Week 8: Properties of Plastics/Polymers Flashcards
What material are relaxation of stresses typical for?
Polymers
Which type of deformation is irreversible in plastics/polymers?
Viscous flow
What model is used to describe the relaxation of stresses in thermoplastics?
Maxwell model
As average relaxation time increases, what happens to the stresses?
Begins to decrease slower
As temperature increases what happens to relaxation times?
They decrease.
This is because the molecules are bumping into each other more as they have more energy.
As temperature decreases what happens to relaxation times?
They increase
This is because the molecules are bumping into each less more as they have less energy.
What model is used to describe the creep in thermoplastics?
Voight-Kelvin Model
What model is able to measure both creep and relaxation time?
Maxwell + Voight-Kelvin Model
This is made up of a V-K model that has the dashpot and spring connected in parallel but also another spring connected to it.
Viscoelasticity
When there is a time dependency on the deformation, unlike in plastic deformation
How much relaxation time occurs in an elastic material?
Very little and is almost instantaneous. When we remove the strain on the object, it returns back to its original position.
What are advantages of using transparent polymeric materials for eyeglass lenses?
- More safe (not as brittle as glass)
- Lower density, so lighter
- Able to easily grind contours vs glass
- Filters more UV
What are disadvantages of using transparent polymeric materials for eyeglass lenses?
- Easily scratched, so not a hard material compared to glass
How does molecular weight affect modulus of elasticity in semi-crystalline materials?
With lower molecular weight, the polymers are shorter, so the ease of crystallization is greater.
E increases
How does degree of crystallinity affect modulus of elasticity in semi-crystalline materials?
E increases
How does deformation by drawing affect modulus of elasticity in semi-crystalline materials?
Plastically deforming leads to ordering of crystalline areas. (orientation)
E increases
