I am currently studying certain materials. Some of these materials have ionic bonds, while some others have covalent bonds. Moreover, each unit of these materials interacts with the adjacent unit via van der Waals interaction. I have observed that the strength of the van der Waals interaction is different in case of materials with different type of bonds.
Is there any correlation between the nature of bonds and the strength of van der Waals interactions between such sub-units of the material?
The term van der Waals force is a catchall for several different kinds of forces. However all of the forces are due to electric dipoles, and the strength of the force is related to the dipole moments of the interacting molecules.
The strongest interaction is when both interacting molecules have a permanent electric dipole. For example water molecules have a large permanent dipole and the dipole-dipole interactions produce a relatively large force between them.
An example of the other extreme would be the interaction between two noble gas atoms. These do not have a permanent dipole, but they have a (very small) oscillating dipole due to random thermal fluctuations in their electron density. These random ocillating dipoles in two atoms interact to produce a relative weak force called the London dispersion force.
And finally we the the case in between where one of the interacting molecules as a permanent dipole and the other doesn't.
So as a general rule a permanent dipole produces a strong VdW force, and we get dipoles when the bonds in the molecules are polar. If there is no permanent dipole the higher the polarisibility of the molecule the larger the London dispersion force.
