polyamide 6 nanocomposites using Mg–Al–taurate LDH as nanofiller

Applied Clay Science 114 (2015) 265–272

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Research paper

Preparation of Mg–Al layered double hydroxide/polyamide 6 nanocomposites using Mg–Al–taurate LDH as nanofiller Dana Lennerová a, František Kovanda b, Jiří Brožek a,⁎ a b

Department of Polymers, University of Chemistry and Technology, Prague, Technická 5, 166 28 Praha 6, Czech Republic Department of Solid State Chemistry, University of Chemistry and Technology, Prague, Technická 5, 166 28 Praha 6, Czech Republic

a r t i c l e

i n f o

Article history: Received 12 February 2015 Received in revised form 3 June 2015 Accepted 5 June 2015 Available online xxxx Keywords: Layered double hydroxides Polymer nanocomposites Polyamide 6 Taurine Anionic polymerization

a b s t r a c t A novel method using Mg–Al layered double hydroxide (LDH) intercalated with taurate (2-aminoethanesulfonate acid) was applied for preparation of LDH/polyamide 6 nanocomposites via in situ intercalative polymerization. Two polymerization mechanisms were used — hydrolytic and anionic polymerization of ε-caprolactam, in which the Mg–Al LDH intercalated with taurate was dispersed. The obtained LDH/polyamide 6 nanocomposites were characterized by a combination of wide- and small-angle X-ray scattering, transmission electron microscopy, differential scanning calorimetry and thermogravimetric analysis. It was found that LDH filler was exfoliated in the polymer matrix and the LDH/polyamide 6 nanocomposites were successfully prepared by anionic polymerization of ε-caprolactam in the presence of dispersed LDH. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The use of layered fillers in polymers has attracted great interest in recent years (Alexandre and Dubois, 2000; Kawasumi, 2004; Kadlecová et al., 2008; Kredatusová and Brožek, 2012; Bergaya et al., 2013; Lambert and Bergaya, 2013; Puffr et al., 2013). The dispersion of individual high-aspect ratio platelets to form nanocomposites has been shown, even at very low filler concentrations, to lead to dramatic improvements in several properties: increased modulus and impact strength, higher heat distortion temperature, better dimensional stability, reduced flammability, and enhanced barrier properties compared to pristine polymer matrix (Alexandre and Dubois, 2000; Sinha Ray and Okamoto, 2003; Paul and Robeson, 2008). The pioneering Toyota research was based on the formation of nanocomposites by the in situ polymerization of ε-caprolactam (CL) in the presence of organo-modified layered silicate-montmorillonite (Mt) by exchanging Na+ with ω-ammonium lauric acid (Kojima et al., 1993; Usuki et al., 1993). Recently, the attention has been turned to another group of layered inorganic materials, layered double hydroxides (LDH), which are also known as hydrotalcite-like compounds or anionic clays (Leroux and Besse, 2001). The chemical composition of LDH can be expressed by the general formula [MII1 − xMIIIx(OH)2]x+[An−x/n·yH2O]x− where MII and MIII are divalent and trivalent metal cations, An− is an n-valent anion, and x has values usually between 0.20 and 0.33. The MII/MIII isomorphous substitution in octahedral sites of hydroxide sheets results ⁎ Corresponding author. Tel.: +420 22044 3190. E-mail address: [email protected] (J. Brožek).

http://dx.doi.org/10.1016/j.clay.2015.06.004 0169-1317/© 2015 Elsevier B.V. All rights reserved.

in a net positive charge, which is compensated by anions localized with water molecules in the interlayer. The cation composition of the hydroxide layers, their charge density given by the MII/MIII molar ratio, as well as interlayer anion composition can be tailored during LDH synthesis. The most common method used for LDH synthesis is coprecipitation, when a solution containing MII and MIII metal cations in adequate proportions reacts with an alkaline solution. Other methods such as the urea method, induced hydrolysis, salt-oxide method, hydrothermal synthesis, or sol– gel method have been also reported (Forano et al., 2006; Kovanda et al., 2006; Forano et al., 2013). A weak bonding between interlayer anions and hydroxide sheets is characteristic for LDH and the interlayer anions can be exchanged under suitable conditions. Anion exchange reactions using coprecipitated LDH containing easily exchangeable an− ions (e.g., NO− 3 or Cl ) are often used, when a direct synthesis of the desired intercalates is difficult or even impossible. The rehydration of mixed oxides obtained after thermal decomposition of the LDH precursors containing volatile interlayer anions represents an alternative way; the rehydration reaction results in the reconstruction of layered LDH structure intercalated with anions from the solution. Modification of Mg–Al LDH with different types of organic species (dodecylsulfate, benzenesulfonate, laurate and bis(2-ethylhexyl)hydrogen phosphate) using the rehydration method has been investigated in (Costa et al., 2008a, 2008b). The use of LDH as nanofillers is advantageous due to their versatility in chemical composition and tunable charge density, allowing multiple interactions with the polymer. All methods mentioned above, i.e., direct coprecipitation, anion exchange, and rehydration/reconstruction can be applied for direct intercalation of polymeric species (especially that with lower molar mass) into the

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LDH hosts; the intercalation of suitable monomers followed by in situ polymerization is also possible. Intercalation of anions containing reactive groups into the LDH interlayer enables anchoring the monomeric units and subsequent formation of polymeric chains results in gradual disordering of the layered LDH structure. Finally, the inorganic nanoparticles (hydroxide nanosheets) are randomly dispersed in the polymer matrix. In comparison with clay minerals used as nanofillers (e.g., Mt), LDH exhibit higher charge density, which can complicate delamination and exfoliation of these compounds (Lan et al., 1995). Most polymers are hydrophobic and their compatibility with hydrophilic LDH filler should be improved by modifying inorganic layers with surfactants (dodecyl sulfate is often used). Such pretreatment leads to better dispersion of inorganic nanoparticles in polymer and, moreover, the intercalation of LDH with bulky organic anions makes easier their exfoliation in some solvents or melts. More detailed information about preparation and properties of various LDH/polymer nanocomposites can be found in formerly published reviews (Leroux, 2006; Costa et al., 2008a, 2008b). LDH/polymer nanocomposites can be synthesized by three basic procedures (Alexandre and Dubois, 2000; Leroux and Besse, 2001; Sinha Ray and Okamoto, 2003): (i) The in situ polymerization method is based on a polymerization of a monomer, in which the inorganic material is dispersed. In this procedure, the polymer chains grow in the space between the layers of the layered filler swelled by the monomer, the layers thus being drifted apart, and finally exfoliated. (ii) The most widespread method is melt intercalation. The organically modified layered filler is mixed with the polymer melt using current processes such as extrusion and injection molding. Good compatibility of the components is a necessary condition for gradual loosening of the LDH layers from LDH crystals and for their exfoliation. The mechanical parameters can influence only the rate of the process. However, complete exfoliation is usually more difficult to achieve by melt intercalation (blending) when compared to other techniques. (iii) In the exfoliated adsorption (or solution induced intercalation), LDH or LDH modified with organic compounds are dispersed and swelled in a polymer solution. With polyamides, the application of this method is limited due to the fact that they are soluble only in aggressive, environmentally harmful and expensive solvents (Kredatusová and Brožek, 2012). Polyamide 6 (PA6) is one of the most important types of aliphatic polyamide having applications such as fibers and engineering plastic. The first commercially produced clay polymer nanocomposites developed as automotive material by Toyota in 1985 were those of PA6 prepared by a hydrolytic polymerization of CL in the presence of Mt modified by 12-aminododecanoic acid (Kojima et al., 1993). Several years later, after the optimum organophilization of Mt and the conditions of the technological process had been found, clay PA6 nanocomposites were successfully prepared by a technique of melt mixing (Fornes et al., 2002; Chavarria et al., 2007). In the case of Mt/PA6 nanocomposites prepared by in situ technique, the polymerization of CL is initiated by carboxylic groups while amino groups are anchored to the surface of the layers (Usuki et al., 1993). When LDH is modified with a proper amino acid, amino groups of tethered amino acids will be available for an aminolytic polymerization of CL or other modifications suitable for an anionic polymerization. Few reports on the preparation of LDH/PA6 nanocomposites by in situ polymerization by a hydrolytic mechanism of CL and melt intercalation can be also found (Chen, 2006; Yan-wu and Jun-qing, 2007; Peng et al., 2009). The LDH intercalated with amino acids (mostly α-, not ω-amino acids) were obtained by direct coprecipitation (Aisawa et al., 2001; Aisawa et al., 2006) or by rehydration/reconstruction procedure (Aisawa et al., 2004; Nakayama et al., 2004). The intercalation of various amino acids was influenced by the solution pH and by the kind of sidechain of the amino acids. The synthesis of Mg–Al LDH intercalated with (D,L)-alanine was reported in Yan-wu and Jun-qing (2007). The presence of alanine in the interlayer did not change the interlayer distance. Low-molar mass

PA6/LDH intercalated nanocomposites were prepared via in situ polymerization of CL initiated with alanine at 150 °C. Preparation of exfoliated LDH/PA6 nanocomposites by in situ hydrolytic polymerization was investigated in Chen (2006) and Peng et al. (2009). The Co/Al-LDH intercalated with dodecyl sulfate, prepared according to the procedure described in Liu et al. (2006), was dispersed in CL melt (80 °C/90 min) and after addition of ε-aminocaproic acid the polymerization proceeded at 250 °C for 6 h. The XRD and TEM results indicated that nanoplatelets of Co/Al-LDH were homogeneously dispersed in the PA6 matrix. Mechanical properties (tensile modulus, yield strength and hardness) of nanocomposites were improved in comparison with neat PA6. Tabuani et al. (2009) studied the influence of nanofiller nature (organically modified Mt and LDH exchanged with a C16–C18 fatty acid) on the morphology and properties of PA6 nanocomposites prepared by twin-screw extrusion. It is interesting that modified LDH behaves as an efficient flame retardant due to a combination of charring promotion and flame quenching. On the other hand, inorganic LDH has almost no role as a flame retardant because of its poor dispersion in the PA6 matrix. The Mg–Al LDH modified with 4-dodecylbenzenesulfonate was used in the preparation of LDH/PA6 nanocomposites by melt blending (Zammarano et al., 2006). It was reported that shear, together with the anionic exchange capacity, are the key factors for the delamination of LDH in PA6. Thermal properties of LDH/PA6 nanocomposites were evaluated in Du et al. (2007). The nanocomposites were prepared by direct melt intercalation of PA6 into Mg–Al LDH modified with dodecyl sulfate. DSC traces showed that the exfoliated LDH layers have nucleating effect on the crystallization of PA6. The TGA results revealed that the added LDH decreases thermal stability of prepared LDH/PA6 nanocomposites. Prepared nanocomposites with exfoliated LDH are promising for the application of flame retardant polymer materials. Due to its high molar mass and high content of crystalline part, PA6 prepared by the anionic polymerization of CL has many advantages over PA6 prepared by hydrolytic polymerization (Rusu et al., 2001). The main advantage of the fast anionic polymerization of lactams lies in the increasingly used technologies of simultaneous polymerization and molding, i.e., monomer casting and reactive injection molding (RIM). However, due to the rapid exchange of the organic cations of organically modified Mt with the counter-cations of the ε-caprolactamate catalyst (e.g., Na+ or Mg2 +), these methods cannot be used for the preparation of exfoliated organically modified Mt/PA6 nanocomposites (Kadlecová et al., 2008). It might, though, be possible to solve this problem by using LDH modified with suitable organic anions. In the present work, we synthesized the Mg–Al LDH intercalated with taurate (2-aminoethanesulfonate acid) and co-intercalated with CL and then used the product for the preparation of LDH/PA6 nanocomposites via in situ polymerization methods. 2. Experimental 2.1. Materials Taurine (2-aminoethanesulfonic acid, Fluka), magnesium nitrate — Mg(NO3)2·6H2O, aluminum nitrate — Al(NO3)3·9H2O, and sodium hydroxide — NaOH (all Penta, Czech Republic) were used as purchased. ε-Caprolactam (CL, DSM) was used as purchased without further purification and stored in a dessicator over P2O5, water content 89 ppm. εCaprolactam magnesium bromide (CLMgBr) concentrate in CL (Brügemann) was stored in a round-bottom flask under the protective argon atmosphere. ε-Caprolactone (CLO, Sigma-Aldrich) was purified by distillation under reduced pressure over powdered calcium hydride, water content 40 ppm. N,N-isophthaloyl-bis-ε-caprolactam (IPBCL) was prepared in accordance with a procedure described in Stehlíček and Šebenda (1980).

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2.2. Synthesis of Mg–Al LDH host The Mg–Al LDH host in nitrate form was prepared by coprecipitation. Carbonate-free distilled water was used for dissolution of chemicals and the preparation was carried under nitrogen. An aqueous solution (450 mL) of Mg(NO3)2 and Al(NO3)3 with Mg/Al molar ratio of 2 and total metal ion concentration of 1.0 mol L−1 was added with a flow rate of 7.5 mL min−1 into 1000 mL batch reactor containing 200 mL of distilled water. A flow rate of simultaneously added NaOH solution (3 mol L−1) was controlled to maintain the reaction pH = 10.0 ± 0.1. Coprecipitation was carried out under vigorous stirring at 75 °C and the resulting suspension was stirred for 1 h at 75 °C. The product was filtered off, washed thoroughly with carbonate-free distilled water and dried at 105 °C. The obtained sample was denoted as Mg–Al–NO3. 2.3. Taurate intercalation The Mg–Al–NO3 sample was heated for 4 h at 450 °C in air and then cooled in a desiccator to room temperature. The obtained Mg–Al mixed oxide (1.4 g) was added into aqueous solution prepared by dissolution of 2.66 g (0.021 mol) taurine in 200 mL of carbonate-free distilled water; aqueous solution with initial pH of 5.6 and taurine excess of 100% with respect to the anion exchange capacity of the resulted LDH was applied. The suspension was sealed in the 500 mL glass bottle under nitrogen and stirred for 6 days at room temperature. Then the solid was separated by centrifugation at 5000 rpm for 15 min, washed with carbonate-free distilled water and subsequently with methanol. The gel-like product after centrifugation in methanol was obtained and denoted as Mg–Al–Tau; the chemical analysis (AAS) showed that the sample contained 14 wt.% of the intercalated Mg–Al LDH. The nondried Mg–Al–Tau product was stored and used for preparation of the nanocomposite. 2.4. Preparation of Mg–Al–Tau dispersion in CL melt The non-dried Mg–Al–Tau sample (3.63 g) was dispersed in 5 mL of methanol in round-bottom flask. Then 28 g of solid CL was added. The mixture was stirred at 90 °C and methanol was removed by striping with argon (400 mL min− 1) for 2 h (water content determined by Karl Fischer coulometric titration was 0.116 wt.%). Residual methanol and a part of CL (approx. 10%) were removed by distillation under reduced pressure (20 Pa) at 110 °C. The prepared Mg–Al–Tau dispersion in CL (water content 319 ppm) was used for polymerization tests. The content of LDH in the dispersion (1.9 wt.%) was calculated from Mg content determined by AAS. 2.5. Preparation of LDH/PA6 nanocomposites 2.5.1. Anionic polymerization Nanocomposites were prepared in an aluminum mold (Kadlecová et al., 2008) with a wall thickness of 4 mm and an internal cavity of 150 (height) × 140 × 4 mm. The CLMgBr initiator (1.0 mol%) was placed to the mixed LDH/CL dispersion equilibrated under dry argon at 90 °C. After complete dissolution (3 min), the activator (IFBCL or CLO) was dosed. The polymerization mixture with dosed CLO was thoroughly stirred and transferred to the aluminum mold, heated to 110 °C, immediately submerged in an oil bath, and kept at 150 °C for 1 h. After demolding, the test specimens were prepared. The mixture with IFBCL was prepared analogously and transferred to the glass ampoule (ID 20 mm), the polymerization proceeded at 150 °C for 0.5 h. 2.5.2. Hydrolytic polymerization A mixed LDH/CL dispersion equilibrated under dry argon at 90 °C was dosed by glass syringe into glass ampoules (ID 10 mm, length 70 mm) under inert atmosphere. After solidification of the charge at

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laboratory temperature, the necks of ampoules were sealed under reduced pressure. Polymerizations were performed at 260 °C. 2.6. Characterization of the samples Powder X-ray diffraction (XRD) patterns of the LDH samples were recorded using a Seifert XRD 3000P instrument with Co Kα radiation (λ = 0.179 nm, graphite monochromator, goniometer with the Bragg–Brentano geometry) in 2θ range from 3.5 to 80° 2θ with the step size of 0.05°. The qualitative analysis was performed with a HighScore software package (PANanalytical, the Netherlands, version 2.0a). Fourier-transform infrared absorption spectra (FTIR) were recorded using the KBr pellet technique on a Nicolet 740 spectrometer in the range from 4000 to 400 cm−1 and the resolution of 2 cm−1. A part of prepared materials was rasped and used for the gravimetric determination of polymer yield by hot-water extraction (three times for 30 min). Extracted samples were dried at laboratory temperature under reduced pressure (30 Pa) over P2O5 to constant weight. DSC measurements were performed with Q 100 DSC (TA Instruments) within the temperature range from 20 to 240 °C in three cycles involving heating–cooling–heating at the constant heating/cooling rate of 10 °C min−1; nitrogen purge flow rate of 50 mL min−1. For the determination of content of crystalline part the enthalpy of fusion — ΔHm(100%) = 190 J g−1 (Inoue, 1963) was used. Thermogravimetric analysis (TGA) was carried out employing TGA Q 500 (TA Instruments) in the temperature range from 20 to 600 °C at heating rate of 10 °C min−1 and nitrogen purge flow rate of 60 mL min−1. Small angle X-ray scattering (SAXS) experiments were performed using a pinhole camera (Molecular Metrology SAXS System) attached to a microfocused X-ray beam generator (Osmic MicroMax 002) operating at 45 kV and 0.66 mA (30 W). The camera was equipped with a multiwire, gas-filled area detector with an active area diameter of 20 cm (Gabriel design). Two experimental setups were used to cover the q range from 0.05 to 11 nm− 1 (q = (4π/λ)sin Θ, where λ is the wavelength and 2Θ is the scattering angle). Wide angle X-ray scattering (WAXS) measurements were performed with an automated, computer-controlled diffractometer HZG/4A (Praezisiontechnik Freiberg GmbH, Germany). The Cu Kα radiation filtered electronically and with a Ni filter was applied. Tensile tests were carried out on beam-shaped specimens (4.0 × 2.0 mm in cross-section) cut from plaques. The measurements were performed using an Instron 1122 with a cross-head speed of 100 mm min−1. The Young modulus of elasticity in tension (Et) was obtained from the linear dependence of the relative elongation on the stress applied. The tensile strength at break (σs) and, deformation at break (εr) were also determined. TEM micrographs were obtained with transmission electron microscope JEM 200CX, JEOL. 3. Results and discussion 3.1. Intercalation of taurate into Mg–Al LDH host The Mg–Al LDH host in the nitrate form was prepared by coprecipitation under nitrogen. Only a well-crystalline hydrotalcite-like phase with d003 basal spacing of 0.88 nm was found in powder XRD pattern of the Mg–Al–NO3 sample (Fig. 1). Rather disordered Mg–Al mixed oxide exhibiting diffraction peaks characteristic for periclase (MgO) was obtained after heating of the Mg–Al–NO3 precursor at 450 °C (not shown here). Rehydration of the calcination product in the taurine aqueous solution resulted in reconstruction of layered LDH structure; the (003) and (006) diffraction peaks at about 8 and 15° 2θ, as well as non-basal diffraction peaks at 2θ angles above 40° 2θ are clearly visible in the powder XRD pattern of the Mg–Al–Tau sample dried at 65 °C (Fig. 1). Relatively low intensity and marked broadening of (003) and (006) diffraction peaks

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467

597

1383

448

448

523

740 741

1512 1458 1388 1345 1304

1383 1365 1305 1214 1214 1183 1184 1112 1114 1046 1038 963 961 894 893 847

Mg-Al-NO3

1512

Mg-Al-Tau 1620

Transmittance / a.u.

1617

Taurine

2000

1800

1600

1400

1200

1000

Wavenumbers / cm

800

600

400

-1

Fig. 1. Powder XRD patterns of the coprecipitated Mg–Al–NO3 sample and the Mg–Al–Tau sample intercalated with taurate (the sample was dried at 65 °C); * — taurine.

Fig. 2. FTIR spectra of the coprecipitated Mg–Al–NO3 sample, the Mg–Al–Tau sample intercalated with taurate (the sample was dried at 65 °C) and taurine.

indicated a disorder in stacking of hydroxide layers in (001) direction. The gel-like product obtained after washing in methanol and centrifugation contained 14 wt.% of the Mg–Al–Tau intercalate. Therefore, a considerable swelling of the Mg–Al–Tau product in methanol, resulting in disorder of the layered LDH structure can be expected. Very likely, this disorder preserved in the dried sample after methanol removal. Approximate d003 basal spacing of 1.29 nm was evaluated for the dried Mg–Al– Tau sample. The increased d003 basal spacing in comparison with the value of 0.77 nm, characteristic for hydroxide form of hydrotalcite formed during rehydration of the Mg–Al mixed oxide in distilled water, indicated intercalation of taurate anions into the Mg–Al LDH interlayer. Trace amount of taurine, which was not completely removed from the rehydrated product during washing, was also detected in the powder XRD pattern of the dried Mg–Al–Tau sample (Fig. 1). Vibrational bands characteristic for taurine were found in the FTIR spectrum of the dried Mg–Al–Tau sample (Fig. 2). The vibrational bands at 1214, 1184 and 1046 cm−1 were ascribed to the SO3 group; the band at 1046 cm− 1 corresponds also to stretch vibrations of the C\\N bond. The bands at 1512, 1305 and 1114 cm− 1 are connected with vibrations of the NH2 group. The vibrations of CH2 groups can be observed at 1305, 963, 894, and 741 cm−1; the latter band corresponds also to stretch vibrations of the C\\S bond (Ohno et al., 1992). The wide band around 1620 cm−1 was ascribed to OH groups in the interlayer water molecules. The sharp vibrational band at 1617 cm−1 was found also in the FTIR spectrum of taurine; it was likely connected with asymmetric deformation vibrations of the NH3 group (Ohno et al., 1992). The sharp and intensive band at 1383 cm−1 observed in the Mg–Al–NO3 sample corresponds to the NO3 group. This band was detected also in the dried Mg–Al–Tau sample; it is likely connected with residual nitrates in the rehydrated sample. The shoulder at 1365 cm− 1 corresponds to CO23 − anions and can indicate slight contamination of the Mg–Al–Tau sample with carbonate during sample washing and drying. The presence of organic component in the Mg–Al–Tau sample was confirmed also by the elemental analysis; following contents of elements were found in the washed and dried sample (in wt.%): C 6.04, H 4.81, N 3.46, S 7.57. Chemical analysis (AAS) of the Mg–Al–Tau sample showed slight decrease in Mg/Al molar ratio in comparison with the coprecipitated Mg–Al–NO3 precursor (from 1.95 to about 1.6). The pH of 5.6 was measured in the used taurine aqueous solution and the observed decrease in Mg/Al molar ratio can be explained by partial leaching of Mg2+ cations

from the solid during rehydration of the Mg–Al mixed oxide in acid solution (Kostura et al., 2007). Based on vibrational spectra, Ohno et al. (1992) showed that taurine in aqueous solution exists as an ionic spe− cies NH+ 3 CH2CH2SO3 not as NH2CH2CH2SO3H. Very likely, such ionic species was intercalated into LDH interlayer during rehydration reaction in the taurine aqueous solution. We tried to rehydrate the Mg–Al mixed oxide in aqueous solution of sodium taurate at pH of 9.0, when the solution was obtained by neutralizing the dissolved taurine with NaOH solution. The rehydrated product exhibited diffraction peaks characteristic for hydroxide form of hydrotalcite (d003 basal spacing of 0.77 nm) and only very slight diffraction peak corresponding to dspacing of about 1.2 nm was found in the powder XRD pattern. No intercalation of taurate anions was observed during anion exchange reaction in taurine and sodium taurate aqueous solutions using Mg–Al–NO3

Mg-Al-Tau

-1

100

Intensity / cm

100

10

10

1

Intensity / cm

-1

0,1

0,01 0,01

0,1

1

-1

q/Å

1

0,1

1C sample 2C sample 4D sample

0,01 0,01

0,1

q/Å

1

-1

Fig. 3. SAXS patterns of the LDH/PA6 nanocomposites obtained by anionic polymerization carried out in ampoule (1C and 2C samples) or mold (4D sample, plaque); the polymerization mixture contained 3 wt.% of LDH filler. Small picture inserted in right corner: SAXS pattern of the Mg–Al–Tau sample dispersed in methanol.

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Scheme 1. Proposed mechanism of LDH/PA6 nanocomposite formation (activated with IFBCL).

precursor. Therefore, only the Mg–Al–Tau sample, prepared by rehydration of Mg–Al mixed oxide in taurine aqueous solution, was used in further experiments. 3.2. Preparation of nanocomposites Preparation of homogeneous dispersion by direct adding of the Mg– Al–Tau sample into CL melt was not successful. Obtaining the homogeneous dispersion is the necessary condition in preparation of PA6 nanocomposites by in situ procedure. Therefore, a dispersion of Mg–Al–Tau in methanol was used for the preparation of polymerization mixture. The SAXS pattern of the Mg–Al–Tau sample dispersed in methanol (Fig. 3) showed no maxima corresponding to non-dispersed big particles or aggregates and the slope of about −1.7 (i.e., close to −2) is characteristic for two-dimensional objects in the measured sample (Schaefer and Justice, 2007). This finding indicated a high-extent delamination/exfoliation of the taurate-intercalated Mg–Al LDH in methanol and formation of a colloidal dispersion can be expected. The crucial point in preparation of the dispersion suitable for polymerization reaction lies in removing of water present in a hydrophilic LDH because water is an inhibitor of the

anionic polymerization. Water was removed by two-step procedure — stripping with argon stream and subsequent distillation at reduced pressure. The presence Mg–Al–Tau in CL melt causes a substantial increase in viscosity and complicates the distillation. Therefore, the prepared dispersion of Mg–Al–Tau LDH in CL contained 319 ppm of water due to the presence of Mg–Al–Tau LDH. Application of the same distillation procedure on CL melt resulted in the water content of 15 ppm (Merna et al., 2006). The LDH/PA6 nanocomposites were prepared by in situ polymerization methods — via hydrolytic and anionic polymerization of CL in LDH/CL dispersion. 3.2.1. Anionic polymerization A formation of LDH/PA6 nanocomposite during in situ anionic polymerization is schematically shown in Scheme 1. Taurate is intercalated in LDH interlayers. The reaction of tethered amine group with IFBCL (polymerization activator) proceeds exclusively via the ring elimination associated with CL formation (Stehlíček and Šebenda, 1980; Maier et al., 2003). In the presence of polymerization initiator (CLMgBr) and monomer (CL), the anionic polymerization of CL consists in a repeated

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Scheme 2. Proposed mechanism of LDH/PA6 nanocomposite formation (activated with CLO).

addition of the CL anion on the carbonyl of the N-acyllactam group (Ncarbamoyl-ε-caprolactam groups) (Aharoni, 1997). The subsequent formation of PA6 chains results in a gradual disordering and exfoliation of the LDH structure. Finally, the hydroxide nanosheets are randomly dispersed in the PA6 matrix. CLMgBr has been used as a polymerization initiator because magnesium initiators are rather exceptional; they are capable to eliminate retardation effect of water on anionic polymerization (Bernat et al., 2008). The rate of polymerization activated with IFBCL was very high; the casting polymerization mixture into the aluminum mold to obtain a plaque was not possible and the nanocomposite was prepared only in ampoule. Therefore, a slower activator, ε-caprolactone (CLO), has been used and two plaques have been successfully prepared (with and without LDH). In the first stage of the process, CLO is converted into poly(ε-caprolactone) (PCLO) — Scheme 2. The polymerization is initiated with CLMgBr. Ester groups of PCLO represent a source of growth centers for the polymerization of CL, as it was confirmed by a preparation of polyesteramide through an anionic polymerization of CL in the presence of PCLO (Bernášková et al., 2004). The formed N-acyllactam structure reacts with amine groups of intercalated taurine or lactam anion in a similar way as illustrated in Scheme 1. The SAXS patterns of the LDH/PA6 nanocomposites obtained by the anionic polymerization (Fig. 3) revealed the power law behavior with the slopes between − 2.1 and − 1.9; such slope values indicate the presence of two-dimensional objects in the samples and exfoliation of the LDH filler can be expected. The samples containing threedimensional objects such as aggregates or small particles exhibit considerably higher slope values between −3 and −4 (Schaefer and Justice, 2007). To access the benefit of hydroxide sheets (coming from the exfoliated LDH) in the polyamide matrix, the tensile strength (σs), modulus (Et) and deformation at break (εr) have been determined (Table 1). Comparing the mechanical properties we have found the same Et but lower σs and εr, even compared with the reference PA6 without added Mg–Al–Tau LDH (the 5D sample in Table 1). These results reflect a balance between softening effect of unreacted monomer (difference in polymer yield) and reinforcing contribution of exfoliated filler. The WAXS patterns of the prepared LDH/PA6 nanocomposites containing 3 wt.% of the Mg–Al–Tau LDH filler showed on Fig. 4 no marked difference in comparison with the reference 5D sample (the PA6 plaque without Mg–Al–Tau LDH). The diffraction maxima at 2θ angle of about 20 and 24° corresponded to the (200) and (200) + (202) planes of a more stable monoclinic α-structure of PA6. The maxima at 2θ angles

of 21° and, less frequently observed, at 22° can be ascribed to the (001) and (200) + (201) planes of the γ-structure of PA6. Both αand γ-structure of PA6 were found in the prepared LDH/PA6 nanocomposites. TEM images (Fig. 5) showed relatively homogeneous dispersion of disordered LDH filler in the PA6 matrix. 3.2.2. Hydrolytic polymerization The hydrolytic polymerization of CL can be described by mechanism characterized by three main equilibrium reactions (Aharoni, 1997): (i) ring-opening (hydrolysis of CL to ε-aminocaproic acid — ACA), (ii) polycondensation (amine and carboxylic acid), and (iii) polyaddition (amine + CL). In our case, polymerization mixture composed of Mg–Al– Tau LDH dispersed in CL melt contained 320 ppm of water (0.2 mol%). Presence of water in polymerization mixture resulted in formation of ACA but its concentration was ten-times lower than in standard polymerization (2 mol%). Further release of water from LDH filler was caused by increased temperature applied during polymerization. As the LDH filler was modified with taurine, amino groups of tethered amino acids underwent polyaddition reactions and the formation of PA6 chains resulted in a gradual disordering and exfoliation of the LDH structure. The results of hydrolytic polymerizations are summarized in Table 2. The yield of polymer increased with polymerization time, but the product became yellow. It can be explained by thermal degradation of LDH filler at polymerization temperature (260 °C). Compared to a standard hydrolytic polymerization initiated with 2 mol% of ACA, the equilibrium content of polymer is lower. The results of thermogravimetric analysis of extracted polymer samples (Table 2) show that the presence of LDH decreases the thermal stability of LDH/PA6 composites. There is decrease of more than 10 °C in the temperature at maximum rate of decomposition (peak of derivative Table 1 Anionic polymerization of CL initiated with 1.5 mol% CLMgBr at 150 °C. Activator

Samplea

wLDH (wt.%)

yw (%)

Et (GPa)

σs (MPa)

εr (%)

IFBCL (0.75 mol%) CLO (5 mol%) CLO (5 mol%) CLO (5 mol%)

1C 2C 4D 5D

3.0 3.0 3.0 0

97.1 94.0 92.4 96.9

ND ND 2.59 ± 0.10 2.61 ± 0.16

ND ND 58 ± 5 76 ± 13

ND ND 2.8 6.1

a — polymerization carried out in ampoule (C) or mold (D); wLDH — content of LDH in polymerization mixture; yw — content of polymer; Et — tensile modulus; σs — tensile strength at break; εr — deformation at break.

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Table 2 Hydrolytic polymerization of CL carried out at 260 °C. wLDH (wt.%)

wACA (mol%)

tpol (h)

yw (%)

Tmax (°C)

Tm (°C)

ΔHm (J g−1)

xc (%)

Tc (°C)

ΔHc (J g−1)

0 1.9 1.9 1.9 1.9 1.9 3.0 3.0

2.0 0 0 0 0 2.0 0.5 1.0

24 2 4 6 24 24 24 24

89.8 16.0 49.1 66.6 87.4 86.1 87.9 87.0

459 438 440 449 448 428 432 421

225 205 216 223 224 222 219 217

79 82 83 100 87 94 95 93

42 43 44 53 46 49 50 49

183 178 181 186 184 186 184 184

70 72 75 74 76 78 80 75

wLDH — content of LDH filler in polymerization mixture; wACA — content of ACA in polymerization mixture; tpol — polymerization time; yw — content of polymer; Tmax — temperature at a maximum rate of decomposition (from TGA); Tm — melting temperature; Tc — crystallization temperature; ΔHm — enthalpy of fusion; ΔHc — enthalpy of cold crystallization; xc — content of crystalline part from DSC.

Fig. 4. WAXS patterns of the LDH/PA6 nanocomposites containing 3 wt.% LDH filler obtained by anionic polymerization carried out in ampoule (1C and 2C samples) or mold (4D sample, plaque) in comparison with the PA6 plaque without added LDH (5D sample).

weights) compared to the neat PA6. Here, the layers of Mg–Al–Tau LDH dispersed in the PA6 matrix are more efficient to catalyze the degradation of polyamide (Du et al., 2007). As reported in Zammarano et al. (2006) a nucleophilic attack mechanism is a possible explanation of the decreasing thermal stability. The release of interlayer water and/or the partial LDH dehydroxylation during polymerization at 260 °C is a possible source of nucleophile. A nucleophilic attack mechanism can also be activated by intercalated taurate as the Mg–Al–Tau LDH was decomposed to form strong mineral acid during heating above 300 °C. Reference PA6 DSC melting peak showed a maximum at 225 °C. On the other hand, the sample containing Mg–Al–Tau LDH showed a slight shift of melting exotherm to lower temperatures, depending on filler content and content of polymer. This may be attributed to the presence of γ-crystalline form of PA6. The content of crystalline part was determined from the first heating scan. The presence of Mg–Al–Tau LDH increases melt enthalpy and, therefore, the content of crystalline part. To elucidate the effect of LDH filler on crystallization behavior of PA6, the DSC cooling scans were measured. The maximum rate of crystallization (exothermic peak temperature) of neat PA6 was observed at 183 °C

(Table 2). The presence of LDH did not increase the exotherm of cold crystallization too much. As reported in Du et al. (2007) the presence of Mg–Al–LDH organophillized with dodecyl sulfate in PA6 increased the crystallization temperature of 13 °C. The crystallization behavior was explained by the fact that organophillized LDH layers act as nucleating agents, which have a heterogeneous nucleating effect on the crystallization of polyamide phase. In our case, the polyamide chains are bonded to hydroxide layers via reaction of tethered amino groups and carboxyl groups of ACA or polyamide chains. 4. Conclusion The LDH/polyamide 6 nanocomposites have been prepared by in situ polymerization of ε-caprolactam in the presence of dispersed Mg– Al LDH intercalated with taurate. The LDH filler was prepared by rehydration of calcined Mg–Al–NO3 precursor in taurine aqueous solution. The increase in d003 basal spacing revealed intercalation of taurate anions into the LDH interlayer. The presence of taurate in the LDH filler was confirmed also by FTIR spectroscopy and elemental analysis. Two mechanism of ε-caprolactam polymerization were applied — anionic and hydrolytic. The preparation of LDH/polyamide 6 nanocomposites by anionic polymerization of CL, i.e., in situ procedure, was not yet reported. The dispersion of exfoliated LDH in polyamide 6 matrix was confirmed by WAXS and SAXS measurements. In case of hydrolytic polymerization, both α- and γ-structures of polyamide 6 were found in the LDH/polyamide 6 composites formed as a result of simultaneous polymerization and crystallization of polyamide phase below melting temperature.

Fig. 5. TEM images of the LDH/PA6 nanocomposites containing 3 wt.% LDH filler obtained by anionic polymerization carried out in ampoule (1C sample — left) and mold (4D sample, plaque — right).

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