Lecture# 3
Molecular Mass and Chain Microstructure
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Mass vs. Weight
Molecular “Weight” and Distribution
Averages
Polydispersity
Property Implications
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Polymer Chain Length
• Polymer Chain Length
– Polymer notation represents the repeating group
• Example, -[A]-n where A is the repeating monomer and n
represents the number of repeating units.
• Molecular Weight
– Way to measure the average chain length of the polymer
– Defined as sum of the atomic weights of each of the atoms in the
molecule. Example:
– Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole
– Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole
– Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000
g/mole
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MOLECULAR WEIGHT
• Molecular weight, M: Mass of a mole of chains.
Low M
high M
• During the polymerization NOT ALL chains in a polymer grow to
the same length, so there is a distribution of molecular weights.
• The molecular weight distribution in a polymer describes the
relationship between the number of moles of each polymer
species and the molar mass of that species.
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MOLECULAR WEIGHT DISTRIBUTION
M n  x i M i
M w  w i M i
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Mn = the number average molecular weight (mass)
Mi = mean (middle) molecular weight of size range i
xi = number fraction of chains in size range i
wi = weight fraction of chains in size range i
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Polydispersity
• By virtue of its definition, Mw cannot be less than Mn. It is influenced
by the high molecular weight fraction of the material to a greater
degree than Mn.
• The ratio of Mw to Mn,
defines the polydispersity
of a molecular weight
distribution.
• Low polydispersity
(PD=Mw/Mn  2) generates
higher melt viscosity, higher
tensile strength and
better toughness in
polyethylene.
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Degree of Polymerization
The degree of polymerization refers to the total
number of repeat units in the chain.
DP = Mn/Mo
Ex. Calculate the degree of polymerization if polyethylene (PE)
has a molecular weight of 56,000 g/mol.
H H
C C
H H
n
Mrepeat unit = 2(atomic wt. of C) + 4(atomic wt. of H)
= 2(12) + 4(1) = 28
Polyethylene (PE) Degree of Polymerization = 56,000/28 = 2,000
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Property Implications of MW
• Higher MW increases
• Tensile Strength, impact toughness, creep resistance,
and melting temperature.
– Due to entanglement, which is wrapping of
polymer chains around each other.
– Higher MW implies higher entanglement which
yields higher mechanical properties.
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and Molecular Weight Distribution
• Broader MWD decreases strength
• Broad MW distribution represents polymer with many
shorter molecules which are not as entangled and slide
easily.
• Broader MWD decreases crystallinity
– Shorter chains are too short to fold into crystalline
domains
• Broader MWD increases melt flow rate
• Shorter chains flow more easily and act as plasticizer.
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Example 1.1:
What is the molecular weight of polypropylene
(PP), with a degree of polymerization of 3×104 ?
Solution:
Structure of the repeating unit for PP
Molecular weight of repeat unit = (3×12 + 6×1) = 42
Molecular weight of polypropylene = 3×104×42 =
1.26×106
Example 1.2:
Nylon 11 has the following structure
If the number-average degree of polymerization, X n
, for nylon is 100 and M w= 120,000, what is its
polydispersity?
Example (3.1): a. To Find:
(a) The number-average molecular weight
(b) The weight-average molecular weight
(c) The degree of polymerization and P.D
for the given polypropylene material
CLASSIFICATION OF POLYMERS
Polymers can be classified in many different ways. The most
obvious classification is based on the origin of the polymer, i.e.,
natural vs. synthetic. Other classifications are based on the
polymer structure, polymerization mechanism, preparative
techniques, or thermal behavior.
A. NATURAL VS. SYNTHETIC
Polymers may either be naturally occurring or purely synthetic.
All the conversion processes occurring in our body (e.g.,
generation of energy from our food intake) aredue to the
presence of enzymes. Life itself may cease if there is a deficiency
of these enzymes. Enzymes, nucleic acids, and proteins are
polymers of biological origin. Their structures, which are
normally very complex, were not under stood until very recently.,
etc. Each family itself has subgroups.
Starch — a staple food in most cultures — cellulose, and
natural rubber, on the other hand, are examples of polymers
of plant origin and have relatively simpler structures than
those of enzymes or proteins. There are a large number of
synthetic (man-made) polymers consisting of
variousfamilies: fibers, elastomers, plastics, adhesives
B. POLYMER STRUCTURE
1. Linear, Branched or Cross-linked.
2.Amorphous or Crystalline
Examples of crystalline polymers include polyethylene ,
polyacrylonitrile
poly(ethylene terephthalate) , and polytetrafluoroethylene
Poly(methyl methacrylate)
polycarbonate
3. Homopolymer or Copolymer
Polymers composed of only one repeating unit in the
polymer molecules are known as homopolymers
Polymers composed of two different repeating units in the
polymer molecule are defined as copolymers. An example is
the copolymer formed when styrene and acrylonitrile are
polymerized in the same reactor..
There are several types of copolymer systems:
See lecture notes