[jkmiller]

I can't seem to figure out which chemical properties govern the physical property that is young's modulus (E). For example, any linear (or with a low degree of branching) polyethylene with no crosslinking is still a somewhat rigid and solid substance (higher E), whereas the most linear possible silicone with lots of crosslinking (VMQ) is super rubbery in the same temperature range (really really low E) . 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. As I mentioned before, it doesn't seem to be crosslinking either (crosslinked hydrocarbons are rigid, crosslinked silicones are rubbery). Originally I thought it was  glass transition issue (that silicon has too low of a glass transition) until I realized that the glass transition is not continuous and there is a range of temperatures at which the substance remains in its "rubber" state with no change in modulus. Everything I can think of would seem to point to silicone products being more rigid than hydrocarbons, not less. Help?

[Enthalpy]

My understanding of E-modulus of polymers (alloys are completely different) is limited.

The ability of macromolecules to fold reduces the stiffness. This happens by heat, most molecules do fold a lot at room temperature, even though they are drawn straight on the screen. Si-O bonds rotate more freely than C-C bonds and make very soft silicone. To make stiff polymers, designers synthesize molecules that can't fold. Examples around E=200GPa (!) in the length direction
https://en.wikipedia.org/wiki/Zylon
compare also para-aramid with meta-aramid.
This is why designers thrive to make polymers of bicyclo[1.1.1]pentane or 1,4-phenyl.

The same happens with polyethylene. As a bulk material, E~1GPa. As a super-stretched fibre it's Dyneema or Spectra etc, extremely stiff and strong, despite weak intermolecular interactions
https://en.wikipedia.org/wiki/Ultra-high-molecular-weight_polyethylene#Fiber

The interaction of neighbour macromolecules is obviously important, because a lone stretched PE molecule would just fold back once released. The same happens with p-aramid
https://en.wikipedia.org/wiki/Aramid
where hydrogen bonds keep the molecule blocks in place, despite lone molecules could rotate along the C-C and C-N bonds. So here, the macromolecules keep stretched collectively.