Polymers II

Question 1

What is the typical crystalline rate of metals?

Answer 1

Metals are typically 100% crystalline

Question 2

What is the range of crystallinity in polymers?

Answer 2

Polymers can be partially crystalline, ranging from entirely
amorphous (noncrystalline) to almost completely crystalline

Question 3

What is the relationship between crystallinity and polymer density? Why?

Answer 3

• For a given composition and molecular weight, increased crystallinity
  increases the density of a polymer due to increased packing

Question 4

What bonds form the polymer chain backbone of amorphous polymers?

Answer 4

– Strong covalent bonds form the polymer
chain backbone

Question 5

What holds the polymer strands of amorphous polymers?

Answer 5

Extensive van der Waals
interactions between chains hold the
polymer strands together, but with no
long-range order

Question 6

What are factors that inhibit crystal formation?

Answer 6

• Bulky side groups
• Branching
• Atacticity
• Random arrangement of mers in copolymers

Question 7

What is atacticity?

Answer 7

Random distribution of sidechain locations

Question 8

What does an increase in crystallinity lead to?:

Answer 8

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)

Question 9

How is the degree of polymerization determined?

Answer 9

By dividing the average
molecular weight by the
the molecular weight of a mer
unit.

Question 10

The mechanical behavior of polymers is highly sensitive to:

Answer 10

• rate of deformation (strain rate)
• temperature, even close to room temperature
• environment (organic solvents,
  water)

Question 11

What are the three types of general stress-strain
behaviour for polymers

Answer 11

1. Brittle
2. Plastic
3. Highly elastic

Question 12

What are the brittle and plastic behaviours of polymers similar to?

Answer 12

The brittle and plastic behaviors are similar to metals

Question 13

What is the highly elastic behaviour of polymers characteristic of and not characteristic of?

Answer 13

Highly elastic behavior is only characteristic of some polymers,
not metals or ceramics

Question 14

What is the impact of increasing the strain (deformation) rate on elastic modulus (stiffness), ductility, and ultimate tensile strength?

Answer 14

Increasing the strain (deformation) rate:
* Increases the elastic modulus (stiffness)
* Decreases ductility
* Increases the ultimate tensile strength

Question 15

What types of deformation are present during the deformation of semi-crystalline polymers?

Answer 15

Elastic and Plastic Deformation

Question 16

What are the steps of deforming semi-crystalline polymers under tension?

Answer 16

2. Elongation of
   amorphous tie chains
3. Tilting of lamellar
   chain folds
4. Separation of crystalline
   block segments
5. Orientation of block
   segments and tie chains
   with tensile axis

Question 17

Which steps of the deformation of semi-crystalline polymers are elastic deformation?

Answer 17

2. Elongation of
   amorphous tie chains
3. Tilting of lamellar
   chain folds

Question 18

Which steps of the deformation of semi-crystalline polymers are plastic deformation?

Answer 18

4. Separation of crystalline
   block segments
5. Orientation of block segments and tie
   chains with tensile axis

Question 19

What are the three types of polymers based on temperature and elasticity?

Answer 19

-Thermoset
-Thermoplastic
-Elastomers

Question 20

What are examples of thermoset polymers?

Answer 20

• Eg. epoxies, most polyurethanes, formaldehyde and polyesters, protein
  molecules

Question 21

What are examples of thermoplastic polymers?

Answer 21

Polyethylene, acrylic, ABS, nylon, and polystyrenes.

Question 22

What are examples of elastomers?

Answer 22

Natural rubber, polyurethanes, neoprene, silicones.

Question 23

How does a thermoset polymer respond to heat?

Answer 23

decomposes upon heating and does not become softer

Question 24

How does a thermoplastic respond to heating?

Answer 24

Becomes more pliable or moldable upon heating

Question 25

How are thermoset polymers formed?

Answer 25

By curing (i.e. hardening) a soft solid or viscous liquid material

Question 26

What is a thermoplastic?

Answer 26

Any resin which can be melted by heat and cooled without appreciable change in properties

Question 27

What can elastomers withstand at ambient temperatures?

Answer 27

Large elastic strains

Question 28

Are most elastomers categorized as thermoset or thermoplastic?

Answer 28

Most elastomers are also thermoset plastic (i.e. cannot heat and melt most elastomers).

Question 29

How can the stability of elastomers be improved?

Answer 29

May be lightly crosslinked for stability

Question 30

What happens during curing of a thermoset polymer?

Answer 30

The addition of catalysts and/or heat result in crosslinking of polymer chains, so that chains cannot freely flow or move

Question 31

What are polymer chains of thermoplastics held by and not interconnected by?

Answer 31

Polymer chains are not interconnected by covalent bonds, but held by weak secondary bonds

Question 32

What are the major characteristics of thermoset polymers?

Answer 32

* Strong and stiff * Can be used at higher temperature (although will decompose at high temperature) * More chemically inert

Question 33

What are the major characteristics of thermoplastic polymers?

Answer 33

* Less strong and stiff * Cannot be used at high temperatures (will melt instead of decompose) * Formable via injection molding, vacuum forming and extrusion

Question 34

What is the relative modulus of elasticity for elastomers?

Answer 34

The modulus of elasticity is small

Question 35

What are elastomers like in its unstressed state?

Answer 35

Elastomers are amorphous, and composed of molecular chains that are highly twisted, kinked and coiled

Question 36

What happens when a tensile load is applied to an elastomer?

Answer 36

There is partial uncoiling, untwisting and straightening of chains in the direction of stress.

Question 37

What happens upon the release of stress for an elastomer?

Answer 37

Entropy increase restores the chains original conformations

Question 38

How does deformation by drawing affect the mechanical properties of a polymer?

Answer 38

Deformation of a polymer via drawing enhances its mechanical properties by increasing alignment of and secondary interactions between polymer chains

Question 39

What happens prior to the deformation of semi-crystalline polymers under tension?

Answer 39

Adjacent chain-folded lamellae, interspaced by amorphous material