Degradation of Polymers: Biological
Course Module for ME 5XX - The Degradation of Polymers
Types of Degradation - Biological
The bio-degradation of polymers on a natural level is familiar to everyone in the form of incomplete degradations such as fossil fuels, and complete degradations such as those of proteins and lipids, for nutritional purposes.
Unfortunately for environmentalists, man-made polymers are theoretically biodegradable but essentially inert. Microbial degradation has been found to be effective for low molecular weight polymers, while macromolecular matrices are scarcely affected. It has been shown that contrary to naturally occuring polymers, synthetic polymers are attacked only from the ends of the chain, which would account for the much slower rate of degradation.
Natural products that are particularly susceptible to biological attack include:
- Industrial gums - hydrolyzable by bacteria and fungi.
- Natural rubber - almost completely consumable by soil microorganisms.
- Starch - degraded easily by bacteria and fungi.
- Cellulose - attack by biological agents, through enzymatic hydrolysis.
Enzymatic Degradation
Enzymes are mostly involved in the chemical mode of polymer degradation, pertaining to the decomposition of polymers that are a part of organized living species. Organized species have evolved to the point where they have enzymes that can break down certain polymers (such as proteins) in their digestive systems, which have become highly specific to their biological processs.
Enzymatic degradation occurs by a catalytic process. Molecular conformation is very important to the specificity of the enzyme, and the enzyme can be rendered inactive (denatured) very quickly by varying the pH, temperature, or solvent. Some enzymes require other enzymes (co-enzymes) to be present in order to be effective, in some cases forming association complexes in which the coenzyme acts as a donor or acceptor for a specific group.
In synthetic polymers, chain ends tend to be deep in the polymer matrix and since enzymes tend to attack only at the chain ends, the process is quite slow.
Microbial Degradation
Microorganisms, such as fungi and bacteria, degrade polymers by ingesting the carbon molecules that are the backbone of the polymer. Given either an aerobic (fungi) or an anaerobic (both fungi and bacteria) environment, microbial degradation is theoretically possible. Due to the hydrophobicity of many plastics, however, optimal growth conditions (water, light) are rarely found and the microorganisms usually don't penetrate deeper than the surface of a part.
Check out the Bio/Environmentally Degradable Polymer Society (bedps.org) webpage or the Journal of Environmental Polymer Degradation for more info.
This page prepared by Kimberly Farrell for Professor Sisson of WPI, for use in ME XXX, Corrosion and assorted fun times.