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Can someone explain the how/why Nylons brittleness is to some degree a function of it's moisture content. I don't understand HOW the water content changes the elongation at break of the material.
Water doesn't bond or change the chemistry of the nylon when absorbed, my understanding is that it just fills some of the voids within the nylon. so then how is this changing the slope of the stress/strain curve?
Nylons are a class of polymers formed from di-acid and di-amine monomers. The amine groups bond with the acid groups to form amide groups, end to end. The resulting amide groups have a positive charge at the amine hydrogen, and negative charge at the acid double-bond oxygen. The result is a polymer with periodic polar moments across the chain. The polar moments align between neighboring chains, creating a strong van der Waals force between them, in the form of hydrogen bonding. This is part of why nylon fibers are so strong: they resist chain sliding due to the strong inter-chain bonding.
Water is also a polar molecule and also exhibits hydrogen bonding, giving it some interesting properties compared with similar molecules. Because of its polar nature, water readily infiltrates nylon materials and is attracted to the polar amide groups of nylons. Water creates space between chains, increasing the ability for chains to move around one another more easily, improving ductility. As Chris Johns noted, it acts as a plasticizer.
It is worth noting that nylon and other amide polymers are somewhat special in that they are hydrophilic and oleophobic, unlike other common engineering polymers.
Water acts as a plasticizer, and plasticizers decrease the attractive forces between molecular chains to make the material more flexible.
