MSE 280: Introduction to Engineering Materials
Polymer Structures
Reading: Callister Ch. 4 & 14.11
•
•
•
•
•
•
•
•
Classification.
Monomers and chemical functional groups.
Nomenclature.
Polymerization methods.
g & degree
g
of p
polymerization.
y
Molecular weight
Molecular structures.
Copolymers.
Crystallinity.
1
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Polymers
-Very large molecules (macromolecules) that are composed of
smaller repeating units (monomers).
A
A+A
A-A
A-A-A
…..
- Examples:
- Textile fibers: polyester, nylon…
- IC packaging materials.
- Resists for photolithography/microfabrication.
- Plastic
Pl ti b
bottles
ttl ((polyethylene
l th l
plastics).
l ti )
- Adhesives and epoxy.
- High-strength/light-weight fibers: polyamides, polyurethanes,
Kevlar…
- Biopolymers: DNA, proteins, cellulose…
2
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
1
Classification
• Thermoplastics: polymers that flow more easily when
squeezed, pushed, stretched, etc. by a load (usually at
elevated
l
t d T)
T).
– Can be reheated to change shape.
• Thermosets: polymers that flow and can be molded
initially but their shape becomes set upon curing.
– Reheating will result in irreversible change or decomposition.
• Other ways to classify polymers.
– B
By chemical
h i l ffunctionality
ti
lit ((e.g. polyacrylates,
l
l t
polyamides,
l
id
polyethers, polyeurethanes…).
– Vinyl vs. non-vinyl polymers.
– By polymerization methods (radical, anionic, cationic…).
– Etc…
3
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Chemical functional groups
H
Ethylene
(ethene)
H
C C
H
H
H
Propylene
(propene)
H
=
C C
H
C H
H
H
1-butene
2-butene
trans
Acetylene
(ethyne)
cis
H C C H
Saturated hydrocarbons
Unsaturated hydrocarbons
4
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
2
Chemical functional groups
5
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Some common polymers
teflon
Polyacrylonitrile (PAN)
H
H
C
C
H
C
N
Note: many of these polymers have
backbone.
Vinyl polymers (one or more H’s of ethylene can
be substituted)
H
H
C C
© 2007, 2008 Moonsub Shim, University of Illinois
H
X
H
H
C
C
H
X
6
nMSE280
3
Nomenclature
Monomer-based naming:
poly________
poly
Monomer name goes here
e.g. ethylene -> polyethylene
if monomer name contains more than one word:
poly(
p
y(_____ ____))
Monomer name in parentheses
e.g. acrylic acid -> poly(acrylic acid)
7
© 2007, 2008 Moonsub Shim, University of Illinois
polymethylene MSE280
Monomers to polymers
polymerization
ethylene
polyethylene
8
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
4
Polymerization methods
A. Free Radical Polymerization
1. Initiation
H
R
H
R
C C
Free radical initiator
(unpaired electron)
H
H
monomer
H
H
C
C
H
H
Radical
t
transferred
f
d
R
R
H
H
H
C
sp2 carbons
C
C
H
C
H
H
H
H
σ bonds
π bond
© 2007, 2008 Moonsub Shim, University of Illinois
sp3 carbon
9
MSE280
Polymerization methods
A. Free Radical Polymerization
H
2. Propagation
H
R
H
C
C
H
H
H
H
R
C C
H
H
H
C C
H
H
H
H
H
C
C
C
C
H
H
H
H
H
R
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H
H
C
H
R
H
H
C
C
C
H
H
H
© 2007, 2008 Moonsub Shim, University of Illinois
Both carbon atoms will
change from sp2 to sp3.
10
MSE280
5
Polymerization methods
A. Free Radical Polymerization
3. Termination
R
R
H
H
C
C
H
H
H
H
C
C
H
H
+
R
+
R
R
H
H
C
C
H
H
H
H
C
C R
H
H
R
H
H
H
H
C
C
C
C
H
H
H
H
R
Intentional or unintentional molecules/impurities can also terminate.
11
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Polymerization methods
Lose water
(condensation)
B. Stepwise polymerization
O
O
H2N
R
C
+
OH
H2N
R
C
O
OH
H2N
R
C
N
H
R
C
OH
+
H
H
N
R
Various R groups…
n
H
O
Proteins (polypeptides have similar composition)
O
H
N H C
C
O
O
R
+ (n-1)
C
H
O
H
n
C. Other methods
Anionic polymerization, cationic
polymerization, coordination
polymerization…
© 2007, 2008 Moonsub Shim, University of Illinois
12
MSE280
6
Molecular Weight
Not only are there different structures (molecular arrangements)
but there can also be a distribution of molecular weights (i.e.
number of monomers p
per p
polymer
y
molecule).
)
16 mers
20 mers
10 mers
Average molecular weight =
20 + 16 + 10
M monomer = 15.3M monomer
3
This is what is called number average molecular weight.
© 2007, 2008 Moonsub Shim, University of Illinois
13
MSE280
Molecular Weight
Number average molecular weight:
∑N M
=
∑N
j
Mn
j
j
j
j
=
mo ∑ N j j
j
∑N
j
j
Nj = number of polymer chains with length j
Mj = jmo = mass of polymer chain with length j
mo = monomer molecular weight
g
Note:
∑N M
j
j
j
= Total weight
∑N
j
= Total # of polymer chains
j
14
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
7
Molecular Weight
Weight average molecular weight:
∑W M
=
∑W
j
Mw
∑N M
=
∑N M
j
j
2
j
j
j
Wj = N jM j
j
j
j
In general:
j
j
∑N Mα
M=
∑N Mα
+1
j
j
j
j
j
j
If α = 0 then M n
If α = 1 then M w
15
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Molecular Weight: Different Notations
In Callister Textbook
In Lecture Notes
∑N M
=
∑N
j
Mn
M n = ∑ xi M i
j
i
j
xi =
j
j
Mw =
∑ N j M 2j
j
∑N
j
M
Ni
∑N j
j
M w = ∑ wi M i
i
j
j
Nj = number of polymer chains with length j
Mj = jmo = mass of polymer chain with length j
© 2007, 2008 Moonsub Shim, University of Illinois
wi =
Ni M i
∑ N jM
j
j
16
MSE280
8
Molecular Weight
Weight average counts the
heavier chains more – certain
properties can be dependent on
molecular weight (i.e. larger MW
polymer chains can contribute to
the overall property differently
than smaller ones).
NOTE:
Mw
is always larger than
Mn
unless polydispersity = 1
17
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Polydispersity & Degree of polymerization
Polydispersity =
Mw
≥1
Mn
When polydispersity = 1, monodisperse.
Degree of polymerization:
Number avg degree of polymerization
Weight avg degree of polymerization
© 2007, 2008 Moonsub Shim, University of Illinois
nn =
nw =
Mn
mo
Mw
mo
Monomer
molecular
weight
18
MSE280
9
Example 1
A. Calculate number and weight avg degrees of polymerization and
polydispersity for a polymer sample with the following distribution.
Avg # of monomers/chain
10
100
500
1000
5000
50,000
Relative abundance
5
25
50
30
10
5
B. If the polymer is PMMA, calculate number and weight average
molecular weights.
C. If we add polymer chains with avg # of monomers = 10 such that
their relative abundance changes from 5 to 10, what are the new
number and weight average degrees of polymerization and
polydispersity?
19
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Isomerism in polymers
So far, we’ve considered polymers as if they were beads on a string,
there is more structural complexity.
Isomers: molecules with same empirical formula but different
structure.
geometrical isomers: different sequence of bonds.
H
Cl
H
C C Cl
H
H
Cl
H
Cl C C
H
H
H
stereoisomers: same bond sequence but different
arrangementt off atoms
t
in
i space.
Cl
H
Cl
H
Cl
C C
C C
Cl
H
H
Similar to molecules polymers also have isomers…
sequence, stereo and structural isomers
© 2007, 2008 Moonsub Shim, University of Illinois
20
MSE280
10
Sequence isomerism
For an asymmetric monomer
T
+
H
T
H
F
H
H
F
H
H
H
C
C
C
C C
C
C
C
H
H
H
F H
H
H
F
H to T
H
T
H
T
H
H
T
H
T
T
H
e.g. PMMA
e.g. poly(vinyl fluoride):
H
T
H
H3C H3C
H3C
O O
O O O O
O
C H C H C H C
C
C
C
C C
C
H
CH3 H
CH3H
CH3 H
H3C
T to T
O
C
C
CH3
H to H
H to T
Random arrangement
H to T
H to T
Exclusive H to T arrangement (Why?)
21
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Stereoisomerism (tacticity)
Even with exclusive head-to-tail arrangement, there can still be variations
R
H
C
H
H
C
C
or
H
X
H
H H
HH
X H X H
meso-diad
© 2007, 2008 Moonsub Shim, University of Illinois
C
H
X
H H
HH
X H
H X
racemic-diad
22
MSE280
11
Stereoisomerism (tacticity)
Isotactic: only meso-diads
X
X
X
X
X
X
X
Syndiotactic: only racemic-diads
X
X
X
Atactic: random mixture
X
X
X
X
X
23
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Structural Isomers
Some polymers contain monomers with more than 1 reactive site
e.g. isoprene
2
1
H2C
4
CH3
C
C
H
CH2
3
trans-isoprene
trans-1,4-polyisoprene
CH3
C
H2
C
C
H
trans-1,2-polyisoprene
H2
C
H2
C
C
n
H3C
n
3,4-polyisoprene
H2
C H
C
CH
H2C
C
H2C
Note: there are also cis-1,4- and cis-1,2-polyisoprene
© 2007, 2008 Moonsub Shim, University of Illinois
n
CH3
24
MSE280
12
Molecular structure
So far, we’ve considered polymers as simple linear chain of one type of
monomer…
Linear structures
a) Linear homopolymer:
…--A—A—A—A—A—A—A—A—A—A—A—A—A—A—A--…
b) Linear copolymer:
…--A—B—A—B—A—B—A—B—A—B--…
A B A B A B A B A B
alternating copolymer
…--A—A—A—A—A—B—B—B—B—B--… block copolymer
…--A—B—B—A—B—A—A—A—B—B--… random copolymer
25
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Molecular structure
Star branching
© 2007, 2008 Moonsub Shim, University of Illinois
-B-B-B
B-B-B-B-…
Long branching
-B-B-B
B-B-B-B-…
Short branching
…A-A-A-A-A-A-A-A-A-A-A-…
-B-B-B
B-B-B-B-…
Branched structures
Graft copolymer
Dendrimers
26
MSE280
13
Molecular structure
Secondary bonding
between chains
secondary
Covalently linked
chains
bonding
Linear
Branched
Cross-Linked
Network
Direction of increasing strength
From Callister resource CD
Not always in this order (e.g. high-density vs. low-density polyethylene).
Why?
27
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Molecular structure
How do crosslinking and branching occur in polymerization?
1. Start with or add in monomers that have more than 2 sites that
bond with other monomers.
e g crosslinking polystyrene with divinyl benzene
e.g.
…
…
…
stryene
Control degree
of crosslinking
by styrenedivinyl benzene
ratio
…
polystyrene
…
…
+
styrene
divinyl benzene
crosslinked polystyrene
Monomers with three or more polymerizable groups lead to network polymers.
28
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
14
Molecular structure
Branching in polyethylene (back-biting)
H2C CH2
R
H2
C
C
H2
H2
C
C
H2
Same as
H
H
C
H
CH2
C
CH2
R
H2
C
C H
H
H
Radical moves to a different
carbon
(H transfer)
C
H H2
H
C
H
CH2
C
R
CH2
C
H H2
Polymerization continues from this carbon
This process is usually difficult to avoid and leads to low-density polyethylene
(highly branched).
When there is small degree of branching: high-density polyethylene.
29
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Molecular structure
copolymers
Block
Alternating
Random
Graft
30
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
15
Example 2
A nitrile rubber copolymer, co-poly(acrylonitrilebutadiene) is found to have
butadiene),
M n = 106,740 g / mol
and
nn = 2000
Calculate the ratio of number of acrylonitrile
y
to number
of butadiene.
31
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Crystalline or Amorphous
We’ve seen crystalline and amorphous materials in metals and ceramics.
What about polymers?
© 2007, 2008 Moonsub Shim, University of Illinois
32
MSE280
16
Polymer crystallinity
Some are amorphous, some are
partially crystalline (semi-crystalline).
-Why is it difficult to have a 100%
crystalline polymer?
%crystallin
t lli ity
it =
ρc ( ρ s − ρ a )
×100%
ρ s (ρc − ρa )
ρs = density of specimen in question
ρa = density of totally amorphous polymer
ρc = density of totally crystalline polymer
33
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Polymer crystallinity
%crystallinity =
M crystalline
M total
×100% =
ρ cVc
ρ
×100% = c f c × 100%
ρ sVs
ρs
What is fc? Volume fraction of crystalline component.
To get fc :
M total= M crystalline + M amorphous
Ms = Mc + Ma
ρ sVs = ρ cVc + ρ aVa
ρ s = ρc
Vc
V
+ ρa a
Vs
Vs
34
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
17
Polymer crystallinity
Using definition of volume fractions:
fc =
Vc
Vs
and
fa =
ρ s = ρ c f c + ρ a f a = ρ c f c + ρ a (1 − f c )
ρ s = fc (ρc − ρa ) + ρa
ρ − ρa
fc = s
ρc − ρ a
Va
Vs
Substituting
g in fc into the original
g
definition,, we get:
g
%crystallinity =
ρc ⎛ ρ s − ρa ⎞
⎜
⎟ ×100%
ρ s ⎜⎝ ρ c − ρ a ⎟⎠
35
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Polymer crystallinity
Degree of crystallinity depends on processing conditions (e.g.
cooling rate) and chain configuration.
Cooling rate: during crystallization upon cooling through MP,
polymers become highly viscous. Requires sufficient time for
random & entangled chains to become ordered in viscous liquid.
Chemical groups and chain configuration:
More Crystalline
Less Crystalline
Smaller/simper
S
ll / i
side
id
groups
L
Larger/complex
/
l side
id groups
Linear
Highly branched
Crosslinked, network
Isotactic or syndiotactic
Random
36
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
18
Semicrystalline polymers
Fringed micelle model: crystalline region embedded in amorphous
region. A single chain of polymer may pass through several
crystalline regions as well as intervening amorphous regions.
37
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
Semicrystalline polymers
Chain-folded model: regularly shaped platelets (~10 – 20 nm thick)
sometimes forming multilayers. Average chain length >> than
platelet thickness.
38
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
19
Semicrystalline polymers
Spherulites: Spherical shape composed of aggregates of chainfolded cystallites.
Natural rubber
Cross-polarized light
through spherulite
structure of polyethylene.
© 2007, 2008 Moonsub Shim, University of Illinois
39
MSE280
Diblock copolymers
F. Bates, Science 1991.
40
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
20
Concepts to remember…
•
•
•
•
•
•
•
•
•
Classification: thermosets and thermoplastics.
Monomers and chemical functional groups.
Nomenclature.
Polymerization methods: free radical, addition, and other
polymerization.
Number & weight average molecular weights,
polydispersity & degree of polymerization.
Sequence isomerism, tacticity, structural isomerism
Molecular structures: linear, branched, crosslinked,
network…
Copolymers: block, alternating, random and graft.
Crystallinity: fringed micelles & chain-folded models and
spherulites.
41
© 2007, 2008 Moonsub Shim, University of Illinois
MSE280
21