The Structure and Properties of Polymers

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The Structure and Properties of Polymers

• BySawsan D.Shubbar

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What is a polymer?

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

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All the same monomer

• Monomers all same type (A)
• A A A A ?
• -A-A-A-A-
• eg poly(ethene) polychloroethene PVC

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Different monomers

• Monomers of two different types A B
• A B A B
• ? -A-B-A-B-
• eg polyamides
• polyesters

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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.

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Thermoplastics (80)

• No cross links between chains.
• Can change shape.
• Can be remoulded.

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Thermosets

• Cross-linking formed by covalent bonds.
• Bonds prevent chains moving relative to each
  other.

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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.
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Molecular Weight of Polymers

• Unlike small molecules, polymers
• are typically a mixture of differently
• size molecules.
• Only an average molecular
  weight can be defined.

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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.

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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.

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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.

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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)

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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.

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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)

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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.

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Plasticizers

• Are small molecules which occupy position between
  polymer chains (like adding water to mud to make
  it easy in molding)
• To increase flexibility, elongation and to
  reduce hardness and stiffness.
• To lower the processing temperature (energy
  saving, decomposition preventing)

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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.

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Stabilizers

• Heat stabilizers (Pd soap, dibasic phosphate).
• Antioxidants (easy to oxidize-phenols).
• UV absorbants.
• Light stabilizers carbon black 2 (1 to 20
  years).

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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.

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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

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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
• Whether it is thermoplastic or thermosetting.
• The geometry and size of the final product.

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Compression moldings

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

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Injection molding

• In injection molding , polymer granules are
• Compressed by a ram or a screw
• Injected until molten.

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Thermoplastic extrusion
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Blow moldings

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

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Thank you