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Polymers

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polymer chapter for engineering students
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  POLYMERS Syllabus i)Introduction ii)Types of polymerization: addition and condensation, iii)Mechanismof polymerization- free radical mechanism taking vinyl chloride as an example. iv) Molecular weight of polymers: number average and weight average numerical problems. v) Glass transition temperature (Tg): Factors influencing Tg-Flexibility, inter molecular forces, molecular mass, branching & cross linking, and stereo regularity. Significance of Tg. vi) Structure propertyrelationship: crystallinity, tensile strength, elasticity, plastic deformation &chemical resistivity. vii) Synthesis, properties and applications of PMMA (plexiglass), Teflon, Polyurethane and polycarbonate. viii)Elastomers: Introduction,synthesis, properties and applications of Silicone rubber. ix) Adhesives:Introduction, synthesis, properties and applications of epoxy resin. x) PolymerComposites: Introduction, synthesis, properties and applications of kevlar an d carbon fiber.  xi ) Conducting polymers: Introduction, mechanism of conduction   in Polyaniline and  applications of conducting polyaniline     Glass transition temperature(6 marks) Defination: Glass transition temperature is the temperature at which a polymer transforms from glassy (hard) to the rubbery state (soft). OR Defination : The temperature below which a polymer is hard, brittle and above which is soft and flexible is called glass transition temperature. It is denoted by T g . The hard and brittle state is known as glassy sate and soft flexible state is called rubbery state or viscoelastic state . At below T g  , molecular rotations are not possible so the polymer is hard (glassy).At above T g ,  molecules can undergo rotation about single bond .So the polymer becomes soft(rubber). When heated beyond T g  ,the polymer melts and reaches a molten state or viscofluid state . Following factors affect the glass transition temperature. a)Flexibility:  Linear polymers containing single bond ( C  C ,C  O and C  N ) have high degree of freedom of rotation and hence low T g  . If rigid and bulky groups such as aromatic , cyclic are present ,T g  increases due to less freedom of rotation. Eg Polythene  110 0 C Flexible backbone PVC +80 0  C restricted rotation due to strong polar attraction. b) Intermolecular forces : Presence of a large number of polar group in the molecular chain lead to strong intermolecular cohesive forces, which restricts the molecular rotation and mobility . This leads to an increase in T g .   For example: Polyproplyene which contains no polar group , has low Tg of ─ 18 0 C compared to nylon-6,6 which contains polar group has high T g  of 57 0 C. c) Stereo regularity:  T g  of an syndiotactic polymer  has higher than atactic polymer  ,which in turn has higher T g  than  isotactic.   syndiotactic polymer > atactic polymer > isotactic polymer NOTE: Syndiotactic polymer  :  functional groups are present on alternate side of chain .  Atactic polymer :  functional groups are randomly present in chain.  Isotactic :  functional groups are present on same side of chain .   d) Branching and cross linking: A small amount of branching will reduce the value of T g  , because the free volume increases with branching and thus decreases the T g . On the other hand , a high density of branching and cross linking brings the polymer chain closer , lowers free volume and thus reduces the chain mobility, thereby increasing T g  value. e) Molecular Mass : Higher the molecular mass more is the restriction in molecular rotation . Generally Tg of a polymer increases with molecular mass upto 250 and beyond that there is no change. SIGNIFICANCE OF Tg 1) It is used to evaluate the flexibility of a polymer and to predict its response to mechanical stress. 2) It is useful in choosing proper temperature range during processing operations such as moulding , extrusion , calendring. 3) Coefficient of thermal expansion , heat capacity,refractive index,electrical properties etc. at Tg determine the usefulness of a polymer over a temperature range. STRUCTURE - PROPERTY RELATIONSHIP (6 marks) 1)Crystallinity 2) Tensile strength 3) Elasticity 4) Plastic deformation 5)Chemical resistivity 1) Crystallinity : The crystallinity of polymer depends upon structure of monomer , nature of groups present and stereo regularity of polymeric chain   1) A polymer may be crystalline or amorphous. A polymer will be crystalline, if the chain has regular and symmetrical molecular structure. Because ploymer chains can pack up closely and orderly. 2)Linear polymer like HDPE is more crystalline than polysterene.This is because polysterene is branched and contains bulky groups like benzene. 3)Presence of polar groups in polymer chain leads to high crystallinity , because of hydrogen bonding with neighbouring chain. Eg: nylon 6,6 4) isotactic and syndiotactic polymers have high degree of crystallinity than actactic ,because in actactic groups are present randomly. Tensile strength: Impact strength ,tensile strength and melt viscosity are mechanical properties of a polymer 1)Polymers with low molecular mass are soft. 2) Polymers with high molecular mass are hard ,tough and heat resistant. 3) Cross-linked polymers are stronger than linear and less branched polymers. 4)Tensile and impact strength increseas with molecular mass up to certain point and then becomes constsnt. 5)The melt viscosity of polymer show gradual increase with molecular mass and a steep increase at high molecular mass. 6) For commercial use , polymer should have low melt viscosity ,high tensile and impact resistant. Elasticity : The property of polymer to uncoil on stretching and recoil on leaving is called elasticity of polymer. A polymer to show elasticity, individual chains should not break on stretching. Usually of breaking occurs when chain slip past each other. This is avoided by 1)introducing cross linking at suitable molecular position. 2) Avoiding bulky side groups such aromatic and cyclic structures in repeating units. 3) introducing more non polar groups in chain so that chain don not separate on stretching.
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