Cellulose is a very abundant polymer naturally available as it is a vital component present in various plant cell walls. Other than that cellulose nanocrystals (CNC) also found in every other species all of which are explained below in detail. It is one of the most essential components as it carries out a lot of important processes and takes part in carrying out the process of life.
Cellulose nanocrystals are the unique form of cellulose that enable a lot of applications as the properties that this component hold are very unique and excellent in their nature. However, all the applications that are carried out by cellulose nanocrystals are authentic in their nature and enhance the overall productivity of this product.
The richest natural available polymer present on the Earth is cellulose and it is the cell wall's significant structural component of numerous plants. Cellulose is not only present in plants but it is available in a broad range of living species like bacteria, fungi, algae, and in some sea animals like tunicates too. Cellulose is a water-insoluble, tough, and fibrous polymer.
Cellulose has a very major role when it comes to maintaining the plant cell wall's structure. Also, cellulose is a renewable, biocompatible, and biodegradable natural polymer. Due to cellulose's remarkable features, cellulose is known to be an alternative to nondegradable fossil fuel-based polymers. Cellulose's chemical structure displays that this polymer is made up of condensation and it consists of monomers that are joined together through the glycosidic oxygen bridges.
β-1,4-linked glucopyranose units make up cellulose and it produces a high-molecular-weight linear homopolymer. In such kind of homopolymer, there is a corkscrew of every monomer unit at 180° to its neighbors. The natural polymer's repeating unit is glucose's dimer which is called cellobiose. The source determines cellulose's degree of polymerization as it can vary. Usually, for cotton-derived cellulose, it is almost 15,000 glucose units and for wood-derived cellulose, it is almost 10,000 glucose units.
3 hydroxyl groups are beared by each of the glucopyranose units, and they give cellulose some of its remarkable features like biodegradability, chirality, hydrophilicity, etc, which are initiated because of the hydroxyl group's high reactivity. Hydroxyl groups possess the capability of forming strong hydrogen bonds and that is the reason for some of its other remarkable characteristics like extremely cohesive nature, hierarchical organization (amorphous and crystalline fractions), and multiscale micro fibrillated structure.
Main sources of cellulose
Plants are the main sources of cellulose but they can also be produced in large quantities by some of the sea animals, bacteria, and algae. Some of the details about these main sources are mentioned below.
During vinegar fermentation, some of the particular bacteria’s species, for instance, Komagataeibacter xylinus, occur as contaminants, whereas they are famous in the production of cellulose by using a huge number of carbon and nitrogen sources. Cellulose microfibrils make up K. xylinus in thick, flat, and clear pellicles form that are floating on the growth medium's surface. Pure cellulose, other ingredients of the medium, and a large proportion of water make up these cellulose pellicles. Dilute alkaline solutions can hydrolyze and eliminate the presence of impurities in the cellulose pellicle. After washing and treatment with alkali, one can dry and process the cellulose pellicles into pure cellulose membranes.
Cell walls of many kinds of algae have native cellulose as its major component, and they are very crystalline. Numerous types of algae like yellow, green, and red are famous for the production of cellulose, although, when it comes to the extraction of cellulose, the most preferred ones are green algae.
The usual algae that produce cellulose belong to the orders Siphonocladales (Boergesenia, Siphonocladus, Dictyosphaeria, and Valonia), and Cladophorales (Microdyction, Rhizoclonium, Chaetomorphia, and Cladophora). A remarkably high degree of crystallinity is possessed by the cellulose derived from Cladophora or Valonia, which can be more than 95 percent. Cellulose microfibrils that are attained have different characteristics as that depends on the biosynthesis process that takes place in various species.
Due to the plants being comparatively cheap and abundant, they are cellulose's most significant source. Cotton fibers and wood pulp is cellulose's main source. Right now, large-scale industrial infrastructures are present for their extraction, processing, and harvesting. A huge range of other plant materials like hemp, flax, sisal, ramie, jute, etc, are also some of the famous sources to produce cellulose. Some parts of the plants like fruit, stem, leaves, etc, and grasses, water plants, are included in the other cellulose-producing plants. Cellulose can also be produced by using agricultural wastes like cotton stables, sawdust, sugarcane baggage, rice and wheat straw, etc.
Cellulose that is made from microbes is rare, remarkable, and has various benefits over the cellulose that is derived from plants as the microbially derived cellulose has better characteristics like 1) Greater capacity to hold water, 2) Greater mechanical strength, 3) higher-dimensional stability, 4) Purity, and 5) unique nanostructure.
Plant cellulose and microbial cellulose are identical in their polymeric structure and molecular formula, but their forms are different in the arrangement of the glycosyl units in crystallite unit cells, which results in the former’s higher crystallinity. Superior characteristics and a high degree of polymerization are displayed by microbial cellulose as compared to the cellulose obtained from plants.
They are marine invertebrate sea animals and are famous to produce cellulose in large quantities. Such animals possess a leathery, thick mantle, which is cellulose's good source. In the tunic tissues, cellulose functions as a skeletal structure, and it is a remarkable integumentary tissue covering up the tunicate's whole epidermis. These animals produce cellulose by utilizing enzyme complexes that are available in the epidermis membrane.
Tunicate species are present in nature in large amounts and the obtained cellulose can have different characteristics as they differ from species to species. One can often compare the characteristics and structure of the cellulose microfibrils that are obtained from different species, however, some of the small differences in the formation process of cellulose microfibril can influence microfibril’s final characteristics.
Cellulose nanocrystals and their isolation
Highly Crystalline and ordered regions along with some disordered (amorphous) regions make up the naturally occurring bulk cellulose. The source determines the varying proportions of those disordered regions. When subjected to a proper combination of the enzyme, chemical, and mechanical treatments, one can extract these highly crystalline regions of the cellulose microfibrils, leading to the production of the cellulose nanocrystals (CNCs).
CNCs are stiff particles that are like rods and they consist of the cellulose chain segments in an almost perfect crystalline structure. Usually, these nanocrystals are called microcrystallites, nanofibers, nanoparticles, whiskers, and so on, but CNCs are the most broadly accepted nomenclature. In comparison to the bulk cellulose that possesses greater amorphous fractions, remarkable liquid crystalline characteristics, high surface area, modulus, and high specific strength, are exhibited by these nanocrystals.
Under the mechanical processes
Various mechanical processes like cryo crushing, micro fluidization techniques, high-intensity ultrasonic treatments, high-pressure homogenization, etc, have been used to extract cellulose microfibrils. Enough shear forces are produced by these mechanical processes for splitting apart the cellulose fibers along the longitudinal axis and aiding in the extraction of the cellulose microfibrils. Each cellulose microfibril is devoid of chain folding and is a string of cellulose crystals, and they are linked alongside microfibril through paracrystalline or disordered regions.
As compared to the mechanical methods, the chemical method to convert the cellulose microfibrils into CNCs is better as the chemical method forms rod-like short nanocrystals with enhanced crystallinity and lessens the energy consumption. Wood-derived cellulose gives ribbon-like nanofiber samples after mechanical refining and they display a lower crystalline fraction (0.05–0.55) in comparison with the rod-like wood CNC (0.6) that are attained after acid hydrolysis.
Strong acid hydrolysis is often utilized to remove amorphous domains that are commonly disturbed along the microfibrils. It is easy for the strong acids to penetrate the amorphous regions that possess a low level of order and hydrolyze those regions, resulting in the crystalline regions unaffected.
In 1951, Ranby first made the colloidal suspensions of cellulose by utilizing a controlled sulfuric acid-catalyzed degradation of cellulose fibers. After treatment for a certain period, the degradation that was induced by boiling the cellulose fibers in the acidic solution came to a limit. The presence of the cellulose particles with the shape of a needle was confirmed by electron diffraction studies and transmission electron microscopy. Those particles have a similar crystalline structure that the original cellulose fibers possess. Followed by ultrasound sonication, cellulose's acid hydrolysis, later resulted in the production of microcrystalline cellulose.
The difference in the kinetics
The selective cleavage of the cellulose chains was because of the differences in the hydrolysis kinetics between crystalline and amorphous domains. It is due to this acid hydrolysis that there were observations of a sudden and fast decrease in polymerization's degree, which reached a level of cutoff that is also referred to as the level-off degree of polymerization (LODP). The cellulose origin determines the LODP's value as it varies, where 250 LODP was possessed for cotton-derived cellulose, 6000 for Valonia-obtained cellulose, 140-200 for bleached wood pull fibers, and 300 for ramie fibers.
A high polydispersity in the molecular weight is exhibited by the CNCs that are attained by cellulose’ in s acid hydrolysis from Valonia, tunicates, or bacteria, with no evidence of the LODP, possibly because of the absence of amorphous domain’s regular distribution.
The new concept for producing cellulose nanomaterials
There were reports of a new concept for the preparation of the cellulose nanomaterials, whereby they used enzymatic hydrolysis in combination with high-pressure homogenization and mechanical shearing, resulting in controlled fibrillation down to the nanoscale, for the production of the cellulose nanomaterials with 5-6 nm of diameter. According to the reports, a commercially available cellulase enzyme was used in combination with mechanical shearing to isolate the bacterial CNCs. In comparison to the nanocrystals that were attained from hydrolysis and sulfuric acid, better thermal and mechanical characteristics were exhibited by the nanocrystals that were obtained through this route.
Characteristics of CNCs
One can describe the significant characteristics of CNCs under three main categories, and they are discussed here briefly.
It is due to the limitations in the measurement of the nanomaterial's mechanical characteristics along multiple axes that have led To the quantitative evaluation of CNC’s strength and tensile modulus very challenging. Other than that, various factors like dimensions of the samples, percentage crystallinity, defects in the nanocrystals, anisotropy, and so on can too have an impact on the results.
CNC’s elastic characteristics were calculated by using Raman scattering, inelastic X-ray scattering, X-ray diffraction analysis, atomic force microscopy (AFM), etc, to use Indirect experimental measurements and theoretical calculations. CNCs theoretical tensile strength is in the 7.5-7.7 GPa range, which is way more as compared to the theoretical tensile strength of Kevlar-49 and steel wire.
CNC’s elastic modulus
Tunicates-obtained CNC’s elastic modulus was determined in another study by utilizing AFM, whereas a three-point bending test was performed by using the AFM tip. According to the findings, ~150 GPa was CNCs elastic moduli. The experimental force-distance curves were compared with 3-D finite elemental calculations to determine the transverse elastic modulus of CNCs by utilizing AFM.
It was proved by this measurement that the individual CNC's transverse modulus lies in the 18-50 GPa range. Raman spectroscopy was used in the deformation micromechanics analysis of tunicate cellulose whiskers and it ensured that tunicate CNC’s calculated modulus is ~143 GPa.
CNC’s Liquid crystalline nature
All asymmetric plate-like or rod-like particles produce ordered structures spontaneously, resulting in the production of a nematic phase at critical concentrations and under appropriate conditions. When dispersed in water, the rod-like CNCs self-align for producing chiral nematic phases with liquid crystalline characteristics. They are ideal to exhibit liquid crystalline behavior because of their aspect ratios, stiffness, and the capability of aligning under some particular conditions. Although, like a screw, cellulose crystallites have a helical twist down the long axis, which can result in a cholesteric phase or a chiral nematic of stacked planes aligned along a perpendicular axis that depends on the concentration.
Factors influencing CNC’s crystallinity
Numerous factors like external stimuli, electrolyte, dispersity, shape, charge, and size can influence CNC's liquid crystallinity. Nanocrystal's liquid crystallinity joined with the birefringent nature results in an exciting optical phenomenon. Liquid crystalline nature can also be affected by the usage of this type of acid for hydrolysis.
Usually, CNC attained by sulfuric acid hydrolysis have a negatively charged surface, that promotes uniform dispersion in water because of the electrostatic repulsions. Despite the strong interactions between the nanocrystals, the highly sulfonated CNC is easily dispersible and results in the formation of the lyotropic behavior. Normally, phosphoric acid- and sulfuric acid-derived CNCs offer chiral nematic structure, but hydrochloric acid-derived CNCs with the post-reaction sulfonation result in a birefringent glassy phase.
Applications of CNCs
CNC is the perfect choice of nanomaterial for a broad variety of applications like drug carrier synthesis in diagnostic and therapeutic medicine, biosensing, green catalysis, synthesis of medical and antimicrobial materials, and enzyme immobilization, etc. It is because of their characteristics like biocompatibility, hydrophilicity, smaller size, etc. that these nanomaterials provide various potential benefits as drug delivery excipients. Large amounts of drugs can be bound to these material's surface because of the probability to acquire negative charge during hydrolysis and their extremely large surface area with the potential for optimal control of dosing.
CNC from softwood
CNCs made from the softwood were utilized in another such report for binding the ionizable drugs like doxorubicin and tetracycline, which can be rapidly released over a period of 1-day. The presence of abundant surface hydroxyl groups in the nanocrystals offers sites for modification of the surface with various chemical groups. Surface modification can be utilized for the modulation of drugs' loading and release, especially those drugs that don't bind to the cellulose normally, like hydrophobic or nonionized drugs.
There has been a rise in interest in the CNC-based aerogels in pharmaceutical and biomedical applications because of their high surface area and open pore structure, which can offer better drug-loading capacity and improved drug bioavailability. There were reports of highly porous nano cellulose aerogel scaffolds attaining sustained drug release, that displayed new potential too as carriers for the controlled drug delivery.
Utilization of CNCs upon two broad types
There are two broad types of utilization of CNCs for numerous applications: one type involves the usage of polymer nanocomposites whereas CNC functions as a reinforcing agent and the other one involved the utilization of nonfunctionalized or functionalized as-synthesized CNC. It is due to their distinctive characteristics that the as-synthesized CNCs possesses the chance of being utilized in diverse and numerous applications varying from products like pH sensors, barrier films, and nano paper, to stabilization of water/oil, to the formation of Pickering emulsions with remarkable stability, etc, however, a lot more applications are possessed by the CNC-containing polymer nanocomposites.
It is a multiphase material whereas the nanomaterial reinforces the polymer phase. Unique characteristics are displayed by these polymer nanocomposites due to their reinforcing material’s increased surface area and nanometric size. In various polymer nanocomposite systems, CNC is utilized as the load-bearing constituent as it forms major enhancements in the mechanical characteristics at extremely low volume fractions too. Moreover, it is a broadly preferred reinforcing component due to its ability to form percolated network-type architecture within the polymer matrix, good dispersion in the hydrophilic systems, and the high aspect ratio.
CNCs are utilized with a defined morphology as model nanofillers for imparting enough modulus and strength. Nanocomposites can be fabricated by using both synthetic and natural polymers. Various natural polymers like soy protein, gelatin, hydroxypropyl methylcellulose, carboxymethyl cellulose, cellulose acetate butyrate, natural rubber, chitosan, and starch, are utilized for preparing nano-composites. Just like that, various synthetic polymers like polyurethane, polypropylene, polycaprolactone, polyethylene, polyvinyl chloride, and polyvinyl alcohol are also utilized.
Attainment of remarkable performance
The major problem in attaining remarkable performance is the attainment of good matrix-filler interaction and the nanocrystal's homogeneous dispersion within the polymer matrix. CNCs good dispersibility in the polymer matrix is the reason the polymer nanocomposites have such great characteristics, as nanocomposite material’s final mechanical characteristics are decreased by the filler’s homogeneous dispersion in the polymer matrix
They are beneficial for numerous biomedicine applications too like hydrogels, tissue engineering scaffolds, and wound healing patches for pharmacological and clinical applications, etc. One should consider CNC's biocompatibility and the possibility of chemical modifications like fluorescent labeling because they are extremely beneficial in the applications of biomedicine fields like bioimaging applications, fluorescence bioassays, bio probes, biosensors, and so on. The usage of fluorescence methods was enabled by the fluorescently labeled CNCs for studying the interactions between the living cells in vivo and CNCs. CNCs can also form highly functional nanocomposites for applications like ultrathin film-coating materials.
Stabilizing nanoparticles of specific functionality
CNC can be utilized to stabilize nanoparticles of particular functionality for some particular applications. CNC-containing polymer nanocomposites are utilized to develop actuators, sensors, electroactive polymers, supercapacitors, batteries, textiles, fibers, and developing membranes too that use electromechanical responses. The biodegradable packaging materials field is the one future area of application in which an impact can be made by the CNC-containing polymer nanocomposites. Optical, barrier characteristics, thermal stability, and mechanical performance can be significantly improved by the incorporation of CNC because of its better interfacial interaction and enhanced crystallinity.
Cellulose nanocrystals being one of the most important products of various industries play a significant role in enhancing the economy of the industries that are constantly using it through various means that is why a lot of applications are being carried out by them. Nonetheless, it is all due to the excellent properties of cellulose nanoparticles that productivity has been enhanced at such a large scale.