Polymers II Flashcards
What is the typical crystalline rate of metals?
Metals are typically 100% crystalline
What is the range of crystallinity in polymers?
Polymers can be partially crystalline, ranging from entirely
amorphous (noncrystalline) to almost completely crystalline
What is the relationship between crystallinity and polymer density? Why?
- For a given composition and molecular weight, increased crystallinity
increases the density of a polymer due to increased packing
What bonds form the polymer chain backbone of amorphous polymers?
– Strong covalent bonds form the polymer
chain backbone
What holds the polymer strands of amorphous polymers?
Extensive van der Waals
interactions between chains hold the
polymer strands together, but with no
long-range order
What are factors that inhibit crystal formation?
- Bulky side groups
- Branching
- Atacticity
- Random arrangement of mers in copolymers
What is atacticity?
Random distribution of sidechain locations
What does an increase in crystallinity lead to?:
A greater degree of secondary interactions, which increases:
* Melting temperature
* Strength
* Stiffness
* Density (ie. Greater packing of polymer chains)
* Resistance to softening (polymer can be used at higher temperatures)
How is the degree of polymerization determined?
By dividing the average
molecular weight by the
the molecular weight of a mer
unit.
The mechanical behavior of polymers is highly sensitive to:
- rate of deformation (strain rate)
- temperature, even close to room temperature
- environment (organic solvents,
water)
What are the three types of general stress-strain
behaviour for polymers
- Brittle
- Plastic
- Highly elastic
What are the brittle and plastic behaviours of polymers similar to?
The brittle and plastic behaviors are similar to metals
What is the highly elastic behaviour of polymers characteristic of and not characteristic of?
Highly elastic behavior is only characteristic of some polymers,
not metals or ceramics
What is the impact of increasing the strain (deformation) rate on elastic modulus (stiffness), ductility, and ultimate tensile strength?
Increasing the strain (deformation) rate:
* Increases the elastic modulus (stiffness)
* Decreases ductility
* Increases the ultimate tensile strength
What types of deformation are present during the deformation of semi-crystalline polymers?
Elastic and Plastic Deformation
What are the steps of deforming semi-crystalline polymers under tension?
- Elongation of
amorphous tie chains - Tilting of lamellar
chain folds - Separation of crystalline
block segments - Orientation of block
segments and tie chains
with tensile axis
Which steps of the deformation of semi-crystalline polymers are elastic deformation?
- Elongation of
amorphous tie chains - Tilting of lamellar
chain folds
Which steps of the deformation of semi-crystalline polymers are plastic deformation?
- Separation of crystalline
block segments - Orientation of block segments and tie
chains with tensile axis
What are the three types of polymers based on temperature and elasticity?
-Thermoset
-Thermoplastic
-Elastomers
What are examples of thermoset polymers?
- Eg. epoxies, most polyurethanes, formaldehyde and polyesters, protein
molecules
What are examples of thermoplastic polymers?
Polyethylene, acrylic, ABS, nylon, and polystyrenes.
What are examples of elastomers?
Natural rubber, polyurethanes, neoprene, silicones.
How does a thermoset polymer respond to heat?
decomposes upon heating and does not become softer
How does a thermoplastic respond to heating?
Becomes more pliable or moldable upon heating
How are thermoset polymers formed?
By curing (i.e. hardening) a soft solid or viscous
liquid material
What is a thermoplastic?
Any resin which can be melted by heat and cooled without appreciable change
in properties
What can elastomers withstand at ambient temperatures?
Large elastic strains
Are most elastomers categorized as thermoset or thermoplastic?
Most elastomers are also thermoset plastic (i.e. cannot heat and melt most
elastomers).
How can the stability of elastomers be improved?
May be lightly crosslinked for stability
What happens during curing of a thermoset polymer?
The addition of
catalysts and/or heat result in
crosslinking of polymer chains,
so that chains cannot freely
flow or move
What are polymer chains of thermoplastics held by and not interconnected by?
Polymer chains are not
interconnected by covalent
bonds, but held by weak
secondary bonds
What are the major characteristics of thermoset polymers?
- Strong and stiff
- Can be used at higher
temperature (although will
decompose at high temperature) - More chemically inert
What are the major characteristics of thermoplastic polymers?
- Less strong and stiff
- Cannot be used at high
temperatures (will melt instead of
decompose) - Formable via injection molding,
vacuum forming and extrusion
What is the relative modulus of elasticity for elastomers?
The modulus of elasticity is small
What are elastomers like in its unstressed state?
Elastomers
are amorphous, and composed of
molecular chains that are highly
twisted, kinked and coiled
What happens when a tensile load is applied to an elastomer?
There is partial uncoiling,
untwisting and straightening of
chains in the direction of stress.
What happens upon the release of stress for an elastomer?
Entropy
increase restores the chains
original conformations
How does deformation by drawing affect the mechanical properties of a polymer?
Deformation of a polymer via drawing enhances its mechanical properties by increasing alignment of and secondary interactions between polymer chains
What happens prior to the deformation of semi-crystalline polymers under tension?
Adjacent chain-folded
lamellae, interspaced by
amorphous material
