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Introduction to Green Molecules, to the Present Situation and to Previous Research on Green Polymer Nanocomposites
Neelamshobha Nirala
Contents
1.1 The Importance of Green Molecules
1.2 The Importance of Green Nanoparticles
1.3 The Importance of Natural Polymers
1.4 The Importance of Nanocomposites
1.5 Types of Natural Polymers and Their Properties
1.6 Why We Combine Natural Polymers with Green Nanoparticles
1.7 The Present Scenario and Previous Research on Green Polymer Nanocomposites
1.8 Comparative Study of the Synthesis Process of PNCs
1.8.1 In-Situ Polymerization
1.8.2 Melt Intercalation
1.8.3 Exfoliation Adsorption
1.9 Comparative Study of the Properties of NPCs
1.10 Comparative Study of the Quality Asessment of NCs
1.10.1 Infrared Spectroscopy
1.10.2 X-Ray Diffraction
1.10.3 Wide-Angle X-Ray Diffraction (WAXD)
1.10.4 Fourier-Transform Infrared
1.10.5 Scanning Electron Microscope
1.10.6 Transmission Electron Microscope
1.10.7 Zeta Potential
1.10.8 Differential Thermal Analysis
1.10.9 Thermo Gravimetric Analysis
1.11 Comparative Study of the Drawbacks of PNCs
1.12 Conclusion
References
1.1 The Importance of Green Molecules
Synthesis of nanomaterials with the desired quality and properties is one of the primary concerns in current nanotechnology. But these synthesized nanomaterials should be environmentally friendly and maintain sustainability. To fulfil this criteria, concepts of green chemistry are incorporated in the synthesis of nanomaterials. Green chemistry involves the utilization of renewable sources, the minimization of waste production and the saving of energy (Jawed & Khan, 2018), thus helping to save the environment from excessive waste created by the accumulation of plastics, which is extremely harmful to living organisms, reduce the financial burden and save non-renewable sources for the future. In nanocomposites, the term “green” defines the materials that are biodegradable and renewable in nature. As a result, synthetic polymers are now being replaced by various degradable polymers (both from natural and synthetic sources) and green synthesis of nanoparticles takes place. The green synthesis of nanoparticles includes the selection of green solvents, nontoxic material as a stabilizer and an eco-friendly benign reducing agent.
1.2 The Importance of Green Nanoparticles
In the past few decades, nanoparticles have gained huge interest due to their vast applications in the field, ranging from food packaging to paint and ceramics; from wastewater treatments to bioanalysis; in the agricultural industry; from diagnostic and therapeutic applications to the design of biosensors; in optical imaging, targeted drug delivery systems, tumor detectors and radiotherapy dose enhancers. Day by day its applications are amplified in various fields. Nanoparticles (NanoPs) are available in a range of sizes from 1 to 100 nms. Morphological diversity of NanoPs can be seen in the form of triangles, hexagons, pentagons, cubes, spheres, ellipsoids, nanowires and nanorods (Nadaroglu, Alayli, & Ince, 2017). Nanoparticles can be divided into three types based on their geometry as nanoparticles, nanoplatelets and nanofibers, nanotubes, nanorods or whiskers. Nanoparticles are particles where all three dimensions are in the order of nanometres. Colloidal silica, noble nanometals and metal oxides are examples. Nanoplatelets are where only the thickness is in the nanoscale range. Layered clay minerals, silicic acid and zirconium phosphates are just a few examples. When two dimensions of nanoparticulates are in the nanoscale range they are called nanofibers or tubes, of which carbon nanotubes are an example (Shchipunov, 2012).
NanoPs show improved properties (physical, chemical, optical, electrical and catalytic) and functions compared to their bulk materials because of their nanoscopic size and large surface area to volume ratio. Further, due to its wide applications, NanoP synthesis has been improved by the use of many physical and chemical methods such as plasma arcing, sputtering, laser ablation, spinning, sol–gel processes and so on. Apart from synthesizing NanoPs of various shapes, sizes and characteristics, these physiochemical processes also suffer from many limitations. These methods are highly expensive, labor intensive and require high temperatures, pressure and radiations, highly toxic reductants and stabilizing agents, which are harmful to the environment and living organisms. To overcome these limitations and to produce NanoPs that utilize inexpensive, environmentally benign reducing agents and non-toxic stabilizing agents that are biocompatible, green synthesis of NanoPs has emerged (Green biosynthesis of nanoparticles mechanism and applications, 2013).
Green synthesis of metal nanoparticles means the reduction of metal ions into NanoPs using biological systems. These biological systems include micro-organisms such as bacteria, fungi, yeast, diatoms and plant tissues (Singh et al., 2018). The metabolites present in the biological systems act both as reducing and stabilizing agents (which is required to avoid the agglomeration of NanoPs) for NanoP synthesis. This not only reduces the cost associated with the chemical process but also saves the environment from the toxic effect of chemicals. Biosynthesis of NanoPs is a one-step bio-reduction method which is quite simple, fast, economical and efficient, and can be easily scaled for industrial applications. Green synthesis of NanoPs showed enhanced antimicrobial, antifungal, antioxidant, anticancer and catalytic activity compared to chemical methods (Alananbeh, Refaee, & Qodah, 2017; Gold, Slay, Knackstedt, & Gaharwar, 2018; Vishwanatha et al., 2018). Among the various available methods of biosynthesis of NanoPs, utilization of plant extracts are comparatively more favorable. Cost of maintenance and cultivation is much less in plants, as waste disposal requires less effort and there is no need to take care of numerous parameters, as in the culture of bacteria. Also, the therapeutic effects of plant extracts are transferred into nanoparticles. NanoP synthesis can use the whole plant but it also has certain limitations: the shape and size of NanoPs may change depending upon their localization in plants, and the proficient extraction, isolation and purification of NanoPs from the whole plant is difficult, whereas phytochemicals from plant extract can reduce metal ions much quicker than bacteria and fungi, which need a longer incubation time. NanoPs synthesized by plant extracts contain a functionalized surface that can attach with organic ligands (Makarov et al., 2014; Green biosynthesis of nanoparticles mechanism and applications, 2013).
1.3 The Importance of Natural Polymers
Natural polymers are polymers obtained from natural resources. Human beings have learned to use these from the very beginning of evolution for different purposes such as shelter, food and clothing. During the Second World War, due to the scarcity of natural polymers and their inherent limitations, synthetic polymers gained major interest. Synthetic polymers are produced by human beings using petroleum products. The use of various chemical methods helps in the economical production and tailoring of synthetic polymers with required properties. The resistance of synthetic polymers to various modes of degradation and vast petroleum reserves further increased its applications. However, the same advantages of synthetic polymers with overuse created many threats to the environment and living organisms. Further, the increasing costs of petroleum and its limited resources, energy demands, chemical toxicity, ingestion by marine life and aggregation of plastic wastes means that the recyclability of synthetic plastics is still in the developmental stage and their non-degradability demands the replacement of synthetic polymers by natural polymers as much as possible.
Natural polymers are degradable by various enzymes and hence it does not produce any adverse effects on the environment. Both the raw materials needed for the production of natural polymers as well as its production costs are economical. They are biocompatible, non-toxic and maintain CO2 neutrality. Due to the natural origin of natural polymers, they are safe and free from any major side effects. Ease of availability, low organic salt content and no regional limitations on industrial production are other advantages of natural polymers. Polymers are called green when they exhibit properties such as eco-friendly, biodegradable and renewable (Jacob John & Thomas, 2012; Olatunji, 2016; Pillai, 2010).
1.4 The Importance of Nanocomposites
Composites are the combination of two or more different materials into a single substance with a distinct interface between them. These physically and chemically different materials form two different phases, called the matrix phase and the reinforcement phase. The purpose of reinforcement material is to bear the load whereas the matrix phase is responsible for binding together and distributing the load among the reinforcement material. Composites usually show superior properties to their composing materials. On the basis of material used as matrix, composites are broadly classified as metal matrix composites, ceramics matrix composites and polymer matrix composites. So, in polymer matrix composites, a polymer is used as a matrix (Shekar & Ramachandra, 2018). In this chapter we will focus our study on polymer matrix composites. Composites are called nanocomposites (NCs) when at least one dimension of the reinforcement material is in the range of 1 to 100 nm. So in simple words, a combination of reinforcement elements as nanofillers (e.g. nanoparticles) with matrix is called a nanocomposite. These nanofillers can be of three types based on their morphology, as shown in Figure 1.1 (Fu, Sun, Huang, Li, & Hu, 2019). Nanocomposites offer many advantages, such as industrial significance in designing different structures and materials with better properties and flexibilities, less interfacial defects due to the size of nanoparticles and large volume to surface area. But the direct ingestion of nanoparticles by higher organisms and their entry into the aquatic ecosystem creates a need for other composites, called bionanocomposites. Bionanocomposites are made up of biopolymers and its reinforcement materials include inorganic solids like metal nanoparticles, hydroxyapatite and clay (Arora, Bhatia, & Attri, 2018). Biopolymers have huge availabilit...
