Polymers are substances made of macromolecules, which are very large molecules composed of many repeating smaller units called monomers. These long, chain-like structures give materials like plastics, rubber, and fibers useful properties, such as elasticity, toughness, and durability. Polymer degradation is the chemical process where these long molecular chains break down into smaller segments, known as chain scission. This irreversible change in chemical structure leads to a loss of the original physical and mechanical properties.
Key Types of Degradation
The breakdown of polymer chains is initiated by specific external energy sources, with different environments triggering distinct chemical mechanisms. Heat energy causes thermal degradation when molecular bonds absorb enough energy to break spontaneously, even without oxygen. This can occur through depolymerization, or “unzipping,” where monomers break away from the chain ends one by one, a common mechanism in polymers like polymethyl methacrylate (PMMA). Alternatively, random chain scission breaks bonds at random points along the chain, leading to a mixture of lower molecular weight fragments.
Sunlight, specifically ultraviolet (UV) radiation, drives photodegradation, often combined with oxidation to form photo-oxidation. UV radiation is energetic enough to excite polymer molecules, leading to the formation of highly reactive free radicals. These radicals react rapidly with oxygen, initiating a chain reaction that results in the breaking of the polymer’s main chains and the introduction of new chemical groups. This mechanism is a primary cause of material failure for plastics exposed to outdoor elements.
Water causes hydrolytic degradation in polymers containing water-reactive bonds, such as polyesters and polyamides. This process involves a water molecule attacking the polymer’s ester or amide linkages, splitting the chain in a reaction that is essentially the reverse of how the polymer was initially formed. The rate of this degradation is highly dependent on factors like temperature, humidity, and the acidity or basicity of the water, with acidic or basic conditions accelerating the cleavage of these bonds.
In biological environments, microorganisms drive biodegradation by secreting specific enzymes, primarily hydrolases and oxidoreductases. These enzymes act as biological catalysts, cleaving the polymer’s main chain into smaller molecules, such as oligomers and monomers. Once fragmented, microorganisms absorb and metabolize them as a source of carbon and energy, eventually converting the material into carbon dioxide and water under aerobic conditions. The susceptibility of a polymer to biodegradation is influenced by its structure, crystallinity, and the specific microbial community present.
Impact of Degradation on Material Properties
Degradation reduces the polymer’s molecular weight as long chains break into shorter segments. This reduction in chain length means fewer intermolecular forces hold the material together, leading to a loss of mechanical integrity. The most noticeable mechanical change is a loss of tensile strength, which is the material’s resistance to being pulled apart, and a decrease in elasticity. The material loses toughness and impact resistance, becoming brittle and prone to cracking under stress.
Degradation results in aesthetic changes that signal chemical breakdown. Photo-oxidation often causes discoloration, such as yellowing or hazing, as new oxygen-containing chemical groups are incorporated. Surface changes like chalking, where a white, powdery layer forms on the exterior, are also common as the surface layer breaks down. Chemical transformations also form smaller, soluble by-products known as oligomers, which can leach out of the material. Furthermore, the breakdown of the polymer’s main chains can release additives, such as colorants or plasticizers, altering the material’s composition.
Controlling Polymer Lifespan
Engineers use chemical additives to stabilize durable goods and interrupt the degradation cycle. Antioxidants are the primary stabilizers used to combat thermal and oxidative degradation by neutralizing free radicals. Stabilization often involves a two-part system: primary antioxidants scavenge initial free radicals, stopping the chain reaction. Secondary antioxidants decompose unstable hydroperoxides into stable, non-radical products, preventing the generation of new radicals that would continue the damage.
UV stabilizers are added to materials intended for outdoor use to counteract the damaging effects of sunlight. These additives work through UV screening, blocking radiation from reaching the polymer, or by quenching, absorbing energy from excited molecules and dissipating it as heat. Conversely, accelerated degradation is used for environmental disposal. Biodegradable polymers are synthesized with hydrolytically-labile links, such as ester bonds, that are easily cleaved by water or microbial enzymes. For resistant conventional plastics, pro-oxidant additives (often metal salts) promote initial oxidative breakdown into smaller fragments accessible to microorganisms for final biodegradation.