Effect of Water Vapor Adsorption on Electrical Properties of Carbon

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Effect of Water Vapor Adsorption on Electrical Properties of Carbon Nanotube/Nanocrystalline Cellulose Composites Salman Safari, and Theo G.M. van de Ven ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02374 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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ACS Applied Materials & Interfaces

Effect of Water Vapor Adsorption on Electrical Properties of Carbon Nanotube/Nanocrystalline Cellulose Composites Salman Safari†,‡ and Theo G. M. van de Ven∗,¶,‡ †Department of Chemical Engineering, McGill University, Montreal, Canada ‡Centre for Self-Assembled Chemical Structures, McGill University, Montreal, Canada ¶Pulp and Paper Research Centre, Department of Chemistry, McGill University, Montreal, Canada E-mail: [email protected],Phone:+1-514-398-6177

Abstract It has been long known that the electrical properties of cellulose are greatly influenced by adsorption of water vapor. Incorporating conductive nanofillers in a cellulose matrix is an example of approaches to tailor their characteristics for use in electronics and sensing devices. In this work, we introduce two new nanocomposites comprising carbon nanotubes (CNT) and conventional or electrosterically stabilized nanocrystalline celluloses matrices. While conventional nanocrystalline cellulose (NCC) consists of a rigid crystalline backbone, electrosterically stabilized cellulose (ENCC) is comprised of a rigid crystalline backbone with carboxylated

1 ACS Paragon Plus Environment


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polymers protruding from both ends. By tuning CNT loading, we can tailor a CNT/NCC composite with minimal electrical sensitivity to the ambient relative humidity, despite the fact that the composite has a high moisture uptake. The expected decrease in CNT conductivity upon water vapor adsorption, due to electron donation, is counterbalanced by an increase in the conductivity of NCC, due to proton hopping at an optimum CNT loading (1 to 2%). Contrary to the CNT/NCC composite, a CNT/ENCC composite at 1% CNT loading shows insulating behavior for relative humidities up to 75%, after which the composite becomes conductive. This interesting behavior can be ascribed to the low moisture uptake of ENCC at low and moderate relative humidities due to the limited number of hydroxyl groups and hydrogen bond formation between carboxyl groups on ENCC, which endow ENCC with limited water molecule adsorption sites.

Keywords carbon nanotubes, nanocrystalline cellulose, conductive films, humidity sensors, impedance spectroscopy

Introduction The annual production capacity of carbon nanotubes (CNT) is currently as high as a few kilotons, which are primarily used in bulk composite materials and thin films. 1 For example conductive CNT plastics are being used in the automotive industry as electrostatic charge dissipators in fuel lines and filters. 1,2 Moreover, transparent CNT composites can be employed as supercapacitors 3,4 or replace indium tin oxide (ITO) films for use in displays, touchscreen devices, and photovoltaics, due to the scarcity and brittleness of ITO films. 5,6 However, one of the major challenges


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of dispersing CNT in a matrix is that at high CNT loading conventional mixing techniques fail to avoid CNT aggregation, 7 which can decrease the electrical conductivity of the composite. 8 Cellulose-based composites are of great importance because of the abundance of cellulose and its biodegradability. 9 Cellulose nanomaterials are increasingly being studied for use in composites either as inclusions 10,11 or matrix, 12,13 along their use as bioimaging agents 14 or drug carriers. 15 In addition to the mentioned characteristics for cellulose, nano-sized cellulose particles have a high surface area with the possibility of introducing functional groups and can be dispersed readily in water, desirable for CNT composites, 16 with potential applications in optoelectronics, 17–19 supercapacitors, 20–23 and vapor sensing. 24,25 The electrical properties of cellulose are sensitive to the concentration of water vapor in the ambient surroundings, 26–28 mainly due to the adsorption of water molecules on hydroxyl groups, formation of conductive paths 29 and proton hopping on adsorbed water molecules. 30–32 Dielectric/impedance spectroscopy has been utilized to monitor water vapor sorption on cellulose nanofibers including bacterial cellulose 33 and nanofibrillar cellulose (NFC), 34 which has shown an exponential increase in conductivity of the films upon an increase in their moisture content. This characteristic can be exploited for use in humidity sensors by incorporating CNTs in cellulose nanofibers matrices to increase the electrical conductivity of the composite and improve the accuracy of acquiring electrical responses. 19 However, interpretation of the results becomes rather more complicated at high relative humidities, since on the one hand water vapor adsorption increases conductivity of the matrix (i.e. cellulose), as stated above, and on the other hand it decreases the conductivity of the inclusions (i.e. CNT) by donating electrons. 35,36 In this study, we use conventional and electrosterically stabilized nanocrystalline celluloses as matrix. While the former, so-called NCC (also called cellulose nanocrys-



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tals (CNC) or cellulose nanowhiskers), can be produced by harsh acid hydrolysis on wood fiber, 37 the latter, so-called ENCC, can be prepared by consecutive periodate and chlorite oxidations of wood fiber. 38 Periodate oxidation can cleave cellulose chains, which attack amorphous region first making their way toward crystalline part of cellulose. 39 So the carboxyl contents in crystalline and amorphous parts will be different and they will be separated from each other when the amorphous region is dissolved due to oxidation reactions. 40 Both NCC and ENCC are highly stable in pure water due to their surface chemistry and for ENCC also the electrosteric effects of the highly charged polymers protruding from both ends. 41 The presence of these charged polymers was evidenced previously by equivalent diameter measurements results using dynamic light scattering and attenuation spectroscopy, which showed the ENCC size of about 200 nm, much larger than the dimensions of ENCC observed with transmission electron microscopy (TEM). 41 Due to their lower aspect ratio and higher mobility as compared to other cellulose nanofibers, a more homogeneous CNT/(E)NCC dispersion can be achieved with minimized entanglement of CNTs. In this paper, the effect of humidity on composite films with varying CNT/NCC ratios is characterized by impedance spectroscopy, to study the increase and decrease in the electrical conductivity of films made of (E)NCC and CNT, respectively.

Materials and methods Materials Lithium chloride (LiCl), magnesium chloride (MgCl2 ), and potassium chloride (KCl) salts were supplied by Sigma-Aldrich, Ontario, Canada. Sodium bromide (NaBr) and sodium chloride salts were purchased from J.T. Baker Chemicals, Pennsylva-


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nia, USA and ACP Chemicals Inc., Quebec, Canada, respectively. Short length single-walled carbon nanotubes with carboxyl content of 0.6 mmol g−1 , and outer diameter and length of 1-2 nm, and 1-3 µm, respectively, were purchased from US Research Nanomaterials Inc., Texas, USA. An aqueous NCC (4% w/v) suspension was provided by Louis Godbout, produced following Revol et al., 42 and a fractionated ENCC (4.8% w/v) suspension and a dicarboxylated cellulose (DCC) solution (3% w/v) were provided by Han Yang, produced according to Yang et al. 38,40

Methods Conductometric titration was carried out to determine the charge contents of NCC, ENCC, and DCC following the procedure outlined in our previous papers. 41,43 Accordingly, the sulfate and carboxyl contents of NCC and ENCC were determined to be 0.41 and 6 mmol g−1 , respectively, while DCC was found to have 5.6 mmol g−1 carboxyl groups. Since most of the charge of ENCC are in the protruding DCC chains, the protruding DCC chains have a similar charge density as the DCC chains in solution (see figure 1). Transmission electron microscopy (TEM, Tecnai 12, 120 kV, Field Emission Inc.) was performed on air-dried specimens of CNT, NCC, and ENCC. Due to the high charge density of ENCC, the TEM grids were treated by glow discharging in the presence of amylamine at 0.26 mbar and 25 mA for 30 sec using a Peclo easiglow discharge cleaning machine. To determine the moisture content of CNT, NCC, ENCC, and DCC samples (≈ 20 mg each) were pretreated at 105 ◦ C for 6 hrs, after which water vapor sorption isotherms of the samples were acquired at 21 ◦ C with a volumetric Belsorp-Max instrument (BEL Japan Inc., Osaka, Japan). The adsorbed volume of water was then converted into mass and was divided by the mass of the dry sample to determine the moisture content. A 25%/75% (w/w) CNT/(E)NCC stock dispersion was prepared by mixing an



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COOCOO-

DCC chains

ENCC’s crystalline backbone

Figure 1: Schematic of electrocsterically stabilized nanocrystalline cellulose (ENCC) with dicarboxylated cellulose (DCC) chains. 38 adequate amount of CNT with a (E)NCC suspension followed by sonication in an ice bath for 30 min (13 mm probe tip, 1500 Watts, Sonics and Materials Inc., Connecticut, USA). 19 To separate the entangled CNTs, the dispersion was centrifuged at 23000 g at room temperature for 40 min using an Avanti J-E centrifuge (Beckman Coulter, USA). The amount of precipitated CNTs after centrifugation was negligible compared to the initial CNT weight. The supernatant was separated as the stock dispersion and a sufficiently concentrated (E)NCC suspension was added to the stock dispersion to lower the CNT loading in the mixture followed by sonication. The final dispersion (figure 2) was cast in a teflon dish at 55 ◦ C overnight to prepare conductive composites, from which specimens with width ≈ 3 mm, length about 1 cm, and thickness ≈ 10–15 µm were cut. The solid-state electrode cell was prepared by gluing the specimen to planar strips of aluminium electrodes with ≈ 1 cm spacing and a teflon sheet as support using conductive epoxy (figure 2). To generate the desired relative humidity (RH) of 11.3, 33, 59, 75, and 85%, 250 mL bottles were half filled with saturated solutions of LiCl, MgCl2 , NaBr, NaCl, and KCl, respectively, 44–46 and covered with parafilm to securely seal the bottle. To observe the effect of humidity on the composites conductivity, the planar electrodes were hung in the bottles with films exposed to the generated water vapor for 24 hours before further characterization with the electrodes free ends lying outside the bottle.


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Cyclic voltammetry ranging from 0 to 1 V and vice versa was performed at scan rate 10 mV s−1 using an Ecochemie Autolab potentiostat/galvanostat/frequency response analyzer (PGSTAT30/FRA2, Metrohm, Canada). The DC conductivity was determined from the current measured at 1 V. Using the same equipment, impedance spectroscopy on the films was conducted in a frequency range 1 kHz– 1 MHz with an alternating current (AC) voltage amplitude of 100 mV at room temperature.

CNT/(E)NCC composite Electrodes

Support

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Figure 2: CNT/NCC dispersion and electrical cell.

Results and discussion Water vapor adsorption isotherm measurement on CNT, NCC, and ENCC TEM images of CNTs show that they are bundled, due to the high aspect ratio (figure 3). NCC and ENCC particles, on the other hand, can be observed individually because of their lower aspect ratio and higher charge content. Both NCC and ENCC have been reported to have length of 100-200 nm and width of 5-15 nm depending on the starting raw material. 37,40 Moisture content measurements shed



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light on a fundamental difference between NCC and ENCC, as seen in figure 4. While CNT powder and NCC film moisture contents increase monotonically with relative humidity up to ≈ 8.5 and 9.5%, respectively, in good agreement with values reported in the literature, 34,47 the ENCC film moisture content reaches only ≈ 3% at the highest relative humidity. Interestingly, the amorphous DCC film has a low moisture uptake (less than 1.5%) until RH 80%, after which it abruptly rises to ≈ 9%. Note the sorption isotherms obtained for NCC and CNT attest to the presence of a strong attraction between them and water vapor, whereas ENCC and DCC sorption isotherms are indicative of a weak interaction with water vapor. 48 The underlying causes of these behaviors originate from the surface chemistry of the nanoparticles. 49 NCC particles have hydroxyl or sulfonate groups, which favor water vapor adsorption. 50,51 On the other hand, ENCC and DCC have many hydroxyl and carboxyl groups, of which many are hydrogen bonded to one another. 52,53 Such bonds decrease the affinity between the ENCC/DCC films and water vapor; hence, ENCC moisture uptake stays below 1.5% until RH 80%, similar to DCC. Part of the crystalline region of ENCC could be covered by the flexible DCC chains. At higher RH, the hydrogen bonds between carboxyl and hydroxyl groups are broken 53 and the moisture uptake increases, but not as much as for DCC. DCC films are purely amorphous whereas ENCC films contain disproportionate crystalline and amorphous polymers which are being held together tightly by the hydrogen bonds. 40 Crystalline part of ENCC can accommodate only ≈ 0.8 mmol g−1 carboxyl groups, as calculated by Yang et al., 38 therefore; DCC chains are carrying most of the charge. One also should note that determining the weight fraction of protruding DCC chains in an ENCC particle is not trivial. This hypothesis was further evaluated by Argon sorption isotherms, which show a weak interaction between probe gas and all cellulose films including NCC, DCC, and ENCC (see the supporting information). The calculated surface area available


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for a monolayer of Argon molecules is low and very similar for all cellulose films, which can be ascribed to low porosity and disconnected pores of the films or continues gas condensation in the narrow gaps between nanoparticles . 54 On the other hand, the calculated surface area occupied by a monolayer of water in NCC film is significantly higher than those of DCC and ENCC films, which supports our hypothesis that the surface chemistry difference between these films has a significant impact on their moisture content.

100 nm

(a) CNT

100 nm

100 nm

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(c) ENCC

Figure 3: TEM images of (a) carbon nanotube (CNT), (b) nanocrystalline cellulose (NCC), and (c) electrosterically stabilized cellulose (ENCC). The hysteresis seen in water vapor adsorption/desorption isotherms has been reported elsewhere for nanofibrillar cellulose, 34 and has been attributed to the diffusion of water molecules in the swollen amorphous region of the fibers. This hypothesis holds true for both ENCC and DCC films, where amorphous chains constitute part of the cellulose in the former and entirely in the latter. On the other hand, NCC has a high crystallinity (> 90%) 55 which rules out the possibility of the effect of water molecules in the disordered region on the desorption measurement. During the adsorption, the adsorbed water molecules can perturb the arrangement of NCC particles resulting in a wider pore size distribution. This structural change is probably responsible for the deviation of desorption branch of NCC film from its adsorption counterpart. 56 Such hysteresis is evident in multiple cycles of the conductivity measurement of NCC films, which will be presented in the next section.



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NCC CNT ENCC DCC

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Figure 4: Moisture content from water vapor adsorption isotherms conducted at 21 ◦ C on CNT, NCC, ENCC, and DCC showing hysteresis.

Electrical properties of CNT/NCC composites The effect of CNT loading on the electrical properties of CNT/NCC composites is presented in figure 5, in which the CNT loading decreases both the real (Z0 ) and imaginary (Z00 ) resistances. When the CNT loading is ≈ 25%, the intrinsic impedance of CNT itself is visible as a maximum in 100-1000 kHz frequency (f) region of the imaginary resistance, probably because of the presence of defects in CNT. 57 As the NCC content increases, for lower loadings, another maximum starts to appear in the 10-100 kHz frequency range corresponding to interfacial polarization effects. 58 The latter maximum becomes more pronounced particularly at CNT 1%, while the former maximum is suppressed. The equivalent resistance (R)/capacitor (C) circuit of the composite impedance spectra are schematically shown in figure 5, in which Rc is the electrical contact resistance, R1 and C1 represent electrical contribution of CNT and R2 and C2 represent the contribution from polarized water


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molecules adsorbed on cellulose. As mentioned earlier, water vapor sorption has an opposite effect on the electrical conductivity of CNT and NCC. To investigate this further, the direct current (DC) conductivity of NCC/CNT composites with varying CNT loading was measured versus the ambient relative humidity using cyclic voltammetry. As seen in figure 6, at high CNT loading the composite conductivity decreases as the relative humidity increases due to the donation of electrons by adsorbed water molecules. 35,36 Lowering the CNT loading not only decreases the composite conductivity but also reduces the sensitivity of the composite to water vapor sorption. At 2% loading the conductivity still decreases with relative humidity but with a smaller slope compared to the higher CNT loadings. At 1% loading the electrical contribution from cellulose dominates and the conductivity starts to increase slightly, more pronounced at RHs higher than 60%, due to the presence of more ions capable to conduct the current, as observed previously for different types of cellulose. 27,28,32,34 Accordingly, one can conclude that there must be a weight fraction between 1% and 2% at which CNT and NCC offset each other’s effect and the composite conductivity remains insensitive to the relative humidity, although both CNT and NCC have a high affinity to water vapor as seen in figure 4. Since at 1% CNT loading the electrical properties of NCC prevail, the effect of relative humidity on its nanocomposite impedance was studied in more detail. The high frequency Nyquist semicircle is almost invisible compared to the moderate frequency semicircle (figure 7). The latter starts to shrink at higher RH due to the higher moisture content. One can note a sharp reduction of the impedance dome when increasing the RH from 59% to 75%, which can be related to presence of unbound water molecules in NCC pores. Since the first few layers of water molecules are tightly bound to the NCC surface, their proton transfer contribution to conducting the current is limited, 30 while the unbound or free ones can easily



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12 Figure 5: Real and imaginaryACS components of impedance of CNT/NCC composites with Paragon Plus Environment different CNT loading (w/w) measured at RH 33% and room temperature and its equivalent electrical circuit.

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Figure 6: DC conductivity of CNT/NCC composites with varying CNT loading versus relative humidity. slide or hop from one binding site to another. 32 The composite impedance behavior during desorption is similar to the NCC water vapor sorption isotherm; both show a strong hysteresis during the desorption. The excess water in the composite during the desorption increases the conductivity and capacitance, because of the excess of polar molecules. These properties may find applications in printed electronics, in which well dispersed conductive inks with minimal sintering and high conductivity are desired most. 59

Electrical properties of CNT/ENCC composites The electrical properties of a CNT/ENCC composite with 1% CNT loading were investigated at different RH. At 1% CNT loading, the composite was sufficiently conductive to enable us to monitor the effect of water vapor sorption on the composite electrical characteristics, as also observed for CNT/NCC. Moreover, at this



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Figure 7: Effect of water vapor adsorption (top) and desorption (bottom) on Nyquist plots of the impedance of CNT/NCC (1% loading) composites.


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weight fraction, the contribution of nanocellulose to the overall conductivity and capacitance of the composite is dominant, which is of more importance in the present study. The conductivity of CNT/ENCC composites, measured by cyclic voltammetry, was near the detection limit of the instrument at RH up to 75%. At 75% and 85% RH, the conductivity was about 2.6 × 10−4 and 0.7 S cm−1 , respectively (figure 8).

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NCC ENCC

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Figure 8: Effect of water vapor sorption on DC conductivity of NCC and ENCC composites with 1% CNT loading. Conductivity of CNT/ENCC below RH 75% is close to the detection limit of the instrument. These results are in good agreement with impedance spectroscopy results (figure 9), in which both real and imaginary components decrease by about 2 orders of magnitude as the RH increases from 59% to 75% and 85%, which is likely related to the amount and state of sorbed water on ENCC. The impedance spectra indicate that the electrodes are polarizable, with no electrochemical charge-transfer reactions taking place and where a constant-phase-element (CPE) represents a double-layer charging in series with small solution resistance (Rsol ), 60 as shown schematically in



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figure 9. This hypothesis is corroborated further by moisture content measurement results in figure 4. The crystalline part of ENCC is likely to host a limited number of water molecules, while its DCC protruding chains act as insulator (moisture content less than 1.5%) and interrupt the flow of current from one CNT to another at low and moderate RH, as schematically shown in figure 10. At RH ≈ 70% or higher, DCC chains become highly hygroscopic, reflected in their high moisture uptake, and, therefore, start to conduct the current. It is noteworthy that ENCC moisture uptake is always lower than its NCC counterpart, which explains its lower conductivity, except for RH 85% at which ENCC ends are conductive enough that CNT/ENCC overall conductivity is higher than that of CNT/NCC. The water at this state is believed be freely moving, since broken hydrogen bonds lead to electrostatic repulsion between carboxyl groups and ultimately rearrangement of ENCC particles. Such an abrupt change in the composite electrical properties can be exploited for use in dew sensors. 61,62

Concluding remarks In this work, two types of nanocomposites were prepared by dispersing carbon nanotubes (CNT) in suspensions of conventional and electrosterically stabilized nanocrystalline celluloses and casting them to fabricate conductive films. Effect of water vapor sorption and CNT loading on the electrical properties of the cast films were characterized using impedance spectroscopy and cyclic voltammetry. It was found that the conductivity of CNT/NCC composites decreased as CNT loading decreased. Increasing the relative humidity resulted in a higher moisture content in the composite and an increase in its conductivity. Subsequently, a critical weight fraction of CNT in range 1–2% was identified as an optimum weight fraction at


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Figure 9: Effect of water vapor sorption on (top) real and (bottom) imaginary components of impedance of CNT/ENCC (1% loading) composite and the equivalent electrical circuit for RH > 75%. 17 ACS Paragon Plus Environment


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CNT/NCC composite Water molecule

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CNT/ENCC composite NCC

CNT

ENCC crystalline part

Possible pathway of current

low moisture content DCC chains

Figure 10: Schematic of the possible pathways of current in CNT/NCC and CNT/ENCC composites at RH below 75%. DCC chains act as insulator due to their low water uptake. which the composite shows minimal sensitivity to variation in the ambient relative humidity. This behavior is ascribed to the opposite effects of water vapor on CNT and NCC electrical properties: on the one hand it decreases the conductivity of CNT by donating electrons to the holes of CNT and on the other hand, it raises the NCC conductivity by the formation of conductive paths and proton hopping on cellulose surfaces, which compensates for the reduction in CNT conductivity. CNT/ENCC composites exhibit an abrupt increase in conductivity at rather high relative humidities, due to the presence of a limited number of hydroxyl groups and hydrogen bond formation between carboxyl and hydroxyl groups. The sudden rise in its conductivity could be explained by breakup of the hydrogen bonds between carboxyl and hydroxyl groups of DCC chains of ENCC, which was supported further by the moisture uptake behavior of DCC. The insensitivity of CNT/NCC composites to relative humidity might find applications in electronics; while CNT/ENCC composites can be utilized as a humidity switch sensor.


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Acknowledgments The authors thank Louis Godbout for providing NCC and Han Yang for supplying ENCC and DCC, and Dr. Sasha Omanovic (Department of Chemical Engineering, McGill University) for the use of the potentiostat. The authors are also grateful to Alemayehu Hailu Bedane and Dr. Mladen Eic (Department of Chemical Engineering, University of New Brunswick) for running water vapor adsorption isotherm measurements. Financial support from NSERC Strategic Research Network on Green Fibres (NSERC NETGP 387004–09d) is gratefully acknowledged.

Supporting information Argon sorption isotherms NCC,ENCC, and DCC films and the corresponding surface areas and affinity constants are included in the supporting information file. This information is available free of charge via the Internet at http://pubs.acs.org/.



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CNT/NCC composite Water molecule

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CNT/ENCC composite NCC

CNT

ENCC crystalline part

Possible pathway of current

low moisture content DCC chains

Table of Contents


Effect of Water Vapor Adsorption on Electrical Properties of Carbon

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