Cellulose is biosynthesised in varieties of woods, plants, algae and microbes, making it the most abundant polymer material on earth. Different pulping methods are used to extract cellulose
CHAPTER2. BACKGROUND ON CELLULOSE
from its sources, yielding variable grades of cellulose pulp. The hydroxyl groups on the surface of cellulose repeat unit alongside the supramolecular orientation of cellulose result in a material that is difficult to dissolve in water. Hence, the swelling and dissolution of cellulose in selected swelling agents and solvents have been matter of contention amongst cellulose researchers. The current hypothesis is that the lack of aqueous dissolution of cellulose is as a result of the sum of all hydrogen bonding forces (from hydroxyl groups) and hydrophobic forces (from CH), as cellulose can be considered as an amphiphilic material. The hydroxyl groups of cellulose apart from contributing to cellulose insolubility in water also allow for the surface functionalisation of cellulose. Indeed, the functionalisation of cellulose opens wide areas of applications, which would not be possible with the pure cellulose material. Moreover, the rapidly developing fields of nanoscience, nanotechnology and research in nano-sized cellulose materials have further widened the spectrum of cellulose applications.
[ Chapter Three \
Background on Cellulose Nanomaterials and Composites
Here a general review of nanomaterials is given before emphasis on nanocellulose, a nano-sized cellulose material. The different types of nanocellulose, their methods of preparation, some of the general characterisation methods and the properties of cellulose nanofibrils (CNF) are discussed. The effects of CNF in composite materials are presented alongside details concerning CNF/alginate-based nanocomposites. The literature available in this research field is extensive and wide-ranging. Therefore, effort has been made towards reviewing the salient, core literature related to the research project using the “funnel” approach as shown in Figure 3.1.
Nanomaterials
Nanocellulose
BNC CNF CNC
CNF
• Methods
• Characterisation
• Challenges
• CNF nanocomposites
Figure 3.1: Funnel approach used in the literature review of nanomaterials and nanocellu-lose materials
CHAPTER3. BACKGROUND ON CELLULOSE NANOMATERIALS AND COMPOSITES
3.1 Introduction to nanomaterials
The branch of science involved with the study, production and application of materials in the nano-field is nanotechnology. The term nanotechnology is generally accepted to have originated following the lecture given by Richard Feynman in 1959 on miniature machinery (Feynman, 1960). Although Feynman focused unsurprisingly on the physical aspects of miniature machines, he offered little or no insights into the contribution of chemistry and the principles of molecular self-assembly to the miniaturisation concepts (Ball, 2009). Currently, the field of nanoscience and nanotechnology is regarded as an allied science made up of chemistry, biology, physics, material science and engineering.
Nanomaterials are defined as materials with one or more dimensions of approximately 100 nm or less, (British Standards Institution, 2007). The nano-size dimension, surface chemistry and high surface area of nanomaterials impart different mechanical, thermal, optical and electrical properties compared with those of the bulk material (Klaessig et al., 2011). Although 100 nm is generally used as the upper limit of the dimension at which the changes in material properties are observed, this upper limit has not yet been generally accepted. Klaessig et al., 2011 reviewed the current perspectives in nanotechnology, terminology and nomenclature.
They identified that the upper limit accorded to nanomaterials depends on the field of study.
For example, material scientists consider the intrinsic properties of the nanomaterials and suggested 30 nm as the upper limit, where changes in properties compared to the bulk material are observed (Klaessig et al., 2011). On the other hand, biological scientists considering the extrinsic properties of the nanomaterial i.e. how these materials interact with the body and have suggested a 1000 nm upper limit.
Nanomaterials can be classified as either having non-polymeric origin or polymeric origin.
Non-polymeric nanomaterials include metals, non-metals, metal oxides, carbon allotropes, clay minerals and quantum dots. Polymeric nanomaterials are nanomaterials having natural origins or synthetic polymeric origins. This classification is illustrated in Figure 3.2 for examples of each class. Regardless of the origin, there are two major methods for the preparation of nanomaterials; the top-down approach or the bottom-up approach (Mijatovic et al., 2005; Yuan and Müller, 2010; Tiwari et al., 2012). The top-down method involves the deconstruction of bulk materials using various methods such as acid hydrolysis, the use of enzymes or fungi,
CHAPTER3. BACKGROUND ON CELLULOSE NANOMATERIALS AND COMPOSITES
mechanical processes (such as cutting, homogenisation and grinding) (Spence et al., 2011), and lithography. The bottom-up method on the other hand involves the use of various deposition techniques and the self-assembly of nanomaterial atomic/molecular precursors, (Mijatovic et al., 2005; Tiwari et al., 2012).
Nanomaterials
Figure 3.2: Classification of nanomaterials based on polymeric origins and non-polymeric origins
Each of these preparation methods presents advantages and disadvantages. While the top-down deconstruction method may be less time consuming, it is demanding in energy consumption and may produce nanomaterials in association with contaminants. The bottom-up method is time consuming but, in most cases, requires less energy and produces high purity materials (Joye and McClements, 2014). The hybrid approach, which uses a mixture of bottom-up methods and top-down methods, is suggested for the fabrication of nanomaterials which cannot be made by separate methods, as in the fabrication of nanochannels for nanofluidic devices, (Mijatovic et al., 2005).
Most of the investigations into the applications of non-polymeric nanomaterials are in en-ergy storage devices, catalysis and in electronic displays (Tiwari et al., 2012; Zhukov et al., 2000).
They are also used in structural nanocomposites (Schaefer and Justice, 2007). Applications of non-polymeric nanomaterials in biomedicine have also been investigated for drug/gene
CHAPTER3. BACKGROUND ON CELLULOSE NANOMATERIALS AND COMPOSITES
delivery, biosensors and imaging devices (Ventola, 2012; Mclaughlin et al., 2016). For example, carbon nanotubes (CNT) (Taghdisi et al., 2011) and CdS quantum dot (Li et al., 2006) have been studied for the delivery of the blood cancer drug daunorubicin. In addition, Ag nanoparticles are now approved as antimicrobial agents (Khundkar et al., 2010) and sterilising agents (Ventola, 2012).
However, the wide acceptance and use of non-polymeric materials in food and drugs has been deterred by the perceived toxicity issues or concerns. These materials are known to accumulate in organs over time, leading to inflammation and chronic toxicity (De Matteis, 2017). Although non-polymeric materials can be coated or functionalised with other less toxic materials, to reduce the toxicity and increase compatibility, the possibility of the toxic components leaching out raises concerns (Saad et al., 2012; De Matteis, 2017).
Polymeric materials however seem to pose less toxicity to the human body. Some polymeric materials such as Poly(ethylene glycol) (PEG)-based nanomaterials, Poly(lactic acid) (PLA), Poly(lactic-glycolic acid) (PLGA), Poly(caprolactone) (PCL), with their well characterised physico-chemical properties, have been approved for use by the United States’ Food and Drug Agency (FDA) (Bobo et al., 2016). These materials are known to be stable and highly reproducible (Saad et al., 2012). In recent times, there has been a great public awareness on the renewability of synthetic polymers from fossil fuel origins (Hopewell et al., 2009). Consequently, naturally synthesised materials (such as cellulose, alginate, chitosan, hyaluronic acid, etc.), which are inherently biocompatible, biodegradable, less toxic to humans and to the environment are now being explored. Intensive studies and reviews of the various applications in drug delivery, tissue regeneration, packaging, etc., abound (Joye and McClements, 2014; Wurm and Weiss, 2014;
Mclaughlin et al., 2016; Voisin et al., 2017; Han et al., 2018). Nanocellulose is one of the natural polymeric nanomaterials that has attracted much research effort. This aspect is considered in the following sections.