[Jonas Andersson]

What intermolecular forces are present between PET-molecules. I've searched a lot and got different answers such as dipole-dipole, and van der Waals forces. I can understand why there are van der Waals forces but I really don't get how that molecule can be a dipole.

[Enthalpy]

Welcome, Jonas Andersson!

"Van der Waals forces" is quite vague.

Local dipoles, not necessarily observable over the whole molecule, for instance if they cancel out, can also act between neighbour molecules, if these have suitable positions and orientations. An example would be the linear O=C=O.

The conformation of a molecule in three dimensions matters too. For instance PVDF
-CF₂-CH₂-
becomes strongly ferroelectric after a polarization step that puts most F at one side and most H at the other.

[pm133]

Jonas,
In order for there to be no dipole, any polar bonds must cancel out over the whole molecule. To do that they must be the same magnitudes, co-linear and in opposing directions.
There are two type of polar bond within each repeating unit.

1) The C=O bonds. Do they oppose each other and cancel out?
2) The C-O bonds. Same question.