Polymers- Intro

Question 1

Features of most polymers

Answer 1

High molecular weight 10^4-10^6
Low density
Cheap
One long dimension is the backbone and two smaller dimensions up and out

Question 2

Definitions of plastic

Answer 2

Any synthetic organic solid that is mouldable. Arguable.

One or more polymers containing one or more additives.

Question 3

How are addition polymers synthesised?

Answer 3

Via chain growth. Monomers add to the polymer backbone one at a time. Reaction always involves a monomer. Requires a catalyst.

Question 4

How are condensation polymers synthesised?

Answer 4

Via step growth. Monomers react with each other to form dimers. These react with each other or monomers to form oligomers. Reaction can involve monomers, dimers or oligomers. The reactions condensé out a small molecule often water.

Question 5

How are thermoplastics and thermosets processed?

Answer 5

Thermoplastic processing involves moulding pre-synthesised polymers. This is simpler but requires someone to synthesise the polymer.
Thermoset processing uses monomers and synthesises the polymer during moulding. More complicated but lower viscosity.

Question 6

What structural features can polymers have?

Answer 6

Can be a homo-polymer (one monomer) or co-polymer (two monomers). Can be linear, branched, crosslinked or a 3D network

Question 7

Tacticity

Answer 7

Backbone has side groups. Isotactic has them all on same side of the backbone. Atactic have them on random sides of the backbone. Isotactic will have higher crystallinity.

Question 8

Features of thermoplastic polymers

Answer 8

Will melt, recyclable, linear, branched or few crosslinks

Question 9

Features of thermoset polymers

Answer 9

Won’t melt, degrade at high temperatures (breaks covalent bonds), highly crosslinked, 3D network

Question 10

Feature of elastomers

Answer 10

Special case with a reversible crosslink network

Like natural rubber or nitrile rubber

Question 11

Crystallinity in polymers

Answer 11

Can be amorphous or semi-crystalline (mixture of crystalline and amorphous). Never fully crystalline due to chain end imperfections. Slower cooling leads to higher crystallinity. Large side groups lead to lower crystallinity. Higher crystallinity means probably stiffer and stronger.

Question 12

Primary phase transition

Answer 12

Melt transition at melting point Tm. Below Tm solid and above liquid. Don’t deal with gas phase. Likely to process as liquid.

Question 13

Secondary phase transition

Answer 13

Glass transition at Tg. Below Tg glassy solid and above a rubbery solid (can process here)

Question 14

Bonding in polymers

Answer 14

Stiff and strong covalent bonds along the long backbone chain (350kJ/mol). Weaker interactions like van der waals forces, hydrogen bonding, ion-ion interactions between chains (3-10kJ/mol). Polymers much stiffer and stronger along the backbone than perpendicular to it.

Question 15

How does viscosity of polymer melts change with shear stress?

Answer 15

They are shear thinning fluids (psuedoplastics). The shear rate increases by more as shear stress increases and viscosity decreases. The random alignment of chains becomes more aligned with the direction of flow under shear which reduces resistance and increases flow.

Question 16

Flow during processing of polymer melts

Answer 16

Force applied to melt results in flow. Flow affected by temperature, pressure and strain rate. Plastic parts are stiffer/stronger in the direction of flow during processing

Question 17

Polymer processing considerations

Answer 17

Big list lecture 1 page 12

Question 18

Commodity thermoplastic polymers

Answer 18

High density polyethylene (HDPE)
Low density polyethylene (LDPE)
Polypropylene (PP)
Polystyrene (PS)
Polyvinyl chloride (PVC)

Question 19

Other thermoplastic polymers (engineering and specialty)

Answer 19

Polyethylene terephthalate (PET), Polyamides (PA), polycarbonates (PC), acrylonitrile butadiene styrene (ABS). All engineering 
Polyurethanes (PU), polytetrafluoroethylene (PTFE), polymethylmethacrylate (PMMA). All speciality

Question 20

HDPE

Answer 20

HDPE: linear, 55-75% crystalline, semi-rigid, moisture/chemical resistance, gas impermeable. Used for packaging, containers, films, pipes, bottle caps
UTS=15MPa, E=1GPa

Question 21

LDPE

Answer 21

LDPE: branched, 30-55% crystalline, flexible, same resistance and impermeability. Used for food bags, squeezy bottles, films
UTS=5MPa, E=0.3GPa

Question 22

Ultra-high molecular weight PE (UHMWPE)

Answer 22

Highly aligned molecules, 95% crystalline, very high specific strength. Used for fishing lines, chopping boards, ballistic armour.
UTS=3500MPa, E=130GPa

Question 23

Polypropylene

Answer 23

Stiffer and stronger than PE. Opaque, cheap, good melting point. Chemical and moisture resistant. Can be brittle at room temp but improve toughness adding impact modifiers like rubber particles (reduces stiffness). Improve stiffness adding glass fibre (increased cost and lowers ductility). Commonly isotactic, 40-70% crystalline. Used for packaging, pipes, textiles, lab equipment, automotive parts.
UTS=25MPa, E=2GPa

Question 24

Polystyrene

Answer 24

Stronger and stiffer than PE, PP. Transparent and thermally stable, chemically resistant, cheap. Commonly atactic, amorphous. Brittle so add impact modifiers (rubber) to improve toughness (HIPS) or co-polymerise with acrylonitrile (ABS). Used for rigid packaging, disposable cups, medical devices, expanded foams (expanded polystyrene EPS).
UTS=30MPa, E=2.5GPa

Question 25

PVC

Answer 25

Stronger and stiffer than PP, PE. Commonly atactic, amorphous, cheap. Less common to injection mould as it degrades above 200°C to give off HCl gas. Naturally rigid so often plasticised to improve flexibility. Unplasticised (uPVC) used for window and door frames, pipes, gutters, credit cards.
Plasticised used for wire coatings bouncy balls, shoes, inflatables
UTS=30MPa, E=2.5GPa

Question 26

What do large side groups do?

Answer 26

Reduce crystallinity and can increase stiffness

Question 27

Polyesters

Answer 27

Most common PET. Excellent strength and stiffness. Thermally stable, gas and liquid impermeable. Semi-crystalline or amorphous. Used for drink bottles, other bottles, packaging films, automotive parts. The backbone phenyl ring imparts very good mechanical properties.
UTS=80MPa, E=4GPa

Question 28

Polyamides (nylons)

Answer 28

Most common nylon 6 (one monomer with 2 different functional groups) and nylon 6,6 (2 monomers with 2 same functional groups each). Stronger and stiffer than PP, PE. Thermally stable, tough, wear resistant. Semi-crystalline. Used for gears, bearings, textiles and fibres, packaging and films, coatings, automotive parts. Extensive hydrogen bonding leads to good properties

Question 29

Polycarbonates

Answer 29

Most common bisphenol-A polycarbonate. Stronger and stiffer than PP, PE. Tough, thermally stable, transparent. Bisphenol-A can leach into water causing health concerns. Used for safety glass, compact discs, windows, medical devices, large water bottles

Question 30

ABS

Answer 30

Acrylonitrile butadiene styrene. Co-polymer which adds toughness (via butadiene groups) to otherwise brittle PS. Used for Lego, car dashboards, keyboards

Question 31

PTFE

Answer 31

Very low coefficient of friction so many non-stick applications

Question 32

Examples and applications of thermosetting polymers

Answer 32

Unsaturated polyesters: common matrix for glass fibre composites
Epoxy resins: common matrix for carbon fibre composites
Thermosetting PUs: wide range properties and applications, often foamed for seat cushioning
Phenolic resins (Bakelite): low flammability
Bismaleimides (BMI) and Cyanate esters: high temp processing and applications, expensive