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Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications...

Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications (Lecture 1) 1. Monomer (1) Monomers are the building blocks of polymers. It forms linkages with other monomer molecules to form the macromolecule. (2) ved from two Greek words namely, (a) (b) (3) The functional groups in the monomer organic molecules can be like : OH, COOH, NH2, C N, COOR, Cl, cyclic amide (lactam) group, cyclic acid anhydride group, etc. A C = C contains a weak -bond and it can easily be broken by the action of heat or a reagent. When the -bond is broken, two positions become available for polymerization. 2. Polymer (1) Definition: A polymer is a large molecule built by the repetition of small and simple chemical unit called monomer. The repeating chemical units are covalently linked to each other in a macromolecule. (2) (a) (b) ans parts. 3. General characteristics of polymers (1) Polymeric molecules are very big molecules. Their average molecular weights may approach10 5 or more. (2) Polymers are semi-crystalline materials. It means they have both amorphous and crystalline regions. (3) The intermolecular forces in polymers can be vander waals forces, dipole-dipole. (4) Hydrogen bonding attraction. (5) Polymers are combustible materials. (6) Generally, polymers are thermal and electrical insulators. Although, number of conducting polymers have been discovered so far. 4. Polymerization reaction: The process with the help of which monomers are transformed into a polymer is called polymerization reaction. Polymerisation n CH2=CH2 [ CH2 CH2 ]n (Ethylene monomer) (Polyethylene) 5. Functionality: Functionality is defined as the number of bonding sites available in a single monomer unit.  Specialty Polymers for Engineering Applications Monomer Unit Functionality 2 2 2 2 2 6. Chemical Bonding in Polymer (a) Covalent Bond: Its an interatomic linkage formed due to equal sharing of electron pair between two atoms. The most prevalent C- -bonds are usually found in polymer molecules. Carbon has higher catenation property which is due to the strength of its covalent bonds that allows two or more carbon atoms to share their valence electrons. (b) Hydrogen Bond (Van der Waals bond): It is a type of dipole-dipole attraction between H and electronegative atoms. Hydrogen bond can be intermolecular and intramolecular. H-bonding in DNA polymer is mentioned below.  Specialty Polymers for Engineering Applications 7. Molecular weight of polymers (a) Number-Average Molecular mass M n Number-average molecular mass M n is defined as the total mass (M) of all the molecules in a polymer sample divided by the total number of molecules present in the sample. Ni Mi N1 M1 + N2 M2 +.... Mn = = Ni N1 + N2 +.... N = Number of molecule in polymer chain. M = Molecular weight of the fraction. (b) Weight-Average Molecular-Mass Mw Weight-average molecular mass M w is defined as the total weight (W) of all the molecules in a polymer sample divided by the weight-fraction of molecules present. WiMi Mw = Wi 2 Ni M i = Ni Mi (Where, W = N × M) (c) Viscosity-Average Molecular mass Mv The average molecular weight of polymer is obtained from viscosity measurement is known as viscosity average molecular weight and it is expressed mathematically as follows; 1/ 1/ Mv = Wi Mi Ni Mi1 = Wi Ni Mi -Hawkin- Mv = M w < < temperature. 8. Polydispersity index (PDI) PDI is defined as the ratio of M w and M n. Mw PDI = PDI = 1 for monodisperse polymer. Mn PDI > 1 for polydisperse polymer 9. Molecular weight distribution curve Mixture of many systems present in polymer which have different molecular weights. In order to characterize polymers, we need use the molecular weight distribution (MWD) curve, which represents a plot of the frequency of occurrence for a particular species against its molecular weight. As a result of the existence of different sizes of molecular species in a polymeric material, we cannot strictly assign of the molecular weight of a polymer. Instead, we use molecular weight averages to express the size of synthetic polymers.  Specialty Polymers for Engineering Applications Molecular Methods of Determination Molecular Methods of Weight Weight Determination End Group Analysis Light Scattering Mn Mw Ebullioscopy Sedimentation Cryoscopy Mv Viscometry Osmometry Gel Permeation Chromatography (GPC) 10. Numerical problems on molecular weights P1: A polymer sample consists of 10 chains with the following molecular weights calculate number-average and weight average molecular weight and PDI of the sample. Number of Molecules (Ni) 2 3 5 Molecular weight of each molecule (Mi) 50000 15000 10000 We know that, Ni Mi 2 50000 + 3 15000 + 5 10000 (I) Mn = = = 19500. Ni 10 (II) Ni Mi = 2 50000 + 3 15000 + 5 10000 = 195000 2 Ni M i 2 2 (50000) + 3 2 (15000) + 5 (10000) 2 (III) M w = = = 31666.666 Ni Mi 195000 Mw 31666.666 (IV) PDI = = = 1.62 Mn 19500 Hence, the polymer is polydisperse. (Some practice problems) Q. 1 In a sample of polymer, there are 3 molecules. 50% molecules have molecular weight of 60000, 20% molecules have molecular weight of 20000 and 30% molecules have molecular weight of 40000. Calculate number average and weight average molecular weight of the sample. (Ans. : Mn = 43000, Mw = 50000) 5 5 5 Q. 2 A polymer has molecules having molecular weights of 2 10 , 3 10 and 1 10. Calculate the number-average molecular weight of the polymer. (Ans. : 200000)  Specialty Polymers for Engineering Applications Q. 3 A polymer sample has 40%, 30%, 20% and 10% molecules of molecular weights of 16000, 14000, 12000 and 10000 respectively. Find number-average molecular weight as well as weight-average molecular weight of the polymer sample. (Ans. : 14000, 14300) Q. 4 A polymer with 10 chains has 3 molecules of molecular weight of 30000, 2 molecules of molecular weight of 60000 and 5 molecules of molecular weight of 10000. Calculate number-average and weight-average molecular weights of the given polymer. (Ans. : M n = 26000, M w = 39999)  Specialty Polymers for Engineering Applications 11. Classification of polymers On the Basis of Types Occurrence (a) Natural polymers (b) Synthetic or organic polymers Structure of (a) Linear polymers (b) Branched polymers (c) Cross linked polymers chain Stereo (a) Isotactic (b) Syndotactic polymers (c) Atactic polymers chemical orientation Repeating (a) Homopolymers (b) Co-polymers units Synthesis (a) Addition polymers (b) Condensation polymers Thermal (a) Thermoplastic polymers behavior (b) Thermosetting polymers or thermosets (I) On the basis of occurrence (a) Natural polymers : Those polymers which are obtained from natural sources (plants and animals) are called natural polymers Example : Starch, cellulose, rubber, DNA protein, silk etc. (b) Synthetic or organic polymers: These polymers are synthesized by men. Most of the synthetic polymers are long chain organic molecules containing thousands of monomer units. Example : Polythene, nylon, PVC etc. (II) On the basis of chain structure (a) Linear polymers : In linear polymers, each monomeric unit is linked with two other monomeric units on either side, forming a continuous straight chain. The monomeric units are added on to each other forming a long chain. Example : High density polythene (HDPE) structure (b) Branched polymers : In branched polymers most of the monomeric units are linked with two other on either side. Some monomeric units are linked with a third monomeric unit. Example : Low density polythene (LDPE) (c) Cross linked polymers: When a bi-functional monomer is mixed in small amount with tri-functional monomers, a three dimension network polymer is formed which is known as cross-lined polymer.  Specialty Polymers for Engineering Applications (III) On the basis of stereo chemical orientation On the basis of tacticity, polymers are of following three types: (a) Isotactic polymer : In this type of polymer all the functional groups are present on the same side of the chain i.e. ordered orientation. Example : Polypropylene (b) Syndiotactic polymers : If the arrangement of functional group is in alternating fashion around the main chain, it is called syndiotactic polymers. Example : Natural rubber (c) Atactic polymers: In this type of polymer the arrangement of functional groups are at random around to the main chain. Example : Polystyrene (IV) On the basis of repeating units (a) Homopolymers: Homopolymers are those polymers where the entire polymer chain is composed of one single repeat unit. Example: Polyethylene, PVC (b) Co-polymers : Co-polymers are those polymer which have more than one type of repeating unit in the polymer backbone. Example: (V) On the basis of synthesis (a) Addition polymer: A polymer formed by direct addition of repeated units of monomer is called addition polymer. E.g. Polyethylene, Orlon, PVC etc. n Ethylene Polyethylene  Specialty Polymers for Engineering Applications (b) Condensation polymers: A polymer formed by condensation of two same or different monomers with the liberation of by-product is known as condensation polymer. E.g. Polyester, Nylon 66, Kevlar, Polycarbonate etc. (VI) On the basis of thermal behavior (a) Thermoplastic polymers: These polymers are flexible, linear chain molecule which once shaped or formed can be soften by the application of heat and can be reshaped repeatedly. Examples: Polyethylene, Nylon, Teflon, polycarbonate etc. (b) Thermosetting polymers: These are rigid and 3D polymeric network, which once shaped or formed Examples: Polyester, Bakelite, Thermoset resin, Melamine formaldehyde polymer etc.  Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications (Lecture 2) 12. Mechanism of addition polymerization Addition polymerization occurs through free radical mechanism which involved the following this process by considering the polymerization ethylene. Step1 : Initiation (i) A radical initiator (usually peroxides and azides) in a decomposition reaction breaks down to free radical R through homolytic scission of covalent bond. These initiators are unstable compounds and their homolytic bond scission occurs through either thermal or photochemical process. This step is known as initiation. 2R (free radical) (ii) The formed free radical comes into closed proximity of olefin and it cases disruption of double bond in ethylene monomer. The free electron in R -bond is formed by cleaving the double bond in ethylene and gives rise to the formation of R-CH2-CH2 radical. R + CH2=CH2 2 2 Examples of free radical initiator Step2 : Propagation The formed R-CH2-CH2 radical obtained in previous step attacks to fresh ethylene molecule to form a dimer free radical with a shift of free radical to the second monomer molecule. The chain length goes in increasing manner and polymeric free radical is formed with a growing unit. n CH2=CH2 2 2 + CH2=CH2 2 2 2 2 2)n 2 2 Step3 : Termination In this step, growing of chain length terminates to give stable polymeric molecule. This chain termination occurs via three probable pathways; (i) By coupling with polymer free radical with another polymer free radical or initiator free radical to form stable final polymeric product. 2)n 2 2 2)n 2 2 2)n 2 2 2 2 2)n (ii) By disproportion of two polymeric radicals. In this method, the radicals of two growing chains are transferred away, resulting in two dead chains. 2)n CH2 2 2)n 2 2 2)n 2 + 2)n 2 3  Specialty Polymers for Engineering Applications (iii) In this termination process, the 1º-radical at the end of a growing chain is converted to a more stable 2º-radical by hydrogen atom transfer. This can lead to the creation of other side chains as it is observed for low density polyethylene (LDPE). o 2)n 2 2 (1 -radical) 2)n 2 3 + 2)n 2 2 2 2 2)n 2)n 2 2 2 2)n o (2 -radical) Addition Polymers Name of Polymer Monomer LDPE & HDPE CH2=CH2 PVC CH2=CHCl Orlon CH2=CHCN Teflon CF2=CF2 PMMA CH2=C(CH3)CO2CH3 Natural rubber CH2=CH C(CH3)=CH2 13. Condensation polymerization (a) Polyester Mechanism (b) Nylon 6,6  Specialty Polymers for Engineering Applications Mechanism Sr. Addition polymerization Condensation polymerization No. 1. It occurs in the presence of multiple bonds in Minimum of two different monomers with one or more monomers. functional groups are required. 2. No liberation of by-products like water, HCl It is always associated with the liberation of by- etc. products like H2O, NH3 etc. 3. Chain growth operates at one of the active Chain growth is observed at two active centers. center. 4. Generally, a thermoplastic is obtained which A thermosetting or thermoplastic plastic is obtained in addition polymerization. in condensation polymerization. 14. Glass transition temperature (Tg) Glass transition temperature (T g) is defined as the temperature below which an amorphous polymer is brittle. hard, glassy and above this temperature it becomes flexible, soft and rubbery. Glassy State Tg Rubbery State (Hard & Brittle) (Soft & Flexible) (a) Significance of Tg (1) It helps to anticipate flow properties / softening temperature of polymers. (2) Knowing T g, different samples can be compared and selection of polymer for desired moulding / article can be done more efficiently. (3) It gives us the idea about thermal expansion, flexibility and heat capacity of polymers.  Specialty Polymers for Engineering Applications (b) Factors affecting Tg (1) Molecular weight : Tg molecular weight of the polymer. It increases up to a particular value after that no changes observed. The molecular weight is related to the glass transition temperature by the Fox Flory Equation: Tg = Tg, (K/Mn) Where Tg, is the limiting glass transition temperature at the very high molecular weight, M n is the number average molecular weight and, K is the empirical parameter called Fox Flory parameter related to the free volume inside the polymer. In reality, it is observed that T g is increased up to the molecular weight of approximately 20,000 gm/ mole and after this limit T g is not affected appreciably. (2) Cross-links : Tg degree of cross-links The cross-links between chains restrict rotational motion and thus cross-linking introduces restriction and stiffness in the polymer. Hence, higher cross-linked molecule will show higher Tg than that with lower cross- linked molecule. 1 (3) Flexibility : Tg flexibility The presence of flexible pendant groups, for example, aliphatic chains, limits the packing of the chains and hence increases the rotational motion leading to less T g value. In polybutylmethacrylate, the presence of large aliphatic chain reduces the Tg value when compared with that of polymethylmethacrylate. 1 (4) Plasticiser : Tg Plasticisation Plasticizers are low molecular weight and non-volatile materials added to polymers. Addition of plasticizer increases the free volume in polymer structure as the plasticizer gets in between the polymer chains and spaces them apart from each other. This causes the polymer chains to sliding past each other more easily. As a result, the polymer chains can move around at lower temperatures resulting in decrease in T g of a polymer (5) Intermolecular force: Tg Intermolecular force Segmental rotations are also affected by intermolecular interactions or secondary bonding such as dipole- dipole interaction, induction forces, van der waals forces and hydrogen bonding, etc. These types of interactions increase the rigidity of polymeric material therefore increase the glass transition temperature. E.g. PVC : Tg = 80 oC Polypropylene: Tg = -18 oC (It lacks dipole dipole forces)  Specialty Polymers for Engineering Applications (6) Bulky pendent group : Tg number of bulky side groups Bulky group attached to polymer backbone also reduces flexibility of chain backbone therefore it increases T g value. (7) Crystallinity: T g crystallinity of polymers In crystalline polymers the polymer chains are arranged in a regular parallel fashion. Each chain is bound to other chain by intermolecular attraction which results higher T g than amorphous polymers. 1 (8) Co-polymerization: Tg Co polymeriza tion Random co-polymerization causes disorder and reduces molecular packing therefore glass transition temperature is often reduced on random copolymerization. The following relation gives the glass transition temperature of a random polymer: 1 WA WB Tg(A, B) Tg(A) Tg(B) Where Tg(A,B), T g(A) and Tg(B) are glass transition temperature of copolymer AB and homopolymer A and B respectively. WA and WB are the weight fraction of monomer A and monomer B in the copolymer AB. Methods of Determination (a) Differential Scanning Calorimetry (DSC) Glass transition temperature (b) Differential Thermal Analysis (DTA) (Tg) (c) Dynamic Mechanical Analysis (DMA) (d) Thermal Mechanical Analysis (TMA) 15. Melting temperature (T m) When a polymer is heated beyond T g, it changes from glassy state to rubbery state. Further heating causes melting of the polymer and it starts flowing. The temperature below which the polymer is in rubbery state and above which it is a liquid is called melting temperature (Tm) of the polymer. The hard and brittle state is the glassy state and soft-flexible state is the viscoelastic state. If viscoelastic state of polymer is heated further, the polymer becomes a viscous liquid and can flow. This state is known as viscofluid state. The effect of temperature on polymer is as follows;  Specialty Polymers for Engineering Applications Sr. No. Glass transition temperature Melting temperature (Tg) (Tm) 1 Glass Transition is a property Melting is the property of the of the amorphous region. crystalline region. 2 At Tg, phase transition happens At Tm, Polymer becomes from brittle glassy state to viscous liquid. rubbery state. 3 Below Tg, there exists a Below Tm it is an ordered disordered amorphous solid crystalline solid which where chain motion is frozen becomes disordered to melt and molecules start wiggling above Tm. around above T g. 16. Crystallinity of polymers (i) A polymer is said to be crystalline if all the molecules are orderly arranged with symmetric orientation by using force of attraction between two chains. E.g. Polyethylene, Polyesters, Nylons etc. (ii) Factors affecting crystallinity of polymers: (a) Polymer should be highly packed in order to achieve (b) Polymer should not be branched. crystallinity. (c) It should have high Tg. (d) Arrangement of pendent group must be regular. Properties (a) Crystalline polymers have high density. (b) These are hard and brittle. (c) Crystalline polymers generally exhibit good (d) A polymer with a higher crystallinity has a high thermal stability and resistance to high refractive index, thus making them translucent in temperature. nature. (e) These are resistive towards chemicals.  Specialty Polymers for Engineering Applications Sr. Amorphous polymer Crystalline polymer No. 1. Amorphous polymers have randomly packed Crystalline polymers have uniformly packed molecules. molecules. 2. Amorphous polymer do not have sharp melting Crystalline polymers have sharp melting point. point. 3. Amorphous polymers are transparent. Crystalline polymers are opaque or translucent 4. These have poor chemical resistance. These have good chemical resistance. 5. These polymers are generally soft. These polymers are generally hard in nature.  Specialty Polymers for Engineering Applications 17. Techniques of addition polymerization (a) Bulk polymerization Monomer (liq. state) + Initiator Homogeneous mixture This method involves only the monomer molecule, an initiator and a chain transfer agent (if necessary). The monomer is taken in the liquid state and the initiator is dissolved in the monomer. The chain transfer agents whenever used to control the molecular weight, is also dissolved in the monomer. Therefore the whole system is therefore in a homogenous phase. The reaction mass is heated or exposed to light source for initiating the polymerization and kept under agitation for proper mass and heat transfer. Once the reaction starts, heating is stopped, since the reaction is exothermic. As the polymerization proceeds, the viscosity of the medium increases and mixing become progressively difficult, leading to a broad molecular weight distribution. The polymer is usually removed in molten state otherwise it is difficult to remove the polymer from reactor. Examples: The free radical polymerization of styrene, methyl methacrylates, vinyl chloride etc.  Specialty Polymers for Engineering Applications Advantages Disadvantages (i) This method is very simple. (i) As the medium gets viscous, the diffusion (ii) There is no solvent contamination in of the growing polymer chain become the product. restricted, collisions becomes less and thereby (iii) No isolation is required. termination becomes difficult. (iv) No requirement of additives other (ii) Reaction is exothermic. than the initiator and the chain transfer (iii) Agitation becomes difficult due to reagent. increased viscosity. (v) Percent conversion of monomer to polymer is high with high purity. (b) Solution polymerization In this process, the monomer is dissolved in a suitable inert solvent along with the chain transfer agents wherever used. The free radical initiator is also dissolved in the solvent medium whereas for ionic and coordination catalyst, can be dissolved or suspended. The solvent enhances the heat capacity, thereby reducing the viscosity and promotes proper heat transfer. Examples: Synthesis of polyacrylonitrile by free radical polymerization, polyisobutylene by cationic polymerization, poly(vinyl acetate) to convert into poly(vinyl alcohol) etc. Advantages Disadvantages (i) Solvent acts as a diluent and aids in (i) Chain transfer to solvent may occurs, removal of heat of polymerization. leading to low molecular weights. (ii) The solvent allows for easy stirring as (ii) Difficult to remove solvent from final it decreases the viscosity of reaction form, causing degradation of bulk properties. mixture, making the synthesis process (iii) Lower yield is obtained and purity is not easier. high. (iii) Thermal control is easier than in the (iv) Environmental pollution due to solvent bulk. release.  Specialty Polymers for Engineering Applications (c) Suspension polymerization (i) Water-insoluble monomers are polymerized by this technique. The monomer is suspended in water in the form of fine droplets, which are then stabilized against coalescence, using stabilizers, surfactants, protective colloids and by stirring. (ii) But the initiators are monomer-soluble. Each monomer droplet is isolated and is independent of other droplets and hence acts as an independent bulk polymerization nucleus (where the polymer chain growth starts and proceeds). The continuous aqueous phase separating the monomer droplets, acts as the efficient heat transfer medium and hence the exothermicity is controlled. (iii) The size of the monomer droplets depends upon the monomer/water ratio, type and concentration of stabilizers and the mode and speed of agitation. This technique is more economical compared to solution polymerization technique, since water is used as the heat transfer medium. (iv) As the entire bulk of the monomer is divided into numerous tiny droplets, the control of the kinetic chain length (chain length of living polymer; that in chain propagation stage) of the formed polymer is quite good and results in a fairly narrow molecular weight distribution of the product. Thus the polymerization proceeds termed as bead or pearl polymerization. Examples: Expandable polystyrene beads (from which polystyrene foams are made), styrene-divinyl benzene copolymer beads(ion exchange resin) by free radical initiators. Typical stabilizer used: Gelatin, methyl cellulose, poly(vinyl alcohol), sodium polyacrylate. Advantages Disadvantages (i) Isolation of product is easy since the (i) Applicable only for the water insoluble product is insoluble in water. monomers and monomer soluble initiator. (ii) Low viscosity due to the suspension. (ii) Control of size is difficult. (iii) Easy heat removal due to the high (iii) Cannot be used for polymers whose glass heat capacity of water. transition temperature is less than the (iv) Polymerization yields finely divided, polymerization temperature or else stable latexes and dispersions to be used aggregation will occur. directly in coatings, paints, and adhesives.  Specialty Polymers for Engineering Applications (d) Emulsion polymerization (i) The system consists of water insoluble monomer, dispersion medium water, emulsifying agent/surfactant (Soaps/detergents), water soluble initiator. (ii) This is the most widely used method of polymerization. In emulsion polymerization, the monomer is dispersed in an aqueous phase as fine droplets which are then stabilized (emulsified) by surface active agents (surfactants- soaps or detergents), protective colloids and also by certain buffers. The surfactants can be cationic (alkali salts of fatty acids and of aryl and alkyl sulfonic acids), or non-ionic (alkyl glycosides). An water soluble initiators are used (persulfates). (iii) Surfactants serves the purpose of lowering the surface tension at the monomer water interface and facilitate emulsification of the monomer in water. The surfactant will form micelles when their concentration exceeds critical micelle concentration (CMC), that are dispersed throughout the solution. In micelle formation, the emulsifier molecule aggregates in such a way that the polar end of the molecule align themselves outward and the hydrocarbon end come close to each other at the interior. Due to the close proximity of the hydrocarbon ends of all emulsifier molecule, the interior of the micelle acts a hydrocarbon phase where the monomer can be stabilized. (iv) When the monomer is added and agitated, emulsification take place. The resultant emulsion is a complex system where polymerization takes place inside the micelle. This gives rise to the formation of latex particle. Examples: (1) Polystyrene synthesis: Monomer (styrene), Initiator (Potassium Persulfate), Buffer (Disodium Hydrogen phosphate), water, and emulsifier( Sodium Lauryl sulfate). (2) Styrene-butadiene copolymer synthesis: Monomer: styrene and butadiene (1:3), Initiator( Potassium Persulfate), and emulsifier( Sodium Lauryl sulfate), Lauryl mercaptan (chain transfer agent).  Specialty Polymers for Engineering Applications Advantages Disadvantages (i) Easy control of heat. (i) Polymer may require additional cleanup (ii) Thermal and viscosity problems are and purification; minimized due to the high heat capacity (ii) Difficult to eliminate entrenched and ease of stirring of the continuous coagulants, emulsifiers and surfactants. aqueous phase. (iii) The latex may be used directly without purification. (iv) High molecular weight polymer is obtained. Note: Critical Micelle Concentration (CMC): The highest concentration wherein all the molecules are in the dispersed state or the concentration beyond which only the micelle formation is possible is known as CMC.  Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications (Lecture 3) 18. Epoxy Resin Epoxy resin belongs to the polymer under the category of thermosetting polymer which is consisting of long extended cross- polymeric chain. These are highly reactive, presumably due to the strained three-membered ring structures, and react with many nucleophilic and electrophilic reagents. Epoxy group: (a) Synthesis The most prevalent epoxy resins synthesis involves the condensation of bisphenol A with epichlorohydrin under basic medium. (b) Mechanism (c) Curing agent in epoxy resin The best performance is attained by cross-linking an epoxy resin with a curing agent or hardener so as to form a three-dimensional insoluble and infusible network. The choice of curing agents depends on the application and on the handling. Important properties such as viscosity, pot life, and gel time; use of properties such as  Specialty Polymers for Engineering Applications mechanical, chemical, thermal, electrical, and environmental depends upon the curing agent attached to epoxy unit of polymer chain. 1o and 2o amines are the most widely employed curing agents in epoxy resin chemistry. (d) Properties (i) Epoxy resin has high tensile, compression, and bond strengths. (ii) Epoxy resin adheres well to many surfaces. (iii) It is resistant to corrosion and solvents. (iv) Epoxy resin can withstand moderate temperatures as it is thermosetting polymer. (v) Epoxy resin has good electrical insulation and retention properties. (vi) It shrinks very little during curing. (e) Applications (i) Epoxy resins are used for surface coating of floors. (ii) Epoxy resins are used in aircraft components, wind turbines, solar panels, and transportation vehicles. (iii) In industry, these are used as adhesives, anti-slip coatings, and the manufacturing of electrical insulators. (iv) Epoxy resins are used as anti-corrosive paint on metal. (v) Epoxy resin is added to asphalt binders to improve pavement performance.  Specialty Polymers for Engineering Applications 19. Acrylonitrile-Butadiene-Styrene (ABS) co-polymer ABS is an engineering thermoplastic. It is made from three monomers: (a) Synthesis ABS co-polymer is synthesized from arylonitrile, 1,3-butadiene and styrene monomer by emulsion technique using peroxide radical imitator and surfactant. It is typically opaque ivory color in nature. (b) Properties (i) ABS is strong and stable polymer with high tensile strength and rigidity. (ii) It is dimensionally stable and less likely to shrink than other plastics during molding process. (iii) ABS is resistant to corrosion and can sustain adverse environmental conditions. (iv) It has a relatively low modulus of elasticity, which gives it flexibility and allows it to absorb vibrations and shocks. (v) ABS has high surface brightness. It's also easy to add color pigment with it with shiny appearance. Glass transition temperature (Tg) Melting temperature (Tm) 105 oC 200 oC (c) Applications (i) ABS is extensively used in the manufacturing automobile parts. (ii) Pipes and fittings made from ABS are widely used as they are easier to install and do not rot, rust or corrode. (iii) ABS plastic is used for electronic applications, including computer keyboards, printer parts, and electronic enclosures, because it is an electrical insulator. (iv) ABS polymer works well for the large vacuum case designs and housings where an economical, lightweight, and rugged material is required.  Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications (Lecture 4) 20. Polypropylene (PP) Polypropylene is a versatile polymer which serves the duty of both plastic and fiber. It is structurally related to vinyl polymer. Generally, polypropylene is isotactic and semi-crystalline in nature. It exhibits the behavior like thermoplastic polymer. (a) Synthesis Polypropylene is synthesized from its monomer propylene by Ziegler-Natta polymerization. For this purpose, highly stereoselective catalyst i.e. combination of TiCl4 and AlEt 3 is used to achieve desired transformation. (b) Properties (i) PP is tough and rigid with good fatigue resistance and elasticity. It's also resistant to stress and cracking, even when flexed. Hence, it a good material for hinges. (ii) Polypropylene can be used at temperatures ranging from 90 150 °C. (iii) Polypropylene is highly resistant to most acids, alkalies, and organic solvents at moderate temperature (below 80 °C). (iv) It is known for its high flammability and thermooxidative degradation. (v) It is semi-crystalline and translucent polymer. (vi) PP is very lightweight with a low density 0.85 gm/cm3. Glass transition temperature (Tg) Melting temperature (Tm) o -18 C 160 oC (c) Applications (i) It is used as air/moisture resistive membrane and insulating wrap. (ii) It can be injected into a mold while molten to create complex shapes at a relatively low cost and high volume. (iii) It is used as packaging material such as bottles, jars, yogurt containers, hot beverage cups etc. (iv) PP has useful application in tissue engineering and vascular surgery. (v) PP is used as separator in Li ion battery.  Specialty Polymers for Engineering Applications 21. Conducting polymer Conducting polymers are known for their electrical conductivity. They have alternating single and double bonds along the polymer backbone (conjugated bonds) or that are composed of aromatic rings. Conducting polymers are two types as follows; 21.1. Structural requirement electron through its crystal lattice. In general conducting polymer is highly crystalline in nature. 21.2 Doping in polymer Purpose of doping is to enhance the electrical conductivity of polymer. There are manly two types of doping these are discussed below; (a) Oxidative or p-doping p-Doping is an oxidation reaction where electron is removed from the valance band and leaving the polymer -network. It effectively increases the mobility of electrons in these delocalized orbitals and the polymer becomes highly conductive. Reagents: I2, AsF5, FeCl3 etc. Behavior of oxidatively doped polymer is similar to p-type semiconductor. Presence of oxidative dopants induces positive charge within polymer network which creates vacancy similar to hole. In this type of doping, acceptor level or charge carrier comes below the Fermi level and closer to valence band. It facilitates the easy electron transfer from balance band to acceptor level and thereby overcomes large band gap. As a result conductivity of the p-doped polymer is significantly increased than is neutral form. This process is illustrated in the following figures.  Specialty Polymers for Engineering Applications (b) Reductive or n-doping: n-Doping is a reduction reaction where electron is added to the conduction band -network. It effectively increases the mobility of electrons in these delocalized orbitals. This gives rise to enhanced conductivity of the polymer. Reagents: Li, Na metal etc. Behavior of reductively doped polymer is similar to n-type semiconductor. Presence of reductive dopants induces negative charge within polymer network which provides excess electrons. In this type of doping, donor level comes above the Fermi level and closer to conduction band. It facilitates the easy electron transfer from donor level to a conduction band and thereby overcomes the large band gap. As a result conductivity of the n-doped polymer is significantly increased than is neutral form. This process is illustrated in the above figures. 21.3 Conducting polymer: Polyacetylene -conjugation having general formula (C2H2)n where n = 15-20. Undoped trans-polyacetylene films have a conductivity of 4.4 × 10 cm , while cis-polyacetylene has a lower conductivity of 1.7 × 10 9 1cm 1. (a) Synthesis One of the most common method uses Ziegler-Natta catalyst Ti(OiPr)4/AlEt3 with gaseous acetylene (C 2H2). This method allows the control over the structure and properties of the final polymer by varying catalyst loading and temperature. At very low temperature -78 oC, treatment of acetylene with Ti(OiPr)4/AlEt3 results in cis-polyacetylene but at higher temperature 150 oC it gives rise to the formation of trans-polyacetylene. The cis-polymer is inter convertible as it is heated at 145 oC converts to its trans-isomer. Using this method, it is possible to synthesize thin film of polyacetylene.  Specialty Polymers for Engineering Applications (b) Properties (i) Films containing the cis form appear coppery, while the trans form is silvery. (ii) Films of cis-polyacetylene are very flexible and can be readily stretched, while trans-polyacetylene is much more brittle. (iii) Polyacetylene has a bulk density of 0.4 g/cm3. (iv) The morphology consists of fibrils, with an average width of 200 Å. These fibrils form an irregular,web- like network. (v) They are insoluble in solvents, making it difficult to process the material. (vi) Polyacetylene shows high thermal stability but exposure to air causes a large decrease in the flexibility and conductivity. When polyacetylene is exposed to air, oxidation of the backbone by O 2 occurs. (c) Applications (i) Optoelectronics : Polyacetylene is a promising material for optoelectronics because it has conductivity comparable to Cu matal after doping. Some of its forms have high interesting non-linear third-order optical susceptibility. (ii) Sensors and actuators : The doping/dedoping process of polyacetylene is actually reversible process that makes it useful in sensors and actuators. (iii) Corrosion control : Doping of polyacetylene makes it useful for corrosion control and protection. (iv) Microwave absorption: Doped polyacetylene can absorb microwaves. (v) Nano-electronic devices : Doped polyacetylene can be used in nano-electronic devices. (vi) Molecular electronics : Conductive polyacetylene could potentially be used in molecular electronics. 21.4 Conducting polymer: Polyaniline Polyaniline is one of the most widely used conducting polymer with diverse applications. Electrical conductivity of polyaniline arises due to extended conjugation of non-bonded electron pair of N with aromatic Ph ring. Its conductivity increases exponentially as the temperature increases. At 28 oC, specific conductivity is observed 0.21 S m-1. (a) Synthesis In chemical oxidation polymerization method, polyaniline is synthesized by using H2SO4 and ((NH4)2S2O8) as an oxidant. For this purpose, aniline is treated with H 2SO4 with vigorous stirring followed by dropwise addition of (NH4)2S2O8 solution. Final products is filtered and washed with dilute H2SO4 and dried at 60 oC for 12 h. Resultant product is grinded and obtained as green powder. It is known as Emeraldine salt.  Specialty Polymers for Engineering Applications (b) Properties (i) It has great electrical conductivity and specific conductivity is found to be 10² s/cm after doping. (ii) It has band gapes of 4.3 and 2.7 ev in its reduced and oxidized forms respectively. (iii) It has high chemical stability which makes it anti-corrosion agent. (iv) Polyaniline-based composition can withstand high temperatures like 230-240 without significant change in electrical properties. (v) It is redox active which exhibits various shapes that are reversibly converted by accepting or removing electrons. (c) Applications (i) The color change of polyaniline in different oxidation states can be used in sensors and electrochromic devices. Printed emeraldine polyaniline-based sensors have wide application in the electronic sector. (ii) It is used in photovoltaic cell due its conducting property. (iii) It is used as gas separation membrane. (iv) It is used in the manufacturing of printed circuit boards. (v) It has significant potential applications in biomedicine due to its high electrical conductivity and biocompatibility caused by its hydrophilic environment, low toxicity, high environmental stability, and nanostructured morphology. (vi) It is used as supercapacitor in electronic devices. (PANI = Polyaniline)  Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications (Lecture 5) 22. Liquid crystalline polymers (LCP) Liquid crystal polymers (LCPs) present a special category of material which straddles the boundary between an ordinary solid and a liquid. These have significantly higher order structure than their liquid phase. Thermotropic - Phase transitions occur as temperature changes. Lyotropic - Phase transitions occurs when the molecules are mixed with a solvent. (a) Mesophase & Mesogens This liquid crystal is a new phase of matter which is in between liquid and solid termed as mesophase. The molecules in liquid crystal are capable of forming a mesophase termed mesogens. These mesogens are structurally rigid in nature. Example: Aromatic ring (b) Arrangement of mesogens Arrangement of mesogens in LCPs are basically three types; (i) Nematic (ii) Smectic A (iii) Smectic C  Specialty Polymers for Engineering Applications Nematic: Nematic liquid crystal exhibits orientational order. Smectic A: These liquid crystals exhibit positional as well as directional order parallel with respect to the smectic layer normal. Smectic C: These liquid crystals exhibit positional as well as directional order tilted to with respect to the smectic layer normal. (c) Structural feature of LCPs Like other polymer, LCPs are composed of repeated units of monomer which are made from extended chain- like molecules. Such monomer consists of hydrophilic polar part, mesogen and flexible non-polar part. These polymeric chains aggregate to form LCPs just as single mesogen molecules do to form liquid crystals. Some examples of LCPs (d) Properties of LCPs (i) These have good chemical and heat resistance. (ii) These have good moldability. (iii) These are flame retardant. (iv) These can absorb the vibration. (v) These have good dimensional stability. (e) Application of LCPs (i) LCPs have significant applications on electronics. (ii) These are applied in automotive parts. (iii) These are applied in distillation tower. (iv) Chemical pumps are made of LCPs. (v) LCPs are applied for manufacturing surgical devices.  Specialty Polymers for Engineering Applications 23. Electroluminescent polymer It is a type of polymer that emits light upon application of electric field. Structurally, this type of polymer is -electron system. (a) Synthesis of polyphenylenevinylene (PPV) PPV is electroluminescent material. It is conventionally synthesized by Witting reaction using bis- phosphonium ylide and terephthalaldehyde. Wittig reaction and its mechanism (b) Applications of PPV (i) It is used as thin films for liquid crystal display. (ii) Automotive instrument panel backlighting. (iii) It is used as electron donating material in organic solar cell. (iv) Light stripes for decoding building and vehicle safety precaution. (v) It has utility in electroluminescent night lamp.  Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications (Lecture 6) 24. Smart polymers Smart polymers are such materials that can respond to external stimuli, such as temperature, pH, light, etc. and thereby changes their properties. Classifications of smart polymers (a) pH sensitive smart polymers (b) Temperature sensitive smart polymers (c) Electroactive smart polymers (d) Light sensitive smart polymers 24.1 pH sensitive smart polymers pH sensitive smart polymers are polyelectrolytes that bear acidic or basic functional groups in their structure that either accept or release protons in response to the changes in pH environment. (i) Sulfonic acid (-SO3H) functional group (ii) Carboxylic acid (-COOH) functional group (iii) Amino acid functional group (iv) Boronic acid (-B(OH)2) functional group  Specialty Polymers for Engineering Applications (v) Basic functional group 24.1(a). Responses of pH sensitive polymers (i) Reverse micelles: The solution of PMEMA-b-PDEA block polymer changes its micelle structure depending upon pH. At pH 6.7, it forms micelle with PMEMA with cationic PDEA shells. But at alkaline medium (pH >8), micelle formation occurs with PDEA core and PMEMA shell. This phenomenon is known as reverse micelle. The difference in behavior for the micelle formation is associated with different basicity i.e. different pKa values of pendent amine groups. (ii) Drug delivery: Coating of pH sensitive polymer helps in drug delivery process. pH responses of PAAC polymer assists the unfolding of loaded drug at higher pH which is illustrated below. In the acidic pH medium, the pendant acidic group remains unionized and retains the drug in the polymer carrier. By increasing the pH, the pendant carboxylic group ionizes and deprotonates, and the polymer swells due to the increase of electron charge density, which induces water diffusion into the polymer network, releasing the incorporated drug.  Specialty Polymers for Engineering Applications 24.1(b). Applications of pH sensitive polymers (i) The most important application for pH-responsive polymers for drug delivery is pH, which is used to determine the pH of the body site where the trapped drugs will be released. (ii) These are used as stabilizers in dispersion and emulsion polymerizations. (iii) pH-Responsive polymer systems can behave as the host for the production of metal nanoparticles. (iv) This type of polymer is used in chromatographic purification and separation technology. (v) More importantly, this type of polymer acts as sensor for the detection of changes caused by variation of pH. 24.2. Electroactive smart polymers Electroactive polymers (EAPs) are smart materials that change shape or size when these are stimulated by electric field. These polymers have the ability to deform when excited by electrical potential. Classifications of EAPs (a) Ionic EAP (b) Electronic EAP 24.2(a). Ionic EAP This type of smart materials changes their shape and properties due to mobility or diffusion of ions. They require a lower voltage to activate compared to other EAPs since the diffusion of ions drives them. Construction: The following diagram shows the facilitate ion exchange sandwiched between two electrodes. Here, the membrane contains dissolved cations. When the ionic EAP is exposed to an electric field, the hydrated cations move toward the negatively charged electrode, causing a localized change in volume. This volume change results in a characteristic bending motion in the ionic EAP. Block copolymer in between the conducting polymers contains the ionic liquids, one kind of electrolyte, comprised of big and bulky cation-anion moiety. These sorts of ionic EAPs can demonstrate a sharp bending angle, which helps them to mimic the natural movement of muscle more  Specialty Polymers for Engineering Applications precisely. Additionally, when the ionic EAP is bent by external means, it generates an electric signal, meaning that the ionic EAP can be used as both an actuator and a sensor. Example: EAP made of a conductive polymer (CP) actuator made of PEDOT:PSS, was able to reach a sharp bending angle above 90. Properties of ionic EAPs (i) Work function of ionic EAPs based on the movement of charged ions. (ii) Ionic EAPs can operate at low voltage ranges of 1 2 volts. (iii) Ionic EAPs have high ionic conductivity. (iv) Ionic EAPs can deform in both directions depending on the polarity of the applied voltage. (v) Ionic EAPs have low electromechanical coupling efficiency. 24.2(b). Electronic EAP Electronic EAPs are driven by strong electric fields. The occurring electrostatic forces lead to an electromechanical change in shape of the material. Construction: This type of EAP exhibits actuation (to put into mechanical action or motion) using the electrostatic forces between two electrodes. They are similar to a capacitor that can change its capacitance in response to an applied voltage. They are capable of very high strain, and due to the applied electric field, they get compressed in thickness while their area gets expanded. Fabrication process for the multi-layer electronic EAPs (using dielectric elastomer actuators) is illustrated below. (ii) Properties of electronic EAPs (i) Electronic EAPs are driven by Coulomb forces and they can be made to hold the induced displacement while activated under a DC voltage. (ii) These have high mechanical energy density. (iii) They require a high activation voltage (typically 100 MV/m). (iv) They can be operated in air with no major constraints. (v) These exhibit rapid response.  Specialty Polymers for Engineering Applications Sr. No. Ionic EAPs Electronic EAPs 1. Ionic EAPs requires ionic liquid. These works due to mobility or ions. diffusion of ions. 2. These require low voltage for These require high voltages for operation. operation (typically ~100 MV/meter). 3. These exhibit relatively low These exhibit high mechanical mechanical energy density as well energy density. as low electromechanical coupling efficiency. 4. Take some times to exhibit Rapid response is observed. response. 5. These require encapsulation or These can operate for a long time protective layer in order to protect in open air room conditions. from air and moisture. 24.3. Applications of EAPs (i) These have significant applicability in robotic arms. (ii) These are used as artificial muscle and organ replacement material. (iii) These are used in drug delivery in human body. (iv) EAP actuators can be used for noise less propulsion. (v) EAPs are also used as the base material for active lens modules in optical imaging. 24.4. Comparison between EAP & other smart materials Property Electroactive polymers Shape memory alloys Electroactive (EAP) (SMA) cermaics (EAC) Actuation strain > 10% < 8% 0.1-0.3% Force (MPa) 0.1 - 3 About 700 30-40 Reaction speed µsec to sec sec to min µsec to sec Density 1-2.5 g/cc 5-6 g/cc 6-8 g/cc Drive voltage 10-100 V NA 50-800 V Consumed Power m-watts watts watts Fracture toughness Resilient, elastic elastic fragile  Specialty Polymers for Engineering Applications Engineering Chemistry Unit 1: Specialty Polymers for Engineering Applications (Lecture 7) 25. Bio-based and biodegradable polymer Biodegradable polymers are a special class of polymer that breaks down after its intended purpose by bacterial decomposition or enzyme-catalyzed reactions to result in natural byproducts. Example of natural bio-polymer: Cellulose, Keratin, Fructose etc. (a) Factors affecting biodegradation (i) Micro-organism : There is a large variety of micro-organism like pseudomonas, balillus, protozoa, azotobactor and various fungi in our environment are very much active to initiate the biodegradation. These microorganisms can convert toxic elements into water, carbon dioxide, and other less toxic compounds. (ii) Environment : The environmental factor such as temperature, moist condition, presence of salts, O 2, and pH are essential for survival and growth of microorganism and these factors play a key role in the biodegradable reactions. (iii) Nature of polymer : The polymer chain should contain bonds which can be hydrolyzed or oxidized by the enzymatic reaction. There should be N, O and S like atom in the polymer chain. Highly aromatic chains are tough for degradation. Amorphous and size of polymer chain also the important parameter for their biodegradation. (b) General applications of biodegradable polymers (i) Biodegradable polymers can be used in different areas of medicine, unlike conventional plastic polymers. Examples of some applications are implants, bone fixations or sutures and other surgical use. (ii) It can be applied as food packaging container. (iii) It can be applied in the agriculture sector. (c) Biodegradable Polymer: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) PHBV is a microbial biopolymer with excellent biocompatible and biodegradable properties that make it a potential candidate for substituting conventional polymers. It is basically the co-polymer of 3-hydroxy butyric acid and 3-hydroxy valeric acid. Synthesis:  Specialty Polymers for Engineering Applications Properties of PHBV: Sr. No. Properties PHBV 1. Density (g/cm3) 1.25 2. Elasticity modulus (GPa) 2.38 3. Traction Resistance (MPa) 25.9 4. Elongation (%) 1.4 5. Melting temperature (°C) 153 6. Glass transition temperature (°C) 5 Biodegradation of PHBV: (i) PHBV can degrade completely in soil with 100% relative humidity (RH) within two weeks. (ii) PHBV films can degrade up to 100% in pilot-scale composting conditions. (iii) Rain water can degrade this polymer. (iv) PHBV biodegradation begins with enzymatic hydrolysis reactions by PHA-degrading enzymes. Applications of PHBV: (i) It is useful for moulded article, films for packaging and lamination. (ii) It is a good material for veterinary application. (iii) It is used for control release of fertilizers and growth hormones to the plants. (iv) It is useful for surgical purpose, organ transplant and orthopedic operation. (v) It is used for control release of drug.