This document discusses natural and synthetic polymers. Natural polymers include collagen, gelatin, silk and wool. Synthetic polymers include polyethylene terephthalate, high density polyethylene, polyvinyl chloride, low density polyethylene and polypropylene. The document then discusses the structure of polymers including that they are made of repeating monomer units and can be formed through chain-reaction or step-reaction polymerization. It also discusses properties of polymers related to their molecular structure and weight.

2. • Collagen
• Gelatin
• Silk
• Wool
• Natural rubber
• DNA
NATURAL POLYMER SYNTHETIC POLYMER
• Polyethylene terephthalate
(PET)
• High Density Polyethylene
(HDPE)
• Polyvinyl Chloride (PVC)
• Low Density Polyethylene
(LPDE)
• Polypropylene (PP)
• Polystyrene (PS)

3. • The word polymer comes from the Greek ‘poly’ meaning many, and
‘meros’ is parts or units.
• A polymer is organic substance made up of many repeating units or
building blocks of molecules called mers.
• Combine many monomers to create a polymer.
• Polymer is often used as a synonym for ‘plastic’. All plastic are
polymers, but not all polymers are plastics.
Poly mers are made up of many Mono mer
↓ ↓ ↓ ↓
Many Units One Unit

4. POLYMERIZATION
chemical process where monomers linked into
polymers in repeating unit to make longer and larger
molecules
• Also called additional
polymerization, with aids of
initiators to form benzene or
paraffin.
Chain –
Reaction
Polymerization
• Also called condensation
polymerization, dissimilar
monomer joined into short
groups that gradually grow
with by product released.
Step –
Reaction
Polymerization

5. The straightforward addition of monomers of the same kind
Homogeneous type : A +A … → A-A-A-A-…
or a different kind
Copolymer type : A +B+A+B… → A-B-A-B-…
 Rapid chain reaction of chemically activated mers
 Each reaction sets up the condition for another to proceed
 Each site need a reactive site (a double carbon bond or unsaturated
molecules)
 Initiator is added to open the double bond between carbon
 3 stages :
Initiation
Propagation
Temination
 The composition of resultant molecule is a multiple of the individual mers
 Most commonly produced linear structure but can produce network
structure

6. • Initiation
free radical – a single unit
that has one unpaired electron
(OH‾ molecule)
• H₂O₂ break up into 2 OH‾
molecules
• Each can act to initiate and to
terminate the reaction
• Termination - recombination
Polymerization of polyethylene

7. Involves a polymerization reaction between two monomers with the expulsion
of a simple by product.
A+B → AB + simple by product
 Individual chemical reactions between reactive mer that occur one step at
a time
 By products (water or carbon ,oxygen or hydrogen gas) is formed and
condensed out
 Polymer molecule growth step by step until all of one reactant is consumed
 Slower than additional polymerization
 Need reactive functional groups
 No reactant species has the chemical formula of a mer repeating unit
 Most commonly produce network structure but can produce linear
structures

8. Condensation polymerization of nylon 6,6

9. The properties of the polymer depends on:
i. Structures of individual polymer molecules
ii. Molecule shape and size
iii. Arrangement of molecules to form a polymer structure
Basic structure of polymer
molecules:
(a) ethylene molecule
(b) polyethylene, a linear chain of
many ethylene molecule
(c) molecular structure of various
polymers

10. • Molecular weight of the polymer is the
sum of the molecular weights of mers in
a representative chain.
• Molecular Weight Distribution (MWD) is
the spread of the molecular weights in a
chain
• Strong influence on the properties:
Increase in molecular weight will
increase:
i. Tensile & impact strength
ii. Resistance to cracking
iii. Viscosity of molten state
higher molecular weight,
greater average chain length
Figure 2 Effect of molecular
weight and degree of
polymerization on the strength
and viscosity of polymers

11. • The ratio of the molecular weight of the polymer to the molecular
weight of the mer (repeating unit)
• Example: Polyvinyl chloride (PVC)
Mer weight: 62.5,
thus DP of PVC with 50,000 molecular weight is:
50,000 / 62.5 = 800
Higher DP → Higher viscosity (resistance to flow) → hard to shape →
increase cost adversely.
Higher DP → stronger polymers

12. • During polymerization, the monomers are linked together by covalent
bond forming a polymer chain (high strength at primary bond)
• The polymer chains are held together by secondary bonds (low
strength):
i. Van der Waals bonds
ii. Hydrogen bonds
iii. Ionic bonds
• In polymer, the increase in strength and viscosity → the longer the
polymer chain → the greater is energy needed to overcome
secondary bonds

13. Linear Polymers (sequential structure)
• Generally a polymer consists of more than
one type of structure (a linear polymer may
contain some branched and cross-linked
chains; properties are changed significantly)
Branched Polymers
• Side-branch chains are attached to the main
chain during the synthesis.
• Interferes with the relative movement of the
molecular chains → increase in resistance to
deformation and stress cracking
• Interferes with the packing efficiency of
chains → density is lower than linear-chain
Branched polymers ~pile of tree branches
Linear-chain polymers ~ bundle of straight logs
Difficult to move branch rather than log.
3D entanglements of branches→ difficult to
move→increase in strength
Schematic illustration of
polymer chains.
(a) Linear structure--
thermoplastics such as
acrylics, nylons,
polyethylene, and
polyvinyl chloride have
linear structures.
(b) Branched structure,
such as in polyethylene

14. Cross-linked polymers
• Thermosets or thermosetting plastics 3D
structure, adjacent chains linked by
covalent bonds
• Increase hardness, strength, stiffness,
brittleness, better dimensional stability
Network polymers
• Spatial, 3D networks of three or more
active covalent bonds
• Highly cross-linked polymers = network
polymer
• Cross-linking thermoplastics polymers
→ by high-energy radiation (UV, X-rays,
e- beams) → increase in strength

15. Polymers are generally amorphous
The chain exist without long-range order (like bowl
of spaghetti, or worms in a bucket, all intertwined
with each other)
Crystallinity in polymers
• modify the characteristics
• fostered during synthesis or deformation in
subsequent process
Crystallites → Crystalline region in polymers
Formed when long molecules arrange themselves in an
orderly
manner
Semicrystalline polymer 2 phase material (crystalline +
amorphous)
Different degree of crystallinity can be impart by controlling:
• Rate of solidification during cooling
• Chain structure
Degree of crystallinity affected by branching:
The higher the
crystallinity, the harder,
stiffer, and less ductile the
polymer.

16. 1. S. Kalpakjian, S. R. Schmid, “Manufacturing Engineering and
Technology”, 6th ed, 2010