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Introduction To Plastics Materials

Introduction To Plastics Materials

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Introduction To Plastics Materials

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  1. Introduction To Plastics Materials

  2. Classification of Polymers

  3. POLYMER “The polymer (poly- many; mer-unit or parts) is a high molecular weight compound, formed by the combination of small molecules of low molecular weight”. OR “Polymers having long chain macromolecules, which are built up by the linking together of a large number of small molecules, called monomer”.

  4. Basic Terms of Polymers • Polymerization: “The process by which, monomer combine to form polymers is known as polymerization”. • Degree of Polymerization (DP): “The numbers of repeating unit present in it call degree of polymerization (DP)”. • Addition Polymerization: : ‘When molecules just add on to form the polymer, the process is called ‘addition polymerization’ • In ‘addition polymerisation’ the molecular weight of the polymer is roughly equal, to that of all the molecules, which combine to form the polymer. • Ex; Polyethylene, polypropylene

  5. Condensation Polymerization: When, however, molecules do not just add on but also undergo some reaction in forming the polymer, the process is called ‘condensation polymerisation’ • The molecular weight of polymer is lesser by the weight of simple molecules eliminated during the condensation process • The condensation takes place between the two reactive functional groups, like the carbonyl group (of an acid) and hydroxyl group (of an alcohol) to form polyesters. • Ex. Nylon, PET

  6. Polymer Material Properties Depends on • Degree of Polymerization • Molecular Weight of the Polymer • Molecular Weight Distribution • Glass Transition Temperature • Percentage of Crystallinity • Structure and Distribution of Chain Branching

  7. Types of Polymers A polymer consist of identical monomers or monomers of different chemical structure and accordingly they are called homopolymer and copolymers respectively. If the main chain is made up of same species of atoms, the polymer is called ‘homochain polymer’ Graft copolymer are branched structures in which the monomer segments on the branches and backbone differ

  8. The monomeric unit in a polymer may be present in a linear, branched or cross-linked (three dimensional) structure

  9. Tacticity 1. The head to tail configuration in which the functional groups are all on the same side of the chain, is called ‘isotactic polymers’. 2. If the arrangements of functional groups are at random around the main, it is called ‘atactic polymers’ e.g. polypropylene. 3. If the arrangements of side groups is in alternating fashion, it is called ‘synditactic polymers’ Where R = Alkyl Group

  10. Functionality

  11. Thermoplastics Polymers Thermoplastics are resins that repeatedly soften when heated and harden when cooled Thermosetting Polymers • Thermosets are resins that undergo reaction during processing to become permanently insoluble and infusible due to they formed three-dimensional cross linked network structure when heat is applied. • Characteristics of thermosetting resins: • During the hardening the cross-links are formed between adjacent molecules, resulting in a complex, interconnected network that can be related to its viscosity and performance • These cross-links prevent slippage of individual chains, thus preventing plastic flow under addition of heat • If excessive heat is added after cross links, degradation rather than melting will occur. • Ex: Phenolic Resin, Epoxy Resin, Polyester resin

  12. Thermoplastics and Thermosetting Polymers Structure of Thermoplastics and Thermosetting Polymers

  13. Difference Between Thermoplastics and Thermosetting Polymers

  14. Molecular weight of Polymers “Molecular weight of a polymer is defined as sum of the atomic weight of each of the atoms in the molecules, which is present in the polymer”. This distribution of molecular weights is caused by the statistical nature of the polymerization process e.g. methane (CH4) molecules have the same molecular weight (16), but all polyethylene do not have the same molecular weight because the statistical distribution of molecular weight may be different for the different grade of the polyethylene and the degree of polymerization may also be different.

  15. Generalization of Concept

  16. Number-average molecular weight Weight-average molecular weight z-Average molecular weight Where, n = Moles of molecules (n1 + n2 + n3 + ----------ni) i.e. weight (w)/molecular weight (M) w = Weight of individual molecules (w1 + w2 + w3 + ---------wi) M = Molecular weight of each molecules

  17. Number Average Weight (Mn) The number average molecular weight is not too difficult to understand. It is just the total weight of all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample. Consider a polymer, which contains four molecular weight polymers in different numbers and weight and these

  18. Total number of polymer in containing each entity of poly-1, poly-2, poly-3 and poly-4 is = 15 Number of Poly-1 present in the polymer = 2 Number of fraction of poly-1 = 2/15 Similarly,Number of fraction of poly-2 = 4/15, Number of fraction of poly-3 = 6/15, Number of fraction of poly-4 = 3/15 Contribution made by poly-1 towards the average weight of polymer = number of fraction of each polymer x weight of each poly entity Therefore, each poly contribution is (2/25) x 10 =1.33g, (4/15) x 20 = 5.33g, (6/25) x 100 =40g, (3/15) x 250 = 50g Summing up the contribution to get Number Average Molecular Weight= 1.33 + 5.33 + 40 + 50 = 96.66g

  19. Generalization of Concept Total number of molecules (n) id given by Number fraction of each molecule is = Number average weight contribution of each entity is = Number average weight molecular weight is

  20. Weight Average Molecular Weight (Mw) Total weight of each poly present in the polymer =1450g Weight of poly-1 present in polymer = 20g Weight fraction of poly-1 = 20/1450, Weight fraction of poly-2 = 80/1450, Weight fraction of poly-3 = 600/1450, Weight fraction of poly-4 = 750/1450 Contribution made by each poly towards average weight of polymer = weight fraction of poly-1x weight of each unit For poly-1 (20/1450) x 10 = 0.14g For poly-2 (80/1450) x 20 = 1.10g For poly-3 (600/1450) x 100 = 41.38g For poly-4 (750/1450) x 250 = 129.31g Summing up the contribution made by each poly to get weight average molecular weight is 0.14 + 1.10 + 41.38 + 129.31 = 171.93g

  21. Generalization of Concept Total number of molecules (n) id given by Total weight of the polymer is =W Weight fraction of each molecule is = Weight average weight contribution of each entity is = Number average weight molecular weight is For synthetic polymers Mw is greater than the Mn. If they are equal than they will consider as perfectly homogeneous. (Each molecule has same molecular weight).

  22. Molecular weight and degree of polymerisation Number of repeating unit in a polymer called as degree of polymerisation (DP). DP provides the indirect method of expressing the molecular weight and the relation is as follows; M = DP x m Where, M is the molecular weight of polymer, DP is the degree of polymerisation and m is the molecular weight of the monomer Each of these averages can be related to the corresponding molecular weight average by the following two equations; Mn = (DP)n.m Mw = (DP)w.m

  23. Influence of Molecular weight of Polymers • The influence of molecular weight on the bulk properties of polyolefin's, an increase in the molecular weight leads to • Increase in: • Melt viscosity • Impact strength • Lowers in: • Hardness • Stiffness • Softening point • Brittle point • High molecular weight polymer does not crystallize so easily as lower molecular weight material crystallizes due to chain entanglement and that reflect in bulk properties of the high molecular weight polymer.

  24. A high molecular weight polymer increases the mechanical properties. Higher molecular weight implies longer polymer chains and a longer polymer chain implies more entanglement thereby they resist sliding over each other. • Increasing the molecular weight and the chain length of the polymer increases impact strength. • Thermal properties can also improved by increasing the molecular weight.

  25. Polydispersity Index for Molecular weight of Polymers Polydispersity is a very important parameter and it gives an idea of lowest and the highest molecular weight species as well as the distribution pattern of the intermediate molecular weight species. Plastics processing are affected by the molecular weight distribution.

  26. Polydispersity Index for Molecular weight of Polymers

  27. To bring into sharper focus the effect of molecular weight on physical properties, a more generalized form of representation is given in figure, mechanical strength is plotted against 'DP. The useful range of DP is from 200 to 2000, which corresponds to its molecular weight 20000 to 200000

  28. Determination of Molecular Weight

  29. Determination of Molecular Weight • Number-average Molecular Weight • a) End-group Analysis: • If functional groups present in a given weight of the sample and this is expressed as a functional group equivalent/100 g. From knowledge of the functional group equivalent and the functionality, The molecular weight is calculated using the equation: • Then the functionality of the polymer sample can be given by the equation • Functionality = molecular weight X functional equivalent

  30. b) Measurement of Colligative Properties • Depression in freezing point (cryoscopy) and elevation in boiling point (ebulliometry). As these two properties are Colligative (i.e. depend only on the number of moles of a solute present in a liquid and not on their nature). • Colligative properties are those that depend on the number of species present rather than on their kind. From thermodynamic arguments it may be shown that for very dilute ideal solutions • Where, 1 is the activity of the solvent in a dilute ideal solution and X2 is the mole fraction of solute. From this relationship the solute molecular weight may be calculated if the weight fraction W2is known.

  31. The suitability of methods based on Colligative properties Vs Molecular Weight Lowering of Vapour Pressure The partial vapor pressure PIof solvent 1 over a solution is lower than the vapor pressure over the pure solvent p10.This is expressed by Raoult's law: Where X1 is the mole fraction of the solvent. For a binary solution containing a mole fraction X2 of solute then,

  32. For a dilute solution, Combining above two equation, Assuming ideal solution behavior, the unknown molecular weight is calculated from

  33. i) Ebulliometry Ebulliometry is another technique for determining the depression of the solvent activity by the solute. In this case the elevation of the boiling point is determined. The boiling-point elevation Tbis measured with sensitive thermocouples or matched thermostats in a Wheatstone bridge. The molecular weight M" is calculated from Where c is the concentration of solute in g/1000g of solvent and is the molal ebullioscopic constant. M is the molecular weight of the solvent and Tbits boiling point; Hvis the molar latent heat of vaporization of the solvent.

  34. ii) Cryoscopy The freezing point of a solution is depressed below that of the pure solvent by an amount proportional to the mole fraction of solute. The value for Mnis obtained from Where c is the concentration of solute in g/1000 g of solvent and is the molal cryoscopy .constant; M is the molecular weight of the solvent and Tm its melting point; Hfus is the molar latent heat of fusion of the solvent.

  35. c) Osmometry I) Vapor-pressure Osmometry The number-average molecular weight of the unknown sample may then be calculated from this equation, Where c is the concentration of solute in g/1000 g of solvent is the straight line equation and the plot intercept of which would provide Mn.

  36. ii) Membrane Osmometry The membrane osmometer apparatus basically measures osmotic pressure of polymer solutions of known concentration, say 1 g dl-1 As already mentioned the van't Hoff law forms the basis for the determination of number average molecular weight, MnFollowing equation gives the relation ship between osmotic pressure, of polymer solution to Mn of the polymer Where, A1, A2and A3are the first, second and third virial coefficients, C2 is the concentration of polymer solution and R and T are the gas constant and temperature respectively. For very dilute solutions (typically less than 1 g dl-I), the concentration terms containing higher order powers can be neglected and hence it can be written that

  37. Working Principle

  38. 2. Viscosity-Average Molecular Weight The molecular weight is related to the intrinsic viscosity by the Mark-Houwink relationship. Where k and  are empirical constants and are characteristic for a polymer ­solvent pair at a given temperature.

  39. Huggins Equation Kraemer Equation

  40. Weight-average Molecular Weight Light Scattering When a beam of light is passed through a colloidal solution, it is scattered. This is well-known Tyndall effect, which results from the scattering of a part of the beam of light by the colloidal particles in all directions. Since polymer solutions can be considered as colloidal (lyophilic) solutions and as the intensity of light scattered depends on the size of colloidal particles (or polymer molecules)

  41. Where B is the second virial coefficient, C is the concentration of the solution, and R90is the Rayleigh ratio at 90° observation angle. This ratio in a generalized case is represented as Ri.e. the Rayleigh ratio is determined at an observation angle of 90°, R= R90'

  42. Ultracentrifuge Polymer solutions are lyophilic colloids arid these can be made to settle down on the application of centrifugal force. The ultracentrifuge is used to determine molecular weights. The rate of the settling (sedimentation rate) of polymer molecules depends on the size of the molecules: Where Rand T are the gas constant and temperature in Kelvin scale. The above equation is known as Svedberg equation. For a polydisperse solute the correct averages for Sand D are combined in the Svedberg equation

  43. Gel Permeation Chromatography (GPC) • The basic principle underlying the separation of different' fractions of a polydispersed sample is based on the size of individual polymer molecules that explore the pore system of the column material. • Large molecules are excluded from small pores and can only diffuse into a restricted part of the pore system within the beads while smaller ones would enter into the pores of the bead. • Thus large molecules would have less residence time and would emerge first.

  44. Working Principle of GPC

  45. GPC Curve for standard polystyrene sample showing elution volumes corresponding to different molecular weight

  46. *Relative methods and need calibration from standard polymers samples **Indirect method and needs values of k and a for particular polymer - solvent system

  47. Glass Transition Temperature (Tg) The temperature below, which a polymer is hard and above which it is soft, is called the glass transition temperature (Tg). Or The molecular mobility is just starts above that temperature or below which mobility arrested called Tg

  48. Glass Transition Temperature (Tg) Tg is unique to amorphous thermoplastics. It occurs at a specific temperature that depends on pressure and specific volume and is lower than melting point.