Crystallization, rheological behavior and mechanical properties of carbon nanotube/metallocene polypropylene composites

In this paper, metallocene polypropylene (mPP) composites filled with carbon nanotubes (CNTs) were prepared using twin-screw extruder. The crystallization behavior, mechanical properties and rheological behavior were characterized by a differential scanning calorimetry (DSC), universal material testing machine and rotational rheometer. The results of DSC indicated that the effect of CNTs on heterogeneous nucleation of mPP was very obvious and the crystallizability of the resin matrix was improved after adding CNTs, especially the initial crystallization temperature (T 0), crystallization temperature (T c) increased by 9.63 °C and 8.28 °C when the CNTs content was 1.25 wt%. The yield stress and elastic modulus increased to 33.98 MPa and 1605.6 MPa as the CNTs concentration increased to 1.0 wt% in contrast to that of the neat mPP. The results of SEM images showed that the better dispersion and adhesion of CNTs into polymer matrix. The results of rotational rheometer proved that interactions increased between CNTs and mPP as the content of CNTs increasing.


Introduction
Utilization of polypropylene in many engineering applications had increased due to their low density [1], easy processability [2], low cost, and reasonable mechanical strength, especially in the automotive industry [3][4][5]. It was mainly used in automobile interior parts, which required the resin matrix having less smell [6,7]. Usually, high molecular weight isotactic polypropylene (iPP) was synthesized using Ziegler-Natta catalyst, and then high flow polypropylene was obtained by peroxide degradation, which had the pungent smell [8]. However, high flow metallocene based iPP overcome this disadvantage. It could be synthesized by hydrogen regulation method, which had low molecular weight, high fluidity and narrower molecular weight distributions [9,10]. Due to technology and equipment, Z-N based iPP occupied a large market share. Thus, most research mainly focused on of Ziegler-Natta based iPP [11][12][13]. What's more, there was less research on metallocene based iPP.
Nanotechnology has been considered as one of the key technologies in the technical field. In the past years, a lot of research has been focused in the field of composites due to their potential as materials with novel properties. CNTs as nano particles were used in polymer composites because of their excellent thermal, mechanical properties and large surface area [14][15][16][17][18]. The large surface area of the CNTs lead to the formation of an interphase between the CNTs and polymer matrix, which was helpful to load transfer and was an important feature for the effective enhancement of composite properties [19].
Based on the literatures, the CNTs/mPP composites could be prepared by three different methods: melting blending, solution blending and in situ polymerization [20]. It was worth noting that melt blending was the most widely used by researchers. Because this method was simpler, lower in cost and had better commercial scale-up feasibility.
Narimani et al [3] had investigated the steady shear rheological behavior, physical and mechanical properties and crystallization behavior of single walled carbon nanotube (SWNT)/PP composites. The results of the rheological behaviors showed pure matrix and its composites displayed non-Newtonian behavior and introduction of SWNT increased the shear viscosity. The results of DSC indicated that SWNT playing the role of heterogeneous nucleation in matrix.
Jayvardhan et al [21] had prepared multiwall carbon nanotube (MWCNTs) reinforced polypropylene composites with 0.4 wt%, 0.8 wt%, 1.2 wt% and 1.5 wt% of MWCNTs by a compression molding machine and studied mechanical and thermal behaviors. The results showed the tensile strength reached the maximum when the MWCNTs content was 1.2 wt% and impact strength was improved by 82.14% when the MWCNTs content was 1.5 wt%. The SEM images of fractured MWCNTs/PP composites indicated that MWCNTs had dispersed uniform in matrix reasons for properties enhancement.
In previous papers, a study on crystallization and melting behaviors and mechanical properties of continuous carbon fiber reinforced CNTs/mPP composite prepared by a melt impregnate method were presented. However, the effect of CNTs on properties of composite matrix resin had not been studied [22].
In this study, the CNTs/mPP composites were prepared by twin-screw extruder. The crystallization behavior, mechanical properties and rheological behavior of the composites were researched, which purpose was to explore the mechanism of the effect of CNTs content on properties of mPP matrix.

Materials
The iPP (mPP powder) using metallocene catalyst was synthesized by the Advanced Synthetic Materials Research Institute of Shandong Qinghe Chemical Technology Co., Ltd., China. Its molecular weight was 21.4×10 4 , molecular weight distribution was 2.82. CNTs were produced by Shandong Exhibition Nanomaterials Co. Ltd., China, the grade was GT-210. The types of antioxidants were 1098 and 168 produced by Suzhou Liangcai Chemical Co. Ltd., China, respectively.

Preparation of composites 2.2.1. mPP pellets
According to the scale shown in table 1, mPP powder and antioxidants (1010 and 168) were mixed dryly by a plastics high-speed blender for 5 min. The plastics high-speed blender was manufactured by Dongguan Huanxin Machinery Co., Ltd., China. And then these powder blends were melted and plasticized through a twin-screw extruder, granulated cut by pelletizer after cooling in water to obtain mPP pellets. The extruder produced by Harbin Hapu Electric Technology Co., Ltd., China. The process temperature of mPP was set at 130°C , 200°C, 220°C, 220°C, 210°C, 210°C, 200°C. The melt mass flow rate (MFR) of matrix resin were shown in table 2.

CNTs/mPP pellets
According to the MFR in table 1, the different content of CNTs (0.25, 0.5, 0.75, 1.0, 1.25 wt%) and the antioxidants 1010 and 168 were added in the mPP. The composites pellets (CMP 1 # , CMP 2 # , CMP 3 # , CMP 4 # , CMP 5 # ) with mPP, CNTs and antioxidants was obtained by the same process as CMP 0 # , as illustrated in figure 1 and table 1. The mixing and dispersion of carbon nanotubes in mPP were divided into two processes. Firstly, mPP powder, CNTs and antioxidants were mixed dryly by a plastics high-speed blender for 5 min. Highspeed mixing enhanced the dispersion of mPP powder, CNTs and antioxidants. And then these powder blends were melted and plasticized through a twin-screw extruder. High screw speed enhanced the dispersion of nanocomposites by imposing high shear during processing [23]. The MFR of matrix resin were shown in table 2. It indicates that the flow characteristics of the composites were decreased than the pure mPP.

Differential scanning calorimetry
The crystallization and melting behaviors of mPP and CNTs/mPP composites were tested with the differential scanning calorimeter (model DSC 3500 Sirius) from Netzsch. The weight of each test sample was 6-7 mg. The samples were first heated from room temperature to 200°C at a rate of 20°C min −1 kept at that temperature for 3 min in nitrogen atmosphere to remove the thermal history. Then, the temperature was cooled to room temperature at a rate of 10°C min −1 and kept the temperature for 3 min, which obtaining the crystallization curves. Finally, the melting curves of the materials were obtained by heating to 200°C again at a rate of 10°C min −1 .

Crystallization morphology
The crystalline morphology of the mPP and CNTs/mPP composites was observed using an optical polarizing microscope (UP200i) equipped with an automatic hot-stage thermal control. Samples were melted completely and pressed softly between the coverslip at 200°C for 5 min and then cooled to the isothermal crystallization temperature of 125°C at a rate of 10°C min −1 .

Tensile tests
The tensile property was measured using universal material testing machine (model TSE503B) was produced by Shenzhen Wance Test Equipment Co., Ltd., according to GB/T 1040. . And the tensile speed was 50 mm min −1 . The tensile strength of every sample was obtained by the average value of 5 effective data.

Microscopy analysis
The surface morphology of CNTs/mPP composites by coating with Au/Pt overlayer was observed using Quanta 250 scanning electron microscope manufactured by FEI Company of the United States.

Rheological behavior
The rheological analyses were carried out using rheometer Kinexus Lab+with parallel plate geometry (diameter of 25 mm and a gap of 1.0 mm) from Netzsch. The samples were tested at 180°C under nitrogen atmosphere to avoid thermal degradation. The frequency range was 10 −3 to 10 2 Hz and the strain was 0.5%.

Results and discussion
3.1. Crystallization and melting behaviors Figures 2(a) and (b) represent crystallization and melting curves of mPP and CNTs/mPP composites, respectively. While the non-isothermal crystallization parameters extracted from DSC curves, such as initial crystallization temperature (T 0 ), crystallization temperature (T c ) and melting temperature (T m1 and T m2 ), melting enthalpy (ΔH m ) and degree of crystallinity (X c ), were summarized in table 1. The X c could be obtained by the following equation (1): here the parameter !H m was melting enthalpy, !H 0 was the enthalpy of fusion of 100% crystalline polypropylene, which was 209 J g −1 .
It could be observed that the T c and T 0 of CMP 0 # were only 110.28°C and 116.00°C, respectively table 3. While the T c and T 0 of CNTs/mPP composites were increased clearly after adding CNTs to mPP. And the T c and T 0 of CMP 1 # and CMP 5 # increased by 119.91°C, 124.28°C from 116.82°C, 121.32°C with the increase of the content of CNTs from 0.25 wt% to 1.25 wt%, which indicated the crystallizability of the resin matrix was improved after adding carbon nanotubes, respectively. It could be attributed to CNTs playing the role of heterogeneous nucleation in mPP, resulting in the molecular chain of mPP induced to crystallize earlier [24]. In addition, the X c of CNTs/mPP composites was increased from 50.38% for CMP 0 # to 52.26% for CMP 5 # with 1.25 wt% CNTs content. This could be due to the effect of CNTs on the nucleation process of crystallization. Thus, the CNTs act as a nucleating agent to initiate crystallization, leading to forming more crystal nucleus around the CNTs.
As shown in figure 2(b), introduction of CNTs had a dramatic impact on the melting behavior of mPP. Clearly, the T m2 of CMP 0 # was 151.38°C. After adding CNTs to mPP, the T m2 of CNTs/mPP composites were increased significantly, which increased with the increase of CNTs content. This was because the added CNTs played the role of heterogeneous nucleation in mPP. It was interesting to note that the melting polts of CMP 0 # exhibited single melting peak, the shape of the melting transition progressively evolves toward double melting behavior with increasing CNTs content. And The T m1 of CNTs/mPP composites was 139.88°C-142.51°C. It can be attributed to CNTs playing the role of heterogeneous nucleation in mPP and limiting the movement of molecular chain after the introduction of the CNTs, leading to the size of the spherulites of mPP becoming smaller and the crystallization becomes unperfect. Figure 3 displayed the microphotographs of the crystalline morphology of CMP 0 # and CMP 1 # during isothermal crystallization at 125°C for 15 min and 5 min, respectively. As can be seen, it showed larger spherulites in CMP 0 # . When the content of CNTs was higher than 0.25 wt%, it was difficult to observe the size and distribution of spherulites (as shown in figure 3(b)). However, nucleation density in the former was also  Table 3. Non-isothermal crystallization parameters of mPP and CNTs/mPP composites.

Samples
T obviously lower than that in the latter. It was demonstrated that CNTs playing the role of heterogeneous nucleation in mPP, which corresponded to the result of DSC.

Tensile properties
The better dispersion and adhesion of CNTs into polymer matrix further enhanced the mechanical properties of the mPP. In general, the stiffness (yield stress, elastic modulus) of the composites increases with increasing CNTs content. The yield stress and elastic modulus of mPP and CNTs/mPP composites were reported in figure 4. As can be seen, the yield stress and elastic modulus of CMP 0 # was 30.91 MPa and 1329 MPa, respectively. And the yield stress and elastic modulus of CNTs/mPP composites inceratured clearly after introduction of CNTs comparing with CMP 0 # . Adding CNTs increased the yield stress and elastic modulus from 30.91 MPa and 1329 MPa to 33.98 MPa, 1605.6 MPa as the CNTs concentration increased to 1.0 wt% (i.e. by 9.93% and 20.81% compared to CMP 0 # ). At lower CNTs content, partial tensile strain could be transferred to CNTs embedded in PP matrix under tensile stress due to the finer dispersion and adhesion of CNTs and mPP, which leads to the increase of tensile strength. However, the yield stress and elastic modulus of CMP 5 # decreased to 32.69 MPa, 1551.1 MPa when CNTs content was 1.25 wt%. It was known that the aggregation of CNTs will significantly affect the mechanical properties of the composites. With further addition of CNTs, the aggregation of CNTs formed in mPP matrix and the internal defects of were introduced into the matrix owing to the difficulty of homogeneously dispersing CNTs by melt mixing, which leading to the decrease of yield stress and elastic modulus.

Morphology
To further confirm the dispersion of CNTs in the mPP matrix, the fracture morphology of fabricated samples of CNTs/mPP composites with different CNTs content was characterized by SEM. According to observations by  . When the content of CNTs was higher than 0.75 wt%, it could be seen that most of the CNTs sarefully dispersed, more or less with a small number of aggregates or bundles, which were observed as small white spots (see figures 5(c), (e), (g)). And the amount of aggregates or bundles increased with increasing of CNTs content. However, CNTs were randomly oriented, locally forming interconnecting structure. It was interesting to note that the surfaces of CNTs were wrapped with a little matrix when the aggregates or bundles were enlarged (see figures 5(d), (f), (h)). The better interfacial adhesion between CNTs and mPP matrix was one of the important reasons for the higher stiffness of the composites.

Rheological behavior
Since the linear viscoelastic behavior was very sensitive to the presence of CNTs, the dispersion state of CNTs might be examined by the viscoelasticity [25]. In general, rheological characteristic could reflect the internal structure of polymers or polymeric composites. The variation of the shear viscosity versus frequency for the mPP and CNTs/mPP composites were shown in figure 6. When the content of CNTs was lower than 0.75 wt%, CNTs/mPP composites had similar frequency dependencies as the mPP, which appearing the Newtonian plateau under lower frequencies. The Newtonian plateau was disappeared when the content of CNTs was higher than 0.75 wt%. The dependence of the shear viscosity on frequency exhibited the phenomenon of shearthinning. The shear viscosity of CNTs/mPP composites increases with a content of the CNTs loading ratio from 0 to 1.25 wt%. Similar to the results showed in table 1. Under lower CNTs level, the shear viscosity increased unobviously. However, the shear viscosity increased obviously and changed abruptly when CNTs content exceed 0.75 wt%. This was because the form of physical interaction between the CNTs and mPP matrix, which restricted the matrix chains mobility, thereby increasing the viscosity. The increase in shear viscosity with CNTs/mPP composites was primarily caused by the increase in the storage modulus (G′), as can be seen in figure 7(a). The corresponding increase in the loss modulus (G″) was much lower as seen in figure 7(b). The G′ and G″ increased with frequency increasing. However, the effect of nanotube content was much higher at G′ than G″. The G′ characterized the elastic deformation of matrix and composites. When the test frequency was higher than 1 Hz, composites had similar rheological behavior to matrix. Under lower frequencies, the G′ was sensitive to the structure of materials, which increased with introduction of CNTs. And the G′ of CMP 4 # and CMP 5 # containing 1.0 and 1.25 wt% were orders of three  magnitude than pure CMP 0 # . These were mainly attributed to the movement of the mPP segment blocked addition of CNTs, which was helpful to enhance the resistance to elastic deformation when the matrix when subjected to shear stress. The G″ represented the viscous deformation of materials, which increased with the content of CNTs, because of the addition of the CNTs enhancing the resistance of the activity of the melt molecular chains and the slip between the adjacent melt layers. During the movement of molecular chains, the motion resistance increased due to internal friction, resulting in more dissipated energy.
The theoretical basis of Cole-Cole curve only accorded with the single system. The Cole-Cole curve can be expressed by the following equation (2): Where the parameter G 0 was constant modulus, ω was angular frequency, η′ was η″ represented the real part and imaginary part of complex viscosity, respectively, which could be obtained by the following equations (3) and (4) [26,27]: For multicomponent systems, the degree of deviation from Cole-Cole curve could be used to confirm the description of viscoelasticity of the materials with a relaxation time distribution such as heterogeneous polymeric systems. Figure 8 displayed the Cole-Cole curves for mPP and CNTs/mPP composites. As shown in figure 8, the differences of these samples were very clear. The Cole-Cole plots of CMP 0 # was close to a semicircle arc which represented relaxation process with a relaxation time distribution [28]. For the CMP 1 # and CMP 2 # , the Cole-Cole curves were also close to a semicircle, and the higher the content of CNTs was, the  bigger the radius was. The result revealed that CNTs and resin matrix formed homogeneous system, which confirmed CNTs dispersed homogeneously into mPP matrix. However, when the content of CNTs reached 0.75 wt%, the Cole-Cole plots of CMP 3 # -CMP 5 # was divided into two parts: a semicircle arc at lower viscosities corresponding to the local dynamic of mPP, and evident upturning at higher viscosity, which was indicative of the long-term relaxation of those restrained mPP chains [25]. With addition of the CNTs, the semicircle arc of CMP 4 # -CMP 5 # nearly disappeared, which indicating that the long-term relaxation of those restrained mPP chains became the dominant one in the whole relaxation behaviors of the composites. This result was related to the formation of CNTs network-like structures and increasing interactions between CNTs and mPP as the content of CNTs increasing.
To further investigate the effect of the content of CNTs on the rheological characteristic, the G′ was plotted against G″ (Han plot) for mPP and CNTs/mPP composites as shown in figure 9. Obviously, the slope of CMP 1 # was close to CMP 0 # indicating that the CNTs/mPP composites was a homogeneous system. In addition, the introduction of CNTs results in a significant decrease of slope for the CNTs/mPP composites when CNTs content exceed 0.5 wt%, which indicated that the composites become heterogeneous [29]. The lower the slope of the curve, the more heterogeneous of the composites. This result of CNTs dispersion was consistent with that shown on the fracture morphology of CNTs/mPP composites with different CNTs content was characterized by SEM. However, the inflection point where the slope was moved to a higher frequency with introduction of CNTs content. This indicated that the strong interactions between CNTs and mPP matrix owing to the formation of CNTs network-like structures extended relaxation behaviors of the mPP chain inevitably.

Conclusion
In this work, the effects of CNTs content on the crystallization, mechanical properties and rheological behavior of mPP and CNTs/mPP composites were examined. The T c , T 0 and T m of CNTs/mPP composites were increased clearly after adding CNTs to mPP, especially the T c and T 0 increased by 9.63°C and 8.28°C introduction of CNTs content of 1.25 wt%. It was difficult to observe the size and distribution of spherulites in the microphotographs of the crystalline morphology of CNTs/mPP composites with the content of CNTs exceeding 0.25 wt%. It showed that the effect of CNTs on 'heterogeneous nucleation' of mPP was very obvious. The yield stress and elastic modulus increased first and then decreased with the increase of CNTs content. The yield stress and elastic modulus increased by 9.93% and 20.81% as the CNTs concentration increased to 1.0 wt. % in contrast to that of the neat mPP. The better dispersion and adhesion of CNTs into polymer matrix, and the form of physical interaction between the CNTs and mPP matrix, which enhanced further the mechanical properties of the mPP. It was also found that the surfaces of CNTs were wrapped with a little matrix and the better interfacial adhesion in the fracture morphology was characterized by SEM. Rheological measurements showed the formation of CNTs network-like structures and increasing interactions between CNTs and mPP as the content of CNTs increasing.