1. Introduction

Microwave irradiation is a form of electromagnetic irradiation, which is between infrared radiation and radio waves. Its wavelengths are 0.001 - 1 m with frequencies ranging from 300 (0.001 m) - 0.3 (1 m) GHz.^[1] Microwaves have multiple applications in communications, navigation, radar technology, radio astronomy, spectroscopy and heating recourse.^[2,3] In addition,  two operating frequencies of micro-waves commonly used in industrial, scientific and medical equipment are 0.915 and 2.450 GHz to avoid the interferences, which are hold by the Federal Communications Commission.^[4] Moreover, one of the commercially available microwave network analyzers is allowed to operate at wide frequencies from 0.1 to 20 GHz.^[5] In 1947, Raytheon built the first commercially available microwave oven. Over the past seven decades, the microwave oven has been an indispensable appliance in almost kitchens possessing advantages of shorter cooking time and energy saving over traditional cooking ways.  Notwithstanding the microwave oven for heating food is broadly used, the application of this machine to process materials is a relatively new investigation. There are multiple benefits from microwave assisted research, including higher production quality, reduced processing times, low costs, new product development, and reduced hazards.^[6] Normally, traditional thermal processing is to transfer energy to the materials via convection, radiation, or conduction of heat through the surface of materials. During this process, the energy is transferred based on thermal gradients. However, microwave irradiation energy is directly transferred to materials through the interaction of microwaves with molecules, and thus displays the principle that the electromagnetic energy converts to thermal energy, rather than heat transfer.^[7,8] Hence, the molecular structure is important in the ability of the objects to interact with the microwave. Furthermore, microwaves will selectively heat the composites or multi-phase materials possessing diverse dielectric properties with a higher dielectric loss.^[6] Therefore, it is a great advantage to fabricate materials with unique and new microstructures via selectively heating distinct phases. Accordingly, microwave technology acquired fast popularity in the fabrication of metallic, ceramics and polymers based products, and composite materials.^[9-11]

Polymer nanocomposites (PNCs) have gained tremendous impetus as a result of their outstanding performances arising from their unique and special physico-chemical properties such as low costs, low manufacture conditions, lightweight, and functional characteristics of nanofillers including outstanding thermal stability, robust flexibility and tunable mechanical, magnetic, rheological and electrochemical properties.^[12-17] In the rapid development of advanced materials, the PNCs with electrical conductivity or semiconductivity have drawn enormous attention because of a great deal of applications like energy storage devices,^[18,19] sensors,^[20-22] kinesiology,^[23-25] aerospace vehicles,^[26] anticorrosion coatings^[27] and microwave absorbers.^[28-30] Generally, electrically conductive PNCs are fabricated by mixing the insulative polymer matrix with conductive nanofillers. Carbon nanotubes (CNTs) are outstanding conductive nanofillers, which already have had great applications in various devices with lightweight,^[31] high aspect ratio,^[32] excellent elastic modulus,^[33,34] and great electrical and thermal conductivity.^[35] In particular, the response of CNTs to electromagnetic waves as the most attractive topic has been investigated by many researchers. Since strong absorptions are obtained with powerful outgassing, heating and light emission when CNTs are heated by microwave irradiation.^[36] In recent years, numerous researchers have used microwaves to purify and functionalize raw CNTs.^[37,38] However, the microwave irradiation used as a green technology to prepare highly electrically conductive polymer/CNT nanocomposites as engineering materials has been rarely reported so far.

In this study, we explored the efficacy of microwave irradiation as an environment-friendly method for the fabrication of electrically conductive PNCs with low loading level CNTs. The formation of CNTs network on the surface of PP granules depends on the microwave heating time, and the final electrical conductivity is related to different processing temperatures. This solvent-free preparation procedure was adjustable and repeatable. The microstructure of the PNCs was studied by both scanning electron microscope (SEM) and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) was utilized to examine the thermal stability of PNCs. The effects of CNTs on the PP were investigated by X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The rheological behaviors of the PNCs were well discussed. Moreover, the influences of CNTs loading changes and hot press temperatures on the electrical conductivity, magnetoresistance, and optical property were studied for all PNCs. The electrical conductivity mechanism was well theoretically analyzed by the variable range hopping model.

2. Experiments

2.1 Materials

Polymer matrix polypropylene (PP) was supplied from the Total Company (Total 3270, M_w: 297000, melt flow rate (MFR): 2.0 g 10min^-1, density: 0.91 g cc^-1). The carbon nanotubes (CNTs, SWeNT SMW 200X) were provided by SouthWest Nano Technologies, Inc with 10.4 nm average diameter and 4.3 μm average length. No further treatment was applied to all the chemicals utilized in this project.

2.2 Nanocomposites Preparation

The PP/CNTs nanocomposites mixed CNTs with loadings of 0.15, 0.50, 1.20 and 1.30 wt% were prepared by microwave solvent-free irradiation as the following procedure. As shown in Scheme 1, 125 mL Buchner flask containing 5 g PP and 2 g CNTs was purged by nitrogen for 15 min, then the vents were sealed by the parafilm immediately. The PP granules were fully surrounded by CNTs after mechanically shaking about 30 s (Vortex-Genie 2, Gear: 6). After that, the PP/CNTs mixture was microwaved (Rival 700 Watts, 2.450 GHz) for 5 s as a period; during each interval, the mixture was taken out and mechanically shaken for 3 s (Gear: 6).  The samples were collected after 4, 5, 6, and 7 periods and marked as 20, 25, 30 and 35 s, respectively. Afterward, the microwave treated mixture was poured into a designed container with ordered pores smaller than granules on the inner wall to collect surplus CNTs (Gear: 8, 3 min). The pure PP was treated as the same procedure without CNTs, which was marked as m_Pure PP. The finally collected pellets were shaped into a disk (diameter: 25 mm) with a series of (140, 160 and 180 °C).

Scheme 1. The procedures of PP/CNTs PNCs fabrication by the microwave solvent-free irradiation.

2.3 Characterizations

The melting and crystallization properties of pure PP and nanocomposites were analyzed by a TA Instruments Q2000 differential scanning calorimeter (DSC). This test was completed in a nitrogen environment with a flow rate of about 50 mL min^-1 and a heating rate of 10 °C min^-1. These samples were about 10 mg for each test. During the test, each sample was heated from 30 to 200 °C at first to get rid of the thermal history, then lower temperate to 30 °C, and reheated again to 200 °C. The data of the first cooling and second heating steps were collected to be analyzed. Thermogravimetric analysis (TGA, TA Instruments Discovery) was used to study the thermal stability of the PP/CNTs PNCs. It was carried out from 30 to 600 °C under both air and nitrogen atmospheres at 10 °C min^-1 heating rate.

The rheometer (TA Instruments Discovery HR-1) examined the storage and loss modulus and loss factor of the PP/CNTs PNCs. An environmental test chamber (ETC) aluminum parallel-plate geometry (25 mm in diameter) was utilized to perform the dynamic rheological measurements with the dynamic oscillation frequency 0.1 - 100 rad s^-1 at 1.0 % strain. The viscosity measurements were completed from 0.1 to 100 s^-1 of shear rate at 220 °C. The PP/CNTs test samples were shaped into disc pellets with a diameter of 25 mm and the average thickness was about 2.0 mm through applying a clamping force of 5 Tons in a benchtop auto presser (Carver Standard Auto CH).

The crystalline structures of pure PP and its PNCs were examined by X-ray diffraction (XRD) scanning from 10 to 30°. The morphology was investigated by emission scanning electron microscope (SEM, Zeiss MERLIN). The micro-structure of CNTs and nanocomposites from the fracture surface of the samples was observed. Meanwhile, the fracture surface was created in liquid nitrogen for characterization.

UV/VIS/NIR spectrophotometer (JASCO, Model V-670) accompanied with a Jasco ISN-723 diffuse reflectance accessory was applied to test Ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) of the PP/CNTs PNCs.

The electrical resistivity was studied using a standard four-probe method. The samples used in this measurement were shaped into disc pellets by hot presser following the same procedure as the samples used in the viscosity measurements. The magnetoresistance (MR) was measured in a 9-Tesla Physical Properties Measurement System (PPMS) by Quantum Design at 290 K, which was carried out using a standard four-probe technique.

3. Results and Discussion

3.1 Thermal stability

The thermal decomposition data of pure PP and its PNCs at both nitrogen and air conditions are shown in Fig. 1a&c; and the corresponding derivative weight loss curves are presented in Fig. 1b&d, respectively. The CNT loadings of  PP/CNT PNCs under different microwave periods are summarized in Table 1.

Table 1. TGA record of the pure PP and its PNCs with CNTs.

For PNCs under N₂ atmosphere, there is a pointy weight loss stage in the temperature range from 400 to 450 °C, which is consistent with the previous observation,^[39] owing to a large scale thermal degradation of PP polymer chains. The onset degradation temperatures (T_on) and the temperatures of the maximum weight loss (T_end) of CNTs/PP PNCs increase slightly with increasing CNTs loading as shown in Table 1. This indicates the stabilization of PP by the introduced CNTs. This result also can be proved by DTG curves, in which the peaks of PP/CNT PNCs move to higher temperatures than that of pure PP (Fig. 1b). The CNT loadings of PP/CNT PNCs treated under a series of microwave periods can be determined by the TGA curves under N₂ condition, which are 0, 0.15, 0.5, 1.20, 1.30 wt.% CNTs for pure PP and 20, 25, 30 and 35 s microwave heating PP/CNTs PNCs, respectively.

        PP/CNT PNCs with 1.20 wt% CNTs have the highest T_on (320.12 °C) under air condition as shown in Table 1, which is about 30 °C higher than that of pure PP. The hardy interfacial particle-polymer interactions is one of the reasons.^[40] Another reason is that the heat is absorbed by neat CNTs transitorily and thus postpones the degradation of PP chains since the neat CNTs begin to be decomposed at around 500 °C, which was observed from TGA of neat CNTs in the insert of Fig. 1c. However, when the loading of CNTs comes to 1.30 wt%, the T_on decreases slightly. Since heat transfers to polymer more deeply and quickly with a good thermal conductivity of mixed CNTs. Therefore, the network structure is destroyed rapidly in the polymer matrix by increasing the CNTs loading. The above phenomena also can be obtained in DTG curves as shown in Fig. 1d.

3.2 Melting and crystalline behaviors

The melting and crystallization behaviors of pure polymer and its PNCs can be depicted using the DSC endothermic and exothermic data as presented in Fig. 2, respectively. The exact crystallization data (T_c: crystalline peak temperature; : crystallization enthalpy; T_m: melting peak temperature; : fusion enthalpy; and F_c: crystalline fraction) are shown in Table 2. The T_c and T_m are easily read from curves. The  and data are simply integrated from the DSC curves. Compare crystallization and fusion enthalpy, the  is smaller than the value for all samples, which is attributed to the sample recrystallization. The  for pure polymer and nanocomposites is calculated from Equation 1,^[41]

Fc=ΔHmΔHm0×(1-x)                                                       (1)

where ΔHm0 is the enthalpy for a theoretical 100% crystalline polymer. As a reference, PP is 209 J g^-1[42] and  is CNTs loadings. The as-received (pure PP) and the treated (m_Pure PP) PP granules are compared in Fig. 2a to reveal the effects of microwave on the PP granules. The cooling and heating curves of these two samples almost overlap with closed T_c and T_m values, and the F_c also stays unchanged. Therefore, it confirms that the microwave has no influence on pure PP granules after 35-s microwave treatment. Since no or negligible energy absorption was obtained in the pure PP granules with a very low dielectric loss factor. Consequently, microwave energy does not work on them.^[43]

Table 2. DSC paraments of pure PP and PNCs with different microwave treatments.

The microwave treatment duration of PP/CNT PNCs was studied in Fig. 2b. The T_c values of PP/CNT PNCs are smaller than those of pure PP. The variation of T_c is caused by different roles of the CNTs during crystallization process of PP, which can be explained by the spherulitic growth rate () as shown by Equation 2:^[44-46]

G=G0exp-U*R(Tc-T∞)exp-KgTcf(Tm0-Tc)                            (2)

where G0 is a factor on the units cm s^-1, U* is the activation energy for the alteration of polymer chain segments to the crystallization sites, T_m⁰ is the equilibrium T_m, R is the gas constant, f is the factor which showed the decrease in heat of fusion with decreasing temperature, T_∞  is the hypothetical temperature where all the motions associated with viscous flow are ceased, the Kg is the nucleation constant, which can be presented by Equation 3:^[45]

Kg=4EEeb0Tm0ΔhfρckB                                       (3)

where Eand Ee are side and fold surface free energies that measure the work required to create a new surface, b0 is the single layer thickness, Δhfρc is the enthalpy of melting and kB is the Boltzmann constant. Hence, the crystallization formation is affected by two parts as shown in Equation 2, the transport of polymer chain segments to the growth front contributes to the first part and the second part is for the nucleation process. After introducing CNTs on the surface of pure PP granules, the CNTs would immobilize partial polymer chains to the growth front, which results in  value increasing and leads the crystallization temperature (T_c) to decrease.^[47] However, the CNTs can be acted as nucleation sites and foster the crystallization of PP by diminishing the surface free energy barrier (E,Ee) towards nucleation. Therefore, the crystallization of  PP/CNT PNCs under 25 s microwave treatment with 0.5 wt.% CNTs was at a slightly higher temperature at the beginning.^[48] In addition, a small shoulder can be obtained in PNCs with 1.20 and 1.30 wt% CNTs. However, pure PP and PNCs with 0.15 and 0.50 wt% CNTs have no shoulder during the crystallization process. The separation of crystallization peaks depicts a variation of the crystal structures. Both α and γ phases of PP were generated during crystallization. Meanwhile, the γ phase can be formed with a lower supercooling. During the cooling proceeding, γ phase PP would appear dominant lamellae at first, then the α phase would form on it.^[49] Thus, the small shoulder is produced owning to γ phase formation of PP by cooling and the γ phase PP promoted by CNTs.

In the interest of studying the effects of hot-pressing process temperature on PP/CNTs PNCs, PNCs treated by 30 s microwave and hot pressed at 140, 160 and 180 °C were investigated. The results are shown in Fig. 2c. From the data summarized in Table 2, the F_c increases slightly after introducing CNTs into the PP host. The higher F_c of these PNCs is due to a higher process temperature which promotes the crystalline structure formation of the PP chains.

3.3 Crystallinity

The effects of the CNT loading and hot press temperature on the crystalline structure of pure PP and its PNCs were detected by XRD. PP possesses a noteworthy complexity of crystal structures including α, β and γ phases, though the simplicity of its chemical structure. Each of α, β and γ phase shows its own characteristic peaks in the XRD patterns. As presented in Fig. 3, the peak intensity of pure PP increases gradually with increasing processing temperature, which is concordant with Zanneti’s results.^[50]

In addition, G" curve has a similar tendency to linearly increase with increasing frequency. The tan δ (the ratio of Gʹʹ to Gʹ) is employed to evaluate the PNCs’ damping property. Compared with PP/CNT PNCs, the tan δ value of pure PP is slightly higher. This is because that the polymer chain relaxation and relative movement of PP/CNT PNCs are hardly hampered by the introduced CNTs, which results in less generation of internal chain-chain friction heat in PNCs than that in pure PP when applying the same oscillation frequency. The viscosity of pure PP and its PNCs was also studied and shown in Fig. 4d at a range of shear rate. All these samples did not present a Newtonian plateau and showed  shear thinning behaviors under melting state, which displayed a decreasing viscosity with increasing shear rate.^[56] Random thermal motion restrains the alignment of CNTs. Meanwhile, the speed of recreating of polymer chain entanglement is slower than that of breaking the physical cross-linked sites with increasing the shear rate. The viscosity of PNCs with the incorporation of CNTs is slightly larger than that of pure PP, which results in the increased melt free volume after adding nanofillers in the entangled and confined systems, and the mobility of polymer chains was restrained due to the dilution effect from nanoparticles.^[57] The stronger shear thinning behavior was also obtained in PP/CNT PNCs made by the masterbatch dilution method.^[58]

3.9 Magnetoresistance (MR)

Fig. 10 presents the magnetoresistance (MR) of PNCs with 30 s microwave treatment processed at 160 °C measured at 290 K, respectively. Equation (7) can be applied for calculating MR,^[79]

MR%=RH,T-R0,T/R(0,T)×100          (7)

where R(H, T) and R(0, T) are the resistance with and without magnetic field H, respectively. The trend of MR reveals different magnetic field dependent behaviors that can be divided into positive and negative groups. After applying a magnetic field, the shrinkage in the overlap of electron wave functions causes the positive MR and thus the average length of hopping reduced.^[80] The orbital magneto-conductivity theory is utilized to study the negative MR effect, which considers the effects of interference among different hopping paths.^[81] For PNCs with 30 s microwave treatment measured at 290 K, a linear negative MR was observed at the low magnetic field. Furthermore, the materials were saturated at high magnetic fields. The ratio of R(H,T)/R(0,T) caused by interference effects is presented as empirical Equation (8):^[82]

R(H,T)R(0,T)≈ 11+Csat(H/Hsat)1+H/Hsat                                   (8)

where the fitting parameters C_sat is a constant and named temperature independent fitting parameter and H_sat is the effective saturation magnetic field. According to the mechanism of Mott VRH electrical conduction, H_sat can be calculated by Equation (9):^[83,84]

_{Hsat≈0.7(83)3/2(1a02)(he)(TT0)3/8}                        (9)

where h is the Planck’s constant, and T₀ is the Mott characteristic temperature (K).

Fig. 10 Magnetoresistance of PNCs with 30 s microwave treatment processed at 160 °C characterized at 290 K, respectively.

Equation (9) can be transformed into Equation (10) in the low field limit:    

R(H, T)R(0,  T)≈1-CsatHHsat                               (10)

Substituting Equation (9) into Equation (10) and rearranging, the Equation (11) can calculate MR values:

MR=R(H,T)-R(0,T)R(0,T)≈-Csat[H/Hsat]= -CsatH0.7(83)3/2(1a02)(he)(TT0)3/8              (11)

The calculated a₀ of PNCs with 1.20 wt.% CNTs measured at 290 K are 636.75, 495.37, and 416.120 nm at H of 2, 4, and 8 T, respectively. Meanwhile, the a₀ value decreases with increasing the H, due to the advance in the polaron population at the expense of bipolarons while the magnetic field is increasing.^[85] The negative MR is also obtained in a nanoporous system composed of bundles of single-walled carbon nanotubes (SWCNTs).^[86] However, Yan et al. reported a positive MR obtained from PNCs with 2.6 and 3.0 wt% CNTs loadings prepared by a solvent surface coating method under the same measurement conditions.^[65] The reason caused the difference is that the loadings and network structures of CNTs of the measured samples were affected by two different sample preparation methods. In the same magnetic field, two different electron transport models exist in the corresponding CNTs PNCs, which can be selected depending on desired MR devices.

4. Conclusions

Microwave radiation can be used for heating materials selectively. Highly electrically conductive PP/CNT PNCs were successfully synthesized by an efficient microwave solvent-free irradiation method under inert atmosphere and controlled treating time. This method gives a new perspective for designing and producing CNTs based conductive polymer materials on an industrial scale with environment-friendly technology. No effects of microwave radiation on PP granules lasting 35 s were confirmed. The thermal stability of PNCs gets enhanced by introducing CNTs which was measured under both air and N₂ atmospheres. The lower internal chain-chain friction heat was obtained in the PNCs than that of pure polymer from melt rheological behaviors, which is caused by the stable interaction between the polymer chains and CNTs. Both XRD and DSC results depicted that higher processing temperate promoted the generation of γ-phase PP. The morphology study showed that CNTs network structure depended on the processing temperature. Well-formed CNTs network resulted in a higher electrical conductivity and a 3-dimension variable range hopping (VRH) was obtained from the electrical conductivity mechanism. The electrical conductivity value of PNCs increased with decreasing the E_g, which means that the PNCs having a better conductivity prosses a lower E_g. Negative MR can be obtained in PNCs with 1.20 wt% CNTs measured at 290 K, providing a favorable approach to design expected MR devices.

Acknowledgments

This work was financially supported by the University of Tennessee, Knoxville. The authors also would like to thank Ramiz Boy, Jihua Chen, Dale K. Hensley, Yangyang Wang from Oak Ridge National Laboratory for the assistance of some sample tests.

Conflict of Interest

There is no conflict of interest.

Supporting Information

Not applicable.

References

1. W. Jakes and D. Cox, Microwave mobile communications, Wiley-IEEE Press, 1994.
2. M. I. Skolnik, Radar Handbook, McGraw-Hill book compaby, 1970.
3. A. A. Metaxas and R. J. Meredith, Industrial microwave heating, IET, 1983.
4. R. J. Meredith, Engineers' handbook of industrial microwave heating, IET, 1998.
5. Q. He, T. Yuan, X. Zhang, X. Yan, J. Guo, D. Ding, M. A. Khan, D. P. Young, A. Khasanov and Z. Luo, J. Phys. Chem. C, 2014, 118, 24784-24796, doi: 10.1021/jp507975r.
6. E. Thostenson and T. W. Chou, Compos. Part A., 1999, 30, 1055-1071, doi: 10.1016/S1359-835X(99)00020-2.
7. D. E. Clark, D. C. Folz and J. K. West, Mater. Sci. Eng. A, 2000, 287, 153-158, doi: 10.1016/s0921-5093(00)00768-1.
8. D. Jiang, V. Murugadoss, Y. Wang, J. Lin, T. Ding, Z. Wang, Q. Shao, C. Wang, H. Liu and N. Lu, Polym. Rev., 2019, 59, 280-337, doi: 10.1080/15583724.2018.1546737.
9. D. K. Agrawal, Curr. Opin. Solid State Mater. Sci., 1998, 3, 480-485, doi: 10.1016/S1359-0286(98)80011-9.
10. L. Zong, S. Zhou, N. Sgriccia, M. Hawley and L. Kempel, J. Microwave Power Electromagn. Energy, 2003, 38, 49-74, doi: 10.1080/08327823.2003.11688487.
11. S. Singh, D. Gupta, V. Jain and A. Sharma, Mater. Manuf. Processes, 2015, 30, 1-29, doi: 10.1080/10426914.2014.952028.
12. X. Guan, G. Q. Zheng, K. Dai, C. Liu, X. Yan, C. Shen and Z. Guo, ACS Appl. Mater. Interfaces, 2016, 8, 14150-14159, doi: 10.1021/acsami.6b02888.
13. H. Zhao, X. Yan, G. Zhao and Z. Guo, Polym. Eng. Sci., 2016, 8, 939-946, doi: 10.1002/pen.24323.
14. L. Zhang, Y. Qin, G. Zheng, K. Dai, C. Liu, X. Yan, J. Guo, C. Shen and Z. Guo, Polymer, 2016, 90, 18-25, doi: 10.1016/j.polymer.2016.02.052.
15. S. Guo, J. Liu, S. Qiu, Y. Wang, X. Yan, N. Wu, S. Wang and Z. Guo, Electrochim. Acta, 2016, 190, 556-565, doi: 10.1016/j.electacta.2015.12.135.
16. X. Yan, Q. He, X. Zhang, H. Gu, H. Chen, Q. Wang, L. Sun, S. Wei and Z. Guo, Macromol. Mater. Eng., 2014, 299, 485-494, doi: 10.1002/mame.201300208.
17. H. Gu, D. Cao, J. Kong, J. Gu, Q. Jiang, Y. Li, B. Wang, X. Yan, Y. Chen and J. E. Ryu, Eng. Sci., 2018, 1, 1-3. doi: 10.30919/es8d128.
18. Y. Liu, M. Shi, C. Yan, Q. Zhuo, H. Wu, L. Wang, H. Liu and Z. Guo, J. Mater. Sci., 2019, 30, 6583-6592, doi: 10.1007/s10854-019-00965-2.
19. H. Wei, C. He, J. Liu, H. Gu, Y. Wang, X. Yan, J. Guo, D. Ding, N. Z. Shen and X. Wang, Polymer, 2015, 67, 192-199, doi: 10.1016/j.polymer.2015.04.064.
20. H. Gu, J. Guo, X. Zhang, Q. He, Y. Huang, H. A. Colorado, N. Haldolaarachchige, H. Xin, D. P. Young and S. Wei, J. Phys. Chem. C, 2013, 117, 6426-6436, doi: 10.1021/jp311471f.
21. S. Zhang, H. Liu, S. Yang, X. Shi, D. Zhang, C. Shan, L. Mi, C. Liu, C. Shen and Z. Guo, ACS Appl. Mater. Interfaces, 2019, 11, 10922-10932, doi: 10.1021/acsami.9b00900.
22. H. Liu, Y. Li, K. Dai, G. Zheng, C. Liu, C. Shen, X. Yan, J. Guo and Z. Guo, J. Mater. Chem. C, 2016, 4, 157-166, doi: 10.1039/C5TC02751A.
23. L. J. Lee, C. Zeng, X. Cao, X. Han, J. Shen and G. Xu, Compos. Sci. Technol., 2005, 65, 2344-2363, doi: 10.1016/j.compscitech.2005.06.016.
24. R. Rahimi, M. Ochoa, W. Yu and B. Ziaie, ACS Appl. Mater. Interfaces, 2015, 7, 4463-4470, doi: 10.1021/am509087u.
25. M. Amjadi, A. Pichitpajongkit, S. Lee, S. Ryu and I. Park, ACS Nano, 2014, 8, 5154-5163, doi: 10.1021/nn501204t.
26. J. Njuguna and K. Pielichowski, Adv. Eng. Mater., 2003, 5, 769-778, doi: 10.1002/adem.200310101.
27. A. Nautiyal, M. Qiao, T. Ren, T. Huang, X. Zhang, J. Cook, M. J. Bozack and R. Farag, Eng. Sci., 2018, 4, 70-78, doi: 10.30919/es8d776.
28. J. Zhu, S. Wei, N. Haldolaarachchige, D. P. Young and Z. Guo, J. Phys. Chem. C, 2011, 115, 15304-15310, doi: 10.1021/jp2052536.
29. Z. Guo, S. E. Lee, H. Kim, S. Park, H. Hahn, A. Karki and D. Young, Acta Mater., 2009, 57, 267-277, doi: 10.1016/j.actamat.2008.09.024.
30. L. Lyu, J. Liu, H. Liu, C. Liu, Y. Lu, K. Sun, R. Fan, N. Wang, N. Lu, Z. Guo and E. Wujcik, Eng. Sci., 2018, 2, 26-42, doi: 10.30919/es8d615.
31. G. Gao, T. Cagin and W. A. Goddard III, Nanotechnology, 1998, 9, 184-191, doi: 10.1088/0957-4484/9/3/007.
32. X. Wang, Q. Li, J. Xie, Z. Jin, J. Wang, Y. Li, K. Jiang and S. Fan, Nano Lett., 2009, 9, 3137-3141, doi: 10.1021/nl901260b.
33. L. Sun, G. Warren, J. O’reilly, W. Everett, S. Lee, D. Davis, D. Lagoudas and H. J. Sue, Carbon, 2008, 46, 320-328, doi: 10.1016/j.carbon.2007.11.051.
34. T. Uchida and S. Kumar, J. Appl. Polym. Sci., 2005, 98, 985-989, doi: 10.1002/app.22203.
35. R. Nie, Q. Wang, P. Sun, R. Wang, Q. Yuan and X. Wang, Eng. Sci., 2019, 6, 22-29, doi: 10.30919/es8d668.
36. T. Imholt, C. A. Dyke, B. Hasslacher, J. M. Pérez, D. Price, J. A. Roberts, J. Scott, A. Wadhawan, Z. Ye and J. M. Tour, Chem. Mater., 2003, 15, 3969-3970, doi: 10.1021/cm034530g.
37. P. X. Hou, C. Liu and H. M. Cheng, Carbon, 2008, 46, 2003-2025, doi: 10.1016/j.carbon.2008.09.009.
38. A. de la Hoz, A. Diaz-Ortiz and A. Moreno, Chem. Soc. Rev., 2005, 34, 164-178, doi: 10.1039/b411438h.
39. Q. He, T. Yuan, X. Yan, D. Ding, Q. Wang, Z. Luo, T. D. Shen, S. Wei, D. Cao and Z. Guo, Macromol. Chem. Phys., 2014, 215, 327-340, doi: 10.1002/macp.201300608.
40. X. Yan, M. Li, J. Long, X. Zhang, H. Wei, Q. He, D. Rutman, D. Cao, S. Wei and G. Chen, Macromol. Chem. Phys., 2014, 215, 1098-1106, doi: 10.1002/macp.201400097.
41. Q. He, T. Yuan, X. Zhang, Z. Luo, N. Haldolaarachchige, L. Sun, D. P. Young, S. Wei and Z. Guo, Macromolecules, 2013, 46, 2357-2368, doi: 10.1021/ma4001397.
42. A. T. Jones, J. M. Aizlewood and D. Beckett, Macromol. Chem. Phys., 1964, 75, 134-158, doi: 10.1002/macp.1964.020750113.
43. D. E. Clark and W. H. Sutton, Annu. Rev. Mater. Sci., 1996, 26, 299-331, doi: 10.1146/annurev.ms.26.080196.001503.
44. J. D. Hoffman, R. L. Miller, H. Marand and D. B. Roitman, Macromolecules, 1992, 25, 2221-2229, doi: 10.1021/ma00034a025.
45. G. Z. Papageorgiou, D. S. Achilias, D. N. Bikiaris and G. P. Karayannidis, Thermochim. Acta, 2005, 427, 117-128, doi: 10.1016/j.tca.2004.09.001.
46. H. Tsuji, H. Takai and S. K. Saha, Polymer, 2006, 47, 3826-3837, doi: 10.1016/j.polymer.2006.03.074.
47. J. Zhu, S. Wei, Y. Li, L. Sun, N. Haldolaarachchige, D. Young, C. Southworth, A. Khasanov, Z. Luo and Z. Guo, Macromolecules, 2011, 44, 4382-4391, doi: 10.1021/ma102684f.
48. H. Li and M. A. Huneault, Polymer, 2007, 48, 6855-6866, doi: 10.1016/j.polymer.2007.09.020.
49. T. Foresta, S. Piccarolo and G. Goldbeck-Wood, Polymer, 2001, 42, 1167-1176, doi: 10.1016/S0032-3861(00)00404-3.
50. R. Zannetti, G. Celotti, A. Fichera and R. Francesconi, Macromol. Chem. Phys., 1969, 128, 137-142, doi: 10.1002/macp.1969.021280111.
51. T. Foresta, S. Piccarolo and G. Goldbeck-Wood, Polymer, 2001, 42, 1167-1176, doi: 10.1016/S0032-3861(00)00404-3.
52. R. Thomann, C. Wang, J. Kressler and R. Mülhaupt, Macromolecules, 1996, 29, 8425-8434, doi: 10.1021/ma951885f.
53. S. H. Lee, E. Cho, S. H. Jeon and J. R. Youn, Carbon, 2007, 45, 2810-2822, doi: 10.1016/j.carbon.2007.08.042.
54. S. H. Lee, M. W. Kim, S. H. Kim and J. R. Youn, Eur. Polym. J., 2008, 44, 1620-1630, doi: 10.1016/j.eurpolymj.2008.03.017.
55. F. Du, R. C. Scogna, W. Zhou, S. Brand, J. E. Fischer and K. I. Winey, Macromolecules, 2004, 37, 9048-9055, doi: 10.1021/ma049164g.
56. P. M. Wood-Adams, J. M. Dealy, A. W. Degroot and O. D. Redwine, Macromolecules, 2000, 33, 7489-7499, doi: 10.1021/ma991533z.
57. A. Tuteja, P. M. Duxbury and M. E. Mackay, Macromolecules, 2007, 40, 9427-9434, doi: 10.1021/ma071313i.
58. J. Zhang, A. Panwar, D. Bello, T. Jozokos, J. A. Isaacs, C. Barry and J. Mead, Environ. Sci.: Nano, 2016, 3, 409-417, doi: 10.1039/c5en00253b.
59. A. Wadhawan, D. Garrett and J. M. Pérez, Appl. Phys. Lett., 2003, 83, 2683-2685, doi: 10.1063/1.1615679.
60. J. Walkiewicz, G. Kazonich and S. McGill, Miner. Metall. Process, 1988, 5, 39-42, doi: 10.1007/BF03449501.
61. S. Frank, P. Poncharal, Z. Wang and W. A. de Heer, Science, 1998, 280, 1744-1746, doi: 10.1126/science.280.5370.1744.
62. Z. Ye, W. D. Deering, A. Krokhin and J. A. Roberts, Phys. Rev. B, 2006, 74, 075425, doi: 10.1103/PhysRevB.74.075425.
63. M. K. Seo, J. R. Lee and S. J. Park, Mater. Sci. Eng. A, 2005, 404, 79-84, doi: 10.1016/j.msea.2005.05.065.
64. K. Wu, Y. Xue, W. Yang, S. Chai, F. Chen and Q. Fu, Compos. Sci. Technol., 2016, 130, 28-35, doi: 10.1016/j.compscitech.2016.04.034.
65. X. Yan, J. Gu, G. Zheng, J. Guo, A. M. Galaska, J. Yu, M. A. Khan, L. Sun, D. P. Young and Q. Zhang, Polymer, 2016, 103, 315-327, doi: 10.1016/j.polymer.2016.09.056.
66. A. H. Alshehri, M. Jakubowska, M. Sloma, M. Horaczek, D. Rudka, C. Free and J. D. Carey, Appl. Phys. Lett., 2011, 99, 153109, doi: 10.1063/1.3651278.
67. J. Stejskal, I. Sapurina, M. Trchová, J. Prokeš, I. Krivka and E. Tobolková, Macromolecules, 1998, 31, 2218-2222, doi: 10.1021/ma970823h.
68. Z. Guo, K. Shin, A. B. Karki, D. P. Young, R. B. Kaner and H. T. Hahn, J. Nanopart. Res., 2009, 11, 1441-1452, doi: 10.1007/s11051-008-9531-8.
69. M. Aggarwal, S. Khan, M. Husain, T. Ming, M. Tsai, T. Perng and Z. Khan, Eur. Phys. J. B, 2007, 60, 319-324, doi: 10.1140/epjb/e2007-00348-3.
70. S. J. Tans, A. R. Verschueren and C. Dekker, Nature, 1998, 393, 49-52, doi: 10.1038/29954.
71. J. Tauc, Mater. Res. Bull., 1970, 5, 721-729, doi: 10.1016/0025-5408(70)90112-1.
72. A. Hjelm, C. G. Granqvist and J. M. Wills, Phys. Rev. B, 1996, 54, 2436-2445, doi: 10.1103/PhysRevB.54.2436.
73. N. Serpone, D. Lawless and R. Khairutdinov, J. Phys. Chem., 1995, 99, 16646-16654, doi: 10.1021/j100045a026.
74. M. B. Tzolov, T. F. Kuo, D. A. Straus, A. Yin and Xu, J. Phys. Chem. C, 2007, 111, 5800-5804, doi: 10.1021/jp068701r.
75. J. Wu, W. Walukiewicz, W. Shan, E. Bourret-Courchesne, J. Ager III, K. Yu, E. Haller, K. Kissell, S. M. Bachilo and R. B. Weisman, Phys. Rev. Lett., 2004, 93, 017404, doi: 10.1103/PhysRevLett.93.017404.
76. O. Gülseren, T. Yildirim, S. Ciraci and Ç. Kılıç, Phys. Rev. B, 2002, 65, 155410, doi: 10.1103/PhysRevB.65.155410.
77. L. Yang, M. P. Anantram, J. Han and J. P. Lu, Phys. Rev. B, 1999, 60, 13874-13878, doi: 10.1103/PhysRevB.60.13874.
78. Z. H. Ni, T. Yu, Y. H. Lu, Y. Y. Wang, Y. P. Feng and Z. X. Shen, ACS Nano, 2008, 2, 2301-2305, doi: 10.1021/nn800459e.
79. B. Hu and Y. Wu, Nat. Mater., 2007, 6, 985-991, doi: 10.1038/nmat2034.
80. T. A. Skotheim and J. Reynolds, Handbook of Conducting Polymers, CRC press, 2007.
81. J. Guo, H. Gu, H. Wei, Q. Zhang, N. Haldolaarachchige, Y. Li, D. P. Young, S. Wei and Z. Guo, J. Phys. Chem. C, 2013, 117, 10191-10202, doi: 10.1021/jp402236n.
82. A. J. Hoffman, L. Alekseyev, S. S. Howard, K. J. Franz, D. Wasserman, V. A. Podolskiy, E. E. Narimanov, D. L. Sivco and C. Gmachl, Nat. Mater., 2007, 6, 946-950, doi: 10.1038/nmat2033.
83. T. I. Su, C. R. Wang, S. T. Lin and R. Rosenbaum, Phys. Rev. B, 2002, 66, 054438, doi: 10.1103/PhysRevB.66.054438.
84. A. Sarkar, P. Ghosh, A. K. Meikap, S. K. Chattopadhyay, S. K. Chatterjee and M. Ghosh, Solid State Commun., 2007, 143, 358-363, doi: 10.1016/j.ssc.2007.05.013.
85. P. Bobbert, T. Nguyen, W. Wagemans, F. van Oost, B. Koopmans and M. Wohlgenannt, Synth. Met., 2010, 160, 223-229, doi: 10.1016/j.synthmet.2009.06.002.
86. Y. Yosida and I. Oguro, J. Appl. Phys., 1999, 86, 999-1003, doi: 10.1063/1.370838.