A set of up to 130 kits for distribution to students contains samples of high-density polyethylene (HDPE) and low density polyethylene (LDPE) so that students can compare properties such as tensile strength. 

Curriculum Notes 

This demo is useful for comparing the structures of polymers to their physical properties. 

One day of lead time is required for this project.

Discussion 

The polymer chains of LDPE are highly branched compared to HDPE. This branching prevents the chains from stacking neatly beside each other, reducing the intermolecular forces of attraction. This results in a plastic that is softer and more flexible, but which also has lower tensile strength. The polymer chains in HDPE, on the other hand, are more linear. They pack closer together, resulting in greater intermolecular forces and a more "crystalline" structure. HDPE has greater tensile strength than LDPE. 

Materials 

Each kit consists of a LDPE bag (Baggie sandwich bag) and a piece of a HDPE bottle (soft plastic milk jug) packaged inside of a HDPE plastic shopping bag. 

Procedure 

Kits are passed out to the students and they investigate the strength and flexibility of the samples.