|MadSci Network: Chemistry|
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
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