Polymer (Greek: poly=many; mer=part) Made up of large molecules characterized by repeating units called monomers held together by covalent bonds
Functionality To act as monomer, it must have at least two reactive sites/bonding sites Number of bonding sites in a monomer is called functionality
Polymerization Fundamental process in which low molecular weight compounds combine to form macromolecules of high molecular weight
Mechanism Initiation: Free radicals formed from H 2 O 2 through the addition of heat Free radical acts to open the C=C double bond by joining to one side of the monomer. This allows the monomers to react with other open monomers on their other side.
Propagation: Process continues with successive addition of monomer units to the chains Termination: Through a combination of two chains
Degree of Polymerization Number of repeating units in the chain is called the degree of polymerization (n). Polyethene: (C 2 H 4 ) n, where n stands for DP Molecular weight of PE, M = nmo, where Mo is molecular weight of monomer Strength of the polymer can be increased by increasing its DP
Data: If MW of PE, M = 28000, MW of repeat unit, Mo = 28, Calculate DP??? M = nmo Thus, n = M/Mo = 28000/28 = 1000 Degree of Polymerization (n) = 1000
Addition Polymerization It yields product that is an exact multiple of the original monomer unit Polyethene (PE)
Condensation/Step Polymerization Formation of polymers from polyfunctional monomers of organic molecules with elimination of small molecules like water, HCl, ammonia Functional group of one monomer unit reacts with functional group of the other
Copolymerization Specific type of addition polymerisation Monomers of more than one type are involved thereby giving variety of polymers Eg. Styrene-Butadiene rubber (Buna-S)
Tacticity: Orientation of monomeric units in polymer takes place in orderly/disorderly fashion w.r.t main chain Isotactic: Functional groups are all on the same side of the main chain
Syndiotactic: Functional groups occupy alternating position. Atactic: Functional groups arranged in random manner
Melting and Glass transition temperatures Hard, brittle glass-like cooling Polymer heat Fluid Tg, Lower T Increasing Temperature Tm Lowest temperature beyond which polymer becomes hard, brittle, glass-like (Tg) Temperature above which it turns out to be flexible, elastic and rubbery (Tm)
Significance Tg and Tm are significant parameters Gives an indication of the temperature region at which a polymeric material transforms from a rigid solid to a soft viscous state Helps in choosing the right processing temperature in which materials are converted into finished products
Factors affecting Tg Tg is directly proportional to the molecular weight of the polymer. Greater the degree of cross-linking, higher the Tg. Polymers with strong intermolecular forces of attraction have greater Tg. Side groups, especially benzene and aromatic groups attached to main chain increases Tg.
Plastics ThermoPlastics/Thermosoftening Polymers Some polymers when heated become soft and can be moulded into any shape that can retain on cooling PVC, PE Thermosetting polymers On heating, polymers undergo a chemical change and become an infusible mass which cannot be reshaped Egg, polyester, resins
Thermoplastic polymers They soften on heating and harden on cooling Can be reshaped and recycled Formed by addition polymerization Linear in structure They are soluble in some organic solvents Moulded articles are taken out after cooling the mould to avoid deformation of the article Thermosetting polymers They are fusible on initial heating, but turn into hard infusible mass on heating further Cannot be reshaped and recycled Formed by condensation polymerization Three dimensional in structure Insoluble in organic solvents Moulded articles are taken out from the mould even when they are hot.
Compounding of Plastics Unusual for a finished high polymeric articles to solely consist of high polymers alone Mixed with ingredients known as additives resulting in useful functions and imparts useful properties to the finished products Main types of compounding ingredients are Resin: Binder, which holds different constituents/additives together. Natural or synthetic resins used in this case Plasticizers: Low MW organic liquids added to polymer to improve its flexibility; Added 8-10% of total bulk of plastics (oils, camphor, dioctyl phthalates)
Stabilizers: Most polymers do not possess chemical stability change colors & decompose Stabilizers are additives which chemically stabilize the polymer and thus arrest degradation Organic, inorganic, organometallic compounds like CaO, BaO, Organo-tin compounds Fillers/Extenders: Inert material added to enhance mechanical strength-- asbestos powder, saw dust, cotton pulp, clay, etc Lubricants: Glossy finish to product, Prevents plastics from sticking to fabrication equipments; oils, waxes, soaps, etc
Catalysts Antioxidants like H 2 O 2, benzoyl peroxide, ZnO, NH 3, Ag, Pb, are added to the polymeric matrix to accelerate the cross linking in thermosetting plastics while moulding process Coloring materials Organic dyes and pigments impart desired color for aesthetic appeal of the finished polymeric material. Some colors are added to impart UV protection to the finished products.
Preparation, Properties and Uses of Commercial Plastics
Phenol Formaldehyde Resin
Properties and Uses Phenolic resins are rigid, hard, water resistant Resistant to acids, salts, organic solvents Easily attacked by alkalies due to the presence of free hydroxy groups Possess electrical insulating properties due to low thermal conductivity Uses: Used to fabricate insulators, plugs, switches Used as cation-exchanger resin in water softening Adhesives in paints and varnishes Propellar shafts for paper industry and mills
Polymethyl Methacrylate O CH 3 n(- CH 3 H 2 C Polymerisation Catalyst CH 2 C O C CH 3 n O O CH 3 PMMA Methyl methacrylate
Properties and Uses Also known as plexiglass Transparent, colorless plastic Easy to mould Used to fabricate artificial eyes (cosmetic surgery), bone splints, dentures, TV screens, aircrafts, adhesives, paints.