NANOCOMPOSITES

 A rapidly growing area of nano-engineered materials provides a new dimension for plastics and composites

Introduction

The definition of nano-composite material has broadened significantly to encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinctly dissimilar components and mixed at the nanometer scale.

The general class of nanocomposite organic/inorganic materials is a fast growing area of research. Significant effort is focused on the ability to obtain control of the nanoscale structures via innovative synthetic approaches. The properties of nano-composite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics.

This rapidly expanding field is generating many exciting new materials with novel properties. The latter can derive by combining properties from the parent constituents into a single material. There is also the possibility of new properties which are unknown in the parent constituent materials.

The inorganic components can be three-dimensional framework systems such as zeolites, two-dimensional layered materials such as clays, metal oxides, metal phosphates, chalcogenides, and even one-dimensional and zero-dimensional materials such as (Mo₃Se₃-)_n chains and clusters. Experimental work has generally shown that virtually all types and classes of nanocomposite materials lead to new and improved properties when compared to their macrocomposite counterparts. Therefore, nanocomposites promise new applications in many fields such as mechanically reinforced lightweight components, non-linear optics, battery cathodes and ionics, nano-wires, sensors and other systems.

The general class of organic/inorganic nanocomposites may also be of relevance to issues of bio-ceramics and biomineralization in which in-situ growth and polymerization of biopolymer and inorganic matrix is occurring. Finally, lamellar nanocomposites represent an extreme case of a composite in which interface interactions between the two phases are maximized. Since the remarkable properties of conventional composites are mainly due to interface interactions, the materials dealt with here could provide good model systems in which such interactions can be studied in detail using conventional bulk sample (as opposed to surface) techniques. By judiciously engineering the polymer-host interactions, nanocomposites may be produced with a broad range of properties.

Inorganic layered materials exist in great variety. They possess well defined, ordered intralamellar space potentially accessible by foreign species. This ability enables them to act as matrices or hosts for polymers, yielding interesting hybrid nano-composite materials.

Polymer Nanocomposites

Materials and their development are fundamental to society. Major historical periods of society are ascribed to materials (i.e., stone age, bronze age, iron age, steel age [industrial revolution]; silicon age and silica age [telecom revolution]). Scientists will open the next societal frontiers not by understanding a particular material, but rather by understanding and optimizing the relative contributions afforded by material combinations.

The nanoscale, and the associated excitement surrounding nanoscience and technology, affords unique opportunities to create these revolutionary material combinations. These new materials promise to enable the circumvention of classic material performance trade-offs by accessing new properties and exploiting unique synergisms between constituents that only occur when the length scale of the morphology and the critical length associated with the fundamental physics of a given property coincide. From a materials perspective, morphologies that exhibit nanoscopic features are necessary but far from sufficient – the key opportunities are afforded either when the physical size of the material’s constituents is engineered to coincide with the onset of nonbulk-like behavior, such as observed for the size-dependent light emission of quantum dots (QDs), or  when a structure-property relationship approaches a singularity or depends nonlinearly on aspects of the morphology, such as the internal interfacial area.

Polymeric nanocomposites (PNCs) have been an area of intense industrial and academic research for the past 20 years. No matter the measure – articles, patents, or R&D funding – efforts in PNCs have been exponentially growing worldwide over the last ten years. PNCs represent a radical alternative to conventional filled polymers or polymer blends – a staple of the modern plastics industry.

The reinforcement of polymers using fillers, whether inorganic or organic, is common in the production of modern plastics. Polymeric nanocomposites or polymer nanostructured materials represent a radical alternative to conventional-filled polymers or polymer blends. In contrast to the conventional systems where the reinforcement is on the order of microns, discrete constituents on the order of a few nanometers (~10,000 times finer than a human hair) exemplify PNCs.

Uniform dispersion of these nanoscopically sized filler particles (or nanoelements) produces ultra-large interfacial area per volume between the nanoelement and host polymer. This immense internal interfacial area and the nanoscopic dimensions between nanoelements fundamentally differentiate PNCs from traditional composites and filled plastics. Thus, new combinations of properties derived from the nanoscale structure of PNCs provide opportunities to circumvent traditional performance trade-offs associated with conventional reinforced plastics, epitomizing the promise of nano-engineered materials.

A literature search provides many examples of PNCs, demonstrating substantial improvements in mechanical and physical properties. However, the nanocomposite properties discussed are generally compared to unfilled and conventional-filled polymers, but are not compared to continuous fiber reinforced composites. Although PNCs may provide enhanced, multifunctional matrix resins, they should not be considered a potential one-for-one replacement for current state-of-the-art carbon-fiber reinforced composites.

From both a commercial and military perspective, the value of PNC technology is not based solely on mechanical enhancements of the neat resin. Rather, it comes from providing value-added properties not present in the neat resin, without sacrificing the inherent processibility and mechanical properties of the resin. Traditionally, blend or composite attempts at multifunctional materials require a trade-off between desired performance, mechanical properties, cost, and processibility.

Considering the number of potential nanoelements, polymeric resins, and applications, the field of PNCs is immense. Development of multicomponent materials, whether microscale or nanoscale, must simultaneously balance four interdependent areas: constituent-selection, fabrication, processing, and performance. This is still in its infancy for PNCs, but ultimately scientists will develop many perspectives dictated by the final application of the PNC. Researchers developed two main PNC fabrication methodologies: in-situ routes and exfoliation. Currently, researchers in industry, government, and academia worldwide are heavily investigating exfoliation of layered silicates, carbon nanofibers/nanotube-polymer nanocomposites, and high-performance resin PNCs.

Developing an understanding of the characteristics of this interphase region, its dependence on nanoelement surface chemistry, the relative arrangement of constituents and, ultimately, its relationship to the PNC properties, is a current research frontier in nanocomposites. Equally important is the development of a general understanding of the morphology-property relationships for mechanical, barrier, and thermal response of these systems. This necessitates the determination of the critical length and temporal scale with which continuum description of a physical process must give way to mesoscopic and atomistic view of these nanoscale systems—a current challenge for computational materials science.

A rapidly growing area of nano-engineered materials is PNCs, providing lighter weight alternatives to conventional-filled plastics with additional functionality associated with nanoscale-specific value-added properties. If the promise and excitement surrounding layered silicates and carbon nanotubes are any indication, the future of PNC technology is truly boundless. The opportunities to extend PNC concepts to other nanoelements and polymer hosts are immense, opening the way to provide tailor-made materials that circumvent current limitations and enable future concepts.

PNC Framework

The initial question when beginning to examine polymer nanocomposites is: how are these materials different from classic filled polymers or traditional composites? As anticipated, there is no simple answer; rather ‘they are related, but bring new opportunities, perspectives, and issues.’ Whether tubes (e.g. single- and multi-walled carbon nanotubes, SWNT and MWNTs, respectively) or plates (e.g. exfoliated graphite, layered silicates), the nanoscopic dimensions and extreme aspect ratios inherent in these nanofillers result in six interrelated characteristics distinguishing the resultant PNCs from classic filled systems:

q Low percolation threshold (~0.1-2 vol%);

q Particle-particle correlation (orientation and position) arising at low volume fractions (φ C < 0.001);

q Large number density of particles per particle volume (106-108 particles/µm3);

q Extensive interfacial area per volume of particles (103-104 m2/ml);

q Short distances between particles (10-50 nm at φ ~1-8 vol%); and

q Comparable size scales among the rigid nanoparticle inclusion, distance between particles, and the relaxation volume of polymer chains.

Properties And Applications of PNC’S

Advantages of Nanosized Additions

Researches have revealed clearly the property advantages that nanomaterial additives can provide in comparison to both their conventional filler counterparts and base polymer. Properties which have been shown to undergo substantial improvements include:

·         Mechanical properties e.g. strength, modulus and dimensional stability

·         Decreased permeability to gases, water and hydrocarbons

·         Thermal stability and heat distortion temperature

·         Flame retardancy and reduced smoke emissions

·         Chemical resistance

·         Surface appearance

·         Electrical conductivity

·         Optical clarity in comparison to conventionally filled polymers

Disadvantages of Nanosized Additions

To date one of the few disadvantages associated with nanoparticle incorporation has concerned toughness and impact performance. Some of the data presented has suggested that nanoclay modification of polymers such as polyamides, could reduce impact performance. Clearly this is an issue which would require consideration for applications where impact loading events are likely. In addition, further research will be necessary to, for example, develop a better understanding of formulation/structure/property relationships, better routes to platelet exfoliation and dispersion etc.

Particle Loadings

In addition it is important to recognise that nanoparticulate/fibrous loading confers significant property improvements with very low loading levels, traditional microparticle additives requiring much higher loading levels to achieve similar performance. This in turn can result in significant weight reductions (of obvious importance for various military and aerospace applications) for similar performance, greater strength for similar structural dimensions and, for barrier applications, increased barrier performance for similar material thickness.

Areas of Application

Such mechanical property improv  ements have resulted in major interest in nanocomposite materials in numerous automotive and general/industrial applications. These include potential for utilisation as mirror housings on various vehicle types, door handles, engine covers and intake manifolds and timing belt covers. More general applications currently being considered include usage as impellers and blades for vacuum cleaners, power tool housings, mower hoods and covers for portable electronic equipment such as mobile phones, pagers etc.

Gas Barriers

The gaseous barrier property improvement that can result from incorporation of relatively small quantities of nanoclay materials is shown to be substantial. Data provided from various sources indicates oxygen transmission rates for polyamide-organoclay composites which are usually less than half that of the unmodified polymer. Further data reveals the extent to which both the amount of clay incorporated in the polymer, and the aspect ratio of the filler contributes to overall barrier performance. In particular, aspect ratio is shown to have a major effect, with high ratios (and hence tendencies towards filler incorporation at the nano-level) quite dramatically enhancing gaseous barrier properties. Such excellent barrier characteristics have resulted in considerable interest in nanoclay composites in food packaging applications, both flexible and rigid. Specific examples include packaging for processed meats, cheese, confectionery, cereals and boil-in-the-bag foods, also extrusion-coating applications in association with paperboard for fruit juice and dairy products, together with co-extrusion processes for the manufacture of beer and carbonated drinks bottles. The use of nanocomposite formulations would be expected to enhance considerably the shelf life of many types of food.

Oxygen Barriers

Honeywell have also been active in developing a combined active/passive oxygen barrier system for polyamide-6 materials. Passive barrier characteristics are provided by nanoclay particles incorporated via melt processing techniques whilst the active contribution comes from an oxygen scavenging ingredient (undisclosed). Oxygen transmission results reveal substantial benefits provided by nanoclay incorporation in comparison to the base polymer (rates approximately 15-20% of the bulk polymer value, with further benefits provided by the combined active/passive system). Akkapeddi suggests that the increased tortuosity provided by the nanoclay particles essentially slows transmission of oxygen through the composite and drives molecules to the active scavenging species resulting in near zero oxygen transmission for a considerable period of time.

Food Packaging

Triton Systems and the US Army are conducting further work on barrier performance in a joint investigation. The requirement here is for a non-refrigerated packaging system capable of maintaining food freshness for three years. Nanoclay polymer composites are currently showing considerable promise for this application. It is likely that excellent gaseous barrier properties exhibited by nanocomposite polymer systems will result in their substantial use as packaging materials in future years. A somewhat more esoteric possibility arising from enhanced barrier performance recently suggested has been blown–films for artificial intestines!

Fuel Tanks

Films

The presence of filler incorporation at nano-levels has also been shown to have significant effects on the transparency and haze characteristics of films. In comparison to conventionally filled polymers, nanoclay incorporation has been shown to significantly enhance transparency and reduce haze. With polyamide based composites, this effect has been shown to be due to modifications in the crystallisation behaviour brought about by the nanoclay particles; spherilitic domain dimensions being considerably smaller. Similarly, nano-modified polymers have been shown, when employed to coat polymeric transparency materials, to enhance both toughness and hardness of these materials without interfering with light transmission characteristics. An ability to resist high velocity impact combined with substantially improved abrasion resistance was demonstrated by Haghighat of Triton Systems.

Environmental Protection

Water laden atmospheres have long been regarded as one of the most damaging environments which polymeric materials can encounter. Thus an ability to minimise the extent to which water is absorbed can be a major advantage. Data provided by Beall from Missouri Baptist College indicates the significant extent to which nanoclay incorporation can reduce the extent of water absorption in a polymer. Similar effects have been observed by van Es of DSM with polyamide based nanocomposites. In addition, van Es noted a significant effect of nanoclay aspect ratio on water diffusion characteristics in a polyimide nanocomposite. Specifically, increasing aspect ratio was found to diminish substantially the amount of water absorbed, thus indicating the beneficial effects likely from nanoparticle incorporation in comparison to conventional microparticle loading. Hydrophobic enhancement would clearly promote both improved nanocomposite properties and diminish the extent to which water would be transmitted through to an underlying substrate. Thus applications in which contact with water or moist environments is likely could clearly benefit from materials incorporating nanoclay particles.

Flammability Reduction

The ability of nanoclay incorporation to reduce the flammability of polymeric materials was a major theme of the paper presented by Gilman of the National Institute of Standards and Technology in the US. In his work Gilman demonstrated the extent to which flammability behaviour could be restricted in polymers such as polypropylene with as little as 2% nanoclay loading. In particular heat release rates, as obtained from cone calorimetry experiments, were found to diminish substantially by nanoclay incorporation. Although conventional microparticle filler incorporation, together with the use of flame retardant and intumescent agents would also minimise flammability behaviour, this is usually accompanied by reductions in various other important properties. With the nanoclay approach, this is usually achieved whilst maintaining or enhancing other properties and characteristics.

Conclusion

Today, nanocomposites are really nanofilled plastics, where the total internal interfacial area becomes the critical characteristic rather than simply the relative volume fraction of constituents. The use of the moniker nano-‘composites’ invokes parallels to traditional fiber-reinforced composite technology and the ability to spatially ‘engineer, design, and tailor’ materials performance for a given application. Currently, the realization of ‘compositing’ PNCs is over the horizon. For the vast majority of investigations, the challenge is still to achieve single-particle dispersions and the subsequent PNCs are treated much as an isotropic, filled polymer. Only recently have examples emerged that consider cost-effective approaches to provide spatial and orientational control of the hierarchical morphology with a precision comparable to that conventionally obtained through fiber plies and weaving – thus transforming ‘nano-filled systems’ to ‘nanocomposite systems’. In parallel, PNCs are moving beyond commodity plastic applications to critical components of active devices, such as fuel cell membranes, photovoltaics, sensors, and actuators.

PNCs have great potential, especially when viewed with respect to the explosion of available functional nanoparticles, enabling never-before-realized properties to be generated within plastics. The underlying framework of PNCs implies that the physics and chemistry of these systems parallels many macromolecular systems, not just filled polymers. By considering the idealized framework, examination of the underlying principles defining structure-property relationships can begin, and the potential pitfalls arising from extrapolating structure-property relationships of classic filled systems can be considered and addressed. The necessary foundation and tools to address system-specific complexities and process-history dependencies, such as nonequilibrium phenomena including irreversible aggregation, nanoparticle network association, percolation, and ultra-long relaxation times of process-induced orientation (glass-like behavior), are beginning to evolve. The topological similarities between PNCs and other mesoscale polymer systems, such as semicrystalline polymers, block-copolymers, liquid crystals, and colloids, are the impetus for many of these current efforts, providing significant guidance toward understanding the role of processing on structure control and the ultimate impact on properties.

So, is the full realization of PNCs technologically here? No, but it is a viable option today when considering the selection of filled or blend polymer systems. Will PNCs deliver the potential currently ascribed? That is still to be determined, especially since realistic estimates of the ultimate potential, which are based on fundamental understanding of the physics at these scales, are still in development. However, the possibilities are engaging communities worldwide, and the scientific literature is being enriched at an increasing rate with works that show great promise and are beginning to establish a pervasive fundamental understanding of PNC structure-property relationships.