MSE 230 - Week 11

How does molecular weight affect thermoplastic properties?

As average MW increase (look at equations too for the final Aaliyah), strength and stiffness go up, but it plateaus.

How does temperature affect TP properties?

If above glass transition temp → pliable and ductile

If below glass transition temp → brittle

As temperature increases, VDW bonds are weakened and ductility increases (E and o lowers, but strain goes up)

In a stress-strain diagram, what are the different regions?

1. Elastic - stretching of covalent and VDW bonds

2. Drawing - amorphous chains and crystalline regions align in the loading direction to increase polymer strength (plastic deformation) → covalent bonds hold the load

3. Increase in strength!! due to covalent bonds holding the load

What is viscoelasticity?

Viscous like a fluid, elastic like a solid

E_r(t) = o(t) / e_o

As temperature increases, stress relaxes faster and the elastic modulus E decreases faster

What are the five regions of viscoelastic behavior?

1. Glassy

2. Leathery - modulus drops three orders of magnitude

3. Rubbery - equal balance between viscous and elastic properties of polymer

4. Rubbery flow - more viscosity

5. Viscous flow - no elastic contribution

How does crystallization affect TP properties?

Polymers with big substituent groups are hard to crystallize, so an increase in crystallization increases strength and modulus due to secondary bonding scheme.

How do substituent groups affect TP properties?

Bulky side groups increase chain stiffness to make it more difficult to move chains relative to one another, and include stronger forms of secondary bonding that influence melting temperature and glass transition temperature.

Can you remelt cross-linked polymers?

NO due to chemical bonds

less sensitive to temperature and stiffer and stronger

What are elastomers?

Crazy behavior due to (light) crosslinking and coiled chains, can elastically deform a lot and get all strain back