Re: Why dosen't silly putty ever dry out?

Reference:  Allcock and Lampe, Contemporary Polymer Chemistry, 2nd edition, 
1990.

I think it is best to answer this question by explaining some things.  
Elementary physics divides matter into 3 distinct categories that you are 
most likely familiar with- solid, liquid and gas.  Solids are substances 
that occupy a fixed shape and volume (i.e. they do not flow).  Liquids flow 
readily but occupy a fixed volume.  Gases flow and change their volume 
easily.  
A class of materials exists called polymers.  Polymers are something you 
use everyday from your toothbrush in the morning to cups you use to drink 
juice to the clothes you wear.  Polymers are very long chains of molecules 
that has repeating unit strung together many times.  These long chained 
molecules are polymers.  The best way to visualize a polymer is to think of 
spaghetti.  When wet, the noodles (chains) can slide past one another.  
When the noodles (chain) dry out or harden after cooking, they become an 
entangled mess that is now one piece (that is, the hundreds of noodle 
became one piece).  This behavior is similar to a polymer, lots of chains 
get entangled and behave like one piece of material.  One of several things 
that can be done to the entangled solid is crosslinking.  The material can 
become crosslinked, meaning that chains (noodles) are connected to 
neighboring chains instead of just being physically entangled.  
Crosslinking can be carried out to various extents and can control the 
behavior of the material.  Too much crosslinking and the material will 
become brittle and can shatter like window glass.  
Most polymers are neither classical solids nor liquids.  They are 
viscoelastic materials.  The viscoelastic state has the characteristics of 
both the solid and liquid states.  This means it can flow, like a liquid or 
maintain its shape like a solid. 
Silly putty is a polymer made from a rubbery material, 
polydimethylsiloxane. Polydimethylsiloxane does something really strange 
when you mix it with boric acid, or B(OH)3. The mixture is soft and 
pliable; you can mold it into any shape easily with your fingers.  The 
addition of boric acid lightly crosslinks the polydimethylsiloxane.  This 
is what is known as silly putty.  After crosslinking, some of the chains 
are tied together, but not so many that the material becomes brittle.  This 
lightly crosslinked rubbery material has characteristics of a solid 
(definite shape, fixed volume, no flow) when sitting there, but when 
pressure is applied to it (through pushing or pulling), it will change its 
shape.  Thus, the material never appears to dry out since it has solid and 
liquid behavior.
You may want to try heating a piece of silly putty in the oven for a while. 
 This should induce more crosslinking in the material and the silly putty 
would then act more like a solid than a viscoelastic material.  But be 
careful when trying this or any experiment.  There are also some web pages 
that give recipes on how to make silly putty at home.  I suggest reading 
the following web pages before trying any experiments:  http://www3.hmc.edu/~j
kurtze/spexperiment.htm http://funstuffusa.com/sillyput.
htm
Or try searching silly putty on the internet.  Hopes this helps.