Overview/Checklist

Overview

Our last lesson of this course is focused on the mechanical properties of polymers. While we will be largely discussing the macroscale properties of polymers, always be mindful of how those bulk properties are actually derived from the molecular scale structure and chemistry of the polymer. In order to motivate how important it is to understand the mechanical properties of polymers and how that relates to molecular scale polymer structure and dynamics, consider the tragic accident of the Challenger explosion. Polymeric O-ring seals, which are flexible and elastic, were used in the space shuttle at joints where it was necessary to contain and compartmentalize explosive propellant. The day of the shuttle launch, it was quite cold, with temperatures dropping below the glass transition temperature of the polymer O-rings. The low temperatures caused the O-rings to become brittle, non-elastic, and glassy, as we well know should happen when a polymer is below $T_g$. Thus, the O-rings could not form a tight seal, and upon launch, the joints failed and the Challenger exploded. This accident highlights how critical it is for you – the future scientists and engineers – to learn the fundamental chemistry and mechanics of the materials you work with. For more information, read the O-ring Concerns section of the Wikipedia Space Shuttle Challenger Disaster page

Polymers are a unique class of materials in terms of their mechanical properties because they may possess some characteristics of a fluid and some characteristics of a solid. While ideal elastic solid materials store all the energy from stress in the bonds, so that the material will restore itself upon release of stress, ideal viscous fluids dissipate all the stress in flow. Polymers, as we will see, tend to be somewhere in the middle. We have a special term to describe this combination of fluid and solid properties: viscoelasticity.

Figure 12.1: Viscoelastic properties

Source: Lauren Zarzar

Learning Outcomes

By the end of this lesson, you should be able to:

• Describe the stress-strain characteristics of elastomers, glassy polymers, semi-crystalline polymers.
• Correlate chemical structure with mechanical properties.
• Explain the changes in entropy and enthalpy that occur during elastomer deformation.
• describe strain induced crystallization and hysteresis.
• Draw time dependent strain plots for viscoelastic material.
• Analyze modulus vs time plots and identify glass region, glass transition, rubbery plateau.
• Compare mechanical properties for low and high molecular weight polymers; crosslinked and non-crosslinked polymers.
• Describe how mechanical properties of polymers depend on temperature.
• Define shear thinning and shear thickening polymers.
• Describe how molecular weight affects viscosity.

Lesson Checklist

Please refer to the Canvas Calendar for specific timeframes.

Questions?

If you have questions, please feel free to post them to the General Questions and Discussion forum. While you are there, feel free to post your own responses if you, too, are able to help a classmate.