the polymer shrinkage is a normal phenomena and the reason behind this is the contraction of polymer. To decrease the degree of shrinkage you may add either nanoparticles or polymeric additives like PEG, PVP. for more explanation you can read the following articles:

http://www.sciencedirect.com/science/article/pii/S1383586616315179

http://www.sciencedirect.com/science/article/pii/S0376738807007995

Article Characteristics, performance and stability of polyethersulfo...

In order to avoid shrinkage you should fix the film in a metal frame.

If you can't solve the problem by improving adhesion to the substrate, you could try thermoforming the sheet after casting.

thank you all for your suggestions