Taking a step back: What is cellulose and where does it come from?
As the main material in focus in this work, this Section will give a fundamen-tal background on cellulose, its basic building blocks and their properties, with special focus on CNCs. This section is divided into the following main topics:
• History and Origin of Cellulose
• Chemistry and Morphology
• From Cellulose Micro- to Nanocrystals
2.2.1 History and Origin of Cellulose
Cellulose looks back upon millennia of extensive use, where most likely the very first “technological” use must have been the ignition of cellulosic material to create fire – a moment that set our very early ancestors on their path to superiority over other species. Other milestones include but are not limited to its production of flax yarns, hemp to produce ropes or clothing as discovered in Asia dated to a few thousand years BC, fabrication of garments and the spinning of cotton in Egypt and India dating as far back as 3000 BC. A more detailed description can be accessed in “Cellulose: a random walk along its historical path” by Hon et al..[2]
Industrially relevant applications include pulps, tissue and packaging mate-rials.[3–5] Indeed, celluloid, generally considered as the first thermoplastic, was developed in 1856 as a combination of nitrocellulose (initially used in explosives) and camphor.[6] More notably however is that cellulose has taken the key role as an information carrier through paper production since ancient Egypt. Not surprising, as paper derives from the wordpapyrus, a thick mat with resemblance
to paper, produced by the Egyptians from the pith of theCyperus Papyrusplant to write and draw. The work presented here shall resume to the topic of cellulose as an information carrier exploiting different aspects of the fundamental building blocks of cellulose in microelectronic devices in Chapter 5 and 6. More technolog-ically advanced applications for cellulose have been seen in its use for propellants, glues, thermoplastic biopolymers, paints, food, pharmaceuticals, construction or even in gunpowder, bringing technological and monetary value to this highly abundant biopolymer.[4] Moreover, through fundamental investigation, cellulose helped to establish some of the most fundamental concepts that are known in polymeric chains research.
Figure 2.2 – Some of the most common cellulose sources.
As a raw source for industrial processing, 93% of cellulosic fibres are obtained from either hardwood (angiosperms) or softwood (gymnosperms), while the rest comes from other sources, including bagasse, algae, straw, bacteria or seeds (see Figure 2.2).[7] Besides the distinction between hard- and softwood with signifi-cant differences in structure, morphology and chemistry both are composed of cell walls known as tracheids and vessels for soft- and hardwood, respectively. The cell walls (as schematized in Figure 2.3) are synthesized by a plasma membrane-localized Cellulose Synthase (CESA) enzyme family, which are organized as six lobes into 25-30 nm symmetrical rosettes (Figure 2.3b), called Cellulose Synthe-sis Complexes (CSCs). Investigations into CSCs and their behaviour is still an ongoing research field, however each CSC is believed to contain up to 36 CESAs.
The CSCs move around while extruding cellulose (as six hydrogen bonded β-1,4-glucan chains - up to 7µm in length) into the extracellular space.[8, 9] It is thus from this extracellular space (cell walls), where the cellulose fibres (either micro or nanofibrils) can be later extracted. Figure 2.3 depicts a schematic of a wood cell and its cell wall structure.
Figure 2.3 – Wood Cell and its constituents. a) Wood cell with individual layers.
b) Cross-section of cell walls until plasma membrane.
Protecting the cell there are several parts, including the middle lamella, the primary and secondary wall. Additionally, the latter is divided into S1, S2 and S3. In general terms the chemical composition of the cell walls can be divided into:[10]
• Cellulose (42 ± 2%)
• Hemicellulose (27 ± 2%)
• Lignin (28 ± 3%)
• Extractives (3 ± 2%)
Consequently, cellulose makes up almost half of the cell wall and can thus be extracted with a fairly high yield. Furthermore, it is found in all the layers of the cell wall, however most frequently throughout S2 in the secondary wall.
2.2.2 Chemistry and Morphology
Figure 2.4 breaks down the cell walls even further to reveal the inner micro- and nanostructure, down to the individual molecular layout. The cell walls are made
up of microfibrils, containing the cellulose nanofibrils, which divide further into individual cellulose chains.
Figure 2.4 – Schematic representation when breaking down plant cells into their micro and nanostructures. a) Moving from plant cell to cellulose macro, micro-and nanofibrils micro-and into individual polymer chains. b) Chemical structure of cellobiose, the repetitive unit of cellulose with twoβ-D-anhydroglucose units.
From a chemical point of view cellulose is a polysaccharide made up of re-peating cellobiose units (made up of two D-glucose units). These units are linked through equatorial acetal bonds that connect the anomeric C1 and C4 carbons of each glucose ring. Resulting inβ-1,4 glucosidic bonds, where each unit is rotated by 180° around the chain axis. These equatorial bonds confer to cellulose a linear outline with a ribbon like structure, where polar hydroxyl groups are located on the sides and the non-polar CH groups on the flat surface (see Figure 2.4b).
Figure 2.5 exemplifies this concept, where 3 D-glucose units are represented. It is further possible to observe that extra hydrogen bonds between the 3-OH and the ring oxygen atoms provide the polymer with considerable intra-molecular stabil-ity. Additionally, from one cellulose chain to the next further hydrogen bonds stabilize the whole structure conferring inter-molecular bonding.[11] Cellulose may appear in at least 4 polymorphs, reaching from type I through IV, where throughout this work the focus lies on Cellulose type I. Cellulose I, which con-stitutes the crystallographic structure of native cellulose, found in plants and bacteria, presents two crystallographic structures; the monoclinicIα and the tri-clinicIβ, which are present in different proportions, depending on the source of the cellulose.[12]
Figure 2.5 – 3 repeating D-glucose units in the chair conformation with indication of intra-molecular bonding (dashed pink lines). Numerous inter-molecular bonds are also formed to neighbouring molecules (not indicated here).
Due to the interplay between polar and non-polar functional groups cellulose is insoluble in commonly used solvents. The strong inter- and intra-molecular hy-drogen bonds give additional chemical and thermal stability, where cellulose does not melt below its degradation temperature, as well as mechanical strength.[13]
Usually strong acidic or alkaline conditions or considerable temperatures are nec-essary before degradation sets in. These two specific properties inhibit however a facile industrial application, as conventional polymers are normally dissolved in suitable solvents or more commonly melt down for extrusion or injection mould-ing. Nevertheless, cellulose has seen an increase in industrial applications with suited chemical modifications.
The -OH group at the C6 carbon is the most exposed one, offering a specific binding site for functionalization. In order to overcome solubility issues, cellulose can be modified to yield cellulose derivatives that are highly soluble in common solvent systems.
The selective addition of specific functional groups (e.g. Cellulose ethers such as Hydroxypropyl cellulose, Carboxymethyl cellulose, Hydroxyethyl cellulose or Methyl cellulose) provide solubility of cellulose derivatives in for instance water.
Due to the chemical and thermal stability of non-modified (native) cellulosic fibres, it is often used in its supramolecular structure, such as in paper, where the morphology of the cell wall is maintained.
Apart from the interesting applications that emerged from non-modified cellu-lose in for instance packaging, cleaning, construction or as an information carrier (books, notebooks etc.), additional interest arose for cellulose on the nanoscale.
In this case the native cellulosic fibres can be broken down into nanofibrils or nanocrystals, conferring a completely different approach to the production of films and membranes from nanocelluloses with added functionalities, such as
high gas barrier properties, increased surface to volume ratios and distinct me-chanical properties from native lignocellulosic fibres. Additionally, when deal-ing with the smallest possible constituent of lignocellulosic fibres – the cellu-lose nanocrystals – interesting liquid crystalline and photonic properties can be achieved, which are in focus in this work and will be explored more in detail in Chapter 3.