Classification, Properties and Applications of Polymer Nanocomposites

What is Nanocomposites?
Polymer nanocomposites (PNCs) are a new class of reinforced hybrid materials that are formed by the dispersion of nanoscale clay particles throughout a polymer matrix. PNCs is an appropriate synonym for nanoparticles in the form of rods, spheres, or sheets dispersed within the polymer matrix, has aroused tremendous interest, both in academia and industry. Nanocomposites consist of an organic polymer matrix embedded with inorganic particles, which have at least one of the dimensions in the nano range. The particles may be spherical (metallic or ceramic), fibrillar (CNT) or lamellar. The polymer nanocomposites combine the concept of both composites and nanomaterials. Though nanocomposite can include porous media, colloids, gels and copolymers, mainly it is considered to mean the solid combination of nano-dimensional phases differing in properties due to dissimilarities in structure and chemistry. There is the presence of nanocomposite in nature, such as the structure of abalone shell, tooth, nacre and bone.

The technology of nanocomposites involves the use of very small amount of Nanofillers. The addition of nanofillers can strongly impact the macroscopic properties of the polymer nanocomposite. The properties of nanocomposites are quite superior to conventional composites as nanoscale organic–inorganic materials are mixed on a nearly molecular level in the former. In nanocomposites, there is a substantial improvement in the following properties when compared with the base polymer as well as conventional filler counterparts:

  • Improved mechanical properties (e.g. strength, modulus and dimensional stability);
  • Reduced permeability to gases, water and hydrocarbons;
  • Improved thermal stability and heat distortion temperature (HDT);
  • Reduced thermal expansion coefficient;
  • Enhanced flame retardancy;
  • Reduced smoke emissions;
  • Improved chemical resistance;
  • Better surface appearance;
  • Higher electrical conductivity and
  • Improved optical clarity.

Nanocomposites are promising for use in various areas such as automotive, aerospace, defense, and biomedicine fields. Nanocomposites allow design and characteristic choices that are impossible with conventional composites. Based on their light weight and multifunctionality, nanocomposites cater the needs without compromising aesthetics and comfort of textiles. In smart textiles, nanocomposites take part in sensors, actuators, mediators, biosensors, thermoregulation, energy storing, and harvesting elements, among others. Nanocomposites are especially promising for sophisticated niche areas. Nanocomposites have already started to be used in a number of applications; nevertheless, there are still various potential areas where nanocomposites can be utilized in the future.

Classification of Nanocomposites:
Composites can be defined as an ensemble of two phases, blended/mixed to obtain the desired properties, tailor-made for specific applications. Nanocomposites constitute a subgroup of this bigger domain of multifunc­tional materials having at least one of the components in the nanoscale range. The arena of nanocomposites with unusual property combinations and design possibilities at a very low concentration of fillers has gained a special status in recent years owing to a high matrix to filler interfacial area (the so-called “Nano effect”) and greater aspect ratio. The best example appears in Mother Nature in the form of bones, shells, and wood.

Nanocomposites can be classified in three groups in terms of their matrices:

  1. Ceramic-matrix nanocomposites,
  2. Metal-matrix nanocomposites, and
  3. Polymer-matrix nanocomposites.

Their flexibility and conformability with textiles render polymer-matrix nanocomposites more suitable for smart textiles use.

A schematic diagram demonstrating the overall classification of nanocomposites is provided in Figure 1. Among these categories, PNCs have reached a phenomenal status in industrial and real-life applications due to their ease of production, lightweight, ductile nature, high strength, better resistance to corrosion, fire, and acids, higher fatigue strength, and much more. Polymer matrices can be broadly divided into two major groups based on their response to heat: (i) thermosetting polymers and (ii) thermoplastic polymers, and are discussed in the following sections.

A schematic diagram displaying the overall classification of nanocomposites
Figure 1: A schematic diagram displaying the overall classification of nanocomposites

The nanocomposites can be divided into three categories according to the strength of interfacial interactions between the polymer matrix and layered silicate, such as-

  1. Intercalated nanocomposites,
  2. Flocculated nanocomposites and
  3. Exfoliated nanocomposites

Intercalated Nanocomposites
In this class of nanocomposites, the polymer matrix is inserted into the layered silicate structure in a crystallographically regular fashion so as to swell the spacing between the platelets. Generally, these nanocomposites are interlayered by a few molecular layers of polymer, and the properties are very similar to ceramics.

Flocculated Nanocomposites
These nanocomposites are very similar to intercalated nanocomposite. Sometimes, in this class of composite, the silicate layers are flocculated because of hydroxylated edge–edge interaction of the silicate layers.

Exfoliated Nanocomposites
In exfoliated nanocomposites, depending upon clay loading, the individual clay layers are separated in a continuous polymer matrix. The clay content of this class of nanocomposite is much lower than intercalated nanocomposite.

Nanoparticles of metals, metal oxides, and nonmetal oxides are utilized in nanocomposites as reinforcement components. These nanocomposites show unique mechanical, thermal, and electrical characteristics. Metal nanoparticles have found use in nanocomposites developed for catalyst and biomedical applications. Nanoparticles of metal oxides are added to nanocomposites to obtain mechanical strength, electrical and thermal conductivity, barrier effect, antibacterial effect, UV protection, and self-cleaning property. Among metal oxide particles, TiO2 and SiO2 are commonly utilized.

Nanoclay-reinforced nanocomposites have been extensively studied in terms of their special properties including thermal resistance, flame retardancy, stiffness, and strength. Nanowire-based nanocomposite have found use in energy storing and harvesting applications.

Characterization of Nanocomposites:
The macroscopic morphology and surface texture of nanocomposites are mainly investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). SEM is very simple and most widely used for morphological analysis. Though TEM is used by many researchers as an essential tool for qualitative analysis, structural defect analysis and spatial distribution study, it is time consuming and does not provide quantitative information. WAXD is also widely used to study the structure by monitoring the position, shape and intensity of basal reflections. WAXD does not provide much information regarding the spatial distribution or any structural defects in nanocomposite.

DSC is used to analyze the thermal behavior of nanocomposites. The molecular structure of nanocomposite can be characterized by various techniques such as nuclear magnetic resonance, Fourier transform infrared, SAXS and optical birefringence. X-ray photoelectron spectroscopy and water contact angle measurement techniques can be used for the analysis of chemical properties.

Properties of Nanocomposites:
The properties of the nanocomposite depend upon the clay and polymer combination, the characteristics of the nanofiller and polymer as well as the structure of the composite produced. The nanocomposite possess noticeable differences in their thermal, mechanical, barrier and electrical properties when compared with traditional composites.

The optimal structure of a nanocomposite for one physical property may not be the best for another physical property. This section highlights the properties of nanocomposites.

Thermal Properties:
The thermal properties of nanocomposites can be analyzed by DSC. From the weight loss on heating the nanocomposites, the thermal stability can be calculated. The heat resistance of nanocomposite on external loading can be measured from the HDT. The dependence of HDT on clay content has been investigated by several researchers. The nanocomposite with good thermal conductivity have multiple applications, such as printed circuit boards, thermal interface materials, heat sinks, connectors and high-performance thermal management systems.

Mechanical Properties:
The mechanical properties of nanocomposites, such as tensile strength, elongation and modulus, are affected by the surface morphology and the material used for production. The improvement of mechanical properties of polymer nanocomposite can be attributed to the good affinity between the polymer and nanofiller along with the high rigidity and high aspect ratio of nanofillers.

Electrical Properties:
The electrical properties of nanocomposites depend on several factors, such as aspect ratio, dispersion and alignment of the conductive nanofillers in the structure. The nanocomposites containing CNTs have superior electrical properties (high energy densities and low driving voltages). The nanocomposite of ether/clay (organically modified) exhibit ionic conductivity that is several orders of magnitude higher than that of the corresponding clay. The electrical conductivity increased by several orders of magnitude with a very small loading (0.1 wt.% or less) of nanotubes to the nanocomposite, without altering other properties such as optical clarity, mechanical properties and low melt flow viscosities. The conductive nanocomposite has found applications in many fields such as electrostatic dissipation, electrostatic painting, electromagnetic interference shielding, printable circuit wiring and transparent conductive coating.

Barrier Properties:
The nanocomposites have very good barrier property against gases because of their high aspect ratio and by the creation of a tortuous path that retards the progress of the gas molecules through the matrix resin. Inside the nanocomposite structure, the presence of the filler introduces a tortuous path for diffusing penetrants. The permeability is reduced because of the longer diffusive path that the penetrants must travel in the presence of filler. The polyimide nanocomposite containing a small fraction of layered silicate exhibit barrier property against small gases such as oxygen, carbon dioxide, helium, nitrogen and ethyl acetate vapors.

Rheological Properties:
The flow behavior of PCL / nylon 6 nanocomposite was significantly different from the corresponding neat matrices. The thermo-rheological properties of the nanocomposite from the behavior of matrices. The viscoelastic properties of nanocomposites are important in relation to composite processing and composite dynamics and microstructure analysis. Krishnamoorti and Giannelis (1997) were the first to describe the rheological properties of in situ polymerized nanocomposite with end-tethered polymer chains.

Application of Nanocomposites:
Polymer nanocomposites with their unprecedented property combinations and exceptional design possibilities are establishing themselves as high-performance materials of the twenty-first century and are used in multifari­ous cutting-edge technologies. A schematic diagram is provided in Figure 2 listing out various applications of polymer nanocomposites. A few of the applications of nanocomposites are briefly discussed here.

A schematic diagram listing various applications of polymer nanocomposites
Figure 2: A schematic diagram listing various applications of polymer nanocomposites

Aerospace:
Projecting heavy lift systems to the earth’s lower atmosphere incurs a huge cost in terms of fuel prices. The fuel cost amounts to about 30% of the oper­ational cost even in general aviation. So the implementation of polymer/CNT nanocomposite in a space shuttle and commercial aircraft such as Boeing 787 and Airbus A380 as shown in Figure 3.

Polymer nanocomposites used in different parts of the Airbus A380.
Figure 3: Polymer nanocomposites used in different parts of the Airbus A380.

Automotive:
With increasing global concerns for low fuel economy and low emissions in the case of land transportation systems, research is trending toward the low cost, high performance, and lightweight polymer nanocomposite. This class of novel materials is expected to increase the speed of production, environ­mental and thermal stability, and recyclability, while reducing the weight.

Infrastructures/Civil Structures:
Polymer composites with nanofillers have always acted as game-changers about their use in structural components (buildings, bridges, and other engi­neered structures) which can be attributed to the high strength-to-weight ratio of the class of materials. They are also highly durable in terms of thermal, mechanical, and barrier properties. One of the important components of civil structures is concrete. But a lot of improvements are expected in it concerning its increased durability, tensile strength, and reduced brittleness. Composites with organo-clays are commonly used as barrier coatings to protect the civil structures against environmental aging and cor­rosion. A coating based on epoxy polymer with nano-ceramic fillers could shield the concrete structures from UV radiations, contamination, and dete­rioration.

Food Packaging:
Polymer nanocomposites, owing to their superior functionality, antibacterial properties, lightweight, and cheap and simple processing techniques, have proved to be better replacements for the conventional packaging materials such as metals, ceramics, and paper. The inherent barrier properties (mechan­ical and thermal), biodegradability, self-healing, and self-cleaning of those composites increase the shelf life of the packaged food items. Polymer/clay has considerable performance in the packaging of processed foods like cheese, meats, confectionary, cereals, boil-in-bag foods, and even for fruit juices and carbonated drinks.

Energy:
Materials with high dielectric constant, optimum piezoelectric properties are often searched for their importance in energy storage and harvesting applica­tion. Low dielectric constant polymers, when blended with dielectric / piezoelectric ceramics nanofillers, form a good combination with requisite properties. Flexible polymer-based nanocomposite suitable for energy harvesting serve as the new generation functional mate­rials.

Bio-Medical:
Polymer nanocomposites form the basic building blocks of life systems starting right from bone (a combination of ceramic phosphate crystallites and collagen fibers forming strong and dense cortical bone or spongy shear resistant cancellous bone), teeth (enamel, cementum, and dentin contain­ing different volume fractions of hydroxyapatite crystals along with collag­enous or non-collagenous proteins), or wood (consisting of cellulose and lignin). Hence, polymers blended with other nanoparticles open up great avenues for a multitude of applications in a biomedical field such as tis­sue engineering, bone replacement/repair, dental applications, controlled drug delivery, and many more. Some of the applications of polymer nanocomposites are shown in a representative Figure 4. Several magnetic polymer nanocom­posite have been used in biomedical and environmental applications.

Polymer nanocomposites for different biomedical applications
Figure 4: Polymer nanocomposites for different biomedical applications

Conclusion:
The polymer nanocomposites are very important in the field of material science for the past two decades. Even though the development of this promising technique is just in its infancy, it has an emerging future in various applications. Polymer nanocomposite have a great potential marking it as a vibrant area of work in the recent years. The improvement and application of nanocomposites will depend upon how effectively we can handle the challenges.

References:

  1. Fibres to Smart Textiles Advances in Manufacturing, Technologies, and Applications Edited by Asis Patnaik and Sweta Patnaik
  2. Smart Textiles: Wearable Nanotechnology Edited by Nazire D. Yilmaz
  3. Hydrothermal Behavior of Fiber- and Nanomaterial-Reinforced Polymer Composites by Ramesh Kumar, Nayak Bankim Chandra Ray, Dibyaranjan Rout, Kishore Kumar Mahato

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