Download Happy/Sad Balls: Effect of Temperature on Polymer Properties

Happy/Sad Balls: Effect of Temperature on Polymer Properties
Desription: This activity visually demonstrates how temperature affects the properties
of crosslinked polymers (rubbery materials).
Objectives:
(1) Students will learn how polymers can be engineered to give different properties
(bouncy or not bouncy termed as elastic and inelastic).
(2) Relate visual demonstrations to real world examples of engineered rubbery
materials (i.e. car tires).
Materials: Happy/sad balls (in the case of this demonstration, we used identical thiolene network balls, one bounces and one doesn’t bounce), microwave, freezer.
Procedure:
Engage – The students pass around thiol-ene balls to feel and make observations (one is
glassy and one is rubbery).
Explore – Ask the students what will happen when the ball is dropped on the floor. After
they make a hypothesis, drop the balls.
Explain – Why does one ball bounce and one doesn’t? This is an explanation of glassy
and rubbery states of crosslinked materials. The harder ball is in a glassy state where
there is little motion of the molecules (frozen in), so they are unable to absorb the force of
the drop (almost all of the energy is returned as it bounces close to its original height).
The softer ball is in a transition region between a glass and a rubber-like material at room
temperature. In this state, the molecules are just starting to wiggle around but want more
motion. The force from dropping the ball gets absorbed resulting in energy being
converted into molecular motion.
Elaborate – Microwave the rubbery ball for ~ 1 minute. Now, the ball bounces because
the molecules have ample molecular motions and can no longer absorb energy from a
bounce. Therefore, it exhibits a rubbery-elastic behavior. Furthermore, the ball can be
frozen before class the next and once again it bounces (elastic behavior). Here, the
molecules exhibit little to no motion below the glass transition temperature resulting in
all the energy being returned as a bounce when the ball is dropped.
Evaluate – Informal questions asking students why they think the balls behave the way
they do.