The Structure of Polymers

Polymers are just a chain of repeat molecular units called ___?

monomers

What conditions is molecular weight impacted by?

1. temperature

2. humidity

3. reactant ratio

4. time/inhibitor

How does high molecular weight affect properties?

• Increases stiffness

• increases toughness

• increases service temperature

• increases chemical durability

Increasing chain length increases _______ between chains, which leads to _______

1. intermolecular bonds

2. changes in properties

what are side groups?

Groups that hang off the polymer backbone that influence the polymer’s properties

Rigid/bulky side groups cause…

• More stiff (less flexible)

• less dense (harder to pack)

• higher Tg

polar side groups cause…

• more stiffness

• higher Tg

Flexible side groups cause…

• less dense (compared to linear)

What ar the 2 different types of polymer structures?

1. Homopolymer - have the same repeat unit

2. Copolymers - have different repeat units

What are the different types of copolymers? (3)

1. Random

2. Alternating

3. Block

4. Branched

What is cross-linking?

• Cross-links adds primary between neighboring polymer chains

• These are stronger than the typical Van der Waals bonds

• Increases strength and rigidity

  • And Tg, service temperature, chemical durability

What is viscoelasticity?

Polymers exhibit time-dependent behavior in their relationship between stress and strain

Deborah Number

De = (time of relaxation) / (time of observation)

• Liquids have a Deborah number that tends towards zero

• Solids have a Deborah number that tends towards infinity

Creep

• Constant stress is applied

• Strain changes with time

Stress relaxation

• Constant strain is applied

• Stress changes with time

What is Tg influenced by?

• How hard is it for chains to slide past each other

  • Molecular weight

  • Side groups

  • % crystallinity

  • Cross links

What happens when polymers are cooled above Tg?

They turn rubbery and moldable

What happens when polymers are cooled below Tg?

They turn brittle and glassy

Thermoplastic Polymers

• Soften (“melt”) when heated and harden when cooled

• Reversible process

• Usually linear or branched polymers

• Easier to recycle, more processing options

Thermosetting polymers

• Harden when formed and do not “melt” upon heating

• Cross linked or network polymers

• Difficult to recycle, fewer processing options

Water Soluble Polymers

• These polymers dissolve in water

• How might we change the structure so this doesn’t happen?

  • Add cross-links?

Hydrogels

Do not dissolve in water. Instead, they absorb water, causing them to swell to many times their original size

• Highly cross-linked or network polymers

Semiconducting Polymers

• Charge transport along conjugated backbone

• Seen in polymers with alternating double bonds and/or aromatic ringlets

• Overall, significantly lower electron mobility than traditional conductors

Natural Polymers

• Includes proteins, DNA, RNA, keratin

• Most natural polymers are very complex

• Many biological polymers are hierachically ordered

What type of polymer can be used in extrusion processes and injection molding?

thermoplastics

What type of polymer can be used in thermoforming?

Best with thermoplastics

What type of polymer can be used in compression molding?

OK for thermosets and thermoplastics

What type of polymer can be used in 3D Printing (“standard”) FDM?

Thermoplastics

What type of polymer can be used in 3D Printing (“resin”) SLA?

Thermosets

How are microplastics formed?

• Secondary bonds in polymers will break down over time

• Primary bond remain, resulting in small plastic particles that can last for decades