From: BretCahill@aol.com   
      
   > > Actually I'm looking for a thin flexible elastic film   
   > > that has low permeability to gas.   
   >   
   > Unobtainium.  As you stretch the film, to alter and *always* increase the   
   gas permeability.   
      
   The film won't ever be stretched much.  It just needs to be able to withstand   
   2-D or 3-D flexing which induces some stretching.   
      
   I keep thinking some clever origami trick could help a lot.  A little extra   
   folding, pleating and material won't bust the bank.   
      
   A helix would be better than a bellows.   
      
   There should be a web page where you enter the basic geometrical shape with   
   material moduli, add the flexing situation and the software tells you where to   
   put what size folds.   
      
   It could be used on a lot of problems, i.e., designing thin shell concrete   
   structures near fault lines, all kinds of thermal loading situations.   
      
   > One, you make the film thinner, and two, you break intermolecular bonds from   
   "relaxed" natural state.   
      
   > ...   
      
   > > One of interest is the old Saran cling wrap,   
   > > polyvinylidene chloride:  With an ultimate yield strain   
   > > of 20% it is 4X stretchier than Mylar yet has a   
   > > permeability an order of magnitude lower than Mylar.   
   >   
   > How about rinsing the film after stretch in shape, with some sodium or   
   potassium silicate?   
      
   Water even humidity acts as a plasticizer on Mylar reducing the strength, but   
   maybe increasing the elasticity.  Strength isn't so much an issue as   
   elasticity so adding a little water might do the trick.   
      
   > You may have to expose the film to corona to get this to stick... Are the   
   final contents moist, or very dry?   
      
   Either way.  Nothing is easier than adding a little water.   
      
      
   Bret Cahill   
      
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