Foundations of Materials Science and Engineering Third Edition

Figure 7.16
Polyethylene-folded chain Figure 7.17
10-14 After F. Rodriguez, “priciples of Polymer Systems,” 2nd ed., McGraw-Hill, 1982,p.42 After R. L. Boysen, Olefin Polymers, in “Encyclopedia of Chemical Technology,” vol. 16, Wiley, 1981, p.405.
Figure 7.12
10-12 After W. E. Driver, “Plastics Chemistry and Technology,” Van Nostrand Reinhold, 1979, p.19
Solidification of Thermoplastics.
• There is no sudden change in specific volume on cooling in noncrystalline thermoplastics.
• Solution polymerization: Monomer dissolved in non-reactive solvent and catalyst. • Suspension polymerization: monomer and catalyst suspended in water. • Emulsion polymerization: Monomer and catalyst suspended in water along with emulsifier.
• Chains are randomly entangled.
Figure 7.4
• Entanglement increases tensile strength. • Branching decreases tensile strength.
Figure 7.5
10-8
Vinyl and Vinylidene Polymers
Raw Materials:
Natural gas, Petroleum and coal
Granules, pellets, Polymerization powders or liquids.
•Bulk polymerization : Monomer and activator mixed in a reactor and heated and cooled as desired
10-10
➢ Random copolymers: Different monomers randomly
arranged in chains. Eg:- ABBABABBAAAAABA
➢ Alternating copolymers: Definite ordered alterations of
Tg = glass transition temperature.
Glass brittle
below Tg abov polyethylene is –1100C For PVC it is 820C
Figure 7.14
• In crystalline thermoplastics, sudden decrease in specific volume occurs due to more efficient packing of polymer chains.
Stereoisomerism in Thermoplastics
• Stereoisomer:- Same chemical composition but different structural arrangements.
➢ Atactic stereoisomer:- Pendent methyl group of polypropylene is randomly arranged on either side of main carbon chain.
M m = average molecular weight of thermoplastics.
Mi = Mean molecular weight of each molecular range selected.
fi = Weight fraction of the material having Molecular weights of a selected molecular
nm wandering successively through a series of disordered and ordered region. • Folded chain model: sections of molecular chains folding on themselves.
• Good insulation. • Light weight. • Noise Reduction.
Remote Control
10-3
Wafer bands Figure 7.1
Air intake manifold
Polymerization
• Chain growth polymerization: Small molecules covalently bond to form long chains (monomers) which in turn bond to form polymers.
10-4
Chain Polymerization - Steps
• Initiation:
➢ A Radical is needed. ➢ Example H2O2
In General
• One of free radicals react with ethylene molecule to form new longer chain free radical.
10-13
Structure of Partly Crystalline Thermoplastics
• Longest dimension of crystalline region is 5-50 nm. • Fringed micelle model: Long polymer chains of 5000
• Energy of system is lowered by polymerization. • Termination:-
➢ By addition of termination free radical. ➢ Combining of two chains ➢ Impurities.
R(CH2 CH2)m + R’(CH2 CH2)n R(CH2 CH2)m R (CH2 CH2)n R’
10-5
Chain Polymerization – Steps (cont..)
• Propagation: Process of extending polymer chain by addition of monomers.
R CH2 CH2 + CH2 CH2 R CH2 CH2 CH2 CH2
10-2
Thermosetting Plastics
Cannot be reformed by reheating.
Set by chemical reaction.
Plastics - Advantages
• Wide range of properties.
• Minimum finishing. • Minimum lubrication.
Figure 7.7 10-9
Homopolymer and Copolymers
• Homopolymers: Polymer chain is made up of single repeating units.
Example: AAAAAAAA
• Copolymers: Polymer chains made up of two or more repeating units.
monomers. Eg:- ABABABABABAB
➢ Block copolymers: Different monomers arranged in long
blocks. Eg:- AAAAA…….BBBBBBBB……
➢ Graft copolymers: One type of monomer grafted to long chain
CHAPTER
10
Polymeric Materials
10-1
Introduction to Polymers
• Polymers
many parts Polymers
Plastics
Elastomers
Thermoplastics
Can be reheated and
formed into new materials
Coupling of two chains
10-6
Average Molecular Weight
• Average molecular weight determined by special physical and chemical techniques.
Mm
fiMi fi
• Example:
Figure 7.10
• Network polymerization:
Chemical reaction takes
place in more than two
reaction sites
(3D network).
Figure 7.11
10-11