Polymer Nanocomposites: Introduction

Transcription

1 Chapter 1 Polymer Nanocomposites: Introduction Downloaded via on April 5, 2019 at 22:23:00 (UTC). See for options on how to legitimately share published articles. Richard A. Vaia 1 and Ramanan Krishnamoorti 2 1 Materials and Manufacturing Directorate, Air Force Research Library, Wright-Patterson Air Force Base, OH Deaprtment of Chemical Engineering, University of Houston, Houston, TX INTRODUCTION Reinforcement of polymers with a second organic or inorganic phase to produce a polymer composite is common in the production of modern plastics. Polymer nanocomposites (PNCs) represent a radical alternative to these conventional polymer composites [1-5]. The most noteworthy effort in the last 15 years has demonstrated a doubling of the tensile modulus and strength without sacrificing impact resistance for nylon-layer silicate nanocomposites containing as little as 2 vol. % inorganic layered silicate. In addition, the heat distortion temperature of the nanocomposites increases by up to 100 C, extending the use of the composite to higher temperature environments, such as for under-the-hood parts in automobiles. Besides their improved properties, these nanocomposites materials are also easily extruded or molded to near-final shape, simplifying their manufacturing. Since high degrees of stiffness and strength are realized with far less high -density inorganic material, they are much lighter compared to conventional polymer composites. This weight advantage could have significant impact on environmental concerns amongst many other potential benefits. For example, it has been reported that widespread use of PNCs by US vehicle manufactures could save 1.5 billion liters of gasoline over the life of one year's production of vehicles and reduce related carbon dioxide emissions by more than 10 billion pounds [6]. In addition, their outstanding combination of barrier and mechanical properties may eliminate the need for a multipolymer layer design in packaging materials, enabling greater recycling of food and beverage packaging American Chemical Society 1

2 2 Even though significant progress has been made in developing polymer nanocomposites with varying polymer matrices and inorganic nanoparticles, a general understanding has yet to emerge. For example, the combination of enhanced modulus, strength and toughness is a unique feature of only a fraction of PNCs fabricated to date. A major challenge in developing nanocomposites for systems ranging from high performance to commodity polymers is the lack of even simple structure - property models. In the absence of such models and the role of processing in affecting the structure and properties, progress in the engineering of nanocomposites has remained largely empirical. Similarly, predicting the ultimate material limits or maximum performance for different classes of nanocomposites is almost impossible at present. This book compiles the current status of research in nanocomposite, with regards to both the established polymer-clay nanocomposite systems for mechanical enhancements and emerging polymer-organic and other novel polymer-inorganic systems for electrical, optical, thermal and magnetic applications. The 'Nanocomposite' Concept What are nanocomposites and what makes them especially interesting? Why are they different and worthy of the rapidly increasing scientific and technological excitement? The answer to these questions, in our opinions, resides in the fundamental length scales dominating the morphology and properties of these materials. The nanoparticles have at least one characteristic length scale that is of the order of nanometers and can range from essentially isotropic to highly anisotropic needle-like to sheet-like elements. Uniform dispersion of these isotropic and anisotropic nanoscopically-sized particles (or nanoelements) can lead to ultra-large interfacial area between the constituents, for example, approaching 700 m2/cm3 in dispersions of layered silicates in polymers. In addition to the large interfacial area, the distance between the nanoelements begins to approach molecular dimensions at extremely low loadings of the nanoparticles. Thus, for a system comprising of 1 nm thick plates, the distance between plates (considered as discs with a diameter I Jim) approaches 10 nm at only 7 vol. % of plates! This large internal interfacial area and the nanoscopic dimensions between constituents differentiate polymer nanocomposites from traditional composites and filled plastics. The dominance of interfacial regions resulting from the nanoscopic phase dimensions implies the behavior of polymer nanocomposites cannot be understood by simple scaling arguments that begin with the behavior of traditional polymer composites.

3 3 Three major characteristics define and form the basis of PNC performance: nanoscopically confined matrix polymer, nanoscale inorganic constituents, and nanoscale arrangement of these constituents. The driver for current research is to develop the tools - synthesis, processing, characterization and theory - to optimize and enable full exploitation of the potential of the combination of these unique characteristics. The proliferation of internal inorganic-polymer interfaces means the majority of polymer chains reside near an inorganic surface. Since an interface limits the conformations that polymer molecules can adopt, the free energy of the polymer in this interfacial region is fundamentally different from that of polymer far removed from the interface (i.e. bulk). The influence of an interface is related to a fundamental length-scale of the matrix adjacent, which for polymers is on the order of the radius of gyration of a chain, Rg (5-20 nm) [7]. Thus, in PNCs with only a few volume percent of dispersed nanoparticles, the entire matrix polymer may be considered as nanoscopically confined interfacial polymer. The restrictions in chain conformations will alter molecular mobility, relaxation behavior and the consequent thermal transitions such as the glass transition temperature. More complicated is the picture for semi-crystalline polymers and mesostructured liquid-crystalline polymers and ordered blockcopolymers, where the interface will alter the degree of ordering and packing perfection and thus crystallite and domain growth, structure and organization. The second major characteristic of PNCs is the dimensions of the added nanoelements. As with the matrix polymers, when the dimensions of the cluster or particle approach the fundamental length scale of a physical property, new mechanical, optical and electrical properties arise, which are not present in the macroscopic counterpart. Examples include superplastic forming of nanograined ceramics, plasmon absorption in metal nanoparticles, quantum confinement in semiconductor nanoparticles, and superparamagnetic response in nanoparticle magnets. Dispersions of nanoelements exhibiting these unique properties create bulk materials dominated by solid-state physics of the nanoscale. A short list of potential nanoparticles includes layered chalcogenides, metal nanoparticles, graphitic layers, carbon nanotubes, metal oxide, nitride and carbide clusters, quantum dots and biological components. Finally, as with any composite, the arrangement of constituents critically determines the material's behavior. Conceptually, spatial ordering of spherical, rod - like or plate - like nanoparticles (0, 1 or 2 dimensional) into positional (1, 2 or 3 dimensional) arrays with varying degrees of orientational order will manifest in an enormous variety of systems. The possibilities are further expanded by varying degrees of particle-particle association, clustering, percolation and heterogeneous distribution of particles. The final properties of the PNC system will depend as much on the individual properties of the

4 4 constituents (characteristics 1 and 2) as on the relative arrangements and subsequent synergy between the constituents. Ultimately, polymer nanocomposites offer the possibility of developing a new class of materials with their own manifold of structure-property relationships, only indirectly related to their components and their micron and macro-scale composite counterparts. Though nanocomposites with inorganics of different dimensionality and chemistry are possible, efforts have only begun to uncover the wealth of possibilities for these new materials. Overview In this book we have compiled the current research in PNCs. We have tried to stress the connections between the developments in PNCs with those in more conventional filled systems such as carbon black filled polymer systems. Further, the collection of chapters provide a snapshot of the current experimental, theoretical and computer simulation tools being used to advance our understanding of polymer nanocomposites. In Chapter 1, Collister discusses the commercial viability of polymer nanocomposites. In Chapters 2-4, the synthesis aspects of nanocomposites development are discussed. Balazs and coworkers examine the thermodynamic considerations in the development of PNCs in Chapter 5. Characterization of such nanofilled polymer composites by scattering, adsorption and NMR measurements are discussed in Chapters 6 through 10. Hjelm discusses the application of small angle neutron scattering (SANS) to examine the structure and aggregation of carbon black (Chapter 6), while Vaia and Lincoln apply small angle x-ray scattering to examine nylon-6 based PNCs (Chapter 8) and Ho and coworkers use SANS to examine the dispersion of organically modified layered silicates (Chapter 10). Thin film properties, viscoelastic properties and crystallization behavior of layered - silicate based PNCs are discussed in Chapters 11 through 13. In Chapter 14, Gerspacher and OTarrell discuss the dispersion and viscoelastic properties of carbon black filled polymers and correlate these fundamental properties to final-use properties in applications such as in rubber tires. Finally, computer simulations have increasingly been used to understand the synthesis and formation of PNCs as well as understand their properties. Manias and Kuppa (Chapter 15) describe via computer simulations the structural and dynamic properties of polystyrene confined between layered-silicates and Bhardwaj et al. describe results from computer simulations to examine the process of nanocomposites formation.

5 5 REFERENCES 1. E.P. Giannelis, Adv. Mater. 8, 29 (1996). 2. Polymer Clay Nanocomposites, T.J. Pinniavaia, G.W. Beal eds Polymer Nanocomposites, R.A. Vaia, R. Kishnamoorti, eds., American Chemical Society, Vol xxx, Washington, D.C Nanocomposites 1999: Polymer Technology for the Next Century. Principia Partners, Exton, PA M. Alexandre, P. Dubois, Mater. Sci. Eng. 28, 2000, "Nanocomposites New Low-Cost, High-Strength Materials for Automotive Parts", National Insititutes of Technology, ATP Project, , Y.S. Lipatov, Polymer Reinforcement, ChemTec Publishing, 1995.