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Conductive carbon black-filled polymethacrylate composites as gas sensing materials: Effect of glass transition temperature
Thin Solid Films 516 (2008) 7886–7890
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Conductive carbon black-filled polymethacrylate composites as gas sensing materials: Effect of glass transition temperature Xian Ming Dong a,b,⁎, Ying Luo a,b, Li Na Xie a, Ruo Wen Fu b, Ming Qiu Zhang b a b
Department of Applied Chemistry, College of Science, South China Agricultural University, Guangzhou 510642, China Materials Science Institute, PCFM laboratory, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
A R T I C L E
I N F O
Article history: Received 28 April 2007 Received in revised form 25 May 2008 Accepted 3 June 2008 Available online 8 June 2008 Keywords: Gas sensors Electrical properties Composite materials Carbon black Polymer
A B S T R A C T To reveal the influence of glass transition temperature of matrix polymer on electric resistivity and electrical response to organic solvent vapors of carbon black (CB) -filled polymer composites, conductive CB/poly (methyl methacrylate), CB/poly(butyl methacrylate) and CB/poly(2-ethylhexyl methacrylate) composites were fabricated by polymerization filling. It is found that the composites obtained exhibit a lower percolation threshold and a slower electrical response rate when the glass transition temperature of the matrix polymer is higher. Because the dispersion status of CB particles in the composites is related to the glass transition temperature or viscosity of the matrix polymer, and influences the composite conductivity. Low viscosity of the matrix polymer would certainly benefit the diffusion of solvent molecules in the matrix polymer and decrease the response time of the composites against organic vapors. These results would help to choose a suitable polymer as the polymer matrix and to understand the electrical response behavior of the composites as promising gas-sensing materials. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Conductive thermoplastic composites comprised of carbon black and insulating polymer matrix as liquid or vapor sensors have attracted widespread interests [1–7]. The polymer matrix of the composites would swell up when the materials are exposed to volatile organic compounds (VOCs). It decreases the connectivity between the conductive particles within the composite film, and the electrical resistance of these composites exhibits a drastic increase. On the basis of this feature, the composites have been exploited for the fabrication of chemisensors and vapor detector arrays or “electronic noses” capable of quantifying and discriminating various organic vapors in chemical and petrochemical industry, food industry, environmental monitoring [8]. So far, it is known that carbon black-filled polymer composites as sensing materials have many advantages, including easy fabrication with cost effectiveness, stability in many different environments, rapid response rate and high sensitivity to the targets, high selectivity, and miniature design. Many factors can influence electrical responsivity, reproducibility and stability of the composite sensors against organic solvent vapors, such as modification of carbon black surface by grafting polymerization [9], species and dispersity of carbon black in ⁎ Corresponding author. Department of Applied Chemistry, College of Science, South China Agricultural University, Guangzhou 510642, China. Tel.: +86 20 38295132; fax: +86 20 85282366. E-mail address: [email protected] (X.M. Dong). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.06.003
the composites [10,11], crystallinity of matrix polymer [12], molecular weight and molecular weight distribution of matrix polymer [13], crosslinking treatment of polymer matrix [14], addition of plasticizer [15], and composite preparation method or process [16]. In addition, changes in environmental conditions (such as temperature, species and concentration or pressure of organic vapors) also affect the sensing performance of the composites [13,17]. To breakdown large carbon black agglomerates into smaller aggregates and to decrease the percolation threshold of the composites, carbon black-filled polymer composites were prepared by polymerization filling [13,18]. It is expected that tiny monomer molecules might be able to penetrate into carbon black agglomerates, so that the subsequent polymerization would produce composites with better dispersity of carbon black particles and lower percolation threshold. To further understand the electric response behavior of the composites in organic vapors, the present work investigates the effect of the glass transition temperature, Tg, of the matrix polymer on the electrical resistivity and response to organic vapors of three conductive carbon black-filled polymer composites, including carbon black/poly(methyl methacrylate) (CB/PMMA), carbon black/poly(butyl methacrylate) (CB/PBMA) and carbon black/poly (2-ethylhexyl methacrylate) (CB/PEHMA) composites prepared by polymerization filling. PMMA, PBMA and PEHMA are selected as matrix polymer because of the difference in molecular structure and characteristics.
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2. Experimental details 2.1. Materials Conductive carbon black (XC-72) supplied by Cabot Inc., with a N2 specific surface area of about 254 m2/g and diameter of 50–70 nm, was dried in vacuum at 110 °C for 48 h before use. Besides, the monomer, methyl methacrylate MMA or butyl methacrylate (BMA), was distilled for two times and the initiator, benzoyl peroxide (BPO), was purified by recrystallization from chloroform and methanol prior to the polymerization filling process. 2-ethylhexyl methacrylate (EHMA) and other reagents or solvents were used as received. 2.2. Preparation of the composites Carbon black-filled polymer composites were prepared as follows. Typically, 2.0 g of CB, 18.0 g of MMA (or BMA, EHMA) and a small amount of BPO were added into a 100 ml flask with a reflux condenser. Having been treated by ultrasonic agitation for 1 h, the mixture was stirred at a certain temperature under nitrogen for 6–8 h. Afterwards, the product was added into 60–80 ml CCl4 and stirred for 1–2 h at room temperature to produce pasty composites. The polymerization conditions had been described in our previous papers in detail [18–20]. 2.3. Characterization The subsequent preparation of the specimens for measuring electrical resistance and electrical responses to saturated organic solvent vapors were carried out as follows. The conventional 7.5 × 2.5 cm2 glass slides were cut into 1.0 × 2.5 cm2 strips. Then two parallel copper wires (0.1 mm in diameter, at intervals of 1 mm) were coiled onto a piece of strip glass serving as electrodes. The above composite paste was uniformly coated onto the strip glass to form composite film 30–50 μm thick. The electric resistance variation of the composites against the organic solvent vapor was measured through a digital multimeter (model: DT890B+) by hanging the composite electrode in a glass container saturated with the organic solvent vapor. When the electrical resistance reached the maximum, the composite electrode was immediately moved out of the container. The parameter that quantifies the variation of composite resistance in saturated solvent vapor is named the maximum responsivity, Rmax/R0, where Rmax is the maximum resistance of the composites in organic vapor, R0 is the initial resistance of the composites in air before meeting the testing vapor. The response time, Δtmax, is defined as the
Fig. 1. Electrical resistivity of CB/PMMA, CB/PBMA and CB/PEHMA composites prepared by polymerization filling as a function of CB content.
Fig. 2. DSC heating curves of three carbon black-filled polymethacrylate composites and pure polymethacrylate.
time needed to reach the maximum responsivity. The responsivity is characterized by Rt/R0, where Rt is the transient resistance. Besides, electrical responses to low concentration organic vapors had been described in the previous paper in detail [17]. That is, the composite sensor hung in a 5.8 L sealed glass vessel, which contained a known concentration of organic vapor prepared by injecting a known quantity of certain organic solvent inside. The electrical resistance changes of the composites against organic vapors were also determined through a digital multimeter. When electrical resistance of the composites reached or approached its equilibrium value (300 s is required), the composite sensor was moved out of the vessel into air. The relative electrical resistance responsivity in low concentration vapor is defined as (Rt − R0) / R0. To observe the dispersion status of the conductive fillers in the matrix polymer, a JSM-6330F scanning electronic microscope (SEM) (produced by JEOL Co., LTD from Japan) was employed. The samples were obtained by freeze-fracture in liquid N2, and then gold sputtering prior to the observation. Differential scanning calorimetric (DSC) traces were recorded on a NETZSCH 204 analyzer at a heating rate of 10 °C/min under N2 atmosphere. 3. Results and discussion 3.1. Electrical properties of the composites As carbon black content exceeds a certain critical value (called percolation threshold), which is generally attributed to percolation phenomenon, the conductive networks are established throughout the matrix resulting in an insulator-conductor transition accordingly. To make a comparison between the composites with different matrix polymer, the CB content dependence of electrical resistivity of CB/ PMMA, CB/PBMA and CB/PEHMA composites prepared by polymerization filling is shown in Fig. 1. It is interesting to note that the percolation threshold (~ 2.1 vol.% or 3.3 wt.%) of CB/ PMMA composites is lower than that of CB/PBMA (~3.5 vol.% or 6 wt.%) or CB/PEHMA (~ 3.3 vol.% or 6 wt.%) composites. The electric resistivity of CB/PMMA composites is about 2–5 orders of magnitude lower than that of CB/PBMA or CB/ PEHMA composites at the same CB concentration. It means that the conductive networks in CB/PMMA composites are built more easily than other two composites. It is believed that the difference in the molecular structure of matrix polymer should take the responsibility. It is known that the viscosity of the matrix polymer decreases with the increase of its side chain length, following the order: PMMA N PBMA N PEHMA. Fig. 2 illustrates the DSC curves of three carbon blackfilled polymer composites and three pure matrix polymers obtained
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Table 1 Characteristics of polymer matrices in carbon black filled polymethacrylate composites Polymer matrix
Mn × 10− 4
PDIa
Tg (°C)
[η]b (ml·g− 1)
PMMA PBMA PEHMA
4.4 3.8 5.4
3.58 3.61 3.64
65 10 −12
4.02 × 107 1.08 × 107
a PDI: polydispersity index; b[η]: intrinsic viscosity (solvent: chloroform) at 20 °C, calculated according to the reference [21].
from the composites by the extraction method. It is seen that the glass transition of CB/PMMA composites or pure PMMA is perceivable while that of CB/PBMA composites, pure PBMA, CB/PEHMA composites or pure PEHMA is weak. The glass transition temperature of the composites or pure matrix polymer appears the same order as the viscosity. Table 1 shows the number-average molecular weight, polydispersity index, glass transition temperature and intrinsic viscosity of polymer matrix of the composites in Fig. 2. Obviously, glass transition temperature and intrinsic viscosity of polymer matrix appear notable decrease from PMMA to PBMA or PEHMA. This result indicates that the thermal property of CB/PMMA composites is different from that of CB/PBMA or CB/PEHMA composites. Usually, carbon black particles dispersed in the low viscosity matrix polymer are easier to agglomerate together in a way of the Brownian
Fig. 4. Response of electric resistance of carbon black-filled polymethacrylate composites against (a) CHCl3, (b) THF, and (c) C6H6 vapors at 25 °C. The dash lines define the vapor absorption and desorption zones.
Fig. 3. SEM micrographs of carbon black-filled polymethacrylate composites prepared by polymerization filling: (a) 11.0 wt.% CB/PMMA; (b) 14.0 wt.% CB/ PEHMA.
movement. Besides, the movement is more active at the temperature of above Tg of matrix polymer. With regard to CB/PMMA composites, it is difficult for well-dispersed carbon black particles to aggregate again through the Brownian movement because of high viscosity and Tg of PMMA Such a even dispersion status of CB particles will help to increase the probability of the formation of conductive paths in the composites at a low CB content. On the contrary, it is relatively easy that carbon black particles in CB/PBMA or CB/PEHMA composites agglomerate together. This makes the partial conductive networks in the composites damaged again and the electrical resistivity of the composites increasing. That is, carbon black has to present itself as
X.M. Dong et al. / Thin Solid Films 516 (2008) 7886–7890
Fig. 5. Electric resistance response of carbon black-filled polymethacrylate composites against ethyl benzene vapor at a concentration of 87 ppm at 25 °C. The dash lines define the vapor absorption and desorption zones.
larger agglomerates, reducing the probability of the formation of conduction networks as compared with smaller particles for the same filler content and arrangement. Therefore, the dispersion status of CB particles in a polymer matrix is a key factor that influences the composite conductivity. To evidence above consideration, the morphological structure of the composites and the dispersion status of CB particles in the matrix polymer should be examined by SEM. As seen in Fig. 3, the morphology for CB/ PMMA composites is quite different from that of CB/PEHMA composites. The fracture surface of the former is rather rough as characterized by “net eyes” and a uniform dispersion of CB particles in PMMA can be observed (Fig. 3(a)). But the fracture surface of the latter is rather flat and most of CB aggregates are surrounded by the matrix polymer (Fig. 3(b)). Carbon black aggregates in CB/PMMA composites are limited in the holes of the “net eyes” of PMMA owing to high Tg or viscosity of the matrix polymer. Feng et al [22] found that increased viscosity of the matrix polymer by crosslinking can stabilize the dispersion of CB particles in the matrix polymer and eliminate the negative temperature coefficient (NTC) effect of carbon black-filled polymer composites. Comparatively, carbon black aggregates in CB/PEHMA composites move easily due to low viscosity of the matrix polymer. It also demonstrates that the flexibility of polymer chains may play a dominant role in the dispersion of CB particles in the matrix polymer.
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in the matrix polymer and decrease the response time. On the base of the same reason, the recovery time of three carbon black-filled polymer composites in organic vapors has the same change as the response time, because absorption and desorption of solvent vapors are reversible process due to the characteristics of physisorption. Returning to Fig. 4, it is worth noting that when the electric resistances of CB/PBMA and CB/PEHMA composites in CHCl3 and C6H6 vapors get to the maxima, there are obvious drops in the curves instead of leveling off (see Fig. 4(a) and (c)). This is referred to as the negative vapor coefficient (NVC) effect [14], Similar to NTC effect. NVC effect results from the newly formed conducting networks. Since the mobility of CB particles is significantly increased in the swollen matrix, some damaged conducting paths due to volume expanding of the matrix polymer can be re-connected. As seen in Fig. 4, the NVC intensity of carbon black-filled polymethacrylate composites in organic vapors increases with the decreasing Tg or viscosity of the matrix polymer, following the order: PEHMA N PBMA N PMMA. Considering the importance of the sensing behavior against low concentration solvent vapors for practical applications, the electrical response of carbon black-filled polymethacrylate composites against ethyl benzene vapor as a function of time at a concentration of 87 ppm at 25 °C is shown in Fig. 5. The same phenomenon as Fig. 4 can be observed, that is, the response time increases with the Tg or viscosity of the matrix polymer: Δtmax [PEHMA] (70 s)b Δtmax [PBMA] (120 s) b Δtmax [PMMA] (300 s). This result indicates once more the Tg or viscosity of the matrix polymer plays an important role in the electrical response of the composites against organic vapors regardless of the vapor concentration. 3.3. Effect of testing temperature on electrical response Testing temperature is another key factor affecting electrical response of carbon black-filled polymer composites against organic vapors. Fig. 6 shows the effect of testing temperature on the maximum responsivity and responsive time of CB/PBMA composites against saturated toluene vapor. It is interesting to note that the maximum responsivity notably increases from 1745 times to 4633 times and the response time decreases from 300 s to 45 s when the testing temperature rises from 15 °C to 20 °C. Obviously, an increase in temperature, especially near or above Tg of PBMA, would promote the structural relaxation of the composites. This helps to accelerate the diffusion of solvent molecules in the composites and the swelling or dissolving of partial matrix polymer, resulting in the increase of response rate of the composites. On the other hand, the partial pressure or concentration of an organic solvent vapor increases with increasing temperature and the vapor absorption of the composites is accelerated. As a result, the maximum responsivity of the composites against organic vapors increases with testing temperature.
3.2. Electrical responses of the composite films in organic vapors Considering that the purpose of developing the current composites lies in the preparation of gas sensing materials, the electrical resistance responses of three carbon black-filled polymethacrylate composites against saturated CHCl3, tetrahydrofuran (THF) and C6H6 vapors as a function of time at 25 °C are shown in Fig. 4. Clearly, there is much difference in the response time and recovery time of three composites in organic vapors, especially between CB/PMMA composites and other two composites. For example, the response time in THF vapor follows the order as a function of the matrix polymer: Δtmax [PMMA] (480 s)N Δtmax [PBMA] (180 s)N Δtmax [PEHMA] (45 s). The authors believe that the difference in the response time or recovery time has a great relation with the Tg or viscosity of the matrix polymer. It has been demonstrated that the migration of solvent molecules in carbon black-filled polymer composites belongs to Case Π diffusion [16]. The matrix polymer must relax to respond to the osmotic swelling pressure and to rearrange the macromolecular chains for accommodating the penetrating molecules. Therefore, low viscosity of the matrix polymer would certainly benefit the diffusion of solvent molecules
Fig. 6. Effect of testing temperature on the maximum responsivity and the responsive time of CB/PBMA composites (CB content: 14 wt.%) against toluene vapor.
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4. Conclusions The electric resistivity of CB/PMMA composites synthesized by polymerization filling is about 2–5 orders of magnitude lower than that of CB/PBMA or CB/PEHMA composites at the same CB content. Because CB particles dispersed in the matrix polymer with low Tg or viscosity are easier to agglomerate together by the Brownian movement. On the other hand, the Tg or viscosity of the matrix polymer also plays an important role in electrical responses of the composites against organic vapors regardless of the vapor concentration, since low Tg or viscosity of the matrix polymer would benefit the diffusion of solvent molecules in the matrix polymer and decrease the response time. Depending on these results, one might understand the electrical response behavior of the composites as promising gas-sensing materials. Acknowledgements The financial support by the National Natural Science Foundation of China (Grant 50133020), the Team Project of the Natural Science Foundation of Guangdong (Grant 20003038), and the Natural Science Foundation of Guangdong (Grant 5300841) are gratefully acknowledged.
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J. Chen, H. Iwata, N. Tsubokawa, Y. Maekawa, M. Yoshida, Polymer 43 (2002) 2201. B.C. Sick, N.S. Lewis, Sens. Actuators B 96 (2003) 268. S.G. Chen, J.W. Hu, M.Q. Zhang, M.W. Li, M.Z. Rong, Carbon 42 (2004) 645. L. Quercia, F. Loffredo, M. Bombace, I. Nasti, G. Di Francia, Sens. Actuators B. 111–112 (2005) 166. K. Arshak, E. Moore, L. Cavanagh, C. Cunniffe, S. Clifford, Prog. Solid State Chem. 33 (2005) 199. K. Zribi, J.F. Feller, K. Elleuch, A. Bourmaud, B. Elleuch, Polym. Adv. Technol. 17 (2006) 727. Y.S. Kim, Sens. Actuators B 114 (2006) 410. K.J. Albert, N.S. Lewis, C.L. Schauer, G.A. Sotzing, S.F. Stitzel, T.P. Vaid, D.R. Walt, Chem. Rev. 100 (2000) 2595. N. Tsubokawa, Y. Shirai, M. Okazaki, K. Maruyama, Polym. Bull. 42 (1999) 425. L. Quercia, F. Loffredo, G. Di Francia, Sens. Actuators B 109 (2005) 153. J.F. Feller, Y. Grohens, Synth. Met. 154 (2005) 193. J.R. Li, J.R. Xu, M.Q. Zhang, M.Z. Rong, Q. Zheng, Polymer 46 (2005) 11051. J.R. Li, J.R. Xu, M.Q. Zhang, M.Z. Rong, Carbon 41 (2003) 2353. S.G. Chen, J.W. Hu, M.Q. Zhang, M.Z. Rong, Q. Zheng, Sens. Actuators B 113 (2006) 361. M.E. Koscho, R.H. Grubbs, N.S. Lewis, Anal. Chem. 74 (2002) 1307. X.M. Dong, R.W. Fu, M.Q. Zhang, Z.P. Qin, B. Zhang, M.Z. Rong, Polym. J. 35 (2003) 1003. X.M. Dong, R.W. Fu, M.Q. Zhang, B. Zhang, M.Z. Rong, Carbon 42 (2004) 2551. X.M. Dong, R.W. Fu, M.Q. Zhang, B. Zhang, J.R. Li, M.Z. Rong, Carbon 41 (2003) 371. X.M. Dong, R.W. Fu, M.Q. Zhang, B. Zhang, J.R. Li, M.Z. Rong, Polym. Bull. 50 (2003) 99. X.M. Dong, R.W. Fu, M.Q. Zhang, B. Zhang, M.Z. Rong, J. Mater. Sci. Lett. 22 (2003) 1057. J. Brandrup, E.H. Immergut, Polymer handbook, John Wiley & Sons. Inc., 1975. J. Feng, C.M. Chan, Polymer 41 (2000) 7279.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Conductive carbon black-filled polymethacrylate composites as gas sensing materials: Effect of glass transition temperature Xian Ming Dong a,b,⁎, Ying Luo a,b, Li Na Xie a, Ruo Wen Fu b, Ming Qiu Zhang b a b
Department of Applied Chemistry, College of Science, South China Agricultural University, Guangzhou 510642, China Materials Science Institute, PCFM laboratory, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
A R T I C L E
I N F O
Article history: Received 28 April 2007 Received in revised form 25 May 2008 Accepted 3 June 2008 Available online 8 June 2008 Keywords: Gas sensors Electrical properties Composite materials Carbon black Polymer
A B S T R A C T To reveal the influence of glass transition temperature of matrix polymer on electric resistivity and electrical response to organic solvent vapors of carbon black (CB) -filled polymer composites, conductive CB/poly (methyl methacrylate), CB/poly(butyl methacrylate) and CB/poly(2-ethylhexyl methacrylate) composites were fabricated by polymerization filling. It is found that the composites obtained exhibit a lower percolation threshold and a slower electrical response rate when the glass transition temperature of the matrix polymer is higher. Because the dispersion status of CB particles in the composites is related to the glass transition temperature or viscosity of the matrix polymer, and influences the composite conductivity. Low viscosity of the matrix polymer would certainly benefit the diffusion of solvent molecules in the matrix polymer and decrease the response time of the composites against organic vapors. These results would help to choose a suitable polymer as the polymer matrix and to understand the electrical response behavior of the composites as promising gas-sensing materials. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Conductive thermoplastic composites comprised of carbon black and insulating polymer matrix as liquid or vapor sensors have attracted widespread interests [1–7]. The polymer matrix of the composites would swell up when the materials are exposed to volatile organic compounds (VOCs). It decreases the connectivity between the conductive particles within the composite film, and the electrical resistance of these composites exhibits a drastic increase. On the basis of this feature, the composites have been exploited for the fabrication of chemisensors and vapor detector arrays or “electronic noses” capable of quantifying and discriminating various organic vapors in chemical and petrochemical industry, food industry, environmental monitoring [8]. So far, it is known that carbon black-filled polymer composites as sensing materials have many advantages, including easy fabrication with cost effectiveness, stability in many different environments, rapid response rate and high sensitivity to the targets, high selectivity, and miniature design. Many factors can influence electrical responsivity, reproducibility and stability of the composite sensors against organic solvent vapors, such as modification of carbon black surface by grafting polymerization [9], species and dispersity of carbon black in ⁎ Corresponding author. Department of Applied Chemistry, College of Science, South China Agricultural University, Guangzhou 510642, China. Tel.: +86 20 38295132; fax: +86 20 85282366. E-mail address: [email protected] (X.M. Dong). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.06.003
the composites [10,11], crystallinity of matrix polymer [12], molecular weight and molecular weight distribution of matrix polymer [13], crosslinking treatment of polymer matrix [14], addition of plasticizer [15], and composite preparation method or process [16]. In addition, changes in environmental conditions (such as temperature, species and concentration or pressure of organic vapors) also affect the sensing performance of the composites [13,17]. To breakdown large carbon black agglomerates into smaller aggregates and to decrease the percolation threshold of the composites, carbon black-filled polymer composites were prepared by polymerization filling [13,18]. It is expected that tiny monomer molecules might be able to penetrate into carbon black agglomerates, so that the subsequent polymerization would produce composites with better dispersity of carbon black particles and lower percolation threshold. To further understand the electric response behavior of the composites in organic vapors, the present work investigates the effect of the glass transition temperature, Tg, of the matrix polymer on the electrical resistivity and response to organic vapors of three conductive carbon black-filled polymer composites, including carbon black/poly(methyl methacrylate) (CB/PMMA), carbon black/poly(butyl methacrylate) (CB/PBMA) and carbon black/poly (2-ethylhexyl methacrylate) (CB/PEHMA) composites prepared by polymerization filling. PMMA, PBMA and PEHMA are selected as matrix polymer because of the difference in molecular structure and characteristics.
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2. Experimental details 2.1. Materials Conductive carbon black (XC-72) supplied by Cabot Inc., with a N2 specific surface area of about 254 m2/g and diameter of 50–70 nm, was dried in vacuum at 110 °C for 48 h before use. Besides, the monomer, methyl methacrylate MMA or butyl methacrylate (BMA), was distilled for two times and the initiator, benzoyl peroxide (BPO), was purified by recrystallization from chloroform and methanol prior to the polymerization filling process. 2-ethylhexyl methacrylate (EHMA) and other reagents or solvents were used as received. 2.2. Preparation of the composites Carbon black-filled polymer composites were prepared as follows. Typically, 2.0 g of CB, 18.0 g of MMA (or BMA, EHMA) and a small amount of BPO were added into a 100 ml flask with a reflux condenser. Having been treated by ultrasonic agitation for 1 h, the mixture was stirred at a certain temperature under nitrogen for 6–8 h. Afterwards, the product was added into 60–80 ml CCl4 and stirred for 1–2 h at room temperature to produce pasty composites. The polymerization conditions had been described in our previous papers in detail [18–20]. 2.3. Characterization The subsequent preparation of the specimens for measuring electrical resistance and electrical responses to saturated organic solvent vapors were carried out as follows. The conventional 7.5 × 2.5 cm2 glass slides were cut into 1.0 × 2.5 cm2 strips. Then two parallel copper wires (0.1 mm in diameter, at intervals of 1 mm) were coiled onto a piece of strip glass serving as electrodes. The above composite paste was uniformly coated onto the strip glass to form composite film 30–50 μm thick. The electric resistance variation of the composites against the organic solvent vapor was measured through a digital multimeter (model: DT890B+) by hanging the composite electrode in a glass container saturated with the organic solvent vapor. When the electrical resistance reached the maximum, the composite electrode was immediately moved out of the container. The parameter that quantifies the variation of composite resistance in saturated solvent vapor is named the maximum responsivity, Rmax/R0, where Rmax is the maximum resistance of the composites in organic vapor, R0 is the initial resistance of the composites in air before meeting the testing vapor. The response time, Δtmax, is defined as the
Fig. 1. Electrical resistivity of CB/PMMA, CB/PBMA and CB/PEHMA composites prepared by polymerization filling as a function of CB content.
Fig. 2. DSC heating curves of three carbon black-filled polymethacrylate composites and pure polymethacrylate.
time needed to reach the maximum responsivity. The responsivity is characterized by Rt/R0, where Rt is the transient resistance. Besides, electrical responses to low concentration organic vapors had been described in the previous paper in detail [17]. That is, the composite sensor hung in a 5.8 L sealed glass vessel, which contained a known concentration of organic vapor prepared by injecting a known quantity of certain organic solvent inside. The electrical resistance changes of the composites against organic vapors were also determined through a digital multimeter. When electrical resistance of the composites reached or approached its equilibrium value (300 s is required), the composite sensor was moved out of the vessel into air. The relative electrical resistance responsivity in low concentration vapor is defined as (Rt − R0) / R0. To observe the dispersion status of the conductive fillers in the matrix polymer, a JSM-6330F scanning electronic microscope (SEM) (produced by JEOL Co., LTD from Japan) was employed. The samples were obtained by freeze-fracture in liquid N2, and then gold sputtering prior to the observation. Differential scanning calorimetric (DSC) traces were recorded on a NETZSCH 204 analyzer at a heating rate of 10 °C/min under N2 atmosphere. 3. Results and discussion 3.1. Electrical properties of the composites As carbon black content exceeds a certain critical value (called percolation threshold), which is generally attributed to percolation phenomenon, the conductive networks are established throughout the matrix resulting in an insulator-conductor transition accordingly. To make a comparison between the composites with different matrix polymer, the CB content dependence of electrical resistivity of CB/ PMMA, CB/PBMA and CB/PEHMA composites prepared by polymerization filling is shown in Fig. 1. It is interesting to note that the percolation threshold (~ 2.1 vol.% or 3.3 wt.%) of CB/ PMMA composites is lower than that of CB/PBMA (~3.5 vol.% or 6 wt.%) or CB/PEHMA (~ 3.3 vol.% or 6 wt.%) composites. The electric resistivity of CB/PMMA composites is about 2–5 orders of magnitude lower than that of CB/PBMA or CB/ PEHMA composites at the same CB concentration. It means that the conductive networks in CB/PMMA composites are built more easily than other two composites. It is believed that the difference in the molecular structure of matrix polymer should take the responsibility. It is known that the viscosity of the matrix polymer decreases with the increase of its side chain length, following the order: PMMA N PBMA N PEHMA. Fig. 2 illustrates the DSC curves of three carbon blackfilled polymer composites and three pure matrix polymers obtained
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Table 1 Characteristics of polymer matrices in carbon black filled polymethacrylate composites Polymer matrix
Mn × 10− 4
PDIa
Tg (°C)
[η]b (ml·g− 1)
PMMA PBMA PEHMA
4.4 3.8 5.4
3.58 3.61 3.64
65 10 −12
4.02 × 107 1.08 × 107
a PDI: polydispersity index; b[η]: intrinsic viscosity (solvent: chloroform) at 20 °C, calculated according to the reference [21].
from the composites by the extraction method. It is seen that the glass transition of CB/PMMA composites or pure PMMA is perceivable while that of CB/PBMA composites, pure PBMA, CB/PEHMA composites or pure PEHMA is weak. The glass transition temperature of the composites or pure matrix polymer appears the same order as the viscosity. Table 1 shows the number-average molecular weight, polydispersity index, glass transition temperature and intrinsic viscosity of polymer matrix of the composites in Fig. 2. Obviously, glass transition temperature and intrinsic viscosity of polymer matrix appear notable decrease from PMMA to PBMA or PEHMA. This result indicates that the thermal property of CB/PMMA composites is different from that of CB/PBMA or CB/PEHMA composites. Usually, carbon black particles dispersed in the low viscosity matrix polymer are easier to agglomerate together in a way of the Brownian
Fig. 4. Response of electric resistance of carbon black-filled polymethacrylate composites against (a) CHCl3, (b) THF, and (c) C6H6 vapors at 25 °C. The dash lines define the vapor absorption and desorption zones.
Fig. 3. SEM micrographs of carbon black-filled polymethacrylate composites prepared by polymerization filling: (a) 11.0 wt.% CB/PMMA; (b) 14.0 wt.% CB/ PEHMA.
movement. Besides, the movement is more active at the temperature of above Tg of matrix polymer. With regard to CB/PMMA composites, it is difficult for well-dispersed carbon black particles to aggregate again through the Brownian movement because of high viscosity and Tg of PMMA Such a even dispersion status of CB particles will help to increase the probability of the formation of conductive paths in the composites at a low CB content. On the contrary, it is relatively easy that carbon black particles in CB/PBMA or CB/PEHMA composites agglomerate together. This makes the partial conductive networks in the composites damaged again and the electrical resistivity of the composites increasing. That is, carbon black has to present itself as
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Fig. 5. Electric resistance response of carbon black-filled polymethacrylate composites against ethyl benzene vapor at a concentration of 87 ppm at 25 °C. The dash lines define the vapor absorption and desorption zones.
larger agglomerates, reducing the probability of the formation of conduction networks as compared with smaller particles for the same filler content and arrangement. Therefore, the dispersion status of CB particles in a polymer matrix is a key factor that influences the composite conductivity. To evidence above consideration, the morphological structure of the composites and the dispersion status of CB particles in the matrix polymer should be examined by SEM. As seen in Fig. 3, the morphology for CB/ PMMA composites is quite different from that of CB/PEHMA composites. The fracture surface of the former is rather rough as characterized by “net eyes” and a uniform dispersion of CB particles in PMMA can be observed (Fig. 3(a)). But the fracture surface of the latter is rather flat and most of CB aggregates are surrounded by the matrix polymer (Fig. 3(b)). Carbon black aggregates in CB/PMMA composites are limited in the holes of the “net eyes” of PMMA owing to high Tg or viscosity of the matrix polymer. Feng et al [22] found that increased viscosity of the matrix polymer by crosslinking can stabilize the dispersion of CB particles in the matrix polymer and eliminate the negative temperature coefficient (NTC) effect of carbon black-filled polymer composites. Comparatively, carbon black aggregates in CB/PEHMA composites move easily due to low viscosity of the matrix polymer. It also demonstrates that the flexibility of polymer chains may play a dominant role in the dispersion of CB particles in the matrix polymer.
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in the matrix polymer and decrease the response time. On the base of the same reason, the recovery time of three carbon black-filled polymer composites in organic vapors has the same change as the response time, because absorption and desorption of solvent vapors are reversible process due to the characteristics of physisorption. Returning to Fig. 4, it is worth noting that when the electric resistances of CB/PBMA and CB/PEHMA composites in CHCl3 and C6H6 vapors get to the maxima, there are obvious drops in the curves instead of leveling off (see Fig. 4(a) and (c)). This is referred to as the negative vapor coefficient (NVC) effect [14], Similar to NTC effect. NVC effect results from the newly formed conducting networks. Since the mobility of CB particles is significantly increased in the swollen matrix, some damaged conducting paths due to volume expanding of the matrix polymer can be re-connected. As seen in Fig. 4, the NVC intensity of carbon black-filled polymethacrylate composites in organic vapors increases with the decreasing Tg or viscosity of the matrix polymer, following the order: PEHMA N PBMA N PMMA. Considering the importance of the sensing behavior against low concentration solvent vapors for practical applications, the electrical response of carbon black-filled polymethacrylate composites against ethyl benzene vapor as a function of time at a concentration of 87 ppm at 25 °C is shown in Fig. 5. The same phenomenon as Fig. 4 can be observed, that is, the response time increases with the Tg or viscosity of the matrix polymer: Δtmax [PEHMA] (70 s)b Δtmax [PBMA] (120 s) b Δtmax [PMMA] (300 s). This result indicates once more the Tg or viscosity of the matrix polymer plays an important role in the electrical response of the composites against organic vapors regardless of the vapor concentration. 3.3. Effect of testing temperature on electrical response Testing temperature is another key factor affecting electrical response of carbon black-filled polymer composites against organic vapors. Fig. 6 shows the effect of testing temperature on the maximum responsivity and responsive time of CB/PBMA composites against saturated toluene vapor. It is interesting to note that the maximum responsivity notably increases from 1745 times to 4633 times and the response time decreases from 300 s to 45 s when the testing temperature rises from 15 °C to 20 °C. Obviously, an increase in temperature, especially near or above Tg of PBMA, would promote the structural relaxation of the composites. This helps to accelerate the diffusion of solvent molecules in the composites and the swelling or dissolving of partial matrix polymer, resulting in the increase of response rate of the composites. On the other hand, the partial pressure or concentration of an organic solvent vapor increases with increasing temperature and the vapor absorption of the composites is accelerated. As a result, the maximum responsivity of the composites against organic vapors increases with testing temperature.
3.2. Electrical responses of the composite films in organic vapors Considering that the purpose of developing the current composites lies in the preparation of gas sensing materials, the electrical resistance responses of three carbon black-filled polymethacrylate composites against saturated CHCl3, tetrahydrofuran (THF) and C6H6 vapors as a function of time at 25 °C are shown in Fig. 4. Clearly, there is much difference in the response time and recovery time of three composites in organic vapors, especially between CB/PMMA composites and other two composites. For example, the response time in THF vapor follows the order as a function of the matrix polymer: Δtmax [PMMA] (480 s)N Δtmax [PBMA] (180 s)N Δtmax [PEHMA] (45 s). The authors believe that the difference in the response time or recovery time has a great relation with the Tg or viscosity of the matrix polymer. It has been demonstrated that the migration of solvent molecules in carbon black-filled polymer composites belongs to Case Π diffusion [16]. The matrix polymer must relax to respond to the osmotic swelling pressure and to rearrange the macromolecular chains for accommodating the penetrating molecules. Therefore, low viscosity of the matrix polymer would certainly benefit the diffusion of solvent molecules
Fig. 6. Effect of testing temperature on the maximum responsivity and the responsive time of CB/PBMA composites (CB content: 14 wt.%) against toluene vapor.
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4. Conclusions The electric resistivity of CB/PMMA composites synthesized by polymerization filling is about 2–5 orders of magnitude lower than that of CB/PBMA or CB/PEHMA composites at the same CB content. Because CB particles dispersed in the matrix polymer with low Tg or viscosity are easier to agglomerate together by the Brownian movement. On the other hand, the Tg or viscosity of the matrix polymer also plays an important role in electrical responses of the composites against organic vapors regardless of the vapor concentration, since low Tg or viscosity of the matrix polymer would benefit the diffusion of solvent molecules in the matrix polymer and decrease the response time. Depending on these results, one might understand the electrical response behavior of the composites as promising gas-sensing materials. Acknowledgements The financial support by the National Natural Science Foundation of China (Grant 50133020), the Team Project of the Natural Science Foundation of Guangdong (Grant 20003038), and the Natural Science Foundation of Guangdong (Grant 5300841) are gratefully acknowledged.
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