Chain Microstructure

1
Chain Microstructure
TOPICS COVERED -

• Linear chains,branching
• Cross - linking and network formation
• Sequence isomerism
• Stereoisomerism in vinyl polymers
• Diene polymers (structural isomerism)
• Copolymers and blends

2
Philosophical Approach
Illustrate the connection between structure and
properties right from the beginning. To do this
we need to remind ourselves about the nature of
crystallinity
RANDOM COIL Like cooked spaghetti
CRYSTALLINE
A bit like "uncooked spaghetti
3
The Effect of Crystallinity on
Properties
We will be asking how crystallinity affects

• Strength
• Stiffness
• Toughness
• Barrier Properties
• Solubility
• Transparency
• Thermal Properties
• Etc

4
Linear and Branched Polymers
Linear
Branched
Which of these is more likely to crystallize?
5
The answer is linear !
Crystallizes more like this
than this
Various grades of polyethylene are produced
commercially and are often referred to as high
density or low density. Which do you think is
the high density polyethylene
A. The linear, more crystalline stuff ?
B. The (somewhat) branched
less crystalline stuff ?
6
The answer is still linear !
Chains that cannot crystallize (e.g., highly
branched ones), or even linear chains that are
heated above their crystalline melting points,
actually look something like cooked spaghetti or
random coils.
They do not pack as closely together as in the
crystalline state.
7
The Effect of Crystallinity on
Properties
The type of polyethylene that goes into milk jugs
is stronger, stiffer, but more opaque (less
optically clear) than the type of polyethylene
that is used to make film wrap (greater optical
clarity,more flexible, but less strong) . Can you
figure out which type of polyethylene is used to
make film wrap ? A. High
density B. Low density
8
Property
Change with Increasing Degree of Crystallinity
Strength
Generally increases with degree of crystallinity
Stiffness
Generally increases with degree of crystallinity
Toughness
Generally decreases with degree of crystallinity
Generally decreases with increasing degree of
crystallinity.Semi-crystalline polymers usually
appear opaque because of the difference in
refractive index of the amorphous and crystalline
domains, which leads to scattering. The
scattering will depend upon crystallite size.
Optical Clarity

Small molecules usually cannot penetrate or
diffuse through the crystalline domains, hence
barrier properties, which make a polymer useful
for things like food wrap, increase with degree
of crystallinity
Barrier Properties

Similarly, solvent molecules cannot penetrate the
crystalline domains, which must be melted before
the polymer will dissolve. Solvent resistance
increases with degree of crystallinity
Solubility
9
Branching and network formation
Long Chain Branching
Star Polymer
10
Network Formation
Reacting Trifunctional Molecules
Reacting Tetrafunctional Molecules
What would happen if you reacted bifunctional
molecules ?
11
Network Formation
Here is what a small part of a phenolic resin
network looks like. We discussed these in
previous lectures.
12
More Network Formation
Networks can also be made by taking linear
polymer chains and linking them using covalent
bonds. We call this cross-linking
13
Network formation by cross-linking
An example of cross linking is the reaction of
natural rubber or poly(isoprene)
with sulfur (or, as we prefer, sulphur) . The
sulfur interconnects the chains by reacting with
the double bonds.
14
Network formation by cross-linking
15
Networks - Summary
We can make networks by

• Linking together small multi-functional monomers
• Cross - linking already formed chains

Note you can change properties dramatically by
changing the cross-link density
Think of the difference between a rubber band and
a rubber tire
16
Isomerism in Polymers
Two molecules are said to be isomers if they are
made up of the same number and types of atoms,
but differ in the arrangement of these atoms.

• Sequence isomerism
• Stereoisomerism (in vinyl polymers)
• Structural isomerism (in diene polymers)

17
Sequence Isomerism
When a monomer unit adds to a growing chain it
usually does so in a preferred direction.
Polystyrene, poly(methyl methacrylate) and
poly(vinyl chloride) are only a few examples of
common polymers where addition is almost
exclusively what we call head-to-tail.
R-CH -CXY (TH)
2
R CH2CXY
R-CXY-CH (HT)
T H
2
18
Sequence Isomerism
In many common polymers, such as polystyrene,
addition occurs almost exclusively in a
head-to-tail fashion.
Head to Tail
Head to Tail
Head to Tail
(TH)
(TH)
(TH)
(TH)
active site
part of growing chain
monomer about to collide with active site

19
Sequence Isomerism
Tail to Tail
Head to Tail
Head to Head
In other polymers, particularly those with
smaller substituents, head-to-head and
tail-to-tail placements can occur.
-
-
-
-
-
-
CH
CXY
CH
CXY
2
2
(TH)
(TH)
(HT)
(TH)
Example poly(vinylidene fluoride), -CH2-CF2- .
All head-to-tail units
Some head-to-head and tail-to-tail units
20
Stereoisomerism in Vinyl Polymers
Polymerization of a vinyl monomer, CH2 CHX,
where X may be a halogen, alkyl or other
chemical group (anything except hydrogen!) leads
to polymers with microstructures that are
described in terms of tacticity.
meso diad
racemic diad
21
Isotactic Chains
Part of an isotactic polypropylene chain
The catalysts developed by Ziegler and Natta
produced predominantly isotactic polypropylene,
but also atactic polypropylene.
22
Syndiotactic Chains
Here are two more polypropylene chains, both
shown as if we were looking down from on top.
One of these consists of units that are all
racemic to one another and is called
syndiotactic. The other has a random arrangement
of units and we call such chains atactic. Which
one is the atactic chain , A or B ?
A
B
23
Tacticity in Some Commercially
Important Polymers
Polystyrene - atactic Polypropylene -
largely isotactic PVC -
largely atactic
(Some syndiotactic sequences ?) PMMA
-atactic
24
Structural Isomerism
Diene Polymers
CH2 CX - CH CH2
X H we have butadiene
X CH3 we have isoprene
X Cl we have chloroprene
Where if
But, what is trans-1,4-polybutadiene,
cis-1,4-polyisoprene ?
25
Structural Isomerism
cis-1,4
trans-1,4
1 2 3 4
Isoprene
1,2 unit
3,4 unit
CH3
H
-
-
-CH2 - C -
-CH2 - C -
-
-
Microstructure of Poly(isoprene)
26
Structural Isomerism
What about cis and trans?
CH2 -
H
cis-1,4
H
trans-1,4
27
Copolymers
-A-B-B-B-A-A-B-A-B-A-A-A-B-A-B-B-A-B-B-A-A-A-B-
Random Copolymers
-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-
Alternating Copolymers
-A-A-A-A-A-A-B-B-B-B-B-B-B-B-B-B-A-A-A-A-A-A-
Block Copolymers
Graft Copolymers
28
Blends
Why are polymer blends important ? A
route to new materials Miscible
Immiscible
Single phase Phase
separated
29
Molecular Weight
Increasing
Molecular Weight
30
Molecular Weight Distributions
The problem with describing the molecular weight
of synthetic polymers is that there is always a
distribution of chain lengths (although certain
polymerizations can give very narrow
distributions).
31
Molecular Weight
Why is it important?
Mol. Wt.
Mol. Wt.
32
Making Plastic Bottles
Molecular Weight is very important in processing.
You dont want the viscosity too high, or the
polymer will be difficult to process. At the same
time, you want a material that is being extruded,
for example, to hold together until it
solidifies. Lets look at making plastic bottles
as an example.
33
Blow Molding
Bottles are made by blow molding. This is a
two-step process. In the first step, a preform is
made either by extrusion or injection molding.
The second step utilizes air pressure to inflate
the preform inside a closed, hollow mold. The
polymer expands to take the shape of the cooled
mold and solidifies while air pressure remains in
the part. After cooling, the mold halves open,
the part is ejected, and the next preform enters
the mold. Several variations of blow molding are
used, depending on the type of product being made.
34
Extrusion Blow Molding
Extrusion blow molding utilizes an extruder to
produce the preform to be blown. In a typical
arrangement for producing juice bottles, the tube
is extruded vertically downward to some
predefined length, as shown opposite. This tube
is commonly called the parison, from the Latin
word for wall. When the parison is the proper
length, the two open mold halves shuttle to a
position surrounding it, as also shown in the
movie. With the parison in position, the mold
closes and the tube is cut from the
extruder. The mold is designed to pinch the tube
and form a seal at the bottom, but leave the top
open. The mold then shuttles back to the blow
position. At this point, a blow pin is inserted
into the hole at the top of the parison,
pressurizing the parison against the mold walls
with air. After sufficient cooling, the blow pin
is removed and the part is ejected. While the
part was cooling, the next parison was being
extruded, so that it is ready to be taken by the
mold upon its return.
35
Extrusion Blow Molding
One of the more important things to understand
about extrusion blow molding involves the freely
suspended, molten parison. In some operations,
the parison may be several feet long and
suspended for many seconds before the mold
captures it. In all cases, the parison is acted
upon by gravity, so the polymer being used must
therefore have a good melt strength. Melt
strength can be defined as a high resistance to
(stretching) flow in the absence of shear, and
can be thought of in terms of an extrudate that
has a consistency that is more like taffy than
water. Certain polymers have been synthesized
specifically for good parison melt strength. For
example, some grades of high molecular weight,
high density polyethylene have good melt
strength.
36
Injection Blow Molding
Injection blow molding utilizes an injection
molder to produce the preform to be blown. In the
example we will consider here, a hollow preform
resembling a test tube is made by conventional
injection molding. The preform is then
transferred to a second mold where it is reheated
and blown into final shape, as shown in the
animation. Injection blow molding has some
advantages over extrusion blow molding. First, it
is capable of greater dimensional accuracy in the
final product because the injection mold can be
machined to produce a very accurate preform.
Second, an injection molded preform contains no
weld lines, unlike an extruded parison that has a
pinch-off at the bottom. If a blow molded product
is to be pressurized, like a soda bottle, the
weakness of a weld line may be unacceptable. Of
course, the fact that two costly molds are
required for injection blow molding is a
significant disadvantage.