Fundamentals of Polymer Engineering

1. Fundamentals of Polymer Engineering
Molecular Weight of Polymers
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Course Code: PE-3107
Credits: 3-0-3
Teacher: Engr. Asra Tariq
Email Id: asra.tariq@ntu.edu.pk

2. 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.
Recall: Periodic Table:
g/ mol
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3. Molecular Weight
 Molecular weight is the sum of the atomic weights of the atoms
that make up the molecule.
Because of their extremely long molecules (8,000-10,000 mers long),
polymers can have extremely high molecular weights.
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4. Molecular Weight
▪ Each monomer has a molecular weight (often called the formula
weight)
▪ Adding the monomers together to make polymers increases the
molecular weight
▪ The longer the chains, the higher the molecular weight
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5. 5
Effect of Molecular Weight
For example, let’s look at hydrocarbons
▪ Very short chain hydrocarbons are the
predominant component of petrol – liquid
at room temperature
▪ Longer chain hydrocarbons are present
in various waxes such as candle wax –
soft, pliable and easy to melt
▪ Polythene is a very long chain
hydrocarbon – tough, strong and very
resistant to heat and solvents
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Molecular Weight

6. 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.
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6Molecular Weight

7. Average Molecular Weight
 Molecular weight of a polymer is defined as sum of the atomic
weight of each of the atoms in the molecules, which is present in the
polymer”
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8. Average Molecular Weight
 The molecules produced by polymerization reaction have chain lengths that
are distributed according to a probability function that is governed by the
polymerization mechanism and by the condition prevailing during the
process.
 A concept of average molecular weight, therefore, assumes importance and
very much relevant.
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9. Average Molecular Weight
 An average molecular weight, M may in fact be generally expressed as
 Here, M1 , M2 , M3 etc. refer to molecular weights of different sizes of
molecules and the coefficients f1 , f2 , f3 etc. are fractions such that their
summation Σ fi equals to unity.
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10. Average Molecular Weight
 The average molecular weight M may otherwise be expressed as
 where, Ni is the number of molecules, each of which is characterized
by the molecular weight Mi and the index ‘a’ may have any real value.
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11. Number Average Molecular Weight
The Number Average Molecular Weight ( ) is the total
weight of the polymer molecules divided by the total number
of polymer molecules.
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12. Number Average Molecular Weight
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Number of Molecules Mass of each Molecule
1 800,000
3 750,000
5 700,000
8 650,000
10 600,000
13 550,000
20 500,000
13 450,000
10 400,000
8 350,000
5 300,000
3 250,000
1 200,000
Number of
Molecules, Ni
Mass of Each
Molecule, Mi

13. Weight Average Molecular Weight
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Number of Molecules Mass of each Molecule
1 800,000
3 750,000
5 700,000
8 650,000
10 600,000
13 550,000
20 500,000
13 450,000
10 400,000
8 350,000
5 300,000
3 250,000
1 200,000
Number of
Molecules, Ni
Mass of Each
Molecule, Mi

14. Viscosity Average Molecular Weight
 The molecular weight of the polymer is measured by using
viscometer and the molecular weight obtained by this technique is
called viscosity average molecular weight. The molecular weight of
the polymer solution is very high so the viscosity of polymer
solution is very high compared to that of pure solvent.
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15. Z-Average Molecular Weight
 The Z-average molecular weight (Mz) is measured in some
sedimentation equilibrium experiments. Mz isn’t common technique
for molecular weight of polymers. The molar mass depends on size
and mass of the molecules. The ultra centrifugation techniques
employ to determine Mz. Mz emphasizes large particles and it
defines the EQ, where Mi is molecular weight and Ni is number of
molecules.
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16. Molecular Weight Distribution
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17. Molecular Weight Distribution
 Since most polymers are polydisperse, the four molecular weight
averages are often used in conjunction with one another to define more
thoroughly the nature of the molecular weight distribution.
 Mn provides information about the molecular weight which is present
at the greatest frequency in the sample. Mw is a weighted average
which favours higher molecular weight molecules and appears on the
high side of the molecular weight distribution. The Mz value has the
term Mi to the third power and so it is even more skewed toward the
highest molecular weights. The value for Mv ranges depending upon
the constant “a” such that when “a” = 1 this average is equal to Mw.
When “a” is less then 1, Mv is found to be between Mn and Mw. A
typical value for “a” is between 0.5-0.9.
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18. Molecular Weight Distribution
 When polymers are provided for sale, the molecular weight reported is
generally Mn or Mw. Mv is reported less frequently. In our experience,
we have never seen the Z-average molecular weight listed apart from
another average.
 Another parameter which is frequently used when describing polymers
is the polydispersity index or PDI. This parameter gives an indication
of how broad a range of molecular weights are in the sample. The PDI
is defined as:
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19. Molecular Weight Distribution
A broader or ‘wide spec’ distribution may make the
material unsuitable for processes like injection
molding, but better suited for processes like
extrusion, blow-molding, or thermoforming.
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.
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20. Effect of Molecular Weight
 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.
 The higher degree of entanglement allows the material to be pulled
further before the chains break.
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21. Properties
A higher molecular weight:
 Increases ductility: 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.
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22. 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.
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23. 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
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Effect of Molecular Weight

24. 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
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Effect of Molecular Weight