Materials T4 - Polymers

Natural Polymers

Wood, natural rubber, proteins, DNA

Saturated Hydrocarbon

Carbon is bonded to four hydrogens

Unsaturated Hydrocarbon

Contains double and/or triple bonds, reactive

Isomers

Two compounds with identical chemical composition but different structures

Addition (Radical) Polymerisation

Free radical attacks double bond, forming new radical out of monomer. Cycle repeats, forms a polymer chain until fully saturated.

Polymers from Addition Polymerisation

Polyethylene, polypropylene, poly(vinyl chloride), polystyrene

Polycondensation (Step) Polymerisation

Chemical reaction occurs between functional group of two monomers, forms new covalent bond to create long polymer chain.

Homopolymer

Composed of the same repeat unit

Co-polymer

Composed of two or more different repeat units

Functionality

Number of bonds a monomer can form

Number-Average Polymer Weight

Favours all molecules equally (lower g/mol)

Weight-Average Polymer Weight

Larger molecules weighted more heavily (higher g/mol)

Polydispersity Index (PDI)

Weight-Avg / Number-Avg

Degree of Polymerisation

Number of repeat units per chain (number-avg / molecular weight of repeat unit)

Number Average Molecular Weight Increases

Melting temperature increases (up to 100g/mol for liquid, 1000g/mol for wax, over 1000g/mol for solid)

Molecular Orientation (Conformation)

Can be changed by rotation around singular bonds without breaking needed

Retricts Molecular Orientation

Double bonds and bulky/side groups

Configuration

The direction monomers are linked together or the order (for copolymers), to change must break bonds

Stereoisomers

Compounds that are mirror images, cannot superimpose without breaking bonds

Isotactic

All R groups are on same side of the chain

Syndiotactic

R groups alternate sides of chain

Atactic

R groups randomly placed on chain

Cis Configuration

Bulky groups on same side of chain

Trans Configuration

Bulky groups on opposite sides of chain

Linear Polymers

Repeat units joined together end to end in single chains

Linear Polymer Properties

Flexible with Van der Waals forces acting between chains

Branched Polymers

Contains side branch chains connected to main ones

Branched Polymer Properties

Reduced chain packing efficiency, lower density

Cross Linked Polymers

Adjacent linear chains joined to one another at various positions via covalent bonds

Network Polymers

Multifunctional monomers forming 3 or more covalent bonds

Strength of Polymer

Greater as number of chains/links increase

Copolymer

Two or more monomers polymerised together

Block Copolymer

Large blocks of monomers alternate

Graft Copolymer

Chains of one monomer grafted onto another monomer backbone

Thermoplastics

Linear and branched polymers (normally additive) that are soft and can be recycled

Thermoplastics when Heated

Secondary bonds break and polymer becomes viscous, bonds reformed when cooled

Thermosets

Cross-linked or network polymers that are harder, stronger and cannot be recyled

Thermosets when Heated

Undergo cross-linking, which is irreversible

Thermoplastic Vs Thermoset Bonding

Thermoplastics are held together by secondary bonds, thermosets are held together by cross-linking bonds

Crystallinity

Molecular chains pack to produce an ordered atomic array, range from entirely amorphous to mostly crystalline

Factors of Crystallinity

Rate of cooling during solidification and simplicity of repeat unit

Crystalisation of Atactic Polymers

Difficult due to random R groups

Crystalisation of Isotactic and Syndiotactic Polymers

Easier

Crystallite

Small crystalline regions in a semicrystalline polymer

Brittle Polymer

Fractures during elastic deformation

Plastic

Elastic then plastic deformation

Elastomer

Experiences large recoverable strains

Effects of Increasing Temperature

Elastic modulus and tensile strength decrease, ductility increases

Viscoelasticity

Behaviour of polymers as rubbery solids at an intermediate temperature

Elastic

Recovers fully

Viscous

Does not recover

Viscoelastic

Recovers somewhat

a

Chain molecules in amorphous regions elongate + align in direction of applied tensile stress

a

Block segments separate from lamellae + slide past one enother to orient w/ tensile axis

Blocks and tie chains become oriented in direction of tensile axis

Higher Degree of Crystallinity

Increased tensile strength and material becomes more brittle

Heat Treating

Increases % of crystallinity and size of crystallites

Carbon bonded to H, O, N, F

Backbone atoms of polymer molecules