Electrical, thermal, and mechanical properties of polyarylene ether nitriles/graphite nanosheets nanocomposites prepared by masterbatch route
- First Online:
- Cite this article as:
- Zhan, Y., Lei, Y., Meng, F. et al. J Mater Sci (2011) 46: 824. doi:10.1007/s10853-010-4823-7
- 165 Views
Graphite nanosheets (GN) reinforced polyarylene ether nitriles (PEN) nanocomposites were successfully fabricated through masterbatch route and investigated for morphological, thermal electrical, mechanical, and rheological properties. The SEM images showed that GN were well coated by phthalonitrile prepolymer (PNP) and dispersed in the PEN matrix. Thermal degradation and heat distortion temperature of PEN/GN nanocomposites increased substantially with the increment of GN content up to 10 wt%. Electrical conductivity of the polymer was dramatically enhanced at low loading level of GN; the electrical percolation of was around 5 wt% of GN. The mechanical properties of the nanocomposites were also investigated and showed significant increase with GN loading. For 10 wt% of GN-reinforced PEN composite, the tensile strength increased by about 18%, the tensile modulus increased by about 30%, the flexural strength increased by about 25%, and the flexural modulus increased by 90%. Rheological properties of the PEN/GN nanocomposites also showed a sudden change with the GN loading content; the percolation threshold was in the range of 3–4 wt% of GN.
As a type of engineering thermoplastic resin, polyarylene ether nitriles (PEN) have attracted much interest both from industry and academia because it has excellent properties similar to polyether ether ketone (PEEK). Owning to its rigid molecular structure, PEN exhibits high tensile strength, good radiation resistance, and high thermo-oxidative stability [1–3], which make it very attractive for the composites to use at elevated temperatures and aggressive chemical environments. Although PEN has the good mechanical properties and thermal stability, development of multifunctional PEN composites is still needed to pursue for special area such as aerospace and military. In some cases, a low electrical resistivity of PEN is required for the electrostatic and/or electromagnetic dissipation of advanced applications.
Polymer-based nanocomposites containing strong, durable, and multifunctional nanoparticles such as silicates [4–6], carbon nanotubes [7–10], and graphite sheets [11–16], have been considered as the promising advanced materials, because greatly improved properties of the nanocomposites can be achieved, compared with conventional composite counterparts, when nanoparticles are homogeneously dispersed in a polymeric matrix. In recent years, many studies on PEN-based nanocomposites have been performed to improve the properties of PEN by incorporating nanoparticles such as carbon nanotubes [17, 18]. However, research on PEN-based nanocomposites containing graphite sheets or graphite nanoplatelets has been carried out limitedly.
Graphite sheets exhibit unique structural features and physical properties. It has been known that graphite sheets have excellent mechanical strength (Young’s modulus of 1060 GPa), electrical conductivity of 104 S/cm, and thermal stability. These properties observed at nanoscale have motivated many researchers to utilize graphite nanosheets (GN) as a new reinforcement in polymer composites [19–22]. Previous researches showed that GN are ideal filler for preparing polymer composites and promising application in polymer multifunctional composites. On these results, the development of GN as reinforcement for PEN matrix is expected to expand various applications with high performance in thermal, mechanical, and electrical properties. However, GN, whose thickness is in nanoscale, tend to accumulate when they are blended with polymer resins directly, and thus it is difficult to achieve good distribution in polymer matrices. In order to obtain high performance for PEN/GN nanocomposites with even low nanofiller content, it is desirable to modify the surface of the GN to enhance the dispersion of GN and PEN/GN′ interfacial adhesion. At present, main approaches for modifying the inorganic fillers are chemical covalent, plasma treatment, and electro-chemical, etc. It is well-known that masterbatch technique is also a simple and effective method to modify the inorganic fillers and presents potential application in thermoplastic industry.
In our lab, phthalonitrile prepolymer (PNP), which had the high initial decomposition temperature (>450 °C) and the low initial melt viscosity at 330 °C (<0.1 Pa·S) [23, 24], was successfully used as a kind of plasticizers to overcome the temperature resistance problem, and the blending materials maintained the high thermal and thermo-oxidative stability of PEN. Previous research also showed that PEN had favorable compatibility with PNP . These excellent properties of PNP make it good candidate for modification of GN by masterbatch technique.
In this study, we first prepared the disorder GN via the intercalation of natural graphite followed by rapid exfoliation in a microwave environment. The as-prepared GN were then coated by PNP in order to prepare modified GN, called GN masterbatches, which contained different weight ratios of GN. The masterbatches were then blended with PEN via the extrusion process, resulting in good dispersion of GN throughout PEN matrix. Morphology, structures, thermal stability, mechanical, electrical, and rheological properties of PEN/GN nanocomposites were systematically investigated. The as-prepared PEN/GN nanocomposites with multifunctional properties are believe to have potential application in automotive area, aerospace, and other places where solvent resistance and/or exposure to high temperature is necessary.
Natural graphite was purchased from Qingdao Yanxin Graphite Co. Ltd., China. The mean diameter is about 500 μm. Diphenyl diamine sulfoxide (DDS) was purchased from Yangzhou Tianchen Meticulous Chemical Co. Ltd., China. All the chemicals and reagents were used without further purification. Phthalonitrile monomer was synthesized in our laboratory, the synthetic procedure and raw materials were reported previously . PEN were provided by Union Laboratory of Special Polymers of UESTC-FEIYA, Chengdu, China. It is a copolymer derived from 2,6-dichlorobenzonitrile with hydroquinone and resorcin with the inherent viscosity of 1.22 dL/g (0.005 g/mL in N-methylpyrrolidone).
Preparation of GN
According to Ref. , GN were prepared by intercalation of natural graphite followed by rapid exfoliation in a microwave environment. The graphite rapidly heats as a result of coupling with the microwave radiation and the entrapped intercalates vaporize. The exfoliated graphite particles undergo significant expansion (500×) forming a worm-like structure. This worm-like structure is then mechanically grounded to form the individual GN.
Preparation of PNP modified GN
Preparation of PEN/GN nanocomposites
A series of nanocomposites consisted of PEN/PNP/GN (92/8/x wt%; x = 0, 1, 3, 5, 8, 10) were melt-mixed in a TSSJ-2S co-rotating twin-screw extruder. The temperatures were maintained at 310, 320, 330, 330, 330, and 325 °C from the hopper to the die and the screw speed was about 120 rpm. Extruded strands of the molten blends were then pelletized and dried in vacuum oven at 120 °C for 24 h, followed by injection molding to prepare standard bars for mechanical tests with an injection and molding machine at 340 °C.
It should be clarified that a PEN/PNP (92/8 wt%) blend was used as the basal polymer (regarded as the neat PEN) for two reasons: (1) the good compatibility between PEN and PNP guarantees no obvious phase separation; (2) the estimation of the GN effect on properties and morphology will be drawn exactly, based on the basal polymer with the same composition.
The morphology of the GN and fracture surfaces of the nanocomposites were observed with scanning electron microscope (JEOL JSM-5900LV) and transmission electron microscopy (Hitach H600). The SEM samples were coated with a thin layer of gold prior to examinations.
Glass transition temperature (Tg) and melting transition were measured on TA instrument DSC Q100, at a heating rate of 10 °C/min. TGA analysis of the composite was carried out under N2 atmosphere at a heating rate of 10 °C/min using TA Q50 series analyzer system combination with data processing station. Heat distortion temperature (HDT) of the samples was measured according to ASTM D648-2007 at ZWK computer controlled HDT vikar tester (Shenzhen New SANS Materials Testing Machine Co., China).
The tensile and flexural tests of the composites were performed with a SANS CMT6104 Series Desktop Electromechanical Universal Testing Machine at room temperature, with a crosshead rate of 5 mm/min for tensile tests and 2 mm/min for flexural tests, respectively. The final results were the average values of six replicate measurements.
Dynamical rheological measurements were carried out on a rheometer (TA Instruments Rheometer AR-G2) equipped with a parallel-plate geometry (25 mm diameter). Disk samples were prepared by compression molding with a thickness of 1.0 mm and diameter of 25 mm. Storage modulus (G′) as a function of angular frequency (ω) range from 0.01 to 100 rad/s at 340 °C were measured. A fixed strain of 1% was used to ensure that measurements were carried out within the linear viscoelastic range of the materials investigated.
Results and discussion
Structure of GN and GN masterbatch
The thermal properties of PEN and PEN/GN nanocomposites
GN content (wt%)
Figure 4b shows the HDT for PEN/GN composites with various GN contents. Compared with the neat PEN, the HDT of the PEN/GN nanocomposite with 10 wt% GN increased by about 10 °C, indicating that the formation maintenance capability of PEN/GN composite was enhanced compared to the neat PEN. It also can be seen from Table 1 that glass transition temperature (Tg) increases slightly with the increasing GN content in the PEN/GN nanocomposites. However, compared with the neat PEN matrix, melting temperature is not influenced by the addition of GN.
Electrical properties of PEN/GN nanocomposites
The percolation threshold for the resistivity depends very much on the dispersion and geometry of the conducting fillers . We believed the low graphite content of electrical percolation threshold for PEN/GN nanocomposites is associated with the result that the GN were dispersed homogeneously in the PEN matrix by wrapping of PNP on GN, in agreement with the SEM observations. On the other hand, fillers with elongated geometry such as sheets can be used to achieve a very low percolation threshold value, due to the fact that sheets with higher aspect ratios have great advantage over spherical or elliptical fillers in forming conducting networks in polymer matrix . The resistivity slowly decreased when the GN content was above 5 wt%. This is because once the conductive network formed; a further increase of the GN loading had no obvious influence on the resistivity of the composite. It is noticeable that the electrical resistivity of 106 Ω cm of PEN/GN nanocomposites is low enough to attain the electrostatic dissipation and/or partial electromagnetic dissipation for thermoplastics and fibers.
According to linear viscoelastic theory, the dynamic storage modulus G′ for homogenous polymer system is proportional to ω2 at low frequencies (terminal zone). However, for heterogeneous polymer systems, G′ is no longer proportional to ω2. Such deviation from the linear viscoelastic properties is significant, especially for particulate-filled polymer system.
GN reinforced PEN nanocomposites with significant improved electrical, thermal, and mechanical properties were successfully fabricated via masterbatch route with a twin-screw extruder. The morphological characterization confirmed that GN were distributed uniformly, indicating a good dispersion of nanosheets in the PEN matrix by coating PNP on the GN surface. The incorporation of GN into PEN showed the significant improvement in thermal stability and HDT of nanocomposites. Electrical conductivity of the polymer was dramatically enhanced at low loading level of GN; the electrical percolation of was around 5 wt% of GN. The addition of GN into the PEN also had great impact on the mechanical properties of the nanocomposites. For 10 wt% of GN-reinforced PEN composite, the tensile strength increased by about 18%, the tensile modulus increased by about 30%, the flexural strength increased by about 25%, and the flexural modulus increased by 90%. The improvements of mechanical properties are due to the good adhesion between GN and PEN matrix and a certain degree of load transfer from resin to the GN. Rheological properties of the PEN/GN nanocomposites also showed a sudden change with the GN loading content; the percolation threshold was in the range of 3–4 wt% of GN.
Furthermore, these results reveal that using PNP/GN masterbatch to manufacture plastic parts is a very promising route. Such a route allows keeping the functional benefits of well-dispersed GN, whereas limiting the handling difficulties in plastics processing industrial workshops.
We gratefully acknowledge the financial support of National Natural Science Foundation of China (20801009) and Special program of National Innovation (2008IM021800).