It is possible to charge a balloon to several thousand volts using electrostatics, for example, by rubbing it on one's hair or on different types of clothing. Does this mean that the balloon is also storing several Joules of energy? Why or why not?

Yes it is possible to charge balloon to several thousand of volts and the balloon will also store several Joules of energy.

Explanation:

By rubbing a balloon on one's hair or on different types of clothing, the balloon either gain or loss electron.

If the balloon gains electron it becomes negatively charged, it contains more electron and subsequently charged to several thousand volts.

Also, if the balloon losses electron, it becomes positively charged. In this case it contains more proton, which makes the balloon positively charged to several thousand volts.

However, amount of joules depends on volts produced in the balloon.

Volt = Joules/coulomb,

Joules = volts*coulomb

Because the charge of the particles (electron and proton) are small, amount of joules will always be small than volts.

So, it is possible to charge balloon to several thousand of volts and the balloon will store several Joules of energy