1 ISSUES TO ADDRESS... • What are the general structural and chemical characteristics of polymer molecules? • What are some of the common polymeric materials, and how do they differ chemically? • How is the crystalline state in polymers different from that in metals and ceramics ? Chapter 14 Polymer Structures
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1
ISSUES TO ADDRESS...
• What are the general structural and chemical characteristics of polymer molecules?
• What are some of the common polymeric materials, and how do they differ chemically?
• How is the crystalline state in polymers different from that in metals and ceramics ?
Chapter 14Polymer Structures
2
What is a Polymer?
Poly mermany repeat unit
Adapted from Fig. 14.2, Callister & Rethwisch 9e.
C C C C C C
HHHHHH
HHHHHH
Polyethylene (PE)
ClCl Cl
C C C C C C
HHH
HHHHHH
Poly(vinyl chloride) (PVC)
HH
HHH H
Polypropylene (PP)
C C C C C C
CH3
HH
CH3CH3H
repeatunit
repeatunit
repeatunit
3
Ancient Polymers
• Originally natural polymers were used
– Wood – Rubber
– Cotton – Wool
– Leather – Silk
• Synthetic polymers
– Synthesized from small organic molecules
– Plastics, rubber, fiber materials
4
Polymer Composition
Most polymers are hydrocarbons– i.e., made up of H and C
• Saturated hydrocarbons
– Each carbon is singly bonded to four other atoms (covalent)
– Example:
• Ethane, C2H6
C C
H
H HH
HH
5
Unsaturated Hydrocarbons
• Double & triple bonds somewhat unstable –can form new bonds– Double bond found in ethylene or ethene - C2H4
– Triple bond found in acetylene or ethyne - C2H2
C C
H
H
H
H
C C HH
6
Paraffin Family
7
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
⇓⇓⇓⇓
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 Polyethylene
Adapted 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)
12
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
What is the averageweight of the students inthis 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?
15
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
ΣNi ΣNiW 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
16
Molecular Weight Calculation (cont.)
weight mean number weight
range weight fraction fractionW i xi wi
mass (lb) mass (lb)
81-120 110 0.2 0.117
121-160 142 0.2 0.150
161-200 184 0.3 0.294
201-240 223 0.2 0.237
241-280 - 0 0.000
281-320 - 0 0.000
321-360 - 0 0.000
361-400 380 0.1 0.202
Chapter 14 - 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
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?
18
19
Adapted from Fig. 14.7, Callister & Rethwisch 9e.
Molecular Structures for Polymers
Branched Cross-Linked NetworkLinear
secondarybonding
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
24
Polymers – Molecular Shape
Molecular Shape (or Conformation) – chain
bending and twisting are possible by rotation
of carbon atoms around their chain bonds
– note: not necessary to break chain bonds
to alter molecular shape
Adapted from Fig. 14.5, Callister &
Rethwisch 9e.
25
Chain End-to-End Distance, r
Fig. 14.6, Callister &
Rethwisch 9e.
26
Copolymers
two or more monomers polymerized together
• random – A and B randomly positioned along chain
• alternating – A and B alternate in polymer chain
• block – large blocks of A units alternate with large blocks of B units
• graft – chains of B units grafted onto A backbone
Thermoplastics and some thermosets• when ram retracts, plastic pellets drop from hopper into barrel• ram forces plastic into the heating chamber (around the
spreader) where the plastic melts as it moves forward• molten plastic is forced under pressure (injected) into the mold
cavity where it assumes the shape of the mold
Barrel
48
Processing Plastics – Extrusion
Fig. 15.25, Callister & Rethwisch 9e. (Reprinted with permission from Encyclopædia