MSE101H1 Lecture Notes - Cross-Link, Poly(Methyl Methacrylate), Copolymer

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MSE101: Introduction to Material Science
Chapter Four: Polymer Structures
4.2 Hydrocarbon Molecules:
Most polymers are organic in origin
Intramolecular bonds within polymers are covalent
Saturated hydrocarbons are those which contain only single bonds and do not allow for the
addition of other atoms without removal of others
Unsaturated hydrocarbons are those which contain double or triple bonds on the carbon atoms
Between the molecules, only weak hydrogen and van der Walls bonds exist
Hydrocarbons have a relatively low melting and boiling point, which increase proportional to the
molecular weight
Isomerism is a phenomenon which occurs when two compounds have the same chemical
composition, but the atoms appear in a different arrangement.
When an isomer is formed, it has different chemical properties then the original molecular
4.3 Polymer Molecules:
Polymers are extremely long in size compared to hydrocarbon molecules, and are therefore
referred to as macromolecules
Polymers are bound together by covalent interatomic bonds
Each polymer is composed of numerous repeating units. Each small molecule from which a
polymer is synthesized is known as a monomer
[1]: Polymer structures to be memorized
Homopolymers are polymers which are formed from a chain of the same type of repeating units
Copolymers are those which are formed from two or more different repeating units
Functionality of a polymer is defined as the number of bonds that a given monomer can
form(Phenol-formaldehyde can form 3)
4.5 Molecular Weight
During the polymerization process, not all polymer chains will grow to the same length, which
results in a distribution of lengths and weights
The molecular weight of a polymer can be defined either through the number average, or
weight average
Number average: Obtained by diving the chains into a series of size ranges and determining the
number fraction of chains within each size range ( = Σx₁m₁ ― x:fraction of total number of
chains, m: mean molecular weight of size range)
Weight average: Obtained through the weight fraction of molecules within the various size
ranges ( = Σw₁m₁ ― m:mean molecular average weight, w:weight fraction of molecules within
the same size interval)
Weight average is generally higher than the number average for a given polymer
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MSE101: Introduction to Material Science
Melting points increase as the length of the polymer increases
To express the average chain size of a polymer, we use the degree of polymerization: DP = /m
where is the number average, and m is the repeating unit molecular weight
At room temperature, shorter polymer chains exist as a liquid
Other properties affected by molecular weight are the elastic modulus and strength
4.6 Molecular Shape
Single chain bonds are capable of rotating and bending in three dimensions
A third carbon atom can bond anywhere along the conical revolution of the two other atoms, at
an angle of about 109°
Due to polymers consisting of long molecular chains, each able to subject to bending and coiling,
polymer networks become intertwined and entangled
4.7 Molecular Structure
Linear polymers: Polymers in which the repeating units are joined end to end in a linear fashion.
Extensive van der Waals and hydrogen bonds are present between the chains. Some examples
of linear polymers are; polyethylene, polystyrene, poly(methyl methacrylate), nylon.
Branched polymers: Polymers containing side branches connected to a main branch. The
branches result from side reactions that occur during synthesis. The packing factor of these
polymers is reduced due to the branches. An example of a branched polymer is LDPE
Crosslinked polymers: Polymers chains which are joined together by smaller chains covalently
bonded to the main chains. This occurs during synthesis or by a nonreversible chemical reaction
Network polymers: Network polymers are formed from monomers which form three or more
active covalent bonds. A highly crosslinked polymer can also be considered a network polymer.
Polymers do not have one distinct structure type
[2]: Diagrams of polymer structures
4.8 Molecular Configurations
Regularity and symmetry of side group arrangements significantly influence the properties of a
Isomerism is the term for the different atomic configurations of the same polymer
Stereoisomerism: atoms which are bonded together in the same order
Isotactic: R groups bonded in the same location on all parts of the chain
Syndiotactic configuration: R groups are located on alternating sides of the chain
Atactic configuration: R groups located at random locations on the chain
Cis: R groups located on the same side of the double bond
Trans: R groups located on opposite sides of the double bond
4.9 Thermoplastic and Thermosetting Polymers
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MSE101: Introduction to Material Science
When a polymer is heated, it can be classified as either a thermoplastic or thermoset.
Thermoplastics soften when heated and will liquefy and solidify as the temperature changes
once more
As temperature is raised in a thermoplast, the secondary bonds are diminished allowing for
more motion.
Thermoplastics are generally polymers with flexible chains such as branched and linear polymers
Thermosetting polymers are network polymers
Thermosets do not soften when heat is introduced. This is due to the fact that the polymer is
formed by crosslinked section. The bonds between the chains keeps the bonds from rotating
and vibrating.
4.10 Copolymers
Random copolymers are copolymers which have the two different units placed as random
along the chain
Alternating copolymers are copolymers which have the two units alternating
Block polymers are copolymers which contain blocks of one unit, followed by a block of
Graft copolymers contain a main chain of one unit, with branches of the other unit
[3] Copolymer drawings
To calculate the degree of polymerization of a copolymer ( = Σfm f is the mole
fraction, m is the molecular weight of repeating unit 1.
4.11 Polymer Crystallinity
Polymer crystallinity is the packing of molecular chains to produce an ordered atomic array
Small molecular polymers are either totally crystalline or totally amorphous
Most polymers are partially crystalline; the range goes from completely amorphous to 95%
As the crystallinity of a polymer increases, so does the density
Crystallinity can be solved mathematically ( %crystallinity = ρᵥ-ρᵩ) /ρᵤ-ρᵩ) where rho v is
the density of the perfectly crystalline polymer, rho u is the density of polymer, and rho phi is
the density of the totally amorphous polymer)
Crystallinity also depends on the rate of cooling during solidification
Crystallization is not favoured in polymers composed of chemically complex repeating units
Linear polymers can easily become crystalline because there is few restrictions on chain
Network and crosslinked polymers rarely form polymers due to the crosslinks
Isotactic and syndiotactic polymers crystallise very easily due to the regularity of the geometry
For copolymers, the more irregular and random the repeat unit configuration is, the more
amorphous the polymer will be
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