LDH reinforced semi-crystalline poly(amide-imide) containing naphthalene ring; study on thermal stability and optical properties

Applied Clay Science 139 (2017) 9–19

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Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Development of new acid-imide modified Mg-Al/LDH reinforced semi-crystalline poly(amide-imide) containing naphthalene ring; study on thermal stability and optical properties Mohsen Hajibeygi a,⁎, Meisam Shabanian b, Mehrdad Omidi-Ghallemohamadi a a b

Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran Young researchers and elite club, Arak Branch, Islamic Azad University, Arak, Iran

a r t i c l e

i n f o

Article history: Received 5 November 2016 Received in revised form 1 January 2017 Accepted 9 January 2017 Available online xxxx Keyword: Poly(amide-imide) Polymer nanocomposite Thermal stability Optical properties LDH

a b s t r a c t New poly(amide-imide) Mg-Al/layered double hydroxide (LDH) nanocomposites (PAINC) were prepared by solution intercalation technique from new semi crystalline poly(amide-imide) (PAI) containing naphthalene with new diacid-diimide modified Mg-Al LDH (DLDH) and sodium dodecylbenzene sulfonate (SDBS) modified Mg-Al LDH (SLDH). The X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results showed a uniform distribution for LDH sheets with in the PAI matrix. Thermal properties of all the samples were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal properties results in both nitrogen and air atmospheres showed that the addition of DLDH resulted substantial increase in the thermal stability and char yields of PAINC and indicated that DLDH had the better effect than the SLDH in thermal properties of PAI. The UV–Vis spectra of all PAINC showed reduction in intensities with increase LDH content. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Polymeric nanocomposite materials derived from the two components, polymers and inorganic materials as fillers have attracted attention from scientists because of their unexpected properties. Different properties of the polymeric nanomaterials such as thermal stability, mechanical properties, flame retardancy and film barrier characteristic could be significantly improved with low nano-filler content loadings (Kotal et al., 2009). Polymers reinforced with layered nanostructure materials due to their unique properties such as diameter in nanometer range, thermal stability and other good properties have shown enhancements in various applications (Wang et al., 2007; Kiliaris and Papaspyrides, 2010; Hajibeygi et al., 2013, 2015a, 2015b; Hajibeygi and Shabanian, 2014). The layered nanostructure materials involved in this field mainly include silicates, manganese oxides, layered phosphates, titanates, molybdenum sulfide, and layered double hydroxides (LDH). Among them, LDH, also called anionic clays, were considered as a kind of clays. They have already been used in polymer nanocomposites for many years, although it is true that they probably still have untapped potential (Shabanian et al., 2014). In contrast to the cationic clay with negatively ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Hajibeygi).

http://dx.doi.org/10.1016/j.clay.2017.01.011 0169-1317/© 2017 Elsevier B.V. All rights reserved.

charged layers, LDH have been kind of layered nanostructure materials consisting of positively charged layers with interlayer exchangeable anions and intercalated water (Acharya et al., 2007; Wang et al., 2011b). Because of their flexible, tunable chemical composition and high anion exchange capacity, LDH have found many potential applications, including in pharmaceuticals, adsorbents, catalysis and inorganic fillers. In addition, LDH has been widely used in various fields such as catalysts (Wang et al., 2012), ion exchange hosts (Millange et al., 2000), drug delivery (Alcantara et al., 2010), hydrothermal reactor (Wang et al., 2013) and eco-friendly flame retardant additives (Manzi-Nshuti et al., 2009b; Gao et al., 2014). Due to high charge density and the hydrogen-bonding between the hydroxide layers and intercalated anions and/or water molecules in LDH, it can be difficult to formation uniform structure with good distribution of the LDH sheets, especially when being used as nanofillers for reinforcing polymer nanocomposites. Therefore, LDH have to organomodified by some surfactants before using as nanofiller in the polymer matrix to obtain a good dispersion in polymer matrices (Wang et al., 2009). LDH can be modified by the replacement of inter2– − layer anions such as NO− 3 , Cl , or CO3 with different organic anionic surfactant (Hsueh and Chen, 2003; Kotal et al., 2011). Recently many researches have been focused on polymer nanocomposite materials containing LDH (Lv et al., 2009; Wang et al., 2011a; Purohit et al., 2012) and significant effects were observed in both thermal properties and

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flame retardancy for formulations containing polymer matrices with modified different LDH as nanofillers (Manzi-Nshuti et al., 2009a; Nyambo et al., 2008; Wang et al., 2012). High performance polymers, such as polyimide, have been considered as a good candidate for matrices in polymer nanocomposites due to their excellent properties namely outstanding dielectric, thermal and mechanical properties at elevated temperatures. For this reason, polyimides are considered to be one of the most useful super-engineering plastics (Agag et al., 2001; Hsueh and Chen, 2003; Huang et al., 2011). One of the problems with high-performance polyimides has been their poor solubility as well as processability, which limited their applications as engineering polymers. However, these problems can be solved or reduced by introducing amide and ester groups in main and side chain of polyimide, by producing poly(amide-imide) and poly(ester-imide) (Hajibeygi et al., 2011; Mallakpour and Dinari, 2013; Hasegawa et al., 2015), some flexible bonds such as aliphatic chains (Shabanian and Basaki, 2013) and pendant bulky groups (Hajipour et al., 2008; Ghaemy and Nasab, 2011) in the polyimide backbone. The aim of present work was preparation of new aliphatic-aromatic and semi crystalline poly(amide-imide) nanocomposites reinforced with two different organo modified LDH included new prepared reactive diacid-diimide modified Mg-Al LDH (DLDH) and SDBS modified Mg-Al LDH (SLDH). In this work, the effect of two different organo modified LDH on the properties of the developed PAINC concluded PAI/ DLDH (PAIDN) and PAI/SLDH (PAISN) was described. Due to the presence of the heterocyclic imide groups in modifier of DLDH, it was expected that corresponding polymer nanocomposites have good improvement in thermal properties. 2. Experimental 2.1. Materials 3,5-dinitrobenzoyl chloride, 1-aminonaphthalene, 3,3′,4,4′benzophenonetetracarboxylic dianhydride, 6-aminohexanoic acid, Nmethyl-2-pyrrolidone (NMP), palladium charcoal (Pd/C), N,Ndimethylformamide (DMF), hydrazine monohydrate, pyridine (Py) and triphenyl phosphite (TPP) from Merck. Commercially available calcium chloride (CaCl2, Merck) was dried under vacuum at 150 °C for 6 h. Aluminum nitrate, magnesium nitrate, sodium hydroxide, and sodium dodecylbenzene sulfonate (SDBS) were used for synthesis of SLDH were also obtained from Merck. 2.2. Measurements 1

H NMR and 13C NMR spectra were recorded using Bruker 300 MHz instrument (Germany). Fourier transform infrared (FTIR) spectra were done by Perkin-Elmer RXI spectrometer. Vibration transition frequencies were reported in wave number (cm−1). Band intensities were assigned (w) for weak, (m) for medium, (sh) for shoulder, (s) for strong and (br) for broad. Inherent viscosity was determined with concentration of 0.5 g/dL in DMF at room temperature by standard procedure using Ostwald viscometer. Molar mass of the synthesized PA concluded mass-average (− Mw) and number-average (‾Mn) molar mass, was performed in size exclusion chromatography (SEC) by Agilent Series (Agilent 1100, USA) system. The molar mass was measured after calibration using poly(2vinylpyrrolidone) as standard. X-ray diffraction (XRD) was described using 2-circle diffractometer XRD 3003 (Germany, Freiberg) with Cu-Kα radiation (λ = 0.154 nm), generated at 30 mA and 40 kV in the range of 2θ = 2°–80°. The surface morphology of the samples was analyzed using FieldEmission Scanning Electron Microscopy (FE-SEM) (Mira 3-XMU). Solid state UV–Visible spectra were recorded at 25 °C in the range of 190–790 nm using a Cary 6000i UV–Visible/Near-infrared spectrophotometer (Varian). Also solution state UV–Visible spectra were recorded

at 25 °C in the range of 190–790 nm using Perkin-Elmer UV–Vis spectrometer Lambda 25. Photoluminescence (PL) emission spectrum of poly(amide-imide) was investigated using Perkin-Elmer Florescence spectrometer LS 55. The thermogravimetric analyses (TGA) were recorded using a STA Instruments 1500 in the range between 25 °C to 800 °C with heating rate of 10 °C/min in nitrogen and air atmospheres. Differential scanning calorimetry (DSC) was used for description of thermal events of PAI and corresponding PAINC using TA instrument Q1000. 2.3. Monomer synthesis 2.3.1. Synthesis of diamine 4 3,5-diamino-N-(naphthalen-1-yl)benzamide 4 as diamine compound containing amide group was synthesized starting from 1aminonaphthalene 1 and 3,5-dinitrobenzoyl chloride 2 by two step reactions as following: 2.3.1.1. Synthesis of N-(naphthalen-1-yl)-3,5-dinitrobenzamide 3. 3.00 g (20.94 mmol) 1-aminonaphthalene 1, 10 mL of triethylamine and 35 mL of dry N,N-dimethylformamide (DMF) were added into 250 mL round-bottomed flask with a magnetic stirrer under N2 atmosphere. The reaction mixture was cooled in an ice water bath and while being stirred, a solution of 3,5-dinitrobenzoyl chloride 2 (7.20 g, 31.44 mmol) in DMF (30 mL) was added dropwise over a 20 min period. The mixture was stirred in ice bath for 1 h and at room temperature for overnight. The mixture of reaction was poured into 150 mL of water. The precipitate was collected by filtration and washed thoroughly with ethanol and dried at 80 °C. Yield = 91%; mp: 267–269 °C. FT-IR (KBr): 3263 (s), 3108 (m), 3095 (m), 1649 (s), 1629 (m, sh), 1597 (w), 1533 (s), 1398 (m), 1342 (s), 1288 (m), 1077 (m), 802 (m), 730 (m) cm− 1. 1H NMR (DMSO-d6, TMS) δ: 11.10 (s, 1H), 9.27 (s, 2H), 9.04 (s, 1H), 7.98–8.01 (m, 2H), 7.90–7.92 (d, 1H), 7.55–7.65 (m, 4H) ppm. 2.3.1.2. 3,5-diamino-N-(naphthalen-1-yl)benzamide 4. 2.00 g (5.93 mmol) of N-(naphthalen-1-yl)-3,5-dinitrobenzamide 3, 0.05 g of Pd/C 10%, 60 mL of ethanol and 5 mL of DMF were added into 250 mL round-bottomed flask with a magnetic stirrer under N2 atmosphere and a dropping funnel, to which 10 mL of hydrazine was added dropwise over a period of 30 min at 70 °C. After the complete addition, the reaction was continued heating at reflex condition for another 3 h. Then, the mixture was filtered to remove the Pd/C, and the filtrate poured in the ice-water mixture. The product was filtered, washed thoroughly with EtOH, and dried at 80 °C. Yield = 91%; mp: 253–255 °C. FTIR (KBr): 3427 (m), 3342 (m), 3237 (m), 3048 (w), 1632 (s, sh), 1593 (s), 1526 (m), 1363 (m), 1276 (m), 1119 (m), 849 (m), 797 (s), 766 (m) cm− 1. 1H NMR (DMSO-d6, TMS) δ: 10.03 (s, 1H), 7.92–7.95 (m, 2H), 7.95–7.92 (d, 1H), 7.48–7.53 (m, 4H), 6.44 (s, 2H), 6.01 (s, 1H), 4.94 (s, 4H) ppm. 13C NMR (DMSO-d6) δ: 167.8, 149.1, 136.3, 134.3, 134.2, 133.7, 129.1, 128.0, 125.9, 125.8, 125.7, 125.5, 123.3, 102.4, 102.3 ppm. 2.3.2. Synthesis of diacid 7 6,6′-(5,5′-carbonylbis(1,3-dioxoisoindoline-5,2-diyl))dihexanoic acid 7 as diacid monomer and also LDH modifier containing imide heterocyclic ring and long aliphatic chain was synthesized by following procedure: 1.00 g (3.10 mmol) of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride 5, 0.81 g (6.20 mmol) of 6-aminohexanoic acid 6, 40 mL mixture of acetic acid and pyridine (3:2) were added into 100 mL round-bottomed flask with a magnetic stirrer under N2 atmosphere. The mixture was stirred at room temperature overnight and heated in reflux condition for 4 h. The solvent of reaction was removed under reduced pressure and the residue was dissolved in 100 mL of water. Then 5 mL of HCl 1 M was added to mixture of reaction. A white precipitate

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was formed, filtered off and dried at 80 °C. Yield = 95%; mp: 191– 193 °C. FT-IR (KBr): 2500–3500 (m, br), 1778 (w), 1708 (s, br), 1649 (m, sh), 1440 (m), 1396 (s), 1322 (m), 1247 (m), 1207 (m), 1073 (s), 939 (m), 706 (m) cm− 1. 1H NMR (DMSO-d6, TMS) δ: 11.99 (s, 2H), 8.16 (d, 2H), 8.05 (d, 4H), 3.60 (d, 4H), 3.34 (d, 4H), 2.19 (quin, 4H), 1.53 (quin, 4H), 1.29 (quin, 4H) ppm. 13C NMR (DMSO-d6) δ: 193.3, 174.5, 167.1, 141.3, 135.6, 135.5, 134.8, 131.9, 123.6, 123.4, 37.4, 33.6, 27.5, 25.6, 23.9 ppm.

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PAIDN 8). Also, the amounts of SLDH were 2 and 5 mass% in the PAI/ SLDH nanocomposites (PAISN 2 and PAISN 5). To prepare PAI nanocomposite containing 2 mass% of DLDH, 0.98 g of PAI was dissolved in 10 mL DMF, followed by the addition of 0.02 g of DLDH. The reaction mixture was agitated to high speed at 25 °C overnight to disperse DLDH platelets uniformly in the PAI matrix. After that, the mixture was irradiated with high-intensity ultrasonic wave for 45 min. The PAIDN 2 was caste by pouring the hybrid solution in Petri dish and removed the solvent at 70 °C for 12 h and was further dried at 85 °C under vacuum.

2.4. Synthesis of poly(amide-imide) 3. Results and discussion 1.00 g (1.82 mmol) of diacid 7, 0.50 g (1.82 mmol) of diamine 4, calcium chloride (0.50 g), triphenyl phosphite (1.00 mL), pyridine (0.30 mL) and N-methyl-2-pyrrolidone (8 mL) were added into 50 mL round-bottomed flask fitted with a magnetic stirrer under N2 atmosphere. 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 and the reaction mixture was poured in 100 mL of methanol and the precipitated PAI was collected by filtration and washed with cold and hot methanol, respectively. Finally, the product was dried at 70 °C for 12 h inside vacuum oven to leave 1.41 g (97.9%) yellow solid of PAI. ηinh (inherent viscosity, measured at a concentration of 0.5 g/dL in DMF at 25 °C): 0.76 (dL/g). Elemental analysis: calculated for C46H39N5O8 (790.84 g/mol), calculated: C, 69.95; H, 4.98; N, 8.87; found: C, 67.99; H, 4.38; N, 9.41. FT-IR (KBr): 3321 (m, sh), 3065 (w), 2937 (m), 2859 (w), 1772 (m), 1712 (s), 1598 (m), 1531 (m), 1497 (m), 1441 (m), 1395 (m), 1366 (m), 1201 (m), 918 (m), 795 (w), 770 (m), 726 (m) cm−1. 13C NMR (DMSO-d6) δ: 171.3, 167.3, 167.1, 166.7, 149.3, 139.5, 135.8, 135.6, 134.7, 133.7, 131.9, 128.9, 128.0, 125.9, 125.5, 123.4, 37.6, 36.1, 27.7, 25.9, 24.6 ppm. 2.5. Preparation of diacid-diimide modified Mg-Al LDH (DLDH) and SDBSmodified Mg-Al LDH (SLDH) Mg-Al/LDH was prepared by the coprecipitation method according to previous work. Briefly, a solution of Mg(NO3)2. 6H2O and Al(NO3)3. 9H2O (with Mg2+:Al3+ equal to 2:1 and a total metal ion concentration of 0.3 M) was added to 30 mL deionized water under N2 atmosphere. During the Preparation of LDH, solution was stirred at 50 °C and the pH value was kept at 10.5 ± 0.2 by adding suitable amount of a solution of sodium hydroxide. After the addition of the metals salt solution, the resulting mixture was stirred at 50 °C for 0.5 h. Then the temperature of mixture was increased to 70 °C and allowed to age for 24 h. Upon cooling final white product was filtered and washed with deionized water until a solution with pH = 7 was obtained. The product was then dried in an oven at 75 °C. The diacid-diimide modified Mg-Al LDH (DLDH) was prepared by an ion-exchange approach between Mg-Al LDH and the carboxylate salt of diacid 7. 2 g Mg-Al LDH was dispersed in 50 mL decarbonated water under N2 atmosphere. The amount of modifier was dissolved in decarbonated water and a stoichiometric amount of concentrated NaOH was added to the solution and the pH value was kept at 10. Dissolved Mg-Al LDH was added to the solution of the modifier and this mixture was agitated vigorously for 24 h. The white precipitate was isolated by suction filtration and washed with 200 mL decarbonated water. This process was repeated thrice to ensure the removal of excess salt of diacid 7, thus ensuring complete removal of nitrate ions. The final white product was filtered and dried in a vacuum oven for 24 h. SDBS-modified Mg-Al LDH (SLDH) was prepared according to the procedure reported elsewhere (Shabanian et al., 2014). 2.6. Preparation of PAI/Mg-Al LDH nanocomposites (PAINC) The PAINC were synthesized by taking the PAI solution in a flask, followed by the addition of a known proportion of DLDH and SLDH for particular concentrations. The amounts of DLDH were 2, 5 and 8 mass% in the PAI/DLDH nanocomposites (PAIDN 2, PAIDN 5 and

3.1. Monomer synthesis 3.1.1. Synthesis of diamine 4 The new aromatic diamine monomer, 3,5-diamino-N-(naphthalen-1yl)benzamide 4, was prepared by two-step synthetic routes. Firstly, the N-(naphthalen-1-yl)-3,5-dinitrobenzamide 3 containing amide group and naphthalene ring was synthesized via substitution nucleophilic reaction from 3,5-dinitrobenzoyl chloride and 1-aminonaphthalene in the presence of triethylamine in DMF. Then, the diamine 4 was obtained by catalytic hydrogenation of the dinitro compound using hydrazine monohydrate and Pd/C as catalyst in ethanol (Scheme 1). The structure of new aromatic dinitro 3 was confirmed by FTIR and 1H NMR spectroscopies and the new aromatic diamine 4 was confirmed by FTIR, 1H NMR and 13C NMR spectroscopies. In 1H NMR spectrum of diamine 4 (Fig. S1), the signals of aromatic protons related to naphthalene ring appeared in the range of 7.48–7.92 ppm and the characteristic resonance signal at 10.03 ppm was due to the amide (N-H) group. Also, the protons related to NH2 groups appeared at 4.94 ppm. Assignments of each proton were also presented in the Fig. S1 and the spectrum proved the proposed structure of aromatic diamine 4. 3.1.2. Synthesis of diacid 7 6,6′-(5,5′-carbonylbis(1,3-dioxoisoindoline-5,2-diyl))dihexanoic acid 7 containing imide groups and long aliphatic chain was prepared from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 6aminohexanoic acid in a mixture of acetic acid and pyridine. The synthetic route of diacid 7 is represented in Scheme 1. The structure of diacid 7 was confirmed by FTIR and 1H NMR spectroscopies. The 1H NMR spectrum of diacid 7 is shown in Fig. S2. The aromatic protons appeared at 8.05–8.16 ppm. The protons related to carboxylic acid groups appeared at 11.99 ppm. Five types of aliphatic protons have created five signals in the region of 1.29–3.60 ppm. 3.2. Synthesis of PAI In order to synthesis the matrix of PAINC, the Yamazaki method (Yamazaki et al., 1975) (triphenyl phosphite (TPP)-activated polycondensation) was used to synthesis PAI containing aliphatic group in the main chain and naphthalene ring in the side chain (Scheme 1). The molar mass of PAI was measured by SEC in DMAc with poly(2vinylpyrrolidone) as standard. The number-average molar mass (‾Mn) and mass-average molar mass (− Mw) of PAI were 2.32 × 104 and 6.02 × 104, (Poly dispersity index (PDI) =2.59). The solubility of PAI was investigated with 0.01 g of polymeric sample in 2 mL of solvent. Due to presence of aliphatic groups in the main chain and naphthalene ring as a bulky pendent group in PAI, it had good solubility in organic solvents at 25 °C. The polymer was dissolved in polar organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF) and N-methyl-2-pyrrolidone (NMP) at 25 °C and was insoluble in methanol, ethanol and water as protic solvents. The structure and purity of newly synthesized PAI were confirmed by elemental analysis, FTIR, 1H NMR and 13C NMR spectroscopies. The elemental analysis results showed a good accordance between

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Scheme 1. Synthesis route of PAI.

calculated and real data for the proposed PAI structure. The 1H NMR spectrum of PAI showed different peaks that confirmed its chemical structure (Fig. S3). The aromatic protons appeared in the region of 7.50–8.17 ppm. Protons related to aliphatic groups were appeared in the region of 1.32–3.62 ppm. Two peaks at 10.06 ppm and 10.41 ppm in 1H NMR spectrum of PAI, were related to two N-H groups in the main chain and one N-H amidic in the side chain of PAI, respectively. Also, the 13C NMR spectrum of PAI confirmed its proposed structure.

3.3. Preparation of PAI/Mg-Al LDH nanocomposites In general, LDH are employed to prepare different organic-inorganic nanomaterials and have received significant attention. LDH needs to be organomodified by some anionic surfactants before employment as

nanofiller in polymer to obtain a good dispersion in the matrix of polymers (Wang et al., 2009). In this study, diacid 7 was used as building blocks for efficient prepare of potentially nanohybrid material of diacid-diimide modified LDH (DLDH). DLDH was prepared by an ion-exchange approach between Mg-Al LDH and the carboxylate salt of diacid 7. As a possible model, the structure of diacid 7 can be arranged vertically and/or horizontally to the basis of LDH layer. For comparison and further studies, SLDH (SDBS modified Mg-Al LDH) was prepared by one-step route according to previous research (Shabanian et al., 2014) and used for preparation of PAI nanocomposites. New PAI nanocomposites containing 2, 5 and 8 mass% of DLDH were prepared using DLDH in dry DMF through solution intercalation technique (Scheme 2). For comparison PAI nanocomposites containing 2 and 5 mass% of SLDH were also prepared by the same method.

Scheme 2. Preparation of nanocomposites.

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4. Characterization of the PAI/LDH nanocomposites 4.1. FT-IR spectra FTIR spectra of unmodified LDH, DLDH, PAI and PAINC are shown in Fig. 1. In the FTIR spectrum of neat Mg-Al LDH, the broad absorption band in the range of 3000–3700 cm−1 related to the O\\H stretching of the metal hydroxide layer and interlayer water molecules of Mg-Al LDH; and the bending vibration of the interlayer H2O was reflected in the broad band centered at 1618 cm− 1. Also, the absorption band at 1378 cm−1 related to NO3 stretching vibrations. The absorption bands in the range of 590–890 cm− 1 can be assigned to Al\\O and Mg\\O stretching modes. In comparison with the neat Mg-Al LDH, the FTIR spectrum of the DLDH revealed the presence of diacid anion 7 as modifier between the layers of LDH. The strong and broad absorption band in the range of 3000–3700 cm− 1 centered at 3460 cm− 1 was due to the O\\H stretching of the metal hydroxide layer and also O\\H stretching vibrations of surface and interlayer water molecules. The characteristic weak band around 1630 cm−1 related to the bending vibration of water molecules. The absorption bands at 2855 cm−1 and 2929 cm−1 were detected for aliphatic groups in diacid anion 7. The absorption bands were detected for carbonyl imide groups (symmetric stretching at 1711 cm−1 and antisymmetric at 1774 cm−1). Also, the absorption bands in the range of 590–890 cm− 1 can be related to metal oxide stretching modes. These FTIR characteristic bands demonstrated that diacid anion 7 had been intercalated into the interlayer space of Mg-Al LDH. The FTIR spectrum of PAI (Fig. 1) showed absorption bands at 3321 cm−1 (N\\H, stretching vibration), 1774 and 1715 cm−1 for the imide heterocyclic ring (antisymmetric and symmetric C _O stretching vibration), 1388 cm−1 (C\\N stretching vibration), 1079 and 727 cm−1 (imide ring). Also the absorption band appeared at 1673 cm−1 related to amide carbonyl group in the main and side chain of PAI. The FTIR spectra of PAINC are also shown in Fig. 1. The FTIR spectra of the all PAINC were shown the characteristic absorption bands of the PAI structure as well as the absorption bands of DLDH or SLDH. The FTIR spectra of the PAINC appeared the absorption bands attributed to the carbonyl groups around 1776 cm−1 due to the antisymmetric imide heterocyclic ring stretching vibration and two overlapped around 1712 cm− 1 related to symmetric imide heterocyclic ring and amide

Fig. 1. FTIR spectra of Mg-Al LDH, DLDH, PAI and PAINC.

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group stretching. The absorption bands in the range of 2850– 2930 cm− 1 were due to aliphatic groups related to the DLDH, SLDH and also PAI. The weak absorption bands related to metal oxide in the LDH structures were appeared around 580 cm− 1 in all of the PAINC FTIR spectra. By comparison of these spectra, it can be concluded that the PAINC not only had absorption bands related to the neat PAI but also had characteristic absorption bands for modified LDH. These data, in sum, confirmed the formation of the PAI/Mg-Al LDH nanocomposites.

4.2. Structural characterization For structural characterization of neat Mg-Al LDH, DLDH, PAI and PAINC, direct evidence can be obtained from wide angle X-ray scattering (WAXS), which is one of the powerful techniques to characterize the layered structure of the polymer nanocomposites (Fig. 2). The changes to lower angles of the maximum reflections in the XRD pattern of DLDH as compared to the neat Mg-Al LDH indicated that modifier anion had been intercalated in the Mg-Al LDH layers (Fig. 2). The XRD pattern of neat LDH revealed a multilayer structure due to the basal reflection maximum (003) at 2θ = 10.84°, while the XRD pattern of DLDH revealed the basal reflection maximum (003) at 2θ = 2.92°. The d-values of the neat LDH and DLDH can be calculated from Bragg's law (Klug and Alexander, 1974). During the intercalation of diacid 7 anion as modifier, the layers of LDH were swelled to host the anions and d003-value was increased from 0.81 nm (for neat LDH) to 3.02 nm (for DLDH). The XRD pattern of PAI (Fig. 2) revealed two reflection maxima at 23.55° and 26.79°, which proved semi crystalline structure of PAI. These peaks were also depicted in all XRD patterns of the PAINC. In comparison to basal reflections of DLDH, did not observe any maxima reflection related to ordered crystal structure of LDH in the XRD patterns of nanocomposites. Therefore, a good dispersion for DLDH in PAI matrix can be expected. This means that the nanolayer structures were either too small or very amorphous (Shabanian et al., 2014). However, the XRD analysis alone can not be reliable to have a final conclusion about the dispersion kind of LDH layers in the polymer matrix, so the investigation by transmission electron microscopy (TEM) is essential. SEM has been a primary instrument for characterizing the fundamental surface morphology and physical properties of material surfaces. It is useful for determining the porosity, particle shape and appropriate size distribution of the adsorbent (Mahmoodi et al., 2011).

Fig. 2. XRD pattern of Mg-Al LDH, DLDH, PAI and PAINC.

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The FE-SEM micrographs of PAI and PAINC are shown in Fig. 3. Also, the complete elemental distribution in PAINC was further investigated by elemental mapping images (Fig. 4). With comparison the FE-SEM images of PAI (Fig. 3a) and PAINC containing different contents of DLDH and SLDH (Fig. 3b–c), it can be concluded that the presence of LDH in the polymer matrix has created some changes in the PAINC shapes. The elemental mapping images of PAINC (Fig. 4) indicated that the LDH nanolayers were uniformly dispersed in the PAI matrix. In the case of PAI having different functional groups, the good compatibility between the LDH layers and the PAI chain was strong enough and led to acceptable dispersion. In order to confirm the distribution of LDH layers in the PAI matrix, TEM investigations were done on PAIDN 2 and PAIDN 5 and the results are presented in Fig. 5. The dark lines represent the LDH sheets, whereas the bright area represents the PAI matrix. The TEM images of PAINC show a good distribution of DLDH layers in the PAI matrix, providing direct evidence of crystal layer exfoliation. These results were supported by the absence of reflections for PAINC in the XRD patterns.

4.3. Optical properties Polymers containing aromatic fused rings such as naphthalene are notable for their unique properties such as thermal and optical properties. Naphthalene and its derivatives chromophoric dyes have been excellent materials of aromatic π-conjugated heterocyclic compounds due to their UV–Vis absorption and photoluminescence emission. Naphthalene chromophores which their rigid structures and strong π–π intermolecular interactions has increased optical and electronical applications (Yu and Wu, 2014; Aleshinloye et al., 2015). UV–Vis spectroscopy and photoluminescence measurement have been two suitable and convenient methods for investigation of the optical properties of polymers and polymer nanocomposites. UV–Vis absorption of neat PAI and PAIDN 2, 5 and 8 mass% were recorded at solid state and corresponding spectra are demonstrated at Fig. 6. UV–Vis absorption spectrum of PAI showed maximum absorption bands in the range of 295–312 nm related to transition electrons in n–π* and π–π* for the nonbonding electrons related to nitrogen and

Fig. 3. FE-SEM micrograph of a) PAI, b) PAIDN 2, c) PAIDN 5 and d) PAISN 2.

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Fig. 4. Mapping images of a) PAIDN 2, b) PAIDN 5 and c) PAISN 2 (Mg: green, Al: red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

oxygen atoms and also for the naphthalene rings in the PAI backbone. The maximum absorption bands related to transition electrons in the LDH nano structure have been detected around 450 nm, which can be related to the linkage to metal charge transfer that arised from the 2p

orbital of oxygen to the 3d orbitals of Mg2 + and Al2 + ions (Li et al., 2013; Nayak et al., 2015). Accordingly, by comparison of UV–Vis spectra of PAI and corresponding PAINC (Fig. 6), it can be concluded that the presence of LDH nanolayers in the PAI matrix led to the maximum

Fig. 5. TEM micrographs of a) PAIDN 2 b) PAIDN 5.

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4.4. Thermal properties

Fig. 6. UV–Vis spectra of PAI and PAIDN in solid state.

absorption bands in the range of 400–460 nm related to transition electrons in the LDH structure have been added to UV–Vis spectra. The UV–Vis absorption spectra of neat PAI and PAINC were recorded in a solution of DMF and are shown in Fig. 7. The solutions of PAI and PAINC were prepared by the addition of 0.02 g of each sample to 10 mL of DMF and were irradiated ultrasonic wave for 15 min. The maximum absorption band of the PAI solution was appeared around 297 nm, which almost like solid state. According to Fig. 7, it is interesting to mention that in the UV–Vis absorption spectra of the PAINC in the solution state did not observe any maximum absorption bands related to the LDH nanolayers. It can be attributed to high dilution of the PAINC solutions. By contrast, by increase LDH content to the PAI matrix, the absorption intensities were decreased and the least absorption intensities were observed in UV–Vis absorption spectra of PAISN 2 and 5 mass%. This change may be attributed to the strong interaction between the PAI and LDH nanolayers in the resulting PAINC. Also, the photoluminescence (PL) spectrum of PAI is also illustrated in Fig. 7. The PL spectrum of PAI was recorded using excitation wavelengths of 370 nm. The main emission band of PAI was located at 439 nm which related to violet emission which can be attributed to the PAI functional groups such as naphthalene ring in its side chain.

Fig. 7. UV–Vis spectra of PAI and PAINC at solution state & PL emission of PAI.

Thermal stability of all the samples was investigated by TGA method in both N2 and air atmospheres. The TGA thermograms of PAI, PAINC in N2 and air atmospheres as well as neat Mg-Al LDH, DLDH and SLDH in N2 atmosphere are illustrated in Fig. 8. The TGA data in two different atmospheres are summarized in Table 1 that include temperatures at which 5% (T5), 10% (T10) degradation occurred, maximum mass loss temperature (Tmax) and the amount of residue at 800 °C. According to the TGA thermograms of neat LDH and DLDH in N2 atmosphere (Fig. 8a), the decomposition of neat LDH occurred into two main steps. In the first one, at around 180 °C, mass loss of 4.1% was due to the loss of water molecules in the interlayer and physically adsorbed of the LDH sheets. In the second one, at around 350 °C, mass loss of 20.5% can be attributed to the dehydroxylation of the LDH layers and the elimination of nitrate anion (Lv et al., 2009). The total weight loss at maximum temperature of 800 °C was 42.2%. In the TGA curve of DLDH (Fig. 8a), there were almost three decomposition areas at around 85, 280 and 520 °C. In the first decomposition, the weight loss from room temperature to almost 100 °C can be attributed to the loss of the absorbed water (Chen and Qu, 2003). After ion-exchange, the surface of the layers LDH changed to an organophilic structure and the hydrogen bond interaction between water molecules may be decreased resulting in a reduction of the decomposition temperature. In the second decomposition step, at around 250–280 °C, the decomposition of DLDH can be ascribed to leaves of diacid anion from the DLDH structure. The third step, around 500 °C, can be due to the dehydroxylation of LDH sheet. The total mass loss at 800 °C was almost 53.9%, which was 11.7% higher than that of neat LDH. The lower amount of char residue of DLDH as compared to the neat LDH could be due to presence of the diacid anion 7 in the DLDH structure. These results are in agreement with the FTIR and XRD measurements. Also, the three decomposition steps related to water molecules, SDBS structure and dehydroxylation of LDH sheet can be observed in the TGA thermogram of SLDH (Fig. 8b). The total mass loss for SLDH at 800 °C was almost 56.8%, which was almost 3% higher than that of DLDH, which had not discernable difference with DLDH. The TGA thermograms in N2 (Fig. 8a) exhibited T5, T10 and char yield for the neat PAI, 271 °C, 365 °C and 44.7%, respectively. By incorporation of 2 mass% of DLDH to PAI, T5, T10 and char yield increased to 393 °C and 57.1%, respectively. These modes in growth thermal stability were observed in PAIDN 5 and PAIDN 8 (Table 1). Considering to the TGA thermograms of PAI nanocomposites, it was concluded that DLDH increased thermal degradation temperature of PAI. The improvement in thermal stability can be attributed to reduction of mobility of polymer chain by incorporation of LDH layers to polymer matrix (Hajibeygi et al., 2015a). Also the increases amount of residue in PAI nanocomposites with increasing DLDH content might be due to the polymer char that was formed from carbonaceous char along with increasing of MgO and Al2O3 residue from products decomposition of LDH (Acharya et al., 2007). This improvement in thermal properties can be related to the relative exfoliation and/or delamination processes as a function of the amount of LDH (Qiu et al., 2006; Shabanian et al., 2016). According to TGA thermograms of PAISN 2 and PAISN 5 (Fig. 8b and Table 1), the effect of DLDH on the thermal properties of PAI has been greater than that of SLDH. With comparison of thermal stability and char yield of PAIDN 2 and PAISN 2 (Table 1), it can be observed that the present of diacid anion 7 with heterocyclic imide and carboxylic groups in the DLDH structure led to appearance of the better interaction between the LDH layers and the PAI chain. The TGA thermograms of PAI, PAIDN 2, PAIDN 5 and PAIDN 8 in air atmosphere are represented in Fig. 8c. Also the TGA thermograms of PAISN 2 and PAISN 5 in air atmosphere are illustrated in Fig. 8d and corresponding data are listed in Table 1. The oxygen effect in decomposition of all samples was noted with comparing TGA curves of the PAI and corresponding PAI nanocomposites

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Fig. 8. TGA thermograms of the all samples in a,b) N2 and c,d) air atmospheres.

in air and nitrogen, due to the reactions of oxygen with radicals that produced from polymer degradation (Manzi-Nshuti et al., 2009a). As shown in Table 1, the neat PAI in air atmosphere had T5 and T10 of 252 °C and 363 °C, respectively with final of amount residue 9.0%. From Table 1, it can be observed that in all of the nanocompsoites by increasing the DLDH content from 2 to 8 mass%, the thermal stability and char yields were increased. This implied that the DLDH loading in PAI matrix was helped to formation of char and the improvement in thermal stability can be attributed to the presence of LDH sheets, which had a barrier effect to decrease the permeability of volatile degradation products and increase the insulation properties (Herrero et al., 2010). Like TGA results in

N2 atmosphere, the effect of DLDH in thermal properties of PAI has been more than that of SLDH. With comparison of TGA data of PAI nanocomposites containing DLDH and SLDH resulted from air atmosphere (Table 1), it can be observed that the present of organic modifier containing imide groups in DLDH led to appearance the better thermal properties.

Table 1 Thermal properties data of PAI and PAINC.

T5(°C)a T10(°C)b CY (%)c

Thermal properties Tgd(°C) Tm(°C)e in air T5(°C) T10(°C) CY (%)

271 293 307 313 293 300

252 267 284 291 258 266

Samples Thermal properties in N2

PAI PAIDN 2 PAIDN 5 PAIDN 8 PAISN 2 PAISN 5

365 393 405 412 387 392

44.7 57.1 58.0 58.1 55.7 55.6

363 377 387 400 370 378

9.0 15.7 18.1 28.8 11.8 12.0

102 102 105 107 102 104

250 251 253 255 253 256

a

Temperature at 5% mass loss. Temperature at 10% mass loss. c CY: Char yield, Weight percentage of material left after TGA analysis at a maximum temperature of 800 °C. d Glass transition temperature and melting temperature data were recorded by DSC at a heating rate of 10 °C/min in N2. e Glass transition temperature and melting temperature data were recorded by DSC at a heating rate of 10 °C/min in N2. b

Fig. 9. DSC curves of PAI and PAINC.

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DSC was used to determine the glass transition temperature values (Tg) of the obtained PAI and PAI nanocomposites with a heating rate of 10 °C/min under N2 atmosphere. The DSC thermograms of PAI and PAINC are shown in Fig. 9. Two thermal events during analysis including glass transition temperature (Tg) and melting temperature (Tm) of PAI and PAINC were reported in Table 1. Tg is related to changes in amorphous region of polymer and depended to mobility of the polymer chains, whereas Tm is depended to kind and nature of the crystalline regions. As shown Fig. 9, the semi crystallinity of PAI and the corresponding PAINC was approved with appearance of two peaks related to Tg and Tm. These peaks in DSC thermograms completed the results from XRD pattern and approved the semi crystalline structure of PAI. From the DSC results, it was clear that the Tg and Tm of PAINC exhibited slight differences from the neat PAI. The Tg and Tm values of all PAINC were higher than that the neat PAI. This can be attributed to the “hydrogen bond barrier” effect due to the hydrogen bond between functional groups of the PAI chains and the modified LDH layers, which increased the intermolecular bonding and indicated that the nano layers help to resist the free motion of the polymer chain.

5. Conclusions A new diacid-diimide modified Mg-Al LDH was prepared from carboxylic anion of a synthesized diacid-diimide and Mg-Al LDH sheets using an ion exchange reaction. The incorporation of diacid-diimide anions in the LDH layers was analyzed by FTIR spectroscopy, XRD patterns and TGA measurements. A new semi-crystalline poly(amide-imide) containing naphthalene chromophor ring and aliphatic groups was synthesized by direct polycondensation reaction and was used as polymeric matrix for the preparation of PAI/LDH nanocomposites. Also two PAI nanocomposites were prepared from PAI and SDBS-modified LDH for further studies on their properties. The XRD pattern, FE-SEM and TEM results revealed a good distribution for LDH layers in the polymer matrix. The UV–Vis spectra of PAINC indicated that by increasing the LDH content, due to high interaction between LDH layers and polymer chain, the absorption intensities decrease. Due to presence of naphthalene ring chromophore in the PAI structure, the photoluminescence emission was observed in violet region. Thermal decomposition results in both N2 and air atmospheres indicated that the addition of LDH in the PAI matrix increased the degradation temperatures of the PAINC. Also the degradation temperatures of PAINC containing diacid-diimide modified LDH were higher than those the PAI nanocomposites containing SDBS modified LDH. According to the all results, it was obvious that due to high physical interactions such as hydrogen bonding and Van der Waals interactions along with desirable compatibility between the modified LDH with an appropriate modifier and PAI chain, an efficient improvement in thermal properties of PAI was observed. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.clay.2017.01.011.

References Acharya, H., Srivastava, S.K., Bhowmick, A.K., 2007. Synthesis of partially exfoliated EPDM/ LDH nanocomposites by solution intercalation: structural characterization and properties. Compos. Sci. Technol. 67, 2807–2816. Agag, T., Koga, T., Takeichi, T., 2001. Studies on thermal and mechanical properties of polyimide–clay nanocomposites. Polymer 42, 3399–3408. 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. Aleshinloye, A.O., Bodapati, J.B., Icil, H., 2015. Synthesis, characterization, optical and electrochemical properties of a new chiral multichromophoric system based on perylene and naphthalene diimides. J. Photochem. Photobiol. A Chem. 300, 27–37.

Chen, W., Qu, B., 2003. Structural characteristics and thermal properties of PE-g-MA/ MgAl-LDH exfoliation nanocomposites synthesized by solution intercalation. Chem. Mater. 15, 3208–3213. Gao, Y., Wu, J., Wang, Q., Wilkie, C.A., O'Hare, D., 2014. Flame retardant polymer/layered double hydroxide nanocomposites. J. Mater. Chem. A 2, 10996–11016. 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. Hajibeygi, M., Shabanian, M., 2014. Poly(amide-imide)/organoclay nanocomposites using glycine as a natural amino acid: study on fire and thermal behavior. Polym. Int. 63, 514–520. Hajibeygi, M., Faghihi, K., Shabanian, M., 2011. Synthesis and characterization of novel heat resistance poly(amide-imide)s from N,N′-[2,5-bis(4-aminobenzylidene) cyclopentanone] bistrimellitimide acid and various aromatic diamines. J. Appl. Polym. Sci. 121, 2877–2885. Hajibeygi, M., Shabanian, M., Madani, M., 2013. New thermally stable nanocomposites reinforced silicate nanoparticles containing phosphine oxide moiety based on poly(amide-imide): synthesis, characterization and flame retardancy study. J. Polym. Res. 20, 1–12. Hajibeygi, M., Shabanian, M., Khonakdar, H.A., 2015a. Zn–AL LDH reinforced nanocomposites based on new polyamide containing imide group: from synthesis to properties. Appl. Clay Sci. 114, 256–264. Hajibeygi, M., Shabanian, M., Moghanian, H., Khonakdar, H.A., Hau, 2015b. Development of one-step synthesized LDH reinforced multifunctional poly(amide-imide) matrix containing xanthene rings: study on thermal stability and flame retardancy. RSC Adv. 5, 53726–53735. 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. Hasegawa, M., Ishigami, T., Ishii, J., 2015. Optically transparent aromatic poly(ester imide)s with low coefficients of thermal expansion (1). Self-orientation behavior during solution casting process and substituent effect. Polymer 74, 1–15. 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. Hsueh, H.-B., Chen, C.-Y., 2003. Preparation and properties of LDHs/polyimide nanocomposites. Polymer 44, 1151–1161. Huang, H.-Y., Huang, T.-C., Yeh, T.-C., Tsai, C.-Y., Lai, C.-L., Tsai, M.-H., Yeh, J.-M., Chou, Y.-C., 2011. Advanced anticorrosive materials prepared from amine-capped aniline trimerbased electroactive polyimide-clay nanocomposite materials with synergistic effects of redox catalytic capability and gas barrier properties. Polymer 52, 2391–2400. Kiliaris, P., Papaspyrides, C.D., 2010. Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy. Prog. Polym. Sci. 35, 902–958. Klug, H.P., Alexander, L.E., 1974. X-Ray Diffraction Procedures: For Polycrystalline and Amorphous Materials. Wiley. 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. Kotal, M., Srivastava, S.K., Bhowmick, A.K., Chakraborty, S.K., 2011. Morphology and properties of stearate-intercalated layered double hydroxide nanoplatelet-reinforced thermoplastic polyurethane. Polym. Int. 60, 772–780. Li, S., Li, J., Wang, C.J., Wang, Q., Cader, M.Z., Lu, J., Evans, D.G., Duan, X., O'Hare, D., 2013. Cellular uptake and gene delivery using layered double hydroxide nanoparticles. J. Mater. Chem. B 1, 61–68. 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. Mahmoodi, N.M., Khorramfar, S., Najafi, F., 2011. Amine-functionalized silica nanoparticle: preparation, characterization and anionic dye removal ability. Desalination 279, 61–68. Mallakpour, S., Dinari, M., 2013. Facile synthesis of nanocomposite materials by intercalating an optically active poly(amide-imide) enclosing (l)-isoleucine moieties and azobenzene side groups into a chiral layered double hydroxide. Polymer 54, 2907–2916. 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. 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. Nayak, S., Mohapatra, L., Parida, K., 2015. Visible light-driven novel g-C3N4/NiFe-LDH composite photocatalyst with enhanced photocatalytic activity towards water oxidation and reduction reaction. J. Mater. Chem. A 3, 18622–18635. 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.

M. Hajibeygi et al. / Applied Clay Science 139 (2017) 9–19 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., Basaki, N., Khonakdar, H.A., Jafari, S.H., Hedayati, K., Wagenknecht, U., 2014. 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., Ardeshir, H., Haji-Ali, S., Moghanian, H., Hajibeygi, M., Faghihi, K., Khonakdar, H.A., Salimi, H., 2016. Efficient poly(methyl-ether-imide)/LDH nanocomposite derived from a methyl rich bisphenol: from synthesis to properties. Appl. Clay Sci. 123, 285–291. 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., Leuteritz, A., Wagenknecht, U., Heinrich, G., 2009. Self-assembling organomodified Co/Al based layered double hydroxides (LDH) via one-step route. Trans. Nonferrous Metals Soc. China 19, 1479–1482. 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.

19

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 N-phosphonium 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. Yu, C.-Y., Wu, C.-Y., 2014. Synthesis, characterization, optical and electrochemical properties of cyclopentadithiophene and fluorene based conjugated polymers containing naphthalene bisimide. Dyes Pigments 106, 81–86.