What Chemical Properties Affect Young's Modulus?

[jkmiller]

TL;DR

Which chemical properties effect the physical property Young's Modulus? I know the answer is not as simple as intermolecular force

I can't seem to figure out which chemical properties govern the physical property that is young's modulus. For example, any linear (or with a low degree of branching) polyethylene with no crosslinking is still a somewhat rigid and solid substance (higher ym), whereas the most linear possible silicone with lots of crosslinking (VMQ) is super rubbery in the same temperature range (really really low ym) .

At their core, silicone and hydrocarbon plastics are chemically different on so many levels, but which differences are the ones that determine the modulus? I would assume it would be intermolecular force (so LDFs for both unless silicone can create temporary dipoles in that case I would predict them to be less fluid), which would make sense that silicones were more fluid because they are less linear, but there are plastics a whole lot less linear than VMQ that are way more rigid.

Help me physics friends I am but a lowly biochemist very lost in the realm of materials physics.

 

[Chestermiller]

Silicone is kind of like an elastomer, right?

 

[Matcon]

Fundamentally, Young's Modulus is largely determined by intermolecular forces. At equilibrium, atoms sit (or, more correctly vibrate around) a point a certain distance from their nearest neighbours at which potential energy is minimised. You can think of it as sitting at the bottom of an energy well. Young's Modulus represents the force required to increase that distance from the unstressed equilibrium value.

In a simple, crystalline material, the Lennard-Jones function gives a pretty good approximation of potential energy versus interatomic distance, so you can do an order of magnitude calculation for Young's Modulus from just the binding energy.

With polymers, the potential energy curve is affected by microstructural features that need not be chemical in nature. For example, long linear molecules can be folded back on one another, so different atoms can be influenced by a varying number of near neighbours.

Perhaps the most dramatic example is the rubber-like elasticity of poly(cis-isoprene), in which rotation of carbon-carbon bonds allows the molecule to "unwind" under tension. By contrast, poly(trans-isoprene) is a stiff, relatively hard polymer going under the name of gutta-percha. It is the same chemistry, but completely different Young's Modulus