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Highly stable polyimide composite films based on 1,2,4-triazole ring reinforced with multi-walled carbon nanotubes: Study on thermal, mechanical, and morphological properties
Progress in Organic Coatings 80 (2015) 142–149
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Highly stable polyimide composite films based on 1,2,4-triazole ring reinforced with multi-walled carbon nanotubes: Study on thermal, mechanical, and morphological properties Mohammad Ali Takassi a , Amin Zadehnazari a,∗ , Asadollah Farhadi a , Shadpour Mallakpour b a b
Department of Science, Petroleum University of Technology, Ahwaz, 6198144471, Iran Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran
a r t i c l e
i n f o
Article history: Received 2 October 2014 Received in revised form 2 November 2014 Accepted 2 December 2014 Keywords: Multi-walled carbon nanotube Polyimide Composite film Thermal stability Transmission electron microscopy
a b s t r a c t A novel diamine bearing aromatic pendant triazole ring, namely, 3,5-diamino-N-(1H-[1,2,4]triazol-3yl)-benzamide, was successfully synthesized. The prepared diamine and a commercial dianhydride were reacted in situ in the presence of carboxylated multi-walled carbon nanotubes (MWCNTs) with stirring to give a homogeneous MWCNT/poly(amic acid) mixture which was then heated under a heating program to give a series of MWCNT/polyimide (PI) composites with different proportions of MWCNT (5, 10, and 15 wt%). The composite films were tested for different properties including spectral, morphological, thermal, and mechanical properties. Scanning and transmission electron microscopy revealed the modified MWCNTs were well dispersed in the PI matrix while the structure of the polymer and the MWCNTs structure were stable in the preparation process. The thermal stability of the films containing MWCNTs was improved as the MWCNT content increased from 5 to 15 wt% due to the improved interfacial interaction between the PI matrix and surface-modified MWCNTs. Tensile tests on the composites showed an increase in the elastic modulus and the yield strength, and decrease in the failure strain. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The engineering materials provide continuous research opportunities for the current industrial applications. Among all the nano functional materials, carbon nanotubes (CNTs) are the frontiers of these researches because of significantly improved and unusual properties. CNTs have exceptional mechanical, electrical, and thermal properties, which are strongly anisotropic [1]. In order to fully utilize these properties, numerous CNT/polymer composites have been produced and investigated [2]. These nanocomposites exhibit the superior physical properties such as enhanced tensile strength [3] and improved electrical conductivity [4]. Many polymers such as epoxy [5], polycarbonate [6], polyamide [7], polyimide (PI) [8], polystyrene [9], and polypropylene [10] have been used to prepare CNT/polymer composites. These materials have potentially superior thermal, electrical, and mechanical properties than the pristine polymers. In polymer-matrix composites made with
∗ Corresponding author. Tel.: +98 611 555 1321; fax: +98 611 555 1321. E-mail addresses: [email protected], [email protected] (A. Zadehnazari). http://dx.doi.org/10.1016/j.porgcoat.2014.12.001 0300-9440/© 2014 Elsevier B.V. All rights reserved.
non-continuous fillers, dispersion of the filler influences nearly all relevant properties of the composite. Unfortunately, pure CNTs are insoluble in the organic solvents and tend to form aggregates due to the extremely high surface energy, which leads to a heterogeneous dispersion in the polymer matrix and has negative effects on the properties of the resulting composites [11]. The degree of dispersion of CNTs and the consequent macroscopic behavior is strongly dictated by the choice of the matrix, the type of CNT, modification of their surface, and the processing method and associated parameters [12,13]. Several methods have already been developed over the last years to achieve an efficient dispersion of individual CNTs in a polymer matrix. The simplest method consists of directly mixing the polymer and the CNTs [14–17]. Stirring in a solvent, polymer solution, or a polymer melt is usually not sufficient to achieve this goal. For this purpose, ultrasound (bath ultrasonication) is frequently applied [18–21]. Like any sound wave, ultrasound propagates via a series of compression and rarefaction waves induced in the molecules of the medium through which it passes. One can also either modify the polymer in such a way that it can interact with the -system of the CNT wall, or modify the walls of the CNTs by functionalization and so improve the wetting of the filler, as well as the dispersion in the medium [22,23].
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In this contribution in order to achieve fine dispersion of carboxyl-functionalized multi-walled CNTs (MWCNTs), we attempted to prepare MWCNT/PI composites using in situ polymerization approach (i.e., solution-casting followed by subsequent imidization). Thermal property, mechanical property and morphology of MWCNT/PI composites were then investigated. The introduction of several functional groups as well as triazole bulky units resulted in increased chain packing distances and decreased intermolecular interactions, leading to better interaction of the PI chains with MWCNTs and better dispersion of MWCNTs in the PI matrix. 2. Experimental 2.1. Materials All materials and solvents were purchased either from Aldrich Chemical Co. (Milwaukee, WI), Merck or Fluka (Germany). 3-Amino-1H-1,2,4-triazole, 3,5-dinitrobenzoylchloride, hydrazine monohydrate, propylene oxide, pyromellitic dianhydride (PMDA), dimethyl sulfoxide (DMSO), and 10% palladium on activated carbon were used as received without further purification. N,N -dimethylacetamide (DMAc) (d = 0.94 g cm−3 at 20 ◦ C) and N,N -dimethylformamide (DMF) (d = 0.94 g cm−3 at 20 ◦ C) were purified by distillation under the reduced pressure over barium oxide. Carboxylated MWCNTs synthesized by chemical vapor deposition (CVD) (the outer-diameter 8–15 nm, the inner-diameter 3–5 nm, length ∼50 m, carboxyl content 2.56 wt% and purity >95 wt%), were achieved from Neutrino Co. (Iran). All other reagents were used as received from commercial sources. 2.2. Methods Proton nuclear magnetic resonance (1 HNMR) spectra were recorded on a Bruker (Rheinstetten, Germany) Avance 500 instrument at room temperature in DMSO-d6 . Multiplicities were given as s (singlet). Chemical shifts 1 HNMR spectra are reported as ı in units of parts per million (ppm) and relative to the signal of DMSO-d6 (ı = 2.50, singlet). The number of protons (n) for a given resonance was indicated by nH. Carbon nuclear magnetic resonance (13 CNMR) spectra were reported as ı in units of ppm downfield and relative to the signal of DMSO-d6 (ı = 39.50). Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Bomem MB-Series 1998 FT-IR spectrophotometer (Quebec, Canada) at a resolution of 4 cm−1 . The samples were examined as a disc, grinded together with potassium bromide (KBr) salt. They were scanned at wavenumber (cm−1 ) range of 400–4000 cm−1 . Band intensities were assigned as weak (w), medium (m), strong (s), and broad (br). Melting points were determined in capillary tubes using a melting-point apparatus (Barnstead Electrothermal 9200, Iowa, USA) without correction. Elemental analysis was run on an Elementar Analysensysteme GmbH (Hanau, Germany). Monitoring of the reactions was accomplished by thin layer chromatography (TLC) on silica gel polygram SILG/UV 254 plates (0.2 mm thickness). Inherent viscosity (inh ) (at a concentration of 0.5 g/dL) was measured with an Ubbelohde suspended-level viscometer at 25 ◦ C using DMF as solvent. Thermogravimetric analysis (TGA) was performed with the DuPont Instruments (TGA 951, 2805 West Frye Road Chandler 85224 AZ, USA) analyzer at a heating rate of 10 ◦ C/min−1 under nitrogen (20 cm3 /min) in temperature range of 25–800 ◦ C. Measurements were carried out on 10 ± 2 mg film samples. The mechanical properties of PI and the composite films were measured at room temperature on a universal testing machine (SANTAM, model STM-20, Iran), according to ASTM D 882 (standards). Tests were carried out with a cross-head speed
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of 12.5 mm/min until reaching a deformation of 20% and then, at a speed of 50 mm/min at break. The dimensions of the test specimens were 35 × 2 × 0.04 mm3 . Property values reported here represented an average of the results for tests run on at least five specimens. Tensile strength, tensile modulus, and strain were obtained from these measurements. The X-ray diffraction (XRD) was used to characterize the crystalline structure of the samples. XRD patterns were collected using a Bruker, D8AVANCE (Rheinstetten, Germany) diffractometer with a copper target at the ˚ a tube voltage of 40 kV, and tube wavelength of CuK␣ = 1.54 A, current of 35 mA. The samples were scanned at a rate of 0.05 ◦ /min from 10◦ to 80◦ of 2. For XRD studies, rectangular pellets prepared by compression molding were used. The morphology of the MWCNTs and the dispersion morphology of the MWCNTs on the PI matrix were observed using field emission scanning electron microscopy (FE-SEM). The images were taken at 15 KV using a HITACHI S-4160 instrument (Tokyo, Japan). Transmission electron microscopy (TEM) images were obtained using a Philips CM 120 microscope (Netherlands) with an accelerating voltage of 100 kV. For TEM studies, ultra-thin sections (30–80 nm) of the composites were prepared using Leica Ultramicrotome. Branson S3200 (50 kHz, 150 W) ultrasonic bath (Americas Headquarters 41 Eagle Road, Danbury CT 06810, USA) was used for better dispersion of MWCNTs. 2.3. Synthesis of 3,5-dinitro-N-(1H-[1,2,4]triazol-3-yl)benzamide (3) The synthetic pathway leading to the synthesis of target diamine is outlined in Scheme 1. Into a 50 mL round-bottomed flask equipped with a condenser, a magnetic stirring bar and a nitrogen gas inlet tube, 3.45 g (15.0 mmol) of 3,5-dinitrobenzoyl chloride and 1.26 g (15.0 mmol) of 3-amino-1H-1,2,4-triazole were dissolved in 15.0 mL of DMAc at 0 ◦ C. After 30 min, 1.00 mL of propylene oxide was dropped into the stirring was continued at 0 ◦ C for 6 h. The reaction mixture was slowly warmed to room temperature, stirred for additional 6 h, and poured into 150 mL of distilled water. The resulting yellowish powder was collected by filtration and washed with boiling ethanol and distilled water several times. The crude product was dried under reduced pressure at 70 ◦ C. The yield of the crude product was 3.67 g (88%) with the melting point of 270–272 ◦ C. FT-IR (KBr; cm−1 ): 3431, 3382 (s, N H stretching), 3073 (w, C H aromatic), 1656 (s, C O amide stretching), 1546, 1332 (s, N O stretching), 1189 (w, C O stretching), 774 (w, N H out of plane bending). 1 HNMR [dimethylsulphoxide-d6 (DMSO-d6 ), ı, ppm]: 7.92 (s, 1H, Ar H, triazole ring), 8.47 (s, 2H, Ar H), 8.69 (s, 1H, ArH), 10.12 (s, 1H, NH, amide), 12.72 (s, 1H, NH, triazole ring). 13 CNMR (DMSO-d6 ; ı, ppm): 181.5 (C, C O), 168.4 (C, triazole ring), 157.2 (C, triazole ring), 149.4 (C, Ar), 137.3 (CH, Ar), 128.5 (CH, Ar), 123.2 (CH, Ar). Elemental analysis: calculated for C9 H6 N6 O5 : C, 38.86%; H, 2.17%; N, 30.21%; found: C, 38.84%; H, 2.15%; N, 30.17%. 2.4. Synthesis of 3,5-diamino-N-(1H-[1,2,4]triazol-3-yl)benzamide (4) To a 100 mL round-bottomed three-necked flask equipped with a dropping funnel, a reflux condenser and a magnetic stirring bar, 3.34 g (12.0 mmol) 3,5-dinitro-N-(1H-[1,2,4]triazol-3yl)-benzamide (3) and 0.05 g palladium on activated carbon (Pd/C, 10%), were dispersed in 40 mL of ethanol. The suspension solution was heated to reflux, and 5 mL of hydrazine monohydrate was added slowly to the mixture. After a further 18 h of refluxing, the solution was filtered hot to remove Pd/C, and the filtrate was cooled to give yellow crystals. The product was collected by filtration and dried in vacuum oven at 80 ◦ C. The yield of the reaction was 78% (2.04 g), and the melting point was 255–257 ◦ C.
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NO2
O2N
C
NO2
O2N
O
Cl
NH2
H2N
1 DMAc
+ NH2 N
Hydrazine, Pd/C
Propylene oxide
NH N 2
C O
HN N
EtOH
C
N
NH
O
HN NH N 4
N 3 Scheme 1. Synthesis of target diamine.
FT-IR (KBr; cm−1 ): 3423, 3330, 3218 (s, N H stretching), 3108 (w, C H aromatic), 1663 (s, C O amide stretching), 1505 (m) 1468 (w), 1383 (w), 1189 (w, C O stretching), 834 (m), 781 (w, N H out of plane bending). 1 HNMR (DMSO-d6 ; ı, ppm): 4.93 (s, 4H, NH), 6.19 (s, 1H, Ar H), 6.50 (s, 1H, Ar H), 6.97 (s, 2H, Ar H), 8.16 (s, 1H, Ar H, triazole ring), 10.04 (s, 1H, NH, amide), 12.63 (s, 1H, NH, triazole ring). 13 CNMR (DMSO-d6 ; ı, ppm): 178.3 (C, C O), 165.7 (C, triazole ring), 152.1 (C, triazole ring), 143.8 (C, Ar), 134.0 (CH, Ar), 122.4 (CH, Ar), 116.1 (CH, Ar). Elemental analysis: calculated for C9 H10 N6 O: C, 49.54%; H, 4.62%; N, 38.51%; found: C, 49.50%; H, 4.65%; N, 38.46%. 2.5. Synthesis of poly(amic acid) (PAA) and MWCNT/PI composites The MWCNTs/PI composite films were prepared by solution casting method. In a typical process, a certain amount of synthesized diamine was dissolved in DMAc completely with stirring; then PMDA (with the same molar ratio of diamine) was added into the solution in four times within 1 h. The viscosity was increased quickly over 2 h. The reaction was continued for another 4 h at room temperature under stirring and the PAA solution was obtained. Separately, 3 mL of DMAc and the appropriate amount (from 5 to 15 wt%) of MWCNT was placed in a beaker and stirred under ultrasonication for 12 h at room temperature to reach a homogenous suspension. This beaker was added to the PAA mixture prepared above and further stirred for 12 h. The composite films were prepared by coating 3 mL of the final mixture solution on a piece of 50 mm × 50 mm glass slide. The resultant tack-free films were dried in an oven and subsequently imidized at high temperature in air. The imidization process was completed in a heating program as follow: ramping from room temperature to 80 ◦ C in 30 min, keeping at 80 ◦ C for 2 h, then ramping from 80 to 200 ◦ C in 5 h, and then from 200 to 300 ◦ C in 2 h, keeping at 300 ◦ C for 45 min, and finally cooling to room temperature. The imidization process was finalized by many experiments in our laboratory. For the preparation of PI, the viscous solution of neat PAA was poured into 40 mL of methanol and the precipitated solid was filtered off and dried at 80 ◦ C for 6 h under vacuum to yield 87% of the solid PAA with inherent viscosity of 0.68 dL/g (measured at a concentration of 0.5 g/dL in DMF at 25 ◦ C). PAA: FT-IR (KBr; cm−1 ): 2548–3645 (br), 1722 (s), 1623 (s), 1566 (s), 1475 (s), 1438 (s), 1321 (s), 1273 (s), 1024 (s), 880 (s), 737 (s), 620 (m). Pure PI: FT-IR (KBr; cm−1 ): 3433 (m), 3067 (w), 1775 (w), 1725 (s), 1661 (w), 1438 (s), 1369 (s), 1260 (w), 1110 (w), 1021 (m), 806 (m), 742 (m), 618 (m).
MWCNT/PI 5 wt%: FT-IR (KBr; cm−1 ): 3463 (br), 3087 (w), 1760 (w), 1732 (s), 1641 (w), 1594 (m), 1476 (m), 1332 (s), 1285 (m), 1140 (w), 819 (w), 627 (m). MWCNT/PI 10 wt%: FT-IR (KBr; cm−1 ): 3460 (br), 3079 (w), 1765 (w), 1737 (s), 1647 (w), 1593 (m), 1475 (m), 1338 (s), 1291 (m), 1146 (w), 820 (w), 627 (m). 3. Results and discussion 3.1. Synthesis of diamine Diamine 4 as a new compound was prepared from the reaction of 3,5-dinitrobenzoylchloride and 3-amino-1H-1,2,4-triazole in DMAc as solvent in the presence of propylene oxide as an acid scavenger by the nucleophilic displacement reaction and then reduction with hydrazine monohydrate in ethanol in the presence of palladium activated carbon as a metallic hydrogenation catalyst. The reaction pathway is illustrated in Scheme 1. 3.2. Synthesis of PI and MWCNT/PI composites In general, aromatic PI is synthesized from the condensation of aromatic diamine and aromatic dianhydride by either a two-step condensation polymerization method, that is, the formation of PAA followed by a thermal or chemical imidization to give PI, or a onestep thermal polycondensation in solution at elevated temperature. In this study, the PI was prepared via two-step condensation using thermal imidization. The polymerization of diamine 4 and PMDA in dry DMAc proceeds at room temperature to avoid the premature imidization of amic acid moieties (Scheme 2). The inherent viscosity of the synthesized PAA was 0.68 dL/g and the yield was 87%. The reaction pathway for preparing MWCNT/PI composites is shown in Scheme 2. The dispersion of MWCNTs in 5, 10, and 15 wt% solutions of PAA in DMAc was attained by a vigorous stirring for 12 h, to form a new series of MWCNT/PI composites. Thin films of the stated crude mixtures were prepared by casting on dust-free glass slides. Then gradual annealing of thin films was performed to 300 ◦ C and continued at this temperature for several minutes to ensure the complete imidization. The morphology of MWCNTs and physicochemical properties of MWCNT/PI composites were investigated by FT-IR spectroscopy, XRD, TGA, FE-SEM, and TEM. The effective use of CNTs in composite applications depends on the ability to disperse the CNTs homogeneously throughout the matrix without reducing their aspect ratio. Due to van der Waals attraction, CNTs are held together as bundles and ropes. Therefore, they have a very low dispersability in solvents and tend to remain as entangled agglomerates. To utilize CNTs as an effective reinforcement into
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3.3. Characterization methods
NH2
H 2N
O
O
HN N
O
O
+
C
O O
O PMDA
NH N 4
DMAc Room temperature 6h O NH C
COOH
HOOC
C
NH
C
O n O
HN N
NH N
PAA solution
Stirring, room temperature, 24 h MWCNT Casting
Δ
O N
O N n
O
O
C O
HN N
NH N
145
MWCNT/PI composite
Scheme 2. Reaction scheme for synthesis of MWCNT/PI composite films.
the polymer composites and ensure proper dispersion and suitable interfacial adhesion between CNTs and the polymer matrix, several techniques were used [24–26]. So, in this study, acid-functionalized MWCNTs were used for the preparation of the composites. In many investigations about the fabrication of CNT/polymer composites, 1–15 wt% of CNT content has been used and the excellent results have been obtained [27–29]. Thus, we selected 5, 10, and 15 wt% of CNT content in the composites. The lower level of aggregation in the modified MWCNTs can be attributed not only to the presence of functional groups such as carboxyl functional groups, but to their high aspect ratio. This transformation should contribute positively to the good dispersion of MWCNTs in the PI matrix. Moreover, the introduction of several functional groups into the backbone of the aromatic polymer performs a hydrogen bonding with CNTs and a composite based on hydrogen bond, by which PI chains are tightly attached to the surface of MWCNT, can be resulted (Fig. 1).
3.3.1. Spectral data All materials were characterized by FT-IR, and Fig. 2 shows the spectra of the representative materials. The FT-IR spectrum of dinitro compound 3 revealed strong peaks at 3431 and 3382 cm−1 , which was assigned to the N H stretching amide group and triazole ring, and two absorption bands at 1546 and 1332 cm−1 , which were characteristic peaks for NO2 asymmetric and symmetric, respectively. The FT-IR spectra of synthesized diamine 4 showed the presence of the characteristic peaks for NH2 functions and the absence of the original peaks, arising from the NO2 groups in the corresponding dinitro intermediate. Absorption of amine NH2 , N H of amide group and triazole ring appeared around 3423, 3330, and 3218 cm−1 and the peak at 1663 cm−1 confirms the presence of carbonyl amide group. Two absorption bands at 1505 and 1468 cm−1 were characteristic peaks for aromatic rings. The infrared spectrum of MWCNT-COOH/KBr pellet showed a strong, broad absorption band centered at 3433 cm−1 , which could be related to the stretching vibration of O H bands of carboxylic acid moieties on the surface of MWCNTs (Fig. 2e). The small peak around 2923 cm−1 was ascribed to aliphatic sp3 C H of MWCNT [30]. Strong band of absorption characteristic for the acid linkage appeared at 1629 cm−1 assigned to C O stretching vibration. FT-IR spectra were used to study the chemical structure of the matrix polymer; for example, FT-IR spectra of the neat PAA, PI, and MWCNT/PI with 5 and 10 wt% of MWCNT are shown in Fig. 2a–d. The FT-IR spectra of PI showed distinct features that clearly indicate imide ring formation and the disappearance of the PAA peak during the thermal cyclization step. The characteristic absorption bands of amic acid and carboxyl groups in the 2548–3645 cm−1 and 1623 cm−1 regions disappear and those of the imide ring appear near 1775 cm−1 (asym. C O stretching) and 1725 cm−1 (sym. C O stretching), 1369 cm−1 (C N stretching), 1021 cm−1 , and at 742 cm−1 (imide ring deformation). The presence of MWCNTs in the polymer matrix showed very few changes in the FT-IR spectrum, presumably due to the low MWCNT composition and the weak vibration signals of MWCNTs. The structure of dinitro 3 and diamine 4 compounds was also identified by 1 H and 13 CNMR spectroscopy. In the 1 HNMR spectrum of dinitro 3, the aromatic protons at 7.92–8.69 ppm showed the expected multiplicity and integration. The aromatic protons at the ortho position to the nitro group had the largest chemical shift (8.69 ppm) due to the inductive and anisotropic deshielding effect of the nitro groups. In this spectrum, the NH protons of the amide bond and triazole ring were observed at 10.12 and 12.72 ppm, respectively. The 13 CNMR spectrum of dinitro 3 showed six signals in the regions of 123.2–168.4 ppm related to aromatic carbons and a signal corresponding to carbonyl carbon at 181.5 ppm. The 1 HNMR spectrum (Fig. 3, bottom) of diamine 4 presented aromatic proton signals in the range of 6.19–8.16 ppm, which were easily attributable and compatible with the proposed structure. In this spectrum, the protons for the amine groups were observed at 4.93 ppm. The 13 CNMR spectrum (Fig. 3, top) of diamine 4 exhibited six peaks of various absorptions for aromatic carbons and a peak for carbonyl group. The elemental analysis results were also in good conformity with calculated percentages of carbon, hydrogen, and nitrogen contents in the synthesized compounds, indicating that the expected compounds were successfully obtained. 3.3.2. XRD Fig. 4 shows the XRD patterns of MWCNTs and the composites having varying amounts of MWCNTs. Difractogram of MWCNT (Fig. 4e) showed three peaks at 2 = 26◦ and 44◦ , which are typically associated with diffraction metal impurities (2 = 26◦ corresponds to the (0 0 2) diffraction plane of the impurity graphite, and 2 = 44◦
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Fig. 1. Typical interactions of H-bonding between MWCNT and the PI chain.
to ␣-Fe (1 1 0) and/or Ni (1 1 1) diffractions [31]. As for the pure PI, an obvious broad peak centered at 20◦ indicates that PI has an amorphous nature. The XRD patterns of the MWCNT/PI composites appeared the characteristic peaks of the pure PI and MWCNTs. Moreover, the intensity of the peaks assigned to the MWCNTs in the composite increased with increasing the MWCNT content. However, the position of the peaks corresponding to the two constituents of the composite was same to the individual PI and MWCNT, which illustrated that either the orientation of the PI chains or the structure of MWCNTs was not affected each other throughout the fabrication process.
in Fig. 5a. It can be seen that the tubes are randomly and loosely entangled together without any particle-like impurities. By dispersing into the extremely diluted solution, the MWCNTs were observed to be several nanometers in diameter and several microns in length. FE-SEM observation (Fig. 5b–d) shows a fine and homogeneous dispersion of MWCNTs throughout PI matrix, as an example,
3.3.3. Microscopy characterization The microstructure morphology of the modified and unmodified CNTs and PI composites were studied by FE-SEM and TEM. The typical FE-SEM images of the modified MWCNTs are presented
Fig. 2. FT-IR spectra of (a) PAA, (b) neat PI, the composites containing (c) 5 wt% and (d) 10 wt% of MWCNT, and (e) MWCNT.
Fig. 3. 13 CNMR (top) and 1 HNMR (bottom) (400 MHz) spectra of diamine 4 in DMSOd6 at room temperature.
M.A. Takassi et al. / Progress in Organic Coatings 80 (2015) 142–149
Fig. 4. XRD patterns for (a) pristine PI, (e) MWCNT, and (b)–(d) MWCNT/PI composites containing 5, 10, and 15 wt% of MWCNT, respectively.
for the fractured surface of the composite film containing 5 wt% of MWCNTs, and no aggregation of MWCNTs is observed. In addition, an intimate adhesion of MWCNTs with the matrix, indicating good wettability between them, is clearly observed. The film crosssection was obtained by breaking the film in liquid nitrogen to expose the intact surface fracture and the intrinsic morphology.
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The TEM images shown in Fig. 6 present the morphology of the MWCNT/PI composite films. For the FE-SEM analysis, three samples with different weight percent of MWCNTs were selected. All samples showed approximately similar observation. So, the composite with 10 wt% of MWCNT content was selected as the representative for TEM study. The aggregation of MWCNTs cannot be observed easily, which reveals the good interaction between the MWCNT and PI. The images were taken at two different magnifications for better observation of the polymer and MWCNT interactions. The greatly improved dispersion of CNTs in the PI matrix might be due to the strong interfacial interactions (hydrogen bonds) as well as to the chemical compatibility between the PI matrix and the modified CNTs. Because modified MWCNT has carboxylic acid groups and the polymer chains have many polar functional groups as well as carbonyl and amine functional groups, they can form hydrogen bonding together to achieve better interaction with PI. These TEM images suggest the in situ polymerization can achieve a fine dispersion method in this study. 3.3.4. Thermal analysis Thermal properties of PI and MWCNT/PI composite films were examined by TGA analysis as listed in Table 1. The TGA curves are shown in Fig. 7. The MWCNT decomposed slowly from 200 ◦ C and showed approximate 5 percent loss in weight before 500 ◦ C in TGA under nitrogen, probably due to the loss of carboxyl groups on the surface of the MWCNTs. Pristine PI exhibited the good thermal stability and prevented itself from thermal decomposition until 385 ◦ C.
Fig. 5. Typical FE-SEM micrographs of (a) modified MWCNT and the fracture surface of the composites containing (b) 5 wt%, (c) 10 wt%, and (d) 15 wt% of MWCNT.
Fig. 6. TEM micrographs of MWCNTs dispersed in 10 wt% of PI matrix.
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Table 1 Thermal properties of PI and MWCNT/PI composites.a Code
Td5 a (◦ C)
Td10 a (◦ C)
CR (%)b
LOI (%)c
Hcomb (kJ/g)d
PI MWCNT/PI 5% MWCNT/PI 10% MWCNT/PI 15%
385 408 430 460
400 426 449 483
57 61 64 68
40.3 41.9 43.1 44.7
19.8 19.1 18.5 17.9
a Temperature at which 5% and 10% weight loss was recorded by TGA at a heating rate of 10 ◦ C min−1 in a nitrogen atmosphere. b Percentage weight of material left undecomposed after TGA analysis at maximum temperature 800 ◦ C in a nitrogen atmosphere. c Limiting oxygen index (LOI) evaluating at char yield at 800 ◦ C. d Heat of combustion, calculated from LOI.
The 5 wt% of decomposition temperature (Td5 ) of the MWCNT/PI composites, measured under nitrogen atmosphere, was increased with increasing MWCNT content, and shifted from 385 ◦ C (neat PI) to 460 ◦ C (5 wt% MWCNT), as shown in Table 1. This could be related to the higher thermal conductivity of CNTs that facilitated heat dissipation within the composites, hence preventing the accumulation of heat at certain points for degradation [32]. The 10 wt% of decomposition temperatures (Td10 ) were also listed in Table 1. The end temperature of decomposition was also retarded with increasing MWCNT content. The highest thermal decomposition temperature of 483 ◦ C can be achieved, when the composite was added with 15 wt% of MWCNT. This indicated that MWCNT reduced the degradation of PI at high temperature as the effect was clearly seen in the curves. Therefore, it could be verified that a small amount of MWCNT acted as effective thermal degradation resistant reinforcement in the PI matrix, increasing the thermal stability of the MWCNT/PI composites. The limiting oxygen index (LOI), also called the critical oxygen index (COI) or oxygen index (OI), is defined as the minimum concentration of oxygen required in the air mixture just to support the downward burning (like a candle) of a vertically placed strip specimen [33]. It can be used to evaluate the flame retardancy of the polymeric materials. LOI is more commonly reported as a percentage rather than as a fraction. Normal atmospheric air has about 20.95% oxygen, so a material with an LOI of less than 20.95% would burn easily in air. Self-sustaining combustion in any oxygen-nitrogen atmosphere is not possible if LOI >100, indeed such values are not physically meaningful. No classification of burning behaviour based upon the LOI has gained wide
Fig. 8. Tensile stress vs. strain (%) of neat PI and MWCNT/PI composites.
acceptance. However, several researchers have suggested that “self-extinguishing polymer” has a LOI greater than 28 [34,35]. This is the definition that we use in this paper. According to Van Krevelen [36], there is a linear relationship between LOI and char residue (CR) or char yield of polymers according to the following equation (Eq. (1)): LOI = 17.5 + 0.4CR
(1)
From this equation, PI and composites, containing 5, 10, and 15 wt%, had LOI values of 40.3, 41.9, 43.1, and 44.7, respectively. On the basis of the LOI values, such materials can be classified as self-extinguishing materials. According to Johnson [37], the LOI values of a lot of materials can also be rationally well predicted by the expression (Eq. (2)): LOI = 8000/Hcomb
(2)
where Hcomb is the specific heat of combustion in J/g. So, in the case of PI and MWCNT/PI composites (5, 10, and 15 wt%), Hcomb is 19.8, 19.1, 18.5, and 17.9 kJ/g, respectively. 3.3.5. Tensile test The tensile stress–strain curve is a tool to provide data on toughness (area under the curve), ultimate tensile strength, ultimate elongation at break and Young’s modulus. Tensile tests were conducted to evaluate the effect of MWCNTs on the mechanical properties of composite samples. An average of five individual measurements was used for each sample. Typical stress-strain curves of PI and the composites are illustrated in Fig. 8 and the corresponding data are summarized in Table 2. From the table, we can see the noticeable trend of tensile strength of specimens increased when increasing the MWCNT content, while the tensile elongation and the area under the curve which is used to measure the fracture Table 2 Mechanical properties from tensile testing for PI and the composites.a
Fig. 7. TGA thermograms of the composites with different MWCNT content by weight.
MWCNT content (%)
TS (MPa)
0 5 10 15
91 102 117 137
a
± ± ± ±
0.9 1.1 1.2 1.1
EB (%) 7.9 5.2 4.5 3.4
± ± ± ±
YM (GPa) 0.5 0.7 0.3 0.2
TS : Tensile strength, EB : elongation at break, YM : Young’s modulus.
2.2 2.5 2.9 3.4
± ± ± ±
0.2 0.3 0.2 0.1
M.A. Takassi et al. / Progress in Organic Coatings 80 (2015) 142–149
energy or static toughness decreased. Compared to neat PI, the addition of 5 wt% of MWCNT improved tensile strength and modulus about 12.0% and 13.6%, respectively, as shown in Table 2. The increasing trend continues by further addition of MWCNTs. The elongation at break decreased from 7.9 to 5.2% at a 5 wt% loading of MWCNT in comparison with neat PI film. These effects can be mainly attributed to the strong interactions between MWCNTs and PI matrix via formation of secondary interactions. Therefore, network formation strengthens the material and increase the modulus while reduce the flexibility and decrease elongation. These results indicate that tensile properties of composites are improved by dispersion of MWCNTs in the PI matrix. In general, the mechanical properties of fiber-reinforced composites strongly depend on the extent of load transfer between the matrix and fiber. CNTs present a particular form of reinforcing fiber with a high aspect ratio and highly flexible elastic behavior during loading and are very different from micrometer-size fibers. So, the fine dispersion and a certain extent of orientation of MWCNTs in the PI matrix can lead to the efficient load transfer from the polymer matrix to MWCNTs and less stress convergence during the elongation process. 4. Conclusions In this study, at first, a novel aromatic diamine monomer with a triazole ring pendant moiety, 3,5-diamino-N-(1H-[1,2,4]triazol3-yl)-benzamide, has been successfully synthesized in high purity and high yield from readily available reagents. In the next step, new classes of the MWCNT/PI hybrid composite films have been fabricated by in situ polymerization and solution blending processes. A homogeneous dispersion of MWCNTs in the PI matrix was visually confirmed by detailed microscopic observations. To obtain homogeneous dispersion of MWCNT in the PI matrix, acidtreated MWCNT was chosen in this study. The presence of MWCNTs in the polymer matrix increased the thermal stability and reduced the thermal deformation of the polymer films. The incorporation of MWCNT improved mechanical properties of the PI composites, whereas the improvement was not linear with the concentration of MWCNT. The maximum tensile properties were obtained at MWCNT concentration of 15 wt% with a uniform dispersion. By comparing the obtained results from thermal and mechanical testing with some of the previously reported CNT/PI composites [38–40], it can be concluded that the thermal and mechanical stability of the prepared composited was significantly improved. The significant improvements of thermal and tensile properties of the composites are due to the reinforcement of finely dispersed MWCNT nanofillers throughout the matrix as well as the strong interfacial interaction between MWCNTs and PI matrix. This study confirmed that the thermal, mechanical, and morphological properties of the composites are strongly dependent on the uniform dispersion of CNTs and the interactions between CNT and PI, which can be improved by using of modified CNT and the introduction of several functional groups in the polymer’s chains. The approach can be broadly used for the fabrication of the composites with better alignment and much higher volume fraction of nanofillers. The good combination of properties and film-forming capability exhibited in this series of the composites demonstrate their potential for future high performance materials as well as potential applications
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in the preparation of stronger lightweight PI-based composites suitable for use as advanced composites. Acknowledgments We are grateful to the Research Affairs Division of Petroleum University of Technology (PUT), Ahwaz Faculty of Petroleum Engineering for the financial support. References ˛ [1] M. Barski, P. Kedziora, M. Chwał, Key Eng. Mater. 542 (2013) 29–42. [2] N. Behabtu, C.C. Young, D.E. Tsentalovich, O. Kleinerman, X. Wang, A.W. Ma, E.A. Bengio, R.F. ter Waarbeek, J.J. de Jong, R.E. Hoogerwerf, Science 339 (2013) 182–186. [3] Z.-L. Zhao, H.-P. Zhao, J.-S. Wang, Z. Zhang, X.-Q. Feng, Mechanical properties of carbon nanotube ropes with hierarchical helical structures, J. Mech. Phys. Solids 71 (2014) 64–83. [4] L.H. Esposito, J.A. Ramos, G. Kortaberria, Prog. Org. Coat. 77 (2014) 1452–1458. [5] N.W. Khun, B.C.R. Troconis, G.S. Frankel, Prog. Org. Coat. 77 (2014) 72–80. [6] T.A. El-Brolossy, S.S. Ibrahim, E.A. Alkhudhayr, Carbon nanotube functionalization effects on thermal properties of multiwall carbon nanotube/polycarbonate composites, Polym. Compos. (2014) (in press). [7] L. Qiu, Y. Yang, L. Xu, X. Liu, Polym. Compos. 34 (2013) 656–664. [8] Q. Jiang, X. Wang, Y. Zhu, D. Hui, Y. Qiu, Compos. Part B 56 (2014) 408–412. [9] X. Wang, S.C. Jana, Polymer 54 (2013) 750–759. [10] J. Zhao, K. Dai, C. Liu, G. Zheng, B. Wang, C. Liu, J. Chen, C. Shen, Compos. Part A 48 (2013) 129–136. [11] K. Kobashi, S. Ata, T. Yamada, D.N. Futaba, M. Yumura, K. Hata, Chem. Sci. 4 (2013) 727–733. [12] T.A. Shastry, J.W.T. Seo, J.J. Lopez, H.N. Arnold, J.Z. Kelter, V.K. Sangwan, L.J. Lauhon, T.J. Marks, M.C. Hersam, Large-Area, Small 9 (2013) 45–55. [13] S.J. Yoo, W.J. Kim, Mater. Sci. Eng. A 570 (2013) 102–105. [14] S. Mallakpour, S. Soltanian, Prog. Org. Coat. 77 (2014) 1023–1029. [15] S. Mallakpour, A. Zadehnazari, Prog. Org. Coat. 77 (2014) 679–684. [16] M. Sheikholeslam, M. Pritzker, P. Chen, Carbon 71 (2014) 284–293. [17] S. Saha, U. Saha, J.P. Singh, T.H. Goswami, J. Appl. Polym. Sci. 128 (2013) 2109–2120. [18] S. Mallakpour, A. Zadehnazari, Carbon 56 (2013) 27–37. [19] S. Mallakpour, A. Zadehnazari, Polymer 54 (2013) 6329–6338. [20] A. Horrocks, J. Soc. Dyers Colour. 99 (1983) 191–197. [21] S. Mallakpour, A. Zadehnazari, Synth. Met. 169 (2013) 1–11. ¨ [22] B. Gebhardt, F. Hof, C. Backes, M. Muller, T. Plocke, J. Maultzsch, C. Thomsen, F. Hauke, A. Hirsch, J. Am. Chem. Soc. 133 (2011) 19459–19473. [23] M. Assali, M.P. Leal, I. Fernández, P. Romero-Gomez, R. Baati, N. Khiar, Nano Res. 3 (2010) 764–778. [24] T.S. Jo, H. Han, L. Ma, P.K. Bhowmik, Polym. Chem. 2 (2011) 1953–1955. [25] Q. Wang, Q. Song, J. Qiao, X. Zhang, L. Zhang, Z. Song, Polymer 52 (2011) 3496–3502. [26] C.M. Voge, J. Johns, M. Raghavan, M.D. Morris, J.P. Stegemann, J. Biomed. Mater. Res. Part A 101 (2013) 231–238. [27] S. Mallakpour, A. Zadehnazari, Polym. Bull. 71 (2014) 207–225. [28] Q.-P. Feng, J.-P. Yang, S.-Y. Fu, Y.-W. Mai, Carbon 48 (2010) 2057–2062. [29] M. Mohiuddin, S. Hoa, Compos. Sci. Technol. 72 (2011) 21–27. [30] H.-J. Lee, S.-J. Oh, J.-Y. Choi, J.W. Kim, J. Han, L.-S. Tan, J.-B. Baek, Chem. Mater. 17 (2005) 5057–5064. [31] M. Perez-Cabero, I. Rodrıguez-Ramos, A. Guerrero-Ruız, J. Catal. 215 (2003) 305–316. [32] S.T. Huxtable, D.G. Cahill, S. Shenogin, L. Xue, R. Ozisik, P. Barone, M. Usrey, M.S. Strano, G. Siddons, M. Shim, Nat. Mater. 2 (2003) 731–734. [33] J. Rychly, L. Costa, Fire Mater. 19 (1995) 215–220. [34] A. Horrocks, M. Tune, L. Cegielka, The burning behaviour of textiles and its assessment by oxygen-index methods, Text. Prog. 18 (1988) 1–186. [35] M. Nelson, Combust. Theory Model. 5 (2001) 59–83. [36] D. Van Krevelen, Polymer 16 (1975) 615–620. [37] P. Johnson, J. Appl. Polym. Sci. 18 (1974) 491–504. [38] S.G. Gunasekaran, S. Devaraju, M. Alagar, M. Dharmendirakumar, High Perform. Polym. 26 (2014) 43–51. [39] T. Ogasawara, Y. Ishida, T. Ishikawa, Compos.: Part A 35 (2004) 67–74. [40] S.H. Lee, S.H. Choi, J.I. Choi, J.R. Lee, J.R. Youn, Korean J. Chem. Eng. 27 (2010) 658–665.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Highly stable polyimide composite films based on 1,2,4-triazole ring reinforced with multi-walled carbon nanotubes: Study on thermal, mechanical, and morphological properties Mohammad Ali Takassi a , Amin Zadehnazari a,∗ , Asadollah Farhadi a , Shadpour Mallakpour b a b
Department of Science, Petroleum University of Technology, Ahwaz, 6198144471, Iran Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran
a r t i c l e
i n f o
Article history: Received 2 October 2014 Received in revised form 2 November 2014 Accepted 2 December 2014 Keywords: Multi-walled carbon nanotube Polyimide Composite film Thermal stability Transmission electron microscopy
a b s t r a c t A novel diamine bearing aromatic pendant triazole ring, namely, 3,5-diamino-N-(1H-[1,2,4]triazol-3yl)-benzamide, was successfully synthesized. The prepared diamine and a commercial dianhydride were reacted in situ in the presence of carboxylated multi-walled carbon nanotubes (MWCNTs) with stirring to give a homogeneous MWCNT/poly(amic acid) mixture which was then heated under a heating program to give a series of MWCNT/polyimide (PI) composites with different proportions of MWCNT (5, 10, and 15 wt%). The composite films were tested for different properties including spectral, morphological, thermal, and mechanical properties. Scanning and transmission electron microscopy revealed the modified MWCNTs were well dispersed in the PI matrix while the structure of the polymer and the MWCNTs structure were stable in the preparation process. The thermal stability of the films containing MWCNTs was improved as the MWCNT content increased from 5 to 15 wt% due to the improved interfacial interaction between the PI matrix and surface-modified MWCNTs. Tensile tests on the composites showed an increase in the elastic modulus and the yield strength, and decrease in the failure strain. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The engineering materials provide continuous research opportunities for the current industrial applications. Among all the nano functional materials, carbon nanotubes (CNTs) are the frontiers of these researches because of significantly improved and unusual properties. CNTs have exceptional mechanical, electrical, and thermal properties, which are strongly anisotropic [1]. In order to fully utilize these properties, numerous CNT/polymer composites have been produced and investigated [2]. These nanocomposites exhibit the superior physical properties such as enhanced tensile strength [3] and improved electrical conductivity [4]. Many polymers such as epoxy [5], polycarbonate [6], polyamide [7], polyimide (PI) [8], polystyrene [9], and polypropylene [10] have been used to prepare CNT/polymer composites. These materials have potentially superior thermal, electrical, and mechanical properties than the pristine polymers. In polymer-matrix composites made with
∗ Corresponding author. Tel.: +98 611 555 1321; fax: +98 611 555 1321. E-mail addresses: [email protected], [email protected] (A. Zadehnazari). http://dx.doi.org/10.1016/j.porgcoat.2014.12.001 0300-9440/© 2014 Elsevier B.V. All rights reserved.
non-continuous fillers, dispersion of the filler influences nearly all relevant properties of the composite. Unfortunately, pure CNTs are insoluble in the organic solvents and tend to form aggregates due to the extremely high surface energy, which leads to a heterogeneous dispersion in the polymer matrix and has negative effects on the properties of the resulting composites [11]. The degree of dispersion of CNTs and the consequent macroscopic behavior is strongly dictated by the choice of the matrix, the type of CNT, modification of their surface, and the processing method and associated parameters [12,13]. Several methods have already been developed over the last years to achieve an efficient dispersion of individual CNTs in a polymer matrix. The simplest method consists of directly mixing the polymer and the CNTs [14–17]. Stirring in a solvent, polymer solution, or a polymer melt is usually not sufficient to achieve this goal. For this purpose, ultrasound (bath ultrasonication) is frequently applied [18–21]. Like any sound wave, ultrasound propagates via a series of compression and rarefaction waves induced in the molecules of the medium through which it passes. One can also either modify the polymer in such a way that it can interact with the -system of the CNT wall, or modify the walls of the CNTs by functionalization and so improve the wetting of the filler, as well as the dispersion in the medium [22,23].
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In this contribution in order to achieve fine dispersion of carboxyl-functionalized multi-walled CNTs (MWCNTs), we attempted to prepare MWCNT/PI composites using in situ polymerization approach (i.e., solution-casting followed by subsequent imidization). Thermal property, mechanical property and morphology of MWCNT/PI composites were then investigated. The introduction of several functional groups as well as triazole bulky units resulted in increased chain packing distances and decreased intermolecular interactions, leading to better interaction of the PI chains with MWCNTs and better dispersion of MWCNTs in the PI matrix. 2. Experimental 2.1. Materials All materials and solvents were purchased either from Aldrich Chemical Co. (Milwaukee, WI), Merck or Fluka (Germany). 3-Amino-1H-1,2,4-triazole, 3,5-dinitrobenzoylchloride, hydrazine monohydrate, propylene oxide, pyromellitic dianhydride (PMDA), dimethyl sulfoxide (DMSO), and 10% palladium on activated carbon were used as received without further purification. N,N -dimethylacetamide (DMAc) (d = 0.94 g cm−3 at 20 ◦ C) and N,N -dimethylformamide (DMF) (d = 0.94 g cm−3 at 20 ◦ C) were purified by distillation under the reduced pressure over barium oxide. Carboxylated MWCNTs synthesized by chemical vapor deposition (CVD) (the outer-diameter 8–15 nm, the inner-diameter 3–5 nm, length ∼50 m, carboxyl content 2.56 wt% and purity >95 wt%), were achieved from Neutrino Co. (Iran). All other reagents were used as received from commercial sources. 2.2. Methods Proton nuclear magnetic resonance (1 HNMR) spectra were recorded on a Bruker (Rheinstetten, Germany) Avance 500 instrument at room temperature in DMSO-d6 . Multiplicities were given as s (singlet). Chemical shifts 1 HNMR spectra are reported as ı in units of parts per million (ppm) and relative to the signal of DMSO-d6 (ı = 2.50, singlet). The number of protons (n) for a given resonance was indicated by nH. Carbon nuclear magnetic resonance (13 CNMR) spectra were reported as ı in units of ppm downfield and relative to the signal of DMSO-d6 (ı = 39.50). Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Bomem MB-Series 1998 FT-IR spectrophotometer (Quebec, Canada) at a resolution of 4 cm−1 . The samples were examined as a disc, grinded together with potassium bromide (KBr) salt. They were scanned at wavenumber (cm−1 ) range of 400–4000 cm−1 . Band intensities were assigned as weak (w), medium (m), strong (s), and broad (br). Melting points were determined in capillary tubes using a melting-point apparatus (Barnstead Electrothermal 9200, Iowa, USA) without correction. Elemental analysis was run on an Elementar Analysensysteme GmbH (Hanau, Germany). Monitoring of the reactions was accomplished by thin layer chromatography (TLC) on silica gel polygram SILG/UV 254 plates (0.2 mm thickness). Inherent viscosity (inh ) (at a concentration of 0.5 g/dL) was measured with an Ubbelohde suspended-level viscometer at 25 ◦ C using DMF as solvent. Thermogravimetric analysis (TGA) was performed with the DuPont Instruments (TGA 951, 2805 West Frye Road Chandler 85224 AZ, USA) analyzer at a heating rate of 10 ◦ C/min−1 under nitrogen (20 cm3 /min) in temperature range of 25–800 ◦ C. Measurements were carried out on 10 ± 2 mg film samples. The mechanical properties of PI and the composite films were measured at room temperature on a universal testing machine (SANTAM, model STM-20, Iran), according to ASTM D 882 (standards). Tests were carried out with a cross-head speed
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of 12.5 mm/min until reaching a deformation of 20% and then, at a speed of 50 mm/min at break. The dimensions of the test specimens were 35 × 2 × 0.04 mm3 . Property values reported here represented an average of the results for tests run on at least five specimens. Tensile strength, tensile modulus, and strain were obtained from these measurements. The X-ray diffraction (XRD) was used to characterize the crystalline structure of the samples. XRD patterns were collected using a Bruker, D8AVANCE (Rheinstetten, Germany) diffractometer with a copper target at the ˚ a tube voltage of 40 kV, and tube wavelength of CuK␣ = 1.54 A, current of 35 mA. The samples were scanned at a rate of 0.05 ◦ /min from 10◦ to 80◦ of 2. For XRD studies, rectangular pellets prepared by compression molding were used. The morphology of the MWCNTs and the dispersion morphology of the MWCNTs on the PI matrix were observed using field emission scanning electron microscopy (FE-SEM). The images were taken at 15 KV using a HITACHI S-4160 instrument (Tokyo, Japan). Transmission electron microscopy (TEM) images were obtained using a Philips CM 120 microscope (Netherlands) with an accelerating voltage of 100 kV. For TEM studies, ultra-thin sections (30–80 nm) of the composites were prepared using Leica Ultramicrotome. Branson S3200 (50 kHz, 150 W) ultrasonic bath (Americas Headquarters 41 Eagle Road, Danbury CT 06810, USA) was used for better dispersion of MWCNTs. 2.3. Synthesis of 3,5-dinitro-N-(1H-[1,2,4]triazol-3-yl)benzamide (3) The synthetic pathway leading to the synthesis of target diamine is outlined in Scheme 1. Into a 50 mL round-bottomed flask equipped with a condenser, a magnetic stirring bar and a nitrogen gas inlet tube, 3.45 g (15.0 mmol) of 3,5-dinitrobenzoyl chloride and 1.26 g (15.0 mmol) of 3-amino-1H-1,2,4-triazole were dissolved in 15.0 mL of DMAc at 0 ◦ C. After 30 min, 1.00 mL of propylene oxide was dropped into the stirring was continued at 0 ◦ C for 6 h. The reaction mixture was slowly warmed to room temperature, stirred for additional 6 h, and poured into 150 mL of distilled water. The resulting yellowish powder was collected by filtration and washed with boiling ethanol and distilled water several times. The crude product was dried under reduced pressure at 70 ◦ C. The yield of the crude product was 3.67 g (88%) with the melting point of 270–272 ◦ C. FT-IR (KBr; cm−1 ): 3431, 3382 (s, N H stretching), 3073 (w, C H aromatic), 1656 (s, C O amide stretching), 1546, 1332 (s, N O stretching), 1189 (w, C O stretching), 774 (w, N H out of plane bending). 1 HNMR [dimethylsulphoxide-d6 (DMSO-d6 ), ı, ppm]: 7.92 (s, 1H, Ar H, triazole ring), 8.47 (s, 2H, Ar H), 8.69 (s, 1H, ArH), 10.12 (s, 1H, NH, amide), 12.72 (s, 1H, NH, triazole ring). 13 CNMR (DMSO-d6 ; ı, ppm): 181.5 (C, C O), 168.4 (C, triazole ring), 157.2 (C, triazole ring), 149.4 (C, Ar), 137.3 (CH, Ar), 128.5 (CH, Ar), 123.2 (CH, Ar). Elemental analysis: calculated for C9 H6 N6 O5 : C, 38.86%; H, 2.17%; N, 30.21%; found: C, 38.84%; H, 2.15%; N, 30.17%. 2.4. Synthesis of 3,5-diamino-N-(1H-[1,2,4]triazol-3-yl)benzamide (4) To a 100 mL round-bottomed three-necked flask equipped with a dropping funnel, a reflux condenser and a magnetic stirring bar, 3.34 g (12.0 mmol) 3,5-dinitro-N-(1H-[1,2,4]triazol-3yl)-benzamide (3) and 0.05 g palladium on activated carbon (Pd/C, 10%), were dispersed in 40 mL of ethanol. The suspension solution was heated to reflux, and 5 mL of hydrazine monohydrate was added slowly to the mixture. After a further 18 h of refluxing, the solution was filtered hot to remove Pd/C, and the filtrate was cooled to give yellow crystals. The product was collected by filtration and dried in vacuum oven at 80 ◦ C. The yield of the reaction was 78% (2.04 g), and the melting point was 255–257 ◦ C.
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NO2
O2N
C
NO2
O2N
O
Cl
NH2
H2N
1 DMAc
+ NH2 N
Hydrazine, Pd/C
Propylene oxide
NH N 2
C O
HN N
EtOH
C
N
NH
O
HN NH N 4
N 3 Scheme 1. Synthesis of target diamine.
FT-IR (KBr; cm−1 ): 3423, 3330, 3218 (s, N H stretching), 3108 (w, C H aromatic), 1663 (s, C O amide stretching), 1505 (m) 1468 (w), 1383 (w), 1189 (w, C O stretching), 834 (m), 781 (w, N H out of plane bending). 1 HNMR (DMSO-d6 ; ı, ppm): 4.93 (s, 4H, NH), 6.19 (s, 1H, Ar H), 6.50 (s, 1H, Ar H), 6.97 (s, 2H, Ar H), 8.16 (s, 1H, Ar H, triazole ring), 10.04 (s, 1H, NH, amide), 12.63 (s, 1H, NH, triazole ring). 13 CNMR (DMSO-d6 ; ı, ppm): 178.3 (C, C O), 165.7 (C, triazole ring), 152.1 (C, triazole ring), 143.8 (C, Ar), 134.0 (CH, Ar), 122.4 (CH, Ar), 116.1 (CH, Ar). Elemental analysis: calculated for C9 H10 N6 O: C, 49.54%; H, 4.62%; N, 38.51%; found: C, 49.50%; H, 4.65%; N, 38.46%. 2.5. Synthesis of poly(amic acid) (PAA) and MWCNT/PI composites The MWCNTs/PI composite films were prepared by solution casting method. In a typical process, a certain amount of synthesized diamine was dissolved in DMAc completely with stirring; then PMDA (with the same molar ratio of diamine) was added into the solution in four times within 1 h. The viscosity was increased quickly over 2 h. The reaction was continued for another 4 h at room temperature under stirring and the PAA solution was obtained. Separately, 3 mL of DMAc and the appropriate amount (from 5 to 15 wt%) of MWCNT was placed in a beaker and stirred under ultrasonication for 12 h at room temperature to reach a homogenous suspension. This beaker was added to the PAA mixture prepared above and further stirred for 12 h. The composite films were prepared by coating 3 mL of the final mixture solution on a piece of 50 mm × 50 mm glass slide. The resultant tack-free films were dried in an oven and subsequently imidized at high temperature in air. The imidization process was completed in a heating program as follow: ramping from room temperature to 80 ◦ C in 30 min, keeping at 80 ◦ C for 2 h, then ramping from 80 to 200 ◦ C in 5 h, and then from 200 to 300 ◦ C in 2 h, keeping at 300 ◦ C for 45 min, and finally cooling to room temperature. The imidization process was finalized by many experiments in our laboratory. For the preparation of PI, the viscous solution of neat PAA was poured into 40 mL of methanol and the precipitated solid was filtered off and dried at 80 ◦ C for 6 h under vacuum to yield 87% of the solid PAA with inherent viscosity of 0.68 dL/g (measured at a concentration of 0.5 g/dL in DMF at 25 ◦ C). PAA: FT-IR (KBr; cm−1 ): 2548–3645 (br), 1722 (s), 1623 (s), 1566 (s), 1475 (s), 1438 (s), 1321 (s), 1273 (s), 1024 (s), 880 (s), 737 (s), 620 (m). Pure PI: FT-IR (KBr; cm−1 ): 3433 (m), 3067 (w), 1775 (w), 1725 (s), 1661 (w), 1438 (s), 1369 (s), 1260 (w), 1110 (w), 1021 (m), 806 (m), 742 (m), 618 (m).
MWCNT/PI 5 wt%: FT-IR (KBr; cm−1 ): 3463 (br), 3087 (w), 1760 (w), 1732 (s), 1641 (w), 1594 (m), 1476 (m), 1332 (s), 1285 (m), 1140 (w), 819 (w), 627 (m). MWCNT/PI 10 wt%: FT-IR (KBr; cm−1 ): 3460 (br), 3079 (w), 1765 (w), 1737 (s), 1647 (w), 1593 (m), 1475 (m), 1338 (s), 1291 (m), 1146 (w), 820 (w), 627 (m). 3. Results and discussion 3.1. Synthesis of diamine Diamine 4 as a new compound was prepared from the reaction of 3,5-dinitrobenzoylchloride and 3-amino-1H-1,2,4-triazole in DMAc as solvent in the presence of propylene oxide as an acid scavenger by the nucleophilic displacement reaction and then reduction with hydrazine monohydrate in ethanol in the presence of palladium activated carbon as a metallic hydrogenation catalyst. The reaction pathway is illustrated in Scheme 1. 3.2. Synthesis of PI and MWCNT/PI composites In general, aromatic PI is synthesized from the condensation of aromatic diamine and aromatic dianhydride by either a two-step condensation polymerization method, that is, the formation of PAA followed by a thermal or chemical imidization to give PI, or a onestep thermal polycondensation in solution at elevated temperature. In this study, the PI was prepared via two-step condensation using thermal imidization. The polymerization of diamine 4 and PMDA in dry DMAc proceeds at room temperature to avoid the premature imidization of amic acid moieties (Scheme 2). The inherent viscosity of the synthesized PAA was 0.68 dL/g and the yield was 87%. The reaction pathway for preparing MWCNT/PI composites is shown in Scheme 2. The dispersion of MWCNTs in 5, 10, and 15 wt% solutions of PAA in DMAc was attained by a vigorous stirring for 12 h, to form a new series of MWCNT/PI composites. Thin films of the stated crude mixtures were prepared by casting on dust-free glass slides. Then gradual annealing of thin films was performed to 300 ◦ C and continued at this temperature for several minutes to ensure the complete imidization. The morphology of MWCNTs and physicochemical properties of MWCNT/PI composites were investigated by FT-IR spectroscopy, XRD, TGA, FE-SEM, and TEM. The effective use of CNTs in composite applications depends on the ability to disperse the CNTs homogeneously throughout the matrix without reducing their aspect ratio. Due to van der Waals attraction, CNTs are held together as bundles and ropes. Therefore, they have a very low dispersability in solvents and tend to remain as entangled agglomerates. To utilize CNTs as an effective reinforcement into
M.A. Takassi et al. / Progress in Organic Coatings 80 (2015) 142–149
3.3. Characterization methods
NH2
H 2N
O
O
HN N
O
O
+
C
O O
O PMDA
NH N 4
DMAc Room temperature 6h O NH C
COOH
HOOC
C
NH
C
O n O
HN N
NH N
PAA solution
Stirring, room temperature, 24 h MWCNT Casting
Δ
O N
O N n
O
O
C O
HN N
NH N
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MWCNT/PI composite
Scheme 2. Reaction scheme for synthesis of MWCNT/PI composite films.
the polymer composites and ensure proper dispersion and suitable interfacial adhesion between CNTs and the polymer matrix, several techniques were used [24–26]. So, in this study, acid-functionalized MWCNTs were used for the preparation of the composites. In many investigations about the fabrication of CNT/polymer composites, 1–15 wt% of CNT content has been used and the excellent results have been obtained [27–29]. Thus, we selected 5, 10, and 15 wt% of CNT content in the composites. The lower level of aggregation in the modified MWCNTs can be attributed not only to the presence of functional groups such as carboxyl functional groups, but to their high aspect ratio. This transformation should contribute positively to the good dispersion of MWCNTs in the PI matrix. Moreover, the introduction of several functional groups into the backbone of the aromatic polymer performs a hydrogen bonding with CNTs and a composite based on hydrogen bond, by which PI chains are tightly attached to the surface of MWCNT, can be resulted (Fig. 1).
3.3.1. Spectral data All materials were characterized by FT-IR, and Fig. 2 shows the spectra of the representative materials. The FT-IR spectrum of dinitro compound 3 revealed strong peaks at 3431 and 3382 cm−1 , which was assigned to the N H stretching amide group and triazole ring, and two absorption bands at 1546 and 1332 cm−1 , which were characteristic peaks for NO2 asymmetric and symmetric, respectively. The FT-IR spectra of synthesized diamine 4 showed the presence of the characteristic peaks for NH2 functions and the absence of the original peaks, arising from the NO2 groups in the corresponding dinitro intermediate. Absorption of amine NH2 , N H of amide group and triazole ring appeared around 3423, 3330, and 3218 cm−1 and the peak at 1663 cm−1 confirms the presence of carbonyl amide group. Two absorption bands at 1505 and 1468 cm−1 were characteristic peaks for aromatic rings. The infrared spectrum of MWCNT-COOH/KBr pellet showed a strong, broad absorption band centered at 3433 cm−1 , which could be related to the stretching vibration of O H bands of carboxylic acid moieties on the surface of MWCNTs (Fig. 2e). The small peak around 2923 cm−1 was ascribed to aliphatic sp3 C H of MWCNT [30]. Strong band of absorption characteristic for the acid linkage appeared at 1629 cm−1 assigned to C O stretching vibration. FT-IR spectra were used to study the chemical structure of the matrix polymer; for example, FT-IR spectra of the neat PAA, PI, and MWCNT/PI with 5 and 10 wt% of MWCNT are shown in Fig. 2a–d. The FT-IR spectra of PI showed distinct features that clearly indicate imide ring formation and the disappearance of the PAA peak during the thermal cyclization step. The characteristic absorption bands of amic acid and carboxyl groups in the 2548–3645 cm−1 and 1623 cm−1 regions disappear and those of the imide ring appear near 1775 cm−1 (asym. C O stretching) and 1725 cm−1 (sym. C O stretching), 1369 cm−1 (C N stretching), 1021 cm−1 , and at 742 cm−1 (imide ring deformation). The presence of MWCNTs in the polymer matrix showed very few changes in the FT-IR spectrum, presumably due to the low MWCNT composition and the weak vibration signals of MWCNTs. The structure of dinitro 3 and diamine 4 compounds was also identified by 1 H and 13 CNMR spectroscopy. In the 1 HNMR spectrum of dinitro 3, the aromatic protons at 7.92–8.69 ppm showed the expected multiplicity and integration. The aromatic protons at the ortho position to the nitro group had the largest chemical shift (8.69 ppm) due to the inductive and anisotropic deshielding effect of the nitro groups. In this spectrum, the NH protons of the amide bond and triazole ring were observed at 10.12 and 12.72 ppm, respectively. The 13 CNMR spectrum of dinitro 3 showed six signals in the regions of 123.2–168.4 ppm related to aromatic carbons and a signal corresponding to carbonyl carbon at 181.5 ppm. The 1 HNMR spectrum (Fig. 3, bottom) of diamine 4 presented aromatic proton signals in the range of 6.19–8.16 ppm, which were easily attributable and compatible with the proposed structure. In this spectrum, the protons for the amine groups were observed at 4.93 ppm. The 13 CNMR spectrum (Fig. 3, top) of diamine 4 exhibited six peaks of various absorptions for aromatic carbons and a peak for carbonyl group. The elemental analysis results were also in good conformity with calculated percentages of carbon, hydrogen, and nitrogen contents in the synthesized compounds, indicating that the expected compounds were successfully obtained. 3.3.2. XRD Fig. 4 shows the XRD patterns of MWCNTs and the composites having varying amounts of MWCNTs. Difractogram of MWCNT (Fig. 4e) showed three peaks at 2 = 26◦ and 44◦ , which are typically associated with diffraction metal impurities (2 = 26◦ corresponds to the (0 0 2) diffraction plane of the impurity graphite, and 2 = 44◦
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Fig. 1. Typical interactions of H-bonding between MWCNT and the PI chain.
to ␣-Fe (1 1 0) and/or Ni (1 1 1) diffractions [31]. As for the pure PI, an obvious broad peak centered at 20◦ indicates that PI has an amorphous nature. The XRD patterns of the MWCNT/PI composites appeared the characteristic peaks of the pure PI and MWCNTs. Moreover, the intensity of the peaks assigned to the MWCNTs in the composite increased with increasing the MWCNT content. However, the position of the peaks corresponding to the two constituents of the composite was same to the individual PI and MWCNT, which illustrated that either the orientation of the PI chains or the structure of MWCNTs was not affected each other throughout the fabrication process.
in Fig. 5a. It can be seen that the tubes are randomly and loosely entangled together without any particle-like impurities. By dispersing into the extremely diluted solution, the MWCNTs were observed to be several nanometers in diameter and several microns in length. FE-SEM observation (Fig. 5b–d) shows a fine and homogeneous dispersion of MWCNTs throughout PI matrix, as an example,
3.3.3. Microscopy characterization The microstructure morphology of the modified and unmodified CNTs and PI composites were studied by FE-SEM and TEM. The typical FE-SEM images of the modified MWCNTs are presented
Fig. 2. FT-IR spectra of (a) PAA, (b) neat PI, the composites containing (c) 5 wt% and (d) 10 wt% of MWCNT, and (e) MWCNT.
Fig. 3. 13 CNMR (top) and 1 HNMR (bottom) (400 MHz) spectra of diamine 4 in DMSOd6 at room temperature.
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Fig. 4. XRD patterns for (a) pristine PI, (e) MWCNT, and (b)–(d) MWCNT/PI composites containing 5, 10, and 15 wt% of MWCNT, respectively.
for the fractured surface of the composite film containing 5 wt% of MWCNTs, and no aggregation of MWCNTs is observed. In addition, an intimate adhesion of MWCNTs with the matrix, indicating good wettability between them, is clearly observed. The film crosssection was obtained by breaking the film in liquid nitrogen to expose the intact surface fracture and the intrinsic morphology.
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The TEM images shown in Fig. 6 present the morphology of the MWCNT/PI composite films. For the FE-SEM analysis, three samples with different weight percent of MWCNTs were selected. All samples showed approximately similar observation. So, the composite with 10 wt% of MWCNT content was selected as the representative for TEM study. The aggregation of MWCNTs cannot be observed easily, which reveals the good interaction between the MWCNT and PI. The images were taken at two different magnifications for better observation of the polymer and MWCNT interactions. The greatly improved dispersion of CNTs in the PI matrix might be due to the strong interfacial interactions (hydrogen bonds) as well as to the chemical compatibility between the PI matrix and the modified CNTs. Because modified MWCNT has carboxylic acid groups and the polymer chains have many polar functional groups as well as carbonyl and amine functional groups, they can form hydrogen bonding together to achieve better interaction with PI. These TEM images suggest the in situ polymerization can achieve a fine dispersion method in this study. 3.3.4. Thermal analysis Thermal properties of PI and MWCNT/PI composite films were examined by TGA analysis as listed in Table 1. The TGA curves are shown in Fig. 7. The MWCNT decomposed slowly from 200 ◦ C and showed approximate 5 percent loss in weight before 500 ◦ C in TGA under nitrogen, probably due to the loss of carboxyl groups on the surface of the MWCNTs. Pristine PI exhibited the good thermal stability and prevented itself from thermal decomposition until 385 ◦ C.
Fig. 5. Typical FE-SEM micrographs of (a) modified MWCNT and the fracture surface of the composites containing (b) 5 wt%, (c) 10 wt%, and (d) 15 wt% of MWCNT.
Fig. 6. TEM micrographs of MWCNTs dispersed in 10 wt% of PI matrix.
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Table 1 Thermal properties of PI and MWCNT/PI composites.a Code
Td5 a (◦ C)
Td10 a (◦ C)
CR (%)b
LOI (%)c
Hcomb (kJ/g)d
PI MWCNT/PI 5% MWCNT/PI 10% MWCNT/PI 15%
385 408 430 460
400 426 449 483
57 61 64 68
40.3 41.9 43.1 44.7
19.8 19.1 18.5 17.9
a Temperature at which 5% and 10% weight loss was recorded by TGA at a heating rate of 10 ◦ C min−1 in a nitrogen atmosphere. b Percentage weight of material left undecomposed after TGA analysis at maximum temperature 800 ◦ C in a nitrogen atmosphere. c Limiting oxygen index (LOI) evaluating at char yield at 800 ◦ C. d Heat of combustion, calculated from LOI.
The 5 wt% of decomposition temperature (Td5 ) of the MWCNT/PI composites, measured under nitrogen atmosphere, was increased with increasing MWCNT content, and shifted from 385 ◦ C (neat PI) to 460 ◦ C (5 wt% MWCNT), as shown in Table 1. This could be related to the higher thermal conductivity of CNTs that facilitated heat dissipation within the composites, hence preventing the accumulation of heat at certain points for degradation [32]. The 10 wt% of decomposition temperatures (Td10 ) were also listed in Table 1. The end temperature of decomposition was also retarded with increasing MWCNT content. The highest thermal decomposition temperature of 483 ◦ C can be achieved, when the composite was added with 15 wt% of MWCNT. This indicated that MWCNT reduced the degradation of PI at high temperature as the effect was clearly seen in the curves. Therefore, it could be verified that a small amount of MWCNT acted as effective thermal degradation resistant reinforcement in the PI matrix, increasing the thermal stability of the MWCNT/PI composites. The limiting oxygen index (LOI), also called the critical oxygen index (COI) or oxygen index (OI), is defined as the minimum concentration of oxygen required in the air mixture just to support the downward burning (like a candle) of a vertically placed strip specimen [33]. It can be used to evaluate the flame retardancy of the polymeric materials. LOI is more commonly reported as a percentage rather than as a fraction. Normal atmospheric air has about 20.95% oxygen, so a material with an LOI of less than 20.95% would burn easily in air. Self-sustaining combustion in any oxygen-nitrogen atmosphere is not possible if LOI >100, indeed such values are not physically meaningful. No classification of burning behaviour based upon the LOI has gained wide
Fig. 8. Tensile stress vs. strain (%) of neat PI and MWCNT/PI composites.
acceptance. However, several researchers have suggested that “self-extinguishing polymer” has a LOI greater than 28 [34,35]. This is the definition that we use in this paper. According to Van Krevelen [36], there is a linear relationship between LOI and char residue (CR) or char yield of polymers according to the following equation (Eq. (1)): LOI = 17.5 + 0.4CR
(1)
From this equation, PI and composites, containing 5, 10, and 15 wt%, had LOI values of 40.3, 41.9, 43.1, and 44.7, respectively. On the basis of the LOI values, such materials can be classified as self-extinguishing materials. According to Johnson [37], the LOI values of a lot of materials can also be rationally well predicted by the expression (Eq. (2)): LOI = 8000/Hcomb
(2)
where Hcomb is the specific heat of combustion in J/g. So, in the case of PI and MWCNT/PI composites (5, 10, and 15 wt%), Hcomb is 19.8, 19.1, 18.5, and 17.9 kJ/g, respectively. 3.3.5. Tensile test The tensile stress–strain curve is a tool to provide data on toughness (area under the curve), ultimate tensile strength, ultimate elongation at break and Young’s modulus. Tensile tests were conducted to evaluate the effect of MWCNTs on the mechanical properties of composite samples. An average of five individual measurements was used for each sample. Typical stress-strain curves of PI and the composites are illustrated in Fig. 8 and the corresponding data are summarized in Table 2. From the table, we can see the noticeable trend of tensile strength of specimens increased when increasing the MWCNT content, while the tensile elongation and the area under the curve which is used to measure the fracture Table 2 Mechanical properties from tensile testing for PI and the composites.a
Fig. 7. TGA thermograms of the composites with different MWCNT content by weight.
MWCNT content (%)
TS (MPa)
0 5 10 15
91 102 117 137
a
± ± ± ±
0.9 1.1 1.2 1.1
EB (%) 7.9 5.2 4.5 3.4
± ± ± ±
YM (GPa) 0.5 0.7 0.3 0.2
TS : Tensile strength, EB : elongation at break, YM : Young’s modulus.
2.2 2.5 2.9 3.4
± ± ± ±
0.2 0.3 0.2 0.1
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energy or static toughness decreased. Compared to neat PI, the addition of 5 wt% of MWCNT improved tensile strength and modulus about 12.0% and 13.6%, respectively, as shown in Table 2. The increasing trend continues by further addition of MWCNTs. The elongation at break decreased from 7.9 to 5.2% at a 5 wt% loading of MWCNT in comparison with neat PI film. These effects can be mainly attributed to the strong interactions between MWCNTs and PI matrix via formation of secondary interactions. Therefore, network formation strengthens the material and increase the modulus while reduce the flexibility and decrease elongation. These results indicate that tensile properties of composites are improved by dispersion of MWCNTs in the PI matrix. In general, the mechanical properties of fiber-reinforced composites strongly depend on the extent of load transfer between the matrix and fiber. CNTs present a particular form of reinforcing fiber with a high aspect ratio and highly flexible elastic behavior during loading and are very different from micrometer-size fibers. So, the fine dispersion and a certain extent of orientation of MWCNTs in the PI matrix can lead to the efficient load transfer from the polymer matrix to MWCNTs and less stress convergence during the elongation process. 4. Conclusions In this study, at first, a novel aromatic diamine monomer with a triazole ring pendant moiety, 3,5-diamino-N-(1H-[1,2,4]triazol3-yl)-benzamide, has been successfully synthesized in high purity and high yield from readily available reagents. In the next step, new classes of the MWCNT/PI hybrid composite films have been fabricated by in situ polymerization and solution blending processes. A homogeneous dispersion of MWCNTs in the PI matrix was visually confirmed by detailed microscopic observations. To obtain homogeneous dispersion of MWCNT in the PI matrix, acidtreated MWCNT was chosen in this study. The presence of MWCNTs in the polymer matrix increased the thermal stability and reduced the thermal deformation of the polymer films. The incorporation of MWCNT improved mechanical properties of the PI composites, whereas the improvement was not linear with the concentration of MWCNT. The maximum tensile properties were obtained at MWCNT concentration of 15 wt% with a uniform dispersion. By comparing the obtained results from thermal and mechanical testing with some of the previously reported CNT/PI composites [38–40], it can be concluded that the thermal and mechanical stability of the prepared composited was significantly improved. The significant improvements of thermal and tensile properties of the composites are due to the reinforcement of finely dispersed MWCNT nanofillers throughout the matrix as well as the strong interfacial interaction between MWCNTs and PI matrix. This study confirmed that the thermal, mechanical, and morphological properties of the composites are strongly dependent on the uniform dispersion of CNTs and the interactions between CNT and PI, which can be improved by using of modified CNT and the introduction of several functional groups in the polymer’s chains. The approach can be broadly used for the fabrication of the composites with better alignment and much higher volume fraction of nanofillers. The good combination of properties and film-forming capability exhibited in this series of the composites demonstrate their potential for future high performance materials as well as potential applications
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in the preparation of stronger lightweight PI-based composites suitable for use as advanced composites. Acknowledgments We are grateful to the Research Affairs Division of Petroleum University of Technology (PUT), Ahwaz Faculty of Petroleum Engineering for the financial support. References ˛ [1] M. Barski, P. Kedziora, M. Chwał, Key Eng. Mater. 542 (2013) 29–42. [2] N. Behabtu, C.C. Young, D.E. Tsentalovich, O. Kleinerman, X. Wang, A.W. Ma, E.A. Bengio, R.F. ter Waarbeek, J.J. de Jong, R.E. Hoogerwerf, Science 339 (2013) 182–186. [3] Z.-L. Zhao, H.-P. Zhao, J.-S. Wang, Z. Zhang, X.-Q. Feng, Mechanical properties of carbon nanotube ropes with hierarchical helical structures, J. Mech. Phys. Solids 71 (2014) 64–83. [4] L.H. Esposito, J.A. Ramos, G. Kortaberria, Prog. Org. Coat. 77 (2014) 1452–1458. [5] N.W. Khun, B.C.R. Troconis, G.S. Frankel, Prog. Org. Coat. 77 (2014) 72–80. [6] T.A. El-Brolossy, S.S. Ibrahim, E.A. Alkhudhayr, Carbon nanotube functionalization effects on thermal properties of multiwall carbon nanotube/polycarbonate composites, Polym. Compos. (2014) (in press). [7] L. Qiu, Y. Yang, L. Xu, X. Liu, Polym. Compos. 34 (2013) 656–664. [8] Q. Jiang, X. Wang, Y. Zhu, D. Hui, Y. Qiu, Compos. Part B 56 (2014) 408–412. [9] X. Wang, S.C. Jana, Polymer 54 (2013) 750–759. [10] J. Zhao, K. Dai, C. Liu, G. Zheng, B. Wang, C. Liu, J. Chen, C. Shen, Compos. Part A 48 (2013) 129–136. [11] K. Kobashi, S. Ata, T. Yamada, D.N. Futaba, M. Yumura, K. Hata, Chem. Sci. 4 (2013) 727–733. [12] T.A. Shastry, J.W.T. Seo, J.J. Lopez, H.N. Arnold, J.Z. Kelter, V.K. Sangwan, L.J. Lauhon, T.J. Marks, M.C. Hersam, Large-Area, Small 9 (2013) 45–55. [13] S.J. Yoo, W.J. Kim, Mater. Sci. Eng. A 570 (2013) 102–105. [14] S. Mallakpour, S. Soltanian, Prog. Org. Coat. 77 (2014) 1023–1029. [15] S. Mallakpour, A. Zadehnazari, Prog. Org. Coat. 77 (2014) 679–684. [16] M. Sheikholeslam, M. Pritzker, P. Chen, Carbon 71 (2014) 284–293. [17] S. Saha, U. Saha, J.P. Singh, T.H. Goswami, J. Appl. Polym. Sci. 128 (2013) 2109–2120. [18] S. Mallakpour, A. Zadehnazari, Carbon 56 (2013) 27–37. [19] S. Mallakpour, A. Zadehnazari, Polymer 54 (2013) 6329–6338. [20] A. Horrocks, J. Soc. Dyers Colour. 99 (1983) 191–197. [21] S. Mallakpour, A. Zadehnazari, Synth. Met. 169 (2013) 1–11. ¨ [22] B. Gebhardt, F. Hof, C. Backes, M. Muller, T. Plocke, J. Maultzsch, C. Thomsen, F. Hauke, A. Hirsch, J. Am. Chem. Soc. 133 (2011) 19459–19473. [23] M. Assali, M.P. Leal, I. Fernández, P. Romero-Gomez, R. Baati, N. Khiar, Nano Res. 3 (2010) 764–778. [24] T.S. Jo, H. Han, L. Ma, P.K. Bhowmik, Polym. Chem. 2 (2011) 1953–1955. [25] Q. Wang, Q. Song, J. Qiao, X. Zhang, L. Zhang, Z. Song, Polymer 52 (2011) 3496–3502. [26] C.M. Voge, J. Johns, M. Raghavan, M.D. Morris, J.P. Stegemann, J. Biomed. Mater. Res. Part A 101 (2013) 231–238. [27] S. Mallakpour, A. Zadehnazari, Polym. Bull. 71 (2014) 207–225. [28] Q.-P. Feng, J.-P. Yang, S.-Y. Fu, Y.-W. Mai, Carbon 48 (2010) 2057–2062. [29] M. Mohiuddin, S. Hoa, Compos. Sci. Technol. 72 (2011) 21–27. [30] H.-J. Lee, S.-J. Oh, J.-Y. Choi, J.W. Kim, J. Han, L.-S. Tan, J.-B. Baek, Chem. Mater. 17 (2005) 5057–5064. [31] M. Perez-Cabero, I. Rodrıguez-Ramos, A. Guerrero-Ruız, J. Catal. 215 (2003) 305–316. [32] S.T. Huxtable, D.G. Cahill, S. Shenogin, L. Xue, R. Ozisik, P. Barone, M. Usrey, M.S. Strano, G. Siddons, M. Shim, Nat. Mater. 2 (2003) 731–734. [33] J. Rychly, L. Costa, Fire Mater. 19 (1995) 215–220. [34] A. Horrocks, M. Tune, L. Cegielka, The burning behaviour of textiles and its assessment by oxygen-index methods, Text. Prog. 18 (1988) 1–186. [35] M. Nelson, Combust. Theory Model. 5 (2001) 59–83. [36] D. Van Krevelen, Polymer 16 (1975) 615–620. [37] P. Johnson, J. Appl. Polym. Sci. 18 (1974) 491–504. [38] S.G. Gunasekaran, S. Devaraju, M. Alagar, M. Dharmendirakumar, High Perform. Polym. 26 (2014) 43–51. [39] T. Ogasawara, Y. Ishida, T. Ishikawa, Compos.: Part A 35 (2004) 67–74. [40] S.H. Lee, S.H. Choi, J.I. Choi, J.R. Lee, J.R. Youn, Korean J. Chem. Eng. 27 (2010) 658–665.