Carbon Nanotube Functionalization and Radiation Induced

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Carbon Nanotube Functionalization and Radiation Induced Enhancements in the Sensitivity of Standalone Chemiresistors for Sensing Volatile Organic Compounds R. K. Mondal, Kumar Abhinav Dubey, Jitendra Kumar, * Jagannath, Yateindra Kumar Bhardwaj, Jose Savio Melo, and Lalit Varshney ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00790 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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ACS Applied Nano Materials

Carbon Nanotube Functionalization and Radiation Induced Enhancements in the Sensitivity of Standalone Chemiresistors for Sensing Volatile Organic Compounds R. K. Mondal1,2, K. A. Dubey1,2,*, Jitendra Kumar1,3, Jagannath4, Y. K. Bhardwaj1,2, J. S. Melo1,3, L. Varshney1,2 1 2

Homi Bhabha National Institute, Mumbai 400094, India

Radiation Technology Development Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

3

Nuclear Agriculture and Biotechnology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

4

Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India (*Corresponding author: [email protected]; Fax No.: 91-022-25505151)

Abstract Chemical stress-induced actalterations in the number-density and the architecture of conducting interjunctions impart chemiresistivity in conducting polymer composites (CPCs). Herein, marked enhancement in the chemiresistive response of CPC is reported by manoeuvring interfacial and percolation characteristics of carbon nanotube (CNT) based chemiresistors, through CNT functional group variation and high energy radiation exposure. Un-functionalized (CNT-UF), hydroxyl functionalized (CNT-OH), amine functionalized (CNT-NH2) and carboxylic functionalized (CNT-COOH) carbon nanotubes, polydimethylsiloxane (PDMS) and gamma radiation were used in different combinations to prepare PDMS/CNT chemiresistors. Even at the same φCNT, PDMS/CNT-NH2 chemiresistors exhibited significantly milder response against toluene vapours than PDMS/CNT-COOH chemiresistors; whereas, PDMS/CNT-OH chemiresistors were unresponsive due to poor nanotube dispersion and low electrical conductivity. PDMS/CNT-COOH chemiresistors had the lowest surface energy, highest entanglement density and highest critical strain for CNT-CNT structure breakdown under shear, suggesting superior interfacial interactions. Dielectric, CNT percolation and XPS analysis also

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revealed marked variations in CPCs’ properties with change in functional groups on CNT. The gel content of all irradiated chemiresistors increased with radiation dose; however, aminefunctionalized CNT was found to inhibit interfacial radical combinations. With the increase in radiation dose, the chemiresistivity increased in a dose-dependent fashion, suggesting the possibility of interfacial grafts and immobilization of PDMS segments on to CNT-COOH surface. The chemiresistivity of PDMS/CNT-COOH chemiresistor was highly dependent on polymer-solvent interaction parameters. The chemiresistor displayed excellent sensitivity, reversibility, and reproducibility, underscoring its potential in monitoring hazardous volatile organic compounds. Keywords: Chemiresistors; Stand-alone sensors; Radiation crosslinking; Percolation; Polymer composite; Functional groups

1

Introduction Conducting polymer composites (CPCs) have a wide applications domain. The major

challenge in CPC development is to assure good dispersion and good interfacial interactions between polymer and the filler. Since the formation of a percolated network is a prerequisite for the development of CPCs, the incorporation of a high amount of conducting filler is often needed 1-2

. Loading of higher amounts of fillers; however, poses challenges in terms of increased

viscosity, agglomeration, and morphological instabilities. Asymmetric nano-fillers such as carbon nanotube, silver nanowires, and graphene provide an opportunity to form a percolated network in a polymer matrix at much lower loading than conventional fillers such as conducting carbon black and metallic powders3. Furthermore, due to their high surface area, asymmetric nano-fillers provide a large polymer-filler interface which enables efficient stress transfer from polymer chains to the filler4. Chemiresistivity is a phenomenon wherein significant change in the resistance is observed when a material is exposed to a particular chemical

5-7

. This chemical concentration-

dependent change in resistance can be utilized to develop a wide range of chemical sensors. Metal oxides and CPCs based chemiresistors have been demonstrated to have high efficacy, costeffectiveness, and portability for detecting volatile organic compounds

3, 8-12

. Particularly, CPC

based chemiresistors, in addition to be cost-effective, can be used at ambient temperature and,


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generally, do not have cross-sensitivity to humidity. The chemiresistivity of CPC based chemiresistors does not depend on the chemical reactions (e.g. redox). Indeed, in CPC based chemiresistors, on exposure to a chemical with good thermodynamic interactions with a polymer, the polymer chains relax, thereby inducing disruptions in the percolated conducting network

13

.

Such disruptions or the chemical induced internal strain lead to change in the resistivity of the matrix, mainly due to the increased separation between conducting aggregates or agglomerates or due to the reduction in conducting network density14. These factors are relatively well documented for the piezoresistivity of CPCs but the underlying mechanism of chemiresistivity and strategies for enhancing chemiresistivity of CPCs are little understood. Particularly, not much attention has been paid on utilizing the functional group on the filler surface and the high energy radiation to strengthen polymer-filler interfacial interactions within CPCs, with a focus on enhancement in the chemiresistivity. Carbon nanotube is a high aspect ratio allotrope of carbon. It can be incorporated in a polymer matrix to improve electrical conductivity and mechanical characteristics be functionalized with different chemical groups

17

15-16

. CNT can

. The functionalized CNTs have been

demonstrated to improve the interfacial compatibility, percolation, and micromechanics

18-20

.

Chemiresistivity depends on the structure of conducting network and the characteristics of polymer chains. And since the functional groups of CNT greatly affect its surface properties, it is also expected to affect chemiresistivity of CPCs. Crosslinking renders intermolecular linkages between polymer chains, profoundly affecting the vapour permeability, solvation and mechanical properties of a polymer

21-24

. For filled systems, such as CPCs, both physical and chemical

crosslinking may take place. We have earlier reported that optimal radiation dose can be used to develop standalone chemiresistors 25-26. High energy radiation such as electron beam and gamma ray is an additive-free room temperature process to develop advanced polymers. It has potential to forms linkages not only between the polymer chains but also at polymer-filler interface 23. This is mainly possible due to the radiolytic efficiency and penetrating power of high energy radiation, enabling the formation of free radicals on the filler surfaces, interface and in the bulk23,

27-28

. Functionalized CNTs

together with optimal radiation dose thus can be useful in improving the interface, integrity and the chemiresistivity of CPCs. Notably, a functional group driven changes in the in the interface



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have been claimed to have a profound effect on the load transfer mechanisms in CNT/polymer composites20. In this work, CNT functionalization and gamma radiation were used to tailor polymerfiller interfacial interactions. PDMS, an elastomeric matrix which is extensively used for different electrical applications, was chosen as polymer matrix

25, 29-31

. PDMS/CNT CPCs were

prepared in different proportions by an additive-free process of shear compounding. Non-linear oscillatory rheology was used to gauge CNT-polymer interactions and the impedance spectroscopy was used to infer electron conduction. Chemiresistivity was investigated with respect to CNT functionalization, CNT volume fraction, and radiation dose. Surface energy, functional groups (x-ray photoelectron spectroscopy), gel content and morphology of the CPCs were also analyzed in the context of observed chemiresistivity.

2 2.1

Materials and Methods Materials Poly(dimethylsiloxane) (PDMS) was procured from M/s DJ silicone, China (Hardness=

60; density= 1.13±0.05 g/cc) containing vulcanizator 2,5-dimethyl-2,5-bis(tert-butyl peroxy) hexane (0.65%). Multiple walled CNTs [unfunctionalized (CNT-UF), amine functionalized (2-3 wt %; CNT-NH2), hydroxyl functionalized (2.36-2.6 wt %’ CNT-OH), and carboxylic functionalized (1.47-1.63 wt %; CNT-COOH); OD: 10-30 nm; Length: 1-2 um SSA: 100-130 m2/g] were purchased from Otto Chemie Pvt. Ltd, Mumbai India. Benzene, toluene, acetone, chloroform, methanol and ethanol used were of Analytical grade (purity >99.9%) and was procured from M/s SD Fine Chemicals, Mumbai. 2.2

Sample preparation PDMS/CNT [un-functionalized, hydroxyl, amine and carboxylic group functionalized]

nanocomposites were obtained by the shear compounding of constituent materials in Brabender plasticorder at 70 oC at 40 rpm for 30 min. Prior to the mixing, all components were carefully chosen considering their bulk densities and vacuum dried for 12h at 60 oC. After mixing, the homogeneous mixture was taken out, cut into small pieces and compress moulded to the different thickness (100 -1000 microns) at 150 kg/cm2 for 20 min at 70 oC. These moulded sheets were crosslinked by high energy Co-60 gamma irradiation (GC-5000, M/s BRIT, India). Before


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irradiation, the dose rate of gamma chamber was ascertained by Fricke dosimetry and was found to be 8.0 kGy/h. The samples in the study are mentioned as CNT-UF (PDMS/unfunctionalized CNT), CNT-COOH (PDMS/carboxyl functionalized CNT), CNT-OH (PDMS/hydroxyl functionalized

CNT)

and

CNT-NH2

(PDMS/amine

functionalized

CNT)

composite/CPC/chemiresistor, depending on the type of CNT used in the synthesis and the context. The respective dose (D) and the volume fraction of CNT (φCNT) are specified in the figures and text. 2.3

Scanning electron microscopy and optical microscopy The morphology of composites with different CNT content was investigated using a

scanning electron microscope (Model PS-230, Pemtron, S. Korea). The cryo-fractured specimen was used to observe fractured morphology and dispersion of CNT. The acceleration voltage of 5 kV was used for recording micrographs.

Optical Microscope (RXL-4T, Radical Scientific

Instruments, India) was used to evaluate the gross morphology of CPCs. 2.4

Impedance spectroscopy The impedance of the samples was measured at room temperature using an LCR meter

(HIOKI IM3570) in the frequency range of 4 Hz to 5 MHz. All samples were of disc shape (Diameter = 15 mm; thickness = 0.5 mm). To minimize contact resistance the surface of the samples were coated with conducting silver paste. For ensuring good electrical contact, the surfaces of the specimens were polished with sandpaper and conductive silver paste was applied between the electrodes and the specimen. For each composition, at least three specimens were tested. All measurements were performed at ~24°C and relative humidity of 55%. 2.5 Surface energy and X-ray photoelectron spectroscopy (XPS) The radiation-induced modification of surface was characterized by its wetting angle measurements. In this work, the Owens and Wendt method was used32. The measurement of contact angles of the sample was carried out by the sessile drop technique using image analysis software(Windrop++, GBX instruments). XPS analysis of the samples was performed at 4354 eV photon energy on the PES-BL14 beamline (BARC) at Indus-2, RRCAT, Indore, India.



2.6

Vapour sensing

For static measurements, a fixed volume of the volatile organic compound (VOC) was placed in a 2000 ml closed glass container with heating and homogenizer arrangement for the vapours 33. The relative change in resistance (Ar=∆R/Ro) was recorded for different concentrations. Baseline corrected peak response was represented as Ar,p. The chemiresistors used in this work were prepared by compression moulding (Thickness~100 µm) followed by crosslinking by high energy radiation to impart sufficient strength to function as a standalone sensing film. The electrodes were prepared by piercing gold coated pins in the 100 µm thick PDMS/CNT composite sheet. PTFE guides were used to hold chemiresistors, without blocking the exposure and to minimize vibrations due to vapour flow. The drawings and schemes regarding sensor geometry and assembly are presented elsewhere26. A schematic representation of CPC fabrication and sensing mechanism is shown in Figure 1.

2.7

Mechanical properties For tensile strength measurements, five specimens were cut from the CPC sheets using a

sharp-edged steel die. The thickness of the samples was determined to the nearest of 0.1 mm. The tensile strength and elongation at break were measured by using a universal testing machine at room temperature.

2.8

Gel fraction and viscoelasticity

Irradiated CPCs were Soxhlet extracted for 24 h to extract any sol content using toluene as solvent. The insoluble gel part was then dried initially under room conditions and later under vacuum at 40°C. Rheology measurements were performed on an MCR 102 Rheometer (Anton Par, Austria) at different angular frequencies (linear) and different amplitudes (non-linear) using a parallel plate fixture. Samples dimensions were 12.5 mm (radius) and 1 mm (thickness). The N2 atmosphere was maintained to avoid oxidative degradation.

3 3.1

Results and Discussion CNT functionalization, percolation, and chemiresistivity Figure 2A-B shows the relative changes in the resistance of PDMS/CNT chemiresistors

with different functional groups on CNT (φCNT=0.06, Dose=100 kGy). All chemiresistors were exposed to the successive cycles of toluene vapour (100 to 1000 µgL-1) and air. Figure 1C


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depicts changes in the imaginary and real part of AC impedance, in the air and in toluene vapours (500 µgL-1). The type of functional groups on CNT was found to play a critical role in the chemiresistive response of PDMS/CNT based chemiresistors. Major distinction was observed between PDMS/CNT-OH (hydroxyl functionalized) and PDMS/CNT-COOH (carboxyl functionalized) chemiresistors. PDMS/CNT-OH chemiresistors showed very poor electrical conductivity and therefore found unsuitable for chemiresistive measurements (Figure S1B, Fig 3A, and section 3.2). Indeed, insulator type behaviour was observed in PDMS/CNT-OH composites even at φCNT=0.06 (Figure S1B). On the other hand, PDMS/CNT-COOH chemiresistors had good chemiresistivity (Figure 2A-B), and electrical conductivity even at φCNT CNT>CNT-NH2. At 1000 µgL-1, about 6 fold higher responses were observed for CNT-COOH chemiresistor than for CNT-NH2 based chemiresistor. The inherent mechanism governing the change in the resistance in CPC based chemiresistors is the perturbations in the conducting network architecture due to the strain imposed by the relaxation of polymer chains because of polymer chain-chemical interactions

13, 33-34

. Since all studied

chemiresistors were percolated by CNTs, which have much higher aspect ratio than the conventional conducting filler such as carbon black and metallic powders, more long-range interconnectivity of conducting CNT-CNT junctions is possible. Such an increased range of interconnectivity allows electrical conduction even at higher concentrations of the toluene, entailing a linear chemiresistive response over a wide concentration range26.

The effect of

functional groups on CNT on the chemiresistivity is explained is explained the section 3.2 to 3.5, considering factors such as electrical conduction, radiation dose polymer-filler interaction, macromolecular entanglements and surface energy. Section 3.6 details sensing characteristics. 3.2 Functional groups on CNT, AC impedance and surface energy Frequency dependence of AC impedance can provide important information about the type of electrical circuitry (conducting network structure) within the matrix and the electrical conduction (electron transport) mechanism19. Figure 2C shows the effect of toluene field on the electrical impedance of the chemiresistors; whereas, Figure 3A and Figure S1A-D, depict the effect of different functional groups on CNT on the AC impedance of CPCs. Impedance Z includes imaginary and real component; Z’ is the real component and reflects the resistance of the composites and Z’’ is the imaginary component and represents the reactance of the composites. The presence of a single semicircle in all chemiresistors is suggestive of single


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relaxation time (Fig 2C). The semi-circle obtained by plotting Z’ and Z’’ can be represented by the following relation ′ −

+ ′′ =

(1)

The electrical conduction in the percolation region of a conducting composite can be approximated as resistor-capacitor series. Contact resistance (Rc), the resistance of CNT aggregates (Ra) and the gap between aggregates (da) are factors determining the electrical conduction in such chemiresistors. In the electron transport within the chemiresistor, the contribution of da can be represented as a parallel plate capacitor. The capacitance of such a capacitor depends on the dielectric constant of the material, the contact area and the separation between the CNT aggregates. It is evident from the Fig 2C that in the initial region both curves overlap but depart significantly in the terminal region (marked with arrows). This implies that by changing the environment from air to toluene there is a significant change in the Rc. It may be noted that Rc in case of an semicircle is gauged from its radius whereas Ra can be estimated from the position of its centre (Eq. 1). Since PDMS and toluene both have similar dielectric constant, it is not expected that, under toluene field, there could be a significant change in the dielectric constant of the medium that could affect capacitive reactance at high frequency. Therefore, the higher value of Rc can be attributed to the observed chemiresistivity i.e. toluene exposure leads to higher contact resistance and increased gap between CNTs, thereby hindering the electron transport 35. The impedance of CPCs over the complete frequency range is presented in the Figure S1 (A-D). CNT-NH2 composites had the least impedance whereas CNT-OH composites manifested the highest impedance. Indeed CNT-OH composites exhibited insulator type behaviour at all loadings (φ=0.009-0.06) over the entire frequency range; whereas all other composites showed a significant decrease in the impedance at φCNT greater than 0.019. This observation was confirmed in DC measurements as well (Figure S3A), wherein no electrical conductivity was noted in CNT-OH composites; though CNT-UF, CNT-COOH, and CNT-NH2 composites were conducting (φ=0.009-0.06). In impedance measurements, after φCNT =0.009, the impedance of CNT-UF, CNT-COOH, and CNT-NH2 composites, showed a plateau followed by a rapid decline after a particular frequency. This behaviour is attributed to the capacitive effect of the matrix at higher frequencies.



Notably, the critical frequency (ωc) i.e. the frequency after which rapid decline in the AC impedance was different for the composites of different functionalities, even at the same φCNT (Figure S1). For example, at φCNT=0.03, for PDMS/CNT-COOH CPC ωc was 12.7 kHz whereas, for PDMS/CNT-UF CPC, it was 76.8 kHz. Interestingly, for PDMS/CNT-NH2 a reverse trend i.e. an increase in the AC impedance after a critical frequency was observed. It suggests that the electron transport mechanism is different in CPCs with different CNT functionalities. This is particularly important, as the emergence of the capacitive effect at an early frequency reflects inefficiency in terms of electron transport via tunnelling or hopping. The chemiresistivity mainly depends on the interaction of a chemical with a polymer matrix and its subsequent impact on the contact resistance and overall architecture conducting network. Therefore, in this work, in addition to measuring the relative change in the resistance, the change in AC impedance at different frequencies was also measured in the presence and absence of chemical vapours (section 3.4 and 3.5). Loss permittivity (ε'') and storage permittivity (ε') were also evaluated to understand electron transport and dielectric loss (Figure 3B). At 1 kHz, CNT-OH CPCs had around 40 fold lower storage permeability than CNT-COOH or CNT-NH2 CPCs. Nonetheless, in comparison to PDMS (storage permittivity~2.3, 1 kHz) the storage permittivity of CNT-OH CPC (storage permittivity~6.2, 1 kHz) was about 3 fold higher. CNT embedded in an insulating matrix is expected to behave as a microcapacitor and with the increase in the number of such microcapacitors, dielectric permeability is expected to increase36. The markedly low storage permittivity of CNT-OH CPCs can be attributed to the poor dispersion of hydroxylfunctionalized CNTs in PDMS matrix, as was evident from the SEM analysis and high impedance. It may be noted that the agglomeration of CNT will reduce the overall surface area and thus the boundary layer responsible for capacitive effects. The loss permittivity increased with increase in φCNT (Fig 3 C). As the dielectric loss is an indicator of leakages through electron transport, the observed increase in the dielectric loss with an increase in φCNT can be attributed to the increased CNT interjunctions. Intriguingly, however, over a wide frequency range, the dielectric loss of PDMS/CNT-NH2 CPC was higher than that of PDMS/CNT-COOH CPC, though the both CPCs had comparable storage permittivity between 1000 Hz- 0.1 MHz (Figure 3B). In the frequency range, 100 Hz- 10MHz, the contribution of


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dipole relaxations and Maxwell–Wagner polarization is expected to be negligible and the dielectric loss is mainly expected to be due to the high mobility of electrons via tunnelling or hopping through CNT

37-38

. Since the insulating layer i.e. the interface between CNT and the

PDMS matrix is a major factor in facilitating the electron transport within CPC, these results indicate better interfacial characteristics in CNT-COOH CPCs than in CNT-NH2 CPCs. At higher frequency (>0.1 MHz), where the capacitive effect due to the matrix are prominent, there was the significantly lower rate of storage permittivity decline in CNT-COOH than in CNT-NH2 CPC. Essentially, the above analysis suggests that the electron transport in PDMS/CNT CPC depends on the functional groups on CNT ; this can be due to functional groups dependent changes in dispersion, (as reported in the previous section for CNT-OH and CNT-COOH based CPCs) and/or due to the variation in the PDMS-CNT interfacial interactions39. It must be pointed out here that variation in the functional groups can affect the intrinsic electrical conductivity of CNT and CNT’s affinity for other chemicals. However, as established later in section 3.6, the major mode of chemiresistivity in PDMS/CNT CPCs is the interaction of chemical vapours with PDMS matrix, ruling out a significant contribution from the functional groups led intrinsic changes in the CNT. Surface energy is an important parameter playing a key role in the interactions of two matrices with different chemical potential, such as a chemiresistor and the chemical vapours 40. The surface energy of the composites was therefore measured to understand the possible effect of (Figure S3C). Figure 3D shows the relative change in the surface energy of the CPCs with the inclusion of functionalized CNT (∆Surface Energy = Surface energy PDMS/CNTfunctionalized

-Surface energy

PDMS/CNT-UF).

It was found that CNT-NH2 composites had the highest

surface energy whereas the CNT-COOH composites had the least. This lowering of surface energy further suggests better polymer-filler interactions in CNT-COOH CPC 41. To understand differences in the functional groups present at the surface PDMS/CNT composites, the XPS spectra of composites with different CNT functionalities (φCNT=0.06) were recorded (Figure 3E, Figure 4A-E, Fig S4A-B). Wide scan XPS spectra of composite showed characteristics peaks of O1s (529.3 eV), C1s (282.4 eV), Si 2s (152.0 eV) and Si 2p (101.0 eV) 4243

. Since C1s is expected to include information about functional groups on CNT, a detailed

analysis of C1s spectra was done from the deconvolution of functional groups peaks which can



be clearly discerned. For Gaussian fit, based on the literature values, C-C, C=C, and C-H were considered at 284.6 eV43 (i), C-O, C-N and C-Si were considered at 285.8eV43-44 (ii), C=O was considered at 286.8 eV43 (iii), and COO was considered at 288.9 eV43(iv). Except for CNT-OH, all composites showed significant additional photoemissions at higher binding energy in addition to the main peak at around 285 eV [(i)+(ii)]. This was evident from the asymmetry of the main peak at higher binding energy and from the deconvolution results. Notably, CNT-OH had the most symmetrical peak whereas CNT-COOH showed an asymmetrical peak with a shoulder at higher binding energy. As discussed in the previous section that, even at the fixed φCNT, the dispersion and distribution of CNT in the PDMS matrix can be greatly varied. Therefore, no attempt was made to correlate the observed peak with the concentration of a functional group in the composites. XPS analysis is however remarkable in pointing out the presence or absence of a carboxylic group (COO, iv) peak on the surface of chemiresistors. The carboxylic group (peak iv) was present at the surface of CNT-UF and CNT-COOH CPCs. Whereas, this peak was absent in CNT-NH2 and CNT-OH based chemiresistors. The presence of COOH group was also observed in O1s spectra of PDMS/CNT-COOH composites45 (Fig 34, Fig S4A). COOH group is expected to entail polar-polar interaction with oxygen atom presented in PDMS, allowing better interfacial characteristics and wrapping of CNT surface by polymer chains. It may be noted that such wrapping of CNT-COOH with PDMS chains will lead to the encapsulation of polar groups of CNT, leaving methyl groups at the outer surface (Figure 4F). Lowering of the surface energy of CNT-COOH based composites, despite high polarity of CNTCOOH, also corroborates this assumption. These results also support the observations made by Khare et al. on the effect of carbon nanotube functionalization in transferring the mechanical load across the matrix-filler interface in a cross-linked nanocomposites20. The chemiresistivity of a CPC, essentially, relies on the efficiency of the stress transfer from the matrix to the filler i.e. the overall disruption of conducting networks due to chemical-induced relaxation of polymer chains. When a matrix-filler interface is very weak, the interface region in the nanocomposites becomes more compressible20, thereby attenuating the effect of chemical-induced relaxation of polymer chains and vice-versa.


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3.3 Macromolecular entanglement, CNT-CNT network and gel content Entanglements and the molecular weight between entanglements (Me) are unique features of a polymer and are governed by factors affecting the stiffness of polymer chains such as interactions with reinforcing filler or interfacial grafting

46-48

. The changes in the molecular

entanglements in different CPCs were gauged from the linear oscillatory shear rheology (4A, Fig S5 A-D). The frequency dependence of the shear storage modulus was recorded for unirradiated (D=0 kGy) CPCs at a temperature much higher than the glass transition temperature of PDMS. The molecular weight between entanglements was calculated using the following relation46 =

(2)

where is mass density, R is gas constant, T is absolute temperature and is the shear storage modulus at angular frequency 0.1 rad/s. The density ( ) of such entanglements was estimated by =

(3)

With the increase in φCNT, all functionalities exhibited an increase in ; though the CNT-OH had

the least enhancement. Furthermore, CNT-NH2 had significantly lower in comparison to

CNT-COOH and CNT-UF CPCs, highlighting the extent of lower degree molecular entanglements in CNT-NH2 CPCs. The above analysis points out the differences in the entanglements due to functional groups and loading CNT. However, it fails to explain the phenomenal difference in the chemiresistivity of CNT-COOH and CNT-UF CPCs (Fig 2A-B). Non-linear rheology was therefore employed to understand finer differences between PDMS/CNT-UF and PDMS/CNT-COOH CPCs. The shear amplitude dependence of the storage modulus was fitted to a phenomenological quantitative Kraus model ( Eq. 3; Figure S6)49-50. This model is based on the agglomeration and deagglomeration of filler networks under shear strain and provides information about the extent of filer-filler contact breakage and polymer-filler interface breakdown51. G , γ = G +

γ #$ γ"

!


(4)


Page 14 of 36

' G , γ is the shear storage modulus at shear strain γ; G& and G are shear storage modulus at

very low and very high strain respectively, γc is the critical strain at which G , γ decreases to a

' value half of G' -G . The number density of CNT interconnection changes in response to applied

strain and depends on γc. The structure factor m is related to the fractal dimension. The experimental data for all composites fitted well with the Kraus model (in all cases: r2>0.99). The fitting of the Kraus model to the experimental data is shown in Figure S6. Critical strain as a function of φCNT is plotted in Figure 4B, for CNT-UF and CNT-COOH CPCs. It is evident that CNT-COOH CPC has higher critical strain than CNT-UF CPC at all φCNT. This result proves better

interfacial

interactions

between

CNT-COOH

and

PDMS,

allowing

more

strain/deformation withstanding capacity in PDMS/CNT-COOH without irreversible disruptions in CNT-CNT network52. The above analysis was done on unirradiated CPCs (D=0 kGy). It, therefore, provides only a measure of physical crosslinks and interfacial characteristics. To evaluate the effect of high energy radiation-induced crosslinking and interfacial modifications, which were ascribed to the observed chemiresistivity of CPCs (section 3.1), the gel content and the mechanical property analysis were done. The gel content of the irradiated chemiresistors containing different CNT functionalities is shown in Figure 5C. At 100kGy, PDMS/CNT-NH2 chemiresistor had the least gel fraction (0.80); whereas, PDMS/CNT-COOH chemiresistor had the highest (0.92). It was also observed that with an increase in radiation dose the gel fraction increases up to 100 kGy and saturates thereafter (Fig S3B). The gel fraction is an indicator of covalent crosslinking of polymer chains. It may be noted that high energy radiation induces covalent crosslinking in PDMS; whereas reflects only physical crosslinking/chain stiffening related factors. However since, within the time frame of radiolytic transformations, the possibility of diffusion/migration of CNT within the polymer matrix is negligible, the inhibition or acceleration of radiationinduced crosslinking by CNT is expected to be a localized effect. The characteristics of polymerfiller interface i.e. PDMS-CNT interface and the formation of crosslinks between polymer chain and the filler interface are the major determinants of micromechanics (i.e. how stress/strain generated in the bulk matrix is transferred to the reinforcing filler such as CNT) of the system53. Since, the overall chemiresistivity depends on the changes in the interconnections of conducting networks within CPCs, nature of interface and its effect on the inter- /intramolecular radical


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recombination (leading to grafting at the interface and crosslinking in the bulk ) is of the fundamental importance. In the case of good interactions at the interface, there is a possibility of formation of polymer-CNT graft structures i.e. covalent linkages between the CNT surface and the polymer (Figure1 and Figure S7)54. These bonds will result in a better transfer of stress (induced due to the relaxation of polymer chains via interacting with vapors of a chemical) between polymer bulk and the CNT-CNT conducting interjections 53, 55. Stress-strain profiles of irradiated PDMS/CNT composites, shown in Figure 5D, corroborate this hypothesis. At a particular strain, a fixed φCNT and radiation dose, the lowest increase in the stress was observed for amine functionalized CNTs (PDMS/CNT-NH2). In these composites, all factors are same, except the functional groups of CNT, confirming a significant role of functional groups on CNT in altering the course of radiolytic transformations in PDMS/CNT composites. It may be noted that at 100 kGy as well as at 200 kGy the same trend was observed i.e. CNT-COOH (PDMS/CNT-COOH) had highest strain dependence whereas CNT-NH2 (PDMS/CNT-NH2) the least. To further delineate the nature of molecular entanglements in irradiated (100 kGy) CNT-COOH and CNT-NH2 CPCs, the departure of elasticity from the ideal behaviour was estimated using Mooney-Rivlin relation: )

+

* # ,

= 2.! + 2

/# *

(5)

where 0 is the tensile stress at the elongation ratio 1 (ratio of final to initial lengths)56. C1 and C2 are constants. The value of C2, which is an indicator of departure from ideal elastomeric behavior due to interfacial grafting and physical crosslinks 57, was found to be 0.70 for PDMS/CNT-COOH and was just 0.28 for PDMS/CNT-NH2composites It may be noted that the data fitting was done in the low strain range, making contribution due to strain hardening negligible(Figure 5E)58. It may however be noted that the strain range applied in tensile measurements is much higher than the shear strain imposed in linear rheology (1%) (Figure S6). Altogether, linear/ non-linear rheology and Mooney-Rivlin analysis reveal better interfacial interaction and higher (due to entanglements) and higher covalent linkages (due to radicalradical recombination after irradiation) in CNT-COOH CPCs.



In effect, the lower tensile stress, lower C2 and lower gel fraction (~12%) in irradiated PDMS/CNT-NH2 composites than in irradiated PDMS/CNT-COOH composites suggest the inhibition of inter-molecular radical recombination at the polymer-filler interface, in PDMS/CNT-NH2 composites. This assumption is based on the fact that diffusion of radicals generated on CNT to bulk polymer matrix is difficult, particularly considering the high viscosity of the matrix and the timeframe of radiolytic transformations. The aromatic amines are known to inhibit the radical recombination59. A possible mechanism could be hydrogen abstraction from the amine group by polymer radical (Figure 5F)

60-61

. Since CNT has a large surface area, such

effects are expected to play a major role and can explain the lowering of gel content in CNT-NH2 CPCs. On the contrary, better gel content together with changes in the surface energy and interfacial interactions are expected to be primarily responsible for the observed enhanced chemiresistivity of CNT-COOH CPCs. The next sections detail the sensing characteristics of PDMS/CNT-COOH chemiresistors. 3.4

CNT volume fraction and chemiresistivity The chemiresistive behaviour of the PDMS/CNT-COOH composites was recorded at two

different φCNT (Figure 6 A-C). The relative increase in resistance, which was observed on exposure to different concentrations of toluene vapours, decreased with an increase in the φCNT from 0.03 to 0.06. It may be noted that the sensitivity (slopes of curve Figure 6B) was three times higher at φCNT=0.03 than at φCNT=0.06. With the increase in φCNT, the cumulative interfacial layers length, as well as, the tendency to CNT to agglomerate is expected to increase. Furthermore, the number of CNT-CNT interconnections is expected to be lower at lower loadings, as was evident from the higher impedance of composites with φCNT =0.03 than that with φCNT =0.06. The relative change in the impedance with an increase in frequency is shown in Figure 6C for φCNT=0.03and φCNT =0.06 chemiresistors. The profile also contains AC impedance of these composites under toluene field. As expected, the critical frequency was much higher at higher loading; interestingly, however, under the toluene field, at φCNT =0.03there was a significant change in critical frequency but at φCNT =0.06the change is not that sharp. Such effect confirms that chemiresistive effect is more pronounced in the φCNT =0.03composites. This can be attributed to the fact that the density of the CNT-CNT network is lower at lower φCNT; such chemiresistors are therefore more prone to substantial change resistance due to the increased gap between CNTs under toluene field (section


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3.2)62. It may be noted that the decrease in gel content with increase in φCNT from 0.03 to 0.06 was negligible (~0.01 i.e. ?,@ = ABCDEF

(7)

where a, b and n are constants 12. The sensing response of PDMS/CNT-COOH (D=100 kGy) is plotted against IEF in Figure 8B Inset. It can be seen that the sensing response decreased with an increase in χ12. The equation 7 fitted well (r2>0.99) for composites, suggesting the dependence of the response of the senor on the polymer-filler interaction parameter. Since polymer filler


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interactions originate from the thermodynamics of the system, the high selectivity towards toluene can be attributed to the relaxation of the PDMS chains under toluene atmosphere. Such a relaxation will build stress within the matrix, increasing inter-aggregate distance within the percolated CNT networks63. Figure 8C-D and Figure S8 depict the sensitivity and reproducibility of the sensor. The chemiresistor was exposed to three successive exposure cycles of a fixed concentration followed by three successive cycles of increased concentration. The signal was reversible and reproducible. The sensor attained 90% of peak response in about 90 seconds and 50% of the signal was recovered in about 30 seconds. The reproducibility of the signal was estimated from the relative standard deviation and was found to be 0.049 (4.9%). The sensitivity (s) of the chemiresistors was found to be 0.008 Lµg-1. The detection limit was gauged from 3.3σ/s and was found to be 1.5µgL-1. 3.7

Conclusion The study establishes that functional groups on CNT have a profound effect on the

chemiresistivity of PDMS/CNT composites. Carboxyl-functionalized CNT based chemiresistors showed the best sensing behaviour, whereas amine functionalized chemiresistors had much lower sensitivity. The better interfacial interactions between the functionalized CNT and PDMS translated in radiation-induced modifications of PDMS/CNT interface, which eventually lead to a marked enhancement in the sensitivity of the chemiresistor. Such factors were evident from the CNT and radiation dose-dependent changes in the gel content of the composites and from the non-linear rheology measurements. The contact resistance between CNTs and the critical frequency both were found to be dependent on the functional groups on CNT. An increase in the CNT volume fraction leads to a decrease in the sensing response. The sensors showed good selectivity for toluene and had a wide detection range and good reproducibility. These results highlight the impact of the functional group driven changes at the CNT/polymer interface, on the electron transport within the matrix and its profound effect on the chemiresistivity. Furthermore, as the sensors exhibit reversible and reproducible response for multiple cycles, such chemiresistors will be useful as the standalone field sensors for volatile organic compounds.



Supporting Information Available: [Electrical impedance of composites, cryofractured SEM, optical micrographs, DC conductivity, gel content, surface energy, XPS spectra (O1s), diffuse reflectance FTIR, reproducibility and signal characteristics and schematic representation.]


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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 1: Fabrication of standalone chemiresistor with radiation-induced interfacial grafts and covalent crosslinking.


Page 28 of 36

Page 29 of 36

-Z'' (Ω) 2

0 . 0

0 0 2

H N T N C

2

0 8 1

r i A

5 . 0

Ar,p

T N C

0 6 3

F U T N C H N T N C

Ar

6

H0 O. O0 C= T N C 0 4 5

φ

F U T N C

0 . 1

0 0 5 0 0 3

5 . 0

4 0 1 x 0 . 8

6

4 0 1 x 0 . 6

H O O C T N C

(F)

(E) (E)

1.0x10

4 0 1 x 0 . 4

Z' (Ω)

))))

(D)

4 0 1 x 0 . 2

µ

0 . 00

((((

0 0 0 1

0 0 8 1 L g 0 0 6 e n e u l o T

)

0 0 4

5.0 µm

0 0 2

s d n o c e S

0 5 7 3

0 5 2 2

e m i T

0 0 0 3

0 0 5 1

0 5 7

0

0

0 . 0 6

(F) 5.0 µm

5

5.0x10

H O T N C

G'C-G'P(Pa)

e n e u l o T

H O O C T N C

5 . 1

0 . 1 (

1.5x10

(C)

(B)

0 5 7

(A)

0 0 0 1

H O O C T N C

5 . 1

3 µm

0.00

0.02

0.04

φ

T N C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


3 µm

0.06

Figure 2: Effect of CNT functionality on the chemiresistivity (A) Relative change in resistance for PDMS/CNT CPCs in successive cycles [Toluene-Air, 5 min each] of increasing concentrations (concentration of toluene is shown in µgL-1, against each peak) (B) Relative peak change in resistance at different concentrations and linear fit of the data (black line) (C) Imaginary and real impedances in air and in the presence of toluene (500 µgL-1) (D) Increase in the shear storage modulus with increase in CNT volume fraction (E) SEM of cryo-fractured PDMS/CNT-OH composite, φCNT=0.06 (dotted circles show agglomerates) (F) SEM of cryo-fractured PDMS/CNT-COOH composite, φCNT=0.06 (arrow shows one of the dispersed CNT).



( ) ε

2 y t 10 i v i t t i m r1 e 10 p e g a r o t0 10 S

(B) 5

10

2

10

CNT-COOH CNT-NH2 CNT-OH

' ' y t i v i t t i m r e p s s o L

6

10

3

10

0

10

(C)

-1

10

0.04

φ

0.06

-3

2

10

3

4

10

10

5

6

10

10

7

10

10

0.01 0.02 0.03 0.04 0.05 0.06 0.07

φ

FREQUENCY(Hz)

T N C

0.02

T N C

10 0.00

7

8

2

(A)

10 '

6

10

10

H N T N C

s 1 O

9

3000

(E)

H O O C T N C

(D)

6

s 1 C

H N T N C 2

3

-3

2000

p 2 i s S 2 i S

0

F U T H N O C T N C

Counts

-1

∆ Surface Energy ( mJ/m2)

Z(Ohm)

H O O C T N C

2

10

5

10

H O T N C

y t i v i t t i m r e p s s o L

5

10

4

10

( ) ε

H O T N C

'

8

10

3

10

H O O C T N C

10

( ) ε

2

F U T N C

1000

-6 CNT-COOH

CNT-OH

CNT-NH2

V e

0 600 500 400 300 200 100 y g r e n E g n i d n i B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 30 of 36

( )

0

Figure 3: Effect of CNT functionality on the electrical impedance, surface energy, filler structural breakdown and gel content (A) Change in AC impedance with increase in the CNT volume fraction in PDMS/CNT composites with different CNT functionalities (100 Hz) (B) Loss and storage permittivity with frequency (C) Loss permittivity as a function of CNT volume fraction (D) Change in the surface energy of different CPCs with functionalized CNT, with respect to CPC with unfunctionalized CNT; φ=0.06 (E) Wide scan XPS spectra of PDMS/CNT composites with different CNT functionalities.


Page 31 of 36


s 1 C

s 1 C

s 1 C

1 2 3 1500 (iv) (iii) (ii) (i) (iv) (iii) (ii) (i) (B) (iii) (ii) (i) 4 (C) (A) 5 1000 1000 6] . ] u ] . . 1000 7. u a u . [ . a a 8y [ [ t i y y t 9s i t i n s s e n 500 t 10 n 500 e n e t I t 500 n 11 n I I 12 13 14 0 0 0 15 300 296 292 288 284 280 300 296 292 288 284 280 300 296 292 288 284 280 16 ( ) ( ) ( ) 17 18 19 600 2490 (iii) (ii) (i) 20 (F ) (D) (E) 21 22 . 23 ] ] . 1660 u 400 u . 24 . a [ a [ 25 y t y i t i s 26 n s n e e 27 t t n 200 I 830 n I 28 29 30 31 0 0 32 300 296 292 288 284 280 540 536 532 528 524 33 ( ) ( ) 34 35 36 37 38 Figure 4: Effect of CNT functionality on the XPS spectra (A) C1s CNT-UF (B) C1s CNT-COOH (C) C1s CNT-NH2 (D) C1s CNT-OH 39 (E) O1s CNT-COOH (F) schematic representation of possible polar-polar interactions between CNT-COOH and PDMS. 40 41 42 43 44 ACS Paragon Plus Environment 45 46 47 2 H N T N C

H O O C T N C

F U T N C

V e

y g r e n E g n i d n i B

V e


V e


s 1 O

s 1 C

H O O C T N C

H O T N C

i S O

O = C

O = C O

V e


V e



Page 32 of 36

H O O C T N C

1.00

2

Gel fraction

H N T N C

γ( )

)

T N C

T N C

F U T N C

F U T N C

υ(

6 0 . 0 =

4 0 . 0 =

s / d a r

H O O C T N C

1 0.4 (A) (C) (B) 2 12 0.95 φ φ 3 ω= 1 0.3 10 4g / 5k 0.90 l % 8 6o 0.2 m c 7 6 0.85 e 8 0.1 9 4 0.80 10 2 0.0 11 12 0.75 0.00 0.01 0.02 0.03 0.04 13 UF COOH NH2 UF COOH NH2 0.01 0.02 0.03 0.04 0.05 0.06 0.07 14 φ φ 15 16 (D) 1.80( ) 17 18 4 19 1.44 20 21 1.08 22 2 23 0.72 24 25 26 0.36 27 0 28 0.80 0.84 0.88 0.92 0.96 0 40 80 120 160 200 29 −1 Strain % λ 30 31 32 33 34 35 36 37 38 39 40 Figure 5: Effect of CNT functionality on the entanglements, filler structural breakdown and gel content (A) Change in entanglement density as a 41 function of CNT volume fraction (B) Change in critical strain with increase in CNT volume fraction (C) Change in gel content with CNT 42 volume fraction and functionalities (Dose 100 kGy) (D) Stress-strain profiles of irradiated (100 kGy and 200 kGy) PDMS/CNT composites 43 44 (E) Mooney-Rivlin plot for PDMS/CNT-COOH CPC irradiated at 100 kGy (F) Schematic representation of amine induced quenching of ACS Paragon Plus Environment 45 radicals via hydrogen abstraction. 46 47 H O T N C

T N C

T N C

2

)

−2

H N T N C

σ/( λ−λ

H N T N C

2222

Stress, σ (MPa)

H O O C T N C

E

F U T N C

H O O C T N C

n e p O : y G k 0 0 2

y G k 0 0 1

d e s o l C : y G k 0 0 1

(F)

Page 33 of 36

Ar,p

6 0 . 0 =

φ

T N C

0 0 5

Ar

3 0 . 0 = T N C

3 2

0 0 3

2

φ

4

T N C

φ

(B)

5

0 5 7

6 0 . 0 =

4

0 0 0 1

3 0 . 0 =

φ

T N C

(A)

6

0 0 2

1

0 0 1

0

0 0

1000 2000 3000 Time (Seconds)

4000

200

400 600 800 -1 Toluene (µgL )

1000

6 0 . 0 = T N C

φ 1

Z / Z0

Exposed: 1000 ppmToluene

0.1

3 0 . 0 =

(C) 1

10

φ 2

10

3

4

T N C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


5

10 10 10 Frequency (Hz)

6

10

Figure 6: Effect of CNT volume fraction on chemiresistivity (A) relative change in resistance for PDMS/CNT-COOH (φ=0.03 and φ=0.06) CPCs in successive cycles [Toluene-Air, 5 min each] of increasing concentrations (concentration of toluene is shown in µg/L, against each peak) (B) Relative peak change in resistance at different concentrations and linear fit of the data (red line) (C) Relative change in impedance in air and in the presence of toluene. ACS Paragon Plus Environment


Sensitivity (Lµg-1)

Ar

3 0 . 0 =

0.004

E

y G k 0 5 , T N C

8

φ

Ar

200

100 kGy: Toluene

0.1

100

3 0 . 0 =

1 10 100 Angular Frequency [rad/s]

50 kGy: Air

φ

0.0 400

800 1200 1600 Time (Seconds)

1

10

100 kGy: Air

T N C

0.1

50 kGy: Toluene

1

0.5

6

CNT-COOH

( )

( )

300

100 150 200 250 300 Dose(kGy)

y G k 0 5 , H O O C T N C

1.0

50

F

140

T N C

φ

Z/Z0

Ar

0.006

y G k 0 0 1 , H O O C T N C

3 0 . 0 = T N C

φ 0 0 . 0 =

Complex Viscosity [mPas]

C

y G k 0 5

D

6 0 . 0 = T N C

9

80 100-1 120 Toluene (µgL ) y G k 0 0 1 , T N C

60

( )

T N C

10

40

500 1000 1500 Time (Seconds)

φ

7

0.008

0.002

10

10

y G k 5 7

0.2 0

10

( ) 0.010

y G k 0 0 1

0.4

0.0

10

3 0 . 0 =

y G k 0 5

0.2

10

T N C

0.6

φ

y G k 0 5 1 y G k 0 y 5 G 2 k 0 0 3

B

A

0.8

y G k 5 7

5 7 0 5

0.4

y G k 0 0 1

0 0 1

0.8 0.6

( ) 1.0

( )

y G k 0 0 3

φ

1.0

5 2 1

3 0 . 0 = T N C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 34 of 36

2

10

3

4

5

10 10 10 Frequency(Hz)

6

10

Figure 7: Effect of radiation dose on chemiresistivity (A) Relative change in resistance of irradiated PDMS/CNT-COOH chemiresistors in successive cycles [Toluene-Air, 5 min each] of increasing concentrations (concentration of toluene is shown in µgL-1, against each peak) (B) Relative peak change in resistance at different concentrations at different doses and linear fit of the data (C) Variation of sensitivity with radiation dose (D) Change in complex viscosity as a function of angular frequency (E) Relative change in resistance for irradiated PDMS/CNT-COOH and PDMS/CNT chemiresistors in successive cycles [TolueneAir, 5 min each, φCNT=0.03] of increasing concentrations of toluene vapour (F) Relative change in impedance in air and in the presence of toluene (500 µgL-1). ACS Paragon Plus Environment

Page 35 of 36

Benzene

Ethanol

φ

Methanol

Chloroform

0

Ar,p

Water

9 9 . 0 ~ 2 r

Toluene

T N C

( 1 )

3 0 . 0 =

3 0 . 0 =

A

φ

T N C

10%

e -1 e -2 e -3 e

Ar

Benzene -1

0

1

χ12 e

e

B

0.1

e

Acetone

( )

Choloroform

1 L g 0 0 5

0

50

0 5

0 0 2

Ar

0 0 0 1

0 0 8 0 0 6

0 0

34 0 . 0 = T N C

φ

1.2

10

5 7

0.46

)

H O O C T N C

100

0.2 0.4 0.6 0.8 1.0 A (VOC) /A (Toluene) r,p r,p

D

0 5 1

(

0 0 1

0.69

µ

0.0

300

5 2 1

φ

H O O C T N C

0.92

250

3 0 . 0 = T N C

( )

100 150 200 Time (Seconds)

Toluene

Ar

0.01

Methanol

Ethanol Acetone

C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


1

0.23 0.00 1100

2200 3300 4400 Time (Seconds)

5500

0.1

0

500

1000 1500 2000 2500 Time (Seconds)

Figure 8: Selectivity and reproducibility of the chemiresistor (A) Relative change in resistance for PDMS/CNT-COOH chemiresistors in the presence of vapors of different VOCs [500 µgL-1] (B) Relative peak change in resistance with respect to the peak response of toluene [Ar,p(VOC)/Ar,p(toluene)] (C) Change in Ar after exposure to successive air-toluene cycles, three cycles of a fixed concentration were used followed by increased concentrations (D) Change in Ar after exposure to successive cycles of toluene-air [high concentrations of toluene]. ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

For Table of Contents Only Table of Contents Graphics (5.0 cm x 4.5 cm)


Page 36 of 36

Carbon Nanotube Functionalization and Radiation Induced

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