Advances in the Synthesis of Preceramic Polymers for the Formation of Silicon-Based and Ultrahigh-Temperature Non-Oxide Ceramics.

Preceramic polymers (PCPs) are a group of specialty macromolecules that serve as precursors for generating inorganics, including ceramic carbides, nitrides, and borides. PCPs represent interesting synthetic challenges for chemists due to the elements incorporated into their structure. This group of polymers is also of interest to engineers as PCPs enable the processing of polymer-derived ceramic products including high-performance ceramic fibers and composites. These finished ceramic materials are of growing significance for applications that experience extreme operating environments (e.g., aerospace propulsion and high-speed atmospheric flight). This Review provides an overview of advances in the synthesis and postpolymerization modification of macromolecules forming nonoxide ceramics. These PCPs include polycarbosilanes, polysilanes, polysilazanes, and precursors for ultrahigh-temperature ceramics. Following our review of PCP synthetic chemistry, we provide examples of the application and processing of these polymers, including their use in fiber spinning, composite fabrication, and additive manufacturing. The principal objective of this Review is to provide a resource that bridges the disciplines of synthetic chemistry and ceramic engineering while providing both insights and inspiration for future collaborative work that will ultimately drive the PCP field forward.

Performance of Advanced Waterborne Wood Coatings Reinforced with Cellulose Nanocrystals.

The main objective of this study was to examine the impact of cellulose nanocrystals (CNCs) in advanced waterborne wood coatings such as polycarbonate urethane (PCU) and hybrid alkyd varnish (HAV) in terms of coating performance, mechanical properties, optical properties, and water permeation and uptake properties. The influence of CNCs on the overall quality of the various waterborne wood coatings was investigated by incorporating different percentages of CNCs. Varying CNC content in coating formulations showed that CNCs are effective for waterborne wood coatings; CNCs offer both higher scratch and impact resistance as compared to neat coatings and have a significant reduction in water vapor permeation through a film with little increase in water vapor uptake at high concentrations. It was observed that the CNC darkened and reduced gloss in the coatings and viscosified the dispersion. These research findings suggest that CNCs are well-dispersed at lower concentrations, but at high concentrations, agglomeration occurred. Thus, while CNCs can give better mechanical and permeation performances at contents of up to 5 wt %, at 1 wt % CNCs can still provide modest scratch and chip resistance improvement without loss of optical properties (gloss and color) while retaining a similar water uptake. Overall, it can be concluded that CNCs have the potential to be used as a reinforcement filler in high-performance waterborne wood coatings.

Recent Developments in Cellulose Nanomaterial Composites.

Cellulose nanomaterials (CNMs) are a class of materials that have recently garnered attention in fields as varied as structural materials, biomaterials, rheology modifiers, construction, paper enhancement, and others. As the principal structural reinforcement of biomass giving wood its mechanical properties, CNM is strong and stiff, but also nontoxic, biodegradable, and sustainable with a very large (Gton yr-1 ) source. Unfortunately, due to the relatively young nature of the field and inherent incompatibility of CNM with most man-made materials in use today, research has tended to be more basic-science oriented rather than commercially applicable, so there are few CNM-enabled products on the market today. Herein, efforts are presented for preparing and forming cellulose nanomaterial nanocomposites. The focus is on recent efforts attempting to mitigate common impediments to practical commercialization but is also placed in context with traditional efforts. The work is presented in terms of the progress made, and still to be made, on solving the most pressing challenges-getting properties that are competitive with currently used materials, removing organic solvent, solving the inherent incompatibility between CNM and polymers of interest, and incorporation into commonly used industrial processing techniques.

Alignment of Cellulose Nanofibers: Harnessing Nanoscale Properties to Macroscale Benefits.

In nature, cellulose nanofibers form hierarchical structures across multiple length scales to achieve high-performance properties and different functionalities. Cellulose nanofibers, which are separated from plants or synthesized biologically, are being extensively investigated and processed into different materials owing to their good properties. The alignment of cellulose nanofibers is reported to significantly influence the performance of cellulose nanofiber-based materials. The alignment of cellulose nanofibers can bridge the nanoscale and macroscale, bringing enhanced nanoscale properties to high-performance macroscale materials. However, compared with extensive reviews on the alignment of cellulose nanocrystals, reviews focusing on cellulose nanofibers are seldom reported, possibly because of the challenge of aligning cellulose nanofibers. In this review, the alignment of cellulose nanofibers, including cellulose nanofibrils and bacterial cellulose, is extensively discussed from different aspects of the driving force, evaluation, strategies, properties, and applications. Future perspectives on challenges and opportunities in cellulose nanofiber alignment are also briefly highlighted.