Structure and Thermal Properties

Question 1

What is melting temperature?

Answer 1

The temperature where polymers change from solid to liquid states

Question 2

What is crystallization temperature?

Answer 2

The temperature where crystallization occurs when cooling a polymer

Question 3

What is glass transition temperature?

Answer 3

The temperature where a polymer changes from a rubber to a glassy state (no longer able to do any long range movement and solids become more brittle)

Question 4

What is free volume?

Answer 4

A measure of how much space of a sample is not occupied by polymer chains

• at low T the chains will back together and the free volume will decrease

Question 5

Why is the glass transition temperature important?

Answer 5

We can mold the polymer in the rubber state and then cool the solid to fix the shape in place

Question 6

What are the routes of forming solid polymers?

The different structures solid polymers can take

Answer 6

1. Chemically crosslink network
2. glass amorphous solid
3. semicrystalline solid

Question 7

What is a chemical crosslink network?

Answer 7

covalent bonds between chains prevents flow - attatched chains turns liquid into a solid

• this is a non-reversible transition
• only goes through glass transition not melt transition

Question 8

What is a glassy amorphous solid

Answer 8

• no crystals have formed to hold chains in place
• cooled passed glass transition regions, so there is insufficient energy from chain movement
• reversible by heating/cooling

Question 9

What is a semicrystalline solid?

Answer 9

• groups of chains packed into crystals interspersed with amorphous regions
• can be reversed by adding head

Question 10

Chemical vs Physical crosslinks

Answer 10

• chemical crosslinks are chemical bonds that are difficult to pull apart while physical crosslinks are temporary bonds

Question 11

Lamella vs Spherulite

Answer 11

Lamella = crystal growth all occurs in 1 direction

Spherulite = crystal growth all starts at a single starting location and grows out (often composed of multiple lamella)

The spherulite crystal growth explains why polymers can only ever be semi-crystalline structures

Question 12

How does the density of crosslinks impact the modulus?

Answer 12

increased crosslinks increases the modulus linearly (in a basic relationship)

Question 13

What factors impact Tg?

Answer 13

• chain flexibility
• bulky pendant groups
• addition of lower Tg comonomers
• molecular weight
• polarity (increase polarity, increases Tg) - harder for chains to move around

(Tg decreases as it becomes easier for chains to move around)