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CHAPTER 1
Polysaccharide and Protein Based Aerogels: An Introductory Outlook
RUBIE MAVELIL-SAM*a, LALY A. POTHANa AND SABU THOMAS,b,c
a Department of Chemistry, Bishop Moore College, University of Kerala, Mavelikara, Alappuzha, Kerala, India
b International and Inter University Centre for Nanoscience and Nanotechnology (IIUCNN), Mahatma Gandhi University, Kottayam, Kerala, India;
c School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India
*Email:
[email protected] 1.1 Introduction
Aerogels have been in use for almost nine decades now. Synthetic aerogels are being used as super capacitors, electrodes, insulators, and so forth. Increased aerospace applications and rising energy costs have increased the prominence of insulating nanoporous materials (Figure 1.1). Nanocrystals based on a wide range of biomaterials have found their way into the production of nanoporous materials by replacing synthetic counterparts. In addition, awareness of using non benign, environmental friendly materials is driving researchers to develop novel green materials. Aerogels based on several polysaccharides and proteins have been developed recently, involving comprehensive research such as theory modelling and life cycle analysis of such materials.
Figure 1.1 The crayons on top of the aerogel are protected from the flame underneath.
Reproduced from ref. 1.
The aim of this book is to bring together the results of research studies in the area of polysaccharide and protein based aerogels. Aerogels based on these biomaterials, their preparation from various sources, characterisation methods and applications in various domains of science and day to day life will be discussed in detail.
It is indeed commendable that all the contributors in this book have done their best to profoundly review recent advances in this growing area of strong international interest, providing a comprehensive idea on the preparation, properties and applications of polysaccharide and protein based aerogels. In-depth studies and reviews on the technological developments concerning processing strategies and structural analyses have also been included, providing a general idea of the individual types. The chapters are designed in such a way as to edify readers from various realms: academics, students, researchers and industrialists in particular.
1.2 Aerogels: A General Overview
Aerogels are porous ultralight materials derived from gels, in which the liquid component of the gel has been replaced with a gas.2 These advanced materials are highly porous solids that hold gas (usually air) within the pores or networks of solid substances. Due to their light weight, low density, large surface area and high mechanical strength, aerogels are useful for many applications, such as heat insulators, particle filters, particle trappers and catalyst supports.3 Aerogels are composed of a network of clustered nanoparticles. The materials usually have unique properties including high strength to density, and high surface area to volume ratios. They are manufactured by subjecting a wet gel precursor to critical point drying in order to remove the liquid through supercritical drying, without disturbing the network.
Well known aerogels are those of silica and metal oxides such as TiO2 and Fe2O3. However, these inorganic aerogels usually lack mechanical strength and tend to collapse easily when subjected to small stresses. In contrast, aerogels made of organic polymers are stronger and can also be used as carbon precursors for pyrolysis, the resorcinolāformaldehyde aerogel being a prominent example. Aerogels fabricated from synthetic polymers are fragile. As a result, such aerogels have limitations to be used in situations that need robustness. As aerogels combine the properties of highly divided solids and metastable characteristics, they can develop very attractive physical and chemical properties that are not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing a sophisticated potential for a range of applications.4
Aerogels can be classified according to their appearance (as monoliths, powders and films), their different microstructural characteristics (as microporous, mesoporous and mixed porous) or by defining their composition (Figure 1.2).
Figure 1.2 Different classification possibilities of aerogels.
Adapted from ref. 5 with permission from Springer Nature, Ā© Springer Science+Business Media New York 2016.
Polysaccharide-based aerogels are commonly obtained in the form of cylindrical monoliths, although many other shapes (Figure 1.3) can be found in the literature (such as beads, microspheres, etc.).6,7
Figure 1.3 Calciumāalginate aerogel obtained in different shapes: (a) monoliths, (b) beads and (c) microparticles.
Adapted from ref. 6 with permission from Elsevier, Copyright 2011.
The size and morphology of aerogels can be customized by means of shaping the gel by moulding, extrusion or any other suitable physical techniques. In general, gels take the shape of the mould in which gelation takes place and this shape is preserved in the monolithic aerogels after supercritical drying (Figure 1.4).
Figure 1.4 Starch aerogel obtained by supercritical drying from a hydrogel prepared in a mould for Christmas cookies.
Adapted from ref. 6 with permission from Elsevier, Copyright 2011.
1.3 Why Bio-based?
The development of innovative materials from renewable and abundant bio-resources is becoming an important area of research as such materials exhibit high physical properties with a low impact on the environment. Increasing demand for products made from sustainable and non-petroleum based resources is also a major driving force for the development of new bio-based products. Polymers derived from non-petrochemical feedstocks are gaining a great deal of momentum from both commercial and scientific points of view. Biopolymers can be derived from natural sources such as plants, exoskeletons of arthropods, skin, silkworm cocoons, spider webbing, hair, and so forth. These materials are carbon neutral, sustainable, renewable, recyclable, nontoxic and environmental friendly, and can replace petroleum based products.8,9
Fundamental research in the production, modification, property enhancement and new applications of these materials is important. The new materials, concepts and utilizations that result from these efforts will shape the future of polymers from renewable resources.10,11 Various biopolymers, predominantly polysaccharides and proteins, are used in aerogel production because they are ecological materials that can transform numerous industrial processes from being petroleum-dependent into biomaterial-dependent. In particular, they have numerous applications in food and non-food industries.
Biopolymers from various sources such as alginate, cellulose, lignin, pectin, chitosan, proteins and others have been tested as precursors. The resulting aerogels exhibit both the specific inheritable functions of the starting polymer and the distinctive features of aerogels (open porous structure with high specific surface and pore volume). This synergy of properties has prompted researchers to view biopolymer aerogels as promising candidates for a wide range of applications.12 More recent reports on biopolymer aerogels, as chronicled in this book, describe their use for thermal insulation, tissue engineering, regenerative medicine, drug delivery systems, functional foods, and as catalysts and sensors.
1.4 Applications of Bio-based Aerogels: Highlights
Some of the most studied uses of aerogels include different aerospace...
