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?