In my most recent research into thermoplastics, I was unable to find any information regarding what actually happens to intramolecular forces when heat is applied.

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    $\begingroup$ They don't weaken. It is just that the molecules start to move faster, so you can't hold them in place using the same force as before. $\endgroup$ – Ivan Neretin May 3 '18 at 11:56
  • $\begingroup$ You can think of intermolecular forces as having an anharmonic and dissociating potential , such as Morse or Lennard -Jones. The potential will have vibrational energy levels and as heat is applied more of these levels become excited till eventually the dissociation limit is reached and the molecules separate. So the intermolecular 'bond' is weaker as energy is suppled as levels close to the dissociation limit are populated and this is also where the separation of molecules is larger. $\endgroup$ – porphyrin May 3 '18 at 13:24

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