Effect of processing procedures and conditions on structural, morphological, and rheological properties of polyethylene/polyamide/nanoclay blends.
Immiscible polymer blends have attracted interest of the scientific and industrial community for a few decades (1). More recently, the influence of nanofiller addition, such as carbon nanotubes (2), silica (3) or layered phyllosilicates (4), on the structure and macroscopic properties of immiscible polymer blends has received much attention. Nevertheless, to get nanocomposites with enhanced mechanical or thermal properties, it is necessary to achieve good dispersion of nanoparticles, which depends on the filler surface treatment and also on the mixing procedure and conditions (5). For instance, in the case of polymer/clay binary nanocomposites, it is well known that a high degree of exfoliation requires a strong filler/polymer affinity (6) as well as optimized processing conditions (7).
Since the early 2000s, polymer blends filled with nanoclay particles have attracted particular attention, especially when clay has a selective affinity toward one polymer of the blend. Such ternary blends are characterized by a two-level structure: the first one, corresponding to the morphology of the biphasic polymer blend, and the second one, associated with the dispersion of nanoparticles in the polymer matrix or in the polymer dispersed phase [8J. Most published studies have focused on the so-called "compatibilizing effect" of clay located at the interface between the two polymers, inducing either a reduction of apparent interfacial tension (9), (10), even though the concept of interfacial tension is quite questionable in the presence of a polymer/clay interphase, or a coalescence inhibition (11). Moreover, the presence of clay nanoparticles within the matrix or in the dispersed phase modifies the viscosity and elasticity ratio between the two polymers, and, consequently, influences the break-up of dispersed phase inclusions. In addition, for a nanocomposite matrix, anisometric particles have been shown to hinder the coalescence process (4). In a recent article, all these mechanisms were studied and discussed as a function of clay fraction, for polyethylene (PE)/polyamide (PA)/organically modified montmorillonite blends exhibiting a nodular morphology (12).
To our knowledge, most studies dealing with the effect of mixing procedures and/or conditions on the structural and macroscopic properties of ternary blends focused on carbon black particles or silica nanospheres. Elias et al. (13), (14) and Yang et al. (15) have shown that the location of silica nanospheres in polymer blends was closely related to the mixing procedure: simultaneous mixing of the three components or use of a polymer/silica master-batch. Indeed, the migration of silica at the interface between the two polymers or from a polymer phase to the other was shown to depend strongly on both thermodynamic interactions and kinetic effects generated by mixing conditions (16). Similar conclusions have been drawn concerning the effect of the mixing sequence on the location of carbon black particles in polymer blends (17). Few studies have focused on the effect of mixing procedure on morphology of polymer blends filled with nanoclays. From microscopy and X-ray diffraction results, Xiao et al. (18) have clearly shown that the component order in the mixing process affected the dispersion state of organically modified montmorillonite and influenced the morphology of compatibilized polymer/clay blends.
This work focused on the study of PE/PA/nanoclay blends, with nodular morphology, prepared either by simultaneous mixing or by using a polymer/clay masterbatch pre-step, aimed at discussing the influence of mixing procedures and conditions on ternary blend's structural, morphological, and rhcological properties. Because of the selective affinity of clay toward PA, two cases were studied: PA as a dispersed polymer phase and PA as a polymer matrix.
Blends were prepared from two immiscible commercial polymers, a linear low-density PE and a PA 12, supplied by Enichem (Flexirene[R] FG 20F) and Arkema (Rilsan[R] AECHVO), respectively. Table 1 shows the main characteristics of the two polymers: the number- and weight-average molecular weight, [M.sub.n] and [M.sub.w], respectively, and the melting point, [T.sub.m]. The Newtonian viscosity and the zero-shear first normal stress difference coefficient, denoted [[eta]*.sub.0] and [[psi].sub.1,0], respectively, were measured at 200[degrees]C. The viscosity ratio, i.e., the ratio of the dispersed phase viscosity to that of the matrix, is approximately 0.2, when the matrix is PE, and 5, when the matrix is PA. Besides, elasticity of PE was shown to be 20 times higher than the elasticity of PA.
TABLE 1. Main characteristics of the polymers used. Material [M.sub.w] (g/mol) [M.sub.n] (g/mol) [T.sub.m] ([degrees]C) PE 140,000 37,000 121 PA 37,000 20,000 183 Material [[eta].sub.0] * (Pa s) [[PSI].sub.(1),(0)] (Pa [s.sup.2]) PE 10,750 3,000 PA 2,000 150
The layered silicate added to these polymer blends was organically modified montmorillonite clay, supplied by Southern Clay Products (Cloisite - C30B). This commercial organoclay is a methyl tallow bis-2-hydroxyethyl ammonium-exchanged montmorillonite with a modifier concentration of 90 mEq per 100 g. It has a good affinity toward PA, leading to the formation of PA-based nanocomposites (19), but a very poor affinity toward PE, leading to the formation of PE-based microcomposites (20). The specific gravity of this organophilic clay is very close to 2. Contrary to numerous previous studies, a recent work by Paul and Robeson (5) suggests that the most probable lateral dimension of an individual silicate platelet is in the range of 100-200 nm; a thickness of ~0.7 nm leads to an average aspect ratio of about 100.
Blending Procedures and Conditions
Blends have been prepared at PE weight fractions of 20% or 80% and at clay weight fractions, [[PHI].cub.C30B], ranging from 1% to 4%, relative to PA. It means that for 1% added C30B, the blend with 80% PE contains 0.2% clay, whereas the blend with 20% PE contains 0.8% clay. All systems were prepared by using a Haake Rheocord internal mixer. The temperature imposed during mixing was 200[degrees]C, chosen to minimize the degradation of the components, especially that of the organic modifier (19).
Two binary polymer blends, that is without clay, were prepared at a blade rotational speed of 32 rpm for 12 min. The blend composed of 20% PE is denoted [B.sub.20%], whereas the blend composed of 80% PE is denoted [B.sub.20%]. Two blending procedures were used for the preparation of ternary systems: the masterbatch procedure and the simultaneous procedure.
First, in the case of masterbatch procedure, the organoclay was precompounded with PA at 200[degrees]C, for 6 min. The PA/C30B masterbatch, denoted generically as M, was then blended at 32 rpm for 6 min with PE, leading to M/ PE ternary blends, named [M.sub.20%] when prepared with 20% PE and [M.sub.80%] when prepared with 80% PE. Moreover, two mixing conditions were used to prepare M master-batches: [M.sup.ls] samples were prepared at a blade rotational speed of 32 rpm (the superscript ls stands for "low speed"), and [M.sub.hs] samples at 100 rpm (the superscript hs stands for "high speed"). In two previous articles, using transmission electron microscopy (TEM), [M.sub.hs] samples were shown to present a better exfoliated structure than [M.sub.ls] samples (7), (21). More precisely, at low clay fractions, [M.sub.ls] samples were shown to exhibit an intercalated structure composed of clay layers and a few large clay stacks with a thickness ~50 nm (Fig. 1a), whereas the higher mixing mechanical energy leads to a near complete exfoliation of clay particles in the case of [M.sub.hs] samples (Fig. lb). At high clay fractions, the presence of a few clay aggregates (~200 nm thick) is evidenced for [M.sup.ls] samples (Insert a, Fig. 2), whereas the presence of numerous isolated clay layers and stacks composed of few layers is indicative of partially intercalated/exfoliated [M.sup.hs] nanocomposites (Insert b, Fig. 2). The better exfoliation of the clay structure in [M.sup.hs] samples leads to better elastic properties of [M.sup.hs] samples, as shown in Fig. 2. For ternary blends prepared from a masterbatch, considering the good affinity of clay toward PA, the contact time between PA and clay particles should be equal to the total mixing time, which is 12 min. For the sake of clarity, samples prepared from a masterbatch will be denoted, for example, as [M.sup.ls] 20% when the ternary blend has been prepared with 20% PE from PA/C30B masterbatch mixed at 32 rpm; hence, [M.sup.hs.sub.80%] denotes the ternary blend prepared with 80% PE from a PA/C30B masterbatch mixed at 100 rpm.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Second, for simultaneous procedure, the three components were loaded into the mixing chamber simultaneously and compounded. All samples were prepared at a blade rotational speed of 32 rpm for 12 min. PE melts at a lower temperature than PA, and, therefore, may incorporate some fraction of clay particles during the mixing process despite the bad affinity of clay toward PE, so that the average contact time between PA and clay fillers during mixing is expected to be shorter than that for blends prepared from a masterbatch ([M.sup.ls] or [M.sup.hs]). Ternary blends with 20% PE and 80% PE are denoted as [S.sub.20%] and [S.sub.80%] respectively, in the text.
All samples were pelletized and processed by compression molding at 200C to get 2-mm-thick sheets. All blends were dried for 4 h at 80[degrees]C in a vacuum oven before performing experiments because of the hydrophilic character of PA.
Structural and Morphological Characterization
The blend morphology was observed by scanning electron microscopy (SEM) using a Hitachi S-3200N scanning electron microscope, with an accelerating voltage of 15 kV. The samples were cryofractured and the surface was vacuum-metallized with gold/palladium. For nodular morphology, the number-average diameter, [D.sub.n], was obtained from the SEM micrographs by measuring at least 350 particle diameters with SigmaScan [R] Pro 5.0 image analysis software.
The location, dispersion, and exfoliation degree of organoclay particles were determined by TEM. Ultrathin sections were cut at--100[degrees]C with an ultracryomicrotome (Reichert-Jung), using a diamond knife. Imaging was performed with a JEOL JEM 1230 transmission electron microscope at 80 kV.
Oscillatory shear measurements were performed using a rheomctrics dynamic analyzer rheometer, equipped with a parallel plate geometry (25 mm diameter and 2 mm spacing). All experiments were carried out at a temperature of 200[degrees]C, under a continuous purge of dry nitrogen, to avoid sample degradation and absorption of moisture. Frequency sweep tests were performed at a fixed strain of 4%, which is smaller than the limit of the linear visco-elastic regime for all the systems studied.
The material stability at 200[degrees]C of all systems studied in this work was systematically investigated by performing time sweep experiments. No significant variation in the rheometrical data was observed, and experiments were shown to be reproducible within [+ or -] 3%.
Preliminary Study of Clay Fraction Effect
In a blend, the morphology of the dispersed phase results from a delicate balance between shear forces, which tend to deform the droplets, and interfacial tension forces, which tend to resist to the droplet deformation. Thus, inclusion shape and size result from continuous competition between break-up and coalescence mechanisms during mixing (22). Moreover, the elasticity of the matrix and dispersed phase may also play a role (23). In this work, all blends, without or with clay, were shown to present a nodular morphology, as illustrated by SEM micrographs (Fig. 3).
[FIGURE 3 OMITTED]
The number-average nodule diameter, [D.sub.n], as a function of C30B weight fraction is shown for [B.sub.80%], [S.sub.80%], and [M.sup.ls.sub.80%] systems in Fig. 4a (PA nodules) and for [B.sub.20%], [S.sub.20%] and [M.sup.ls.sub.20%] systems in Fig. 4b (PE nodules). First, [D.sub.n] is smaller for PA nodules of [B.sub.80%] blends than for PE nodules of [B.sub.20%] blends. This result can be explained by the lower viscosity and also by the weaker elasticity of PA, which makes PA droplets easier to deform and break than PE droplets, as suggested by Starita's analysis (23). Second, for all systems, a strong decrease in the nodule size was observed with increasing clay fraction, up to a clay fraction threshold, [[PHI].sub.c]. Beyond [[PHI].sub.c], the nodule diameter reaches a value of ~1.5 [micro]m, independent of clay fraction and of mixing procedure. In a previous article (12), in the case of [S.sub.80%] blends, C30B was shown to be exclusively located at the interface, except at the highest clay fraction, i.e., 4%. Then, the nodule size reduction was attributed to the coalescence inhibition because of steric repulsions between PA/layered clay nanocomposite shells forming the interphase. In the same article, we have shown that, for [S.sub.20%] blends, C30B entities were observed, not only at the interphase, but also in the PA matrix, increasing PE nodule size reduction attributed to both coalescence barrier effect and increase of the matrix viscosity.
[FIGURE 4 OMITTED]
Besides, [[PHI].sub.c] is slightly higher for PA matrix blends (Fig. 4b) than for PE matrix blends (Fig. 4a). Moreover, Fig. 4a and b also shows that the clay fraction needed to get a stabilized morphology, [[PHI].sub.c] is affected by the mixing procedure. Indeed, [[PHI].sub.c] is lower for all ternary blends prepared from a masterbatch: [[PHI].sub.c]. is ~1% for [M.sup.ls.sub.80%] whereas it is ~2% for [S.sub.80%]. The same tendency is also observed for [M.sup.ls.sup.20%] ([[PHI].sub.c] ~1.5%) and [S.sub.20%] ([[PHI].sub.c] > 2%). At clay fractions [[phi].sub.C30B] [greater than or equal to] [[PHI].sub.c] viscoelastic properties, as well as morphology, were shown to be independent of mixing procedures. In this work, which focused on the effect of mixing procedures, [[phi].sub.C30B] = 1%, which is inferior to [[PHI].sub.c] for all blends, was chosen to prepare all samples. At this clay fraction, it should be noted that the number-average nodule diameter in ternary blends is larger for PE nodules than for PA nodules, whatever the mixing procedure.
Effect of Mixing Process on Morphology
The number-average nodule diameter for [S.sub.80%], [M.sup.ls.sup.80%], and [M.sup.hs.sub.80%] is reported in Table 2 and for [S.sub.20%], [M.sup.ls.sub20%], and [M.sup.hs.sup.20%] in Table 3. From a quantitative point of view, Tables 2 and 3 show that the morphology of blends filled with 1% C30B is clearly dependent on mixing procedure. Indeed, systems that result from simultaneous mixing of the three components ([S.sub.20%] or [S.sub.80%]) exhibit much larger nodules than systems with the same composition but prepared from a masterbatch, underlining the effects of mixing procedure. Besides, [M.sup.ls.sub.80%] and [M.sup.hs.sub.80%] present slight differences in average nodule diameter, highlighting the effects of mixing conditions: [M.sup.hs.sub.80%] has larger nodules than [M.sup.ls.sub.80%]. Contrary to [M.sup.ls.sub.80%] and [M.sup.hs.sub.80%] blends, morphologies of [M.sup.ls.sub.80%] and [M.sup.hs.sub.20%] samples are the same, which is independent of the mixing conditions.
TABLE 2. Schematic description of [S.sub.80%], [M.sup.ls.sub.80%], and [M.sup.hs.sub.80%] structure and morphology. Simultaneous mixing Masterbatch procedure [S.sub.80%] (a) [M.sup.ls.sub.80%] (b) [M.sup.hs.sub.80%] (c) [**] [**] [**] The organoclay weight fraction is 1%, relative to PA phase. (a) [D.sub.n] = 2.24 [micro]m, r = 75%, and e = 7 nm. (b) [D.sub.n] = 1.43 [micro]m, r = 75%, and e = 7 nm. (b) [D.sub.n] = 1.83 [micro]m, r = 75%, and e = 7 nm. TABLE 3. Schematic description of [S.sub.20%], [M.sup.ls.sub.20%] and [M.sup.hs.sub.20%] structure and morphology. Simultaneous mixing, Masterbatch procedure, [M.sup.ls.sub.20%] [S.sub.20%] (a) and [M.sup.hs.sub.20%] (b) [**] [**] The organoclay weight fraction is 1%, relative to PA phase. (a) [D.sub.n] = 2.77 [micro]m, r = 94%, e = 9 nm, t = 4 nm, [[empty set].sub.[right arrow]int] = 0.30%, and [[empty set].sub.[right arrow]PA] = 0.70%. (b) [D.sub.n] = 2.22 [micro]m, r = 90% for [M.sup.ls.sub.80%] and 96% for [M.sup.hs.sub.80%], e = 12 nm, t = 3 nm, [[empty set].sub.[right arrow]int] = 0.45%, and [[empty set].sub.[right arrow]PA] = 0.55%.
Effect of Mixing Process on Clay Location and Structure
Representative TEM micrographs of ternary blends filled with 1% clay are shown in Fig. 5a-c for blends with 80% PE, and in Fig. 6a-c for blends with 20% PE. The PE phase is easily identifiable because of the presence of crystallites, which makes it slightly noisy on the micrographs. TEM micrographs clearly show that clay is not dispersed in the PE matrix phase (Fig. 5a-c) or in the PE dispersed phase (Fig. 6a-c), which can be explained by the selective affinity of clay particles toward PA. In the case of simultaneously mixed blends (Figs. 5a and 6a), the clay entities initially trapped in the melt PE phase had to migrate to the matrix/nodule interface, suggesting that the mixing time is long enough to allow the migration of clay entities out of PE phase during the process (8), (10).
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
For [S.sub.80%] and [M.sup.ls.sub.80%] systems, the clay is only located at the matrix/nodule interface, creating a discontinuous PA-intercalated clay interphase (Fig. 5a and b). For the [M.sup.hs.sub.80%] blend, prepared from the best exfoliated master-batch, in addition to a discontinuous clay interphase, a few individual clay layers were observed in the PA nodules (Fig. 5c). The presence of individual clay nanoparticles in PA nodules for [M.sup.hs.sub.80%] blend might be due to difficult migration of isolated clay layers to the matrix/ nodule interface because of the large clay layer aspect ratio, even though the precise mechanisms governing clay migration during processing are still unclear. To characterize the clay interphase, its average thickness, e, was determined using 12 measurement points regularly distributed along the nodule perimeter, and the interface coverage, r, which quantifies the interphase discontinuity, was inferred. For the three blends ([S.sub.80%], [M.sup.ls.sub.80%] and [M.sup.hs.sub.80%]), the discontinuous clay interphase has an average thickness, e ~ 7 nm, and an interface coverage, r ~ 75% (Table 2). Thus, in this study, the characteristics of the interphase (thickness and continuity) are independent of mixing procedure and conditions. In Table 2, a schema illustrates the morphology of [S.sub.80%], [M.sup.ls.sub.80%], and [M.sup.hs.sub.80%] samples, as well as the structure and location of clay.
In the case of PE-dispersed phase blends, a continuous clay interphase is observed whatever sample elaboration mode, but the dispersion of clay particles in PA matrix is slightly affected by the mixing procedure. Indeed, a few small clay stacks are observed in PA phase for the simultaneous mixed blend (Fig. 6a), whereas numerous individual clay layers are present in PA phase for blends prepared from a mastcrbatch (Fig. 6b and c). Values of the interface coverage, r, and interphase thickness, e, determined from TEM observations as well as a schema illustrating the morphological and structural aspect of [S.sub.20%], [[M.sup.ls.sub 20%], and [M.sup.hs.sub.20%] samples are reported in Table 3. The interface coverage of [S.sub.20%], [M.sup.ls.sub.2o%], and [M.sup.hs.sub.20%] samples being superior to 90%, the interphase of these three samples can be considered as continuous. However, the interphase average thickness is significantly lower for [S.sub.2o%], for which e ~ 9 nm, than for [M.sup.ls.sub.20%] and [M.sup.hs.sub.20%] blends, for which e ~ 12 nm, showing that it is strongly dependent on mixing procedure, but independent of mixing conditions. Knowing the number-average diameter of PE nodules, Dn, the thickness of a clay particle, 0.7 nm, the interphase thickness, e, and the interlayer distance for [M.sup.hs] and Mhs nanocomposites, [d.sub.i] = 3 nm , the fraction of clay covering nodules of PE, [[empty set].sub.right arrow int]-, and, consequently, the fraction of clay within PA phase, [[empty set].sub.right arrowPA], can be inferred (Table 3). Table 3 shows that [[empty set].sub.right arrow int] is higher for [M.sup.ls,sub.20%] and [M.sup.hs.sub.20%] blends than for [S.sub.20%] blend. Indeed, materials prepared from a masterbatch exhibit a thicker continuous clay interphase and a higher interfacial area (smaller PE nodules), leading to a larger clay fraction within the interphase. Table 3 also presents the average thickness of clay entities dispersed in PA matrix, denoted as t. For both [M.sup.ls.sub.20%] and [M.sup.hs.sup.20%] blends, t = 3 nm, corresponding to two stacked layers on average, indicative of near-complete exfoliation of clay particles. For S20% blend, t = 4 nm, which corresponds to stacks composed of more than two layers, underlining the lower efficiency of simultaneous mixing in terms of clay dispersion. To sum up, clay entities dispersed in PA phase are thinner and more numerous (Fig. 6b and c) for blends prepared from a masterbatch. Finally, Fig. 6a-c does not show any large clay stacks in the matrix of these ternary systems, contrary to what was observed in the case of the weakly filled binary Mls nanocomposites (presence of ~50 nm large clay entities in Fig. lb). We suggest that the presence of PE nodules, covered with a rigid intercalated PA/clay interphase, favors the break-up of large clay stacks initially present in the PA matrix during the mixing process.
Effect of Mixing Process on Rheological Properties
As far as rheological properties are concerned, the critical strain, [[gamma].sub.c], defining the limit of the linear viscoelastic regime, is close to 40% for all blends studied in this work, except for ternary PE-dispersed phase blends. Indeed, for [S.sub.20%], [M.sup.hs.sub.20%], and [M.sup.ls.sub.20%] blends, [[gamma].sub.c] ~ 25%. For these ternary blends, the lower value of [[gamma].sub.c] can be attributed to the presence of clay within PA phase (Fig. 5a-c). On the other hand, the critical strain seems to be independent of mixing procedure and conditions.
The storage modulus, G', of pure PE, [B.sub.80%] neat blend, and [S.sub.8o%], [M.sup.ls.sub.80%], and [M.sup.hs.sub.80] ternary blends is presented as a function of frequency in Fig. 7. Figure 7 shows no significant difference in G' at high frequencies between all these materials, which is consistent with the absence of clay in PE matrix , in agreement with TEM observations. Moreover, Fig. 7 shows that G' ([S.sub.80] < [G.sup.t]([M.sup.hs.sub.80%]) < G' ([M.sup.ls. sub.805]) over the whole frequency range, in connection with the number-average diameter of PA nodules, [D.sub.n]([S.sub.80%]) > [D.sub.n]([M.sup.hs.sub.80%]) > [D.sub.n] ([M'.sup.ls.sub.80%]). as shown by SEM investigation. Indeed, at a fixed PA dispersed phase volume, the total interfacial area, which contributes to the blend elasticity, is inversely proportional to the nodule diameter, generated during mixing. Thus, the increase in total interfacial area, ~60% for the [M.sup.ls.sub.80%] blend relative to the [S.sub.80%] blend, is accompanied by a strong increase in the storage modulus, over an extended frequency range. [S.sub.80%] and [M.sup.hs.sub.80%] storage moduli are close to each other, from moderate to high frequencies, because the difference in total interfacial area between these systems is small (~20%). However, the G' versus frequency curve of the [M.sup.hs.80%] blend exhibits a shoulder at a much lower frequency and much more marked than that observed for [S.sub.80%] and M/s"8o% systems. The relaxation time associated with this type of shoulder is usually attributed to form relaxation process of the nodules (25). The characteristic time of this form relaxation mechanism is ~30s for [B.sub.8o%]; it is shifted toward higher values (~50 s) for [S.sub.80] and [M.sup.ls.sub.80% ] blends, which could be attributed to the presence of the discontinuous clay interphase. The large order of magnitude of this characteristic time (~150 s), for the [M.sup.hs.sub.80%] blend, could be due to the presence of some exfoliated clay nanoparticles in the PA nodular phase, as revealed by TEM micrographs, which could enhance the viscoelastic properties of the nodules.
[FIGURE 7 OMITTED]
The storage modulus, G', of pure PA, [B.sub.20%] neat blend without clay, and [S.sub.20%], [M.sup.ls.sub.20%], and [M.sup.hs.sub.20%] blends filled with 1 % clay fraction is shown as a function of frequency in Fig. 8. Figure 8 shows that the storage modulus of ternary systems ([S.sub.20%], [M.sup.hs.sub.2o%], and [M.sup.hs.sub.20%]), at high frequencies, is higher than that of samples without clay (pure PA and [B.sub.20%]) because of the presence of clay in PA phase (Fig. 6a-c). Figure 8 also shows a significant difference in low-frequency storage modulus of the simultaneously mixed blend ([S.sub.20%]) and blends prepared from a master-batch ([M.sup.ls.sub.20%] and [M.sup.hs.sub.20%]). Indeed, the shoulder observed in the low-frequency elastic modulus curve of [S.sub.20%] blend corresponds to a characteristic time (~40 s, close to the characteristic time determined for Sgo% blend) superior to the PE nodule form relaxation time (~20 s), determined for the [B.sub.20%] polymer blend (Fig. 8). The low-frequency viscoelastic behavior of the simultaneously mixed blends is governed by the contribution of PE nodules covered by a continuous clay interphase. On the other hand, the thicker interphase of blends prepared from a masterbatch hinders the coalescence through strong steric repulsions; in parallel, the finer dispersion of clay in the matrix improves the barrier effect to coalescence and also increases the viscosity of the PA/clay matrix. The combination of these three mechanisms favors the reduction of nodule size. The similarity in morphology and exfoliated clay structure of [M.sup.ls.sub.20%] and [M.sup.hs.sub.20%] blends leads to the same Theological behavior for the two blends: that of a PA/exfoliated clay nanocomposite (Fig. 2). It has to be noticed that the value of the low-frequency plateau storage modulus of the ternary blends prepared from a masterbatch is rather high (~50 Pa) for a so weak clay fraction. Such a result would tend to show that the rheological behavior is governed by the numerous clay nanoplatelets, which have a large aspect ratio and are well dispersed in the PA matrix (Table 3, schematic representation). Moreover, the rather high value of the elastic modulus, compared with that obtained for binary nano-composites at the same clay fraction (19), could be attributed to synergistic effects of the two structure levels: that associated with PE nodules and that associated with clay layers (8).
[FIGURE 8 OMITTED]
This part aims at discussing the above results in terms of the different process-dependent parameters, which seem to govern the structural, morphological, and rheological properties of the ternary blends.
Effect of PEIPA Contact Time
In this work, it is shown that mixing procedure affects the morphology of all blends studied, in particular, that of PE matrix blends filled with 1% clay. Indeed, PA nodules are shown to be larger for the [S.sub.80%] blend than for [M.sup.ls.sub.80%] and [M.sup.hs.sub.80%] blends prepared from a masterbatch. Such an effect echoes that observed with immiscible polymer blends compatibilized by a block copolymer: increasing the contact time between the two polymer phases during mixing can lead to an increase in coalescence because of a weak interfacial coverage of the interface by the compatibilizer (26). In this work, PA nodules are partially covered by clay, which can be considered as a compatibilizer (12); therefore, the contact time between PE and PA polymer phases could explain the differences in morphology and rheology induced by the mixing procedure. Indeed, the contact time of the two polymer phases is longer in the case of simultaneous mixing (12 min) than in the case of two-step mixing, i.e., when using a masterbatch (6 min).
In opposite, for ternary PE-dispersed phase blends, although PE nodules are larger for the S20% blend than for [M.sup.ls.sub. 20%] and [M.sup.hs.sub.20%], blends, the difference in total interfacial area is only ~20% between the simultaneously mixed system and systems prepared from a masterbatch.
Compared with ternary blends with 80% PE, the role played by the PE/PA contact time in the nodule final size in the case of ternary blends with 20% PE is certainly less important because of the formation of a continuous clay interphase during mixing.
Effect of PAIClay Contact Time
In the case of PE matrix blends, the structure of clay particles, located in a discontinuous interphase, depends on mixing procedure. Indeed, as the interface coverage by clay, r ~ 75%, and interphase thickness, e ~ 7 nm, do not seem to depend on the mixing procedure, it means that clay particles most likely adopt a more exfoliated structure in the systems prepared from a masterbatch, for which the nodules are smaller, meaning a larger accessible total interfacial area. This effect underlines the influence of the contact time between PA and clay during mixing: a longer contact time between PA and clay particles, for systems prepared from a masterbatch, leads to a better intercalation of PA chains into clay galleries, making easier exfoliation of clay particles.
For ternary PE-dispersed phase blends, clay/PA contact time also affects the structure of clay particles not only located in a continuous interphase covering PE nodules, but also dispersed in the PA phase. A longer contact time between PA and clay particles, for systems prepared from a masterbatch, leads to a thicker continuous PA intercalated clay interphase and a finer dispersion of better exfoliated clay particles within PA phase.
Effect of Mixing Energy
The mixing conditions also influence the structural state of clay, and, therefore, the rheology of the ternary blends, as evidenced by the differences between [M.sup.hs.sub.80%] and [M80% systems. Increasing the average shear rate, and, therefore, the mixing energy, during the mixing process leads to a better exfoliation of particles (7), (21), which allows some individual nanoclays to stay in PA nodules, thus increasing the viscoelasticity of the nodules. This increase tends to limit the nodule break-up, leading to larger nodules and to increase the form relaxation time of the nodules.
In the case of ternary PE-dispersed phase blends, the difference in structure between Mls and Mhx nanocompo-sites, produced by the difference in mixing mechanical energy, seems to vanish during the second step of mixing (incorporation of PE). Indeed, M/v2o% and M 20% blends exhibit the same characteristics, in terms of clay fractions at the interphase, thickness of clay interphase, and quality of clay dispersion in the PA matrix. Moreover, for both blends, PE nodules can be considered as rather rigid entities, because of the presence of a polymer/clay nanocom-posite interphase. Therefore, PE nodules can help the breaking up of clay stacks during the mixing process, which adds to the direct effect of mixing mechanical energy. Both effects certainly contribute to clay exfoliation within the PA/C30B nanocomposite matrix.
This work focused on the effect of processing procedures and conditions on structural, morphological, and rheological properties of ternary blends composed of PE, PA-12, and organically modified nanoclay with selective affinity toward the polymer couple. Ternary blends with 20% and 80% PE were prepared either by simultaneous mixing or by using a two-step procedure including the preparation of PA/clay masterbatch.
The first result is that mixing procedure and conditions only affect morphological and rheological properties of polymer blends at low clay fractions, [[PHI].sub.c308] < [[PHI].sub.c] where [[PHI].sub.c]. is a weight fraction threshold, which depends on mixing procedure. Even if the interpretation is somewhat delicate, this result, which was not necessarily expected, is interesting from a practical point of view.
Most results of the study highlight the significant role of two parameters that depend on processing procedures and/or conditions: the contact time between PE and PA phases and the contact time between clay and PA. The PE/PA contact time, which depends on the mixing procedure, is a parameter influencing the size of PA nodules partially covered by a PA intercalated/clay interphase and, therefore, the rheology of the ternary systems with 80% PE. However, the presence of isolated clay layers in PA nodules, mainly due to a high mixing shear combined with a long PA/clay contact time that favors clay exfoliation, strongly affects the morphology and rheology of these systems, underlining the importance of mixing conditions in ternary blends with PE matrix.
The PA/C30B contact time, which depends on the mixing procedure, seems to play a key role in morphological and rheological properties of ternary blends composed of a PA/C30B nanocomposite matrix and PE nodules covered by a continuous clay interphase. Indeed, for ternary blends of this type prepared from a masterbatch, the clay interphase thickness is increased and the dispersion quality of clay nanoparticles within the matrix is improved, favoring mechanisms that lead to nodule size reduction. For the ternary blends with 20% PE prepared from a masterbatch, the mixing conditions do not seem to affect the morphology and rheology of the systems.
Future work on such PE/PA/nanoclay ternary blends will focus on the influence of mixing procedures and conditions on particle migration, which occurs during mixing and governs the formation of the interphase.
The authors thank Dr. BenoTt Brule (Arkema, France) for fruitful discussion and Dr. Nicolas Dufaure (Arkema, France) for his precious help concerning TEM experiments. The authors also thank Gerard Sinquin (Plateforme dTmagerie et de Mesures en Microscopie, Universite de Bretagne Occidentale, France) for SEM experiments.
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Correspondence to: Pascal Mederic; e-mail: email@example.com
Contract grant sponsor: Region Bretagne.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[C]2011 Society of Plastics Engineers
Pascal Mederic, Julien Vide, Jacques Huitric, Michel Moan, Thierry Aubry
LIMATB, Equipe Rheologie, Universite Europeenne de Bretagne (UBO), Brest, France
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|Author:||Mederic, Pascal; Ville, Julien; Huitric, Jacques; Moan, Michel; Aubry, Thierry|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 2011|
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