As suggested by the posts before the situation is more complicated than you might think. A hydrophilic polymer is soluble in water if it is not bonded to the surface or if it is cross-linked. In the latter case a good adhesion to the substrate is needed in order not to lift off the coating. Only covalent bonds will offer an adequate adhesion since water molecule are small and diffuse readily to the interface and disrupt any non-covalent interaction. If the polymer also contains hydrophobic parts it might not be lsoluble in water. But in this case you will find surfactant effects.

If the hydrophilic polymer is bonded at the surface the density and the free length of the polymer chains will influence the contact angle. At this point it is important to consider if you measure the advancing contact angle (wetting) or the receding contact angle (de-wetting). The latter is more sensitive to a small fraction of a hydrophilic surface than the other. In cases when there are islands of hydrophilic coatings or hydrophilic polymer chains flowting in the liquid and exposing a hydrophobic substrate the difference between the two angles (hysteresis) can be very large.

The analysis depth of contact angle measurements was investigated in the past (doi: 10.1021/ja00225a050). For water it is well below 1 nm. For other liquids it is even smaller.

Maybe you can clarify your question more. I guess you are talking about a substrate which has been modified with a coating to change the wettability. What type of substrate are you talking about ?  You are talking about polymer thickness. It sounds you are talking about rather thick layers and not monolayers ?

C.M.J. Mutsaers Thanks, asking for clarification

The changes in wettability due to the combined effect of a hydrophobic (flat) substrate coated with a hydrophilic polymer having thickness ranges from 60 nm to 5 nm.

I am looking for references to defend the situation explained below.

Flat polyethylene shows water contact angle of 100° and with a hydrophilic polymer (carboxylic group) coating having a thickness in 30 nm range it changes to 15° WCA.

For thickness ranges 10 to 5 nm of hydrophilic polymer coating on a hydrophobic substrate, WCA change to 70° due to the domination of hydrophobic effect of the substrate over the hydrophilic effect of polymer coatings.

Although this situation is straightforward, but failed to find some references directly that explained wettability effect of hydrophobic polymer substrate on hydrophilic coatings for various coating thicknesses.   

Would you clarify your question more : what kind of link between substrate and hydrophilic polymer?, this could help you to better understand the phenomenon

all depends  on the orientation and density of the molecules of the hydrophilic polymeer at the substrate

As suggested by the posts before the situation is more complicated than you might think. A hydrophilic polymer is soluble in water if it is not bonded to the surface or if it is cross-linked. In the latter case a good adhesion to the substrate is needed in order not to lift off the coating. Only covalent bonds will offer an adequate adhesion since water molecule are small and diffuse readily to the interface and disrupt any non-covalent interaction. If the polymer also contains hydrophobic parts it might not be lsoluble in water. But in this case you will find surfactant effects.

If the hydrophilic polymer is bonded at the surface the density and the free length of the polymer chains will influence the contact angle. At this point it is important to consider if you measure the advancing contact angle (wetting) or the receding contact angle (de-wetting). The latter is more sensitive to a small fraction of a hydrophilic surface than the other. In cases when there are islands of hydrophilic coatings or hydrophilic polymer chains flowting in the liquid and exposing a hydrophobic substrate the difference between the two angles (hysteresis) can be very large.

The analysis depth of contact angle measurements was investigated in the past (doi: 10.1021/ja00225a050). For water it is well below 1 nm. For other liquids it is even smaller.

I believe Hollander summarized the issues very well. My experience has also taught me to be aware of coating inhomgeneities especially in very thin coatings. Substrate variabilities can influence the coating patency all which affect contact angle.

Thank you very much all for comprehensive answers. Your views provided a better understanding of this issue.

in addition to the above I would be also very skeptical whether a really 'ideal' (homogeneous) nanometer layer of a hydrophilic polymer can be obtained on a hydrophobic substrate.

Taking Andreas Holländer remarks into account about surface wettability measurements, I can predict less homogeneity of hydrophilic polymer coatings on the hydrophobic substrate for thickness less than 10 nm in my case.

This prediction I made due to observations like the difference in left and right contact angles, sudden increases in contact angle with high contact angle angles hysteresis. 

To prove “islands of hydrophilic coatings on hydrophobic surface” speculations and check the level of homogeneity by lowering thickness I need AFM images along with ToF-SIMS spectra.

thank you very much for this valuable discussion.