Chapter 15 Flashcards
What are the three types of elasticities of polymers?
- Brittle
- Plastic
- Elastic
What type of stress-strain behavior do brittle polymers exhibit?
Fractures during elastic deformation
What type of stress-strain behavior do plastic polymers exhibit?
Deformation is initially elastic followed by yielding and plastic deformation until eventual fracture
What type of stress-strain behavior do elastic polymers exhibit?
Completely elastic deformation, but still will eventually fracture
Where is the yield point on a stress-strain curve for a plastic polymer?
The local max of the curve where plastic deformation begins
What is tensile strength?
The amount of stress at which fracture occurs
What is yield strength?
The amount of stress at the onset of plastic deformation
What mechanism of deformation occurs with crosslinked and network polymers?
Slight elongation of polymer chains then brittle failure
What is the order of deformation in semicrystalline (plastic) polymers?
- Undeformed
- Elongation of amorphous regions
- Crystalline regions align (necking begins)
- Crystalline block segments separate
- Fibrillar structure forms
- Plastic failure
What is the purpose of pre-deformation by drawing a plastic material?
- Increases the elastic modulus (stiffness) and tensile strength in the stretching direction
- Decreases ductility
How does drawing affect the structure of a plastic material?
- Stretches the polymer before use
- Aligns chains in the stretching direction
How does annealing affect a drawn semicrystalline polymer?
- Decreases chain alignment
- Reduces elastic modulus and tensile strength in the stretching direction
- Increases ductility
What is the order of deformation in elastomers?
- Amorphous chains are kinked and cross-linked
- Chains are straighter, elongated, and cross-linked
- Failure
What is a thermoplastic?
- A polymer that melts into a soft, pliable form above a certain temperature and solidifies upon cooling
- Can be reshaped/melted an indefinite amount of times
- Often in pellets
What is a thermoset?
- A polymer that will remain in a permanent solid state after being cured (melted and cooled) one time
- Cannot be reshaped/melted
What are some characteristics of thermoplastics?
- Little crosslinking
- Ductile
- Soften with heating
Ex: polyethylene, polypropylene, polycarbonate, polystyrene
What are some characteristics of thermosets?
- Significant crosslinking (10-50% of units)
- Hard and brittle
- Do not melt/soften when heated, instead degrades
Ex: vulcanized rubber, epoxies, polyester/phenolic resin
How does increasing temperature or strain rate affect thermoplastics?
As temperature decreases or strain increases: the elastic modulus (stiffness) increases, tensile strength increases, and ductility decreases
How does chain stiffness affect melting and glass temperatures?
Increasing chain stiffness increases Tm and Tg
What increases chain stiffness?
- Bulky side groups
- Polar groups or side groups
- Chain double bonds and aromatic chain groups
What is the relaxation modulus?
The ratio between the time-dependent stress and a constant strain value
What is the stress relaxation test?
A material is strained in tension to a predetermined, low-strain level. The stress needed to maintain the strain decreases over time due to molecular relaxation.
How does the relaxation modulus relate to viscosity?
Low for rigid solids and high for viscous liquids
What happens during craze formation in a fracturing thermoplastic polymer?
- The plastic deformation of spherulites
- Formation of micro voids and fibrillar bridges
- Crack/fracture
