1. Chapter 7: Polymers Part 1
DMT125 Materials Science
Chapter 7: Polymers
Part 1

2. Contents Overview Polymerization reactions
Crystallinity & stereoisomerism
Thermoplastics
Thermosetting plastics (thermosets)
Deformation & strengthening
Creep & fracture

3. Overview Polymer means “many parts”
Contains many chemically bonded parts or units that themselves are bonded together to form a solid
2 industrial important polymeric materials:
Plastics: large & varied group of synthetic materials that are processed by forming or molding into shape
Thermoplastics
Thermosetting plastics (thermosets)
Elastomers: can be elastically deformed a large amount when a force is applied to them & can return to their original shape when the force is released

4. Overview Thermoplastics
Require heat to make them formable & after cooling, retain the shape they were formed into
Can be reheated & reformed into new shapes a number of times without significant change in their properties
Consist of very long main chains of carbon atoms covalently bonded together
Long molecular chains are bonded to each other by secondary bonds

5. Overview
Thermosets
Cannot be remelted & reformed into another shape but degrade or decompose upon being heated too high a temperature
Cannot be recyled
Heat is required to permanently set the plastic
Consist of a network of carbon atoms covalently bonded to form a rigid solid

6. Overview Advantages Relatively low in cost
Elimination of parts through engineering design with plastics
Elimination of many finishing operations
Simplified assembly
Weight savings
Noise reduction
Elimination of the need for lubrication of some parts

7. Polymerization reactions
Most thermoplastics are synthesized by the process of chain-growth polymerization
Small molecules are covalently bonded together to form very long molecular chains
Monomers – simple molecules that are covalently bonded into long chains
Polymer – long chain molecule formed from monomer units

8. Polymerization reactions
Ethylene molecule, C2H4 is chemically bonded by a double covalent bond between the carbon atoms & by 4 single covalent bonds between the carbon & hydrogen atoms
Chain polymerization – covalently bonded together to form a polymer
Polyethylene – polymer produced by the polymerization of ethylene

9. Polymerization reactions
Mer – repeating subunit in the polymer chain
Degree of polymerization (DP)
N in the equation
Number of subunits or mers in the polymer molecular chain
Average – 3500 to 25,000 corresponding to average molecular masses ranging from 100,000 to 700,000 g/mol

10. Polymerization reactions
3 steps of chain polymerization
initiation
Propagation
Termination
Initiation
Free radical – atom that has an unpaired electron that can covalently bond to an unpaired electron of another atom or molecule

11. Polymerization reactions
Example: Hydrogen peroxide, H2O2
Upon heating, this peroxide can decompose into 2 free radicals
Benzoyl peroxide – organic peroxide used to initiate some chain polymerization reactions
Propagation
Process of extending the polymer chain by the successive addition of monomer units
Termination
Can occur by the addition of a terminator free radical or when 2 chains combine
Trace amounts if impurities may terminate the polymer chain

12. Polymerization reactions
Thermoplastics consist of chains of polymers of many different lengths, each of which has its own molecular weight & degree of polymerization
For a monomer to polymerize, it must have at least 2 active chemical bonds
When it has 2 active bonds, it can react with 2 other monomers & by repetition of the bonding, others can form a long chain or linear polymer
3D network molecules can be built up
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13. Polymerization reactions
Functionality – number of active bonds a monomer has
Bifunctional – a monomer that uses 2 active bonds for the polymerization of long chains
Example: ethylene
Trifunctional – uses 3 active bonds to form a network polymeric material
Example: phenol
Bonding between the long molecular chains in polyethylene consists of weak, permanent dipole secondary bonds

14. Polymerization reactions

15. Polymerization reactions
Homopolymers – polymeric materials that consist of polymer chains made up of single repeating units
Example: AAAAAAA
Copolymers – consist of polymer chains made up of 2 or more chemically different repeating units that can be in different sequences

16. Polymerization reactions
4 types
Random – randomly arranged within polymer chains
AABABBBAAABBB…
Alternating – ordered alternation
ABABABABAB…
Block – arranged in relatively long blocks of each monomer
AAAA---BBBB---
Graft – appendages of one type of monomer are grafted to the long chain of another
AAAAAAAAAAAAAAAAAA
B B

17. Polymerization reactions

18. Polymerization reactions
Other methods of polymerization
Stepwise
Monomers chemically react with each other to produce linear polymers
Network
Involving a chemical reactant with more than 2 reaction sites, a 3D network material can be produced
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19. Crystallinity & stereoisomerism
Thermoplastic when solidified from liquid state will form either
Noncrystalline
Partly crystalline solid
When noncrystalline thermoplastics solidify, there is on sudden decrease in specific volume as the temperature is lowered
The liquid upon solidification, changes to a supercooled liquid that is in the solid state & shows a gradual decrease in specific volume with decreasing temperature
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20. Crystallinity & stereoisomerism
Upon cooling this material to lower temperatures, a change in slope of the specific volume versus temperature curve occurs
Glass transition temperature, Tg
Average temperature within the narrow temperature range over which slope in the curve changes
Above Tg, viscous behavior is displayed (rubbery)
Below Tg, glass-brittle behavior is displayed (due to molecular chain motion is very restricted)

21. Crystallinity & stereoisomerism
Tg might be considered a DBT
Example: polypropylene
Partly crystalline
When this material solidifies & cool, sudden decrease in specific volume occurs
This is due to more efficient packing of the polymer chains into crystalline regions
As cooling continued, glass transition is encountered
In going through glass transition, supercooled liquid matrix transforms to glassy state
Structure of thermoplastics consists of crystalline regions in a glassy noncrystalline matrix
Example: polyethylene

22. Crystallinity & stereoisomerism

23. Crystallinity & stereoisomerism

24. Crystallinity & stereoisomerism
Degree of crystallinity in partly crystalline ranges from 5 to 95 percent of their total volume
Complete crystallization is not achievable even with polymeric materials that are highly crystallizable due to:
Molecular entanglements
Crossovers
Degree of crystallinity increase, strength of material increase

25. Crystallinity & stereoisomerism
Stereoisomers
Molecular compounds that have the same chemical compositions but different structural arrangements
3 types:
Atactic: pendant methyl group of polypropylene is randomly arranged on either side of the main carbon chain
Isotactic: always on the same side of the main carbon chain
Syndiotactic: alternates from one side to the other side
13 January 2014
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26. Crystallinity & stereoisomerism

27. END
End of Part 1
Copyright © Mr.Mohd. Azarulsani b. Md. Azidin (2014)