Scholarly article on topic 'Polyolefin/layered silicate nanocomposites with functional compatibilizers'

Polyolefin/layered silicate nanocomposites with functional compatibilizers Academic research paper on "Chemical sciences"

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European Polymer Journal
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{"Polyolefin nanocomposites" / "Layered silicates" / "Polymeric compatibilizer" / Intercalation / Exfoliation}

Abstract of research paper on Chemical sciences, author of scientific article — K. Chrissopoulou, S.H. Anastasiadis

Abstract Polymer nanocomposites containing layered silicates have been considered as a new generation of composite materials due to their expected unique properties attributed to the high aspect ratio of the inorganic platelets. Nevertheless, addition of layered silicates to polyolefins mostly results in phase separated systems because of the incompatibility of the silicates with the non-polar polyolefins. Functional compatibilizers are required to enhance the interactions and alter the structure from phase separated micro-composites to intercalated and exfoliated nanocomposites. Commercial macromolecular compatibilizers (mainly maleic-anhydride-functionalized polyolefins) are most commonly used to improve the interfacial bonding between the fillers and the polymers whereas specifically synthesized functional homopolymers or copolymers have been utilized as well. In this article, we are reviewing a number of investigations, which studied the influence on the composite structure of various parameters like the compatilizer to inorganic ratio, the type and content of the functional groups and the molecular weight of the functional additive, the miscibility between the matrix polymer and the compatibilizer, the kind of surfactants modifying the inorganic surface, the processing conditions, etc. The most important results obtained utilizing maleic-anhydride-functionalized polyolefins are discussed first, whereas a summary is presented then of the studies performed utilizing other functional oligomers/polymers. X-ray diffraction and transmission electron microscopy studies supported by rheology indicate that the most important factor controlling the structure and the properties is the ratio of functional additive to organoclay whereas the miscibility between the matrix polymer and the compatibilizer is a prerequisite.

Academic research paper on topic "Polyolefin/layered silicate nanocomposites with functional compatibilizers"

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Feature Article - Macromolecular Nanotechnology

Polyolefin/layered silicate nanocomposites with functional compatibilizers

K. Chrissopouloua, S.H. Anastasiadis

a Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, P.O. Box 1527, 711 10 Heraklion Crete, Greece b Department of Chemistry, University of Crete, 710 03 Heraklion Crete, Greece


Article history:

Available online 1 October 2010

Dedicated to Professor Nikos Hadjichristidis on the occasion of his retirement from the Department of Chemistry of the University of Athens.


Polyolefin nanocomposites Layered silicates Polymeric compatibilizer Intercalation Exfoliation


Polymer nanocomposites containing layered silicates have been considered as a new generation of composite materials due to their expected unique properties attributed to the high aspect ratio of the inorganic platelets. Nevertheless, addition of layered silicates to polyolefins mostly results in phase separated systems because of the incompatibility of the silicates with the non-polar polyolefins. Functional compatibilizers are required to enhance the interactions and alter the structure from phase separated micro-composites to intercalated and exfoliated nanocomposites. Commercial macromolecular compatibilizers (mainly maleic-anhydride-functionalized polyolefins) are most commonly used to improve the interfacial bonding between the fillers and the polymers whereas specifically synthesized functional homopolymers or copolymers have been utilized as well. In this article, we are reviewing a number of investigations, which studied the influence on the composite structure of various parameters like the compatilizer to inorganic ratio, the type and content of the functional groups and the molecular weight of the functional additive, the miscibility between the matrix polymer and the compatibilizer, the kind of surfactants modifying the inorganic surface, the processing conditions, etc. The most important results obtained utilizing maleic-anhydride-functionalized polyolefins are discussed first, whereas a summary is presented then of the studies performed utilizing other functional oligomers/ polymers. X-ray diffraction and transmission electron microscopy studies supported by rheology indicate that the most important factor controlling the structure and the properties is the ratio of functional additive to organoclay whereas the miscibility between the matrix polymer and the compatibilizer is a prerequisite.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Composite materials that consist of a polymer matrix and various inorganic fillers have been widely used in order to improve the mechanical, thermal, barrier, and other properties of the polymer. However, important compromises are often required in material design since, for example, an increase in strength is often accompanied by

* Corresponding author at: Department of Chemistry, University of Crete, 710 03 Heraklion Crete, Greece. Tel.: +30 2810 391466; fax: +30 2810 391305.

E-mail address: (S.H. Anastasiadis).

0014-3057/$ - see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.09.028

a loss in toughness and/or a loss of optical clarity. It is anticipated, that such problems can be overcome if the inorganic additive exists in the form of a fine dispersion within the polymeric matrix producing a nanocomposite

[1]. In these cases, the final properties of the hybrid are determined mainly by the existence of many interfaces

[2]. Moreover, the addition of highly anisotropic nanoscop-ic fillers to polymer matrices is even more interesting for the polymer industry because the large surface-to-volume ratio of the high-aspect-ratio additives can lead to enhanced reinforcement.

A special case of nanocomposites is obtained [3-9] by mixing polymers with layered silicates (nanoclays); three

different kinds of structure are identified depending on the interactions between the polymer and the inorganic particles: phase separated, where the polymer and the inorganic are mutually immiscible, intercalated, where the polymer chains reside between the layers of the inorganic material, and exfoliated, where the periodic stacking of the inorganic material is destroyed and the inorganic platelets are dispersed within the polymeric matrix. Obtaining the optimum structure is highly desirable since it controls the properties of the micro- or nano-composites. In such nanocomposites various properties like strength and heat resistance [10], toughness [11], gas permeability [12-15], flammability [16,17] and biodegradability [18] are significantly enhanced. It has been anticipated that large surface area, high aspect ratio and good interfacial interactions are essential to fully exploit the advantages of polymer/layered silicate nanocomposites; thus, the silicate should be exfoliated and the platelets should strongly adhere to the polymer [19,20].

The layered silicate particles are usually hydrophilic and their interactions with non-polar polymers are unfavorable. Thus, hydrophilic polymers are able to intercalate within Na-activated montmorillonite clays [21-25], whereas hydrophobic polymers can result in intercalated [26-29] or exfoliated [30] structures only with organo-philic clays, which are produced when proper cationic surfactants (e.g., alkylammonium) have replaced the hydrated Na+ ions within the galleries by a cation exchange reaction. Enthalpic and entropic contributions to the free energy [31-33] have been considered in discussing the thermodynamics of intercalation or exfoliation. It has been recognized that the entropy loss due to the confinement of chains within the galleries is compensated by the entropy gain associated with the increased conformational freedom of the surfactant tails as the interlayer distance increases upon polymer intercalation [31,34], whereas the favorable enthalpic interactions are extremely critical in determining the nanocomposite structure [35].

Polyolefins constitute the most widely used group of commodity thermoplastics. They are prepared by polymerization of simple olefins such as ethylene, propylene, butenes, isoprenes, and pentenes, as well as their copoly-mers, whereas they are the only class of macromolecules which can be produced catalytically with precise control of stereochemistry and, to a large extent, of (co)monomer sequence distribution. An inherent characteristic common to all polyolefins is a nonpolar, nonporous, low-energy surface that is not receptive to inks, and lacquers without special oxidative pretreatment. Polyolefin-based materials can be tailor-made for a wide range of applications: from rigid thermoplastics to high-performance elastomers. These vastly different properties are achieved by a variety of molecular structures, whose common features are low cost, excellent performance, long life cycle and ease of recycling.

Since the first mass production of polyolefins with the development of Ziegler-type catalysts, commercial exploitation has been very rapid because of their attractive characteristics. However, polyolefins are notch sensitive and brittle on exposure to severe conditions, such as low temperature or high rate of impact. In order to improve the competitiveness of polyolefins in engineering applications,

it is important to simultaneously increase stability, heat distortion temperature, stiffness, strength and impact resistance without sacrificing their processability. Modification of the polymers by the addition of fillers, reinforcements, or blends of special monomers or elastomers can render them more flexible with a variety of other properties, and their competitiveness in engineering resin applications can be greatly improved [36-38].

A great number of research publications have appeared reporting on efforts to develop intercalated or exfoliated nanocomposite structures with polyolefins like polyethylene [39-44] or polypropylene [17,20,45-59] with moderate success. It has been recognized that, due to the strong hydrophobic character of the polymers and the lack of favorable interactions with the silicate surfaces, polyethylene or polypropylene lead to phase separated systems even when mixed with hydrophobically modified clays. It has, thus, been clear that for the synthesis of polyolefin/layered silicate nanocomposites one has to modify the interactions between the polymer and the inorganic surfaces. Synthetic efforts have focused on the introduction of functional groups to the polyolefin chains, on altering the organoph-ilization of the inorganic or on the use of suitably functional compatibilizers. There seem to be two important factors in terms of the structure of the functional additive in order to prepare a polyolefin nanocomposite using a compatibilizer; first, it should include a certain percentage of polar groups to interact favorably with the inorganic layers via, e.g., hydrogen bonding to the oxygen groups of the silicate layers and, second, it should be miscible with the polymer. Since the content of polar functional groups of the additive will affect the miscibility, there must be an optimum content of polar functional groups in the compatibilizer.

Various compatibilizers with different functionalities have been employed and the effect of parameters like their molecular weight, the type and the content of the functional groups, the compatibilizer to organoclay ratio, the processing method, etc., have been studied in order to optimize the structure and achieve the desired dispersion of the inorganic material. Among the different compatibiliz-ers, maleic-anhydride functionalized polypropylenes [20,45-47,53-59] or polyethylenes [39-44] are most commonly used to improve the interfacial bonding between the fillers and the respective polymers, as will be reviewed below. Moreover, hydroxyl-terminated [60], hydroxyl-functional [45,52], chlorosulfonated [61], or diethyl maleate grafted [50] polypropylenes, oxidized polyethylenes [44], functionalized polyolefins with ammonium end-groups [43,62] or ammonium functionalities along the chain [43] as well as diblock copolymers with one polyole-fin and one polar block [43,45,58,63] have been utilized as well. These will be reviewed below as well.

In this feature article, we are going to refer to polyolefin/ layered silicate nanocomposites focusing on the attempts to achieve the desired nanohybrid structure. Emphasis will be placed on the effects of various types of functional additives introduced either as macromolecular surfactants or as com-patibilizers on the micro- or nano-composite structure. We will first discuss the most important results obtained utilizing maleic-anhydride-functionalized polyolefins.

Then, we will summarize studies performed utilizing other functional oligomers/polymers. This is by no means a complete review of the field of polyolefin nanocomposites since it does not discuss the effects of structure on the properties (mechanical, thermal, barrier, etc.) whereas it discusses in more detail results from the work of the authors as well as comparison with the current literature.

2. Compatibilizers based on maleic-anhydride-functionalized polyolefins

As discussed in Section 1, attempts to develop nano-composite structures with polyolefins have not been particularly successful because of the strong hydrophobic character of the polymers and the absence of attractive interactions with the inorganic surfaces; when polyethylene or polypropylene is mixed even with organophilized clays, neither intercalation of the polymer chains nor exfoliation of the silicate platelets could be achieved: phase separated micro-composite structures are obtained. This is illustrated clearly in Fig. 1, which shows X-ray diffraction (XRD) data for mixtures of an impact polypropylene copolymer (Carmel Olefins S.A.; melt flow index, MFI, 25), PP, with either a natural hydrophilic montmorillonite (Southern Clay), Na+-MMT (Fig. 1b), or with an organophil-ized analogue (Southern Clay, Cloisite20A), C20A (Fig. 1c), each containing 10 wt.% additive [59]. The diffractograms of the pure inorganic materials are shown in the respective

2000 1500 1000 500 0

2000 ^ 1500

^ 1000

6000 4000 2000


26 (o)

Fig. 1. X-ray diffraction data of (a) polypropylene (PP), (b) a hydrophilic clay, Na+-MMT (dash line) and a micro-composite with PP:Na+-MMT 90:10 (solid line) and (c) an organoclay C20A (dash line) and a microcomposite with PP:C20A 90:10 (solid line). Adopted from Ref. [59].

plots as are the XRD data for the polymer (Fig. 1a). It is noted that in all cases the mixing was performed in a 5 cm3 DSM micro-mixer and micro-extruder at 180 °C under nitrogen flow [59].

PP shows a number of diffraction peaks, the position of which and their relative intensities indicate that the corresponding structure is that of the isotactic a-form of polypropylene [64]. The data for Na+-MMT show a main peak at 20 = 8.9°, which corresponds to an interlayer distance of d001 = 9.9 A whereas the diffractogram for C20A shows its main peak at 20 = 3.4° that corresponds to d0o1 = 25.9 A. Comparison between the diffractograms of either clay with the corresponding hybrid reveals that the peaks of the silicates appear at exactly the same position following the mixing with PP. It is, thus, evident that neither intercalation of the polymer chains nor exfoliation of the clay layers has occurred with either the natural clay or the organophilized one. The immiscibility of polypropylene with a hydrophilic clay is, of course, anticipated and it is shown here as a reference. However, it is also clear that the organophilization of the clay is not sufficient to alter the situation for the case of the non-polar and very hydro-phobic PP; the composites are phase separated systems leading only to micro-composites. Additionally, the peaks corresponding to PP appear very similar with those of the pure polymer, except from two small peaks that appear at 16.0° and 19.6° that could be related to the (3 0 0) reflection of the p-phase and the (1 3 0) plane of the y-phase of polypropylene [52,64]. Similar are the results when a linear low density polyethylene (SABIC; MFI 37), PE, was utilized in the place of PP, mixed either with a hydrophilic or with an organophilic clay [43] (the mixing was performed in the 5 cm3 DSM micro-mixer at 150 °C); for all the different clay concentrations, phase separated micro-composites were obtained. It is, thus, clear that, in order to successfully synthesize polyolefin nano-composites with intercalated or exfoliated structure, an appropriate compatibilizer should be utilized, which would act as a macromolecular surfactant or would modify the surface polarity.

An additive that has been widely utilized for the synthesis of polyolefin/layered silicate nanocomposites is maleic-anhydride-functionalized polyethylene or polypropylene. It is generally believed that the polar character of the grafted maleic anhydride results in favorable interactions and, thus, a special affinity for the silicate surfaces, so that the maleat-ed polyolefins can serve as a compatibilizer between the matrix and the filler. In various studies, the effect of parameters like the molecular weight, the content of the functional groups, the compatibilizer to organoclay ratio, the processing method, etc., have been investigated to optimize the micro- or nano-structure and achieve the desired dispersion of the inorganic material.

Among such studies, the authors of the present paper and co-workers had attempted to understand the role of the maleated compatibilizer and to obtain the rules that would allow control of the micro- or nano-structure of the hybrids; such rules would concern the optimum concentration of the compatibilizer as well as the most appropriate specimen preparation procedure [43,59]. In one of these studies, a maleic anhydride grafted polypropylene, PP-g-MAH (MFI 115; maleic anhydride content wMAH ~

0.6%) was utilized as a compatibilizer for the system of PP with organoclay C20A, of Fig. 1 [59].

Binary hybrids of just the "compatibilizer" and C20A were first prepared to examine the ability of PP-g-MAH to intercalate between the layers of the inorganic material or to exfoliate the structure. Fig. 2 shows the X-ray diffraction data of these composite materials containing PP-g-MAH and C20A (solid lines in Fig. 2b-d); the diffrac-togram of the pure C20A is shown in Fig. 2a for comparison. One should focus on two different regimes of the XRD data. At high scattering angles, the diffractograms of all the composites containing PP-g-MAH show a series of scattering peaks that are very similar to the crystalline pattern of PP (Fig. 1); nevertheless, a more quantitative study of the composite crystalline structure is beyond the scope of the present review and will not be discussed further. At low angles, however, the scattering from the organosili-cates should be discussed. The diffraction peak corresponding to the periodic structure of C20A (Fig. 2a) is observed at exactly the same scattering angle even in the composite containing 60 wt.% polymer PP-g-MAH and 40 wt.% C20A; this signifies a predominantly phase separated structure; it is only the increase of the scattered intensity at low angles that indicates some percentage of platelet exfoliation. As the ratio of PP-g-MAH to organoclay increases, the characteristic peak decreases in intensity but

101 103

15 26 (o)

Fig. 2. X-ray diffraction data of (a) an organoclay C20A and (b-d) binary PP-g-MAH:C20A and ternary PP:PP-g-MAH:C20A hybrids of different compatibilizer to clay ratio, a. (b) Ternary hybrid with 10 wt.% C20A and PP:PP-g-MAH 85:15 (circles) and binary hybrid PP-g-MAH:C20A 60:40 (solid line). (c) Ternary hybrid with 10 wt.% C20A and PP:PP-g-MAH 50:50 (diamonds) and binary hybrid PP-g-MAH:C20A 80:20 (solid line). (d) Ternary hybrid with 10 wt.% C20A and PP:PP-g-MAH 15:85 (inverted triangles) and binary hybrid PP-g-MAH:C20A 90:10 (solid line). Adopted from Ref. [59].

remains at the same position and it is only for PP-g-MAH to C20A ratios 9:1 by weight and higher that the diffraction peak vanishes.

In order to prove that it is indeed the ratio of compati-bilizer to organoclay, a, that fully controls the final structure of the micro- or nano-composites, ternary mixtures of PP, PP-g-MAH and C20A were prepared in such a way that a was similar with the one in the respective binary mixtures (curves with symbols in Fig. 2b-d); here the XRD data are shown for a « 1.4 (Fig. 2b), a « 4.4 (Fig. 2c) and a « 7.7 (Fig. 2d). It is noted that the diffractograms of the three component systems are shown shifted for clarity. Comparison between each pair of curves with similar a shows that the specimens exhibit almost the same behavior, with the diffraction curves having exactly the same shape especially in the low angle regime, where the peaks corresponding to the organoclay structure appear; at high angles, the observed differences are probably due to the effect of both clay and compatibilizer to the crystallization of the polymer. For the composite with a « 1.4, the main diffraction peak appears at 20 = 3.3°, i.e., at the same position with that of the corresponding binary mixture and that of the organoclay, leading to the conclusion that the lamellar structure of the organoclay is preserved and that the system is a phase separated micro-composite. An increase of the ratio a leads to an increase of the degree of exfoliation of the system, manifested by the decrease of the intensity of the main peak and the concurrent increase at low angles [65].

It is, thus, evident that the structure is indeed determined by the value of the ratio of PP-g-MAH to C20A, a, irrespectively of the presence of the extra PP matrix polymer [59]. This result is useful especially in view of industrial applications, since it allows to first prepare a masterbatch (e.g., the binary PP-g-MAH/organoclay mixture) with the desired a and then further mixed it with PP. In this case the PP-g-MAH weakens the interactions between the organoclay layers and exfoliates its structure creating ''hairy particles" (with PP-g-MAH being the "hair" chains) that would be friendlier for the PP polymer; such hairy particles can be homogeneously mixed with PP dispersing the silicate layers even more. This explanation is in accordance with similar descriptions of the observed behavior in such systems [46].

X-ray diffraction is not the most appropriate technique to discuss exfoliation since there are other reasons for the disappearance of the peak besides the dispersion of the silicate layers [66]. Transmission electron microscopy, TEM, was, thus, utilized to verify the results of XRD. Fig. 3 shows representative TEM images of three binary PP-g-MAH/ C20A hybrids with ratios a = 1.5 (Fig. 3a), a = 4 (Fig. 3b) and a = 9 (Fig. 3c). The dark lines represent the edges of the silicate layers and the white regions the polymeric matrix. Clear differences are observed among the three systems; nevertheless, in all cases there is good dispersion of the clay particles (or platelets) and there are no particle aggregates. It is evident that coexistence is observed in Fig. 3a with clay particles retaining the layered structure and clay platelets dispersed within the polymer matrix. By increasing a, the stacking order of the layers is progressively lost up to the higher concentration of PP-g-MAH in Fig. 3c, where, uniform dispersion of exfoli-

Fig. 3. TEM images of (a) a binary hybrid PP-g-MAH:C20A 60:40, (b) a binary hybrid PP-g-MAH:C20A 80:20, (c) a binary hybrid PP-g-MAH:C20A 90:10 and (d) a ternary hybrid containing 50 wt.% of the binary PP-g-MAH:C20A 80:20 and 50 wt.% of PP. Adopted from Ref. [59].

ated platelets is observed. These results are in excellent agreement with the X-ray diffraction data. It is interesting to note that whereas the platelets in Fig. 3b apparently exhibit a more-or-less parallel orientation, probably due to the extrusion process [11], the distances between the layers are larger than the larger distance that can be measured with X-ray diffraction and, thus, the measurement indicates a "pseudo-exfoliated" structure.

Fig. 3d shows a hybrid that is prepared utilizing the PP-g-MAH:C20A binary mixture with a = 4, as a master-batch, mixed with PP in a 1:1 ratio in the micro-mixer. Thus, it derives from the specimen of Fig. 3b (same a) but it possesses the same total polymer and clay concentration as the specimen of Fig. 3c (with a = 9). It is clear that the composites prepared with the masterbatch procedure utilizing the a = 4 masterbatch as the additive to PP show an even higher degree of dispersion than the respective binary masterbatch and, apparently, resemble the degree of exfoliation that is observed in a binary system with much higher a but the same clay concentration. This finding was quite general: mixing the masterbatch with the polymer resulted in even higher degrees of exfoliation evidenced by the disappearance of even small peaks present in the diffractogram of the masterbatch; this was reproducible and held for every composition of the masterbatch [59].

The influence of the ratio of the maleated compatibilizer to organoclay on the hybrid micro- or nano-structure was investigated for polyethylene/layered silicate nanocompos-ites as well [43]. In this case, a maleic-anhydride-grafted-polyethylene, PE-g-MAH (MFI 1.5; maleic anhydride content wMAH ~ 0.85%) was mixed with an injection grade linear low density polyethylene (SABIC; MFI 37), PE, and the organoclay C20A. Fig. 4 shows the X-ray diffractograms of a series of ternary composites (prepared in a micromixer) where the concentration of the organoclay was kept constant at 9 wt.% whereas the concentration of PE-g-MAH (with respect to the total polymer content) was varied from 2.5 wt.%, where PE-g-MAH can be considered as an additive, up to 70 wt.% where PE-g-MAH is a significant part of the polymer matrix; in this way, the ratio a of compatibilizer to organoclay varies from 0.25 to 7.1. For low concentrations of PE-g-MAH, i.e., for low values of a (a <0.5), the data exhibit the characteristic peak of the parent C20A organoclay signifying a phase separated micro-composite. As the relative ratio a increases above 0.81, the diffraction peak appears to shift gradually to 20 « 2.9°, which indicates the existence of intercalated structures with interlayer distances d001 up to 30 A. At low angles, the intensity increases with decreasing scattering angle, which signifies the coexistence of exfoliated layers (together with intercalated ones). For the highest values of a, there is no indication for the existence

104 г

в 10zr

101 г

0 5 10 15 20 25 30

29 (o)

Fig. 4. X-ray diffraction data of ternary polyethylene hybrids containing PE-g-MAH and 9 wt.% organoclay C20A with different relative ratios PE:PE-g-MAH. The solid lines from bottom to top correspond to PE:PE-g-MAH = 30:70, 50:50, 70:30, 80:20, 92:8, 95:5, and 97.5:2.5. The data for C20A are shown for comparison on the top. The curves are shifted vertically for clarity. The vertical line indicates the position of the main diffraction peaks of C20A.

of a scattering peak of either the montmorillonite or of an intercalated nanohybrid. The diffracted intensity shows a continuous increase with decreasing scattering angle, which indicates that the ordered organoclay structure has been destroyed due to the interactions of the polymer with the inorganic nanoparticles. This behavior is observed for PE-g-MAH concentrations corresponding to a values higher than ~5. Note that exactly the same behavior was reported by us before [43] when a Dellite 72T (dimethyl dihydroge-nated tallow ammonium chloride modified montmorillon-ite, Laviosa Chimica Mineraria) organoclay was used instead of C20A; in that case exfoliated nano-structures were obtained for a values higher than ~4.5. In both cases the XRD results were further supported by TEM images [43].

The observed micro- or nano-structures correlate with changes in the macroscopic rheological behavior of the composites [43]. A progressive change of the frequency dependencies of the elastic and loss moduli, G and G", respectively, and of the complex viscosity was demonstrated with increasing PE-g-MAH:organoclay ratio a, i.e., with progressively modifying the structure from a phase separated to a completely exfoliated one. For the immiscible micro-composites the G and G" moduli exhibit the expected x 2 and x 1 dependencies on frequency in the flow regime, whereas the viscosity shows a low frequency plateau; this behavior resembles the one of the pure polymer. In contrast, an exfoliated system shows a solid-like behavior at low frequencies with a very weak dependence of

both moduli and an increase of the complex viscosity with decreasing frequency. These results will be discussed further in the next section in relation to the behavior of composites utilizing a block copolymer compatibilizer [43].

The importance of the ratio of maleated compatibilizer to the organoclay, a, was discussed in an earlier study by Hotta and Paul as well [42]. An increasing degree of exfoliation with increasing a was observed with XRD and TEM for composites of linear low density polyethylene (LLDPE; MFI 2) with C20A in the presence of a maleated compatibilizer (Fusabond MB226D - Dupont; MFI 1.5), LLDPE-g-MAH. It was found that the diffraction peak at 26 = 3.6°, which corresponds to the interlayer distance of the organoclay, disappears when a increases from 0 to 11 indicating a change of the structure from phase separated to completely exfoliated. Moreover, TEM analysis showed that the aspect ratio of the resulting "particles" increase from 5 (for a = 0) to ~42 (for a = 11). In a following study, a PP (MFI 37) and a PP-g-MAH (1 wt.% MAH content) were melt blended with a dimethyl-di(hydrogenated tallow)-modified MMT with PP-g-MAH:organoclay ratio a of 0.5,1 and 2 [57]. Complete exfoliation was not observed for any of the composites containing 1-7 wt.% organoclay, probably because of the small value of a; however, quantitative TEM analysis showed an increase of the aspect ratio of the clay particles, which is one of the main parameters for the polymer reinforcement. It was found that, although the rheological properties suggest that the extent of a percolation network can be enhanced by increasing a, the mechanical and thermal expansion behavior do not improve correspondingly because of the reduction of matrix properties by the addition of PP-g-MAH, e.g., lower crystallinity.

In early works that utilized PP-g-MAH oligomers as compatibilizers, the effects of the number of polar groups as well as of the miscibility of the compatibilizer with the pure polymer were investigated. Two PP-g-MAH oligomers with different number of polar groups were mixed with octadecyl-amine-modified montmorillonite, C18-MMT, in binary mixtures [46a]. When C18-MMT was mixed with the oligomer possessing one carboxyl group per ~25 units of propylene (PP-g-MA-1010; acid value: 52 mg KOH/g) with a =1, the main diffraction peak shifted from 26 = 4° to 26 = 2.3° and, thus, the interlayer distance increased from 21.7 to 38.2 A. On the contrary, the oligomer with only one carboxyl group per ~190 units of propylene (PP-g-MA-110TS) with a =1 resulted in a phase separated micro-composite. The effect of the miscibility of the polymer with the compatibilizer was examined by utilizing similar PP-g-MAH oligomers with different number of polar groups. PP-g-MA-1001 (acid value: 26 mg KOH/g) and PP-g-MA-1010 were mixed with PP in a 77:23 ratio of PP:PP-g-MAH [46b]; polarized optical micrographs of the blends in the melt state (200 °C) showed that the compatibilizer with fewer polar groups is more miscible with the polymer than the one with more. When ternary systems with PP were synthesized, the results were understood in light of a strong driving force for the intercalation of PP-g-MAH via strong hydrogen bonding between the maleic anhydride group (or the carboxyl groups generated from the hydrolysis of the maleic anhydride groups) and the oxygen atoms of the silicates, which led to the in-

crease of the interlayer spacing of the clay and the weakening of the interactions between the layers. If the miscibility of PP-g-MAH with PP is good enough to mix at the molecular level, the exfoliation of the intercalated clay takes place; otherwise, there is phase separation of the two polymers, which appears as an intercalated nano-structure. Thus, there is an optimum amount of polar functional groups to achieve the desired structure.

The effect of the molecular weight of PP-g-MAH on the resulting structure has been investigated as well [54]. PP (molecular weight Mw = 250 kg/mol) was mixed with Cloisite 15A and with two PP-g-MAH's with different molecular weights and different maleic anhydride content. Use of the PP-g-MAH with low Mw and high maleic content (Mw = 9.1 kg/mol, wMAH = 3.8%) leads to relatively good and uniform intercalation evidenced by a main diffraction peak in the XRD pattern and a regularity in the layer stacking in the TEM images but not to any profound signs of exfoliation; this was attributed to the interactions of PP-g-MAH with the clay particles and to the lack of miscibility with the matrix polymer. On the other hand, when PP-g-MAH with high molecular weight and low maleic content (Mw = 330 kg/mol, wMAH = 0.5%) was utilized, no intercalation was observed but there were signs of exfoliation indicated by TEM images showing more disordered and distanced layered structure. It was proposed that this compatibilizer could interact less with the clay but had enhanced miscibility with PP resulting in a higher degree of exfoliation. It is noted that, in that study, a was kept equal to 2.

Along the same lines, four PP-g-MAH's with different Mw and different wMAH (two of which are the same with the ones studied above [54]) were utilized as compatibiliz-ers [55] between an injection-grade isotactic PP homopolymer (Yungsox 1040) and octadecylamine-modified montmorillonite clay (Nanomer I.30 P). The ratio a was varied from 1 to 10. It was found that all the compatibiliz-ers, but the one with the lower Mw and higher wMAH, were equally efficient in compatibilizing PP and organoclay especially for ratios a >3. Nevertheless, no indication for intercalation was reported. Similarly, two different PP-g-MAH's were utilized with different molecular weights and different maleic anhydride content [58]. The compati-bilizer with the high Mw and low MAH content was ineffective in modifying the interlayer spacing of the organoclay. On the contrary, the one with the lower Mw and high MAH content showed an increase of d001. It is noted that, the interlayer spacing did not increase linearly as a function of the weight fraction of PP-g-MAH for constant concentration of organoclay; apparently, there is a threshold amount of compatibilizer above which the weakly held platelets are intercalated significantly.

The role of the silicate modification on the morphology development and mechanical and rheological properties was investigated by Mulhaupt and co-workers [47a] for hybrids containing PP (Borealis PP HC 001 A-B1, MFI 3.2) with synthetic sodium fluoromica (SOMASIF ME100) organophil-ized with various alkyl ammonium surfactants with different alkyl length in the presence of a maleic-anhydride PP-g-MAH oligomer (Mn = 4000, wMAH = 4.2 wt.%). The synthesized hybrids contained 10 wt.% organoclay and

20 wt.% compatibilizer. It was shown that the values ofboth Young modulus and yield stress made a jump when orga-noclays with surfactant chains carrying more than twelve carbon atoms were utilized whereas they exhibited lower values for shorter surfactants; this increase was attributed to the increase in the degree of exfoliation due to the enhanced interfacial coupling. Moreover, it was demonstrated that annealing the samples (200 min at 220 °C) is necessary in order for the hybrids to reach their equilibrium nano-structure, which is described by a higher degree of exfoliation and a recovery of their thermorheological simplicity manifested via time-temperature superposition of the rhe-ological data [47b].

Besides the formation of hydrogen bonds between the oxygen groups of the silicate and the functionalized polymer, alternative explanations concerning possible mechanisms of interaction between the different components in maleic-anhydride functionalized polypropylene composites have been discussed [56]. Organophilized montmorill-onites were utilized and the surfactants used were 1-hexadecylamine (HDA) and cetylpyridinium chloride monohydrate (CPCl). The authors demonstrated that a reaction between HDA and PP-g-MAH can take place, where anhydride groups are consumed and amide groups are mainly formed. On the contrary, CPCl that contains no active hydrogen atoms does not react with the specific compatibilizer. XRD data showed that, when the HDA surfactants are used, the characteristic diffraction peak of the organoclay disappears (for a hybrid containing 20 wt.% PP-g-MAH and 2 wt.% silicate) while, when CPCl is utilized, a decrease in the thickness of the interlayer galleries is observed. The authors explained the results with the assumption that chemical reactions remove the surfactants from the surface of MMT and hydrogenated silicate sites are left behind. The high energy surface interacts either with the anhydride or with the amide groups by dipole-dipole interactions, whereas even the unmodified polypropylene may adhere stronger to such surface by London dispersion forces than to the silicate covered by aliphatic chains.

Nevertheless, the usefulness of utilizing PP-g-MAH or PE-g-MAH has been doubted in the past; it was claimed that, when such an oligomer is used as a compatibilizer, the polymer and the MAH-treated organoclay are effectively at ^-conditions and the extrusion is the only parameter that promotes the mixing because of the imposed mechanical shear [45]. As a result, the structure and the properties of the resulting hybrid materials would depend strongly on the processing conditions; they would vary from a fair dispersion with moderate property improvements to a good dispersion with better performing hybrids. This explanation together with the large number of material and processing parameters that play a role when synthesizing such three component hybrids (e.g., the molecular weight of the polyolefin and of the compatibiliz-er, the content of its polar groups, the miscibility of the two polymers, the type of surfactants modifying the inorganic surface, the processing conditions, etc.) may partly explain the quite often contradictory results in the literature. At the same time, however, it can be safely concluded that a certain ratio of compatibilizer to organoclay and adequate miscibility of the maleated polyolefin and the polymer are

the absolute prerequisites in order to accomplish a certain degree of exfoliation.

3. Non-maleated functional compatibilizers

Substantial research efforts have appeared that go beyond the use of maleic-anhydride-functionalized polyole-fin oligomers or polymers for controlling the structure of polyolefin/organoclay nanocomposites; these additives are specifically synthesized homopolymers and/or copoly-mers that could modify the interactions and effectively compatibilize the blend.

In a systematic investigation, Manias et al. [45] introduced random copolymers of polypropylene with typically 1 mol.% of functionalized monomers containing hydroxyl or maleic anhydride functional groups. The functionalized polypropylenes were derived from the same random polypropylene copolymers synthesized by metallocene catalysis, which contained 1 mol % p-methylstyrene (p-MS) comonomers [45]. Subsequently, the p-MS's were interconverted to functional groups containing hydroxyl (OH) or maleic anhydride moieties by lithiation or free-radical reactions, respectively. These functionalized polypro-pylenes were melt-blended under static conditions with dimethyl-dioctadecyl-ammonium-modified montmoril-lonite, which readily blends with the styrenic comonomers and their functionalized derivatives. XRD measurements of the resulted hybrids indicated an intercalated structure with ~10 A increase of the interlayer distance for all the differently functionalized polymers, whereas, bright field TEM images showed the coexistence of both intercalated tactoids and disordered/exfoliated stacks of layers. Moreover, polypropylene-block-poly(methyl methacrylate) diblock copolymers were synthesized, containing 1 and 5 mol.% of PMMA; the synthesis involved preparation of polypropylene by metallocene catalysis, hydroboranation of the olefinic chain end, and subsequent free radical polymerization of the PMMA block [67]. The diblock copolymers were utilized as compatibilizers between polypropylene and an octadecylammonium-modified MMT, which interacts favorably with PMMA; the mixing resulted in composites containing approximately ~20% exfoliated/disordered layers whereas the rest of the organoclays existed as intercalated tactoids.

Polyethylene-based model macromolecules were especially designed and synthesized to act as surfactants and/ or compatibilizers to alter the interactions and control the structure in PE/organoclay nanocomposites [43]. Three types of additives were synthesized by Pitsikalis and Hadjichristidis [43] utilizing anionic polymerization under high vacuum [68] followed by the appropriate post-polymerization reactions in order to introduce or reveal the desired functional moieties. Polyethylene chains func-tionalized by dimethyl ammonium chloride either as a single end-group, NPE, or as multiple functional groups grafted along the chain, PE-g-NPE, were synthesized by an-ionic polymerization of butadiene (using benzene solvent in order to obtain polybutadiene with highly 1,4-micro-structure), subsequent hydrogenation to produce polyethylene followed by quaternization of the dimethylamine

end-groups using excess of concentrated HCl; the functional amino groups were introduced either with the dimethylaminopropyl lithium initiator (for NPE) or with the dimethylaminopropyl chloride used as a terminating agent for the grafted short chains (for PE-g-NPE). The third type of additive, a diblock copolymer of polyethylene-block-poly(methacrylic acid), PE-b-PMAA was also synthesized anionically followed by hydrogenation and deprotection of the methacrylic acid; polybutadiene-block-poly(t-butyl methacrylate) was synthesized first followed by hydrogenation of the predominantly 1,4-polybutadiene to polyethylene and the hydrolysis of the poly(t-butyl meth-acrylate) to poly(methacrylic acid) at 85 °C in the presence of concentrated HCl. Details concerning the synthesis can be found elsewhere together with the molecular characteristics of all the synthesized compatibilizers [43].

The functional polyethylene chains with the quaternized amine end-groups were used to synthesize composites with either hydrophilic or organophilic montmorillonite. Fig. 5a shows a hybrid that was synthesized utilizing, PE (SABIC; MFI 37), organoclay Dellite 72T (dimethyl dihydrogenated tallow modified MMT, Laviosa Chimica Mineraria), D72T, and NPE (Mw = 9700, Mw/Mn = 1.04) as an additive. The hybrid consisted of 15 wt.% D72T whereas the ratio of PE:NPE

8000 -

6000 -

4000 -

2000 -

8000 -

6000 -

4000 -

2000 -

10 15 20

29 (o)

Fig. 5. X-ray diffraction data of (a) a ternary hybrid containing 15 wt.% Dellite 72T and functional NPE additive with PE:NPE 90:10 (open circles) and (b) a ternary hybrid containing 13 wt.% C20A and functional PE-g-NPE additive with PE:PE-g-NPE 90:10 (open circles). The data for the respective organoclays are shown by solid lines. The vertical lines indicate the position of the main diffraction peaks of the organoclays. Adopted from Ref. [43].

was 90:10. It is clear that the main diffraction peak of the hybrid remains at exactly the same position with the one of the pure organoclay and thus the composite shows a phase separated micro-structure. Similar results were obtained [43] when an NPE with higher molecular weight was utilized or when different concentration of NPE was used. Nevertheless, it is noted that the ratio a of the compatibilizer to organoclay was in all cases very small due to the specialized synthesis that produced a limited amount of the functional polymers. Moreover, attempts to utilize the specific NPE additive as a macromolecular surfactant mixed with a hydrophilic Na+-MMT led to phase separated structures, as well, which can be possibly attributed to the high molecular weight of the functional polymer chains or to the unfavorable interactions of the long PE tail of the "surfactant" with the surfaces.

Since one functional quaternary ammonium end group was, apparently, not enough to alter the clay structure or act as compatibilizer, the functional additive PE-g-NPE, which was specifically synthesized to contain multiple functional ammonium groups distributed along the polymer chains, was utilized [43]. Fig. 5b shows XRD measurements of a hybrid containing the specific additive with a composition similar to the one of Fig. 5a (13 wt.% organoclay and ratio of PE:PE-g-NPE 90:10) and C20A as the organoclay (which is very similar to D72T). A significant drop of the intensity of the main peak is evident, which shows a small shift towards lower angles as well that correspond to an increase of the interlayer distance by 3.9 A. At the same time, there is an increase of the intensity at low angles, which indicates that there must be a degree of disorder and possible exfoliation in the hybrid. It is noted, however, that in this case the ratio of the compati-bilizer to organoclay was kept low (a = 0.67) as well, because of the limited amount of PE-g-NPE due to its specialized synthesis.

End-functionalized polypropylene with ammonium terminal groups, PP-NH3+, with well-controlled molecular weight and narrow molecular weight distribution, was similarly utilized as a compatibilizer for PP nanocompos-ites with both pristine Na+-MMT and dioctadecyl-ammonium-modified organophilic clay [62]. Binary mixtures with compositions 90:10 were prepared by melt intercalation and in both cases featureless XRD patterns were obtained indicating the formation of exfoliated structures probably via a cation exchange reaction between the alkali or the dioctadecylammonium cations, respectively, and the ammonium terminated PP. The exfoliated structure observed by TEM was maintained after further mixing with isotactic polypropylene, i-PP. Apparently i-PP chains serve as diluents in the ternary PP-NH3+/MMT/i-PP system, with the thermodynamically stable PP-NH3+/MMT exfoliated structure dispersed in the i-PP matrix.

The PE-b-PMAA diblock copolymer was synthesized [43] so that it will be able to intercalate within the galleries of the montmorillonite because of the polarity of the car-boxyl groups of poly(methacrylic acid). This would either bring the polyethylene block into the galleries or lead to hairy plates [69] making, thus, the environment much friendlier for the polyethylene homopolymer [43]. Fig. 6 shows X-ray diffraction results for hybrids where the

c 1000

- J 1 1 1 1 1 1 1 1 1 1 \ (a):

- i i i i i i i i i i (b) ..........

_ i i i i i i i i i i (c)

* \ i i i i i i i i i i (d):

.........; v (e)-

5 10 15 20 25 30

29 (o)

Fig. 6. X-ray diffraction data of pure C20A (a) and ternary polyethylene hybrids with 13 wt.% C20A utilizing PE-b-PMAA additive with PE:PE-b-PMAA ratios of 98:2 (b), 94:6 (c), 90:10 (d), and 85:15 (e). The vertical line shows the position of the main diffraction peak of C20A. Adopted from Ref. [43].

concentration of the C20A clay is kept constant (13 wt.%) but the amount of the copolymer additive is varied between 2 and 15 wt.% with respect to the total polymer. This way the ratio of copolymer to organoclay was varied from a = 0.13 to a = 1. It can be seen that, for the lower copoly-mer concentration, there is not any significant change of the interlayer distance of the organoclay C20A; the main diffraction peak is observed at 20 = 3.15° leading to d001 = 28.0 A. As the concentration of the additive increases, there is a gradual shift of the main diffraction peak to lower angles. At the higher concentration of the additive, i.e., at 15 wt.%, a very weak peak is observed at 20 = 2.45° that corresponds to d0o1 = 36.0 A, which means an increase of the interlayer distance by ~10 A. There is a significant decrease of the intensity of the peak as well, which is accompanied by an increase of the scattering intensity at lower angles. Based on these results, it was concluded that, as the concentration of the PE-b-PMAA increases, both intercalated and exfoliated regions exist. It is noted that, this degree of exfoliation is even higher than the corresponding result utilizing PE-g-MAH at the same ratio a (discussed in relation to Fig. 4 above) although such a conclusion is only qualitative.

The changes in the structure of the composite observed by XRD are expressed in the rheological behavior or, alternatively, one can utilize the rheological data to support the obtained micro- or nano-structures. Fig. 7 shows the elastic and loss moduli, G and G", respectively, for a hybrid that contains 13 wt.% C20A and PE:PE-b-PMAA ratio 98:2,

o 104 r

10-1 100 101

aT œ (rad/s)

Fig. 7. Dynamic frequency sweep measurements of a ternary polyethylene PE hybrid with 13 wt.% C20A containing PE-b-PMAA additive with PE:PE-b-PMAA ratio of 98:2 at different temperatures following time-temperature superposition. Storage modulus G' (solid symbols), loss modulus G" (open symbols). The inset shows the respective frequency sweep measurements for the pure PE, which exhibit the and behavior shown by the solid lines.

■ □ 150oc '

■ o 140oc -

▲ A 130oc :

102 10

i.e., with a ratio a = 0.13; G and G" were obtained by dynamic frequency sweep experiments. The measurements were performed at temperatures higher than the crystallization temperature of polyethylene (Tc = 103 °C measured by rheology and differential scanning calorimetry) to ensure that the results are not influenced by crystallization and are only due to the effect of the inorganic material. It is clear that time-temperature superposition holds indicating that the system remains thermorheologically simple despite the presence of the silicate. The inset show the respective G and G" data for the PE homopolymer in the absence of clay; in this case the G and G" moduli exhibit the expected x-2 and x-1 dependencies on frequency in the flow regime, as indicated by the lines with slopes -1 and -2, whereas the viscosity exhibits a low frequency plateau (not shown). The rheological behavior of the hybrid in the main figure deviates significantly from this picture; both moduli exhibit much smaller slopes than the -1 and -2 obeyed by a polymer melt. Similar measurements were performed for the hybrids discussed with respect to Fig. 1 (no compatibilizer) and Fig. 4 (containing the PE-g-MAH as a compatibilizer) [43]. It has been found that the data for the immiscible micro-composites are very similar to those for the PE homopolymer with the G and G" moduli exhibiting the expected x-2 and x-1 frequency dependencies in the flow regime. Nevertheless, the frequency dependence of G and G" for the exfoliated system with a ~ 4 was significantly different; the storage modulus, G', attained values that were higher than the loss modulus, G'', whereas both G and G' displayed weak frequency dependencies indicative of a solid-like behavior. The dynamic data for a hybrid, which contains both intercalated and exfoliated platelets (a ~ 1.4), showed an intermediate behavior: G' was slightly higher than G with both exhibiting very weak frequency dependencies. The latter result correlates very well with the results of Fig. 7 for the PE-b-PMAA additive

despite the fact that this contains even less additive, i.e., it possesses a much lower a.

The change in the behavior can be quantified by evaluating the shear-thinning exponent n by analyzing the low frequency complex viscosity data in terms of a g* = Axn expression [43,70]. For liquid-like behavior n « 0 whereas for a solid-like response n 1. Wagener and Reisinger [70] proposed to use the value of n as a measure of the degree of exfoliation since it was explicitly assumed that exfoliation leads to a percolated structure, which results to a solid-like behavior. Fig. 8 shows the frequency dependence of the complex viscosity measured at 140 °C for different micro- and nano-hybrids containing compatibilizer or not, i.e., for pure PE polymer, a binary phase separated microcomposite containing PE:Dellite 72T 85:15, two hybrids containing PE-g-MAH as compatibilizer with different degrees of exfoliation (a = 1.4 and a = 4) and the hybrid of Fig. 7 containing PE-b-PMAA with a = 0.13. An increase of the exponent n is observed with the increase of the degree of exfoliation, when the rheological data are correlated to the X-ray diffraction results. n is zero for the homopolymer PE and n = -0.1 for the immiscible PE:Dellite 72T 85:15 hybrid whereas n = -0.4 for the slightly exfoliated three-component hybrid containing 10 wt.% Dellite 72T and PE-g-MAH with PE:PE-g-MAH 85:15 at a =1.4 and n = -0.65 for the completely exfoliated three-component hybrid containing 10 wt.% Dellite 72T and PE-g-MAH with PE:PE-g-MAH 51:49 at a = 4.4. It is noted that, in all cases, the frequency dependence of g* was much weaker for the respective blend of polymers (i.e., without the organoclay). Therefore, the shear-thinning exponent can be indeed correlated with the structure of the system since it essentially describes the transition from a liquid-like behavior of the immiscible micro-composites to the solid- or gel-like behavior of the exfoliated nanocomposites due to the percolated structure of the nanohybrids. More importantly,


♦ ♦/

° ° O o o O

O ° o o o O o c

□ □ □ □ □

' ° O O t

100 101 Frequency (rad/s)

Fig. 8. Frequency dependence of the complex viscosity of various systems at 140 "C: polyethylene PE (open squares), a binary mixture containing 85 wt.% PE and 15 wt.% Dellite 72T (open circles), a three-component hybrid containing PE-g-MAH and 10 wt.% Dellite 72T with PE:PE-g-MAH 85:15 and a = 1.4 (open triangles), a three-component hybrid containing PE-g-MAH and 10 wt.% Dellite 72T with PE:PE-g-MAH 51:49 and a = 4.4 (open inverted triangles) and a three-component hybrid containing PE-b-PMAA and 13 wt.% C20A with PE:PE-b-PMAA 98:2 and a = 0.13 (filled diamonds).

the shear-thinning exponent for the PE-b-PMAA hybrid (a = 0.13) is n = -0.38, i.e., it resembles the situation of PE-g-MAH with almost 10 times larger a. This means that the specific PE-b-PMAA diblock may be a much more effective compatibilizer that the widely used maleated polyethylene. Of course, its specialized synthesis should be always kept in mind.

Similar results were found when a polypropylene-block-poly(propylene glycol), PP-b-PPG, diblock copolymer was utilized as compatibilizer; its ability to increase the interlayer distance of PP/dimethyl-dioctadecyl-ammo-nium-modified montmorillonite hybrids was studied and compared with the corresponding behavior of PP-g-MAH [58]. The ratio of compatibilizer to organoclay used was low but nevertheless 2 wt.% of PP-b-PPG resulted in a ~4 A increase of interlayer distance, which was better than what was observed utilizing maleated PP under the same conditions. Polyethylene-block-poly(ethylene glycol), PE-b-PEG, with 33 methylene groups and 2.6 ethylene oxide units per molecule on average (more like end-functional-ized PE with a small polar head group rather than a block copolymer), and random copolymers of poly(ethylene-co-vinyl alcohol), PE-co-PVOH and poly(ethylene-co-meth-acrylic acid), PE-co-PMAA were utilized to prepare polyethylene nanocomposites with dimethyldioctadecyl-ammonium-modified montmorillonite as well [63]. The morphology varied between phase separated and intercalated depending on the dispersing agent with the best copolymer (in terms of increasing interlayer distance) being PE-co-PVOH; a linear dependence of the interlayer distance on the copolymer weight fraction was observed. It is noted, however, that the copolymer concentration was kept very low in this case as well (copolymer/organo-clay weight ratio of 0.17).

In one of the first attempts to synthesize polypropylene/ clay nanocomposites, a distearyldimethylammonium-modified montmorillonite was first mixed in toluene solu-

tions with a polyolefinic oligomer (polyolefin diol, carbon number = 150-200), possessing two OH end groups, in different ratios of oligomer to organoclay [60]. XRD measurements demonstrated that a ratio of 1:1 ratio of additive to organoclay simply increased the interlayer distance, whereas the XRD data for a >3 did not show any clear diffraction peaks; this was attributed to the interactions of the OH groups with the silicate layer via hydrogen bonding. Further blending of binary hybrids even with ratio 1:1 with PP resulted in exfoliation of the silicate and in the dispersion of the platelets in the polymer matrix.

In another case, comparison between LLDPE/organoclay nanocomposites that contained either a low molecular weight oxidized polyethylene (Mn = 2950, acid number = 30 mg KOH/g) or PE-g-MAH (MFI = 4, wMAH = 1.6 wt.%, acid number = 18.3 g KOH/g) was performed [44]. Comparison of the obtained morphology of the compatibilized hybrids at constant a = 3 showed that the use of oxidized polyethylene results in intercalated nanocomposites with an ~1 nm increase of the interlayer distance of the organoclay, whereas PE-g-MAH leads to a higher degree of exfoliation despite of its lower functionality. All nanocomposites showed a solid-like rheological behavior with increasing clay content, whereas the estimated percolation threshold was higher in the hybrids with oxidized polyethylene, which, however, contained clay tactoids with smaller aspect ratio values.

A hydroxyl-functionalized PP was synthesized using metallocene catalysts and its efficiency as a compatibilizer with respect to PP-g-MAH (maleic anhydride content: 0.5 wt.%) was investigated as well in composites with compatibilizer to organoclay ratio 1:1 and 2:1 [52]. It was only the latter that showed exfoliated structure for both compatibilizers based on XRD data whereas rheological measurements showed that the composites with hydroxyl-functionalized PP exhibited smaller viscosity compared to the pure polymer indicating probably improved processabil-ity despite the addition of the filler. Nevertheless, composites

with PP-g-MAH showed higher viscosity compared with the pure polymer and a deviation from the low frequency plateau indicating a higher degree of exfoliation.

Altering the type of surfactants, which are used to make the clay organophilic, was proposed to modify the polymer-clay interactions as well [45]; semi-fluorinated surfactants were utilized. Specifically, the MMT cations were first exchanged by octadecyllammonium, and, subsequently, semi-fluorinated alkyltrichlorosilane surfactants were introduced; these were tethered to the surface through a reaction of the trichlorosilane groups with hydroxyl groups in the cleavage plane of the MMT. The resulting organoclay contained octadecylammonium at full CEC and ~60% additional semi-fluorinated surfactants. Hybrids were synthesized by melt intercalation that exhibited an intercalated structure with a 12 A increase of the interlayer distance of the fluorinated montmorillonite. Moreover, use of mechanical shear promoted further the dispersion.

In an alternative route to increase polypropylene polarity to make it more compatible with clay, -Cl and SO2Cl-groups were introduced by reaction with sulfuryl chloride, SO2Cl2, under UV irradiation in the presence of small amounts of pyridine [61]. An organophilized silicate (Cloi-site 15A) and three chlorosulfonated polypropylenes with different degrees of functionalization were utilized and a mixture of intercalated and exfoliated structure was obtained in all cases. The highest degree of intercalation was observed for systems with the compatibilizer possessing a medium amount of SO2Cl-, but a distinctly higher amount of -Cl indicating that chlorine is more efficient in organoclay delamination. Nevertheless, full exfoliation of clay platelets was not achieved.

In summary, the need for effective compatibilizers for the preparation of polyolefin/layered silicate nanocompos-ites has lead to the synthesis of various model functional polymers. Molecules with functional ammonium or hydro-xyl groups as well as block or random copolymers, among others, have been tested for their ability to compatibilize the components. Phase separated, intercalated, exfoliated or mixed structures have been observed depending on the kind, the molecular characteristics and the degree of func-tionalization of the additive as well as on its concentration in the mixtures. In certain cases, enhanced dispersion in comparison to the widely used maleated polyolefins has been reported. Nevertheless, despite the number and the quality of the reported studies, it is not clear yet what is the best compatibilizer for such systems.

4. Concluding remarks

In the present article we have provided a short review of the various efforts to synthesize micro- and nano-com-posites based on polyolefins and high-aspect-ratio layered silicates as additives. The synthesis requires the use of functional oligomers/polymers to modify the unfavorable interactions between the strongly non-polar polyolefins and the inorganic additive. To achieve the dispersion of the inorganic material within the polymer matrix and obtain the desired structure, compatibilizers like maleic-anhydride-functionalized polyolefins, polymers with func-

tional hydroxyl or ammonium groups as well as block or random copolymers with one polar monomer have been utilized.

X-ray diffraction and transmission electron microscopy have been mainly used to identify the structure, whereas, in certain cases, rheology can provide complementary information. Many of the compatibilizers used were found effective in exfoliating the clay particles whereas the most important parameters that control the structure are the ratio of the compatibilizer to organoclay and the miscibility of the compatibilizer with the polyolefin; organophiliza-tion of the inorganic material by utilizing surfactants with long enough alkyl chains is certainly a prerequisite whereas attention should be paid to whether equilibrium is established. It is understood that the strong interaction between the functional groups of the compatibilizer and the organoclay particles can lead to the formation of a mas-terbatch of ''hairy particles", which can in turn favorably mix with the polyolefin matrix polymer.


We would like to acknowledge I. Altintzi, I. Andrianaki, R. Shemesh, H. Retsos for their involvement in the experimental work of our group reviewed herein. We would also like to thank M. Pitsikalis and N. Hadjichristidis, who synthesized the functional NPE, PE-g-NPE and PE-b-PMAA polymers utilized in [43]. E.P. Giannelis is acknowledged for introducing S.H.A. to the area of polymer nanocompos-ites as well as for his involvement in some of the works of our group reviewed herein. N. Theophilou is acknowledged for his collaboration in the project. The authors would like to acknowledge that part of this research was sponsored by the European Union in the framework of GROWTH Programme (NANOPROP Project No. G5RD-CT-2002-00834), by the Greek General Secretariat of Research and Technology (PENED Programme 03ED581) and by NATO Scientific Affairs Division (Science for Stability Programme).


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Kiriaki Chrissopoulou studied Physics at the University of Crete, Greece and obtained a PhD in 2000 from the Physics Department of the University of Patras, Greece studying the effect of thermodynamic incompatibility and macromolecular architecture on the dynamics of block copolymers. In 2001, she moved to Collège de France, Paris, where she worked as ''Maître de Conférences" in Laboratoire de Physique de la Matière Condensée investigating the adhesion between a solid surface and an elastomer. Currently, she is an associated researcher at the Institute of Electronic Structure and Laser of the Foundation for Research and Technology-Hellas. Her current research focuses on the investigation of the structure and dynamics of polymer nanocomposites. More specifically, she is interested on understanding and controlling the interactions between the constituents, investigating the crystallinity and chain conformation close to solid surfaces as well as studying the dynamics under severe confinement. In 2009, she was awarded the best oral presentation award in the XXV Panhellenic Conference on Solid State Physics & Materials Science.

Spiros H. Anastasiadis is a Professor of Polymer Science and Engineering at the Department of Chemistry of the University of Crete and an affiliated researcher at the Foundation for Research and Technology - Hellas. He studied Chemical Engineering at the Aristotle University of Thessaloniki, Greece whereas he received his PhD in Chemical Engineering from Princeton University in 1988. He has been a Visiting Scientist at the IBM Almaden Research Center in 1988-1989. He was awarded the John H. Dillon Medal of the American Physical Society in 1998 and was elected Fellow of the American Physical Society in 2000. He has received the Materials Research Society Graduate Student Award in 1987 and the Society of Plastics Engineers - Plastics Analysis Division Best Paper Award during ANTEC 1985. He has been an Editor of the Journal of Polymer Science: Part B: Polymer Physics (20062010), a Professor of Materials at the Department of Physics of the University of Crete (until 2005) and a Professor of Materials Science and Engineering at the Aristotle University of Thessaloniki (2005-2008). He has been the Alternate Chairman of the Department of Chemical Engineering (09/2007-10/2008) and a Member of the Scientific Council of the Institute of Electronic Structure and Laser of the Foundation for Research and Technology - Hellas (01/2009-now). His research interests are in the areas of polymer surfaces/interfaces and thin films, block copolymers, polymer blends and homopolymer/copolymer blends, dynamics and diffusion in multi-constituent systems, organic/inorganic nanohybrid materials and responsive polymer systems.