CHE 333 Class 21
Polymers
Reference W.D.Callister
Materials Science and Engineering
Applications
Applications Details
Packaging:
Light weight, flexibility and easy to process are responsible for the plastics for remaining
the material of choice for packing
Eg: packaging of food, medicines etc…
Building and Construction:
Used for insulation to piping, windows to interior design.
Popularity of them in these sector is because of their durability, strength, resistance to
corrosion, low maintenance and aesthetically pleasing finish
Electrical and Electronics:
LCD (liquid crystal display) flat screens made of liquid crystalline plastics save energy
day after day using 2/3 less power than ordinary screens with cathode ray tubes!
Recyclable
Types of Polymers
Organic Polymers – based on use of carbon atom as building block
Inorganic Polymers – based on silicon atom as building block
Same region of periodic table – same number of valence electrons in outer shell - 4.
Hydrocarbon Structures
Covalent bonding between atoms and secondary between molecules, so gases
at room temperature in some cases. Cannot polymerize these molecules.
Other Structures.
Isomers
Bonding of Carbon
Carbon has a valence of four
In some cases, these are all used in
bonding to another atom, such as hydrogen.
Carbon also has the ability to bond to itself and
so it can form other molecules. These are
unsaturated bonds which give it
functionality. A double carbon bond
is shown along with a triple carbon bond.
These are covalent bonds between the atoms.
These can be polymerized as a multifunctional
bond is needed for the process.
Hence “Polyethylene” – sandwich bags , labware.
Ethylene
Acetylene
Carbon Chains
Polymers have a carbon chain backbone. All four of the covalent carbon bonds are
used. In the case above two go to other carbon atoms and two are available for
bonding to either atoms or groups.
109o angle
Building Blocks - Mers
Mers are the repeat units on the carbon chain
More Mer Structures
Addition Polymerization
Free radical used to break c=c bond in ethylene, leaves electron for covalent bond.
A second mer does the same and a chain forms. Goes until terminated by radical
Condensation Polymerization
Bi-product
Nylon 6.6
Molecular Weight
Due to the manufacturing processes, a range of polymers chain lengths occurs and so
There will be a range of molecular weights. Two methods for calculating this is available.
Number average Molecular Weight.
The number of chains lengths of a certain range is measured and expressed as a
number fraction. The total of all the fractions equals 1. The molecular weight is then
the number fraction multiplied by the molecular weight of the chain.
Mw = SnfMnf
Weight average Molecular Weight
Within a weight range the fraction of polymer chains of this weight is measured. Again
the weight fraction should equal 1. The molecular weight is then the weight fraction
multiplied by the molecular weight of the chain.
Mw = SwfMwf
Crystallinity
Folding of chains produces crystal like properties. More crystallinity stiffer
and stronger the material
Microstructures
Thermosetting and Thermoplastics
One way of categorizing polymers is to use the terms Thermosetting and
Thermoplastic to differentiate them
Thermoplastic polymers soften upon heating and harden upon cooling. These
secondary bonding types to hold these chains together which are weak, but
easily broken and reformed at relatively low temperature ranges. Chain and
branched polymer are usually thermoplastics – weak with plasticity
Thermosetting polymers form interchain covalent links which is a primary and
strong bond, requiring a higher temperature to break. It usually non reversible,
so the polymer is destroyed upon heating. It does not soften with heat.
Crosslinked and network polymers are usually thermosets . Strong but brittle