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Improving thermal durability and mechanical properties of poly(ether ether ketone) with single-walled carbon nanotubes
Polymer 176 (2019) 60–65
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Improving thermal durability and mechanical properties of poly(ether ether ketone) with single-walled carbon nanotubes
T
Seisuke Ataa,∗, Yoshihiro Hayashib, Thanh Binh Nguyen Thia, Shigeki Tomonoha, Susumu Kawauchib, Takeo Yamadaa, Kenji Hataa a b
CNT-Application Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8552, Japan
H I GH L IG H T S
resistance can be improved by adding carbon nanotube (CNT) to polyetheretherketone (PEEK). • Heat heat resistance improvement of PEEK is because CNT functions as an antioxidant even in a high-temperature environment. • The • The addition of CNTs can also improve other physical properties such as mechanical properties.
A B S T R A C T
In this study, adding carbon nanotubes (CNTs) to poly (ether ether ketone) (PEEK) not only improved the mechanical properties, but also improved the thermal resistivity of PEEK owing to the CNTs’ function as an antioxidant. The PEEK–CNT composite did not show large weight loss at 450 °C. The stabilization energies (ΔG) of CNTs for 1,4-diphenoxybenzene radical and 4-phenoxyphenol radical, which are decomposition products of PEEK at 450 °C, were calculated by density functional theory, and they were estimated to be −80.3 and 150.2 kJ mol−1, respectively. It was confirmed that 1,4-diphenoxybenzene radical could be trapped in a hightemperature environment of 450 °C by the CNTs. The addition of CNTs yielded almost no change in formability, but the mechanical properties such as tensile strength and bending strength were greatly improved. The PEEK–CNT composite is expected to have improved heat resistivity and mechanical properties, as well as higher reliability, so that it can be used in the future in various fields such as aerospace and automotive.
1. Introduction Poly (ether ether ketone) (PEEK, Figure S1) is a thermoplastic resin with particularly high crystalline melting temperature and glass-transition temperature when compared to other super-engineering plastics. The recommended temperature for general continuous use (related to the long-term heat resistance) of PEEK is 240 °C, but the heat-deflection temperature (related to the short-term heat resistance) is about 156 °C. Therefore, it can only be used at temperatures up to 156 °C for applications that require long-term strength. Since PEEK is a highly heatresistant thermoplastic, it can be molded by injection molding and can be expected to be suitable for a wide range of applications. By taking advantage of its characteristics, PEEK has been used for electronic parts, automobile parts, industrial machines, etc.; it is also expected that its application in semiconductors, liquid crystals, and automobiles will increase in the future. PEEK has attracted attention as an alternative to aluminum in the fields of automobiles and space industry ets., where the increasingly
∗
strenuous demand for material reliability necessitates an increase in the temperature for safe continuous use. The deflection temperature of PEEK under a load can be increased from about 156 to about 334 °C by forming a composite with 30 wt% of carbon fibers (CFs) or glass fibers (GFs). On the other hand, because the continuous-use temperature is not improved by the addition of CFs or GFs, the heat-deflection temperatures of the PEEK–CF and PEEK–GF composites are higher than their respective continuous-use temperatures (240). Therefore, it is necessary to increase the continuous-use temperature of PEEK. Because PEEK shows a difference between the entropies of its amorphous and crystalline regimes, and it has a large enthalpy of fusion, it is difficult to improve its heat resistance by changing the chemical structure. In other approaches, the heat deterioration of PEEK can be suppressed with an antioxidant or the like. However, the limit to the useable temperature of an ordinary antioxidant is around 300 °C, and it is not easy to improve the heat resistance of PEEK by adding a general antioxidant [1]. There have been some reports which PEEK has been mixed with carbon fiber [2], and the heat resistance has been improved
Corresponding author. E-mail address: [email protected] (S. Ata).
https://doi.org/10.1016/j.polymer.2019.05.028 Received 28 January 2019; Received in revised form 4 May 2019; Accepted 11 May 2019 Available online 16 May 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Surface resistivity of PEEK/SG (1w%) with different rotation speed of screw. (a) surface resistance change with rotation speed of screw at 10 min. (b) surface resistance kneading time with rotation speed of screw of 500 rpm.
PEEK–CNT composites has been confirmed, possibly because it is challenging to uniformly disperse CNTs in PEEK. In general, as-grown CNTs form bundles, and it is preferable to use an organic solvent to disperse them. However, there is no organic solvent that is capable of dissolving PEEK (which is known to dissolve in acids and freon), and because PEEK itself has very high viscosity, it is problematic for it to penetrate the voids in CNT bundles. In this study, CNTs and PEEK were defibrated using a twin-screw kneader with strong shearing force and long residence time to prepare a composite material in which the CNTs were dispersed as uniformly as possible in PEEK. Furthermore, an evaluation of the composite material confirmed that CNTs suppressed thermal decomposition of PEEK, and it was shown that the CNTs functioned as an antioxidant for the resin with high heat resistance.
Table 1 IDs and its composition of each samples. ID
SG-SWNT (wt%)
PEEK (wt%)
PEEK PEEK/CNT (1 wt%) PEEK/CNT (2 wt%) PEEK/CNT (5 wt%)
0 1.0 2.0 5.0
100.0 99.0 98.0 95.0
by electron beam crosslinking [3], but these were not due to the antioxidant function. In the study reported here, we attempted to improve the heat resistance of PEEK by adding long single-walled carbon nanotubes (CNTs) [4]. CNTs can remain sufficiently stable under the kneading and molding conditions in the operating environment of PEEK because they constitute a material with high thermal resistance and can withstand high temperatures of about 450 °C in air and 3000 °C under vacuum. Fullerenes have a structure very similar to that of CNTs, and they are known to have radical-scavenging tendencies [5–8]; even though CNTs are a similar nanocarbon material, its radical-scavenging ability has received little attention. Through quantum chemical calculations, however, Galano and co-workers showed the possibility of radical supplementation of CNTs [9–11]. We also succeeded in improving the heat resistance of fluorinated rubber by adding long single-walled CNTs on its surface [12]. Furthermore, Puértolas and co-workers [13] and Hsu and co-workers [14] successfully improved the heat resistance of resins by incorporating CNTs. Nevertheless, no great improvement in the heat resistance of
2. Experimental Single-walled carbon nanotubes were synthesized by the supergrowth chemical vapor deposition (CVD) method (SG-SWCNTs) [15], and PEEK (Victrex 150P, Victrex Plc., UK) was kneaded with a lab-scale compounding mixer (MC15, DSM Xplore, The Netherlands). The kneading temperature was set at 400 °C, and the revolution speed of the kneading screw was set at 100 rpm. The CNT concentration was controlled to remain in the range of 0–5 wt% by adjusting the amount of CNTs added to PEEK. The prepared PEEK–CNT compound was molded by an injection-molding machine (Micro 12 cc injection molding machine, DSM Xplore, The Netherlands). The conditions for injection molding are shown in Table S1, but most of the conditions are constant regardless of the concentration of CNTs, and were set at a slightly
Fig. 2. Fig. 2. (a)(b) TEM image of cross-section of PEEK/CNT (1 wt%) knead by optimized condition. The fibrous one shown by the arrow is CNT/CNT bundles. On the other hand, the black shadow is the crystal part of PEEK, or well-defoliated CNT. Inset figure in left image is as Grown CNT structure observed by SEM. (c) Raman shift of CNT of as-grown and composites. 61
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Fig. 3. Relation between heat-flow and temperature of PEEK and PEEK/CNT composites with 5 K/min heating process in N2 atmosphere. (left) whole temperature range (right) magnitude 300–350 °C. Arrows indicate starting points of melting crystals.
K 6250 A, PG - 20, Teclock Corporation, Japan). The crystallinity of PEEK was determined by differential scanning calorimetry (DSC-7000X, Hitachi High-Tech, Japan) at a heating rate of 5 K/min in N2. The mechanical strength of the PEEK–CNT composite at various temperatures was tested by thermal-mechanical analysis (TMA; TMA-7000X, Hitachi High-Tech, Japan). The tensile strength and elongation-at-break of the PEEK–CNT composite were measured with a tensile testing machine (Autograph AG-IS, Shimadzu Corporation, Japan) according to procedures outlined in the international standard ASTM D 638 (tensile speed: 10 mm min−1; initial check length: 20 mm). The heat resistance of PEEK and PEEK/CNT were evaluated based on the weight change in 3 h in a range of 300 °C–450 °C, and the generated gas in this temperature range was measured by thermogravimetry-mass spectrometry TG-MS (TG-DSC, STA 449 F1 Jupiter, Netzsch and quadrupole mass spectrometer, JMS-Q1500GC-AC28, JEOL Ltd.). Furthermore, the reactivity of the desorbed molecules from PEEK and the CNTs was verified by quantum chemical calculations. All calculations were performed by the long-range and dispersion-corrected ωB97X-D functional [16] combined with the 6-31G(d) basis set [17] using Gaussian 16 program [18]. The optimized molecular structures were verified by vibrational analysis; the equilibrium structures did not have imaginary frequencies. Gibbs energies were calculated using the unscaled vibrational frequencies. Atomic coordination of all of the stationary points are shown in the appendix.
Table 2 Melting heat of PEEK and PEEK/CNT composites around 340 °C (PEEK crystal). Crystallinities were calculated using the value of melting heat of perfect crystal of PEEK, 130 J/g [17]. ID
peak area (J/g)
Estimated crystallinity of PEEK (%)
PEEK PEEK/CNT (1 wt%) PEEK/CNT (2 wt%) PEEK/CNT (5 wt%)
44.7 39.9 41.8 40.7
34.4 31.0 32.8 33.0
3. Results and discussion Using the conductivity of the PEEK–CNT (1 wt%) composite as an index, the kneading conditions of all of the PEEK–CNT composites were optimized. The relationship between the kneading conditions and the surface resistance is shown in Fig. 1. As RPM increases, the required torque increases and the sheer force on the CNT also increases. The conductivity dropped greatly between 300 and 500 rpm, and it was nearly constant at ≥500 rpm, indicating that there was a threshold value for the energy required for fiber disintegration of the CNTs. On the other hand, with the kneading speed fixed at 500 rpm, the kneading time reached the minimum value after 10 min and then increased. These results were expected because both disintegration and cleavage occurred during the entire CNT-dispersion process, but defibration proceeded predominantly for the first 10 min, and the effect of cleavage became conspicuous after the completion of disintegration. Therefore, since the defibration of CNT bundles was almost completed at a kneading time of 10 min and a screw-rotation speed of 500 rpm [19–21], all future samples, such as PEEK–CNT (1 wt%), PEEK–CNT (3 wt%), and PEEK–CNT (5 w%), were prepared under these conditions.
Fig. 4. Tensile strength (black bar) and bending strength (gray bar) of PEEK and PEEK/CNT (1, 5 wt%) composites.
higher value of PEEK/CNT 5 wt% with only the holding pressure. Also the change of storage modulus, loss modulus and loss tangent (tanδ) of each sample were shown in Figure S2. The structure of as-grown CNT was observed by SEM (S4800, Hitachi), and the structure of CNT in the composite was observed by TEM (EM-002B, TOPCON) cut with an ultratoming. Moreover, damage to CNT was evaluated by Raman scattering (inVia Reflex RE04, Renishaw). Each composite sample was molded into a sheet with thickness of 500 μm by a press molding machine (H400-15, AS-ONE). The surface resistance was obtained by using the four-terminal fourprobe method (Loresta, Mitsubishi-Chemical Analytech, Japan) to measure the film conductivity and by measuring the film thickness (JIS 62
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Fig. 5. Sample thickness versus temperature of PEEK and PEEK/CNT composites at various pressure (a) 102 Pa, (b) 103 Pa, (c) 104 Pa, (d) 105 Pa. Initial sample thickness of all samples were 1000 mm (1 mm).
crystallinity of each sample from each endothermic peak. The estimated values of crystallinity of each sample are shown in Table 2. The degree of crystallization was the largest for neat PEEK; it slightly decreased as the amount of added CNTs increased and began to increase when the amount of added CNTs reached 2 wt%. Although the change was very small, the starting temperature of crystal melting shifted to the lowertemperature side as the amount of added CNTs increased, and the final melting peak shifted to a lower temperature. This result is presumed to be due to the apparent increase in crystallinity and micro-crystallization due to the crystal-nucleation effect of the CNTs [23,24], as the crystal growth was slightly more inhibited when the amount of added CNTs increased. The mechanical properties, such as the tensile strength and bending strength, of each sample are shown in Fig. 4; data for the injectonmolded PEEK and the PEEK–CNT composites are shown in Figure S3, and the S-S curves of tensile test are shown in Figure S4. By adding CNTs to PEEK, the mechanical properties were greatly improved. Since there was no big difference in the degree of crystallinity of PEEK, it can be said that the improvement in mechanical properties with increasing CNT concentration resulted mainly from the effect of fiber reinforcement by CNTs.
To maintain the same thermal history for PEEK, neat PEEK was also kneaded under the same conditions. The ID and composition of each sample are shown in Table 1. The CNT morphology in the PEEK–CNT composite that was kneaded under the optimized condition is shown in Fig. 2. Although the bundle diameter of CNTs reached several hundreds of micrometers, the CNTs in PEEK were observed to have a diameter of several tens of nanometers, and it was confirmed that the CNTs were well dispersed. Moreover, because the CNTs maintained their linearity, they did not appear to have suffered serious damage by the kneading process. In addition, the G/D ratio, measured by Raman spectroscopy, of CNTs was lowered to 1.3 to 1.5 in the PEEK, compared to 4.8 at the as-grown. This means that the crystallinity of the CNT was decreased, but the value was not so bad compared to other dispersion methods [20]. Although the crystallinity of a polymer greatly affects its mechanical properties, it is known that both CNTs and nanofillers affect the crystal nucleation and crystal growth of a resin. The crystallization degree of PEEK was estimated from the heat of fusion, obtained using DSC, of the PEEK resin (Fig. 3). Exothermic peaks in the heating process were observed near 340 °C in the data of all samples. Since the heat of complete crystallization of PEEK is 130 J/g [22], it is possible to estimate the 63
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Fig. 6. Fig. 6(a) Weight loss of PEEK and PEEK/CNT composites with annealing at 350, 400 and 450 °C for 3h. (b)Counts of m/z = 44 in degradation gas from PEEK and its CNT composites at heating process measured by TG-MS.
Fig. 7. Diagram of stabilization energy calculation of (a) 1,4-diphenoxybenzene and (b) 4-phenoxyphenol. Electronic state of CNT (7,0) and CNT/reactant were singlet diradical state and doublet state, respectively.
measured (Fig. 5). In the case of neat PEEK, when a pressure of 102 Pa was applied, the film thickness decreased greatly at ≥330 °C. On the other hand, the PEEK–CNT composite could gradually withstand high pressure as more CNTs were added. Even when a pressure of about 105 Pa was applied to the PEEK–CNT (5 wt%) composite, a small reduction in film thickness was recorded at a temperature of 340 °C, which is the melting point of PEEK. This result indicates that the CNTs improved the deflection temperature under load, which is an index of the short-term heat resistance of PEEK. In the second evaluation, the weight loss of CNTs, which is an index of long-term heat resistance, was verified. Each sample was heat-treated in air at 350, 400, and 450 °C in a ring furnace for 3 h, and the weight of
Table 3 Stabilized energy of 1,4-diphenoxybenzene radical and 4-phenoxyphenol radical with CNT (7,0) calculated by the ωB97X-D/6-31G (d,p) at 723 K (450 °C).
1,4-diphenoxybenzene radical 4-phenoxyphenol radical
ΔH (723 K) (kJ/mol)
ΔG (723 K) (kJ/mol)
−205.9 25.5
−80.3 150.2
Next, two types of evaluations were conducted on the heat resistance of CNTs. First, using TMA, the temperature was raised at various values of applied pressure, and the changes in film thickness were 64
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Appendix A. Supplementary data
the sample before and after the heat treatment was measured. After annealing at temperatures of 350 and 400 °C, large weight changes were not observed for each sample. However, after annealing at 450 °C, pure PEEK, PEEK–CNT (1 wt%), PEEK–CNT (2 wt%), and PEEK–CNT (5 wt%) showed weight loss of 18.3%, 2.3%, 0.5%, and 0.5%, respectively (Fig. 6 (a)). The thermal-decomposition temperature (Td) of PEEK was in the range of 550–570 °C (Figure S5), and the thermal decomposition progressed little by little even at 450 °C, which is 100 °C lower than Td of PEEK. The weight-loss phenomenon due to the gentle thermal decomposition was suppressed as the CNT fraction increased in the samples, indicating that the CNTs functioned as an antioxidant of PEEK, even at a very high temperature of 450 °C. In addition, TG-MS measurement of PEEK and PEEK/CNT was performed in order to detect the amount of generation of CO2, which is a major decomposition product during pyrolysis of PEEK. The temperature change with respect to the amount of generation of m/z = 44 from each sample was shown in Fig. 6 (b). From the fact that the amount of generation of m/z = 44 decreases as the amount of CNT addition increases, it can be concluded that the suppression of the thermal decomposition of PEEK resulted in the reduction of the weight loss. Also, considering that PEEK thermal decomposition occurred gradually from around 300 °C, the effect of CNT addition in Fig. 5 was considered to be due to both suppression of degradation of PEEK and physical reinforcement by CNT network. The thermal decomposition of PEEK at each temperature was studied in detail by Hull and co-workers. It was reported that the generation of 1,4-diphenoxybenzene and 4-phenoxyphenol (Figure S6) progressed at 450 °C [25]. When the gas released during PEEK decomposition was measured by thermogravimetric mass spectroscopy (TG-MS), phenol was recorded around 450 °C. For the CNTs to show an aging-prevention function, it is necessary to suppress the generation of phenoxybenzene and phenoxyphenol. Therefore, the stabilization energy in the reaction of decomposition products and CNTs at 450 °C was estimated by quantum chemical calculations. As reported by Hod and Scuseria, the most stable state of CNTs is the antiferromagnetic singlet (diradical) rather than the closedshell singlet [24]; therefore, we defined the reference state of CNTs as the singlet diradical state (table S2). It is necessary to consider whether the stabilized reaction with CNTs spontaneously occurred at 450 °C when the thermal-degradation test was performed in this research. To estimate the free energy of the reaction at 450 °C, the temperature dependence of the enthalpy of the reaction was also calculated. The results confirmed that even at 450 °C, the free energy had values of −80.2 and 150.2 kJ mol−1 for the reaction between CNTs and the 1,4-diphenoxybenzene radicals and 4-phenoxyphenol radicals, respectively (Fig. 7 and Figure S7). In other words, quantum chemical calculations suggest that CNTs can function as an antioxidant of PEEK at 450 °C. Even for 4-phenoxyphenol, which has been shown to be difficult to trap, the reaction was possible by changing the chirality and length of the CNTs. Obviously, because this conclusion was drawn as a result of our calculations, we cannot be sure whether the same reaction takes place in an actual system. However, we can be certain that at least at such a high temperature, the CNTs can be used to stabilize the decomposition products of PEEK (see Table 3).
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Acknowledgments The numerical calculations were carried out on the TSUBAME3.0 supercomputer at the Tokyo Institute of Technology, Tokyo, Japan, and on the supercomputer at the Research Center for Computational Science, Okazaki, Japan. This computational work was supported by a Grant-in-Aid supported for Young Scientists (B) (JSPS KAKENHI Grant Number JP17K17720 to Y. H.), a Grant-in-Aid supported for Specially Promoted Research (JSPS KAKENHI Grant Number JP17H06092 to S. K.), and a JST CREST (Grant Number JPMJCR1522 to S·K.).
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Improving thermal durability and mechanical properties of poly(ether ether ketone) with single-walled carbon nanotubes
T
Seisuke Ataa,∗, Yoshihiro Hayashib, Thanh Binh Nguyen Thia, Shigeki Tomonoha, Susumu Kawauchib, Takeo Yamadaa, Kenji Hataa a b
CNT-Application Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8552, Japan
H I GH L IG H T S
resistance can be improved by adding carbon nanotube (CNT) to polyetheretherketone (PEEK). • Heat heat resistance improvement of PEEK is because CNT functions as an antioxidant even in a high-temperature environment. • The • The addition of CNTs can also improve other physical properties such as mechanical properties.
A B S T R A C T
In this study, adding carbon nanotubes (CNTs) to poly (ether ether ketone) (PEEK) not only improved the mechanical properties, but also improved the thermal resistivity of PEEK owing to the CNTs’ function as an antioxidant. The PEEK–CNT composite did not show large weight loss at 450 °C. The stabilization energies (ΔG) of CNTs for 1,4-diphenoxybenzene radical and 4-phenoxyphenol radical, which are decomposition products of PEEK at 450 °C, were calculated by density functional theory, and they were estimated to be −80.3 and 150.2 kJ mol−1, respectively. It was confirmed that 1,4-diphenoxybenzene radical could be trapped in a hightemperature environment of 450 °C by the CNTs. The addition of CNTs yielded almost no change in formability, but the mechanical properties such as tensile strength and bending strength were greatly improved. The PEEK–CNT composite is expected to have improved heat resistivity and mechanical properties, as well as higher reliability, so that it can be used in the future in various fields such as aerospace and automotive.
1. Introduction Poly (ether ether ketone) (PEEK, Figure S1) is a thermoplastic resin with particularly high crystalline melting temperature and glass-transition temperature when compared to other super-engineering plastics. The recommended temperature for general continuous use (related to the long-term heat resistance) of PEEK is 240 °C, but the heat-deflection temperature (related to the short-term heat resistance) is about 156 °C. Therefore, it can only be used at temperatures up to 156 °C for applications that require long-term strength. Since PEEK is a highly heatresistant thermoplastic, it can be molded by injection molding and can be expected to be suitable for a wide range of applications. By taking advantage of its characteristics, PEEK has been used for electronic parts, automobile parts, industrial machines, etc.; it is also expected that its application in semiconductors, liquid crystals, and automobiles will increase in the future. PEEK has attracted attention as an alternative to aluminum in the fields of automobiles and space industry ets., where the increasingly
∗
strenuous demand for material reliability necessitates an increase in the temperature for safe continuous use. The deflection temperature of PEEK under a load can be increased from about 156 to about 334 °C by forming a composite with 30 wt% of carbon fibers (CFs) or glass fibers (GFs). On the other hand, because the continuous-use temperature is not improved by the addition of CFs or GFs, the heat-deflection temperatures of the PEEK–CF and PEEK–GF composites are higher than their respective continuous-use temperatures (240). Therefore, it is necessary to increase the continuous-use temperature of PEEK. Because PEEK shows a difference between the entropies of its amorphous and crystalline regimes, and it has a large enthalpy of fusion, it is difficult to improve its heat resistance by changing the chemical structure. In other approaches, the heat deterioration of PEEK can be suppressed with an antioxidant or the like. However, the limit to the useable temperature of an ordinary antioxidant is around 300 °C, and it is not easy to improve the heat resistance of PEEK by adding a general antioxidant [1]. There have been some reports which PEEK has been mixed with carbon fiber [2], and the heat resistance has been improved
Corresponding author. E-mail address: [email protected] (S. Ata).
https://doi.org/10.1016/j.polymer.2019.05.028 Received 28 January 2019; Received in revised form 4 May 2019; Accepted 11 May 2019 Available online 16 May 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Surface resistivity of PEEK/SG (1w%) with different rotation speed of screw. (a) surface resistance change with rotation speed of screw at 10 min. (b) surface resistance kneading time with rotation speed of screw of 500 rpm.
PEEK–CNT composites has been confirmed, possibly because it is challenging to uniformly disperse CNTs in PEEK. In general, as-grown CNTs form bundles, and it is preferable to use an organic solvent to disperse them. However, there is no organic solvent that is capable of dissolving PEEK (which is known to dissolve in acids and freon), and because PEEK itself has very high viscosity, it is problematic for it to penetrate the voids in CNT bundles. In this study, CNTs and PEEK were defibrated using a twin-screw kneader with strong shearing force and long residence time to prepare a composite material in which the CNTs were dispersed as uniformly as possible in PEEK. Furthermore, an evaluation of the composite material confirmed that CNTs suppressed thermal decomposition of PEEK, and it was shown that the CNTs functioned as an antioxidant for the resin with high heat resistance.
Table 1 IDs and its composition of each samples. ID
SG-SWNT (wt%)
PEEK (wt%)
PEEK PEEK/CNT (1 wt%) PEEK/CNT (2 wt%) PEEK/CNT (5 wt%)
0 1.0 2.0 5.0
100.0 99.0 98.0 95.0
by electron beam crosslinking [3], but these were not due to the antioxidant function. In the study reported here, we attempted to improve the heat resistance of PEEK by adding long single-walled carbon nanotubes (CNTs) [4]. CNTs can remain sufficiently stable under the kneading and molding conditions in the operating environment of PEEK because they constitute a material with high thermal resistance and can withstand high temperatures of about 450 °C in air and 3000 °C under vacuum. Fullerenes have a structure very similar to that of CNTs, and they are known to have radical-scavenging tendencies [5–8]; even though CNTs are a similar nanocarbon material, its radical-scavenging ability has received little attention. Through quantum chemical calculations, however, Galano and co-workers showed the possibility of radical supplementation of CNTs [9–11]. We also succeeded in improving the heat resistance of fluorinated rubber by adding long single-walled CNTs on its surface [12]. Furthermore, Puértolas and co-workers [13] and Hsu and co-workers [14] successfully improved the heat resistance of resins by incorporating CNTs. Nevertheless, no great improvement in the heat resistance of
2. Experimental Single-walled carbon nanotubes were synthesized by the supergrowth chemical vapor deposition (CVD) method (SG-SWCNTs) [15], and PEEK (Victrex 150P, Victrex Plc., UK) was kneaded with a lab-scale compounding mixer (MC15, DSM Xplore, The Netherlands). The kneading temperature was set at 400 °C, and the revolution speed of the kneading screw was set at 100 rpm. The CNT concentration was controlled to remain in the range of 0–5 wt% by adjusting the amount of CNTs added to PEEK. The prepared PEEK–CNT compound was molded by an injection-molding machine (Micro 12 cc injection molding machine, DSM Xplore, The Netherlands). The conditions for injection molding are shown in Table S1, but most of the conditions are constant regardless of the concentration of CNTs, and were set at a slightly
Fig. 2. Fig. 2. (a)(b) TEM image of cross-section of PEEK/CNT (1 wt%) knead by optimized condition. The fibrous one shown by the arrow is CNT/CNT bundles. On the other hand, the black shadow is the crystal part of PEEK, or well-defoliated CNT. Inset figure in left image is as Grown CNT structure observed by SEM. (c) Raman shift of CNT of as-grown and composites. 61
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Fig. 3. Relation between heat-flow and temperature of PEEK and PEEK/CNT composites with 5 K/min heating process in N2 atmosphere. (left) whole temperature range (right) magnitude 300–350 °C. Arrows indicate starting points of melting crystals.
K 6250 A, PG - 20, Teclock Corporation, Japan). The crystallinity of PEEK was determined by differential scanning calorimetry (DSC-7000X, Hitachi High-Tech, Japan) at a heating rate of 5 K/min in N2. The mechanical strength of the PEEK–CNT composite at various temperatures was tested by thermal-mechanical analysis (TMA; TMA-7000X, Hitachi High-Tech, Japan). The tensile strength and elongation-at-break of the PEEK–CNT composite were measured with a tensile testing machine (Autograph AG-IS, Shimadzu Corporation, Japan) according to procedures outlined in the international standard ASTM D 638 (tensile speed: 10 mm min−1; initial check length: 20 mm). The heat resistance of PEEK and PEEK/CNT were evaluated based on the weight change in 3 h in a range of 300 °C–450 °C, and the generated gas in this temperature range was measured by thermogravimetry-mass spectrometry TG-MS (TG-DSC, STA 449 F1 Jupiter, Netzsch and quadrupole mass spectrometer, JMS-Q1500GC-AC28, JEOL Ltd.). Furthermore, the reactivity of the desorbed molecules from PEEK and the CNTs was verified by quantum chemical calculations. All calculations were performed by the long-range and dispersion-corrected ωB97X-D functional [16] combined with the 6-31G(d) basis set [17] using Gaussian 16 program [18]. The optimized molecular structures were verified by vibrational analysis; the equilibrium structures did not have imaginary frequencies. Gibbs energies were calculated using the unscaled vibrational frequencies. Atomic coordination of all of the stationary points are shown in the appendix.
Table 2 Melting heat of PEEK and PEEK/CNT composites around 340 °C (PEEK crystal). Crystallinities were calculated using the value of melting heat of perfect crystal of PEEK, 130 J/g [17]. ID
peak area (J/g)
Estimated crystallinity of PEEK (%)
PEEK PEEK/CNT (1 wt%) PEEK/CNT (2 wt%) PEEK/CNT (5 wt%)
44.7 39.9 41.8 40.7
34.4 31.0 32.8 33.0
3. Results and discussion Using the conductivity of the PEEK–CNT (1 wt%) composite as an index, the kneading conditions of all of the PEEK–CNT composites were optimized. The relationship between the kneading conditions and the surface resistance is shown in Fig. 1. As RPM increases, the required torque increases and the sheer force on the CNT also increases. The conductivity dropped greatly between 300 and 500 rpm, and it was nearly constant at ≥500 rpm, indicating that there was a threshold value for the energy required for fiber disintegration of the CNTs. On the other hand, with the kneading speed fixed at 500 rpm, the kneading time reached the minimum value after 10 min and then increased. These results were expected because both disintegration and cleavage occurred during the entire CNT-dispersion process, but defibration proceeded predominantly for the first 10 min, and the effect of cleavage became conspicuous after the completion of disintegration. Therefore, since the defibration of CNT bundles was almost completed at a kneading time of 10 min and a screw-rotation speed of 500 rpm [19–21], all future samples, such as PEEK–CNT (1 wt%), PEEK–CNT (3 wt%), and PEEK–CNT (5 w%), were prepared under these conditions.
Fig. 4. Tensile strength (black bar) and bending strength (gray bar) of PEEK and PEEK/CNT (1, 5 wt%) composites.
higher value of PEEK/CNT 5 wt% with only the holding pressure. Also the change of storage modulus, loss modulus and loss tangent (tanδ) of each sample were shown in Figure S2. The structure of as-grown CNT was observed by SEM (S4800, Hitachi), and the structure of CNT in the composite was observed by TEM (EM-002B, TOPCON) cut with an ultratoming. Moreover, damage to CNT was evaluated by Raman scattering (inVia Reflex RE04, Renishaw). Each composite sample was molded into a sheet with thickness of 500 μm by a press molding machine (H400-15, AS-ONE). The surface resistance was obtained by using the four-terminal fourprobe method (Loresta, Mitsubishi-Chemical Analytech, Japan) to measure the film conductivity and by measuring the film thickness (JIS 62
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Fig. 5. Sample thickness versus temperature of PEEK and PEEK/CNT composites at various pressure (a) 102 Pa, (b) 103 Pa, (c) 104 Pa, (d) 105 Pa. Initial sample thickness of all samples were 1000 mm (1 mm).
crystallinity of each sample from each endothermic peak. The estimated values of crystallinity of each sample are shown in Table 2. The degree of crystallization was the largest for neat PEEK; it slightly decreased as the amount of added CNTs increased and began to increase when the amount of added CNTs reached 2 wt%. Although the change was very small, the starting temperature of crystal melting shifted to the lowertemperature side as the amount of added CNTs increased, and the final melting peak shifted to a lower temperature. This result is presumed to be due to the apparent increase in crystallinity and micro-crystallization due to the crystal-nucleation effect of the CNTs [23,24], as the crystal growth was slightly more inhibited when the amount of added CNTs increased. The mechanical properties, such as the tensile strength and bending strength, of each sample are shown in Fig. 4; data for the injectonmolded PEEK and the PEEK–CNT composites are shown in Figure S3, and the S-S curves of tensile test are shown in Figure S4. By adding CNTs to PEEK, the mechanical properties were greatly improved. Since there was no big difference in the degree of crystallinity of PEEK, it can be said that the improvement in mechanical properties with increasing CNT concentration resulted mainly from the effect of fiber reinforcement by CNTs.
To maintain the same thermal history for PEEK, neat PEEK was also kneaded under the same conditions. The ID and composition of each sample are shown in Table 1. The CNT morphology in the PEEK–CNT composite that was kneaded under the optimized condition is shown in Fig. 2. Although the bundle diameter of CNTs reached several hundreds of micrometers, the CNTs in PEEK were observed to have a diameter of several tens of nanometers, and it was confirmed that the CNTs were well dispersed. Moreover, because the CNTs maintained their linearity, they did not appear to have suffered serious damage by the kneading process. In addition, the G/D ratio, measured by Raman spectroscopy, of CNTs was lowered to 1.3 to 1.5 in the PEEK, compared to 4.8 at the as-grown. This means that the crystallinity of the CNT was decreased, but the value was not so bad compared to other dispersion methods [20]. Although the crystallinity of a polymer greatly affects its mechanical properties, it is known that both CNTs and nanofillers affect the crystal nucleation and crystal growth of a resin. The crystallization degree of PEEK was estimated from the heat of fusion, obtained using DSC, of the PEEK resin (Fig. 3). Exothermic peaks in the heating process were observed near 340 °C in the data of all samples. Since the heat of complete crystallization of PEEK is 130 J/g [22], it is possible to estimate the 63
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Fig. 6. Fig. 6(a) Weight loss of PEEK and PEEK/CNT composites with annealing at 350, 400 and 450 °C for 3h. (b)Counts of m/z = 44 in degradation gas from PEEK and its CNT composites at heating process measured by TG-MS.
Fig. 7. Diagram of stabilization energy calculation of (a) 1,4-diphenoxybenzene and (b) 4-phenoxyphenol. Electronic state of CNT (7,0) and CNT/reactant were singlet diradical state and doublet state, respectively.
measured (Fig. 5). In the case of neat PEEK, when a pressure of 102 Pa was applied, the film thickness decreased greatly at ≥330 °C. On the other hand, the PEEK–CNT composite could gradually withstand high pressure as more CNTs were added. Even when a pressure of about 105 Pa was applied to the PEEK–CNT (5 wt%) composite, a small reduction in film thickness was recorded at a temperature of 340 °C, which is the melting point of PEEK. This result indicates that the CNTs improved the deflection temperature under load, which is an index of the short-term heat resistance of PEEK. In the second evaluation, the weight loss of CNTs, which is an index of long-term heat resistance, was verified. Each sample was heat-treated in air at 350, 400, and 450 °C in a ring furnace for 3 h, and the weight of
Table 3 Stabilized energy of 1,4-diphenoxybenzene radical and 4-phenoxyphenol radical with CNT (7,0) calculated by the ωB97X-D/6-31G (d,p) at 723 K (450 °C).
1,4-diphenoxybenzene radical 4-phenoxyphenol radical
ΔH (723 K) (kJ/mol)
ΔG (723 K) (kJ/mol)
−205.9 25.5
−80.3 150.2
Next, two types of evaluations were conducted on the heat resistance of CNTs. First, using TMA, the temperature was raised at various values of applied pressure, and the changes in film thickness were 64
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Appendix A. Supplementary data
the sample before and after the heat treatment was measured. After annealing at temperatures of 350 and 400 °C, large weight changes were not observed for each sample. However, after annealing at 450 °C, pure PEEK, PEEK–CNT (1 wt%), PEEK–CNT (2 wt%), and PEEK–CNT (5 wt%) showed weight loss of 18.3%, 2.3%, 0.5%, and 0.5%, respectively (Fig. 6 (a)). The thermal-decomposition temperature (Td) of PEEK was in the range of 550–570 °C (Figure S5), and the thermal decomposition progressed little by little even at 450 °C, which is 100 °C lower than Td of PEEK. The weight-loss phenomenon due to the gentle thermal decomposition was suppressed as the CNT fraction increased in the samples, indicating that the CNTs functioned as an antioxidant of PEEK, even at a very high temperature of 450 °C. In addition, TG-MS measurement of PEEK and PEEK/CNT was performed in order to detect the amount of generation of CO2, which is a major decomposition product during pyrolysis of PEEK. The temperature change with respect to the amount of generation of m/z = 44 from each sample was shown in Fig. 6 (b). From the fact that the amount of generation of m/z = 44 decreases as the amount of CNT addition increases, it can be concluded that the suppression of the thermal decomposition of PEEK resulted in the reduction of the weight loss. Also, considering that PEEK thermal decomposition occurred gradually from around 300 °C, the effect of CNT addition in Fig. 5 was considered to be due to both suppression of degradation of PEEK and physical reinforcement by CNT network. The thermal decomposition of PEEK at each temperature was studied in detail by Hull and co-workers. It was reported that the generation of 1,4-diphenoxybenzene and 4-phenoxyphenol (Figure S6) progressed at 450 °C [25]. When the gas released during PEEK decomposition was measured by thermogravimetric mass spectroscopy (TG-MS), phenol was recorded around 450 °C. For the CNTs to show an aging-prevention function, it is necessary to suppress the generation of phenoxybenzene and phenoxyphenol. Therefore, the stabilization energy in the reaction of decomposition products and CNTs at 450 °C was estimated by quantum chemical calculations. As reported by Hod and Scuseria, the most stable state of CNTs is the antiferromagnetic singlet (diradical) rather than the closedshell singlet [24]; therefore, we defined the reference state of CNTs as the singlet diradical state (table S2). It is necessary to consider whether the stabilized reaction with CNTs spontaneously occurred at 450 °C when the thermal-degradation test was performed in this research. To estimate the free energy of the reaction at 450 °C, the temperature dependence of the enthalpy of the reaction was also calculated. The results confirmed that even at 450 °C, the free energy had values of −80.2 and 150.2 kJ mol−1 for the reaction between CNTs and the 1,4-diphenoxybenzene radicals and 4-phenoxyphenol radicals, respectively (Fig. 7 and Figure S7). In other words, quantum chemical calculations suggest that CNTs can function as an antioxidant of PEEK at 450 °C. Even for 4-phenoxyphenol, which has been shown to be difficult to trap, the reaction was possible by changing the chirality and length of the CNTs. Obviously, because this conclusion was drawn as a result of our calculations, we cannot be sure whether the same reaction takes place in an actual system. However, we can be certain that at least at such a high temperature, the CNTs can be used to stabilize the decomposition products of PEEK (see Table 3).
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Acknowledgments The numerical calculations were carried out on the TSUBAME3.0 supercomputer at the Tokyo Institute of Technology, Tokyo, Japan, and on the supercomputer at the Research Center for Computational Science, Okazaki, Japan. This computational work was supported by a Grant-in-Aid supported for Young Scientists (B) (JSPS KAKENHI Grant Number JP17K17720 to Y. H.), a Grant-in-Aid supported for Specially Promoted Research (JSPS KAKENHI Grant Number JP17H06092 to S. K.), and a JST CREST (Grant Number JPMJCR1522 to S·K.).
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