4.2 Amorphous Solids - polymers

Polymers are just a chain of repeat molecular units called…

monomers

n represents…

the amount of monomers in the polymers

What is molecular weight impacted by?

Temperature, reactant ratios, catalysts, inhibitors, polymer chemistry

A high molecular weight =

high elastic modulus, tensile strength, service temp, toughness, chemical resistant to dissolution

a low molecular weight =

low elastic modulus, tensile strength, service temp, toughness, chemical resistant to dissolution

more vanderwaals bonds between chains means…

a higher molecular weight

The monomer molecular weight…

gets skewed towards heavier molecules

What are homopolymers?

They have the same repeat unit

What are copolymers?

They have different repeat units

What do side groups do?

Hang off the polymer backbone

Rigid/bulky styrene side group means

a denser polymer property, higher Tg

Polar side group

higher Tg (stronger bonding)

Flexible side group

lower Tg (easier to pack/slide)

What is tacticity?

The way side groups are arranged along the backbone

What does atactic mean?

Randomly arranged, less dense, harder to crystallize

What does isotactic mean?

Same side arranged, more dense & easier to crystallize

What does syndiotactic mean?

Every other arranged

What’s a signature feature of a comb polymer?

The branches don’t have branches

What’s a signature feature of a randomly branched polymer?

The branches have branches

What do the different colors in a grafted polymer structure mean?

The side branch is one type of polymer, and the main chain is one type of monomer

What’s a defining feature of a blend polymer?

A physical mixture of 2 types of polymers

What kind of structure is this?

A grafted copolymer

What type of structure is this?

A random linear copolymer

What do chemical cross links involve?

Covalent bonds between 2 polymer chains

What do physical cross links involve?

Ionic or vanderwaals bonds between 2 polymer chains

What’s an example of a chemical cross link?

Tires

What’s an example of a physical cross link?

Slime

What happens when you increase cross-links?

Strength and rigidity increases as well

What are some cross linking methods?

Catalysts (vulcanization or photo-curing) and network formers

Which polymer will pack more denser?

Polymer B because there’s no side group in the way

Which polymer will be mechanically stronger?

Polymer A because there’s more secondary bonds between chains

What happens when you increase chain length?

It increases 2nd degree bonding between chains, which leads to increased strength

What is visco-elasticity?

Time dependent behavior between stress and strain

What is the deborah number?

time of relaxation/time of observation

What does it mean if you have a smaller deborah number?

the material changes faster

What deborah numbers do liquids have?

0

What deborah numbers do solids have?

infinity (much > 1 is a solid)

What is creep?

Constant stress is applied

What is stress?

Constant strain is applied

What’s an example of a viscoelastic polymer?

Slime

True/False: Polymers have a glass transition

True

What is Tg influenced by?

Molecular weight, types of side groups, % crystalinity

What happens if your polymer is above Tg?

It’s rubbery, moldable, and flowable

What happens if your polymer is below Tg

The polymer is glassy and brittle

Which polymer will have a higher Tg?

Polymer A because the side group makes it harder for the chains to move past each other

True/False: Thermosets have a high degree of crosslinks compared to thermoplastics

true

True/False: Thermoplastics are stronger/tougher but also more brittle than thermosets

True

Why can’t thermosets be melted and remelted?

Because they degrade

What is special about thermoplastics?

They can be recycled and be shaped using more different processing methods

How does the thermoplastic change as the temperature gets hotter?

glassy (below Tg) —> rubbery moldable <—> Melt

How does the thermoset polymer change as the temperature increases?

glassy (below Tg) —> rubbery tough —> degrade before they ever melt

How can you change polymers so they don’t dissolve in water?

By adding crosslinks to the structure

True/False: Hydrogels are soluble in water

False, they are not soluble instead they absorb water

Are solid state polymers 100% crystalline?

No, they can be completely amorphous, partially crystalline, or almost completely fully crystalline

What are crystalline regions of polymers called?

Crystallite

What are crystallites composed of?

Chains folded onto themselves called lamellae which organize into spherulite

What structural features make a polymer more likely to crystallize?

Smaller side groups, lower branching density, isotactic or syndiotactic, fewer repeat units, hydrogen bonding or polar side groups

What does higher crystallinity lead to?

Higher elastic modulus, melting (service) temp, opacity, difficulty of dyeing, better vapor barrier properties

Where do semiconducting polymers charge transport?

Along the conjugated backbone

what’s a defining feature of a semiconducting polymer?

Alternating double bonds or aromatic cycles

What is significant about electron mobility in semiconducting polymers?

They have much lower electron mobility than traditional conductors

Why are semiconducting polymers useful?

Flexibility, biocompatibility, cheaper, better for enivronment

True/False: You can use thermosets in the extrusion process or in injection molding

False, you can only use thermoplastics because you’re melting it

True/False: Compression molding can be done with thermosets

True because you’re not melting them

What happens in chain growth?

Growth only happens from one side

What happens in step growth?

2 or more reactive functional groups where growth can happen from either side

If it has double bonds what can we conclude about the synthesis route?

It will proceed via chain growth

If it has 2+ function groups what can we conclude about the synthesis route>

It will proceed via step growth

What is the route if there are 3+ functional groups?

The route is able to make a network polymer

Addition reactants have…

no byproducts

Condensation reactions have…

a byproduct