Nanocellulose Polymer Nanocomposites: Fundamentals and Applications

By Vijay Kumar Thakur

Biorenewable polymers based nanomaterials are rapidly emerging as one of the most fascinating materials for multifunctional applications. Among biorenewable polymers, cellulose based nanomaterials are of great importance due to their inherent advantages such as environmental friendliness, biodegradability, biocompatibility, easy processing and cost effectiveness, to name a few. They may be produced from biological systems such as plants or be chemically synthesised from biological materials.

This book summarizes the recent remarkable achievements witnessed in green technology of cellulose based nanomaterials in different fields ranging from biomedical to automotive. This book also discusses the extensive research developments for next generation nanocellulose-based polymer nanocomposites. The book contains seventeen chapters and each chapter addresses some specific issues related to nanocellulose and also demonstrates the real potentialities of these nanomaterials in different domains.

The key features of the book are:

• Synthesis and chemistry of nanocellulose from different biorenewable resources
• Different  characterization of nanocellulosic materials and their respective polymer nanocomposites
• Physico-chemical, thermal and mechanical investigation of nanocellulose based polymer nanocomposites
• Provides elementary information and rich understanding of the present state-of- art of nanocellulose-based materials
• Explores the full range of applications of different nanocellulose-based materials.

• Language: English
• Publisher: Wiley
• Release date: Oct 28, 2014
• ISBN: 9781118872345

Vijay Kumar Thakur

Vijay Kumar Thakur is Permanent Faculty in the School of Aerospace, Transport and Manufacturing Engineering, Cranfield University, UK. Previously he was a Staff Scientist in the School of Mechanical and Materials Engineering at Washington State University, USA, Research Scientist in Temasek Laboratories at Nanyang Technological University Singapore and Visiting Research Fellow in the Department of Chemical and Materials Engineering at LHU Taiwan. He spent his postdoctoral study in Materials Science & Engineering at Iowa State University, USA. He has extensive expertise in the synthesis of polymers, nano materials, nanocomposites, biocomposites, graft copolymers, high performance capacitors and electrochromic materials. He sits on the editorial board of several SCI journals.

Book preview

Preface

The increasing environmental awareness has resulted in a renewed interest in polymer nanocomposites that are procured from biorenewable polymers such as nanocellulose. These polymer nanocomposites offer higher thermal and mechanical properties, transport barrier, thermal resistivity and flame retardance in comparison with the conventional biocomposites. Nanocomposite describes a two-phase material where one of the phases has at least one dimension in nanometre range (1–100 nm). They differ from conventional composites by the exceptionally high surface to volume ratio of the reinforcing phase and/or its exceptionally high aspect ratio. The reinforcing material can be made up of particles (e.g. minerals), sheets (e.g. exfoliated clay stacks) or fibers (e.g. carbon nanotubes, electrospun fibers or cellulose nanofibers). Large reinforcement surface area means that a relatively small amount of nanoscale reinforcement can have an observable effect on the macroscale properties of the composites. The ability to control the material features at the nanoscale and evaluation of their influence on the micro and macroscopic properties provides a new aspect to the development of nanocomposite systems. There has been enormous interest in the commercialization of nanocomposites for a variety of applications, and a number of these applications are already found in the market. Nanocomposites are currently used in a number of fields and new applications are continuously sought after.

In line with the development of nanotechnology and recent concern about environmental issues, more attention is being paid to utilizing bio-based nano-materials. In this regard, nanocellulose has gained much more interest because of the promising characteristics such as biodegradable nature, renewability and lower price. Nanocellulose-based materials are showing significant interest as potential nanofillers for nanocomposites due to their nanoscale dimension (very high surface area-to-volume ratio), high aspect ratio and impressive mechanical properties (or nano-strength) imparting to desired nanocomposites. Advantages in the use of nanosize cellulosic materials are related not only to these properties, in fact, its dimensions, in the nanometer scale, open a wide range of possible properties to be discovered. Nanosize cellulosic materials can be isolated from a variety of cellulosic resources, including plants, animals (tunicates), bacteria, algae, and in principle could be extracted from almost any cellulosic material by using different procedures. Remarkable achievements have been witnessed in green technology of cellulose nanomaterials in the field of materials science including the development of bio-nanocomposites. The growing interest in green product and unsurpassed physical and chemical properties of nanocellulose has resulted in increased academic and industrial interests towards development of cellulose nanocomposites. However, there are still some issues to be overcome and main challenges in the field are related to an efficient separation of nanosize cellulosic materials from the natural resources. The non-compatible nature of nanocellulose with most of the polymers is also a crucial issue for its application in nanocomposites. In addition, the drying process of nanocellulose for application in polymer composite is another challenge. Last but not least is that we need to find a process for obtaining higher yields in nanocellulose isolation. All these challenges and drawbacks have become the strong driving forces for discovering more efficient processes and technologies to produce nanocelluloses for application in nanocomposites, and for inventing new applications as well.

This book is aimed to provide a detailed knowledge on the issues mentioned above. It also provides a comprehensive overview on the synthesis and applications of nanocellulose-based nanocomposites materials. This book discusses extensive developments for the next generation research in the field of nanocellulose-based nanocomposites. The book contains seventeen chapters and each chapter addresses some specific issues related to nanocellulose and also demonstrates the real potentialities of these materials in different domains.

The principal credit of this goes to the authors of the chapters for summarizing the science and technology in the exciting area of nanocellulose. I would also like to thank Martin Scrivener of Scrivener Publishing along with Dr. Srikanth Pilla (Series Editor) for their invaluable help in the organisation of the editing process.

Finally, I would like to thank my parents and wife Manju for their continuous encouragement and support.

Vijay Kumar Thakur, Ph.D.

Washington State University, U.S.A.

August 30, 2014

Part 1

SYNTHESIS AND CHARACTERIZATION OF NANOCELLULOSE-BASED POLYMER NANOCOMPOSITES

Chapter 1

Nanocellulose-Based Polymer Nanocomposites: An Introduction

Manju Kumari Thakur*,¹, Vijay Kumar Thakur² and Raghavan Prasanth³

¹Division of Chemistry, Govt. Degree College Sarkaghat, Himachal Pradesh University, Summer Hill, Shimla, India

²School of Mechanical and Materials Engineering, Washington State University, Washington, U.S.A.

³Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas, USA.

*Corresponding author: [e-mail]

Abstract

Rising environmental awareness and the high demand for alternatives to non-renewable petroleum resources has led to extensive research focused on the concept of biomass-based biorenewable materials. Natural cellulosic polymers are such materials of prime choice for different applications due to their inherent advantages which include the fact that they are easily available, environmentally-friendly and have lower health risks; and also that they are economical, biodegradable, easily processed, have acceptable specific properties and have excellent insulating/noise absorption properties to name a few. These cellulosic materials in nano form, i.e., as nanocellulose, are rapidly emerging as one of the most promising future materials with outstanding physical, chemical, mechanical and thermal properties for multifunctional applications in different fields. Keeping in mind the promising characteristics of nanocellulosic materials, the present chapter gives an overview of the recent progress in the structure and applications of nanocellulose procured from different resources.

Keywords: Nanocellulose, natural fibers, structure, processing, applications

1.1 Introduction

Polymer-based materials derived from both natural and petrochemical resources are currently being extensively used in a wide range of products and in numerous applications [1-3]. These polymers have superseded the use of other materials such as metals, glasses and ceramics in a number of fields [4-6]. Compared to their natural counterparts, synthetic polymers have been widely used in a vast number of applications such as films, flexible plastic bags, composites and rigid containers to name a few [7-9]. Properties such as light weight, strength, chemical inertness and inexpensive production make them a favorable candidate for most present day applications [10-12]. However some of their other properties also cause considerable environmental problems, with their high molecular weight, chemical stability and relatively low surface area-to-volume ratio making them resistant to degradation by microbial attack, and causing them to persist in the environment long after disposal [5, 13, 14]. In addition, these polymers are produced by oil-based technology, which raises a number of pertinent issues related to increasing oil prices and dwindling resources, so the impetus to replace these polymers with renewable materials is increasing [15-17]. In order to conserve resources and avoid adding increased carbon emissions, materials must be developed that consume less energy and use raw materials that are derived from renewable resources [18].

Indeed, rising environmental awareness around the world has resulted in a renewed interest in materials procured from biorenewable resources [19, 20]. One of the common practices to prepare new environmentally-friendly materials is the incorporation of a least one component that is derived from renewable resources [21, 22]. Green materials have attracted great attention and interest in the development of biodegradable or natural polymer-derived green composites, while minimizing the generation of pollution [23]. Natural polymers, or biopolymers, are polymers that are produced from renewable resources [24, 25]. They may be produced by biological systems such as plants or animals, or be chemically synthesized from biological materials [26]. It is also desirable to make use of natural materials which do not, for example, compete with the food chain [27, 28]. A biodegradable polymer can be defined as a material in which degradation results from the action of microorganisms such as bacteria, fungi and algae [29, 30]. Therefore the use of biopolymers to replace synthetic polymers is attractive due to their obvious environmental advantages of being sustainable, renewable and biodegradable, being broken down into carbon dioxide and water when exposed to microbial flora [16, 31, 32]. In this advancement, the development of high-performance polymer biocomposite materials made from natural resources has been increasing worldwide due to environmental and sustainability issues [9, 27, 33]. The use of renewable materials such as natural cellulose (most abundant biopolymer) is becoming impellent because of the great demand for alternatives to non-renewable petroleum materials and good reinforcing material due to its availability, low cost, low density, nontoxicity, low abrasiveness, biocompatibility and biodegradability [28, 34, 35]. Biocomposites consisting of the polymer matrix and natural cellulose fibers are environmentally-friendly materials which can replace glass fiber-reinforced polymer composites, and are currently used in a wide range of fields such as the automotive and construction industries, electronic components, sports and leisure, etc. [36, 37].

Recently, the research on biobased nanocomposites which are reinforced with both natural fibers and nanofillers is actively proceeding in order to offer higher thermal and mechanical properties, transport barrier, thermal resistivity and flame retardance in comparison with the conventional biocomposites [20, 38]. Nanocomposite describes a two-phase material where one of the phases has at least one dimension in nanometer range (1–100 nm) [39]. They differ from conventional composites by the exceptionally high surface-to-volume ratio of the reinforcing phase and/or its exceptionally high aspect ratio. The reinforcing material can be made up of particles (e.g., minerals), sheets (e.g., exfoliated clay stacks) or fibers (e.g., carbon nanotubes, electrospun fibers or cellulose nanofibers) [40]. Large reinforcement surface area means that a relatively small amount of nanoscale reinforcement can have an observable effect on the macroscale properties of the composites. The ability to control the material features at the nanoscale and evaluation of their influence on the micro- and macroscopic properties provides a new aspect to the development of nanocomposite systems. There has been enormous interest in the commercialization of nanocomposites for a variety of applications, and a number of these applications are already found in the market [41]. Nanocomposites are currently used in a number of fields and new applications are continuously sought after.

In line with the development of nanotechnology and recent concern about environmental issues, more attention has been paid to the utilization of biobased nano-materials. In this regard, nanocellulose has gained much more interest because of its promising characteristics such as biodegradable nature, renewability and lower price [19]. Nanocellulose-based materials are gaining significant interest as potential nano-fillers for nanocomposites due to their nanoscale dimension (very high surface area-to-volume ratio), high aspect ratio and impressive mechanical properties (or nanostrength), which are imparted to the desired nanocomposites [42]. The advantages for the use of nanosize cellulosic materials are not only related to these properties; in fact, its dimensions, in the nanometer scale, open a wide range of possible properties yet to be discovered. Nanosize cellulosic materials can be isolated from a variety of cellulosic resources, including plants, animals (tunicates), bacteria and algae, and in principle could be extracted from almost any cellulosic material by using different procedures. Remarkable achievements have been witnessed in the green technology of cellulose nanomaterials in the field of materials science, including the development of bio-nanocomposites. The growing interest in green product and unsurpassed physical and chemical properties of nanocellulose have resulted in increased academic and industrial interest towards the development of cellulose nanocomposites. However, there are still some issues to be overcome and the main challenges in the field are related to an efficient separation of nanosize cellulosic materials from the natural resources [43]. The incompatible nature of nanocellulose with most polymers is also a crucial issue for its application in nanocomposites. In addition, the drying process of nanocellulose for application in polymer composite is another challenge. The last but not least point is related to finding a process for obtaining a higher yield in nanocellulose isolation. All these challenges and drawbacks have become the strong driving force for discovering more efficient processes and technologies to produce nanocelluloses for application in nanocomposites, and for inventing new applications as well [15]. Chapters 2–9 of this book discuss in detail the synthesis and characterization of different types of nanocellulose-based polymer composites, while Chapters 10–17 discuss in detail the processing and multifunctional applications of cellulose-based polymer nanocomposites.

1.2 Nanocellulose: Source, Structure, Synthesis and Applications

Human society has used natural cellulose-based materials for thousands of years, both knowingly and unknowingly [44]. All the industries around the world are looking for materials that can be easily procured from renewable and sustainable resources. However, although cellulose-based materials offer a number of advantages, for advanced applications some of the imperative properties such as functionality, uniformity and durability are not achieved using traditional cellulosic materials. Fortunately, the use of cellulose in nanoform can solve these issues. By suitable extraction of cellulose from different biorenewable resources at the nanoscale, next generation of multifunctional polymer nanocomposites can be obtained by employing a new cellulose-based building block known as nanocellulose. Nanocellulose offers a number of advantages such as high aspect ratio, low density (1.6 g cm−3), and a reactive surface of –OH side groups compared to the parental cellulose, and these functional groups also facilitate the attachment of desired functional groups onto these nanocellulose surface to achieve different surface properties. Nanocellulose can be obtained in different forms depending upon the source material and the intended applications. Chapters 2–9 of this book discuss in detail the different types of nanocellulosic materials. Natural cellulosic fibers are one of the most promising resources for the synthesis of nanocellulose. Natural cellulosic fibers can be divided into different types (Figure 1.1) [45].

Figure 1.1 Classification of cellulosic fibers.

Reprinted with permission from [45]. Copyright 2010 Wiley.

It has been reported that cellulose fibrils (micro/nano size) or cellulose whiskers can be easily procured from cotton fibers as well as cellulose filter papers [46]. Nanocellulose can be obtained from different resources such as wood, plants, tunicates, algae and bacteria. Figure 1.2 shows the structure of cellulose. Cellulose is a non-branched linear polysaccharide molecule that is comprised of two anhydroglucose rings (C6H10O5) n; n = 10000 to 15000, where n depends upon the source of cellulose. These rings are linked together through the β 1–4 glucosidic bond [44].

Figure 1.2 Schematic structure for carbon atoms in an anhydroglucose unit of cellulose.

Cellulose is one of the most abundant natural polymers on earth and provides strength/stability to the plant cell walls [45]. The properties and economics of fiber production for various applications are influenced by the amount of cellulose in a fiber. In natural cellulosic fibers, stiff semicrystalline cellulose microfibrils have been found to be embedded in a pliable amorphous matrix (Figure 1.3) [45].

Figure 1.3 Plant plasma membrane and cell wall structure: a) Cell wall containing cellulose microfibrils, hemicellulose, pectin, lignin and soluble proteins; b) Cellulose synthase enzymes are in the form of rosette complexes, which float in the plasma membrane; c) Lignification occurs in the S1, S2 and S3 layers of the cell wall.

Reprinted with permission from [45, 52]. Copyright 2010 Wiley and 2008 Nature.

Nanocellulose can be obtained through different processes [47]. The length of these nanocellulosic materials depends upon the resource from which they are obtained [52]. For example, in the nanocellulose obtained from tunicates and green algae, crystallites have lengths in the range of a few micrometers, while those obtained from wood and cotton have lengths of the order of a few hundred nanometers. These nanocellulose are referred to by different names such as microcrystalline cellulose (MCC), nanocrystalline cellulose (NCCs), nanowhiskers, bacterial cellulose, etc., depending upon their specific characteristics and synthesis procedures [48]. Chapters 2 and 5 of this book are solely focused on the bacterial cellulose-based polymer nanocomposites. Bacterial cellulose (BC) is generally biosynthesized by numerous bacteria as a 3D network of nano- and micro-fibrils. It has received greater attention during the last few years because of its unique features such as crystallinity, high purity, water-holding capacity, tensile strength and Young’s modulus, which can be successfully exploited in the development of innovative nanostructured composite materials. Different synthesis and characterization aspects of the bacterial cellulose are discussed in Chapters 2 and 4. Chapter 3 discusses the synthesis and chemistry of cellulose whiskers, nanofibrillated cellulose and the synthesis of nanocomposites using polyurethane as the polymer matrix. Chapter 4 comprehensively discusses the structure, properties and methods of characterization along with the growth conditions for bacterial cellulose. Different modification strategies to alter the properties of bacterial cellulose for certain specific applications are also discussed in this chapter. These modification strategies include both physical and chemical modifications. Chapter 6 focuses on the synthesis of multifunctional ternary polymer nanocomposites using cellulosic nano-reinforcement with an emphasis on nanocrystalline cellulose (NCC), microfibrillated cellulose nanofibers (MFC) and bacterial cellulose (BC). Chapter 8 of the book comprehensively discusses the nanocellulose-based liquid crystalline composite systems in detail. The main emphasis of this chapter is on nanocrystalline cellulose, micro crystalline cellulose, composites, films and electrospun fibers. Chapter 11 describes in detail the isolation of nanocellulose from numerous sources and its utilization for fabrication methods, its characterization, drying processes and modification. The chapter also discusses the application of nanoscale cellulosic materials in polymer nanocomposites. Chapter 13 is focused on the cellulose whiskers procured from kenaf fibers. Different thermal and dynamic mechanical properties of the nanocomposites are also discussed in this chapter. Chapter 4 focuses on the processes in cellulose derivative structures. The main steps that are generally involved in the preparation of cellulose nanocrystals and microfibrillated celluloses are shown in reference [49].

Figure 1.4 shows the transmission electron micrographs (TEM) of microcrystalline cellulose obtained from dilute suspensions of cotton, sugar beet pulp, and tunicin (the cellulose extracted from tunicate) whiskers [48].

Figure 1.4 Transmission electron micrograph from a dilute suspension of hydrolyzed (a) cotton, (b) sugar beet pulp and (c) tunicin.

Reprinted with permission from [48]. Copyright 2005 American Chemical Society.

Microcrystalline cellulose has been found to be insoluble in common solvents generally used in the preparation of nanocomposites. The MCC has been found to form colloidal suspensions when suspended in water (Figure 1.5). Different parameters of MCC such as dimensions of the dispersed particles, surface charge and their size polydispersity control the stability of these suspensions [48].

Figure 1.5 Photograph of an aqueous suspension of tunicin whiskers observed between cross nicols, showing the formation of birefringent domains.

Reprinted with permission from [48]. Copyright 2005 American Chemical Society.

Nanocrystalline cellulose (NCCs) are general referred to as rigid rod-like crystals having a diameter in the range of 10–20 nm and lengths of a few hundred nanometers [47]. Figure 1.6 depicts the location and extraction of nanocrystalline cellulose [50]

Figure 1.6 Location and extraction of nanocrystalline cellulose.

Reprinted with permission from [50]. Copyright 2012 John Wiley.

Figure 1.7 shows the TEM images of some of the nanocrystalline cellulose obtained using sulfuric acid hydrolysis. Nanocellulose contains an abundance of hydroxyl groups susceptible to various chemical reactions. The nanocellulosic materials such as nanofibers are also processed to produce the micro/nanocrystal using several pretreatments. Some of the common treatments include the removal of the amorphous regions at the interface of microcrystalline domains in these fibers by acid treatment [46]. For a number of applications nanocellulose is modified using different techniques. Some of the commonly used techniques include carboxylation, esterification, silylation, cationization, and polymer grafting [47, 53].

Figure 1.7 TEM micrographs of nanocrystals obtained by sulfuric acid hydrolysis of (a) cotton (b) avicel and (c–e) tunicate cellulose. The insets of (a) and (b) provide higher resolution images of some characteristic particles.

Reprinted with permission from [47, 53]. Copyright 2010 John Wiley and 2008 American Chemical Society.

A summary of the different chemical modification techniques used to alter the surface characteristics of nanocellulose can be found in reference [47].

There has been Intense ongoing research to avoid the complex surface functionalization techniques. One of the new techniques is to combine the synthesis and functionalization of nanocellulose in a single step (see reference [47]).

Nanocellulose and its derivatives can be processed into different forms. Bacterial celulose is one such important type of nanocellulose. It has been processed into nanofibers for different applications. Figure 1.8 shows the different applications of cellulose acetate nanofibers [51]. Chapters 2 and 4 discuss the different perspectives of bacterial cellulose-based materials and their different applications. In these chapters the authors discuss in detail a vast collection of BC nanocomposites prepared using different polymer matrices such as natural polymers and thermoplastic matrices. In addition to this, the effect of inorganic nanophases are also addressed to demonstrate the real potentialities of bacterial cellulose in the polymer nanocomposites. Chapter 2 also discusses in detail the bacterial cellulose-based hybrid nanocomposite materials. Chapter 3 summarizes the new trends in the use of nanocellulose (nanowhiskers and nanofibrillated cellulose) as reinforcement of different types of polyurethane systems.

Figure 1.8 Schematic representation of electrospinning cellulose acetate nanofibers (at the center) and myriad biotechnological applications.

Reprinted with permission from [51]. Copyright 2013 Elsevier.

Chapter 4 discusses in detail the bacterial cellulose-reinforced renewable polymer-matrix-based composites. The techniques used to prepare the nanocomposites include impregnating bacterial cellulose, solution blending and casting, electrospinning and melt blending, in-situ composites, along with several other methods. Chapter 5 focuses solely on the preparation of nanocomposites using the in-situ synthesis technique. It also summarizes the applications of the in-situ synthesized nanocomposites in bone defect repair and bone tissue engineering, electrically active paper, nanostructured porous materials for drug delivery or as bioactive compounds, surface coating applications, and biobased green nanocomposites. The effect of in-situ polymerization on the biodegradation behavior of cellulose nanocomposites is also discussed in this chapter. Chapter 6 reviews selected approaches for the modulation of the final properties of a polymeric nanocomposite containing cellulosic nano-reinforcement combined with a second filler of different chemical nature. Different properties of the synthesized nanocomposites are analyzed and reported in this chapter, taking into account the required functionality of the device in the appropriate final application. The effect of the incorporation of other fillers on the properties of nanocellulose-based nanocomposites is also discussed in detail. Chapter 9 reviews the recent advances in nanocomposites based on biodegradable polymers and nanocellulose. Different kinds of biodegradable polymers were used as the matrix material in the preparation of cellulose-based nanocomposites. The different ways to obtain nanocellulose from several sources (micro-crystalline cellulose, natural fibers and agro-wastes) have been reviewed in this chapter along with the recent advances in biodegradable polymers/cellulose nanocomposites for packaging applications. Chapter 10 describes the fundamental problems faced in the development of cellulose nanocomposite and the methods adopted to overcome them. Chapter 12 is solely focused on the electrospinning of cellulose. It discusses in detail the fundamental processing aspect and utilization of different solvents for electrospinning of cellulose, along with the preparation of cellulose composites. Chapter 15 of the book comprehensively discusses cellulose nanocrystals and their biomedical applications. In this chapter, the extraction and characterization of cellulose nanocrystals are discussed along with their functionalization as well as industrial and biological applications. Chapter 16 also focuses on the biomedical applications of cellulose and its derivatives. The last chapter focuses on recent advances in the multifunctional nanocomposites based on nanocellulose. In this chapter different types of nanocomposites ranging from magnetic to electrically-conductive nanocomposites are discuses, with a particular emphasis on the structure and chemistry.

1.3 Conclusions

Among various biobased nanomaterials, nanocellulose is one of the most economical and environmentally-friendly biorenewable materials that can be easily procured from different resources. Different kinds of eco-friendly polymer nanocomposite materials with outstanding thermal, morphological and mechanical properties can be obtained using nanocellulose as potential reinforcement. The versatile applications of nanocellulose ranges from biomedical to high-performance structural nanocomposites. One of the biggest challenges in the use of nanocellulose is its large-scale production. To extensively use the nanocellulose for multifunctional applications, active research participations from the academic and industrial sectors is highly desired to overcome some of the shortcomings associated with nanocellulose.

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Chapter 2

Bacterial Cellulose-Based Nanocomposites: Roadmap for Innovative Materials

Ana R. P. Figueiredo, Carla Vilela, Carlos Pascoal Neto, Armando J. D. Silvestre and Carmen S. R. Freire*

Department of Chemistry, CICECO, University of Aveiro, Aveiro, Portugal

*Corresponding author: [e-mail]

Abstract

In the last decades there has been an increasing awareness in the search for biobased alternatives as sources of novel nanocomposites for application in several fields such as packaging, biomedical products and devices, as well as in high-technology domains. Nanocellulose forms like bacterial cellulose (BC), biosynthesized by several bacteria as a 3D network of nano- and micro-fibrils, have gained particular attention in this context because of their unique features, namely high purity, water-holding capacity, crystallinity, tensile strength and Young’s modulus, that can be successfully exploited in the development of innovative nanostructured composite materials. In this chapter, a comprehensive overview on the production, processing, properties and applications of bacterial cellulose-based nanocomposites is compiled and discussed. A vast collection of BC nanocomposites such as those with other natural polymers, thermoplastic matrices and inorganic nanophases will be addressed, aiming to demonstrate the real potentialities of BC in this domain.

Keywords: Bacterial cellulose, nanocomposites, polymer composites, hybrid materials, inorganic nanoparticles

2.1 Introduction

Cellulose is the most abundant biological macromolecule on Earth, with about 1.5 × 10¹² tons produced each year and a high economic importance in the pulp and paper as well as textile industries [1-3]. Most cellulose is obtained from plants, where it represents the main structural element of cell walls; but it is also produced by a family of sea animals called tunicates, several species of algae and some aerobic nonpathogenic bacteria, as well as through enzymatic and chemical methods [1]. Regardless of its origin, cellulose is a linear homopolymer of β-D-glucopyranose units linked by β-(1→4) glycosidic bonds, varying essentially on purity, degree of polymerization and crystallinity index [4]. Bacterial cellulose (BC) was first reported by Adrien Brown in 1886. While studying acetic fermentations, he noticed the formation of a white gelatinous pellicle on the surface of a liquid medium, which had the capability to grow to a thickness of 25 mm and proved to be very strong and tough. Brown also verified that this membrane was generated by a bacterium, initially named Bacterium xylinum, but later classified as Acetobacter xylinum and currently termed Gluconacetobacter xylinus. Further research studies showed that this material had the same chemical composition as the cellulose produced by plants, and until today bacterial cellulose remains as the most pure existing natural form of cellulose [5, 6].

Several other species of bacteria of the genera Gluconacetobacer, Sarcina and Agrobacterium have been reported as cellulose producers [4]. However, only Gluconacetobacter species can produce cellulose at commercial levels. Gluconacetobacter xylinus remains as a model strain and is used in research and commercial production [7]. It is a nonpathogenic, rod-shaped, obligate aerobic Gram-negative bacterium capable of producing relatively high amounts of cellulose from several carbon and nitrogen sources [4, 8]. Such bacteria are ubiquitous in Nature, being naturally present wherever the fermentation of sugars takes place, for example, on damaged fruits and unpasteurized juices, beers and wines [8]. Recently, we have also reported the production of BC by a Gluconoacetobacter sacchari strain using different carbon sources with yields comparable to those obtained with G. xylinum [9]. In the latter decades, the use of BC gained considerable attention in the global scenario of increasing awareness and demand for biobased environmentally-friendly functional materials because of its inherent abundance, renewability, biodegradability, biocompatibility and specific features (particularly the nanometric dimensions and nanostructured network). The creation of nanocomposites with diverse partners, as synthetic and natural polymers, bioactive compounds as well as inorganic NPs constitutes a wide field of BC research and development, as tentatively revised by some authors in the latter years [10–13]. For instance, Shah et al. [11] compiled representative methodologies for BC composites preparation, some classes of BC composites and their applications, Fu et al. [12] summarized the current investigation on BC–based materials for skin tissue repair and Hu et al. [13] collected relevant results on functionalized BC derivatives and composites. However, the domain of BC-based composites is exceptionally vast and in continuous innovation. Therefore, the aim of this chapter is to present a comprehensive, yet detailed, overview of most relevant results obtained on the production and properties of BC, and in particular on the design and applications of BC composites with different partners.

2.2 Bacterial Cellulose Production, Properties and Applications

2.2.1 Bacterial Cellulose Production

Bacterial cellulose synthesis by G. xylinus starts with the production of individual β-(1→4) chains between the outer and plasma membranes of the bacterial cell. A single G. xylinus cell may polymerize up to 200000 glucose molecules per second into β-(1→4) glucan chains, followed by their release outwards through pores in the cell surface [14]. BC chains then assemble into protofibrils, with approximately 2-4 nm of diameter, that further gather into microfibrils of approximately 3-15 nm thick and 70-80 nm wide [1, 4, 15, 16]. Microfibrils, in turn, entangle into a ribbon of crystalline cellulose whose interwoven produces the BC fibrous network (Figure 2.1) [5, 8, 17, 18]. The reason why these bacteria generate cellulose is still unclear, but it has been suggested that it is a mechanism that bacteria use to maintain their position close to the surface of the culture medium, where there is a high oxygen content; and also serves as a protective coating against ultraviolet radiation, prevents the entrance of enemies and heavy-metal ions whereas nutrients diffuse easily throughout the pellicle [1, 19].

Figure 2.1 Scanning electron microscopy (SEM) images of Gluconacetobacter xylinus and BC network of micro and nano fibrils; and schematic description of the formation of bacterial cellulose.

Reproduced with permission from [7].

BC has many applications which have triggered high interest in its production at a commercial scale. However, the main problems that hamper this process are the low yield and production costs, especially for low added value applications. Therefore, some attempts have been made in terms of optimization of culture conditions and medium composition, as well as the scaling-up process [14]. Bacterial cellulose is commonly produced using the Hestrin-Schramm (HS) medium, that uses glucose as the carbon source and a combination of peptone and yeast extract as nitrogen sources [14]. However, the use of glucose as carbon source for BC production is quite expensive and causes the formation of by-products such as gluconic acid that decreases the pH of the culture medium and ultimately declines the production of BC [4, 8]. Therefore, researchers have investigated the capability of G. xylinus to grow and produce BC using different carbon sources. Besides glucose and sucrose (the most commonly used), other carbohydrates such as fructose, maltose, xylose and starch, and polyols as glycerol have also been successfully tested [4].

In addition, other efforts have been also devoted to the identification of cheap feedstocks as alternatives to the expensive conventional culture media, with pure compounds, for the economically viable production of BC [20]. In this context, several industrial wastes have already been effectively explored for the production of BC, as for example tea infusions [21], wheat straw acid hydrolysate [22], grape bagasse and crude glycerol [23], beet molasses [24], sugar-cane molasses and corn steep liquor [20], Konjac powder [25], fruit juices, including orange, pineapple, apple, Japanese pear and grape [26, 27], grape skins aqueous extract and sulfite pulping liquor [28], and dry olive mill residue [29]. The development of culture media based on cheaper feedstocks will simultaneously allow the production of BC at lower price and the valorization of the residues themselves [29].

Enhancement of BC production has also been attempted through supplementation of the culture medium with different additives. Several chemicals including alcohols [30], vitamin C [31], lignosulfonates [32], water-soluble polysaccharides [33–35], thin stillage from rice wine distillery [36] have been investigated in this perspective. For instance, Lu et al. [30] investigated the stimulatory effects of six different alcohols, added at different concentrations, during static fermentation of G. xylinus 186. All alcohols tested improved BC production and could be ranked as n-butanol > mannitol > glycerol > ethylene glycol > methanol > n-propanol. However, results showed that n-butanol only improves BC production when added at concentrations lower than 1.5% v/v (maximum production of 132.6 mg/100 ml, 56.0% higher than the control), while mannitol stimulates BC production at any concentration, with a maximum effect at a concentration of 4% v/v (maximum production of 125.2 mg/100 ml, 47.3% above the control).

Another purpose of adding chemicals to the culture medium is the chemical modification of the structural and physical properties of bacterial cellulose, allowing the preparation of composites directly during biosynthesis and broadening the applications of cellulose [18], as will be discussed latter.

Another important point to be taken into account in BC production is the cultivation method employed, once this affects the structure, physical and mechanical properties of the final material. Therefore, the selection must be made according to BC intended applications [37]. BC has been synthesized through a number of different routes, which are broadly classified into static and agitated processes.

Static cultivation is the most common method, from which a highly hydrated BC membrane (or pellicle) on the air-culture medium interface is obtained (Figure 2.2) [19, 38]. As cellulose is synthesized, a membrane with increasing thickness is generated and, once oxygen is required for bacteria growth and cellulose production, it is assumed that the mature BC membrane is constantly pushed down as new cellulose is produced on the interface [8, 15].

Figure 2.2 Photographs of a static culture (with a visible BC pellicle in the interface air-culture medium) and a purified bacterial cellulose wet membrane produced in static conditions.

Reproduced with permission from [29, 39].

Under static conditions, and using suitable molds, it is possible to obtain uniform and smooth BC products with defined shapes, which can be employed for instance in the biomedical field [12] as artificial blood vessels [8] or artificial skin [40]. The moldability of BC during biosynthesis and shape retention is a feature that may enable the development of designed shape products directly in the culture media [8, 41], increasing the application range of BC.

However, production under static conditions requires more working space and workload, turning the potential industrial scale production more expensive [7]. Therefore, a major goal on BC research has been centered on the optimization of BC production through the design of efficient static culture reactors. Kralisch et al. [42] developed a novel, efficient bioreactor named Horizontal Lift Reactor (HoLiR) (Figure 2.3) that allows the semi-continuous cultivation of planar bacterial cellulose. This process combines the advantages of static cultivation with the continuous harvesting under steady state conditions. Furthermore, the resulting material has similar properties to those of BC produced under traditional static conditions, but with a significant cost reduction.

Figure 2.3 HoLiR—experimental scale, experimental plant (left), BNC fleece (right).

Reproduced with permission from [42].

In a different study, Serafica et al. [43] reported the production of BC using a rotating disk bioreactor (Figure 2.4), consisting of a cylindrical trough with inoculated medium into which flat circular disks mounted on a rotating central shaft are dipped. The disks alternately dip into liquid medium and travel into air, making the transport of nutrients and oxygen to the growing cells nearly ideal, and creating a product that has twice the water holding capacity of a typical BC pellicle from static cultures. Moreover, this rotating disk bioreactor is more efficient than common surface cultures (BC production 86.78% higher than in traditional static conditions) and reduces the time of a run to about 3.5 days instead of the usual 12–20 days.

Figure 2.4 Schematic diagram of a rotary disk reactor (reproduced with permission from [43]) and photographs of the apparatus.

Reproduced with permission from [7].

The aerosol bioreactor (Figure 2.5) is another novel system that involves the generation of an aerosol spray of glucose and its even distribution to bacterial cells on the culture medium-air interface. This process results in homogeneous pellicles of BC with superior mechanical properties than those produced under traditional static conditions. BC can also be generated, not as one thick membrane but, as several 3-4 cm thick slices, by interruption of glucose supply. Furthermore, this new bioreactor eliminates the problems inherent to traditional static cultivation of BC: the hindering of BC growth by the wall effect, the rate of mass transfer limitation of the glucose supply and the culture medium enrichment with by-products; and offers the theoretical prospect of an unlimited, continuous production rate [44].

Figure 2.5 Mechanism of BC production using an aerosol bioreactor and the formation of BC slices by interrupting the glucose supply.

Reproduced with permission from [44].

An alternative approach to BC production is through agitated cultivation, which generates small pellets, fibers, irregular masses or spherical particles instead of membranes (Figure 2.6) [45–47].

Figure 2.6 Photograph of spherical BC particles produced in agitated culture under different rotational speeds: (a) 125 rpm (particle diameter of ~ 8mm), (b) 150 rpm (particle size ~ 2.5 mm), (c) 175 rpm (particle size ~ 1 mm), (d) 200 rpm (particle size < 1 mm) (reproduced with permission from [47]); and BC produced in a biofilm reactor.

Reproduced with permission from [46].

BC produced in agitated cultures shows a microscopic structure similar to that obtained under static conditions (Figure 2.7). However, its nanofibers are curved and entangled with one another, in contrast with the highly extended ones attained under static conditions, resulting in a denser structure. In addition, agitated BC has a lower degree of polymerization and crystallinity index, and higher water holding capacity than the one obtained under static conditions [38].

Figure 2.7 Scanning electron micrographs (SEM) of bacterial cellulose produced in (a) agitated and (b) static cultures.

Reproduced with permission from [38].

Agitated culture is considered as the most suitable cultivation technique for the commercial production of BC once, in comparison with static culture, requires less space and work force and higher production rates may be possibly achieved [45, 48].

Several attempts have been made to produce BC using the stirred-tank reactor. However, cellulose production under intensive agitation encounters many problems, including the spontaneous appearance of mutations in the bacterial strains, which causes a decline in the biopolymer synthesis; accumulation of BC in fibrous form during cultivation which increases the viscosity of the broth enabling proper oxygen supply; and an easy attachment of BC to the shaft of reactors, making it hard to collect and also to clean up the reactors [45, 49].

In order to overcome such drawbacks, several effective agitated culture bioreactors have recently been designed and tested. The airlift reactor was firstly described by Chao et al. [50] for the production of BC (Figure 2.8). Accordingly, oxygen-enriched air is supplied to increase the amount of dissolved oxygen in the broth and also promote the mixing of the culture medium. This way, no mechanical powered agitation is required, granting it with low power requirements. Furthermore, the homogenous shear stress throughout the bioreactor and mild agitation resulted in unique elliptical pellets of BC, instead of the fibrous materials attained when using the stirred-tank reactor. The results reported a BC production of 3.8 g.L−1 using normal air supply. However, the concentration of BC doubled (8 g.L−1) when oxygen–enriched gas was used in the system [51]. Furthermore, Chao et al. also discovered that the addition of 0.1% (w/w) agar to the culture medium increased BC production from 6.3 g.L−1, in the control sample, to 8.7 g.L−1 [52].

Figure 2.8 Different types of bioreactors used to produce BC: airlift bioreactor (left) (reproduced with permission from [37]); and spherical type bubble column bioreactor (right) (reproduced with permission from [54]).

Several modifications of the air-lift reactor have been proposed. For instance, Cheng et al. [53] produced a modified air-lift reactor containing a rectangular wire-mesh draft tube that was easier to scale up and construct. These new draft tubes are capable of subdividing the bubbles into smaller ones which resulted in a higher volumetric oxygen transfer coefficient and mixing capability than conventional reactors. In fact, without using oxygen-enriched air, the dissolved oxygen in the modified airlift reactor could be maintained above 35% throughout the cultivation process. As a result of the use of such reactor, after 72 h of cultivation, the final concentration of the bacterial cellulose was 7.72 g.L−1, which was three times higher than that reported for the stirred-tank reactor [53].

The spherical type bubble column bioreactor (Figure 2.8) is another modified airlift reactor with spherical shape instead of cylindrical. Through the addition of small amounts of agar to the culture medium, low shear stress is achieved as well as high oxygen transfer rates. As a result, 5.6 g.L−1 of BC were produced after 72 hours of cultivation [54, 55].

2.2.2 Bacterial Cellulose Properties and Applications

Bacterial cellulose is characterized by specific and extraordinary properties which allow applications other than those of plant cellulose. First of all, it is obtained in a highly pure form, completely free of hemicelluloses, lignin and pectins [4], making it easier to extract and purify as compared to plant cellulose [7].

BC is characterized by an ultrafine network structure composed of ribbon-shaped fibrils with an average diameter 100 times thinner than those of plant cellulose fibers (Figure 2.9) [4]. As a result, BC membranes are a highly porous material with substantial permeability for liquids and gases and high water uptake (water content >90%) [8].

Figure 2.9 SEM images of the surface (left) (reproduced from [56]) and cross-section (right) (reproduced with permission from [57]) of a BC membrane.

BC nanofibers have low density [58] and high degree of polymerization (about 2000-6000) [4, 59]. In addition, their large aspect ratio and high surface area leads to strong interactions with surrounding components, resulting, for example, in the retention of high amounts of water, strong interactions with other polymers and biomaterials, and fixation of different types of nanoparticles [60], these are fundamental aspects on the development of novel composite materials as will be discussed latter. BC is also characterized by a high crystallinity index (60–80%) [1, 4, 61, 62], high mechanical strength, with a tensile strength of 200-300 MPa [1, 4] and a Young’s modulus of up to 15 GPa [1, 4, 8, 63]; as well as high thermal stability (with a maximum decomposition temperature ranging between 340-370°C) [64].

The resistance to in vivo degradation, due to the absence of cellulases in the human body, and low solubility of BC may also be advantageous for some tissue engineering applications [65].

The biocompatibility and nontoxicity of BC has also been accessed, through in vitro and in vivo studies. Several reports indicated that BC is not cytotoxic to Chinese hamster ovary (CHO) cells, fibroblasts and endothelial cells, in vitro. The in vivo toxicity of BC was investigated through its subcutaneous implantation into rats and the implants evaluation with respect to any sign of inflammation, foreign body responses and cell viability [66]. The results attained revealed no macroscopic signs of inflammation around the implants and allowed concluding that BC was beneficial to cell attachment and proliferation [66]. Another approach to these tests is through the intraperitoneal injection of various doses of BC nanofibers into mice. After several days of exposure, blood samples were collected and the results showed no effect on the biochemical profile between the control and mice exposed to BC [65].

These unique properties of BC have inspired attempts to use it in a number of distinct applications. One of the first uses of BC was as a raw material for the production of an indigenous dessert in Philippines called nata de coco. BC is produced from coconut water fermentation and then cut into pieces and immersed in a sugar syrup [1, 19, 60]. In addition, BC has also been investigated as potential thickening, stabilizing, gelling and suspending agent in the food industry. For instance, ice cream containing BC can retain its shape for at least 60 minutes after removing from freezer while control ice cream melts away completely over the same time [7].

BC can also be employed as support for enzymes and cells immobilization. Glucoamylase has been immobilized in BC beads (0.5-1.5 mm) which demonstrated to increase its stability towards pH and temperature changes [67]. Wine yeast has also been immobilized in BC, showing higher metabolic activity and resistance to unfavorable conditions during wine fermentation in comparison with non-immobilized yeasts. In addition, the application of immobilized yeast to repeated batch fermentation in wine-making enhances the economic effectiveness of the production-line because of the cost reduction in inoculum preparation and the simple separation of yeast at the end of the fermentation [68, 69].

Bacterial cellulose has been also explored in a series of technical applications. For instance, SONY Corporation and Ajinomoto developed an audio speaker diaphragm membrane using a compressed low thickness (~20 μm) BC membrane, that is currently utilized in audio headphones (Figure 2.10) [4, 58].

Figure 2.10 Bacterial cellulose diaphragm used in SONY headphones.

Reproduced with permission from [58].

In another study, Iguchi et al. described that the addition of disintegrated bacterial cellulose to wood pulp fibers allowed the creation of a paper sheet with increased tensile strength and folding endurance [19].

The properties of BC, such as the high water-retention capacity, mechanical strength and biocompatibility encouraged also the development of several products for biomedical applications, especially as wound dressings (Figure 2.11) [61], temporary skin substitutes [61] and vascular implants (Figure 2.11) [8]. Biofill®, a temporary human skin substitute for second and third degree burns [8], and Nexfill®, a BC dry bandage for burns and wounds [59], are examples of already commercialized BC products.

Figure 2.11 (a) BC dressing applied on a wounded hand (reproduced with permission from [61]); (b) BC synthesized as tubes with different diameters for potential microvessel endoprotesis (reproduced with permission from [8]) and (c) 3D BC implant prototype for potential ear cartilage replacement (reproduced with permission from [70]).

Furthermore, the properties of BC, namely its favorable mechanical properties, biocompatibility, in situ moldability and porosity (that favors cell proliferation), gives BC excellent perspectives as scaffold for tissue engineering. Several works have focused on designing ideal biomedical devices from BC, such as artificial blood vessels [8, 71], artificial cornea [72], heart valve prosthesis [73], artificial bone [74] and artificial cartilage [70, 75].

BC membranes are likewise promising nanostructured topical drug release systems for different drugs or active compounds, such as lidocaine hydrochloride, ibuprofen and caffeine, while at the same time serving as an efficient physical barrier against any external infection [76–78].

In a similar vein, BC has also been described as an excellent non-allergic biomaterial for the cosmetic industry where it can be employed as facial masks for the treatment of dry skin [79], in the formulation of natural facial scrubs [80] or as a structuring agent in personal cleansing compositions [81].

Finally, the remarkable mechanical properties and reinforcing potential, renewability, biobased nature, biodegradability and unique nanostructured porous network of BC make it a perfect candidate for polymer and hybrid nanocomposites development. In this sense, extensive research has been carried on the design of innovative BC nanocomposite materials with improved and functional properties, by combination with several natural and synthetic polymers as well as inorganic nanophases, for a wide range of biomedical and technological applications. This will be the object of the two coming sections.

2.3 Bacterial Cellulose-Based Polymer Nanocomposites

Bacterial cellulose (BC)-based polymer composites could be prepared by several methodologies, however the in situ approach and the post impregnation or blending are the most frequently described [11, 82]. The in situ method involved the addition of water soluble polymers or insoluble polymeric particles to the BC culture medium at the beginning of the biosynthesis; in this way, the entrapped materials became part of the cellulose micro and nanofibrillar network. In the post impregnation or blending approaches, BC is impregnated or blended with polymeric solutions or melted pure polymers, respectively, followed by suitable processing (e.g., solvent casting, freeze-drying, injection-molding or pressing).

2.3.1 BC/Natural Polymers Nanocomposites

In the last two decades, a broad range of natural and synthetic polymeric matrices have been successfully combined with BC to produce innovative nanocomposite materials with applications in several areas. However, one of the first BC polymer composite classes described in literature, pioneered and essentially explored by the research group of Gindley, refer to BC-hemicelluloses and/or pectin nanocomposites [83–92], prepared by incubation of Gluconacetobacter xylinum in the presence of hemicelluloses (xyloglucans, xylans, glucomannans, among others) or pectin, in both stationary and agitated conditions, that mimicked the primary plant cell wall assembly and served as basic models to study the molecular interactions and structure [83–87, 89, 90, 92], mechanical behavior [88] and hydraulic conductivity [91] of the plant cell walls. Some of these studies [85, 87] indicated that these nanocomposite networks showed no evidence of direct molecular interaction between the components, but pectin composites became more aggregated followed by cellulose deposition. However, in a recent study Gu and Catchmark [92] suggested, by means of a sphere-like BC assembly (Figure 2.12), that xyloglucan had the dominant impact on cellulose synthesis and xyloglucan and pectin may interact with cellulose at different points in the assembly process, or in distinct regions.

Figure 2.12 Images of cellulose assemblies harvested from 0.5% xyloglucan and pectin blend media after 7 days of incubation (a-e; xyloglucan:pectin; 1:0, 3:1, 1:1, 1:3, 0:1).

Reproduced with permission from [47].

In the groundbreaking work of Iwata et al. [84], BC nanocomposites with different neutral and acidic lignin-carbohydrate complexes have likewise been considered. Their resistance against alkali, in contrast with the high lability of their delignified counterparts, clearly indicated the importance of lignin on the formation