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
What are the two types of polymerization?
- Addition/chain polymerization
- Condensation/step polymerization
What are the steps of addition/chain polymerization?
- Initiation: a free radical is added to a monomer, opening up the double bonds
- Propagation: more repeating units are added with single bonds
- Termination: Polymerixation stops through combination (combining with another chain) or disproportionation (free radical moves)
What is condensation/step polymerization?
A form of step-growth polymerization where smaller molecules or monomers react with each other to form larger structural units (usually polymers) while releasing by-products.
What are the different types of polymer additives?
- Fillers
- Plasticizers
- Stabilizers
- Lubricants
- Colorants
- Flame retardants
What do fillers do?
- Improve tensile strength, abrasion resistance, toughness, and decrease cost
Ex: carbon black, silica gel, wood flour, glass, limestone, talc
What do plasticizers do?
- Reduce the glass transition temperature
- Turns brittle polymers into ductile polymers
Ex: commonly added to PVC to increase ductility
What do stabilizers do?
- Counteract deterioration processes
Ex: antioxidants, UV protectants
What do lubricants do?
- Reduce friction for easier processing (in dies)
Ex: sodium stearate
What do colorants do?
- Add color to materials
Ex: dyes and pigments
What do flame retardants do?
- Increase flammability resistance
Ex: substances with chlorine, fluorine, and boron
What are the different types of polymer processing techniques?
- Compression
- Injection
- Extrusion
- Blown-film
What is compression molding?
- Used for thermoplastics and thermosets
1. Polymer and additives are placed in a mold
2. Mold is heated and hydraulic pressure is applied
3. Polymer assumes the shape of the mold
What is injection molding?
- Used for thermoplastics and some thermosets
1. A ram forces pellets into a heating chamber
2. Pellets melt while moving around a spreader
3. Molten plastic is injected into a mold where it cools and takes the shape of the mold
What is extrusion?
- Used for thermoplastics
1. Plastic pellets drop from a hopper onto a screw
2. Pellets melt as the screw pushes them forward
3. Molten plastic is extruded through a die to form the extrudate
What is blown-film extrusion?
- Plastic pellets drop from a hopper onto a screw
- Pellets melt as the screw pushes them forward
- Molten plastic is extruded through a tubing die
- Air is blown through the die forming an air bubble
- Bubbled polymer is collected into pinch rolls
- Film roll is formed
What are the different types of polymers?
- Fibers
- Coatings
- Adhesives
- Films
- Foams
What are fibers used for?
Primarily textiles
What are the characteristics of polymers?
- High tensile strengths
- High degrees of crystallinity
- Structures containing polar groups
How are fibers formed?
- Formed by spinning
1. Extrude polymer through a spinneret
2. Spun fibers are drawn under tension
3. Results in highly aligned chains/fibrillar structure
What are coatings?
Thin polymer films applied to surfaces
- Protect against corrosion/degradation
- Decorative
- Electrical insulation
What are adhesives?
Bonds two solid materials (adherends) using either van de Walls forces (secondary bonding) or penetration into pores/crevices (mechanical)
What are foams?
A polymer that has been made porous/spongelike through the incorporation of gas bubbles
What are films?
Thin polymer films produced by blown-film extrusion
What are the characteristics of ultra-high-molecular-weight polyethylene?
- High molecular weight
- High impact strength
- Resistance to wear and abrasion
- Low coefficient of friction
- Self-lubricating surface
- Used in bullet-proof vests, golf ball covers, and joint implants
