Make a polymer chain ten times longer and its melt gets over a thousand times thicker
Plastic bags, silk, DNA and rubber tyres share one design: enormous molecules built from small units repeated thousands of times. Chain length changes everything. Beyond the point where chains start tangling, a tenfold increase in length can make the molten material more than a thousand times harder to pour.
The word joins Greek terms for many and part, and Jöns Jacob Berzelius coined it in 1833, though with a meaning different from today's. The modern idea, that polymers are giant molecules held together by covalent bonds, came from Hermann Staudinger in 1920, and he spent roughly ten years gathering the experimental proof. Humans had long used natural examples without knowing it: hemp, shellac, amber, wool, silk and natural rubber, plus the cellulose in wood and paper.
Living things are polymer factories. Proteins, nucleic acids and polysaccharides are all long chains, and cells assemble them with enzymes such as DNA polymerase. DNA is built from four kinds of nucleotide units, which makes it a varied chain rather than a simple repeat. Even space has one: hemoglycin, found in meteorites, is the first polymer of amino acids identified there.
Industry makes more than 330 million tons of synthetic polymers a year, led by polyethylene, polypropylene, polystyrene and PVC. Chemists build them two ways. In chain growth, monomers join one at a time, as with polystyrene. In step growth, short chains can link to each other, as with polyester, sometimes shedding small molecules; making PET releases two water molecules per repeat unit. Other plastics began as modified natural materials, such as nitrocellulose from cellulose and vulcanised rubber made by heating rubber with sulfur.
Shape matters as much as ingredients. Straight or branched chains usually give plastics that soften when heated, loosely cross-linked chains give stretchy elastomers, and tight cross-linking yields rigid thermosets. Longer chains tangle and attract one another more strongly, which makes the material tougher and raises the temperature at which it turns glassy. Backbones are mostly carbon, but silicon gives silicones such as Silly Putty.
Source: Polymer
