Remember the purpose of this reading assignment is to prepare you for class. Reading for
familiarity not mastery is expected.
After completing this reading assignment and reviewing the intro video you should be comfortable
either 1) Describing the following key concepts in your own words, or 2) asking focused questions
regarding the following key concepts.
How much chemistry is needed to understand the formation of polymers.
How long chain molecules based on carbon can form.
How materials made up of long chain molecules based on carbon can support mechanical
loads.
I.
The Structure and Properties of Polymers
Polymers are long chain molecules based on carbon. Like metals and ceramics, a single polymer
molecule may contain millions of individual atoms, all bonded together. Just as it is not possible to
determine how many aluminum atoms are in one molecule of Al(s); it is not possible to determine
how many carbon (C) and hydrogen (H) are in a single molecule of polyethylene -(CH2CH2)n–. The
average molecular weight is often on the order of 100,000 or more, indicating that there are
approximately 8000 carbon atoms and 16,000 hydrogen atoms in a single molecule of polyethylene.
Also, as in the case of solid aluminum, each of the molecules in the solid material will have a
different molecular weight.
A.
Basic Chemistry Review1
Polymers, like metals and ceramics, solidify. Metals and ceramics are usually solidified by cooling
from the melt, polymers typically solidify through a (or a series of) chemical reactions. Because of
this it is important to understand the basic chemistry responsible for polymers, which is the
chemistry of carbon.
Carbon has an atomic number of 6, and is the smallest
atom with 4 valence electrons. Carbon will form covalent
bonds with itself, oxygen, hydrogen, nitrogen, and the
halogens (F, Cl, Br, and I). Covalent bonds arise when a
pair of electrons is shared between two atoms. This is
shown for a single molecule of ClF in Figure 1. Note the
Figure 1: Covalent Bond between
two electrons occupy a region of space between the centers
Chlorine and Fluorine
of the two atoms. This, space or orbital, is well-defined for
each bond. The bond shown in Figure 1 is a single bond. In
some cases, 2 pairs of electrons are shared between two atoms. This is called a double bond. Each
pair of electrons occupies a distinct region of space. Thus there are two separate bonds in a double
bond. Triple bonds can also form.
In order to form a covalent bond it must be possible to form an electron pair
between two atoms. In order to predict the structure of an organic (organic means based on carbon) molecule, one needs to first determine how many
unpaired electrons exist in each atom. These unpaired electrons are available
for bonding. To draw the Lewis structure for an element one counts the
number of valence electrons in an element and places them around the
elemental symbol in the order shown in Figure 2. Note that the first four
electrons will be unpaired, the next four will form pairs. The actual placement
of electrons is not critical, however what is critical is that the first four
electrons are unpaired and only the latter four are paired. Thus, oxygen with 6
valence electrons, would have two sets of paired electrons, and two unpaired
electrons. The Lewis structures for the most important elements in polymers
are shown below.
1
Figure 2: Lewis
Dot Structure for
Ne showing the
placement of
electrons
The Chemistry presented in this review is intended to be a brief review of material
presented in a fundamental chemistry course as it applies to polymers. Nothing presented in this
section should be taken outside this context. The information presented in most Chemistry texts will
be more complete.
Carbon - 4
valence electrons
Hydrogen - 1
valence electron
Nitrogen - 5
valence electrons
Oxygen - 6
valence electrons
Chlorine - 7
valence electrons
Hydrogen therefore can only form one covalent bond, meaning that hydrogen can only bond to one
other atom. Chlorine also has only one unpaired electron, so chlorine can only bond to one other
atom. Oxygen has two unpaired electrons, thus it can form two single bonds, bonding to two other
atoms, or it can form a double bond with a single atom. Nitrogen has three unpaired electrons, which
means it can form 3 single bonds, a single bond and a double bond, or a triple bond. Carbon has four
unpaired electrons. Carbon can therefore form four single bonds, two double bonds, or a single and a
triple bond.
When determining the structure of an organic molecule, one
needs to be aware of the number of bonds. For example, the
structure of CH3COOH is shown in Figure 3. Note in each case,
•
Hydrogen (H) is bonded to only one atom. Three
hydrogen atoms are bonded to a carbon(C) atom, and one
to an oxygen (O) atom.
•
Oxygen (O) forms two bonds. In one case an oxygen atom
forms a double bond with a single carbon atom. In the
other case oxygen is bonded to two other atoms - carbon
and hydrogen.
•
Figure 3: Lewis Structure of Acetic
Acid
Carbon (C) forms four bonds. In one case carbon forms
four single bonds, bonding with three hydrogen atoms
and one other carbon atom. In the other, carbon forms one double bond and two single bonds.
It forms a double bond with an oxygen atom, and a single bond with both another carbon
atom and another oxygen atom.
Also note that the structure shown in Figure 3 resembles the chemical formula, CH3COOH.
The molecule shown in Figure 3 has a functional group - COOH. This is an acid group. Functional
groups contain the atoms which participate in chemical reactions. We will limit our consideration to
three types of functional groups. These are shown below.
Alcohol
Amine
Acid
Note, R is used to indicate an “organic root”. It can be anything for example in Figure 3, “R” refers to
the “CH3" group.
Organic molecules react and form new organic molecules, that it the focus of organic chemistry. The
functional groups shown above, can react with each other forming water and a “linkage”. Two of
these reactions are shown below. The formation of an amide is shown in Figure 4 and that of an ester
is shown in Figure 5. Note water is a by-product of both reactions.
Figure 4: Reaction between an amine and an
acid to form an polyamide
B.
Figure 5: Reaction between an alcohol and an
acid to form an ester
Polymerization Reactions
The reactions shown in Figure 4 and Figure 5 would not form polymers. The product would be a new
molecule and water. However, in the cases shown in Figure 4 and Figure 5, each reactant only has
one functional group.
1.
Condensation
If each reactant has two functional groups then it is
possible to form a polymer. This type of reaction, like
the reactions shown in Figure 4 and Figure 5 are
condensation reactions. Water is a product of the
chemical reaction. The same is true for condensation
polymerization. Water is a product of the reaction.
While, in industrial practice it is common for the byproduct to be a chemical other than water, we are going
to restrict our discussion to reactions where water is a
by-product.
Figure 6: Two reactants which could form
Consider the potential product of reacting,
a polyamide
HOOC(CH2)6COOH and H2N(CH2)6NH2 as shown in
Figure 6. This reaction is very similar to the
one shown in Figure 4. The amine
functional group (NH2) and the acid
functional group (COOH) will react and
form an amide “linkage” -NHCO- and
water. This is shown below in Figure 7. The
reaction will not cease in this case. There is
no possibility that one and only one
HOOC(CH2)6COOH molecule reacted with
one and only one H2N(CH2)6NH2 molecule.
Therefore, more than one of the product
molecules formed. The product molecule
has an amine functional group (NH2) and
Figure 7:The reaction of a difunctional alcohol and a
an acid functional group (COOH). These
difunctional amine.
can react and form an amide “linkage” NHCO- and water. This is shown in Figure
8. The reaction will continue and a molecule
of “n” units will form. The single unit
enclosed in [] is called a mer, and because
the molecule has many mers, it is called a
polymer. The []n is important it clearly
denotes that the material is a polymer.
Figure 8: Formation of a polyamide from the product
of a difunctional alcohol and a difunctional amine
2.
Addition
Some polymers are formed without producing a by-product. There are called addition polymers.
Before discussing addition polymers, or the reactions by which they are formed, two facts must be
made clear.
First, a carbon-carbon double bond is not two carbon-carbon single bonds. The second bond, is not as
strong as the first bond. It is called a B-bond and overlaps the first bond which forms. Therefore,
energy is reduced if a carbon-carbon double bond is replaced by two carbon-carbon single bonds.
Second, it is possible for a molecule to exist with an unpaired electron, although only for a short time.
Such a molecule is electrically neutral, it has no charge. These molecules are extremely reactive.
Molecules containing carbon-carbon double bonds can be attacked by free-radicals and form
polymers. This is because, the product of the reaction is a free radical, and can react with another
molecule containing another carbon-carbon double bond. Just like the product of the condensation
reaction had two functional groups and could react with another molecule, so can the free-radical.
An addition polymer is formed by adding a small (maybe 1 part per
million) amount of initiator and monomer ( a molecule with a carboncarbon double bond). In the first step the initiator forms two free radicals.
This is shown in Figure 9, where the O-O bond in H2O2 splits and forms
two HO free radicals. Note, there is no charge on the HO free radical, it is
not an OH- ion. This occurs because the O-O bond is weak, requiring only
142 kJ/mol of energy to break. In contrast C-H single bonds require 347
kJ/mol of energy to break. In the organic reactions we will discuss
C-H, C-C and C=O bonds will not break.
The free-radical will react with the second bond in the C=C bond. The
product which forms is a free-radical as shown in Figure 10. The freeradical (the HO molecule with an un-paired electron) will form a C-O bond
leaving an un-paired electron on the molecule. Note, this free-radical is
also unstable. The reaction can thus continue and a polymer will form as
shown in Figure 11. Because only a small amount of initiator is used, it is
unlikely that the same double bonded molecule will be attacked by two
free-radicals. If this happened the reaction would terminate and no
polymer would form. Because the product of the reaction shown in Figure
10 and Figure 11 are reactive (just like the product shown for condensation
polymerization) the chemical reaction will continue and a polymer will
form. The polymer which forms is shown in Figure 12.
Note, the HO groups are not shown as part of the polymer.
This is because they are present in very small amounts.
Figure 9: Formation of
two Free Radicals from
Hydrogen Peroxide
Figure 10:Reaction of
Free-Radical and
Molecule Containing
C=C Bond
Figure 11: Continuation of Reaction
Shown in Figure 10
Figure 12: Polymer
Which Forms from
Reaction shown in
Figure 11
3.
Cross-linking
Cross-linked polymers are polymers where the chains are
physically connected by other polymer chains. Such
molecules are really three-dimensional networks.
Crosslinked polymers can form through either addition or
condensation polymerization.
Condensation polymers from because the product of a
chemical reaction has two functional groups which can
keep reacting with each other. If the product contains more
than two functional groups than it is possible to connect the
chains which form. This is shown schematically in Figure
13. The polymer which forms will have extra functional
groups as shown below in Figure 14
Figure 13: Reactants to Form a
Crosslinked Polymer through
Condensation Polymerization
Figure 14: Polymer Formed from Reactants Shown in Figure
13
The arrows in Figure 14 show the extra functional groups which could continue to react. This
reaction is shown in Figure 15.
Figure 15: Reaction of Extra Functional Groups Prior to the Formation of a Crosslink
The extra OH group shown in Figure 15 can react with the extra COOH group in Figure 15. The
result is shown in Figure 16
Figure 16: Crosslinked Polymer formed by Condensation Polymerization. This is the Product of the
Reaction Shown in Figure 15
Note the polymer chains are physically connected, it is essentially one molecule.
While crosslinking polymerization as shown in Figure 13
through Figure 16 is easy to visualize, in most cases
crosslinking occurs through addition polymerization. To
form a crosslinked polymer through addition
polymerization it is required that the polymer have an
extra reactive group, just like an extra functional group
was required to form a crosslinked polymer through
condensation polymerization. Thus, a polymer with a C=C
bond in the chain can crosslink. The most common example
is polybutadiene. This is shown in Figure 17. The monomer
CH2CHCHCH2 reacts with a small amount of H2O2.
Figure 17: Formation of
Addition polymerization occurs and the polymer forms as
PolyButadiene
shown. As was shown for crosslinking through
condensation polymerization in Figure 15 and Figure 16 ,
when the polymer contains reactive groups crosslinking can occur.
The cross linking of polybutadiene can also occur with the
addition of sulfur. This is called vulcanization. Sulfur has
two unbonded electrons. This is shown in Figure 18. The
reaction is more complicated than S2 (which does not exist
in nature) reacting with the polymer shown in Figure 17.
However, it is clear in Figure 18, that two double bonds in
adjacent polymer molecules have been replaced by a series
of single bonds, which connect the two molecules. It should
be noted that many epoxies are also crosslinked. The epoxy
group is shown in Figure 19. The
bonds in the COC group have a
bond angle of 60o. The
equilibrium angle is 109.5o. Thus
there is a lot of excess energy in
the molecule and it is very
reactive. Polymers with epoxy
groups in the chain can be
Figure 19: Epoxy
crosslinked.
Group
C.
Figure 18: Cross linking of
Polybutadiene.
Mechanical Properties of Polymers
As in all materials the mechanical strength of polymers depends on structure. The strength of
polymers can vary greatly, and the origin of this variation is the structure
of the molecule.
1.
Origin of Strength - Inhibition of Chain Sliding
Consider that a polymeric material (the size of which you could hold in
your hand) contains millions of long chain molecules. Each of these long
chain molecules could be a polymer where the degree of polymerization “n”
is 105-106. This means that the molecule contains several hundred
thousand C-C bonds in the chain. There is no way (except in very
exceptional cases) that the chain will be one straight line. Rather each
individual chain will be jumbled, and the combination of chains will be
entangled. This is shown in Figure 20.
Figure 20:
Arrangement of
Polymer Chains
When the polymer is subjected to a mechanical load the chains shown in Figure 20 disentangle. If
this disentanglement is difficult the polymer is strong. If not the polymer is weak. In order for
disentanglement to occur the chains must slide past each other. Thus polymers in which chain
sliding is easy are weak, those in which chain sliding is difficult are strong.
All polymers have some strength because of the large number of intermolecular forces between the
polymer chains. Recall from chemistry, that the magnitude of intermolecular forces, effect the
melting and boiling point of water, alcohol, chlorine and many other chemicals. These forces are also
weak compared to the covalent, ionic, or metallic bonding which holds the molecules together.
However, because polymers are such large molecules the large number of these interactions inhibit
chain sliding and give the polymer strength.
There are 4 features which inhibit chain sliding
•
Connecting the chains through crosslinking will be the most effective method of inhibiting
chain sliding. Therefore, crosslinked polymers will be among the strongest.
•
Hydrogen bonding is the strongest of the intermolecular forces. If the polymer has a chemical
structure where hydrogen bonding can occur between the polymer chains, chain sliding will
be inhibited and the polymer will be strong. This is the case for nylons.
•
If the polymer has benzene rings (aromatic) groups in the chain, chain sliding will be
inhibited.
•
If the polymer has large side groups chain sliding will be inhibited as the bulky side groups
will interfere with chain sliding.
Hydrogen bonding can occur when there are F-H, O-H, or N-H bonds in the
polymer. Note, C-H bonds do not participate in hydrogen bonding. This is
shown in Figure 21. The hydrogen attached to the nitrogen becomes a bare
proton, which is an extremely concentrated unit of positive charge. This
can interact with the electron cloud from the adjacent molecule. Because
there will be an extremely large number of hydorgen bonds between the
polymer chains, the chain sliding is inhibited and the polymer is very
strong. Hydrogen bonding cannot occur in molecules such as polyethylene,
polyvinyl chloride, or polycarbonate. As shown in Figure 22, there are no
F-H, O-H, or N-H bonds in any of the polymers.
Figure 21: Hydrogen
Bonding Between two
Kevlar Polymer Chains
Figure 22: Polymers in which Hydrogen Bonding is Not Possible
Polycarbonate is a very strong polymer. As shown in Figure 22 polycarbonate has two benzene rings
in the polymer chain. It also has two CH3 side groups. The benzene ring in the polycarbonate chain,
-C6H4- , although symbolized by a circle inside a hexagon, is quite large. Note, also that it is not
possible for polycarbonate as shown in Figure 22 or Kevlar as shown in Figure 21 to crosslink. There
are no carbon-carbon double bonds in the polymer chain. Benzene rings do not contain carbon-carbon
double bonds. The six electrons, symbolized by the circle, are shared equally among the six carbon
atoms. This makes the ring itself, very stable, and thus it will not break enabling crosslinking.
Two of the polymers shown in Figure 22, polyvinyl chloride and
polycarbonate, have side groups. The Cl in polyvinyl chloride and
the two CH3 groups in polycarbonate. These are moderate size side
groups and will inhibit chain sliding. Some polymers such as
polymethylmethacrylate, which is shown in Figure 23 have very
large side groups. It would be very difficult to slide chains of this
polymer past each other.
2.
Thermoplastics and Thermosets
Polymers are often classified as thermoplastics or thermosets. The
structure of the polymer determines whether it is a thermoplastic
or a thermoset.
Figure 23:
Polymethylmethacrylate
Thermosets cannot be reformed when heated. In fact, they do not
melt, they sublime. Crosslinked polymers are thermosets. Because the chains are connected it is not
possible to use thermal energy to enable chain sliding.
Thermoplastics can be reformed on heating. Thermal energy enables chain sliding at moderate loads.
Polymers which are not crosslinked are thermoplastics.
3.
Crystallinity
There are two types of polymers: amorphous and semicrystalline. No polymer is completely
crystalline. Semicrystalline polymers are those where there are crystalline and amorphous regions in
the polymer chain. Crystalline regions are those where the chain can neatly fold over on itself.
Amorphous regions are those where chain folding does not occur. Bulky side groups and benzene
rings in the chain inhibit chain folding and increase the likelihood that the polymer will be
completely amorphous.
D.
Temperature Dependent Mechanical Properties
The mechanical strength of polymers changes dramatically between temperatures between -50oC and
200oC. Polypropylene loses 30% of its strength when raised from 25oC to 50oC. This means that
normal weather conditions can cause marked decrease in the strength of polymers. This must be
considered when using them in designs. Most metals and ceramics which would be considered for
load bearing applications do not show this behavior at temperatures we commonly experience.
As stated earlier no polymer is completely crystalline. Thus the glass transition temperature Tg is a
critical design consideration. There are four cases to consider.
•
An amorphous polymer at temperatures below the glass transition temperature will behave
as a rigid solid. It will be brittle and fairly strong. This is shown in Figure 24.
•
An amorphous polymer at temperatures above the glass transition temperature will be have
as viscous liquid. Strength will be minimal and the material will creep (flow under constant
load) readilly. This is shown in Figure 26.
•
A semicrystalline polymer at temperatures below the glass transition temperature will
behave as 2 rigid solids. It will be brittle and fairly strong. This is shown in Figure 24.
•
A semicrystalline polymer at temperatures above the glass transition temperature will
behave as a rigid solid surrounded by a viscous liquid. The strength will be significantly