- Email: [email protected]
carbon nanotube nanocomposite films with a segregated structure
Composites: Part A 91 (2016) 77–84
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Thermal and electrical properties of poly(phenylene sulfide)/carbon nanotube nanocomposite films with a segregated structure Tae Jong Yoo, Eun-Byeol Hwang, Young Gyu Jeong ⇑ Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 May 2016 Received in revised form 28 September 2016 Accepted 30 September 2016 Available online 30 September 2016 Keywords: A. Carbon nanotubes A. Nanocomposites B. Electrical properties B. Thermal properties
a b s t r a c t Structurally segregated poly(phenylene sulfide) (PPS)-based nanocomposite films containing multiwalled carbon nanotube (MWCNT) of 0.5–10.0 wt% were manufactured by solid-mixing and following melt-compression. The cross-sectional optical and electron microscopic images of the nanocomposite films revealed that MWCNTs form a segregated and percolated conductive network structure in the PPS matrix. DSC and TGA data demonstrated that melt-crystallization temperatures and thermal degradation temperatures of the nanocomposite films are slightly increased with the MWCNT content, which are owing to the nucleating agent and thermal protection effects of MWCNTs, respectively. The electrical conductivity of the nanocomposite films increased significantly from 1010 to 0.11 S/cm with the increment of the MWCNT content from 0.0 to 10.0 wt%, especially at a low percolation threshold of 0.33 wt% MWCNT. Accordingly, PPS-based nanocomposite films with 1.0–10.0 wt% MWCNT exhibited high performance in electrical resistive heating behavior under applied voltages of 5–100 V by achieving steady-state maximum temperatures of 30–190 °C within a relatively short period of time of 10 s. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Poly(phenylene sulfide) (PPS) consisting of aromatic rings linked with sulfides is one of leading engineering thermoplastic polymers. Owing to its excellent heat resistance, chemical resistance, fatigue resistance, dimensional stability, thermal stability, flame retardancy, electrical insulation, wear resistance, and processability, PPS has been used in a variety of fibers, specialty membranes, electronic products and industries [1,2]. PPS, which is otherwise electrically insulting, is also considered as the precursor to a conducting polymer of the semi-flexible rod polymer family, because it can be converted to the semiconducting form by oxidation or use of dopants [3]. To expand the applications of PPS by improving the physical properties, carbon nanotube (CNT), graphene and their derivatives have been chosen as reinforcing nanofillers for PPS [4–6]. Especially, CNTs have been extensively selected as one-dimensional reinforcing nanofillers for advanced polymeric nanocomposites due to their excellent electrical conductivity of 104–105 S/cm, thermal conductivity of 3000–6600 W/m K, carrier mobility of 1000–4000 cm2/V s, Young’s modulus of 0.27–1.25 TPa, and thermal stability up to 700 °C in air [7–9]. Diez-Pascual and ⇑ Corresponding author. E-mail address: [email protected] (Y.G. Jeong). http://dx.doi.org/10.1016/j.compositesa.2016.09.022 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved.
Naffakh investigated that PPS-based nanocomposites incorporating a polymer derivative covalently grafted single-walled CNT (SWCNT) have highly improved thermal and thermomechanical properties in aspects of high thermal stability, flame retardancy, glass transition/ heat deflection temperatures, tensile strength/modulus, and low thermal expansion coefficient, which are ascribed to strong matrixfiller interfacial interactions combined with a compatibilization effect [4]. Gao et al. reported that hot-stretched PPS composite fibers reinforced with multi-walled CNT (MWCNT) have remarkably enhanced tensile strength and modulus owing to the presence of the strong interfacial adhesion via p-p interaction between the benzene ring of PPS and the surface of MWCNTs [5]. In most of the cases of thermoplastic polymer nanocomposites including MWCNT, melt mixing is the preferred composite compounding method [10], since aggregate formation can be minimized by suitable application of shear during the melt mixing [11]. Melt mixing for PPS/MWCNT nanocomposites is more simple and handy than other processing techniques, and can achieve the production on a large scale in the PPS industries [12]. Many researchers also reported the degree of MWCNT dispersion in the PPS matrix below and above the electrical percolation threshold [13–16]. In general, polymer nanocomposites with random conductive networks require high conductive filler loadings at the insulator/conductor transition. To date, forming a controlled segregated structure in a conductive polymer composite has remained
78
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
the most promising strategy for attaining low electrical percolation threshold. Accordingly, the morphological control of conductive network in polymer composites and their use as electroactive multifunction materials have been also investigated [17,18]. In the polymer composite materials with a segregated structure, the conductive nanofillers are primarily located at the interfaces between the polymeric matrix particles instead of being randomly arranged throughout the entire polymer composite system [19–23]. In this study, we fabricated a series of PPS/MWCNT nanocomposite films with a segregated structure via an efficient solidmixing and following melt-compression, and investigated their morphological features, thermal and electrical properties as a function of MWCNT content (0.5–10.0 wt%). The segregated but percolated structure of MWCNTs in the PPS matrix was identified by using optical and electronic microscopes. The electrical percolation threshold of the nanocomposite films was analyzed by using a power law relation. The electrical resistive heating performance of the PPS nanocomposite films with different MWCNT contents was characterized in terms of temperature responsiveness, steady-steady maximum temperature, and electric power efficiency under different applied voltages. 2. Experimental 2.1. Materials PPS chips (Fortron 0309, Ticona, Celanese) with a density of 1.350 g/cm3 was selected as the polymeric matrix to fabricate the nanocomposites films. Pristine MWCNT (CM-95, Hanwha Chemical) with 10–15 nm diameter and 10–20 lm length, which was manufactured by thermal chemical vapor deposition (CVD), was used as a reinforcing nanofiller for PPS-based nanocomposite films. 2.2. Preparation of PPS/MWCNT nanocomposite films A series of PPS/MWCNT nanocomposite films were fabricated by a solid-mixing and following melt-compression process. First, PPS chips were pulverized into powders with a particle size of 259.6 ± 96.7 lm. Second, predetermined amounts of PPS powders were mixed mechanically with pristine MWCNT for 3 min. In the solid mixtures, the pristine MWCNT content was controlled to be 0.5, 1.0, 3.0, 5.0, 7.0, and 10.0 wt%. Third, each solid mixture was melt-compressed in a hot press at 300 °C and 20 MPa for 10 min and then cooled to room temperature. The thickness of the nanocomposite films during the melt-compression was adjusted to be 200 lm. Finally, the nanocomposite films were dried in a vacuum oven at 80 °C. The final nanocomposite films were named as PPS/MWCNT-x, where x denotes the content of pristine MWCNT by weight percent (wt%). 2.3. Characterization The particle size and its distribution of PPS powders was analyzed by using a laser scattering particle size analyzer (HELOS/ KR, Sympatec GmbH). The dispersion state of the pristine MWCNT in the PPS matrix was identified by examining the cross-section morphology of the nanocomposite films with aids of a scanning electron microscope (SEM, S-4800, HITACHI), a transmission electron microscope (TEM, Tecnai G2 F30, FEI) and an optical microscope (OM, S38, Bimience). For the TEM and OM experiments, thin specimens with 50–500 nm thickness were prepared by using a ultramicrotome (Leica EM UC6). The thermal transition properties of the nanocomposite films were investigated by using a differential scanning calorimeter (DSC, Mettler-Toledo). About 5 mg samples were heated from 0 to 300 °C at a heating rate of 10 °C/min under nitrogen atmosphere. The thermal stability of
neat PPS and its nanocomposite films was investigated by using a thermogravimetric analyzer (TGA, Mettler-Toledo). About 4– 5 mg samples were heated from 25 to 900 °C at a heating rate of 20 °C/min under nitrogen atmosphere. The electrical properties of the nanocomposite films with a variety of MWCNT contents were characterized by obtaining voltage-dependent current density and electric power density curves with an electrometer (2400, Keithley). The electrical resistive heating performance of the nanocomposite films at different applied voltages of 1–100 V was characterized with an infrared camera (SE/A325, FLIR system Inc.) and an electrometer (2400, Keithley). For the electrical property measurement and the electrical resistive heating experiment, the sample distance between electrical test probes was fixed to be 10.0 mm for all the nanocomposite films with 10.0 mm width and 30.0 mm length. 3. Results and discussion 3.1. Structural characterization To identify the morphological features of neat PPS and associated nanocomposite films with different MWCNT contents, crosssectional SEM images of the film specimens were obtained, as can be seen in Fig. 1. The SEM images of neat PPS film show relatively smooth and compact cross-sectional morphology without any micropores, which indicates that the solid-mixing and meltcompression processing was well controlled. In cases of the nanocomposite films, the SEM images exhibit regularly segregated and percolated morphology of MWCNTs in the PPS matrix, which was also confirmed by typical OM and TEM images of PPS/ MWCNT-3.0, as shown in Fig. 2. As represented schematically in Fig. 3, it is highly conjectured that the irregularly segregated structure of MWCNTs can be driven by the manufacturing processes. At the melt-compression condition (300 °C, 20 MPa, 10 min), PPS chains above the melting point can diffuse into neighboring PPS powder domains surrounded by MWCNTs, which eventually leads to rather unclear PPS domain size in the nanocomposite films but contribute to good welding among PPS powders by minimizing micropores for the final nanocomposite films and by remaining interconnected MWCNTs around PPS domains. 3.2. Thermal transition and thermal stability The influence of the pristine MWCNT on the melting and crystallization transition behavior of PPS-based nanocomposite films was investigated by obtaining DSC heating and cooling thermograms, as shown in Fig. 4. In the DSC heating thermograms, the melting transition temperatures (Tm) of neat PPS and nanocomposite films were found to be 285 °C, irrespective of the MWCNT content, within the experimental error. It means that the melting transition is not affected by the presence of the pristine MWCNT in the manufactured nanocomposite films. However, it is noticeable that the melt-crystallization exothermic peaks are shifted to higher temperatures from 238 °C for the neat PPS film to 247 °C for the PPS/MWCNT-10 with the increase of the MWCNT content, which reveals that MWCNTs segregated regularly in the nanocomposite films can serve as nucleating agents for the meltcrystallization of PPS. Fig. 5 shows TGA and DTG curves of neat PPS and nanocomposite films with different MWCNT content. It is found that neat PPS and its nanocomposite films maintain mainly their thermal stability up to 450 °C and are rapidly pyrolyzed in the temperature range of 500–650 °C. In addition, the residues above 650 °C are largely higher for the nanocomposite films with higher MWCNT loading, as can be seen in Fig. 5A. On the other hand, the thermal decomposition of the nanocomposite films is slightly delayed,
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
79
Fig. 1. Cross-sectional SEM images of neat PPS and its nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (A) OM and (B, C) TEM images of PPS/MWCNT-3 with a segregated structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
compared with neat PPS film, as shown in DTG curves of Fig. 5B, which is believed to stem from the thermal protection effect of MWCNTs segregated regularly in the PPS matrix. 3.3. Electrical properties Fig. 6 shows the current density-voltage (J-V) and electric power density-voltage (E-V) curves of neat PPS and its nanocomposite
films. The current density (J) and electric power density (E) imply the current (I) and electric power (P) divided by the crosssectional area (A) of a specimen, respectively. In case of neat PPS film, no current density or electric power vary with the applied voltage. It means that neat PPS film is electrically insulating. On the other hand, the current density and electric powder density of the nanocomposite films increase with increasing the applied voltages and the voltage-dependent increments are steeper for
80
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
Fig. 3. Schematic procedure for manufacturing PPS/MWCNT nanocomposite films with a segregated structure via solid-mixing and following melt-compression. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. (A) DSC heating and (B) cooling thermograms of neat PPS and nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the nanocomposite films with higher MWCNT content. From the linearity of the J-V curves in Fig. 6A, the electrical transport properties of PPS/MWCNT nanocomposite films obey the Ohm’s law of V = IR. Accordingly, the electrical resistance (R) of the
Fig. 5. (A) TGA and (B) DGA curves of neat PPS and nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
nanocomposite films could be evaluated from the slopes of J-V curves in Fig. 6A and it is plotted as a function of MWCNT content, as shown in Fig. 7A. By using the relation of R ¼ r1 AL where L is the sample length between electrodes and A is the cross-sectional area
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
Fig. 6. (A) Current density-voltage (J-V) and (B) electric power density-voltage (E-V) curves of neat PPS and nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of a film sample, the electrical conductivity (r) of the nanocomposite films could be calculated and the result is also represented as a function of MWCNT content, as can be seen in Fig. 7A. The electrical conductivity is strongly dependent on MWCNT content by showing a typical percolation behavior. The electrical conductivity increases significantly from 1010 S/cm for neat PPS to 104 S/ cm for PPS/MWCNT-1 to 0.11 S/cm for PPS/MWCNT-10 and the electrical percolation threshold is conjectured to be formed at a certain MWCNT content between 0.0 and 1.0 wt%. It is generally known that electrically conductive paths owing to the interconnected network of CNTs form in the nanocomposite matrix, when the CNT content reaches the electrical percolation threshold. To evaluate the electrical percolation threshold, the following power law relation can be adopted [24]
r / ðp pc Þa
ð1Þ
where r is the electrical conductivity, p is the MWCNT volume fraction, pc is the critical volume fraction at electrical percolation, and a is the critical exponent as an index of the system dimensionality. As a result, the experimental data of logr versus log(p pc) are well fitted by a linear line with pc = 0.003 (0.33 wt%) and a = 2.141, as shown in Fig. 7B. Theoretical a values for ideal 2-D and 3-D systems are predicted to be 1.3 and 1.94, respectively.[24] Since the percolation theory associated with the power law relation of Eq. (1) does not consider either of particle shape, dispersion, orientation, and
81
Fig. 7. (A) Electrical resistance and resistivity of PPS/MWCNT nanocomposite films as a function of the MWCNT content and (B) log r versus log(p pc) plot from the power relation r (p pc)a with the experimental electrical conductivity data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
interaction, the value of a can vary with particular composite systems. A computer modeling on a system of randomly placed rigid rods connected by free joints predicted a critical exponent value of 2 [25]. Other experimental studies of conductive polymer nanocomposites with segregated dispersion of SWCNT or MWCNT have reported a values of 1.04–3.55 [18]. Thus, the a value of 2.141 evaluated for PPS/MWCNT nanocomposite films supports the fact that MWCNTs form a 3-dimensionally interconnected network in the PPS matrix at a percolation threshold of 0.33 wt%, although they are segregated regularly in the nanocomposite films. It is also believed that the low pc value of 0.33 wt% MWCNT and the relatively high electrical conductivity of the nanocomposite films originate from the uniformly segregated dispersion of MWCNTs in the PPS matrix. As discussed above, for the nanocomposite films including >0.33 wt% MWCNT, regularly segregated MWCNTs are more densely interconnected in the PPS matrix, leading to facilitating the electron tunneling among MWCNTs for high electrical conduction. It needs to mention that PPS/MWCNT nanocomposite films with 1.0–10.0 wt% MWCNT have quite high electrical conductivity of 104–101 S/cm, which is high enough to be utilized as electrostatic dissipation (ESD) and/or electromagnetic insulation (EMI) shielding materials for electronic devices and components [26]. In the E-V curves of Fig. 6B, electric power density (E = P/A) increases quadratically with respect to applied voltage (V), which
82
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
values as an adjusting parameter could be calculated by fitting the experimental time-dependent temperature ramping curves of Fig. 8 and the results are summarized in Table 1. The relatively low average sg value of 9.2 ± 2.2 s reveals that PPS/MWCNT nanocomposite films have rapid temperature responsiveness to applied voltages by reaching steady-state maximum temperatures in a relatively short time. At the second equilibrium region of steady-state maximum temperature, the heat gain of PPS/MWCNT nanocomposite films by applied electric power is supposed to be equal to the heat loss by radiation and convection based to the conservation law of energy. The heat transferred by radiation and convection, hr+c, is expressed as
hrþc ¼
Fig. 8. Typical time-dependent temperature changes of a PPS-based nanocomposite film with 7 wt% MWCNT under different applied voltages of 2–18 V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
is consistent with the relation of P = V2/R. This electric power can be dissipated as heat, allowing a form of heating known as electrical resistive heating or Joule heating. It is thus expected that PPS/ MWCNT nanocomposite films have electrical resistive heating performance at certain applied voltages. In order to characterize the electrical resistive heating behavior of PPS/MWCNT nanocomposite films, temperature changes of the nanocomposite films under constant applied voltages were monitored as a function of time. Fig. 8 represents typical time-dependent temperature changes of a nanocomposite film with 7.0 wt% MWCNT, which are subjected to different applied voltages of 2–18 V. When a certain voltage above 6 V was applied, the temperature increases quickly to a maximum value within 90 s, remains constant over the time under the applied voltage, and decreases to room temperature as soon as the applied voltage was off at 5 min. This electrical resistive heating behavior was found to be almost identical for other PPS/MWCNT nanocomposite films. The time-dependent temperature curves in Fig. 8 can be divided into three regions of temperature ramp (heating), steady-state temperature (equilibrium), and temperature decay (cooling). At the first region of temperature ramp, the time-dependent temperature profile can be expressed as [27–29]
Tt Ti ¼ 1 expðt=sg Þ T max T i
ð2Þ
where Ti is an initial sample temperature, Tmax is a steady-state maximum temperature, Tt is an arbitrary temperature at time t, and sg is the characteristic growth time constant. Accordingly, for PPS/MWCNT nanocomposite films with 1.0–10.0 wt% MWCNT, sg
P IV ¼ T max T i T max T i
ð3Þ
where P is the electric power, I is the steady-state current, and V is the applied voltage. As a result, hr+c values for PPS/MWCNT nanocomposite films can be calculated, as listed in Table 1. The average hr+c value of 21.2 ± 4.6 mW/°C indicates that PPS/MWCNT nanocomposite films have relatively high electric power efficiency by consuming low electrical energy to maintain steady-state maximum temperatures at given applied voltages. At the third region of time-dependent temperature decay, the state-state maximum temperatures of PPS/MWCNT nanocomposite films decrease to the ambient temperature by obeying Newton’s law of cooling. Accordingly, the time-dependent temperature decay can be described by the following relation
Tt Tf ¼ expðt=sd Þ T max T f
ð4Þ
where Tf is a final ambient temperature and sd is the characteristic decay time constant. By fitting the experimental time-dependent temperature decay curves with Eq. (4), the sd values as adjusting parameter can be obtained, as summarized in Table 1. For PPS/ MWCNT nanocomposite papers, the average sd value of 11.2 ± 2.0 s is found to be quite low, which means that electrically heated PPS/MWCNT nanocomposite films can be cooled effectively to room temperature in a dozen seconds. The steady-state maximum temperatures (Tmax) are strongly dependent on the applied voltage as well as the MWCNT content, as shown in Fig. 9. Higher Tmax values are achieved for the nanocomposite films with higher MWCNT loading at a given applied voltage or for a nanocomposite film under higher applied voltages. For instance, Tmax of 121 °C for PPS/MWCNT-7 is attained at an applied voltage of 20 V, which is 80 °C higher than that of PPS/MWCNT-5. The quadratic increases of Tmax with applied voltages in Fig. 9 are found to be consistent well with the trends of E-V curves in Fig. 6B. This result demonstrates that the electric power supplied to PPS/MWCNT nanocomposite films is transformed effectively to thermal energy, which is thus dissipated as heat to the environment by radiation and convection. Accordingly, it is found that Tmax increases linearly with increasing the electric power input per unit volume (Pin, W/cm3) for PPS/MWCNT nanocomposite films, can be seen in Fig. 10. This result indicates that well-controlled Tmax values for a nanocomposite film heater
Table 1 Characteristic parameters for electrical resistive heating behavior of PPS/MWCNT nanocomposite films with a segregated structure. Sample code
Applied voltage (V)
sg (s)
hr+c (mW/°C)
sd (s)
PPS/MWCNT-1 PPS/MWCNT-3 PPS/MWCNT-5 PPS/MWCNT-7 PPS/MWCNT-10 Average
70–100 40–100 20–50 8–20 6–12
11.2 ± 3.2 8.8 ± 0.6 8.1 ± 0.8 7.5 ± 0.8 10.4 ± 0.6 9.2 ± 2.2
18.8 ± 3.4 25.0 ± 3.5 17.4 ± 3.1 20.1 ± 1.1 24.8 ± 3.2 21.2 ± 4.6
11.7 ± 1.1 12.5 ± 2.3 10.4 ± 0.9 9.4 ± 2.0 12.1 ± 0.1 11.2 ± 2.0
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
83
films are thermally stable up to 450 °C and the thermal decomposition of the nanocomposite films are slightly delayed by the thermal protection effects of MWCNTs. The electrical conductivity of the nanocomposite films increased significantly from 1010 S/ cm for neat PPS to 104 S/cm for PPS/MWCNT-1 to 0.11 S/cm for PPS/MWCNT-10. The electrical conductivity of the nanocomposite films with 1.0–10.0 wt% MWCNT was found to be high enough for EDS/EMI shielding applications. By using the power law relation, the electrical percolation threshold was predicted to be 0.33 wt% MWCNT. Accordingly, the nanocomposite films with 1.0–10.0 wt% MWCNT exhibited effective electrical resistive heating performance under applied voltages of 6–100 V by attaining steady-state maximum temperatures (Tmax) of 30–190 °C within a relatively short period of time of 10 s. The Tmax values of the nanocomposite films could be controlled by adjusting electric power input per unit volume (Pin) based on the relation of Tmax = 3.656 Pin + 17.3 with relatively high energy power efficiency of 21.2 ± 4.6 mW/°C. Fig. 9. Steady-state maximum temperature (Tmax) of PPS/MWCNT nanocomposite films under different applied voltages. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements This research was financially supported by the Fundamental R&D Program for Technology of the Graduate Student Education Program for Research of Hybrid and Super Fiber Materials through the Ministry of Trade, Industry & Energy (MOTIE), and Korea Institute for Advancement of Technology (KIAT) (N0000993). References
Fig. 10. Steady-state maximum temperatures (Tmax) of PPS/MWCNT nanocomposite films as a function of electric power input per unit volume (Pin). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
can be efficiently attained by adjusting Pin according to the relation of Tmax 3.656 Pin. 4. Conclusion In summary, a series of PPS/MWCNT nanocomposite films with a segregated structure were manufactured by a simple and effective solid-mixing and following melt-compression. The thermal and electrical properties of the nanocomposite films were investigated as a function of MWCNT content of 0.5–10.0 wt%. The cross-sectional optical and electron microscopic images of the nanocomposite films exhibited that pristine MWCNTs form a segregated conductive network structure in the PPS matrix. Although the melting transition temperatures (Tm) of the nanocomposite films were found to be 285 °C, irrespective of the MWCNT content, the melt-crystallization exothermic peak temperatures (Tc) were shifted to higher temperatures from 238 °C for neat PPS film to 247 °C for PPS/MWCNT-10, which is associated with the nucleating agent effects of MWCNTs on the melt-crystallization of PPS. TGA and DTG curves revealed that neat PPS and its nanocomposite
[1] Silvestre C, Di Pace E, Napolitano R, Pirozzi B, Cesario G. Crystallization, morphology, and thermal behavior of poly(p-phenylene sulfide). J Polym Sci: Polym Phys 2001;39(4):415–24. [2] Rahate AS, Nemade KR, Waghuley SA. Polyphenylene sulfide (PPS): state of the art and applications. Rev Chem Eng 2013;29(6):471–89. [3] Parker D, Bussink J, van der Grampel HT, Wheatley GW, Dorf E-U, Ostlinning E, et al. Polymers, high-temperature. Ullmann’s encyclopedia of industrial chemistry. Weinheim: Wiley-VCH; 2002. [4] Diez-Pascual AM, Naffakh M. Enhancing the thermomechanical behaviour of poly(phenylene sulphide) based composites via incorporation of covalently grafted carbon nanotubes. Composites Part A 2013;54:10–9. [5] Gao Y, Fu Q, Niu LY, Shi ZM. Enhancement of the tensile strength in poly(pphenylene sulfide) and multi-walled carbon nanotube nanocomposites by hotstretching. J Mater Sci 2015;50(10):3622–30. [6] Zhang M, Wang H, Li Z, Cheng B. Exfoliated graphite as a filler to improve poly (phenylene sulfide) electrical conductivity and mechanical properties. RSC Adv 2015;5(18):13840–9. [7] Baughman RH, Zakhidov AA, de Heer WA. Carbon nanotubes-the route toward applications. Science 2002;297:787–92. [8] Zhu Y, Murali S, Cai W, Li X, Suk JW, Potts JR, et al. Graphene and graphene oxide: synthesis, properties, and applications. Adv Mater 2010;22:3906–24. [9] Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: past, present and future. Prog Mater Sci 2011;56:1178–271. [10] Yang J, Xu T, Lu A, Zhang Q, Fu Q. Electrical properties of poly(phenylene sulfide)/multiwalled carbon nanotube composites prepared by simple mixing and compression. J Appl Polym Sci 2008;109(2):720–6. [11] Tang W, Santare MH, Advani SG. Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 2003;41(14):2779–85. [12] Hedrick JL, Twieg R. Poly(aryl ether oxadiazoles). Macromolecules 1992;25 (7):2021–5. [13] Potschke P, Bhattacharyya AR, Janke A. Morphology and electrical resistivity of melt mixed blends of polyethylene and carbon nanotube filled polycarbonate. Polymer 2003;44(26):8061–9. [14] Kim YJ, Shin TS, Choi HD, Kwon JH, Chung Y-C, Yoon HG. Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites. Carbon 2005;43(1):23–30. [15] Du F, Scogna RC, Zhou W, Brand S, Fischer JE, Winey KI. Nanotube networks in polymer nanocomposites: rheology and electrical conductivity. Macromolecules 2004;37(24):9048–55. [16] Wu D, Wu L, Zhang M. Rheology of multi-walled carbon nanotube/poly (butylene terephthalate) composites. J Polym Sci: Polym Phys 2007;45 (16):2239–51. [17] Deng H, Lin L, Ji M, Zhang S, Yang M, Fu Q. Progress on the morphological control of conductive network in conductive polymer composites and the use as electroactive multifunctional materials. Prog Polym Sci 2014;39:627–55. [18] Pang H, Xu L, Yan D-X, Li Z-M. Conductive polymer composites with segregated structures. Prog Polym Sci 2014;39:1908–33.
84
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
[19] Du F, Guthy C, Kashiwagi T, Fisher JE, Winey KI. An infiltration method for preparing single-wall nanotube/epoxy composites with improved thermal conductivity. J Polym Sci, Part B: Polym Phys 2006;44(10):1513–9. [20] Mu M, Walker AM, Torkelson JM, Winey KI. Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes. Polymer 2008;49(5):1332–7. [21] Jurewicz I, Worajittiphon P, King AAK, Sellin PJ, Keddie JL, Dalton AB. Locking carbon nanotubes in confined lattice geometries – a route to low percolation in conducting composites. J Phys Chem B 2011;115(20):6395–400. [22] Pang H, Yan D-X, Bao Y, Chen J-B, Chen C, Li Z-M. Super-tough conducting carbon nanotube/ultrahigh-molecular-weight polyethylene composites with segregated and double-percolated structure. J Mater Chem 2012;22:23568–75. [23] Lee T-W, Jeong YG. Enhanced electrical conductivity, mechanical modulus, and thermal stability of immiscible polylactide/polypropylene blends by the selective localization of multi-walled carbon nanotubes. Compos Sci Technol 2014;103:78–84.
[24] Stauffer D, Aharon A. Introduction to percolation theory. Revised 2nd ed. London: Taylor & Francis; 1994. [25] Obukhov SP. First order rigidity transition in random rod networks. Phys Rev Lett 1995;74(22):4472–5. [26] Goyal RK, Kadam A. Polyphenylene sulphide/graphite composites for EMI shielding applications. Adv Mater Lett 2010;1(2):143–7. [27] El-Tantawy F. Joule heating treatments of conductive butyl rubber/ceramic superconductor composites: a new way for improving the stability and reproducibility? Eur Polym J 2001;37(3):565–74. [28] El-Tantawy F, Kamada K, Ohnabe H. In situ network structure, electrical and thermal properties of conductive epoxy resin-carbon black composites for electrical heater applications. Mater Lett 2002;56(1–2):112–26. [29] Park J, Jeong YG. Investigation of microstructure and electric heating behavior of hybrid polymer composite films based on thermally stable polybenzimidazole and multiwalled carbon nanotube. Polymer 2015;59 (24):102–9.
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Thermal and electrical properties of poly(phenylene sulfide)/carbon nanotube nanocomposite films with a segregated structure Tae Jong Yoo, Eun-Byeol Hwang, Young Gyu Jeong ⇑ Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 May 2016 Received in revised form 28 September 2016 Accepted 30 September 2016 Available online 30 September 2016 Keywords: A. Carbon nanotubes A. Nanocomposites B. Electrical properties B. Thermal properties
a b s t r a c t Structurally segregated poly(phenylene sulfide) (PPS)-based nanocomposite films containing multiwalled carbon nanotube (MWCNT) of 0.5–10.0 wt% were manufactured by solid-mixing and following melt-compression. The cross-sectional optical and electron microscopic images of the nanocomposite films revealed that MWCNTs form a segregated and percolated conductive network structure in the PPS matrix. DSC and TGA data demonstrated that melt-crystallization temperatures and thermal degradation temperatures of the nanocomposite films are slightly increased with the MWCNT content, which are owing to the nucleating agent and thermal protection effects of MWCNTs, respectively. The electrical conductivity of the nanocomposite films increased significantly from 1010 to 0.11 S/cm with the increment of the MWCNT content from 0.0 to 10.0 wt%, especially at a low percolation threshold of 0.33 wt% MWCNT. Accordingly, PPS-based nanocomposite films with 1.0–10.0 wt% MWCNT exhibited high performance in electrical resistive heating behavior under applied voltages of 5–100 V by achieving steady-state maximum temperatures of 30–190 °C within a relatively short period of time of 10 s. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Poly(phenylene sulfide) (PPS) consisting of aromatic rings linked with sulfides is one of leading engineering thermoplastic polymers. Owing to its excellent heat resistance, chemical resistance, fatigue resistance, dimensional stability, thermal stability, flame retardancy, electrical insulation, wear resistance, and processability, PPS has been used in a variety of fibers, specialty membranes, electronic products and industries [1,2]. PPS, which is otherwise electrically insulting, is also considered as the precursor to a conducting polymer of the semi-flexible rod polymer family, because it can be converted to the semiconducting form by oxidation or use of dopants [3]. To expand the applications of PPS by improving the physical properties, carbon nanotube (CNT), graphene and their derivatives have been chosen as reinforcing nanofillers for PPS [4–6]. Especially, CNTs have been extensively selected as one-dimensional reinforcing nanofillers for advanced polymeric nanocomposites due to their excellent electrical conductivity of 104–105 S/cm, thermal conductivity of 3000–6600 W/m K, carrier mobility of 1000–4000 cm2/V s, Young’s modulus of 0.27–1.25 TPa, and thermal stability up to 700 °C in air [7–9]. Diez-Pascual and ⇑ Corresponding author. E-mail address: [email protected] (Y.G. Jeong). http://dx.doi.org/10.1016/j.compositesa.2016.09.022 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved.
Naffakh investigated that PPS-based nanocomposites incorporating a polymer derivative covalently grafted single-walled CNT (SWCNT) have highly improved thermal and thermomechanical properties in aspects of high thermal stability, flame retardancy, glass transition/ heat deflection temperatures, tensile strength/modulus, and low thermal expansion coefficient, which are ascribed to strong matrixfiller interfacial interactions combined with a compatibilization effect [4]. Gao et al. reported that hot-stretched PPS composite fibers reinforced with multi-walled CNT (MWCNT) have remarkably enhanced tensile strength and modulus owing to the presence of the strong interfacial adhesion via p-p interaction between the benzene ring of PPS and the surface of MWCNTs [5]. In most of the cases of thermoplastic polymer nanocomposites including MWCNT, melt mixing is the preferred composite compounding method [10], since aggregate formation can be minimized by suitable application of shear during the melt mixing [11]. Melt mixing for PPS/MWCNT nanocomposites is more simple and handy than other processing techniques, and can achieve the production on a large scale in the PPS industries [12]. Many researchers also reported the degree of MWCNT dispersion in the PPS matrix below and above the electrical percolation threshold [13–16]. In general, polymer nanocomposites with random conductive networks require high conductive filler loadings at the insulator/conductor transition. To date, forming a controlled segregated structure in a conductive polymer composite has remained
78
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
the most promising strategy for attaining low electrical percolation threshold. Accordingly, the morphological control of conductive network in polymer composites and their use as electroactive multifunction materials have been also investigated [17,18]. In the polymer composite materials with a segregated structure, the conductive nanofillers are primarily located at the interfaces between the polymeric matrix particles instead of being randomly arranged throughout the entire polymer composite system [19–23]. In this study, we fabricated a series of PPS/MWCNT nanocomposite films with a segregated structure via an efficient solidmixing and following melt-compression, and investigated their morphological features, thermal and electrical properties as a function of MWCNT content (0.5–10.0 wt%). The segregated but percolated structure of MWCNTs in the PPS matrix was identified by using optical and electronic microscopes. The electrical percolation threshold of the nanocomposite films was analyzed by using a power law relation. The electrical resistive heating performance of the PPS nanocomposite films with different MWCNT contents was characterized in terms of temperature responsiveness, steady-steady maximum temperature, and electric power efficiency under different applied voltages. 2. Experimental 2.1. Materials PPS chips (Fortron 0309, Ticona, Celanese) with a density of 1.350 g/cm3 was selected as the polymeric matrix to fabricate the nanocomposites films. Pristine MWCNT (CM-95, Hanwha Chemical) with 10–15 nm diameter and 10–20 lm length, which was manufactured by thermal chemical vapor deposition (CVD), was used as a reinforcing nanofiller for PPS-based nanocomposite films. 2.2. Preparation of PPS/MWCNT nanocomposite films A series of PPS/MWCNT nanocomposite films were fabricated by a solid-mixing and following melt-compression process. First, PPS chips were pulverized into powders with a particle size of 259.6 ± 96.7 lm. Second, predetermined amounts of PPS powders were mixed mechanically with pristine MWCNT for 3 min. In the solid mixtures, the pristine MWCNT content was controlled to be 0.5, 1.0, 3.0, 5.0, 7.0, and 10.0 wt%. Third, each solid mixture was melt-compressed in a hot press at 300 °C and 20 MPa for 10 min and then cooled to room temperature. The thickness of the nanocomposite films during the melt-compression was adjusted to be 200 lm. Finally, the nanocomposite films were dried in a vacuum oven at 80 °C. The final nanocomposite films were named as PPS/MWCNT-x, where x denotes the content of pristine MWCNT by weight percent (wt%). 2.3. Characterization The particle size and its distribution of PPS powders was analyzed by using a laser scattering particle size analyzer (HELOS/ KR, Sympatec GmbH). The dispersion state of the pristine MWCNT in the PPS matrix was identified by examining the cross-section morphology of the nanocomposite films with aids of a scanning electron microscope (SEM, S-4800, HITACHI), a transmission electron microscope (TEM, Tecnai G2 F30, FEI) and an optical microscope (OM, S38, Bimience). For the TEM and OM experiments, thin specimens with 50–500 nm thickness were prepared by using a ultramicrotome (Leica EM UC6). The thermal transition properties of the nanocomposite films were investigated by using a differential scanning calorimeter (DSC, Mettler-Toledo). About 5 mg samples were heated from 0 to 300 °C at a heating rate of 10 °C/min under nitrogen atmosphere. The thermal stability of
neat PPS and its nanocomposite films was investigated by using a thermogravimetric analyzer (TGA, Mettler-Toledo). About 4– 5 mg samples were heated from 25 to 900 °C at a heating rate of 20 °C/min under nitrogen atmosphere. The electrical properties of the nanocomposite films with a variety of MWCNT contents were characterized by obtaining voltage-dependent current density and electric power density curves with an electrometer (2400, Keithley). The electrical resistive heating performance of the nanocomposite films at different applied voltages of 1–100 V was characterized with an infrared camera (SE/A325, FLIR system Inc.) and an electrometer (2400, Keithley). For the electrical property measurement and the electrical resistive heating experiment, the sample distance between electrical test probes was fixed to be 10.0 mm for all the nanocomposite films with 10.0 mm width and 30.0 mm length. 3. Results and discussion 3.1. Structural characterization To identify the morphological features of neat PPS and associated nanocomposite films with different MWCNT contents, crosssectional SEM images of the film specimens were obtained, as can be seen in Fig. 1. The SEM images of neat PPS film show relatively smooth and compact cross-sectional morphology without any micropores, which indicates that the solid-mixing and meltcompression processing was well controlled. In cases of the nanocomposite films, the SEM images exhibit regularly segregated and percolated morphology of MWCNTs in the PPS matrix, which was also confirmed by typical OM and TEM images of PPS/ MWCNT-3.0, as shown in Fig. 2. As represented schematically in Fig. 3, it is highly conjectured that the irregularly segregated structure of MWCNTs can be driven by the manufacturing processes. At the melt-compression condition (300 °C, 20 MPa, 10 min), PPS chains above the melting point can diffuse into neighboring PPS powder domains surrounded by MWCNTs, which eventually leads to rather unclear PPS domain size in the nanocomposite films but contribute to good welding among PPS powders by minimizing micropores for the final nanocomposite films and by remaining interconnected MWCNTs around PPS domains. 3.2. Thermal transition and thermal stability The influence of the pristine MWCNT on the melting and crystallization transition behavior of PPS-based nanocomposite films was investigated by obtaining DSC heating and cooling thermograms, as shown in Fig. 4. In the DSC heating thermograms, the melting transition temperatures (Tm) of neat PPS and nanocomposite films were found to be 285 °C, irrespective of the MWCNT content, within the experimental error. It means that the melting transition is not affected by the presence of the pristine MWCNT in the manufactured nanocomposite films. However, it is noticeable that the melt-crystallization exothermic peaks are shifted to higher temperatures from 238 °C for the neat PPS film to 247 °C for the PPS/MWCNT-10 with the increase of the MWCNT content, which reveals that MWCNTs segregated regularly in the nanocomposite films can serve as nucleating agents for the meltcrystallization of PPS. Fig. 5 shows TGA and DTG curves of neat PPS and nanocomposite films with different MWCNT content. It is found that neat PPS and its nanocomposite films maintain mainly their thermal stability up to 450 °C and are rapidly pyrolyzed in the temperature range of 500–650 °C. In addition, the residues above 650 °C are largely higher for the nanocomposite films with higher MWCNT loading, as can be seen in Fig. 5A. On the other hand, the thermal decomposition of the nanocomposite films is slightly delayed,
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
79
Fig. 1. Cross-sectional SEM images of neat PPS and its nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (A) OM and (B, C) TEM images of PPS/MWCNT-3 with a segregated structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
compared with neat PPS film, as shown in DTG curves of Fig. 5B, which is believed to stem from the thermal protection effect of MWCNTs segregated regularly in the PPS matrix. 3.3. Electrical properties Fig. 6 shows the current density-voltage (J-V) and electric power density-voltage (E-V) curves of neat PPS and its nanocomposite
films. The current density (J) and electric power density (E) imply the current (I) and electric power (P) divided by the crosssectional area (A) of a specimen, respectively. In case of neat PPS film, no current density or electric power vary with the applied voltage. It means that neat PPS film is electrically insulating. On the other hand, the current density and electric powder density of the nanocomposite films increase with increasing the applied voltages and the voltage-dependent increments are steeper for
80
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
Fig. 3. Schematic procedure for manufacturing PPS/MWCNT nanocomposite films with a segregated structure via solid-mixing and following melt-compression. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. (A) DSC heating and (B) cooling thermograms of neat PPS and nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the nanocomposite films with higher MWCNT content. From the linearity of the J-V curves in Fig. 6A, the electrical transport properties of PPS/MWCNT nanocomposite films obey the Ohm’s law of V = IR. Accordingly, the electrical resistance (R) of the
Fig. 5. (A) TGA and (B) DGA curves of neat PPS and nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
nanocomposite films could be evaluated from the slopes of J-V curves in Fig. 6A and it is plotted as a function of MWCNT content, as shown in Fig. 7A. By using the relation of R ¼ r1 AL where L is the sample length between electrodes and A is the cross-sectional area
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
Fig. 6. (A) Current density-voltage (J-V) and (B) electric power density-voltage (E-V) curves of neat PPS and nanocomposite films with different MWCNT contents. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of a film sample, the electrical conductivity (r) of the nanocomposite films could be calculated and the result is also represented as a function of MWCNT content, as can be seen in Fig. 7A. The electrical conductivity is strongly dependent on MWCNT content by showing a typical percolation behavior. The electrical conductivity increases significantly from 1010 S/cm for neat PPS to 104 S/ cm for PPS/MWCNT-1 to 0.11 S/cm for PPS/MWCNT-10 and the electrical percolation threshold is conjectured to be formed at a certain MWCNT content between 0.0 and 1.0 wt%. It is generally known that electrically conductive paths owing to the interconnected network of CNTs form in the nanocomposite matrix, when the CNT content reaches the electrical percolation threshold. To evaluate the electrical percolation threshold, the following power law relation can be adopted [24]
r / ðp pc Þa
ð1Þ
where r is the electrical conductivity, p is the MWCNT volume fraction, pc is the critical volume fraction at electrical percolation, and a is the critical exponent as an index of the system dimensionality. As a result, the experimental data of logr versus log(p pc) are well fitted by a linear line with pc = 0.003 (0.33 wt%) and a = 2.141, as shown in Fig. 7B. Theoretical a values for ideal 2-D and 3-D systems are predicted to be 1.3 and 1.94, respectively.[24] Since the percolation theory associated with the power law relation of Eq. (1) does not consider either of particle shape, dispersion, orientation, and
81
Fig. 7. (A) Electrical resistance and resistivity of PPS/MWCNT nanocomposite films as a function of the MWCNT content and (B) log r versus log(p pc) plot from the power relation r (p pc)a with the experimental electrical conductivity data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
interaction, the value of a can vary with particular composite systems. A computer modeling on a system of randomly placed rigid rods connected by free joints predicted a critical exponent value of 2 [25]. Other experimental studies of conductive polymer nanocomposites with segregated dispersion of SWCNT or MWCNT have reported a values of 1.04–3.55 [18]. Thus, the a value of 2.141 evaluated for PPS/MWCNT nanocomposite films supports the fact that MWCNTs form a 3-dimensionally interconnected network in the PPS matrix at a percolation threshold of 0.33 wt%, although they are segregated regularly in the nanocomposite films. It is also believed that the low pc value of 0.33 wt% MWCNT and the relatively high electrical conductivity of the nanocomposite films originate from the uniformly segregated dispersion of MWCNTs in the PPS matrix. As discussed above, for the nanocomposite films including >0.33 wt% MWCNT, regularly segregated MWCNTs are more densely interconnected in the PPS matrix, leading to facilitating the electron tunneling among MWCNTs for high electrical conduction. It needs to mention that PPS/MWCNT nanocomposite films with 1.0–10.0 wt% MWCNT have quite high electrical conductivity of 104–101 S/cm, which is high enough to be utilized as electrostatic dissipation (ESD) and/or electromagnetic insulation (EMI) shielding materials for electronic devices and components [26]. In the E-V curves of Fig. 6B, electric power density (E = P/A) increases quadratically with respect to applied voltage (V), which
82
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
values as an adjusting parameter could be calculated by fitting the experimental time-dependent temperature ramping curves of Fig. 8 and the results are summarized in Table 1. The relatively low average sg value of 9.2 ± 2.2 s reveals that PPS/MWCNT nanocomposite films have rapid temperature responsiveness to applied voltages by reaching steady-state maximum temperatures in a relatively short time. At the second equilibrium region of steady-state maximum temperature, the heat gain of PPS/MWCNT nanocomposite films by applied electric power is supposed to be equal to the heat loss by radiation and convection based to the conservation law of energy. The heat transferred by radiation and convection, hr+c, is expressed as
hrþc ¼
Fig. 8. Typical time-dependent temperature changes of a PPS-based nanocomposite film with 7 wt% MWCNT under different applied voltages of 2–18 V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
is consistent with the relation of P = V2/R. This electric power can be dissipated as heat, allowing a form of heating known as electrical resistive heating or Joule heating. It is thus expected that PPS/ MWCNT nanocomposite films have electrical resistive heating performance at certain applied voltages. In order to characterize the electrical resistive heating behavior of PPS/MWCNT nanocomposite films, temperature changes of the nanocomposite films under constant applied voltages were monitored as a function of time. Fig. 8 represents typical time-dependent temperature changes of a nanocomposite film with 7.0 wt% MWCNT, which are subjected to different applied voltages of 2–18 V. When a certain voltage above 6 V was applied, the temperature increases quickly to a maximum value within 90 s, remains constant over the time under the applied voltage, and decreases to room temperature as soon as the applied voltage was off at 5 min. This electrical resistive heating behavior was found to be almost identical for other PPS/MWCNT nanocomposite films. The time-dependent temperature curves in Fig. 8 can be divided into three regions of temperature ramp (heating), steady-state temperature (equilibrium), and temperature decay (cooling). At the first region of temperature ramp, the time-dependent temperature profile can be expressed as [27–29]
Tt Ti ¼ 1 expðt=sg Þ T max T i
ð2Þ
where Ti is an initial sample temperature, Tmax is a steady-state maximum temperature, Tt is an arbitrary temperature at time t, and sg is the characteristic growth time constant. Accordingly, for PPS/MWCNT nanocomposite films with 1.0–10.0 wt% MWCNT, sg
P IV ¼ T max T i T max T i
ð3Þ
where P is the electric power, I is the steady-state current, and V is the applied voltage. As a result, hr+c values for PPS/MWCNT nanocomposite films can be calculated, as listed in Table 1. The average hr+c value of 21.2 ± 4.6 mW/°C indicates that PPS/MWCNT nanocomposite films have relatively high electric power efficiency by consuming low electrical energy to maintain steady-state maximum temperatures at given applied voltages. At the third region of time-dependent temperature decay, the state-state maximum temperatures of PPS/MWCNT nanocomposite films decrease to the ambient temperature by obeying Newton’s law of cooling. Accordingly, the time-dependent temperature decay can be described by the following relation
Tt Tf ¼ expðt=sd Þ T max T f
ð4Þ
where Tf is a final ambient temperature and sd is the characteristic decay time constant. By fitting the experimental time-dependent temperature decay curves with Eq. (4), the sd values as adjusting parameter can be obtained, as summarized in Table 1. For PPS/ MWCNT nanocomposite papers, the average sd value of 11.2 ± 2.0 s is found to be quite low, which means that electrically heated PPS/MWCNT nanocomposite films can be cooled effectively to room temperature in a dozen seconds. The steady-state maximum temperatures (Tmax) are strongly dependent on the applied voltage as well as the MWCNT content, as shown in Fig. 9. Higher Tmax values are achieved for the nanocomposite films with higher MWCNT loading at a given applied voltage or for a nanocomposite film under higher applied voltages. For instance, Tmax of 121 °C for PPS/MWCNT-7 is attained at an applied voltage of 20 V, which is 80 °C higher than that of PPS/MWCNT-5. The quadratic increases of Tmax with applied voltages in Fig. 9 are found to be consistent well with the trends of E-V curves in Fig. 6B. This result demonstrates that the electric power supplied to PPS/MWCNT nanocomposite films is transformed effectively to thermal energy, which is thus dissipated as heat to the environment by radiation and convection. Accordingly, it is found that Tmax increases linearly with increasing the electric power input per unit volume (Pin, W/cm3) for PPS/MWCNT nanocomposite films, can be seen in Fig. 10. This result indicates that well-controlled Tmax values for a nanocomposite film heater
Table 1 Characteristic parameters for electrical resistive heating behavior of PPS/MWCNT nanocomposite films with a segregated structure. Sample code
Applied voltage (V)
sg (s)
hr+c (mW/°C)
sd (s)
PPS/MWCNT-1 PPS/MWCNT-3 PPS/MWCNT-5 PPS/MWCNT-7 PPS/MWCNT-10 Average
70–100 40–100 20–50 8–20 6–12
11.2 ± 3.2 8.8 ± 0.6 8.1 ± 0.8 7.5 ± 0.8 10.4 ± 0.6 9.2 ± 2.2
18.8 ± 3.4 25.0 ± 3.5 17.4 ± 3.1 20.1 ± 1.1 24.8 ± 3.2 21.2 ± 4.6
11.7 ± 1.1 12.5 ± 2.3 10.4 ± 0.9 9.4 ± 2.0 12.1 ± 0.1 11.2 ± 2.0
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
83
films are thermally stable up to 450 °C and the thermal decomposition of the nanocomposite films are slightly delayed by the thermal protection effects of MWCNTs. The electrical conductivity of the nanocomposite films increased significantly from 1010 S/ cm for neat PPS to 104 S/cm for PPS/MWCNT-1 to 0.11 S/cm for PPS/MWCNT-10. The electrical conductivity of the nanocomposite films with 1.0–10.0 wt% MWCNT was found to be high enough for EDS/EMI shielding applications. By using the power law relation, the electrical percolation threshold was predicted to be 0.33 wt% MWCNT. Accordingly, the nanocomposite films with 1.0–10.0 wt% MWCNT exhibited effective electrical resistive heating performance under applied voltages of 6–100 V by attaining steady-state maximum temperatures (Tmax) of 30–190 °C within a relatively short period of time of 10 s. The Tmax values of the nanocomposite films could be controlled by adjusting electric power input per unit volume (Pin) based on the relation of Tmax = 3.656 Pin + 17.3 with relatively high energy power efficiency of 21.2 ± 4.6 mW/°C. Fig. 9. Steady-state maximum temperature (Tmax) of PPS/MWCNT nanocomposite films under different applied voltages. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements This research was financially supported by the Fundamental R&D Program for Technology of the Graduate Student Education Program for Research of Hybrid and Super Fiber Materials through the Ministry of Trade, Industry & Energy (MOTIE), and Korea Institute for Advancement of Technology (KIAT) (N0000993). References
Fig. 10. Steady-state maximum temperatures (Tmax) of PPS/MWCNT nanocomposite films as a function of electric power input per unit volume (Pin). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
can be efficiently attained by adjusting Pin according to the relation of Tmax 3.656 Pin. 4. Conclusion In summary, a series of PPS/MWCNT nanocomposite films with a segregated structure were manufactured by a simple and effective solid-mixing and following melt-compression. The thermal and electrical properties of the nanocomposite films were investigated as a function of MWCNT content of 0.5–10.0 wt%. The cross-sectional optical and electron microscopic images of the nanocomposite films exhibited that pristine MWCNTs form a segregated conductive network structure in the PPS matrix. Although the melting transition temperatures (Tm) of the nanocomposite films were found to be 285 °C, irrespective of the MWCNT content, the melt-crystallization exothermic peak temperatures (Tc) were shifted to higher temperatures from 238 °C for neat PPS film to 247 °C for PPS/MWCNT-10, which is associated with the nucleating agent effects of MWCNTs on the melt-crystallization of PPS. TGA and DTG curves revealed that neat PPS and its nanocomposite
[1] Silvestre C, Di Pace E, Napolitano R, Pirozzi B, Cesario G. Crystallization, morphology, and thermal behavior of poly(p-phenylene sulfide). J Polym Sci: Polym Phys 2001;39(4):415–24. [2] Rahate AS, Nemade KR, Waghuley SA. Polyphenylene sulfide (PPS): state of the art and applications. Rev Chem Eng 2013;29(6):471–89. [3] Parker D, Bussink J, van der Grampel HT, Wheatley GW, Dorf E-U, Ostlinning E, et al. Polymers, high-temperature. Ullmann’s encyclopedia of industrial chemistry. Weinheim: Wiley-VCH; 2002. [4] Diez-Pascual AM, Naffakh M. Enhancing the thermomechanical behaviour of poly(phenylene sulphide) based composites via incorporation of covalently grafted carbon nanotubes. Composites Part A 2013;54:10–9. [5] Gao Y, Fu Q, Niu LY, Shi ZM. Enhancement of the tensile strength in poly(pphenylene sulfide) and multi-walled carbon nanotube nanocomposites by hotstretching. J Mater Sci 2015;50(10):3622–30. [6] Zhang M, Wang H, Li Z, Cheng B. Exfoliated graphite as a filler to improve poly (phenylene sulfide) electrical conductivity and mechanical properties. RSC Adv 2015;5(18):13840–9. [7] Baughman RH, Zakhidov AA, de Heer WA. Carbon nanotubes-the route toward applications. Science 2002;297:787–92. [8] Zhu Y, Murali S, Cai W, Li X, Suk JW, Potts JR, et al. Graphene and graphene oxide: synthesis, properties, and applications. Adv Mater 2010;22:3906–24. [9] Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: past, present and future. Prog Mater Sci 2011;56:1178–271. [10] Yang J, Xu T, Lu A, Zhang Q, Fu Q. Electrical properties of poly(phenylene sulfide)/multiwalled carbon nanotube composites prepared by simple mixing and compression. J Appl Polym Sci 2008;109(2):720–6. [11] Tang W, Santare MH, Advani SG. Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 2003;41(14):2779–85. [12] Hedrick JL, Twieg R. Poly(aryl ether oxadiazoles). Macromolecules 1992;25 (7):2021–5. [13] Potschke P, Bhattacharyya AR, Janke A. Morphology and electrical resistivity of melt mixed blends of polyethylene and carbon nanotube filled polycarbonate. Polymer 2003;44(26):8061–9. [14] Kim YJ, Shin TS, Choi HD, Kwon JH, Chung Y-C, Yoon HG. Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites. Carbon 2005;43(1):23–30. [15] Du F, Scogna RC, Zhou W, Brand S, Fischer JE, Winey KI. Nanotube networks in polymer nanocomposites: rheology and electrical conductivity. Macromolecules 2004;37(24):9048–55. [16] Wu D, Wu L, Zhang M. Rheology of multi-walled carbon nanotube/poly (butylene terephthalate) composites. J Polym Sci: Polym Phys 2007;45 (16):2239–51. [17] Deng H, Lin L, Ji M, Zhang S, Yang M, Fu Q. Progress on the morphological control of conductive network in conductive polymer composites and the use as electroactive multifunctional materials. Prog Polym Sci 2014;39:627–55. [18] Pang H, Xu L, Yan D-X, Li Z-M. Conductive polymer composites with segregated structures. Prog Polym Sci 2014;39:1908–33.
84
T.J. Yoo et al. / Composites: Part A 91 (2016) 77–84
[19] Du F, Guthy C, Kashiwagi T, Fisher JE, Winey KI. An infiltration method for preparing single-wall nanotube/epoxy composites with improved thermal conductivity. J Polym Sci, Part B: Polym Phys 2006;44(10):1513–9. [20] Mu M, Walker AM, Torkelson JM, Winey KI. Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes. Polymer 2008;49(5):1332–7. [21] Jurewicz I, Worajittiphon P, King AAK, Sellin PJ, Keddie JL, Dalton AB. Locking carbon nanotubes in confined lattice geometries – a route to low percolation in conducting composites. J Phys Chem B 2011;115(20):6395–400. [22] Pang H, Yan D-X, Bao Y, Chen J-B, Chen C, Li Z-M. Super-tough conducting carbon nanotube/ultrahigh-molecular-weight polyethylene composites with segregated and double-percolated structure. J Mater Chem 2012;22:23568–75. [23] Lee T-W, Jeong YG. Enhanced electrical conductivity, mechanical modulus, and thermal stability of immiscible polylactide/polypropylene blends by the selective localization of multi-walled carbon nanotubes. Compos Sci Technol 2014;103:78–84.
[24] Stauffer D, Aharon A. Introduction to percolation theory. Revised 2nd ed. London: Taylor & Francis; 1994. [25] Obukhov SP. First order rigidity transition in random rod networks. Phys Rev Lett 1995;74(22):4472–5. [26] Goyal RK, Kadam A. Polyphenylene sulphide/graphite composites for EMI shielding applications. Adv Mater Lett 2010;1(2):143–7. [27] El-Tantawy F. Joule heating treatments of conductive butyl rubber/ceramic superconductor composites: a new way for improving the stability and reproducibility? Eur Polym J 2001;37(3):565–74. [28] El-Tantawy F, Kamada K, Ohnabe H. In situ network structure, electrical and thermal properties of conductive epoxy resin-carbon black composites for electrical heater applications. Mater Lett 2002;56(1–2):112–26. [29] Park J, Jeong YG. Investigation of microstructure and electric heating behavior of hybrid polymer composite films based on thermally stable polybenzimidazole and multiwalled carbon nanotube. Polymer 2015;59 (24):102–9.