Polymers

Question 1

Polymers

Answer 1

The basic building blocks of metals and ceramics are single atoms or clusters of a few atoms. The basic building blocks of polymers are long chain molecules that can contain hundreds of thousands of atoms.

Question 2

Polymers Bulk Form Properties

Answer 2

Lowest density of any class of materials

lowest melting temperature of any class of materials

highest corrosion resistance of any class of materials

Question 3

Polymers Fiber form

Answer 3

Polymers have strength and stiffness competitive with metals, especially specific strength and stiffness

Question 4

Polymer Applications

Answer 4

Typical polymer applications are intended to make use of one or more of their three main advantages: extreme ease of forming and machining because of low strength, superb corrosion resistance, and extreme light weight

Question 5

Polymer Chemistry

Answer 5

chains of carbon atoms covalently bonded to each other, with hydrogen atoms, other atoms, or side groups of atoms attached to each carbon

chains can be covalently “cross-linked” to each other or not. If they are cross-linked, they constitute the groups of polymers called thermosets and elastomers. If they are not cross-linked, the only thing holding the chains together are weak Van der Waals forces, and the polymers are thermoplastic

Question 6

Thermosets

Answer 6

Moderately to heavily cross-linked, operating below their glass transition temperature Tg. Always amorphous.

Once set (by the first application of heat or by a chemical hardener), they cannot be remelted. Therefore they are not recyclable.

Elastic/brittle mechanical behavior (no plastic deformation). Thermosets have the highest strength and stiffness of any bulk polymer (i.e. not counting fiber forms).

Question 7

Elastomers

Answer 7

Lightly to moderately cross-linked, operating above Tg. Always amorphous.

Cross-links often created by a process called “vulcanization.” The more heavily crosslink, the stiffer, harder and less flexible the elastomer. Cannot be recycled.

Viscoelastic mechanical behavior (no plastic deformation)

Examples: natural rubber, polybutadiene

Question 8

Thermoplastics

Answer 8

Linear or branched chains with no cross-links. Can be amorphous or partially crystalline. The greater the degree of crystallinity, the higher the density, stiffness and strength.

Can be melted repeatedly, therefore recyclable

Elastic/plastic mechanical behavior. Can be deformed plastically.

On stretching in tension, chains align and become vastly stronger than the bulk material, resulting in a stable neck (as opposed to the unstable necking and fracture in metals)

Question 9

Isotactic

Answer 9

all side groups on same side

Question 10

Syndiotactic

Answer 10

side groups alternate sides, back and forth

Question 11

Atactic

Answer 11

random arrangement of side groups (not likely to crystallize)