an over view about polymers in general

1. POLYMERS

2. The Outline
Summery
 Reactions of polymers
Addition Polymerization
Step Growth Polymerization
 Chemical and Physical Structures of Polymers
 Polymer’s molecular structures
Configuration and conformation of polymers
Chain structures of polymers
 Physical Structures of Polymers
Polymer crystallinity
Crystallinity and amorphousness of polymers

3. The Outline
Summery
 Types of Polymers and Polymer Processing
 Members of Polymers
Definition of Thermosets & Thermoplastics
Common products and their properties
 Forming Techniques of Polymers
Extrusion of polymers
Injection Molding
Blow Molding
Thermoforming
Compression Molding
Casting

4. A polymer is a large molecule made by linking together
repeating units of small molecules called monomers

5. Addition Polymerization (Chain Growth)
Step Growth Polymerization (Condensation)

7. Differences between step-growth polymerization and
chain-growth polymerization
Step-growth polymerization Chain-growth polymerization
Growth throughout matrix Growth by addition of monomer only at
one end of chain
Rapid loss of monomer early in the
reaction
Some monomer remains even at long
reaction times
Same mechanism throughout Different mechanisms operate at
different stages of reaction (i.e.
Initiation, propagation and
termination)
Average molecular weight increases
slowly at low conversion and high
extents of reaction are required to
obtain high chain length
Molar mass of backbone chain increases
rapidly at early stage and remains
approximately the same throughout the
polymerization
Ends remain active (no termination) Chains not active after termination
No initiator necessary Initiator required

8. Step of Radical Chain Polymerization
 Initiation
 Propagation
 Termination

9. INITIATION

10. PROPAGATION

11. TERMINATION
Dead Polymer
i.) Coupling or Combination;
ii.) Disproportionation

12. CHAIN TRANSFER REACTIONS
Transfer to monomer reaction
Transfer to initiator reaction
Transfer to solvent reaction

13. IONIC CHAIN POLYMERIZATION
 Using catalyst, not initiator
 Highest reaction rate
 Termination step is just disproportionation
 Environment must be pure
 Reaction occurs in the cold

14. Anionic Polymerization=Living Polymerization
If the starting reagents are pure and the
polimerization reactor is purged of all oxygen and
traces of water, polimerization can proceed until
all monomer is consumed.

15. CONDENSATION POLYMERIZATION
 Using catalyst
 Minumum two functional groups required
 Usually linear
 Molecular weight increases slowly at low conversion
 High extents of reaction are required to obtain high
chain length

16. Chemical Structure of Polymers
Molecular configuration of polymers
 Side groups atoms or molecules with free bonds, called free-radicals, like H, O,
methyl affects polymer properties.
Stereoregularity describes the configuration of polymer chains :
 Isotactic is an arrangement where all substituents are on the same side of the
polymer chain.
 Syndiotactic polymer chain is composed of alternating groups
 Atactic the radical groups are positioned at random
Figure 2: Isotactic Syndiotactic and Atactic combinations of a stereoisomers of polymer chain
(http://www.microscopy-uk.org.uk/mag/imgsep07/atactic.png)

17. Molecular configuration of polymers
FIGURE.3. Diagrams of (a) isotactic, (b) syndiotactic, and (c) atactic configuration in a vinyl polymer.
The corresponding Fischer projections are shown on the right.
(Plastic Technolgoy Handbook)

18. Table 1. Properties of Polypropylene Stereoisomers
(Plastic Technology Handbook)

19. Molecular configuration of polymers
Geometrical isomerism:
 The two types of polymer configurations are cis and trans. These structures
can not be changed by physical means (e.g. rotation).
 The cis configuration → substituent groups are on the same side of a carbon-
carbon double bond.
 Trans → the substituents on opposite sides of the double bond.
Figure4.cis trans configurations of polyisoprene
( http://openlearn.open.ac.uk/file.php/2937/T838_1_019i.jpg )

20. Conformations of a Polymer Molecule
 Conformation →The two atoms have other atoms or groups attached
to them configurations which vary in torsional angle are known as
conformations (torsional angle:The rotation about a single bond which
joins two atoms )
 Polymer molecule can take on many conformations.
 Different conformation →different potential energies of the
molecule→Some conformations: Anti (Trans), Eclipsed (Cis), and Gauche (+
or -)

21. Other Chain Structures
 Copolymers →polymers that incorporate more than one kind of
monomer into their chain (nylon)
 Three important types of copolymers:
 Random copolymer contains a random arrangement of the multiple
monomers.
 Block copolymer contains blocks of monomers of the same type
 Graft copolymer contains a main chain polymer consisting of one type
of monomer with branches made up of other monomers.
 Figure 5 :Block Copolymer Graft Copolymer Random Copolymer
http://plc.cwru.edu/tutorial/enhanced/FILES/Polymers/struct/struct.htm

22. Physical Characteristics of Polymers
 The melting or softening temperature ↑ molecular weight ↑
 The molecular shape of the polymer has influence on the elastic
properties. ↑ coils the ↑ elasticity of the polymer
 The structure of the molecular chains has an effect on the strength
and thermal stability. ↑ crosslink and network structure within the
molecule ↑ the strength and thermal stability.

23. Polymer Crystallinity
 Crystallinity is indication of amount of crystalline region in polymer
with respect to amorphous content
 X-ray scattering and electron microscopy have shown that the
crystallites are made up of lamellae which,in turn, are built-up of
folded polymer chains
 Figure.6 Schematic representation of (a) fold plane showing regular chain folding, (b) ideal stacking oflamellar
crystals, (c) interlamellar amorphous model, and (d) fringed micelle model of randomly distributed crystallites
 (Plastic Technology Handbook)

24. Polymer crystallinity
 Crystallinity occurs when linear polymer chains are structurally
oriented in a uniform three dimensional matrix. Three factors that
influence the degree of crystallinity are:
 i) Chain length
ii) Chain branching
iii) Interchain bonding
Figure 7: Crystalline chain
http://plc.cwru.edu/tutorial/enhanced/FILES/Polymers/orient/Orient.htm

25. Polymer cristallinity
Crystallinity influences:
Hardness,modulus tensile, stiffness, crease, melting point of polymers.
 Most crystalline polymers are not entirely crystalline. The chains, or
parts of chains, that aren't in the crystals have no order to the
arrangement of their chains
 Crystallinity makes a polymers strong, but also lowers their impact
resistance
 Crystalline polymers are denser than amorphous polymers, so the
degree of crystallinity can be obtained from the measurement of
density → Wc=Φcρc/ ρ
ρ → density of entire sample
ρc → density of the crystalline fraction.
Φc→ volume fraction
Wc→ mass fraction

26. Determinants of Polymer Crystallinity
 The degree of crystallinity of a polymer depends on the rate of cooling
during solidification as well as on the chain configuration.
 In most polymers, the combination of crystalline and amorphous
structures forms a material with advantageous properties of strength
and stiffness.
Figure 8: Mixed amorphous crystalline macromolecular polymer structure
(http://web.utk.edu/~mse/Textiles/Polymer%20Crystallinity.htm)

27. Polymer cristallinity
 Polymer molecules are very large so it might seem that they could not
pack together regularly and form a crystal. Regular polymers may form
lamellar crystals with parallel chains that are perpendicular to the face
of the crystals.
 A crystalline polymer consists of the crystalline portion and the
amorphous portion. The crystalline portion is in the lamellae, and the
amorphous portion is outside the lamellae .
Figure 9. Arrangement of crystalline and amorphous portions
http://pslc.ws/mactest/crystal.htm#structure

28. Cristillanity and amorphousness
 An amorphous solid is formed when the chains have little orientation
throughout the bulk polymer. The glass transition temperature is the point
at which the polymer hardens into an amorphous solid.
 In between the crystalline lamellae,regions with no order to the
arrangement of the polymer chains → amorphous regions
 Polyethylene can be crystalline or amorphous. Linear polyethylene is nearly
100% crystalline. But the branched polyethylene is highly amorphous.
Figure 10.Linear and Branched Polyethylene
(http://pslc.ws/macrog/kidsmac/images/pe03.gif )

29. Examples...
 Highly crystalline polymers:
Polypropylene, Nylon, Syndiotactic polystyrene..
 Highly amorphous polymers:
Polycarbonate, polyisoprene, polybutadiene
 Polymer structure and intermolecular forces has a major role of a
polymer’s crystallinity.

30. Classification of Polymers
…with regard to their thermal processing behavior ;
 Thermoplastic Polymers (Thermoplastics)
soften when heated and harden when cooled
 Thermosetting Polymers (Thermosets)
once having formed won’t soften upon heating

31. Thermoplastics
 have linear or branched structure
chains are flexible and can slide past each other

32.  have strong covalent bonds and weak intermolecular van
der Waals bonds
 elastic and flexible above glass transition temperature
 can be heat softened, remolded into different forms
 reversible physical changes without a change in the
chemical structure

33. Thermosets
 chains chemically linked by covalent bonds
 hardening involves a chemical reaction which
connects the linear molecules together to form a
single macromolecule.

34. Thermosets
 once polymerization is complete, cannot be softened, melted
or molded non-destructively.
 have higher thermal, chemical and creep resistance than
thermoplastics
 Thermosets suitable materials for
Composites
Coatings
Adhesive applications

35. Common thermoplastics
Commodity Polymers
POLYETHYLENES
POLYPROPYLENE
POLYSTYRENE
POLYVINYLCHLORIDE-PVC
POLYMETHYLMETHACRYLATE-PMMA
Engineering Polymers(have a thermal resistance 100-150°C)
POLYCARBONATE
NYLON(POLYAMIDE)
POLYETHYLEN TEREPHATALATE-PET
High Performance Polymers (have a thermal resistance >150°C)
POLYTETRAFLUOROETHYLENE-teflon
POLYARYLETHERKETONES-PEEK

36. POLYETHYLENE
 prepared directly from the polymerization of ethylene (C2H4).
 two main types are; low-density (LDPE) and high-density
polyethylene (HDPE)
 Advantages
cheap
good chemical resistance
high impact strength

37.  Limitations
low heat resistance (upper temperature limit is 60°)
degrade under UV irradiation.
high gas permeability, particularly CO2
 Applications
extensively for piping and packaging
chemically resistant fittings, garbage bags
containers, cable covering

38. POLYPROPLYLENE
 improved mechanical properties compared to polyethylene;
has a low density (900–915 kg/m3), harder, and has a higher
strength
Good chemical and fatigue resistance
 Disadvantages
Oxidative degradation, high thermal expansion, high creep
poor UV resistance
 Applications
medical components, films for packaging (e.g. cigarette
packets)reusable containers, laboratory equipment

39. POLYSTYRENE
 a light amorphous thermoplastic
 Advantages
low cost, easy to mould, rigid, transparent
no taste, odor, or toxicity, good electrical insulation
 Disadvantages
sensitive to UV irradiation (e.g. sunlight exposure)
chemical resistance is poor, brittle
 Applications
CD-DVD cases, electronic housings, food packaging, foam
drink cups and egg boxes

40. POLYVINYLCHLORIDE-PVC
 was the first thermoplastic used in industrial applications
 very resistant to strong mineral acid and bases, good electrical
insulators, flame-retardant
 Two grades of the PVC material are available:
rigid PVC is used in the construction industry for piping
cold water and chemicals
flexible PVC is used in wire and cable coating, paints, signs

41. Common thermosets
 EPOXIES
 UNSATURATED POLYESTERS
 PHENOL FORMALDEHYDE (PHENOLIC)
 POLYURETHANES

42. EPOXIES
 Advantage
mechanically strong, highly adhesive
good chemical and heat resistance
electrical insulators
 Disadvantage
expensive
 Applications
as industrial adhesives, coatings or as matrices in advanced
reinforced plastics and also as encapsulation media

43. UNSATURATED POLYSTERS
 Advantage
hard, high strength
cheap compared to Epoxy
good electrical insulator
high heat resistance
 Disadvantage
poor solvent resistance compared to other thermosets
 Applications
molding or casting materials for a variety of electrical
applications, matrix for composites such as fiberglass
boats, fences, helmets, auto body components

44. PHENOLICS
 most commonly used thermosets
 high hardness, excellent thermal stability; low
tendency to creep
 Applications
wiring devices, bottle caps, automotive parts, plugs
and switches, as adhesives coatings and molded
components for electrical applications

45. POLYURETHANES
 depending on the degree of cross-linking they behave as
thermosets or thermoplastics
 low cost, high impact strength, high adhesion properties
 be processed into coatings, adhesives, binders, fibers and
foams

46. Methods of polymer fabrication
▪ Extrusion of polymers
▪ Injection Molding
▪ Blow Molding
▪ Thermoforming
▪ Compression Molding
▪ Casting