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A covalent route for efficient surface modification of ordered mesoporous carbon as high performance microwave absorbers Hu Zhou,ab Jiacheng Wang,*a Jiandong Zhuanga and Qian Liu*a A covalent route has been successfully utilized for the surface modification of ordered mesoporous carbon (OMC) CMK-3 by in situ polymerization and grafting of methyl methacrylate (MMA) in the absence of any solvent. The modified CMK-3 carbon particles have a high loading of 19 wt% poly(methyl methacrylate) (PMMA), named PMMA-g-CMK-3, and also maintain their high surface area and mesoporous structure. The in situ polymerization technique endows a significantly enhanced electric conductivity (0.437 S m1) of the resulting PMMA-g-CMK-3/PMMA composite, about two orders of magnitude higher than 1.34 103 S m1 of PMMA/CMK-3 obtained by the solvent mixing method. A minimum reflection loss (RL) value of 27 dB and a broader absorption band (over 3 GHz) with RL values 99%), acetone, a,a0 -azoisobutyronitrile (AIBN), sucrose, hydrouoric acid, sulfuric acid, and ethyl acetate were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Ordered mesoporous silica SBA-15 powders were purchased from Fudan University, China. All chemicals were used as received without any further purication. Distilled water was used in all experiments. 2.2
Synthesis of CMK-3 carbon
The ordered mesoporous carbon CMK-3 was prepared using mesoporous silica SBA-15 as the solid template and sucrose as the carbon source, following the procedure described in the literature.30 Briey, SBA-15 (1.00 g) was mixed with an aqueous solution (5 mL) consisting of sucrose (1.25 g) and sulfuric acid (0.14 g) under continuous stirring. The mixture was heated in an oven at 100 C for 6 hours, followed by further treatment at 160 C for another 6 hours to allow partial carbonization. This heat treatment was repeated aer the as-carbonized silica powders were impregnated again with additional sucrose of 0.80 g, sulfuric acid of 0.09 g, and deionized water of 5 mL. The CMK-3/SBA-15 composite powder was achieved by further pyrolysis at 900 C for 4 hours under a N2 ow to induce complete carbonization. The SBA-15 silica template was then removed by dissolving in 5 wt% hydrouoric acid. The nal CMK-3 product was washed and then dried at 100 C overnight. 2.3 Fabrication of CMK-3/polymer composites by two different procedures 2.3.1 In situ polymerization technique. In a typical experiment, 0.5 g of CMK-3 carbon was dispersed in 4.5 g of methyl methacrylate (MMA) monomer by ultrasonication for 30 min at room temperature. Then, the polymerization of MMA monomers was conducted using 2,20 -azoisobutyronitrile (AIBN) as an initiator in a capped glass tube. The mixture was agitated with a magnetic stirrer at 75 C. When the monomer conversion reached 25%, the reaction was terminated by cooling and the viscous sample was poured into a mold before the polymerization process was completed in an oven at 50 C for 10 hours. The resulting bulky composite was named PMMA-g-CMK-3/PMMA and the weight percent of CMK-3 was 10 wt% in the composite. Here PMMA-g-CMK-3 means PMMA-graed CMK-3 carbon particles with a loading of 19 wt% PMMA. 2.3.2 Solvent mixing technique. The solvent mixing method has been the most usually applied dispersion technique.29,31 Mixing two or more substance materials together forms a mixture which are physically but not chemically combined. The bulky CMK-3/PMMA composite was also prepared using the traditional solvent mixing method.20 Typically, 4.5 g of PMMA were dissolved in 10 mL of ethyl acetate before the required amount (0.5 g) of CMK-3 carbon was added. Aer 2 hours of ultrasonic treatment, a uniform mixture
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was formed and then most of the solvent was evaporated at 55 C under continuous ultrasonication. Aerwards, the viscous mixture was transformed into a mold and dried in an oven at 40 C. The nal composite obtained by the solvent mixing technique was simply denoted as CMK-3/PMMA, in which the weight percent of CMK-3 addition was also 10 wt% for a comparison purpose with the above in situ polymerization technique result. 2.4
Characterizations
The morphology and mesopore structure were observed using a SEM (JSM-6700F JEOL, 10.0 KV) equipped with an EDS elemental analysis system, and a TEM (JEM-2100F JEOL, 200 KV). Thermogravimetric (TG) analysis was carried out using a thermal analysis system (STA 449/C, Netzsch) with the samples heated in nitrogen at a heating rate of 10 C min1. The chemical structures were characterized using a Fourier transform infrared spectrometer (FT-IR, Bruker, Vertex 70, Germany) using a KBr pellet. The nitrogen adsorption measurements were performed at 196 C to investigate the pore structure of pristine CMK-3 and PMMA-g-CMK-3 using a Micromeritics ASAP2010 System (Micromeritics, Norcross, GA). A Hall Effect Measurement System (model 7704A) was used to measure the electric conductivity of the composites at room temperature. The relative permittivity and permeability were measured by using a vector network analyzer (HP 8722ES) in the X-band. The composites were accurately cut to a size of 22.86 mm 10.16 mm to t the waveguide sample holder of the network analyzer. The microwave absorbing properties were deduced by calculating based on the transmission line theory related to the permittivity and permeability measured experimentally.
3.
Results and discussion
In the current study, a surface covalent route was adopted to modify the surface of CMK-3 carbon via in situ graing and growth of PMMA chains. This procedure is expected to occur during the polymerization step, resulting in the formation of PMMA-graed CMK-3 carbon (PMMA-g-CMK-3) particles. Efficient surface modication of mesoporous carbons is benecial for the fast, complete dispersion of carbon particles in the polymer matrix. In order to characterize CMK-3 carbon aer in situ graing of PMMA, the viscous CMK-3/PMMA liquid prepared through in situ polymerization was dispersed in acetone and the carbon particles were recovered by centrifugation (3000 rpm, 15 min). The rinsing procedure was repeated more than 5 times to completely eliminate non-graed PMMA molecules as well as un-reacted MMA monomers from CMK-3 particles. Aer drying, the modied CMK-3 was obtained, which was denoted as PMMA-g-CMK-3. The formation of PMMA chains in situ graed onto CMK-3 particles was conrmed by FT-IR spectra. As shown in Fig. 1, the FT-IR spectrum of the original CMK-3 carbon shows the characteristic bands at wavenumbers ranging from 600 to 800 cm1, which are attributed to the C–H vibrations.32 The bands appearing at 1010–1260 cm1 are assigned to the OH bending,
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Fig. 1
FT-IR spectra of pristine CMK-3 and PMMA-g-CMK-3 carbon.
whereas those at 1370–1440 cm1 are derived from the alcoholic, phenolic, and carboxylic groups. The band at around 1600 cm1 is due to the olenic C]C bonds. The band at 3500 cm1 is attributable to surface OH groups and/or adsorbed moisture. Obviously, CMK-3 carbon aer in situ graing of PMMA has more surface oxygen functional groups compared to original CMK-3. A signicant increase in band intensities is observed because the PMMA polymer was graed onto the carbon surface. Moreover, there are several additional bands due to new functional groups in the spectrum of modied CMK-3 carbon. The C]O bending and C–O stretching vibration in PMMA chains are found at 760 and 1150 cm1, respectively. The absorption bands at 1700–1760 cm1 are the characteristics of the C]O stretching vibration, directly proving the presence of aldehyde groups.33 The symmetric and asymmetric stretching of methyl (CH3) groups are also observed at 2880–2980 cm1. The formation of new peaks ascribed to PMMA with respect to the pristine CMK-3 carbon strongly indicates the existence of chemically bonded PMMA chains on CMK-3 carbon. It is believed that the active MMA monomers reacted with the surface functional groups of CMK-3 during the polymerization procedure, thus forming the covalent bonds linking PMMA chains with the surface of CMK-3 carbon.34 Ying et al. reported that the radicals of small molecules could be directly graed onto the surface of carbon nanotubes through an addition reaction with double bonds.35 Thus the graing mechanism of PMMA chains on the surface of CMK-3 carbon is suggested as shown in Scheme 1. During the in situ polymerization process,
Scheme 1 Schematic principle of the radical grafting of PMMA onto OMC via an in situ polymerization procedure.
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the MMA monomers generate short chain radicals of PMMA in the presence of AIBN as the initiator. Some of the radical chains are chemically bonded to CMK-3 carbon through the addition reaction of the surface defects and C]C double bonds on the surface of CMK-3, producing PMMA-g-CMK-3. Meanwhile, most of the radical chains continue to polymerize together and then terminate the polymerization procedure, resulting in the formation of free PMMA chains that are un-graed onto CMK-3 carbon. Notably, this graing process has potential to extend the utilization with other vinyl monomers, such as styrene, acrylates and reactive dienes, which can be polymerized by free radical polymerization. The nitrogen adsorption measurements were performed at 196 C to investigate the pore structure of pristine CMK-3 and PMMA-g-CMK-3. The textural properties are listed in Table 1. Fig. 2 shows nitrogen sorption isotherms and pore size distributions of CMK-3 carbon before and aer surface graing. Both pristine CMK-3 and PMMA-g-CMK-3 exhibit representative type IV isotherms with a H1 hysteresis loop, which presents typical adsorption in mesoporous materials.36,37 In the relative pressure region of P/P0 ¼ 0.4 0.6, there is a well-dened step, indicating a uniform mesopore size distribution.9,38 Indeed, a narrow pore size distribution centred at 4.2 nm is observed for pristine CMK-3. With the surface modication via in situ polymerization of PMMA, the mesopore size decreases to 4.0 nm. The BET surface area and pore volume of the PMMA-g-CMK-3 sample are slightly reduced to 1076 m2 g1 and 1.34 cm3 g1, respectively (Table 1). These results give a hint that the chemically bonded PMMA chains exist on the surface and also in the mesopore of CMK-3 carbon, thus leading to a decrease in pore size, surface area, and pore volume. Notably, PMMA-g-CMK-3 still possesses high porosity, which will decrease the density of PMMA-g-CMK3-based bulk composites, making it possible to design a lighter and thinner electromagnetic wave absorber. The pristine CMK-3 carbon particle is composed of many aggregated rod-like particles in the sub-micrometer size (Fig. 3a), which maintains the structural morphology of its parent SBA-15 template.36,37,39 The transmission electron microscopy (TEM) micrograph indicates the 2D ordered arrangements of the carbon nanowires (Fig. 3b). Aer in situ graing of the PMMA polymer, the CMK-3 carbon particles still retain a macroscopic rod-like morphology (Fig. 3c), similar to original CMK-3. This observation indicates that in situ graing of the PMMA polymer does not change the appearance of CMK3 particles. These results are to some extent consistent with those of nitrogen adsorption analysis. The in situ polymerization of the PMMA polymer on CMK-3 carbon does not collapse the mesostructure, and just cause a little decrease in surface area and pore volume. The corresponding energy dispersive spectrum (EDS) (Fig. 3d) conrms that the main elements in
Table 1
Fig. 2 (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of pristine CMK-3 and PMMA-g-CMK-3.
modied CMK-3 carbons are C and O. The weight content of O reaches 4.8 wt%, implying that the carbon particles contain some organic matter. This elemental analysis matches well with the results of FT-IR which shows the existence of oxygen-containing functional groups derived from PMMA chains in modied CMK-3 carbon. Fig. 3d also shows the existence of a small amount of Si element, which is due to the residual SiO2 derived from the SBA-15 template. Fig. 4 shows TGA curves of pristine CMK-3, pure PMMA, and PMMA graed CMK-3 collected in a nitrogen atmosphere. The
Textural properties of pristine CMK-3 and PMMA-g-CMK-3
Sample
SBET (m2 g1)
VBJH (cm3 g1)
Dpore (nm)
CMK-3 PMMA-g-CMK-3
1329 1076
1.45 1.34
4.2 4.0
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Fig. 3 (a) SEM micrograph and (b) TEM micrograph of an original CMK-3 particle; (c) SEM micrograph and (d) EDS elemental analysis of a PMMA-g-CMK-3 particle.
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Fig. 4 TGA plots of original CMK-3, pure PMMA, and PMMA-g-CMK-3 measured in flowing nitrogen.
original CMK-3 is stable with negligible weight loss in the tested temperature range. Pure PMMA decomposes completely above 440 C and the remained weight is 5.3 wt%. In PMMA-functionalized CMK-3, the PMMA chains are degraded at 250– 500 C, then leaving the thermally stable CMK-3 carbon. This temperature range is clearly higher than that for the degradation of pure PMMA, indicating a higher thermal stability of the PMMA polymer graed onto the carbon surfaces. The TGA plot of PMMA-g-CMK-3 reveals a weight loss of 18.3 wt%. Therefore, the graing ratio, dened as the weight percent of the covalently graed polymer with respect to carbon, can be calculated to be 19 wt% by comparing and analyzing the TGA plots. The critical step for designing high performance EM wave absorbers lies in the homogeneous dispersion of CMK-3 carbon within the polymer matrix.32 In present research, two different procedures are adopted to prepare CMK-3/polymer composites with a loading of 10 wt% CMK-3. One is the solvent mixing method and another is the in situ polymerization procedure. The paragraphs mentioned before indicate that PMMA can be chemically graed onto the surface of CMK-3 carbon particles during the in situ polymerization procedure. The in situ surface modication is expected to improve the dispersion of CMK-3 carbon particles in the polymer matrix, thus resulting in high electric conductivity and enhanced microwave absorbing performance. The cross-sectional SEM micrograph shows that pristine rodlike CMK-3 carbon particles are completely wrapped by the PMMA matrix in the composite prepared by the solvent mixing procedure (Fig. 5a), and thus the carbon particles are easily aggregated, resulting in an un-uniform dispersion in the PMMA matrix. Comparatively, the CMK-3 carbons are easily dispersed within the PMMA matrix through the in situ polymerization (the inset in Fig. 5b, one pull-out particle at the fracture surface has the same morphology as the carbon rod illustrated in Fig. 3c), because these surface-modied carbon particles linked by PMMA chains can form bonding with the molecule chains of the PMMA matrix and can be easily pulled-out from the matrix undergoing fracture. The rather uniform dispersion of modied-carbon particles in PMMA further ensures the formation of
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Fig. 5 Cross-sectional SEM micrograph of the PMMA-based bulky composite containing 10 wt% CMK-3, prepared by (a) solvent mixing procedure, with pristine CMK-3 carbon particles (rod-like ones) wrapped by the PMMA matrix, and (b) in situ polymerization method. The inset shows surface-modified and well dispersed carbon particles in the PMMA matrix.
an electric conducting network within the matrix. Thus the PMMA-g-CMK-3/PMMA composite prepared by the in situ polymerization method will demonstrate a higher electric conductivity than the CMK-3/PMMA composite prepared by a simple solvent mixing procedure, discussed in the later part. The electric conductivity measurement shows that the in situ polymerization method is more efficient to prepare CMK-3 composited PMMA having a higher electric conductivity compared to the solvent mixing method (Fig. 6). The electric conductivity of the CMK-3/PMMA composite obtained by solvent mixing is 1.34 103 S m1, while the electric conductivity of the composite prepared by the in situ polymerization technique is as high as 0.437 S m1, about two orders of magnitude higher than that obtained by the solvent mixing
Fig. 6 DC conductivity (sdc) of 10 wt% CMK-3/PMMA composites prepared by solvent mixing and in situ polymerization techniques, respectively, measured at room temperature.
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Paper process. The solvent mixing process is not benecial for the mesoporous carbon's uniform dispersion since the carbon rods are completely wrapped by the matrix. The wrapping or isolation of the carbon rods by the polymer is detrimental to establishing the conductive network.40 The PMMA-g-CMK-3 conductive llers are sufficiently well dispersed in the PMMA matrix and keep connecting together, which promotes the development of an interconnected conductive network. More importantly, the suitable conductivity range of the composite, normally 0.1–10 S m1, is in favour of the absorption of electromagnetic waves. In contrast, the electric insulation (low conductivity) can result in very poor absorbance of electromagnetic energy and too high conductivity leads to much electromagnetic reection on the surface of the material.41 Thus the conductivity (0.437 S m1) of the composite prepared by our in situ polymerization procedure exactly meets the requirement for electric conductivity of microwave absorbing materials, with an optimal addition of 10 wt% CMK-3 to balance the absorbance and reection of electromagnetic energy. So it can be believed that the PMMA-g-CMK-3/PMMA composite has better ability than CMK-3/PMMA in microwave absorbing. Either the in situ polymerization or solvent mixing method has a similar purpose to disperse carbon particles:27 separation of the aggregated CMK-3 carbon particles through permeation of the polymer and then dispersion of the separated carbon particles within the polymer matrix. However, it is suggested that there are two major different dispersing effects between these two procedures. First, during the in situ polymerization the permeation occurs via MMA monomers and/or pre-polymerized short chain PMMA, thus this kind of permeation of short chains is more efficient than that through direct long chain PMMA molecules during the solvent mixing method. Second, it has been conrmed that the growing radical chains are able to gra onto the surface of carbon particles via the formation of a covalent bond in the in situ polymerization procedure.42 The PMMA functionalized CMK-3 particles are more benecial for further dispersion in the polymer matrix, leading to a signicant improvement in electric conductivity. The complex permittivity of the composite has direct inuence on the microwave absorbing performance, in which the imaginary part (300 ) of complex permittivity represents the electromagnetic power loss ability. Fig. 7 shows the complex permittivity of two CMK-3 lled PMMA bulky composites prepared by different methods, in a frequency range of 8.2– 12.4 GHz. It can be found that the real part (30 ) of the complex permittivity value of the pristine CMK-3/PMMA decreases from 11.5 to 11.0, while its imaginary part of the complex permittivity's (300 ) value is ca. 1.8 in the measured frequency range. The permittivity values of the PMMA-g-CMK-3/PMMA composite exhibit a similar trend with respect to frequency as that of pristine CMK-3/PMMA, but the PMMA-g-CMK-3/PMMA composite has higher complex permittivity values due to higher electric conductivity. In particular, the 300 value of PMMA-grafted-CMK-3/PMMA shows a dramatic increase compared to that of pristine CMK-3/PMMA. Thus the enhanced microwave absorbing efficiency will be obtained for the PMMA-graedCMK-3/PMMA composite due to high dielectric loss.
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Fig. 7 Frequency dependencies of complex permittivity for two PMMA-based composites prepared by solvent mixing (CMK-3/PMMA) and in situ polymerization (PMMA-g-CMK-3/PMMA).
Furthermore, in contrast to the features of 30 and 300 , both the real (m0 ) and imaginary (m00 ) parts of complex permeability for these two composites demonstrate little change as a function of frequency in the X-band because of the non-magnetism property of carbon (not shown here). The m0 is 1 and the m00 is nearly 0 in both systems. It is usually believed that an important mechanism of absorbing an electromagnetic wave by a dielectric loss material arises from the relaxation process.43 In the polarization process in an electromagnetic eld, a fraction of EM energies is irreversibly transformed to Joule thermal energies. According to the Debye relaxation equation, the complex permittivity can be written as44 3r ¼ 3N þ
3s 3N ¼ 30 ð f Þ þ j300 ð f Þ 1 þ j2pf s
(1)
where f is the frequency of the EM wave, s is the relaxation time, 3s is the static dielectric constant, and 3N is the dielectric constant at innite frequency. From eqn (1), it can be deduced that 30 ð f Þ ¼ 3N þ
300 ð f Þ ¼
3s 3N 1 þ ð2pf Þ2 s2
2pf sð3s 3N Þ 1 þ ð2pf Þ2 s2
(2)
(3)
The Debye relaxation process can be expressed as follows aer frequency f is eliminated according to eqn (2) and (3): (30 3N)2 + (300 )2 ¼ (3s 3N)2
(4)
Thus, the plot of 30 versus 300 is a single semicircle, which can be dened as the Cole–Cole or Debye semicircle.45,46 Fig. 8b shows the curve characteristic of 30 versus 300 , which presents a clear segment of two distorted Cole–Cole semicircles and one overlapped small semicircle for the in situ polymerized CMK3/PMMA composite. However, only a big irregular semicircle with one overlapped semicircle for the CMK-3/PMMA composite
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Fig. 9 The plots of 30 vs. B0(B0 ¼ 30 0 /f) for CMK-3/PMMA and PMMA-g-CMK3/PMMA composites, in which the slope equation is k ¼ 1/2ps.
Fig. 8 Cole–Cole semi-circles for (a) CMK-3/PMMA and (b) PMMA-g-CMK3/PMMA composites.
prepared by solvent mixing is found in Fig. 8a. It suggests that multiple relaxation processes occurred for PMMA-g-CMK3/PMMA. The additional relaxation could be ascribed to the polarization relaxation resulted from the graed PMMA on the surface of CMK-3. Moreover, by rearranging eqn (2) and (3), it can be indicated that 30 is a function of 300 and f, namely, 30 ¼ 300 /2pfs + 3N.47 Thus, if the dielectric loss is only a consequence of dipolar polarization, the plot of 30 versus B0(B0 ¼ 300 /f) would be linear. The in situ polymerized CMK-3/PMMA composite presents approximately ternary linear variations whereas the CMK-3/PMMA composite prepared by solvent mixing exhibits binary linear variations, as shown in Fig. 9. And each line corresponds to one Debye relaxation process, the slopes (k ¼ 1/2ps) of which can be tted. Thus the calculated relaxation time for the curve of the CMK3/PMMA composite is s1 ¼ 6.09 1012 s and s2 ¼ 1.1.89 1011 s, which correspond to the frequencies (8.41 GHz and 26.1 GHz) of Debye polarization ( fr ¼ 1/2ps), respectively. However there are three relaxation times s1 ¼ 5.94 1012 s, s2 ¼ 1.77 1011 s, and s3 ¼ 3.28 1011 s for the curve of PMMAg-CMK-3/PMMA, and the corresponding frequencies of Debye polarization are 4.85 GHz, 8.97 GHz and 26.78 GHz, respectively. The multi-dielectric polarization of the in situ polymerized CMK-3/PMMA composite probably results from the following three reasons: (1) the conductivity (s < 1.4 S cm1) of pure mesoporous carbon additive in the composite is in the range of semiconductor's conductivity.34 Thus the weakly bound
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electrons under an external electromagnetic eld can absorb some energy and then transit from a lower energy level to a higher energy level in an excited state. These electrons directly transfer from a cation node to another cation node in succession, forming the electronic polarization relaxation; (2) the defective sites and residual chemical groups can act as permanent dipolar polarization centres under the altering electromagnetic eld, forming another kind of dielectric polarization; (3) in the electric eld, the heterogeneous interface between mesoporous carbon and polymer matrix prevents the movement of free electrons and thus induces space charge accumulation at the interfaces, resulting in interfacial polarization. Furthermore, loading PMMA chains on the surface of CMK-3 carbon through the in situ polymerization process further increases surface dangling bonds, and induces the improved interfacial polarization. Under electromagnetic eld, these dielectric relaxations can transfer the electromagnetic energy to heat energy, causing the profound absorbance loss of microwave. Due to its high dielectric loss, the in situ polymerized PMMA/CMK-3 composite could be an excellent candidate as a microwave absorption material. According to the transmission line theory for a single-layer absorber, the measured values of 30 , 300 , m0 , and m00 are used to determine the reection loss of the asprepared composites based on a model for a single-layer planewave absorber. The input impedance (Zin) in the air to absorber surface is given by48 rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffi tan h j (5) Zin ¼ mr 3r 3r c where mr ¼ m0 jm00 and 3r ¼ 30 j300 are the relative complex permeability and permittivity of the absorber medium,
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respectively, while Z0 ¼ 377 U is the wave impedance, f is the frequency of the electromagnetic wave and d is the thickness of an absorber. The reection loss (RL) in decibels (dB) is then determined as Zin Z0 R ¼ 20 log (6) Zin þ Z0 The microwave absorption properties of the as-prepared composites lled with original CMK-3 and PMMA-g-CMK-3 are displayed in Fig. 10, where the minimum reection loss is equivalent to the maximum absorption of incident microwave power. As a function of frequency with a matching thickness of 2 mm, the 10 wt% CMK-3/PMMA composite prepared by the mixing technique shows a minimum reection loss of 10 dB and there is no absorption lower than 10 dB. However, for the CMK-3/PMMA composite prepared by in situ polymerization with a same weight fraction of CMK-3 and matching thickness, a superior microwave absorption performance is clearly observed. The minimum reection loss of 27 dB is obtained at 10.7 GHz, which means that only 0.2% of the incident radiation is reected. This means that the PMMA-g-CMK-3/PMMA composite can be commercially used as an efficient microwave absorbing material. The maximum microwave absorbance efficiency for the in situ polymerized sample signicantly increases compared to that for the pristine sample. The effective absorption bandwidth with a reection loss below 10 dB ranges from 9.2 to 12.4 GHz (3.2 GHz in width). The low reection loss is a direct consequence of the high dielectric loss as well as impedance matching of the PMMA-g-CMK-3/PMMA composite.22 It is well known, the main requirements for an electromagnetic wave absorber are as follows: rst, it should minimize the reection at the interface between air and absorber; second, it should increase the absorption of the electromagnetic wave by improving the dielectric and magnetic losses.49,50 In general, the microwave absorption mechanism of composites lled with conductive components is mainly due to dielectric loss of the electromagnetic wave.51 In other words, the absorption of microwaves is mainly due to dielectric dispersion properties.22 A homogeneous dispersion of the conducting llers is benecial for the formation of a conductive network.
Fig. 10 Frequency dependence of reflection loss (RL) values of 10 wt% CMK-3 PMMA-based composites with a matching thickness of 2 mm.
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Nanoscale The higher conductivity of the PMMA-g-CMK-3/PMMA composite enhances the electronic polarization, thus improving the microwave absorption performance. Meanwhile, the interface polarization, resulting from the heterogeneous interfaces in the in situ polymerized CMK-3/PMMA composite, could also lead to further loss of electromagnetic energy. Moreover, in order to prepare highly efficient electromagnetic wave absorbers, the reection of the incident radiation on the surface of the absorber must be kept as low as possible.52 This prerequisite is hardly satised because the dispersed conductive ller dramatically increases the conductivity of the polymer matrix, at the same time introducing a signicant microwave reection. In comparison with other OMC/resin composites with high amount of 20–40 wt% OMCs,53,54 an evidently low amount (10 wt%) of CMK-3 carbon is lled into the PMMA matrix in our CMK-3/PMMA composite prepared by the in situ polymerization method. Therefore our composite maintains a relatively low dielectric constant while the dielectric loss is signicantly enhanced, leading to both a better impedance matching and a lower microwave reection. So the PMMAg-CMK-3/PMMA composite has excellent microwave absorption properties. According to one of the parameter requirements for zeroreection, the microwave frequency at maximum absorbance and the thickness of the absorbing layer gives a relationship as the following:55 f ¼
c 2pm00 d
(7)
where d is the thickness of the absorbing layer. The frequency range at the maximum absorbance performance can be affected by m00 , but our CMK-3-based composites have a very low, but constant m00 value in the measured frequency range. Therefore, the frequency at maximum absorbance performance is highly sensitive to the thickness of the PMMA-g-CMK-3/PMMA composite. Fig. 11 shows the frequency dependence of the reection loss values of the PMMA-g-CMK-3/PMMA composite at different matching thicknesses. The maximum attenuation peak of the incident microwave regularly moves from 8.5 to 12.2 GHz when the matching thickness decreases from 2.4 to
Fig. 11 Frequency dependence of the RL values of the CMK-3/PMMA composite prepared by in situ polymerization at different matching thicknesses.
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4.
Conclusions
In summary, a CMK-3/PMMA composite was successfully prepared by in situ polymerization of MMA monomers in the presence of CMK-3 carbon particles under solvent-free conditions. Due to the radical nature of the polymerization, some growing PMMA short chains can be graed onto CMK-3 carbon particles. This process leads to efficient surface modication on carbon particles by the formation of chemical covalent bonds. With a high loading of 19 wt% of PMMA on CMK-3 carbon, the in situ modied CMK-3 still maintains a high surface area and mesoporous structure which are benecial for increasing the microwave absorbing performance. This in situ polymerization technique, much less solventconsuming than the mixing process, affords the PMMA-g-CMK3/PMMA composite with a higher electric conductivity (0.437 S m1), about two orders of magnitude higher than that (1.34 103 S m1) for the composite prepared by the solvent mixing method. The surface modication of CMK-3 evidently enhances the electronic polarization relaxation, dielectric polarization, and interfacial polarization, compared to pristine CMK-3. As a result, the in situ prepared CMK-3/PMMA composite exhibits excellent microwave absorption performance compared to CMK-3/PMMA prepared by the solvent mixing method. The minimum RL value for PMMA-g-CMK3/PMMA is 27 dB at a matching thickness of 2 mm and the bandwidth (RL lower than 10 dB) is over 3 GHz in the X-band. The maximum microwave absorbance efficiency for the in situ polymerized sample increases markedly compared to that (10 dB) for the sample prepared by the solvent mixing method. Changing the thickness of the absorber can efficiently adjust the frequency corresponding to the best microwave absorbance ability, which may meet the requirements for the microwave absorbing materials at different frequencies. This in situ polymerization for surface modication of mesoporous carbons opens up a new method and idea for developing light-weight and high-performance polymer-based microwave absorbing materials.
Acknowledgements This work was nancially supported by the Technological Innovation Fund of Shanghai Institute of Ceramic (Project no. Y21ZC8180G) and the One Hundred Talents Plan of Chinese Academy of Sciences.
Notes and references 1 Z. Chen, C. Xu, C. Ma, W. Ren and H. M. Cheng, Adv. Mater., 2013, 25, 1296. 2 Z. Liu, G. Bai, Y. Huang, F. Li, Y. Ma, T. Guo, X. He, X. Lin, H. Gao and Y. Chen, J. Phys. Chem. C, 2007, 111, 13696.
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Nanoscale 42 J.-M. Thomassin, I. Huynen, R. Jerome and C. Detrembleur, Polymer, 2010, 51, 115. 43 C. Wang, X. Han, P. Xu, X. Zhang, Y. Du, S. Hu, J. Wang and X. Wang, Appl. Phys. Lett., 2011, 98, 072906. 44 J. Frenkel and J. Dorfman, Nature, 1930, 126, 274. 45 P. Xu, X. Han, C. Wang, D. Zhou, Z. Lv, A. Wen, X. Wang and B. Zhang, J. Phys. Chem. B, 2008, 112, 10443. 46 X. Dong, X. Zhang, H. Huang and F. Zuo, Appl. Phys. Lett., 2008, 92, 013127. 47 X. Zhang, P. Guan and X. Dong, Appl. Phys. Lett., 2010, 96, 223111. 48 J. Liu, R. Che, H. Chen, F. Zhang, F. Xia, Q. Wu and M. Wang, Small, 2012, 8, 1214. 49 T. Giannakopoulou, L. Kompotiatis, A. Kontogeorgakos and G. Kordas, J. Magn. Magn. Mater., 2002, 246, 360. 50 M. Cao, R. Qin, C. Qiu and J. Zhu, Mater. Des., 2003, 24, 391. 51 C. Yan, X. Cheng, Y. Zhang, D. Yin, C. Gong, L. Yu, J. Zhang and Z. Zhang, J. Phys. Chem. C, 2012, 116, 26006. 52 J.-M. Thomassin, C. Pagnoulle, L. Bednarz, I. Huynen, R. Jerome and C. Detrembleur, J. Mater. Chem., 2008, 18, 792. 53 J. H. Zhou, J. P. He, G. X. Li, T. Wang, D. Sun, X. C. Ding, J. Q. Zhao and S. C. Wu, J. Phys. Chem. C, 2010, 114, 7611. 54 T. Wang, J. He, J. Zhou, J. Tang, Y. Guo, X. Ding, S. Wu and J. Zhao, J. Solid State Chem., 2010, 183, 2797. 55 P. Xu, X. Han, X. Liu, B. Zhang, C. Wang and X. Wang, Mater. Chem. Phys., 2009, 114, 556.
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A covalent route for efficient surface modification of ordered mesoporous carbon as high performance microwave absorbers Hu Zhou,ab Jiacheng Wang,*a Jiandong Zhuanga and Qian Liu*a A covalent route has been successfully utilized for the surface modification of ordered mesoporous carbon (OMC) CMK-3 by in situ polymerization and grafting of methyl methacrylate (MMA) in the absence of any solvent. The modified CMK-3 carbon particles have a high loading of 19 wt% poly(methyl methacrylate) (PMMA), named PMMA-g-CMK-3, and also maintain their high surface area and mesoporous structure. The in situ polymerization technique endows a significantly enhanced electric conductivity (0.437 S m1) of the resulting PMMA-g-CMK-3/PMMA composite, about two orders of magnitude higher than 1.34 103 S m1 of PMMA/CMK-3 obtained by the solvent mixing method. A minimum reflection loss (RL) value of 27 dB and a broader absorption band (over 3 GHz) with RL values 99%), acetone, a,a0 -azoisobutyronitrile (AIBN), sucrose, hydrouoric acid, sulfuric acid, and ethyl acetate were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Ordered mesoporous silica SBA-15 powders were purchased from Fudan University, China. All chemicals were used as received without any further purication. Distilled water was used in all experiments. 2.2
Synthesis of CMK-3 carbon
The ordered mesoporous carbon CMK-3 was prepared using mesoporous silica SBA-15 as the solid template and sucrose as the carbon source, following the procedure described in the literature.30 Briey, SBA-15 (1.00 g) was mixed with an aqueous solution (5 mL) consisting of sucrose (1.25 g) and sulfuric acid (0.14 g) under continuous stirring. The mixture was heated in an oven at 100 C for 6 hours, followed by further treatment at 160 C for another 6 hours to allow partial carbonization. This heat treatment was repeated aer the as-carbonized silica powders were impregnated again with additional sucrose of 0.80 g, sulfuric acid of 0.09 g, and deionized water of 5 mL. The CMK-3/SBA-15 composite powder was achieved by further pyrolysis at 900 C for 4 hours under a N2 ow to induce complete carbonization. The SBA-15 silica template was then removed by dissolving in 5 wt% hydrouoric acid. The nal CMK-3 product was washed and then dried at 100 C overnight. 2.3 Fabrication of CMK-3/polymer composites by two different procedures 2.3.1 In situ polymerization technique. In a typical experiment, 0.5 g of CMK-3 carbon was dispersed in 4.5 g of methyl methacrylate (MMA) monomer by ultrasonication for 30 min at room temperature. Then, the polymerization of MMA monomers was conducted using 2,20 -azoisobutyronitrile (AIBN) as an initiator in a capped glass tube. The mixture was agitated with a magnetic stirrer at 75 C. When the monomer conversion reached 25%, the reaction was terminated by cooling and the viscous sample was poured into a mold before the polymerization process was completed in an oven at 50 C for 10 hours. The resulting bulky composite was named PMMA-g-CMK-3/PMMA and the weight percent of CMK-3 was 10 wt% in the composite. Here PMMA-g-CMK-3 means PMMA-graed CMK-3 carbon particles with a loading of 19 wt% PMMA. 2.3.2 Solvent mixing technique. The solvent mixing method has been the most usually applied dispersion technique.29,31 Mixing two or more substance materials together forms a mixture which are physically but not chemically combined. The bulky CMK-3/PMMA composite was also prepared using the traditional solvent mixing method.20 Typically, 4.5 g of PMMA were dissolved in 10 mL of ethyl acetate before the required amount (0.5 g) of CMK-3 carbon was added. Aer 2 hours of ultrasonic treatment, a uniform mixture
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was formed and then most of the solvent was evaporated at 55 C under continuous ultrasonication. Aerwards, the viscous mixture was transformed into a mold and dried in an oven at 40 C. The nal composite obtained by the solvent mixing technique was simply denoted as CMK-3/PMMA, in which the weight percent of CMK-3 addition was also 10 wt% for a comparison purpose with the above in situ polymerization technique result. 2.4
Characterizations
The morphology and mesopore structure were observed using a SEM (JSM-6700F JEOL, 10.0 KV) equipped with an EDS elemental analysis system, and a TEM (JEM-2100F JEOL, 200 KV). Thermogravimetric (TG) analysis was carried out using a thermal analysis system (STA 449/C, Netzsch) with the samples heated in nitrogen at a heating rate of 10 C min1. The chemical structures were characterized using a Fourier transform infrared spectrometer (FT-IR, Bruker, Vertex 70, Germany) using a KBr pellet. The nitrogen adsorption measurements were performed at 196 C to investigate the pore structure of pristine CMK-3 and PMMA-g-CMK-3 using a Micromeritics ASAP2010 System (Micromeritics, Norcross, GA). A Hall Effect Measurement System (model 7704A) was used to measure the electric conductivity of the composites at room temperature. The relative permittivity and permeability were measured by using a vector network analyzer (HP 8722ES) in the X-band. The composites were accurately cut to a size of 22.86 mm 10.16 mm to t the waveguide sample holder of the network analyzer. The microwave absorbing properties were deduced by calculating based on the transmission line theory related to the permittivity and permeability measured experimentally.
3.
Results and discussion
In the current study, a surface covalent route was adopted to modify the surface of CMK-3 carbon via in situ graing and growth of PMMA chains. This procedure is expected to occur during the polymerization step, resulting in the formation of PMMA-graed CMK-3 carbon (PMMA-g-CMK-3) particles. Efficient surface modication of mesoporous carbons is benecial for the fast, complete dispersion of carbon particles in the polymer matrix. In order to characterize CMK-3 carbon aer in situ graing of PMMA, the viscous CMK-3/PMMA liquid prepared through in situ polymerization was dispersed in acetone and the carbon particles were recovered by centrifugation (3000 rpm, 15 min). The rinsing procedure was repeated more than 5 times to completely eliminate non-graed PMMA molecules as well as un-reacted MMA monomers from CMK-3 particles. Aer drying, the modied CMK-3 was obtained, which was denoted as PMMA-g-CMK-3. The formation of PMMA chains in situ graed onto CMK-3 particles was conrmed by FT-IR spectra. As shown in Fig. 1, the FT-IR spectrum of the original CMK-3 carbon shows the characteristic bands at wavenumbers ranging from 600 to 800 cm1, which are attributed to the C–H vibrations.32 The bands appearing at 1010–1260 cm1 are assigned to the OH bending,
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Fig. 1
FT-IR spectra of pristine CMK-3 and PMMA-g-CMK-3 carbon.
whereas those at 1370–1440 cm1 are derived from the alcoholic, phenolic, and carboxylic groups. The band at around 1600 cm1 is due to the olenic C]C bonds. The band at 3500 cm1 is attributable to surface OH groups and/or adsorbed moisture. Obviously, CMK-3 carbon aer in situ graing of PMMA has more surface oxygen functional groups compared to original CMK-3. A signicant increase in band intensities is observed because the PMMA polymer was graed onto the carbon surface. Moreover, there are several additional bands due to new functional groups in the spectrum of modied CMK-3 carbon. The C]O bending and C–O stretching vibration in PMMA chains are found at 760 and 1150 cm1, respectively. The absorption bands at 1700–1760 cm1 are the characteristics of the C]O stretching vibration, directly proving the presence of aldehyde groups.33 The symmetric and asymmetric stretching of methyl (CH3) groups are also observed at 2880–2980 cm1. The formation of new peaks ascribed to PMMA with respect to the pristine CMK-3 carbon strongly indicates the existence of chemically bonded PMMA chains on CMK-3 carbon. It is believed that the active MMA monomers reacted with the surface functional groups of CMK-3 during the polymerization procedure, thus forming the covalent bonds linking PMMA chains with the surface of CMK-3 carbon.34 Ying et al. reported that the radicals of small molecules could be directly graed onto the surface of carbon nanotubes through an addition reaction with double bonds.35 Thus the graing mechanism of PMMA chains on the surface of CMK-3 carbon is suggested as shown in Scheme 1. During the in situ polymerization process,
Scheme 1 Schematic principle of the radical grafting of PMMA onto OMC via an in situ polymerization procedure.
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the MMA monomers generate short chain radicals of PMMA in the presence of AIBN as the initiator. Some of the radical chains are chemically bonded to CMK-3 carbon through the addition reaction of the surface defects and C]C double bonds on the surface of CMK-3, producing PMMA-g-CMK-3. Meanwhile, most of the radical chains continue to polymerize together and then terminate the polymerization procedure, resulting in the formation of free PMMA chains that are un-graed onto CMK-3 carbon. Notably, this graing process has potential to extend the utilization with other vinyl monomers, such as styrene, acrylates and reactive dienes, which can be polymerized by free radical polymerization. The nitrogen adsorption measurements were performed at 196 C to investigate the pore structure of pristine CMK-3 and PMMA-g-CMK-3. The textural properties are listed in Table 1. Fig. 2 shows nitrogen sorption isotherms and pore size distributions of CMK-3 carbon before and aer surface graing. Both pristine CMK-3 and PMMA-g-CMK-3 exhibit representative type IV isotherms with a H1 hysteresis loop, which presents typical adsorption in mesoporous materials.36,37 In the relative pressure region of P/P0 ¼ 0.4 0.6, there is a well-dened step, indicating a uniform mesopore size distribution.9,38 Indeed, a narrow pore size distribution centred at 4.2 nm is observed for pristine CMK-3. With the surface modication via in situ polymerization of PMMA, the mesopore size decreases to 4.0 nm. The BET surface area and pore volume of the PMMA-g-CMK-3 sample are slightly reduced to 1076 m2 g1 and 1.34 cm3 g1, respectively (Table 1). These results give a hint that the chemically bonded PMMA chains exist on the surface and also in the mesopore of CMK-3 carbon, thus leading to a decrease in pore size, surface area, and pore volume. Notably, PMMA-g-CMK-3 still possesses high porosity, which will decrease the density of PMMA-g-CMK3-based bulk composites, making it possible to design a lighter and thinner electromagnetic wave absorber. The pristine CMK-3 carbon particle is composed of many aggregated rod-like particles in the sub-micrometer size (Fig. 3a), which maintains the structural morphology of its parent SBA-15 template.36,37,39 The transmission electron microscopy (TEM) micrograph indicates the 2D ordered arrangements of the carbon nanowires (Fig. 3b). Aer in situ graing of the PMMA polymer, the CMK-3 carbon particles still retain a macroscopic rod-like morphology (Fig. 3c), similar to original CMK-3. This observation indicates that in situ graing of the PMMA polymer does not change the appearance of CMK3 particles. These results are to some extent consistent with those of nitrogen adsorption analysis. The in situ polymerization of the PMMA polymer on CMK-3 carbon does not collapse the mesostructure, and just cause a little decrease in surface area and pore volume. The corresponding energy dispersive spectrum (EDS) (Fig. 3d) conrms that the main elements in
Table 1
Fig. 2 (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of pristine CMK-3 and PMMA-g-CMK-3.
modied CMK-3 carbons are C and O. The weight content of O reaches 4.8 wt%, implying that the carbon particles contain some organic matter. This elemental analysis matches well with the results of FT-IR which shows the existence of oxygen-containing functional groups derived from PMMA chains in modied CMK-3 carbon. Fig. 3d also shows the existence of a small amount of Si element, which is due to the residual SiO2 derived from the SBA-15 template. Fig. 4 shows TGA curves of pristine CMK-3, pure PMMA, and PMMA graed CMK-3 collected in a nitrogen atmosphere. The
Textural properties of pristine CMK-3 and PMMA-g-CMK-3
Sample
SBET (m2 g1)
VBJH (cm3 g1)
Dpore (nm)
CMK-3 PMMA-g-CMK-3
1329 1076
1.45 1.34
4.2 4.0
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Fig. 3 (a) SEM micrograph and (b) TEM micrograph of an original CMK-3 particle; (c) SEM micrograph and (d) EDS elemental analysis of a PMMA-g-CMK-3 particle.
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Fig. 4 TGA plots of original CMK-3, pure PMMA, and PMMA-g-CMK-3 measured in flowing nitrogen.
original CMK-3 is stable with negligible weight loss in the tested temperature range. Pure PMMA decomposes completely above 440 C and the remained weight is 5.3 wt%. In PMMA-functionalized CMK-3, the PMMA chains are degraded at 250– 500 C, then leaving the thermally stable CMK-3 carbon. This temperature range is clearly higher than that for the degradation of pure PMMA, indicating a higher thermal stability of the PMMA polymer graed onto the carbon surfaces. The TGA plot of PMMA-g-CMK-3 reveals a weight loss of 18.3 wt%. Therefore, the graing ratio, dened as the weight percent of the covalently graed polymer with respect to carbon, can be calculated to be 19 wt% by comparing and analyzing the TGA plots. The critical step for designing high performance EM wave absorbers lies in the homogeneous dispersion of CMK-3 carbon within the polymer matrix.32 In present research, two different procedures are adopted to prepare CMK-3/polymer composites with a loading of 10 wt% CMK-3. One is the solvent mixing method and another is the in situ polymerization procedure. The paragraphs mentioned before indicate that PMMA can be chemically graed onto the surface of CMK-3 carbon particles during the in situ polymerization procedure. The in situ surface modication is expected to improve the dispersion of CMK-3 carbon particles in the polymer matrix, thus resulting in high electric conductivity and enhanced microwave absorbing performance. The cross-sectional SEM micrograph shows that pristine rodlike CMK-3 carbon particles are completely wrapped by the PMMA matrix in the composite prepared by the solvent mixing procedure (Fig. 5a), and thus the carbon particles are easily aggregated, resulting in an un-uniform dispersion in the PMMA matrix. Comparatively, the CMK-3 carbons are easily dispersed within the PMMA matrix through the in situ polymerization (the inset in Fig. 5b, one pull-out particle at the fracture surface has the same morphology as the carbon rod illustrated in Fig. 3c), because these surface-modied carbon particles linked by PMMA chains can form bonding with the molecule chains of the PMMA matrix and can be easily pulled-out from the matrix undergoing fracture. The rather uniform dispersion of modied-carbon particles in PMMA further ensures the formation of
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Fig. 5 Cross-sectional SEM micrograph of the PMMA-based bulky composite containing 10 wt% CMK-3, prepared by (a) solvent mixing procedure, with pristine CMK-3 carbon particles (rod-like ones) wrapped by the PMMA matrix, and (b) in situ polymerization method. The inset shows surface-modified and well dispersed carbon particles in the PMMA matrix.
an electric conducting network within the matrix. Thus the PMMA-g-CMK-3/PMMA composite prepared by the in situ polymerization method will demonstrate a higher electric conductivity than the CMK-3/PMMA composite prepared by a simple solvent mixing procedure, discussed in the later part. The electric conductivity measurement shows that the in situ polymerization method is more efficient to prepare CMK-3 composited PMMA having a higher electric conductivity compared to the solvent mixing method (Fig. 6). The electric conductivity of the CMK-3/PMMA composite obtained by solvent mixing is 1.34 103 S m1, while the electric conductivity of the composite prepared by the in situ polymerization technique is as high as 0.437 S m1, about two orders of magnitude higher than that obtained by the solvent mixing
Fig. 6 DC conductivity (sdc) of 10 wt% CMK-3/PMMA composites prepared by solvent mixing and in situ polymerization techniques, respectively, measured at room temperature.
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Paper process. The solvent mixing process is not benecial for the mesoporous carbon's uniform dispersion since the carbon rods are completely wrapped by the matrix. The wrapping or isolation of the carbon rods by the polymer is detrimental to establishing the conductive network.40 The PMMA-g-CMK-3 conductive llers are sufficiently well dispersed in the PMMA matrix and keep connecting together, which promotes the development of an interconnected conductive network. More importantly, the suitable conductivity range of the composite, normally 0.1–10 S m1, is in favour of the absorption of electromagnetic waves. In contrast, the electric insulation (low conductivity) can result in very poor absorbance of electromagnetic energy and too high conductivity leads to much electromagnetic reection on the surface of the material.41 Thus the conductivity (0.437 S m1) of the composite prepared by our in situ polymerization procedure exactly meets the requirement for electric conductivity of microwave absorbing materials, with an optimal addition of 10 wt% CMK-3 to balance the absorbance and reection of electromagnetic energy. So it can be believed that the PMMA-g-CMK-3/PMMA composite has better ability than CMK-3/PMMA in microwave absorbing. Either the in situ polymerization or solvent mixing method has a similar purpose to disperse carbon particles:27 separation of the aggregated CMK-3 carbon particles through permeation of the polymer and then dispersion of the separated carbon particles within the polymer matrix. However, it is suggested that there are two major different dispersing effects between these two procedures. First, during the in situ polymerization the permeation occurs via MMA monomers and/or pre-polymerized short chain PMMA, thus this kind of permeation of short chains is more efficient than that through direct long chain PMMA molecules during the solvent mixing method. Second, it has been conrmed that the growing radical chains are able to gra onto the surface of carbon particles via the formation of a covalent bond in the in situ polymerization procedure.42 The PMMA functionalized CMK-3 particles are more benecial for further dispersion in the polymer matrix, leading to a signicant improvement in electric conductivity. The complex permittivity of the composite has direct inuence on the microwave absorbing performance, in which the imaginary part (300 ) of complex permittivity represents the electromagnetic power loss ability. Fig. 7 shows the complex permittivity of two CMK-3 lled PMMA bulky composites prepared by different methods, in a frequency range of 8.2– 12.4 GHz. It can be found that the real part (30 ) of the complex permittivity value of the pristine CMK-3/PMMA decreases from 11.5 to 11.0, while its imaginary part of the complex permittivity's (300 ) value is ca. 1.8 in the measured frequency range. The permittivity values of the PMMA-g-CMK-3/PMMA composite exhibit a similar trend with respect to frequency as that of pristine CMK-3/PMMA, but the PMMA-g-CMK-3/PMMA composite has higher complex permittivity values due to higher electric conductivity. In particular, the 300 value of PMMA-grafted-CMK-3/PMMA shows a dramatic increase compared to that of pristine CMK-3/PMMA. Thus the enhanced microwave absorbing efficiency will be obtained for the PMMA-graedCMK-3/PMMA composite due to high dielectric loss.
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Fig. 7 Frequency dependencies of complex permittivity for two PMMA-based composites prepared by solvent mixing (CMK-3/PMMA) and in situ polymerization (PMMA-g-CMK-3/PMMA).
Furthermore, in contrast to the features of 30 and 300 , both the real (m0 ) and imaginary (m00 ) parts of complex permeability for these two composites demonstrate little change as a function of frequency in the X-band because of the non-magnetism property of carbon (not shown here). The m0 is 1 and the m00 is nearly 0 in both systems. It is usually believed that an important mechanism of absorbing an electromagnetic wave by a dielectric loss material arises from the relaxation process.43 In the polarization process in an electromagnetic eld, a fraction of EM energies is irreversibly transformed to Joule thermal energies. According to the Debye relaxation equation, the complex permittivity can be written as44 3r ¼ 3N þ
3s 3N ¼ 30 ð f Þ þ j300 ð f Þ 1 þ j2pf s
(1)
where f is the frequency of the EM wave, s is the relaxation time, 3s is the static dielectric constant, and 3N is the dielectric constant at innite frequency. From eqn (1), it can be deduced that 30 ð f Þ ¼ 3N þ
300 ð f Þ ¼
3s 3N 1 þ ð2pf Þ2 s2
2pf sð3s 3N Þ 1 þ ð2pf Þ2 s2
(2)
(3)
The Debye relaxation process can be expressed as follows aer frequency f is eliminated according to eqn (2) and (3): (30 3N)2 + (300 )2 ¼ (3s 3N)2
(4)
Thus, the plot of 30 versus 300 is a single semicircle, which can be dened as the Cole–Cole or Debye semicircle.45,46 Fig. 8b shows the curve characteristic of 30 versus 300 , which presents a clear segment of two distorted Cole–Cole semicircles and one overlapped small semicircle for the in situ polymerized CMK3/PMMA composite. However, only a big irregular semicircle with one overlapped semicircle for the CMK-3/PMMA composite
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Fig. 9 The plots of 30 vs. B0(B0 ¼ 30 0 /f) for CMK-3/PMMA and PMMA-g-CMK3/PMMA composites, in which the slope equation is k ¼ 1/2ps.
Fig. 8 Cole–Cole semi-circles for (a) CMK-3/PMMA and (b) PMMA-g-CMK3/PMMA composites.
prepared by solvent mixing is found in Fig. 8a. It suggests that multiple relaxation processes occurred for PMMA-g-CMK3/PMMA. The additional relaxation could be ascribed to the polarization relaxation resulted from the graed PMMA on the surface of CMK-3. Moreover, by rearranging eqn (2) and (3), it can be indicated that 30 is a function of 300 and f, namely, 30 ¼ 300 /2pfs + 3N.47 Thus, if the dielectric loss is only a consequence of dipolar polarization, the plot of 30 versus B0(B0 ¼ 300 /f) would be linear. The in situ polymerized CMK-3/PMMA composite presents approximately ternary linear variations whereas the CMK-3/PMMA composite prepared by solvent mixing exhibits binary linear variations, as shown in Fig. 9. And each line corresponds to one Debye relaxation process, the slopes (k ¼ 1/2ps) of which can be tted. Thus the calculated relaxation time for the curve of the CMK3/PMMA composite is s1 ¼ 6.09 1012 s and s2 ¼ 1.1.89 1011 s, which correspond to the frequencies (8.41 GHz and 26.1 GHz) of Debye polarization ( fr ¼ 1/2ps), respectively. However there are three relaxation times s1 ¼ 5.94 1012 s, s2 ¼ 1.77 1011 s, and s3 ¼ 3.28 1011 s for the curve of PMMAg-CMK-3/PMMA, and the corresponding frequencies of Debye polarization are 4.85 GHz, 8.97 GHz and 26.78 GHz, respectively. The multi-dielectric polarization of the in situ polymerized CMK-3/PMMA composite probably results from the following three reasons: (1) the conductivity (s < 1.4 S cm1) of pure mesoporous carbon additive in the composite is in the range of semiconductor's conductivity.34 Thus the weakly bound
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electrons under an external electromagnetic eld can absorb some energy and then transit from a lower energy level to a higher energy level in an excited state. These electrons directly transfer from a cation node to another cation node in succession, forming the electronic polarization relaxation; (2) the defective sites and residual chemical groups can act as permanent dipolar polarization centres under the altering electromagnetic eld, forming another kind of dielectric polarization; (3) in the electric eld, the heterogeneous interface between mesoporous carbon and polymer matrix prevents the movement of free electrons and thus induces space charge accumulation at the interfaces, resulting in interfacial polarization. Furthermore, loading PMMA chains on the surface of CMK-3 carbon through the in situ polymerization process further increases surface dangling bonds, and induces the improved interfacial polarization. Under electromagnetic eld, these dielectric relaxations can transfer the electromagnetic energy to heat energy, causing the profound absorbance loss of microwave. Due to its high dielectric loss, the in situ polymerized PMMA/CMK-3 composite could be an excellent candidate as a microwave absorption material. According to the transmission line theory for a single-layer absorber, the measured values of 30 , 300 , m0 , and m00 are used to determine the reection loss of the asprepared composites based on a model for a single-layer planewave absorber. The input impedance (Zin) in the air to absorber surface is given by48 rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffi tan h j (5) Zin ¼ mr 3r 3r c where mr ¼ m0 jm00 and 3r ¼ 30 j300 are the relative complex permeability and permittivity of the absorber medium,
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respectively, while Z0 ¼ 377 U is the wave impedance, f is the frequency of the electromagnetic wave and d is the thickness of an absorber. The reection loss (RL) in decibels (dB) is then determined as Zin Z0 R ¼ 20 log (6) Zin þ Z0 The microwave absorption properties of the as-prepared composites lled with original CMK-3 and PMMA-g-CMK-3 are displayed in Fig. 10, where the minimum reection loss is equivalent to the maximum absorption of incident microwave power. As a function of frequency with a matching thickness of 2 mm, the 10 wt% CMK-3/PMMA composite prepared by the mixing technique shows a minimum reection loss of 10 dB and there is no absorption lower than 10 dB. However, for the CMK-3/PMMA composite prepared by in situ polymerization with a same weight fraction of CMK-3 and matching thickness, a superior microwave absorption performance is clearly observed. The minimum reection loss of 27 dB is obtained at 10.7 GHz, which means that only 0.2% of the incident radiation is reected. This means that the PMMA-g-CMK-3/PMMA composite can be commercially used as an efficient microwave absorbing material. The maximum microwave absorbance efficiency for the in situ polymerized sample signicantly increases compared to that for the pristine sample. The effective absorption bandwidth with a reection loss below 10 dB ranges from 9.2 to 12.4 GHz (3.2 GHz in width). The low reection loss is a direct consequence of the high dielectric loss as well as impedance matching of the PMMA-g-CMK-3/PMMA composite.22 It is well known, the main requirements for an electromagnetic wave absorber are as follows: rst, it should minimize the reection at the interface between air and absorber; second, it should increase the absorption of the electromagnetic wave by improving the dielectric and magnetic losses.49,50 In general, the microwave absorption mechanism of composites lled with conductive components is mainly due to dielectric loss of the electromagnetic wave.51 In other words, the absorption of microwaves is mainly due to dielectric dispersion properties.22 A homogeneous dispersion of the conducting llers is benecial for the formation of a conductive network.
Fig. 10 Frequency dependence of reflection loss (RL) values of 10 wt% CMK-3 PMMA-based composites with a matching thickness of 2 mm.
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Nanoscale The higher conductivity of the PMMA-g-CMK-3/PMMA composite enhances the electronic polarization, thus improving the microwave absorption performance. Meanwhile, the interface polarization, resulting from the heterogeneous interfaces in the in situ polymerized CMK-3/PMMA composite, could also lead to further loss of electromagnetic energy. Moreover, in order to prepare highly efficient electromagnetic wave absorbers, the reection of the incident radiation on the surface of the absorber must be kept as low as possible.52 This prerequisite is hardly satised because the dispersed conductive ller dramatically increases the conductivity of the polymer matrix, at the same time introducing a signicant microwave reection. In comparison with other OMC/resin composites with high amount of 20–40 wt% OMCs,53,54 an evidently low amount (10 wt%) of CMK-3 carbon is lled into the PMMA matrix in our CMK-3/PMMA composite prepared by the in situ polymerization method. Therefore our composite maintains a relatively low dielectric constant while the dielectric loss is signicantly enhanced, leading to both a better impedance matching and a lower microwave reection. So the PMMAg-CMK-3/PMMA composite has excellent microwave absorption properties. According to one of the parameter requirements for zeroreection, the microwave frequency at maximum absorbance and the thickness of the absorbing layer gives a relationship as the following:55 f ¼
c 2pm00 d
(7)
where d is the thickness of the absorbing layer. The frequency range at the maximum absorbance performance can be affected by m00 , but our CMK-3-based composites have a very low, but constant m00 value in the measured frequency range. Therefore, the frequency at maximum absorbance performance is highly sensitive to the thickness of the PMMA-g-CMK-3/PMMA composite. Fig. 11 shows the frequency dependence of the reection loss values of the PMMA-g-CMK-3/PMMA composite at different matching thicknesses. The maximum attenuation peak of the incident microwave regularly moves from 8.5 to 12.2 GHz when the matching thickness decreases from 2.4 to
Fig. 11 Frequency dependence of the RL values of the CMK-3/PMMA composite prepared by in situ polymerization at different matching thicknesses.
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Nanoscale 1.8 mm. Therefore, the microwave absorbing properties can be modulated simply by changing the matching thickness of the PMMA-g-CMK-3/PMMA composite in practical applications at different frequency bands.
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4.
Conclusions
In summary, a CMK-3/PMMA composite was successfully prepared by in situ polymerization of MMA monomers in the presence of CMK-3 carbon particles under solvent-free conditions. Due to the radical nature of the polymerization, some growing PMMA short chains can be graed onto CMK-3 carbon particles. This process leads to efficient surface modication on carbon particles by the formation of chemical covalent bonds. With a high loading of 19 wt% of PMMA on CMK-3 carbon, the in situ modied CMK-3 still maintains a high surface area and mesoporous structure which are benecial for increasing the microwave absorbing performance. This in situ polymerization technique, much less solventconsuming than the mixing process, affords the PMMA-g-CMK3/PMMA composite with a higher electric conductivity (0.437 S m1), about two orders of magnitude higher than that (1.34 103 S m1) for the composite prepared by the solvent mixing method. The surface modication of CMK-3 evidently enhances the electronic polarization relaxation, dielectric polarization, and interfacial polarization, compared to pristine CMK-3. As a result, the in situ prepared CMK-3/PMMA composite exhibits excellent microwave absorption performance compared to CMK-3/PMMA prepared by the solvent mixing method. The minimum RL value for PMMA-g-CMK3/PMMA is 27 dB at a matching thickness of 2 mm and the bandwidth (RL lower than 10 dB) is over 3 GHz in the X-band. The maximum microwave absorbance efficiency for the in situ polymerized sample increases markedly compared to that (10 dB) for the sample prepared by the solvent mixing method. Changing the thickness of the absorber can efficiently adjust the frequency corresponding to the best microwave absorbance ability, which may meet the requirements for the microwave absorbing materials at different frequencies. This in situ polymerization for surface modication of mesoporous carbons opens up a new method and idea for developing light-weight and high-performance polymer-based microwave absorbing materials.
Acknowledgements This work was nancially supported by the Technological Innovation Fund of Shanghai Institute of Ceramic (Project no. Y21ZC8180G) and the One Hundred Talents Plan of Chinese Academy of Sciences.
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