Transcript What is a Polymer? - Department of Chemistry

Macromolecules
as Engineering Materials
CY 1050
Dr. Debashis Chakraborty
Room 201, Department of Chemistry
Phone: 4223
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Macromolecule ?
A macromolecule is a molecule with a large molecular mass
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Polymer Synthesis ?
The deliberate synthesis of complex
mixtures of large molecules with
recognizable repeat units from smaller
molecular precursors for fun and profit.
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What is a Polymer?
Polymer is a term used to describe large molecules
consisting of repeating structural units, or monomers,
connected by covalent chemical bonds. The term is
derived from the Greek words: polys meaning many, and
meros meaning parts. A key feature that distinguishes
polymers from other molecules is the repetition of many
identical, similar, or complementary molecular subunits in
these chains. These subunits, the monomers, are small
molecules of low to moderate molecular weight, and are
linked to each other during a chemical reaction called
polymerization.
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The quantity x represents the number of repeat units in the chain,
and is called the degree of polymerization. There are end groups on
the chain that are different from the repeat units, but these usually
represent a negligible portion of the molecule, so they are seldom
drawn.
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Monomer ?
A monomer (from Greek mono "one" and meros "part") is
a small molecule that may become chemically bonded to
other monomers to form a polymer.
Examples of monomers are hydrocarbons such as the
alkene and arene homologous series. Here hydrocarbon
monomers such as phenylethene and ethene form
polymers used as plastic like poly(phenylethene)
(commonly known as polystyrene) and polyethylene.
Other commercially important monomers include acrylic
monomers such as acrylic acid, methyl methacrylate,
acrylamide, propylene oxide, lactams, lactones and
lactides.
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Amino acids are natural monomers, and polymerize to form
proteins. Glucose monomers can also polymerize to form
starches, amylopectins and glycogen polymers. In this case
the polymerization reaction is known as a dehydration or
condensation reaction (due to the formation of water as one
of the products) where a H atom and a (-OH) group are lost to
form H2O and an oxygen molecule bonds between each
monomer unit.
Lower molecular weight compounds built from
monomers are also referred to as dimers, trimers, tetramers,
pentamers, octamers, 20-mers, etc. if they have 2, 3, 4, 5, 8, or
20 monomer units, respectively. Any number of these
monomer units may be indicated by the appropriate prefix,
eg, decamer, being a 10-unit monomer chain or polymer.
Polymers with relatively low number of units are called
oligomers.
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Polymer and Nature
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Modern History
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Like the old story about the elephant and the blind
men, the way people classify polymers depends
their experience. For example, an organic chemist
is interested in the detailed arrangement of atoms
in the chain, while a structural engineer only
considers a table of physical attributes such as
tensile strength or density. There is no uniform
system of classification of polymers. The
terminology has evolved along with polymer
science, and there are numerous exceptions to
categories, as well as widely used historical terms
or trade names lacking information content.
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Polymer Classification based On Repeat Units
The repeat units can all be identical, in which case the compound is
a homopolymer.
If the repeat units are different, the result is a copolymer. For
copolymers, the situation gets complicated quickly. At the limit of
extreme complexity, there are biopolymers such as proteins and
DNA that have exactly defined sequences of many repeat units to
serve the purposes of the organism. Synthetic copolymers are
seldom so elegant; however, there are still many possibilities for
even simple systems. Consider a copolymer made from just two
ingredients. There are infinite ways in which the two can be
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sequenced along the backbone. Here are some limiting cases:
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Classification based on Overall Structure
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Molecular Classification
The structures of polymers are conveniently represented by the repeating
chain formula, which shows the arrangement of bonds and atoms. The
repeat units often contain recognizable functional groups that can be
used to describe the polymer. This terminology often emphasizes the
functional groups that were involved in the synthesis of the polymer from
its monomers, although the usage is seldom exact. Some examples:
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A polymer is often named according to the monomer that was
used to form it. This is why the polymer consisting of only a
long chain of CH2 groups is called polyethylene, not
polymethylene.
The polyamide containing 6 carbons is known as polycaprolactam.
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Butvar (TMMonsanto)
Dacron (TMDuPont)
Lexan (TM General Electric)
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Nylon 6 (TM DuPont)
Noryl (TM General Electric)
Kapton (TM DuPont)
Dowlex (TM Dow)
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Classification Based on Physical Properties
Processability: Some polymers can be readily melted and then molded
into any shape. These are known as thermoplastics, and they usually
have linear or branched architectures. Most thermoplastics can also be
dissolved in suitable solvents. Some other polymers decompose on
heating before they can melt. These are known as thermosets, and they
are usually crosslinked and therefore insoluble. To form a part out of a
thermoset, one usually synthesizes the polymer in the mold itself.
Once the polymer has cured, the only way to reshape the part is by
machining (e.g., grinding, drilling, etc.)
Physical performance: Polymers that stretch and rebound are
called rubbers or elastomers. These materials are usually
crosslinked, either by covalent bonds, or, in several modern cases,
by noncovalent forces such as H-bonds. Other solid polymers are
known simply as plastics, adhesives, or fibers, depending on their
application. The word resin is a generic term for polymer, although
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occasionally it indicates a thermoset.
A common physical measurement that provides distinctive curve
shapes for the different physical classes of polymers is called
tensile testing. The measurement is carried out by stretching a
sample of polymer at a controlled rate, and measuring the amount
of force required. The curve stops when the sample breaks, and
the area under the curve is a measure of the energy required to
make this sample fail.
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