Unit-4 PDF 2 - Polymer Science PDF

Unit-4 pdf 2 Natural Rubber is a high molecular weight hydrocarbon polymer represented by the formula (C5H8)x. It is obtained from a milk emulsion called latex by tapping the bark of the tree “Hevea brasiliensis.” The main composition of natural rubber is polyisoprene which is in the form of long c...

Unit-4 pdf 2 Natural Rubber is a high molecular weight hydrocarbon polymer represented by the formula (C5H8)x. It is obtained from a milk emulsion called latex by tapping the bark of the tree “Hevea brasiliensis.” The main composition of natural rubber is polyisoprene which is in the form of long coiled chains. The isoprene units polymerise to form rubber. Isoprene in natural rubber exits in two geometrical isomeric forms, cis and trans. Advantages of vulcanization: 1. Vulcanized rubber has good tensile strength and load bearing capacity. 2. Vulcanized rubber has excellent resilience. 3. It has better resistance to moisture, oxidation & abrasion. 4. It is resistance to organic solvents like CCl4, benzene, petrol etc. 5. It has good elasticity. 6. It is a good electrical insulator. Applications 1. The major application of natural rubber is in the manufacture of tyres. 2. In heavy duty tyres, the major portion of the rubber used is natural rubber. 3. The tank linings in chemical plants where corrosive chemicals are stored are prepared from rubber. 4. To reduce machine vibrations, rubber is used for sandwiching between two metal surfaces. 5. Foam rubber is used for making cushions’, matrices, padding etc. toys and sports items are manufactured from natural rubber. 6. Gutta percha is used for making submarine cables, golf ball covers, tissue or adhesive etc.  Neoprene Neoprene, often known as polychloroprene, is a class of synthetic rubbers made from chloroprene polymerization. Neoprene Extrusions Neoprene Sheets Properties High tensile strength, which makes it ideal for use in industrial settings and other places with a lot of movement. Resistant to chemicals, water, heat, fires, oxygen, and ozone. When compared to natural rubber, it is more resistant to water, chemicals, and heat. resistant to weather and sunlight, so it lasts long and does not need to be replaced frequently. Neoprene has a low oxidation rate and is UV and ozone resistant. Uses of Neoprene Neoprene is more resistant to degradation than natural or synthetic rubber. Because of its relative inertness, it is ideal for high-stress applications including gaskets, hoses, and corrosion-resistant coatings. used as a basis for adhesives, noise isolation in power transformer installations. Electrical and Electronic Applications: Neoprene is frequently used as an insulator in power transformers, light bulbs, cables, and other electrical applications due to its fire resistance. Safety Equipment Uses: gloves and other protective gear Marine Uses: in wetsuits and diving suits Automotive Uses: used to make vehicle parts since it is abrasion, tear, solvent, oil, and weather-resistant, as well as heat and fireproof. It can be found in window and door seals, hose covers, belts etc. Medical Uses: used to manufacture supports and braces, such as hand, knee, and elbow braces, because of its flexibility and wearability. ISOPRENE Fibers: Fibers are a class of materials that are continuous filaments or discrete elongated pieces. They are crystalline, present in both plants & animals. They are used for making textiles, ropes, utilities, strings etc. These are of two types (1) Natural Fibers (2) Synthetic fibers 1. Natural fibers: Produced by plants, animals & geological materials. a. Vegetable fibers: Cellulosic material Ex: cotton, jute etc. used for making textiles, ropes, mats, paper, bags etc. b. Wood fiber: The strength of a plant is due to presence of wood fiber. Wood pulp is used in making paper and wood fibers like jute are used for making bags. c. Animal fibers: They are largely made of protein. E.g. Pure silk, wool, hair are animal fibers. Spider silk is used for making special bullet proof jackets. d. Mineral fibers: Asbestos is a typical example of mineral fiber. Mica & other minerals are used as fibers. 2. Synthetic fibers: This type fiber can be produced in large quantities and are cheaper than some of the natural fibers like pure silk. Polyamide nylons, polyesters, PVC, phenolformaldehyde resin, polyethylene are often used for making textiles. Polyester (or) Terylene (or) Polyethylene terephthalate: This category of polymers has ester linkages in the main chain. It takes 18% of market share of synthetic polymers. Preparation: Terylene is a polyester fiber made from ethylene glycol and terephthalic acid. Terephthalic acid required for the manufacture of terylene is produced by the catalytic atmospheric oxidation of p-xylene. Properties: 1. This occurs as a colorless rigid substance. 2. This is highly resistant to mineral & organic acids but is less resistant to alkalis. 3. This is hydrophobic in nature. 4. This has high melting point due to presence of aromatic ring. Uses: 1. It is mostly used for making synthetic fiber. 2. It can be blended with wool, cotton for better use and wrinkle resistance. 3. Other application of polyethylene terephthalate film is in electrical insulation. NYLON (POLYAMIDE RESIN): Nylon is a polyamide resin containing recurring amide groups in its structure produced by copolymerization of di-amine with di- acid. Depending on the number of C atoms in di-amine & di-acid there are different types of nylons like nylon 6,6, nylon 6,10 etc., where the first number indicates number of carbon atoms in di-amine & the second number indicates the number of ‘C’ atoms in di-acid. 1. Nylon 6: Nylon 6 or polycaprolactam is a polymer, in particular semicrystalline polyamide. nylon 6 is not a condensation polymer, but instead is formed by ring-opening polymerization; this makes it a special case in the comparison between condensation and addition polymers. Synthesis: Nylon 6 is synthesized by ring-opening polymerization of caprolactam. Caprolactam has 6 carbons, hence Nylon 6. When caprolactam is heated at about 533 K in an inert atmosphere of nitrogen for about 4–5 hours, the ring breaks and undergoes polymerization. Properties: Nylon 6 fibres are tough, possessing high tensile strength and elasticity. They are wrinkleproof and highly resistant to abrasion and chemicals such as acids and alkalis. The fibres can absorb up to 2.4% of water. Used in making tire cords, textile, in automobile industry etc. 2. Nylon 6,6: Preparation: It is prepared by condensation polymerization of adipic acid and hexamethylene diamine in the absence of air. 6 Properties: (1) Translucent, whitish, high melting point. (2) possess high temperature stability and good abrasion-resistance. (3) Have good strength. (4) Insoluble in common organic solvents. Applications: The major application is in textile industry. Because of its high thermal & abrasion resistance nylons are used in mechanical engineering applications like gears, bearings, machine parts where greater friction is there. Flexible tubing’s for conveying petrol etc are made from nylons Used as electrical insulators. Nylons are used in automobile industry and telecommunication industry for making radiator parts and coil formers. Biodegradable Polymers ▪ Generally polymers are not affected by the environment. ▪ To some extent polymers and plastics are degraded slowly by oxidation, UV radiations, extreme temperature etc. ▪ Natural polymers are biodegradable, but synthetic polymers and plastics are not prone to biodegradation and cause pollution of environment. ▪ The growing awareness on the hazards caused by the disposal of plastics has resulted in the search for plastics which are biodegradable. ▪ Biodegradable polymers are defined as the degradable polymers in which degradation is caused by the action of naturally occurring micro organisms such as bacteria, fungi and algae. During compositing they yield CO2, H2O, inorganic compounds and biomass without leaving toxic residue. Naturally occurring biodegradable polymers: The naturally biodegradable polymers classified in to four groups as given below. Synthesized biodegradable polymers: There are many polymers produced from derived from petrochemical or biological sources that are biodegradable. There are a number of biodegradable synthetic resins that are: 1) Poly-lactic acid 2) Polyvinyl acetate Polylactic acid or Polylactide (PLA): An aliphatic thermoplastic polyester Derived from renewable resources, such as corn starch (in the United States), tapioca roots, chips or starch (mostly in Asia), or sugarcane (in the rest of the world). Lactic acid is obtained by the bacterial fermentation of sugarcane or from the starch obtained from corn. Polylactic acid is obtained by the condensation polymerization of lactic acid in the presence of acid or base catalyst. Properties: 1. It has a crystallinity of around 37%, 2. melting temperature between 173-178 °C 3. It is soluble in chlorinated solvents, benzene (heated benzene), tetrahydrofuran Applications 1. Used in biomedical applications such as drug delivery devices and dialysis media. 2. Used in the production of compost bags, food packaging, and disposable tableware. 3. Can also be use in the form of fibers and non-woven textiles.  2) Polyvinyl acetate: Preparation: Polyvinyl acetate is obtained by the addition polymerization of vinyl acetate in the presence of benzoyl peroxide Properties: It is a rubbery synthetic polymer with the formula (C4H6O2)n. It belongs to the polyvinyl esters family. It is water soluble, colorless and transparent. Has excellent mechanical properties and good heat resistance It is harmless if taken orally. Applications and uses: 1. As an emulsion in water, a PVAc emulsions are used as adhesives for porous materials, particularly for wood, paper, and cloth. 2. As wood glue PVAc is known as "white glue" and the yellow "carpenter's glue" or PVA glue. 3. As paper adhesive during paper packaging. 4. It is used for making of chewing gums, paint emulsions etc. Conducting polymers are classified into two types. a) Intrinsic conducting polymers: These are characterized by intensive conjugation of π-bonds in their structure. This is a polymer whose back bones or associated groups consisting of delocalized electron pair or residual charge, which increases their conductivity to a large extent. The conduction process is due to the overlapping of orbitals containing conjugated π- electrons, resulting in the formation of valence bands as well as conduction bands separated by significant Fermi energy gap. The electrical conductivity is due to thermal or photolytic activation of the electrons, which gives them sufficient energy to cross the Fermi gap and cause conduction. e.g. polyacetylene, polythiophene, polyaniline. 2. Doped conducting polymers: The conducting polymers having e-s in their backbone can easily be oxidized or reduced because they possess low ionization potential and high electron affinities. Hence their conductance can be increased by introducing a positive charge or negative charge on polymer backbone by oxidation or reduction. This process is similar to semiconductor technology and is called doping. Doping is again two types. (1) Creating a positive site on the polymer backbone called p-doping. (2) Creating a negative site on the polymer backbone called n-doping. P-doping: P-doping is carried out by oxidation process by the removal of an electron from conducting polymer like polyacetylene with a Lewis acid or iodine. This is called oxidative doping. N-doping: N-doping is carried out by reduction process by the addition of an electron to conducting polymer like polyacetylene with reducing agents like sodium napthalide Na(C10H8).  Classification of polymer based on tacticity: = stereochemical arrangement of atoms Isotactic polymers : In isotactic macromolecules all the substituents are located on the same side of the macromolecular backbone. Syndiotactic polymers: In syndiotactic or syntactic macromolecules the substituents have alternate positions along the chain. Atactic polymers In atactic macromolecules the substituents are placed randomly along the chain.  Some extra points to remember:  Some extra points to remember: