Strategies for Optimizing Polypropylene−Clay Nanocomposite

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Strategies for Optimizing Polypropylene-Clay Nanocomposite Structure Darrell Marchant and Krishnamurthy Jayaraman* Department of Chemical Engineering and Materials Science, 2527 Engineering Building, Michigan State University, East Lansing, Michigan 48824

The structure of nanocomposites produced by melt-mixing polypropylene and 5 wt % of organically modified layered silicates with varying amounts of different maleated polypropylene compatibilizer grades has been analyzed with X-ray diffraction, transmission electron microscopy (TEM), and melt rheology. The extent of delamination in different nanocomposites has been quantified from analysis of several TEM images for each specimen by a product of the single particle volume fraction and the intrinsic viscosity of such particles. This measure correlated directly with the low shear relative viscosity of the molten composites (relative to the silicate free mixture of polypropylene and compatibilizer). The results also indicate that the acid number alone is not a good predictor of compatibilizer effectiveness, i.e., the amount required for exfoliation. The molar ratio of functional groups to compatibilizer chains is found to be a better parameter for ranking compatibilizer effectiveness than the acid number, which is a weight ratio. With higher values of the molar ratio, lower concentrations of compatibilizer are required for significant exfoliation. With the highest molar ratio compatibilizer, an optimum compatibilizer concentration of about 10 wt % leads to a composite with the most exfoliated structure. 1. Introduction The dispersion of layered silicates, a natural form of many types of clay, in thermoplastic matrices is of great interest in the polymer industry. This interest stems from the fact that nanolayer-reinforced polymers can exhibit greatly enhanced mechanical, thermal, and gas barrier properties at low weight fractions.1,2 These enhancements are a consequence of the large surfaceto-volume ratio of the nanoscopic particles. For example, montmorillonite clay exists naturally in a tactoid structure comprised of several tens of stacked layers3 with a typical lateral dimension of 100-200 nm, a layer thickness of 1 nm, and an interlayer spacing of about 1 nm. While the tactoid aspect ratio is of order 1, the individual nanolayer aspect ratio is 100-200. If the matrix polymer infiltrates the space between layers while preserving the layered structure, we obtain an intercalated nanocomposite. Dispersal of the nanolayers produces a disordered structure or an exfoliated nanocomposite with the desired filler aspect ratio. The closely stacked structure of silicate layers in clay and the chemical incompatibility between the clay surface and the polymers pose significant challenges in the production of polymer-clay nanocomposites. The chemical incompatibility may be overcome by organically modifying the clays, i.e., exchanging the hydrophilic cations on the clay surfaces with short-chain organic cations such as alkylammonium ions. This exchange also leads to a larger basal spacing between the layers because of the bulkiness of the organic ions. In addition to modifying the clay, it is frequently necessary to blend in a functionalized polymer that wets the modified clay surface more readily while being miscible with the bulk polymer, especially when the bulk polymer is nonpolar. This has been established by * Corresponding author. E-mail: [email protected]. Phone: (517) 355-5138. Fax: (517) 432-1105.

the work of several investigators, especially Okada and co-workers4,5 and Reichert et al.6 with polypropylene (PP) and Ishida et al.7 with a wide range of polymers and modifiers. It is important to have a significant level of polar functionality that can interact with the silicate structure. Okada and co-workers4,5 showed that polymer nanocomposites can be produced by melt-mixing the modified clay with the functionalized polymer and the bulk polymer. They added 3 times as much maleated PP (PPMA) as the clay by weight in preparing well-mixed PP-clay nanocomposites with a disordered or exfoliated structure. The acid number (AN) of PPMA is defined as the amount of potassium hydroxide that will react with the anhydride groups in PPMA (mg of KOH/g of PPMA). Kato et al.5 concluded from their results with the 3:1 proportion of PPMA to clay that PPMA should have an AN of more than 7 to be effective. They also pointed out that an AN ) 52 compatibilizer at this loading leads to phase separation of the compatibilizer from the bulk PP. Reichert et al.6 also used a fixed 20 wt % PPMA compatibilizer throughout their study. It is well-known that functionalization with maleic anhydride leads to chain scission of the PP so that large amounts of low or modest molecular weight PPMA will lower the mechanical properties of the final composite drastically.8 Hence, it is important to investigate the effect of smaller compatibilizer loading on the structure of the composite experimentally for various degrees of chain functionalization. There are difficulties in structural characterization also that need to be resolved, especially for exfoliated nanocomposites. Both X-ray diffraction (XRD) and transmission electron microscopy (TEM) have limitations. The XRD technique is most suited for intercalated structures and interrogates regions close to the surface of the sample. Distinct peaks in the XRD patterns may be used to determine the spacing between the clay

10.1021/ie011022c CCC: $22.00 © 2002 American Chemical Society Published on Web 07/30/2002

Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6403 Table 1. Characteristics of Compatibilizers compatibilizer E-43 G-3015 G-3003

AN

Mn

Mw

mol of MA/mol of PPMA

45 15 8

3 900 24 800 27 200

9 100 47 000 52 000

3.1 6.3 4.1

layers, and the area under the peak provides an index of the concentration of intercalated structures. TEM is a useful complement, especially when distinct peaks are missing from XRD patterns, but this technique focuses on a much smaller area than the area probed by the XRD test. This technique also requires extensive sample preparation. For these reasons, a more accessible and global characterization technique is desired. Solid-state mechanical properties of the composite such as the modulus have been used as an indirect measure of the extent of dispersion; these properties reflect not only the state of dispersion but also the improvement in phase adhesion brought about by the compatibilizer. The rheology of polymer nanocomposites has been investigated by Krishnamoorti and co-workers,3,9 Solomon et al.,10 and Galgali et al.11 with particular emphasis on intercalated nanocomposites; for these systems, the dynamic storage modulus curves are quite sensitive to the degree of intercalation and to flowinduced alignment. It is shown in the present paper that for polymer nanocomposite samples with a more delaminated and disordered particle structure the low shear melt viscosity, relative to that of the silicate-free melt, provides a more representative index. The object of this work is to examine the effect of adding varying amounts of different PPMA compatibilizers in melt mixing of PP and clay and to develop a new measure of the extent of delamination in nanocomposites. The questions addressed in the present paper are as follows. Is it possible to develop a representative index of the extent of delamination from rheological measurements? Do we need to add such a high level of compatibilizer to get good delamination and dispersion? Is the total maleic anhydride functionality for a fixed amount of clay (5 wt %) a useful guide to determining required levels of compatibilizer with different grades? Will the extent of delamination and the dispersion of nanolayers continue to improve with increasing ratio of compatibilizer to clay for different AN grades of PPMA? 2. Experimental Details 2.1. Materials. The starting material for the clay used in this study is sodium montmorillonite with a cation-exchange capacity (CEC) between 140 and 150 mequiv/100 g. This material is modified by the exchange of 95-98% of the sodium cations in the clay gallery with octadecylammonium ions. The modified clay, referred to as C18-M in the rest of the paper, was used as received from Nanocor and has a specific gravity of 1.9 and a gallery spacing of 2.3 nm. The PP matrix is Basell Polyolefin’s PP 6323, which has a weight-average molecular weight of 280 000 and a melt flow index of 12. In view of the study by Kawasumi et al.,4 three grades of maleic anhydride modified PP (PPMA) compatibilizers with ANs of 45, 15, and 8 have been chosen from a set of Eastman Epolenes, and the number-average and weight-average molecular weights are identified in Table 1. These specifications have been drawn from publications by Eastman Chemical Co. and confirmed by personnel from Eastman Chemical. The molar ratio

Table 2. Compatibilizer Loading for Fixed Ratios of Maleic Anhydride Functionality to Clay wt ratio of maleic anhydride to C18-M

E-43 wt %

G-3015 wt %

G-3003 wt %

0.0681 0.050 0.025

4.3 3.2 1.6

13.6 10.0 5.0

22.7 16.7 8.3

Table 3. Composition of Various Composite Samples by Weight sample ID PPCN0 PPCN1 PPCN2 PPCN3 PPCN4 PPCN5 PPCN6 PPCN7 PPCN8 PPCN9

E-43 wt %

G-3015 wt %

G-3003 wt %

C18-M wt %

PP 6323 wt %

8.3 16.7 22.7

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

95.0 93.4 91.8 90.7 90.0 85.0 81.4 86.7 78.3 72.3

1.6 3.2 4.3 5.0 10.0 13.6

of polar functionality to PPMA has been evaluated and presented in the last column; this is significant for the discussion of results presented here. These materials were used to prepare 10 different composites with 5 wt % clay (or 2 vol % in the melt state). One of them is a control material comprised of just PP 6323 and the clay. The amounts of compatibilizer in the nine other composites were chosen to provide three different weight ratios of total maleic anhydride functionality to clay. This was done to examine whether the concentrations required of different compatibilizers to get equivalent extents of delamination might scale inversely with the AN. The compatibilizer concentrations are lower than those used in previous work by previous researchers, to pursue some of the questions posed above. The selected ratios by weight of total maleic anhydride to clay are listed in Table 2 with the corresponding ratios of PPMA to clay for the three different grades. Table 3 shows the composition of all 10 composites that were prepared here. 2.2. Melt Mixing. These composites were prepared by melt compounding. A dry batch of 47 g containing all three components in powder or pellet form was first combined in a bag, and the bag was shaken to ensure that the materials were well dispersed. The mixture was then added to a laboratory-scale Banbury mixer, where it was compounded for 10 min at 180 °C and 150 rpm while purging the mixer with nitrogen. Providing a sufficient residence time is key to obtaining adequate dispersion and delamination, as shown by the work of Dennis et al.12 and Fornes et al.13 Indeed Fornes et al. have suggested the sliding of platelets subjected to sufficient shear as a mechanism for exfoliation. The compounded material was then put into a granulator to create uniform pieces small enough to compression mold. 2.3. Structural Characterization. The characterization of the structure by XRD was performed on a compression-molded disk with a Rigaku Rotaflex Ru200BH X-ray diffractometer. The diffractometer was equipped with a Ni-filtered Cu KR radiation source and operated at 45 kV and 100 mA. The emitted radiation had a wavelength of 1.54 Å. The sample was scanned over a 2θ range of 0.5-15° at a rate of 0.5°/min, and measurements were recorded at equal increments of 0.01°. When the XRD pattern exhibits a peak, it

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Figure 1. XRD patterns of composites without PPMA and with two different loadings of the AN ) 45 compatibilizer: (A) PPCN3, (B) PPCN1, (C) PPCN0, and (D) C18-M.

indicates that there exists a significant fraction of material with a regular structure such as clay layers in a tactoid structure. The corresponding spacing between the clay layers can then be ascertained by applying Bragg’s diffraction law. If an intercalated structure is formed, the concentration of the intercalated clay layers with a given spacing is proportional to the area under the corresponding XRD peak.14 The desired result, however, is a delaminated structure that would be indicated by a diffraction pattern without any peaks. The nanostructure of the composite was viewed in a JEOL 100 CX transmission electron microscope with a 120 keV electron accelerator. Sections of 90 nm were prepared from disks that were molded for the rheological characterization. The sections were prepared using a cryomicrotoming procedure, where the sample was held at a constant temperature of -125 °C. The cryogenic procedure was necessary to provide adequate rigidity to the polymer in order to produce uniform thin sections required to obtain clear reproducible images. Each image covered 2.29 µm2 and was analyzed using an image analysis software package (SigmaScan). The total area analyzed for each sample varied because of variations in the particle distribution and the need to analyze roughly the same number of particles (about 150) for each sample. 2.4. Rheological Characterization. The granules were compression-molded in a hydraulic press at 200 °C and 20 tons of pressure for 15 min, into disks that were 50 mm in diameter and 1.0-1.2 mm in thickness. The samples were subjected to oscillatory shear in a Rheometrics RMS 800 over a frequency range of 0.05100 rad/s. A strain amplitude of 2% was applied during the measurement, and the data were recorded only when the torque amplitude was above the recommended threshold of 2 g‚cm. The measurements were recorded within 30 min of loading the sample on the platen and were reproducible. A strain sweep was performed to verify that 2% strain was within the linear viscoelastic regime. This would also ensure that the structure was unaltered during the tests. It has been found with nanocomposites that larger amplitude shear will align the particles3,9 and thus defeat the intended purpose. 3. Results and Discussion 3.1. XRD Patterns. An XRD pattern of the composite prepared without any compatibilizer is presented in Figure 1, along with XRD patterns for two of the composites prepared with the AN ) 45 compatibilizer and for the organoclay itself. The basal spacing of the C18-M clay is 2.3 nm before compounding, as seen from its XRD pattern. Each of the three patterns for the nanocomposites shows a distinct intercalation peak,

Figure 2. XRD patterns of composites prepared with three different loadings of the AN ) 15 compatibilizer: (A) PPCN6, (B) PPCN5, (C) PPCN4, and (D) C18-M.

Figure 3. XRD patterns of composites prepared with three different loadings of the AN ) 8 compatibilizer: (A) PPCN9, (B) PPCN8, (C) PPCN7, and (D) C18-M.

with the basal spacing increasing progressively from 3.05 nm for the uncompatibilized composite to 3.71 nm for the composite with nearly 5 wt % AN ) 45 compatibilizer. The increase of the basal spacing obtained even without any compatibilizer indicates that adequate shear was imposed over a sufficient residence time during the mixing process employed here. Because of the high acid value, the miscibility of this PPMA with bulk PP was checked by looking at optical micrographs taken under polarized light of the mixture of PP and PPMA that forms the matrix for the PPCN3 material, above the melting point with 180× magnification. These optical micrographs showed no visible phase separation. Reichert et al.6 have reported intercalated structures in PP-clay nanocomposites prepared with 5 wt % C12 modified SOMASIF clay and as much as 20 wt % E-43 compatibilizer (although the molecular weight and anhydride content they report for the E-43 compatibilizer are off from the values listed in the company literature and in this paper). Figure 2 presents XRD patterns for composites prepared with various amounts of the AN ) 15 compatibilizer, along with the pattern for the organoclay by itself. With 5 wt % compatibilizer, the pattern shows a slight peak with a small area; the pattern with the 10 wt % compatibilizer is the most featureless, indicating perhaps the greatest degree of exfoliation, while the pattern with the highest amount shows a distinct shoulder. It appears likely that all three of these composites have significant extents of exfoliation. The structure obtained by using the AN ) 8 PPMA is quite sensitive to the concentration of the compatibilizer added, as illustrated in Figure 3. The sharpness of the peak and the peak area diminish with increasing compatibilizer loading, and the pattern for the highest loading shows only a shoulder at the same location. This indicates that the structure changes from a noticeably intercalated structure to a more exfoliated structure as the AN ) 8 compatibilizer concentration increases from 8.3 to 22.7 wt %. Figures 1-3 taken together indicate

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Figure 4. Transmission electron micrograph of the PPCN4 composite with 5 wt % AN ) 15 compatibilizer.

Figure 6. Transmission electron micrograph of the PPCN6 composite with 13.6 wt % AN ) 15 compatibilizer.

Figure 5. Transmission electron micrograph of the PPCN5 composite with 10 wt % AN ) 15 compatibilizer.

Figure 7. Transmission electron micrograph of the PPCN9 composite with 22.7 wt % AN ) 8 compatibilizer.

that the AN of the compatibilizer alone does not correlate directly with the effectiveness of the compatibilizer in producing delaminated structures. The molar ratio of functional groups to compatibilizer chains presented in Table 1 appears to be a better indicator: it is lowest for the E-43 compatibilizer and highest for the AN ) 15 compatibilizer. Further elucidation of the structure is required to differentiate among the composites displaying patterns with shoulders; this has been accomplished with image analysis of transmission electron micrographs. 3.2. TEM Analysis. Transmission electron micrographs at a magnification of 50 000× are presented in Figures 4-7 respectively for the composites which appear from the XRD results to be significantly exfoliated: PPCN4, PPCN5, PPCN6, and PPCN9. It is readily seen that Figures 5 and 7 have the maximum number of distinct particle structures in a single image. Staggered arrangements of platelets sliding out of a stack when subjected to shear are seen in both of these figures, like the formation termed “skewed stack” by Fornes et al.13 in their analysis of nylon-6-layered silicate nanocomposites. We have identified three different types of particle structures on several such images of these four composites; these are illustrated schematically in Figure 8. A structure where no gaps can be discerned and that is at least half a particle length away from neighboring particles is termed a single particle. The orientation of a single particle

Figure 8. Different types of particle structures observed in the TEM images.

relative to the cutting direction will change the projected width or thickness of the particle. A stack refers to a structure containing particles arranged with significant face-to-face overlap and several discernible gaps ranging from 3 to 20 nm. The remaining structures can be categorized as agglomerates. A quantitative comparison of the degree of delamination in these composites can be obtained from an analysis of the fraction of single particles and the distribution of lengths of such particles in these samples. Particles were catalogued and counted in several images for each composite so that a total of close to 150 particles were counted for each composite. Four images were required for the composite with 5 wt % AN ) 15 compatibilizer and two or three images for the rest. As

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Figure 9. Length distributions of single particles for composites with three different concentrations of the AN ) 15 compatibilizer: (- -) PPCN4, (s) PPCN5, and (- - -) PPCN6.

Figure 10. Length distributions of single particles for composites with the AN ) 15 and 8 compatibilizers: (s) PPCN5 and (- -) PPCN9.

Table 4. Distribution of Particle Structures from TEM Image Analysis

Table 5. Structural Parameters and Relative Viscosity for Nanocomposites

volume fraction of number fraction particle population sample ID single stacked agglomerate single stacked agglomerate

estimated volume average aspect [η]IφI from single ratio of single fraction φI of particles ηrel single particles particles sample ID

PPCN4 PPCN5 PPCN6 PPCN9

0.50 0.71 0.57 0.57

0.12 0.15 0.36 0.23

0.38 0.14 0.07 0.20

0.2 0.5 0.2 0.3

0.2 0.2 0.4 0.32

0.6 0.3 0.4 0.38

a result of the detailed image analysis, the number fractions of different structures in the particle population of the four different composites have been presented in Table 4. The ratio of particle volumes present in each of three types of structures to the total particle volume for each composite has been estimated from the ratio of projected areas for the respective particles and is also presented in Table 4. Similar trends are seen in both measures. Each of these four composites had at least 50% by number and 20 vol % of single particles. The greatest fraction of single particles was obtained in the composite made with 10 wt % AN ) 15 compatibilizer. The composite with 13.6 wt % AN ) 15 compatibilizer had the largest fraction of stacked particles in addition to a substantial fraction of single particles. The composite made with 5 wt % AN ) 15 compatibilizer had the largest fraction of agglomerates. The distribution of single particle lengths relative to the total particle population is presented for the three composites prepared with the AN ) 15 compatibilizer in Figure 9. The smallest resolvable dimension on the TEM images is over 2 nm; hence, the recorded lengths would have an error bound of 2 nm. It is clear that the greatest extent of exfoliation is obtained in PPCN5, the composite prepared with 10 wt % AN ) 15 compatibilizer. Figure 10 presents a comparison of the single particle length distribution for PPCN5 with PPCN9, the composite prepared with 22.7 wt % AN ) 8 compatibilizer; this comparison indicates that PPCN5 has the more delaminated structure. In the following discussion, we have calculated the mean aspect ratios of single particles or platelets from these distributions, taking the thickness of the clay platelets to be 1 nm (cf. Grim15). The mean aspect ratios of single particles are presented in Table 5 along with the estimated volume fraction of such particles for four composites. 3.3. Rheology and Microstructure. The particle aspect ratio has a strong influence on the key properties of suspensions. In particular, the intrinsic viscosity of a suspension of square platelets of side b and thickness

PPCN4 PPCN5 PPCN6 PPCN9

0.004 0.010 0.004 0.006

69 79 69 65

0.12 0.34 0.12 0.17

2.4 4.0 2.6 3.4

t is proportional to the aspect ratio (b/t) for aspect ratios greater than 20. This has been confirmed experimentally by Luciani et al.,16 who recorded the relative viscosity of dilute suspensions of randomly aligned, square, model platelets having aspect ratios ranging from 5 to 170 in a viscous medium. They found that, for aspect ratios greater than 20, the intrinsic viscosity [η] of the platelets, expressed in dimensionless terms, is linearly related to the particle aspect ratio p with a slope of 4/3π.

[η] =

(3π4 )p

(1)

This is valid when the particles are randomly aligned. Utracki et al.17 have reported a similar linear relation between the intrinsic viscosity and the average aspect ratio for mica flakes with a polydisperse size distribution suspended in PP melts. The overlap particle volume fraction φ*, at which hydrodynamic particle interactions influence the viscosity of the suspension, is inversely proportional to the intrinsic viscosity and thus to the aspect ratio as well. Utracki et al.17 have correlated the relative viscosity of nondilute suspensions with an exponential function of the product of intrinsic viscosity and particle volume fraction. Other relations for relative viscosity of concentrated suspensions also involve the same product (cf. Goodwin and Hughes18). Although the composites in the present study contain other types of particle structures in addition to the single particles, the single-particle contribution should be the greatest. Hence, the product of the intrinsic viscosity and volume fraction from the single particles in each of the four composites is presented in Table 5. This product represents the contribution of these particles to the relative viscosity of the molten composites, relative to the silicate-free melts. Figure 11 presents example plots of the magnitude of complex viscosity against frequency for some composites and the corresponding silicate-free melts with

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Figure 11. Complex viscosity curves for four composites and the corresponding silicate-free mixtures of PP and PPMA: (a) PPCN0, (b) PPCN2, (c) PPCN5, and (d) PPCN9.

PP and PPMA in the same proportion as the composites. The composites prepared without compatibilizer and with 3.2% E-43 compatibilizer have melt viscosity curves very close to the corresponding matrix viscosity curves. Recalling that the XRD patterns for these samples revealed predominantly intercalated structures, the effect of intercalated structures on the complex viscosity curve is seen to be modest. The viscosity curves for two other composites presented in parts c and d of Figure 11 show large enhancements in the complex viscosity magnitude at low frequencies, relative to the corresponding silicate-free melts. These curves represent the effect of exfoliated structures in PPCN5 and PPCN9. A low shear relative viscosity can be determined from the ratio of the viscosity for the composite and the silicate-free melt at frequencies corresponding to a fixed low value of shear stress or |G*|. This has been evaluated at a magnitude of |G*| ) 600 Pa.

ηrel )

|ηc*|(ωc)

]

|ηm*|(ωm)

(2)

|G*| fixed

The low shear relative viscosity is tabulated for the four composites in Table 5 along with the product of intrinsic viscosity and volume fraction of single particles present in these composites. The exfoliated structure with the highest fraction of randomly aligned single particles turns out to have the highest melt viscosity relative to the silicate-free melt containing both PP and PPMA in the same proportion. More precisely, Table 5 reveals that the relative viscosity follows the same trend as the product [η]IφI identified from the structural analysis. Hence, the relative viscosity defined here may be used as a measure of the extent of delamination. The low-frequency relative viscosity is plotted against the compatibilizer concentration in Figure 12 for composites prepared with the AN ) 8 and 15 compatibilizers. The curve for the AN ) 8 compatibilizer shows a monotonic increase with the compatibilizer concentration. The other curve for AN ) 15 displays higher extents of delamination at lower concentrations as well as a maximum. The optimum concentration of this

Figure 12. Effect of compatibilizer loading on the relative viscosity of nanocomposites prepared with two different compatibilizers: (A) AN ) 15 and (B) AN ) 8.

compatibilizer is about 10 wt % and has the most exfoliated structure. This trend is also apparent in the product [η]IφI tabulated in Table 5 that was obtained from image analysis of the micrographs. The drop in extent of exfoliation above a certain loading of this compatibilizer is accompanied by a sharp increase in the fraction of stacked particles with face-to-face overlap (see Table 4). This could be caused by a combination of two factors. Concentrations greater than a threshold of the high-functionality compatibilizer between platelets could give rise to an attractive force between the platelets similar to the results of computations by Balazs et al.19 Also, when the PPMA content of the melt outside the particles is increased beyond a certain amount, lower shear stresses would be imposed in the mixing process. The question of how much of the added PPMA is segregated within the galleries and how much is mixed with the bulk PP during the mixing process is unresolved and must be investigated with further research to examine these factors quantitatively. 4. Conclusions This study reveals important effects of compatibilizer characteristics and concentrations on the nanostructure

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of PP composites with organically modified layered silicates. The extent of delamination in different nanocomposites has been quantified from analysis of several TEM images for each specimen by a product of the single particle volume fraction and the intrinsic viscosity of such particles. This product correlates directly with the ratio of the low shear complex viscosity magnitude for the molten composite to that of the silicate-free mixture. Thus, the low-frequency relative viscosity has been shown to be an effective rheological probe for distinguishing between structures with different extents of exfoliation. The molar ratio of functional groups to compatibilizer chains is a better parameter for ranking the compatibilizer effectiveness than the AN, which is a weight ratio. With higher values of the molar ratio, lower concentrations of compatibilizer are required for significant exfoliation. With the highest molar ratio compatibilizer (AN ) 15), an optimum compatibilizer concentration of about 10 wt % leads to a composite with the most exfoliated structure. The last observation is a subject for further research. Acknowledgment This research was supported in part by a GAANN award from the U.S. Department of Education and by a grant from Ford Motor Co. The authors are pleased to acknowledge donations of material by Nanocor and Eastman Chemical Co., expert assistance with cryomicrotoming from Dr. Richard Shalek of Michigan State University, and discussions with Dr. T. Lan of Nanocor and with Professors T. Pinnavaia, P. M. Duxbury, and L. T. Drzal of Michigan State University. Nomenclature b ) radius of the clay tactoid G* ) complex dynamic shear modulus, mL-1 t-2 t ) thickness of the clay tactoid, L Mn ) number-average molecular weight, M Mw ) weight-average molecular weight, M n ) number density of clay particles in the composite p ) mean particle aspect ratio [η] ) intrinsic viscosity ηc* ) complex viscosity of the composite, mL-1 t-1 ηm* ) complex viscosity of the matrix, mL-1 t-1 ηrel ) relative viscosity θ ) diffraction angle φ ) volume fraction of clay in the composite φ* ) overlap particle volume fraction ωc ) frequency where the magnitude of G* is 600 Pa for the composite, t-1 ωm ) frequency where the magnitude of G* is 600 Pa for the silicate free mixture, t-1

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Received for review December 17, 2001 Revised manuscript received May 31, 2002 Accepted June 6, 2002 IE011022C