Rheological Percolation Behavior and Isothermal Crystallization of

ARTICLE pubs.acs.org/IECR

Rheological Percolation Behavior and Isothermal Crystallization of Poly(butyene Succinte)/Carbon Nanotube Composites Lijuan Yuan,†,‡ Defeng Wu,*,†,‡ Ming Zhang,‡,§ Weidong Zhou,§ and Dongpo Lin†,‡ †

School of Chemistry & Chemical Engineering, Yangzhou University, Jiangsu 225002, People's Republic of China Provincial Key Laboratory of Environmental Material & Engineering, Jiangsu 225002, People's Republic of China § Testing Center, Yangzhou University, Jiangsu 225002, People's Republic of China ‡

ABSTRACT: Carbon nanotube (CNT) filled poly(butylene succinate) composites (PBSCNs) were prepared by melt compounding. The oscillatory rheological properties and crystallization behavior and kinetics were then investigated. The results show that the percolation network of CNTs in the small amplitude oscillatory shear flow is temperature dependent and the values of percolation thresholds reduce gradually with an increase of temperature. Therefore, the principle of time
temperature superposition is invalid on the dynamic rheological responses of those percolated PBSCNs. Besides, the presence of CNTs highly promotes the crystallization of PBS, increasing the overall crystallization rate. But the nucleation mechanism of PBS is not altered with addition of CNTs because the PBS itself is nucleated heterogeneously.

1. INTRODUCTION Most of the biodegradable synthetic polymers are mainly aliphatic polyesters, which are promising materials for the production of high performance and environmentally friendly plastics.1,2 As one of those aliphatic polyesters, poly(butylene succinate) (PBS) has the most excellent comprehensive performance. It exhibits a melting point similar to that of low-density polyethylene (LDPE), tensile strength between that of polyethylene (PE) and poly propylene (PP), and stiffness between that of LDPE and high density polyethylene (HDPE).3,4 Consequently, PBS is considered highly promising as a commercial commodity polymer with biodegradable characteristics. However, other properties of PBS, such as softness, strength, and gas barrier properties, are often not sufficient for its further processing and end-use applications. Several techniques, including copolymerization and blending with other polymers,5
10 have been developed to further improve the physical properties of PBS, aiming at extending its enduse applications. Besides, compounded with nanofiller, such as layer silicate,11,12 is also an effective way of fabricating PBS materials with high performance. In recent years, carbon nanotubes (CNTs) have been used as the new-generation nanofiller to prepare polymer composites because of their fascinating properties, such as high modulus and strength, and high electrical conductivity.13
15 Many polymer/CNTs composites16 have hitherto been prepared successfully via various approaches, also including PBS/CNTs composites.17
21 Ray et al.20 prepared the PBS/CNTs nanocomposites by melt mixing, and they found that with the addition of 3 wt % CNTs, the storage flexural modulus increased from 0.64 to 1.2 GPa, and the in-plane conductivity values increased by about 6 orders of magnitude. Chen et al.21 found that the CNTs were successfully modified by N,N0 -dicyclohexylcarbodiimide (DCC) and the modified CNTs-C18 could be well dispersed in the PBS matrix, and as a result, the PBS/CNTs-C18 nanocomposites with enhanced thermal, electrical conductivity, and mechanical r 2011 American Chemical Society

properties could be obtained. Some other properties of the PBS/ CNTs nanocomposites, such as thermal17 and crystallization properties,19 have also been explored preliminarily. It is well-known that rheology is a powerful tool to examine mesoscopic structure of the filled polymer systems because the viscoelastic properties are highly related to the dispersion state of filler and the interactions between filler and polymers.22 Hitherto numerous rheological studies have been reported on various polymer composites with CNTs.23
33 Similar to those observed on the polymer composites containing clay,34,35 a solid-like flow behavior can also be observed on the polymer/CNTs composites, which is believed to be attributed to the formation of a filler network. The rheological behaviors of the PBS/CNTs composites have also been studied preliminarily. Ali and co-workers17 examined the rheology of the PBS/CNTs systems and found a typical nonterminal rheological response. They reported a rheological percolation threshold of about 5 wt % for their systems. However, a lower threshold value of about 2 wt % was further reported by Choi et al.18 This is very interesting. It is believed that the difference in the reported threshold values is mainly attributed to the bulk property differences of the used PBS and CNTs. However, are there any other aspects also influencing the percolation behavior of a polymer/CNTs system? This is worthy of further study. In addition, both the processing and the application also require further information on rheological responses of the PBS/CNTs. Therefore, in this work, the PBS/ CNT composites were prepared via melt mixing for the rheological study. The oscillatory rheology was then used as a probe to explore the rheological percolation behavior and its temperature dependence. The crystallization of the composites was also studied, aimed at relating the macroscopic performance to the Received: September 6, 2011 Accepted: November 3, 2011 Revised: October 18, 2011 Published: November 03, 2011 14186

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mesoscopic and microscopic structures of the CNTs in the PBS matrix.

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Poly(butylene succinate) (Bionnlle#1001) with a melt flow index of 2.5 g/10 min (190 °C, 2.16 kg) was supplied by SHOWA Highpolymer Co. Ltd. (Japan). The purified multiwalled carbon nanotubes (Purity g98%), which were supplied by the Department of Chemical Engineering, Tsinghua University, are the chemical vapor deposition material with average outside diameter of 8.6 nm, inside diameter of 3.5 nm, and length of 100
300 μm, presenting a special surface area of about 250
300 m2/g. PBS/CNTs composites (PBSCNs, where s is the weight ratio of CNTs) were prepared by melt compounding the purified CNTs with the PBS directly in a HAAKE Polylab Rheometer (Thermo Electron, USA) at 150 °C and 50 rpm for 8 min. All of the materials were dried at 80 °C under vacuum for 24 h before use. The CNTs loadings are 0.5, 1, 2, 3, 4, and 5 wt %, respectively. For better comparison, the pure PBS sample was also processed in the rheometer to keep identical thermal histories with those of the PBSCNs. The sheet samples with the thickness of about 1 mm used for the following measurements were prepared by compression molding at 150 °C and 15 MPa. 2.2. Microstructure Characterizations. The dispersion of CNTs was explored using a Tecnai 12 transmission electron microscope (TEM, PHILIPS, Netherlands) with 120 kV accelerating voltage. The microtomed sections are about 80
100 nm in thickness. The crystallographic structure of the neat PBS and PBSCNs samples was determined by a D8 ADVANCE X-ray diffractometer (XRD, BRUKER AXS, Germany) with Cu target and a rotating anode generator operated at 40 kV and 200 mA. The scanning rate was 4° /min from 10° to 80°. The crystallization morphology at the temperature of 96 °C was studied using a polarized optical microscope (POM, LEICA BX51, Germany) equipped with a hot stage (Linklam LTM350, England). 2.3. Thermal Characterizations. The crystallization was carried out on a differential scanning calorimeter (DSC, NETZSCH 204F1, Germany). The samples of about 5 mg in weight for DSC were cut from the film. In the nonisothermal crystallization process, the samples were molten at 150 °C for 5 min to eliminate the previous thermal histories, and then cooled to the room temperature at the rate of 10 °C/min, finally heating up to 150 °C at the rate of 10 °C/min. In the isothermal crystallization process, the samples were cooled rapidly to the preset temperature (98
101 °C) for crystallization after eliminating thermal histories. The exothermal signals of heat flow as a function of temperature were recorded in those processes. 2.4. Rheological Measurements. Rheological measurements were carried out on the rheometer (HAAKE RS600, Thermo Electron USA) equipped with a parallel plate geometry using 20 mm diameter plates. The sheet samples in thickness of 1.0 mm were molten at 150 °C for 5 min in the fixture to eliminate residual thermal histories, and then experienced dynamic strain sweep to determine their linear viscoelastic region. In the dynamic frequency sweep, the small amplitude oscillatory shear (SAOS) was applied at the strain level of 1%. In the temperature sweep, the SAOS responses were recorded at the strain level of 5% and the frequency of 1 Hz.

Figure 1. TEM images of (a) PBSCN1 and (b) PBSCN5 samples with a scale bar of 200 nm.

Figure 2. Dynamic storage modulus (G0 ) for the neat PBS and PBSCNs samples obtained from dynamic frequency sweep at the temperature of 150 °C.

3. RESULTS AND DISCUSSION 3.1. Microstructure of PBSCNs. Figure 1 gives the TEM images of the PBSCNs samples with various CNT loadings. It is clear that at low loading level (Figure 1(a)), CNTs are randomly oriented as the single nanotube or small bundles, showing good dispersion throughout the PBS matrix. At high loading level (Figure 1(b)), however, CNTs are mainly dispersed as big bundles or small aggregates. This indicates that the primary CNT aggregates cannot be fully detached during melt processing because of strong van der Walls interaction among those entangled nanotubes. Therefore, the CNTs show poor dispersion at high loading levels. 3.2. Rheological Percolation Behavior of PBSCNs. Figure 2 shows the dependence of storage modulus (G0 ) on frequency for pure PBS and PBSCNs samples. Similar with those already observed on other polymer/CNTs systems,23
35 the low-frequency G0 of PBSCNs also increases with addition of CNTs significantly. It is seen that the neat PBS sample shows typical terminal behavior at low frequencies with the scaling properties of G0 µ ϕ2, which is in accordance with the Cox
Merz rule.36 However, this terminal behavior disappears gradually with increase of CNT loadings. As the CNT loadings achieves up to 4 wt %, the PBSCNs show evident solid-like response in the lowfrequency region, indicating occurrence of an elastic deformation dominated flow. At this loading level, the particle
particle interactions among the CNTs are strong enough, and as a result 14187

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Figure 4. Dynamic storage modulus (G0 ) for the PBSCNs samples obtained from temperature sweep at the frequency of 1 Hz and strain level of 5%.

Figure 3. Han plots of dynamic storage modulus (G0 ) versus dynamic loss modulus (G00 ) at various temperatures for (a) PBSCN0.5 and (b) PBSCN5 samples.

lead to the formation of percolated CNT networks.26
30 In this case, the large-scale relaxations of PBS chain coins are highly restrained by the presence of CNTs, and hence the composite system shows a solid-like flow behavior at the low-frequency region. Therefore, the rheological percolation threshold of PBSCNs is between 3 and 4 wt %. This threshold value is lower than that reported by Ali and co-workers (5 wt %),17 but higher than that (2 wt %) by Choi et al.18 The dispersion of CNTs is believed to play an important role on the percolation behavior of CNTs.37 As discussed on the TEM observations, the CNTs show poorer dispersion at higher loading levels in comparison to those at lower loading levels. The existence of big bundles or small aggregates of CNTs decreases their volume fraction they should be if they were fully detached.32 Accordingly, the percolated CNT network can merely form at higher loading levels. It should be further pointed out here that the flocculated structure, such as bundles, is not the percolated network one. The percolation network in the dynamic flow is generally defined as a hydrodynamic structure, which is a transient structure caused by particle
particle interactions in the oscillatory process.23,29 This means that the rheological percolation network of CNTs is not a real structure, that is, the particle
particle interactions are strong enough to lead to additional elastic contribution during SAOS flow as the volume fraction of CNTs achieves to a critical value because the distance among the bundles or aggregates of CNTs is smaller than their hydrodynamic radius in this case. Therefore,

Figure 5. Master curves (to a reference temperature of 190 °C) of dynamic storage modulus (G0 ), dynamic loss modulus (G00 ) and complex viscosity (η*) for (a) PBSCN0.5 (b) PBSCN5 samples.

besides the dispersion of CNTs, the aspects influencing the flow, such as the temperature, matrix viscosity, etc., may also affect the particle
particle distance, and finally determine the percolation behavior of CNTs in the PBS matrix. Han38 believed that the temperature independence of G0 ∼ G00 for homogeneous polymer systems could be also applied to the filled polymer composites. Figure 3 gives Han plots of G0 versus G00 for the composite systems. PBSCN0.5 and PBSCN5 systems obtained at various temperatures, respectively. It is clear that the curves at various temperatures can coincide well for the 14188

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Figure 7. POM images of (a) neat PBS and (b) PBSCN1 samples with the scale bar of 100 μm.

Figure 6. The percolation thresholds (jc) at various temperatures for the PBSCNs samples.

unpercolated PBSCN0.5 system, while poor for the percolated PBSCN5 system, especially at low frequencies. This indicates that the evolution of mesoscopic structure of CNTs, such as percolation network, could be dependent on the temperature, which is further confirmed by the observation on the temperature sweep. Figure 4 shows the dependence of low-frequency modulus on temperature for the two composite systems. For the unpercolated PBSCN0.5 sample, the modulus decreases with increasing temperature. In this system, the rheological responses are still controlled by the viscous PBS matrix. Thus, the viscosity and elasticity of the system reduce with increase of temperature because of the enhanced movements of PBS chains. For the PBSCN5 system, however, the modulus increases with increase of temperature evidently. As mentioned above, the oscillatory flow behavior in this system is dominated by the percolated CNT networks. On the one hand, the enhanced Brownian motion of CNTs may lead to the increase of network elasticity. But it has been reported that Brownian motion is not the major force for the reorganization of the nanofiller network,34,35 also including the CNT network structure.29 Therefore, the enhancement of modulus in the percolated system is still attributed to the decrease of the viscosity, because with reduced viscosity (increased temperature), the collision and friction among the CNTs become more and more easily, leading to the increase of network elasticity. The results above suggest that the principle of time
temperature superposition (TTS) may be invalid for those percolated PBSCNs because of the temperature dependence of the percolation structure. Figure 5 gives the master curves of dynamic rheological responses for the composite systems. Clearly, a good superposition at both high and low frequencies can be seen on the unpercolated PBSCN0.5 sample (Figure 5(a)), indicating that the relaxation behaviors of this composite system have an active energy with almost the same order as those of the neat PBS. However, TTS is invalid for the percolated PBSCN5 sample especially at low frequencies (Figure 5(b)), suggesting that the relaxation behaviors of percolation network structure could change with temperature. Similar temperature dependence has also been observed on the PC/CNTs 23,30 and the PBT/CNTs29 systems. Potschke and co-workers23 proposed that the density of the networks changes with temperature because of the mobility and characteristic structure of the

Figure 8. XRD patterns of the neat PBS and PBSCNs samples.

polymer chains. In other words, the shortest distances among CNTs are temperature dependent. This is in accordance with the discussion in Figure 4. Since the relaxation behaviors of the percolated CNT network structure show the temperature dependence, the percolation threshold of the network may also be temperature dependent. Plotting the slopes of low-frequency modulus curves versus the CNT loadings, the turning point can be consider as the percolation thresholds (jc) approximately.32 Figure 6 gives the values of percolation thresholds (jc) at various temperatures for the PBSCNs systems. As expected, the percolation thresholds reduce with increase of temperature, indicating that the percolation network can form at the lower CNT loading levels in the SAOS flow at high temperatures. This is attributed to the reduced particle
particle distance and enhanced interactions among CNTs because of reduced matrix viscosity. 3.3. Crystallization Behavior of PBSCNs. It is well accepted that the mechanical and physical properties of the crystalline polymers are governed by their supermolecular structure, which in turn is controlled by the crystallization process. It has been reported that CNTs have large influence on the crystallization of polymer matrix in the polymer/CNTs composites.39,40 Figure 7 gives the POM images of the neat PBS and its composite samples after isothermal crystallization. A typical Maltese-cross spherulite can be observed on the neat PBS (Figure 7(a)). In the presence of CNTs, the spherulite size reduces evidently (Figure 7(b)), indicating that the CNTs may act as heterogeneous nuclei. Figure 8 gives XRD patterns of the neat PBS and its composites. All samples present four characteristic 2θ peaks (19.6°, 22.6°, 26.1°, and 34.0°), which are assigned to (020), (110), (111), and (121) planes of α crystal of PBS.41 The same locations 14189

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Table 1. Crystallite Dimensions of the Neat PBS and PBSCNs Samples L020 (nm)

L110 (nm)

L121 (nm)

PBS

18.0

10.4

14.2

PBSCN0.5 PBSCN1.5

15.9 14.5

9.8 9.3

11.0 7.9

PBSCN3

16.4

8.9

9.7

samples

Figure 10. Plots of relative crystallinity (X(t)) versus time (t) for (a) neat PBS and (b) PBSCN1 samples.

Figure 9. DSC thermograms of the neat PBS and PBSCNs samples for (a) crystallization and (b) melting processes.

of diffraction peaks indicate that with the presence of CNTs does not alter the crystal structure of PBS. Scherrer equation42 is generally used to estimate the crystallite size basing on the diffraction pattern. The crystallite dimension (Lhlk) can be calculated by the following: Lhlk ¼

Kλ βhlk cos θhlk

ð1Þ

where Lhlk is the crystallite dimension or coherence length perpendicular to the (hkl) plane, K the Scherrer constant, λ the wavelength of the X-rays, θ the Bragg angle, and βhlk the diffraction half-width. The calculated results are listed in Table 1. Clearly, the crystallite size of the (020), (110), and (121) planes of PBSCNs is smaller than that of the neat PBS, confirming the heterogeneous nucleating effect of CNTs. This agrees with the observations reported by Qiu et al.19 Such a nucleating effect leads to an evident high-temperature shift of the crystallization temperature (Tc) of the composite

systems, as can be seen in Figure 9(a). However, it also leads to the formation of more defect ridden crystalline lamella and less ordered crystals of PBS, finally reducing the melting temperature (Tm) of the composites (Figure 9(b)). Besides, all samples present two melting endotherms. Tm1 corresponds to the melt of the crystals formed during nonisothermal melt crystallization, and Tm2 corresponds to the melt of the crystals formed through melting and recrystallization during DSC heating scans.19,43 Clearly, with increase of the CNT loadings, Tm1 shifts to the low-temperature region, indicating that the presence of CNTs reduces the ability of PBS chains to be fully incorporated into growing crystallite lamella and finally leads to formation of the less ordered PBS crystals. However, Tm2 is almost constant. This is because the recrystallization is not of CNT loadings dependence. 3.4. Crystallization Kinetics of PBSCNs. Since the presence of CNTs has large influence on the crystallization of PBS, it is necessary to further explore the crystallization kinetics of PBSCNs. The Avrami equation44 can be used to describe isothermal crystallization: XðtÞ ¼ 1
expð
kt n Þ

ð2Þ

where X(t) is the relative crystallinity at crystallization time, k is the crystallization rate constant, n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystal. The plots of relative crystallinity (X(t)) versus crystallization time (t) are given in Figure 10. It is clear that the PBSCN1 sample shows reduced crystallization time compared with the neat PBS at the 14190

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crystallization time t1/2 decreases with addition of CNTs, confirming the heterogeneous nucleating effect of CNTs. However, compared with that of the neat PBS, the Avrami exponent n values of the PBSCN1 have nearly no evident alteration, and both are around 3. The same observation has also been reported by Qiu et al.19 This suggests that the crystallization mechanism of PBS nearly does not change with the addition of CNTs. It is reported that for the neat PBS, the residual catalysts and impurities may provide nucleation sites in the crystallization process, and hence the crystallization of PBS is a spherulite growth with heterogeneous nucleation.45 In this case, the presence of CNTs merely provides additional nucleation sites, enhancing the nuclei density but not changing the mechanism of crystal growth.

4. CONCLUSIONS PBS/CNT composites (PBSCNs) prepared by melt mixing show a typical solid-like dynamic rheological response at the lowfrequency range because of the percolation of CNTs. But the time
temperature superposition (TTS) is invalid for those percolated PBSCNs because the percolation network is temperature dependent, and the values of percolation threshold reduce with increase of temperature. With addition of CNTs, the spherulite size of PBS reduces evidently and the overall crystallization rate of the composite increases in contrast to those of the neat PBS due to the nucleating effect of CNTs. But the presence of CNTs does not change the mechanisms of nucleation and spherulite growth of the PBS. Figure 11. Plots of
log[ln(1
X(t))] vs. log t for (a) neat PBS and (b) PBSCN1 samples.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Table 2. Kinetic Parameters for the Neat PBS and PBSCNs Samples samples PBS

PBSCN1


1

Tc (°C)

ΔHc (J/g)

n

98

44.09

3.03

0.0120

3.81

99

39.18

2.76

0.0098

4.63

100

42.69

2.98

0.0023

6.96

101

38.80

2.76

0.0018

8.53

98 99

49.02 47.84

2.88 2.86

0.0420 0.0330

2.67 2.90

100

45.51

2.95

0.0091

4.36

101

48.46

3.07

0.0028

6.16

k (min

)

t1/2 (min)

’ REFERENCES

same Tc. This indicates that the presence of CNTs facilitates crystallization kinetics of PBS. The Avrami exponent n and the crystallization rate constant k can then be obtained by plotting log[
ln(1
X)(t))] versus log t. In addition, the half crystallization time, t1/2, characterizing the crystallization rate, can be calculated from the following equation: t1=2 ¼ ðln 2=kÞ1=n

’ ACKNOWLEDGMENT This work was supported by research grants from the National Natural Science Foundation of China (51173156) and the Natural Science Foundation of Jiangsu Province (BK2010040).

ð3Þ

Figure 11 shows the Avrami plots of log[
ln(1
X)(t))] versus log t for the crystallization of the neat PBS and PBSCN1 samples at various temperatures. The as-obtained kinetics parameters of isothermal crystallization are listed in Table 2. It is seen that the crystallization rate constant k increases and the half

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