- Email: [email protected]
Comparative study of the structural and rheological properties of PA6 and PA12 based synthetic talc nanocomposites
Polymer 62 (2015) 109e117
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Comparative study of the structural and rheological properties of PA6 and PA12 based synthetic talc nanocomposites ro ^ me Crepin-Leblond, Pascal Mederic, Thierry Aubry* Quentin Beuguel, Julien Ville, Je LIMATB, Equipe Rh eologie, Universit e de Bretagne Occidentale, UFR Sciences et Techniques, 6 Avenue Victor le Gorgeu, CS 93 837, 29 238 Brest Cedex 3, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2014 Received in revised form 16 February 2015 Accepted 18 February 2015 Available online 25 February 2015
The structural and rheological properties of synthetic talc/polyamide (ST/PA) composites, elaborated by simultaneous melt mixing of a PA6 or PA12 matrix and a hydrogel filled with talc, were studied, focusing on the difference between PA6 and PA12 based nanocomposites. The structure of ST/PA composites is shown to be largely composed of nanometric particles. However, the presence of few micrometric aggregates is shown at moderate solid volume fractions, for both PA6 and PA12. The results also show that PA6 matrix is more efficient than PA12 matrix to disperse synthetic talc nanoparticles; the difference in matrix effect is discussed in terms of matrix polarity, PA chain radius of gyration and role played by water brought by the hydrogel. Finally, the results of the present study suggest that the structural and rheological properties of ST/PA are close to those of organically modified montmorillonite/PA nanocomposites, especially when PA6 is the matrix. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Synthetic talc Polyamides Nanocomposites
1. Introduction Since the early 1990s, many studies have dealt with polymer layered silicate nanocomposites, from preparation to characterization, as thoroughly reviewed by Sinha Ray and Okamoto [1] or by Krishnamoorti and Yurekli [2]. Many different polymer matrices were used to prepare such nanocomposites, as shown by the numerous references cited by Sur et al. [3]. In the case of a polar matrix, such as polyamide, the layered silicate classically used for the preparation of nanocomposites is montmorillonite which has been organically modified by ion-exchange reactions with cationic surfactants like quaternary alkylammonium cations [4]. Addition of such high aspect ratio organoclay nanoplatelets in polyamide thermoplastic matrices, even at very low volume fractions, may lead to the improvement of flame retardancy [5e7], barrier properties [8e10], thermal properties [7,11] and mechanical properties [11e13], explaining the potential commercial interest of these nanostructured materials. However, good macroscopic properties require a high exfoliation degree of clay particles, which is dependent on processing conditions [14], polyamide molecular weight [15] but also polyamide polarity governing matrix/
* Corresponding author. Tel.: þ33 (0)2 98 01 66 86; fax: þ33 (0)2 98 01 79 30. E-mail address: [email protected] (T. Aubry). http://dx.doi.org/10.1016/j.polymer.2015.02.031 0032-3861/© 2015 Elsevier Ltd. All rights reserved.
organoclay interactions [16]. Most experimental studies on linear viscoelastic behavior of layered silicate nanocomposites show that there exists a transition from liquid-like behavior on short time scales to solid-like behavior on longer time scales, at a low silicate volume fraction fp usually ranging from 1 to 2% [17]. This result is attributed to the formation of a three-dimensional percolation network at very low solid contents, because of the high aspect ratio of clay entities [18e21]. For organically modified montmorillonite/ polyamide 12 nanocomposites, the network was described as constituted of mesoscopic domains composed of correlated silicate layers, with a characteristic size independent of the solid content [22]. For many years, thermoplastic matrices reinforced by natural talc have been widely used in various industrial applications, such as household appliances and automotive industry [23e25]. Recently, talc industry strives to produce talc nanoparticles, which could challenge exfoliated montmorillonite nanoparticles. However, the processes used to elaborate talc based nanocomposites are different from those used to prepare polymer layered silicate nanocomposites, mainly because of the lack of exchangeable cations in the interlayer space of talc particles. The first attempts to reduce the size of natural talc particles were based on delamination techniques, which led to particles with average thickness of a few hundreds of nanometers, which were referred to as high aspect ratio particles [26]. More recently, an innovative and much more
110
Q. Beuguel et al. / Polymer 62 (2015) 109e117
efficient process, based on hydrothermal synthesis of talc nanoparticles, was proposed [27e30]. This process leads to the formation of a hydrogel which contains synthetic talc particles with controlled physical characteristics identical to those of natural talc particles in terms of lamellarity and crystallinity [31]. The average length of these synthetic talc particles was shown to lie between a few tens and a few hundreds of nanometers, and their average thickness between 6 nm and 10 nm [32]. Finally, using an extruder, synthetic talc particles, prepared from hydrothermal synthesis and dispersed in PA6 matrix, were shown to lead to nanocomposites [33]. The aim of the present work is to investigate the influence of the polyamide matrix physicochemical properties on the structural and rheological properties of nanocomposites elaborated by simultaneous melt mixing of a polyamide 6 or polyamide 12 matrix and a hydrogel filled with synthetic talc particles. Structure and rheology of these innovative nanocomposites are compared to those of delaminated natural talc/polyamide microcomposites as well as organically modified montmorillonite/polyamide nanocomposites.
As mentioned above, structural and rheological properties of ST/ PA composites have been compared to those of organically modified montmorillonite/PA nanocomposites and natural talc/PA conventional microcomposites:
2. Experimental part 2.1. Materials Composites were prepared from two commercial polyamide matrices: a polyamide 12 (PA12) and a polyamide 6 (PA6), supplied by Arkema (referenced as Rilsan® AECHVO) and Rhodia (referenced as Technyl® S-27-BL), respectively. Table 1 gives the main characteristics of the two polymers: the number and weight average molar weights, Mn and Mw, the radius of gyration, Rg, determined from the freely jointed chain model, and the melting point, Tm. The number and weight average molar weights of both polyamides are rather similar, even if the radius of gyration of PA6 chains is somewhat smaller than that of PA12 chains. Moreover, it is worth pointing out that the water absorption of PA12 is less than 0.5% after 45 min at 220 C whereas it is close to 2% for the PA6 matrix, which is more polar [34], under the same conditions [35]. The hydrogel filled with Synthetic Talc (ST) particles was prepared at GET Laboratory (University Paul Sabatier, Toulouse, France) according to a hydrothermal process patented [30]. The ST particles of the study were obtained after a hydrothermal treatment duration for 6 h and were previously characterized by Dumas et al. [31]. The particle size was shown to vary from ~30 nm to ~3000 nm, leading to an average length of 150 nm, meaning that micronic particles are few in number, contrary to what is observed for natural talc. Dumas et al. [31] also reported that ST particles had (i) a specific surface area of 150 m2/g, higher than that of natural talc, (ii) a number of stacking defects higher than that observed in natural talc particles and (iii) a hydrophilic character, contrary to particles of natural talc, which have a hydrophobic character. Representative Transmission Electron Microscopy (TEM) micrograph of the hydrogel, diluted ten times in water, is shown in Fig. 1. Synthetic talc particles have a length, ranging from 20 nm to 300 nm, close to the values obtained by Dumas et al. [31], and an average thickness of ~8 nm.
Table 1 Characteristics of polyamide matrices.
Mn (g/mol) Mw (g/mol) Rg (nm) Tm ( C)
Fig. 1. TEM micrograph of hydrogel filled with ST particles, diluted ten times in water.
PA12
PA6
38,000 60,000 11 183
39,000 79,000 8 222
The organically modified montmorillonite clay (OMMT), namely Cloisite® 30B, has been supplied by Southern Clay Products. It is a methyl tallow bis-2-hydroxyethyl ammonium exchanged montmorillonite clay, with a modifier concentration of 90 milliequivalent per 100 g and has a good affinity towards polyamide [22,36]. This organophilic clay is characterized by a specific area of about 700 m2 g1. The primary OMMT particles are composed of about ten stacked layers, whose thickness is ~0.7 nm, and length ~200 nm [37], corresponding to an aspect ratio of ~350. The interlayer distance, estimated from XRD measurements, is close to 1.2 nm and the specific gravity of OMMT is close to 2. The High Aspect Ratio® Natural Talc, referred to as HARNT in this work, is supplied by Imerys Talc (Toulouse, France). HARNT results from the delamination of a coarse natural talc, leading to thinner, but still micrometric, particles. The average equivalent diameter of HARNT talc particles, determined by laser diffraction, is about 10 mm. The interlayer distance, estimated from XRD measurements, is close to 0.3 nm. The specific area of HARNT talc is 19.5 m2/g and the specific gravity of HARNT is close to 2.8.
2.2. Sample elaboration PA12 and PA6 samples were vacuum dried at 80 C, during 4 h for PA12 and 24 h for PA6, whose hygroscopic character is more marked [35]. All samples used in this study were prepared by simultaneous melt mixing of components using DSM Xplore lab twin screw extruder. Because of the difference of matrix melting point (Table 1), the mixing temperature was chosen at 220 C and 240 C for PA12 and PA6, respectively. The screw rotational speed was fixed at 50 rpm for 6 min. The melt mixing of a polymer matrix and a hydrogel of synthetic talc leads to the formation of a very small number of almost millimetric ST agglomerates in the extrudate, at least at ST volume fractions higher than 5% (Fig. 2a). The existence of these micrometric particles was attributed to fast drying of part of the talc filling the hydrogel, during its incorporation into the heated extruder chamber (Fig. 2b). After mixing, all samples were pelletized and moulded by compression into 2 mm thick plates, at 200 C for PA12 samples and
Q. Beuguel et al. / Polymer 62 (2015) 109e117
111
Fig. 2. Highly filled ST/PA12 composites: formation of micrometric aggregates in the extrudate with ~2 mm diameter (a) and of a whitish talc deposit on chamber wall and blade surface (b).
240 C for PA6 samples; pressure was increased by steps, from 5 MPa to 25 MPa, in order to avoid the formation of air bubbles during the compression molding. Following this route, samples were prepared at filler volume fraction, f, ranging from 0 to 9.2%, from 0 to 21.5% and from 0 to 11.3 %, for OMMT, HARNT and ST particles, respectively. It is worth pointing out that such volume fractions have been calculated from the weight fraction measured after calcination. 2.3. Structural characterization The nanostructure of the samples was investigated from 40 nm thick ultrathin sections cut using a diamond knife, at 130 C, with an ultracryomicrotome (Reichert Jung); imaging was achieved with a JEOL 1400 Transmission Electron Microscope at 100 kV. On TEM micrographs, length and thickness of fillers were measured for at least 200 particles, using SigmaScan® Pro 5.0 image analysis software. From these values, the number average length, L, and thickness, e, were calculated. The specific particle density, dsp, that is the number of particles per mm2 divided by the filler weight fraction, as defined by Fornes et al. [15], was also estimated. Moreover, on a micrometric length scale, cryofractured samples with vacuummetallized surface were observed using a Hitachi S-3200N Scanning Electron Microscope (SEM), with an accelerating voltage of 15 kV. Confocal microscopy characterization, used because of the natural fluorescence of clay fillers, completes SEM and TEM characterizations, providing a 3D description of the samples. It was carried out by spectral deconvolution, using a Zeiss confocal microscope LSM 780 equipped with a laser diode of wavelength 405 nm, a 32 GaAsP PMT detector and a 63 oil lens. Clay structure was also investigated using wide-angle X-ray scattering at ambient temperature. It was performed using a PANalytical Empyrean X-ray diffractometer with Cu Ka radiation of wavelength 0.15 nm, generated at 45 kV and 40 mA. XRD experiments were carried out over a 2q range of 2 e10 , using steps of 0.015 and counting times of 10 s at each angular position. 2.4. Rheological characterization Oscillatory shear measurements were performed using a controlled strain rheometer (Gemini) equipped with parallel disks
of 25 mm diameter and 2 mm spacing. All experiments were carried out under a continuous purge of dry nitrogen in order to avoid sample degradation. Rheological tests were performed at a temperature of 200 C for PA12 matrix samples and at a temperature of 240 C for PA6 matrix samples, because of their different melting point (Table 1). First, the so-called critical strain, gc, which defines the extent of the linear viscoelastic regime, was determined by strain sweep experiments, carried out at a fixed frequency of 1 Hz. Then, all frequency sweep tests were performed at a strain amplitude within the linear viscoelastic regime. Special attention was paid to material thermal stability during rheological tests, using time sweep experiments. The thermal stability was shown to be enhanced by addition of clay particles, as reported by many research works [5,22]. More precisely, variations of rheological properties inferior to 10% were observed at 200 C for PA12 based composites and at 240 C for PA6 based composites, over 30 min. At last, all rheometrical data obtained within 20 min were shown to be reproducible within ±5%. Steady-state viscosity measurements at high shear rates were €ttfert capillary rheometer in order to performed on a RT 1000 Go verify that the matrix viscosity had the same order of magnitude under the processing conditions.
3. Results and discussion 3.1. Structural investigation Fig. 3 shows typical TEM micrographs of OMMT/PA12 (Fig. 3a) and OMMT/PA6 (Fig. 3b) nanocomposites with 0.5% and 0.6% OMMT, respectively. Numerous individual OMMT particles and few nanometer thick OMMT stacks are well-dispersed within PA12 or PA6 matrix (Fig. 3). The average particle length, L ~ 65 nm, calculated from TEM micrographs, is the same for the two PA matrices and does not depend on OMMT volume fraction (Table 2). Besides, Table 2 shows that using PA6 matrix tends to slightly decrease the average thickness of the particles, therefore improving the exfoliation degree of OMMT particles [16]. Finally, for both PA matrices, an increase of average particle thickness, hence a decrease of specific particle density, with increasing OMMT volume fraction is observed (Table 2). This classical result is explained by the presence of a few layer stacks,
112
Q. Beuguel et al. / Polymer 62 (2015) 109e117
Fig. 3. TEM micrographs of nanocomposites filled with OMMT: (a) PA12 matrix and (b) PA6 matrix.
Table 2 Characteristic size of OMMT, HARNT and ST particles and specific particles density.
OMMT
Matrice
f (%)
L (nm)
e (nm)
dsp
PA12
0.5 2.6 0.6 2.9 0.8 2.2 6.6 2.4 5.3 3.9 10.8 4.4 12
~65
3.6 4.3 3.1 3.9 8
5
23 11 33 19 9 7 6 16
~55
<0.2
PA6 ST
PA12
PA6 HARNT
PA12 PA6
~65 ~65
~450
with a characteristic size of ~500 nm, in highly filled nanocomposites [14,22]. In order to illustrate the structural state of HARNT/PA composites, Fig. 4 presents SEM micrograph (Fig. 4a) and TEM micrograph (Fig. 4b) of the PA12 matrix filled with 3.9% HARNT. Electron microscopy observations show that most HARNT particles are micrometric, in accordance with the results from measurements carried out by laser diffraction for the neat HARNT. Finer particles (~50 nm thick), probably ripped from micrometric particles during mixing, are also shown. Average particle dimensions, L ~ 450 nm and e ~ 55 nm, estimated from TEM micrographs, are the same for both PA matrices and do not depend on HARNT volume fraction. The large average particle dimensions and the very low specific particle density (Table 2) characterizing HARNT/PA composites mean that these systems should be considered as microcomposites. Fig. 5 presents TEM micrograph and SEM micrograph of ST/PA12 composites (Fig. 5a and b) and of ST/PA6 composites (Fig. 5c and d). ST volume fractions are 2.2% and 2.4% in PA12 and PA6, respectively. A nanostructure, mainly composed of fine ST particles and a few layer stacks, with submicronic thickness (Fig. 5a and c), is
highlighted for composites based on PA12 or PA6 matrix. However, for both PA matrices, SEM micrographs reveal the presence of a few micrometric aggregates from 2% ST volume fraction (Fig. 5b and d), and also the presence of some millimetric agglomerates formed during mixing (Fig. 2a and b). Indeed, dried micrometric ST aggregates were shown to be difficult to disperse in PA matrix. Above 2% ST, the size and proportion of micrometric ST entities increase with increasing ST volume fraction, for the two PA matrices used. In order to characterize more quantitatively the multi-scale structural state, the proportion of micrometric ST entities was estimated from confocal micrographs, in the case of the PA12 matrix filled with 6.6% synthetic talc (Fig. 6). Synthetic talc particles with thickness above 1 mm were considered as micrometric entities. Using this criterion, the volume fraction of micrometric ST entities dispersed within the PA12 matrix is close to 25% in the case of ST/PA12 composite with 6.6% synthetic talc, confirming that the structure is mainly nanometric, even for highly filled ST/PA composites (Fig. 6). Moreover, even though the volume fraction of micrometric entities is significant, their number is still very small compared to the number of nanometric ST particles. The volume fraction of micrometric ST particles determined by confocal microscopy seems to be consistent with the volume fraction of micrometric particles determined from electron microscopy observations (Fig. 5). Even though the micrometric structure of ST/PA composites seems to be insensitive to the matrix used, either PA6 or PA12, their nanostructure is but affected. Indeed, although the average length of submicronic particles, L ~ 65 nm, close to the value estimated by Dumas et al. [31], is independent of ST volume fraction and PA matrix (Table 2), their average thickness depends on PA matrix. Indeed, the average thickness of nanoparticles dispersed in PA12 matrix is 8 nm, i.e. close to the one obtained for the ST filled hydrogel, but it is only 5 nm, when ST particles are dispersed in PA6 matrix (Table 2). The PA12 matrix does not refine the structure of ST particles present in the hydrogel. On the contrary, the lowest value of ST particles average thickness in PA6 matrix suggests that this
Fig. 4. SEM micrograph (a) and TEM micrograph (b) of HARNT/PA microcomposites.
Q. Beuguel et al. / Polymer 62 (2015) 109e117
113
Fig. 5. ST/PA12 composite: TEM micrograph (a) and SEM micrograph (b). ST/PA6 composite: TEM micrograph (c) and SEM micrograph (d).
polyamide divides ST particles, as confirmed by the higher specific particle density in ST/PA6 composites (Table 2). Nevertheless, specific particle density, which is sensitive to the presence of micrometric aggregates (Fig. 5b and d), decreases with increasing ST volume fraction. Fig. 7 presents thickness distribution of OMMT, ST and HARNT particles dispersed within PA12 or PA6 matrix, at least for particles with thickness up to 1 mm, at filler volume fractions ~0.5%, ~2% and ~4% for OMMT, ST and HARNT, respectively. Fig. 7 shows that the thickness of OMMT nanoparticles does not exceed 50 nm, for both PA matrices. Besides, OMMT thickness distribution is slightly narrower for nanoparticles dispersed in the PA6 matrix; still, the thickness distribution slightly broadens with increasing OMMT volume fraction. Besides, Fig. 7 shows that the minimum thickness of HARNT particles is superior to 10 nm. In addition, the flattened thickness distribution proves that submicronic particles of HARNT/PA microcomposites are thicker than OMMT nanoparticles. Moreover, HARNT thickness distribution is the same for both PA matrices and does not depend on volume fraction.
At last, for all ST fractions studied, Fig. 7 shows that 80% of ST nanoparticles dispersed in PA12 matrix have a thickness inferior to 10 nm. Moreover, the thickness distribution of submicronic ST particles dispersed in PA6 matrix is significantly narrower, and quite comparable with the thickness distribution of OMMT nanoparticles. Wide-angle X-ray diffraction measurements were carried out in order to characterize the structure of OMMT/PA nanocomposites, HARNT/PA microcomposites and ST/PA composites (Fig. 8). The peak at 2q ¼ 5.5 is the signature of the semi-crystalline structure of PA12 matrix [12], whereas no peak is observed for PA6 matrix over the angle range explored in the study [38]. Fig. 8a shows a well-defined peak at 2q ¼ 2.7, corresponding to a d-spacing of 3.3 nm, meaning that PA12 chains are intercalated between OMMT layers [22,36]. Besides, in the case of OMMT/PA6 nanocomposites, the absence of peak at low angles investigated suggests a better exfoliation of OMMT nanoparticles in PA6 matrix, in agreement with electron microscopy observations (Fig. 3). Indeed, from interlayer spacing and average thickness values, we can infer that OMMT particles dispersed in PA6 matrix are
Fig. 6. Confocal micrograph in the case of ST/PA12 composite with 6.6% synthetic talc.
114
Q. Beuguel et al. / Polymer 62 (2015) 109e117
Fig. 7. Thickness distribution of OMMT, ST and HARNT particles within PA12 or PA6.
and only ~5 layers in PA6 matrix. Again, like in the case of OMMT, the better efficiency of PA6 to separate ST particles could be attributed to the higher polarity of PA6 [34] and to its smaller radius of gyration. Moreover, the difference in diffractograms between ST/ PA12 and ST/PA6 composites, within the range of small angles (<5 ) corresponding to interfoliar distances superior to 1.1 nm, could be the signature of the presence of stacking defects, as mentioned by Dumas et al. [31]. We suggest that the presence of water coming from the hydrogel and the defects present in hydrophilic ST particles could ease the penetration of PA chains within ST galleries. These mechanisms could be favored in the case of the most hydrophilic and most polar matrix, i.e. PA6, and together with a smaller radius of gyration of PA6 could facilitate the separation of the smallest ST particles. Therefore, the decrease of number of defects is in agreement with the weakest intensity of diffraction signal for the PA6 based composite. To summarize, specific particle density and average thickness of ST/PA composites (Table 2), as well as their thickness distribution (Fig. 7) are systematically between those of OMMT/PA nanocomposites and HARNT/PA microcomposites. More precisely, the structure of ST/PA composites is mainly a nanostructure, with the presence, at moderate ST volume fractions, of large aggregates, which most likely form into the extruder chamber, during the incorporation step. The nanostructure is close to that of partially exfoliated OMMT/PA nanocomposites, especially in the case of PA6 matrix, which is more polar, therefore more hydrophilic, and whose chains have a smaller radius of gyration. However, even though PA6 matrix seems to be more efficient than PA12 to finely disperse ST nanoparticles, it seems to have no effect on the presence of micrometric ST defaults.
composed of 2 layers, on average. The better efficiency of PA6 to exfoliate OMMT particles cannot be attributed to higher shear induced exfoliation during the mixing process since PA6 and PA12 viscosities have the same order of magnitude under the mixing conditions used. However, the higher polarity of PA6 matrix, which confers a better affinity towards OMMT, could partly explain its efficiency to improve exfoliation of OMMT particles [16]. Exfoliation might also be improved by the smaller radius of gyration of PA6 chains, which makes their intercalation into OMMT galleries easier. In Fig. 8b, the peaks at 2q ¼ 6 and 2q ¼ 9.3 , due to chlorite and talc respectively [39], are identical for the two matrices (PA6 or PA12), suggesting that no intercalation of PA chains occurs between HARNT galleries. Fig. 8c, presenting diffractograms of ST/PA nanocomposites, also shows the characteristic peak of talc, at 2q ¼ 9.3 . As suggested by Yousfi et al. [33], the weak intensity of this peak is indicative of a structure mainly composed of stacks of a few ST layers, as shown by electron microscopy (Fig. 5a and b). At close volume fractions, this peak seems to be less marked for composites based on PA6 matrix, confirming the better efficiency of PA6 matrix to disperse ST particles. In fact, ST stacks are composed of ~8 layers in PA12 matrix
The critical strain, gc, determined by the classical tangent analysis, defines the limit of the linear viscoelastic regime. It is unambiguously determined when the number of fillers is sufficient. In Fig. 9, it is plotted as a function of filler volume fraction. Regardless of the matrix, the critical strain is a decreasing function of filler volume fraction. At any given filler volume fraction, the critical strain of ST/PA systems with a multi-scale structure (Fig. 6), lies systematically between that of OMMT/PA nanocomposites and that of HARNT/PA microcomposites, in agreement with structural characterization results.
Fig. 8. Wide-angle X-ray diffractograms of OMMT/PA nanocomposites (a), HARNT/PA microcomposites (b) and ST/PA composites (c).
Fig. 9. Critical strain as a function of filler volume fraction for OMMT/PA, ST/PA nanocomposites and HARNT/PA microcomposites.
3.2. Rheological investigation
Q. Beuguel et al. / Polymer 62 (2015) 109e117
On the one hand, the critical strain is higher for ST/PA nanocomposites than for OMMT/PA nanocomposites. This could be attributed to larger interparticle distances in the case of ST/PA systems, due to the presence of a few micrometric aggregates of synthetic talc (Fig. 5b and d) and also to a nanostructure slightly less fine than that of OMMT exfoliated nanoparticles (Table 2, Fig. 7). On the other hand, the critical strain of ST/PA nanocomposites is drastically lower than that of HARNT/PA microcomposites, which is simply due to a much better dispersion state of ST particles. As far as PA matrix effect is concerned, in the case of HARNT/PA microcomposites, the critical strain is about the same for the two PA matrices, because of quite similar dispersion states, as suggested by structural characterization (Table 2). On the contrary, at a given filler volume fraction, the critical strain of ST/PA6 nanocomposites is significantly smaller than that of ST/PA12 nanocomposites and tends towards that of nanocomposites based on OMMT particles. This matrix effect on gc can be explained by the better state of dispersion of ST nanoparticles, when PA6 matrix is used, as discussed above. Following the above interpretation based on the relationship between critical strain and dispersion state, the results concerning OMMT/PA nanocomposites could seem paradoxical. Indeed, although the structure of OMMT nanoparticles is finer in the presence of PA6 matrix, the critical strain of OMMT/PA6 nanocomposites is higher than that of OMMT/PA12 nanocomposites. This apparently surprising result could be attributed to a possible degradation of quaternary alkylammonium modifier at higher mixing temperatures [40] (240 C, in the case of PA6 matrix), which could result in reduced clay/matrix interactions. The critical strain versus filler volume fraction can be described by a power law for all composites of this study [41]. In the case of
115
OMMT/PA nanocomposites, the critical strain is inversely proportional to OMMT volume fraction (Fig. 9), as obtained by Aubry et al. [22] for OMMT/PA12 nanocomposites prepared using an internal mixer, or for other layered silicate/polymer nanocomposites [42]. It is worth pointing out that the exponents of the power law dependence of the critical strain versus volume fraction obtained for ST/PA nanocomposites are close to those of OMMT/PA nanocomposites, i.e. 1, whereas an exponent close to 2 is obtained for HARNT/PA microcomposites (Fig. 9). Fig. 10 shows the complex viscosity jh*j, as a function of frequency, for OMMT/PA12, ST/PA12 nanocomposites and HARNT/ PA12 microcomposites, with different filler volume fractions. PA6 matrix composites qualitatively exhibit the same rheological behaviors (not presented here). For both matrices (PA6 and PA12) and their weakly filled composites, at low frequencies, the complex viscosity exhibits a plateau, defining a Newtonian viscosity. However, it is not possible to determine a Newtonian viscosity for sufficiently filled nanocomposites. Indeed, low frequency complex viscosity depends on frequency, above a filler volume fraction, fp, which is specific to each composite: fp ~ 1% for OMMT nanocomposites [22] (Fig. 10a), fp ~ 10% for HARNT microcomposites (Fig. 10b) and fp ~ 6% for ST/ PA nanocomposites (Fig. 10c). This specific filler volume fraction corresponds to a percolation threshold attributed to the formation of a percolated network [17e22]. It is worth noticing that percolation threshold of ST/PA nanocomposites is between that of OMMT/PA nanocomposites and that of HARNT microcomposites, in agreement with structural characterization results. Below percolation threshold (f < fp), the relative viscosity, hr, defined as the ratio of Newtonian complex viscosity of the composite to that of the PA matrix, is presented as a function of filler volume fraction in Fig. 11.
Fig. 10. Complex viscosity as a function of frequency for OMMT/PA12 nanocomposites (a), HARNT/PA12 microcomposites (b) and ST/PA12 nanocomposites (c).
116
Q. Beuguel et al. / Polymer 62 (2015) 109e117
Fig. 11. Relative viscosity as a function of filler volume fraction for OMMT/PA12, ST/ PA12 nanocomposites and HARNT/PA12 microcomposites; experimental data are fitted with Einstein model (1).
As proposed by Utracki and Lyngaee-Jorgensen [20], these results were discussed in terms of intrinsic viscosity [h], determined by fitting the curve with the second-order Einstein-type equation:
hr ¼ 1 þ ½hf þ kð½hfÞ
2
(1)
where k is an interaction constant. Knowing the intrinsic viscosity [h], the aspect ratio p of the disklike clay particles can be inferred:
½h ¼ 2:5 þ 0:025 1 þ p1:47
(2)
Intrinsic viscosity, interaction constant and aspect ratio of all systems studied are reported in Table 3. Firstly, the results in Table 3 show that the aspect ratio of synthetic talc particles, p ~ 80, is between that of OMMT and HARNT particles (Table 3), in agreement with structural results. Moreover, the aspect ratio of HARNT particles does not seem to depend on the nature of PA, whereas it strongly depends on the matrix for the two other particles: an increase of 20% of the aspect ratio for OMMT nanoparticles, and 40% for ST nanoparticles, is observed when the PA6 matrix is used. Such matrix effects on the aspect ratio are in agreement with the thickness decrease of OMMT and ST nanoparticles when PA6 is the matrix (Table 2, Fig. 7), which was attributed to the relatively weak improvement of the exfoliation mechanism in the case of OMMT/PA6 nanocomposites and to the significant efficiency of separation mechanism in the case of ST/PA6 nanocomposites. As far as the interaction constant is concerned, its value depends on the composite type, but not on the nature of PA matrix.
Table 3 Intrinsic viscosity, interaction constant and aspect ratio values for OMMT, ST and HARNT particles.
OMMT ST HARNT
Matrices
[h]
k
p
PA12 PA6 PA12 PA6 PA12 PA6
65 82 18 27 8
0.8
200 240 80 110 40
0.7 0.25
Fig. 12. Yield shear stress as a function of filler volume fraction for OMMT/PA12, ST/ PA12 nanocomposites and HARNT/PA12 microcomposites.
Above percolation threshold (f > fp), as suggested by Lertwimolnun et al. [43], the flow curves plotted in Fig. 10 can be correctly described by a phenomenological CarreaueYasuda Model with a yield stress:
h* ¼
n1 t0 þ h*0 1 þ ðluÞa a u
(3)
with t0, the apparent yield stress, h*0 the Newtonian complex viscosity, l a characteristic time, n the pseudoplasticity index and a a fitting parameter. The apparent yield stress, t0, has been plotted versus filler volume fraction in Fig. 12. The apparent yield stress is shown to be strongly dependent on filler volume fraction, close to the percolation threshold. For sufficiently high OMMT volume fractions, the f dependence of yield stress is shown to be correctly fitted by a quadratic expression, in agreement with the results by Aubry et al. [22]. It is worth noticing that, at a given filler volume fraction, the yield stress estimated for ST/PA nanocomposites lies between that of OMMT/PA nanocomposites and that of HARNT/PA microcomposites, regardless of PA matrix. As observed for the critical strain, the yield stress of HARNT/PA microcomposites is independent of PA matrix. Besides, at a fixed filler volume fraction, the apparent yield stress of ST/PA6 nanocomposites is significantly higher than that of ST/PA12 nanocomposites, in accordance with the higher aspect ratio of ST nanoparticles within the PA6 matrix. At last, the yield shear stress of OMMT/PA6 nanocomposites, which exhibit the most exfoliated structure, is paradoxically smaller than that of OMMT/PA12 nanocomposites, at the same OMMT volume fraction. We suggest that the clay network could be weakened by the degradation of quaternary alkylammonium modifier occurring at the mixing temperature of 240 C.
4. Conclusion Structural and rheological properties of innovative synthetic talc/polyamide composites were shown to lie between those of organically modified montmorillonite/polyamide nanocomposites and those of delaminated natural talc/polyamide microcomposites, for both PA12 and PA6 matrices. More precisely, synthetic talc particles were shown to be mainly nanometric in size in both PA6
Q. Beuguel et al. / Polymer 62 (2015) 109e117
and PA12 matrices, but they were shown to be even smaller in PA6. We suggest that PA6 chains could more easily penetrate the synthetic talc particles present in the hydrogel used during the sample elaboration, favoring their separation and therefore leading to synthetic talc nanoplatelets whose dimensions are closer to those of organically modified nanoparticles. Indeed, the intercalation of matrix chains within their stacking defects in presence of water coming from the hydrogel is favored by the higher polarity, and therefore higher hydrophilicity, of PA6, together with the smaller radius of gyration of PA6 chains. However, the presence of a few very large aggregates, formed during the incorporation of synthetic talc hydrogel into the extruder, somewhat affects the rheological behavior of synthetic talc/polyamide nanocomposites. Nevertheless, the results of the present work suggest that, despite the existence of a few large aggregates, nanocomposites based on synthetic talc could be potential challengers of nanocomposites based on organically modified clays. In addition, synthetic talc/ polyamide nanocomposites have the advantage to be insensitive to thermal degradation at relatively high temperatures, contrary to organically modified montmorillonite/polymer nanocomposites, because of degradation of the modifier. Acknowledgments The authors are grateful to Imerys Talc (Toulouse, France) for thermogravimetry measurements, logistic and financial support. rard Sinquin We thank Philippe Elies, François Michaud and Ge de Bretagne Occidentale) for practical assistance in (Universite structural characterization. References [1] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites, a review from preparation to processing. Prog Polym Sci 2003;28:1539e41. [2] Krishnamoorti R, Yurekli K. Rheology of polymer layered silicate nanocomposites. Curr Opin Colloid Interface Sci 2001;6:464e70. [3] Sur GS, Sun HL, Lyu SG, Mark JE. Synthesis, structure, mechanical properties, and thermal stability of some polysulfone/organoclay nanocomposites. Polymer 2001;42:9783e9. chelle et stabilite colloïdale de suspensions [4] Gherardi B. Organisation multi e d'Orle ans; 1998. d'argiles en milieu organique [Ph.D. thesis]. Universite [5] Gilman JW. Flammability and thermal stability studies of polymer-layered silicate (clay) nanocomposites. Appl Clay Sci 1999;15:31e49. [6] Zanetti M, Camino G, Thomann R, Mulhaupt R. Synthesis and thermal behavior of layered silicate-EVA nanocomposites. Polymer 2001;42:4501e7. [7] Tang Y, Hu Y, Wang S, Gui Z, Chen Z. Preparation of poly(propylene)/clay layered nanocomposites by melt intercalation from pristine montmorillonite (MMT). Polym Adv Tech 2003;14:733e57. [8] Massersmith PB, Giannelis EP. Synthesis and barrier properties of poly(εcaprolactone)-layered silicate nanocomposites. J Polym Sci A Polym Chem 1995;33:1047e57. [9] Ke Z, Yongping B. Improve the gas barrier properties of PET film with montmorillonite by in-situ interlayer polymerization. Mater Lett 2005;59: 3348e51. [10] Krook M, Albertsson AC, Gedde UW, Hedenqvist MS. Barrier and mechanical properties of montmorillonite/polyesteramide nanocomposite. Polym Eng Sci 2004;42:1238e46. [11] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng 2000;28: 1e63. €ppelmann G. Nanocomposites [12] Reichert P, Kressler J, Thoann R, Mülhaupt R, Sto based on synthetic layer silicate and polyamide-12. Acta Polym 1998;49: 116e23. [13] Paul DR, Newman S. Polymer blends, vol. 1 and 2. New-York: Academic; 1976. de ric P, Razafinimaro T, Aubry T. Influence of melt-blending conditions on [14] Me structural, rheological and interfacial properties of polyamide-12 layered silicate nanocomposites. Polym Eng Sci 2006;46:986e94. [15] Fornes TD, Yoon PJ, Keskkula H, Paul DR. Nylon 6 nanocomposites: the effect of matrix molecular weight. Polymer 2001;42:9929e40. [16] Fornes TD, Paul DR. Structure and properties of nanocomposites based on nylon-11 and -12 compared with those based on nylon-6. Macromolecules 2004;37:7698e709.
117
[17] Krishnamoorti R, Giannelis EP. Rheology of end-tethered polymer layered silicate nanocomposites. Macromolecules 1997;30:4097e102. €ppelmann G, Friedrich C, Kim GM. Rheology of [18] Hoffmann BJ, Kressler G, Sto nanocomposites based on layered silicates and polyamide-12. Colloid Polym Sci 2000;278:629e36. [19] Ren J, Silva AS, Krishnamoorti R. Linear viscoelasticity of disordered polystyrene polyisoprene block copolymer bases layered-silicate nanocomposites. Macromolecules 2000;33:3739e46. [20] Utracki LA, Lyngaae-Jorgensen J. Dynamic melt flow of nanocomposites based on poly-ε-caprolactam. Rheol Acta 2002;41:394e407. [21] Ayyer RK, Leonov AI. Comparative rheological studies of polyamide-6 and its low loaded nanocomposite based on layered silicates. Rheol Acta 2004;43: 283e92. de ric P. Rheological investigation of the melt state [22] Aubry T, Razafinimaro T, Me elastic and yield properties of a polyamide-12 layered silicate nanocomposite. J Rheol 2005;49:425e40. [23] Hong CH, Lee YB, Bae JW, Jho JY, Nam JO, Hwang TW. Preparation and mechanical properties of polypropylene/clay nanocomposites for automotive parts application. J Appl Pol Sci 2005;98:427e33. [24] Duska J, Finestone A, Maher J. Plastic ovenware containing talc; Us. Pat. 1986; US 4626557. [25] Chabrol K, Gressier M, Pebere N, Menuy MJ, Martin F, Bonino JP, et al. Functionalization of synthetic talc-like phyllosilicates by alkoxyorganosilane grafting. J Mat Chem 2010;20:9695e706. [26] Leterme P, Gayot A, Finet G, Bizi M, Flament MP. Influence of the morphogranulometry and hydrophobicity of talc on its antisticking power in the production of tablets. Int J Pharm 2005;289:109e15. bre C, Petit S, Grauby O, Bonino JP, Arseguel D, Decarreau [27] Martin F, Ferret J, Le A, Ferrage E. Method for preparing talcose compositions comprising synthetic mineral particles containing silicon, germanium and metal; Fr. Pat. 2008; WO 2008009799A2. bre C, Petit S, Grauby O, Bonino JP, Arseguel D, Decarreau [28] Martin F, Ferret J, Le A, Ferrage E. Preparation of a composition of a swelling t.o.t.-t.o.t. interlayer; Fr. Pat. 2008; WO 2008009800A2. bre C, Petit S, Grauby O, Bonino JP, Arseguel D, Decarreau [29] Martin F, Ferret J, Le A, Ferrage E. Method for preparing a synthetic talc composition from a kerolite composition; Fr. Pat. 2008; WO 2008009801A2. [30] Le Roux C, Martin F, Micoud P, Dumas A. Process for preparing a composition comprising synthetic inorganic mineral particles and composition; Fr. Pat. 2013; WO 004979A1. [31] Dumas A, Martin F, Le Roux C, Micoud P, Petit S, Ferrage E, et al. Phyllosilicates synthesis: a way of accessing edges contributions in NMR and FTIR spectroscopies. Example of synthetic talc. Phys Chem Min 2013;40:361e73. bre C. Elaboration et caracte risation de talcs synthe tiques pour [32] Le lioration des proprie te s physiques des mate riaux composites industriels l'ame ^tements de surface, plastique, peintures, …) incorporant du talc comme (reve rale [Ph.D. thesis]. Universite de Toulouse; 2007. charge mine pin Leblond J, Greenhill-Hooper M, [33] Yousfi M, Livi S, Dumas A, Le Roux C, Cre et al. Use of new synthetic talc as reinforcing nanofillers for polypropylene and polyamide 6 systems: thermal and mechanical properties. J Colloid Interface Sci 2013;403:29e42. [34] Cabrera Alvarez EN, Ramos de Valle LF, Rodriguez Gonzalez FJ, SorianoCorral F, Diaz De Leon RE. Influence of laurylactam content on the clay intercalation of polyamide 6,12/clay nanocomposites synthesized by open ring anionic polymerization. J Nanomater 2012. Article ID 487948. [35] Elf, Elf-Atochem. Rilsan® A: Step into the future, brochure. Elf Atochem SA; 1997. de ric P, Razafinimaro T, Aubry T, Moan M, Klopffer MH. Rheological and [36] Me structural investigation of layered silicate nanocomposites based on polyamide or polyethylene: influence of processing conditions and volume fraction effects. Macromol Symp 2005;221:75e84. [37] Paul DR, Robeson LM. Polymer nanotechnology: nanocomposites. Polymer 2008;49:3187e204. [38] Liu TX, Liu ZH, Ma KX, Shen L, Zeng KY, He CB. Morpholgy, thermal and mechanical behavior of polyamide6/layered-silicate nanocomposites. Com Sci Tec 2003;63:331e7. [39] Blount AM, Vassiliou AH. Identification of chlorite and serpentinite in cosmetic of pharmaceutical talc. Environ Health Perspect 1983;51:379e85. [40] Xie X, Gao Z, Liu K, Pan WP, Vaia E, Hunter D, et al. Thermal characterization of organically modified montmorillonite. Thermochim Acta 2001;367e368: 339e50. [41] Durmus A, Kasgoz A, Macosko CW. Linear low density polyethylene (LLDPE)/ clay nanocomposites. Part I: structural characterization and quantifying clay dispersion by melt rheology. Polymer 2007;48:4492e502. [42] Cassagnau P. Melt rheology of organoclay and fumed silica nanocomposites. Polymer 2008;49:2183e96. [43] Lertwimolnun W, Vergnes B. Influence of compatibilizer and processing conditions on the dispersion of nanoclay in a polypropylene matrix. Polymer 2005;46:3462e71.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Comparative study of the structural and rheological properties of PA6 and PA12 based synthetic talc nanocomposites ro ^ me Crepin-Leblond, Pascal Mederic, Thierry Aubry* Quentin Beuguel, Julien Ville, Je LIMATB, Equipe Rh eologie, Universit e de Bretagne Occidentale, UFR Sciences et Techniques, 6 Avenue Victor le Gorgeu, CS 93 837, 29 238 Brest Cedex 3, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2014 Received in revised form 16 February 2015 Accepted 18 February 2015 Available online 25 February 2015
The structural and rheological properties of synthetic talc/polyamide (ST/PA) composites, elaborated by simultaneous melt mixing of a PA6 or PA12 matrix and a hydrogel filled with talc, were studied, focusing on the difference between PA6 and PA12 based nanocomposites. The structure of ST/PA composites is shown to be largely composed of nanometric particles. However, the presence of few micrometric aggregates is shown at moderate solid volume fractions, for both PA6 and PA12. The results also show that PA6 matrix is more efficient than PA12 matrix to disperse synthetic talc nanoparticles; the difference in matrix effect is discussed in terms of matrix polarity, PA chain radius of gyration and role played by water brought by the hydrogel. Finally, the results of the present study suggest that the structural and rheological properties of ST/PA are close to those of organically modified montmorillonite/PA nanocomposites, especially when PA6 is the matrix. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Synthetic talc Polyamides Nanocomposites
1. Introduction Since the early 1990s, many studies have dealt with polymer layered silicate nanocomposites, from preparation to characterization, as thoroughly reviewed by Sinha Ray and Okamoto [1] or by Krishnamoorti and Yurekli [2]. Many different polymer matrices were used to prepare such nanocomposites, as shown by the numerous references cited by Sur et al. [3]. In the case of a polar matrix, such as polyamide, the layered silicate classically used for the preparation of nanocomposites is montmorillonite which has been organically modified by ion-exchange reactions with cationic surfactants like quaternary alkylammonium cations [4]. Addition of such high aspect ratio organoclay nanoplatelets in polyamide thermoplastic matrices, even at very low volume fractions, may lead to the improvement of flame retardancy [5e7], barrier properties [8e10], thermal properties [7,11] and mechanical properties [11e13], explaining the potential commercial interest of these nanostructured materials. However, good macroscopic properties require a high exfoliation degree of clay particles, which is dependent on processing conditions [14], polyamide molecular weight [15] but also polyamide polarity governing matrix/
* Corresponding author. Tel.: þ33 (0)2 98 01 66 86; fax: þ33 (0)2 98 01 79 30. E-mail address: [email protected] (T. Aubry). http://dx.doi.org/10.1016/j.polymer.2015.02.031 0032-3861/© 2015 Elsevier Ltd. All rights reserved.
organoclay interactions [16]. Most experimental studies on linear viscoelastic behavior of layered silicate nanocomposites show that there exists a transition from liquid-like behavior on short time scales to solid-like behavior on longer time scales, at a low silicate volume fraction fp usually ranging from 1 to 2% [17]. This result is attributed to the formation of a three-dimensional percolation network at very low solid contents, because of the high aspect ratio of clay entities [18e21]. For organically modified montmorillonite/ polyamide 12 nanocomposites, the network was described as constituted of mesoscopic domains composed of correlated silicate layers, with a characteristic size independent of the solid content [22]. For many years, thermoplastic matrices reinforced by natural talc have been widely used in various industrial applications, such as household appliances and automotive industry [23e25]. Recently, talc industry strives to produce talc nanoparticles, which could challenge exfoliated montmorillonite nanoparticles. However, the processes used to elaborate talc based nanocomposites are different from those used to prepare polymer layered silicate nanocomposites, mainly because of the lack of exchangeable cations in the interlayer space of talc particles. The first attempts to reduce the size of natural talc particles were based on delamination techniques, which led to particles with average thickness of a few hundreds of nanometers, which were referred to as high aspect ratio particles [26]. More recently, an innovative and much more
110
Q. Beuguel et al. / Polymer 62 (2015) 109e117
efficient process, based on hydrothermal synthesis of talc nanoparticles, was proposed [27e30]. This process leads to the formation of a hydrogel which contains synthetic talc particles with controlled physical characteristics identical to those of natural talc particles in terms of lamellarity and crystallinity [31]. The average length of these synthetic talc particles was shown to lie between a few tens and a few hundreds of nanometers, and their average thickness between 6 nm and 10 nm [32]. Finally, using an extruder, synthetic talc particles, prepared from hydrothermal synthesis and dispersed in PA6 matrix, were shown to lead to nanocomposites [33]. The aim of the present work is to investigate the influence of the polyamide matrix physicochemical properties on the structural and rheological properties of nanocomposites elaborated by simultaneous melt mixing of a polyamide 6 or polyamide 12 matrix and a hydrogel filled with synthetic talc particles. Structure and rheology of these innovative nanocomposites are compared to those of delaminated natural talc/polyamide microcomposites as well as organically modified montmorillonite/polyamide nanocomposites.
As mentioned above, structural and rheological properties of ST/ PA composites have been compared to those of organically modified montmorillonite/PA nanocomposites and natural talc/PA conventional microcomposites:
2. Experimental part 2.1. Materials Composites were prepared from two commercial polyamide matrices: a polyamide 12 (PA12) and a polyamide 6 (PA6), supplied by Arkema (referenced as Rilsan® AECHVO) and Rhodia (referenced as Technyl® S-27-BL), respectively. Table 1 gives the main characteristics of the two polymers: the number and weight average molar weights, Mn and Mw, the radius of gyration, Rg, determined from the freely jointed chain model, and the melting point, Tm. The number and weight average molar weights of both polyamides are rather similar, even if the radius of gyration of PA6 chains is somewhat smaller than that of PA12 chains. Moreover, it is worth pointing out that the water absorption of PA12 is less than 0.5% after 45 min at 220 C whereas it is close to 2% for the PA6 matrix, which is more polar [34], under the same conditions [35]. The hydrogel filled with Synthetic Talc (ST) particles was prepared at GET Laboratory (University Paul Sabatier, Toulouse, France) according to a hydrothermal process patented [30]. The ST particles of the study were obtained after a hydrothermal treatment duration for 6 h and were previously characterized by Dumas et al. [31]. The particle size was shown to vary from ~30 nm to ~3000 nm, leading to an average length of 150 nm, meaning that micronic particles are few in number, contrary to what is observed for natural talc. Dumas et al. [31] also reported that ST particles had (i) a specific surface area of 150 m2/g, higher than that of natural talc, (ii) a number of stacking defects higher than that observed in natural talc particles and (iii) a hydrophilic character, contrary to particles of natural talc, which have a hydrophobic character. Representative Transmission Electron Microscopy (TEM) micrograph of the hydrogel, diluted ten times in water, is shown in Fig. 1. Synthetic talc particles have a length, ranging from 20 nm to 300 nm, close to the values obtained by Dumas et al. [31], and an average thickness of ~8 nm.
Table 1 Characteristics of polyamide matrices.
Mn (g/mol) Mw (g/mol) Rg (nm) Tm ( C)
Fig. 1. TEM micrograph of hydrogel filled with ST particles, diluted ten times in water.
PA12
PA6
38,000 60,000 11 183
39,000 79,000 8 222
The organically modified montmorillonite clay (OMMT), namely Cloisite® 30B, has been supplied by Southern Clay Products. It is a methyl tallow bis-2-hydroxyethyl ammonium exchanged montmorillonite clay, with a modifier concentration of 90 milliequivalent per 100 g and has a good affinity towards polyamide [22,36]. This organophilic clay is characterized by a specific area of about 700 m2 g1. The primary OMMT particles are composed of about ten stacked layers, whose thickness is ~0.7 nm, and length ~200 nm [37], corresponding to an aspect ratio of ~350. The interlayer distance, estimated from XRD measurements, is close to 1.2 nm and the specific gravity of OMMT is close to 2. The High Aspect Ratio® Natural Talc, referred to as HARNT in this work, is supplied by Imerys Talc (Toulouse, France). HARNT results from the delamination of a coarse natural talc, leading to thinner, but still micrometric, particles. The average equivalent diameter of HARNT talc particles, determined by laser diffraction, is about 10 mm. The interlayer distance, estimated from XRD measurements, is close to 0.3 nm. The specific area of HARNT talc is 19.5 m2/g and the specific gravity of HARNT is close to 2.8.
2.2. Sample elaboration PA12 and PA6 samples were vacuum dried at 80 C, during 4 h for PA12 and 24 h for PA6, whose hygroscopic character is more marked [35]. All samples used in this study were prepared by simultaneous melt mixing of components using DSM Xplore lab twin screw extruder. Because of the difference of matrix melting point (Table 1), the mixing temperature was chosen at 220 C and 240 C for PA12 and PA6, respectively. The screw rotational speed was fixed at 50 rpm for 6 min. The melt mixing of a polymer matrix and a hydrogel of synthetic talc leads to the formation of a very small number of almost millimetric ST agglomerates in the extrudate, at least at ST volume fractions higher than 5% (Fig. 2a). The existence of these micrometric particles was attributed to fast drying of part of the talc filling the hydrogel, during its incorporation into the heated extruder chamber (Fig. 2b). After mixing, all samples were pelletized and moulded by compression into 2 mm thick plates, at 200 C for PA12 samples and
Q. Beuguel et al. / Polymer 62 (2015) 109e117
111
Fig. 2. Highly filled ST/PA12 composites: formation of micrometric aggregates in the extrudate with ~2 mm diameter (a) and of a whitish talc deposit on chamber wall and blade surface (b).
240 C for PA6 samples; pressure was increased by steps, from 5 MPa to 25 MPa, in order to avoid the formation of air bubbles during the compression molding. Following this route, samples were prepared at filler volume fraction, f, ranging from 0 to 9.2%, from 0 to 21.5% and from 0 to 11.3 %, for OMMT, HARNT and ST particles, respectively. It is worth pointing out that such volume fractions have been calculated from the weight fraction measured after calcination. 2.3. Structural characterization The nanostructure of the samples was investigated from 40 nm thick ultrathin sections cut using a diamond knife, at 130 C, with an ultracryomicrotome (Reichert Jung); imaging was achieved with a JEOL 1400 Transmission Electron Microscope at 100 kV. On TEM micrographs, length and thickness of fillers were measured for at least 200 particles, using SigmaScan® Pro 5.0 image analysis software. From these values, the number average length, L, and thickness, e, were calculated. The specific particle density, dsp, that is the number of particles per mm2 divided by the filler weight fraction, as defined by Fornes et al. [15], was also estimated. Moreover, on a micrometric length scale, cryofractured samples with vacuummetallized surface were observed using a Hitachi S-3200N Scanning Electron Microscope (SEM), with an accelerating voltage of 15 kV. Confocal microscopy characterization, used because of the natural fluorescence of clay fillers, completes SEM and TEM characterizations, providing a 3D description of the samples. It was carried out by spectral deconvolution, using a Zeiss confocal microscope LSM 780 equipped with a laser diode of wavelength 405 nm, a 32 GaAsP PMT detector and a 63 oil lens. Clay structure was also investigated using wide-angle X-ray scattering at ambient temperature. It was performed using a PANalytical Empyrean X-ray diffractometer with Cu Ka radiation of wavelength 0.15 nm, generated at 45 kV and 40 mA. XRD experiments were carried out over a 2q range of 2 e10 , using steps of 0.015 and counting times of 10 s at each angular position. 2.4. Rheological characterization Oscillatory shear measurements were performed using a controlled strain rheometer (Gemini) equipped with parallel disks
of 25 mm diameter and 2 mm spacing. All experiments were carried out under a continuous purge of dry nitrogen in order to avoid sample degradation. Rheological tests were performed at a temperature of 200 C for PA12 matrix samples and at a temperature of 240 C for PA6 matrix samples, because of their different melting point (Table 1). First, the so-called critical strain, gc, which defines the extent of the linear viscoelastic regime, was determined by strain sweep experiments, carried out at a fixed frequency of 1 Hz. Then, all frequency sweep tests were performed at a strain amplitude within the linear viscoelastic regime. Special attention was paid to material thermal stability during rheological tests, using time sweep experiments. The thermal stability was shown to be enhanced by addition of clay particles, as reported by many research works [5,22]. More precisely, variations of rheological properties inferior to 10% were observed at 200 C for PA12 based composites and at 240 C for PA6 based composites, over 30 min. At last, all rheometrical data obtained within 20 min were shown to be reproducible within ±5%. Steady-state viscosity measurements at high shear rates were €ttfert capillary rheometer in order to performed on a RT 1000 Go verify that the matrix viscosity had the same order of magnitude under the processing conditions.
3. Results and discussion 3.1. Structural investigation Fig. 3 shows typical TEM micrographs of OMMT/PA12 (Fig. 3a) and OMMT/PA6 (Fig. 3b) nanocomposites with 0.5% and 0.6% OMMT, respectively. Numerous individual OMMT particles and few nanometer thick OMMT stacks are well-dispersed within PA12 or PA6 matrix (Fig. 3). The average particle length, L ~ 65 nm, calculated from TEM micrographs, is the same for the two PA matrices and does not depend on OMMT volume fraction (Table 2). Besides, Table 2 shows that using PA6 matrix tends to slightly decrease the average thickness of the particles, therefore improving the exfoliation degree of OMMT particles [16]. Finally, for both PA matrices, an increase of average particle thickness, hence a decrease of specific particle density, with increasing OMMT volume fraction is observed (Table 2). This classical result is explained by the presence of a few layer stacks,
112
Q. Beuguel et al. / Polymer 62 (2015) 109e117
Fig. 3. TEM micrographs of nanocomposites filled with OMMT: (a) PA12 matrix and (b) PA6 matrix.
Table 2 Characteristic size of OMMT, HARNT and ST particles and specific particles density.
OMMT
Matrice
f (%)
L (nm)
e (nm)
dsp
PA12
0.5 2.6 0.6 2.9 0.8 2.2 6.6 2.4 5.3 3.9 10.8 4.4 12
~65
3.6 4.3 3.1 3.9 8
5
23 11 33 19 9 7 6 16
~55
<0.2
PA6 ST
PA12
PA6 HARNT
PA12 PA6
~65 ~65
~450
with a characteristic size of ~500 nm, in highly filled nanocomposites [14,22]. In order to illustrate the structural state of HARNT/PA composites, Fig. 4 presents SEM micrograph (Fig. 4a) and TEM micrograph (Fig. 4b) of the PA12 matrix filled with 3.9% HARNT. Electron microscopy observations show that most HARNT particles are micrometric, in accordance with the results from measurements carried out by laser diffraction for the neat HARNT. Finer particles (~50 nm thick), probably ripped from micrometric particles during mixing, are also shown. Average particle dimensions, L ~ 450 nm and e ~ 55 nm, estimated from TEM micrographs, are the same for both PA matrices and do not depend on HARNT volume fraction. The large average particle dimensions and the very low specific particle density (Table 2) characterizing HARNT/PA composites mean that these systems should be considered as microcomposites. Fig. 5 presents TEM micrograph and SEM micrograph of ST/PA12 composites (Fig. 5a and b) and of ST/PA6 composites (Fig. 5c and d). ST volume fractions are 2.2% and 2.4% in PA12 and PA6, respectively. A nanostructure, mainly composed of fine ST particles and a few layer stacks, with submicronic thickness (Fig. 5a and c), is
highlighted for composites based on PA12 or PA6 matrix. However, for both PA matrices, SEM micrographs reveal the presence of a few micrometric aggregates from 2% ST volume fraction (Fig. 5b and d), and also the presence of some millimetric agglomerates formed during mixing (Fig. 2a and b). Indeed, dried micrometric ST aggregates were shown to be difficult to disperse in PA matrix. Above 2% ST, the size and proportion of micrometric ST entities increase with increasing ST volume fraction, for the two PA matrices used. In order to characterize more quantitatively the multi-scale structural state, the proportion of micrometric ST entities was estimated from confocal micrographs, in the case of the PA12 matrix filled with 6.6% synthetic talc (Fig. 6). Synthetic talc particles with thickness above 1 mm were considered as micrometric entities. Using this criterion, the volume fraction of micrometric ST entities dispersed within the PA12 matrix is close to 25% in the case of ST/PA12 composite with 6.6% synthetic talc, confirming that the structure is mainly nanometric, even for highly filled ST/PA composites (Fig. 6). Moreover, even though the volume fraction of micrometric entities is significant, their number is still very small compared to the number of nanometric ST particles. The volume fraction of micrometric ST particles determined by confocal microscopy seems to be consistent with the volume fraction of micrometric particles determined from electron microscopy observations (Fig. 5). Even though the micrometric structure of ST/PA composites seems to be insensitive to the matrix used, either PA6 or PA12, their nanostructure is but affected. Indeed, although the average length of submicronic particles, L ~ 65 nm, close to the value estimated by Dumas et al. [31], is independent of ST volume fraction and PA matrix (Table 2), their average thickness depends on PA matrix. Indeed, the average thickness of nanoparticles dispersed in PA12 matrix is 8 nm, i.e. close to the one obtained for the ST filled hydrogel, but it is only 5 nm, when ST particles are dispersed in PA6 matrix (Table 2). The PA12 matrix does not refine the structure of ST particles present in the hydrogel. On the contrary, the lowest value of ST particles average thickness in PA6 matrix suggests that this
Fig. 4. SEM micrograph (a) and TEM micrograph (b) of HARNT/PA microcomposites.
Q. Beuguel et al. / Polymer 62 (2015) 109e117
113
Fig. 5. ST/PA12 composite: TEM micrograph (a) and SEM micrograph (b). ST/PA6 composite: TEM micrograph (c) and SEM micrograph (d).
polyamide divides ST particles, as confirmed by the higher specific particle density in ST/PA6 composites (Table 2). Nevertheless, specific particle density, which is sensitive to the presence of micrometric aggregates (Fig. 5b and d), decreases with increasing ST volume fraction. Fig. 7 presents thickness distribution of OMMT, ST and HARNT particles dispersed within PA12 or PA6 matrix, at least for particles with thickness up to 1 mm, at filler volume fractions ~0.5%, ~2% and ~4% for OMMT, ST and HARNT, respectively. Fig. 7 shows that the thickness of OMMT nanoparticles does not exceed 50 nm, for both PA matrices. Besides, OMMT thickness distribution is slightly narrower for nanoparticles dispersed in the PA6 matrix; still, the thickness distribution slightly broadens with increasing OMMT volume fraction. Besides, Fig. 7 shows that the minimum thickness of HARNT particles is superior to 10 nm. In addition, the flattened thickness distribution proves that submicronic particles of HARNT/PA microcomposites are thicker than OMMT nanoparticles. Moreover, HARNT thickness distribution is the same for both PA matrices and does not depend on volume fraction.
At last, for all ST fractions studied, Fig. 7 shows that 80% of ST nanoparticles dispersed in PA12 matrix have a thickness inferior to 10 nm. Moreover, the thickness distribution of submicronic ST particles dispersed in PA6 matrix is significantly narrower, and quite comparable with the thickness distribution of OMMT nanoparticles. Wide-angle X-ray diffraction measurements were carried out in order to characterize the structure of OMMT/PA nanocomposites, HARNT/PA microcomposites and ST/PA composites (Fig. 8). The peak at 2q ¼ 5.5 is the signature of the semi-crystalline structure of PA12 matrix [12], whereas no peak is observed for PA6 matrix over the angle range explored in the study [38]. Fig. 8a shows a well-defined peak at 2q ¼ 2.7, corresponding to a d-spacing of 3.3 nm, meaning that PA12 chains are intercalated between OMMT layers [22,36]. Besides, in the case of OMMT/PA6 nanocomposites, the absence of peak at low angles investigated suggests a better exfoliation of OMMT nanoparticles in PA6 matrix, in agreement with electron microscopy observations (Fig. 3). Indeed, from interlayer spacing and average thickness values, we can infer that OMMT particles dispersed in PA6 matrix are
Fig. 6. Confocal micrograph in the case of ST/PA12 composite with 6.6% synthetic talc.
114
Q. Beuguel et al. / Polymer 62 (2015) 109e117
Fig. 7. Thickness distribution of OMMT, ST and HARNT particles within PA12 or PA6.
and only ~5 layers in PA6 matrix. Again, like in the case of OMMT, the better efficiency of PA6 to separate ST particles could be attributed to the higher polarity of PA6 [34] and to its smaller radius of gyration. Moreover, the difference in diffractograms between ST/ PA12 and ST/PA6 composites, within the range of small angles (<5 ) corresponding to interfoliar distances superior to 1.1 nm, could be the signature of the presence of stacking defects, as mentioned by Dumas et al. [31]. We suggest that the presence of water coming from the hydrogel and the defects present in hydrophilic ST particles could ease the penetration of PA chains within ST galleries. These mechanisms could be favored in the case of the most hydrophilic and most polar matrix, i.e. PA6, and together with a smaller radius of gyration of PA6 could facilitate the separation of the smallest ST particles. Therefore, the decrease of number of defects is in agreement with the weakest intensity of diffraction signal for the PA6 based composite. To summarize, specific particle density and average thickness of ST/PA composites (Table 2), as well as their thickness distribution (Fig. 7) are systematically between those of OMMT/PA nanocomposites and HARNT/PA microcomposites. More precisely, the structure of ST/PA composites is mainly a nanostructure, with the presence, at moderate ST volume fractions, of large aggregates, which most likely form into the extruder chamber, during the incorporation step. The nanostructure is close to that of partially exfoliated OMMT/PA nanocomposites, especially in the case of PA6 matrix, which is more polar, therefore more hydrophilic, and whose chains have a smaller radius of gyration. However, even though PA6 matrix seems to be more efficient than PA12 to finely disperse ST nanoparticles, it seems to have no effect on the presence of micrometric ST defaults.
composed of 2 layers, on average. The better efficiency of PA6 to exfoliate OMMT particles cannot be attributed to higher shear induced exfoliation during the mixing process since PA6 and PA12 viscosities have the same order of magnitude under the mixing conditions used. However, the higher polarity of PA6 matrix, which confers a better affinity towards OMMT, could partly explain its efficiency to improve exfoliation of OMMT particles [16]. Exfoliation might also be improved by the smaller radius of gyration of PA6 chains, which makes their intercalation into OMMT galleries easier. In Fig. 8b, the peaks at 2q ¼ 6 and 2q ¼ 9.3 , due to chlorite and talc respectively [39], are identical for the two matrices (PA6 or PA12), suggesting that no intercalation of PA chains occurs between HARNT galleries. Fig. 8c, presenting diffractograms of ST/PA nanocomposites, also shows the characteristic peak of talc, at 2q ¼ 9.3 . As suggested by Yousfi et al. [33], the weak intensity of this peak is indicative of a structure mainly composed of stacks of a few ST layers, as shown by electron microscopy (Fig. 5a and b). At close volume fractions, this peak seems to be less marked for composites based on PA6 matrix, confirming the better efficiency of PA6 matrix to disperse ST particles. In fact, ST stacks are composed of ~8 layers in PA12 matrix
The critical strain, gc, determined by the classical tangent analysis, defines the limit of the linear viscoelastic regime. It is unambiguously determined when the number of fillers is sufficient. In Fig. 9, it is plotted as a function of filler volume fraction. Regardless of the matrix, the critical strain is a decreasing function of filler volume fraction. At any given filler volume fraction, the critical strain of ST/PA systems with a multi-scale structure (Fig. 6), lies systematically between that of OMMT/PA nanocomposites and that of HARNT/PA microcomposites, in agreement with structural characterization results.
Fig. 8. Wide-angle X-ray diffractograms of OMMT/PA nanocomposites (a), HARNT/PA microcomposites (b) and ST/PA composites (c).
Fig. 9. Critical strain as a function of filler volume fraction for OMMT/PA, ST/PA nanocomposites and HARNT/PA microcomposites.
3.2. Rheological investigation
Q. Beuguel et al. / Polymer 62 (2015) 109e117
On the one hand, the critical strain is higher for ST/PA nanocomposites than for OMMT/PA nanocomposites. This could be attributed to larger interparticle distances in the case of ST/PA systems, due to the presence of a few micrometric aggregates of synthetic talc (Fig. 5b and d) and also to a nanostructure slightly less fine than that of OMMT exfoliated nanoparticles (Table 2, Fig. 7). On the other hand, the critical strain of ST/PA nanocomposites is drastically lower than that of HARNT/PA microcomposites, which is simply due to a much better dispersion state of ST particles. As far as PA matrix effect is concerned, in the case of HARNT/PA microcomposites, the critical strain is about the same for the two PA matrices, because of quite similar dispersion states, as suggested by structural characterization (Table 2). On the contrary, at a given filler volume fraction, the critical strain of ST/PA6 nanocomposites is significantly smaller than that of ST/PA12 nanocomposites and tends towards that of nanocomposites based on OMMT particles. This matrix effect on gc can be explained by the better state of dispersion of ST nanoparticles, when PA6 matrix is used, as discussed above. Following the above interpretation based on the relationship between critical strain and dispersion state, the results concerning OMMT/PA nanocomposites could seem paradoxical. Indeed, although the structure of OMMT nanoparticles is finer in the presence of PA6 matrix, the critical strain of OMMT/PA6 nanocomposites is higher than that of OMMT/PA12 nanocomposites. This apparently surprising result could be attributed to a possible degradation of quaternary alkylammonium modifier at higher mixing temperatures [40] (240 C, in the case of PA6 matrix), which could result in reduced clay/matrix interactions. The critical strain versus filler volume fraction can be described by a power law for all composites of this study [41]. In the case of
115
OMMT/PA nanocomposites, the critical strain is inversely proportional to OMMT volume fraction (Fig. 9), as obtained by Aubry et al. [22] for OMMT/PA12 nanocomposites prepared using an internal mixer, or for other layered silicate/polymer nanocomposites [42]. It is worth pointing out that the exponents of the power law dependence of the critical strain versus volume fraction obtained for ST/PA nanocomposites are close to those of OMMT/PA nanocomposites, i.e. 1, whereas an exponent close to 2 is obtained for HARNT/PA microcomposites (Fig. 9). Fig. 10 shows the complex viscosity jh*j, as a function of frequency, for OMMT/PA12, ST/PA12 nanocomposites and HARNT/ PA12 microcomposites, with different filler volume fractions. PA6 matrix composites qualitatively exhibit the same rheological behaviors (not presented here). For both matrices (PA6 and PA12) and their weakly filled composites, at low frequencies, the complex viscosity exhibits a plateau, defining a Newtonian viscosity. However, it is not possible to determine a Newtonian viscosity for sufficiently filled nanocomposites. Indeed, low frequency complex viscosity depends on frequency, above a filler volume fraction, fp, which is specific to each composite: fp ~ 1% for OMMT nanocomposites [22] (Fig. 10a), fp ~ 10% for HARNT microcomposites (Fig. 10b) and fp ~ 6% for ST/ PA nanocomposites (Fig. 10c). This specific filler volume fraction corresponds to a percolation threshold attributed to the formation of a percolated network [17e22]. It is worth noticing that percolation threshold of ST/PA nanocomposites is between that of OMMT/PA nanocomposites and that of HARNT microcomposites, in agreement with structural characterization results. Below percolation threshold (f < fp), the relative viscosity, hr, defined as the ratio of Newtonian complex viscosity of the composite to that of the PA matrix, is presented as a function of filler volume fraction in Fig. 11.
Fig. 10. Complex viscosity as a function of frequency for OMMT/PA12 nanocomposites (a), HARNT/PA12 microcomposites (b) and ST/PA12 nanocomposites (c).
116
Q. Beuguel et al. / Polymer 62 (2015) 109e117
Fig. 11. Relative viscosity as a function of filler volume fraction for OMMT/PA12, ST/ PA12 nanocomposites and HARNT/PA12 microcomposites; experimental data are fitted with Einstein model (1).
As proposed by Utracki and Lyngaee-Jorgensen [20], these results were discussed in terms of intrinsic viscosity [h], determined by fitting the curve with the second-order Einstein-type equation:
hr ¼ 1 þ ½hf þ kð½hfÞ
2
(1)
where k is an interaction constant. Knowing the intrinsic viscosity [h], the aspect ratio p of the disklike clay particles can be inferred:
½h ¼ 2:5 þ 0:025 1 þ p1:47
(2)
Intrinsic viscosity, interaction constant and aspect ratio of all systems studied are reported in Table 3. Firstly, the results in Table 3 show that the aspect ratio of synthetic talc particles, p ~ 80, is between that of OMMT and HARNT particles (Table 3), in agreement with structural results. Moreover, the aspect ratio of HARNT particles does not seem to depend on the nature of PA, whereas it strongly depends on the matrix for the two other particles: an increase of 20% of the aspect ratio for OMMT nanoparticles, and 40% for ST nanoparticles, is observed when the PA6 matrix is used. Such matrix effects on the aspect ratio are in agreement with the thickness decrease of OMMT and ST nanoparticles when PA6 is the matrix (Table 2, Fig. 7), which was attributed to the relatively weak improvement of the exfoliation mechanism in the case of OMMT/PA6 nanocomposites and to the significant efficiency of separation mechanism in the case of ST/PA6 nanocomposites. As far as the interaction constant is concerned, its value depends on the composite type, but not on the nature of PA matrix.
Table 3 Intrinsic viscosity, interaction constant and aspect ratio values for OMMT, ST and HARNT particles.
OMMT ST HARNT
Matrices
[h]
k
p
PA12 PA6 PA12 PA6 PA12 PA6
65 82 18 27 8
0.8
200 240 80 110 40
0.7 0.25
Fig. 12. Yield shear stress as a function of filler volume fraction for OMMT/PA12, ST/ PA12 nanocomposites and HARNT/PA12 microcomposites.
Above percolation threshold (f > fp), as suggested by Lertwimolnun et al. [43], the flow curves plotted in Fig. 10 can be correctly described by a phenomenological CarreaueYasuda Model with a yield stress:
h* ¼
n1 t0 þ h*0 1 þ ðluÞa a u
(3)
with t0, the apparent yield stress, h*0 the Newtonian complex viscosity, l a characteristic time, n the pseudoplasticity index and a a fitting parameter. The apparent yield stress, t0, has been plotted versus filler volume fraction in Fig. 12. The apparent yield stress is shown to be strongly dependent on filler volume fraction, close to the percolation threshold. For sufficiently high OMMT volume fractions, the f dependence of yield stress is shown to be correctly fitted by a quadratic expression, in agreement with the results by Aubry et al. [22]. It is worth noticing that, at a given filler volume fraction, the yield stress estimated for ST/PA nanocomposites lies between that of OMMT/PA nanocomposites and that of HARNT/PA microcomposites, regardless of PA matrix. As observed for the critical strain, the yield stress of HARNT/PA microcomposites is independent of PA matrix. Besides, at a fixed filler volume fraction, the apparent yield stress of ST/PA6 nanocomposites is significantly higher than that of ST/PA12 nanocomposites, in accordance with the higher aspect ratio of ST nanoparticles within the PA6 matrix. At last, the yield shear stress of OMMT/PA6 nanocomposites, which exhibit the most exfoliated structure, is paradoxically smaller than that of OMMT/PA12 nanocomposites, at the same OMMT volume fraction. We suggest that the clay network could be weakened by the degradation of quaternary alkylammonium modifier occurring at the mixing temperature of 240 C.
4. Conclusion Structural and rheological properties of innovative synthetic talc/polyamide composites were shown to lie between those of organically modified montmorillonite/polyamide nanocomposites and those of delaminated natural talc/polyamide microcomposites, for both PA12 and PA6 matrices. More precisely, synthetic talc particles were shown to be mainly nanometric in size in both PA6
Q. Beuguel et al. / Polymer 62 (2015) 109e117
and PA12 matrices, but they were shown to be even smaller in PA6. We suggest that PA6 chains could more easily penetrate the synthetic talc particles present in the hydrogel used during the sample elaboration, favoring their separation and therefore leading to synthetic talc nanoplatelets whose dimensions are closer to those of organically modified nanoparticles. Indeed, the intercalation of matrix chains within their stacking defects in presence of water coming from the hydrogel is favored by the higher polarity, and therefore higher hydrophilicity, of PA6, together with the smaller radius of gyration of PA6 chains. However, the presence of a few very large aggregates, formed during the incorporation of synthetic talc hydrogel into the extruder, somewhat affects the rheological behavior of synthetic talc/polyamide nanocomposites. Nevertheless, the results of the present work suggest that, despite the existence of a few large aggregates, nanocomposites based on synthetic talc could be potential challengers of nanocomposites based on organically modified clays. In addition, synthetic talc/ polyamide nanocomposites have the advantage to be insensitive to thermal degradation at relatively high temperatures, contrary to organically modified montmorillonite/polymer nanocomposites, because of degradation of the modifier. Acknowledgments The authors are grateful to Imerys Talc (Toulouse, France) for thermogravimetry measurements, logistic and financial support. rard Sinquin We thank Philippe Elies, François Michaud and Ge de Bretagne Occidentale) for practical assistance in (Universite structural characterization. References [1] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites, a review from preparation to processing. Prog Polym Sci 2003;28:1539e41. [2] Krishnamoorti R, Yurekli K. Rheology of polymer layered silicate nanocomposites. Curr Opin Colloid Interface Sci 2001;6:464e70. [3] Sur GS, Sun HL, Lyu SG, Mark JE. Synthesis, structure, mechanical properties, and thermal stability of some polysulfone/organoclay nanocomposites. Polymer 2001;42:9783e9. chelle et stabilite colloïdale de suspensions [4] Gherardi B. Organisation multi e d'Orle ans; 1998. d'argiles en milieu organique [Ph.D. thesis]. Universite [5] Gilman JW. Flammability and thermal stability studies of polymer-layered silicate (clay) nanocomposites. Appl Clay Sci 1999;15:31e49. [6] Zanetti M, Camino G, Thomann R, Mulhaupt R. Synthesis and thermal behavior of layered silicate-EVA nanocomposites. Polymer 2001;42:4501e7. [7] Tang Y, Hu Y, Wang S, Gui Z, Chen Z. Preparation of poly(propylene)/clay layered nanocomposites by melt intercalation from pristine montmorillonite (MMT). Polym Adv Tech 2003;14:733e57. [8] Massersmith PB, Giannelis EP. Synthesis and barrier properties of poly(εcaprolactone)-layered silicate nanocomposites. J Polym Sci A Polym Chem 1995;33:1047e57. [9] Ke Z, Yongping B. Improve the gas barrier properties of PET film with montmorillonite by in-situ interlayer polymerization. Mater Lett 2005;59: 3348e51. [10] Krook M, Albertsson AC, Gedde UW, Hedenqvist MS. Barrier and mechanical properties of montmorillonite/polyesteramide nanocomposite. Polym Eng Sci 2004;42:1238e46. [11] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng 2000;28: 1e63. €ppelmann G. Nanocomposites [12] Reichert P, Kressler J, Thoann R, Mülhaupt R, Sto based on synthetic layer silicate and polyamide-12. Acta Polym 1998;49: 116e23. [13] Paul DR, Newman S. Polymer blends, vol. 1 and 2. New-York: Academic; 1976. de ric P, Razafinimaro T, Aubry T. Influence of melt-blending conditions on [14] Me structural, rheological and interfacial properties of polyamide-12 layered silicate nanocomposites. Polym Eng Sci 2006;46:986e94. [15] Fornes TD, Yoon PJ, Keskkula H, Paul DR. Nylon 6 nanocomposites: the effect of matrix molecular weight. Polymer 2001;42:9929e40. [16] Fornes TD, Paul DR. Structure and properties of nanocomposites based on nylon-11 and -12 compared with those based on nylon-6. Macromolecules 2004;37:7698e709.
117
[17] Krishnamoorti R, Giannelis EP. Rheology of end-tethered polymer layered silicate nanocomposites. Macromolecules 1997;30:4097e102. €ppelmann G, Friedrich C, Kim GM. Rheology of [18] Hoffmann BJ, Kressler G, Sto nanocomposites based on layered silicates and polyamide-12. Colloid Polym Sci 2000;278:629e36. [19] Ren J, Silva AS, Krishnamoorti R. Linear viscoelasticity of disordered polystyrene polyisoprene block copolymer bases layered-silicate nanocomposites. Macromolecules 2000;33:3739e46. [20] Utracki LA, Lyngaae-Jorgensen J. Dynamic melt flow of nanocomposites based on poly-ε-caprolactam. Rheol Acta 2002;41:394e407. [21] Ayyer RK, Leonov AI. Comparative rheological studies of polyamide-6 and its low loaded nanocomposite based on layered silicates. Rheol Acta 2004;43: 283e92. de ric P. Rheological investigation of the melt state [22] Aubry T, Razafinimaro T, Me elastic and yield properties of a polyamide-12 layered silicate nanocomposite. J Rheol 2005;49:425e40. [23] Hong CH, Lee YB, Bae JW, Jho JY, Nam JO, Hwang TW. Preparation and mechanical properties of polypropylene/clay nanocomposites for automotive parts application. J Appl Pol Sci 2005;98:427e33. [24] Duska J, Finestone A, Maher J. Plastic ovenware containing talc; Us. Pat. 1986; US 4626557. [25] Chabrol K, Gressier M, Pebere N, Menuy MJ, Martin F, Bonino JP, et al. Functionalization of synthetic talc-like phyllosilicates by alkoxyorganosilane grafting. J Mat Chem 2010;20:9695e706. [26] Leterme P, Gayot A, Finet G, Bizi M, Flament MP. Influence of the morphogranulometry and hydrophobicity of talc on its antisticking power in the production of tablets. Int J Pharm 2005;289:109e15. bre C, Petit S, Grauby O, Bonino JP, Arseguel D, Decarreau [27] Martin F, Ferret J, Le A, Ferrage E. Method for preparing talcose compositions comprising synthetic mineral particles containing silicon, germanium and metal; Fr. Pat. 2008; WO 2008009799A2. bre C, Petit S, Grauby O, Bonino JP, Arseguel D, Decarreau [28] Martin F, Ferret J, Le A, Ferrage E. Preparation of a composition of a swelling t.o.t.-t.o.t. interlayer; Fr. Pat. 2008; WO 2008009800A2. bre C, Petit S, Grauby O, Bonino JP, Arseguel D, Decarreau [29] Martin F, Ferret J, Le A, Ferrage E. Method for preparing a synthetic talc composition from a kerolite composition; Fr. Pat. 2008; WO 2008009801A2. [30] Le Roux C, Martin F, Micoud P, Dumas A. Process for preparing a composition comprising synthetic inorganic mineral particles and composition; Fr. Pat. 2013; WO 004979A1. [31] Dumas A, Martin F, Le Roux C, Micoud P, Petit S, Ferrage E, et al. Phyllosilicates synthesis: a way of accessing edges contributions in NMR and FTIR spectroscopies. Example of synthetic talc. Phys Chem Min 2013;40:361e73. bre C. Elaboration et caracte risation de talcs synthe tiques pour [32] Le lioration des proprie te s physiques des mate riaux composites industriels l'ame ^tements de surface, plastique, peintures, …) incorporant du talc comme (reve rale [Ph.D. thesis]. Universite de Toulouse; 2007. charge mine pin Leblond J, Greenhill-Hooper M, [33] Yousfi M, Livi S, Dumas A, Le Roux C, Cre et al. Use of new synthetic talc as reinforcing nanofillers for polypropylene and polyamide 6 systems: thermal and mechanical properties. J Colloid Interface Sci 2013;403:29e42. [34] Cabrera Alvarez EN, Ramos de Valle LF, Rodriguez Gonzalez FJ, SorianoCorral F, Diaz De Leon RE. Influence of laurylactam content on the clay intercalation of polyamide 6,12/clay nanocomposites synthesized by open ring anionic polymerization. J Nanomater 2012. Article ID 487948. [35] Elf, Elf-Atochem. Rilsan® A: Step into the future, brochure. Elf Atochem SA; 1997. de ric P, Razafinimaro T, Aubry T, Moan M, Klopffer MH. Rheological and [36] Me structural investigation of layered silicate nanocomposites based on polyamide or polyethylene: influence of processing conditions and volume fraction effects. Macromol Symp 2005;221:75e84. [37] Paul DR, Robeson LM. Polymer nanotechnology: nanocomposites. Polymer 2008;49:3187e204. [38] Liu TX, Liu ZH, Ma KX, Shen L, Zeng KY, He CB. Morpholgy, thermal and mechanical behavior of polyamide6/layered-silicate nanocomposites. Com Sci Tec 2003;63:331e7. [39] Blount AM, Vassiliou AH. Identification of chlorite and serpentinite in cosmetic of pharmaceutical talc. Environ Health Perspect 1983;51:379e85. [40] Xie X, Gao Z, Liu K, Pan WP, Vaia E, Hunter D, et al. Thermal characterization of organically modified montmorillonite. Thermochim Acta 2001;367e368: 339e50. [41] Durmus A, Kasgoz A, Macosko CW. Linear low density polyethylene (LLDPE)/ clay nanocomposites. Part I: structural characterization and quantifying clay dispersion by melt rheology. Polymer 2007;48:4492e502. [42] Cassagnau P. Melt rheology of organoclay and fumed silica nanocomposites. Polymer 2008;49:2183e96. [43] Lertwimolnun W, Vergnes B. Influence of compatibilizer and processing conditions on the dispersion of nanoclay in a polypropylene matrix. Polymer 2005;46:3462e71.