Lecture 6 - Mechanical Properties of Materials

Question 1

Below Tg, why do amorphous solids not undergo plastic deformtion?

Answer 1

No crystalline structure = no dislocations = no plastic deformation

Question 2

What are polymers?

Answer 2

Materials made up of many repeating units.

Question 3

What is the molecular weight of a polymer?

Answer 3

The mass of one mole of polymer chains.

Question 4

What is the difference between weight-average and number-average of a polymer?

Answer 4

The weight-average is calculated by summing the products of each molecular weight (Mi) and the number of molecules (Ni) with that molecular weight, then dividing by the total weight of the sample. Mw = Σ(Mi * Ni) / w_total.

The number-average is calculated by dividing the product by the total number of molecules (N_total). Mn = Σ(Mi * Ni) / Σ(N_total)

Question 5

What is the degree of polymerization (n)?

Answer 5

The number of repeats within a polymer chain

Question 6

What is a copolymer?

Answer 6

Polymers that have more than one repeating pattern within the chain.

Question 7

What are the four types of copolymers?

Answer 7

a) Random
b) Alternating
c) Block
d) Graft

Question 8

What are the stereoisomer polymers?

Answer 8

Isotactic, Syndiotactic, and Atactic

Question 9

What is an isotactic polymer?

Answer 9

A polymer with all side chains on the same side of the molecule.

Question 10

What is a syndiotactic polymer?

Answer 10

A polymer with all side chains on alternating sides of the molecule.

Question 11

What is an atactic polymer?

Answer 11

A polymer with random position of side chains on the molecule.

Question 12

What are the geometric isomer polymers?

Answer 12

cis and trans

Question 13

What is a cis polymer?

Answer 13

A polymer with the R group and hydrogen on the same side.

Question 14

What is a trans polymer?

Answer 14

A polymer with the R group and hydrogen on opposite sides.

Question 15

what is a spherulite?

Answer 15

A type of crystalline structure found within polymers.

Question 16

How does temperature affect the types of bonds within a polymer?

Answer 16

At T>Tg, the polymer is only held together with covalent bonds and the structure is amorphous

At T<Tg, Van Der Waals bonds hold the polymer together more densely and increase density and decrease mobility.

Question 17

What is a thermoplastic?

Answer 17

A polymer that becomes pliable or moldable when heated and solidifies upon cooling, retaining its shape

ex: polypropylene, polystyrene, polyethylene

Question 18

What is a thermosetting polymer?

Answer 18

A polymer that becomes hard and brittle at increased temperature (opposite for thermoplastic) and does not melt.

ex: epoxy resins, polyesters, polyurethane…

Question 19

When subject to a strain force, how do polymers react?

Answer 19

Viscoelastic(ally): they exhibit both flow-like (viscous) and recovery-like (elastic) properties when subject to a deformation