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Molecular Weight
Molecular Weight
Plastics 001
Molecular Weight
Plastics 001
KEY POINTS:
After reviewing the Molecular Weight presentation,
students should:
• Understand what molecular weight means when dealing with
polymers
• Understand the effect of molecular weight on material properties
• Understand entanglement
Molecular Weight
Plastics 001
Molecular Weight
By this point in your education, you should be aware of what atoms,
molecules, protons, electrons, and a periodic table are. If you are
not, you should probably review some of your basic chemistry before
continuing with this lesson.
On a periodic table of the elements, you will
see something that looks like this:
for each element.
Molecular Weight
Plastics 001
Molecular Weight
At the top of the cell for each element it will
commonly list the name of the element.
Next, you will normally find the atomic number
which is the number of electrons present in the
atom of the element.
The symbol for the element
Lastly the atomic weight of the element.
Molecular Weight
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Molecular Weight
Molecular weight is the sum of the atomic weights of the atoms that
make up the molecule.
Carbon has an atomic weight of 12.011 grams/mole. 12 is close enough
for what we’re doing
A mole is 6.0221415 × 1023 atoms or molecules
This is known as Avogadro's number, it is a count or number of
units – like a dozen
Because of their extremely long molecules (8,000-10,000 mers long),
polymers can have extremely high molecular weights.
Molecular Weight
Plastics 001
Molecular Weight
To put it into perspective:
A mole of water (1 oxygen [16] and 2 hydrogen [1]) weighs 18 grams –
that is barely enough to cover the bottom of a glass.
A mole of polypropylene (3 carbons [12] and 6 hydrogen [1]) weighs
378,000 grams (assuming 9,000 mers have linked up)
• That converts to around 830 pounds
or almost a full gaylord.
Gaylord
Molecular Weight
Plastics 001
Molecular Weight
If you had Avogadro’s number of pennies and you could stack them up,
you would have a pile of pennies
579,783,000,000,000,000 miles high that weighs
1,660,000,000,000,000,000 tons
The earth is 92,956,952 miles from the sun
Molecular Weight
Plastics 001
Molecular Weight
The atomic weights of the atoms most commonly found in polymers are:
Hydrogen (H) = 1 g/mole (1.0079)
Carbon (C) = 12 g/mole (12.011)
Nitrogen (N) = 14 g/mole (14.007)
Oxygen (O) = 16 g/mole (15.999)
Chlorine (CL) = 35.5 g/mole (35.453)
Molecular Weight
Plastics 001
Molecular Weight
When we talk about molecular weight in terms of polymers, we are
really talking about the length of the individual chains.
The polymerization process is subject to variation so there is no single
chain length, there is actually a
wide range of lengths, so when
we discuss molecular weight, we
really mean the average molecular
weight of the material. This
average is found by measuring
samples of the material as it is
produced.
Molecular Weight
Plastics 001
Molecular Weight
There are two different categories of molecular weight average that
are commonly used:
The first is the Number Average Molecular Weight (Mn )
The second is the Weight Average Molecular Weight (
)
Another important aspect of the molecular weight distribution is the
shape of the curve.
Molecular Weight
Plastics 001
Molecular Weight
The figure to the right represents
a typical molecular weight
distribution.
The vertical axis represents
the number of chains at that
length.
The horizontal axis represents the
different chain lengths.
Notice that the longer chains are to the left on the graph and the
shorter chains are to the right.
Molecular Weight
Molecular Weight
The Number Average Molecular
Weight (
) is the total weight
of the polymer molecules
divided by the total number
of moles.
Plastics 001
Molecular Weight
Molecular Weight
Number Average Molecular Weight (
) Example:
We have:
10 moles of Polyethylene (PE) that are 500 monomers long
5 moles of PE that are 100 monomers long
5 moles of PE that are 800 monomers long
What is
?
Plastics 001
Molecular Weight
Molecular Weight
Number Average Molecular Weight (
) Example:
Each monomer is 2 Carbons and 4 Hydrogens
10 moles X 500 monomers = 5,000
5 moles X 100 monomers = 500
5 moles X 800 monomers = 4,000
Total number of moles = 20 (10 + 5 + 5)
Total number of monomers = 9,500
What is
?
= 475 monomers
The average chain is 475 monomers long
Plastics 001
Molecular Weight
Molecular Weight
The Weight Average Molecular
Weight (
) takes into account
that the larger molecules
contain a much higher amount
of the molecular mass of the
polymer.
The Weight Average Molecular
Weight is almost always higher
than the Number Average
Molecular Weight (
).
Plastics 001
Molecular Weight
Molecular Weight
An example that points out why the
would be important.
You have:
100 cherries at 9 g
6 bananas at 180 g
4 watermelons at 11,350 g
What is the average weight of a piece of fruit?
Plastics 001
Molecular Weight
Plastics 001
Molecular Weight
The average weight of a piece of fruit in this example is around 431 g
(100 x 9 g) + (6 X 180 g) + (4 X 11,350 g) = 430.73 g
(100 + 6 + 4)
This would be equivalent to the
value we calculated earlier, but
the watermelons contain a much higher percentage of the weight.
The average gram of fruit found in a fruit salad made up of the fruits
we used didn’t come from a fruit that
weighed 431 g. It was much heavier.
Molecular Weight
Plastics 001
Molecular Weight
To calculate the
from the example, you need to find the weight
fraction of the group.
The weight fraction is determined by taking the weight of one of the
components and dividing it by the total weight.
100 cherries at 9 g = 900 g
6 bananas at 180 g = 1,080 g
4 watermelons at 11,350 g = 45,400 g The total weight = 47,380 g
The weight fractions are:
Cherries: 900/47,380 = 1.9 % (The cherries make up 1.9% of the total weight)
Bananas: 1,080/47,380 = 2.3%
Watermelons: 45,400/47,380 = 95.8%
Molecular Weight
Plastics 001
Molecular Weight
To calculate the Weight Average Molecular Weight (
), you take the
sum of the weight fractions times their respective weights.
Mw = (0.019 x 9 g) + (0.023 x 180 g) + (.958 x 11,350 g)
= 0.171 g + 4.10 g + 10,876 g = 10,880 g
This turns out to be much higher than the 431 g (
Another good example of the differences between
found at: http://pslc.ws/mactest/weight.htm
)found earlier
and
can be
Molecular Weight
Molecular Weight
Look at the numbers from the previous example:
10 moles of PE that are 500 monomers long
5 moles of PE that are 100 monomers long
5 moles of PE that are 800 monomers long
Calculate the
Plastics 001
Molecular Weight
Plastics 001
Molecular Weight
Calculate the
Find the weight fractions:
10 x 500 / 9500 = 52.6%
5 x 100/ 9500 = 5.3%
5 x 800 / 9500 = 42.1%
= (0.562 x 500) + (0.053 x 100) + (0.421 x 800) = 605.3 monomers
long
Molecular Weight
Molecular Weight
Just knowing the averages is not
enough, the distribution of
the molecular weights also
has a large effect on how the
material will process and its
properties.
A broader or ‘wide spec’
distribution may make the
material unsuitable for
processes like injection
molding, but better suited for
processes like extrusion, blowmolding, or thermoforming.
Plastics 001
Molecular Weight
Molecular Weight
For injection molding grades of
material, a narrower
distribution is better.
When the distribution is
narrow, the polymer chains
will melt and flow at around
the same temperature.
The longer the chains, the
higher the viscosity or
resistance to flow.
Plastics 001
Molecular Weight
Molecular Weight
When you have a broad or even a bi-modal
distribution, the shorter chains melt
more quickly and allow some flow, while
the longer chains hold the material
together.
This gives the polymer mixture melt
strength which allows it to be used for
some of the other processes mentioned
other than injection molding.
Plastics 001
Molecular Weight
Molecular Weight
You can have virtually an infinite
number or distributions with
the same number average
molecular weight.
All of these materials will process
differently and have at least
slightly different properties.
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Molecular Weight
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Properties
When making polymers, the goal is to make a material with the ideal
properties.
The longer the molecules (or the higher the molecular weight) the
higher the entanglement forces:
• Longer hair is harder to get untangled than shorter hair
Molecular Weight
Plastics 001
Properties
Increasing the molecular weight of the material increases many of the
properties of the material by increasing the entanglement of the
molecules.
A higher molecular weight:
•Increases the chemical resistance - to a point
– It takes more damage to the main chains of the molecules before
it will affect the strength of the material
– The big loophole to this is if you have a chemical
that is very similar to the chemical makeup of the
main chain, it will dissolve it much more easily
»Like Dissolves Like
Molecular Weight
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Properties
A higher molecular weight:
•Increases how far the material can stretch before rupturing
– The higher degree of entanglement allows the material to be
pulled further before the chains break
Molecular Weight
Plastics 001
Properties
A higher molecular weight:
– A candle and Polyethylene (PE) have basically the same molecular
structure. The chain length of the candle is just much shorter than
that of the PE. If you bend a bar of PE in half – it will bend, if you
bend a candle in half, it will fracture.
Molecular Weight
Plastics 001
Properties
A higher molecular weight:
•Increases the impact resistance of the material
–The higher degree of entanglement means that in order to
rupture, more polymer bonds need to be broken, this means that
the polymer can absorb more energy before failing.
Molecular Weight
Plastics 001
Properties
A higher molecular weight:
•Increases the weather resistance of the material
– Same type of reasoning behind the increase in chemical resistance,
the chains are longer, so they can withstand more damage before
the mechanical properties will start to diminish
Image by Alex Cheek
Creative Commons
OAR/ERL/National Severe Storms
Laboratory (NSSL)
Molecular Weight
Plastics 001
Properties
A higher molecular weight:
•Increases the viscosity of the material – makes it harder to process
the material using conventional methods
–The longer the chains, the harder it is to get them to flow
» More tangled
Image of Honey by Balakov Creative Commons
Molecular Weight
Plastics 001
Properties
Processors want materials that will flow easily in order to form
complex geometries, but that can affect the properties of material
used to create the product.
Many times it turns out to be a trade-off between the required
properties and processability of the material.
CD’s and DVD’s are made from the same material as most safety
glasses, Polycarbonate.
Safety glasses require a higher molecular weight in order to provide
the necessary property of impact resistance.
CD’s and DVD’s require a lower molecular weight material in order to
fill out the thin walls. CD’s and DVD’s can shatter, safety glasses don’t.
Molecular Weight
Properties
Plastics 001
Molecular Weight
Molecular Weight
Questions?
Plastics 001

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