Polymers - strawberrydevelopers.weebly.comstrawberrydevelopers.weebly.com/.../5/.../polymers.pdf · ... monomers with functional groups form polymers with ... thus leading to high

Engineering Chemistry Dr. Payal Joshi

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Polymers

Introduction: Polymers (Greek: poly=many; mer=part) are made up large molecules characterized by repeating units called monomers held together by covalent bonds, depicted as,

Monomer is defined as a simple molecule with two or more bonding sites through which it forms covalent linkages with other monomer molecules to form macromolecule. Monomers are thus, building blocks of polymers. All simple molecules cannot behave as monomers, but only those with two or more bonding sites can act as monomers. Hence, molecules like ammonia, water, ethanol, etc are not monomers. Alkenes, vinyl chloride, adipic acid, glycol with two bonding sites act as monomers. This is termed as functionality of a monomer. Polymerization: It is the fundamental process in which low molecular weight compounds combine to form giant molecules/ macromolecules of high molecular weight. The number of repeating units in the chain is called the degree of polymerization. Polymers with high degree of polymerization are called high polymers and those with low degree of polymerization are called oligomers. High polymers that have very high molecular weights (104 to 106) are called macromolecules. Three types of polymerization are generally distinguished, viz, addition, condensation and copolymerization. In addition polymerization, polymer is formed from the monomer, without the loss of any byproduct, like small molecules. Monomers with double or triple bonds tend to polymerize without the liberation of small molecules. Example: Polyethylene (PE).

In condensation reaction, monomers with functional groups form polymers with elimination of small molecules like water, ammonia, methanol, etc. Example: Nylon-66.

Contents: Introduction and definition of important terms – monomer, polymer, polymerization, degree of polymerization, tacticity, and melting-glass transition temperature. Plastics: Thermosetting & Thermoplastics, Compounding of plastics, Preparation, properties and applications of commercial plastics (PF, PMMA) Elastomers: Natural rubber, drawbacks of natural rubber, Vulcanization of rubber. Preparation, properties and applications of commercial elastomers (Buna-S, Isocyanate rubber). Speciality polymers: Conducting polymers, Self-healing polymers. Applications of speciality polymers.


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Copolymerization: Polymers are formed from the different types of monomers without loss of any small molecules, thus leading to high molecular weight compounds. Example: Buna-S Rubber.

Addition polymerization Condensation polymerization

1. Reaction proceeds in a fast manner under favorable conditions.

1. Reaction proceeds comparatively slow.

2. It proceeds by chain-growth mechanism. 2. It proceeds by step-growth mechanism. 3. None of the by-products are formed. 3. By-products are formed. 4. Monomers with carbon-carbon unsaturation (-C=C-, alkynes) are involved.

4. Monomers with reactive functional groups are involved.

5. Number of monomers decrease throughout the course of the reaction.

5. Concentration of monomers decreases much faster in the early stages of the reaction.

General Mechanism of Polymerization: There are three stages in polymerization namely, initiation, propagation and termination. Initiation: Initiation involves production of free radicals by homolytic dissociation of initiator. Free radicals are formed from hydrogen peroxide, H2O2 by application 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: Chain reaction process continues with successive addition of monomer units to the chains.


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Termination: Once the desired polymer is obtained, the polymerization reaction is terminated by combining two chains, as shown,

ThermoPlastics/Thermosoftening Plastics: Those polymers when heated become soft and can be moulded into any shape that can retain on cooling, eg, PVC, PE, nylon sealing wax, etc. Thermosetting polymers: On heating, polymers undergo a chemical change and become an infusible mass which cannot be reshaped, eg, Egg yolk, polyester, formaldehyde 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.

Ingredients of Plastic (Compounding of Plastics): It is unusual for finished high polymeric articles to solely consist of high polymers alone. They are generally mixed with ingredients known as additives resulting in useful functions and impart useful properties to the finished products. Main types of compounding ingredients are: Resin: It acts as a binder which holds different constituents/additives together. Natural or synthetic resins used in this case. Plasticizers: Low molecular weight organic liquids are added to polymer to improve its plasticity and flexibility. It is added 8-10% of total bulk of plastics, not used in thermosets, (oils, camphor, dioctyl phthalates). The small molecules penetrate into the polymer matrix and neutralize a part of intermolecular forces of attraction between macromolecules and increase mobility of polymer segments so that chains can slide over each other. Hence, plasticizers act as an internal lubricant. Stabilizers: Most polymers do not possess chemical stability. They change colors & decompose. Stabilizers are additives which chemically stabilize the polymer and thus arrest degradation. Organic, inorganic, compounds like CaO, BaO, Organo-tin compounds are generally used.


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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. 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 which thereby enhances the aesthetic appeal of the finished polymeric material. Some colors are added to impart UV protection to the finished products. Tacticity: Orientation of monomeric units in polymer can take place in an orderly or disorderly fashion with respect to the main chain. The difference in configuration affects their physical properties. Various types of tacticity can be described as follows, 1) Isotactic polymer: Functional groups are all on the same side of the chain, as depicted below,

2) Syndiotactic polymer: Arrangement of the functional groups (side chains) is in alternating fashion,

3) Atactic polymer: if orientation of the functional groups is at random around the main polymeric chain,

Example: Vinyl polymers that have a single substituent (eg, propylene) or two unsymmetrical substituents (eg, methyl methacrylate) have pseudoasymmetric carbon atoms in the backbone. There are many possible relative placements of the groups. Different stereoisomers may have very different physical properties. For example, atactic polypropylene is a useless, gummy solid, while isotactic version is highly crystalline & tough.

Melting and Glass Transition temperatures: Amorphous polymers do not possess melting points, but softening point. At low temperatures, polymers exist as glassy materials. In this state, solid tends to shatter if it is hit, since the molecular chains cannot move easily. Lowest


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temperature beyond which polymer becomes hard, brittle, glass-like (Tg) and temperature above which it turns out to be flexible, elastic and rubbery (Tm).

Behaviour of polymers with respect to flow properties is temperature sensitive. Temperature at which polymer experiences transition from rubbery to rigid states is called glass transition temperature denoted as Tg. Beyond glass transition temperature, crystalline and amorphous polymer behaves differently as shown in the diagram below.

Example: Rubber band (rubber like state, very flexible) into a container of liquid N2. When removed rubber band is solid and inflexible (glass state) and can be shattered. Upon standing to room temperature rubber band will again become flexible and rubbery (rubber like state). Tg and Tm are significant parameters. These parameters give an indication of the temperature region at which a polymeric material transforms from a rigid solid to a soft viscous state. It also helps in choosing the right processing temperature in which materials are converted into finished products.

Factors influencing Tg:

1. Tg is directly proportional to the molecular weight of the polymer. 2. Greater the degree of cross-linking, higher the Tg. This is due to the fact that crystalline

polymer chains are arranged in a regular manner and each chain is bound by strong forces like H-bonding. Polymers with strong intermolecular forces of attraction have greater Tg.


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3. Side groups, especially benzene and aromatic groups hinder free rotation when attached to main chain thereby increasing Tg.

4. Stereoregularity of polymers increase Tg. Thus Tg of an isotactic polymer is greater than that of syndiotactic polymer whose Tg is greater than atactic polymer.

Importance/Significance of Tg: Tg value is a measure of flexibility of a polymer and also gives an idea of thermal expansion, heat capacity, electrical and mechanical properties of a polymer. Thus the workability and usefulness of a polymer over a range of temperature can be obtained from its Tg.

Preparation, Properties and Applications of Commercial Plastics Phenol Formaldehyde Resin: Phenol and formaldehyde react with each other in presence of acid or a base as a catalyst to undergo polymerization to give products whose nature depends on the catalyst and ratio of phenol and formaldehyde. Novolac resin is a linear thermoplastic polymer, whereas Bakelite is a cross-linked thermosetting polymer.

Properties: Phenolic resins are rigid, hard, and water resistant. They are resistant to non-oxidizing acids, salts, organic solvents. It can be easily attacked by alkali due to the presence of free hydroxy groups. They possess electrical insulating properties due to low thermal conductivity.


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Applications: They are used to fabricate insulators, plugs, switches. They are used as cation-exchanger resin in water softening. They can be used as adhesives in paints and varnishes and propeller shafts for paper industry and mills Polymethyl methacrylate (PMMA)

Preparation: Methyl methacrylate, on polymerization in presence of a catalyst such as acetyl peroxide results in polymethyl methacrylate (PMMA). It is also known as plexiglass. Properties: It is transparent, colorless plastic and easy to mould. Applications: It is used to fabricate artificial eyes, bone splints, dentures, TV screens, aircrafts, adhesives, paints. Elastomers or Rubbers: They are amorphous polymer with numerous cross-linkages. They possess high degree of elasticity which get deformed on stretching & regain original form when stretching force is removed. Rubber has no crystallinity. Their extension and contraction are due to temporary movements of segments of polymer chain. The chains do not slip past each other due to cross linkages. 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 transported to a natural-rubber processing plant, where acetic acid is added to the latex to precipitate out the rubber, which then hardens (coagulates). After being washed and dried, rubber is cured in special smokehouses to protect it against mold. The purer the rubber, the higher the grade – it is ready for delivery to rubber companies worldwide. Natural Rubber is a polymerized form of isoprene (2-methyl-1,3-butadiene).

It possesses low tensile strength, elasticity over a narrow range of temperature. Properties: Natural rubber is an amorphous solid, translucent, impervious to gases, elastic in nature. Rubber slowly oxidizes when exposed to air. On heating it softens and liquefies. It burns to form CO2 and H2O. Raw rubber powder catalytically reacts

with hydrogen gas. It decolorizes bromine water, forms ozonide with ozone, and reacts with HCl. Limitations of natural rubber: It softens at high temperature and becomes brittle at low temperature. Natural rubber is attacked by acids, oxidizing agents, non-polar solvents and oxidized by air. To overcome these limitations rubber is vulcanized.

CH2 C

CH3

C O

O CH3

nCH2

CH3

O O

CH3

n(-

Methyl methacrylate

Polymerisation

Catalyst


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Applications of rubber: a) Due to remarkable resistance to electricity, it is used as an insulating coating on wires and cables, used for electric power transmission. b) Due to its elasticity, it is used to fabricate rubber bands, rubber goods, golf balls, tubes for automobiles, etc c) It acts as an excellent adhesive d) Foam-rubber is used for making pillows, cushions, mattresses, automotive pads, etc. e) Polysulfide rubber is used as a solid-propellant fuel for rocket motors. Vulcanization of Rubber:

1. Vulcanization is a process in which natural rubber is compounded with different chemicals like sulphur, H2S, benzoyl chloride etc. The method is basically heating raw rubber with sulphur at 1000C-1400C.

2. When sulphur enters into the double bonds of rubber forming crosslinks between the chains and this gives the structure toughness.

3. The toughness of vulcanized rubber depends on the amount of sulphur added. For flexible tyre rubber, sulphur content is from 3-5% whereas for tougher variety like ebonite, content of sulphur is 32%. Ebonite is so tough that it can be machined and has very good electrical insulation property.


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Synthetic Rubbers: Synthetic rubber is any vulcanizable man-made rubber-like polymer which can be stretched to twice its length and on releasing the stress, it returns to its original shape and size. Preparation, properties and applications of commercial elastomers Buna-S Rubber/ Cold Rubber/SBR:

Preparation: Styrene-Butadiene rubber is prepared by copolymerization of butadiene & styrene carried out at 5oC. Properties: SBR possesses high resilience and good and tough mechanical properties. It is easily attacked by oxidizing agents, mainly ozone. It also swells in organic solvents. It can be vulcanized as natural rubber. Uses: manufacture of tyres, soles and other components of shoes, for insulating wires and cables, as adhesives and lining of vessels. Polyurethane Rubber/Isocyanate Rubber:

Preparation: Ethylene glycol polymerizes with ethylene di-isocyanate to form polyurethane rubber. Properties: Highly resistant to oxidation because of its saturated character, resistant to many organic solvents, but attacked by acid and alkali. Polyurethane foams are light, tough and resistant to heat, abrasion, chemicals and weathering. Uses: surface coatings and manufacture of foams and fibers.


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Conducting Polymers:

1. Due to non-availability of free electrons, polymers behave as poor conductors of electricity. 2. In 1977, it was discovered that polyacetylene, a poor conductor in its pure state could be

turned into a highly conductive polymer by conversion to salt on reacting it with iodine. The result was a dramatic increase of over 1010 in conductivity.

3. As conduction appears to be due to movement of electrons through the polymers, this discovery was exciting adding new dimension. Polymers with polyconjugated structures are insulators in pure state, but when treated with oxidizing or reducing agents can be converted into polymer salts with electrical conductivities comparable to metals.

4. Conductivity of polymers can be increased by decreasing the energy band gap with increase in the amount of energy needed to promote an electron from valence to empty band (conduction band).

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

6. There are two conditions by which polymers can be made conducting, those are; a) Polymer should consist of alternating single and double bonds called conjugated double

bonds. b) Polymer matrix has to be disturbed - either by removing electrons from (oxidation), or

inserting them into (reduction) the material. The process is known as doping. By doping with electron donor like alkali-metal ion or electron acceptor like AsF5, Iodine, etc polymers turn conductive materials.

7. Conducting polymers are classified as a) π-electron conducting polymers: In these polymers, backbone of the polymer is made

up of molecules that contain conjugated π-electrons which extend the entire polymer and make the polymer conducting.

b) Conducting element-filled polymer: Here, polymer acts as a binder that binds the conducting elements like carbon black, metal oxides, metallic fibres that conduct electricity.

c) Inorganic polymer: A metal atom with polydentate ligand, which is a charge transfer complex is bound to the polymer to make it conducting.

d) Doped- conducting polymer: Polymer is made conducting by exposing the surfaces to charge transfer agents in gas or in solution phase.

e) Blended conducting polymer: This polymer is made by blending a conventional polymer with a conducting polymer.

Industrially conducting polymers are: Polyquinoline, polyanthrylene, polythiophene, polybutadienylene, etc. Uses: Making button cells, photovoltaic devices, sensors, biomedical devices.


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Example: 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, the double bond successively moves along the molecule. Positive charge, on the other hand, is fixed by electrostatic attraction to the iodide ion, which does not move readily. Applications of conducting polymers: Conducting polymers are finding increased use due to their light weight, easy to process and good mechanical properties. Some of the important applications are,

1. In rechargeable light weight batteries based on perchlorate-doped polyacetylene-lithium system.

2. Contex, a fibre is coated with a conductive polymer polypyrrole can be woven to create an anti-static fabric which is used in carpet industry.

3. Used as anti-radiation coatings, batteries, catalysts, electrochromic windows, fuel cells, non-linear optics, radar dishes

4. Used in producing photovoltaic devices, eg, in Al/polymer/Au photovoltaic cells.

Self-Healing Polymers

1. Self-healing polymers are a new class of smart materials that have the capability to repair themselves when they are damaged without the need for detection or repair by manual intervention of any kind.

2. The concept of ‘self-healing’ stems from wound healing observed in human beings. Hence, it is considered as a biomimetic approach to solve engineering problems. One way to extend the lifetime of a material is to mitigate the mechanism leading to failure.

3. In brittle polymers, failure occurs through crack formation and propagation and the ability to repair these cracks when they are still very small will prevent further propagation thus extending the lifetime of the material.

4. Emerging self-healing technologies are designed to give polymeric materials the capability to arrest crack propagation at an early stage thereby preventing catastrophic failures.


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Healing Process: It is accomplished by incorporating a microencapsulated healing agent (or monomer) and a chemical trigger (typically a catalyst or initiator) within an epoxy matrix. 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, an additional external stimuli such as heat or uv-radiation is required for complete healing to occur.

Example: Terminator polymer (T-100) is a self-healing polymer. The material comprises of a poly (urea-urethane) elastomeric matrix, a network of complex molecular interactions that will spontaneously cross-link to “heal” almost any break. In this context, the word “spontaneous” means that the material needs no outside intervention to begin its healing process, no catalyst or extra reactant. Experimentally, a sample cut in half with a razor blade at room temperature healed the cut with 97% efficiency in two hours. Applications: Currently, self-healing materials development is at a preliminary level. Few applications that are developed to date are mainly in automotive, aerospace, and building industries. Nissan Motor Co. Ltd has commercialized world’s first self-healing clear coat for car surfaces. The trade name of this product is ‘Scratch Guard Coat.’ In construction industry, self-healing concretes may become a reality soon. Self-healing corrosion resistant coatings can be beneficial for structural metallic components. Self-healing materials are now used as composite materials in aircrafts.

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Polymers - strawberrydevelopers.weebly.comstrawberrydevelopers.weebly.com/.../5/.../polymers.pdf · ... monomers with functional groups form polymers with ... thus leading to high