(c) Dr. Payal B. Joshi - Strawberry -...

(c) Dr. Payal B. Joshi

• 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 H2O2 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: (C2H4)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

Polymer Fluid heat cooling Hard, brittle glass-like

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 Thermosetting polymers They soften on heating and harden on cooling

They are fusible on initial heating, but turn into hard infusible mass on heating further

Can be reshaped and recycled Cannot be reshaped and recycled Formed by addition polymerization

Formed by condensation polymerization

Linear in structure Three dimensional in structure They are soluble in some organic solvents

Insoluble in organic solvents

Moulded articles are taken out after cooling the mould to avoid deformation of the article

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 stabilityÆ 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 H2O2, benzoyl peroxide, ZnO, NH3,

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

CH2 C

CH3

C O

O CH3

nCH2

CH3

O O

CH3

n(-

Methyl methacrylate

Polymerisation

Catalyst

PMMA


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.


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