CHE 333 Class 21 Polymers Reference W.D.Callister Materials Science and Engineering

MONDAY and Wednesday LAB ONLY No lab this Monday and Wednesday afternoon. LAB IS ON FOR TUESDAY.

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. Acetylene Ethylene

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. 109 o 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 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 =  n f M nf 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 =  w f M wf

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