Polyacrylonitrile

Lore & Background

However, because PAN is non-fusible and did not dissolve in any industrial solvents then in use, further research was halted. Commercial introduction was delayed due to wartime stresses, inability to melt the polymer without degradation, and lack of known solvents for solution processing. These carbon fibers are found in civil and military aircraft primary and secondary structures, missiles, solid propellant rocket motors, pressure vessels, fishing rods, tennis rackets, and bicycle frames.

Reader's Guide

Polyacrylonitrile is significant primarily as the precursor for 90% of carbon fiber production, a material essential to modern aerospace and high-performance composites. Approximately 20–25% of Boeing and Airbus wide-body airframes are carbon fibers derived from PAN. Its high tensile strength and modulus of elasticity, established by fiber sizing, coatings, and production processes, make it critical in composite structures for military and commercial aircraft. Beyond carbon fiber, PAN is used in ultrafiltration membranes, hollow fibers for reverse osmosis, and textiles. Acrylic fibers, which consist of at least 85% acrylonitrile, are low-cost alternatives to natural fibers with better sunlight resistance and resistance to moths. Modacrylics, containing halogen comonomers, offer flame resistance for sleepwear, tents, and blankets. PAN also serves as a support polymer for ion exchange resins; divinylbenzene-crosslinked PAN is a precursor to weakly acidic resins with high affinities for divalent metal ions. Despite its utility, applications are limited by PAN's high price of around $15/lb. Its legacy includes enabling lightweight, strong materials for both high-tech and everyday products, from aircraft to fishing rods.

Did You Know?

• PAN is the precursor for 90% of carbon fiber production.

Position Among Synthetic Polymers

Polyacrylonitrile occupies a notable position in the hierarchy of synthetic polymers ranked by worldwide demand. It appears in the list alongside polyethylene, polypropylene, polystyrene, polyvinyl chloride, synthetic rubber, phenol formaldehyde resin, neoprene, nylon, PVB, and silicone. Like most synthetic polymers used for plastics, polyacrylonitrile's continuously linked backbone consists mainly of carbon atoms, distinguishing it from materials where silicon or oxygen form the primary structural framework. As a synthetic polymer, it stands in contrast to naturally occurring materials such as hemp, shellac, amber, wool, silk, natural rubber, and cellulose, which have served human needs for centuries. The broad spectrum of properties that synthetic polymers like polyacrylonitrile offer has made them essential and ubiquitous in everyday life, filling roles that natural materials alone could not satisfy.

The Chemistry of Its Formation

The creation of polyacrylonitrile falls within the broader process of polymerization, in which many small molecules called monomers are combined into a covalently bonded chain or network. During this process, certain chemical groups may be lost from each monomer, as seen in other polymerizations where water molecules are eliminated. The distinct fragment of each monomer that becomes incorporated into the final polymer is termed a repeat unit or monomer residue. Synthetic polymerization methods are generally divided into two principal categories: chain polymerization, where monomers are added to a growing chain one at a time, and step-growth polymerization, where chains of monomers may combine directly with one another. The latter can be further subdivided into polycondensation, which produces a low-molar-mass by-product at each reaction step, and polyaddition. These reactions may proceed with or without a catalyst, and newer techniques such as plasma polymerization do not fit neatly into either traditional category.

Structural Characteristics and Physical Properties

As a polymer, polyacrylonitrile inherits a set of physical properties that emerge from its large molecular mass relative to small-molecule compounds. These properties include toughness, high elasticity, viscoelasticity, and a characteristic tendency to form amorphous and semicrystalline structures rather than true crystals. The structure of any polymeric material, including polyacrylonitrile, can be described across a hierarchy of length scales, ranging from the sub-nanometer level up to the macroscopic. Each structural stage provides the foundation for the next, creating a nested architecture that ultimately determines the material's behavior. This structural complexity is what allows polymers to serve such diverse applications. The study of these properties falls within polymer science, encompassing both polymer chemistry and polymer physics, as well as materials science and engineering. The broad spectrum of properties available in synthetic polymers like polyacrylonitrile is precisely what makes them essential and ubiquitous in everyday life.

Intellectual Heritage and Scientific Context

The very concept of what polyacrylonitrile is—a macromolecular structure built from repeating subunits—rests on intellectual foundations laid over nearly a century. The term 'polymer' itself derives from the Greek words polus, meaning many or much, and meros, meaning part. Today, the science surrounding materials like polyacrylonitrile is pursued across multiple disciplines, including polymer chemistry, polymer physics, biophysics, and materials science and engineering. Historically, the primary focus has been on products arising from the linkage of repeating units by covalent chemical bonds, though an emerging area now examines supramolecular polymers formed by non-covalent links, broadening the field's scope considerably.