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Introduction to Engineering Materials
ENGR2000
Chapter 14: Polymer Structures
Dr. Coates
14.1 Introduction
• Naturally occurring polymers
– Wood, rubber, cotton, wool, leather, silk
• Synthetic polymers
– Plastics, rubbers, fibers
14.2 Hydrocarbon Molecules
• Many organic materials are hydrocarbons
• Composed of hydrogen & carbon
• Covalent bonding
Saturated & Unsaturated hydrocarbons
Saturated
• Molecules with all single
bonds
• No new atoms may be
joined without the
removal of others that are
already bonded
Unsaturated
• Molecules with double or
triple covalent bonds
5
Paraffin Family
14.3 Polymer Molecules
• Polymers are often called macromolecules
• Covalent bonds
• Molecules in the form of long & flexible chains
Mers, polymers & monomers
• Mer units - structural entities which are repeated
along the chain
• Polymer – many mers
• Monomer – a stable molecule from which a
polymer is synthesized
8
Polymerization and
Polymer Chemistry
• Free radical polymerization
• Initiator: example - benzoyl peroxide
C
H
H
O O C
H
H
C
H
H
O2
C C
H H
HH
monomer(ethylene)
R +
free radical
R C C
H
H
H
H
initiation
R C C
H
H
H
H
C C
H H
HH
+ R C C
H
H
H
H
C C
H H
H H
propagation
dimer
R= 2
propagation
Free radical
9
Chemistry and Structure of PolyethyleneAdapted from Fig.
14.1, Callister &
Rethwisch 9e.
Note: polyethylene is a long-chain hydrocarbon
- paraffin wax for candles is short polyethylene
10
Bulk or Commodity Polymers
11
Bulk or Commodity Polymers (cont)
Isomerism
• Isomerism
– two compounds with same chemical formula can
have quite different structures
for example: C8H18
• normal-octane
• 2,4-dimethylhexane
C C C C C C C CH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=
H3C CH
CH3
CH2 CH
CH2
CH3
CH3
H3C CH2 CH3( )6
13
MOLECULAR WEIGHT
• Molecular weight, M: Mass of a mole of chains.
Low M
high M
Not all chains in a polymer are of the same length
— i.e., there is a distribution of molecular weights
14
xi = number fraction of chains in size range i
MOLECULAR WEIGHT DISTRIBUTIONFig. 14.4, Callister & Rethwisch 9e.
wi = weight fraction of chains in size range i
Mi = mean molecular weight of size range i
15
Molecular Weight Calculation
Example: average mass of a class
Student Weight
mass (lb)
1 104
2 116
3 140
4 143
5 180
6 182
7 191
8 220
9 225
10 380
What is the average
weight of the students in
this class:
a) Based on the number
fraction of students in
each mass range?
b) Based on the weight
fraction of students in
each mass range?
16
Molecular Weight Calculation (cont.)
Solution: The first step is to sort the students into weight ranges.Using 40 lb ranges gives the following table:
weight number of mean number weight
range students weight fraction fractionN i W i xi wi
mass (lb) mass (lb)
81-120 2 110 0.2 0.117
121-160 2 142 0.2 0.150
161-200 3 184 0.3 0.294
201-240 2 223 0.2 0.237
241-280 0 - 0 0.000
281-320 0 - 0 0.000
321-360 0 - 0 0.000
361-400 1 380 0.1 0.202
SNi SNiW i
10 1881
total
number
total
weight
Calculate the number and weight
fraction of students in each weight
range as follows:
For example: for the 81-120 lb range
17
Degree of Polymerization, DP
DP = average number of repeat units per chain
C C C C C C C CH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C C C C
H
H
H
H
H
H
H
H
H( ) DP = 6
mol. wt of repeat unit iChain fraction
14.7 Molecular Structure
• Characteristics of a polymer depend on
– Its molecular weight
– Molecular shape
– Structure of molecular chains
Linear Polymers
• Mer units are joined together end to end in single
chains
• Flexible long chains (like a mass of spaghetti)
• Van der Waals & hydrogen bonds between the
chains
• Polyethylene, polyvinyl chloride, nylon, etc.
Branched Polymers
• Side branch chains are connected to the main ones
• Linear polymers form branched polymers when
side reactions that occur during synthesis form the
branches
• Chain packing efficiency reduced->lower density
• Ex. low density polyethylene
Crosslinked Polymers
• Adjacent linear chains are joined one to another at
various positions by covalent bonds
• Crosslinking may be accomplished by additives
• Vulcanized rubber
Network Polymers
• Multifunctional mer units, with three or more
active covalent bonds, form 3-D networks
• Epoxies, phenol-formaldehydes
Questions
• How might the length of the chain affect the likely
phase (hard solid, waxy solid, liquid) at room
temperature? Melting temperature? Why?
• How might molecular weight affect modulus and
strength? Why?
23
14.9 Thermoplastic and Thermosetting
Polymers
• Thermoplastic polymers
– Soften when heated
– Eventually liquefy
– Harden when cooled
– Processes are totally reversible & may be repeated
– Linear & branched polymers
Thermoplastic polymers
• The structure of polyethylene (a) the basic monomer (b)
the double bond in the monomer is opened (c) monomers
are linked together (d) secondary bonds between the
polymer chains
14.9 Thermoplastic and Thermosetting
Polymers
• Thermosetting polymers
– Become permanently hard when heated
– Do not soften or liquefy
– Harder & stronger than thermoplastic polymers
– Cross-linked & network polymers
Thermoset polymers
• The structure of crosslinked rubber (a) double bonds
along the length of the polymer chains (b) formation of
crosslinks (primary bonds) between the chains
14.11 Polymer Crystallinity
• Packing of molecular chains so as to produce an
ordered atomic array
• Linear polymers
– crystallization is easily obtained
– No restrictions to prevent chain alignment
• Crosslinked and network polymers
– amorphous
Practice Problems:
14.3, 14.5, 14.6, 14.13, 14.16