Cellulose Chemistry and Properties: Fibers, Nanocelluloses and Advanced Materials

Cellulose, a fascinating biopolymer and the most common organic compound on earth, is comprehensively reviewed. Details of its crystalline phases are given, starting with a description of molecular and supramolecular structures, including the hydrogen bond systems. Sources of this ubiquitous biopolymer are mentioned, with attention to the special properties of bacterially synthesized nanofibrous cellulose. Nanostructures obtained by disintegration of cellulose fibers (top-down approach) yielding nano- or microfibrillated cellulose and cellulose whiskers are the basis for novel materials with extraordinary properties. Moreover, nanofibers and nanoparticles can be made by special techniques applying the bottom-up approach. Efficient systems to dissolve cellulose by destruction of the hydrogen bond systems using ionic liquids and systems based on polar aprotic solvent and salt are described. Novel cellulose derivatives are available by chemical modification under heterogeneous or homogeneous conditions, depending on the cellulose reactivity. In particular, unconventional nucleophilic displacement reactions yielding products for high-value applications are highlighted. Novel amino cellulose derivatives showing fully reversible aggregation behavior and nanostructure formation on various materials are the focus of interest. Finally, “click chemistry” for the synthesis of novel cellulose derivatives is discussed.

This review provides a general overview of preparation, separation, and analytical methods for cello- and xylooligosaccharides. Arising as side-stream products of different biorefinery processes, these compounds have increasingly gained the interest of researchers and engineers in the last few decades. Beside their application as additives in the food, feed, and pharmaceutical industries, these oligomeric carbohydrates are of key importance as model compounds for studying the dependence of physicochemical properties on the degree of polymerization (DP). First, different preparation methods for mixtures of oligosaccharides with DPs between 1 and 30 are discussed. These methods include acetolysis, acid and enzymatic hydrolysis, and glycoside synthesis. Then, separation techniques, including size exclusion chromatography, normal phase and hydrophilic interaction chromatography, and chromatography on cation exchange resins, are presented. Analysis of oligosaccharides by different techniques is described.

This contribution summarizes achievements in the understanding of cellulose accessibility, structure, and function with a particular focus on its interactions with deuteration. This review is the first to explicitly devote a discussion to deuteration of cellulose and highlights remarkable new findings in cellulose research as a result of the development of new experimental approaches, from simple weighing of deuterated samples to sophisticated techniques such as small angle neutron scattering and ²H-NMR spectroscopy.

Although the cellulose crystallinity index (CI) is used widely, its limitations have not been adequately described. In this study, the CI values of a set of reference samples were determined from X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and infrared (IR), Raman, and vibrational sum frequency generation (SFG) spectroscopies. The intensities of certain crystalline peaks in IR, Raman, and SFG spectra positively correlated with the amount of crystalline cellulose in the sample, but the correlation with XRD was nonlinear as a result of fundamental differences in detection sensitivity to crystalline cellulose and improper baseline corrections for amorphous contributions. It is demonstrated that the intensity and shape of the XRD signal is affected by both the amount of crystalline cellulose and crystal size, which makes XRD analysis complicated. It is clear that the methods investigated show the same qualitative trends for samples, but the absolute CI values differ depending on the determination method. This clearly indicates that the CI, as estimated by different methods, is not an absolute value and that for a given set of samples the CI values can be compared only as a qualitative measure.

The constant worldwide increase in consumption of goods will also affect the textile market. The demand for cellulosic textile fibers is predicted to increase at such a rate that by 2030 there will be a considerable shortage, estimated at ~15 million tons annually. Currently, man-made cellulosic fibers are produced commercially via the viscose and Lyocell™ processes. Ionic liquids (ILs) have been proposed as alternative solvents to circumvent certain problems associated with these existing processes. We first provide a comprehensive review of the progress in fiber spinning based on ILs over the last decade. A summary of the reports on the preparation of pure cellulosic and composite fibers is complemented by an overview of the rheological characteristics and thermal degradation of cellulose–IL solutions. In the second part, we present a non-imidazolium-based ionic liquid, 1,5-diazabicyclo[4.3.0]non-5-enium acetate, as an excellent solvent for cellulose fiber spinning. The use of moderate process temperatures in this process avoids the otherwise extensive cellulose degradation. The structural and morphological properties of the spun fibers are described, as determined by WAXS, birefringence, and SEM measurements. Mechanical properties are also reported. Further, the suitability of the spun fibers to produce yarns for various textile applications is discussed.

This review updates the most relevant advances achieved in the field of surface and in-depth modification of cellulose fibers during the last 5 years. It reports work dealing with cellulose substrates on the nano- to micrometer scale, namely cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), microfibrillated cellulose (MFC), and bacterial cellulose (BC), as well as conventional lignocellulosic fibers. Several approaches have been applied for surface modification of these substrates, namely hydrophobization and oleophobization, physico-chemical adsorption, oxidation, cationization, esterification, urethane and siloxane formation, and grafting-from and grafting-onto macromolecular sequences. In-depth modification can be achieved by both partial esterification and partial oxypropylation.

Nanocellulose has been used with promising results as reinforcement material in composites, many of which include hydrophobic polymers. However, the hydrophilic nature of nanocellulose can be better exploited in composites that incorporate high surface energy systems as well as in applications that can benefit from such properties. In fact, proteins can be ideal components in these cases. This paper reviews such aspects, which are based on the remarkable mechanical properties of nanocellulose. This material also exhibits low density, high aspect ratio, high surface area, and can be modified by substitution of its abundant hydroxyl groups. It also shows biocompatibility, low toxicity, and biodegradability. Convenient biotechnological methods for its production are of interest not only because of the possible reduction in processing energy but also because of positive environmental aspects. Thus, enzymatic treatments are favorable for effecting fiber deconstruction into nanocellulose. In addition to reviewing nanocellulose production by enzymatic routes, we discuss incorporation of enzyme activity to produce biodegradable systems for biomedical applications and food packaging. Related applications have distinctive features that take advantage of protein–cellulose interactions and the possibility of changing nanocellulose properties via enzymatic or protein treatments.

Coating with polyelectrolyte multilayers has become a generic way to functionalize a variety of materials. In particular, the layer-by-layer (LbL) technique allows the coating of solid surfaces to give them several functionalities, including controlled release of bioactive agents. At present there are a large number of applications of the LbL technique; however, it is still little explored in the area of textiles. In this review we present an overview of LbL for textile materials made from synthetic or natural fibers. More specifically, LbL is presented as a method for obtaining new bioactive cotton (as in cellulosic fibers) for potential application in the medical field. We also review recent progress in the embedding of active agents in adsorbed multilayers as a novel way to provide the system with a “reservoir” where bioactive agents can be loaded for subsequent release.

This article surveys progress in both fundamental and applied research related to cellulosic liquid crystals, mainly of chiral nematic order. These liquid crystals are divided into two different classes, namely cellulosic macromolecules and cellulose nanocrystals (CNCs), depending on the mesogenic constituent. We start with a review of the fundamental and chiroptical characteristics of molecular liquid crystals of representative cellulose derivatives and then discuss recent efforts on the design and construction of functional material systems (such as stimuli-sensitive optical media and novel hybrids with minerals). These systems make use of the liquid crystalline molecular assembly of cellulosics. The survey of the other class of cellulosic liquid crystals deals with colloidal suspensions of CNCs obtained by acid hydrolysis of native cellulose fibers. Following the review of fundamental aspects related to the isotropic–anisotropic phase separation behavior of CNC suspensions, attention is directed to current applications of free-standing colored films, polymer composites reinforced with CNCs as mesofiller, and inorganic hybridizations using CNC chiral nematics as template. Some comments and the outlook for future explorations are also offered.

Cellulose nanocrystals (CNCs) are renewable, sustainable nanomaterials, typically produced by strong sulfuric acid hydrolysis of lignocellulosic biomass. CNCs can self-assemble in aqueous, and other, suspensions at a critical concentration, or under evaporation, into chiral nematic organization to exhibit anisotropic structural color. The degree of sulfation is critical for producing both stable colloidal suspensions and iridescent films by evaporation-induced self-assembly. CNCs also possess electromagnetic and piezoelectric properties, as well as active surface groups that render them suitable for tailored functionalization. This chapter presents a framework of how CNCs can be used to (i) template in/organic mesoporous photonic and electronic materials and structures, and (ii) develop sustainable, flexible electronics. Using a novel supramolecular co-templating approach, the first example of functional, mesoporous, photonic cellulose films, or nanopaper, has been produced. The CNC-templating approach is a scalable, effective tool for imparting long-range chirality in a number of distinct materials (polymer, silica, metal oxides, carbon) with promising applications in, for example, optoelectronics, biosensors, actuators, functional membranes, 3D printing, and tissue engineering.