CHE 333 Class 21

PolymersReference W.D.Callister

Materials Science and Engineering

Applications

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!

Recyclable

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.

Hydrocarbon StructuresCovalent bonding between atoms and secondary between molecules, so gasesat room temperature in some cases. Cannot polymerize these molecules.

Other Structures.

Isomers

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

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

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

Nylon 6.6

Bi-product

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

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, 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