Polymers Flashcards
(29 cards)
What is a Polymer?
Polymer is high molecular weight substance formed by joining a large number of small repeating units called as Monomers.
What are the three types of polymers?
Isotactic, Atactic, and Syndiotactic
Elaborate on the three types of Polymers
Isotactic polymers: Functional groups are arranged on the same side.
Atactic polymers: No regular arrangement of functional groups on the back bone of the polymer chain.
Syndiotactic Polymers: Alternate arrangement of functional groups .
Condensation Polymers
Step Growth Polymers.
Takes place in monomers having reactive functional groups.
Elimination of simple molecules.
Repeat units are different.
Large change in molecular mass of polymer.
Polymer formed in gradual steps
Product can be thermoplastic or setting.
Eg : Polyester, Bakelite
Addition Polymers
Chain Growth Polymers
No Elimination of simple molecules
Takes place in monomers having multiple bonds
Little change in the mass of the polymer
Repeating unit is the same
Polymer formed instantly, fast reaction
Thermoplastic
Eg : PVC and Polyethylene
Thermoplastic Polymers
Addition Polymerization
Long chains with weak Vander Waals forces
Soften on heating and harden on cooling and no chemical change
Can be remoulded
Scrap can be reused
Soft, weak and less brittle
No cross linkage
Can be dissolved in organic substances
Eg : PVC and polyethylene
Thermosetting Polymers
Condensation Polymerization
3D network structure and connected bonds
Harden on heating, no change on cooling and chemical alteration
Cant be remoulded
Hard, stiff and very brittle
Scrap cant be reused
Cant be dissolved in organic substances
Eg : Nylon, Bakelite
Significance of Molecular Mass of Polymers
Molecular Mass influences a lot of properties of a polymer, the higher the mass more resilient and strong the polymer is, its viscosity and softening temperature is also higher and that’s why they have specific applications.
The molecular mass of a polymer is not fixed as it depends on the polymerization reaction, which further depends on the availability of functional groups, charge carrier and life time of carrier.
Hence polymers are poly disperse mixture of various molecular mass polymers.
Classification of Polymers based on source
- Naturally occurring Polymers: These occur in plants and animals and are very essential for life e.g. starch, cellulose, amino
acids, etc. - Synthetic polymers: These polymers are prepared in laboratory they are man made polymers e.g. plastics, synthetic rubbers, etc.
- Semi synthetic polymers: These are derived from naturally occurring polymers by chemical modification. e.g. vulcanized
rubber, Cuprammonium silk and Cuprammonium rayon, etc.
Classification of polymers based on Molecular forces
- Elastomers: In these polymers, chain are held by weakest intermolecular forces which permits the polymers to be stretched. The polymer regains its original position when forces are released.
- Fibers: In these polymers the inter molecular forces are strong due to hydrogen bonding, cross linking, cyclic structure
- Thermoplastics: These are polymers for which inter molecular forces between elastomers and fibers. Due to this they can be easily molded by heating.
- Thermosetting polymers: Thermosetting polymers undergo chemical changes and cross linking on heating and become permanently hard and infusible.
Plastics
- Low fabrication cost, low thermal & electrical conductivities, high resistance to corrosion, Plastics are resistant to chemicals & solvents
- Plastics reduce noise & vibrations in machines
- Plastics are bad conductors of heat are useful for making handles used for hot objects, most plastics are inflammable.
- Plastics are electrical insulators & find large scale use in the electrical industry.
- Plastics are clear & transparent so they can be given beautiful colors.
Compounding of Plastics
The process of mechanical mixing of various additives with
polymers (resin) to impart some special properties to the plastics.
The additives gets incorporated with resins to give homogeneous mixture.
- Resin or Binder
- Fillers or extenders
- Plasticizers
- Pigments or Dyes
- Activators, catalysts or accelerators
- Lubricants
- Stabilizers
Compression Molding
Raw materials or softened resinous mass compressed under heat and pressure.
Done in a mold, molten mass fills the cavities in this.
This is then cured by heating(Thermosetting) or cooling(Thermoplastic)
Injection Molding
Used for thermoplastics
Granular or powdered resin melted and poured into mold using a nozzle and piston, after cooling mold removed to obtain final product.
Disadvantage is that air bubbles can form inside the finished product.
Transfer Molding
Used for thermosets.
Granular or powder form of resin heated at low temp and introduced into mold using an orifice.
Once in mold, the material is cured at high temps for certain amt of time, after cooling article is removed.
Intricate and detailed shapes can be achieved, no blistering, small wires or glass fibers can be introduced in the mold.
Extrusion Molding
Used for thermoplastics
Used to form long continuous objects.
Granular/powder form introduced through hopper using a screw by heating, into a vertical/horizontal mold.
Mold surrounded by water for cooling of mold.
Proper temperature control of heating chamber, and speed of screw are important.
Conducting Polymers
- Generally polymers are insulators because of the absence of free electrons.
- But they can be made conductive in certain cases by the process called doping.
- Two conditions for the polymer to become conducting are:
1. Polymers should possess conjugated double bonds
2. Polymer structure has to be disturbed either by adding or removing electrons by the process of doping.
Intrinsically conducting polymers
Consist Conjugated pi-electrons in the backbone of their macromolecules, which are responsible for high electric conductivity
Conjugated pi-electrons get excited in presence of electric field and can be transported through the solid polymer.
Overlapping of orbitals of conjugated pi-electrons over the entire backbone of the polymer results in the formation of valence bands and conduction bands. This induces conductivity in the presence of electric field.
Poor mechanical properties, environmental
sensitivity, moderate stability of electrical properties with temperature significantly limit the industrial applicability of ICPs.
Doped conducting polymer
Step one is formation of a cation or anion, called a soliton or a polaron.
Because it is not until an electron is removed from the valence band (p-doping) or added to the conducting band (n-doping) does a polymer become highly conductive.
Doping generates charge carriers which move in an electric field. Positive charges and negative charges move to the opposite electrodes and this movement is responsible for electrical conductivity.
Elaborate on p-doping
Involves treating intrinsically conducting polymer with a Lewis acid, oxidation takes place and positive charges on the polymer backbone are created.
Elaborate on n-doping
Involves treating intrinsically conducting polymers with a Lewis Acid, reduction takes place and negative charges on the polymer backbone are created.
Extrinsically Conducting Polymer
2 types
1. Conductive element filled polymer - Non conductive polymer as binder, holding conductive substance like carbon black, metals, metal oxides etc. Cheap, good conductivity, light weight.
2. Blended conducting polymer: Obtained by
blending conventional polymers with conducting polymers
either through physical or chemical process.
Applications of conducting polymers
- In rechargeable light weight batteries
- In optical display devices
- In aircrafts and aerospace components
- In diodes and transistors
- In solar cells
Biodegradable polymers
Polymers in which monomers are joined to one another by functional group linkage and has unstable link in backbone.
They are broken down to biologically accepted
molecules that are metabolized and removed from
the body via normal metabolic pathway.
