- Email: [email protected]
Zn–AL LDH reinforced nanocomposites based on new polyamide containing imide group: From synthesis to properties
Applied Clay Science 114 (2015) 256–264
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Zn–AL LDH reinforced nanocomposites based on new polyamide containing imide group: From synthesis to properties Mohsen Hajibeygi a,⁎, Meisam Shabanian b, Hossein Ali Khonakdar c a b c
Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), P.O. Box 31745-139, Karaj, Iran Department of Polymer Engineering, Faculty of Engineering, South Tehran Branch, Islamic Azad University, P.O. Box 19585-466, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 3 April 2015 Received in revised form 4 June 2015 Accepted 8 June 2015 Available online xxxx Keywords: Polyamide Nanocomposite Layered double hydroxide Thermal stability Flame retardant Mechanical properties
a b s t r a c t A new series of polyamide/Zn–Al layered double hydroxide (LDH) nanocomposites (PANC) was prepared by solution intercalation technique under ambient condition in dimethylformamide as solvent. The polyamide (PA) containing pyridine ring and imide group was synthesized using direct polycondensation reaction with good solubility and desired molar mass. Organo-modified Zn–Al LDH (OLDH) was produced by one-step method and used to improve mechanical, thermal and flame properties of PA. The extent of dispersion of OLDH sheets was quantified by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and the results showed a good dispersion for OLDH in the PA matrix. According to the results of mechanical tests, the tensile strength and the Young's modulus of PANC enhanced with increasing OLDH content. Thermal properties of PANC were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal property results in both nitrogen and air atmospheres showed that the addition of OLDH resulted in a substantial increase in the thermal stability and char yields of PANC as compared to the neat PA. Significant improvements in flame retardancy performance were observed for PANC from microscale combustion calorimeter (MCC) (reducing both the heat release rate and the total heat released). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polymeric nanocomposite materials have attracted increasing attention because of their potential in terms of improving thermal stability, flame retardancy, mechanical properties and film barrier characteristics with small nanoparticles loading in organic polymers. In recent years, many studies were reported about layered nanostructure materials as one of the potential candidates for polymeric nanocomposite preparation because of their large value of aspect ratio, diameter in nanometer range, thermal stability and other good properties even at low concentrations (Sinha Ray and Okamoto, 2003; Wang et al., 2007; Katiyar et al., 2010; Shabanian et al., 2014a). Layered double hydroxides (LDHs), also called anionic clays, were considered as a class of clays which have a promising future in the field of nanocomposites due to their various unique properties. LDH can be prepared from a wide range of various metallic compositions (Kutlu et al., 2014). Chemical structure of LDH was found to be a stack of positively charged metal hydroxide layers with intercalated counteranions and water molecules (Wang et al., 2011b). LDH was used in various fields such as catalysts (Lopez et al., 2010; Wang and O'Hare, 2012), ion exchange hosts (Millange et al., 2000), drug delivery ⁎ Corresponding author. Tel.: +98 9122391983; fax: +98 2188257969. E-mail addresses: [email protected], [email protected] (M. Hajibeygi).
http://dx.doi.org/10.1016/j.clay.2015.06.008 0169-1317/© 2015 Elsevier B.V. All rights reserved.
(Alcantara et al., 2010), hydrothermal reactor (Wang et al., 2013) and fire retardant additives (Nyambo et al., 2008; Manzi-Nshuti et al., 2009a). Recently many researches have been focused on polymer nanocomposites containing LDH (Lv et al., 2009; Wang et al., 2011a, 2011b; Purohit et al., 2012) and significant synergistic effects were observed in both thermal stability and flame retardancy for formulations containing polymer matrices with modified Mg–Al and Zn–Al LDH as nanofillers (Nyambo et al., 2008; Manzi-Nshuti et al., 2009a; Wang et al., 2012). Due to the increasing demand for high-performance polymers as a good candidate for matrix nanocomposites, thermally stable polymers such as polyamides have attracted increasing interest over the past decade (Díez-Pascual et al., 2012; Shabanian et al., 2013b). Polyamides are one of the most versatile classes of high performance polymers which are used in wide range of applications and display unique properties, but they suffer from disadvantages such as low solubility in organic solvents and having high melting and/or glass transition temperatures, which cause some restrictions in their processing. However, these problems can be solved or reduced by introducing some flexible bonds such as aliphatic chains (Shabanian and Basaki, 2013), pendant bulky group (Hajipour et al., 2008; Ghaemy and Nasab, 2011) and heteroaromatic rings in the polyamide backbone (Liu et al., 2013). Among these approaches, introduction of heteroaromatic rings into the backbone of polyamides has been considered in some reports
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
(Mehdipour-Ataei and Heidari, 2003; Faghihi and Mozaffari, 2008; Kausar et al., 2010; Shabbir et al., 2010; Shabanian et al., 2014b). Pyridine is a heteroaromatic molecule with polarizability and rigidity. Some kinds of heteroaromatic structures containing pyridine unit have been designed and synthesized, and the heteroaromatic polymers have been synthesized from those monomers containing pyridine nucleus structures at the same time (Liu et al., 2004; Ma et al., 2010). In general, the heteroaromatic rings in the polymer backbone would impart certain properties to it, while pyridine with heteroaromatic structure would have excellent stabilities derived from its structural symmetry and aromaticity of pyridine ring (Madhra et al., 2002). Also polarizability resulting from nitrogen of pyridine ring could be suitable to improve solubility of polymer in organic solvents (Kurita and Williams, 1974; Butuc and Gherasim, 1984). The aim of present work was the preparation of new reinforced nanocomposites based on OLDH and a polyamide containing pyridine and imide heterocyclic rings through solution intercalation technique. From the presence of the heterocyclic derivatives and the aliphatic groups in the polyamide structure it was expected to provide acceptable glass transition and good solubility in organic solvents. In this work, the effect of OLDH on the mechanical properties, thermal stability and flame retardancy of the developed nanocomposites was also described.
2. Experimental
257
0.05° and slit aperture of 1°. XRD patterns of the samples were measured randomly in a continuous mode with a scan speed of 0.005°s−1. Elemental analyses were performed by Vario EL equipment. The morphological analysis was carried out using a LEO 912 transmission electron microscopy (TEM) operated at room temperature with an acceleration voltage of 200 kV in a brightfield illumination. Mechanical properties were performed at room temperature on a Testometric Universal Testing Machine M350/500 (Mainz, Germany); rate, 3 mm/min; according to ASTM D882 (standards). The mechanical properties were repeated five times. The average values along with standard deviations were reported. Thermal gravimetric analysis (TGA) data for polymers were taken on a Mettler TA4000 System under N2 and air atmosphere at rate of 10 °C/min. Differential scanning calorimeter (DSC) was conducted with DSC, Mettler 110 (Switzerland) at heating rate of 10 °C/min in nitrogen atmosphere. The flame retardancy properties were analyzed by microscale combustion colorimeter (MCC) that is a convenient and relatively new technique. The average values along with standard deviations were reported. The detailed procedure was as follows: about 5 mg of samples was heated to 700 °C with a heating rate of 1°Cs− 1 in a stream of nitrogen flowing at 80 cm3/min. Then, the volatile anaerobic thermal degradation products were mixed with 20 cm3/min gas stream containing 20% oxygen and 80% nitrogen, respectively, prior to entering to a 900 °C combustion furnace. All measurements were repeated at least in duplicate.
2.1. Materials 2.3. Monomer synthesis Glutamic acid, phthalic anhydride, 4-hydroxy-3-methoxybenzaldehyde (vanillin), 4-nitroacetophenone, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), palladium charcoal (Pd/C), hydrazine monohydrate, pyridine (Py) and triphenyl phosphite (TPP) from Merck were used without further purification. Commercially available calcium chloride (CaCl2, Merck) was dried under vacuum at 150 °C for 6 h. Zinc nitrate, aluminum nitrate, sodium hydroxide, and sodium dodecylbenzene sulfonate (SDBS) used for synthesis of OLDH by one-step route were also obtained from Merck.
2.2. Measurements 1
H-NMR spectra were recorded by a Bruker 300 MHz instrument (Germany). Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer RXI spectrometer. The KBr pellet technique was applied for monitoring changes in the range of 400–4000 cm−1 with a resolution of 2 cm−1. Approximately 3 mg of the powdered components was mixed with 100 mg KBr and pressed into pellets and used for further characterization. Vibration transition frequencies were reported in wavenumber (cm− 1). Band intensities were assigned as weak (w), medium (m), shoulder (sh), strong (s) and broad (br). Inherent viscosity was measured at a concentration of 0.5 g/dL in DMF at 25 °C by a standard procedure using a Technico Regd Trade Mark Viscometer. Molar mass (mass-average (Mw ) and number-average (Mn )) determination was performed in size exclusion chromatography (SEC) using Agilent Series 1100 (Agilent, USA) system consisting of a pump, degasser and differential refractive index (RI) detector. Two Zorbax PSM Trimodal-S 250 mm × 6.2 mm columns (Rockland Tech, USA) were used. The measurements were performed using a mixed eluent DMAc with 2 vol.% water and 3 g/L LiCl at a flow rate of 0.5 mL/min. The molar mass was calculated after calibration with poly(2vinylpyrrolidone) standards. Wide angle X-ray scattering (WAXS) was performed using 2-circle diffractometer XRD 3003 (GE Inspection Technologies/Seifert-FPM, Freiberg, Germany) using Cu–Kα radiation (λ = 0.154 nm) generated at 30 mA and 40 kV in the range of 2θ = 2°–12° with a step length of
2.3.1. Synthesis of diacid 2-(1,3-Dioxoisoindolin-2-yl)pentanedioic acid as a diacid monomer was synthesized by the condensation reaction of phthalic anhydride and glutamic acid in an acetic acid solution according to previous research (Faghihi et al., 2010). 2.3.2. Synthesis of diamine 4-(2,6-Bis(4-aminophenyl)pyridine-4-yl)-2-methoxyphenol as a new diamine compound containing pyridine ring was synthesized by two step reactions according to previous researches (Tamami and Yeganeh, 2001; Hajipour et al., 2008; Shabanian et al., 2014b) as follows; Synthesis of 4-(2,6-bis(4-nitrophenyl)pyridin-4-yl)-2-methoxyphenol; 1.08 g (6.56 mmol) of 4-nitroacetophenone, 0.5 g (3.28 mmol) of vanillin, 6 g of ammonium acetate and 12 mL of glacial acetic acid were added into 100 mL round-bottom flask. The reaction mixture was heated in an oil bath at 120 °C for 18 h. Upon cooling, the precipitated yellow solid was collected by filtration and washed with ethanol. FTIR (KBr): 3531 (m), 1593 (s), 1547 (m), 1513 (s), 1439 (m), 1347 (s), 1271 (m), 1209 (m), 1031 (m, sh), 846 (s), 815 (m), 735 (m), and 691 (m) cm−1. 1H-NMR (DMSO-d6, TMS) δ: 9.50 (s, 1H), 8.60 (d, 4H), 8.36 (d, 4H), 8.35 (s, 2H), 7.61 (s, 1H), 7.56 (d, 1H), 6.93 (d, 1H), and 3.94 (s, 3H) ppm. Synthesis of 4-(2,6-bis(4-aminophenyl)pyridin-4-yl)-2-methoxyphenol; 1.62 g (3.67 mmol) of dinitro, 0.1 g of 10% Pd/C, 20 mL of ethanol, 5 mL of DMF and 3 mL of hydrazine monohydrate were added into a 100 mL round-bottomed flask. The reaction mixture was heated in an oil bath at 90 °C for 5 h. Then, the mixture was filtered to remove the Pd/C and the filtrate was poured into water and dried. FTIR (KBr): 3434 (m, sh), 3352 (m), 3212 (w), 3031 (w), 2934 (w), 1607 (s), 1596 (s), 1513 (s), 1437 (m), 1393 (m), 1269 (m), 1179 (m), 1126 (m), 831 (m), and 581 (m) cm−1. 1H-NMR (DMSO-d6, TMS) δ: 9.30 (s, 1H), 7.97–8.00 (d, 4H), 7.72 (s, 2H), 7.43 (s, 1H), 7.35–7.38 (d, 1H), 6.88–6.91 (d, 1H), 6.65–6.68 (d, 4H), 5.39 (s, 4H), and 3.91
258
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
(s, 3H) ppm. Elemental analysis: calculated for C 24H21 N3 O 2 (348.4 g/mol), calculated: C, 75.18; H, 5.52; and N, 10.96; found: C, 75.17; H, 5.52; and N, 10.96. 2.3.3. Synthesis of polyamide 0.941 g of diacid (3.39 mmol), 1.20 g of 4-(4-hydroxyphenyl)-2,6bis(4-aminophenyl)pyridine (3.39 mmol), calcium chloride (0.90 g), triphenyl phosphite (1.76 mL), pyridine (0.54 mL) and N-methyl-2pyrrolidone (10 mL) were added into 100 mL round-bottom flask. The reaction mixture was heated in an oil bath at 60 °C for 1 h, 90 °C for 3 h and 120 °C for 6 h. Subsequently, the reaction mixture was poured into 100 mL of methanol and the precipitated PA was collected by filtration and washed thoroughly with hot methanol. Finally, the product was dried at 70 °C for 12 h inside a vacuum oven to leave 2.51 g (97.6%) yellow solid PA. ηinh (inherent viscosity, measured at a concentration of 0.5 g/dL in DMF at 25 °C): 0.71 (dL/g). Elemental analysis: calculated for C36H26N4O5 (594.62 g/mol), calculated: C, 72.72; H, 4.41; and N, 9.42; found: C, 71.89; H, 4.38; and N, 9.41. 2.3.4. Preparation of organo-modified Zn–Al LDH (OLDH) OLDH was prepared in one step according to the procedure reported elsewhere (Wang et al., 2009). Briefly, a solution of Zn(NO3)2 and Al(NO3)3 (with Zn2+:Al3+ equal to 2:1 and a total metal ion concentration of 0.3 M) was added to a SDBS solution slowly. During the preparation of OLDH, solution was stirred at 50 °C and the pH value was kept at 10.5 ± 0.2 by adding a suitable amount of solution of sodium hydroxide. After the addition of the metal salt solution, the resulting mixture was stirred at 50 °C for half an hour. Then the temperature was increased to 70 °C and allowed to age for 24 h. Upon cooling the final white product was collected by filtration and washed thoroughly with distilled water until a solution with pH = 7 was obtained. The product was then dried in an oven at 75 °C. 2.3.5. Synthesis of PA/Zn–Al LDH nanocomposites The nanocomposites were synthesized by taking the PA solution in a flask, followed by the addition of a known proportion of OLDH for particular concentrations. The amounts of OLDH were 2, 5 and 8 mass% in the nanocomposites (PANC 2, PANC 5 and PANC 8). To prepare nanocomposite containing 2 mass% OLDH, 0.98 g of PA was dissolved in 10 mL DMF, followed by the addition of 0.02 g of OLDH. The reaction mixture was agitated to high speed at 25 °C overnight to disperse OLDH platelets uniformly in the PA matrix. The nanocomposites were caste by pouring the hybrid solution in Petri dishes and removed the solvent at 80 °C for 10 h and were further dried at 80 °C under vacuum to a constant mass.
Fig. 1. 1H-NMR spectrum of aromatic diamine.
presented in Fig. 1, and the spectrum proved the proposed aromatic diamine structure.
3.2. Polymer synthesis In order to prepare the matrix of nanocomposites, the Yamazaki method (Yamazaki et al., 1975) (triphenyl phosphite (TPP)-activated polycondensation) was used to synthesize PA containing imide group in the side chain and pyridine moiety (Scheme 1). The solubility of PA was investigated with 0.01 g of polymeric sample in 2 mL of solvent. Due to the presence of bulky pendant groups in PA, it had good solubility in organic solvents at room temperature. The polymer was dissolved in polar organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc), and N-methyl-2-pyrrolidone (NMP) at room temperature and was insoluble in protic solvents such as methanol, ethanol, and water. Also, newly synthesized PA owned number-average molar mass ( Mn ) and mass-average molar mass ( Mw ); 2.5 × 104 and
3. Results and discussion 3.1. Diamine synthesis The new aromatic diamine monomer, 4-(2,6-bis(4aminophenyl)pyridin-4-yl)-2-methoxyphenol was prepared by a twostep synthetic route. Firstly, the dinitro compound containing pyridine heterocyclic and methoxy pendant group was synthesized via a modified Chichibabin pyridine synthesis (Tamami and Yeganeh, 2001; Hajipour et al., 2008) from vanillin and 4-nitroacetophenone in the presence of ammonium acetate. Then, the aromatic diamine was obtained by catalytic hydrogenation of the dinitro compound using hydrazine monohydrate and Pd/C catalyst in ethanol. The structure of new aromatic diamine was confirmed by elemental analysis, FTIR and 1H-NMR spectroscopies. In the 1H-NMR spectrum of diamine (Fig. 1), the signals of aromatic protons appeared in the range of 6.65–8.00 ppm, and the characteristic resonance signal at 5.39 ppm was due to the amino group. Assignments of each proton were also
Scheme 1. Synthesis route of PA.
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
259
Fig. 2. 1H-NMR spectrum of PA.
5.7 × 104 (poly dispersity index (PDI) = 2.28) respectively, as measured by SEC. The structure and purity of newly synthesized PA were confirmed by elemental analysis, FTIR and 1H-NMR spectroscopies. The elemental analysis of PA was in good agreement with the calculated values for the proposed structure (see Experimental section). The 1H-NMR spectrum of PA showed peaks that confirm its chemical structure (Fig. 2). The aromatic protons appeared in the region of 6.91–8.26 ppm. The proton related to chiral center appeared at 4.97 ppm. The O\\H group related to diamine appeared as a broad peak at around 9.9 ppm. Two peaks at 10.05 ppm and 10.15 ppm in 1H-NMR spectrum of PA were related to two antisymmetric N\\H groups in the main chain of PA. 3.3. Preparation of PA/Zn–Al LDH nanocomposites In general, LDH is employed to prepare different organic–inorganic nanohybrid materials and has received significant attention. The organic/LDH nanohybrids have been studied because the resulting
Fig. 3. FTIR spectra of unmodified LDH, OLDH, PA and PANC.
intercalation compounds are expected to possess a new nanostructure (Gasser, 2009). In this study SDBS was used as building blocks for efficient preparation of potentially nanohybrid material of OLDH by one-step route according to previous research (Shabanian et al., 2014a). As a possible model, SDBS was considered to be arranged vertically and/or horizontally to the LDH basal layer. In comparison to the neat LDH, the layers of OLDH changed from hydrophilic to organophilic and its interlayer space increased. These phenomena caused enlargement in the d-value which facilitates the exfoliation of the plates of the OLHD in the polymer matrix. New nanocomposites containing 2, 5 and 8 mass% of OLDH were prepared using OLDH and PA containing imide groups in the main chains in dry DMF through solution intercalation technique (Scheme 2).
Scheme 2. Preparation of PANC.
260
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
stretching vibration), and 1071 and 718 cm−1 (imide ring). Due to the presence of O\\H group in the polymer chains and hydrogen band formation, the peaks related to N\\H and O\\H groups in the polymer chain were appeared as a broad peak. Fig. 3 also showed the FTIR spectra of PANC. The FTIR spectra of nanocomposites are dominated by the bands of the polyamide, but the presence of the absorption bands of OLDH is also seen. FTIR spectra of the nanocomposites showed the characteristic absorption bands for the carbonyl groups around 1776 cm− 1 due to the antisymmetric imide ring stretching vibration and two overlapped around 1717 cm−1 related to symmetric imide ring and amide group stretching. The characteristic bands around 2850–2930 cm−1 are due to aliphatic groups related to the OLDH. By comparison of these spectra, it can be concluded that the nanocomposites not only have characteristic neat PA bands but also have characteristic bands for OLDH. These data, in sum, confirmed the formation of the PA/Zn–Al LDH nanocomposites. 4.2. Structural characterization
Fig. 4. XRD pattern of OLDH, PA and PANC.
4. Characterization of the PA/LDH nanocomposites 4.1. FT-IR spectra FTIR spectra of unmodified LDH, OLDH, PA and PANC are shown in Fig. 3. In the spectrum of unmodified LDH, the broad characteristic band centered at 3421 cm−1 related to the O\\H stretching vibrations of surface and interlayer water molecules. Also, the characteristic band at 1352 cm−1 was related to NO3 stretching vibrations. In the spectrum of OLDH, the characteristic band related to NO3 was vanished and the characteristic vibration bands for SO3 were appeared. Also, the broad and strong bands in the range of 3200–3650 cm−1 centered at 3475 cm−1 were due to the O\\H stretching vibrations of surface and interlayer water molecules. The band observed near 1601 cm−1 was related to the bending vibration of water molecule. A strong absorption band at 2854 cm−1, 2926 cm−1 and 2956 cm−1 was detected for alkyl groups (CH3 and CH2) in SDBS. The characteristic vibration bands were detected for SO3 stretching (symmetric stretching at 1037 cm−1 and antisymmetric at 1175 cm−1), the benzene group (C\\C stretching at 1466 cm− 1, C\\H in plane bending at 1010 and 1130 cm− 1). The bands at 760 and 618 cm− 1 can be assigned to Al\\O stretching modes. These FTIR characteristic bands demonstrated that the SDBS chains had been intercalated into the interlayer space of Zn–Al LDH. The FTIR spectrum of PA (Fig. 3) showed absorption bands around 3258 cm−1 (N\\H), 1776 and 1717 cm−1 for the imide ring (antisymmetric and symmetric C_O stretching vibrations), 1385 cm−1 (C\\N
Wide angle X-ray scattering (WAXS) and TEM analyses are generally used to analyze the structure and morphology of the layered structure of nanomaterials. X-ray diffraction is one of the techniques to characterize the layered structure of the polymer nanocomposites, an easy way to access the interlayer spacing of the LDH in polymer nanocomposites. In comparison, TEM analysis gives direct evidence on the dispersion of the nanoparticles in a nanocomposite XRD patterns of the one-step prepared OLDH, PA and PANC are shown in Fig. 4. The interlayer distance of OLDH can be calculated from Bragg diffraction law. The XRD pattern of OLDH pointed out to a multilayer structure due to the basal reflection maximum (003) at 2θ = 3.06° (d-value: 2.88 nm). In the XRD pattern of PA, no sharp characteristic reflections were observed and this indicated that this polyamide is amorphous (Fig. 4). In comparison to basal reflections of OLDH for nanocomposites, there were almost no reflections corresponding to crystal structure of OLDH. Therefore, dispersion of OLDH sheets in polyamide matrix can be expected (Herrero et al., 2010). In addition, the disappearance of the reflections related to OLDH in the XRD patterns of the PANC was probably due to the loss of crystalline symmetry in the stacking direction of the hydroxide layers, lowering the number layers of hydroxide and much insertion of polymer chains into the interlayer space of OLDH (Kotal et al., 2009). Disappearance of characteristic reflections in XRD pattern does not always confirm the good dispersion and exfoliated structure in nanocomposites, because XRD is unable to detect regular stacking exceeding 8.8 nm (Kornmann et al., 2001), therefore, more direct information can be obtained from TEM. In order to confirm the dispersion of OLDH sheets in PA matrix, TEM investigations were done on PANC 2 and PANC 5 samples and the results are presented in Fig. 5. The dark lines
Fig. 5. TEM micrographs of a) PANC 2 and b) PANC 5.
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
261
represent the LDH layers, whereas the bright area represents the polyamide matrix. The TEM images of PANC showed a good dispersion of OLDH sheets in the polyamide matrix, providing a direct evidence of crystal layer exfoliation. These results are supported by the absence of reflections for PANC in XRD pattern. 4.3. Mechanical properties Mechanical properties of PA and PANC, including the tensile strength and Young's modulus, were measured and the results are listed in Table 1. Also, the relations between tensile strength and Young's modulus of nanocomposites and OLDH contents are shown in Fig. 6. The increase of OLDH content up to 5 mass% to PA and tensile strength of PANC had a linear relationship. The tensile strength increased from 54 to 61 MPa, which was about 13% higher than that of neat PA, with the addition of OLDH content within 5 mass% in comparison with the neat PA. This can be attributed to the reinforcement effect attained by the OLDH through random dispersion in the PA matrix and/or the PA chains in the nanocomposites were restricted by the OLDH sheets, resulting in the decreased degree of freedom (Wang et al., 2004). The Young's modulus values performed the same significant changes of the tensile strength values. The Young's modulus of PANC 5 is the highest one among them and reached 3.4 GPa, which is 47% and 36% higher than those of neat PA and PANC 2 samples, respectively. The maximum increase in Young's modulus was noted with 5 mass% OLDH, which then decreased with OLDH content up to 8 mass% in PA matrix. This behavior in mechanical properties was also observed in tensile strength of neat PA as compared to PANC. This phenomenon might be due to the increased amount of OLDH in PA matrix. At high OLDH loading, OLDH sheets may stack together and interlayer space does not increase well, as it should, thus deteriorating the properties of composites (Zulfiqar and Sarwar, 2009). Nevertheless, all of these results showed a good improvement in mechanical properties of PANC in low concentration of OLDH as compared to the neat PA. 4.4. Thermal properties Thermal stability is a very important factor for characterization of polymeric materials, and normally is indicated by thermogravimetric analysis (TGA). The TGA thermograms of neat PA and PANC in nitrogen and air atmospheres are presented in Figs. 7 and 8, respectively, and the resulting TGA data are summarized in Table 2 including temperatures at which 5% (T5) degradation and 10% (T10) degradation occur and the residue at 800 °C. The TGA thermogram exhibited T5, T10 and char yield for neat PA, 275 °C, 322 °C and 63.1% under N2 atmosphere, respectively. Considering the TGA thermograms of nanocomposites, it was evident that OLDH had good effect on the thermal properties of PA. By incorporation of 2 mass% of OLDH to PA, T5 increased by about 20 °C reaching to 298 °C. T10 also shifted to higher temperature by about 25 °C in PANC 2 compared to neat PA. These thermal properties were similar to previous reports for different nanocomposites, and it was explained by the relative extent of exfoliation and/or delamination processes as a function of the amount of OLDH (Qiu et al., 2006; Shabanian et al., 2014a). Also, the results suggested that an excess of OLDH in the polymer matrix might restricted the mobility of PA chains.
Fig. 6. Tensile strength and Young's modulus of PANC with various OLDH contents.
By increasing the OLDH content to 5 and 8 mass%, no further improvements were observed with these parameters, insofar as, T10 shifted to lower temperature in PANC 8 as compared to PANC 5. The reason could be that OLDH has a role for catalytic degradation of PA. This behavior in thermal properties was observed in char yields of neat PA as compared to PANC. TGA thermograms of PA and PANC under air atmosphere are shown in Fig. 8 and the corresponding data are summarized in Table 2. The destabilization effect of oxygen is noted comparing TGA curves of the polymeric materials in air and nitrogen, due to the reactions of the degrading polymer radicals with oxygen in air (Manzi-Nshuti et al., 2009b). As shown in Table 2, neat PA in air atmosphere has T5 and T10 of 226 and 256 °C, respectively with final residue of 5.15%. Also, it can be observed that the PANC have lower T5 than neat PA, but in all PANC by increasing the OLDH content from 2 to 8 mass%, T10 and char yields were increased. This implied that the incorporation of OLDH in PA matrix helped in char formation and the improvement can be attributed to the presence of OLDH, which plays barrier effect to maximize the insulation and to minimize the permeability of volatile degradation products (Herrero et al., 2010). Also, the mass loss (%) at 200 °C indicated that OLDH delayed thermal degradation process of PA under N2 atmosphere (Table 2). Unlike of this effect, under air atmosphere, it is seen that the thermo-
Table 1 Tensile properties of PA and PANC and their standard deviations.
PA PANC 2 PANC 5 PANC 8 a b
Tensile strength. Young's modulus.
TS (MPa)a
YM (GPa)b
54 ± 1.71 57 ± 1.53 61 ± 1.23 58 ± 1.42
2.3 ± 0.10 2.5 ± 0.09 3.4 ± 0.05 2.6 ± 0.07 Fig. 7. TGA thermograms of PA and PANC in nitrogen atmosphere.
262
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
Fig. 9. DSC curves of PA and PANC.
Fig. 8. TGA thermograms of PA and PANC in air atmosphere.
oxidative degradation was increased in the nanocomposites as compared to the neat PA. Such variations could be related to catalytic and oxidative degradation effects of LDH on the PA matrix under reactive atmosphere. DSC was used to determine the glass transition temperature values (Tg) of the samples obtained with a heating rate of 10 °C/min under nitrogen atmosphere. The DSC curves of PA and PANC are shown in Fig. 9 and the results are summarized in Table 2. It was found that the addition of OLDH in PA caused a slight increase in the Tg. As shown in Table 2, as the content of OLDH increased to PA matrix from 2 mass% to 5 mass%, the Tg value increased from 138 °C to 143 °C. In general, the enhancement in Tg values by incorporation of LDH can be attributed to the well dispersion of LDH in the PA matrix. On the other hand, the bulky LDH core might restrict the segmental motion of the PA chains, and thus higher temperature was required to provide the thermal energy for the occurrence of glass transition in PANC (Hu et al., 2011). The order of the Tg of samples was PANC 8 = PANC 5 N PANC 2 N PA.
4.5. Combustion properties by MCC Microscale combustion calorimeter (MCC) measures the flammability of materials on milligram quantities and is a small scale flammability testing technique to screen polymer flammability prior to scale-up and is a convenient and relatively new technique, developed in recent years. It was regarded as one of the most effective methods for investigating the combustion properties of polymer materials (Wang et al., 2010;
Shabanian et al., 2013a). The heat release rate, HRR, and its peak value, pHRR, are useful parameters. The peak HRR (pHRR) values of PANC decreased with the increasing OLDH content. As shown in Fig. 10 and Table 3, the neat PA showed a sharp heat release rate region, and exhibited a peak heat release rate (pHRR) of 83.6 W/g. In the HRR curves of PA and the nanocomposites, two peaks related to maximum combustions can be observed. The first (major) peak can be related to PA decomposition and the second peak of HRR can be attributed to the char cracking and oxidation of PA. It was obvious that the pHRR values of both PANC 2 and PANC 5 were lower than that of the neat PA. pHRR of PANC 2 and PANC 5, which were 58.2 W/g and 51.1 W/g, respectively, indicated an improvement in flame retardancy of the material. This reduction in heat release rate was related to the decomposition of OLDH and releases the vapor of water that could cool the pyrolysis area at the combustion surface. In order to find out the real effect of OLDH on combustion properties of PA, the calculated curves of PANC 2 and PANC 5 based on MCC data of OLDH and PA were constructed (Fig. 11). The pHRR value of OLDH was 122.2 W/g. Moreover, the calculated pHRR of the nanocomposites was almost the same as the neat PA. For a comparison purpose the experimental and calculated MCC data are summarized in Table 3. From Table 3, a big difference between experimental and calculated data of the nanocomposites was seen. This indicated that there was a strong interaction between PA and OLDH in the nanocomposites leading to improve the combustion properties.
Table 2 Thermal property data of PA and PANC. Samples
PA PANC 2 PANC 5 PANC 8
Thermal properties in air
Thermal properties in N2
ML (%)b
T5 (°C)c
T10 (°C)d
CY (%)e
ML (%)b
T5 (°C)c
T10 (°C)d
CY (%)e
3.14 3.72 3.38 3.34
226 217 219 220
256 258 263 269
5.15 8.28 10.16 11.35
1.59 1.26 1.16 0.95
275 298 303 311
322 348 349 342
63.10 64.22 65.02 65.03
Tga(°C)
138 141 143 143
a Glass transition temperature (Tg) data were recorded by DSC at a heating rate of 10 °C/min in N2. b Mass loss (%) at 200 °C. c Temperature at 5% mass loss. d Temperature at 10% mass loss. e CY: char yield, mass content of material left after TGA analysis at a maximum temperature of 800 °C.
Fig. 10. HRR curves of PA, PANC 2 and PANC 5.
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264 Table 3 Microscale combustion calorimetry data of OLDH, PA and PANC. Samples
pHRR [W/g]a
pHRR [W/g]b
THR [kJ/g]c
THR [kJ/g]b
OLDH PA PANC 2 PANC 5
122.2 83.6 58.2 51.1
– – 82.1 79.5
11.7 11.1 10.1 8.5
– – 10.2 10.2
a b c
Peak heat release rate. Calculated. Total heat release.
The total heat release (THR) calculated from the area under the HRR curve was also an important parameter for fire hazard evaluation. Compared to the THR value of 11.1 kJ/g in the neat PA, PANC 2 and PANC 5 gave 10.1 and 8.5 kJ/g, respectively. The calculated THR for both PANC 2 and PANC 5 was 10.2 and 10.3 kJ/g, respectively. Considering the experimental and calculated THR data of the nanocomposites it was evident that 2 mass% OLDH had substantial effect on the THR value of PA, whereas, by increasing OLDH content up to 5 mass%, the THR value showed an obvious improvement and decreased to 8.5 kJ/g. All the above results indicated that newly synthesized PA had a good flammability and the introduction of OLDH led to improve flame retardancy of PA. This suggested that the lower HRR of nanocomposites was caused by the reduction of THR and the improvement of flame retardancy of PANC was due to modifications taking place in the condensed phase during polymer combustion (Manzi-Nshuti et al., 2009b). 5. Conclusions New polyamide containing pyridine ring in the main chain and imide group in the side chain with good yield and desired molar mass was synthesized through direct polycondensation reaction. Due to the presence of heterocyclic unit in the polyamide chain, the newly synthesized polyamide had good solubility in organic solvents. New polyamide/LDH nanocomposites with three compositions of the OLDH were prepared via solution intercalation method. TEM and XRD results have revealed the formation of exfoliated OLDH in the PA matrix. The addition of OLDH increased the tensile modulus and tensile strength of the nanocomposites for 2 and 5 mass% content of OLDH as compared to the neat PA. Thermal degradation results in air and nitrogen atmospheres showed that the addition of OLDH in the PA matrix increases the thermal stability and char yields of the resulting PANC. The MCC analysis suggested that the presence of OLDH imposes a positive effect
Fig. 11. Calculated HRR curves of PANC 2 and PANC 5.
263
on the flame retardancy of the neat polyamide, and a decreasing effect on pHRR and THR of the nanocomposites. By increasing the concentration of OLDH, the PANC showed further improvements of flame retardancy. All of the results revealed that the incorporation of OLDH was very efficient in improving the mechanical properties, thermal stability and flame retardancy of PA. Acknowledgments The authors thank the Research Affairs Division of Kharazmi University (project no. 4/2027) for financial support. References Alcantara, A.C.S., Aranda, P., Darder, M., Ruiz-Hitzky, E., 2010. Bionanocomposites based on alginate–zein/layered double hydroxide materials as drug delivery systems. J. Mater. Chem. 20, 9495–9504. Butuc, E., Gherasim, G.M., 1984. Ordered heterocyclic copolymers. Polyamide–imides with S-triazine rings. J. Polym. Sci., Polym. Chem. Ed. 22, 503–507. Díez-Pascual, A.M., Naffakh, M., Marco, C., Ellis, G., Gómez-Fatou, M.A., 2012. Highperformance nanocomposites based on polyetherketones. Prog. Mater. Sci. 57, 1106–1190. Faghihi, K., Mozaffari, Z., 2008. New polyamides based on 2,5-bis[(4-carboxyanilino) carbonyl] pyridine and aromatic diamines: synthesis and characterization. J. Appl. Polym. Sci. 108, 1152–1157. Faghihi, K., Shabanian, M., Izadkhah, A., 2010. Synthesis and characterization of optically active polyamides based on 2-(1,3-isoindolinedione-2-yl)glutaric acid by direct polycondensation. Chin. J. Polym. Sci. 28, 589–596. Gasser, M.S., 2009. Inorganic layered double hydroxides as ascorbic acid (vitamin C) delivery system—intercalation and their controlled release properties. Colloids Surf. B: Biointerfaces 73, 103–109. Ghaemy, M., Nasab, S.M.A., 2011. Synthesis and characterization of organo-soluble polyamides containing triaryl imidazole pendant and ether linkage moieties: thermal, photophysical, and chemiluminescent properties. Polym. Adv. Technol. 22, 2311–2318. Hajipour, A.R., Zahmatkesh, S., Ruoho, A.E., 2008. Microwave-assisted synthesis and characterization of heterocyclic, and optically active poly(amide-imide)s incorporating L-amino acids. Polym. Adv. Technol. 19, 1710–1719. Herrero, M., Benito, P., Labajos, F.M., Rives, V., Zhu, Y.D., Allen, G.C., Adams, J.M., 2010. Structural characterization and thermal properties of polyamide 6.6/Mg, Al/adipateLDH nanocomposites obtained by solid state polymerization. J. Solid State Chem. 183, 1645–1651. Hu, L., Yuan, Y., Shi, W., 2011. Preparation of polymer/LDH nanocomposite by UV-initiated photopolymerization of acrylate through photoinitiator-modified LDH precursor. Mater. Res. Bull. 46, 244–251. Katiyar, V., Gerds, N., Koch, C.B., Risbo, J., Hansen, H.C.B., Plackett, D., 2010. Poly l-lactidelayered double hydroxide nanocomposites via in situ polymerization of l-lactide. Polym. Degrad. Stab. 95, 2563–2573. Kausar, A., Zulfiqar, S., Ahmad, Z., Ilyas Sarwar, M., 2010. Novel thermally stable poly(thiourea–amide–imide)s bearing CS moieties and pyridine units in the backbone: synthesis and properties. Polym. Degrad. Stab. 95, 2611–2618. Kornmann, X., Lindberg, H., Berglund, L.A., 2001. Synthesis of epoxy–clay nanocomposites: influence of the nature of the clay on structure. Polymer 42, 1303–1310. Kotal, M., Kuila, T., Srivastava, S.K., Bhowmick, A.K., 2009. Synthesis and characterization of polyurethane/Mg–Al layered double hydroxide nanocomposites. J. Appl. Polym. Sci. 114, 2691–2699. Kurita, K., Williams, R.L., 1974. Heat-resistant polymers containing bipyridyl units. III. Polyimides having o-, m-, or p-phenylenedioxy linkage. J. Polym. Sci., Polym. Chem. Ed. 12, 1809–1822. Kutlu, B., Schröttner, P., Leuteritz, A., Boldt, R., Jacobs, E., Heinrich, G., 2014. Preparation of melt-spun antimicrobially modified LDH/polyolefin nanocomposite fibers. Mater. Sci. Eng. C 41, 8–16. Liu, J.-G., Wang, L.-F., Yang, H.-X., Li, H.-S., Li, Y.-F., Fan, L., Yang, S.-Y., 2004. Synthesis and characterization of new polybenzimidazopyrrolones derived from pyridine-bridged aromatic tetraamines and dianhydrides. J. Polym. Sci., Part A: Polym. Chem. 42, 1845–1856. Liu, X.-L., Wu, D., Sun, R., Yu, L.-M., Jiang, J.-W., Sheng, S.-R., 2013. Synthesis and properties of novel soluble fluorinated polyamides containing pyridine and sulfone moieties. J. Fluor. Chem. 154, 16–22. Lopez, M.S.-P., Leroux, F., Mousty, C., 2010. Amperometric biosensors based on LDHalginate hybrid nanocomposite for aqueous and non-aqueous phenolic compounds detection. Sensors Actuators B Chem. 150, 36–42. Lv, S., Zhou, W., Miao, H., Shi, W., 2009. Preparation and properties of polymer/LDH nanocomposite used for UV curing coatings. Prog. Org. Coat. 65, 450–456. Ma, T., Zhang, S., Li, Y., Yang, F., Gong, C., Zhao, J., 2010. Synthesis and characterization of novel polyimides derived from 4-phenyl-2, 6-bis [3-(4-aminophenoxy)-phenyl]pyridine diamine and aromatic dianhydrides. Polym. Degrad. Stab. 95, 1244–1250. Madhra, M.K., Salunke, A.K., Banerjee, S., Prabha, S., 2002. Synthesis and properties of fluorinated polyimides, 2. Derived from novel 2,6-bis(3′-trifluoromethyl-paminobiphenyl ether)pyridine and 2,5-bis(3′-trifluoromethyl-p-aminobiphenyl ether)thiophene. Macromol. Chem. Phys. 203, 1238–1248.
264
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
Manzi-Nshuti, C., Hossenlopp, J.M., Wilkie, C.A., 2009a. Comparative study on the flammability of polyethylene modified with commercial fire retardants and a zinc aluminum oleate layered double hydroxide. Polym. Degrad. Stab. 94, 782–788. Manzi-Nshuti, C., Songtipya, P., Manias, E., Jimenez-Gasco, M.M., Hossenlopp, J.M., Wilkie, C.A., 2009b. Polymer nanocomposites using zinc aluminum and magnesium aluminum oleate layered double hydroxides: effects of LDH divalent metals on dispersion, thermal, mechanical and fire performance in various polymers. Polymer 50, 3564–3574. Mehdipour-Ataei, S., Heidari, H., 2003. Synthesis and characterization of novel soluble and thermally stable polyamides based on pyridine monomer. Macromol. Symp. 193, 159–168. Millange, F., Walton, R.I., Lei, L., O'Hare, D., 2000. Efficient separation of terephthalate and phthalate anions by selective ion-exchange intercalation in the layered double hydroxide Ca2Al(OH)6·NO3·2H2O. Chem. Mater. 12, 1990–1994. Nyambo, C., Songtipya, P., Manias, E., Jimenez-Gasco, M.M., Wilkie, C.A., 2008. Effect of MgAl-layered double hydroxide exchanged with linear alkyl carboxylates on fireretardancy of PMMA and PS. J. Mater. Chem. 18, 4827–4838. Purohit, P.J., Wang, D.-Y., Emmerling, F., Thünemann, A.F., Heinrich, G., Schönhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer 53, 2245–2254. Qiu, L., Chen, W., Qu, B., 2006. Morphology and thermal stabilization mechanism of LLDPE/MMT and LLDPE/LDH nanocomposites. Polymer 47, 922–930. Shabanian, M., Basaki, N., 2013. New photosensitive semi aramid/organoclay nanocomposite containing cinnamoyl groups: synthesis and characterization. Compos. Part B 52, 224–232. Shabanian, M., Kang, N.-J., Wang, D.-Y., Wagenknecht, U., Heinrich, G., 2013a. Synthesis of aromatic–aliphatic polyamide acting as adjuvant in polylactic acid (PLA)/ammonium polyphosphate (APP) system. Polym. Degrad. Stab. 98, 1036–1042. Shabanian, M., Kang, N.-J., Wang, D.-Y., Wagenknecht, U., Heinrich, G., 2013b. Synthesis, characterization and properties of novel aliphatic–aromatic polyamide/functional carbon nanotube nanocomposites via in situ polymerization. RSC Adv. 3, 20738–20745. Shabanian, M., Basaki, N., Khonakdar, H.A., Jafari, S.H., Hedayati, K., Wagenknecht, U., 2014a. Novel nanocomposites consisting of a semi-crystalline polyamide and Mg–Al LDH: morphology, thermal properties and flame retardancy. Appl. Clay Sci. 90, 101–108. Shabanian, M., Kang, N., Liu, J., Wagenknecht, U., Heinrich, G., Wang, D.-Y., 2014b. Biobased semi-aromatic polyamide/functional clay nanocomposites: preparation and properties. RSC Adv. 4, 23420–23427. Shabbir, S., Zulfiqar, S., Ahmad, Z., Sarwar, M.I., 2010. Pyrimidine based carboxylic acid terminated aromatic and semiaromatic hyperbranched polyamide–esters: synthesis and characterization. Tetrahedron 66, 7204–7212.
Sinha Ray, S., Okamoto, M., 2003. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog. Polym. Sci. 28, 1539–1641. Tamami, B., Yeganeh, H., 2001. Synthesis and characterization of novel aromatic polyamides derived from 4-aryl-2,6-bis(4-aminophenyl) pyridines. Polymer 42, 415–420. Wang, Q., O'Hare, D., 2012. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 112, 4124–4155. Wang, Y.-C., Fan, S.-C., Lee, K.-R., Li, C.-L., Huang, S.-H., Tsai, H.-A., Lai, J.-Y., 2004. Polyamide/SDS–clay hybrid nanocomposite membrane application to water– ethanol mixture pervaporation separation. J. Membr. Sci. 239, 219–226. Wang, H.-W., Dong, R.-X., Liu, C.-L., Chang, H.-Y., 2007. Effect of clay on properties of polyimide–clay nanocomposites. J. Appl. Polym. Sci. 104, 318–324. Wang, D.-Y., Costa, F.R., Vyalikh, A., Leuteritz, A., Scheler, U., Jehnichen, D., Wagenknecht, U., Häussler, L., Heinrich, G., 2009. One-step synthesis of organic LDH and its comparison with regeneration and anion exchange method. Chem. Mater. 21, 4490–4497. Wang, D.-Y., Leuteritz, A., Wang, Y.-Z., Wagenknecht, U., Heinrich, G., 2010. Preparation and burning behaviors of flame retarding biodegradable poly(lactic acid) nanocomposite based on zinc aluminum layered double hydroxide. Polym. Degrad. Stab. 95, 2474–2480. Wang, D.-Y., Das, A., Leuteritz, A., Boldt, R., Häußler, L., Wagenknecht, U., Heinrich, G., 2011a. Thermal degradation behaviors of a novel nanocomposite based on polypropylene and Co–Al layered double hydroxide. Polym. Degrad. Stab. 96, 285–290. Wang, D.-Y., Leuteritz, A., Kutlu, B., Landwehr, M.A.d, Jehnichen, D., Wagenknecht, U., Heinrich, G., 2011b. Preparation and investigation of the combustion behavior of polypropylene/organomodified MgAl-LDH micro-nanocomposite. J. Alloys Compd. 509, 3497–3501. Wang, D.-Y., Das, A., Leuteritz, A., Mahaling, R.N., Jehnichen, D., Wagenknecht, U., Heinrich, G., 2012. Structural characteristics and flammability of fire retarding EPDM/layered double hydroxide (LDH) nanocomposites. RSC Adv. 2, 3927–3933. Wang, Q., Tang, S.V.Y., Lester, E., O'Hare, D., 2013. Synthesis of ultrafine layered double hydroxide (LDHs) nanoplates using a continuous-flow hydrothermal reactor. Nanoscale 5, 114–117. Yamazaki, N., Matsumoto, M., Higashi, F., 1975. Studies on reactions of the Nphosphonium salts of pyridines. XIV. Wholly aromatic polyamides by the direct polycondensation reaction by using phosphites in the presence of metal salts. J. Polym. Sci., Polym. Chem. Ed. 13, 1373–1380. Zulfiqar, S., Sarwar, M.I., 2009. Investigating the structure–property relationship of aromatic–aliphatic polyamide/layered silicate hybrid films. Solid State Sci. 11, 1246–1251.
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Zn–AL LDH reinforced nanocomposites based on new polyamide containing imide group: From synthesis to properties Mohsen Hajibeygi a,⁎, Meisam Shabanian b, Hossein Ali Khonakdar c a b c
Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), P.O. Box 31745-139, Karaj, Iran Department of Polymer Engineering, Faculty of Engineering, South Tehran Branch, Islamic Azad University, P.O. Box 19585-466, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 3 April 2015 Received in revised form 4 June 2015 Accepted 8 June 2015 Available online xxxx Keywords: Polyamide Nanocomposite Layered double hydroxide Thermal stability Flame retardant Mechanical properties
a b s t r a c t A new series of polyamide/Zn–Al layered double hydroxide (LDH) nanocomposites (PANC) was prepared by solution intercalation technique under ambient condition in dimethylformamide as solvent. The polyamide (PA) containing pyridine ring and imide group was synthesized using direct polycondensation reaction with good solubility and desired molar mass. Organo-modified Zn–Al LDH (OLDH) was produced by one-step method and used to improve mechanical, thermal and flame properties of PA. The extent of dispersion of OLDH sheets was quantified by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and the results showed a good dispersion for OLDH in the PA matrix. According to the results of mechanical tests, the tensile strength and the Young's modulus of PANC enhanced with increasing OLDH content. Thermal properties of PANC were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal property results in both nitrogen and air atmospheres showed that the addition of OLDH resulted in a substantial increase in the thermal stability and char yields of PANC as compared to the neat PA. Significant improvements in flame retardancy performance were observed for PANC from microscale combustion calorimeter (MCC) (reducing both the heat release rate and the total heat released). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polymeric nanocomposite materials have attracted increasing attention because of their potential in terms of improving thermal stability, flame retardancy, mechanical properties and film barrier characteristics with small nanoparticles loading in organic polymers. In recent years, many studies were reported about layered nanostructure materials as one of the potential candidates for polymeric nanocomposite preparation because of their large value of aspect ratio, diameter in nanometer range, thermal stability and other good properties even at low concentrations (Sinha Ray and Okamoto, 2003; Wang et al., 2007; Katiyar et al., 2010; Shabanian et al., 2014a). Layered double hydroxides (LDHs), also called anionic clays, were considered as a class of clays which have a promising future in the field of nanocomposites due to their various unique properties. LDH can be prepared from a wide range of various metallic compositions (Kutlu et al., 2014). Chemical structure of LDH was found to be a stack of positively charged metal hydroxide layers with intercalated counteranions and water molecules (Wang et al., 2011b). LDH was used in various fields such as catalysts (Lopez et al., 2010; Wang and O'Hare, 2012), ion exchange hosts (Millange et al., 2000), drug delivery ⁎ Corresponding author. Tel.: +98 9122391983; fax: +98 2188257969. E-mail addresses: [email protected], [email protected] (M. Hajibeygi).
http://dx.doi.org/10.1016/j.clay.2015.06.008 0169-1317/© 2015 Elsevier B.V. All rights reserved.
(Alcantara et al., 2010), hydrothermal reactor (Wang et al., 2013) and fire retardant additives (Nyambo et al., 2008; Manzi-Nshuti et al., 2009a). Recently many researches have been focused on polymer nanocomposites containing LDH (Lv et al., 2009; Wang et al., 2011a, 2011b; Purohit et al., 2012) and significant synergistic effects were observed in both thermal stability and flame retardancy for formulations containing polymer matrices with modified Mg–Al and Zn–Al LDH as nanofillers (Nyambo et al., 2008; Manzi-Nshuti et al., 2009a; Wang et al., 2012). Due to the increasing demand for high-performance polymers as a good candidate for matrix nanocomposites, thermally stable polymers such as polyamides have attracted increasing interest over the past decade (Díez-Pascual et al., 2012; Shabanian et al., 2013b). Polyamides are one of the most versatile classes of high performance polymers which are used in wide range of applications and display unique properties, but they suffer from disadvantages such as low solubility in organic solvents and having high melting and/or glass transition temperatures, which cause some restrictions in their processing. However, these problems can be solved or reduced by introducing some flexible bonds such as aliphatic chains (Shabanian and Basaki, 2013), pendant bulky group (Hajipour et al., 2008; Ghaemy and Nasab, 2011) and heteroaromatic rings in the polyamide backbone (Liu et al., 2013). Among these approaches, introduction of heteroaromatic rings into the backbone of polyamides has been considered in some reports
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
(Mehdipour-Ataei and Heidari, 2003; Faghihi and Mozaffari, 2008; Kausar et al., 2010; Shabbir et al., 2010; Shabanian et al., 2014b). Pyridine is a heteroaromatic molecule with polarizability and rigidity. Some kinds of heteroaromatic structures containing pyridine unit have been designed and synthesized, and the heteroaromatic polymers have been synthesized from those monomers containing pyridine nucleus structures at the same time (Liu et al., 2004; Ma et al., 2010). In general, the heteroaromatic rings in the polymer backbone would impart certain properties to it, while pyridine with heteroaromatic structure would have excellent stabilities derived from its structural symmetry and aromaticity of pyridine ring (Madhra et al., 2002). Also polarizability resulting from nitrogen of pyridine ring could be suitable to improve solubility of polymer in organic solvents (Kurita and Williams, 1974; Butuc and Gherasim, 1984). The aim of present work was the preparation of new reinforced nanocomposites based on OLDH and a polyamide containing pyridine and imide heterocyclic rings through solution intercalation technique. From the presence of the heterocyclic derivatives and the aliphatic groups in the polyamide structure it was expected to provide acceptable glass transition and good solubility in organic solvents. In this work, the effect of OLDH on the mechanical properties, thermal stability and flame retardancy of the developed nanocomposites was also described.
2. Experimental
257
0.05° and slit aperture of 1°. XRD patterns of the samples were measured randomly in a continuous mode with a scan speed of 0.005°s−1. Elemental analyses were performed by Vario EL equipment. The morphological analysis was carried out using a LEO 912 transmission electron microscopy (TEM) operated at room temperature with an acceleration voltage of 200 kV in a brightfield illumination. Mechanical properties were performed at room temperature on a Testometric Universal Testing Machine M350/500 (Mainz, Germany); rate, 3 mm/min; according to ASTM D882 (standards). The mechanical properties were repeated five times. The average values along with standard deviations were reported. Thermal gravimetric analysis (TGA) data for polymers were taken on a Mettler TA4000 System under N2 and air atmosphere at rate of 10 °C/min. Differential scanning calorimeter (DSC) was conducted with DSC, Mettler 110 (Switzerland) at heating rate of 10 °C/min in nitrogen atmosphere. The flame retardancy properties were analyzed by microscale combustion colorimeter (MCC) that is a convenient and relatively new technique. The average values along with standard deviations were reported. The detailed procedure was as follows: about 5 mg of samples was heated to 700 °C with a heating rate of 1°Cs− 1 in a stream of nitrogen flowing at 80 cm3/min. Then, the volatile anaerobic thermal degradation products were mixed with 20 cm3/min gas stream containing 20% oxygen and 80% nitrogen, respectively, prior to entering to a 900 °C combustion furnace. All measurements were repeated at least in duplicate.
2.1. Materials 2.3. Monomer synthesis Glutamic acid, phthalic anhydride, 4-hydroxy-3-methoxybenzaldehyde (vanillin), 4-nitroacetophenone, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), palladium charcoal (Pd/C), hydrazine monohydrate, pyridine (Py) and triphenyl phosphite (TPP) from Merck were used without further purification. Commercially available calcium chloride (CaCl2, Merck) was dried under vacuum at 150 °C for 6 h. Zinc nitrate, aluminum nitrate, sodium hydroxide, and sodium dodecylbenzene sulfonate (SDBS) used for synthesis of OLDH by one-step route were also obtained from Merck.
2.2. Measurements 1
H-NMR spectra were recorded by a Bruker 300 MHz instrument (Germany). Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer RXI spectrometer. The KBr pellet technique was applied for monitoring changes in the range of 400–4000 cm−1 with a resolution of 2 cm−1. Approximately 3 mg of the powdered components was mixed with 100 mg KBr and pressed into pellets and used for further characterization. Vibration transition frequencies were reported in wavenumber (cm− 1). Band intensities were assigned as weak (w), medium (m), shoulder (sh), strong (s) and broad (br). Inherent viscosity was measured at a concentration of 0.5 g/dL in DMF at 25 °C by a standard procedure using a Technico Regd Trade Mark Viscometer. Molar mass (mass-average (Mw ) and number-average (Mn )) determination was performed in size exclusion chromatography (SEC) using Agilent Series 1100 (Agilent, USA) system consisting of a pump, degasser and differential refractive index (RI) detector. Two Zorbax PSM Trimodal-S 250 mm × 6.2 mm columns (Rockland Tech, USA) were used. The measurements were performed using a mixed eluent DMAc with 2 vol.% water and 3 g/L LiCl at a flow rate of 0.5 mL/min. The molar mass was calculated after calibration with poly(2vinylpyrrolidone) standards. Wide angle X-ray scattering (WAXS) was performed using 2-circle diffractometer XRD 3003 (GE Inspection Technologies/Seifert-FPM, Freiberg, Germany) using Cu–Kα radiation (λ = 0.154 nm) generated at 30 mA and 40 kV in the range of 2θ = 2°–12° with a step length of
2.3.1. Synthesis of diacid 2-(1,3-Dioxoisoindolin-2-yl)pentanedioic acid as a diacid monomer was synthesized by the condensation reaction of phthalic anhydride and glutamic acid in an acetic acid solution according to previous research (Faghihi et al., 2010). 2.3.2. Synthesis of diamine 4-(2,6-Bis(4-aminophenyl)pyridine-4-yl)-2-methoxyphenol as a new diamine compound containing pyridine ring was synthesized by two step reactions according to previous researches (Tamami and Yeganeh, 2001; Hajipour et al., 2008; Shabanian et al., 2014b) as follows; Synthesis of 4-(2,6-bis(4-nitrophenyl)pyridin-4-yl)-2-methoxyphenol; 1.08 g (6.56 mmol) of 4-nitroacetophenone, 0.5 g (3.28 mmol) of vanillin, 6 g of ammonium acetate and 12 mL of glacial acetic acid were added into 100 mL round-bottom flask. The reaction mixture was heated in an oil bath at 120 °C for 18 h. Upon cooling, the precipitated yellow solid was collected by filtration and washed with ethanol. FTIR (KBr): 3531 (m), 1593 (s), 1547 (m), 1513 (s), 1439 (m), 1347 (s), 1271 (m), 1209 (m), 1031 (m, sh), 846 (s), 815 (m), 735 (m), and 691 (m) cm−1. 1H-NMR (DMSO-d6, TMS) δ: 9.50 (s, 1H), 8.60 (d, 4H), 8.36 (d, 4H), 8.35 (s, 2H), 7.61 (s, 1H), 7.56 (d, 1H), 6.93 (d, 1H), and 3.94 (s, 3H) ppm. Synthesis of 4-(2,6-bis(4-aminophenyl)pyridin-4-yl)-2-methoxyphenol; 1.62 g (3.67 mmol) of dinitro, 0.1 g of 10% Pd/C, 20 mL of ethanol, 5 mL of DMF and 3 mL of hydrazine monohydrate were added into a 100 mL round-bottomed flask. The reaction mixture was heated in an oil bath at 90 °C for 5 h. Then, the mixture was filtered to remove the Pd/C and the filtrate was poured into water and dried. FTIR (KBr): 3434 (m, sh), 3352 (m), 3212 (w), 3031 (w), 2934 (w), 1607 (s), 1596 (s), 1513 (s), 1437 (m), 1393 (m), 1269 (m), 1179 (m), 1126 (m), 831 (m), and 581 (m) cm−1. 1H-NMR (DMSO-d6, TMS) δ: 9.30 (s, 1H), 7.97–8.00 (d, 4H), 7.72 (s, 2H), 7.43 (s, 1H), 7.35–7.38 (d, 1H), 6.88–6.91 (d, 1H), 6.65–6.68 (d, 4H), 5.39 (s, 4H), and 3.91
258
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
(s, 3H) ppm. Elemental analysis: calculated for C 24H21 N3 O 2 (348.4 g/mol), calculated: C, 75.18; H, 5.52; and N, 10.96; found: C, 75.17; H, 5.52; and N, 10.96. 2.3.3. Synthesis of polyamide 0.941 g of diacid (3.39 mmol), 1.20 g of 4-(4-hydroxyphenyl)-2,6bis(4-aminophenyl)pyridine (3.39 mmol), calcium chloride (0.90 g), triphenyl phosphite (1.76 mL), pyridine (0.54 mL) and N-methyl-2pyrrolidone (10 mL) were added into 100 mL round-bottom flask. The reaction mixture was heated in an oil bath at 60 °C for 1 h, 90 °C for 3 h and 120 °C for 6 h. Subsequently, the reaction mixture was poured into 100 mL of methanol and the precipitated PA was collected by filtration and washed thoroughly with hot methanol. Finally, the product was dried at 70 °C for 12 h inside a vacuum oven to leave 2.51 g (97.6%) yellow solid PA. ηinh (inherent viscosity, measured at a concentration of 0.5 g/dL in DMF at 25 °C): 0.71 (dL/g). Elemental analysis: calculated for C36H26N4O5 (594.62 g/mol), calculated: C, 72.72; H, 4.41; and N, 9.42; found: C, 71.89; H, 4.38; and N, 9.41. 2.3.4. Preparation of organo-modified Zn–Al LDH (OLDH) OLDH was prepared in one step according to the procedure reported elsewhere (Wang et al., 2009). Briefly, a solution of Zn(NO3)2 and Al(NO3)3 (with Zn2+:Al3+ equal to 2:1 and a total metal ion concentration of 0.3 M) was added to a SDBS solution slowly. During the preparation of OLDH, solution was stirred at 50 °C and the pH value was kept at 10.5 ± 0.2 by adding a suitable amount of solution of sodium hydroxide. After the addition of the metal salt solution, the resulting mixture was stirred at 50 °C for half an hour. Then the temperature was increased to 70 °C and allowed to age for 24 h. Upon cooling the final white product was collected by filtration and washed thoroughly with distilled water until a solution with pH = 7 was obtained. The product was then dried in an oven at 75 °C. 2.3.5. Synthesis of PA/Zn–Al LDH nanocomposites The nanocomposites were synthesized by taking the PA solution in a flask, followed by the addition of a known proportion of OLDH for particular concentrations. The amounts of OLDH were 2, 5 and 8 mass% in the nanocomposites (PANC 2, PANC 5 and PANC 8). To prepare nanocomposite containing 2 mass% OLDH, 0.98 g of PA was dissolved in 10 mL DMF, followed by the addition of 0.02 g of OLDH. The reaction mixture was agitated to high speed at 25 °C overnight to disperse OLDH platelets uniformly in the PA matrix. The nanocomposites were caste by pouring the hybrid solution in Petri dishes and removed the solvent at 80 °C for 10 h and were further dried at 80 °C under vacuum to a constant mass.
Fig. 1. 1H-NMR spectrum of aromatic diamine.
presented in Fig. 1, and the spectrum proved the proposed aromatic diamine structure.
3.2. Polymer synthesis In order to prepare the matrix of nanocomposites, the Yamazaki method (Yamazaki et al., 1975) (triphenyl phosphite (TPP)-activated polycondensation) was used to synthesize PA containing imide group in the side chain and pyridine moiety (Scheme 1). The solubility of PA was investigated with 0.01 g of polymeric sample in 2 mL of solvent. Due to the presence of bulky pendant groups in PA, it had good solubility in organic solvents at room temperature. The polymer was dissolved in polar organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc), and N-methyl-2-pyrrolidone (NMP) at room temperature and was insoluble in protic solvents such as methanol, ethanol, and water. Also, newly synthesized PA owned number-average molar mass ( Mn ) and mass-average molar mass ( Mw ); 2.5 × 104 and
3. Results and discussion 3.1. Diamine synthesis The new aromatic diamine monomer, 4-(2,6-bis(4aminophenyl)pyridin-4-yl)-2-methoxyphenol was prepared by a twostep synthetic route. Firstly, the dinitro compound containing pyridine heterocyclic and methoxy pendant group was synthesized via a modified Chichibabin pyridine synthesis (Tamami and Yeganeh, 2001; Hajipour et al., 2008) from vanillin and 4-nitroacetophenone in the presence of ammonium acetate. Then, the aromatic diamine was obtained by catalytic hydrogenation of the dinitro compound using hydrazine monohydrate and Pd/C catalyst in ethanol. The structure of new aromatic diamine was confirmed by elemental analysis, FTIR and 1H-NMR spectroscopies. In the 1H-NMR spectrum of diamine (Fig. 1), the signals of aromatic protons appeared in the range of 6.65–8.00 ppm, and the characteristic resonance signal at 5.39 ppm was due to the amino group. Assignments of each proton were also
Scheme 1. Synthesis route of PA.
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
259
Fig. 2. 1H-NMR spectrum of PA.
5.7 × 104 (poly dispersity index (PDI) = 2.28) respectively, as measured by SEC. The structure and purity of newly synthesized PA were confirmed by elemental analysis, FTIR and 1H-NMR spectroscopies. The elemental analysis of PA was in good agreement with the calculated values for the proposed structure (see Experimental section). The 1H-NMR spectrum of PA showed peaks that confirm its chemical structure (Fig. 2). The aromatic protons appeared in the region of 6.91–8.26 ppm. The proton related to chiral center appeared at 4.97 ppm. The O\\H group related to diamine appeared as a broad peak at around 9.9 ppm. Two peaks at 10.05 ppm and 10.15 ppm in 1H-NMR spectrum of PA were related to two antisymmetric N\\H groups in the main chain of PA. 3.3. Preparation of PA/Zn–Al LDH nanocomposites In general, LDH is employed to prepare different organic–inorganic nanohybrid materials and has received significant attention. The organic/LDH nanohybrids have been studied because the resulting
Fig. 3. FTIR spectra of unmodified LDH, OLDH, PA and PANC.
intercalation compounds are expected to possess a new nanostructure (Gasser, 2009). In this study SDBS was used as building blocks for efficient preparation of potentially nanohybrid material of OLDH by one-step route according to previous research (Shabanian et al., 2014a). As a possible model, SDBS was considered to be arranged vertically and/or horizontally to the LDH basal layer. In comparison to the neat LDH, the layers of OLDH changed from hydrophilic to organophilic and its interlayer space increased. These phenomena caused enlargement in the d-value which facilitates the exfoliation of the plates of the OLHD in the polymer matrix. New nanocomposites containing 2, 5 and 8 mass% of OLDH were prepared using OLDH and PA containing imide groups in the main chains in dry DMF through solution intercalation technique (Scheme 2).
Scheme 2. Preparation of PANC.
260
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
stretching vibration), and 1071 and 718 cm−1 (imide ring). Due to the presence of O\\H group in the polymer chains and hydrogen band formation, the peaks related to N\\H and O\\H groups in the polymer chain were appeared as a broad peak. Fig. 3 also showed the FTIR spectra of PANC. The FTIR spectra of nanocomposites are dominated by the bands of the polyamide, but the presence of the absorption bands of OLDH is also seen. FTIR spectra of the nanocomposites showed the characteristic absorption bands for the carbonyl groups around 1776 cm− 1 due to the antisymmetric imide ring stretching vibration and two overlapped around 1717 cm−1 related to symmetric imide ring and amide group stretching. The characteristic bands around 2850–2930 cm−1 are due to aliphatic groups related to the OLDH. By comparison of these spectra, it can be concluded that the nanocomposites not only have characteristic neat PA bands but also have characteristic bands for OLDH. These data, in sum, confirmed the formation of the PA/Zn–Al LDH nanocomposites. 4.2. Structural characterization
Fig. 4. XRD pattern of OLDH, PA and PANC.
4. Characterization of the PA/LDH nanocomposites 4.1. FT-IR spectra FTIR spectra of unmodified LDH, OLDH, PA and PANC are shown in Fig. 3. In the spectrum of unmodified LDH, the broad characteristic band centered at 3421 cm−1 related to the O\\H stretching vibrations of surface and interlayer water molecules. Also, the characteristic band at 1352 cm−1 was related to NO3 stretching vibrations. In the spectrum of OLDH, the characteristic band related to NO3 was vanished and the characteristic vibration bands for SO3 were appeared. Also, the broad and strong bands in the range of 3200–3650 cm−1 centered at 3475 cm−1 were due to the O\\H stretching vibrations of surface and interlayer water molecules. The band observed near 1601 cm−1 was related to the bending vibration of water molecule. A strong absorption band at 2854 cm−1, 2926 cm−1 and 2956 cm−1 was detected for alkyl groups (CH3 and CH2) in SDBS. The characteristic vibration bands were detected for SO3 stretching (symmetric stretching at 1037 cm−1 and antisymmetric at 1175 cm−1), the benzene group (C\\C stretching at 1466 cm− 1, C\\H in plane bending at 1010 and 1130 cm− 1). The bands at 760 and 618 cm− 1 can be assigned to Al\\O stretching modes. These FTIR characteristic bands demonstrated that the SDBS chains had been intercalated into the interlayer space of Zn–Al LDH. The FTIR spectrum of PA (Fig. 3) showed absorption bands around 3258 cm−1 (N\\H), 1776 and 1717 cm−1 for the imide ring (antisymmetric and symmetric C_O stretching vibrations), 1385 cm−1 (C\\N
Wide angle X-ray scattering (WAXS) and TEM analyses are generally used to analyze the structure and morphology of the layered structure of nanomaterials. X-ray diffraction is one of the techniques to characterize the layered structure of the polymer nanocomposites, an easy way to access the interlayer spacing of the LDH in polymer nanocomposites. In comparison, TEM analysis gives direct evidence on the dispersion of the nanoparticles in a nanocomposite XRD patterns of the one-step prepared OLDH, PA and PANC are shown in Fig. 4. The interlayer distance of OLDH can be calculated from Bragg diffraction law. The XRD pattern of OLDH pointed out to a multilayer structure due to the basal reflection maximum (003) at 2θ = 3.06° (d-value: 2.88 nm). In the XRD pattern of PA, no sharp characteristic reflections were observed and this indicated that this polyamide is amorphous (Fig. 4). In comparison to basal reflections of OLDH for nanocomposites, there were almost no reflections corresponding to crystal structure of OLDH. Therefore, dispersion of OLDH sheets in polyamide matrix can be expected (Herrero et al., 2010). In addition, the disappearance of the reflections related to OLDH in the XRD patterns of the PANC was probably due to the loss of crystalline symmetry in the stacking direction of the hydroxide layers, lowering the number layers of hydroxide and much insertion of polymer chains into the interlayer space of OLDH (Kotal et al., 2009). Disappearance of characteristic reflections in XRD pattern does not always confirm the good dispersion and exfoliated structure in nanocomposites, because XRD is unable to detect regular stacking exceeding 8.8 nm (Kornmann et al., 2001), therefore, more direct information can be obtained from TEM. In order to confirm the dispersion of OLDH sheets in PA matrix, TEM investigations were done on PANC 2 and PANC 5 samples and the results are presented in Fig. 5. The dark lines
Fig. 5. TEM micrographs of a) PANC 2 and b) PANC 5.
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
261
represent the LDH layers, whereas the bright area represents the polyamide matrix. The TEM images of PANC showed a good dispersion of OLDH sheets in the polyamide matrix, providing a direct evidence of crystal layer exfoliation. These results are supported by the absence of reflections for PANC in XRD pattern. 4.3. Mechanical properties Mechanical properties of PA and PANC, including the tensile strength and Young's modulus, were measured and the results are listed in Table 1. Also, the relations between tensile strength and Young's modulus of nanocomposites and OLDH contents are shown in Fig. 6. The increase of OLDH content up to 5 mass% to PA and tensile strength of PANC had a linear relationship. The tensile strength increased from 54 to 61 MPa, which was about 13% higher than that of neat PA, with the addition of OLDH content within 5 mass% in comparison with the neat PA. This can be attributed to the reinforcement effect attained by the OLDH through random dispersion in the PA matrix and/or the PA chains in the nanocomposites were restricted by the OLDH sheets, resulting in the decreased degree of freedom (Wang et al., 2004). The Young's modulus values performed the same significant changes of the tensile strength values. The Young's modulus of PANC 5 is the highest one among them and reached 3.4 GPa, which is 47% and 36% higher than those of neat PA and PANC 2 samples, respectively. The maximum increase in Young's modulus was noted with 5 mass% OLDH, which then decreased with OLDH content up to 8 mass% in PA matrix. This behavior in mechanical properties was also observed in tensile strength of neat PA as compared to PANC. This phenomenon might be due to the increased amount of OLDH in PA matrix. At high OLDH loading, OLDH sheets may stack together and interlayer space does not increase well, as it should, thus deteriorating the properties of composites (Zulfiqar and Sarwar, 2009). Nevertheless, all of these results showed a good improvement in mechanical properties of PANC in low concentration of OLDH as compared to the neat PA. 4.4. Thermal properties Thermal stability is a very important factor for characterization of polymeric materials, and normally is indicated by thermogravimetric analysis (TGA). The TGA thermograms of neat PA and PANC in nitrogen and air atmospheres are presented in Figs. 7 and 8, respectively, and the resulting TGA data are summarized in Table 2 including temperatures at which 5% (T5) degradation and 10% (T10) degradation occur and the residue at 800 °C. The TGA thermogram exhibited T5, T10 and char yield for neat PA, 275 °C, 322 °C and 63.1% under N2 atmosphere, respectively. Considering the TGA thermograms of nanocomposites, it was evident that OLDH had good effect on the thermal properties of PA. By incorporation of 2 mass% of OLDH to PA, T5 increased by about 20 °C reaching to 298 °C. T10 also shifted to higher temperature by about 25 °C in PANC 2 compared to neat PA. These thermal properties were similar to previous reports for different nanocomposites, and it was explained by the relative extent of exfoliation and/or delamination processes as a function of the amount of OLDH (Qiu et al., 2006; Shabanian et al., 2014a). Also, the results suggested that an excess of OLDH in the polymer matrix might restricted the mobility of PA chains.
Fig. 6. Tensile strength and Young's modulus of PANC with various OLDH contents.
By increasing the OLDH content to 5 and 8 mass%, no further improvements were observed with these parameters, insofar as, T10 shifted to lower temperature in PANC 8 as compared to PANC 5. The reason could be that OLDH has a role for catalytic degradation of PA. This behavior in thermal properties was observed in char yields of neat PA as compared to PANC. TGA thermograms of PA and PANC under air atmosphere are shown in Fig. 8 and the corresponding data are summarized in Table 2. The destabilization effect of oxygen is noted comparing TGA curves of the polymeric materials in air and nitrogen, due to the reactions of the degrading polymer radicals with oxygen in air (Manzi-Nshuti et al., 2009b). As shown in Table 2, neat PA in air atmosphere has T5 and T10 of 226 and 256 °C, respectively with final residue of 5.15%. Also, it can be observed that the PANC have lower T5 than neat PA, but in all PANC by increasing the OLDH content from 2 to 8 mass%, T10 and char yields were increased. This implied that the incorporation of OLDH in PA matrix helped in char formation and the improvement can be attributed to the presence of OLDH, which plays barrier effect to maximize the insulation and to minimize the permeability of volatile degradation products (Herrero et al., 2010). Also, the mass loss (%) at 200 °C indicated that OLDH delayed thermal degradation process of PA under N2 atmosphere (Table 2). Unlike of this effect, under air atmosphere, it is seen that the thermo-
Table 1 Tensile properties of PA and PANC and their standard deviations.
PA PANC 2 PANC 5 PANC 8 a b
Tensile strength. Young's modulus.
TS (MPa)a
YM (GPa)b
54 ± 1.71 57 ± 1.53 61 ± 1.23 58 ± 1.42
2.3 ± 0.10 2.5 ± 0.09 3.4 ± 0.05 2.6 ± 0.07 Fig. 7. TGA thermograms of PA and PANC in nitrogen atmosphere.
262
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
Fig. 9. DSC curves of PA and PANC.
Fig. 8. TGA thermograms of PA and PANC in air atmosphere.
oxidative degradation was increased in the nanocomposites as compared to the neat PA. Such variations could be related to catalytic and oxidative degradation effects of LDH on the PA matrix under reactive atmosphere. DSC was used to determine the glass transition temperature values (Tg) of the samples obtained with a heating rate of 10 °C/min under nitrogen atmosphere. The DSC curves of PA and PANC are shown in Fig. 9 and the results are summarized in Table 2. It was found that the addition of OLDH in PA caused a slight increase in the Tg. As shown in Table 2, as the content of OLDH increased to PA matrix from 2 mass% to 5 mass%, the Tg value increased from 138 °C to 143 °C. In general, the enhancement in Tg values by incorporation of LDH can be attributed to the well dispersion of LDH in the PA matrix. On the other hand, the bulky LDH core might restrict the segmental motion of the PA chains, and thus higher temperature was required to provide the thermal energy for the occurrence of glass transition in PANC (Hu et al., 2011). The order of the Tg of samples was PANC 8 = PANC 5 N PANC 2 N PA.
4.5. Combustion properties by MCC Microscale combustion calorimeter (MCC) measures the flammability of materials on milligram quantities and is a small scale flammability testing technique to screen polymer flammability prior to scale-up and is a convenient and relatively new technique, developed in recent years. It was regarded as one of the most effective methods for investigating the combustion properties of polymer materials (Wang et al., 2010;
Shabanian et al., 2013a). The heat release rate, HRR, and its peak value, pHRR, are useful parameters. The peak HRR (pHRR) values of PANC decreased with the increasing OLDH content. As shown in Fig. 10 and Table 3, the neat PA showed a sharp heat release rate region, and exhibited a peak heat release rate (pHRR) of 83.6 W/g. In the HRR curves of PA and the nanocomposites, two peaks related to maximum combustions can be observed. The first (major) peak can be related to PA decomposition and the second peak of HRR can be attributed to the char cracking and oxidation of PA. It was obvious that the pHRR values of both PANC 2 and PANC 5 were lower than that of the neat PA. pHRR of PANC 2 and PANC 5, which were 58.2 W/g and 51.1 W/g, respectively, indicated an improvement in flame retardancy of the material. This reduction in heat release rate was related to the decomposition of OLDH and releases the vapor of water that could cool the pyrolysis area at the combustion surface. In order to find out the real effect of OLDH on combustion properties of PA, the calculated curves of PANC 2 and PANC 5 based on MCC data of OLDH and PA were constructed (Fig. 11). The pHRR value of OLDH was 122.2 W/g. Moreover, the calculated pHRR of the nanocomposites was almost the same as the neat PA. For a comparison purpose the experimental and calculated MCC data are summarized in Table 3. From Table 3, a big difference between experimental and calculated data of the nanocomposites was seen. This indicated that there was a strong interaction between PA and OLDH in the nanocomposites leading to improve the combustion properties.
Table 2 Thermal property data of PA and PANC. Samples
PA PANC 2 PANC 5 PANC 8
Thermal properties in air
Thermal properties in N2
ML (%)b
T5 (°C)c
T10 (°C)d
CY (%)e
ML (%)b
T5 (°C)c
T10 (°C)d
CY (%)e
3.14 3.72 3.38 3.34
226 217 219 220
256 258 263 269
5.15 8.28 10.16 11.35
1.59 1.26 1.16 0.95
275 298 303 311
322 348 349 342
63.10 64.22 65.02 65.03
Tga(°C)
138 141 143 143
a Glass transition temperature (Tg) data were recorded by DSC at a heating rate of 10 °C/min in N2. b Mass loss (%) at 200 °C. c Temperature at 5% mass loss. d Temperature at 10% mass loss. e CY: char yield, mass content of material left after TGA analysis at a maximum temperature of 800 °C.
Fig. 10. HRR curves of PA, PANC 2 and PANC 5.
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264 Table 3 Microscale combustion calorimetry data of OLDH, PA and PANC. Samples
pHRR [W/g]a
pHRR [W/g]b
THR [kJ/g]c
THR [kJ/g]b
OLDH PA PANC 2 PANC 5
122.2 83.6 58.2 51.1
– – 82.1 79.5
11.7 11.1 10.1 8.5
– – 10.2 10.2
a b c
Peak heat release rate. Calculated. Total heat release.
The total heat release (THR) calculated from the area under the HRR curve was also an important parameter for fire hazard evaluation. Compared to the THR value of 11.1 kJ/g in the neat PA, PANC 2 and PANC 5 gave 10.1 and 8.5 kJ/g, respectively. The calculated THR for both PANC 2 and PANC 5 was 10.2 and 10.3 kJ/g, respectively. Considering the experimental and calculated THR data of the nanocomposites it was evident that 2 mass% OLDH had substantial effect on the THR value of PA, whereas, by increasing OLDH content up to 5 mass%, the THR value showed an obvious improvement and decreased to 8.5 kJ/g. All the above results indicated that newly synthesized PA had a good flammability and the introduction of OLDH led to improve flame retardancy of PA. This suggested that the lower HRR of nanocomposites was caused by the reduction of THR and the improvement of flame retardancy of PANC was due to modifications taking place in the condensed phase during polymer combustion (Manzi-Nshuti et al., 2009b). 5. Conclusions New polyamide containing pyridine ring in the main chain and imide group in the side chain with good yield and desired molar mass was synthesized through direct polycondensation reaction. Due to the presence of heterocyclic unit in the polyamide chain, the newly synthesized polyamide had good solubility in organic solvents. New polyamide/LDH nanocomposites with three compositions of the OLDH were prepared via solution intercalation method. TEM and XRD results have revealed the formation of exfoliated OLDH in the PA matrix. The addition of OLDH increased the tensile modulus and tensile strength of the nanocomposites for 2 and 5 mass% content of OLDH as compared to the neat PA. Thermal degradation results in air and nitrogen atmospheres showed that the addition of OLDH in the PA matrix increases the thermal stability and char yields of the resulting PANC. The MCC analysis suggested that the presence of OLDH imposes a positive effect
Fig. 11. Calculated HRR curves of PANC 2 and PANC 5.
263
on the flame retardancy of the neat polyamide, and a decreasing effect on pHRR and THR of the nanocomposites. By increasing the concentration of OLDH, the PANC showed further improvements of flame retardancy. All of the results revealed that the incorporation of OLDH was very efficient in improving the mechanical properties, thermal stability and flame retardancy of PA. Acknowledgments The authors thank the Research Affairs Division of Kharazmi University (project no. 4/2027) for financial support. References Alcantara, A.C.S., Aranda, P., Darder, M., Ruiz-Hitzky, E., 2010. Bionanocomposites based on alginate–zein/layered double hydroxide materials as drug delivery systems. J. Mater. Chem. 20, 9495–9504. Butuc, E., Gherasim, G.M., 1984. Ordered heterocyclic copolymers. Polyamide–imides with S-triazine rings. J. Polym. Sci., Polym. Chem. Ed. 22, 503–507. Díez-Pascual, A.M., Naffakh, M., Marco, C., Ellis, G., Gómez-Fatou, M.A., 2012. Highperformance nanocomposites based on polyetherketones. Prog. Mater. Sci. 57, 1106–1190. Faghihi, K., Mozaffari, Z., 2008. New polyamides based on 2,5-bis[(4-carboxyanilino) carbonyl] pyridine and aromatic diamines: synthesis and characterization. J. Appl. Polym. Sci. 108, 1152–1157. Faghihi, K., Shabanian, M., Izadkhah, A., 2010. Synthesis and characterization of optically active polyamides based on 2-(1,3-isoindolinedione-2-yl)glutaric acid by direct polycondensation. Chin. J. Polym. Sci. 28, 589–596. Gasser, M.S., 2009. Inorganic layered double hydroxides as ascorbic acid (vitamin C) delivery system—intercalation and their controlled release properties. Colloids Surf. B: Biointerfaces 73, 103–109. Ghaemy, M., Nasab, S.M.A., 2011. Synthesis and characterization of organo-soluble polyamides containing triaryl imidazole pendant and ether linkage moieties: thermal, photophysical, and chemiluminescent properties. Polym. Adv. Technol. 22, 2311–2318. Hajipour, A.R., Zahmatkesh, S., Ruoho, A.E., 2008. Microwave-assisted synthesis and characterization of heterocyclic, and optically active poly(amide-imide)s incorporating L-amino acids. Polym. Adv. Technol. 19, 1710–1719. Herrero, M., Benito, P., Labajos, F.M., Rives, V., Zhu, Y.D., Allen, G.C., Adams, J.M., 2010. Structural characterization and thermal properties of polyamide 6.6/Mg, Al/adipateLDH nanocomposites obtained by solid state polymerization. J. Solid State Chem. 183, 1645–1651. Hu, L., Yuan, Y., Shi, W., 2011. Preparation of polymer/LDH nanocomposite by UV-initiated photopolymerization of acrylate through photoinitiator-modified LDH precursor. Mater. Res. Bull. 46, 244–251. Katiyar, V., Gerds, N., Koch, C.B., Risbo, J., Hansen, H.C.B., Plackett, D., 2010. Poly l-lactidelayered double hydroxide nanocomposites via in situ polymerization of l-lactide. Polym. Degrad. Stab. 95, 2563–2573. Kausar, A., Zulfiqar, S., Ahmad, Z., Ilyas Sarwar, M., 2010. Novel thermally stable poly(thiourea–amide–imide)s bearing CS moieties and pyridine units in the backbone: synthesis and properties. Polym. Degrad. Stab. 95, 2611–2618. Kornmann, X., Lindberg, H., Berglund, L.A., 2001. Synthesis of epoxy–clay nanocomposites: influence of the nature of the clay on structure. Polymer 42, 1303–1310. Kotal, M., Kuila, T., Srivastava, S.K., Bhowmick, A.K., 2009. Synthesis and characterization of polyurethane/Mg–Al layered double hydroxide nanocomposites. J. Appl. Polym. Sci. 114, 2691–2699. Kurita, K., Williams, R.L., 1974. Heat-resistant polymers containing bipyridyl units. III. Polyimides having o-, m-, or p-phenylenedioxy linkage. J. Polym. Sci., Polym. Chem. Ed. 12, 1809–1822. Kutlu, B., Schröttner, P., Leuteritz, A., Boldt, R., Jacobs, E., Heinrich, G., 2014. Preparation of melt-spun antimicrobially modified LDH/polyolefin nanocomposite fibers. Mater. Sci. Eng. C 41, 8–16. Liu, J.-G., Wang, L.-F., Yang, H.-X., Li, H.-S., Li, Y.-F., Fan, L., Yang, S.-Y., 2004. Synthesis and characterization of new polybenzimidazopyrrolones derived from pyridine-bridged aromatic tetraamines and dianhydrides. J. Polym. Sci., Part A: Polym. Chem. 42, 1845–1856. Liu, X.-L., Wu, D., Sun, R., Yu, L.-M., Jiang, J.-W., Sheng, S.-R., 2013. Synthesis and properties of novel soluble fluorinated polyamides containing pyridine and sulfone moieties. J. Fluor. Chem. 154, 16–22. Lopez, M.S.-P., Leroux, F., Mousty, C., 2010. Amperometric biosensors based on LDHalginate hybrid nanocomposite for aqueous and non-aqueous phenolic compounds detection. Sensors Actuators B Chem. 150, 36–42. Lv, S., Zhou, W., Miao, H., Shi, W., 2009. Preparation and properties of polymer/LDH nanocomposite used for UV curing coatings. Prog. Org. Coat. 65, 450–456. Ma, T., Zhang, S., Li, Y., Yang, F., Gong, C., Zhao, J., 2010. Synthesis and characterization of novel polyimides derived from 4-phenyl-2, 6-bis [3-(4-aminophenoxy)-phenyl]pyridine diamine and aromatic dianhydrides. Polym. Degrad. Stab. 95, 1244–1250. Madhra, M.K., Salunke, A.K., Banerjee, S., Prabha, S., 2002. Synthesis and properties of fluorinated polyimides, 2. Derived from novel 2,6-bis(3′-trifluoromethyl-paminobiphenyl ether)pyridine and 2,5-bis(3′-trifluoromethyl-p-aminobiphenyl ether)thiophene. Macromol. Chem. Phys. 203, 1238–1248.
264
M. Hajibeygi et al. / Applied Clay Science 114 (2015) 256–264
Manzi-Nshuti, C., Hossenlopp, J.M., Wilkie, C.A., 2009a. Comparative study on the flammability of polyethylene modified with commercial fire retardants and a zinc aluminum oleate layered double hydroxide. Polym. Degrad. Stab. 94, 782–788. Manzi-Nshuti, C., Songtipya, P., Manias, E., Jimenez-Gasco, M.M., Hossenlopp, J.M., Wilkie, C.A., 2009b. Polymer nanocomposites using zinc aluminum and magnesium aluminum oleate layered double hydroxides: effects of LDH divalent metals on dispersion, thermal, mechanical and fire performance in various polymers. Polymer 50, 3564–3574. Mehdipour-Ataei, S., Heidari, H., 2003. Synthesis and characterization of novel soluble and thermally stable polyamides based on pyridine monomer. Macromol. Symp. 193, 159–168. Millange, F., Walton, R.I., Lei, L., O'Hare, D., 2000. Efficient separation of terephthalate and phthalate anions by selective ion-exchange intercalation in the layered double hydroxide Ca2Al(OH)6·NO3·2H2O. Chem. Mater. 12, 1990–1994. Nyambo, C., Songtipya, P., Manias, E., Jimenez-Gasco, M.M., Wilkie, C.A., 2008. Effect of MgAl-layered double hydroxide exchanged with linear alkyl carboxylates on fireretardancy of PMMA and PS. J. Mater. Chem. 18, 4827–4838. Purohit, P.J., Wang, D.-Y., Emmerling, F., Thünemann, A.F., Heinrich, G., Schönhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer 53, 2245–2254. Qiu, L., Chen, W., Qu, B., 2006. Morphology and thermal stabilization mechanism of LLDPE/MMT and LLDPE/LDH nanocomposites. Polymer 47, 922–930. Shabanian, M., Basaki, N., 2013. New photosensitive semi aramid/organoclay nanocomposite containing cinnamoyl groups: synthesis and characterization. Compos. Part B 52, 224–232. Shabanian, M., Kang, N.-J., Wang, D.-Y., Wagenknecht, U., Heinrich, G., 2013a. Synthesis of aromatic–aliphatic polyamide acting as adjuvant in polylactic acid (PLA)/ammonium polyphosphate (APP) system. Polym. Degrad. Stab. 98, 1036–1042. Shabanian, M., Kang, N.-J., Wang, D.-Y., Wagenknecht, U., Heinrich, G., 2013b. Synthesis, characterization and properties of novel aliphatic–aromatic polyamide/functional carbon nanotube nanocomposites via in situ polymerization. RSC Adv. 3, 20738–20745. Shabanian, M., Basaki, N., Khonakdar, H.A., Jafari, S.H., Hedayati, K., Wagenknecht, U., 2014a. Novel nanocomposites consisting of a semi-crystalline polyamide and Mg–Al LDH: morphology, thermal properties and flame retardancy. Appl. Clay Sci. 90, 101–108. Shabanian, M., Kang, N., Liu, J., Wagenknecht, U., Heinrich, G., Wang, D.-Y., 2014b. Biobased semi-aromatic polyamide/functional clay nanocomposites: preparation and properties. RSC Adv. 4, 23420–23427. Shabbir, S., Zulfiqar, S., Ahmad, Z., Sarwar, M.I., 2010. Pyrimidine based carboxylic acid terminated aromatic and semiaromatic hyperbranched polyamide–esters: synthesis and characterization. Tetrahedron 66, 7204–7212.
Sinha Ray, S., Okamoto, M., 2003. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog. Polym. Sci. 28, 1539–1641. Tamami, B., Yeganeh, H., 2001. Synthesis and characterization of novel aromatic polyamides derived from 4-aryl-2,6-bis(4-aminophenyl) pyridines. Polymer 42, 415–420. Wang, Q., O'Hare, D., 2012. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 112, 4124–4155. Wang, Y.-C., Fan, S.-C., Lee, K.-R., Li, C.-L., Huang, S.-H., Tsai, H.-A., Lai, J.-Y., 2004. Polyamide/SDS–clay hybrid nanocomposite membrane application to water– ethanol mixture pervaporation separation. J. Membr. Sci. 239, 219–226. Wang, H.-W., Dong, R.-X., Liu, C.-L., Chang, H.-Y., 2007. Effect of clay on properties of polyimide–clay nanocomposites. J. Appl. Polym. Sci. 104, 318–324. Wang, D.-Y., Costa, F.R., Vyalikh, A., Leuteritz, A., Scheler, U., Jehnichen, D., Wagenknecht, U., Häussler, L., Heinrich, G., 2009. One-step synthesis of organic LDH and its comparison with regeneration and anion exchange method. Chem. Mater. 21, 4490–4497. Wang, D.-Y., Leuteritz, A., Wang, Y.-Z., Wagenknecht, U., Heinrich, G., 2010. Preparation and burning behaviors of flame retarding biodegradable poly(lactic acid) nanocomposite based on zinc aluminum layered double hydroxide. Polym. Degrad. Stab. 95, 2474–2480. Wang, D.-Y., Das, A., Leuteritz, A., Boldt, R., Häußler, L., Wagenknecht, U., Heinrich, G., 2011a. Thermal degradation behaviors of a novel nanocomposite based on polypropylene and Co–Al layered double hydroxide. Polym. Degrad. Stab. 96, 285–290. Wang, D.-Y., Leuteritz, A., Kutlu, B., Landwehr, M.A.d, Jehnichen, D., Wagenknecht, U., Heinrich, G., 2011b. Preparation and investigation of the combustion behavior of polypropylene/organomodified MgAl-LDH micro-nanocomposite. J. Alloys Compd. 509, 3497–3501. Wang, D.-Y., Das, A., Leuteritz, A., Mahaling, R.N., Jehnichen, D., Wagenknecht, U., Heinrich, G., 2012. Structural characteristics and flammability of fire retarding EPDM/layered double hydroxide (LDH) nanocomposites. RSC Adv. 2, 3927–3933. Wang, Q., Tang, S.V.Y., Lester, E., O'Hare, D., 2013. Synthesis of ultrafine layered double hydroxide (LDHs) nanoplates using a continuous-flow hydrothermal reactor. Nanoscale 5, 114–117. Yamazaki, N., Matsumoto, M., Higashi, F., 1975. Studies on reactions of the Nphosphonium salts of pyridines. XIV. Wholly aromatic polyamides by the direct polycondensation reaction by using phosphites in the presence of metal salts. J. Polym. Sci., Polym. Chem. Ed. 13, 1373–1380. Zulfiqar, S., Sarwar, M.I., 2009. Investigating the structure–property relationship of aromatic–aliphatic polyamide/layered silicate hybrid films. Solid State Sci. 11, 1246–1251.