http://web.umr.edu/~wlf/CHEM381/index.html
Historical Introduction and Definitions
What is a polymer?
A polymer is a large molecule comprised of repeating structural units
joined by covalently bonds. Poly comes the Greek word for "many"
and mer comes from the Greek word for "parts."
We could arbitrarily say a polymer has to have at least 100 repeats
for now, but later we will change this definition to account for physical
properties of polymers. The repeat unit of a polymer is a group of
atoms covalently bonded together in a specific spacial arrangement.
Diana Chiang at MIT has written a monograph that discusses
metals,
polymers and
ceramics,
semiconductors.
's Introduction to Polymers.
Introductory Organic texts talk about a homologous series that runs
methane, ethane, propane, ..., decane, ..., and so on...
This homologous series can be thought of as two methyl groups, CH3, tied together by many methylene groups, -CH2-. As you extend
this series, you create polyethylene.
CH3 - CH2 - CH2 - CH2 - ... - CH2 - CH2 - CH3
or
CH3 [ - CH2 - ] - CH3 (a shorthand method)
n
or finally
[ - CH2 - ] n
n is a subscript to count the number of repeat units. This last
shorthand method requires that you remember to add a hydrogen to
each end group (CH3 on the end rather than CH2.)
Flowchart of Polymeric Materials
Inorganic

Natural
o Clays
Bricks
 Cement
 Pottery
o Sands
 Glass
Synthetic
o Fibres


Organic


Synthetic
o Adhesives
o Fibres
o Coatings
o Plastics
o Rubbers
Natural
o Polysaccharides
 Adhesives
 Fibres
o Proteins
 Adhesives
 Fibers
o Polyisoprene
 Rubbers
Polymer Classification (addition vs. condensation)
Polymers can be classified as addition polymers or condensation
polymers. This comes from a time when it was simple enough to ask
"Did you break a double bond to make your polymer (addition
polymerization) or did you eliminate a by-product such as water,
methanol, or hydrogen chloride (condensation polymerization)?"
There is good reason to segregate polymers into these two
classifications because the method of polymerization dictates what
problems the engineer will encounter in manufacturing the product.
This will be discussed in more detail further on, along with examples
of polymers that fit neither classification such as epoxies and
polyurethanes.
Polymerization Process (chain vs. step)
A polymerization process can be described as either a chain reaction
or a step reaction. This focuses the discussion to some polymers that
don't fit into either of the two strict definitions proposed above.
Your discussion of free radical processes in Organic I lecture ties
provides ideas needed to understand addition polymerization, and
and your coverage of condensation reactions in Organic II lectures
ties in to condensation polymerization.
Throughout the notes there are times when a phenomenon may be
described, but the notes don't say if it applies to both chain and step
polymerization, or just one of the two. I'll try to clear this up.
Chain polymerization polymers
Examples include:

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





polyethylene
polystyrene
poly(vinyl chloride)
poly(vinyl acetate)
poly(methylmethacrylate)
polypropylene
poly(tetrafluoroethylene) [ a.k.a. teflon]
poly(isoprene)
Step polymerization polymers:
Examples include:




polyurethane
PET and other polyesters
epoxies
silicones
The preparation of polyurethane by the reaction of an isocyanate with
an alcohol illustrates a "non-addition" polymerization that does not
give off a by product. The hydrogen of the alcohol bonds to the
carbon of the isocyanate.
Fundamental Definitions:
CopolymerA mixture of two polymers. It may be
composed of two bifunctional units and may alternate to give a welldefined recurring unit or the two different monomers may be joined in
a random fashion in which no recurring unit can be defined. A
copolymer contrasts with a homopolymer.
I've never heard anyone say "the term copolymer applies
only to addition polymers" but this seems to be the case.
Later on, you will discussion the synthesis of polyethylene
terephthalate (PET), the polymer used for plastic two liter soda
bottles (see illustration to the left), which involves two chemicals,
terephthalic acid and ethylene glycol by a condensation
reaction. No one ever calls PET a copolymer.
A copolymerization that results in A-B-A-B-A- etc., is called an
alternating copolymer.
A copolymerization where the sequence of A's and B's is random, AA-B-A-B-B-A-B-A-B-B-B-A etc is an example, is called a random
copolymer.
You may wonder at this point, "what if it isn't alternating, but it isn't
completely random either?" This would get into the idea of reactivity
ratios, which are discussed in the sequel to this class when
copolymerization is covered.
A block copolymer is built from first one polymer, and then another,
as in A-A-A-A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B-B-B-B.
A graft copolymer is shown below, where a polymer of 'B' was
grafted onto a polymer of 'A'.
-A-A-A-A-A-A-A-A-A-A|
B
|
B
|
B
|
shows model graphics of random, block, and graft copolymers
(type 'copolymers' into the find function when you arrive at the site.)
Linear polymer- a straight chain species, i.e., the units are
connected to each other in a chain arrangement. Linear polymer
contrasts with branched polymer and crosslinked polymer.
for more on crosslinked polymers:
(type 'linking' into the find
function when you arrive at the site.)
will talk about star polymers and dendrimers (type
'branch' into the find function when you arrive at the site)
Monomer- the building block or structural unit of the polymer. For
polyethylene, the building block or structural unit is
H H
| |
C-C| |
H H
Homopolymer- a polymer containing a single repeat unit. A
homopolymer contrasts with a copolymer.
Degree of Polymerization (n)- the number of monomer units that
have polymerized together. D.P. values can be as high as 10,000.
Please contact me if you are aware of any polymers which can be
synthesized with higher D.P. values.
(type 'degree' into the find function when you arrive at the
site)
Crosslinkage- The formation of crosslinks. Long polymer chains
form because each bifunctional monomer unit has two "bonding
sites" so it can link to two other monomers. You should be able to see
how C=C gives two bonding sites in addition polymerization. Now, if
you include monomers which possess three bonding sites (see A*
below), then when one of these trifunctional monomers is
incorporated into a polymer chain, it has a third site that monomers
can attach to:
- A - A - A* - A - A - A - A |
A
|
A
Monomers which make the top 50 chemicals list
April 10, 1995 Chemical and Engineering News
CHEMICAL
RANK production in tons
Ethylene
Propylene
Styrene
Terephthalic Acid
Acrylonitrile
Vinyl Acetate
Adipic Acid
4th
7th
20th
24th
39th
40th
46th
48,530,000,000
28,840,000,000
11,270,000,000
8,640,000,000
3,089,000,000
3,020,000,000
1,800,000,000
Bisphenol A
48th
1,480,000,000
Physical Properties of Polymers
The Physical Properties of a polymer material are largely determined
by:
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


molecular weight
strength of intermolecular forces
regularity of the polymer structure
flexibility of the polymer molecule
A list of physical properties might include:
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melting point
boiling point
solubility
melt viscosity
tensile strength
Melting point- the melting point of a polymer does not occur over a
sharp temperature range (1- 2 degrees C) as is observed for small
organic molecules. If a polymer becomes a melt, there is usually a
range of as much as 50 degrees C over which the viscosity of the
polymer slowly changes from that of a solid to that of a liquid. Note
that for a polymer to melt, a polymer must be a thermoplastic.
Using the term "melting point" is missleading, because
polymers never "melt." The term "softening point" is sometimes
used. A polymer does have a glass transition temperature (Tg)
and a crystalline melt temperature (Tm.) The glass transition
temperature refers to a point where there is a change in
polymer molecular chain motion which has drastic effects on
strength. Tg is sometimes called the "glass-rubber" transition.
The crystalline melt temperature (Tm) is higher than Tg, and at
Tm the crystalline domains of a polymer melt to become
amorphous.
(the glossary definition of 'glass transition')
Melting pt. vs. # of carbons for straight chain alkane homologs
Boiling point- Polymers never boil.
Boiling pt. vs. # of carbons for straight chain alkane homologs
You may recall from Organic that as the number of carbons in the
alkane homologous series increases, the boiling point increases
asymptotically. We might chose to define the a polymer as a growing
chain of sufficient length such that the mechanical properties are
about 80% that of the apparent assomptotic limit. The number 80 is
arbitrary. I've seen no reference that set a standard number for this.
In the example below, chains of sufficient length to give a minimum
performance for some mechanical property in the yellow-green zone
are called polymers.
(type "increasing" into the find function when you arrive at
the site)
Solubility
Most polymers are insoluble in water.
Some polymers can be soluble in strong organic solvents. Polymer
nonsolubility is an advantage for a finished product. However, it may
present a tiresome problem for the engineer who is trying to
manufacture a product.
Melt viscosity- ??
Tensile strength- Tensile strength measures how difficult it is to
break a substance when stress is applied to pull it apart. Tensile
strength generally increases with molecular weight. An Instron grips a
sample and pulls it apart. The instron creates a plot of where the y
axis is the force exerted between the two grips, and the x axis is the
separation distance between the two clamps. Usually, the electronics
cause the Instron to increase the distance between the two clamps
(pulling) at a constant rate, and the force or "strength" required is
measured. This is somewhat analogous to when you were a little kid
at the supermarket, and you pulled down on a scale in the Produce
section, and watched the scale readout increase.




A- Stress at the Yield Point: At this point the load (i.e., the
force of the tensile pull) is sufficient to cause polymer chains to
slip past each other, and yield occurs.
B- Elongation at the Yield Point: This is how far you can
stretch the polymer before yield failure takes place.
C- Stress at Failure: At this level of stress the polymer
molecules can no longer maintain cohesive integrity and the
sample breaks.
D- Elongation at Failure: This is how much the sample can
stretch before failure occurs. Elongation Failure is often
reported as follows: a 10 cm specimen that breaks at 15 cm is
reported as a sample with 50% elongation. For this test you
take a sample and clamp it with 10 cm between the two clamps,
and then pull. Thus, you would need a sample with a length of
10 cm plus twice whatever length is required to go between the
clamps.
Tensile strength numbers (psi)- 145 psi = 1 MPa
polyethylene (low to medium density)
1,000- 2,400
poly(tetrafluoroethylene) (a.k.a. teflon) 3,500
polyethylene (high density)
4,400
poly(dimethylsiloxane)
5,000
polypropylene
5,000
poly(vinylidene chloride)
8,000
polystyrene
8,000
polyamides
9,000 to 12,500
polycarbonate
9,500
polyesters (cast- as opposed to molded) ~10,000
polysulfone
10,200- 12,000
poly(phenylene oxide)
10,500
Table from Allcocke and Lampe: edition 1, chapter 21, "The Testing
of Polymers" (Excellent reading!)
This is a tensile instrument.
Full picture availableUniversity of Southern Mississippi Polymer Science Department
photo
What holds molecules together?

Van der Waals forces- These forces are about 1-3 kcal/mole,
and are considered "weak." Van der waal forces are


responsible for hydrogen gas (H - H) becoming a liquid when
close to absolute zero.
Electrostatic- Ionic bonds are examples of electrostatic forces
Hydrogen bonding- these are relatively weak, with values of 3-5
kcal/mole
Molecular cohesion
The higher the chain interaction, the higher the molecular cohesion.
Solubility os determined by intermolecular forces. If you have small
forces, then the molecular cohesion energy is low.
Molecular cohesion per 5 angstroms of polymer chain. Values around
5 or higher on the scale below should be considered "high."
polyethylene
1.0 kcal/mole
polyisobutylene
1.1 kcal/mole
rubber
1.3 kcal/mole
poly(vinyl chloride) 2.6 kcal/mole
poly(vinyl acetate) 3.2 kcal/mole
poly(styrene)
4.0 kcal/mole
poly(vinyl alcohol) 4.2 kcal/mole
The notes list a polyamide at 5.8 but don't specify which one.
Last Update- July 8, 1995- wld