CHEE 317 Test 3

What does a phase diagram show?

Phases present in a material under different temperature, pressure, and composition conditions.

Why do solid solution alloys increase strength?

They strengthen the material by changing the microstructure.

What does annealing do to ductility?

Annealing increases ductility.

What is a phase diagram?

A graphical representation of temperature versus composition.

What is a phase?

A physically distinct, chemically homogeneous, mechanically separable portion of a system with unique properties.

What is the triple point?

The point where three different phases coexist.

What is Gibbs phase rule used for?

It describes system flexibility at equilibrium.

What does Gibbs phase rule tell us?

How many variables can change independently without changing the number of phases present.

What is the Gibbs phase rule equation?

F = C - P + 1.

What does F stand for in Gibbs phase rule?

Degree of freedom.

What does C stand for in Gibbs phase rule?

Number of components.

What does P stand for in Gibbs phase rule?

Number of phases.

For a pure substance, what are C, P, and F?

C = 1, P = 1, F = 1.

For a pure substance with 2 phases, what are C, P, and F?

C = 1, P = 2, F = 0.

For a binary alloy, what are C, P, and F?

C = 2, P = 2, F = 1.

What is a cooling curve?

A temperature vs. time plot used to determine phase transition temperature.

What is a binary isomorphous phase diagram?

A binary system with complete solid solubility and the same crystal state.

What is the lever rule used for?

To determine composition and mass fraction of phases in an alloy.

What is a tie line?

A horizontal line that connects solid and liquid phase compositions.

What does rapid cooling do in alloy solidification?

It delays solidification.

What is a polymer?

A material made of many repeat units called monomers.

What does "poly" mean?

Many.

What does "mer" mean?

Repeat unit.

What are monomers?

Small organic molecules with active bonds.

Why are polymers used?

They are flexible, low cost, lightweight, insulating, and tunable.

What are thermoplastics?

Polymers that soften when heated and can be remelted and reused.

What are thermosets?

Polymers that cure permanently through extensive cross-linking.

What are elastomers?

Lightly cross-linked polymers that are highly stretchable.

What is the monomer of polyethylene?

Ethylene.

How is polyethylene formed?

By chain polymerization.

What is functionality in polymers?

The number of active bonds on a monomer.

What is the minimum functionality for polymerization?

2.

What happens if functionality is 1?

No polymer forms.

What happens if functionality is 2?

A linear thermoplastic polymer forms.

What happens if functionality is 3?

A cross-linked network forms, producing thermosets or elastomers.

Why do polymers have a molecular weight distribution?

Because a single polymer sample contains chains of different lengths.

Why do polymers fold in crystals?

To maximize secondary bonding interactions.

What is a lamella?

A polymer crystal structure where chains fold back inside the crystal.

Why can polymers not reach 100% crystallinity?

Not all chains pack efficiently; maximum is about 60%.

What decreases polymer crystallinity?

Bulky side groups and chain branching.

What increases polymer crystallinity?

Better chain packing and fewer structural obstacles.

What is low-density polyethylene like?

Highly branched and used for plastic bags.

What is high-density polyethylene like?

Less branched and used for toys.

What is ultra-high-density polyethylene like?

No branching and used for car parts.

What are random copolymers?

Copolymers with monomers arranged randomly.

What are alternating copolymers?

Copolymers with alternating monomer units.

What are block copolymers?

Copolymers made of blocks of one monomer followed by blocks of another.

What are graft copolymers?

Copolymers with side chains attached to a main chain.

What are the three polymer stereoisomers?

Atactic, isotactic, and syndiotactic.

What is atactic polymer structure?

Side groups are arranged randomly.

Why is atactic polymer least crystalline?

It fits least compactly in the lamella.

What is isotactic polymer structure?

All side groups are on the same side of the chain.

Why is isotactic polymer most crystalline?

It fits most compactly in the lamella.

What is syndiotactic polymer structure?

Side groups alternate sides along the chain.

What is glass transition temperature, Tg?

The temperature where amorphous polymer chains change behavior.

What happens below Tg?

The polymer behaves as a rigid, hard, brittle glass.

What happens above Tg?

The polymer behaves as soft, flexible, and ductile rubber.

What motions do organic molecules have?

Translational, rotational, and vibrational motion.

What increases Tg?

Bulky side groups, aromatic groups, strong C-C bonding, polar groups, high molecular weight, cross-linking, and crystallinity.

What lowers Tg?

Plasticizers that increase spacing between polymer chains.

What increases polymer strength?

High molecular weight, high crystallinity, polar atoms, and reinforcing composites.

What causes deformation in polymers?

Chain sliding, disentanglement, and uncoiling.