This name hints at how polymers are made

Transcription

1 Chapter- I

2

3

4 Many + Parts This name hints at how polymers are made

5 POLYMERS (the whole train) are made out of MONOMERS (individual cars of the train) joined together.

6 repeat unit H H H H H H C C C C C C H H H H H H Polyethylene (PE) repeat unit H H H H H H C C C C C C H Cl H Cl H Cl Poly vinyl chloride (PVC) repeat unit H H H H H H C C C C C C H CH 3 H CH 3 H CH 3 Polypropylene (PP) Carbon chain backbone

7 Cellulose After collection, the latex is coagulated to obtain the solid rubber. Natural rubber is thermoplastic, with a glass transition temperature of 70 C.

8

9 Cotton fiber is mostly cellulose, and cellulose is made of chains of the sugar, glucose linked together a certain way.

10 Polymers are macro molecules formed by the repeated linking of large number of small molecules. These small molecules called monomers.

11 Polymerization: Polymerization is a process in which large number of small molecules combine to give a big molecules with or without elimination of small molecules like water. HOW: This happens when a carbon to carbon double bond in a monomer is broken and new single bonds are formed creating a polymer.

12 Polymerization of polypropylene (propane)

13 Polymers Polymerization: (of polyethylene)

14 Degree of Polymerization, (n) n= average number of repeat units per chain DP( n) M m where m repeat unit molecular weight n Ex. problem 4.1b, for PVC: m = 2(carbon) + 3(hydrogen) + 1(Clorine) (from front of book) = 2(12.011) + 3(1.008) + 1(35.45) = g/mol DP = 21,150 / =

15 The number of bonding sites or reactive sites or functional groups present in a monomer is known as its functionality Types Functionality Bifunctional Monomers: Functionality of monomer=2 Forms linear chains or straight chain polymer Trifunctional monomers: Functinality of monomer=3 Forms linear and branched chain polymer Polyfunctinal monomers: Functinality of monomer= more than 3 Forms three dimentional network polymer

16 Classifications - based on synthesis Inorganic Organic Natural Clays Sand tc. Synthetic Silicones Silanes Phosphazenes etc. Natural Synthetic Polysaccharides (Carbohydrates) Proteins DNA Natural Rubber Silk etc. Polyolefins Polyesters Polyurethanes Nylons etc.

17 Polymer Structure Polymers can exist with various skeletal structures - such as linear, branched or cross-linked or network polymers. Linear Branched Network

18 Molecular Structures for Polymers secondary bonding Linear Branched Cross-Linked Network The physical characteristics of a polymer depend also on differences in the structure of the molecular chains (other variables are shape and weight). Linear polymers have repeat units joined end to end in single chains. There may be extensive van der Waals and hydrogen bonding between the chains. Examples: polyethylene, PVC, nylon. 18

19 Where side-branch chains have connected to main chains, these are termed branched polymers. Linear structures may have side-branching. HDPE high density polyethylene is primarily a linear polymer with minor branching, while LDPE low density polyethylene contains numerous short chain branches. Greater chain linearity and chain length tend to increase the melting point and improve the physical and mechanical properties of the polymer due to greater crystallinity. 19

20 In cross-linked polymers, adjacent linear chains are joined to one another at various positions by covalent bonding of atoms. Examples are the rubber elastic materials. Small molecules that form 3 or more active covalent bonds create structures called network polymers. Examples are the epoxies and polyurethanes.cam 20

21 Copolymers two or more monomers polymerized together random A and B randomly positioned along chain alternating A and B alternate in polymer chain block large blocks of A units alternate with large blocks of B units graft chains of B units grafted onto A backbone random alternating block A B graft 21

22 Plastics Plastics are high molecular weight organic materials, that can be moulded into any desired shape by the application of heat and pressure in the presence of a catalyst Plastics Thermoplastics Crystalline Amorphous Ex: Polyethylene Polyvinyl chloride Polystyrene Thermosets Ex: Bakelite Polyester Epoxyresin

23 Thermoplastics Often referred to as just Plastics are linear or branched polymers which soften upon heating. They can be moulded (and remoulded) into virtually any shape injection moulding, extrusion and constitute the largest portions of the polymers used in industry Thermoplastics never achieve 100% crystallinity, but instead are semicrystalline with both crystalline and amorphous domains.

24 Thermoplastics The crystalline phases of such polymers are characterized by their melting temperature (T m ). Many thermoplastics are completely amorphous and incapable of crystallization, these amorphous polymers (and amorphous phases of semicrystalline polymers) are characterized by their glass transition temperature (T g ). the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft).

25 Thermosets Thermosets - normally rigid materials - network polymers in which chain motion is greatly restricted by a high degree of crosslinking As with elastomers, they are intractable once formed and degrade rather than melt upon the application of heat.

26

27 Polymerization is classified based on (i) the mechanism of the process and (ii) practical aspects of the process. Based on the mechanism, polymerization is broadly classified into (1) Addition or Chain polymerization and (2) Condensation or Step polymerization. Based on practical considerations, polymerization is of types such as solution polymerization, emulsion polymerization, bulk polymerization etc.

28 Addition polymerization 1 The monomer must have atleast one multiple bond 2 Number of monomer units increases steadily throughout the reaction. Condensation polymerization The monomer must have atleast two identical or different functional groups Monomer disappears early in the reaction. 3 High polymer is formed at once. Polymer molecular weight (degree of polymerization) rises steadily throughout the reaction. 4 Longer reaction times have very little effect on molecular weight but gives higher yields. 5 The reaction mixture contains only monomers, high polymers and very small amount (10-8 ) of growing chains. 6 E.g. polymerization of ethylene, styrene, vinyl chloride, propylene etc. Longer reaction times are essential to obtain higher molecular weights i.e. reaction time influences molecular weight of the polymer. All types of molecular species are present at any stage. Polymerization to get nylon, PET, polycarbonate, polyurethane etc.

29 Chain-Growth Polymers Chain-growth polymers proceed by one of three mechanisms: radical polymerization cationic polymerization anionic polymerization All the above mechanisms occur in three major steps namely, 1. Initiation 2. Propagation and 3. Termination

30 Ethylene has two carbons; plus, instead of the two carbons sharing just one electron each, they share two electrons each. High temperature or UV light can cause two of these shared (paired) electrons to become unshared (unpaired). H C H H C H These HunpairedHelectrons are eager to pair up with another electron. If this ethylene C molecule C bumps another ethylene molecule, the unpaired electrons will cause the one it Hbumped into H to lend one of its inner electrons

31 Here s another way to see the chain reaction. These are the carbon atoms with their double-bond (2 shared electrons each). The hydrogen atoms are not shown. A collision breaks the first bond. Once the first double bond is broken, a chain reaction will occur. In about a second an entire chamber of compressed ethylene gas turns into the polymer, polyethylene.

32 1. Initiation Free Radical Polymerization It is considered to involve two reactions a) First reaction involves production of free radicals by homolytic dissociation of an initiator to yield a pair of free radicals (R*) b) Second reaction involves addition of this free radical to first monomer to produce chain initiating species

33

34 2. Propagation: It involves the growth of chain initiating species by successive addition of large number of monomers

35 3.Termination: Termination of the growing chain of polymer may occur either by coupling reaction or disproportionation.

36

37

38 Faster than Free radical polymerization Chain polymerization of olefinic monomers which can occur with charged radicals. For example styrene. Cationic polymerization when the radical is positively charge. CH 2 -C + H X - Anionic polymerization when the radical is negatively charge CH 2 -C -- H X +

39 Cationic Polymerization This type of polymerization takes place when electron donating groups like CH 3, C 6 H 5 are present in a monomer. These groups stabilize the carbonium ion formation. Ex: Styrene, Isobutylene, Isoprene The catalysts (initiators) used to initiate the reaction are Lewis acids like AlCl 3, BF 3 with a co-catalyst water

40 1.Initiation

41 2.Propagation It involves the growth of chain initiating species by successive addition of large number of monomers and the positive charge simultaneously shifts to the newly added monomer

42 3.Termination Termination of the growing chain involves removal of the catalyst by the addition of proton to the counter ion [AlCl 3 OH] -

43 Monomers that are best able to undergo cationic polymerization are those with electron-donating substituent

44 Anionic Polymerization This type of polymerization takes place when electron withdrawing groups like Cl -, CN - are present in a monomer. These groups stabilize the carbanion formation. Ex: Vinyl chloride, Styrene, Acrylonitrile The catalysts (initiators) used to initiate the reaction are Lewis bases like NaNH 2, KNH 2, LiNH 2 etc.,

45 1.Initiation

46 2.Propagation It involves the growth of chain initiating species by successive addition of large number of monomers and the negative charge simultaneously shifts to the newly added monomer

47 3.Termination Termination of the growing chain occurs by adding ammonia

48

49 Glass Transition Temperature (T g ) : A polymeric sample on heating may transform itself from a state of softness to a state of hardness or brittleness. Thus the temperature below which a polymer is hard and above this temperature, it becomes so soft that it transforms from glassy (brittle) state to a rubbery (elastic) or a viscoelastic state is termed as glass transition temperature (T g ).

50 Factors influencing Tg Chain length, cross linking and internal rotation Rate of heating or cooling Linear polymer network

51 Stereospecific polymer (or)tacticity: Polymers may be synthesized as stereo regular compounds. Stereo regularity is a phenomenon of regular or orderly, spatial arrangement of groups or radicals of the monomer on either side of the main polymeric chain is known as Tacticity

52 POLY(PROPENE) ISOTACTIC The functional groups on same side most desirable properties SYNDIOTACTIC The functional groups alternate sided ATACTIC The functional groups arranged in randomly most likely outcome

53 The molecular weight distribution in a polymer describes the relationship between the number of moles of each polymer species and the molar mass of that species. Molecular Weight Low MW high MW Polymers can have various lengths depending on the number of repeat units. During the polymerization process not all chains in a polymer grow to the same length, so there is a distribution of molecular weights. There are several ways of defining an average molecular weight.

54 Number-average Molecular Weight ( N ) based on methods of counting the number of molecules in a given weight of polymer the total weight of a polymer sample, w, is the sum of the weights of each molecular species present 1 1 i i i i i M N w w i i i i i i i n N M N N w M N = number of molecules M = molecular weight

55 Weight-average Molecular Weight ( W ) determination of molecular weight based on size rather than the number of molecules the greater the mass, the greater the contribution to the measurement M w i 1 i 1 w M i w i i i 1 i 1 N M i N M i 2 i i w = weight fraction M = molecular weight N = number of molecules

56 Molecular Weight Distribution M M n w x M w M i i i i M n = the number average molecular weight (mass) M i x i w i = mean (middle) molecular weight of size range i = number fraction of chains in size range i = weight fraction of chains in size range i 56

57 Number-average molecular weight ( n ) Example - a polymer sample consists of 9 molecules of mw 30,000 and 5 molecules of mw 50,000 M n i 1 (9 30,000) (5 50,000) (9 5) i i i 1 M N N i 37,000

58 Weight-average molecular weight ( w ) Consider the previous example - 9 molecules of molecular weight 30,000 and 5 molecules of molecular weight 50,000 M w 2 9(30,000) 9(30,000) 5(50,000) 5(50,000) 2 40,000

59 In measurements of colligative properties, each molecule contributes regardless of weight, whereas in light scattering, the larger molecules contribute more because they scatter light more effectively. For this reason, w are greater than n, except when all molecules are of the same weight and w = n

60 Poly Dispersity Index (PDI) Poly Dispersity Index (PDI) of a polymer is defined as the ratio of the weight average molecular weight to the number average molecular weight i.e. PDI = (M w ) / (M n ) Typical PDI values for the synthetic polymers are given below for reference: Free radical polymers: obtained by solution / suspension / emulsion methods: obtained by bulk polymerization method: 2 5 obtained by auto-acceleration methods: 8 10 Polymers by cationic / anionic mechanism: Using homogenous catalysts: < 1.5 (less than) Using heterogenous catalysts: > 10 (higher than) Polymers obtained by poly-condensation / poly-addition / ring opening mechanisms: 2 3 Branched chain polymers: > 20 (greater than 20)

61 Techniques for Polymerization There are four commonly used techniques for polymerization 1. Bulk polymerization 2. Solution polymerization 3. Suspension polymerization 4. Emulsion polymerization

62 The simplest technique Bulk Polymerization It gives the highest-purity polymer Ingredients : monomer, monomer-soluble initiator, perhaps a chain transfer agent Advantages High yield per reactor volume Easy polymer recovery Final product form Disadvantages Difficult of removing the lost traces of monomer Dissipating heat produced during the polimerization

63 Solution Polymerization Heat can be removed by conducting the polymerization in an organic solvent or water Initiator or monomer must be soluble in solvent Solvents have acceptable chain-transfer characteristics Solvents have suitable melting or boiling points for the conditions of polymerization Ingredients : monomer initiator solvent Advantages Temperature control is easy Easy removed Disadvantages Small yield per reactor volume Solvent recovery

64 Suspension Polymerization Coalescense of sticky droplets is prevented by PVA Near the end of polymerization, the particles harder and they can be removed by filtration, then washing Ingredients : water-insoluble monomer, water-insoluble initiator, sometimes chain transfer agent suspention medium (water-usually) Advantages (according to bulk polymerization) Forming process not using Stirring is easy Disadvantages Polymer purity is low Reactor capital costs are higher than for solution polymerization Separation process is easy

65 Emulsion Polymerization Particles are formed monosize with emulsion polymerization Polymerization is initiated when the water-soluble radical enters a monomer-containing micelles. Ingredients : water-insoluble monomer, water-soluble initiator, chain transfer agent, dispersing medium (water), fatty acid, surfactant such as sodium salt of a long chain

66

67 Two ingredients are mixed and a solid begins to form at the junction between the two layers of liquid. We say certain polymers are man-made, but the truth is they make themselves. Humans only have to get the ingredients near each other. The chemicals will assemble themselves. Hot nylon spaghetti can be extracted.

68 Preparation, properties and uses of nylon 6,6: Nylon 6,6 is a polyamide prepared by the condensation polymerization between / poly-condensation of the monomers hexamethylene diamine and adipic acid. Here two water molecules are removed from each set of monomers.

69 Tetramethylene dicarboxylic acid (adipic acid) Hexamethylene diamine methylene x 6 (hexa) amine x 2 (di) Nylon is actually a copolymer because is it made from two monomers. When these two monomers are in the same beaker, they combine and give off a molecule of water. This is called a dehydration reaction because taking away (de) water (hydra). (regarding odor: amines smell like fish or worse. Adipic acid is odorless )

70

71 Epoxy resins These are basically poly-ethers. The common types of epoxy resins are prepared from the condensation polymerization between epichlorohydrin and bis-phenol A. The reaction is carried out with excess of epichlorohydrin.

72 Epichlorohydrin bis-phenol

73 Epoxy resins find enormous applications due to their remarkable chemical resistance and good adhesion properties. They are used as excellent structural adhesives. On proper curing, they yield tough materials as used in industrial floorings, as foaming materials, as pottering materials in electrical insulation etc. A principal constituent of most of the fibre reinforced plastic (FRP) materials is the epoxy resins. Also, the EPI coating in lorries carrying corrosive chemicals contains the epoxy resin formulations.

74 Conducting Polymers Introduction What is conductivity? What makes amaterial conductive? How can plastic become conductive? Doping process. Factors that affect the conductivity. Applications. Conclusion.

75 Introduction Polymers (or plastics as they are also called) are known to have good insulating properties. Polymers are one of the most used materials in the modern world. Their uses and application range from containers to clothing. They are used to coat metal wires to prevent electric shocks.

76 Yet Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa have changed this view with their discovery that a polymer, polyacetylene, can be made conductive almost like a metal.

77 What is conductivity? Conductivity can be defined simply by Ohms Law. V= IR Where R is the resistance, I the current and V the voltage present in the material. The conductivity depends on the number of charge carriers (number of electrons) in the material and their mobility.in a metal it is assumed that all the outer electrons are free to carry charge and the impedance to flow of charge is mainly due to the electrons "bumping" in to each other. Insulators however have tightly bound electrons so that nearly no electron flow occurs so they offer high resistance to charge flow. So for conductance free electrons are needed.

78 What makes the material conductive? Three simple carbon compounds are diamond, graphite and polyacetylene. They may be regarded as three- two- and onedimensional forms of carbon materials. Diamond, which contains only σ bonds, is an insulator and its high symmetry gives it isotropic properties. Graphite and acetylene both have mobile π electrons and are, when doped, highly anisotropic metallic conductors.

79 How can plastic become conductive? Plastics are polymers, molecules that form long chains, repeating themselves. In becoming electrically conductive, a polymer has to imitate a metal, that is, its electrons need to be free to move and not bound to the atoms. Polyacetylene is the simplest possible conjugated polymer. It is obtained by polymerisation of acetylene, shown in the figure.

80 Two conditions to become conductive: 1-The first condition for this is that the polymer consists of alternating single and double bonds, called conjugated double bonds. In conjugation, the bonds between the carbon atoms are alternately single and double. Every bond contains a localised sigma (σ) bond which forms a strong chemical bond. In addition, every double bond also contains a less strongly localised pi (π) bond which is weaker.

81 2-The second condition is that the plastic has to be disturbed - either by removing electrons from (oxidation), or inserting them into (reduction), the material. The process is known as Doping. There are two types of doping 1-oxidation with halogen (or p-doping). 2- Reduction with alkali metal (called n-doping).

82 Doping process The halogen doping transforms polyacetylene to a good conductor. Oxidation with iodine causes the electrons to be jerked out of the polymer, leaving "holes" in the form of positive charges that can move along the chain.

83 The iodine molecule attracts an electron from the polyacetylene chain and becomes I3. The polyacetylene molecule, now positively charged, is termed a radical cation, or polaron. The lonely electron of the double bond, from which an electron was removed, can move easily. As a consequence, the double bond successively moves along the molecule. The positive charge, on the other hand, is fixed by electrostatic attraction to the iodide ion, which does not move so readily.

84 DOPING - FOR BETTER MOLECULE PERFORMANCE Doped polyacetylene is, e.g., comparable to good conductors such as copper and silver, whereas in its original form it is a semiconductor. Conductivity of conductive polymers compared to those of other materials, from quartz (insulator) to copper (conductor). Polymers may also have conductivities corresponding to those of semiconductors.

85 Factors that affect the conductivity 1-Denesity of charge carriers. 2- Thier mobility. 3-The direction. 4-presence of doping materials (additives that facilitate the polymer conductivity) 5-Temperature.

86 The conductivity of conductive polymers decreases with falling temperature in contrast to the conductivities of typical metals, e.g. silver, which increase with falling 7/31/2015 temperature.

87 Applications Conducting polymers have many uses. The most documented are as follows: anti-static substances for photographic film Corrosion Inhibitors Compact Capacitors Anti Static Coating Electromagnetic shielding for computers "Smart Windows" A second generation of conducting polymers have been developed these have industrial uses like: Transistors Light Emitting Diodes (LEDs) Lasers used in flat televisions Solar cells Displays in mobile telephones and mini-format television screens

88 Shield for computer screen against electromagnetic "smart" windows radiation smart" windows Photographic Film Solar cell Light-emitting diodes 7/31/2015

89 Conclusion For conductance free electrons are needed. Conjugated polymers are semiconductor materials while doped polymers are conductors. The conductivity of conductive polymers decreases with falling temperature in contrast to the conductivities of typical metals, e.g. silver, which increase with falling temperature. Today conductive plastics are being developed for many uses.

90 We've mentioned high density polyethylene (HDPE) and low density polyethylene (LDPE). It is made by causing the long chains of ethylene to branch. That way they cannot lie next each other, which reduces the density and strength of the polyethylene. This makes the plastic lighter and more flexible.

91 Low density polyethylene is used to make plastic bags, plastic wrap, and squeeze bottles, plus many other things.

92 Another polymer, which is almost the same as polyethylene, is PolyVinyl Chloride or PVC. The difference is that every other hydrogen is replaced with a chlorine atom (green sphere).

93 The favorite properties of plastics are that they are inert and won't react with what is stored in them. They also are durable and won't easily decay, dissolve, or break apart. These are great qualities for things you keep, but when you throw them away, they won't decompose.

94 Since they don t decompose, the answer is to recycle the plastics so they can be remade into something else. Here we see a bunch of CDs getting recycled.

95 The mile long boardwalk at Yellowstone National Park was made from recycled plastic.