The effect of benzothiazolium surfactant modified montmorillonite content on the properties of polyamide 6 nanocomposites

Applied Clay Science 185 (2020) 105417

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Research paper

The effect of benzothiazolium surfactant modified montmorillonite content on the properties of polyamide 6 nanocomposites

T

Mohamed El Mehdi Mekhzouma, Marya Rajia, Denis Rodrigueb, Abou el kacem Qaissa, ⁎ Rachid Bouhfid (Writing - review & editing and supervision)a, a Moroccan Foundation for Advanced Science, Innovation and Research (MAScIR), Composites and Nanocomposites Center, Rabat Design Center, Rue Mohamed El Jazouli, Madinat El Irfane, 10100 Rabat, Morocco b Department of Chemical Engineering and CERMA, Laval University, Quebec G1V0A6, Canada

A R T I C LE I N FO

A B S T R A C T

Keywords: Benzothiazolium surfactant Montmorillonite Polyamide 6 Nanocomposites Mechanical properties

The study of organoclay is a vital subject in current research since various organo-modified clays have become an attractive class of organic–inorganic hybrid materials owing to their potential application as precursors in the field of polymer nanocomposites. In the present investigation, a new benzothiazolium salt, N-dodecyl-2-methylbenzothiazolium iodide (Mbzt) was designed and synthesized for use as sodium montmorillonite (NaeMt) modification by the traditional cation exchange reaction. The successful intercalation of Mbzt surfactant into the montmorillonite interlayer has been confirmed by FTIR, XRD and TGA analysis. The XRD analysis demonstrated that the intercalated surfactant adopted a tilted monolayer arrangement. The resulting benzothiazolium-modified (Mbzt-Mt) displayed a high thermal stability as compared to the unmodified (NaeMt). The desired organoclay was then subsequently mixed with polyamide 6 (PA6) by melt extrusion in a twin-screw extruder. The thermal and morphological properties of the resulting PA6/Mbzt-Mt nanocomposites were evaluated by thermogravimetric analysis (TGA) and scanning electronic microscopy (SEM). TGA analysis reveal that the thermal stability of PA6 nanocomposites slightly decrease (393–391 °C) with particles loading from 1 to 5 wt%, against raw PA6 (407 °C), while an increase in the charred residue up to 5.1% was clearly observed. In addition, a good overall dispersion of the organoclay particles in the PA6 matrix was readily observed by SEM images. The mechanical and rheological properties of nanocomposites were also evaluated. A linear increase in Young's modulus and tensile strength from the use of particles when compared to the neat PA6. In particular, a 26% increase in Young's modulus, a 11.5% increase in tensile strength for the case of PA6 nanocomposites containing 5 wt% of clay as compared to that of pure PA6. From a rheological point of view, the addition of organoclay affect the linear viscoelastic behavior of the PA6 matrix, the viscosity of nanocomposites are higher than that of pure PA6, the organoclays particles restrict the movement of PA6 macromolecular chains, which increases the viscosity. These findings are important and relevant to the elaboration of low-cost organoclay as nano-reinforcement for advanced nanocomposite materials in various application.

1. Introduction Due to its abundance in nature and its peculiar structure and chemical composition, clay has attracted important interest from both engineering and scientific viewpoints. Clay minerals are frequently used to prepare nanocomposites and nowadays many reviews are available about this topic(Sinha Ray and Okamoto, 2003; Morgan, 2006; Sengupta et al., 2007; Panwar et al., 2011; Azeez et al., 2013; Abedi and Abdouss, 2014). In recent years, polymer-layered silicate nanocomposites have attracted a great interest, both in academic institutions and industrial ⁎

sectors (Park et al., 2003; Sinha Ray and Okamoto, 2003; Eckel et al., 2004; Rodlert et al., 2004; Mekhzoum et al., 2017; Leszczyńska et al., 2007; Akat et al., 2008; Paul and Robeson, 2008), since they exhibit dramatic improvements in physical properties (Singh and Balazs, 2000; Lee and Han, 2003; Zhang et al., 2006; Rao and Pochan, 2007; Cipriano et al., 2009; Hennous et al., 2013). The incorporation of clay in polymer matrices leads to polymer nanocomposites with enhanced properties such as dimensional stability, high heat resistance, reduced gas permeability, high surface finish and colorability and to some extent enhanced mechanical properties (Shen et al., 2002; Fischer, 2003; Negrete Letoffe, 2004; Bur et al., 2005; Zeng et al., 2005; Ray, 2006;

Corresponding author. E-mail address: r.bouhfi[email protected] (R. Bouhfid).

https://doi.org/10.1016/j.clay.2019.105417 Received 28 May 2019; Received in revised form 25 November 2019; Accepted 23 December 2019 0169-1317/ Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved.

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commonly used imidazolium-based ionic liquids as organic modifier for clays, could offer cost savings making them attractive to explore (Bottino et al., 2003). Recently, (Nadeem, 2011) have prepared a series of N-benzothiazolium salts studied their physical properties and performed metathesis reactions. They demonstrated that the use of anion − types PF− 6 , BF6 and among others associated to N-benzothiazolium salt increased the thermal stability compared to iodide one. The originality and relevance of the present work lies in the combination of an engineer polymer (PA6) reinforced with a new organoclay using a benzothiazolium surfactant (Mbzt) to develop polymer/ layered silicate nanocomposites with high performance. To the best of our best knowledge, benzothiazolium-modified montmorillonite has not yet been applied as a nanofiller in nanocomposite materials. Herein, we first report the preparation and characterization of benzothiazolebased quaternary ammonium surfactant, we apply the synthesized salt as organic modifier for montmorillonite clay to produce a newly hydrophobic organoclay. The obtained organoclay was investigated using different techniques such as FTIR, XRD as well TGA analysis. The MbztMt was then incorporated into PA6 matrix via melt mixing technique to manufacturePA6/Mbzt-Mt nanocomposites. To this end, the resulting nanocomposites were characterized and discussed on the basis of their structural, thermal, rheological and mechanical properties.

Jankoviuboš et al., 2011; Theng, 2012). Such enhancement of properties is obtained through dispersion / distribution and favorable specific interactions of the nanosized clay lamellae with the polymeric matrix (Olad, 1996; Malwitz et al., 2003; Herrera et al., 2004; Jang et al., 2005; Zhang et al., 2006; Trulove et al., 2007; Chen et al., 2008; Kiliaris and Papaspyrides, 2010; Huskić et al., 2013; Suter et al., 2015). The literature reported that the addition of silicates nanolayers especially montmorillonite into polymer matrix has been extensively studied (Di Gianni et al., 2008; Mishra et al., 2009; Yebra-Rodríguez et al., 2009; Gârea et al., 2010; Zhao et al., 2010; Silva et al., 2011; Zhu et al., 2011; Huskić et al., 2013; Natkański et al., 2013). Generally, three main methods used to incorporate inorganic additives to polymers; including in situ polymerization of monomer/clay intercalates, solution-intercalation, and melt compounding (Leu et al., 2002). The last route is privileged for its industrial advantage (Bousmina, 2006). In this technique, no solvent is required and the layered silicate is mixed within the polymer matrix in the molten state (Shelly and Burnside, 1995; Vaia et al., 1996; Rehab and Salahuddin, 2005). In this sense, a thermoplastic polymer is mechanically mixed by conventional methods such as extrusion and injection molding with organophillic clay at an elevated temperature. The polymer chains are then intercalated or exfoliated to form nanocomposites (Kornmann et al., 2001). A key factor in the nanocomposite preparation is to compatibilize the matrix and the filler, which can affect the nanostructure and, consequently, the factors controlling the interface polymer/nanoclay, contributing to the design of systems for valued-added applications (Akbari and Bagheri, 2012). The first step in the fabrication process of polymer nanocomposites consists of switching the hydrophilic nature of clay to hydrophobic to ensure favorable specific interactions with the polymer host. This is usually obtained by replacing the small counter ions atoms namely Na+, K+, Li+ by bigger amphiphilic molecules anchoring a cation head and an alkyl chain tail that expands the d-spacing between the galleries (Marras et al., 2007). Because most polymers are organophilic, organomodified layered silicates are used to obtain better affinity between the filler and matrix. One of the more common modification methods is the introduction of a commercial available, surfactants ammonium/phosphonium salt, bearing a suitable organic functionality, inside the interlayer space through a cation exchange reaction (Giannini et al., 2001; de Paiva et al., 2008). Unfortunately, the low thermal stability of these surfactants presents a problem for melt compounding and processing of polymer nanocomposites, where high processing temperatures exceeding 180 °C are commonly encountered (Xie et al., 2001). For some polymers this is acceptable, but for engineering polymers, for example polyamides (PA6, PA66), poly(ethyleneterephthalate) (PET), and polycarbonate (PC), more thermally stable clays are required to prepare nanocomposites with high properties by melt-compounding. To enhance the thermal stability of the organically modified clays, the use of more thermally stable compounds can offer a new alternative to the ammonium salts (Gilman et al., 2002). Previous studies in our laboratory have shown that the use of several kinds of heterocyclic surfactants bearing long alkyl chains linked to aromatic groups as organic modification of Mt. clay, provide high thermally stable organclays, with significant expansion of the (d001) basal spacing. These hydrophobic organclays were used to manufacture polymer–organoclay nanocomposites with improved thermal, mechanical and rheological properties (El Achaby et al., 2013; Fardioui et al., 2016; Mekhzoum et al., 2016a, 2016b; Raji et al., 2018; El Bourakadi et al., 2019; El Bourakadi et al., 2019). These remarkable results encourage us to investigate a new surfactant based on benzothiazole moiety. Benzothiazole can be viewed as a homologue of thiazole, therefore it may be possible to produce a cationic surfactant with similar, or superior, thermal properties to thiazolium salt by the quaternization of benzothiazole heterocyclic nitrogen (Yan et al., 2015). Most Literature reports of the use of the benzothiazolium cation are limited (Sutoris et al., 1983; Zahradnik et al., 1996; Tatay et al., 2006; Krajnáková et al., 2010; Zhou et al., 2011).Besides, the use benzothiazolium-based salts over the now

2. Experimental 2.1. Materials Reagents and solvents were purchased from commercial suppliers and were used without further purification. The unmodified nanoclay (sodium montmorillonite, NaeMt) used in this study was obtained from Southern Clay Products, Inc., USA. The cation exchange capacity (CEC) of the NaeMt is 92 mequiv/100 g of clay. The organic modifier, benzothiazolium surfactant was synthesized from 2-methylbenzothiazole and 1-iodododecane, procured from Sigma-Aldrich. Polyamide 6 (PA6) was obtained from Ultramid O-BASF the chemical company, (density: 1.13 g/mL and melting temperature: 220 °C). 2.2. Characterization techniques 1

H and 13C NMR spectra were recorded in DMSO‑d6 solution with TMS as an internal reference using an Avance 300 (Bruker) instrument, chemical shifts. Multiplicities of 13C NMR resources were assigned by distortion less enhancement by polarization transfer (DEPT) experiments. Accurate mass measurement ESI-MS spectrum for compound was acquired in the positive ion mode using a SYNAPT G2 HDMS quadrupole orthogonal time-of-fligh (QqToF)-MS/MS mass spectrometer (WATERS).Thermogravimetric analysis (TGA) were performed on a Q500 (TA instrument) using a heating rate of 10 °C/min from room temperature to 800 °C under air. IR spectra were recorded on an ABB Bomem FTLA 2000–102 FTIR instrument (ATR: SPECAC GOLDEN GATE). Morphology of the samples was evaluated from a cryofracture surface using a scanning electron microscope (FEI, Quanta 200-ESEM) operating at 25 kV. The tensile properties were determined according to ISO 527-1:2012. The X-ray diffraction patterns are recorded on a Bruker D2-Phaser diffractometer (Bruker Corp), using Cu Kα radiation, (λ = 1.54 nm) Å (30 kV, 10 mA). The tests were performed on a universal testing machine INSTRON 8821S (Instron, USA) at a crosshead speed of 5 mm/min using a 5 kN load cell. Young's modulus and tensile strength were obtained from the machine software. A RSA II rheometer (TA Instruments) was used to evaluate the dynamic mechanical properties (DMA) of the neat polymer and the nanocomposites. The samples were cut to 70 mm length, 8.5 mm width, and 0.83 mm thickness. The geometry used was a dual cantilever configuration. Torsion tests were performed on an ARES-LS Rheometer using the rectangular torsion mode with specimens having required dimensions made on the machine conception: 70 mm length, 8.5 mm width, and 2

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analysis of the product obtained from the quaternization reaction also supports the formation of benzothiazole surfactant. Yield 63%. mp 116 °C. IR (ATR) 2918, 2852, 1581, 1517, 1465, 1440, 771. 1H NMR (300 MHz, DMSO‑d6) δ 0.81 (t, J = 6.1 Hz, 3H), 1.15–1.30 (m, 16H), 1.24–1.50 (m, 2H), 1.73–1.91 (m, 2H), 3.22 (s, 3H), 4.70 (t, J = 7.8 Hz, 2H), 7.71–7.95 (m, 2H), 8.34 (d, J = 8.4 Hz, 1H), 8.46 (dd, J = 8.1, 1.3 Hz, 1H)·13C NMR (75 MHz, DMSO) δ 14.4, 17.6, 22.6, 26.3, 28.3, 29.1, 29.2, 29.3, 29.4, 29.5, 29.5, 31.8, 49.8, 117.4, 125.1, 128.5, 129.5, 129.8, 141.3, 177.4. HRMS-ESI(+): calculated for [M-H]+ C20H32NS+: 319.2849; found: 319.2504.

0.83 mm thickness. The torsion modulus was obtained at a constant strain 0.002 by small amplitude oscillatory tests performed via frequency sweeps (0.1–10 Hz). All tests were carried out on a minimum of three samples and the reported results are average values. Prior to rheological measurements, the samples were molded into disc of 0.83 mm thickness and 25 mm diameter. Oscillatory melt rheology measurements were carried out on a MCR 500 (Physica, Anton Paar) rheometer equipped with a CTD600 device, at 220 °C with 25 mm parallel plate geometry. The dynamic viscoelastic properties were determined using low strain values (within the linear viscoelastic region) for these materials with a frequency range from 0.05 to 500 Hz.

2.4. Mbzt-Mt salt preparation 2.3. Benzothiazolium quaternary salt (Mbzt) preparation

For organophillization, Mt clay was treated with N-dodecyl-2-methylbenzothiazolium salt according to the procedure reported by (Vaia et al., 1994). Namely, in a two-neck round-bottom flask of 1 L equipped with a condenser, a mixture of 500 ml of water: ethanol (1:1) and 6 g of NaeMt was vigorously stirred for 3 h and sonicated at high speed for30 min. Sonication is necessary to homogenously disperse the Mt. in the mixture. The amount of surfactant in the interlayer space was 1.5 times the CEC of NaeMt (92 meq/100 g). The suspension was heated at 80 °C after which a solution of benzothiazolium salt (3.7 g) in ethanol (100 mL) was added dropwise over 1 h. The stirring was continued for 24 h at 80 °C. The organoclay was isolated by centrifugation, washed first by water–ethanol (1:1) solution and by deionized water (three times) and then dried in oven overnight at 80 °C. The montmorillonite Mt. modified with N-dodecyl-2-methylbenzothiazolium iodide was named Mbzt-Mt.

Literature survey has revealed that benzothiazolium derivatives are currently used as antimalarial agents, ionic liquids as well as coupling agents for bio-composite materials (Takasu, 2016; He et al., 2017; Hassani et al., 2020). In this work, the synthesized surfactant (Mbzt) possess long alkyl chain attached to the N-thiazole moiety. The alkyl chain length of the Mbzt salt improve the organophilicity of Mt. clay, increase the gallery of silicate layer as well as enhance the compatibility between the nano-charge and the polymer matrix. The synthesis of the desire salt N-dodecyl-2-methylbenzothiazolium iodide is shown in Scheme 1. The general method used for the synthesis of N-dodecyl-2-methylbenzothiazolium iodide was as follows (Mekhzoum et al., 2019): to an acetonitrile solution (25 mL) of 2-methylbenzothiazole (2.86 mL; 22.60 mmol) was added 1-iodododecane (12.25 mL; 42.20 mmol). The mixture was refluxed for 24 h. After cooling, the mixture was poured into ether (150 mL) to allow complete precipitation. The resulting quaternary salt collected by filtration under reduced pressure and washed with ether. Drying under reduced pressure to afford spectroscopically pure salt. The product was fully characterized by IR, NMR and mass spectroscopy. The most intense and characteristic bands in FTIR spectra of the Mbzt core appear around 3000 cm−1 (CeH from AreH stretch), 2924–2852 cm−1 (CeH from CH2 stretch), 1635–1578 (C=C), 1524–1509 cm−1 (C=N), 1467–1428 cm−1(CH3 asymmetric deformation and CH2 deformation) and 781–757 cm−1 (skeletal “out of plane" vibration). The 1H NMR absorption of the 2-CH3 group typically observed at 3.22 ppm. While, the 13C NMR chemical shift of 2-C was appeared at 17.6 ppm region. According to NMR spectroscopy, the product was of satisfactory purity and, therefore, no attempts at further purification by recrystallization were necessary. Mass spectrometry

2.5. PA6/Mbzt-Mt nanocomposites preparation PA6 nanocomposites containing different modified clay contents (1, 2, 3, 4 and 5 wt%) were prepared via melt-compounding method using a minilab twin-screw mixer (Haake Minilab II) at 220 °C with a screw rotating speed of 60 rpm. Before processing, the PA6 granules are grinded in a precision grinder (FRITSCH Pulverisette 19) equipped with a 1 mm sieve size were dried overnight at 80 °C, while the organoclay powder was ground in a mortar and sifted through 200-meshsieves (D = 100 μm) and then dried at 80 °C before mixing. Samples for characterizations and measurements were prepared by hot-press molding machine using a press carver at a temperature of 220 °C. Hereafter, the nanocomposite samples are designated as PA6, PA6/ Mbzt-Mtx, where x indicates the weight percent of Mbzt clay (1, 2, 3, 4

Scheme 1. The synthetic route for the preparation of the benzothiazolium surfactant (Mbzt). 3

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Fig. 1. Fourier transform infrared (FTIR) spectra of Mbzt, Mbzt-Mt and NaeMt. Fig. 2. The XDR patterns of NaeMt and Mbzt surfactant intercalated-montmorillonite clay.

and 5 wt%). 3. Results and discussion

upon two factors: one is the presence of large hydrophobic group on surfactant and second is the decrease in surface energy of montmorillonite. As the size of hydrophobic group increases the basal spacing increases to a large extent (Benbayer et al., 2015; Taleb et al., 2018). According to the literature, the molecular conformation of the intercalated cationic surfactant is strongly dependent on the size, concentration of the organic cations as well as the CEC values of the clay minerals (Zawarh et al., 2014). The 3D structure of Mbzt was created using ChemDraw 15.1 and exported to Chem3D for energy minimization. The interlayer spacing (clearance space) was 0.53 nm, obtained by subtracting the thickness of an individual layer of Mt. (0.96 nm) from the experimental basal spacing. Taking into account the height of both the alkyl chain and the 2-methylbenzothiazolium head group (Fig. 3), we can expect that the cationic surfactant was arranged in a lateralmonolayer with head group tilted with respect to the alkyl chain (Fig. 4). The preference of a tilted position may be favored by the suggested repelling action of the sulphur atom present in the Mbzt regarding the surface oxygens of the Mt. structure. This is considered to

3.1. Mbzt-Mt characterization 3.1.1. FTIR analysis The FTIR was performed on the modified clay to confirm the attachment of the reactive surfactant onto the clay. The successful modification of Mt. by benzothiazolium salt (Mbzt) has been evidenced by FTIR analysis. Fig. 1 shows the spectra of Mbzt, Mbzt-Mt and NaeMt. A longer hydrocarbon chain in Mbzt gives significantly stronger peaks in the ranges of 3000–2800 cm−1 and 1650-1440 cm−1. Beside the framework stretching vibration of the Si–O–Si at 1000 cm−1, two new peaks at 2925 and 2850 cm−1 can be attributed to the CeH band appeared in infrared spectra of Mbzt-Mt. The introduction of the salt in the interlayer space caused band shift from 2919 to 2853 cm−1 in the neat Mbzt salt to 2925–2850 cm−1, which reflected a liquid-like molecular environment of the intercalated surfactant (Zhang et al., 2005). Furthermore, 1465 cm−1 and 771 cm−1 peaks are attributed to –CH2– scissoring vibration absorption and to –CH2– vibration absorption, respectively. Consequently, the presence of commons peaks in Mbzt and Mbzt-Mt confirmed the presence of the organic guest molecule in the clay by cation exchange. These results also indicated that the presence of characteristic peaks of montmorillonite revealed the maintained chemical structure of clay after being intercalated by Mbzt surfactant (Pandey et al., 2014). 3.1.2. X-ray diffraction The X-ray diffraction patterns of the eNa-Mt and Mbzt-Mt in the range of 2–17° (2θ) were presented in Fig. 2. The d-spacing or gallery spacing between Mt. sheets after organic treatment was determined by XRD. The eNa-Mt shows the peak corresponding to the reflexion of the (001) plane at 2θ = 7.53°, the basal spacing was calculated to be 1.17 nm. This value is in good agreement with similar hydrated clays reported in the literature (Mallakpour and Dinari, 2013). After modification of sodium-montmorillonite by treating it with the corresponding benzothiazolium surfactant, the Mbzt-Mt showed that the layered structure is retained. The movement of the diffraction peak of Mbzt-Mt to lower angle (2θ = 5.93°) indicated that intercalation structure has been formed. The basal spacing of Mbzt-Mt was estimated to be 1.49 nm. Therefore, the increase in gallery spacing from 1.17 to 1.49 nm, confirmed the synthesis of the benzothiazolium-based montmorillonite. In general, the increase in value of basal spacing depends

Fig. 3. Molecular conformation and dimension of Mbzt surfactant. 4

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Fig. 4. Possible orientation of Mbzt surfactant within Mt. clay layers.

Fig. 5. (a) Thermal degradation weight loss, (b) Derivative plot of thermal degradation of profiles Mbzt, Mbzt-Mt and NaeMt.

5

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Fig. 6. (a) TGA decomposition curves for the samples, (b) DTG curves for the samples.

rate of the organo-Mt decomposition process (Ding et al., 2005). Thermal stability under air atmosphere of pure Mbzt salt, Mbzt-Mt and NaeMt is investigated by thermogravimetric analysis (TGA). The TGA and derivative thermogravimetric (DTG) curves of all samples are showed in Fig. 5.For the parent NaeMt, the first mass loss below 200 °C is assigned to the decomposition of physically adsorbed water and water molecules around the exchangeable sites in Mt (Chen and Yang, 2002; Vazquez et al., 2008). Whereas, the second mass loss step is observed at 658 °C with a mass loss of 8.43% assigned to the dehydroxylation of the structure water of the Mt. It can be concluded that the structure of NaeMt is relatively stable and will maintain the layer structure up to 800 °C. Previous studies presented different decomposition steps for organo-modified Mt. including water desorption and dehydration, surfactant decomposition and dehydroxylation (Xi et al., 2007; Zhou et al., 2009). In this sence, the presence of organic cations increases the number of decomposition steps. The steps of decomposition/degradation were observed for the Mbzt-Mt and compared to pure NaeMt, as shown by DTG curve in Fig. 5b. The major difference

Table 1 Thermal decomposition results of PA6 nanocomposites. Samples

Tonseta (°C)

T5wt% (°C)

Tmax(°C)

Char yield at 700 (%)

PA6 PA6/Mbzt-Mt 1% PA6/Mbzt-Mt 3% PA6/Mbzt-Mt 5%

407.20 393.96 394.87 391.74

362.87 341.83 334.59 332.45

448.04 435.53 430.16 425.23

0 2.91 3.02 5.10

a Tonset: Temperature corresponding to the cross section of the two tangents around the main degradation point in TGA thermogram.

be the factor responsible for prevention of insertion of the Mbzt perpendicular to the layers (Horvath and Luptakova, 1991). 3.1.3. Thermal analysis Thermal stability plays an essential role in determining both technological applications and processing conditions of organo-Mt. At a given temperature of the organo-Mt weight loss is directly related to the 6

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its intercalated montmorillonite, the Tonset of Mbzt-Mt is 225 °C and the T5wt%is 245 °C. These two temperatures are higher than those observed for pure Mbzt by 47 °C and 77 °C respectively. According to these data, the Mbzt-Mt exhibit dramatic improvement in terms of thermal stability compared to benzothiazolium iodide salt which may attribute to the removal of the halide effect. This reflects the importance of getting rid of the entire halide residue that may contaminate the intercalated product after the ion exchange. The TGA results corroborate well with obtained XRD and FT-IR data and show that the intercalation of cationic salt has been really achieved. Moreover, the weight loss of the Mbzt-Mt is approximately negligible at 220 °C (mass loss ≈2.86% < 5%), the temperature in which the nanocomposites are processed. Therefore, the new Mbzt-Mt formulation seems to be a promising candidate for designing PA6 nanocomposite materials. 3.2. PA6/Mbzt-Mt nanocomposites characterization 3.2.1. Thermal analysis The thermal behavior of the PA6 nanocomposites has been investigated by thermogravimetric analysis (TGA). The TGA curves and the corresponding derivative curves (DTG) of virgin PA6, PA6/Mbzt-Mt 1%, PA6/Mbzt-Mt 3% and PA6/Mbzt-Mt 5% under air atmosphere are shown in Fig. 6. The thermogravimetric analyses were made at a rate of 10 °C/min to measure four parameters: the onset temperature of thermal degradation Tonset, the 5% weight loss temperature T5wt%, the maximum degradation temperature Tmax, and the charred residue at 700 °C. The results are listed in Table 1. First, all of the samples displayed a minor weight loss observed above 70 °C. It can be assigned to the loss of physically sorbed water, while double stage decomposition (as evidenced by a shoulder/peak) occurs in the range between 300 and 600 °C, attributed to the structural decomposition of PA6 matrix. On further heating, all simples lead to the formation of a stable carbonaceous material which then decomposes above 600 °C leading to a stable charred residue. From the TGA graphs, the curves profiles are apparently similar. However, the course of the decomposition seems to have been affected by the structure of the Mbzt-Mt. Except for PA6, there are no outstanding thermal degradation differences between all PA6 nanocomposites but the residues left behind after decomposition are different. It can be seen that when 1 wt% Mbzt-Mt was added to PA6, both Tonset and T5wt%decrease and the charred residue at 700 °C increases. While 5 wt% of organoclay was added, the differences become larger. T5wt%decrease by 30.4 °C and the maximum charred residue was reached at 5.1%. These results indicate that the PA6 nanocomposites have somewhat lower thermal stability than neat PA6. This could be caused by the thermal degradation of the organic treatment on the Mt. (VanderHart et al., 2001; Davis et al., 2003; Fornes et al., 2003). Meanwhile, the clay itself could also catalyze the degradation of polymer matrix (Zhao et al., 2005). On the other hand, layered silicate PA6 nanocomposites, even in fully exfoliated systems, typically do not improve the thermal stability of the polymer(Jang and Wilkie, 2005). In light of these facts, (Pramoda et al., 2003) observed that the degradation onset temperature is 12 °C higher for PA6 with 2.5% clay loading than that of virgin PA6 as well the onset temperature for the higher clay loading remained unchanged, which is contrary to our results. Additionally, literature reported TGA experiment on the PA6/clay nanocomposites in which no significant changes were observed in the onset of degradation (Kashiwagi et al., 2004). In another work, (Gilman et al., 1997) reported that the fire retardant properties of nylon 6 nanocomposites were improved. In this case, the individual layers of clay act as an insulator and a mass transport barrier against oxygen or volatile degradation products generated as the PA6 decomposes. Alternatively, they did not find any differences in thermal stability.

Fig. 7. SEM images of cryofracture surface of PA6 and PA6/Mbzt-Mt nanocomposites with the content of Mbzt-Mt (a) 1 wt%, (b) 3 wt% and (c) 5 wt%.

between the thermal decomposition of NaeMt and organically modified clay is in the range of 240–500 °C. Apart from the peak before 200 °C and the peak at about 589 °C that have been identified previously, one peak has been appeared at around 273 °C, attributed to the decomposition of the ion-exchanged Mbzt surfactant, as intercalation of the surfactant caused a relatively high phase-transformation temperature with respect to the free surfactant (Jianxi et al., 2003; He et al., 2005). It is obviously evident that the low amount of free water trapped in the Mbzt-Mt salt ends at 150 °C: lost about ≈1.53% of water as moisture content, would be a result of its hydrophobic nature compared to NaeMt (approximately 5.64%). For pure Mbzt salt, the 5% weight loss temperature T5wt% is 168 °C and the onset temperature of thermal degradation Tonset is 178 °Cas shown in Fig. 5a. Interestingly, the synthesized N-dodecyl-2-methylbenzothiazolium iodide was slightly more thermally stable than the Ndodecylbenzothiazolium iodide (Nadeem et al., 2010). This may be due to the high acidic character of the C-2 proton (i.e. between the N and S atoms) (Carlin and Fuller, 2002). As in the case of benzimidazole (Costache et al., 2007), if the fusion of benzene and thiazole ring increases the charge delocalization, one might expect further enhanced thermal stability. One of the most interesting findings of this study is the enhancement in the thermal stability of Mbzt salt after being intercalated into Mt. For an accurate comparison between pure salt and

3.2.2. SEM characterization The dispersion of the nanofillers and the compatibility between the nanofillers and the matrix are two main factors affecting the final 7

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Fig. 8. Plot of strain-stress of the PA6 nanocomposites at different Mbzt-Mt clay contents.

Fig. 9. Young's modulus and tensile strength of PA6 nanocomposites.

Table 2 Tensile properties of PA6/Mbzt-Mt nanocomposites. Samples PA6 PA6/Mbzt-Mt PA6/Mbzt-Mt PA6/Mbzt-Mt PA6/Mbzt-Mt PA6/Mbzt-Mt

1% 2% 3% 4% 5%

Young's modulus (MPa)

% Inc.

Tensile strength (MPa)

% Inc.

1202 1214 1302 1386 1442 1512

– 1 8.3 15.3 20 26

40.8 ± 1.7 41.9 ± 0.8 43.23 ± 8.7 44.60 ± 2.4 45.52 ± 1.6 45.54 ± 6.5

– 2.7 6 9.3 11.6 11.6

± ± ± ± ± ±

7 11 22 9 10 15

Data presented here are averages of three independent measurements; % Inc. percentage increase (%).

micrographs, the Mbzt-Mt is uniformly dispersed in the matrix, which indicates the good processing conditions. Furthermore, the absence of the pullout phenomena and the lack of the voids around the organoclay particles indicate the good affinity/interfacial adhesion between PA6 polymer matrix and the Mbzt-Mt. It can be seen that there is no evidence of clay aggregates within the two first samples (1, 3 wt%).

properties of the nanocomposite materials (Adhikari and Michler, 2009). Micrographs of the fractured samples were used to determine the degree of nanoclay dispersion/distribution and their affinity with the matrix. Fig. 7 shows the SEM images of the cryofracture surface of the neat PA6 as well as the PA6 nanocomposites with differing Mbzt-Mt contents (1, 3 and 5 wt%) manufactured by melt compounding. For all 8

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Fig. 10. The evolution of (a) torsional modulus, (b) loss modulus of PA6 nanocomposite as a function of clay contents.

properties of PA6 matrix. Fig. 9 illustrates the variation of tensile properties of the PA6 nanocomposites in terms of Young's modulus and tensile strength. Table 2 lists these parameters obtained with different Mbzt-Mt clay content. It is observed from the Figure 9that the addition of organoclay leads to substantial improvement in stiffness of all nanocomposites compared to that of bulk PA6. Young's modulus, plotted against clay content shows an increase as the clay content increases, more pronounced them for the highest clay filled sample. For instance, the Young's modulus is significantly increased by 26% from 1207 MPa for pure PA6 to a maximum value 1512 MPa for the nanocomposite that contains 5 wt% of Mbzt-Mt. The enhancement of the tensile properties can be attributed to the high resistance exerted by the clay nanoparticles themselves, their good dispersion and the strong interaction between polymer and organo-modified Mt. (Carrado and Xu, 1998; Százdi et al., 2007). For the tensile yield strength, the addition of clay particles to the PA6 matrix resulted in changes in the mechanical properties. It is worth to notice that the tensile strengths for PA6/nanocomposite specimens

However, small amount of agglomerates were observed with some visible clay particles, especially at high loading of Mbzt-Mt 5 wt%. In all selected samples, no presence of bulk aggregation or separated phases were observed witnessing thus on a good homogeneity within the whole material. 3.2.3. Tensile properties Mechanical properties of the materials indicate how the material would respond to forces being applied in tension Semlali Aouragh Hassani et al., 2019. A good dispersion and interfacial adhesion between the matrix and nanofiller are both critical factors for the resulting materials to achieve improved mechanical properties (Essabir et al., 2013b). The effect of clay content on the mechanical properties of PA6 nanocomposites has been studied by tensile testing. Typical stress–strain curves obtained by uniaxial tensile testing of the neat PA6 and PA6/Mbzt-Mt nanocomposites are presented in Fig. 8. It is clear that for a fixed value of strain, an increase of tensile stress is observed with the addition of Mbzt-Mt content, which affects in turn some tensile 9

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Fig. 11. Evolution of dynamical mechanical properties as the functions of clay content and frequency: (a) Complex modulus, (b) Loss factor.

between 0.1 and 10 Hz. It is illustrated from the plots that the torsional modulus is affected and shows a remarkable increase with increasing clay loading as stiff filler until a threshold is reached at 3 wt%, due to the good interfacial adhesion between the PA6 matrix and the clay nanoparticle. After which a drop-in modulus becomes very significant. This could be explained by the presence of agglomerate at high clay content. It can clearly be seen that the evolution of the torsion modulus saves the same allure with varying the frequency from 0.1 to 10 Hz, leading to conclude that the PA6 nanocomposites response is like an elastic solid (Essabir et al., 2016a,b). More than a few studies reported that the molecular time response is responsible for changes in the rheological properties of nanocomposites as a function of frequency. In practically, the elastic character of the material at higher frequency prevails over a viscous behavior. To investigate the elastic and viscous behaviors of PA6 nanocomposites, the evolution of tan δ versus frequencies and clay particles loading is demonstrate in Fig. 10b. It can be observed that the ratio of the loss modulus and the storage modulus is decreasing from the 0.1 Hz to 10 Hz when clay nanoparticles were added. This is the common situation in solid state as the elastic

correspondingly increase with increasing Mbzt-Mt content (up to 4 wt %). The curves show that tensile strength is the lowest when no clay is added. PA6/Mbzt-Mt 4 wt% specimen displayed the highest strength value, then stabilized at 5 wt% of clay content. The improvement of tensile strength is more extensive and reached (45.54 MPa), a gain of 11.6% compared to the un-reinforced PA6 sample. This is because yield strength is very sensitive to the degree of clay dispersion in polymer matrix (Liu et al., 2003). The enhancement in tensile strength can be explained by the enhanced adhesion of interfacial properties between PA6 and layered silicates. PA6 is a polar polymer and contains polar groups such as –NH, –CONH–, –NH2 and –COOH. These polar groups have favorable interactions with the polar silicate surface showing a higher compatibility between filler and matrix, hence higher reinforcement effect (Shishan et al., 2004).

3.2.4. Torsional properties Torsional testing is used to describe the nanocomposites response to shear stress. Fig. 10a shows the effects of Mbzt-Mt charges on torsional modulus (G⁎) of the nanocomposite systems in the frequency range 10

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Fig. 12. (a) Storage modulus (G'), (b) loss modulus (G") and (c) complex viscosity (η*) versus frequency for neat PA6 and PA6 nanocomposites containing 1, 2, 3, 4 and 5 wt% of Mbzt-Mt. 11

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space providing an increase in the basal spacing and affording an inorganic–organic material. The size characteristics of surfactant with the clearance space structure affect its arrangement / orientation. A monolayer-type arrangement was obtained with tilted position of the benzothiazolium head group inside of the montmorillonite gallery. In addition, a significant enhancement in thermal stability of the intercalated surfactant was observed compared to the neat one. The PA6/ Mbzt-Mt nanocomposites have been prepared with different organoclay contents by melt blending. Morphological, thermal, mechanical and rheological behaviors of the nanocomposites were studied. A good dispersion/distribution of organoclay within the polymer matrix was achieved as observed by SEM images. However, increasing the clay nanoparticules loading up 5 wt%, tends to increase aggregation/agglomeration. It can be noticed that the thermal stability of PA6 was decreased by the addition of Mbzt-Mt content within the nanocomposite. On the other hand, the largest improvement in nanocomposite mechanical properties occurred at high organoclay loading level 5 wt%. Rheological behavior of the nanocomposites shows higher storage modulus and complex viscosity than the unfilled PA6. These enhanced properties demonstrate the reinforcing effect of nano-scale dispersed layered silicates in PA6 matrix. Finally, these results, in turn, highlight the importance of the role of the prepared Mbzt-Mt, opening the door to further investigations of new heterocyclic surfactant-modified clays as potential nanofiller for clay polymer nanocomposites with improved physical and mechanical properties.

character of the material at room temperature prevails over a viscous behavior (Arrakhiz et al., 2013). 3.2.5. Dynamic mechanical properties Fig. 11 shows the effect of clay content and frequency (0.15 Hz, 1.5 Hz and 15 Hz) on viscoelastic properties of nanocomposites. It is clear that a change in both complex modulus and loss factor occurs due to clay content and frequencies. Therefore, clay particles cause an increase in the modulus until it reaches a maximum at 3 wt% assigned to changes in nanocomposite's dynamic mechanical properties. Above this content, a rapid decrease has been noticed. The incorporation of organoclay into the PA6 matrix significantly increased the complex modulus (E*) of the resulting nanocomposites compared to the neat PA6, this increase is attributed to the addition of rigid particles leading to limited the polymer chain mobility with respect to molecular dynamics (Essabir et al., 2013a). In addition, the complex modulus increases also with increasing frequencies from 0.15 to 15 Hz (Fig. 11a). Fig. 11b shows the evolution of the loss factor (tanδ), which represents the ratio between the loss (E") and storage modulus (E'), as a function of clay content. It is clear that the loss factor modulus is slightly decreases with the increase in clay content. On the other hand, a decrease with increasing frequency for all nanocomposite systems has been observed. Thus, this result can be explained by the elastic character of the material prevails over the viscous behavior (Karbhari and Wang, 2004). 3.2.6. Rheological properties The rheological properties of PA6/Mbzt-Mt nanocomposites versus frequency and Mbzt-Mt content were illustrated in Fig. 12. As can be seen from Fig. 12a, b, both of the storage (G') and loss modulus (G") of the PA6 nanocomposites increase with increasing clay nanoparticles content and frequency and are generally higher than of the neat matrix. This behavior can be explained by structural changes in the polymer nanocomposites; the storage modulus of the PA6 nanocomposites present a plateau at low frequency or called also the Newtonian behavior indicating a percolated network of clay particles (Bensalah et al., 2017). Furthermore, because of the highly sensitive of storage modulus to nanocomposites structural changes, the PA6 nanocomposites filled by 3 wt% of clay particles shows a high storage moduli value at low frequencies as compared to that of other nanocomposites which attributed to the strong interaction between clay layers and polymer matrix chains. Then at high frequencies the elastic character prevails over the nanocomposites viscous behavior with rigid particle addition which attributed to the formation of a volume spanning mesoscale Mbzt-Mt network, (Raji et al., 2016) moreover the polymer exhibit a chains dynamic more and more a solid-like response in molten state that is generally associated to insufficient time to chain relax allowing to increase the elastic nature of the nanocomposites melt (Raji et al., 2017). Fig. 12c show the complex viscosity (η*) of the nanocomposites as a function of clay nanoparticles content and frequency. As expected, the addition of Mbzt-Mt affects the polymer microstructure chain response in the melt state. It can be observed that the complex viscosity of PA6 nanocomposites increases with increasing clay nanoparticles content (Banerjee et al., 2013). This behavior can be attributed to the particlematrix interactions resulting from strong hydrogen bond interactions which have an important role in increasing the viscosity (Hamid Essabir et al., 2016). Another fact is that at high frequencies there is no enough time for the polymer chain to respond to the applied oscillation which leads to the slight decrease in complex viscosity (El Achaby et al., 2013).

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by MAScIR Foundation, MESRSFC and CNRST, Morocco Grant No. 1970/15. The authors would like to thank Mr. Mehdi Ait Dahi for his fruitful technical support and assistance. References Abedi, S., Abdouss, M., 2014. A review of clay-supported Ziegler-Natta catalysts for production of polyolefin/clay nanocomposites through in situ polymerization. Appl. Catal. A Gen. 475, 386–409. https://doi.org/10.1016/j.apcata.2014.01.028. Adhikari, R., Michler, G.H., 2009. Polymer nanocomposites characterization by microscopy. Polym. Rev. 49, 141–180. https://doi.org/10.1080/15583720903048094. Akat, H., Tasdelen, M.A., Du Prez, F., Yagci, Y., 2008. Synthesis and characterization of polymer/clay nanocomposites by intercalated chain transfer agent. Eur. Polym. J. 44, 1949–1954. https://doi.org/10.1016/j.eurpolymj.2008.04.018. Akbari, B., Bagheri, R., 2012. Influence of compatibilizer and processing conditions on morphology, mechanical properties, and deformation mechanism of PP/clay nanocomposite. J. Nanomater. 2012. https://doi.org/10.1155/2012/810623. Arrakhiz, F.Z., Malha, M., Bouhfid, R., Benmoussa, K., Qaiss, a., 2013. Tensile, flexural and torsional properties of chemically treated alfa, coir and bagasse reinforced polypropylene. Compos. Part B Eng. 47, 35–41. https://doi.org/10.1016/j. compositesb.2012.10.046. Azeez, A.A., Rhee, K.Y., Park, S.J., Hui, D., 2013. Epoxy clay nanocomposites - Processing, properties and applications: a review. Compos. Part B Eng. 45, 308–320. https://doi. org/10.1016/j.compositesb.2012.04.012. Banerjee, S., Joshi, M., Ghosh, A.K., 2013. Investigations on clay dispersion in polypropylene/clay nanocomposites using rheological and microscopic analysis. J. Appl. Polym. Sci. 130, 4464–4473. https://doi.org/10.1002/app.39590. Benbayer, C., Saidi-Besbes, S., Taffin De Givenchy, E., Amigoni, S., Guittard, F., Derdour, A., 2015. Synergistic effect of organoclay fillers based on fluorinated surfmers for preparation of polystyrene nanocomposites. J. Appl. Polym. Sci. 132, 1–12. https:// doi.org/10.1002/app.42347. Bensalah, H., Gueraoui, K., Essabir, H., Rodrigue, D., Bouhfid, R., Qaiss, A.E.K., 2017. Mechanical, thermal, and rheological properties of polypropylene hybrid composites based clay and graphite. J. Compos. Mater. https://doi.org/10.1177/ 0021998317690597. 002199831769059. Bottino, F., Fabbri, E., Fragalà, I.L., Malandrino, G., Orestano, A., Pilati, F., Pollicino, A., 2003. Polystyrene-clay nanocomposites prepared with polymerizable imidazolium surfactants. Macromol. Rapid Commun. 24, 1079–1084. https://doi.org/10.1002/ marc.200300054. Bousmina, M., 2006. Study of intercalation and exfoliation processes in polymer nanocomposites. Macromolecules 39, 4259–4263. https://doi.org/10.1021/ma052647f.

4. Conclusion A new organo-modified montmorillonite (Mbzt-Mt) has been prepared using benzothiazolium surfactant bearing long alkyl chain via cationic exchange reaction. Based on characterization results, the synthesized surfactant has been successfully intercalated into interlayer 12

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