High Performance Polymers

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Thermoplastic polyurethane/single-walled carbon nanotube composites with low electrical resistance surfaces R. Murali Sankar, K. Seeni Meera, Asit Baran Mandal and S.N. Jaisankar High Performance Polymers published online 2 October 2012 DOI: 10.1177/0954008312459545 The online version of this article can be found at: http://hip.sagepub.com/content/early/2012/09/24/0954008312459545

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Article

Thermoplastic polyurethane/ single-walled carbon nanotube composites with low electrical resistance surfaces

High Performance Polymers 1–12 ª The Author(s) 2012 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954008312459545 hip.sagepub.com

R. Murali Sankar, K. Seeni Meera, Asit Baran Mandal and S.N. Jaisankar

Abstract Thermoplastic polyurethane (TPU)/single-walled carbon nanotube (SWCNT) nanocomposite films were prepared using 1,6-hexane diisocyanate and hydroxyl-terminated polybutadiene (HTPB) in tetrahydrofuran with various concentrations of SWCNTs. The interaction between polyurethane (PU) and SWCNTs in nanocomposite was studied using different methods. The film turns yellowish to grayish-black in colour upon increasing the concentration of SWCNTs in PU matrix. This may be due to the formation of p–p interaction between polyurethane amide functional group and SWCNTs. Differential scanning calorimetric results show that the soft segment of nanocomposite interacts much stronger than hard segment, which results in lowering melting transition temperature of soft segments. The activation energy and thermal stability parameters were determined from thermogravimetric and differential scanning calorimetric analyses. The x-ray photoelectron spectroscopic results show the intermolecular interaction between HTPB-based PU and SWCNT. Mesoporous morphology of the nanocomposites was observed by scanning electron microscopy. The average diameter of the pores was calculated using Gaussian method. The TPU films exhibit about 3.5 times greater resistivity than nanocomposite films. All the analysed data prove that the SWCNTs were well distributed in PU matrix and exhibited as tough films with low electrical resistivity. Keywords Nanocomposites, proximity, polyurethane, coating, activation energy

Introduction The single-walled carbon nanotubes (SWCNTs) play a vital role in the field of nanoscience owing to their exceptional properties.1 SWCNTs range from 0.4 to 2 nm in diameter and are classified into three types, namely, armchair, zigzag and chiral, based on their electronic properties. The armchair SWCNTs are metallic in nature, while both zigzag and chiral SWCNTs are semiconductor in nature.2 It is well-known that the SWCNTs have the capacity to form excellent reinforcement in polymer nanocomposites due to their: (i) high aspect ratio, (ii) increase in modulus and (iii) tensile strength.2 The use of SWCNTs along with polymer results in improving physical properties, and it may help in engineering applications like coatings.3 These nanocomposite materials offer a wide range of benefits over traditional materials and are used in the high thermal aircraft coatings.4 There are several disadvantages in using nanocomposite materials for aircraft coatings, namely, high electrical

resistance, lower thermal conductivity, moisture absorption and aging over time. Most aerospace applications require durable coatings that will be exposed to charge environments during flight. Hence, it is necessary to have outstanding nanocomposite materials to overcome the above mentioned drawbacks. The potential carbon nanotube (CNT) as conducting filler with very small loading in polymer matrix may exhibit excellent performance with low electrical resistance and high thermal stability.5

Polymer Division, Council of Scientific and Industrial Research (CSIR) Central Leather Research Institute (CLRI), Adyar, Chennai, India Corresponding author: S.N. Jaisankar, Polymer Division, Council of Scientific and Industrial Research (CSIR) - Central Leather Research Institute (CLRI), Adyar, Chennai 600020, India Email: [email protected]


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High Performance Polymers

Thermoplastic polyurethane (TPU) may be described as the linear structural block copolymer consisting of both soft and hard segments forming a two phase microstructure.5 TPU is one of the most versatile materials with high wear resistance and excellent tailor made properties,6 because of their excellent mechanical properties, light weight and cost effectiveness.7 Generally, polyurethane (PU) consists of a soft segment with high-molecular-weight polyester or polyether polyol and the hard segment, which is the combination of diisocyanate and low-molecular-weight diol or diamine.8 The introduction of inorganic fillers, such as nanoclay, carbon black, and so on, into the PU matrix improves the mechanical and thermal properties of PU.9 However, the improvement by conventional filler is only limited. A better method in improving the physical properties of PU coating is by introducing SWCNT fillers into the PU matrix.10 To obtain good coating properties, it is essential to have strong interfaces between PU and SWCNTs in nanocomposites. This could be achieved by functionalization of CNTs or by making molecular interaction on nanotubes, which further can be chemically bonded to the polymer chains.11,12 To the best of our knowledge, there is no report available on hydroxyl-terminated polybutadiene (HTPB)-based PUembedded SWCNT nanocomposites for aerospace coating application using TPU. In this study, the interaction between HTPB-based PU and SWCNT in PU/SWCNTs nanocomposite is reported for the first time. This interaction brings the urethane amide units to close proximity of PU and SWCNTs. Such interactions may help to disperse the SWCNTs in PU matrix to form PU/SWCNTs nanocomposite films. Here, we report the preparation of PU/SWCNTs nanocomposites by one-step process using the 1,6-hexane diisocyanate (HDI) and HTPB in tetrahydrofuran (THF) through chemically cross-linked moisture-cured approach.4 The preparation of HTPB-based PU with SWCNTs as filler was obtained without disrupting the wall structure of CNTs. The incorporation of SWCNTs in PU matrix exhibits mesoporous morphology. The SWCNTs were used without purification. Since the purification of SWCNTs might introduce functionalization onto the nanotubes, the polymer chain surface interaction with the SWCNTs could be blocked.13 Hence, we dispersed the SWCNTs as such into the polymer matrix by sonication in dry THF. The prepared polyurethane composite (PUC) films were characterized by using different spectral, thermal, morphological and electrical resistivity studies.

Experimental Materials SWCNTs, HTPB (Mw: 2400, OH value: 47 mg KOH g1) and HDI were purchased from Aldrich (Sigma-Aldrich Co., St Louis, MO, USA) and used as received. The HTPB contains 20% vinyl, 20% cis and 60% trans isomer, which

Table 1. The sample code and compositions of HTPB/HDI/ SWCNT. Code

NCO:OH ratio

SWCNT (mg)

SWCNTs (wt%)

PU PUC1 PUC2 PUC3 PUC4

2:1

– 3.30 4.80 6.70 8.10

– 0.10 0.15 0.20 0.25

HTPB: hydroxyl terminated polybutadiene; HDI: 1,6-hexane diisocyanate; PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

was heated at 100 C under vacuum for 1 h to remove moisture and then brought down to room temperature. The THF solvent (GR grade) was obtained from Merck (Laboratory Chemicals, Mumbai, India) and distilled before using. The dibutyltin dilaurate (DBTDL) catalyst was obtained from Fluka (Switzerland) and used as such.

Preparation of HTPB/SWCNT mixture The different weight percentage (wt%) of PU/SWCNT nanocomposite was prepared by unmodified SWCNTs in a 100-mL beaker with dried THF. These unmodified SWCNTs were subjected to ultrasonic irradiation using probe sonicator at 750 W with 60% amplitude for 30 min by maintaining at 0 C to achieve a uniform and good dispersion of SWCNT in 25 mL THF solvent. Oligomeric diol (HTPB) of 3.2 g (1.33 mM) was taken in THF (30 mL) along with nitrogen gas as inlet. To this oligomer, the dispersed SWCNTs with different wt% were taken in THF solvent (Table 1) and added at room temperature (25 C) to obtain the HTPB/SWCNT mixture. The HTPB/SWCNT mixture was further subjected to ultrasonication process using water bath sonicator at 25 C for 30 min to achieve homogenous mixing. This homogenous HTPB/SWCNT in THF solvent shows uniformity due to the low viscosity of HTPB, which prevents the SWCNT aggregation. Such behaviour of HTPB helps to prepare uniform PU/SWCNT nanocomposite films.

Preparation of PU/SWCNT nanocomposites The HTPB/SWCNT mixture was further reacted with 0.30 mL (2.67 mM) of HDI in THF to obtain the final product, that is, PU/SWCNT nanocomposites through one-step process (Figure 1). The NCO:OH ratio is 2:1 and 0.05 wt% of DBTDL was used as the catalyst. The reaction was carried out in a magnetic stirrer with continuous purging of nitrogen gas for 3 h at 25 C.14 After 3 h, the entrapped nitrogen gas bubbles were removed from the viscous nanocomposite solution by vacuum evacuation. This viscous solution was further casted on a mould and moisture cured for overnight to obtain a tough film.15 The film appears initially in yellow colour and in the course of reaction turns


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Figure 1. Preparation of PU/SWCNT nanocomposite. PU: polyurethane; SWCNT: single-walled carbon nanotube.

Figure 2. Photograph of PU and PU/SWCNT nanocomposites. PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

grayish-black colour with increase in SWCNT concentration, as shown in Figure 2. The sample code and composition of PU/SWCNT nanocomposites were given in Table 1.

Characterization The probe sonicator Sonics VCX 750 (750 W, 20 KHz, 60% amplitude; Sonics & materials, Inc., Newtown, USA) and water bath sonicator Equitron (42 KHz, 120 W, The Science House, Chennai, India) were used for the dispersion of SWCNT in THF solvent. The SWCNTs in polymer matrix were characterized by ultraviolet–visible–nearinfrared (UV–Vis–NIR) spectrometer (Cary-5E), Raman spectrometer (Witch confocal Raman microscopy alpha 300 R) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR; ABB MB3000, Canada) to identify the nanocomposite functional groups. The surface morphology of the prepared films was analysed by scanning electron microscopy (SEM) of FEI Quanta 200 Microscope. Thermogravimetric analyses (TGA) were carried out using TG Analyser (Model Q50, TA Instruments Waters Pvt Ltd, Bangalore, India), with a heating rate of 10 C min1 from 30 C to 800 C under nitrogen (N2) atmosphere. Differential scanning calorimetric (DSC) analyses were performed using DSC (Model Q200, TA Instruments) at a heating rate of 5 C min1 with N2 flow of 50 mL min1 from 90 C to 300 C. The binding energy of PU/SWCNT

nanocomposites were measured using the MS Omicron nanotechnology x-ray photoelectron spectroscopy (XPS) with 1382 eV operated at 300 V. The resistivity was measured using the Keithley instrument 2600 (Keithley Instrument International Co., Bangalore, India). The tensile strength and percentage elongation at break were measured using a universal testing machine (Instron 3369, Instron, Norwood, MA, USA) at a cross-head speed of 50 mm min1 with 0.24 mm of thickness for all the samples, as per the ASTM D635 test procedure. The data for mechanical properties reported are the averages of five measurements for each sample.

Results and discussion ATR-FTIR spectroscopy The intermolecular interactions between polyurethane amide (PUA) and SWCNT in PU/SWCNT nanocomposites were examined using ATR-FTIR spectroscopic techniques. FTIR instrument helps us to study the interaction of amide carbonyl of PUA on the surface of SWCNTs. Infrared (IR) study shows that the arrest of hydrogen bonding within PUA due to the intermolecular interaction and functional group proximity of amide carbonyl to the SWCNTs in PU matrix.16 The FTIR spectra of PU/SWCNTs nanocomposite films with different wt% of SWCNTs ranging from 0.1% to 0.25% were shown in Figure 3. The peaks in the


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Figure 3. ATR-FTIR spectra of PU and PU/SWCNT nanocomposites. ATR-FTIR: attenuated total reflectance Fourier-transform infrared spectroscopy; PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube

Figure 4. ATR-FTIR spectra of PU and PU/SWCNT nanocomposite: expanded regions indicate (a) 1800–1500 cm1 and (b) 4000– 3000 cm1. ATR-FTIR: attenuated total reflectance Fourier-transform infrared spectroscopy; PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

range of 2800–3059 cm1 and 3300–3400 cm1 are the characteristics of C–H and N–H stretching vibrations, respectively.17 The HTPB-based PU has the olefinic conjugation in the back bone chain, which shifts the amide C¼O absorption to a lower frequencies. Hence, the peak at 1702 cm1 can be clearly attributed to amide carbonyl stretching vibration. A sharp peak at 1445 cm1 is a typical absorption for C¼C stretch and a peak at 1257 cm1 is identified as C–N stretching vibration.18 The characteristic peak of –NCO at 2275 cm1 was not observed in the spectra, which confirms the formation of PU. Thus, FTIR spectra provide the chemical evidence for the successful formation of PU at room temperature.

The IR spectra of PU/SWCNT nanocomposite (Figure 3) showed that the amide carbonyl at 1702 cm1 shifts with increase in wt% of SWCNTs in the PU films. The urethane carbonyl peak of nanocomposites were shifted to 1689 cm1 with lower intensity compare with neat PU, as shown in Figure 4(a), which is due to p–p interaction between PUA and SWCNT in PU/SWCNT nanocomposite. The other peaks at 1627 cm1 and 1577 cm1 were identified as N–H bending vibration of amides and the broad shoulder peak at 3342 cm1 attributed to the presence of secondary amine in PU polymer (Figure 4(b)). Due to the complete arrest of inter- and intramolecular hydrogen bonding in PU/SWCNT nanocomposite by SWCNT, both


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Figure 5. Raman spectra of (a) SWCNT and (b) PUC1, insert D band. SWCNT: single-walled carbon nanotube; PUC: polyurethane composite.

N–H broad shoulder and vibrational bending peaks become sharper and their intensity also increased (3341 cm1, 1627 cm1 and 1577 cm1).19 Hence, the interfacial interaction between PUA and SWCNT in the polymer matrix was confirmed. As a result, SWCNTs embedded PU nanocomposites films were obtained.

Raman spectra To confirm the presence of SWCNTs in the PU matrix and their interaction by functional group proximity, we performed Raman spectroscopy for unmodified SWCNT and PUC1 samples, as shown in Figure 5. The typical SWCNT gives strong and well-defined Raman spectrum, which consists of four characteristic bands: the radial breathing mode (RBM), a defect-induced mode (D band), the tangential mode due to C–C stretching vibrations (G band) and the overtone of D band (D*). The Raman spectrum of unmodified SWCNT gives a typical G and D* band at 1579 cm1 and 2650 cm1. However, the RBM band is not shown in Figure 5. In the PU/SWCNT nanocomposite (PUC1), a significant blue shift of the G band at 1593cm1 (shifted by 14 cm1), D* band at 2670 cm1 (shifted by 20 cm1) and a predominant D band at 1306 cm1 were observed in comparison with unmodified SWCNTs.20,21 This blue shift of the Raman bands may be due to the cause of strain effect generated by the interaction of PUA moiety. Thus, the lattice and electronic properties of SWCNTs have been changed in the PU/SWCNTs nanocomposites.22 Hence, the interaction between PUA and SWCNTs in the nanocomposite causes a compressive stress on SWCNT, which shifts the G band to higher frequencies.23 These kinds of compressive stress on SWCNT were also reported in the literature by Ni et al.20 A predominant D band at 1306 cm1 was observed in PU/SWCNT nanocomposite due to the presence of p-orbital electrons of PUA. These electrons may spread out to form interaction with SWCNTs surface and lead to p–p

Figure 6. UV–Vis–NIR spectra of (a) SWCNT and (b) PUC1. UV: ultraviolet; Vis: visible; NIR: near-infrared; SWCNT: singlewalled carbon nanotube; PUC: polyurethane composite.

interactions.23 Such intermolecular interaction in PU/ SWCNT nanocomposites gives rise to a strong D band. The oxygen atom of the amide carbonyl interacts more than the amide nitrogen due to proximity of carbonyl amide on SWCNT.16,17 In case of unmodified SWCNTs, the D band peak was not observed in the Raman spectrum, due to bundling of the nanotubes. The D, D* and G bands (upshift) for PU/SWCNT nanocomposites confirm the PUA association with SWCNT, which make a compression on SWCNTs. This molecular level interaction shows good dispersion of SWCNTs in the PU matrix.24 The peak at 1670 cm1 was assigned to PU amide moiety of PUC1 sample, which is similar to the observation of Parnell et al.24 Hence, the Raman study clearly proves the physicochemical interactions in PU/SWCNT nanocomposites. The UV–Vis–NIR spectra measurements were carried out to confirm the interaction of PUA on the surface of nanocomposites (Figure 6). The UV–Vis–NIR spectrum of SWCNT and PU/SWCNT nanocomposite film shows the absorption peak in the range of 400–1400 nm.25 The spectrum (PUC1) confirmed the strong interaction of PUA on the surface of SWCNT by close proximity of functional groups. The characteristic absorption bands of van Hove singularities were suppressed upon increasing the wt% of the SWCNT in the PU matrix. This indicates that the PU forms a very strong interfacial interaction with SWCNTs through a urethane-amide functional group.

Thermal studies The thermal properties of PU and PU/SWCNT nanocomposites were studied by DSC and are depicted in Figure 7(a) to (b). DSC of PU and PU/SWCNT nanocomposite shows a melting temperature (Tm) at 64 C and 60 C, respectively, corresponding to the soft segments of PU matrix. The Tm of soft segment was about 4 C lower than that of PU, which may due to intermolecular interaction between PUA and SWCNT.14 This intermolecular interaction leads to prevent the hydrogen


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Figure 7. DSC thermograms of PU and nanocomposites. DSC: differential scanning calorimetric; PU: polyurethane; PUC: polyurethane composite.

Figure 8. Arrhenius plot from DSC thermogram (a) PU, (b) PUC1 and (c) PUC4]. PU: polyurethane; PUC: polyurethane composite; DSC: differential scanning calorimetric. Table 2. Thermal parameters of PU and PU/SWCNT using DSC. Code

Tm ( C)

Tg ( C)

DH (J/g)

Ea (kJ/mol)

PU PUC1 PUC4

64.10 60.21 60.31

73.71 76.72 77.03

4.179 1.088 1.256

301 1919 1803

PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube; DSC: differential scanning calorimetry; Tm: melting temperature; Tg: glass transition temperature; Ea: activation energy; DH: change in enthalpy.

bonding within the PU matrix, which resulted in lower Tm value for PUC1 and PUC4. The soft segment interacts much stronger on SWCNT, evidenced by DSC

Figure 9. TGA thermograms of (a) SWCNT, (b) PU, (c) PUC1 and (d) PUC4. TGA: thermogravimetric analyses; PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

thermograms given in Figure 7(b). The PUC1 and PUC4 show the soft segment second glass transition temperature (Tg) at 25.37 C and 28.51 C, respectively, with change in enthalpy (DH) values of 0.407 J g1 and 0.851 J g1, respectively. However, in the neat PU soft segment, no changes were observed in the region (Figure 7(b)). This confirms that the SWCNT has more interaction with soft segment than hard segment.26 The activation energy (Ea) and Arrhenius plot for the film samples derived from DSC thermograms using Arrhenius equation (1) are shown27 in Figure 8 and the results are presented in Table 2. The Ea values of nanocomposites were six times higher than PU due to better dispersion of SWCNTs in polymer matrix. The DH values of PUC1 (1.088 J g1) and PUC4 (1.256 J g1) were lower


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Table 3. Tensile strength of PU and PU/SWCNT nanocomposites. Sample code

Tensile strength (MPa)

Elongation at break (%)

Modulus (MPa)

PU PUC1 PUC2 PUC3 PUC4

1.82 2.40 2.60 2.93 3.24

62.50 70.83 94.01 93.90 128.56

0.60 1.00 1.05 1.08 1.12

PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

than that of the neat PU (Table 2) and acts as a proof of interaction between the soft segment of PU and SWCNT, which reflects in increased tensile strength. Qt Ea 1 1 ln ln ð1Þ ¼ Qt Q R Tm T The thermal stability of nanocomposites was determined by TGA and is shown in Figure 9. TGA thermograms of PUC1 and PUC4 samples show two-stage decomposition temperatures (Td): the first stage decomposition occurs at 317 C and the second stage at 473 C. The initial decomposition temperature for both PU and nanocomposites exhibits 10% weight loss at 317 C. At 525 C, the nanocomposite was completely dissociated, which may be due to the presence of SWCNTs in the polymer. Hence, the SWCNTs can improve thermal stability and toughness of PU/SWCNT nanocomposite films.

Tensile strength measurements The tensile strength, percentage elongation at break of PU and PU/SWCNT nanocomposites were measured and are presented in Table 3. The tensile strength of PUC1 and PUC4 increases with increasing SWCNT content. This may be due to the interaction and dispersion of SWCNT in the PU matrix, and the stress–strain data for both PU and PU/SWCNT nanocomposites show the nonlinear elastic behaviour in low-stress region and plastic at high-stress region. In comparison with neat PU, the nanocomposite (PUC4) shows 178% improvement in tensile strength from 1.82 to 3.24 MPa with linearity in modulus.26 This enhancement in mechanical properties is due to effective load transfer to SWCNTs in the nanocomposite matrix by urethane amide interaction. A similar effect was also reflected in the Raman D* band shift of PUC1 from 2650 cm1 to 2670 cm1 (Figure 5(b)).21 The significant reinforcement may be due to better dispersion and well alignment of SWCNT in the PU matrix. As a result, high interfacial interaction and high aspect ratio were obtained, which help in the

formation of uniform tough and flexible PU/SWCNT films.

XPS spectra The XPS spectra of unmodified SWCNTs and PU/ SWCNT (PUC1 and PUC2) nanocomposites were shown in Figure 10. The XPS is an important method to investigate the surface chemistry of nanotubes in polymer matrix. The peak at 284.2 eV of C1s in SWCNT corresponds to the sp2 C–C bond in the graphite-like structure of CNTs, which may shifts its position on disordering.28 The peaks observed in PU/SWCNT nanocomposite at 285.3 eV and 286.3 eV correspond to C2s and C3s for PUC1 and PUC2, respectively. Here, the peaks are shifted to higher binding energy for nanocomposites compared with SWCNTs. The reason for the higher shift may be due to the interaction of urethane amide moiety on the surface of SWCNT. This interaction causes higher shifting thereby forming the p–p interaction and disordering the graphite-like structure in nanocomposites. In addition to the carbon spectra, XPS-oxygen spectrum of PU/SWCNT also gives us the valuable information about the interaction of SWCNTs with PU matrix through PUA moiety (Figure 10(c)).29 The peaks for PUC1-O1s and PUC2-O2s appeared at 532.7 eV and 534.2 eV, respectively. This shows a higher binding energy shift from 532.7 eV to 534.2 eV on increasing the SWCNTs content in PU matrix. The interaction of carbonyl-oxygen atom in PUA with sp2 C–C bond of SWCNT and the close proximity of urethane amide moiety toward SWCNTs surface may cause this shift. Hence, the peaks were shifted to a higher binding energy and form the p–p interaction in the PU/SWCNT nanocomposites. The XPS spectra of nitrogen atom give us an important evidence for the interaction of urethane amide groups on the nanotube surface (Figure 10(d)). Bayazit et al.30 reported that the pyridine-like nitrogen atom contributes p electron pair to the SWCNTs carbon atoms. Similarly, the amide nitrogen atom of PUA in nanocomposites forms the p–p interaction on the surface of nanotubes, which gives two peaks at 400 eV and 401.2 eV corresponding to PUC1-N1s and PUC2-N2s, respectively. The p electrons of nitrogen atom only bonded with the carbon atoms of SWCNTs. As a result, the binding energy was shifted to higher value on increasing the nanotube content in the matrix.31,32 The XPS helps to observe and confirm the intermolecular interaction by functional groups in nanocomposites.

Morphological analysis The surface morphology of SWCNT and nanocomposite film was investigated using SEM. The morphology of


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Figure 10. XPS spectra of (a) SWCNT-C1s, (b) PUC1-C2s, PUC2-C3s, (c) PUC1-O1s, PUC2-O2s and (d) PUC1- N1s, PUC2-N2s. XPS: x-ray photoelectron spectroscopy; PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

SWCNT revealed that an extensive network of overlapping nanotubes is present in bundles and PU shows continuous phase (Figure 11). The morphology of PUC3 (0.20%) and PUC4 (0.25%) nanocomposite sample shows mesoporous morphology,33 as shown in Figure 11. These mesopores formation might be due to the stacking interaction of PU with SWCNT surface through PUA moiety by close proximity, thereby allowing electronic interactions between them. The pore diameter of PU/SWCNT nanocomposites was measured and fitted in Gaussian function, shown in Figure 12. From Gaussian function, the average diameters of mesopores were found to be 19.22 + 0.67 nm and 15.26 + 0.41 nm for PUC3 and PUC4, respectively. The Gaussian fit illustrates (a) uniform distribution of mesopores and (b) decrease in the diameter of pores on increasing the SWCNT content in PU matrix. Sun et al.34 and Feng et al.35 reported that CNT films with a combination of meso-, micro- and nanostructures can provide superhydrophobic surfaces, which are used in textile and

coatings.36 As the SEM image of nanocomposite samples shows a mesopores structure, it may offer superhydrophobic properties required for aerospace coatings.34 The transmission electron microscopic (TEM) image of PUC1 nanocomposites was shown in Figure 13, which shows the presence of SWCNTs in the PU matrix. This clearly shows that the SWCNTs are embedded in the PU matrix. Thus, the SEM and TEM images of nanocomposites support our claim that the SWCNTs have interaction with PU matrix.

Electrical resistivity studies The electrical resistivities of PU/SWCNT nanocomposites films were measured by the two-probe method with pellet type and the results are shown in Figure 14. The resistivity of TPU film exhibits 1.48 1013 Ωcm, whereas the nanocomposite film containing 0.25% of SWCNTs exhibits 5.8 109 Ωcm.37 The decrease in the resistivity of


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SWCNT

PU

5 µm

2 µm

PUC3

PUC4

5 µm

5 µm

Figure 11. SEM images of SWCNT, PU and nanocomposites (PUC3 and PUC4). SEM: scanning electron microscopy; PU: polyurethane; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

Figure 12. Bar chart of pores in nanocomposites PUC3 and PUC4. PUC: polyurethane composite.

PU/SWCNT nanocomposites is because of the formation of p–p interaction of SWCNTs in PU matrix.38,39 Hence,

nanocomposite material exhibits low electrical resistance and more flexibility.6


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Conclusions In this work, we have successfully prepared tough and low electrical resistances of HTPB-based PU/SWCNTs nanocomposites films. The interaction on the surface of SWCNT via HTPB-based PU by proximity was established through PUA moiety. The spectral and thermal studies provide the evidence for intermolecular interaction of oxygen and nitrogen atom by preventing hydrogen bonding between PUA and SWCNTs. The shift in Raman bands evidenced the compressive strain on SWCNTs because of the close proximity of urethane amide moiety. The stress–strain property of the nanocomposite samples shows about threefold enhancement in tensile strength when compared with neat PU. The binding energy of the nanocomposites proves the presence of p–p interaction between the SWCNTs and PUA functional group. SEM and TEM images reveal that SWCNTs are well dispersed in PU matrix, which lead to the formation of mesopores with an average diameter of 15–20 nm. Electrical resistance study shows the lower resistivity upon increasing the SWCNTs content in the nanocomposite matrix. These PU nanocomposite films may have applications in the field of aircraft coatings. Acknowledgements

Figure 13. TEM image of PUC1 nanocomposite in different scale bar (red arrow show the SWCNTs). TEM: transmission electron microscopic; PUC: polyurethane composite; SWCNT: single-walled carbon nanotube.

The authors thank National Centre for Nanoscience and Nanotechnology, University of Madras, Chennai, for XPS analyses and Toshniwal, Bangalore, for Raman spectral analyses. The authors also acknowledge Dr Debasis Samanta for valuable discussion.

Funding The author RMS was supported by a fellowship (grant no. 31/ 6(328)/2010-EMR-I) from Council of Scientific and Industrial Research (CSIR), India.

References

Figure 14. Plots of resistivity against PU/SWCNT nanocomposite. PU: polyurethane; SWCNT: single-walled carbon nanotube.

1. Iijima S. Helical microtubules of graphitic carbon. Nature 1991; 354: 56–58. 2. Saito R, Dresselhaus G and Dresselhaus MS. Physical properties of carbon nanotubes. London: Imperial College Press, 1998. 3. Smith JG Jr, Connell JW, Delozier DM, Lillehei PT, Watson KA, Lin Y, et al. Space durable polymer/carbon nanotube films for electrostatic charge mitigation. Polymer 2004; 45: 825–836. 4. Chattopadhyay DK, Sreedhar B and Raju KVSN. Thermal stability of chemically crosslinked moisture-cured polyurethane coatings. J Appl Polym Sci 2004; 95: 1509–1518. 5. Chattopadhyay DK and Raju KVSN. Structural engineering of polyurethane coatings for high performance applications. Prog Polym Sci 2007; 32: 352–418. 6. Bierwagen GP and Tallman DE. Choice and measurement of crucial aircraft coating system properties. Prog Org Coat 2001; 41: 201–216.


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High Performance Polymers

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