.% GnP composite increases by 60%; i.e., the yield stress of the neat PTFE and the composite with 3 vol.% GnP content are 13.4 MPa and 21.5 MPa, respectively. It is assumed that the dispersed GnPs restrict the reorientation of the PTFE chains, causing an increase in the yield stress of the composites. In spite of the high reinforcing effect of the GnPs, the increasing tendency is attenuated with increasing GnP loading. When the GnP content increases from 2 vol.% to 3 vol.%, the yield stress is only slightly changed from 19.8 MPa to 21.5 MPa. Increased GnP loading results in partial accumulation and restacking and causes the slippage between the GnPs during the tensile test. It also affects the deformation behavior of the composites that the strain hardening and the elongation at break gradually decreases with an increase in the
Table 1 Mechanical properties of neat PTFE and PTFE/GnP composites.
Fig. 5. ATR-FTIR spectra of neat PTFE and PTFE/GnP composites.
Specimen
sy (MPa)
sUTS (MPa)
3b (%)
E (GPa)
Neat PTFE PTFE/1 vol.% GnP PTFE/2 vol.% GnP PTFE/3 vol.% GnP
13.4 18.3 19.8 21.5
19.6 27.2 20.0 21.5
250 300 208 96
0.81 1.81 2.36 2.62
± ± ± ±
0.01 0.03 0.04 0.02
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Fig. 7. (a) Stressestrain curves of PTFE/GnP composites and Young's moduli of (b) PTFE/GnP composites, (c) PTFE/GnP composites and HalpineTsai predictions, and (d) PTFE/GnP composites and polymer composites in other literatures [16e20].
GnP loading. The deformation of the polymer composites mainly depends upon the polymer matrix, such as the alignment of polymer chains during the tensile test. For the PTFE/GnP composites, the strain hardening and the elongation are weakened since the GnPs interfere with the movement of the PTFE chains and reduce the PTFE network. The Young's moduli of the PTFE/GnP composites are exhibited in Fig. 7(b). The Young's modulus increases with a similar trend as that of the yield stress. On the addition of 3 vol.% GnPs, Young's modulus increases by 223% as compared to the neat PTFE from 0.81 GPa to 2.62 GPa. It is caused by the high aspect ratio of GnPs and the interaction between the PTFE and the GnPs. The Young's moduli of the composites could be explained by the HalpineTsai equation. The HalpineTsai equation is modified for applying graphene as follows [15]:
Ec ¼ EP
3 1 þ hL xVG 5 1 þ 2hT VG þ 8 1 hL VG 8 1 hT VG
(1)
Ek ¼ EP
1 þ hL xVG 1 hL VG
(2)
hL ¼
EG Ep EG Ep
1 þx
(3)
EG Ep
1
EG Ep
þ2
hT ¼
x¼
2lG 3tG
(4)
(5)
where Ec and Ek stand for the Young's moduli of the composites containing randomly and unidirectionally oriented GnPs, respectively. EP and EG are the Young's moduli of the neat PTFE and the GnPs, respectively. The Young's modulus of the GnPs is replaced by that of the monolayer graphene, which is ~1 TPa [3]. VG is the GnP volume fraction in the PTFE composite. lG and tG represent the length and thickness of the GnP, respectively. The values of 5 mm and 8 nm are considered for the GnP as provided from the suppliers (XG science) for the values of lG and tG, respectively as the GnPs are used without any pre-treatment in this study. Fig. 7(c) shows the predicted and experimentally obtained Young's moduli of the PTFE/ GnP composites. The experimental values are slightly lower than the HalpineTsai model that considers randomly oriented GnPs. It is caused by the intrinsic property and the dispersion of the GnPs. Young's modulus of the GnPs would be lower than that of the monolayer graphene because the used GnPs are not exfoliated to monolayer graphene. Besides, HalpineTsai equation is based on a perfect adhesion between the matrix and the reinforcement. As described above, the GnPs slip each other when the GnPs with
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Fig. 8. Fractured surfaces of (a) neat PTFE and (b) PTFE/3 vol.% GnP. (c) and (d) Display magnified images of (a) and (b), respectively.
multilayers are dispersed in the matrix, interrupting the perfect adhesion with the PTFE. Nonetheless, the experimental values show a similar trend to the expected values, and it is concluded that the GnPs are randomly distributed in the composites. In Fig. 7(d), Young's moduli of composites with various polymer matrices and graphene structures are plotted as a function of graphene volume fraction [16e20]. Graphene oxide (GO) and functionalized graphene are commonly used to ameliorate the mechanical properties of the neat polymer. GO and functionalized graphene have advantages in the load transfer due to functional groups that can have covalent, non-covalent, and hydrogen bonding with the polymer matrix [21]. Besides, for semi-crystalline polymers, enhancement on the mechanical properties by graphene is facilitated as graphene act as a nucleating source to generate the crystalline interfaces in the semi-crystalline polymer [22]. Due to the reasons explained above, even though a direct comparison for the reinforcing efficiency is complicated, the addition of GnPs clearly shows significant enhancement in the Young's modulus of the PTFE/GnP composites, compared to other polymer composites reinforced with graphene. To scrutinize results of the tensile test, fractured surfaces of the neat PTFE and the PTFE/3 vol.% GnP composite after the tensile test are observed, as shown in Fig. 8. For the typical fracture surface of the PTFE, brittle fracture and ductile fracture coexist. The tensile fracture of the neat PTFE is initiated from the upper left corner of the specimen in Fig. 8(a). The initiation region is largely flat and shows broad hackle marks [23]. At a mirror zone on the corner, the crack slowly initiates, and the crack propagation abruptly accelerates from the boundary of the mirror zone, remaining the hackle region. The hackle region is produced by the shear yielding as the crack velocity rapidly increases [22]. The ductile fracture region commences around the mirror zone. In the ductile region, flat surface by the shear yielding partially appears among dimples (Fig. 8(c)). The region is covered with dimples of hundreds of nanometers in sizes. A few voids are present with submicron sizes. The PTFE/3 vol.% GnP composite shows a completely different fracture morphology. According to the overview of the fractured surfaces in Fig. 8(a) and (b), the fracture surface of the PTFE is
relatively smooth without any PTFE fibrils. In contrast, the fracture surface of the PTFE/3 vol.% GnP composite is rough and sharpened. The fracture morphology also reveals the ductile failure throughout the PTFE/3 vol.% GnP composite. The PTFE fibrils encompass the fracture surface of the PTFE/3 vol.% GnP composite, without any particular direction. The fibrils are tangled with each other, and the entanglement is shown as a fibril network. The GnPs do not exhibit strong chemical adhesion with the PTFE, but the GnPs are strongly entwined by the PTFE fibrils, as seen from Fig. 8(d). It alludes that the GnPs mechanically impede the movement of the PTFE chains that are elongated during the tensile test. It leads to a drastic decrease in the strain hardening because the strain hardening occurs when the polymer chain is reoriented along the tensile direction. Meanwhile, the behavior is attributed to the improvement in the mechanical properties of the composites with increasing GnP loading. 4. Conclusions In this study, we attempt to use the solid-state processing including mechanical milling and hot-pressing for the fabrication of PTFE/GnP composites. The high-energy ball milling results in random dispersion of the GnPs, with attached on and inserted in the PTFE powder, by the powder deformation and tearing without making use of any solvents. The mechanical properties of the hotpressed PTFE/GnP composites are enhanced compared to the neat PTFE. The yield stress of the composites is gradually improved with increasing GnP volume fraction, showing the increase of 60% at 3 vol.% GnP loading. In addition, a significant improvement of 223% is accomplished on Young's modulus for the PTFE/3 vol.% GnP composite. Further work requires to be continued in order to study key issue such as the strong adhesion in polymer composites. Acknowledgments This work is supported by Brain Korea 21 (BK 21) Program.
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