- Email: [email protected]
layered silicate nanocomposites
Current Opinion in Solid State and Materials Science 6 (2002) 205–212
New advances in polymer / layered silicate nanocomposites Daniel Schmidt, Deepak Shah, Emmanuel P. Giannelis* Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853, USA Received 3 April 2002; accepted 12 June 2002
Abstract This review discusses some recent advances in polymer silicate nanocomposites. In particular, we highlight the properties of specific nanocomposites while emphasizing the lack of properties trade-offs in these systems. We also present our work on the structure and dynamics of the polymer / nanofiller interface and attempt to relate them to macroscopic nanocomposite properties. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Work in polymer nanocomposites has exploded over the last few years. The prospect of a new materials technology that can function as a low-cost alternative to high-performance composites for applications ranging from automotive to food packaging to tissue engineering has become irresistible to researchers around the world [*1,2,*3,*4]. The essence of nanotechnology is the ability to work at the molecular level to create large structures with fundamentally new molecular organization. Materials with features on the scale of nanometers often have properties different from their macroscale counterparts. Important among nanoscale materials are nanohybrids or nanocomposites, materials in which the constituents are mixed on a nanometer-length scale. They often exhibit properties superior to conventional composites, such as strength, stiffness, thermal and oxidative stability, barrier properties, as well as unique properties like self-extinguishing behavior and tunable biodegradability. Another unique aspect of nanocomposites is the lack of properties trade-offs. For the first time, there is an opportunity to design materials without the compromises typically found in conventionally filled polymer composites. Uses for this new class of materials can be found in aerospace,
*Corresponding author. Tel.: 11-607-255-9680; fax: 11-607-2552365. E-mail address: [email protected] (E.P. Giannelis).
automotive, electronics and biotechnology applications, to list only a few.
2. Motivation Though significant progress has been made in developing nanocomposites with different polymer matrices, a general understanding has yet to emerge. For example, what allows nanocomposites to be both stiffer and tougher than conventional composites, without sacrificing other properties? Why do they display better thermal stability versus unfilled polymers? How can we utilize specific molecular interactions to control structure and morphology? How can materials properties be predicted from nanostructural data and molecular dynamics simulations? A major challenge to further development of nanocomposites is the lack of even simple structure–property models. Without such models, progress in nanocomposites has remained largely empirical. Similarly, predicting ultimate materials properties or maximum theoretical performance for different classes of nanocomposites is almost impossible at present. At Cornell, our group has pursued new synthetic approaches, molecular-level physical measurements and molecular modeling and theory. Our objective is to build an understanding that will permit the prediction and control of nanocomposite properties. The unique nature of nylon nanocomposites has been reported in pioneering work by a group at Toyota, and has been featured in several review articles [5,6,*7,8]. Okada and Usuki, for instance, have
1359-0286 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 02 )00049-9
206
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
reviewed such hybrids, and have described a number of polymer / silicate nanocomposites as having ‘excellent mechanical, thermal and chemical properties compared with unfilled polymers and / or conventional composites’. They go on to present NMR data addressing materials behavior, and discuss the mechanisms of reinforcement in these systems. It is known, then, that nylon nanocomposites can attain significant improvements in stiffness, strength and heat distortion temperature with much lower inorganic content compared to corresponding macrocomposites, making them lightweight as well. This combination of enhanced performance and reduced weight can contribute to more fuel efficient and environmentally friendly automobiles, for example. More importantly, these nylon nanocomposites combine enhanced stiffness with improved toughness. Skeptics have wondered whether nylon would be the only system to exhibit this set of properties. Recently, we have observed similar behavior in other polymer systems, including thermoplastic olefins (TPOs) and polyimide. In this review, we highlight the properties of specific nanocomposite systems while emphasizing the theme of a lack of properties trade-offs. We also present our work on the structure and dynamics at the polymer / nanofiller interface, in an attempt to better understand the unique behavior of nanocomposites.
3. Background We start by defining a few terms. In general, nanocomposites consist of a nanometer-scale phase in combination with another phase. Classified by nanofiller dimensionality, there are a number of types of nanocomposites. Zero-dimensional (nanoparticle), one-dimensional (nanofiber), two-dimensional (nanolayer), and three-dimensional (interpenetrating network) systems can all be imagined. Our work focuses on two-dimensional systems, consisting of silicate layers of nanometer thickness and high aspect ratio (30–10,000), dispersed in a polymer matrix; we will generically refer to these systems as ‘nanocomposites’. The silicates they contain may be naturally occurring (montmorillonite and other smectites) or synthetic (fluorohectorite, fluoromica, layered double hydroxides, etc.), so long as they have the appropriate dimensions and chemistry. As the inorganic nanolayers that make up these materials tend to be hydrophilic and polymers tend to be hydrophobic, the nanolayers must be compatibilized with the polymer matrix. Modifications of the nanolayer surface are generally achieved either by surfactant cation exchange, as well as by introduction of copolymers containing at least one component with high affinity for the silicate surface. Edge modifications are also possible, and may be accomplished by anion exchange or silane treatment. In general, we refer to organically modified silicate nanolayers as ‘nanoclays’ or ‘organosilicates’.
As an aside, it is also worth noting the existence of other nanofillers that are of interest for use in other types of polymer nanocomposites. Alumina nanofibers and carbon nanotubes are examples of one-dimensional nanofillers, while carbon black, fumed silica, and polyhedral oligomeric silsesquioxanes (POSS) can be classified as nanoparticle reinforcing agents. In a most general sense, the principle is the same; these systems simply shift the balance between aspect ratio and surface area, versus layered inorganics.
4. Biodegradable nanocomposites Applications for biodegradable polymers include disposable food service items, food packaging, health care products, packing foams and agricultural mulch film. Most of these items are currently produced from polyethylene or polystyrene. Biodegradable polymers have also become indispensable in a wide range of implantable applications, including orthopedic and dental devices, drug delivery systems and tissue engineering scaffolds [9–11,*12,13]. Well over 10 million Americans carry at least one major implanted medical device with the medical device industry topping $50 billion in annual sales. Amass and Tighe [9] review the uses of a number of biodegradable polymers and polymer blends with respect to a number of the aforementioned applications, while discussing the properties of some of the system that have attracted our interest as nanocomposites. Despite their attractive degradation characteristics and significant demand for such materials, the lack of structural and functional stability prevents currently available biodegradable polymers from having widespread commercial impact. In fact, Demirgoz et al. [14] present these issues as impetus for their own work on improving the performance of these otherwise attractive biodegradable polymers. One of our objectives is to develop biodegradable nanocomposites that combine the superior properties of non-degradable plastics with biodegradability. We have recently developed a series of poly(l-lactic acid) (PLA) nanocomposites based on different layered inorganics. Dynamic mechanical analysis shows that the nanocomposites exhibit higher moduli compared to the pure PLA. The enhancement is most prominent above T g , significantly extending the working temperature of the nanocomposites versus neat PLA. Furthermore, the increases in mechanical performance do not hinder biodegradation. Marine respirometry and measurements of crystallinity in solution simulating a physiological environment show that the rate of biodegradation is enhanced six to ten times in the nanocomposites (Fig. 1). In addition, choice of organosilicate type and loading level affects biodegradation kinetics, suggesting that the biodegradation can be fine-tuned. The combination of improved mechanicals with
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
207
Fig. 2. Compressive strength of poly(N-isopropyl acrylamide) (NIPAM) nanocomposite hydrogels. The unfilled NIPAM is too weak to test by this method.
Fig. 1. Biodegradation studies on poly(l-lactic acid) (PLA) and PLA nanocomposites by marine respirometry (upper plot) and crystallinity measurements in a Ringer’s solution (lower plot).
controlled biodegradability makes these materials uniquely attractive for the applications described above.
5. Thermoreversible gels Thermoreversible gels can find applications in a number of areas, including materials for artificial muscles, drug delivery systems, and actuators and chemical valves in microfluidic devices [15–17]. This is due to the large volume change they undergo as they are cycled between a high and low temperature. Gels based on poly(N-isopropyl acrylamide) (NIPAM) are one of the most well-known systems. Their volume contracts as much as 100 times when heated a few degrees above room temperature, the change being reversible on cooling. The need to attain large volume changes and fast (de-)swelling kinetics
dictates a low crosslink density. As a result, the gels suffer from a lack of mechanical robustness. NIPAM nanocomposites combine improved mechanical robustness with enhanced release characteristics. Fig. 2 shows the mechanical properties of a series of nanocomposite gels. In our work, we have also found that optimized NIPAM nanocomposites show swelling capacity and release rates on par with or better than unfilled NIPAM hydrogels. The origin of this behavior has not been established, but one explanation involves the lowering of the crosslink density due to the presence of the silicate layers (as crosslinks may not form through these layers), combined with changes in behavior at the polymer / silicate interface. While our understanding of these systems is still developing, the properties observed make these materials very attractive for the applications mentioned above. Note that Liang et al. [16] find that the release characteristics of NIPAM nanocomposite hydrogels improve compared to the neat gels, while Messersmith et al. [17] report degraded properties. Liang et al. use an organosilicate, while the nanocomposite hydrogels studied by Messersmith et al. are based on a pristine (unmodified) silicates. This emphasizes the critical role that compatibilization of the silicate layers and differences in hydrophobic / hydrophilic character play with respect to the release characteristics of the nanocomposite hydrogels.
6. Flammability of nanocomposites Over the years, numerous additives have been developed for use as fire-retardants in polymers [18–20]. Kashiwagi et al. have addressed the use of silica, for instance, while Wilkie covers a broad range of fire-retardant polymer additives. One example is hydrated alumina, which liberates water in an endothermic reaction, cooling the material
208
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
Fig. 3. Cone calorimetry data for poly(styrene) (PS) and a PS nanocomposite.
and increasing the amount of energy needed to maintain the flame. Another class of flame-retardants involves halogens, which quench the radicals that make up the flame. The third group consists of phosphorous containing additives, which function either in the vapor phase, like the halogens, or in the solid phase, by either changing the degradation mechanism of the polymer or by enhancing char formation. Char insulates the underlying polymer from the heat of the flame, preventing production of new fuel and further burning. While each of these additives decreases flammability, they also present their own disadvantages. For example, to be effective, very high loading levels of hydrated alumina must be used, adversely affecting the physical and mechanical properties of the polymer. Halogenated materials present environmental problems, while char-formers are very system-specific. In collaboration with Gilman and others at the National Institute of Standards and Technology (NIST), we have shown that that the peak heat release rate of polymer / silicate nanocomposites containing only 2 to 6 wt% nanoclay decreases by 40 to 60%, without an increase in the amount of carbon monoxide or soot produced during the combustion [21]. Fig. 3 shows the heat release rate of a poly(styrene) nanocomposite (a measure of the flammability of the polymer), as compared to an unfilled poly(styrene). In addition, we also find, in such systems, that the physical and mechanical performance of the polymer is not adversely impacted, which is typically the case with conventional flame retardants.
7. Compatibilization of immiscible polymers Several applications require blending of polymers with dissimilar properties for optimum performance. Due to the
diverse chemistry of polymers and the low entropy of mixing, blends tend to be immiscible. In addition to variations in polymer chemistry, copolymers and other amphiphiles have frequently been used as compatibilizers, to lower the interfacial tension between two polymers [22–24]. Kim, Rafailovich, and Sokolov, for instance, address the role of block copolymers in improving interfacial adhesion [24]. Our group has demonstrated that nanoclays can be used to improve compatibilization and wetting of one polymer on the surface of another, despite immiscibility between the two. In this work, the compatibilization of poly(styrene) (PS) on poly(methyl methacrylate) (PMMA) surfaces was studied. The presence of nanolayers affects the domain evolution in polymer blend films. The PS phase appears to wet the PMMA surface more effectively in films containing organosilicates (Fig. 4), indicating that they are acting as compatibilizers. The work of Voulgaris and Petridis supports this conclusion, showing that organically modified layered silicates can act as emulsifying agents in immiscible polymer blends [25]. How can nanoparticles act as compatibilizers? Both PS and PMMA are individually miscible with nanoclays, intercalating into the inter-layer region as indicated by X-ray diffraction. In the blend, it is reasonable to assume that both PS and PMMA intercalate randomly between the silicate layers, improving the wetting between the two phases. Phase contrast AFM shows that the nanolayers are present in both the PS and PMMA phases; more importantly, they appear to segregate along the PS / PMMA interface. An alternative explanation has to do with kinetics; addition of the nanolayers increases the viscosity of the blend, which can retard droplet formation. Regardless of the reasons, the nanolayers do affect the phase stability of the blend. Further studies are currently under way to elucidate the underlying mechanism.
8. Structure and dynamics at the polymer / inorganic interface All composites share a common feature: the presence of an interphase polymer layer near the inorganic surface, with properties differing dramatically from the bulk polymer. Because of this interphase layer, composite properties are more than the sum of the constituent bulk properties. Due to the large surface area of the inorganic nanofiller, the interphase polymer is expected to dominate the properties of the nanocomposites. In addition, because of their structural regularity, nanocomposites are excellent model systems to probe the structure and dynamics of the interphase polymer, and relate them to physical and mechanical properties. From our experiments combined with atomistically detailed computer simulations, a picture of the structure and dynamics of the interphase polymer in nanocomposites has started to emerge.
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
209
Fig. 4. AFM-based topographical cross-sections of poly(styrene) / poly(methylmethacrylate) (PS / PMMA) blends compatibilized with varying amounts of organosilicate. Differentiation of the two polymers was accomplished by imaging of both, selective dissolution of the PS, and re-imaging of the remaining PMMA.
A unique insight into the structure at the interface is provided by computer simulation studies [26]. We start with the structure of an octadecylammonium-exchanged fluorohectorite silicate, abbreviated C 18 FH, a typical precursor to many nanocomposites. A density profile reveals the tri-layer structure of surfactant molecules present in the silicate galleries. We see that molecular layering of the gallery species is not limited to the small surfactant chains, but is also observed in much longer intercalated polymer chains. Fig. 5 (top) shows a snapshot of the gallery structure of poly(ethylene oxide) (PEO) / silicate nanocomposites. The silicate structure is denoted using coordination polyhedra, while Li 1 is represented by blue spheres and PEO chains are represented by chains of gray and red spheres. Because of its polar nature, intercalation of PEO is achieved using the unmodified (Li 1 ) form of the silicate [27]. In contrast, intercalation of PS can only be achieved if the silicate galleries are organically modified, in this case with octadecylammonium cations. Fig. 5 (middle) shows the gallery structure and the corresponding density profile for PS nanocomposites. The aromatic character of the
styrene rings leads to the formation of partial charges. These partial charges interact with the negatively charged silicate surface and the positively charged surfactant head groups, leading to electrostatic stabilization of the system with styrene rings preferentially occupying the spaces adjacent to silicate layers [28]. This is in agreement with the conclusions of the Lattice Model theory of melt intercalation proposed by Vaia et al. [29], which suggests that the driving force for polymer intercalation arises from strong polymer / silicate interactions that offset the unfavorable entropy changes due to the polymer confinement upon intercalation. The behavior of copolymers has also been of interest. Poly(isoprene) (PI) is non-polar, and therefore does not interact favorably with the silicate surface; thus, pure PI does not intercalate into either pristine or alkylammoniummodified silicates. In order to achieve intercalation, a block copolymer approach is employed, wherein favorable interactions of one block with the silicate help to pull the other block inside the galleries. We have experimentally shown that the PS–PI copolymers can intercalate between the silicate layers.
210
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
gallery. The surfactant chains still favor the center of the silicate gallery, but there is a greater accumulation of surfactant atoms next to the silicate as compared with the PS case. In addition, a comparison of the density profiles, as the PS to PI ratio is decreased, shows the gradual movement of the surfactants from the center of the gallery to the silicate surface. This is due to the fact that surfactant / silicate interactions are more favorable than PI / silicate interactions [28].
9. Dynamics
Fig. 5. Molecular dynamics simulations of polymer / silicate nanocomposites. Shown are three systems: poly(ethylene oxide) (PEO) with unmodified silicate (top), poly(styrene) (PS) with octadecylammoniummodified fluorohectorite (middle), and poly(styrene-block-isoprene) (PSPI) with octadecylammonium-modified fluorohectorite (bottom). In all cases, the lowest energy conformation is shown, at left, with the silicate layers denoted by coordination polyhedra. Cationic species (sodium atoms or surfactant head groups) appear as spheres. Where applicable, surfactant tails are shown in tan. Polymers are colored according to functionality: Black (methylenes) and red (oxygens) for PEO, blue (backbone) and green (rings) for PS, and additionally, for PSPI, red (unsaturated carbons) and cyan (saturated carbons), respectively. To the right of the PS and PSPI conformational diagrams, atomic density plots are shown, and colored accordingly; for instance, surfactant carbon density is plotted in tan, while total carbon density is plotted in black. Density variations (horizontal) are shown versus position within the silicate gallery (vertical).
The equilibrium structure and corresponding density profile are illustrated in Fig. 5 (bottom). The density profiles illustrate the segmental ordering of the diblock inside the silicate gallery. It can be seen that PS resides close to the silicate, while PI occupies the center of the
Our tools for the measurement of polymer dynamics include NMR techniques, dielectric relaxation spectroscopy, inelastic neutron scattering and rheological measurements [30,31,*32,33]. Using surface-sensitive cross polarization with spin-echo NMR measurements in collaboration with D. Zax from the Department of Chemistry, we have shown that the polymer chains in the nanocomposite exhibit a rather unique behavior (Fig. 6) [30]. The bulk polymer (right-hand side, Fig. 6), as expected, displays solid-like character (on-diagonal signal) at T , T g and liquid-like character (off-diagonal signal), at T . T g . In contrast, the nanocomposite possesses both liquid- and solid-like character (both off- and on-diagonal signals) at all temperatures (left-hand side, Fig. 6). In other words, the polymer chains in the nanocomposite exhibit liquid-like mobility even at temperatures below the bulk polymer T g . Similar behavior was observed in a poly(methylphenylsiloxane) (PMPS) nanocomposite, using dielectric relaxation spectroscopy. In collaboration with S. Anastasiadis from IESL, Greece, we have shown that the polymer in this nanocomposite exhibits two relaxation modes [31]. The new relaxation mode seen in the nanocomposite, which dominates the relaxation spectrum, is much faster than the bulk-polymer a-relaxation, and exhibits much weaker temperature dependence as well, suggesting that the polymer in the nanocomposite is more mobile at all temperatures. Intuitively, this is very different from what one might expect; at first glance, it seems that the polymer in the nanocomposite should be less mobile compared to the bulk polymer, as confinement of the polymer chains within a few nanometers should increase their solid-like character and decrease their mobility. In fact, a decrease in mobility of polymers near the interface has been suggested to account for the presence of two glass transitions (the second at a higher temperature) in polymer composites [34]. Polymer dynamics in nanocomposites can be rationalized using the computer simulations discussed earlier, which provide an atomistically detailed view of the interface. Our simulations show that the polymer at the interface organizes into discrete layers, with areas of high and low density coexisting in close proximity to each other
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
211
Fig. 6. Surface-sensitive cross-polarization spin-echo NMR spectroscopy of poly(styrene) (PS), at right, and a PS nanocomposite (left), both below (upper) and above (lower) the glass-transition temperature of bulk poly(styrene). On-diagonal and off-diagonal signal denotes low-mobility, solid-like polymer and high-mobility, liquid-like polymer behavior, respectively.
(Fig. 5) [26–28]. The experimentally observed coexistence of liquid- and solid-like character reflects local variations in density (density plots, Fig. 5), as segments of low and high density are expected to exhibit fast and slow dynamics, respectively. The increased toughness in the nanocomposites may be due to this increased mobility of the polymer at all temperatures.
10. Conclusions From mechanical and physical reinforcement to enhancing the applicability of biodegradable and biocompatible systems to producing self-extinguishing polymers or even compatibilizing otherwise immiscible polymer blends, there are numerous applications for nanocomposites. As new systems are explored and new areas of use are targeted, it is at the same time very important to ensure that we emphasize not only what is possible, but also why it is possible. Furthering our understanding of the fundamentals of these nanocomposites, including structure and dynamics, will allow us to produce materials while avoiding the trade-offs of conventional systems. In addition, the unique behavior found in nanocomposites will allow us to apply these novel materials in wholly new and different applications. These are some of the most exciting prospects offered by nanocomposites.
Acknowledgements This work was supported by AFOSR, ONR, NSF (MRSEC at Cornell), CAT, NIST. We thank our colleagues and collaborators S. Anastasiadis, S. Bandyopadhyay, F. Clement, J. Gilmann, E. Hackett, G. Ji, K. Tyner, and D. Zax.
References Papers of particular interest have been highlighted as: * of special interest. [*1] Giannelis EP. Adv Mater 1996;8:29. [2] LeBaron PC, Wang Z, Pinnavaia TJ. Appl Clay Sci 1999;15(1– 2):11. [*3] Alexandre M, Dubois P. Mater Sci Eng Rep 2000;28(1–2):1. [*4] Vaia RA, Giannelis EP. MRS Bulletin 2001;26(5):394. [5] Okada A, Kawasumi M, Kurauchi T, Kamigaito O. Polym Prepr 1987;28:447. [6] Usuki A, Kawasumi M, Kojima Y, Okada A, Kurauchi T, Kamigaito O. J Mater Res 1993;8(5):1174. [*7] Yano K, Usuki A, Okada A, Kurauchi T, Kamigaito O. J Poly Sci (Part A) 1993;31:2493. [8] Okada A, Usuki A. Mater Sci Eng C-Biomimet Mater Sensors Syst 1995;3(2):109. [9] Amass W, Amass A, Tighe B. Polym Int 1998;47(2):89. [10] Kumar N, Ravikumar MNV, Domb AJ. Adv Drug Delivery Rev 2001;53(1):23.
212
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
[11] Ratto JA, Stenhouse PJ, Auerbach M, Mitchell J, Farrell R. Polymer 1999;40(24):6777. [*12] Mayer JM, Kaplan DL. Trends Polym Sci 1994;2:227. [13] Jacoby M. Chem Eng News 2001;79(6):30. [14] Demirgoz D, Elvira C, Mano JF, Cunha AM, Piskin E, Reis RL. Polym Degradation Stabil 2000;70(2):161. [15] Cypes SH, Saltzman WM, Gemeinhart RA, Giannelis EP. Abstracts of papers of the American Chemical Society 2001;221:657. [16] Liang L, Liu J, Gong XY. Langmuir 2000;16(25):9895. [17] Phillip B, Messersmith, Znidarsich F. Materials research. In: Society symposium proceedings, 1997, pp. 457–507. [18] Nelson GL. In: ACS symposium series 599: fire and polymers II, 1995. [19] Kashiwagi T, Gilman JW, Butler KM, Harris RH, Shields JR, Asano A. Fire Mater 2000;24(6):277. [20] Wilkie CA. J Vinyl Additive Tech 1999;5(4):172. [21] Gilman JW, Jackson CL, Morgan AB, Harris R, Manias E, Giannelis EP, Wuthenow M, Hilton D, Phillips SH. Chem Mater 2000;12(7):1866.
[*22] Kim CK, Paul DR. Macromolecules 1992;25(12):3097. [23] Karim A, Douglas JF, Satija SK, Han CC, Goyette RJ. Macromolecules 1999;32(4):1119. [24] Kim HJ, Rafailovich MH, Sokolov J. J Adhesion 2001;77(1):81. [25] Voulgaris D, Petridis D. Polymer 2002;43(8):2213. [26] Hackett E, Manias E, Giannelis EP. J Chem Phys 1998;108(17):7410. [27] Hackett E, Manias E, Giannelis EP. Chem Mater 2000;12(8):2161. [28] Shah D, Bitsanis IA, Natarajan U, Hackett E, Giannelis EP. MRS Proceedings 2002 (in press). [29] Vaia RA, Giannelis EP. Macromolecules 1997;30(25):7990. [30] Zax DB, Yang DK, Santos RA, Hegemann H, Giannelis EP, Manias E. J Chem Phys 2000;112(6):2945. [31] Anastasiadis SH, Karatasos K, Vlachos G, Manias E, Giannelis EP. Phys Rev Lett 2000;84(5):915. [*32] Manias E, Chen H, Krishnamoorti R, Genzer J, Kramer EJ, Giannelis EP. Macromolecules 2001;33(21):7955. [33] Krishnamoorti R, Giannelis EP. Langmuir 2001;17(5):1448. [34] Tsagaropoulos G, Eisenberg A. Macromolecules 1995;28:6067.
New advances in polymer / layered silicate nanocomposites Daniel Schmidt, Deepak Shah, Emmanuel P. Giannelis* Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853, USA Received 3 April 2002; accepted 12 June 2002
Abstract This review discusses some recent advances in polymer silicate nanocomposites. In particular, we highlight the properties of specific nanocomposites while emphasizing the lack of properties trade-offs in these systems. We also present our work on the structure and dynamics of the polymer / nanofiller interface and attempt to relate them to macroscopic nanocomposite properties. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Work in polymer nanocomposites has exploded over the last few years. The prospect of a new materials technology that can function as a low-cost alternative to high-performance composites for applications ranging from automotive to food packaging to tissue engineering has become irresistible to researchers around the world [*1,2,*3,*4]. The essence of nanotechnology is the ability to work at the molecular level to create large structures with fundamentally new molecular organization. Materials with features on the scale of nanometers often have properties different from their macroscale counterparts. Important among nanoscale materials are nanohybrids or nanocomposites, materials in which the constituents are mixed on a nanometer-length scale. They often exhibit properties superior to conventional composites, such as strength, stiffness, thermal and oxidative stability, barrier properties, as well as unique properties like self-extinguishing behavior and tunable biodegradability. Another unique aspect of nanocomposites is the lack of properties trade-offs. For the first time, there is an opportunity to design materials without the compromises typically found in conventionally filled polymer composites. Uses for this new class of materials can be found in aerospace,
*Corresponding author. Tel.: 11-607-255-9680; fax: 11-607-2552365. E-mail address: [email protected] (E.P. Giannelis).
automotive, electronics and biotechnology applications, to list only a few.
2. Motivation Though significant progress has been made in developing nanocomposites with different polymer matrices, a general understanding has yet to emerge. For example, what allows nanocomposites to be both stiffer and tougher than conventional composites, without sacrificing other properties? Why do they display better thermal stability versus unfilled polymers? How can we utilize specific molecular interactions to control structure and morphology? How can materials properties be predicted from nanostructural data and molecular dynamics simulations? A major challenge to further development of nanocomposites is the lack of even simple structure–property models. Without such models, progress in nanocomposites has remained largely empirical. Similarly, predicting ultimate materials properties or maximum theoretical performance for different classes of nanocomposites is almost impossible at present. At Cornell, our group has pursued new synthetic approaches, molecular-level physical measurements and molecular modeling and theory. Our objective is to build an understanding that will permit the prediction and control of nanocomposite properties. The unique nature of nylon nanocomposites has been reported in pioneering work by a group at Toyota, and has been featured in several review articles [5,6,*7,8]. Okada and Usuki, for instance, have
1359-0286 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 02 )00049-9
206
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
reviewed such hybrids, and have described a number of polymer / silicate nanocomposites as having ‘excellent mechanical, thermal and chemical properties compared with unfilled polymers and / or conventional composites’. They go on to present NMR data addressing materials behavior, and discuss the mechanisms of reinforcement in these systems. It is known, then, that nylon nanocomposites can attain significant improvements in stiffness, strength and heat distortion temperature with much lower inorganic content compared to corresponding macrocomposites, making them lightweight as well. This combination of enhanced performance and reduced weight can contribute to more fuel efficient and environmentally friendly automobiles, for example. More importantly, these nylon nanocomposites combine enhanced stiffness with improved toughness. Skeptics have wondered whether nylon would be the only system to exhibit this set of properties. Recently, we have observed similar behavior in other polymer systems, including thermoplastic olefins (TPOs) and polyimide. In this review, we highlight the properties of specific nanocomposite systems while emphasizing the theme of a lack of properties trade-offs. We also present our work on the structure and dynamics at the polymer / nanofiller interface, in an attempt to better understand the unique behavior of nanocomposites.
3. Background We start by defining a few terms. In general, nanocomposites consist of a nanometer-scale phase in combination with another phase. Classified by nanofiller dimensionality, there are a number of types of nanocomposites. Zero-dimensional (nanoparticle), one-dimensional (nanofiber), two-dimensional (nanolayer), and three-dimensional (interpenetrating network) systems can all be imagined. Our work focuses on two-dimensional systems, consisting of silicate layers of nanometer thickness and high aspect ratio (30–10,000), dispersed in a polymer matrix; we will generically refer to these systems as ‘nanocomposites’. The silicates they contain may be naturally occurring (montmorillonite and other smectites) or synthetic (fluorohectorite, fluoromica, layered double hydroxides, etc.), so long as they have the appropriate dimensions and chemistry. As the inorganic nanolayers that make up these materials tend to be hydrophilic and polymers tend to be hydrophobic, the nanolayers must be compatibilized with the polymer matrix. Modifications of the nanolayer surface are generally achieved either by surfactant cation exchange, as well as by introduction of copolymers containing at least one component with high affinity for the silicate surface. Edge modifications are also possible, and may be accomplished by anion exchange or silane treatment. In general, we refer to organically modified silicate nanolayers as ‘nanoclays’ or ‘organosilicates’.
As an aside, it is also worth noting the existence of other nanofillers that are of interest for use in other types of polymer nanocomposites. Alumina nanofibers and carbon nanotubes are examples of one-dimensional nanofillers, while carbon black, fumed silica, and polyhedral oligomeric silsesquioxanes (POSS) can be classified as nanoparticle reinforcing agents. In a most general sense, the principle is the same; these systems simply shift the balance between aspect ratio and surface area, versus layered inorganics.
4. Biodegradable nanocomposites Applications for biodegradable polymers include disposable food service items, food packaging, health care products, packing foams and agricultural mulch film. Most of these items are currently produced from polyethylene or polystyrene. Biodegradable polymers have also become indispensable in a wide range of implantable applications, including orthopedic and dental devices, drug delivery systems and tissue engineering scaffolds [9–11,*12,13]. Well over 10 million Americans carry at least one major implanted medical device with the medical device industry topping $50 billion in annual sales. Amass and Tighe [9] review the uses of a number of biodegradable polymers and polymer blends with respect to a number of the aforementioned applications, while discussing the properties of some of the system that have attracted our interest as nanocomposites. Despite their attractive degradation characteristics and significant demand for such materials, the lack of structural and functional stability prevents currently available biodegradable polymers from having widespread commercial impact. In fact, Demirgoz et al. [14] present these issues as impetus for their own work on improving the performance of these otherwise attractive biodegradable polymers. One of our objectives is to develop biodegradable nanocomposites that combine the superior properties of non-degradable plastics with biodegradability. We have recently developed a series of poly(l-lactic acid) (PLA) nanocomposites based on different layered inorganics. Dynamic mechanical analysis shows that the nanocomposites exhibit higher moduli compared to the pure PLA. The enhancement is most prominent above T g , significantly extending the working temperature of the nanocomposites versus neat PLA. Furthermore, the increases in mechanical performance do not hinder biodegradation. Marine respirometry and measurements of crystallinity in solution simulating a physiological environment show that the rate of biodegradation is enhanced six to ten times in the nanocomposites (Fig. 1). In addition, choice of organosilicate type and loading level affects biodegradation kinetics, suggesting that the biodegradation can be fine-tuned. The combination of improved mechanicals with
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
207
Fig. 2. Compressive strength of poly(N-isopropyl acrylamide) (NIPAM) nanocomposite hydrogels. The unfilled NIPAM is too weak to test by this method.
Fig. 1. Biodegradation studies on poly(l-lactic acid) (PLA) and PLA nanocomposites by marine respirometry (upper plot) and crystallinity measurements in a Ringer’s solution (lower plot).
controlled biodegradability makes these materials uniquely attractive for the applications described above.
5. Thermoreversible gels Thermoreversible gels can find applications in a number of areas, including materials for artificial muscles, drug delivery systems, and actuators and chemical valves in microfluidic devices [15–17]. This is due to the large volume change they undergo as they are cycled between a high and low temperature. Gels based on poly(N-isopropyl acrylamide) (NIPAM) are one of the most well-known systems. Their volume contracts as much as 100 times when heated a few degrees above room temperature, the change being reversible on cooling. The need to attain large volume changes and fast (de-)swelling kinetics
dictates a low crosslink density. As a result, the gels suffer from a lack of mechanical robustness. NIPAM nanocomposites combine improved mechanical robustness with enhanced release characteristics. Fig. 2 shows the mechanical properties of a series of nanocomposite gels. In our work, we have also found that optimized NIPAM nanocomposites show swelling capacity and release rates on par with or better than unfilled NIPAM hydrogels. The origin of this behavior has not been established, but one explanation involves the lowering of the crosslink density due to the presence of the silicate layers (as crosslinks may not form through these layers), combined with changes in behavior at the polymer / silicate interface. While our understanding of these systems is still developing, the properties observed make these materials very attractive for the applications mentioned above. Note that Liang et al. [16] find that the release characteristics of NIPAM nanocomposite hydrogels improve compared to the neat gels, while Messersmith et al. [17] report degraded properties. Liang et al. use an organosilicate, while the nanocomposite hydrogels studied by Messersmith et al. are based on a pristine (unmodified) silicates. This emphasizes the critical role that compatibilization of the silicate layers and differences in hydrophobic / hydrophilic character play with respect to the release characteristics of the nanocomposite hydrogels.
6. Flammability of nanocomposites Over the years, numerous additives have been developed for use as fire-retardants in polymers [18–20]. Kashiwagi et al. have addressed the use of silica, for instance, while Wilkie covers a broad range of fire-retardant polymer additives. One example is hydrated alumina, which liberates water in an endothermic reaction, cooling the material
208
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
Fig. 3. Cone calorimetry data for poly(styrene) (PS) and a PS nanocomposite.
and increasing the amount of energy needed to maintain the flame. Another class of flame-retardants involves halogens, which quench the radicals that make up the flame. The third group consists of phosphorous containing additives, which function either in the vapor phase, like the halogens, or in the solid phase, by either changing the degradation mechanism of the polymer or by enhancing char formation. Char insulates the underlying polymer from the heat of the flame, preventing production of new fuel and further burning. While each of these additives decreases flammability, they also present their own disadvantages. For example, to be effective, very high loading levels of hydrated alumina must be used, adversely affecting the physical and mechanical properties of the polymer. Halogenated materials present environmental problems, while char-formers are very system-specific. In collaboration with Gilman and others at the National Institute of Standards and Technology (NIST), we have shown that that the peak heat release rate of polymer / silicate nanocomposites containing only 2 to 6 wt% nanoclay decreases by 40 to 60%, without an increase in the amount of carbon monoxide or soot produced during the combustion [21]. Fig. 3 shows the heat release rate of a poly(styrene) nanocomposite (a measure of the flammability of the polymer), as compared to an unfilled poly(styrene). In addition, we also find, in such systems, that the physical and mechanical performance of the polymer is not adversely impacted, which is typically the case with conventional flame retardants.
7. Compatibilization of immiscible polymers Several applications require blending of polymers with dissimilar properties for optimum performance. Due to the
diverse chemistry of polymers and the low entropy of mixing, blends tend to be immiscible. In addition to variations in polymer chemistry, copolymers and other amphiphiles have frequently been used as compatibilizers, to lower the interfacial tension between two polymers [22–24]. Kim, Rafailovich, and Sokolov, for instance, address the role of block copolymers in improving interfacial adhesion [24]. Our group has demonstrated that nanoclays can be used to improve compatibilization and wetting of one polymer on the surface of another, despite immiscibility between the two. In this work, the compatibilization of poly(styrene) (PS) on poly(methyl methacrylate) (PMMA) surfaces was studied. The presence of nanolayers affects the domain evolution in polymer blend films. The PS phase appears to wet the PMMA surface more effectively in films containing organosilicates (Fig. 4), indicating that they are acting as compatibilizers. The work of Voulgaris and Petridis supports this conclusion, showing that organically modified layered silicates can act as emulsifying agents in immiscible polymer blends [25]. How can nanoparticles act as compatibilizers? Both PS and PMMA are individually miscible with nanoclays, intercalating into the inter-layer region as indicated by X-ray diffraction. In the blend, it is reasonable to assume that both PS and PMMA intercalate randomly between the silicate layers, improving the wetting between the two phases. Phase contrast AFM shows that the nanolayers are present in both the PS and PMMA phases; more importantly, they appear to segregate along the PS / PMMA interface. An alternative explanation has to do with kinetics; addition of the nanolayers increases the viscosity of the blend, which can retard droplet formation. Regardless of the reasons, the nanolayers do affect the phase stability of the blend. Further studies are currently under way to elucidate the underlying mechanism.
8. Structure and dynamics at the polymer / inorganic interface All composites share a common feature: the presence of an interphase polymer layer near the inorganic surface, with properties differing dramatically from the bulk polymer. Because of this interphase layer, composite properties are more than the sum of the constituent bulk properties. Due to the large surface area of the inorganic nanofiller, the interphase polymer is expected to dominate the properties of the nanocomposites. In addition, because of their structural regularity, nanocomposites are excellent model systems to probe the structure and dynamics of the interphase polymer, and relate them to physical and mechanical properties. From our experiments combined with atomistically detailed computer simulations, a picture of the structure and dynamics of the interphase polymer in nanocomposites has started to emerge.
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
209
Fig. 4. AFM-based topographical cross-sections of poly(styrene) / poly(methylmethacrylate) (PS / PMMA) blends compatibilized with varying amounts of organosilicate. Differentiation of the two polymers was accomplished by imaging of both, selective dissolution of the PS, and re-imaging of the remaining PMMA.
A unique insight into the structure at the interface is provided by computer simulation studies [26]. We start with the structure of an octadecylammonium-exchanged fluorohectorite silicate, abbreviated C 18 FH, a typical precursor to many nanocomposites. A density profile reveals the tri-layer structure of surfactant molecules present in the silicate galleries. We see that molecular layering of the gallery species is not limited to the small surfactant chains, but is also observed in much longer intercalated polymer chains. Fig. 5 (top) shows a snapshot of the gallery structure of poly(ethylene oxide) (PEO) / silicate nanocomposites. The silicate structure is denoted using coordination polyhedra, while Li 1 is represented by blue spheres and PEO chains are represented by chains of gray and red spheres. Because of its polar nature, intercalation of PEO is achieved using the unmodified (Li 1 ) form of the silicate [27]. In contrast, intercalation of PS can only be achieved if the silicate galleries are organically modified, in this case with octadecylammonium cations. Fig. 5 (middle) shows the gallery structure and the corresponding density profile for PS nanocomposites. The aromatic character of the
styrene rings leads to the formation of partial charges. These partial charges interact with the negatively charged silicate surface and the positively charged surfactant head groups, leading to electrostatic stabilization of the system with styrene rings preferentially occupying the spaces adjacent to silicate layers [28]. This is in agreement with the conclusions of the Lattice Model theory of melt intercalation proposed by Vaia et al. [29], which suggests that the driving force for polymer intercalation arises from strong polymer / silicate interactions that offset the unfavorable entropy changes due to the polymer confinement upon intercalation. The behavior of copolymers has also been of interest. Poly(isoprene) (PI) is non-polar, and therefore does not interact favorably with the silicate surface; thus, pure PI does not intercalate into either pristine or alkylammoniummodified silicates. In order to achieve intercalation, a block copolymer approach is employed, wherein favorable interactions of one block with the silicate help to pull the other block inside the galleries. We have experimentally shown that the PS–PI copolymers can intercalate between the silicate layers.
210
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
gallery. The surfactant chains still favor the center of the silicate gallery, but there is a greater accumulation of surfactant atoms next to the silicate as compared with the PS case. In addition, a comparison of the density profiles, as the PS to PI ratio is decreased, shows the gradual movement of the surfactants from the center of the gallery to the silicate surface. This is due to the fact that surfactant / silicate interactions are more favorable than PI / silicate interactions [28].
9. Dynamics
Fig. 5. Molecular dynamics simulations of polymer / silicate nanocomposites. Shown are three systems: poly(ethylene oxide) (PEO) with unmodified silicate (top), poly(styrene) (PS) with octadecylammoniummodified fluorohectorite (middle), and poly(styrene-block-isoprene) (PSPI) with octadecylammonium-modified fluorohectorite (bottom). In all cases, the lowest energy conformation is shown, at left, with the silicate layers denoted by coordination polyhedra. Cationic species (sodium atoms or surfactant head groups) appear as spheres. Where applicable, surfactant tails are shown in tan. Polymers are colored according to functionality: Black (methylenes) and red (oxygens) for PEO, blue (backbone) and green (rings) for PS, and additionally, for PSPI, red (unsaturated carbons) and cyan (saturated carbons), respectively. To the right of the PS and PSPI conformational diagrams, atomic density plots are shown, and colored accordingly; for instance, surfactant carbon density is plotted in tan, while total carbon density is plotted in black. Density variations (horizontal) are shown versus position within the silicate gallery (vertical).
The equilibrium structure and corresponding density profile are illustrated in Fig. 5 (bottom). The density profiles illustrate the segmental ordering of the diblock inside the silicate gallery. It can be seen that PS resides close to the silicate, while PI occupies the center of the
Our tools for the measurement of polymer dynamics include NMR techniques, dielectric relaxation spectroscopy, inelastic neutron scattering and rheological measurements [30,31,*32,33]. Using surface-sensitive cross polarization with spin-echo NMR measurements in collaboration with D. Zax from the Department of Chemistry, we have shown that the polymer chains in the nanocomposite exhibit a rather unique behavior (Fig. 6) [30]. The bulk polymer (right-hand side, Fig. 6), as expected, displays solid-like character (on-diagonal signal) at T , T g and liquid-like character (off-diagonal signal), at T . T g . In contrast, the nanocomposite possesses both liquid- and solid-like character (both off- and on-diagonal signals) at all temperatures (left-hand side, Fig. 6). In other words, the polymer chains in the nanocomposite exhibit liquid-like mobility even at temperatures below the bulk polymer T g . Similar behavior was observed in a poly(methylphenylsiloxane) (PMPS) nanocomposite, using dielectric relaxation spectroscopy. In collaboration with S. Anastasiadis from IESL, Greece, we have shown that the polymer in this nanocomposite exhibits two relaxation modes [31]. The new relaxation mode seen in the nanocomposite, which dominates the relaxation spectrum, is much faster than the bulk-polymer a-relaxation, and exhibits much weaker temperature dependence as well, suggesting that the polymer in the nanocomposite is more mobile at all temperatures. Intuitively, this is very different from what one might expect; at first glance, it seems that the polymer in the nanocomposite should be less mobile compared to the bulk polymer, as confinement of the polymer chains within a few nanometers should increase their solid-like character and decrease their mobility. In fact, a decrease in mobility of polymers near the interface has been suggested to account for the presence of two glass transitions (the second at a higher temperature) in polymer composites [34]. Polymer dynamics in nanocomposites can be rationalized using the computer simulations discussed earlier, which provide an atomistically detailed view of the interface. Our simulations show that the polymer at the interface organizes into discrete layers, with areas of high and low density coexisting in close proximity to each other
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
211
Fig. 6. Surface-sensitive cross-polarization spin-echo NMR spectroscopy of poly(styrene) (PS), at right, and a PS nanocomposite (left), both below (upper) and above (lower) the glass-transition temperature of bulk poly(styrene). On-diagonal and off-diagonal signal denotes low-mobility, solid-like polymer and high-mobility, liquid-like polymer behavior, respectively.
(Fig. 5) [26–28]. The experimentally observed coexistence of liquid- and solid-like character reflects local variations in density (density plots, Fig. 5), as segments of low and high density are expected to exhibit fast and slow dynamics, respectively. The increased toughness in the nanocomposites may be due to this increased mobility of the polymer at all temperatures.
10. Conclusions From mechanical and physical reinforcement to enhancing the applicability of biodegradable and biocompatible systems to producing self-extinguishing polymers or even compatibilizing otherwise immiscible polymer blends, there are numerous applications for nanocomposites. As new systems are explored and new areas of use are targeted, it is at the same time very important to ensure that we emphasize not only what is possible, but also why it is possible. Furthering our understanding of the fundamentals of these nanocomposites, including structure and dynamics, will allow us to produce materials while avoiding the trade-offs of conventional systems. In addition, the unique behavior found in nanocomposites will allow us to apply these novel materials in wholly new and different applications. These are some of the most exciting prospects offered by nanocomposites.
Acknowledgements This work was supported by AFOSR, ONR, NSF (MRSEC at Cornell), CAT, NIST. We thank our colleagues and collaborators S. Anastasiadis, S. Bandyopadhyay, F. Clement, J. Gilmann, E. Hackett, G. Ji, K. Tyner, and D. Zax.
References Papers of particular interest have been highlighted as: * of special interest. [*1] Giannelis EP. Adv Mater 1996;8:29. [2] LeBaron PC, Wang Z, Pinnavaia TJ. Appl Clay Sci 1999;15(1– 2):11. [*3] Alexandre M, Dubois P. Mater Sci Eng Rep 2000;28(1–2):1. [*4] Vaia RA, Giannelis EP. MRS Bulletin 2001;26(5):394. [5] Okada A, Kawasumi M, Kurauchi T, Kamigaito O. Polym Prepr 1987;28:447. [6] Usuki A, Kawasumi M, Kojima Y, Okada A, Kurauchi T, Kamigaito O. J Mater Res 1993;8(5):1174. [*7] Yano K, Usuki A, Okada A, Kurauchi T, Kamigaito O. J Poly Sci (Part A) 1993;31:2493. [8] Okada A, Usuki A. Mater Sci Eng C-Biomimet Mater Sensors Syst 1995;3(2):109. [9] Amass W, Amass A, Tighe B. Polym Int 1998;47(2):89. [10] Kumar N, Ravikumar MNV, Domb AJ. Adv Drug Delivery Rev 2001;53(1):23.
212
D. Schmidt et al. / Current Opinion in Solid State and Materials Science 6 (2002) 205–212
[11] Ratto JA, Stenhouse PJ, Auerbach M, Mitchell J, Farrell R. Polymer 1999;40(24):6777. [*12] Mayer JM, Kaplan DL. Trends Polym Sci 1994;2:227. [13] Jacoby M. Chem Eng News 2001;79(6):30. [14] Demirgoz D, Elvira C, Mano JF, Cunha AM, Piskin E, Reis RL. Polym Degradation Stabil 2000;70(2):161. [15] Cypes SH, Saltzman WM, Gemeinhart RA, Giannelis EP. Abstracts of papers of the American Chemical Society 2001;221:657. [16] Liang L, Liu J, Gong XY. Langmuir 2000;16(25):9895. [17] Phillip B, Messersmith, Znidarsich F. Materials research. In: Society symposium proceedings, 1997, pp. 457–507. [18] Nelson GL. In: ACS symposium series 599: fire and polymers II, 1995. [19] Kashiwagi T, Gilman JW, Butler KM, Harris RH, Shields JR, Asano A. Fire Mater 2000;24(6):277. [20] Wilkie CA. J Vinyl Additive Tech 1999;5(4):172. [21] Gilman JW, Jackson CL, Morgan AB, Harris R, Manias E, Giannelis EP, Wuthenow M, Hilton D, Phillips SH. Chem Mater 2000;12(7):1866.
[*22] Kim CK, Paul DR. Macromolecules 1992;25(12):3097. [23] Karim A, Douglas JF, Satija SK, Han CC, Goyette RJ. Macromolecules 1999;32(4):1119. [24] Kim HJ, Rafailovich MH, Sokolov J. J Adhesion 2001;77(1):81. [25] Voulgaris D, Petridis D. Polymer 2002;43(8):2213. [26] Hackett E, Manias E, Giannelis EP. J Chem Phys 1998;108(17):7410. [27] Hackett E, Manias E, Giannelis EP. Chem Mater 2000;12(8):2161. [28] Shah D, Bitsanis IA, Natarajan U, Hackett E, Giannelis EP. MRS Proceedings 2002 (in press). [29] Vaia RA, Giannelis EP. Macromolecules 1997;30(25):7990. [30] Zax DB, Yang DK, Santos RA, Hegemann H, Giannelis EP, Manias E. J Chem Phys 2000;112(6):2945. [31] Anastasiadis SH, Karatasos K, Vlachos G, Manias E, Giannelis EP. Phys Rev Lett 2000;84(5):915. [*32] Manias E, Chen H, Krishnamoorti R, Genzer J, Kramer EJ, Giannelis EP. Macromolecules 2001;33(21):7955. [33] Krishnamoorti R, Giannelis EP. Langmuir 2001;17(5):1448. [34] Tsagaropoulos G, Eisenberg A. Macromolecules 1995;28:6067.