Strength
Strength is a mechanical property that you should be able to relate to, but you might not know exactly what we mean by the word "strong" when we're talking about polymers. First, there is more than one kind of strength. There is tensile strength. A polymer has tensile strength if it is strong when one pulls on it like this:
Tensile strength is important for a material that is going to be stretched or under tension. Fibers need good tensile strength.
Then there is compressional strength. A polymer sample has compressional strength if it is strong when one tries to compress it, like this:
Concrete is an example of a material with good compressional strength. Anything that has to support weight from underneath has to have good compressional strength.
There is also flexural strength. A polymer sample has flexural strength if it is strong when one tries to bend it, like this:
There are other kinds of strength we could talk about. A sample torsional strength if it is strong when one tries to twist it. Then there is impact strength. A sample has impact strength if it is strong when one hits it sharply and suddenly, as with a hammer.
What is Strength?
But what does it mean to be strong? We have a very precise definition. Let's use tensile strength to illustrate. To measure the tensile strength of a polymer sample, we take the sample and we try to stretch it just like in the picture above. We usually stretch it with a machine such as an Instron. This machine simply clamps each end of the sample, then, when you turn it on it stretches the sample. While it is stretching the sample, it measures the amount of force (F) that it is exerting. When we know the force being exerted on the sample, we then divide that number by the cross-sectional area (A) of our sample. The answer is the stress that our sample is experiencing.
Likewise, one can imagine similar tests for compressional or flexural strength. In all cases, the strength is the stress needed to break the sample.
Since tensile stress is the force placed on the sample divided by the cross-sectional area of the sample, tensile stress, and tensile strength as well, are both measured in units of force divided by units of area, usually N/cm2. Stress and strength can also be measured in megapascals (MPa) or gigapascals (GPa). It's easy to convert between the different units, because 1 MPa = 100 N/cm2, 1 GPa = 100,000 N/cm2, and of course 1 GPa = 1,000 MPa.
Other times, stress and strength are measured in the old English units of pounds per square inch, or psi. If you ever have to convert psi to N/cm2, the conversion factor is 1 N/cm2 = 1.45 psi.
But there's more to understanding a polymer's mechanical properties than
merely knowing how strong it is. All strength tells us is how much stress
is needed to break something. It doesn't tell us anything about what
happens to our sample while we're trying to break it. That's where it
pays to study the elongation behavior of a polymer sample.
Elongation is a type of deformation. Deformation is simply a change in
shape that anything undergoes under stress. When we're talking about tensile stress, the sample deforms by stretching, becoming longer.
We call this elongation, of course.
Usually we talk about percent elongation, which is just the length the
polymer sample is after it is stretched (L), divided by the
original length of the sample (L0), and then multiplied
by 100.
Ultimate elongation is important for any kind of material. It is nothing
more than the amount you can stretch the sample before it breaks. Elastic
elongation is the percent elongation you can reach without permanently
deforming your sample. That is, how much can you stretch it, and still
have the sample snap back to its original length once you release the
stress on it. This is important if your material is an elastomer. Elastomers have to be able to stretch a
long distance and still bounce back. Most of them can stretch from 500 to
1000 % elongation and return to their original lengths without any
trouble.
Elastomers need to show high elastic elongation. But for some other types
of materials, like plastics, it usually better
that they not stretch or deform so easily. If we want to know how well a
material resists deformation, we measure something called modulus.
To measure tensile modulus, we do the same thing as we did to measure
strength and ultimate elongation. This time we measure the stress we're
exerting on the material, just like we did when we were measuring tensile
strength. We slowly increase the amount of stress, and then we measure
the elongation the sample undergoes at each stress level. We keep doing
this until the sample breaks. Then we make a plot of stress versus
elongation, like this:
There are times when the stress-strain curve isn't nice and straight, like
we saw above. For some polymers, especially flexible plastics, we get odd curves that look like this:
The slope isn't constant as stress increases. The slope, that is the
modulus, is changing with stress. In a case like this we usually that the
initial slope as the modulus, as you can see in the stress-strain curve
above.
In general, fibers have the highest tensile moduli, and elastomers have
the lowest, and plastics have tensile moduli somewhere in between fibers
and elastomers.
Modulus is measured by calculating stress and dividing by elongation,
and would be measured in units of stress divided by units of elongation.
But since elongation is dimensionless, it has no units by which we can
divide. So modulus is expressed in the same units as strength, such as
N/cm2.
How is toughness different from strength? From a physics point of view,
the answer is that strength tells how much force is needed to break a
sample, and toughness tells how much energy is needed to break a sample.
But that doesn't really tell you what the practical differences are.
What is important is knowing that just because a material is strong, it
isn't necessarily going to be tough as well. We'll look at some more
graphs to show this. Take a look at the one below, the one with three
plots, one blue, one red, and one pink.
On the other hand, the red plot is a stress-strain curve for a sample that is both strong and tough. This material is not as strong as the sample in the blue plot, but the area underneath its curve is a lot larger than the area under the blue sample's curve. So it can absorb a
lot more energy than the blue sample can.
So why can the red sample absorb so much more energy than the blue plot? Take a look at the two. The red sample elongates a lot more before breaking than the blue sample does. You see, deformation allows a sample to dissipate energy. If a sample can't deform, the energy won't be
dissipated, and will cause the sample to break.
In real life, we usually want materials to be tough and strong. Ideally,
it would be nice to have a material that wouldn't bend or break, but this
is the real world. You have to make trade-offs. Take a look at the plots
again. The blue sample has a much higher modulus than the red sample. While it's good for materials in a lot of applications to have high moduli and resist deformation, in the real world it's a lot better for a material to bend than to break, and if bending, stretching or deforming in some other way prevents the material from breaking, all the better. So when we design new polymers, or new composites, we often sacrifice a little bit of strength in order to make the material tougher.
This is why there's a big graph on your right. It compares typical stress-strain curves for different kinds of polymers. You can see in the green plot that a rigid plastics such as polystyrene, poly(methyl
methacrylate or polycarbonate can withstand a good deal of stress, but they won't withstand much elongation before breaking. There isn't much area under the stress-strain curve at all. So we say that materials like this are strong, but not very tough. Also, the slope of the plot is very steep, which means that it takes a lot of force to deform a rigid plastic. (I suppose that's what it means to be rigid, now doesn't it?) So it's easy to see that rigid plastics have high moduli. In short, rigid plastics tend to be strong, resist deformation, but they tend not to be very tough, that is, they're brittle.
Flexible plastics like polyethylene and polypropylene are different from rigid plastics in that they don't resist deformation as well, but they tend not to break. Of course, the ability to deform is what keeps them from breaking. Initial modulus is high, that is it will resist deformation for awhile, but if enough stress is put on a flexible plastic, it will eventually deform. You can try this at home with a piece of a plastic bag. If you try to stretch it, it will be very hard at first, but once you've stretched it far enough it will give way and stretch easily. The
bottom line is that flexible plastics may not be as strong as rigid ones,
but they are a lot tougher.
It is possible to alter the stress-strain behavior of a plastic with
additives called plasticizers. A plasticizer is a small molecule
that makes plastics more flexible. For example, without plasticizers, poly(vinyl chloride), or PVC for short, is a rigid
plastic. Rigid unplasticized PVC is used for water pipes. But with
plasticizers, PVC can be made flexible enough to use to make things like
inflatable swimming pool toys.
Fibers like KevlarTM
, carbon fiber and nylon tend to have stress-strain curves like the
aqua-colored plot in the graph above. Like the rigid plastics, they are
more strong than tough, and don't deform very much under tensile stress.
But when strength is what you need, fibers have plenty of it. They are
much stronger than plastics, even the rigid ones, and some polymeric
fibers, like KevlarTM, carbon fiber and ultra-high
molecular weight polyethylene have better tensile strength than steel.
Elastomers like polyisoprene, polybutadiene
and polyisobutylene have completely different
mechanical behavior from the other types of materials. Take a look at the
pink plot in the graph above. Elastomers have very low moduli. You can
see this from the very gentle slope of the pink plot, but you probably
knew this already. You already knew that it is easy to stretch or bend a
piece of rubber. If elastomers didn't have low moduli, they wouldn't be
very good elastomers, now would they?
But it takes more than just low modulus to make a polymer an elastomer.
Being easily stretched is not much use unless the material can bounce back
to its original size and shape once the stress is released. Rubber bands
would be useless if they just stretched and didn't bounce back. Of
course, elastomers do bounce back, and that's what makes them so amazing.
They have not just high elongation, but high reversible elongation.
Going Beyond Tensile Properties
Ok, this is all well and good, but this little discussion which types of
polymers have which mechanical properties has focused mostly on tensile
properties. When we look at other properties, like compressional
properties or flexural properties things can be completely different.
For example, fibers have very high tensile strength and good flexural
strength as well, but they usually have terrible compressional strength.
They also only have good tensile strength in the direction of the fibers.
An example of a copolymer that combines the properties of two materials is
spandex. It is a copolymer containing
blocks of elastomeric polyoxyethylene and blocks of a rigid fiber-forming
polyurethane. The result is a fiber that
stretches. Spandex is used to make stretchy clothing like bicycle pants.
High-impact polystyrene, or HIPS for
short, is
an immiscible blend that combines the properties
of two polymers, styrene and polybutadiene. Polystyrene is a rigid plastic. When mixed with polybutadiene, an elastomer, it forms a phase-separated mixture which
has the strength of polystyrene along with toughness
supplied by the polybutadiene.
For this reason, HIPS is far less brittle than regular polystyrene.
In the case of a composite material, we're usually using a fiber to reinforce a thermoset. Thermosets are
crosslinked materials whose stress-strain behavior is often similar to
plastics. The fiber increases the tensile strength of the composite,
while the thermoset gives it compressional strength and toughness.
Jang, B. Z.; Advanced Polymer Composites: Principles and
Applications, ASM International, Materials Park, OH, 1994.
Elongation
Modulus
Toughness
That plot of stress versus strain can give us another very valuable piece
of information. If one measures the area underneath the stress-strain
curve, colored red in the graph below, the number you get is something we
call toughness.
Mechanical Properties of Real Polymers
We've been talking abstractly for a long time, now, so it would probably
be a good idea to talk about which polymers show which kinds of mechanical
behavior, that is, which polymers are strong, which are tough, and so
forth.
Pool Your Strengths
We've talked a lot about how some polymers are tough, while others are
strong, and how one often has to make trade-offs when designing new
materials. One may have to sacrifice strength for toughness for example.
But sometimes we can combine two polymers with different properties to get
a new material with some of the properties of both. There are three main
ways of doing this, and they are copolymerization, blending, and making composite materials.
References
Odian, George; Principles of Polymerization, 3rd ed., J. Wiley, New
York, 1991.
