I would like to suggest coating your petri dish with polyvinyl alcohol (PVA). It is water soluble so you can remove it with water if it doesn't just come off on it's own. It is also resistant to a large number of organic solvents which will melt polystyrene petri dishes. We use this material as a mold release for a number of different casting and crosslinking processes that we do in our lab.

Application is quite simple, you can just pour it on making sure to cover the entire area and then pour of the excess and dry. I like to put my PVA coated substrates in a 60oC oven for an hour to dry. You can also spin it on if you want a very thin coating. We do this for making thin polymer membranes which are also spin coated.

sir...if i coated petridish with PVA and dry it in an oven den is it will not effect my chitosan film...?and if it will not effect my chitosan film den am i able to remove dat film easily?kindly give response...

Without knowing the chemistry that you are using, I can't say for sure whether or not the PVA will effect your films. PVA is soluble in water, so if your film precursor is largely water, it may partially dissolve the PVA and allow it to interfere with film formation. It is possible to get a high molecular weight PVA that requires hot water to dissolve.

PVA is very resistant to most organic solvents except some of the small alcohols.

If you are using something like 2g chitosan, 2mL acetic acid, mixed to 100mL in water, then you will need to get very high molecular weight PVA and do not use heat to dry the film. Most protocols that I have seen for this are using glass as the substrate and then they use sodium hydroxide or some other base as a release promoter which cause the chitosan to shrink and delaminate from the glass.

Since you are using crosslinkers, you may be using a very different chemistry. If you can provide a list of chemicals, I could give you more information about compatibility.