Molecular deformation processes in aromatic high modulus polymer fibres

Assembling nanocelluloses into fibrous materials and their emerging applications

Nanocelluloses, derived from various plants or specific bacteria, represent the renewable and sophisticated nano building blocks for emerging functional materials. Especially, the assembly of nanocelluloses as fibrous materials can mimic the structural organization of their natural counterparts to integrate various functions, thus holding great promise for potential applications in various fields, such as electrical device, fire retardance, sensing, medical antibiosis, and drug release. Due to the advantages of nanocelluloses, a variety of fibrous materials have been fabricated with the assistance of advanced techniques, and their applications have attracted great interest in the past decade. This review begins with an overview of nanocellulose properties followed by the historical development of assembling processes. There will be a focus on assembling techniques, including traditional methods (wet spinning, dry spinning, and electrostatic spinning) and advanced methods (self-assembly, microfluidic, and 3D printing). In particular, the design rules and various influencing factors of assembling processes related to the structure and function of fibrous materials are introduced and discussed in detail. Then, the emerging applications of these nanocellulose-based fibrous materials are highlighted. Finally, some perspectives, key opportunities, and critical challenges on future research trends within this field are proposed.

Methylcellulose-Cellulose Nanocrystal Composites for Optomechanically Tunable Hydrogels and Fibers

Chemical modification of cellulose offers routes for structurally and functionally diverse biopolymer derivatives for numerous industrial applications. Among cellulose derivatives, cellulose ethers have found extensive use, such as emulsifiers, in food industries and biotechnology. Methylcellulose, one of the simplest cellulose derivatives, has been utilized for biomedical, construction materials and cell culture applications. Its improved water solubility, thermoresponsive gelation, and the ability to act as a matrix for various dopants also offer routes for cellulose-based functional materials. There has been a renewed interest in understanding the structural, mechanical, and optical properties of methylcellulose and its composites. This review focuses on the recent development in optically and mechanically tunable hydrogels derived from methylcellulose and methylcellulose-cellulose nanocrystal composites. We further discuss the application of the gels for preparing highly ductile and strong fibers. Finally, the emerging application of methylcellulose-based fibers as optical fibers and their application potentials are discussed.

Ice-Templated Cellulose Nanofiber Filaments as a Reinforcement Material in Epoxy Composites

Finding renewable alternatives to the commonly used reinforcement materials in composites is attracting a significant amount of research interest. Nanocellulose is a promising candidate owing to its wide availability and favorable properties such as high Young’s modulus. This study addressed the major problems inherent to cellulose nanocomposites, namely, controlling the fiber structure and obtaining a sufficient interfacial adhesion between nanocellulose and a non-hydrophilic matrix. Unidirectionally aligned cellulose nanofiber filament mats were obtained via ice-templating, and chemical vapor deposition was used to cover the filament surfaces with an aminosilane before impregnating the mats with a bio-epoxy resin. The process resulted in cellulose nanocomposites with an oriented structure and a strong fiber–matrix interface. Diffuse reflectance infrared Fourier transform and X-ray photoelectron spectroscopy studies revealed the presence of silane on the filaments. The improved interface, resulting from the surface treatment, was observable in electron microscopy images and was further confirmed by the significant increase in the tan delta peak temperature. The storage modulus of the matrix could be improved up to 2.5-fold with 18 wt% filament content and was significantly higher in the filament direction. Wide-angle X-ray scattering was used to study the orientation of cellulose nanofibers in the filament mats and the composites, and the corresponding orientation indices were 0.6 and 0.53, respectively, indicating a significant level of alignment.

Nanocellulose, a Versatile Green Platform: From Biosources to Materials and Their Applications

With increasing environmental and ecological concerns due to the use of petroleum-based chemicals and products, the synthesis of fine chemicals and functional materials from natural resources is of great public value. Nanocellulose may prove to be one of the most promising green materials of modern times due to its intrinsic properties, renewability, and abundance. In this review, we present nanocellulose-based materials from sourcing, synthesis, and surface modification of nanocellulose, to materials formation and applications. Nanocellulose can be sourced from biomass, plants, or bacteria, relying on fairly simple, scalable, and efficient isolation techniques. Mechanical, chemical, and enzymatic treatments, or a combination of these, can be used to extract nanocellulose from natural sources. The properties of nanocellulose are dependent on the source, the isolation technique, and potential subsequent surface transformations. Nanocellulose surface modification techniques are typically used to introduce either charged or hydrophobic moieties, and include amidation, esterification, etherification, silylation, polymerization, urethanization, sulfonation, and phosphorylation. Nanocellulose has excellent strength, high Young's modulus, biocompatibility, and tunable self-assembly, thixotropic, and photonic properties, which are essential for the applications of this material. Nanocellulose participates in the fabrication of a large range of nanomaterials and nanocomposites, including those based on polymers, metals, metal oxides, and carbon. In particular, nanocellulose complements organic-based materials, where it imparts its mechanical properties to the composite. Nanocellulose is a promising material whenever material strength, flexibility, and/or specific nanostructuration are required. Applications include functional paper, optoelectronics, and antibacterial coatings, packaging, mechanically reinforced polymer composites, tissue scaffolds, drug delivery, biosensors, energy storage, catalysis, environmental remediation, and electrochemically controlled separation. Phosphorylated nanocellulose is a particularly interesting material, spanning a surprising set of applications in various dimensions including bone scaffolds, adsorbents, and flame retardants and as a support for the heterogenization of homogeneous catalysts.

Injectable Highly Loaded Cellulose Nanocrystal Fibers and Composites

Cellulose nanocrystals (CNC)/poly(ethylene oxide) (PEO) composite fibers were successfully produced in situ by injection into a hydrophobic solvent. Using a similar principle, a single step manufacturing method of injectable composites was developed by injection of a CNC solution into a hydrophobic resin. Molecular orientation and deformation of the fibers and composites were obtained using Raman spectroscopy. CNCs were found to be highly aligned along the fiber's axes, as confirmed by 2-fold symmetry of polar plots and second and fourth order orientation parameters. A shift in the position of a characteristic Raman band, initially located at ∼1095 cm^-1, corresponding to vibrations of the cellulose backbone polymer chains was followed under tensile deformation. Using this shift, it was possible to estimate the fiber modulus as being ∼33 GPa, which is remarkably high. Stress transfer between the hydrophobic resin and the injected CNC fibers was quantified in this new type of composite using a modified shear-lag theory showing that appreciable reinforcement occurs. Our approach presents a new way to introduce highly loaded CNC fibers in situ into a composite structure.

In situ Raman spectroscopic study of Al-infiltrated spider dragline silk under tensile deformation

Natural materials consisting of protein structures impregnated with a tiny amount of metals often exhibit impressive mechanical behavior, which represents a new design paradigm for the development of biomimetic materials. Here, we produced Al-infiltrated silks by applying a modified Al2O3 atomic layer deposition process to the dragline silk of the Nephila pilipes spider, which showed unusual mechanical properties. The deformation behavior of the molecular structure of the Al-infiltrated silk was investigated by performing in situ Raman spectroscopy, where Raman shifts were measured concurrently with macroscopic mechanical deformations. For identifying the role of the infiltrated Al atoms, the study was performed in parallel with untreated silk, and the results were compared. Our experimental results revealed that superior mechanical properties of the Al-infiltrated silk are likely to be caused by the alterations of the sizes of the β-sheet crystals and their distribution.

New secrets of spider silk: exceptionally high thermal conductivity and its abnormal change under stretching

Two new significant discoveries are reported: i) the dragline silk of N. clavipes spider has an exceptionally high thermal conductivity: up to 416 W/m·K that beats most materials; ii) contrary to normal materials, its thermal conductivity increases with strain (19% increase under ∼20% strain). These new findings will revolutionize the design of polymer fibers to increase their thermal conductivity by orders of magnitude.

Determination of mechanical strength properties of hemp fibers using near-infrared fourier transform Raman microspectroscopy

Fourier transform near-infrared (FT-NIR) Raman microspectroscopy was adopted for analyzing the micro mechanical tensile deformation behavior of cellulosic plant fibers. Mechanical strength parameters such as tensile strength, failure strain, and Young's modulus of diversified hemp fibers were determined within the range of single fiber cells and fiber filaments. The analysis of fiber deformation at the molecular level was followed by the response of a characteristic Raman signal of fiber cellulose that is sensitive to the tensile load applied. The frequency shift of the Raman signal at 1095 cm(-1) to lower wavenumbers was observed when the fibers were subjected to tensile strain. Microstructural investigations using electron microscopy under environmental conditions supported the discussion of mechanical properties of hemp fibers in relation to several fiber variabilities. Generally, mechanical strength properties of diversified hemp fibers were discussed at the molecular, microstructural, and macroscale level. It was observed that mechanical strength properties of the fibers can be controlled in a broad range by appropriate mercerization parameters such as alkali concentration, fiber shrinkage, and tensile stress applied to the fibers during the alkaline treatments.

Elastic modulus and stress-transfer properties of tunicate cellulose whiskers

Experimental deformation micromechanics of natural cellulose fibers using Raman spectroscopy and X-ray diffraction have been widely reported. However, little has been published on the direct measurements of the mechanical properties, and in particular the elastic modulus, of the highly crystalline material in the native state. Here we report on measurements of the elastic modulus of tunicate cellulose using a Raman spectroscopic technique. A dispersed sample of the material is deformed using a four-point bending test, and a shift in a characteristic Raman band (located at 1095 cm(-1)) is used as an indication of the stress in the material. Relatively little intensity change of the Raman band located at 1095 cm(-1) is shown to occur for samples oriented parallel and perpendicular to the polarization direction of the laser, as compared to a highly oriented flax sample. This indicates that the tunicate sample is a two-dimensional in-plane random network of fibers. By use of this result, the Raman shift, and calibrations with strain from other materials, it is shown that the modulus of the material is very high, at about 143 GPa, and a lack of Raman band broadening is thought to be due to the fact that there is pure crystalline deformation occurring without the effect of crystalline/amorphous fractions. A strain sensitivity of the shift in the 1095-cm(-1) Raman peak for this specimen is shown to be -2.4 +/- 0.2 cm(-1)/%. A molecular mechanics approach, using computer simulation and an empirical force field, was used to predict the modulus of a highly oriented chain of the material, and this is found to be 145 GPa, which is in agreement with the experimental data. However, by use of a normal-mode analysis, it is found that a number of modes have positions close to the central positions of the experimental Raman band. One in particular is found to shift at a rate of 2.5 cm(-1)/%, but due to the complex nature of the structure, it is not entirely conclusive that this band is representative of the experimental findings.

Modeling Crystal and Molecular Deformation in Regenerated Cellulose Fibers

Experimental deformation micromechanics of regenerated cellulose fibers using Raman spectroscopy have been widely reported. Here we report on computer modeling simulations of Raman band shifts in modes close to the experimentally observed 1095 cm(-1) band, which has previously been shown to shift toward a lower wavenumber upon application of external fiber deformation. A molecular mechanics approach is employed using a previously published model structure of cellulose II. Changing the equilibrium c-spacing of this structure and then performing a minimization routine mimics tensile deformation. Normal-mode analysis is then performed on the minimized structure to predict the Raman-intensive vibrations. By using a dot-product analysis on the predicted eigenvectors it is shown that some Raman active modes close to the 1095 cm(-1) band interchange at certain strain levels. Nevertheless, when this is taken into account it is shown that it is possible to find reasonable agreement between theory and experiment. The effect of the experimentally observed broadening of the Raman bands is discussed in terms of crystalline and amorphous regions of cellulose, and this is compared to the lack of X-ray broadening to explain why discrepancies between theory and experiment are present. A hybrid model structure with a series-parallel arrangement of amorphous and misaligned amorphous-crystalline domains is proposed which is shown to agree with what is observed experimentally. Finally, the theoretical crystal modulus for cellulose II is reported as 98 GPa, which is shown to be in agreement with other studies and with an experimental measurement using synchrotron X-ray diffraction.

Biological liquid crystal elastomers

Liquid crystal elastomers (LCEs) have recently been described as a new class of matter. Here we review the evidence for the novel conclusion that the fibrillar collagens and the dragline silks of orb web spiders belong to this remarkable class of materials. Unlike conventional rubbers, LCEs are ordered, rather than disordered, at rest. The identification of these biopolymers as LCEs may have a predictive value. It may explain how collagens and spider dragline silks are assembled. It may provide a detailed explanation for their mechanical properties, accounting for the variation between different members of the collagen family and between the draglines in different spider species. It may provide a basis for the design of biomimetic collagen and dragline silk analogues by genetic engineering, peptide- or classical polymer synthesis. Biological LCEs may exhibit a range of exotic properties already identified in other members of this remarkable class of materials. In this paper, the possibility that other transversely banded fibrillar proteins are also LCEs is discussed.

Liquid crystalline spinning of spider silk

Spider silk has outstanding mechanical properties despite being spun at close to ambient temperatures and pressures using water as the solvent. The spider achieves this feat of benign fibre processing by judiciously controlling the folding and crystallization of the main protein constituents, and by adding auxiliary compounds, to create a composite material of defined hierarchical structure. Because the 'spinning dope' (the material from which silk is spun) is liquid crystalline, spiders can draw it during extrusion into a hardened fibre using minimal forces. This process involves an unusual internal drawdown within the spider's spinneret that is not seen in industrial fibre processing, followed by a conventional external drawdown after the dope has left the spinneret. Successful copying of the spider's internal processing and precise control over protein folding, combined with knowledge of the gene sequences of its spinning dopes, could permit industrial production of silk-based fibres with unique properties under benign conditions.