Unit 2 - Polymers, Failure Modes, and Phase Diagrams

What is an oligomer?

A pre-polymer (usually in liquid form) that transforms into a polymer

What is GPC, how does it work, and what does it find?

• Gel Permeation Chromatography

• Polymer solution passes through beads, separating polymer chains and measuring concentration

• Molecular weight

Why does the durability of a polymer system increase with increasing molecular weight?

• Fewer chain ends → fewer weaknesses

• More covalent bonds → more entanglements

What are addition polymers, and how are they formed?

What are condensation polymers, and how are they formed?

List features of addition polymers compared to condensation polymers

cheaper → continuous process, uses simple hydrocarbons

faster → chain growth reactions propagate quickly

List features of condensation polymers over addition polymers

slower → gradual process means the viscosity increases

When the crystallinity of a polymer is increased, what changes at the atomic level and how are its properties affected?

The structure becomes more regular → covalent bonds dominate, chain motion is reduced

• Increases stiffness & transparency

• Decreases toughness

Sketch and label an oriented crystallite. How does process affect the crystallite?

Stretching pulls some of the disordered domains into taut cables that lock the blocks of folded chains together → higher strength along the fiber axis only, lower ductility and toughness

Sketch and describe the stress-strain curve of an elastomer

Shallow slope (lower stress required) → Initially, elastomers chains are tangled and disoriented. These tangled chains can be easily uncoiled and reoriented

Steep slope (higher stress required) → Pulling these chains taut aligns them, making them much stronger due to covalent bonds and crosslinks.

Describe elastic and plastic deformation for a semi-crystalline polymer

Elastic → covalent bonds and VDWs stretching

Plastic → chains slide past each other and lamellae unravel to form row structure

Why do some materials (metals?) become brittle at a certain low temperature?

Atomic mobility is reduced to the point that the slip mechanism fails

How do larger surface defects affect fatigue resistance?

They decrease resistance by causing weak points where cracks can initiate and failure can occur

What is creep at the atomic level?

High heat ( ≥ 0.4T_m ) increases the motion of atoms, giving them enough energy to break weak bonds and relocate. Under constant stress, these movements combine and stretch the entire part.