Some
elastic polymers heat up when they are stretched (and cool down again when they
are allowed to relax back to their original position). These temperature
changes can be demonstrated by stretching thin polymer strips such as rubber
bands, deflated balloons and even plastic grocery bags. These temperature
changes can be detected with thermometers that can monitor the temperature of
surfaces or by placing the polymer strip against your upper lip - a temperature-sensitive
body part.
The
origin of this thermal behavior lies within the attractions between the
chainlike molecules of the polymer. In an unstretched polymer, the
molecular chains are tangled like a plate of spaghetti. In this tangled
arrangement the molecules do have optimum contact (or optimum attraction)
between the chains. When the polymer is stretched, the molecular chains
are pulled into alignment in the direction of stretching and have greater
contact (and greater attraction) with each other. When attractive forces
are satisfied, energy is released. In this case the energy is released in
the form of heat. When the polymer is released and the chains move back
to their original position against their attractive forces, energy is consumed
and the polymer cools down.
This
polymer strip behavior can also be explained using more in-depth thermodynamic
concepts such as enthalpy (H), temperature (T), entropy (S), and Gibbs free
energy (G). These concepts can be connected
by the mathematical relationship that states that the change in Gibbs energy (G)
for a process is equal to its change in enthalpy (H) minus the absolute
temperature (T) multiplied by the change in entropy (S). Stretching the strip is nonspontaneous, with
a positive value of for its change in free energy, and its change in enthalpy
must be negative, since the strip heats up and indicates an exothermic process. Mathematically, its change in entropy must be
negative. This happens because as the strip
is stretched and the molecular chains are pulled into alignment, the disorder
within the plastic strip decreases.
Releasing the strip and allowing it to contract to its original shape is
spontaneous with a negative change in free energy, and the change in enthalpy
must be positive, since the strip cools down as it contracts. Therefore the change in entropy must be
positive. This happens because as the
strip relaxes and the molecular chains are moved out of alignment, the disorder
within the elastic strip increases.
The pictures show a non-contact thermometer measuring the temperature of a relaxed and stretched deflated balloon. Sorry about the rotated picture - there is a glitch in my software.
