Introduction, Characteristics, Application & Processing of Polymers

Polymers

"Very large molecules made when hundreds of monomers join together to form long chain".

• Repeat Unit  ⇔  group of atoms that repeat itself in polymer chain.
• Used in automobiles, sports, toys, household items, etc.

Advantages:

1. Lightweight
2. Strong and durable 
3. Easy to manufacture

Disadvantages:

1. Don't easily biodegrade 
2. End up producing large amount of waste

Related Terms

• Saturated Hydrocarbons  ⇔  in which each carbon is singly bonded to four other atoms.
• Unsaturated Hydrocarbons  ⇔  in which double or triple bond between carbon atoms present.
• Isomerism  ⇔  Two compounds with same composition but have different atomic arrangements.
• Polymorphism  ⇔  Existence of more than one crystal structure of a material (like BCC, FCC iron).
• Monomer  ⇔  small and unreacted single molecule for a polymer.
• Mer  ⇔  reacted single repeat unit of a polymer.
• Polymer or Macro-Molecule  ⇔  A polymer chain built on carbon backbone.
• Functional Group  ⇔  atom or group of atoms within a molecule that has similar chemical properties
  when appear in various compounds.
• Polymerization  ⇔  process by which monomer molecules are linked to form a big polymer molecule.

Classification of Polymers 

a. By Origin

1. Natural Polymer  ⇔  originated from plant or animal (like wool, silk, rubber, wood)
2. Synthetic Polymer  ⇔  Man made polymer.
   
   

b. By Monomer

1. Homo-Polymer  ⇔  consist of identical monomers.
2. Co-Polymer  ⇔  polymer consist of monomers of different chemical structure.

Copolymer are of different types:

1. Random  ⇔  A and B randomly positioned along chain.
2. Alternating  ⇔  A and B alternately positioned.
3. Block  ⇔  Large blocks A units alternate with large blocks of B units.
4. Graft  ⇔  Chains of B units grafted onto A backbone.

c. By Thermal Response

1. Thermoplastic Polymers  ⇔  easily molded in different shapes by heating and hardened on cooling. They may be linear or branched chain polymer (Eg: polyethene, PCV, polystyrene, polypropylene).
2. Thermosetting Polymers  ⇔  polymers which are hard and not softened on heating under pressure. They are cross-linked polymer (Eg: Bakelite).

d. By Mode of Formation

1. Addition Polymers  ⇔  Polymers formed by the addition of monomers repeatedly without the removal of by-product (Eg: Teflon, PE, PP, PVC).
2. Condensation Polymer  ⇔  formed by the combination of two monomers by the removal of small molecules of by-product (like water, alcohol, ammonia). It has Ester & Amide linkage (Eg: Nylon, polyester).

e. By Structure

1. Linear Polymers  ⇔  Monomers linked with each other and form a long straight chain, no side chain, molecules closely packed, high density, strength and melting point. Eg: HDPE, Nylons.
2. Branched Polymers  ⇔  in which straight long chain with different side chains are present, irregularly packed molecules (low density, tensile strength and melting point). Eg: LDPE, LLDPE.
3. Cross-Linked Polymers  ⇔  monometric units are linked together to constitute a 3D network, are hard rigid due to network (cross-links) structure. Eg: Bakelite (mountain devices), Formaldehyde resins, Vulcanized rubber (automobiles tyres).

f. By Applications & Physical Properties

1. Fibers  ⇔  If polymer is drawn into long filament whose length is at least 100 times its diameter. It has high tensile strength, highly crystalline. Eg: Nylon, Terylene.
2. Plastics  ⇔  Polymer is shaped into hard and tough utility articles by application of heat and pressure, partially crystalline. Eg: Polystyrene, PVC, PMMA.
3. Elastomers  ⇔  Solids (with rubber like properties) polymeric chains held together by weak intermolecular forces, highly amorphous. Eg: Natural rubber, BUNA-S, BUNA-N, Vulcanized rubber.
4. Resins  ⇔  Low molecular weight polymer used as adhesive, sealants, manufacture of composites. Eg: Epoxy adhesive, polysulphides sealant.

g. By Crystallinity

1. Crystalline  ⇔  Monomers arranged in ordered way.
2. Amorphous  ⇔  Monomers arranged in random way.

h. By Backbone Atom

1. Organic  ⇔  Polymer backbone is made up of carbon atoms.
2. Inorganic  ⇔  Polymer backbone is made up of other atoms.

Polymer Crystallinity

• Chain Folded  ⇔  if layer of crystalline are folded (or placed) over one other (thickness: 10-20 micrometer).
• Whole material is not amorphous or crystalline, rather there are regions of amorphous or crystallinity.
  

i. By Tacticity

It is defined as, "deposition of side groups in space".

• Stereoisomerism  ⇔  The situation in which atoms are linked together in the same order but differ in spatial arrangement (3D orientation of their atoms in space).

• Geometric Isomerism  ⇔  non-active electrons of atoms called side groups.

Geometric Isomers are of 2 types:

1. Cis-Isomers  ⇔  in which functional group is present at the same side.
2. Trans-Isomers  ⇔  in which functional group is present on opposite sides of chain.

Molecular Weight

Degree of Polymerization (DP)

It is defined as, "average number of repeat units per chain".

Polymers-Molecular Shape (or Conformation)

It is defined as, "chain bending and twisting are possible by rotating carbon atoms around their chain bonds".

Defects in Polymers

You can get more info about defects in polymers by clicking here

1. Point defects
2. Screw dislocation
3. Non-crystalline region  ⇔  irregularity in crystallinity.
4. Interfacial defects

Processing of Polymers

• Extrusion (Extruder ⇔ Feed section, Compression section, Metering section)
• Molding (Blow molding, Compression molding, Transfer molding, Injection molding)
• Extrusion blow molding
• Injection blow molding 
• Compression molding

Mechanical Properties of Polymers

• Fracture strength of polymers  ⇔  10 % of that metals.
• Deformation strains for polymers  >  1000 %

Predeformation by:

1. Drawing  ⇔  stretches polymer, increase elastic modulus and tensile strength, decrease ductility.
2. Annealing  ⇔  reverse effect that of metal (hot working) while for polymer it works as cold working.

References:

• Material from Class Lectures + Book named Materials Science and Engineering: An Introduction by Callister and Rethwick + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube (Engineering Sights).