Dear Amrita Rath

First, I think that what you really want to know is the diffusion coefficient of water in the polymer film. In that case, you can obtain the diffusion coefficient by fitting the weight versus time dependence by a suitable diffusion equation, e.g., Ficks law. The direct approach depends on the degree of water uptake by the film, i.e., the film swelling. Because if the swelling is large, the film volume changes significantly during the experiment and you have to explicitly account for this in your calculations. In other words, you cant use a constant diffusion path / constant dimensions of the system.

If you would provide more information, I could suggest some more details about the possible evaluation approach.

Regards

Josef

PS: you can have a look at our paper (link below), but your system will probably require a slightly different approach.

https://www.researchgate.net/publication/263983191_Experimental_Study_of_Sorption_and_Diffusion_of_n-Pentane_in_Polystyrene

Article Experimental Study of Sorption and Diffusion of n-Pentane in...

Dear Dr. Rath,

If the polymer swelling is insignificant you may use the solution for the diffusion in a plane sheet at uniform initial distribution zero and surface concentrations equal at both sides of the sheet, given in the J.Crank's book "The mathematics of diffusion", Oxford at the Clarendon Press, 1967, p.45. The solution predicts the ratio of the current increase of the weight with the time and the increase at an infinite time as a function of diffusivity coefficient. You will need numerical calculations.

Be careful if you film is in contact with water at both of its sides! This is not clear from your question. If swelling is significant the problem is more complicated.  

 Dear Georgi,

Any number to tell that polymer swelling is significant? Initial film weight is 5 mg (dry film) and final weight after in contact with water for 100 min is 63 mg. Film is free standing and it is  kept on water surface for around 100 min. So, both ends are in contact with water. 

Dear Amrita,

At the swelling you have reported the solution which I have recommended is not applicable. The thickness of the film has been increased very much as compared to the initial state. In such systems the interpretation of the results is complicated. It is hardly to believe that the diffusion coefficient is constant. You may find information in Google searching for diffusion of water in your concrete polymer.