Effect of carbon nanotubes on the mechanical properties and crystallization behavior of poly(ether ether ketone)

Composites Science and Technology 70 (2010) 380–386

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Effect of carbon nanotubes on the mechanical properties and crystallization behavior of poly(ether ether ketone) Changru Rong a, Gang Ma a, Shuling Zhang a, Li Song b, Zheng Chen a, Guibin Wang a,*, P.M. Ajayan b a b

Alan G. MacDiarmid Laboratory, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China Department of Mechanical Engineering and Materials Science, Rice University, 6100 Main Street, Houston, TX 77005, USA

a r t i c l e

i n f o

Article history: Received 4 July 2009 Received in revised form 23 October 2009 Accepted 30 November 2009 Available online 4 December 2009 Keywords: A. Carbon nanotubes A. Polymer-matrix composites (PMCs) B. Mechanical properties D. Differential scanning calorimetry (DSC)

a b s t r a c t Pristine carbon nanotubes (CNTs) and noncovalently functionalized carbon nanotubes (f-CNTs) were used to prepare poly(ether ether ketone) (PEEK) composites (CNTs/PEEK and f-CNTs/PEEK) via melt blending. Noncovalently functionalized multiwalled nanotubes were synthesized using hydrogen-bonding interactions between sulfonic groups of sulfonated poly(ether ether ketone) (SPEEK) and carboxylic groups of nanotubes treated by acid (CNTs–COOH). The effects of these two kinds of nanotubes on the mechanical properties and crystallization behavior of PEEK were investigated. CNTs improved mechanical properties and promoted the crystallization rate of PEEK as a result of heterogeneous nucleation. Better enhancement of mechanical properties appeared in the f-CNTs/PEEK composites, which is ascribed to the good interaction between f-CNTs and PEEK. However, the strong interaction of f-CNTs and PEEK chains decreased the crystallization rate of PEEK for high content of f-CNTs. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Poly(ether ether ketone) (PEEK) has been widely applied in aerospace technology, energy field and chemical industry, which is a semi-crystalline engineering thermoplastic with outstanding properties, such as superior mechanical properties, thermal stability and solvent resistance [1,2]. Lot of efforts have been devoted to further improving the properties of PEEK via incorporation with fillers such as carbon fiber, glass fibers [3,4], and nanoparticles [5–11]. Carbon nanotubes are excellent candidate nanofillers for composites on account of their unique mechanical and physical properties [12,13]. Recently, poly(aryl ether ketones) (PAEKs) composites with carbon nanotubes have been demonstrated to show good properties. In situ polymerization is an efficient method to uniformly attach PAEKs chains onto the surface of nanotubes, thereby increasing nanotubes solubility, reactivity, and potential applicability in composites [14–16]. Song et al. [17] prepared the sandwich-like PEEK composites with single-walled carbon nanotubes (SWNTs) paper by hot-pressing. The mechanical properties of such composites deeply depended on the layer of SWNTs paper embedded in the sandwich-like composites. Deng et al. [18] investigated the effective reinforcement of multi-walled carbon nanotubes (MWNTs) in PEEK. In situ SEM and TEM revealed that the weak bonding between MWNTs and PEEK matrix were not efficient in transferring load from PEEK matrix to added nanotubes.

* Corresponding author. Tel./fax: +86 431 85168889. E-mail address: [email protected] (G. Wang). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.11.024

The properties of carbon nanotubes composites are mostly dependent on the chosen polymeric matrix and the processing used to make the composites [19]. It is a tough problem to disperse carbon nanotubes well in matrix and to form good interfaces with polymer chains due to their smooth surfaces and agglomeration [20]. Therefore, covalent and noncovalent functionalization have been used to modify carbon nanotubes [21,22]. Functionalization of carbon nanotubes can efficiently improve their dispersion in polymers and obtain composites with strong interfacial interactions [23–26]. It is well known that crystallization behavior plays a crucial role in the properties of semi-crystalline polymers. Therefore, it is necessary to pay attention to this issue. Grady et al. [27] reported nanotubes modified with octadecylamine accelerated crystallization of polypropylene, whereas the amount of nucleation sites had a percolation threshold that could not further produce increment in the crystallization with the increasing loadings of nanotubes. Jin et al. [28] investigated the crystallization behavior of poly(ethylene oxide) (PEO) influenced by the unmodified and modified MWCNTs, and the addition of nanotubes decreased nucleating sites and reduced spherulite growth rate of PEO. They interpreted that the interactions of stiff nanotubes with molecular chains of the polymer in composites inhibit PEO chains mobility for crystallization. In this present work, sulfonated poly(ether ether ketone) (SPEEK) was regarded as the functionalizing agent, which can form hydrogen-bonding interactions with carboxylic groups of acid treated nanotubes (CNTs–COOH) and decrease agglomeration of

C. Rong et al. / Composites Science and Technology 70 (2010) 380–386

nanotubes. Pristine carbon nanotubes (CNTs) and noncovalently functionalized nanotubes (f-CNTs) were used to prepare PEEK composites by direct melt compounding. As a semi-crystalline high performance polymer, the properties of PEEK composites mostly depend on fillers and the compounding processing. Hence, the effects of carbon nanotubes on the mechanical properties and crystallization behavior of PEEK were evaluated.

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tained on the second heating scan. The isothermal crystallization kinetics of the composites were first held at 420 °C for 5 min, then decreased to the desired crystallization temperature at a 100 °C/ min and held for 30 min. 3. Results and discussion 3.1. Characterization of f-CNTs

2. Experimental 2.1. Materials The super fine PEEK powders were supplied by Changchun Jilin University Super Engineering Plastics Research Co. Ltd. (PR China). Melt flow index (MFI) of PEEK is 12 g/10 min. CVD grown multiwall carbon nanotubes were purchased from Chengdu Organic Chemicals Co. Ltd. The outside diameter of the nanotubes is about 10–20 nm, length about 10–30 lm, and purity more than 95 wt%. 2.2. Preparation of functional carbon nanotubes (f-CNTs) Carbon nanotubes with carboxylic groups (CNTs–COOH) were prepared in strong mixing acid by using pristine nanotubes (CNTs) referred [29]. CNTs was treated in mixing acid (H2SO4:HNO3 = 3:1) under sonication for 4 h. This mixture was diluted and washed by deionized water, the CNTs–COOH were obtained after dried in vacuum oven at 50 °C. SPEEK was synthesized at 60 °C under vigorous stirring for 6 h according to the Ref. [30]. CNTs–COOH (10 g) was dispersed in 100 ml N,N-dimethylacetamide and ultrasonicated for 0.5 h, then 0.5 g SPEEK was put into the above solution. After the solution was stirred for 1 h, the functionalized nanotubes (f-CNTs) solution for composites was obtained. 2.3. Preparation of composites Super fine PEEK powders were added into f-CNTs solution. This method effectively avoids the f-CNTs agglomeration to get the predispersion mixture when the solvent is removed at 100 °C in the vacuum oven. The gray pre-dispersion mixture was further blended by using Haake PTW16/25p co-rotating twin-screw extruder at the screw speed of 180 rpm and temperature profile of 320/ 340/350/350/350/340 °C. The mass fraction of CNTs and f-CNTs in composites were 0, 0.5, 1, 3 and 5 wt% respectively. The obtained product were cut into granules, dried at 120 °C for 4 h and molded by an SZ15 injection-molding machine.

FTIR spectra of CNTs and CNTs–COOH are shown in Fig. 1. The characteristic peaks appear at 1200 and 1720 cm1 for CNTs– COOH spectrum, which results from the C–O and C@O stretching vibration of the carboxylic groups [31,32]. This shows that the carboxylic acids have been formed on the surface of nanotubes. The degree of sulfonation is 86% by 1H NMR spectrum of SPEEK. Fig. 2 shows a typical 1H NMR spectrum. Thermogravimetric analysis of f-CNTs in Fig. 3 exhibits that the low weight loss at 350 °C meets the requirement for melt compounding processing of PEEK [33]. Fig. 4 indicates that CNTs exist in the entangled state, whereas f-CNTs exist in the dispersed state. It is expected that f-CNTs were formed by hydrogen-bonding interactions between sulfonic groups of SPEEK and carboxylic groups of CNTs–COOH, as shown in Scheme 1. SPEEK can strengthen interface properties between fCNTs and PEEK due to the similar structure of SPEEK and PEEK. 3.2. Mechanical properties of composites Fig. 5 shows the tensile stress–strain curve of PEEK and PEEK composites with 5%-CNTs and 5%-f-CNTs. The mechanical properties of PEEK composites with nanotubes loadings from 0.5% to 5% are listed in the Table 1. Increasing the nanotube loadings resulted in the enhanced tensile strength, and more enhancement of tensile strength appeared in f-CNTs/PEEK composites owing to good interaction between f-CNTs and PEEK. Moreover, the trend for tensile modulus, flexural strength and flexural modulus of composites was almost same as the trend for tensile strength of composites with increasing nanotubes loadings. It could be found that fCNTs/PEEK composites exhibit better mechanical properties in comparison with CNTs/PEEK composites, derived from better interfacial adhesion between PEEK and f-CNTs. There is good compatibility for PEEK and SPEEK with f-CNTs because of their similar structure. Results show that load was efficiently transferred from matrix to f-CNTs. However, this transferring action is reduced

2.4. Characterization

Transmittance

The structure of CNTs and CNTs–COOH was measured by FTIR (Bruker Vector-22, Germany). The thermo-stability of CNTs, CNTs–COOH and f-CNTs was tested with Netzsch Sta449C thermogravimetric analysis under N2 atmosphere. The morphology of CNTs and f-CNTs was observed by Scanning Electron Microscope (FEI Quanta 400 ESEM FEG). Mechanical testing was carried out with a Shimadzu AG-I instrument (without strain gauge type extensometer) at room temperature. The test speed was 2 mm/ min. Every mass fraction of the composite was tested for at least five specimens. Specimens of composites were quenched in liquid nitrogen to get the fractured surface for observation by SEM. A Mettler Tolledo DSC821e instrument was used to evaluate the crystallization behavior of composites. Cooling scans were performed at a rate of 2 °C/min whereas heating scans were carried out at a rate of 10 °C/min. Crystallization peak temperature (Tc) and crystallization enthalpy (DHc) were determined on the first cooling scan, while the temperature of melting peaks (Tm) were ob-

CNTs-COOH

CNTs

2200 2000 1800 1600 1400 1200 1000 800 600

wavenumber (cm-1) Fig. 1. FT-IR spectra of CNTs and CNTs–COOH.

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Fig. 2. 1H NMR spectrum of SPEEK.

because the aspect ratio of nanotubes were cutted down. Hence, the improvement of mechanical properties is lower than that expected by theory of fiber reinforced composites [13]. Functionalization of nanotubes not only enhanced the wetting of fillers with PEEK chains but also prompted the homogeneous dispersion of fillers in PEEK matrix, as can be seen in the SEM image of fracture surface of the composite material. Some CNTs formed a big aggregation as shown in Fig. 6a and b. Individual white end of nanotube can be found in the Fig. 6c and d, namely f-CNTs have much better dispersion than CNTs in the PEEK matrix.

100

Weight (%)

95

90 CNTs f-CNTs CNTs-COOH

85

3.3. Crystallization behavior of composites

80 100

200

300

400

500

o

Temperature ( C ) Fig. 3. TGA curves of CNTs, f-CNTs and CNTs–COOH.

600

Besides reinforcement and interfacial interactions, nanotubes had an effect on the crystallization behavior of PEEK. Fig. 7a shows DSC thermograms of the composites on the first cooling scan. It is clear that the temperature of crystallization peaks (Tc) for CNTs/ PEEK composites shifted to higher temperature and similar trend is seen for f-CNTs/PEEK composites at low content of f-CNTs. However, Tc of f-CNTs/PEEK composites shifted to lower temperature

Fig. 4. SEM micrographs of: (a) CNTs and (b) f-CNTs.

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CNTs-COOH

O

C

C OH O

OH

HO

OH

O S

O

O

S

O

O

O

C O

O

O

C O

SPEEK

O

O

C O

O

O

C O

PEEK

Scheme 1. The structure of PEEK and f-CNTs were formed by hydrogen-bonding interactions between sulfonic groups of SPEEK and carboxylic groups of CNTs–COOH.

120

Stress (MPa)

100 80 60 40 PEEK 5% CNTs/PEEK 5% f-CNTs/PEEK

20 0 0

10

20

30

40

50

Strain (%) Fig. 5. The tensile stress–strain curve of PEEK and PEEK composites with 5%-CNTs and 5%-f-CNTs.

with further increasing the content of f-CNTs. We suggest that CNTs increased amount of nuclei crystallizing per unit volume of the PEEK matrix and rarely blocked the PEEK chains mobility due to weak interaction, thus accelerating the nucleation process on PEEK crystallization at low content. Similar effects had been observed for composites in other matrices like polypropylene system [34]. With increasing content of CNTs, some nanotubes agglomerate into bulks which restrict the mobility of PEEK chains. Therefore, the nucleation process is accelerated slightly. Namely the nucleation ability of PEEK is obvious at low content of CNTs. It is also

shown in f-CNTs/PEEK composites, the good dispersion of f-CNTs easily accelerated the nucleation process at lower loadings. Whereas Tc of f-CNTs/PEEK composites decreased with the higher loadings of f-CNTs possibly due to increased interaction between f-CNTs and PEEK chains. Modified carbon nanotubes interacting with polymer matrices could increase the inhibited effect on polymer mobility for crystallization [28,35]. Fig. 7b shows DSC thermograms of the composites on the second heating scan. It could be seen that the temperature of melting peaks (Tm) for CNTs/PEEK composites showed a little shift to high temperature compared with pure PEEK, whereas Tm of f-CNTs/PEEK composites first increased then decreased. This decreased result indicated much more restricting of f-CNTs on PEEK chains induced imperfect crystallization. The inhibited mobility of polymer chains caused imperfect crystallization has been reported [36]. In general, the larger DT (DT = Tm  Tc) was, more difficult the crystallization was (see Table 2). There was a DHc decreasing trend in both CNTs/PEEK and fCNTs /PEEK composites as the loadings of carbon nanotubes increased (see Table 2). It was possibly from the reduced concentration of PEEK in the composites. This trend was generally in agreement with other polymer systems [37,38]. The decreased nucleating effect may be the other reason for the reduction of DHc for f-CNTs/PEEK composites. The crystallization kinetics of composites under isothermal condition was studied in a temperature range from 292 to 298 °C. The result was analyzed by means of the Avrami equation [39].

1  Xt ¼ exp kt

n

ð1Þ

Eq. (1) is often written in the logarithmic form:

lnð1  XtÞ ¼ kt

n

ð2Þ

Table 1 The mechanical properties of PEEK composites. Samples

Young’s modulus (MPa)

Tensile strength (MPa)

Flexueral modulus (MPa)

Flexueral strength (MPa)

PEEK 0.5%-CNTs 1%-CNTs 3%-CNTs 5%-CNTs 0.5%-f-CNTs 1%-f-CNTs 3%-f-CNTs 5%-f-CNTs

1019.12 ± 5.10 1060.70 ± 5.30 1078.99 ± 5.39 1154.46 ± 5.77 1222.75 ± 6.11 1088.17 ± 5.44 1122.31 ± 5.61 1269.18 ± 6.35 1333.10 ± 6.67

99.54 ± 0.50 100.12 ± 0.50 100.49 ± 0.50 101.46 ± 0.50 102.71 ± 0.51 100.58 ± 0.50 102.50 ± 0.51 103.98 ± 0.51 105.23 ± 0.52

3203.00 ± 16.02 3267.91 ± 16.34 3303.12 ± 16.52 3434.97 ± 17.17 3574.63 ± 17.87 3303.06 ± 16.52 3354.87 ± 16.77 3581.14 ± 17.90 3686.03 ± 18.43

162.09 ± 0.81 162.77 ± 0.81 165.45 ± 0.83 168.56 ± 0.84 170.59 ± 0.85 165.26 ± 0.83 167.51 ± 0.84 172.48 ± 0.86 174.18 ± 0.87

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Fig. 6. SEM micrographs of the cryogenic fractured surface for (a and b) 3%-CNTs/PEEK, (c and d) 3%-f-CNTs/PEEK.

where k is the rate constant of crystallization and n is the Avrami exponent, which can be related to the type of nucleation and the geometry of crystal growth. The log[ln(1  Xt)] vs log t for composites showed an initial liner portion, followed by a gradual rolloff, it is plotted in Fig. 8a. From the intercept and slop of the linear part, the values of k and n were calculated, respectively (see Table 3). The half time of crystallization t1/2, can be calculated by using the follow equations:

t 1=2 ¼ ðln 2=kÞ1=n

ð3Þ

The half time of crystallization of experimental measurement (t1/ values matched with the t1/2 values suggesting that the Avrami equation was suited to describe the initial crystallization of PEEK and PEEK modified with nanotubes. The values of n were around 2 or 3, and nearly independent of the components and crystallization temperature. The values of k were affected by components and crystallization temperature. This implies that nanotubes may influence the crystallization rate of PEEK but not the crystallization mechanism. The higher t1/2 demonstrates slower rate of crystallization, the rate of crystallization ((t1/2)1) decreased with increasing the crystallization temperature (Fig. 8b). The (t1/2)1 of CNTs/PEEK composites were higher than that of pure PEEK at a given crystallization temperature (Table 3). It is well known that the rate of nucleation controls the rate of crystallization at high temperature, 2exp)

but nuclei are not stabilized under this condition. Therefore, the rate of crystallization decreases with the increasing crystallization temperature. The variation of (t1/2)1 is proof for the nucleation that we have discussed in the aforementioned part. The (t1/2)1 of CNTs/PEEK composites increased with increasing CNTs content at lower loadings, whereas remained almost constant at higher loadings. The (t1/2)1 of f-CNTs/PEEK composites increased with increasing f-CNTs content and changed little at low loadings, while decreased at higher loadings. Above all the non-isothermal and isothermal analysis demonstrated that f-CNTs exhibited two kinds of effect on the crystallization behavior of PEEK. As the loading was lower, f-CNTs induced heterogeneous nucleation that prompted the increment of Tc. As the content was higher, f-CNTs probably restricted the regular arrangement of PEEK chain segments, resulting in a gradual reduction of Tc.

4. Conclusions Pristine and functionalized carbon nanotubes resulted in different effects on the mechanical properties and crystallization behavior of PEEK. f-CNTs more effectively improved the mechanical properties of PEEK in comparison with pristine CNTs. The addition of CNTs promoted the crystallization process of PEEK and Tc of CNTs/PEEK (5%) composite was increased 8 °C compared to that of pure PEEK. However, the addition of f-CNTs first accel-

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(a)

(a) 1

5%-f-CNTs 3%-f-CNTs 1%-f-CNTs

Endo

0.5%-f-CNTs 5%-CNTs 3%-CNTs 1%-CNTs 0.5%-CNTs

log [-ln(1-Xt)]

0

-1

-2

PEEK

270

280

290

300

310

320

330

340

-3

350

-1.0

-0.5

O

Temperature ( C )

0.5

(b) 100

(b)

Endo

3%-f-CNTs 1%-f-CNTs 0.5%-f-CNTs 5%-CNTs 3%-CNTs

300

310

320

330

340

350

360

370

Xt (%)

5%-f-CNTs

1%-CNTs 0.5%-CNTs PEEK

50

O

292 C O 294 C O 296 C O 298 C

0 0.0

380

0.5

1.0

1.5

O

2.0

2.5

3.0

t (min)

Temperature ( C) Fig. 7. DSC curves of CNTs/PEEK and f-CNTs/PEEK composites: (a) first cooling at 2 °C/min and (b) second heating at 10 °C/min.

Table 2 The non-isothermal crystallization behavior of PEEK composites.

a

0.0

lg t

Samples

Tc (°C)

Tm (°C)

DHc (J/g)

DTa (°C)

PEEK 0.5%-CNTs 1%-CNTs 3%-CNTs 5%-CNTs 0.5%-f-CNTs 1%-f-CNTs 3%-f-CNTs 5%-f-CNTs

300.24 304.24 306.66 308.28 308.67 308.77 308.06 303.49 299.38

336.68 339.07 340.47 341.01 341.25 341.18 341.54 339.64 337.26

42.78 38.22 34.53 32.82 33.66 36.51 34.58 31.53 32.36

36.44 34.83 33.81 32.73 32.58 32.41 33.48 36.15 37.88

Fig. 8. (a) Avrami plot of PEEK with different content of carbon nanotubes at 296 °C: (j) PEEK, ( ) 0.5%-CNTs, ( ) 1%-CNTs, ( ) 3%-CNTs, ( ) 5%-CNTs, ( ) 0.5%-f-CNTs, ( ) 1%-f-CNTs, ( ) 3%-f-CNTs, ( ) 5%-f-CNTs and (b) plots of relative degree of crystallization (Xt) vs time t of PEEK with 3%-CNTs composite at different crystallization temperature.

Table 3 The isothermal crystallization kinetics parameters of PEEK composites. Samples

Crystallization temperature (°C)

n

k (min1)

t1/2cal (min)

t1/2exp (min)

(t1/2)1 (min1)

PEEK

292 294 296 298

2.7 2.7 2.9 2.7

2.11 2.06 1.59 0.97

0.66 0.67 0.75 0.88

0.67 0.68 0.76 0.88

1.49 1.47 1.32 1.14

0.5%-CNTs

292 294 296 298

2.1 2.4 1.9 2.2

3.75 3.02 1.59 1.45

0.45 0.54 0.65 0.72

0.45 0.52 0.62 0.69

2.22 1.92 1.60 1.45

1%-CNTs

292 294 296 298

1.9 1.9 2.4 2.4

3.80 3.44 3.28 2.28

0.41 0.44 0.52 0.61

0.40 0.44 0.52 0.61

2.50 2.27 1.92 1.64

3%-CNTs

292 294 296 298

1.7 1.9 1.9 2.1

4.03 3.55 2.75 2.25

0.35 0.42 0.48 0.57

0.35 0.41 0.46 0.55

2.86 2.44 2.17 1.82

DT = Tm  Tc.

erated and then inhibited the crystallization process of PEEK. Tc of f-CNT/PEEK composites decreased with further increasing f-CNTs loadings that demonstrated the restriction on the mobility of PEEK chains with increasing the filler content. The interaction between f-CNTs filler and PEEK matrix was the reason for the increased mechanical properties and the restricted crystallization behavior.

(continued on next page)

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Table 3 (continued) 1

Samples

Crystallization temperature (°C)

n

k (min1)

t1/2cal (min)

t1/2exp (min)

(t1/2) (min1)

5%-CNTs

292 294 296 298

1.6 1.7 1.7 1.8

3.89 2.90 2.86 2.16

0.35 0.43 0.44 0.54

0.35 0.43 0.44 0.52

2.86 2.33 2.27 1.93

0.5%-f-CNTs

292 294 296 298

1.8 1.9 1.9 1.8

3.29 2.38 2.05 2.02

0.42 0.53 0.56 0.55

0.42 0.52 0.55 0.57

2.38 1.92 1.82 1.76

1%-f-CNTs

292 294 296 298

1.7 1.8 1.8 1.8

3.20 2.78 2.40 2.12

0.42 0.46 0.51 0.54

0.40 0.45 0.49 0.54

2.50 2.22 2.04 1.85

3%-f-CNTs

292 294 296 298

2.9 3.0 3.0 2.8

1.38 0.84 0.72 0.54

0.79 0.94 0.99 1.09

0.79 0.95 0.99 1.10

1.27 1.05 1.01 0.91

5%-f-CNTs

292 294 296 298

2.4 2.8 2.9 2.7

0.43 0.42 0.20 0.19

1.21 1.20 1.55 1.60

1.20 1.20 1.53 1.60

0.83 0.83 0.65 0.63

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