Withrow University High School May 2008 What is Cross-linking ? Withrow University High School May 2008

Cross-linking is a ______________ ___________ between two polymer chains. The _____ _________________ ____________ are linked together by a ____________.

Polymer Chains The “M” stands for________________. M M

Add cross-linking agent Before Cross-linking: After Cross-linking: Add cross-linking agent M M M M

The Cross-Linking Reaction Add cross-linking agent Before After

The Cross-Linking Reaction Add cross-linking agent After (Highly cross-linked and really stiff) Before (already cross-linked)

How does cross-linking affect elasticity? Low Optimum High Bounciness Elasticity Memory

How does cross-linking affect elasticity? Cross-linking improves the of the material up to a point. When polymer chains are cross-linked, they cannot slide away from each other when they are stretched. This is a . If there is cross-linking between the chains the ball loses its elasticity and therefore its bounce. This is a bad thing.

Polymers make the best elastics A polymer chain at rest M A force pulls and stretches the polymer chain. A force pulls and stretches the polymer chain M When the force is removed, the polymer chain goes back to its original shape. M

Elasticity makes the ball bouncy An elastic object is able to change shape and return to its original shape. Floor 4. The ball bounces back up. 2. The ball changes shape when it hits the floor. 3a. The ball returns to its original shape. 3b. The ball pushes off the ground 1. The ball drops