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The effect of nanoclays on the nucleation, crystallization, and melting mechanisms of isotactic polypropylene.

INTRODUCTION

Polymeric clay-based nanocomposites are one of the newest polymeric systems. They consist of a polymeric matrix and a disperse nanometric layerlike additive, such as montmorillonite, the combination representing the possibility of enhancing both the physicochemical and mechanical properties of the product [1]. Homogeneously exfoliated clay nanocomposites are the most useful, because they maximize polymer-filler interactions. However, obtaining the exfoliated condition is difficult in practice due to the intrinsic polypropylene-clay incompatibility. Such condition can be achieved, however, with the use of a compatibilizer, which substantially improves mechanical [2], dynamic-mechanical [2, 3], and flammability [4] properties.

Isotactic polypropylene (iPP) is able to develop polymorphism [5], the formation of the different crystalline structures, [alpha], [beta], and [gamma], depending on the particular iPP, and also on the crystallization conditions. The [alpha]-(monoclinic) structure is the most thermodynamically stable, and, as a consequence, the most frequent. It has also the highest thermal and mechanical resistance. This crystal form is directly obtained through quiescent crystallization from the melt, or from uniaxial orientation, as in melt spinning [6]. The [beta]-(hexagonal) crystal form is favored by a number of conditions, namely, the presence of nucleating agents [7], quenching from the molten state [8], crystallization temperature [9, 10], glass fiber [11], ethylene comonomers [12], and blending with ethyl vinyl alcohol [13]. The [gamma]-triclinic form is the least frequently observed, although it has been obtained after crystallizing samples at high pressures [14-16].

Even though there are several methods available to produce polymeric nanocomposites, it is desirable to use simple preparation techniques for industrial applications. The use of compatibilizers has the disadvantage of producing volatile secondary products during processing. Therefore, the best option is to use simple preparation methods where only polymer and filler are involved [3, 17].

Nanoclay nanocomposites are relatively new materials, and few studies have been published regarding their morphological behavior within polymeric matrices. An example is the influence of nanoclay on the nucleation process of iPP, which, in practice, motivates higher crystallization rates [18, 19]. In this study, we have purposely eliminated the use of compatibilizers and used low molecular weight iPP for the purpose of efficiently dispersing the nanoclay platelets. In this form, several new morphological findings are reported for these polymeric systems.

EXPERIMENTAL

Materials and Sample Preparations

Neat iPP homopolymers were obtained from Exxon, USA. The three selected resins had weight-average molecular weights of 158,970 g/g-mol (low molecular weight); 368,930 g/g-mol (intermediate molecular weight), and 680,000 g/g-mol (high molecular weight), respectively. Films of both iPP homopolymers and nanocomposites with 2, 4, and 6 wt% nanoclay were prepared using a recycling extrusion method. For this purpose, a single screw extruder was used with four heating zones set at 146, 160, 170, and 180[degrees]C. The extruder rendered shear rates of 100 [s.sup.-1] and an average flow speed of 20 g/min. The organomodified montmorillonite, I-28, was supplied by Nanocor, USA. After mechanical mixing of the clay powders and iPP pellets, the mixtures were recycled into the extruder thrice.

Wide Angle X-ray Diffraction

A Siemens D-50 diffractometer was used to collect the corresponding X-ray diffraction patterns. Thick plaques were prepared by stacking several film layers within stainless steel molds. These plaques were inserted into a Mettler FP82HT hot-stage and melted at 190[degrees]C for 3 min. The mold was quickly transferred to a second hot-stage, which was already set at the specific crystallization temperature. The crystallization time was 15 min, after which samples were quenched on a cold surface at 5[degrees]C to preserve thermal history. The diffraction patterns of the recovered samples were obtained at room temperature within the 2[theta] range 5[degrees]-40[degrees] using a scanning rate of 0.6[degrees]/min, a filament intensity of 25 mA, and an accelerating voltage of 35 kV.

Thermal Analyses

Thermal experiments of iPP neat polymer and the nanocomposites were made in a PerkinElmer DSC-7. This was calibrated with the indium standard in nitrogen flux. All samples weighted 8 [+ or -] 1 mg and were sealed within aluminum sample pans before experiments. Two different crystallization experiments were made, isothermal, and dynamic. For isothermal crystallization, samples were heated up to 190[degrees]C and then cooled down at 500[degrees]C/min (nominal rate) to the specific crystallization temperatures ([T.sub.c]) of: 125, 128, 131, and 134[degrees]C. The isothermal crystallization time ranged between 15 and 30 min and was followed by linear heating at 10[degrees]C/min up to 190[degrees]C. Nonisothermal crystallization traces were obtained after melting at 190[degrees]C (3 min) and linear cooling at 10[degrees]C/min from 190 to 25[degrees]C. The linear heating traces of these last experiments were also obtained at 10[degrees]C/min from 25[degrees]C up to 190[degrees]C.

Polarized Optical Light Microscopy

An Olympus BX60 transmission optical microscope was used, with crossed polars and a 20x objective (resolution of 1.84 [micro]m) to monitor in situ the heating process of the isothermally crystallized samples. The microscope was coupled to a Mettler FP82HT hot-stage controlled by a FP90 digital controller. A photographic camera was attached to the microscope and in one of the eyepieces there was a Mettler ZU FP 82 photo monitor. This last was used with the purpose to detect the amount of light transmitted as the isothermal crystallization process progressed. Isothermal crystallization experiments were made at 125 and 134[degrees]C.

Fourier Transform Infrared Spectroscopy

A Bruker-Vector 22 Fourier transform infrared spectroscopy (FTIR) spectrophotometer was used to study the conformational changes occurring in the nanoclays. Amorphous films were used for this purpose after quenching samples from the melt (190[degrees]C; 3 min). All films were studied by attenuated total reflectance (ATR). The IR spectra were collected at a resolution of 4.0 [cm.sup.-1] within the range 400-4000 [cm.sup.-1]. All experiments were made at room temperature.

Small Angle X-ray Scattering

Small angle X-ray scattering (SAXS) was used in this study to quantify the morphology of the crystalline phase. For this purpose, bidimensional corrected scattering patterns were obtained on Fuji imaging plates using a sample-to-detector distance of 660 mm, and a wave length of 0.155 nm. The SAXS measurements were performed at the X27C X-ray synchrotron beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory.

RESULTS AND DISCUSSION

Isothermal Crystallization and Melting

In practice, it is common to use compatibilization agents in polymeric systems whose molecular characteristics do not allow mixing or hybridization with other components. In this study, however, a different strategy was used that did not require compatibilizers. Direct iPP nanocomposites were prepared by using commercial iPP of different molecular weights as carriers in addition to three re-extrusion cycles. Through this method, high molecular weight samples rendered nontransparent clay spotted films with melt fractured zones. Intermediate molecular weight samples still displayed spots of clay agglomeration under the optical microscope, although they had better transparency. The iPP nanocomposites with the lowest molecular weight rendered the best optical microscope transparency and so were used for the rest of the study.

[FIGURE 1 OMITTED]

SAXS patterns of reference nanoclay and three nanocomposites, with different clay concentrations (2, 4, and 6 wt%), are shown in Fig. 1. In these results, nanoclay shows at least three scattering peaks, indicating the presence of periodically ordered structures. In the nanocomposites, the basic dispersion pattern is practically the same, although the diffracting peaks are greatly diminished in intensity depending on nanoclay concentration. There is also a tendency of the lowest dispersion peak to displace to lower values of the reciprocal space scattering vector, and then to higher values depending on the nanoclay concentration in the mixture. These results were a primary indication that groups of layers (i.e., so-called tactoids) rather than exfoliated nanoclay platelets were giving rise to the observed scattering patterns since, otherwise, there would only be a single, layer-to-layer spacing-related scattering peak. Figure 2 shows the TEM micrographs of molten and fast-quenched samples for the highest (6 wt%) and lowest (2 wt%) nanoclay concentrations. Here, it is indeed observed that tactoids, rather than exfoliated layers, are the dominant morphology, although there are also some individually exfoliated layers (see arrows), particularly at low nanoclay concentrations (Fig. 2a). The exfoliated layers were rather expected due to the higher dispersion volume available at low nanoclay concentrations. On the contrary, with high nanoclay concentrations (Fig. 2b), there is lower dispersion volume and it becomes more difficult for the individual layers to succeed toward exfoliation; therefore, the dominant morphology mainly involves tactoids. Other polymeric systems have also rendered relatively similar results. For example, Morgan and Gilman [20] reported a mixed morphology of intercalated and exfoliated structures in polystyrene (PS) and maleated polypropylene (PP-g-MA) clay nanocomposites.

iPP is able to develop a number of crystal structures depending on crystallization conditions and molecular parameters [5], although, the [alpha]- and [beta]-crystalline forms are the most common. Zheng et al. [21] reported that nanoclays inhibit the formation of the [beta]-crystal phase in highly exfoliated maleic anhydride modified iPP [MAH-iPP] clay nanocomposites. The [beta]-inhibition was assigned to the presence of exfoliated and oriented clay sheets in the nanocomposites, which, as proposed by the authors, speeds up the crystallization process of the [alpha]-phase, whereas it does not have a significant effect on the crystallization rate of the [beta]-form. There are some reports regarding the induction of crystal habits by nanoclays in other polymers such as Nylon 6 [22, 23]. Wide angle X-ray diffraction (WAXD) results of isothermally crystallized samples as a function of nanoclay concentration are shown in Fig. 3. Here, an inverse relationship between nanoclay concentration and the formation of the [beta]-crystalline structure is observed. If we consider that the neat homopolymer is not able to form the [beta]-structure under identical crystallization conditions (see, for example, the diffraction pattern of the reference homopolymer, A), we can conclude that nanoclays are influencing the observed behavior, particularly at low nanoclay concentrations. The results are in relative disagreement with Zheng et al. [21], who proposed that nanoclays inhibit the formation of the [beta]-structure. This is, however, the case at high nanoclay concentrations. Therefore, we propose that in the case of individual exfoliated layers, which prevail at low nanoclay concentrations, it is likely that the crystallographic requirements are satisfied to give nucleation and formation of the [beta]-structure. The WAXD crystallinity remains rather constant because one crystal phase is converted into another phase as shown in Table 1.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The uncommon inverse relationship between nanoclay concentration and the formation of the [beta]-crystal structure also manifests itself after isothermal crystallization and melting, as shown in the DSC results of Fig. 4. Here, the inverse evolution of the first melting endotherm and nanoclay concentration is observed. If a correlation is made of these results with the WAXD patterns in Fig. 3, the first melting endotherm should be assigned to melting of the [beta]-crystals due to the same inverse relationship with nanoclay content. The low melting endotherm has been observed before with neat iPP resins and has been assigned to the fusion of [beta]- [12, 24] and [gamma]-crystals [12]. The results presented here for the iPP nanocomposites support the first proposition. As for the second-melting endotherm, it is assigned to melting of the more stable [alpha]-crystals.

Using polarized light optical microscopy, we have determined that the [beta]-crystals develop their own morphology as shown in Fig. 5. Here, the melting process of a sample with identical thermal history as that in the DSC results (Curve B) was registered in micrographs. It is clearly observed in the polarized optical microscopy (POM) results that melting of the [beta]-crystals takes place first, that is, when the heating process passes through the first melting endotherm. Although not shown here, we have also determined that the DSC traces maintain this behavior (double melting) when isothermal crystallization is made at other temperatures. The promotion of crystal structures by nanoclays has been reported by Medellin-Rodriguez et al. [22] and Wu et al. [23] in Nylon 6 nanoclay hybrids. Other than the results using modified iPP [21], there have not been reports of this kind using nanoclays and neat iPP.

[FIGURE 4 OMITTED]

Infrared spectroscopy was used with the purpose to understand the effect of nanoclays at the molecular level. Figure 6 shows the FTIR results of amorphous samples with increasing nanoclay content. These were left for 3 min in the melt and then were quenched to 5[degrees]C to preserve molecular characteristics of the amorphous specimens at room temperature. Zhu et al. [25] associated a vibration peak at 973 [cm.sup.-1] with head-to-tail sequences of the repeating units, and also with short isotactic helical sequences of the macromolecule. These authors reported that the indicated peak remained practically constant on heating iPP up to the melting point. Another vibration peak at 998 [cm.sup.-1] was associated with longer helical sequences, although this was reported to decrease with temperature. This last effect was considered an indication of reduction of local order in the melt. In the results shown in Fig. 6, we observed a decrease of both the 973 and the 998 [cm.sup.-1] vibration peaks when the nanoclay content increased (see arrows); although, the one associated with the short isotactic helices (973 [cm.sup.-1]) decreased faster (see also Table 2). These effects were taken as an indication of reduction of local order related to both the short and long helical sequences once nanoclay was introduced in the system. In a random polymeric coil, the number of short sequences is statistically higher than that of the long sequences. Therefore, it was rather expected that the higher the amount of nanoclay introduced in the system the higher the proportional destruction of both types of sequences due to the consequent increase of polymer/substrate interactions. Molecular interactions between iPP and the exfoliated platelets must be succeeding through a combination of molecular contacts between the platelets exposed surface area and/or their chemically exchanged surfactants as shown in Fig. 7. Such new interactions will finally lead to the overall destruction of local order associated to both short- and long-helical polymer sequences. It will actually be shown in the following section that the destruction of local order leads to the enhancement of heterogeneous contact.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Nonisothermal Crystallization and Melting

Most of the polymer processing operations involve nonisothermal, rather than isothermal, crystallization. Therefore, studies were carried out in this work to determine the effect of nanoclays under nonisothermal crystallization conditions. The corresponding DSC cooling traces from the melt are shown in Fig. 8. Here, the effect of nanoclays as nucleating agents of iPP is clear. For example, at the experimental cooling rate of 10[degrees]C/min, the induction time for crystallization decreased from 7'36' to 7'10" (see also Table 3) for neat and 6 wt% clay nanocompounded iPP, respectively. These results are therefore in correlation with those of FTIR, where a proportional increase in nanoclay content enhanced destruction of local molecular order. In other words, the destruction of molecular local order leads to the construction of heterogeneous molecular affinity between the iPP macromolecules and nanoclay. If we consider that the commercial nanoclay was ion exchanged with surfactantlike molecules, the mechanism of heterogeneous interaction could involve molecular contacts between the surfactant chemical groups and the polymer, without discarding molecular contacts with the nanoclay surface as shown before in Fig. 7.

[FIGURE 7 OMITTED]

The melting traces of samples, after nonisothermal crystallization, were also obtained with the purpose of determining the effect of nucleation on the melting process. The results shown in Fig. 9 indicate some similarities and differences compared with those obtained after isothermal crystallization (see Fig. 4). Low nanoclay concentrations (2 wt%) promoted crystallization of the [beta]-structure, as a consequence, melting of this crystal phase is first observed although now in terms of two melting endotherms. The melting process of the main [alpha]-crystal structure is observed in sequence although it also involves double melting. Vleeshouwers [26] reported similar double-melting behavior of iPP samples, which were crystallized at high cooling rates, although the results were obtained with purposely added [beta]-nucleating agents.

[FIGURE 8 OMITTED]

Bulk Isothermal Crystallization

Bulk isothermal crystallization experiments are useful to determine the crystallization kinetics of semicrystalline polymers. In this type of studies, important information regarding nucleation, crystallization kinetics, and secondary crystallization are obtained. Figure 10 shows the POM results, after isothermal crystallization at 125[degrees]C, of nanocompounded iPP with different nanoclay contents. Figure 11 shows the results at 134[degrees]C. It is observed the influence of nanoclays in decreasing the induction time for crystallization (nucleation effect), particularly at the higher crystallization temperature of 134[degrees]C. In this last case, it is not possible to observe the complete polymer crystallization of the neat resin, which is highly retarded. As for the overall crystallization kinetics at 134[degrees]C, there is clearly an influence of nanoclay in the crystallization curve with a gradual tendency to form a sigmoidal curve at high nanoclay concentrations.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

The overall crystallization characteristics are best interpreted when an Avrami [27] type of analysis is applied. This states that the crystalline fraction as a function of time, [theta](t), during the isothermal process follows the equation

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[theta](t) = 1 - exp[-k[t.sup.n]] (1)

where k is a rate constant containing the nucleation and growth parameters and n is a constant whose value depends on the mechanism of nucleation and on the crystal geometry. In terms of POM experiments, [theta](t) can be expressed as

[theta] = [[I.sub.t] - [I.sub.o]]/[[I.sub.o] - [I.sub.[infinity]]] (2)

[FIGURE 13 OMITTED]

where [I.sub.t] is the transmitted light intensity at a particular time, and [I.sub.[infinity]] and [I.sub.o] are the initial and infinite time intensities. In this form, Eq. 1 becomes

log[- log [[[I.sub.[infinity]] - [I.sub.t]]/[[I.sub.[infinity]] - [I.sub.o]]]] = log k + n log t (3)

To apply Eq. 1, it is necessary to determine [I.sub.o], [I.sub.[infinity]], and [I.sub.t] from the normalized plots shown in Figs. 10 and 11. This process generates Avrami's curve and, as a consequence, the overall crystallization parameters can be calculated. Figure 12 shows that the transition from primary to secondary crystallization (inflection in the curve) is sharper with an increase in nanoclay concentration; in other words, secondary crystallization is accelerated by the nanoclay presence. If the crystallization temperature is increased up to 134[degrees]C, as shown in Fig. 13, the same, although less clear, effect is observed. Table 4 shows the corresponding Avrami parameters for each temperature. At 125[degrees]C, the evolution of crystals is associated with instantaneously nucleated spheres, as the growth geometry, and the expected interface growth control due to the high crystallization temperature. When the crystallization temperature is increased to 134[degrees]C, several changes are observed. Secondary crystallization is developed at low levels, the crystallization process of the neat polymer does not occur within the frame time of the experiment, and the overall growth geometry remains basically the same as that of the lower temperatures.

The Avrami curves show that at 125[degrees] high nanoclay concentrations motivate secondary crystallization (break in the plot). The lack of secondary crystallization, at 134[degrees]C, was expected even in the absence of nanoclay considering that high temperature crystals are more perfect. As for the growth geometry [28], some variations were expected due to the nanoclay presence as indicated in Table 3; however, these were not significantly different from those expected for spherulites.

CONCLUSIONS

It was determined that clay stacks (i.e., the so-called tactoids) and exfoliated layers are the main components in the morphology of relatively low molecular weight iPP clay nanocomposites.

After isothermal crystallization of samples, an inverse relationship between nanoclay concentration and the formation of the [beta]-crystalline structure was determined. It was therefore proposed that the exfoliated individual layers, which are present on relatively high amount at low nanoclay concentrations, must meet the crystallographic requirements as for giving rise to the nucleation and formation of the [beta]-structure. The isothermal crystals developed double-melting behavior and the low-melting endotherm was associated with the fusion of [beta]-crystals. The second-melting endotherm was assigned to melting of the more stable [alpha]-crystals. It was also determined that [alpha]- and [beta]-crystals develop their own crystal morphology.

Nanoclays changed the molecular order of chain sequences regarding neat isotropic iPP. The destruction of molecular order led to the construction of heterogeneous nucleation.

Nonisothermal crystallization experiments indicated that nanoclays are nucleating agents for iPP. The melting behavior of such dynamic crystallization indicated that low nanoclay concentrations promote crystallization (and as a consequence melting) of the [beta]-crystalline structure, although the melting behavior was doubled.

The overall crystallization kinetics was affected by the amount of nanoclay and by the isothermal crystallization temperature. It was corroborated that nanoclays act as nucleating agents of iPP and that secondary crystallization is enhanced with nanoclay concentration. Changes in the growth geometry in the presence of naoclays were not significant.

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F.J. Medellin-Rodriguez, (1) J.M. Mata-Padilla, (1) B.S. Hsiao, (2) M.A. Waldo-Mendoza, (1) E. Ramirez-Vargas, (3) S. Sanchez-Valdes (3)

(1) CIEP-FCQ, UASLP., Av. Dr. Manuel Nava 6, Zona Universitaria, San Luis Potosi, S. L. P., 78210, Mexico

(2) Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400

(3) Centro de Investigacion en Quimica Aplicada, Blvd. Enrique Reyna 140 Saltillo Coahuila, Mexico

Correspondence to: F.J. Medellin-Rodriguez; e-mail: francmr@uaslp.mx

Contract grant sponsor: National Research Council of Science and Technology of Mexico (CONACYT); contract grant numbers: 39638-Y, U40177-Y; Contract grant sponsors: Fund for Research Support (FAI) of UASLP, Concurrent Fund (FRC) of UASLP, Academic Group CA9 of FCQ/UASLP.
TABLE 1. WAXD crystallinity of iPP homopolymer and the indicated
nanocomposites.

Sample %[X.sub.c]

iPP Homopolymer 63.6
iPP+2 wt% nanoclay 65
iPP+4 wt% nanoclay 63.8
iPP+6 wt% nanoclay 63.2

TABLE 2. Transmittance and ratio between IR bands 998 and 973
[cm.sup.-1] of iPP homopolymer and the indicated nanocomposites.

 [T.sub.998]/
Sample [T.sub.998] (%) [T.sub.973] (%) [T.sub.973]

iPP Homopolymer 31.58 40.13 0.787
iPP+2 wt% nanoclay 33.55 40.13 0.836
iPP+4 wt% nanoclay 39.47 39.47 1.000
iPP+6 wt% nanoclay 41.78 38.16 1.095

TABLE 3. Relative induction time of crystallization obtained from the
nonisothermal crystallization DSC traces of iPP homopolymer and the
indicated nanocomposites.

Sample Relative induction time

iPP Homopolymer 7'36"
iPP+2 wt% nanoclay 7'24"
iPP+4 wt% nanoclay 7'12"
iPP+6 wt% nanoclay 7'10"

TABLE 4. Avrami indices of iPP nanocomposites isothermally crystallized
at (a) 125[degrees]C and (b) 134[degrees]C.

 Experimental
 Avrami index (a); (b) Growth Growth
Sample (theoretical) (a) Nucleation type geometry control

Homopolymer 3.1; none(3) Instantaneous Sphere Interface
2% clay 3.2; 3.2(3) Instantaneous Sphere Interface
4% clay 3.2; 2.9(3) Instantaneous Sphere Interface
6% clay 2.9; 3.2(3) Instantaneous Sphere Interface

(a) Schultz [28].
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Author:Medellin-Rodriguez, F.J.; Mata-Padilla, J.M.; Hsiao, B.S.; Waldo-Mendoza, M.A.; Ramirez-Vargas, E.;
Publication:Polymer Engineering and Science
Article Type:Technical report
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Date:Nov 1, 2007
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