Chapters 14 & 15: Study Guide and Self–Assessment
The utility of a polymer for a particular application is determined by its mechanical properties, while its ability to be processed from the melt (the preferred method) is determined by its rheological properties. As you will see in this chapter, these are related, because polymers display what are called viscoelastic properties to a much greater extent than most other materials. As background, we start with the basic laws used to describe linear behavior (Hooke’s Law for elastic properties and Newton’s law for the viscosity of simple fluids) and then consider how real behavior deviates from this, maintaining as far as possible a molecular perspective. The elastic properties of polymer materials and polymer matrix composites are considered first, followed by the rheological properties of polymer melts culminating in a discussion of viscoelasticity.
Objectives
Upon successfully completing this chapter you should be able to:
- Understand the concepts of stress, strain, modulus and viscosity and have a broad feel for the strength, stiffness and toughness of polymers relative to other materials.
- Sketch the form of the stress-strain plots obtained from various general classes of polymers (elastomers, glassy polymers, etc.).
- Understand yielding phenomena, crazing and modes of failure in polymer materials.
- Understand how the elastic properties, toughness and modes of failure of polymers are affected by the inclusion of particles and fibers to form composites.
- Appreciate the complex dependence of mechanical properties of polymer matrix composites on the nature of fiber and particle fillers, their aspect ratios ( e.g., fiber length), interfacial interactions, etc.
- Describe the theory of rubber elasticity and know the limitations imposed by the assumption that the chains display Gaussian behavior.
- Describe the basic properties of polymer melts, the concept of reptation, the dependence melt viscosity on molecular weight and phenomena like jet swelling and melt fracture.
- Understand the nature of viscoelasticity, particularly creep and stress relaxation and how these properties can be described using simple mechanical models.
- Describe the time-temperature superposition principle and the WLF equation.
Self-Assessment Questions