The Structure and Properties of Polymers

By:Sawsan D.Shubbar

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What is a polymer?

• A long molecule made up from lots of small molecules called

• monomers.

All the same monomer

• Monomers all same type (A)

• A + A + A + A • -A-A-A-A-• eg poly(ethene)

polychloroethene PVC

Different monomers

• Monomers of two different types A + B

• A + B + A + B -A-B-A-B-• eg polyamides • polyesters

Copolymers

• Copolymers are like polymer alloys. Different mers are joined to form a mixture in the backbone, eg. ABS.

• they can be tailored to obtain specific properties.

Thermoplastics (80%)

• No cross links between chains.

• Can change shape.

• Can be remoulded.

Thermosets

• Cross-linking formed by covalent bonds.

• Bonds prevent chains moving relative to each other.

What Makes Polymers Unique?

• Really big molecules (macromolecules) like polymers have very different properties than small molecules. When polymer is melted, the chains can flow past each other.

Chain entanglement: Long polymer chains get entangled with each other.

Molecular Weight of Polymers

• Unlike small molecules, polymers

• are typically a mixture of differently

• size molecules.

• Only an average molecular weight can be defined.

# o f m o l e c u l e s

Mn

Mw

increasing molecular weight

Mv

Longer chains make stronger polymers.

• There is a critical length needed before strength increases.

• An average No. of 100 repeating units is necessary for HC polymers but only 40 for nylons.

Crystalline polymers• Crystallinty is areas in

polymer where chains packed in a regular way.

• Both amorphous and crystalline areas can exist in the same polymer.

• More crystalline polymer causes stronger and less flexible polymer.

Melting temperature (Tm)

• The (Tm) when applied to polymers suggests not a solid-liquid phase transition, but a transition from a crystalline phase to a solid amorphous phase. Crystalline melting is only discussed with thermoplastics, as thermosets will decompose at high temperatures rather than melt.

Glass transition temperature (Tg)

The glass transition temperature (Tg) describes the temperature at which amorphous polymers undergo a second order phase transition from a rubbery, viscous amorphous solid (fresh spaghetti) to a brittle, glassy amorphous solid (3 days old Spaghetti)

Tensile strength• The tensile strength of a material quantifies how much

stress the material will endure before failing. In general tensile strength increases with polymer chain length.

Tensile strength

• Mechanical behavior of amorphous and semi-crystalline polymers is strongly affected by Tg

• In general • Polymers whose Tg is above the service

temperature are strong, stiff and sometimes brittle• e.g. Polystyrene (cheap, clear plastic drink cups)

– Polymers whose Tg is below the service temperature are weaker, less rigid, and more ductile

• Polyethylene (milk jugs)

Polymer additives

Before its conversion into plastic products, polymer resins is almost always compounded with various additives of different nature, meant to improve processing , stability, or mechanical specifications.

Plasticizers

Are small molecules which occupy position between polymer chains (like adding water to mud to make it easy in molding)

1. To increase flexibility, elongation and to reduce hardness and stiffness.

2. To lower the processing temperature (energy saving, decomposition preventing)

Plasticizers

Plasticizer properties:

• Low viscosity• High stable towards water and oils.• Low vapor pressure (Bpt is high).• Stable towards light and heat.• Low toxic.• Compatible.• Colorless.

Stabilizers

• Heat stabilizers (Pd soap, dibasic phosphate).

• Antioxidants (easy to oxidize-phenols).

• UV absorbants.

• Light stabilizers: carbon black 2% = (1 to 20 years).

Fillers

Improves the attitude and lowers the cost (fiber , powder).• Max. improvement for physical properties.• Low water absorbance.• Low specific gravity.• High polymer wetting.• Free from abrasives.• Cheap and available.• Odorless.• Color compatible.

Polymer additives

• Blowing agents: physical, chemical.• Antistatic agents: ethoxilated ammins.• Lubricants: to allow easier processing and to

slides through dies easier {stearates}.• Colorants: dyes or pigments.• Reinforcing agents: (glass fiber, kevler).• Flame retardants: (Cl/F and Br).• Odorants

Forming of polymers

• Polymeric materials are normally fabricated at elevated temperatures and often by application of high pressures.

• The technique used to form a particular polymer depends on :

1. Whether it is thermoplastic or thermosetting.

2. The geometry and size of the final product.

Compression moldings

Both thermoplastic and thermosets can be formed by compressing molding. Squeeze molten polymer between

hydraulic press.

Injection molding

In injection molding , polymer granules are

Compressed by a ram or a screw

Injected until molten.

Thermoplastic extrusion

Blow moldings

• Not to be confused with film blowing ( which is an extrusion-based process).

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