I think your question is not very specific and much too general.
You have to give more details like what kind of polymer, how much pressure etc.? Or do you mean compression force instead of pressure? Films on the surface of a substrate or membranes?
In our case we relate the term pressure to system pressure which is absolutely different from the mechanical pressure through compression. We have shown that constant compression forces of 2.5 MPa at 160°C on MEAs could lead to some polymer "creeping".
In my earlier work we have seen that high compression on PTFE sealings at temperatures around 200 °C also led to severe creeping behaviour and possible leakages in the experimental setup.
first of all which of polymer are using that is important, there are tonnes of polymer whose properties change by applying pressure in different rate. Piezoelectric polymers like PVDF if you apply pressure it generates voltage, Some conducing polymer if you apply stress there resistance is changed (Piezoresistive effect), So it depends upon polymer what are using.