Branching (polymer Chemistry)
In polymer chemistry, branching occurs by the replacement of a substituent, e.g., a hydrogen atom, on a monomer subunit, by another covalently bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type. In crosslinking rubber by vulcanization, short sulfur branches link polyisoprene chains (or a synthetic variant) into a multiply branched thermosetting elastomer. Rubber can also be so completely vulcanized that it becomes a rigid solid, so hard it can be used as the bit in a smoking pipe. Polycarbonate chains can be crosslinked to form the hardest, most impact-resistant thermosetting plastic, used in safety glasses.
Branching may result from the formation of carbon-carbon or various other types of covalent bonds. Branching by ester and amide bonds is typically by a condensation reaction, producing one molecule of water (or HCl) for each bond formed.
Polymers which are branched but not crosslinked are generally thermoplastic. Branching sometimes occurs spontaneously during synthesis of polymers; e.g., by free-radical polymerization of ethylene to form polyethylene. In fact, preventing branching to produce linear polyethylene requires special methods. Because of the way polyamides are formed, nylon would seem to be limited to unbranched, straight chains. But "star" branched nylon can be produced by the condensation of dicarboxylic acids with polyamines having three or more amino groups. Branching also occurs naturally during enzymatically-catalyzed polymerization of glucose to form polysaccharides such as glycogen (animals), and amylopectin, a form of starch (plants). The unbranched form of starch is called amylose.
The ultimate in branching is a completely crosslinked network such as found in Bakelite, a phenol-formaldehyde thermoset resin.
Other articles related to "branching":
... The branchingindex measures the effect of long-chain branches on the size of a macromolecule in solution ... It is defined as g ,where sb is the mean square radius of gyration of the branched macromolecule in a given solvent,and sl is the mean square radius of gyration of an otherwise identical linear ... indicates an increased radius of gyration due to branching ...
Famous quotes containing the word branching:
“The moose will, perhaps, one day become extinct; but how naturally then, when it exists only as a fossil relic, and unseen as that, may the poet or sculptor invent a fabulous animal with similar branching and leafy horns ... to be the inhabitant of such a forest as this!”
—Henry David Thoreau (18171862)