Rubbers/Elastomers

• Raw material from rubber tree (Hevea brasiliensis) is tapped every second day for its sap, known as latex, by making slanting cuts in the bark of the tree.

• Latex is collected and acetic acid is added to it so as to precipitate out the rubber, which then hardens/coagulates.

• After being washed and dried, rubber is cured in special smokehouses to protect it against microbial attack.

• Natural Rubber (Polymer of isoprene-2-Methyl-1,3-butadiene)

Vulcanization

Property Raw Rubber Vulcanized Rubber

Elasticity Very high Low, depending on % S

Tensile Strength 200 kg/cm2 2000 kg/cm2

Chemical resistance Very poor Higher

Durability Less Higher

Quality Inherent Can be controlled by vulcanization

Preparation, Properties and Uses of Commercial Rubbers

Copolymerization of butadiene & styrene carried out at 5oC Good and tough mechanical properties Easily attacked by oxidizing agents, mainly ozone, organic solvents Uses: Manufacture of tyres, insulating wires and cables, adhesives

Buna-S Rubber/ Cold Rubber

Polyurethane rubber/Isocyanate rubber

• Ethylene glycol polymerizes with ethylene diisocyanate to form polyurethane rubber.

• Highly resistant to oxidation • Resistant to organic solvents, attacked by acids and alkali Uses: surface coatings and manufacture of foams and fibers

Conducting Polymers

• Polymers are poor conductors of electricity, due to non-availability of large number of free electrons

• Today, they are synthesized at par to conductivities of metallic conductors

• Conductivity increases by decreasing the energy band gap

• Amount of energy needed to promote an electron from valence to empty band (conduction band)

• Polymers have large band gaps, with careful design of chemical structure of polymeric backbone Æ band gap = 0.5 to 1eV

Conditions

• Polymer should consist of alternating single and double bonds called conjugated double bonds

• Polymer matrix has to be disturbed – – Either by removing electrons from them, – Or inserting electrons into the material. – The process is known as doping – By doping with electron donor like alkali-metal ion

or electron acceptor like Iodine & polymers turn conductive materials

• Iodine molecule attracts an electron from polyacetylene chain and becomes I3

- • Polyacetylene molecule, now positively charged, is termed a radical

cation, or polaron • Lonely electron of the double bond, from which an electron was

removed, can move easily. • As a consequence, double bond successively moves along the

molecule– Conducting Polymers

Conducting Polymers Telecommunication

Aerospace

Battery technology

Smart Materials

Self healing polymers

• Inspired from biological systems Æ ‘Wound healing’ • Inherent ability of polymers to repair damage caused by

mechanical usage over time • Terminator Polymers

• Chemistry World posted a video of the product in action, showing someone cutting a piece of the polymer in two with a scalpel, pressing the pieces back together and leaving it on a table for two hours at room temperature. The person is unable to pull the material apart with their hands upon returning.

• This is the next generation breakthrough in polymers.

• Healing Process: A propagating crack ruptures the microcapsules, releasing the healing agent into the crack plane by capillary action. Polymerization is initiated by contact with the embedded catalyst or initiator, bonding the crack faces, and restoring structural continuity.

• At times, additional external stimuli such as heat or UV-radiation is required for the healing to occur.

Applications

• Nissan Motor Co. Ltd has commercialized world’s first self-healing clear coat for car surfaces-trade name of this product is ‘Scratch Guard Coat’

• Self healing concretes– in progress • Self-healing materials are now used as composites

in aircrafts.