Engineering Chemistry Polymer Notes PDF

SFIT/BSH/ECH/24-25 Engineering Chemistry POLYMERS Introduction to polymers: - A large molecule or macromolecule, which is formed by repeated linking of small molecules, called monomers, is a polymer. n (CH2=...

SFIT/BSH/ECH/24-25 Engineering Chemistry POLYMERS Introduction to polymers: - A large molecule or macromolecule, which is formed by repeated linking of small molecules, called monomers, is a polymer. n (CH2=CH2) → - (CH2-CH2)-n Degree of polymerization: The no. of repeating units in the chain so formed in a polymer is called degree of polymerization. In the above example, it is ‘n’. Functionality: It is the number of reactive sites or bonding sites in a monomer. A double bond is a site for two free valencies. Depending upon the functionality of a monomer it is possible to obtain different types of structures. If functionality is 2: -linear polymers are formed If functionality is more than 2: -3-dimensional polymer is formed. Classification of polymers: ∙ Based on availability: Natural, Semi-synthetic and synthetic ∙ Based on types of monomers involved: Homopolymer and copolymer ∙ Based on chain structure: Linear, branched and cross-linked ∙ Based on Tacticity of polymer:Isotactic,Atactic,Syndiotactic ∙ Based on effect of heating: Thermoplastic and thermosetting plastic ∙ Based on polymerization reaction: Polymers made from addition or condensation polymerization. 1. Natural polymers like protein, natural rubber, silk, polysaccharides. Semi – synthetic: Cellulose acetate, vulcanized rubber Synthetic polymers like nylon, terylene, PVC etc. 2. Homopolymer are polymers, which consist of monomers of identical chemical structure, e.g. polyethylene, PMMA etc. A-A-A-A The monomeric units may combine with each other into a macromolecule to form polymers of linear, branched or cross-linked structures.  SFIT/BSH/ECH/24-25 Copolymers or mixed polymers are formed when the macromolecules consist of monomers of different chemical structures. ---A-B-A-B-A-B---- -A-B-A-A-A-B Copolymers may also be linear, branched or cross linked.eg: BUNA-S, nylon 66 3. Linear polymers the monomeric units in the polymer (identical or different chemical structure) form a fairly long continuous sequence. They can be both homopolymer and copolymer. -A-A-A-----A-A-A-A---- or -----A-B-A-B-A-B ------- (PE, PTFE) Branched polymer the monomeric units are not only arranged in a long sequence in a row but also branched out chains of monomers exist. They can be homopolymer and copolymer. (PVC) A-A-A-A -- A-A Cross linked polymers are usually thermosetting polymers, which have strong interlinked structure consisting of covalent bonds (Bakelite) Tacticity Polymers are also classified on the basis of configuration of macromolecule known as Tacticity of polymers. Generally, tacticity applies to addition polymers with one non-hydrogen substitute attached to the carbon chain for each monomer unit. Tacticity represents spatial arrangement of “R” group with respect to carbon chain. Tacticity is a polymer's stereochemistry, or the arrangement of chiral centers in a polymer's backbone. H H | | -(- C - C -)- | | H R  SFIT/BSH/ECH/24-25 Isotactic Notice that all of the R groups line up on the same side. (Poly styrene) Syndiotactic Notice that the R groups alternate from side to side. (Gutta Percha) Atactic Notice that the side-to-side positioning is random. (Polypropylene) 4. Thermoplastic and thermosetting polymers Thermoplastics polymers ∙ are those which become soft on heating and hard on cooling. ∙ Hardness is their temporary property, and therefore they can be processed again and again. ∙They are long chain, linear polymers with negligible crosslinks which can be reshaped and reused. ∙ They are formed by addition polymerization. ∙ Soluble in most organic solvents. ∙ They are soft, weak and less brittle. E.g. PE, PVC, PMMA. ∙ Thermosetting polymers Polymers which become harder on applying heat and cannot be softened again, Thus they are permanent setting polymers. They consist of a three-dimensional network of strong covalent bonds and cross links. They cannot be reshaped and reused as they retain their shape and structure on heating. They are formed by condensation polymerization. They are insoluble in most organic solvents. E.g. Urea formaldehyde, phenol formaldehyde, epoxy resins, polyester (Terylene).  SFIT/BSH/ECH/24-25 Difference between Thermoplastics and Thermosetting polymers: Thermoplastic polymers Thermosetting polymers They soften on heating They are fusible on initial heating and harden on cooling but turn into hard fusible mass on further heating and on prolonged heating, they burn. Long chain, linear polymers It consists of a three-dimensional with negligible cross links. network of strong covalent bonds and cross links. They can be reshaped They cannot be reshaped and and reused. reused as they retain their shape and structure on heating. formed by addition formed by condensation polymerization. polymerization. Can be reclaimed from Can’t be reclaimed from the waste the waste. Soluble in most organic Insoluble in most organic solvents solvents Low molecular weight polymer High molecular weight polymer Soft, weak, brittle Hard,strong,more brittle E.g. PE, PVC, PMMA E.g. Urea formaldehyde, phenol formaldehyde, epoxy resins, TYPES OF POLYMERISATIONS: a) Addition polymerization:  It is known as chain polymerization.  Monomer molecule must contain unsaturation (double or triple bond)  This process yields a polymer which is an exact multiple of the original monomeric molecule without loss of any material.  It is initiated by application of heat, light, pressure or catalyst leading to breaking down of double covalent bonds of monomers.  Polymerization proceeds in 3 steps: chain initiation, chain propagation, chain termination. ∙ n (CH2=CH2) → - (CH2-CH2)-n  SFIT/BSH/ECH/24-25 b) Condensation polymerization:  It is also known as step-growth polymerization.  It is a reaction occurring between simple polar group containing monomers with the formation of polymer and elimination of small molecules like H2O, HCl etc.  In case of polyfunctional groups monomer molecules are connected to each other by covalent bonds, resulting in the formation of 3-D network or cross linked structure polymer.  e.g. Urea formaldehyde, phenol formaldehyde, Nylon 6,6 Effect of heat on polymer: Glass Transition Temperature (Tg) and melting point (Tm):  The temperature (actually a broad range of temperatures) above, which a glassy/brittle polymer softens into a viscous liquid or rubbery phase is known as Glass transition temperature.  On the molecular level, it is the temperature at which chains in amorphous (i.e., disordered) regions of the polymer gain enough thermal energy to begin sliding past one another at a noticeable rate.  Tg is the temperature below which amorphous polymers lose their segmental mobility and exist in a supercooled liquid or glassy state.  Glassy state is hard and brittle state of material which consist of short range vibrational and rotational motion of atoms in polymer chain while rubbery state is soft and flexible state of material which is a long range rotational motion of polymer chain segments.  SFIT/BSH/ECH/24-25  Above the Tg the amorphous polymer regains its rubbery state and is soft with the ability to move and twist its chains, it is called viscoelastic state. Hard plastics like PS, PMMA are used below their Tg which is >100oC. (means we use it in glassy state at room temperature) Rubber elastomers like polyisoprene etc. are used above their Tg in soft, rubbery state. Factors affecting Tg and Tm 1. Crystallinity: The crystalline characters of polymers are represented as percentage crystallinity increases, Tg increases. 2. Mobility of polymer chain: Tg depends on mobility of polymer chain. The polymer having an immobile chain shows high Tg. The polymer chain having more mobility can be converted into rubbery state easily from a glassy solid state so will show lower Tg. 3. Chain Stiffness: Stiffening groups in polymer chains reduces chain flexibility and shows high Tg.  SFIT/BSH/ECH/24-25 Presence of aromatic rings in polymer chain causes stiffness, polymer need more heat to convert in to flexible state, so Tg increases. 1. The presence of bulky side groups restricts molecular motion due to the stearic effect. These groups catch on the neighboring chain like a hook and restrict chain movement which reduces flexibility of polymeric chains. So Tg increases. The magnitude of this effect depends on the size of the side group. Polystyrene has higher Tg than polypropylene. 2. Presence of multiple bonds, aromatic groups increase chain stiffness, lowers chain flexibility and increases Tg and Tm. (In case of Poly styrene, Kevlar presence of aromatic rings cause an increase in Tg)  SFIT/BSH/ECH/24-25 3. Presence of polar side atoms or groups causes increase in intermolecular attractive interactions between chains which hinder molecular motion thus causing an increase in Tg. PVC has polar bonds due to presence of -Cl group, it shows higher Tg than polypropylene. 4. Increasing molecular weight increases Tg and Tm. 5. Co-polymerization: Random co-polymerization causes disorder and reduces molecular packing therefore glass transition temperature is often lower. 6. Cross-linking: Cross-linking introduces restriction and stiffness in the polymer therefore cases the increase in glass transition temperature. 7. Plasticizer: Plasticizers are generally low molecular weight substances and when added to polymer, cause separation of polymer chains, reduction of co-hesive forces and overall increase molecular mobility. Plasticizers reduces brittleness of polymers and reduces glass transition temperature. 8. Presence of flexible pendant groups: 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 Tg value. In polybutylmethacrylate, the presence of large aliphatic chain reduces the Tg value when compared with that of polymethylmethacrylate. 9. Water or moisture content of the polymer: Increase in water or moisture content leads to the formation of hydrogen bonds with polymeric chains, thereby increasing the distance between polymeric chains. Hence increases the free volume and decreases Tg.  SFIT/BSH/ECH/24-25 Conducting polymers Most conventional polymers have insulating properties.They possess completely filled valence bands and empty conduction bands. They have wide gaps between these bands, i.e. the energy gap is prohibitive. E.g. polyethylene, polymethyl methacrylate, polypropylene etc. But there are some polymers which have electrical properties which resemble those of conventional semiconductors. Polymeric materials having conductivities on par with metallic conductors are called conducting polymers. Polymers having conjugated double bond are conducting in nature due to presence of delocalized pi electrons and show conductivity as high as 1.5 X 107 ohm-1 m-1.. The conductivity in pure semiconductors is known to increase (exponentially) with increasing temperature and decreasing energy gap, whereas the conductivity in metals decreases with increasing temperature. Interestingly enough, most conducting polymers have a temperature dependence of the conductivity similar to that of semiconductors. This suggests that certain aspects of semiconductor theory may be applied to conducting polymers. Different types of conducting polymers are: Intrinsic Conducting Polymers: (ICP) Intrinsic conducting polymers have extensive conjugation (alternate double bonds) in the backbone which is responsible for conduction of electrons. In an electric field, conjugated 𝜋-electrons of the polymer get excited and overlapping of orbitals over the entire backbone results in the formation of valence bands as well as conduction bands, which extends over the entire polymer molecule.  SFIT/BSH/ECH/24-25 Conductivity of such polymers is of the order 10-10 Scm-1. Conductivity of these polymers, having conjugated π- electrons in the backbone is not sufficient for their use in different applications. Doped conducting polymers: ( DCP) In some polymers when charge carriers are introduced (in the form of addition or removal of electrons) into the conduction or valence, band the electrical conductivity increases dramatically. Intrinsic conducting polymers can be easily oxidized or reduced on account of their low ionization energy and high electron affinities. Doping is a process by which conductivity of the polymers can be increased by creating negative or positive charge on the polymer backbone by oxidation or reduction.  SFIT/BSH/ECH/24-25 (A) p-doping: When the polymer is treated with a Lewis acid, its oxidation takes place and holes (positive charges) are created on the polymer backbone. Commonly used dopants are I2, FeCl3, Br2 etc. (C2H2) n + 3I2→ 2[(C2H2) n + I3 ‒] In p – DCP, delocalized positive charges are current carriers for conduction. Conductivity of ICP increases with the introduction of positive charge. (B). n-doping: When the polymer is treated with Lewis base (electron donor), reduction takes place and negative charges are added on the polymeric chain. Some common n-type dopants are Li, Na naphthylamine, sodium naphthalide, etc. (CH) x + NH2 (C10H7) − → NH+ (CH)x − + C10H8 In case of n-DCP, conductivity of ICP increases as introduction of electrons results in increased delocalization. Extrinsically conducting polymers: (ECP) The polymers whose conductivity is due to the presence of externally added ingredients in them. This can be of the following types: A) Conductive element filled Polymer: A polymer filled with conducting elements like carbon black, metallic fibres, and metal oxides. Polymer acts as a binder and it holds the conducting element. The resultant compound is of low weight, mechanically durable, strong and conductive material. B) Blended conducting polymer is a product obtained by blending a conventional polymer (insulator) with a conducting polymer.  SFIT/BSH/ECH/24-25 D. Coordination conducting (Inorganic) polymers: a polymer is combined with coordination complex containing obtained by combining a metal atom with a polydentate ligand. An insulator polymer is mixed with a coordination compound, due to electron transition it behaves as conductive polymer. Applications of Conducting polymers. 1. Current applications are light weight rechargeable batteries: Some prototype cells are comparable to, or better than nickel-cadmium cells now on the market. The polymer battery, such as a polypyrrole- lithium cell operates by the oxidation and reduction of the polymer backbone. 2. In optical display devices based on polythiophene 3. In wiring in aircrafts and aerospace components. 4. In telecommunication systems. 5. In antistatic clothing, carpets. 6. In electronic devices such as transistors and diodes. 7. In drug delivery system for human body. 8. In molecular wires and molecular switches 9. Polyaniline is used as sensor for ammonia. Polyacetylenes: The polyacetylene film forms at the gas-liquid interface when acetylene gas passes through the Ziegler- Natta catalyst. Cis polymer forms at low temperature (-78 °C). Isomerization to the more stable transform takes place on rising the temperature of the film. Conductivity of doped cis films is two or three times greater than the trans analogues. Although the application of polyacetylene is limited but the discovery of this conductive organic polymer lead to the development in the area of research, of material science.  SFIT/BSH/ECH/24-25 Applications: Doped Polyacetylene offers good electrical conductivity, hence can be used as electrode material in light weight rechargeable batteries. I3 -dopped acetylene can also be used in biosensors. Compounding of plastics/Ingredients of plastics: - The addition of important additives in molten plastic to get desirable characteristics in resultant product is called compounding of plastic. The additive categories are as follows: a) Resin: - hold the other constituents together during manufacturing. Usually natural or synthetic resins or cellulosic derivatives are used as binders, which are high molecular weight substances. Binders influence the properties of plastics. The type of treatment during moulding operation also depends upon binders. If the binder used has comparatively low molecular weight, then plastic article gets moulded easily and vice versa. (moulding is converting molten plastic into an object of desirable shape.) b) Plasticizers: - are added to resins to increase their plasticity and flexibility. These additives neutralize intermolecular forces of attraction between the polymeric chains. It helps to remove cross links between polymeric chains. Thus, they impart greater freedom of movement between the polymeric macromolecules of resins but at the same time, reducing their strength and chemical resistance. Increase in plasticizers affects glass transition temperature of polymer. More amount of plasticizers reduces glass transition temperature.. e.g. vegetable oils, camphor, tricresyl , tributyl phosphate. c) Fillers: - They are added to give the final plastic better hardness, tensile strength, opacity, finish and workability besides reducing its cost, shrinkage on setting and brittleness. e.g. carborundum, quartz, mica provides hardness, barium salts make polymer impervious to X rays, and asbestos for heat resistance. Other important fillers are glass fibres, cotton fabric and paper pulp.  SFIT/BSH/ECH/24-25 d) Lubricants: - help during moulding operations, especially during low or room temperature moulding. The use of lubricants imparts flawlessness, and glossy finish to the plastic products. This is because the lubricants tend to get dispersed towards the outer surface of the finished product, hence during moulding, they form a layer between the article and mould. This layer prevents the plastic material from sticking to the surface of the mould, and thereby facilitating the moulding operations. Commonly used lubricants are soaps, or esters of fatty acids such as oleic and stearic acids or waxes. e) Catalysts or accelerators: -They are added only to thermosetting plastics with the objective of accelerating the polymerization of fusible resin, during moulding operation, into cross linked infusible form.e.g. hydrogen peroxide, benzoyl peroxide, metals like Ag, Cu and Pb, metallic oxides like ZnO , ammonia and its salts. f) Antioxidants: These additives inhibit the undesirable oxidation at high temperature during moulding or storage of polymer.eg. phenyl derivatives. g) Pigments: They are added to increase appearance of polymer. They provide suitable colour to the polymer. Dyes are used to provide bright colours to transparent polymers.eg chromium trioxide to give green, antimony sulphide gives crimson red. h) Stabilizers: -They are added to improve thermal stability during processing. It provides protection against degradation caused by heat, oxidation and solar radiation. e.g salts of Pb like white lead, lead chromate, litharge, read lead, lead silicate and lead naphthenate. Liquid Crystal The term 'liquid crystal' is both intriguing and confusing; while it appears self-contradictory, the designation really is an attempt to describe a particular state of matter of great importance today, both scientifically and technologically. Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as in a solid. The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points. In his experiments, Reinitzer increased the temperature of a solid sample and watched the crystal change into a hazy liquid. As he increased the temperature further, the material changed again into a clear, transparent liquid. Because of this early work, Reinitzer is often credited with discovering a new phase of matter - the liquid crystal phase. A liquid crystal is a thermodynamic stable phase characterized by anisotropy of properties without the existence of a three-dimensional crystal lattice, generally lying in the temperature range between the solid and isotropic liquid phase, hence the term used is mesophase for liquid crystals.  SFIT/BSH/ECH/24-25 Mesogens- A mesogen is rigid rodlike or disclike molecules which are components of liquid crystalline material The distinguishing characteristic of the liquid crystalline state is the tendency of the molecules (mesogens) to point along a common axis, called the director (the molecular direction of preferred orientation in liquid crystalline mesophases). Liquid crystal materials generally have several common characteristics as: a. rod-like molecular structure, b. rigidness of the long axis, c. and strong dipoles. Classification of Liquid Crystals A. Depending upon the factor which is responsible for phase transition: (i) Thermotropic LC: Thermotropic behavior means the compounds are liquid crystalline within a defined temperature range, below this range compounds are crystalline and above it compounds are isotropic liquids. Thermotropic liquid crystalline compounds also require no solvent. (ii) Lyotropic LC: Lyotropic liquid crystals are dependent on solvents, where solvent concentration affects aggregation and liquid crystal behaviour. B. Depending upon different arrangements in phases:  SFIT/BSH/ECH/24-25 1. Nematic Phases The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point in the same direction (along the director). Nematic crystals have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field. Aligned nematics have the optical properties of uniaxial crystals and this makes them extremely useful in liquid-crystal displays (LCD) 2. Smectic Phase In smectic ("soap-like") phases the molecules are arranged in layers, with the long molecular axes approximately perpendicular to the laminar planes. The smectic phases, which are found at lower temperatures than the nematic, form well-defined layers that can slide over one another in a manner similar to that of soap. Within a layer there is a certain amount of short-range order. The orderly arrangement means that the smectic state is more "solid-like “than the nematic phase.  SFIT/BSH/ECH/24-25 In the Smectic A phase, the molecules are oriented along the normal layer, while in the Smectic C phase they are tilted away from it. There are many different smectic phases, all characterized by different degrees of positional and orientational order. 3. Cholesteric Phase: These liquid crystals are made up of rod-like molecules that self-organize into a helical structure. The LC directors rotate continuously along the helical axis in planes perpendicular to it. CLCs are chiral and have a periodic helical structure that causes them to selectively reflect circularly polarized light. This phase is often called the cholesteric phase because it was first observed for cholesterol derivatives. Application of Liquid Crystals 1. Cholesteric liquid crystal substances, when applied to the surface of the skin, have been used to locate veins, arteries, infections, tumors and the fetal placenta which are warmer than the surrounding tissues. 2. Chiral nematic (cholesteric) liquid crystals reflect light with a wavelength equal to the pitch. Because the pitch is dependent upon temperature, the color reflected also is dependent upon temperature. Liquid crystals make it possible to accurately gauge temperature just by looking at the color of the thermometer. 3. Nematic liquid crystals are useful research tools in the application of magnetic resonance 4. Liquid crystals have been used in chromatographic separations. 5. Liquid crystals are widely used in cosmetic industry in manufacturing of liquid crystal makeup removers, lipsticks and lip glasses containing cholesteric liquid crystals. 6. Liquid crystals are using extensively in pharmaceutical industries. 7. Liquid crystal displays are common in calculators, digital watches, oscillaographic systems, television displays using L.C. screens has also been developed. Cholesteric liquid crystals have also been used for novelty items such as toys and decorative materials. 8. Liquid crystal polymers also gained much interest on industrial applications. polyester liquid crystals were developed for fire resistant, and are used as coating for multifibre, optical cables due to good surface roughness, low coefficient of friction.  SFIT/BSH/ECH/24-25 Liquid crystal display (LCD) A Liquid crystal display is a passive device, which means it doesn’t produce any light to display characters, images, video and animations. But it simply alters the light traveling through it. Consider a single pixel area in LCD, in which there are two polarization filters oriented at 90-degree angle to each other as shown in figure 1.1. These filters are used to polarize the unpolarized light. The first filter (Vertical-polarized filter in figure 1.1) polarizes the light with one polarization plane (Vertical). When the vertically polarized light passes through the second filter (Horizontal polarized filter) no light output will produce. The vertically polarized light should rotate 90 degrees in order to pass through the horizontal polarized light. This can be achieving by embedding liquid crystal layer between two polarization filters. The liquid crystal layer consists of rod-shaped tiny molecules and ordering of these molecules creates directional orientation property. These molecules in the liquid crystal are twisted 90 degrees. The vertically polarized light passes through rotation of the molecules and twisted to 90 degrees. When the orientation of light matches with the outer polarization filter light will pass it and brightens the screen.  SFIT/BSH/ECH/24-25 Two glass transparent electrodes (made up of Indium Tin Oxide (ITO) ) are aligned front and back of the liquid crystal in order to change the orientation of the crystal molecules by applying voltage between them. If there is no voltage applied between the electrodes, the orientation of molecules will remain twist at 90 degrees and the light passes through the outer polarization filter thus pixel appears as complete white. Fig: Showing no voltage applied condition If the voltage is applied large enough the molecules in the liquid crystal layer changes its orientation (untwist) so that light orientation also changes and then blocked by the outer polarization filter thus the pixel appears black. In this way, black and white images or characters are produced.  SFIT/BSH/ECH/24-25 Fig: Showing voltage applied condition By controlling the voltage applied between liquid crystal layers in each pixel, light can be allowed to pass through outer polarization filter in various amounts, so that it can be possible to produce different gray levels on the LCD screen. In order to produce colour images a colour filter is placed in front of the outer polarization plate. The red, green and blue are the three standard colors filters are placed for every three pixels to produce different color images by varying the intensity of each colour. Liquid-Crystal Polymer A liquid crystal polymer is a material that retains molecular order in both liquid and solid states. Liquid crystal polymers are capable of forming regions of highly ordered structure while in the liquid phase. However, the degree of order is somewhat less than that of a regular solid crystal. Liquid crystalline polymers are a relatively unique class of partially crystalline aromatic polyesters. LCPs have outstanding mechanical properties at high temperatures, excellent chemical resistance, inherent flame retardancy, and good weatherability. For example: Kevlar It is a synthetic fibre with many properties that make it useful for a variety of applications, including:  Strength: Kevlar has a high tensile strength, which is five times greater per unit weight than steel.  Thermal stability: Kevlar has a low coefficient of thermal expansion and is thermally stable.  Corrosion and flame resistance: Kevlar is resistant to corrosion and flames.  Impact absorption: Kevlar is good at absorbing impact.  Cut resistance: Kevlar is resistant to cutting.  SFIT/BSH/ECH/24-25 Kevlar is used in many applications, including:  Bulletproof vests  Airplane components  Motorcycle safety clothing  Personal armor