Branching (polymer chemistry) 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. Branched polymers have more compact and symmetrical molecular conformations, and exhibit intra-heterogeneous dynamical behavior with respect to the unbranched polymers. 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
Rubber toughening makes epoxies a candidate of interest for rubber toughening research to fine-tune the toughening processes. Some of the factors affecting the toughness of epoxy nanocomposites include the chemical identity of the epoxy curing agent, entanglement density, and interfacial adhesion. Curing epoxy 618 with piperidine, for example, produces tougher epoxies than when boron trifluoride-ethylamine is used. Low entanglement density increases the toughness. Bisphenol A can be added to lower the crosslinking density of epoxy 618, thereby increasing the fracture toughness. Bisphenol A and a rubber filler increase toughness synergistically. In textbooks and literature before 2002 it was assumed that there is