[...] with respect to bulk moduli of polymers, it is sometimes difficult to make comparisons, because polymer properties depend so much on the relative molecular weight and degree of branching (density). E.g., I'd expect the compressibility of polyethylene to change dramatically depending on whether it is HDPE, LDPE, etc.
Sadly I didn't take my measures with me when leaving this employer because my work belongs to him. I do remember that polyolefins are more compressible than polyamides, polyesters... but clearly less than Ptfe, with silicone oils and rubbers being the most compressible, with K<1GPa near room pressure.
Between Hdpe and Ldpe, I didn't notice an important difference. The E modulus changes measurably, the K modulus little. Generally for soft materials, E drops more quickly than K - for the extreme case of rubbers, K sticks around 1GPa while E drops without limit. I haven't measured Pmp as it was too expensive. I can't exclude neither that after a few kilobar, all polyolefins get the same properties.
At that time I couldn't find a qualitative way to estimate how compressible a compound would be. Liquids tend to be more compressible and solids get a bit more compressible before melting, which suggests that "vacuum between the molecules" (whatever this means) contributes - but long paraffins far from melting are quite compressible as well, and I saw no overwhelming relation with the density at identical gross formula.
What I believe to have seen is that some atoms are more compressible, and that the stiffness of hydrogen depends fundamentally on the polarity of its bond: water and acids stiff, alkanes compressible.
Too little data is available about volume compressibility, and is often at 1atm, hence my measures then. The best tables are for sound velocity, since v
2 = K/ρ, but then at 1atm only.
Particularly for students, I think a classical approach is easiest. It's easy to visualize momentary charge asymmetry when picturing electrons as having absolute trajectories. Who knows what "quantum fluctuations" really means?
Sure. QM is abstract on this point. While the orbital of a single electron is reasonably concrete, a wavefunction of several particles is not. Personally, I prefer a wording like "
if one electron is in this vicinity, the other is probably not because they repel an other" and "this is adequately represented by Ψ(r
1, r
2) ≠ Ψ(r
1) * Ψ(r
2)" since at least, it does not suggest the wrong idea of evolution over time, and sticks better to the math formulation. But then, I don't have an intuitive representation of it neither.