Chapter 11: Polymers and Composites

Question 1

Polymers

Answer 1

Entangled long chain molecules (molecular spaghetti) with a high molecular weight (MW) that simulate the large molecules of nature that occur in plants and animals.

Question 2

Polymer generalizations (3)

Answer 2

1. Low density (1g/cm^3), float on water
2. Easy to process
3. Corrosion resistant

Question 3

Mers

Answer 3

Analogous to a unit cell in a crystal, the smallest indivisible unit of a molecule that retains the structure and that can be repeated ad infinitum to produce a larger molecule. Many thousands of molecules (chains), of varying length (different number of mers) are bound together to form a bulk polymeric material.

Question 4

Thermoplasts (thermoplastic polymers)

Answer 4

The largest class of polymers with a structure consisting of long covalently bonded chain molecules held together with secondary bonds. Because these bonds are weak, the materials melt at low temperatures. They are linear or branched polymers.

Question 5

Thermoplasts examples (3)

Answer 5

polyethylene (PE), disposable grocery bags, polyvinylchloride (PVC)

Question 6

Thermosets

Answer 6

Form primary bonds between the molecules so the whole structure is held together by a 3D network of primary covalent bonds. Once these polymers are set they cannot be melted, they are stronger and more stable than thermoplasts, but cannot be formed after setting and cannot be recycled. Network polymers that start out as branched mer units which are then crosslinked.

Question 7

Thermosets example

Answer 7

Two part epoxy (a resin and a hardener). When mixed the hardener initiates the polymerization reaction to form a high density of crosslinks, or primary bonds.

Question 8

Elastomers

Answer 8

Polymers that can elongate more than 100% and will recover their original length quickly and completely. Molecules are tacked together with primary bonds (a crosslink) and can uncoil on application of load but not slide permanently past one another. When the load is removed the molecules recoil to their low energy configuration. These polymers have very low stiffness.

Question 9

Elastomers examples (3)

Answer 9

Elastic bands, hoses, conveyer belts

Question 10

Molecular weight (MW) formula

Answer 10

MW = nM
n = degree of polymerization
M = molar mass

Question 11

Molecular weight and strength

Answer 11

As the degree of polymerization increases, the molecular weight, tensile strength, stiffness and melting point increase; and the processibility decreases. This is caused by primarily chain entanglement, long chains interact more and become entangled, making it hard for the chains to move past one another when stressed.

Question 12

Glass point (Tg)

Answer 12

Characteristic temperature of polymers below which molecular rearrangement is not possible.

Question 13

Vulcanization

Answer 13

Partial crosslinking of polymers to form elastomers.

Question 14

Viscoelasticity

Answer 14

The uncoiling and recoiling of polymers under load. There are no dislocations.

Question 15

Effects of temperature on polymers

Answer 15

Below Tg, polymers are brittle, above Tg, polymers are viscoelastic.

Question 16

Effects of time on polymers

Answer 16

As loading time increases, Tg decreases.

Question 17

Crystallinity of thermoplasts

Answer 17

Some Thermoplast molecules are able to arrange their atoms so that a high percentage of regular, crystalline packing is achieved. Crystallinity is not complete as there are still amorphous regions.

Question 18

Crystalline polymer packings

Answer 18

Higher density, strength, stiffness, melting point than amorphous regions. Scatter light at the interface making them more opaque.

Question 18

Crystallization of polymers and side groups

Answer 18

The ability to crystallize depends on the regularity and size of the side groups, and is ideal with small, regular side groups so the chains can be arranged with regular secondary bonds. Polymers with bulky side groups can be crystalline if the side groups are arranged in a regular pattern.

Question 18

Crystallinity of polymers and temperature

Answer 18

Crystallinity is enhanced in crystallizable polymers by the time spent at temperatures where “spherulites” form spontaneously (at temperatures below the melting point and above the glass point). It is enhanced by hot working strongly in one direction (e.g. extruding through a die).

Question 19

Glass-rubber transition

Answer 19

For a semi-crystalline thermoplastic, only the amorphous phase experiences a glass-rubber transition, the crystalline phase melts.

Question 20

Relaxation modulus formula

Answer 20

Er(t) = σ(t)/ε

Question 21

Polymers compared to metals

Answer 21

Polymers are less stiff/strong, higher percentage of amorphous structure, decreasing temperature in a polymer (rubbery to glassy) is equivalent to increasing strain in a metal.

Question 22

Composites

Answer 22

Mechanically mixed materials

Question 23

Components of a composite (2)

Answer 23

Matrix and reinforcing component

Question 24

Alloys

Answer 24

Formed from melting/chemically combining elements.

Question 25

Types of particle-reinforced composites (2)

Answer 25

Dispersion strengthened and large-particle

Question 26

Dispersion strengthened composites

Answer 26

Tiny particles, usually 10-100 nm. Similar to precipitation strengthening.

Question 27

Large-particle composites

Answer 27

Particles are too big to interact with dislocations, and uses continuum mechanics to model them instead (EX: Concrete).

Question 28

Dispersion strengthening

Answer 28

Better high temperature stability than precipitates due to no coarsening or dissolution, however, it is expensive,

Question 29

Portland cement

Answer 29

Mixed materials containing clay and limestone that needs to be bonded together with the right amount of water.

Question 30

Components of Concrete (2)

Answer 30

Portland cement and aggregate

Question 31

Calcine

Answer 31

Portland cement heated to 1400°C
CaCO3 (s) → CaO(s) + CO2 (g)

Question 32

Primary constituents of Portland cement (2)

Answer 32

Tricalcium silicate (3CaO-SiO2 ) and dicalcium silicate (2CaO-SiO2 ).

Question 33

Hydration of Portland cement

Answer 33

2Cao-SiO2 + xH2O = 2CaO-SiO2-xH2O

Question 34

Reinforced concrete

Answer 34

Concrete composite made by adding steel rebar to increase tensile and bending strength to prevent failure.

Question 35

Rule of mixtures lower limit

Answer 35

Emix = E1V1+E2V2
Equivalent to resistors in series. kmix = V1k1+V2k2

Question 36

Rule of mixtures upper limit

Answer 36

E1E2/(E2V1+E1V2)
Equivalent to resistors in parallel. kmix = 1/(V1/k1 + V2/k2)

Question 37

Critical fiber length formula

Answer 37

Ic = σd/2𝜏
d = fiber diameter
If l»lc, fibers are continuous. If l<lc, fibers are discontinuous.

Question 38

Influences of fiber strength

Answer 38

1. Length, short fibers can’t support applied loads.
2. Orientation, fibers parallel to the longitudinal direction have a higher failure strength.

Question 39

Sandwich composites

Answer 39

Lightweight and strong

Question 40

Creation of carbon-carbon composites

Answer 40

1. Embed carbon fiber in a polymer such as a phenolic or epoxy
2. Pyrolyze the matrix: Convert it to carbon by heating in an inert atmosphere
3. Heat treat to densify