CHE 333 Class 21
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Transcript of CHE 333 Class 21
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CHE 333 Class 21
PolymersReference W.D.Callister
Materials Science and Engineering
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Applications
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Applications DetailsPackaging:Light weight, flexibility and easy to process are responsible for the plastics for remaining
the material of choice for packingEg: 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!
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Recyclable
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Types of PolymersOrganic Polymers – based on use of carbon atom as building blockInorganic Polymers – based on silicon atom as building blockSame region of periodic table – same number of valence electrons in outer shell - 4.
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Hydrocarbon StructuresCovalent bonding between atoms and secondary between molecules, so gasesat room temperature in some cases. Cannot polymerize these molecules.
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Other Structures.
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Isomers
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Bonding of CarbonCarbon has a valence of fourIn some cases, these are all used inbonding to another atom, such as hydrogen.Carbon also has the ability to bond to itself andso it can form other molecules. These are unsaturated bonds which give itfunctionality. A double carbon bondis shown along with a triple carbon bond.These are covalent bonds between the atoms.These can be polymerized as a multifunctionalbond is needed for the process.Hence “Polyethylene” – sandwich bags , labware.
Acetylene
Ethylene
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Carbon Chains
Polymers have a carbon chain backbone. All four of the covalent carbon bonds areused. In the case above two go to other carbon atoms and two are available forbonding to either atoms or groups.
109o angle
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Building Blocks - Mers
Mers are the repeat units on the carbon chain
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More Mer Structures
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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
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Condensation Polymerization
Nylon 6.6
Bi-product
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Molecular WeightDue 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
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Crystallinity
Folding of chains produces crystal like properties. More crystallinity stiffer and stronger the material
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Microstructures
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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, buteasily 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 andstrong 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