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BY-NC-ND 3.0 license Open Access Published by De Gruyter April 2, 2014

Formation of aramid-silica-grafted-multi-walled carbon nanotube-based nanofiber via the sol-gel route: thermal and mechanical profile of hybrids with poly(methyl methacrylate)

Ayesha Kausar
From the journal e-Polymers

Abstract

In this study, thermally and mechanically stable poly(methyl methacrylate) (PMMA)-based nanocomposites were produced through the reinforcement of electrospun aramid-silica-grafted multi-walled carbon nanotube-based nanofibers (MWCNT-Ar-Si). The multi-walled carbon nanotube was initially modified to prepare an isocyanatopropyltriethoxysilane-grafted MWCNT via the sol-gel route using 3-isocyanatopropyl-triethoxysilane and tetraethoxysilane (TEOS). The silica network was developed and linked to MWCNT by hydrolysis and condensation of TEOS. The said isocyanatopropyltriethoxysilane-grafted MWCNT was electrospun with the aramid solution. The electrospun MWCNT-Ar-Si nanofibers (0.1–1 wt.%) were then reinforced in a PMMA matrix. For comparative analysis, PMMA was also reinforced with 0.1–1 wt.% of aramid nanofibers. The tensile modulus of PMMA/MWCNT-Ar-Si 0.1 was 5.11 GPa, which was increased to 13.1 GPa in PMMA/MWCNT-Ar-Si 1. The 10% decomposition temperature of PMMA/MWCNT-Ar-Si 0.1–1 hybrids was in the range of 479–531°C. The glass transition temperature, determined from the maxima of tan δ data using dynamic mechanical thermal analysis, showed an increase with the filler loading and was maximum (301°C) for PMMA/MWCNT-Ar-Si 1 with 1 wt.% of MWCNT-Ar-Si nanofibers. In contrast, PMMA/Ar 0.1–1 hybrids showed lower values in the thermal and the mechanical profile depicting the combined effect of nanotube and aramid in electrospun nanofibers.

1 Introduction

Polymer composites have gained much consideration because the properties of the original matrix may be considerably improved by the addition of even diminutive filler content (1, 2). Polymeric materials are not suited for high-performance applications since they are mechanically feeble and thermally unstable. In this regard, incorporating appropriate filler materials may provide enhancement to the mechanical and thermal properties. Structural strength and toughness have been observed in the composite materials, in which inorganic components have been distributed at a very fine level in the organic matrix (3–5). Ceramics are an important type of inorganic filler, and their micro-level mixing with polymers may create high-performance hybrid materials. Common inorganic components/metal oxides such as silica, titania, zirconia, etc., have also been used to reinforce the organic polymers. Recently, poly(methyl methacrylate), poly(dimethyl siloxane), polyacrylates, polyimides, polyamides, polybenzoxazoles, epoxies, etc., have been frequently used in the preparation of hybrid materials (6–9). Consequently, a successful synthetic approach for the preparation of organic/inorganic materials has been the in situ polymerization of metal alkoxides via the sol-gel process (10, 11). The sol-gel technique has been extensively employed to prepare a variety of silica-based materials such as tubes, fibers, powders, etc. Generally, in polymer solution, inorganic metal oxide precursor is mixed and hydrolysis and condensation are slowly carried out at moderate temperature to incorporate an inorganic network structure (ceramers). Such type of hybrid materials may exhibit heat resistance, preservation of mechanical properties at high temperature and pressure, easy processability, etc. Incidentally, silica has been commonly used as an effective inorganic component to enhance the mechanical and thermal properties of the matrix. Moreover, the strong interface adhesion between the organic matrix and the silica is the key to the application of this filler (12–14). The properties of hybrid composites are affected by many factors such as filler size, distribution, and filler content. Additionally, the surface structure and mechanical properties of the filler play a crucial function in the organic/inorganic composite material. Especially, the properties may be enhanced via improving the bond strength between the inorganic filler and the polymer matrix (15–17). Polyamide/silica nanocomposite films have been synthesized via condensation and interfacial polymerization, which show superior thermal and mechanical stability (18). Similarly, various approaches have been used to synthesize polymer/silica hybrid nanofibers via a one-step electrospinning process. Afterwards, these fibers have been utilized to reinforce and improve the properties of the matrix (19, 20). Electrospinning is a well-known technique for drawing fine fibers from polymer solution or polymer melt under electrostatic field (21). Electrospinning performs an economical and swift single-step processing to fabricate continuous nanofibers. The resulting fibers have a high surface area-to-volume ratio, which makes them applicable for use as matrices for high-performance composites or as functional materials in microelectronic devices, sensors, wire insulation applications, etc. In the current work, a multi-walled carbon nanotube (MWCNT) was modified to prepare an isocyanatopropyltriethoxysilane-grafted MWCNT using 3-isocyanatopropyltriethoxysilane (ICTOS) and tetraethoxysilane (TEOS) via the sol-gel route. The said isocyanatopropyltriethoxysilane-grafted MWCNT was electrospun with the aramid solution prepared from 1,3-phenylenediamine, 2,4-diaminophenol, and terephthaloyl chloride. The choice and design of the fabrication technique for the desired materials were an important part of this research. We have successfully prepared homogeneous organic-inorganic hybrid electrospun fibers by incorporating organic polymer into a sol-gel process-modified isocyanatopropyltriethoxysilane-grafted MWCNT reaction mixture of alkoxysilanes. During the electrospinning, the isocyanate groups in MWCNTs and the hydroxyl groups in polymer developed covalent bonding, which helped in the uniform dispersion of each segment. Moreover, the residual silanol groups in the silica gel and the amide groups in the polymers may develop hydrogen-bonding interactions, which also promote the uniform dispersion of segments during this process. After the surface modification process using ICTOS and TEOS, the sol-gel network showed better compatibility with nanotubes. The sol-gel network actually acted as a bridge between the nanotubes and the polyamide. The results also indicate that polyamide-silica-modified nanotubes played an important role in the preparation of high-strength nanofibers and nanocomposites. As a precondition for the formation of homogeneous polymeric hybrid fibers, polymers should be dispersed uniformly in the sol-gel reaction solution. The electrospun aramid-silica-grafted multi-walled carbon nanotube-based nanofibers (MWCNT-Ar-Si) were then reinforced in poly(methyl methacrylate) (PMMA) matrix. All the samples were solvent-cast films for determining the properties. For comparative analysis, PMMA was also reinforced with 0.1–1 wt.% of aramid nanofibers. The hybrids with different compositions were investigated by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy, thermogravimetric analysis (TGA), dynamic mechanical analysis, and tensile measurement. The effect of increased bonding between MWCNT-Ar-Si nanofiber phases on the morphology of the resulting hybrids and their thermal and mechanical properties have been described. The mechanical stability of PMMA matrix was considerably improved, and the decomposition temperature was also increased with rising amounts of MWCNT-Ar-Si due to hydrogen bonding between the fibers and PMMA.

2 Experimental

2.1 Materials

Poly(methyl methacrylate) with a molecular weight of 100,000 was obtained from BDH Chemicals (Birmingham, UK). The MWCNT (short, outer diameter×wall thickness×length 20–30 nm×1–2 nm×0.5–2 μm), 1,3-phenylenediamine, TEOS, and dimethyl acetamide (DMAc) were AR-grade products of Aldrich Chemicals (CA, USA). Terephthaloyl chloride (99%), 2,4-diaminophenol (96%), and ICTOS (95%) were obtained from Fluka, Monte Carlo, Germany.

2.2 Instrumentation

Infrared spectra were recorded using a Fourier transform infrared spectrometer (model no. FTSW 300 MX, BIO-RAD; 4-cm-1 resolution). The stress-strain response of the samples (strips), with dimensions of ca. 14×5.0×0.39 mm, was monitored according to the Deutsches Institut für Normung 53455 procedure using the INSTRON 4206 universal testing machine at 30°C at a crosshead speed of 5 mm/min. Standard procedures and formulae were applied to calculate various tensile properties including stress, strain, Young’s modulus, and toughness. For the morphological studies, films were cryogenically fractured in liquid nitrogen. Field emission scanning electron microscopy of freeze-fractured samples was performed using the JSM5910 scanning electron microscope (JEOL, Japan). Thermal stability was verified by a METTLER TOLEDO TGA/SDTA 851e thermo gravimetric analyzer using 1–5 mg of the sample in an Al2O3 crucible at a heating rate of 10°C/min. Dynamic mechanical thermal analysis was performed on hybrid materials in the temperature range of 0–300°C with a dynamic mechanical analyzer (DMTA Q800; frequency of 5 Hz), at a heating rate of 10°C/min.

2.3 Preparation of the aramid precursor (Ar)

In a 250-ml, two-necked, round-bottom flask, 1,3-phenylenediamine and 2,4-diaminophenol were charged in a 95:5 mole ratio, respectively, with an equivalent amount of TPC in DMAc. The reaction mixture was stirred at room temperature for 24 h. The hydroxyl groups remained unreacted in the said reaction, leading to polyamide formation (Scheme 1).

Scheme 1 Synthesis of the aramid precursor.

Scheme 1

Synthesis of the aramid precursor.

2.4 Preparation of isocyanatopropyltriethoxysilane-grafted MWCNT (sol-gel)

A 0.5-g MWCNT was dispersed in 50 ml of DMAc with sonication of 8 h. A total of 0.5 mol of TEOS was added to 10 ml of DMAc, and then the mixture was poured to the said MWCNT solution followed by stirring for 8 h. Afterwards, 0.5 mol of ICTOS was also added to the reaction mixture. A stoichiometric amount of water in DMAc was then added to carry out the sol-gel process. HCl produced during the polymerization reaction acted as a catalyst. The reaction mixture was allowed to stir at 60°C for 12 h to complete the hydrolysis and condensation of the inorganic network. The hydrolysis and condensation of ICTOS and TEOS via the sol-gel process yielded a silica network structure that bonded with the nanotubes (Scheme 2). The reaction mixture was then filtered, and modified nanotubes were dried at 80°C (22).

Scheme 2 Formation of isocyanatopropyltriethoxysilane-grafted MWCNT.

Scheme 2

Formation of isocyanatopropyltriethoxysilane-grafted MWCNT.

2.5 Preparation of electrospun MWCNT-Ar-Si

The precursor solution of aramid was synthesized as described previously. The solid content of pristine polyamide solution was maintained at 25 wt.%. For the preparation of nanotube solution, 5 wt.% of modified MWCNT/DMAc solution was sonicated for 2 h to disperse the carbon nanotube homogeneously. Afterwards, both the solutions were mixed and sonicated for 2 h (Scheme 3). Electrospinning was carried out using a syringe with a spinneret (diameter 0.5 mm), and 25 kV of voltage was applied at 30°C. The feeding rate was 0.25 mL/h, and the spinneret-collector distance was set to be 10 cm. MWCNT-Ar-Si nanofibers were collected using a rotating disk collector (diameter 0.30 m; width 10 mm). During electrospinning, the linear speed of the rotating collector was about 10 ms-1. All the electrospun nanofibers were dried at 80°C for 4 h to remove the residual solvent, followed by annealing for 1 h (23).

Scheme 3 Structure of chemically bonded MWCNT/aramid-silica in nanofiber.

Scheme 3

Structure of chemically bonded MWCNT/aramid-silica in nanofiber.

2.6 Fabrication of PMMA/MWCNT-Ar-Si hybrid films

For the formation of PMMA/MWCNT-Ar-Si hybrid films, PMMA was reinforced with different MWCNT-Ar-Si fiber contents (0.1–1 wt.%). Initially, 1 g of PMMA was dissolved in 10 ml of DMAc. The desired amount of as-prepared electrospun nanofibers (0.1–1 wt.%) was placed in a Teflon mold. Afterwards, PMMA solution was poured over the MWCNT-Ar-Si nanofibers. The nanofibers were soaked in PMMA solution for 1 h and then heated to 80°C to obtain the nanofiber-reinforced nanocomposite films. Table 1 shows the structural characterization of the MWCNT-Ar-Si nanofiber and the PMMA/MWCNT-Ar-Si nanofiber-reinforced nanocomposite.

Table 1

Structural characteristics of MWCNT-Ar-Si nanofibers and PMMA/MWCNT-Ar-Si hybrids.

CompoundFTIR
Type of vibrationFrequency (cm-1)
MWCNT-Ar-Si nanofiberN-H stretch3311
Aromatic C-H stretch3001
Aliphatic C-H stretch2866
Amide C=O1656
N-H bend1598
Si-O1111
PMMA/MWCNT-Ar-Si 1N-H stretch3302
Aromatic C-H stretch3012
Aliphatic C-H stretch2878
Methacrylate C=O1721
Amide C=O1652
N-H bend1597
Aliphatic C-H bend1387
Si-O1101

3 Results and discussion

3.1 Spectroscopic analysis

Table 1 illustrates the FTIR data of the MWCNT-Ar-Si nanofiber and the PMMA/MWCNT-Ar-Si 1 hybrid material. The FTIR spectrum of the MWCNT-Ar-Si nanofiber showed aromatic and aliphatic C-H stretching vibrations at 3001 and 2866 cm-1, respectively. The amide linkage displayed N-H stretching and bending vibrations at 3311 and 1598 cm-1, respectively. Moreover, the amide carbonyl appeared at 1656 cm-1. Si-O bond was also found to emerge at 1111 cm-1. In the case of the PMMA/MWCNT-Ar-Si 1 hybrid material, aromatic and aliphatic C-H stretching vibrations were found at 3012 and 2878 cm-1, respectively. The characteristic vibration around 3302 and 1597 cm-1 was observed for N-H stretching. Here the amide carbonyl also appeared at 1652 cm-1. In the spectrum of pure PMMA, a strong band usually appeared at 1732 cm-1 for methacrylate carbonyl (C=O). However, in the PMMA/MWCNT-Ar-Si 1 hybrid composition, there was a shift in the methacrylate carbonyl to a lower wavenumber (1721 cm-1), showing a close contact between the matrix and the filler due to hydrogen bonding.

3.2 Mechanical studies

Table 2 illustrates the tensile test results of pure PMMA and PMMA/MWCNT-Ar-Si 0.1–1 and PMMA/Ar 0.1–1 hybrid materials. The corresponding stress-strain curves obtained are given in Figure 1. In general, tensile stress, tangent modulus, and toughness increased with the filler addition. Whereas elongation at break may decrease with the reinforcement addition. Consequently, the maximum strain at rupture (elongation at break) of the hybrids decreased from 3.92 to 3.41, with increasing nanofiller content from 0.1 up to 1 wt.%. A higher value of tensile stress was found for the PMMA/MWCNT-Ar-Si 1 hybrid (44.2 MPa) relative to the PMMA/MWCNT-Ar-Si 0.1–0.5 composites with lower filler content (41.1–43.5 MPa) and pure PMMA (33.8 MPa). The increase was in agreement with better cohesion between MWCNT-Ar-Si nanofibers and the matrix as presented in the scanning electron micrographs in the subsequent section. There was a sharp increase in the tangent modulus for PMMA/MWCNT-Ar-Si 0.1 (5.11 GPa) relative to neat polymer (0.51 GPa). Further addition of nanofibers established an enhancement in rigidity and presence of interaction between the matrix and the filler. Accordingly, PMMA/MWCNT-Ar-Si 0.2 had a tensile modulus of 7.12 GPa, PMMA/MWCNT-Ar-Si 0.5 had 9.32 GPa, and PMMA/MWCNT-Ar-Si 1 showed 13.1 GPa. The toughness of these materials also increased from 129 to 151 J/m3, because better compatibilized blends are tougher. Toughness of the composites was also much higher relative to neat PMMA (41.2 J/m3). The notable increase in tensile strength also suggested that there subsists an intermolecular interaction between the carbonyl of the methacrylate group and the hydrogen of amide linkage. In contrast, PMMA/Ar 0.1–1 hybrid materials showed relatively lower values in the tensile properties, demonstrating the synergetic effect of aramid and MWCNT in nanofibers. The tensile stress of PMMA/Ar 0.1–1 was found to be lower than that of PMMA/MWCNT-Ar-Si hybrids, around 34.2–39.5 MPa. Similarly, tensile modulus values were relatively lower in the range 3.43–7.23 GPa. Therefore, it was observed that the property improvement was not due to multi-walled carbon nanotubes alone or to pure aramid; rather, the electrospinning of MWCNT and aramid via the sol-gel method created a chemical linkage between the nanotubes and the aramid, improving the properties of the resulting material. The overall improvement in the mechanical properties was due to the interaction (hydrogen binding) between modified MWCNT and PMMA and to the subsequent stress transfer between the filler and the polymer matrix The mechanical profile of PMMA/MWCNT-Ar-Si was also found to be superior relative to several reported PMMA-based systems (24, 25).

Figure 1 Stress-strain curves of PMMA and PMMA/MWCNT-Ar-Si hybrids.

Figure 1

Stress-strain curves of PMMA and PMMA/MWCNT-Ar-Si hybrids.

Table 2

Mechanical properties of pristine polymer and PMMA/MWCNT-Ar-Si and PMMA/Ar hybrids.

SampleMaximum stress

(MPa)±0.2
Maximum

strain±0.01
Tangent

modulus(GPa)±0.05
Toughness

(J/m3)±0.1
PMMA33.81.590.5141.2
PMMA/MWCNT-Ar-Si 0.141.13.925.11129
PMMA/MWCNT-Ar-Si 0.242.73.717.12138
PMMA/MWCNT-Ar-Si 0.543.53.649.32143
PMMA/MWCNT-Ar-Si 144.23.4113.1151
PMMA/Ar 0.134.22.783.4399
PMMA/Ar 0.236.12.924.44101
PMMA/Ar 0.538.33.015.77106
PMMA/Ar 139.53.117.23110

3.3 Morphology investigation

The cryo-fracture surface images of PMMA/MWCNT-Ar-Si 0.1–1 nanocomposite films are shown in Figure 2. Figure 2A–D shows the alignment of various nanofiber contents in the matrix. According to the scanning electron micrograph in Figure 2A, nanofibers were embedded and coated with PMMA. Some portions of the aligned MWCNT-Ar-Si (0.1 wt.% sample) nanofibers were visible on the surface. The partially embedded nanofibers were well aligned, forming some sort of exquisitely beautiful floral patterns. The pattern was more obvious in the cryo-fracture surface in PMMA/MWCNT-Ar-Si 0.2 and PMMA/MWCNT-Ar-Si 0.5 films. Furthermore, the nanofibers seemed to be dispersed without any agglomeration in all the samples analyzed. However, the fully aligned fibers were not discernible in any of the samples; rather, the nanofibers were integrated to form a floral morphology. Figure 2E depicts the morphology of a MWCNT-Ar-Si electrospun fiber, and the modification seemed to be uniform throughout the nanotube.

Figure 2 Field emission scanning electron microscopy images of (A) PMMA/MWCNT-Ar-Si 0.1 film; (B) PMMA/MWCNT-Ar-Si 0.2 film;  (C) PMMA/MWCNT-Ar-Si 0.5 film; (D) PMMA/MWCNT-Ar-Si 1 film; (E) MWCNT-Ar-Si nanofiber.

Figure 2

Field emission scanning electron microscopy images of (A) PMMA/MWCNT-Ar-Si 0.1 film; (B) PMMA/MWCNT-Ar-Si 0.2 film; (C) PMMA/MWCNT-Ar-Si 0.5 film; (D) PMMA/MWCNT-Ar-Si 1 film; (E) MWCNT-Ar-Si nanofiber.

3.4 Thermal analysis

Figure 3 shows the thermograms of the different PMMA/MWCNT-Ar-Si nanocomposite films, when 0.1–1 wt.% of nanofiller was added into the PMMA matrix (Table 3). Some differences were found in the weight loss curves as shown in Figure 3. When the temperature was increased beyond 450°C, the PMMA/MWCNT-Ar-Si nanocomposite began to decompose. Compared with pristine PMMA, the decomposition temperature was increased significantly after the addition of the MWCNT-Ar-Si filler. The results indicated that PMMA had an initial degradation temperature (T0) of 295°C, a 10% degradation temperature (T10) of 354°C, and a maximum degradation temperature (Tmax) of around 392°C. The T0 of nanocomposite films increased with increasing MWCNT-Ar-Si dosage. PMMA/MWCNT-Ar-Si 0.1 (0.1 wt.% nanofiller) had a T0 of 465°C, which was increased to 492°C in PMMA/MWCNT-Ar-Si 1 (1 wt.% nanofiller). The results showed that MWCNT-Ar-Si improved the thermal stability of polymer matrix. In addition, the 10% decomposition temperature of films increased with nanofiller loading from 479°C to 531°C in PMMA/MWCNT-Ar-Si 0.1–1. When the amount of MWCNT-Ar-Si added to the silica was increased from 0.1 to 1 wt.%, the maximum decomposition temperature of the samples increased from 491°C to 509°C, 535°C, and 543°C, respectively. The char yield of the nanocomposites was also improved with filler loading from 33% to 40% relative to PMMA (5%). The interaction between the silica-modified aramid-bonded nanotubes improved not only the dispersal state of the filler in the PMMA matrix, but also the thermal stability of the composite film. The glass transition of the nanocomposites was studied from loss tangent (tan δ) vs. temperature as shown in Figure 4. Yet again, the addition of nanofiber increased the segmental Tg from 267°C to 301°C. The higher glass transition for the system with nanofibers depicted the increase in structural rigidity relative to pure PMMA (97°C). In contrast, neat MWCNT-Ar-Si nanofibers had a Tg of 180°C. PMMA/Ar 0.1–1 hybrids showed relatively lower Tg (181–212°C) and thermal properties (T0 423–467°C; Tmax 480–533°C) than PMMA/MWCNT-Ar-Si 0.1–1, showing the combined effect of aramid and MWCNT in nanofibers. The glass transition recorded from the DMTA curve was due to the combined effect of electrospun fibers and PMMA developed by the interaction between the two components. Thus, the improved Tg of the hybrids was not because of the fiber or matrix alone. New nanofibrous membranes had considerably higher heat constancy relative to previous electrospun PMMA nanocomposites (26, 27).

Figure 3 TGA curves of PMMA/MWCNT-Ar-Si hybrids at a heating rate of 10°C/min in N2.

Figure 3

TGA curves of PMMA/MWCNT-Ar-Si hybrids at a heating rate of 10°C/min in N2.

Figure 4 Variation of loss tangent with temperature for pure PMMA, MWCNT-Ar-Si, and PMMA/MWCNT-Ar-Si.

Figure 4

Variation of loss tangent with temperature for pure PMMA, MWCNT-Ar-Si, and PMMA/MWCNT-Ar-Si.

Table 3

Thermal analyses data of pristine polymer and PMMA/MWCNT-Ar-Si and PMMA/Ar hybrids.

SampleTg (°C)T0 (°C)T10 (°C)Tmax (°C)Yc at 600°C (%)
PMMA972953543925
PMMA/MWCNT-Ar-Si 0.126746547949133
PMMA/MWCNT-Ar-Si 0.227147348950935
PMMA/MWCNT-Ar-Si 0.528948552053538
PMMA/MWCNT-Ar-Si 130149253154340
PMMA/Ar 0.118142345148030
PMMA/Ar 0.219844446449332
PMMA/Ar 0.520245649851234
PMMA/Ar 121246750553336

Tg, Glass transition temperature; T0, initial decomposition temperature; T10, temperature for 10% weight loss; Tmax, maximum decomposition temperature; Yc, char yield; weight of polymer remained.

4 Conclusion

In the current work, electrospun, sol-gel process-modified, aramid-silica-grafted multi-walled carbon nanotube-based nanofibers were successfully prepared and reinforced into a PMMA matrix via a simple solution blending method. A 0.1–1 wt.% dosage of MWCNT-Ar-Si nanofiber was used. The fracture surface micrographs of the composites showed that the nanofibers were well dispersed in the matrix and formed some well-aligned floral patterns. The tensile strength of PMMA/MWCNT-Ar-Si 0.1–1 was found to be reasonably high in the range 41.1–44.2 MPa. In the TGA plots of PMMA/MWCNT-Ar-Si 0.1–1 nanocomposites, the thermal stability and the decomposition temperature obviously increased, with increasing electrospun nanofiber dosage. Similarly, the glass transition temperature as determined from the maxima of tan δ data using dynamic mechanical thermal analysis was increased with filler loading from 267°C to 301°C. Considering the higher thermal and mechanical stability and lower cost, PMMA/MWCNT-Ar-Si 0.1–1 can be widely used as high-performance functional materials in various fields including microelectronic devices, sensors, wire insulation, etc.


Corresponding author: Ayesha Kausar, Nanosciences and Catalysis Division, National Centre for Physics, Quaid-i-Azam University Campus, 44000, Islamabad, Pakistan, Tel.: +92 51 2077300, Fax: +92 51 2077395, e-mail:

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Received: 2014-1-21
Accepted: 2014-3-4
Published Online: 2014-4-2
Published in Print: 2014-5-1

©2014 by Walter de Gruyter Berlin/Boston

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