Polymers

Question 1

Which polymers are considered plastics, and which are considered rubbers?

Answer 1

Thermosets and Thermoplasts are plastics

Elastomers are rubbers

Question 2

Describe Thermoplastic Polymers

Answer 2

Melt to liquids at a few hundred degrees - easily shaped. Can be melted and cooled many times without significant effect.

Question 3

Describe Themosetting Polymers

Answer 3

Melt the first time, but after that they harden at high temp and degrade if reheated.

Question 4

Describe Elastomers

Answer 4

Extreme extensibility under low stress.

Question 5

What is the most common polymer produced?

Answer 5

Thermoplastics (70%)

Question 6

What are common examples of Thermoplasts?

Answer 6

Polyethylene
Polyvinylchloride
Polypropylene
Polystyrene
Nylon

Question 7

What are common examples of Thermosets?

Answer 7

Phenolics
Epoxies
some Polyesters

Question 8

Why are polymers significant?

Answer 8

Can make intricate shapes
Are relatively cheap
Use energy to process than metals

Question 9

What are some general properties of polymers?

Answer 9

Low density
Good strenght-to-weight ratio often
High corrosion resistance
Low electrical and thermal conductivity
Low strength and stiffness
Can only work at low temps
Sometimes degrade in sunlight and radiation

Question 10

What is the structure of polyethylene (PE)

Answer 10

C-C

Question 11

What is the structure of Polyvinylchloride (PVC)

Answer 11

C - C
|
Cl

Question 12

What is the structure of Polytetrafluoroethylene (PTFE)

Answer 12

F    F
 |      |
C  -  C
 |      |
 F     F

Question 13

What is the structure of polypropylene (PP)

Answer 13

C - C
|
C

Question 14

What is the structure of polystyrene (PS)

Answer 14

C - C
|
(ring)

Question 15

What is the structure of polymethylmethacrylate (PMMA)

Answer 15

C
        |
C  -  C
        |
        C - O - C
        ||
        O

Question 16

Describe Addition Polymerization

Answer 16

Starts with a C = C bond, then a catalyst opens the double bond so that mers can start joining together from either end.
Has only the monomers and polymer present during reaction.

Question 17

Describe Step Polymerization

Answer 17

Two mers come together to form a new molecule, then those molecule join. Chains form and join together.
Adds functional groups, and sometimes has byproducts.

Question 18

What is the degree of polymerization? What does it mean? What process creates a higher DP?

Answer 18

n
The average number of mers per chain.
Higher n increases strength and viscosity as a fluid, making manufacturing harder.
Is higher in addition than step polymerization.

Question 19

What is the structure of Nylon6,6

Answer 19

-N-C-C-C-C-C-C-N-C-C-C-C-C-C-
|| ||
O O

Question 20

Define Molecular Weight. How do you calculate molecular weight of a polymer?

Answer 20

M : mass of a mole of chains

MW = n(molar weight of each mer)

Question 21

What polymer types are linear? Branched? Cross-linked?

Answer 21

Linear - TP
Branched - TP
CrossLinked - TS, Elastomers
TS are highly cross-linked, elastomers are loosely cross-linked

Question 22

What is crystallization in polymers?

Answer 22

When long polymer chains fold back on themselves.

Question 23

Can all polymers form crystals?

Answer 23

Nope. And those that can never have 100% crystallinity.

Question 24

How can you increase %Crystallinity in a polymer?

Answer 24

Heat treat
Cool slowly
Mechanical deformation

Plasticizers decrease %CR

Question 25

What is a spherulite structure?

Answer 25

Regularly alternating crystalline and amorphous regions in a polymer.

Question 26

How does %Crystallinity affect density, strength, heat resistance, and transparency?

Answer 26

As %CR increases,
Density, strength, and heat resistance increase
Material becomes more opaque if previously transparent.

Question 27

What do additives do to a polymer?

Answer 27

Alter the molecular structure

or Add a second phase, making it a composite

Question 28

Describe the physical characteristics of TSs. What causes their cross-linking?

Answer 28

Cross-linked structure that occurs from curing
From temp, catalysts, or mixing
Always amorphous
Brittle

Question 29

What can elastomers stretch?

Answer 29

Loose cross-linking

‘Kinked’ structure that can ‘uncoil’.

Question 30

Define vulcanization

Answer 30

The curing of natural rubbers (and some synthetics)

Question 31

Where does natural rubber come from? What are it’s properties?

Answer 31

Latex (plant milk) - polyisoprene
High strength among rubbers
High resilience
Resistance to wear and fatigue
Degrades easily in heat, oil, and oxygen

Question 32

What is synthetic rubber made from?

Answer 32

Petroleum

Question 33

Describe the ductility of crosslinked and networked polymers.
How about semicrystalline polymers?

Answer 33

Crosslinked and networked - almost no ductility (brittle failure)
Semicrystalline - amorphous region elongates until crystalline regions align (yeild), then crystalline regions seperate into blocks.

Question 34

Describe drawing of polymers.

Answer 34

Stretches polymer to align crystalline regions. It increases E, TS, and decreases %EL.
Annealing reverses drawing.

Question 35

How does strain rate effect mechanical properties?

Answer 35

Increasing Strain Rate increases E, TS, and decreases %EL.

Question 36

How can you change chain stiffness? What does chain stiffness effect?

Answer 36

increased by:
bulky sidegroups
polar groups
double bonds / aromatic groups

Increasing chain stiffness increases Tm and Tg.

Question 37

Define viscoelastic deformation

Answer 37

A material that is elastic at low temps and viscous at high temps. This creates a non-linear strain curve.

Question 38

Define Time Dependant Deformation

Answer 38

When stress releases over time, but strain remains constant. Er is the modulus of relation.
Er = stress(time) / strain(constant)
Er goes down as temp goes up.

Question 39

Define Crazing

Answer 39

Defect formation prior to cracking. Spherulites pull apart and create voids and bridges.

Question 40

What is Crystallization Temperature? What happens?

Answer 40

It’s the temperature at which the structure of a polymer becomes more ordered, and therefore the polymer gives off heat.