Polymers- Intro Flashcards
Features of most polymers
High molecular weight 10^4-10^6
Low density
Cheap
One long dimension is the backbone and two smaller dimensions up and out
Definitions of plastic
Any synthetic organic solid that is mouldable. Arguable.
One or more polymers containing one or more additives.
How are addition polymers synthesised?
Via chain growth. Monomers add to the polymer backbone one at a time. Reaction always involves a monomer. Requires a catalyst.
How are condensation polymers synthesised?
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.
How are thermoplastics and thermosets processed?
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.
What structural features can polymers have?
Can be a homo-polymer (one monomer) or co-polymer (two monomers). Can be linear, branched, crosslinked or a 3D network
Tacticity
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.
Features of thermoplastic polymers
Will melt, recyclable, linear, branched or few crosslinks
Features of thermoset polymers
Won’t melt, degrade at high temperatures (breaks covalent bonds), highly crosslinked, 3D network
Feature of elastomers
Special case with a reversible crosslink network
Like natural rubber or nitrile rubber
Crystallinity in polymers
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.
Primary phase transition
Melt transition at melting point Tm. Below Tm solid and above liquid. Don’t deal with gas phase. Likely to process as liquid.
Secondary phase transition
Glass transition at Tg. Below Tg glassy solid and above a rubbery solid (can process here)
Bonding in polymers
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.
How does viscosity of polymer melts change with shear stress?
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.
Flow during processing of polymer melts
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
Polymer processing considerations
Big list lecture 1 page 12
Commodity thermoplastic polymers
High density polyethylene (HDPE) Low density polyethylene (LDPE) Polypropylene (PP) Polystyrene (PS) Polyvinyl chloride (PVC)
Other thermoplastic polymers (engineering and specialty)
Polyethylene terephthalate (PET), Polyamides (PA), polycarbonates (PC), acrylonitrile butadiene styrene (ABS). All engineering Polyurethanes (PU), polytetrafluoroethylene (PTFE), polymethylmethacrylate (PMMA). All speciality
HDPE
HDPE: linear, 55-75% crystalline, semi-rigid, moisture/chemical resistance, gas impermeable. Used for packaging, containers, films, pipes, bottle caps
UTS=15MPa, E=1GPa
LDPE
LDPE: branched, 30-55% crystalline, flexible, same resistance and impermeability. Used for food bags, squeezy bottles, films
UTS=5MPa, E=0.3GPa
Ultra-high molecular weight PE (UHMWPE)
Highly aligned molecules, 95% crystalline, very high specific strength. Used for fishing lines, chopping boards, ballistic armour.
UTS=3500MPa, E=130GPa
Polypropylene
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
Polystyrene
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
PVC
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
What do large side groups do?
Reduce crystallinity and can increase stiffness
Polyesters
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
Polyamides (nylons)
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
Polycarbonates
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
ABS
Acrylonitrile butadiene styrene. Co-polymer which adds toughness (via butadiene groups) to otherwise brittle PS. Used for Lego, car dashboards, keyboards
PTFE
Very low coefficient of friction so many non-stick applications
Examples and applications of thermosetting polymers
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
