UNIT 1 – POLYMERS & COMPOSITES

Introduction: Classification of polymers – Natural and synthetic; Thermoplastic and thermosetting. Functionality – Degree of polymerization. Types and mechanism of polymerization: Addition (Free radical, cationic and anionic); condensation and copolymerization. Properties of polymers: Tg, Tacticity, Molecular weight – weight average, number average and polydispersity index. Techniques of polymerization: Bulk, emulsion, solution and suspension. Preparation, properties and uses of Nylon – 6,6 and Epoxy resin.

1.1 Introduction

In this modern world, polymers are an integral part of every one’s life style. They have different structures and applications ranging from domestic articles to sophisticated scientific and medical instruments. These materials are used as fibres, rubbers, plastics, adhesives, paints, etc. Infact, the existence of life is virtually the formation, transformation and decomposition of bio-polymers viz. Carbohydrates, Proteins and Nucleic acids. Hence, in view of their importance, a proper understanding of polymeric materials is very essential.

1.1.1 Polymer

The word Polymer is derived from two Greek words, Poly - Many and Meros - Units.

A polymer is a macro-molecule (giant molecule) which is formed by the repeated linkage of a large number of small molecules (monomers). It is defined as, “a macromolecule of high molecular weight which is formed by the combination of large number of small molecules of low molecular weight”.

Example

Polyvinyl chloride (PVC) is a polymer formed by repeated linkages of a large number of vinyl chloride (VC) molecules. Where n – degree of polymerization, it can be 104 or more.

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1.1.2 Monomer

A large number of small molecules repeatedly combine to give a macro molecule (polymer) of high molecular weight are called monomers.

1.1.3 Degree of polymerization

The number of monomeric units contained in a polymer is called degree of polymerization.

1.1.4 Functionality

A substance needs at least two reactive sites or bonding sites to acts as a monomer. The number of bonding sites in a monomer is called functionality. For example in ethylene, the double bond can be considered as a site for two free vacancies. When the double bond is broken, two single bonds become available for combination. Thus, ethylene is considered as bifunctional.

1.2 Classification of polymers

1.2.1 Classification based on source:

1.2.1.1 Natural polymers

Natural polymers are found in nature in animals and plants. For example, starch (a polymer of α – D – Glucose), cellulose (a polymer of β – D – Glucose), proteins (polypetides and polyamides), nucleic acids, natural rubber (a polymer of cis-isoprene).

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1.2.1.2 Synthetic polymers

Synthetic polymers (man – made polymers) like polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinylchloride (PVC), nylon, terylene and bakelite, etc

1.2.2 Classification based on molecular force

1.2.2.1 Thermoplastics

They have either linear or branched structures. Their polymeric chains are held together by weak Vander Waals’ forces. There is no cross-link. On heating, they soften readily and become hard and rigid on cooling. That’s why they can be remoulded, reshaped and reused. They are soft, weak and less brittle; they are soluble in organic solvents. Example: Polyethylene (PE), Polyvinylchloride (PVC), Polypropylene (PP), Polystyrene (PS) and Polymethylmethacrylate (PMMA), etc.

1.2.2.2 Thermosetting plastics

They have three dimensional, cross-linked, networked structures. Their polymeric chains are held together by strong covalent bonds. There are many cross-links. Heating does not soften them, since softening would require breaking of covalent bonds. That’s why they can’t be remoulded, reshaped and reused. They are hard, strong and more brittle. They are insoluble in organic solvents. Example: Bakelite & Polyester, etc.

Sl.No THERMOPLATICS THERMOSETTING PLASTICS

1They are formed by addition polymerization.

They are formed by condensation polymerization.

2They have either liner or branched structure.

They have three dimensional, cross-linked network structures.

3Adjacent polymer chains are held together by Vander Waal’s forces.

Adjacent polymer chains are held together by strong covalent bond.

4They soften on heating and stiffen on cooling.

They do not soften on heating.

5They can be re-moulded, re-shaped and re-used.

They can’t be re-moulded, re-shaped and re-used.

6 They can be recycled. They can’t be recycled.7 They are soluble in organic solvents. They are insoluble in organic solvents.8 They are soft, weak and less brittle. They are hard, strong and more brittle.9 There is no change in their chemical

composition during the moulding There is a change in their chemical composition during the moulding

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process. process.

1.3 Polymerisation

The chemical process which leads to the formation of polymer is called polymerization.

1.3.1 Types

Polymerization is classified into addition, condensation and co-polymerization.

1.3.1.1 Addition polymerization

It is also called chain polymerization. The process in which large number of identical monomers repeatedly combines to give a polymer without the elimination of any by-products is called addition polymerization. Example for addition polymers:Polythene (PE), Polyvinyl chloride (PVC), Polystyrene (PS), Polypropylene (PP), etc

1.3.1.2 Condensation polymerization

It is also called step polymerization. The process in which large number of two or more different types of monomers combines to give a polymer with the elimination of by-products like H2O, HCl, methanol, etc is called condensation polymerization. Example for condensation polymers: Nylon, Bakelite, Polyethylene terephthalate (PET), etc

Sl.No ADDITION CONDENSATION

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POLYMERISATION POLYMERISATION1 It requires the presence of double

bond in monomer.It requires two reactive functional (same / different) groups to be present at both ends of the monomer.

2 No by-product is formed. By-product is formed.3 Homo-chain polymer is formed. Hetero-chain polymer is formed4 Thermoplastics are formed. Thermosetting plastics are formed.5 The growth of chain is at one

active centre.The growth of chain occurs at minimum of two active centres.

6 The molecular weight of the polymer is a multiple of that of the monomer.

The molecular weight of the polymer is not a multiple of that of the monomer.

1.3.1.3 Co-polymerisation

The process in which large number of different monomers combines to give a polymer without the elimination of any by-products is called co-polymerization. Example: GR-S rubber (SB-Rubber or Buna-S)

Properties

1. It has high abrasion resistance, high load – bearing capacity and resilience.2. It oxidizes in presence of ozone.3. It swells in oils and solvents.4. It can be vulcanized like natural rubber using S / Sulphur monochloride (S2Cl2).

Applications

1. It is used for the manufacture of motor cycle tyres, floor tiles, shoe soles, gaskets, and foot-wear components, etc2. It is used for making wire and cable insulations, carpet backing, adhesives and tank-lining, etc.

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1.4 Mechanism for addition polymerisation

1.4.1 Free radical mechanism

It involves the following steps, chain initiation, chain propagation and chain termination. The chain initiation includes the production of free radicals by the homolytic cleavage of an initiator followed by the addition of first monomer to radical to produce the chain initiating species.

Chain initiation step: It involves two reactions.

The production of free radicals by the homolytic cleavage of an initiator to give a pair of radicals, Ro. The addition of first monomer (M) to the radical to produce the chain initiating species, M1

o. Thus, the polymerization of monomer, CH2=CHY, takes in the form as follows.

Chain propagation step:

The growth of M1o occurs by the addition of large number of monomers.

Chain termination step

It involves coupling and disproportionation. The hydrogen atom of one radical centre is transferred to another radical centre. This results two polymers, one is saturated and another is unsaturated.

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1.4.2 Cationic mechanism

It involves the following steps, chain initiation, chain propagation and chain termination.

Chain initiation: In this step the formations of carbocation takesplace.

Chain propagation

Chain termination: It involves the attack of nucleophile takes place.

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1.4.3 Anionic mechanism

It involves the following steps, chain initiation, chain propagation and chain termination.

Chain initiation: In this step the formations of carboanion takesplace

Chain propagation:

Chain termination: It involves the atack of electrophile takes place.

1.5 Properties of polymers

1.5.1. Glass transition temperature (Tg)

The temperature at which the amorphous state (rubber like state) of a polymer is changes to rigid state is called glass transition temperature. Below the glass transition temperature the polymer is hard and above which is soft. The hard brittle state is called glassy state and soft flexible state is called rubbery state. For example, Nylon (Tg = 57 oC) and PVC (Tg = 87 oC) are hard and brittle when the temperature above their respective glass transition temperature. Also Polycarbonate

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(Tg = 150 oC) and polypropylene (Tg = –18 oC) are soft and rubbery when the temperature below their respective glass transition temperature.

Factors affecting Tg

1. The presence of bulky side groups increases the Tg value.2. The presence of polar side atom or groups increases the Tg value.3. The presence of double-chain bonds and aromatic chain groups which stiffen the polymer so that Tg value increases.4. Increasing the molecular weight is increases the Tg

5. A high density of branches reduces chain mobility so that Tg value increases.

1.5.2 Tacticity

The fashion of orientation of monomeric units in a polymer with respect to the main chain is called tacticity. The different in tacticity affect the properties of the polymer. More over, based on the orientation of monomeric units the polymer can be classified into isotactic, atactic and syndiotactic polymers.

1.5.2.1 Isotactic polymer

The arrangement, in which the functional groups are all on the same side of the main chain is called isotactic polymer.

1.5.2.2 Atactic polymer

The arrangement, in which the functional groups are at random around main chain is called atactic polymer.

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1.5.2.3 Syndiotactic polymer

The arrangement, in which the functional groups are alternate side of the main chain is called isotactic polymer.

1.6 Molecular mass (weight) of a polymer

Polymers are polydisperse (different molecular weight) and quite heterogeneous in molecular mass. They are mixtures of molecules of different molecular masses. The properties such as viscosity, softening temperature, tensile strength and thermal resistance, etc are related with molecular mass. For example low molecular weight polymers are soft and gum like resins where as high molecular weight polymers are hard and high thermal resistance. Hence molecular weight is very essential in deciding the property of a polymer.

1.6.1 Types of molecular mass

1. Number – Average molecular mass 2. Weight – Average molecular mass

1.6.1.2 Number – Average molecular mass

It is defined as the total mass (w) of all the molecules in a polymer sample divided by the total number of molecules present. It is determined by measurement of colligative properties such as elevation of boiling point, depression of freezing point, osmotic pressure, etc. It is very useful to know the physical properties such as tensile strength and compactness of a polymer.

Where Ni is the number of molecules of mass Mi

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1.6.1.3 Weight – Average molecular mass

It is defined as the mathematical formula

Where wi is the weight-fraction of molecules, whose mass is Mi. It is determined from light-scattering and ultra-centrifugation techniques.

It is also defined as the mathematical formula

Where ci is the weight-concentration of Mi molecules; c is the total-weight concentration of all polymer molecules.

Polydispersity Index (PDI)

The ratio of weight – average molecular mass to number – average molecular mass is called polydispersity index.

The weight – average molecular mass is always greater than the number – average molecular mass.

1.7 Techniques of polymerisation

1.7.1 Types of polymerization

1. Bulk polymerization2. Solution polymerization3. Suspension polymerization4. Emulsion polymerization

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1.7.1.1 Bulk polymerization

It is the simplest method. The monomer (as liquid) is taken in a flask and the initiator, chain transfer agents are dissolved in it. The flask is placed in a thermostat under constant agitation which leads to the formation of a polymer. By increasing the temperature, the reaction becomes fast, then after a while the content is poured in a methanol and finally the polymer precipitated out.

Example. PVC, PMA and polystyrene, etc

Advantages

1. Quite simple and requires simple equipments2. High pure polymers can be obtained3. The polymer has optical clarity4. As the monomer is solvent, excess monomer can be removed by evaporation.

Disadvantages

1. During process, viscosity of the medium increases hence mixing and control of heat is difficult2. It is highly exothermic

Application

1. Used in casting formulations2. Low molecular weight polymers are used as adhesives, plasticizers and lubricant additives

1.7.1.2 Solution polymerisation

The monomer, initiator and the chain transfer agents are taken in a flask and dissolved in an inert solvent. Then the mixture is kept under constant agitation. After certain time the polymer is precipitated by pouring in a suitable non-solvent. The solvent helps to control heat and reduces viscosity build up. Example. Polyacrylic acid, polyisobutylene, etc.

Advantages

1. Heat control is easy2. Viscosity build up is negligible3. Mixture can be agitated easily

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Disadvantages

1. Removal of last traces of solvent is difficult2. Process requires solvent recovery and recycling3. Difficult to get very high molecular weight polymer4. The polymer must be isolated from the solution either by evaporation of the solvent or by precipitation in a non-solvent.

Application

1. It is used as adhesives 2. It is used in lamination coatings.

1.7.1.3 Suspension polymersation

It is used only for water insoluble monomers and heterogeneous systems. At the end of the polymerization, polymer is separated out as spherical beads. The water insoluble monomer is suspended in water as tiny droplet and an initiator is dissolved in it by continuous agitation. The suspending agents like PVA, gelatin is added to prevent coagulation. The whole content is taken in a flask and heated at a constant temperature with vigorous agitation in a thermostat. The beat like polymers is obtained, filtered and washed by water.

Example. Polystyrene.

Advantages

1. Water is used as solvent, so the method is economical2. Highly pure polymer is obtained3. Isolation of polymer is very easy.

Disadvantages

1. It is applicable only for water insoluble monomer2. Controlling of polymer particle size is difficult.

Applications

1. Polystyrene beads are used as an ion exchanger2. It is used in heterogeneous system.

1.7.1.4 Emulsion polymerisation

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It is used for insoluble monomer and water soluble initiator like potassium persulphate, hydrogen peroxide. The monomer is dispersed in a large amount of water and emulsified by the addition of soap followed by the addition of initiator. The whole content is taken in a flask and heated at a constant temperature with vigorous agitation in a thermostat. The pure polymer is isolated from the emulsion by the addition of de-emulsifier like 3% solution of aluminium sulphate.

Advantages

1. Polymer is obtained in short time2. Heat can be controlled easily to avoid viscosity build up3. Polymer of high molecular weight can be obtained

Disadvantages

1. Polymer needs purification2. Very difficult to remove entrapped emulsifier and de-emulsifier3. Requires rapid agitation

Applications

1. It is used in production of large scale water based paints, adhesives, plastics, etc2. It is used to manufacture butadiene and chloroprene polymers.

1.8 NYLON 6, 6

Preparation

It is obtained by the condensation polymerization of hexamethylene diamine and adipic acid in 1:1 molar ratio.

It is also prepared from 1, 3 – butadiene.

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Properties

1. It is translucent, whitish and horny, etc.2. It has high melting point, 160 – 240oC.3. It has an excellent resistance to high temperature and abrasion.4. It is insoluble in solvents like C6H6 & acetone, but soluble in phenol & HCOOH.5. It has good moulding and extrusion properties.

Properties of nylon fibres

1. They are light, horny and have high melting.2. They are insoluble in common solvents and absorb little moisture.3. They have good strength and resistant to abrasion.4. They are very flexible and retain to original shape after use.5. On blending with wool, the strength and abrasion resistance of later increases.

Applications

1. Nylon 6, 6 is primarily used for fibres in making socks, carpets and dresses, etc.2. It is used to make filaments for ropes, bristles for tooth-brushes and tyre-cords, etc.

1.9 Epoxy Resin

It is obtained by the condensation polymerization of bisphenol and epichlorohydrin.

Preparation of bisphenol: It is obtained by reacting phenol and acetone.

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Preparation of epichlorohydrin: It is obtained by reacting propylene with chlorine followed by CaO.

Preparation of Epoxy Resin:

Properties

1. It has high chemical resistance to water and various solvents due to very stable linkage.2. It has high flexibility.

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3. It shows excellent toughness and heat resistance.4. It has excellent adhesion quality.

Applications

1. It is used to make adhesives like araldite, etc2. It is used as skid-resistance surface for high ways.3. It is used to make mould which is used in production of components of aircrafts and automobiles.4. It is used as a laminating material in electrical equipments.

QUESTION BANK

PART – A

1. Define polymer with an example.

2. What is meant the degree of polymerization?

3. Write any two examples for homopolymer and heteropolymer.

4. What is meant by polymerization?

5. Distinguish addition and condensation polymerization.

6. Define monomer with any two examples.

7. Define thermoplastics. Give any one example.

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8. Define thermosetting plastics. Give any one example.

9. Write the monomers of Nylon 66.

10. Write the monomers of Epoxy Resin.

11. Name any two synthetic polymers which are used for making textile fibres.

12. Why cannot thermosetting plastics be reused and reshaped?

13. Why are plastics indispensable in everyday life?

14. What is addition polymerization? Give one example.

15. What is condensation polymerization? Give one example.

16. Write down the steps involved in the free radical polymerization.

17. What are plastics? Give any two examples.

18. What are the advantages of plastics?

19. Mention any four drawbacks of plastics.

20. Differentiate thermoplastics from thermosetting plastics.

21. Differentiate between homo chain polymer and hetero chain polymers.

22. Give the preparation of Nylon 66.

23. Give the preparation of SB rubber.

24. Define Homochain and heterochain polymer

25. Define Tacticity. Give its types.

26. Define Number – average molecular weight

27. Define Weight – average molecular weight

28. Define Polydispersity index (PDI).

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PART – B

1. Distinguish between thermoplastics and thermosetting plastics.

2. Differentiate addition and condensation polymerizations.

3. Discuss the free radical mechanism of addition polymerization.

4. Discuss the cationic and anionic mechanism of addition polymerization.

5. Explain addition and condensation polymerization with suitable examples.

6. Describe the method of preparation, properties and applications of the following polymers.

a) Nylon-66 b) Epoxy Resin c) Co-polymer

7. Explain the following polymerization techniques: a) Bulk b) Emulsion.

8. Explain the following polymerization techniques: a) Solution b) Suspension.

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