Polymers Flashcards
What are monomers?
macromolecules consisting of a large number of repeating units in a continuous chain. The repeating units are called monomers.
Classification of polymers.
Based on source/origin
▪️NATURAL▪️
•Found in nature
•Eg:- natural silk, cellulose, cotton etc
▪️SEMI-SYNTHETIC▪️
•Chemically modified natural fibers.
•Eg:- Halogenated natural rubbers, esters and ethers of cellulose.
▪️ SYNTHETIC▪️
•Artifically prepared.
•Eg:- polyethene, nylon6-6
Classification of polymers
Based on structure
▪️LINEAR▪️
• Monomeric units are linked to each other in a long continuous chain.
• Eg:- HDPE, PVC, polystyrene
▪️BRANCHED▪️
•Polymers with a sidechain extended from the main backbone.
•Eg:-
▪️CROSS-LINKED▪️
•A polymer in which adjacent linear molecular chain are joined at various positions by covalent bonds.
•Eg:- thermosetting polymers
Classification of polymers
Based on the number of monomers and their arrangements
▪️HOMOPOLYMER▪️
•A polymer made up of all identical monomer molecules.
•Eg:- polyethene, polystyrene, PVC etc
▪️ COPOLYMER ▪️
•The polymer is made by polymerisation of different monomers which give the different characteristics to the polymer.
• They are of 4 types
1) Alternating
-They contain alternating arrangements of monomers.
-Eg: ABABABAB…..
2) Random
- The will contain random arrangements of monomers.
-Eg: ABBABAAABABBA…
3) Block
-They contain block of one monomer follow by the block of other monomer.
-Eg: AAABBBAAABBB….
4) Graft
-The polymer contains the main chain of one monomer and branches of other monomer.
-Eg:
B
B
AAAAAAAAAAA
B
B
-
Classification of polymers
Based on types of polymerisation
▪️ ADDITION (chain)▪️
•These are formed by simple addition of monomer molecules to each other one after the other in a chain mechanism.
•They are usually formed from olefenic to diolefenic, or vinyllic and their related monomers.
•Eg:- polystyrene
▪️ CONDENSATION (step-wise)▪️
• They are formed when the monomers react with each other in a step-wise manner to produce condensed structured linkage with the elimination of small molecules like water.
•The monomers consists of reactive functional or bifunctional groups.
•Eg:- polyesters
Classification of polymers
Based on Thermal response
▪️THERMOPLASTING▪️
•These polymers are mostly linear polymers.
•They can be softened or plasticised repeatedly on application of heating without much change in their properties.
•They are soluble and fusible after many cycles of heating and cooling.
▪️ THERMOSETTING ▪️
•They are having cross-linked polymers.
•They cannot be softened.
•In their stages of synthesis they are soluble and fusible but later on they become infusible as the cross-links are broken.
What is the degree of polymerisation?
The number of repeating units in a polymeric chain is called its degree of polymerisation.
(–CH–CH–)n ; here n is the degree of polymerisation
How would you prepare a polymer which is to be obtained from rods? ( BULK POLYMERISATION)
Diagram
•PRE-POLYMERISATION•
The monomer is introduced to a vessel of nitrogenous atmosphere. The vessel is heated using heating coils as heat acts as the initiation for polymerisation. When the monomer is 30% polymerised i.e it forms a viscous fluid.
•POST-POLYMERISATION•
This fluid is passed to the long tower having different temperatures zones.
Post-polymerisation
The 30% polymerised monomer is then polymerised to different percentages. Each zones are kept at desired temperature by means of super heated steams, except the lowest zone which is electrically heated to get maximum heat necessary for complete conversion of monomers. The average residance time in the respective zones decided the successfulness of the process because the possible conversion is carried out in the lowest zone. The resulting molten monomer is taken out with the help of the extruder unit conveyed through the convier belt over an air or water cooling system followed by cheaping, grinding of the solid polymer. Alternatively it is extruded into rods or fabricated into sheets.
Eg:- polystyrene, PVC are prepared by this method.
How would you prepare a polymer which is to be obtained from spherical pears? (SUSPENSION)
DIAGRAM
The monomer is dispersed as relatively large droplets in water. It is kept in suspension by mechanical agitation. The catalyst is then added and initiation takes place. As the polymerisation proceeds, the viscosity of dispersed drops increases and they become sticky.
At this stage, suspension is stabilized by addition of small amount of suspension stabilizer so that they agglomerate together and drip away. This breaking the suspension when the reaction is complete. The reaction product (in the form of pearls is washed to remove stabilizer traces) is dried and used.
This method is used to prevent PVC and polystyrene.
Explain the Bulk and Suspension methods of polymerisation. (6mks)
1)Bulk polymerisation
——-DIAGRAM——-
The monomer is introduced in a vessel of nitrogenous atmosphere. The vessel is heated using heating coils as heat acts as the initiator for polymerisation. when the monomer is 30% polymerised i.e it forms a viscous fluid, it is passed to the long tower having different temperature zones and 30% polymerised monomer is then polymerised to different percentages. Depending upon the desired percentages of polymerisation which is required for different use causes.
2) Suspension
——DIAGRAM—–
Monomer+ water can also be an emulsion or solution but it is mechanically agitated to such an extent that the molecules are transferred to that of suspension.
Discuss the following structure property relationship in polymers.
i) chemical
ii) mechanical
i) CHEMICAL PROPERTY:-
- Solubility and swelling behaviour:
The solution of a polymer in a solvent involves diffusion of solvent into the polymer. When this happens the polymer will swell down and finally disintegrate.
A polymer will be soluble in a solvent which is chemically similar to it.
- Chemical Reactivity:
Depends upon the nature of chemical groups present on it.
Presence of double bonds, tertiary carbons, amide linkages, ester linkages make the polymer reactive. - Diffusion and Permeability:
Diffusion can occur only if gaps and voids are present in the polymer. In amorphous polymers there are gaps and voids so diffusion can take place.
Permeability can take place only if there is complete diffusion taking place. - Ageing:
Ageing and weathering happens in polymers because of the exposition to light, UV radiation and chemical environment.
It results in the change of physical and chemical properties.
If the bond energy is high then it can withstand ageing for some time.
ii) MECHANICAL PROPERTY:-
- Glass transition temperature (Tg):
Amorphous polymers when heated go from solid state to semi solid state and finally to the liquid state. The temperature from semi-solid state to liquid state is called glass transition temperature.
a) Toughness: below the Tg values the amorphous polymers are brittle and break, as they approach the Tg values they become tough.
b) Strength: longer chain polymers will be stronger as the intermolecular forces of attraction will be more. The greater the molecular weight, the more will be the strength.
c) Elasticity: elasticity of the polymer will depend on the Tg value, presence of cross link and the amorphous nature.
Discuss the following structure property relationship in polymers.
i)Electrical properties
ii) optical properties
i) ELECTRICAL PROPERTIES:
- Synthesized materials can be made conducting by doping it with an electron donor or an electron acceptor.
- Polymer exhibit conductivity if it has conjugated pi electron backbone.
- What happens is the conjugated pi electrons can move from one carbon atom to the other, thus when such a polymer is placed in an electric field the conjugated pi electrons get excited and move throughout the polymeric material and that is how electricity is conducted.
- In this type of polymer, the conductivity is increased by creating positive or negative charges on the polymer backbone either by oxidation or reduction called p-doping and n-doping.
ii) OPTICAL PROPERTIES:-
- They are seen in the polymers when there is a constant refractive index throughout the material.
- Amorphous polymers are generally transparent because of the constancy of the refractive index throughout the sample in the line of direction between the object in view and the eye.
- Crystalline polymers may or may not be transparent. If the wavelength of light is bigger than the crystalline structures then the polymer is transparent.
Discuss the crystallinity in polymers with regard to The and Tm.
- The degree of which molecules of a polymer are arranged in an orderly pattern with respect to each other is the measure of crystallinity.
- Longer molecules can be arranged in an orderly manner, hence are more prone to crystallinity than the shorter polymeric molecules.
- Crystallinity in a polymer is favoured by symmetrical chain structure as closed packing of the polymer is possible.
- If there are interchange interaction it favours crystallinity.
- Low Tm values and Tg values crystallinity is favoured in polymers.
Discuss the degradation in polymers due to oxidation and ESC.
Oxidation degradation:
-Oxidation degradation is caused when the polymer comes in contact with an oxidizing agent or is in contact with prolonged excessive heat.
- when oxidation takes place the polymer undergoes embrittlement and stress cracking.
- Deterioration of the electrical insulating properties take place.
- There is loss of clarity of the polymeric material.
- Usually thermoplastic polyethylene PVC nylon unsaturated polymer like rubber undergoes this type of degradation.
ESC (Environmental Stress Cracking):
- Polymers like polyethylene when subjected to mechanical stress in an active environment like alcohol, liquid hydrocarbons, metallic soap, etc undergo environmental stress cracking.
- Fractures occur much faster to these Polymers than it will be without the active environment.
- This degradation is due to the reduction of the molecular cohesive force is of the Polymers by active liquids.
Discuss the degradation in polymers due to weathering and thermal degradation.
Weathering:
- Weathering occurs due to the combined effects of absorption of water and the exposure to UV radiation.
- Water absorption by the polymer has a plasticizing effect and it increases flexibility.
- Thus it reduces the dimensional stability of molded particles.
- Finally on loss of water molecules the plastic undergoes and embrittlement.
- There is also loss of colour and clarity in the polymer.
- Eg: cellulose derivatives, polyethylene etc
Thermal degradation:
- When a polymer is subjected to heat then thermal degradation takes place.
- for eg: polyvinyl chloride when heated to 50⁰C will undergo dehydrohalogenation and form polyene.
PVC→polyene+HCl
The HCl that is produced reacts further, and further degradation takes place.
-Eg: PVC when formed is water white and it degrades as follow;
Water white→pale yellow→orange→ brown→black.
