MMEE 323
Lecturer: Dr V. Manisa (Block 109, Room 006)
Instructor:
Date: 9th February 2024
Goals
Upon completion of this lecture, the student should:



have an understanding of how polymerized organic materials
form engineering materials
have an understanding of the chemical makeup of polymeric
materials
have an understanding of the techniques that are used to
strengthen polymers
Introduction


Natural polymers – exists in nature; animal proteins,
and others like lignin, shellac, rosin, natural rubber, etc.
Manufactured polymers – used today in different
applications

Chronology of plastics

Definition of a polymer

Definition of polymerization
History of Plastics
Figure 1: Chronological development of important engineering polymers
Definition
 Polymer is a molecule produced by linking together a large number of
monomers, which could be the same or different ones.
 Mer is the basic unit of long molecular chain.
 E.g. Methane has the simplest mer – single unit;
 Ethane contains …?
 Propane …?
 Mers – monomer, dimers, trimers, tetramers (oligomers)
Typical Monomers and their structures:





Ethylene, C2H4
Vinylchloride, C2H3Cl
Vinyldiene chloride, C2H2Cl2
Vinylacetate, C4H6O2
Styrene – contains the benzene ring, C8H8
Definition
 Polymerization is a chemical reaction by which monomers combine
together to produce long/large molecules of repeating molecular units.
 The name of the polymer is derived from that of the monomer forming
it. E.g.
 Ethylene – Polyethylene (PE)
 Vinylacetate – Polyvinyl acetate (PVA)
 Vinylchloride – Polyvinyl chloride (PVC)
 Iso-propylene – Polypropylene (PP)
 Styrene – Polystyrene (PS)
 Three main mechanisms are involved in polymerization process:
 Addition polymerization – chain growth process
 Condensation polymerization – step-wise reaction
 Ring-opening polymerization
Definition
 Oligomer can be described as a molecular complex comprised of a few
monomer units.
 Acrylate
 Silane
 Hydroxyl
 Urethane,
 Epoxy,
 Polyester, etc.
 Oligomer can be used to decrease volatile organic compound (VOC)
content and application viscosity.
 Properly chosen oligomer provides the following advantages in coating
applications:
 Improved adhesion
 Improved chemical resistance
 Improved substrate wetting
 Improved weathering
Table 1: Acrylate oligomer types, their performance effects and use in inks
and coatings
Table 2. Examples of Oligomers Employed in Various Coating Applications
Table 3. Examples of Functionalities in Oligomeric Reactive Diluents
Definition
 Degree of Polymerization, DP, can be defined as the number of
subunits or monomers in the polymer molecular chain. That is, it
specifies the length of polymer molecules.
 It is calculated as below:
𝑴𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 𝒐𝒇 𝒕𝒉𝒆 𝒑𝒐𝒍𝒚𝒎𝒆𝒓 𝒄𝒉𝒂𝒊𝒏
𝑫𝑷 =
𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 𝒐𝒇 𝒊𝒕𝒔 𝒎𝒐𝒏𝒐𝒎𝒆𝒓
 The unit is mers per molecule, mers/mol.
Example:
 A type of polyethylene has a molecular mass of 150,000 g. What is its
degree of polymerization?
 What is the molecular weight of polypropylene (PP), with a degree of
polymerization of 3×104?
Definition
 Molecular weight can be defined as the length of the polymer chains.
 For polymers, we use average molecular weight, 𝑴𝒘 , because it is made
up of polymers of many different lengths each with its own characteristic
molecular weight and degree of polymerization.
 This average is determined by using special physical-chemical
techniques. One method commonly used is to:
 Determine the weight fractions of the molecular wt. range and sum
them.
 Multiply the sum by their mean molecular wt. for each particular
range.
 Divide the result by the sum of the wt. fractions using the formula
below:
𝑴𝒘 =
𝒇𝒊 𝑴𝒊
𝒇𝒊
Where:
Mi = mean molecular wt. for each molecular wt. range selected
fi = weight fraction of the material having molecular wt. of a related molecular wt.
range.
Definition
 Molecular weight distribution curve: Two types of molecular weight
averages are most commonly considered: the number-average
molecular weight represented by 𝑴𝒏 , and the weight-average
molecular weight, 𝑴𝒘 .
 The number-average molecular weight is derived from measurements
that, in effect, count the number of molecules in the given sample.
 The weight-average molecular weight is based on methods in which the
contribution of each molecule to the observed effect depends on its size.
Figure 2: Molecular weight distribution curve.
Definition
 Molecular weight distribution curve: Two types of molecular weight
averages are most commonly considered:
 the number-average molecular weight represented by 𝑴𝒏 , and
 the weight-average molecular weight, 𝑴𝒘 .
 In addition to the information on the size of molecules given by the
molecular weights 𝑴𝒘 and 𝑴𝒏 , their ratio, 𝑴𝒘 /𝑴𝒏 is an indication of
just how broad the differences in the chain lengths of the
constituent polymer molecules in a given sample are.
 This ratio is a measure of polydispersity, and consequently it is often
referred to as the heterogeneity index.
 For synthetic polymers, the numerical value of 𝑴𝒘 is always greater than
that of 𝑴𝒏 . Hence, as the ratio P increases, molecular weight distribution
is broader.
Definition
 𝑴𝒏 = 𝑿𝒏 𝑴𝒓
 𝑿𝒏 , can be calculated from:
𝑿𝒏 =
𝑵
𝒊=𝟏 𝒏𝒊 𝑴𝒓
𝑵
Where:
N = total number of molecules in the polymer mass.
Mr = molecular weight of repeating unit.
ni = DP of molecule i.
Example
Nylon 11 has the following structure:
If the number-average degree of polymerization, 𝑿𝒏 , for nylon is 100 and
𝑴𝒘 = 120,000, what is its polydispersity?
Solution:
We note that 𝑿𝒏 and DP define the same quantity for two slightly different
entities. The degree of polymerization for a single molecule is n. But a
polymer mass is composed of millions of molecules, each of which has a
certain degree of polymerization. Xn is the average of these.
Thus,
Definition
 Homopolymer are polymeric materials which comprises polymer chains
of a single repeating monomer unit. That is only one molecular species
repeating itself in a polymer.
 Co-polymer – polymers with chains from two chemically different
repeating monomer (can be arranged in various sequence).
 Random copolymer – monomer are randomly located in chain. E.g.
AABABABBBAABBBBA….
 Alternating copolymer – monomer arranged in alternating order in
chain. E.g. ABABABABABABABA….
 Block copolymer – different monomer in the chain located in
relatively long block of each monomer unit. E.g. AAAA–BBBB–AAAA–
BBBB–….
 Graft copolymer – appendages of one type of monomer grafted
(branched) to the long chain of another monomer units. E.g. …?
 Terpolymer – polymer containing repeating molecules from three
different molecules.
Definition
 Functionality of a monomer is the number of bonds a monomer has
or the number of sites on a monomer at which polymerization can occur.
 Mono-functional – monomer which uses one active bond for
polymerization, e.g. alcohol (R–OH)
 Bi-functional – monomer which uses two active bonds for
polymerization, e.g. ethylene.
 Tri-functional - monomer which uses three active bonds for
polymerization, e.g. phenol (C6H5OH).
 Latent functional – Monomers containing functional groups that
react under different conditions.
Classification of Polymers
Polymers can be classified in many different ways:
 Origin:
 Natural – may either be naturally occurring or purely synthetic. E.g.
Enzymes, nucleic acids, and proteins (animal or biological origin),
lignin (cellulose), shellac, rosin and natural rubber (plant origin).
 Synthetic – manufactured polymers used in different applications.
Fibers, elastomers, plastics, adhesives, etc.
 Structure:
 Linear  Branched
 Cross-linked
 Network
 Polymerization mechanism:
 Addition
 Condensation
 Ring opening
Classification of Polymers
 Preparation techniques:
 Bulk
 Solution
 Suspension
 Emulsion
 Thermal behavior – response to heat.
 Thermoplastic
 Thermosetting
 Elastomers or rubber
 Applications:
 Plastics
 Elastomers
 Adhesives
 Coatings
 Fibers
 Natural polymers
 Bio-systems
Basic Types of Polymers
Figure 3: Spectrum of polymeric materials and some of the important thermoplastic
and thermosetting plastic families.
Principles of Polymeric Materials
Goals
Upon completion of this lecture, the student should:



have an understanding of how polymerized organic materials
form engineering materials
have an understanding of the chemical makeup of polymeric
materials
have an understanding of the techniques that are used to
strengthen polymers
Structures of Polymers
Linear structure:
 Built strictly from di-functional monomers.
 Polymer with 2-dimensional molecular chains.
 Simplest structure in polymers.
 Contains little breadth of chain but significant
length, that is, they have long chains.
 Bond between chains is due to such things as
van der Waals forces (total intermolecular
force), hydrogen bonding, or interaction of
polar groups.
 Chains usually are flexible to the degree that
they intertwine and lie in single plane in space.
 Molecules don’t necessarily assume a
geometrically linear conformation or
configuration.
Figure 1. Schematic of
polymer chains in a linear
polymer
Structures of Polymers
Branched structure:
 Consists of a linear backbone chain with a
pendent side chain.
 It occurs when a chain continues to grow
concurrently as two chains (could have
happened either by chance or intentionally
during polymerization).
 Branched structures result in good strength
and stiffness in polymers.
 Branching reduces the ability of a polymer to
crystallize and also affects the flow behavior of
molten polymer.
 Typically found in elastomers.
Figure 2. Polymer chains in
a branched polymer
 Branching can have tremendous influence on
the properties of polymer through steric
(geometric) effects.
Structures of Polymers
 The relative location of the
methyl pendent group in PP
greatly affects its properties
especially its strength and
stiffness.
 The relative location of the
pendent groups is called
stereoregularity,
stereospecificity,
stereoisomerism, or
tacticity.
 Polymers with greater
tacticity (isotactic) tends to
be crystalline.
 Atactic forms are mostly
amorphous.
Structures of Polymers
Networked/Cross-linked structure:
 All atoms are connected to one another by
covalent bonds.
 It occurs when growing polymer chains
become chemically linked to each other.
 The resulting structure is very strong and
rigid.
 The polymer formed usually cannot be
remelted because the bonds between chains
are too strong.
 The greater the degree of cross-linking, the
greater is the rigidity of the material.
Figure 3. Cross-linked
polymer chains
Structures of Polymers
Crystalline polymers:
 Polymers with long regular straight-chain structures with small (lower
level) pendent groups.
 Have defined melting point, Tm, this is where a liquid phase starts upon
heating.
 Most contains some degree of amorphous polymer.
 Amorphous phase present will have a profound effect on its mechanical
properties.
 Some have ring structures in the main chain but no large pendent
groups.
Figure 4. Crystalline polyethylene polymer. The straight chain and absence of
pendent groups allows the chain to fold into a regular, repeating structure.