Subject: Why does iodine pass through a plastic bag?

I found an experiment to perform and it works, but I don't understand
why.  Mix cornstarch and water, and place it inside a 
plastic bag, then seal it. Place the sealed bag in a beaker of
water plus 15 drops of iodine.  After 15min or so, the cornstarch
mixture becomes purple, and the water and iodine remain orange/brown.

Why does the iodine permeate the plastic bag?  I know it has to 
do with osmosis, but I don't know what chemical properties iodine
has that it can permeate the plastic.

Thanks for your help.
Sincerely,
Sharon

Re: Why does iodine pass through a plastic bag?