MWCNT composite sensing film by in situ dispersed polymerization

Synthetic Metals 157 (2007) 390–400

Preparation, fabrication and response behavior of a HTBN/TDI/MWCNT composite sensing film by in situ dispersed polymerization Yanling Luo ∗ , Chao Wang, Zhanqing Li School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, PR China Received 9 October 2006; received in revised form 27 February 2007; accepted 12 April 2007 Available online 4 June 2007

Abstract A polyurethane inserted multi-wall carbon nanotube (MWCNT) composite conductive film was prepared by in situ dispersed polymerization reaction using hydroxyl-terminated poly(butadiene-acrylonitrile) liquid rubber as a linear diol, toluene diisocynate as a curative, ethylene glycol or glycerine or triethanolamine as a chain-extending agent and MWCNT as a conducive filler. The effect of various curing temperatures and chain-extending agents on vapor-induced electrical responsiveness of the conductive films was investigated. The structural characterization of the cured film was conducted by Fourier transformation infrared spectrophotometer (FTIR), differential scanning calorimeter (DSC), polarization microscope (POM) and wide angle X-ray diffraction (WAXD). The experimental results showed that the conductive composite film obtained in the present work exhibited a microphase separation resulting from the soft-hard segment domains, and possessed some crystalline behavior from the hard segment. The response intensity was enhanced with the curing temperature increased, while the reversibility could be improved at a relatively low curing temperature. The responsivity of the film produced by a linear difunctional group chain-extending agent was lower than that prepared by trifunctional group curatives, and the reversibility was vice versa. The experimental phenomena were explained from the viewpoint of the microphase separation, crystalline behavior, the structural characteristics of the soft-hard segment, and the electronic properties of multi-wall carbon nanotubes as well as a weak electrostatic or noncovalent interaction between polymer or analyte molecules and MWCNTs. © 2007 Elsevier B.V. All rights reserved. Keywords: Conductive composites; Carbon nanotubes; Vapor sensitive response; Electric properties; Characterization

1. Introduction Recently, the outstanding performances of electroconductive polymer-based composites have been achieving international recognition since 1990s. Since some of the characteristic physical parameters of this material such as electric resistance, capacitance, dielectric constant, volume etc. change with the environmental atmospheres or conditions, gas sensors can be fabricated. The sensors can exhibit an extraordinary sensory characteristic analogous to the olfaction system of human or any other beings, so we call it an electronic nose. They are expected to find wide applications in detection of chemical or environmental gases or odors and solvent leaking, identification of chemical structure of the polymers and so on

∗

Corresponding author. Tel.: +86 029 85308442; fax: +86 029 85307774. E-mail addresses: [email protected], [email protected] (Y. Luo). 0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.04.009

[1–3]. This kind of sensing material directly have an impact on sensitivity or responsive rate, selectivity, linear correlativity and stability or life-time of the sensors, so the development of the sensing materials with ideal vapor or humidity sensitive features becomes the highlight in this field. Functional composite materials sensitive to electric conduction prepared by blending polymer matrices with conductive fillers such as carbon black (CB), metal powder, carbon fiber, graphite and carbon nanotube have been studied as an interesting subject [4,5]. In particular, the small size effect and unique molecular structure of carbon nanotubes (CNTs) make them have excellent mechanical, electronic and chemical properties. The quasi-one-dimensional tubular molecular structure and their potential applications determine their indispensable position in conductive polymer-based composite materials research. One of the greatest advantages of the materials prepared using CNTs as conductive filler is easy to process. The high conductivity of CNTs yields much low percolation threshold, and then can be achieved by adding a small amount of CNTs into polymer

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matrices. CNTs are curled from the layered graphite, and are in the range of 0.4 nm to tens of nanometers in diameters and up to several microns in length. They have, therefore, a great aspect ratio (100–1000) [6]. The mechanical intensity of such materials is 2 orders of magnitude higher than the known materials at present [7–10]. The composite materials prepared by multi-walled carbon nanotubes (MWCNTs) with the polymer matrix are also provided with toughness. Moreover, there is a huge interface in carbon nanotubes that can offer a large number of gas channels, thereby the sensitivity is greatly increased. Furthermore, they can reduce the operating temperature and the size of the sensors. Its special electronic features, especially its electrical conductivity, can switch between metals and semiconductors. All of these make CNTs as a kind of fillers of conductive polymer composite materials bear incomparable advantages, compared with any other filling materials. Currently, a universal problem faced with in CNTs/polymer composite materials research is that the high aspect ratio of CNTs makes it easily assembled into bundles or intertwisted [1,2,11]. Thus, it is difficult for CNTs to disperse in the matrix. On the other hand, the ordered arrangement structure of CNTs in the matrix has an important impact on performances of the conductive polymer composite materials. Therefore, the challenge confronted with in the preparation of high performance conductive polymer composite materials is how to improve the dispersion of CNTs in polymer matrices and the arrangement orderliness. Many research interests in recent years have been aroused in order to improve its compatibility with various polymers and find a variety of applications. The polymer-based carbon nanotubes (CNTs) composites were typically prepared by a straightforward dispersion way, an in situ polymerization method and a mixing compositing process [11]. Dai [12] designed and fabricated multifunctional chemical vapor sensors of aligned carbon nanotube-PVAc or/and PI composites. Surface functionalization of the carbon nanotube achieved through a graft reaction of functional groups from the CNTs with polymers, etc, can overcome the agglomeration shortcoming and show significant improvements in performances [13–15]. In general, the functionalities of original pristine CNTs can be provided by introducing chemically-functional groups such as –OH and/or–COOH on the surface and side walls of CNTs and /or opening holes at the ends of CNTs. Consequently, the interfaces’ interactions between CNTs and polymer molecules would be changed in polymeric nanocomposites. This has become an important subject in CNTs research in applications [14–16]. These studies open opportunities to use CNTs as substrates for molecule self-assemblies and as materials for sensors with high performances. The molecule structures and thus, atomic interactions at CNT interfaces are also expected to significantly influence many properties in the above-mentioned nanosystems. For example, molecule structure ordering enhanced interfacial actions in CNT composites and conformation-dependent diffusivity of gases or analytes in CNTs. Based on the aforementioned standpoint, we introduced an in situ dispersed or embedded polymerization method in our work, combined with an ultrasonic process to disperse non-

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functional CNTs sufficiently, so that the stabilities of the CNTs can be maintained and the disadvantages that the CNTs are easily reunited and enlaced can also be avoided effectively. Thereby, a novel polyurethane composite conductive film with excellent vapor-sensitive response was prepared by the reaction of hydroxyl-terminated poly(butadiene-acrylonitrile) liquid rubber (HTBN) and toluene diisocynate in the presence of chinaextending agents and multi-wall carbon nanotubes conducive fillers. 2. Experimental details 2.1. Materials and reagents The hydroxyl-terminated polybutadiene/acrylonitrile liquid rubber (HTBN), with molar mass of 2.96 × 103 , polydispersity index of 1.77 and hydroxyl group value of 0.5672 mmol g−1 , available from Lanzhou Chemical Industry Corp. of PetroChina, was deaerated and dewatered in vacuo at 110 ◦ C for 1 h before use. The multi-walled carbon nanotubes (MWCNTs) with a nominal outside diameter (OD) of 30–50 nm provided by Chengdu Institute of Organic Chemistry, Chinese Academy of Science, were ground to powders in advance and then dewatered in the oven at 120 ◦ C before use. Toluene diisocyanate (TDI), a 80/20 mixture of 2,4 and 2,6 isomers supplied by Chengdu Union Chemical Reagent Research Institute, analytically pure (AP), was purified by vacuum distillation (8.638 × 103 Pa, 148 ◦ C). Ditin n-butyl dilaurate was obtained from Shanghai Shanpu Chemical Industry Corp, which was chemically pure (CP). Glycerine was provided by Xi’an Baqiao Chemical Reagent Factory. Both triethanolamine (CP) and ethylene glycol (AP) were commercially available. Benzene, which was analytically pure, was from Xi’an Chemical Reagent Factory, used as received without further purification. 2.2. Experimental procedures To get sensing films, 0.2 g HTBN was dissolved in toluene of 20 ml (in a 50 ml cuvette) at room temperature and a pellucid solution was formed. Then a calculated amount of MWCNTs was introduced into the solution. The mixture solution was dispersed with a probed ultrasonic cleaner (supplied by Ningbo Scientz Biotechnology Co Ltd., SB-5200D) for 10 min. Subsequently the solution was plunged into a 50 mL flask with three hatches and a calculated amount of TDI was added in the light of the following expression [17]: mTDI = mHTBN r(E−OH /E−NCO )

(1)

where mTDI and mHTPB are the mass of TDI and HTBN, respectively; ENCO and EOH are the mole numbers of isocynate groups in TDI and hydroxyl groups in HTBN, respectively; r is a mole coefficient, r = −NCO/−OH. After the temperature of the mixing solution was elevated up to 85 ◦ C to react for 2 h, the chain-extending agent (CEA) glycerine or triethanolamine or ethylene glycol was introduced into the mixture. If necessary, three drops of the ditin n-butyl

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Scheme 1. Reaction principle of HTBN and TDI in the presence of chain-extending agents.

dilaurate were added as a catalyst and the mixture was stirred to react for 15 min. The mass (me ) of the CEA was determined by formula (2) [17]: me =

0.85 mp − NCO% Me 84

(2)

where mp is the mass of the pre-polymer; Me is the relative molecular weight of the CEA; –NCO% is the content of free isocynate groups in pre-polymers (non-reacted) and theoretically is:   42(r − 1) −NCO% = 100% (3) mt where mt is the total mass of the stuff. The ultimate reaction mixture was cast or spray-coated and deposited onto a spotless glass plate or a comb-like electrode, a copper electrode with a dimension of 6 mm × 6 mm with a ceramic substrate that was prepared by a screen-printing technique, and then a thin film formed. The sensing film specimens obtained above were placed in air at room temperature for 7 d or at different elevated temperatures for 36 h to vaporize the solvent. The curing reaction was carried out until the formation of a complete sensing film. The thickness of the films was approximately close to 1 ␮m, as evaluated by scanning electron microscopy.

2.4. Measurements and characterization To eliminate the influence of down-lead and contact resistance on measurements, the bulk resistance of the composite film samples at room temperature was measured by using a fourpoint probe (4PP) method in our present work [19]. Electrical contact points were located at both sides of the specimens with regular distance apart using copper wire and silver paste. For the electrical measurements, a DC current is passed between the outer probes. The potential is then measured between the inner probes. The circuit of the probe attachment is shown in Fig. 1a. Electrical resistivity was obtained from the measured electrical bulk resistance, cross-sectional area of the composites, Av , and electrical contact length, Lec of the testing specimen connecting to copper wire. The relationship between electrical volumetric resistivity, ρv and electrical bulk resistance, Rv is as ρv = (Av /Lec ) × Rv ( cm)

(4)

The four-point probe measurement is a universal comparison method that has a high accuracy and can be used for various

2.3. Reaction principle An isocyanate (–NCO) group is a highly unsaturated functional group. Its electron resonance structure proposed by Baker et al. can commendably interpret the reaction activity between –NCO groups and the prepolymer containing active hydrogen [18]. Herein the –NCO group becomes an electrophilic center. Since ␣ –NCO functional groups in TDI shifted from a strong electron-withdrawing character to a weak electron-bestowing character, the second –NCO activity is deduced. Therefore the –NCO group at site 2 preferentially reacts. The reaction principle described above is represented in Scheme 1. Because there is no functional group to be used to bond on the original pristine MWCNTs studied, MWCNTs are covered or embedded in polymer networks.

Fig. 1. (a) The circuit of the probe attachment for four point probes, and (b) A schematic setup and a comb-like electrode for the electrical resistance measurement of sensors.

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ranges of electrical resistance values. This kind of measuring technique eliminates the influence of contact resistance at the current lead or probe-film specimens interface on the measurement. Simultaneously, the effect of the contact resistance from voltage probes on the measurement results is considered negligible since input impedance of the digital voltmeter is very high. Therefore, the method has lessened even excluded the contact resistance at the probe-film interfaces. The electrical contact resistivity, ρv is as ρv = Ac × Rc (Ω cm2 )

(5)

where Ac and Rc are electrical contact area and electrical contact resistance, respectively. In order to measure the response pattern as a function of time, a digital multimeter with a maximal range of 109  (manufactured by Shenzhen Victory Instruments Factory, Victor VC9808) was adopted to connect the above thin film samples to record bulk resistance change. Fig. 1 displays an elaborated setup for the electrical resistance measurement of the sensors examined. The initial bulk resistance (R0 ) was measured in dry air at room temperature until it was stabilized. Then the thin film element was immediately transferred into the airtight conical flask full of pure solvent vapors at the bottom and the bulk resistance change was recorded. The distance between the thin film element and the solvent surface is about 2 cm. The measurement of the electrical resistance-time response patterns of the thin film element to various solvent vapors was carried out at room temperature. After being stabilized for 5 min, the element was taken out rapidly and the bulk resistance variation was observed and recorded for 2–5 min. 2.4.1. Fourier transformation infrared spectrophotometer (FTIR) The structural characterization of HTBN/TDI/MWCNT composite PU conductive thin films was carried out on an EQUINX55 FTIR spectroscopy manufactured by Blucker Corporation of Germany using the methods of the potassium bromide tabletting and the attenuated total reflection (ATR), respectively.

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2.4.4. Wide angle x-ray diffraction (WAXD) In order to inspect the crystalline properties of the cured samples, WAXD studies were performed on a Rigalcu Co. D/Max-2550 VB + /PC automatic X-ray diffraction equipment made in Japan employing Cu radiation at a voltage of 40 kV and current of 40 mA, with scanning range from 5 to 50 ◦ and scanning rate 4 ◦ min−1 . 3. Results and discussion 3.1. FTIR analysis In order to confirm whether or not there is any functionalized group existing on CNTs and the cure reaction, we have given the FTIR spectra of original pristine nanotubes and HTBN/TDI/MWCNT composite thin films obtained using different CEAs at various curing temperatures, and have analyzed them so as to shed light on the nature of bonding between carbon nanotubes and polymers molecules, as shown in Fig. 2. Unless otherwise specified, the content of MWCNTs is 10 wt%, and curing time is 36 h. It is clear that there is no significant functional group, for example –OH or/and –C O, detected on the FTIR spectra of original pristine nanotubes (curve h). This is in agreement with many researchers’ reports [20–22]. A broadened absorption peak at ca 3400 cm−1 (curve f) is assigned to the hydroxyl group (–OH) stretching vibration in the HTBN molecule. A sharp peak with a moderate intensity at about 2240 cm−1 is vested in the characteristic absorption of –CN nitrile groups (curve f). The vibration absorptions at 2920 and 2840 cm−1 are attributable to the existence of CH2 – groups. The strong absorption bands at 3070, 1640 and 980 cm−1 are ascribed to the –C C– vibration modes in the HTBN molecule. A broad and strong vibration mode at 2240 cm−1 corresponds to the characteristic absorption of the isocyanate group (–NCO) in TDI (curve g), which is overlapped by the above –CN peak.

2.4.2. Differential scanning calorimeter (DSC) A Q1000DSC + LNCS + FACS Q600SDT thermal analyzer made by the TA Co., USA, was used to measure the glass transition temperatures and thermal properties of the cured samples. The differential scanning calorimetry (DSC) scans of all samples were carried out over a range of temperature from −100 to 220 ◦ C at a heating rate of 10 ◦ C min−1 in N2 atmosphere (40 ml min−1 ). 2.4.3. Polarization microscope (POM) A cold/heat polarization microscope (model LEICA-DMLP EC600) manufactured by Leica Corp. of Germany was used to examine the soft-hard segment microphase separation behavior and crystalline configuration of the cured samples.

Fig. 2. FTIR spectra of HTBN/TDI/MWCNTs composite polyurethane films at curing temperatures (a) 20 ◦ C, (b) 70 ◦ C and (c) 115 ◦ C (CEA: glycerine) in the presence of CEAs (d) Triethanolamine and (e) Ethylene glycol (Curing temperature: 20 ◦ C), here (f) HTBN, (g) TDI and (h) original pristine CNTs.

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After a hydrogen transference reaction between –NCO and –OH takes place, the absorption bonds at 3400 and 2240 cm−1 almost disappear in the composite thin films (curves a–e). However, small peaks at 2240 cm−1 seems to exist in the case of various CEAs (curves d, e and a) and different cured temperatures (curves a–c) owing to the existence of the –CN groups. Additionally, a weak adsorption band at 3200 cm−1 related to the stretching mode of imido groups (–NH–) and a peak of a –NH– in-plane bending vibration at ca. 1520 cm−1 occur. These suggest the formation of a –NH–C(O)–O– (carbaminate) group in the presence of different CEAs and cure temperatures. A strong vibration peak at about 1700 cm−1 belongs to the characteristic adsorption of –C O (carbonyl) groups in –NH–C(O)–O groups. The vibration peaks at 1200 and 1100 cm−1 or so jointly prove the existence of –C(O)–O–C– groups [23]. The foregoing results indicate that an ester linkage exists in the composite sensing films while –CN groups at 2240 cm−1 and –C C– at 3070 and 980 cm−1 still remain in the IR spectra of the composites. It is worthy noting that the stretching vibration peaks of –C C– groups at 1640 cm−1 disappear owing to longer molecular chains. The FTIR results fully show that a HTBN/TDI/MWCNT composite polyurethane (PU) conductive thin film in which the MWCNTs are dispersed through in situ is obtained by a chain-extending reaction at various temperatures. However, the question whether a strong covalent bond between MWCNTs and the cured resultants takes place cannot be confirmed on the basis of present FTIR spectra. 3.2. WAXD analysis Fig. 3 presents the WAXD spectra of HTBN/TDI/MWCNT composite conductive thin films in the presence of the CEAs such as glycerine, triethanolamine and ethylene glycol at a cured temperature of 115 ◦ C for 36 h. A diffusive amorphous peak appeared at 20◦ and an obvious crystalline diffraction pattern

gave birth to at 26◦ . With the r value (i.e., the content of the hard segment) increased, the crystallization diffraction intensities at 26◦ were enhanced (see also diffraction curves a–c). It is interesting to note that at 22.5◦ (curves b and d) and about 32◦ (curve a) also exist two small crystalline diffraction designs of the composite films formed by glycerine and triethanolamine, respectively. This may be caused by different crystallographic forms yielded by varying degrees of order in hard segment domains of the cured thin film. Their diffraction intensities were higher than the ethylene glycol cured system. These results indicated that the crystallization phenomena brought about by the hard segment domains exist in the conductive composite thin films obtained from the three CEAs to some extent [24] and the crystallized degrees of order of the hard segments formed by extending of glycerine and triethanolamine is slightly higher than the ethylene glycol cured system. 3.3. POM analysis Fig. 4 depicts the POM micrographs (×20) of HTBN/TDI/CEAs/MWCNT composite thin films. Herein, the black regions belong to hard segment domains and the white districts are soft segment ones. It is clear that the optical microphotographs without a polarized light (Fig. 4 a–e) assumed a microphase separation status for the cured thin films formed by various CEAs. A distinct microphase separation interface was situated between soft segment and hard segment domains, and the phase interfaces were relatively lubricous. Moreover, the domain sizes were larger and one cannot easily observe the transition region among the interphase domains. Accordingly the thin films exhibited a very high degree of microphase separation. With the hard segment content (namely r value) decreased the black regions representing the hard segment domain and the white area denoting the soft segment domain gradually became small, which made the microphase domain interfaces got thin (Fig. 4 a–c). On observing the samples in the same visual field with a single polarized light (Fig. 4 f–h), many small bright dots presented themselves in hard segment domains corresponding to the black areas in Fig. 4 a, d and e. This proves that the crystallization phenomena resulted from the existence of ordered structures occurred in hard segment domains of the composite materials, further confirming the validity of the WAXD results. 3.4. DSC analysis

Fig. 3. WAXD spectra of HTBN/TDI/MWCNTs PU composite films doped with curatives (a-glycerine, r = 1.9; b-glycerine, r = 1.05; c-glycerine, r = 1.55; d- triethanolamine, r = 1.55 and e-ethylene glycol, r = 1.55).

Fig. 5 displays the DSC curves of HTBN/TDI/CEAs/ MWCNT composite conductive thin films prepared by the three CEAs at varying cured temperatures for 36 h. The main transitions at the range from −36 to −47 ◦ C correspond to the glass transition temperatures (Tg ) of HTBN soft segment domains (see also Table 1). The Tg of the films obtained at a higher cured temperature was lower than the specimens cured at room temperature. The Tg (−36.81 ◦ C) of the films in the presence of ethylene glycol as a CEA was higher than the glycerine (−43.23 ◦ C) and triethanolamine (−46.18 ◦ C) systems. This is not different from the Tg of −44.4 ◦ C produced by HTBN chains in the literature

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Fig. 4. Polarization microscope photos (×20) of HTNN/TDI/MWCNTs PU composite films with various curatives and TDI contents (a-glycerine, r = 1.9; b-glycerine, r = 1.55; c-glycerine, r = 1.05; d-triethanolamine, r = 1.55; e-ethylene glycol, r = 1.55; f-glycerine, r = 1.9; g-triethanolamine, r = 1.55 and h-ethylene glycol, r = 1.55).

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Fig. 5. DSC curves of HTBN/TDI/MWCNTs cured films in the presence of various chain-extending agents at different curing temperatures (Glycerine: (a) r = 1.55, 20 ◦ C; (b) r = 1.55, 70 ◦ C and (c) r = 1.9, 115 ◦ C. Triethnolamine: (d) r = 1.9, 115 ◦ C; Ethylene glycol: (e) r = 1.9, 115 ◦ C).

[25]. The obvious endothermic peaks at the range from 62 to 65 ◦ C are attributable to the crystallite fusion character of hard segment domains of the thin film materials. In addition, a smaller crystalline fusion peak also appeared at ca. 100 ◦ C (see curves b and d), and the corresponding crystallization enthalpy was 1.85 and 2.81 J g−1 , respectively. This is in line with the XRD and POM results that the crystallization phenomena produced in hard segment domains. A clear broadened exothermal peak ranging from 220 to 400 ◦ C may result from the recrystallization of some amorphous regions or incomplete crystalline areas of the cured films and cross linking reactions of the cured PU materials having not undergone the depolymerization at elevated temperatures [26]. With the cured temperature elevated, the reaction became more complete. Meanwhile, the increase in reaction rates led to the molecules in hard segment domains to be able to acquire sufficient energy to arrange regularly, forming a more perfect crystallite structure. Therefore, the rearrangement of the hard-segment molecular backbone and a hydrogen bond effect existing in the domains further raised the energy required on the regular arrangement and crystallization during the elevated temperature, so that the recross-linking reaction or crystallized exothermic peak shifted to high temperatures (curves a–c). This is not in line with the results that the fusion endothermic peaks from hard segments occurred at the range from 200 to 370 ◦ C reported by Ishihara, Yang et al. [27] and Luo et al. [28]. In the case of glycerine as a CEA, an exothermic peak of the thin film Table 1 DSC data of HTBN/TDI/MWCNTs cured films in the presence of various chainextending agents at different curing temperatures Specimen number

Tg , (◦ C)

Hf , (J g−1 )

Tm

Curve a Curve b Curve c Curve d Curve e

−37.29 −43.23 −45.91 −46.18 −36.81

50.92 35.23 + 1.85 52.78 46.08 + 2.81 52.00

64.75 62.78 63.21 62.67 62.49

◦C

103.3 100.6

Fig. 6. Relationship plot between logarithm resistivity and MWCNT contents for HTBN/TDI/MWCNTs cured film samples in the presence of CEAs.

appeared at about 400 ◦ C (curve c), higher than the one at ca. 320 ◦ C of triethanolamine and ethylene glycol cured systems (curves d and e). This is in accordance with the aforementioned WAXD results that the cured resultants in the presence of glycerine possessed a higher crystallinity. The endothermic peaks after the broadened exothermic peaks are assigned to the mutual effect of the –NH–C(O)–O– group in hard segment domains and biuret linkage and thermolysis of the cross-linking networks. 3.5. Dependence of the CEAs on response of HTBN/TDI/MWCNT thin films It is well known that carbon nanotubes have a special electricity behavior and adsorption performance, which therefore makes the resistance response of the HTBN/CEAs/TDI conduction thin film filled with carbon nanotubes become complex. However, from the viewpoint of electric conduction, the formation of the electric conduction networks in this composite sensing film is still mainly accomplished through a conductive pathway formed by the conglomeration of MWCNT nanometer granules, or through leaping among the tunnel potential barriers resulted from insulating polymers [29–32]. This is not in agreement with the electricity or adsorption responsive mechanism behaving in typical CNTs. In our present work, we have demonstrated that the percolation thresholds (Φc ) for HTBN/TDI/CEAs/MWCNT composite films were a weight fraction of 0.08, as shown in Fig. 6. It was evident that the electrical resistances of the three film samples strikingly dropped with the increased MWCNT contents (unless otherwise specified, hereinafter the resistance means bulk resistance). When the content reached over 0.08, the three films transited from insulators to semi-conductors, which indicated a percolation threshold Φc of about 0.08. Of course, this value was marked by slight discrepancy for the three systems. And it was lower than that of some systems doped with carbon black [33,34]. Meanwhile, it is worth noting that other researchers obtained a lower percolation threshold than our present result

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[3,35]. This can be assigned to the fact that the resistivities are influenced by the different shapes, sizes of the conducting aggregates and the uniformity of their dispersion within the non-conductive polymer matrices, processing methods and the interaction between the conducting fillers and the polymer matrices [34,36]. In our present investigations, the formation of crossing networks in the presence of CEAs caused viscosity to be increased. The diffusion coefficient is deduced, and thus, the MWCNT cluster agglomeration is controlled, which makes the resistivity value higher than that of a majority of polymer composites [37]. When Φ exceeded Φc , e.g. in the range of 0.08 to 0.15, the resistances lay in around 103 to 102 . The change tended to be stabilized at about 10  while Φ surpassed 0.15. The experimental results verify that the percolation thresholds for both types of carbon particles cannot be explained by statistical percolation theories based on an excluded volume approach. Such a theoretical approach considers a random distribution of anisotropic particles within a closed system and determines the likelihood of contact between neighboring particles. For a composite containing a randomly oriented carbon nanotube in an insulating matrix with a large aspect ratio, the percolation behavior is not a purely geometrical problem but rather relates to local changes in nanotube contacts. The shearinduced aggregation of the initially well-separated high-aspect ratio particles leads to the formation of a much more efficient network than can occur by random positioning of the particles (although the network remains apparently isotropic). Thus, the strong particle–particle interactions as well as interactions between nanotubes and the polymer matrix are crucially important. This mechanism is in good agreement with results obtained for carbon black filled epoxy and shows that, such systems should be treated by colloid theory [38]. Although this process does not reflect true, geometric percolation, it nevertheless shares the key features, namely the generation of a coherent network of particles, leading to a rapid change of properties following a standard sigmoidal curve. We took Φ = 0.10 to inspect the influence of different CEAs on response intensities or responsivity I of HTBN/TDI/MWCNT thin films (I = Rmax /R0 , where Rmax is a maximal resistance value of a film in solvent vapor and R0 is an initial resistance which is defined as an average value of six group resistance data measured in dry air). The experimental results are shown in Fig. 7. It can be seen that the responsivities of the thin films to some solvent vapors like benzene, toluene, acetone, chloroform vapor are quite high, especially in the case of the triethanolamine solidified system, displaying a selective response to these four solvents. However, the response intensities to others such as carbon tetrachloride, cyclohexane, methanol, formaldehyde are relatively low. Thereinto, the films nearly does not respond to formaldehyde and cyclohexane vapors, with the change in relative resistance smaller than two times. This is contrary to the experimental results reported before [35]. In previous works, we prepared a HTPB/TDI/CB composite prepolymeric sensing film composed of non-polar polybutadiene soft segments and polar hard segments. This film displayed certain responsiveness to strong polar and non-polar solvent vapors, and gave selective response to non-polar solvent vapor. The difference in nature of

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Fig. 7. Effect of chain-extending agents on responsivities of HTBN/TDI/ MWCNTs conductive composite films (Temperature: 70 ◦ C; curing time: 36 h; r = 1.55).

response may dominantly be controlled by the swelling model originated from the principle “like dissolves like” [39–41], associated with the different interaction between the MWCNT with a high polar surface energy or carbon black particles and the film samples. For the HTPB/TDI/CB composite film, only the swelling or dissolvation ability to polymers is a sufficient criterion to throw light on the observed phenomena [35]. In the case of HTBN/TDI/MWCNT thin films, we suggest that a weak electrostatic or noncovalent interactions, for example through a Van der Waals force or any other forms of forces (for example, the charge transfer reaction), between the highly delocalized electron system of MWCNTs and polymers exists in the absence of chemical bonding between CNT and the matrix. This can be testified by a large amount of related investigations [42–45], combined with FTIR results in Fig. 2. This non-bond interaction may affect molecular recognition of polymers to solvent molecules. Just like the descriptions in POM and DSC analyses, the microstructures of HTBN/TDI/CEAs/MWCNT solidified composites consisted of soft segments containing polar butadiene-acrylonitrile chains and hard segments with strong polar carbaminate groups and CEAs chains. There exists a certain amount of crystallinity in hard segment domains. According to the principle “like dissolves like”, the polar solvent vapor may simultaneously cause the soft and hard segment regions to swell. Furthermore, the naked or covered MWCNT with centrallyhollow core structures permits the analytes to permeate the polymers from both inner tubes and outer tubes, thus swelling the polymers rapidly, and shutting off or destroying the conductive tunneling path formed by MWCNT. Additionally, it is likely that when the interlayer interactions of nanotubes were introduced, the MWCNT could be induced changes from metal to semiconductor, resulting in the increase in resistance [46,47]. The evidence of the existence of tunneling currents in the composites is also given by alternating-current property studies [48,49]. In

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general, the interval width among CB particles assumes an exponential relation with resistivity. The latter is sharply increased as long as the former is slightly enhanced [48,50]. It seems that the tunneling current between particles in thin layers of the matrix decreases because the polymer matrix absorbs the solvent molecules separating the carbon particles from each other. In other words, a hopping mechanism is also responsible for an intertube charge transfer reaction between CNTs and an intertube modulation of the CNTs network [51]. The intertube modulation is similar to that of the interaction between conductive polymers and physically adsorbed molecules. It is known that MWCNTs are typically present in inner cavities, in interlayer pores and as aggregates of isolated CNTs and CNT bundles, forming aggregated pores. When the analyte is sufficiently absorbed, these solvent molecules compensates for the hole carriers in p-type MWCNTs (whether it is naked or covered MWCNT), and the barrier offered to charge transport is enhanced, causing the electrical resistance of composite thin films to increase. Therefore, the improvement in the response may be caused by capillary condensation of MWCNTs [52]. In so far as a low response against methanol and formaldehyde, the reason may similarly be attributable to different interactions of solvent vapor-carbon nanotube composites for different solvents [48,51], that is, to different intertube charge transfer between CNTs and an intertube modulation of the CNTs network. No swelling phenomena appear upon exposed to weak polar or nonpolar solvent vapor like cyclohexane and CCl4 etc. Carbon black particles are, nevertheless, not provided with this trait. Fig. 8 shows the dependence of response of the HTBN/TDI/MWCNT composite thin film to acetone vapor on the CEAs. When the conductive film was exposed to acetone vapor, the volume resistance abruptly increased. The change in maximal resistance was 8.6 × 101 , 5.4 × 103 and 1.1 × 104 k in order of ethylene glycol, glycerine and triethanolamine. The resistance immediately reduced as the film was taken out of the acetone vapor and transferred into dry air. After 300 s, the

Fig. 8. Response patterns of HTBN/TDI/MWCNTs conductive composite films in the case of different chain-extending agents upon exposure to acetone vapor. The inset shows response patterns in the presence of ethylene glycol (temperature: 70 ◦ C; curing time: 36 h; r = 1.55).

resistance dropped to 1603, 358 and 6.4 times the original value in the mentioned—above same order. This demonstrates that the composite film with ethylene glycol is provided with the best reversibility. In the case of triethanolamine, the reversibility is the poorest. This is because the cure composite formed from ethylene glycol is basically a linear structure. A crosslinking structure is obtained in the presence of triethanolamine and glycerine. This structure gives rise to larger volume expansion in amorphous regions than the linear structure owing to the swelling caused by the diffusion of analytes. As a result, a larger amount of electric conduction paths are cut off and the resistance increase more rapidly, as described before. Similarly, the desorption degree is also affected by such structure. The desorption rate and the completion degree deduces in order of linear to cross-linking structure. On the other hand, a certain elasticity deformation may exist in soft and hard segment domains of swollen cure composites, and partial electric conduction path of MWCNTs is destroyed. Moreover, the change is unable to recover from the original to some extent. Consequently, the recovery rate and degree of the electric resistance of the composite thin films in trifunctional cure systems are relatively poor. 3.6. Effect of curing temperature on response Fig. 9 displays the relative resistance change (i.e., responsivity Rmax /R0 ) of HTBN/TDI/ethylene glycol/MWCNT thin films at various curing temperatures upon exposure to a variety of solvents (The curing time is 36 h except that the time is 7 d at 20 ◦ C). It is clear that the responsivities of the thin film to several solvents are remarkably enhanced along with curing temperature elevated. For example, the responsivity surpasses 103 orders of magnitude at 115 ◦ C, and the relative resistance change reduces to about 100 times at 70 ◦ C. While at room temperature the value is only in the range of 20 to 60 times. This implies that the curing temperature has a great impact on the responsivity.

Fig. 9. Influence of curing temperatures on response of HTBN/TDI/ethylene glycol/MWCNTs conductive composite films (r = 1.55; curing time: 36 h).

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Similar to the discussion before, although in the case of the ethylene glycol the polymer obtained takes on an approximate linear structure at room temperature, the carbaminate groups in the resultant polymers may further react with unreacted isocyanate groups. Then, a cross-linking structure gives birth to when the temperature is enhanced to 115 ◦ C. The absorption and diffusion of solvent vapors make the networks expand more greatly. Furthermore, the agglomeration process of MWCNT is confined owing to a high density of network formed at high temperatures [34]. It is likely that even if the MWNCT particles have an increased kinetic energy at higher temperatures, they cannot help to surmount the remaining potential barrier caused by the network. Consequently, the electric conduction path of MWCNT is destroyed, leading to the increase in resistance value. On the other hand, the non-bond interactions consisting of electrostatic and Van der Waals forces at CNT-polymer interfacial result in MWNCT disaggregation following expansions of polymer networks. It is believed to be attributed to intimate contact between the two solid phases at the molecular scale. Additionally, the sensitivity of d-band of MWCNT to strain in graphite skeleton is expected to significantly influence the properties of sensor systems [53]. Since the cross-linked polymer network can form a coating on remaining carbon particles and cause field disturbance and physical strain in the graphite skeleton. This results in their disaggregation and changes in electronic properties of the MWCNT [44]. And that the effect is more obvious at a high cure temperature. This makes local non-uniformity of MWCNT and mismatch of the expansion coefficients between MWCNTs and polymer matrices more efficient, the interstitial space between MWCNTs separated by the inserted polymers is broadened, and thus destroying a network of touching particles. Accordingly, the interactions between the MWCNT and the polymer network layer or the cross-linked polymer network has crucial impact on the electronic properties of the MWCNT. Fig. 10 presents responsive patterns of HTBN/TDI/MWCNT composite films with ethylene glycol at various curing tem-

399

Table 2 Resistivity change of HTBN/TDI/Ethylene glycol/MWCNTs cured films at different curing temperatures Curing temperature (◦ C) Average resistivity, × 10−3 ( m)

20 1.13

70 1.00

115 534

peratures to saturated acetone vapor. The initial resistivity and responsivity of the thin films (see also Table 2) at curing temperature of 115 ◦ C were higher than those at 70 and 20 ◦ C, respectively. The value sharply dropped from 115 to 70 and 20 ◦ C, and the resistivity at 115 ◦ C was 110 to 1760 times at 70 and 20 ◦ C. Their responsivities were 106, 85 and 35, respectively. The resistivity reversibility became good in the same order as these films were taken out of the analyte vapors, and could recover to 1160, 6.4 and 2.6 times the original value within 300 s, respectively. This is not in accordance with the conclusion that with the curing temperature increased the initial resistance of HTPB/TDI/CB films reduced [35,54,55]. Just like the description in DSC analysis, because the arrangement orderliness of molecular chains in hard segment domains was enhanced at a high curing temperature, the crystallinity is somewhat increased (see Table 1). Thus, the proportion occupied by amorphous regions in the solidified composites increases. Meanwhile, the possibility for MWCNTs particles distributing in the amorphous region or on the interphase to form a continual electric conduction network increases, the resistance at room temperature should drop. The experimental results, nevertheless, is on the contrary. Obviously, this should be concerned with the special structural characteristics endowed by MWCNTs. Considering that the cure composites would firmly occupy the intra- or intertube, the terminal and the border outside MWCNTs and the wall sites, the layer-to-layer mutual effect would be enhanced with the high temperature solidification. Thus, the metal of the MWCNTs becomes poorer, leading to a high resistance [56]. Although the cross-linking structure formed at high temperature is advantageous to the swelling, the desorption capacity is low, compared with the thin films at 20 and 70 ◦ C. Therefore, the resistance is very difficult to return to the original value. From the two aspects of the response intensity and the reversibility, the electrical conduction composites formed at 70 ◦ C are provided with the application value. 4. Conclusions

Fig. 10. Response patterns of HTBN/TDI/ethylene glycol/MWCNTs composite films at various curing temperatures to saturated acetone vapor. The inset shows response patterns at 115 ◦ C (r = 1.55; curing time: 36 h).

We have prepared and fabricated a conductive polyurethane/ MWCNT composite thin film with a selective vapor-induced sensing performance through in situ dispersion polymerization. FTIR results demonstrated that there may be no formation of strong covalent bonds with the polyurethane matrix. But noncovalent interactions, consisting of a weak electrostatic and a Van der Waals force, exist between the highly delocalized electron system of MWCNTs and polymers instead. WAXD, POM and DSC findings indicated that the curing thin film produced obvious phase disengagement behavior between the two domains. The ordered arrangement in hard segment domains caused the thin film to have certain crystallization liability. This

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kind of thin films yielded higher responsivity to some solvent vapors like benzene, toluene, acetone and chloroform than to others such as carbon tetrachloride, cyclohexane, methanol and formaldehyde. The experimental results reveal that the interaction between polymers or adsorbed solvent vapor molecules and carbon nanotubes, including the charge transfer reaction, may play a predominant role in determining the electrical response of the composites to solvent vapor. Furthermore, the response behavior was associated with the structural characteristics of the soft-hard segment, for instance the microphase separation, and crystalline behavior based on the swelling theory model. With the curing temperature elevated, the response intensity of the thin film was enhanced, but the reversibility was poor. The responsivity of the thin film in the case of the CEAs containing trifunctional groups was higher than that with difunctional groups, but the reversibility was opposite. The electric conduction thin film manufactured using glycol as a CEA at 70 ◦ C had a comparatively suitable responsivity and reversibility, prefiguring an underlying development value. Therefore, it is possible to construct sensor elements based on composites of MWCNTs covered with polyurethanes, which could be prepared by the simple avenue described in this work, for the selective detection of organic vapors. References [1] G.H. Yu, F.D. Zeng, Chin Eng Plast Appl 33 (6) (2005) 11–13. [2] E. Kymakis, I. Alexandou, G.A.J. Amaratunga, Synth. Met. 127 (1) (2002) 59–62. [3] S. Chen, J. Hu, M. Zhang, M. Li, Z. Rong, Carbon 42 (3) (2004) 645–651. [4] X.S. Yi, Functionalized principle of compositely conductive polymeric materials, Beijing, National Defence Ind. Press, 2004. [5] P.X. Wu, J. Shen, Resin based composites with special features, Beijing, Chem. Ind. Press, 2003. [6] W. Li, Y. He, T.H. Liu, J. Yang, Eng. Plast. Appl. 33 (12) (2005) 62–64. [7] X. Wang, H. Cai, Acta Mater. 54 (8) (2006) 2067–2074. [8] X. Wang, B. Sun, H.K. Yang, Nanotechnol. 17 (3) (2006) 815–823. [9] H. Cai, X. Wang, Nanotechnol. 17 (1) (2006) 45–53. [10] X. Wang, H.K. Yang, Phys. Rev. B 73 (8) (2006) 1–8, 085409. [11] J.Q. Liu, Y.C. Wu, R.J. Xue, X.H. Hu, Chin. Ordnance Mat. Sci. & Eng. 29 (6) (2005) 64–69. [12] C. Wei, L.M. Dai, A. Roy, T.B. Tolle, JACS 128 (5) (2006) 1412–1413. [13] J. Liu, M.R. Zubiri, B. Vigolo, M. Dossot, Y. Fort, J.J. Ehrhardt, E. McRae, Carbon 45 (4) (2007) 885–891. [14] M. Grujicic, Y.P. Sun, K.L. Koudela, Appl. Surf. Sci. 253 (6) (2007) 3009–3021. [15] D. Wei, C. Kvarnstr¨om, T. Lindfors, A. Ivaska, Electrochem. Commun. 9 (2) (2007) 206–210. [16] H. Kitano, K. Tachimoto, Y. Anraku, J Colloid Interface Sci. 306 (1) (2007) 28–33. [17] P.L. Xu, S.Q. Zhang, Handbook of polyurethane materials, Chem. Ind. Press, Beijing, 2002, P214. [18] Y.S. Fang, Polyurethane foam plastics, Chem. Ind. Press, Beijing, 1984, P236. [19] J.M. Park, S.J. Kim, D.J. Yoon, G. Hansen, K.L. DeVries, Compos. Sci. Technol. 67 (10) (2007) 2121–2134. [20] E.N. Konyushenko, J. Stejskal, M. Trchov´a, et al., Polymer 47 (16) (2006) 5715–5723. [21] Y.S. Kim, J.H. Cho, S.G. Ansari, et al., Synth. Met. 156 (14-15) (2006) 938–943.

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