7/27/2019 Lecture 6 - Polymers
1/20
POLYMERSTRUCTURES
7/27/2019 Lecture 6 - Polymers
2/20
Polymers
Naturally occurring Wood, rubber, cotton, wool, leather
Proteins, enzymes, starches, cellulose
Synthetic (from organic molecules) Plastics, synthetic rubbers, Fibers
7/27/2019 Lecture 6 - Polymers
3/20
3
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 ?
Polymer Structures
7/27/2019 Lecture 6 - Polymers
4/20
Hydrocarbon molecules
Most polymers are organicin origin
Many organic materials are
hydrocarbons Covalent bonds
Each C has 4 electrons thatmay participate in covalent
bonding
Single electron pair sharing single bond
7/27/2019 Lecture 6 - Polymers
5/20
5
Polymer Composition
Saturated hydrocarbons Each carbon singly bonded to four other atoms
Example:
Ethane, C2H6
C C
H
H HH
HH
7/27/2019 Lecture 6 - Polymers
6/20
6
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
7/27/2019 Lecture 6 - Polymers
7/20
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
7/27/2019 Lecture 6 - Polymers
8/20
Polymer Molecules
Much larger in comparison to the hydrocarbonmolecules Macromolecules
A chain of carbon atoms is the backbone
Mer
The repeating unit in a polymer chain Monomer a stable unit from which the
polymer is synthesized
7/27/2019 Lecture 6 - Polymers
9/20
9
Polymerization andPolymer Chemistry
Free radical polymerization
Initiator: example - benzoyl peroxide
C
H
H
O O C
H
H
C
H
H
O2 R= 2
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
7/27/2019 Lecture 6 - Polymers
10/20
Chemistry of Polymer molecules
C C
H H
HH
Monomer
7/27/2019 Lecture 6 - Polymers
11/20
Types of Polymers
HomopolymerAll repeating units along a chain are of the
same type; polymer formed from a singlemonomer
Eg. polyethylene, formed by polymerization ofethylene
Copolymer
2 or more different mer units involved
PMMA
http://en.wikipedia.org/wiki/Image:Ethylene-2D.png7/27/2019 Lecture 6 - Polymers
12/20
12
Bulk or Commodity Polymers
7/27/2019 Lecture 6 - Polymers
13/20
13
Bulk or Commodity Polymers (cont)
7/27/2019 Lecture 6 - Polymers
14/20
14
Bulk or Commodity Polymers (cont)
7/27/2019 Lecture 6 - Polymers
15/20
VMSE: Polymer Repeat Unit Structures
15
Manipulate and rotate polymer structures in 3-dimensions
7/27/2019 Lecture 6 - Polymers
16/20
16
MOLECULAR WEIGHT
Low M
high M
Not all chains in a polymer are of the same length
i.e., there is a distribution of molecular weights
Large molecular weights for polymers with very longchains
Molecular weight, M: Mass of a mole of chains.
7/27/2019 Lecture 6 - Polymers
17/20
Molecular weights
Number average molecular weight Mn M
n= x
iM
i
Mi represents mean (middle) molecular weight of size
range i
xi is the fraction of the total number of chains within
the corresponding size range
Weight average molecular weight Mw
Mw = wi Mi
wi is the weight fraction of molecules within the same
size interval
7/27/2019 Lecture 6 - Polymers
18/20
Example
Find the molecular weight
7/27/2019 Lecture 6 - Polymers
19/20
Mol. Wt. range
(g/mol)
Mean Mi
(g/mol) wi wiMi xi xiMi5000 10,000 7500 0.12 900 0.2 1500
10,000
15,000 12500 0.18 2250 0.1 1250
15,000
20,000 17500 0.26 4550 0.3 525020,000
25,000 22500 0.21 4725 0.1 2250
25,000
30,000 27500 0.14 3850 0.1 2750
30,000
35,000 32500 0.09 2925 0.2 6500
= 1.0
Mw
=wiMi = 1.0
Mw
=xiMi
=
19,200 19500
Example solution
7/27/2019 Lecture 6 - Polymers
20/20
20
Degree of Polymerization, DP
DP= average number of repeat units per chain
iimfm
m
S=
=
:followsascalculatedisthiscopolymersfor
unitrepeatofweightmolecularaveragewhere
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
m
MDP
n
=