Revitalizing Traditional Phenolic Resin toward a Versatile Platform for Advanced Materials

ConspectusPhenolic resin, the first synthetic plastic with a history of more than a century, is synthesized by polycondensation of phenols and aldehydes. Phenolic resin has been extensively explored and once used in every aspect of life such as civil equipment, construction materials, decorations, and military industry. Although the continuous surge of novel high-performance engineering plastics since the last century has accelerated the displacement of phenolic resin, it is still well-known for its admirable properties including mechanical robustness, electrical insulation, fire resistance, and chemical stability. Fortunately, booming nanotechnologies offer new opportunities to unearth the treasures buried deep beneath this centuries-old phenolic chemistry and have ushered phenolic resin into the age of nanomaterials. Leveraging the phenolic chemistry (high activity of phenols, strong eletrophilicity of phenolic hydroxyl groups, or reducing capacity of aldehydes) at the microscopic scale allows precise design and control of the nano/microstructures and compositions. In the past several decades, phenolic resin has entered its second rejuvenation and flourishment with the hallmark of a range of emerging functional nanomaterials. The merit of easy and controllable synthesis is brought into the fullest play to create a huge amount of unprecedented exquisite microstructures. The good thermal stability and high carbon yield also render wide use as protection for other vulnerable materials or as a carbon source. Engineering phenolic resin has produced a series of novel materials spanning from zero-dimensional (0D) nanomaterials to three-dimensional (3D) macroscopic assemblies with outstanding properties far beyond the capabilities of traditional phenolic bulk products. All these properties confer applications in energy, biomedical engineering, thermal insulation, fire resistance, environment, and many other aspects.The intentions of this Account therefore relate to three levels of content: (i) a call for more attention to this traditional phenolic chemistry and material which can bring us new surprises under the light of emerging technologies, (ii) a summary of the advances in novel phenolic materials in terms of synthesis, properties, and applications, and (iii) inspiring more explorations on phenolic chemistry toward the broader interdisciplinary applications. The Account begins with a brief introduction and basic properties of phenolic resin. It then describes the evolution of phenolic resins toward multiscale functional materials and applications. Novel phenolic materials can be categorized into low-dimensional (0D, 1D, 2D) nanomaterials and macroscopic 3D monoliths based on methods such as hydrothermal synthesis, self-assembly, and freeze-casting. Tuning the synthesis at multiscales leads to various sophisticated structures, such as core–shell nanospheres, nanocables, and wood-like cellular structures that are suitable for applications in energy, biomedical engineering, fire resistance, thermal insulation, and environment. At the end, the remaining challenges and promising directions are proposed from the viewpoints of green synthesis, large-scale fabrication, recycling, and biodegradation. It is expected that this Account would attract more attention to this traditional topic and provide a profound understanding of engineering functional phenolic materials, which finally are expected to provide inspiration for designing other polymeric nanomaterials and monoliths.