Re: Why does the surface of a wrinkled balloon shrink when heated?

Michael,

What you are observing is the very force that makes rubber elastic in the first 
place  --  entropy, the thermodynamic tendency towards disorder in all things.

Let's start with a simple rubber band. At rest and unstressed, it is a loosely 
crosslinked collection of coiled-up rubber [cis-polyisoprene] molecules. It is 
analogous to cooked spaghetti tossed in a bowl. When you stretch the rubber 
band, the molecules uncoil and begin to straighten out parallel to one another, 
analogous to uncooked spaghetti in the box. [The stretching can't go on forever 
because of the crosslinks between the molecules]

At very high elongations the rubber actually begins to crystallize [strain 
induced crystallization] which is much more ordered [lower entropy] than rubber 
is in the relaxed state. When the band is released, it snaps back to its 
original, disordered state. This is entropic elasticity.

Now, on to the balloon.

But first, one more concept --  creep. When rubber is held at a stretch for a 
long time [days, e.g.] the polymer chains start to slide parallel to each other 
and the band or balloon actually becomes longer than it started out as. A 5 
inch long rubber band will 'creep' to a new length of, say, 5.5 inches after 
being stretched to 10 inches for several days. If one heats the 5.5 inch band, 
it will return to the original 5 inch dimension [i.e., creep in a crosslinked 
system is reversible].

By now you should be able to figure out what happened to the balloon....

On inflation the rubber molecules stretch and begin to align with each other 
creating a relatively ordered structure. After a few days at full inflation 
some creep has occurred between the chains. When some air escapes and the 
diameter lessens, the stretched rubber can't quite retract as fast as it might 
like [due to the creep that has occurred]. Your hand provides enough energy 
[heat] to accelerate the entropic elasticity and the rubber 'shrinks' faster 
where it was warmer. 

It would be interesting to see what would happen if you used a hair dryer 
instead of your hand. The higher heat will accelerate the rubbers contraction 
while at the same time expanding the air inside the balloon.

regards,

Ken Johnsen, Your MADScientist