Polymers. Historical Classification

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Dylan Barber
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1 Polymers The term polymer implies many "mers" or the building blocks...similar to the unit cell in metals. A polymer is a chemical compound or mixture of compounds formed by a process called polymerization, a chemical reaction in which two or more molecules combine to form larger molecules. Generally speaking, polymers refer to the intermediate stage before the final plastic product is produced. istorical lassification Natural polymers derived from plants and animals wood, rubber, cotton wool, leather and silk biological polymers protein, enzymes, starches, cellulose Synthetic polymers huge expansion since WWII

2 Basic Building Blocks Plastics are derived from organic materials and are in abundance. Raw materials commonly used in the production of polymers are coal, air, water, wood, petroleum, limestone, and salt. Most common material used is petroleum. These materials contain the basic elements that are used in forming polymers...carbon, hydrogen, oxygen, nitrogen, chlorine, and fluorine.

3 Most polymers are organic composed of and each has 4 bonds each has 1 bond bonds are covalent Bonds between carbons can single (e.g. ethane) double (e.g. ethylene or ethene) triple (e.g. acetylene or ethyne) ydrocarbon Molecules methane, simplest hydrocarbon form = Single Bond

5 Ethane Unsaturated ydrocarbon Molecules single bond Ethylene 2 4 -double bond Acetylene 2 2 -triple bond Molecules that have double or triple bonds are termed unsaturated

6 hemistry of Polymer Molecules Unsaturated hydrocarbons may permit the addition of another atom or group of atoms. Example ethylene 2 4, which is a gas. initiation growth termination R + R- R- + R- R-. + R- R -R R - free radical (unpaired electron) in initiator - catalyst

7 Structure of Polymer Molecules Polymer composed of mers (repeat unit) Single unit called monomer Example [ ] n mer monomer n = the degree of polymerization Ethylene ( 2 4 ) - gas Polyethylene (PE) - solid polymeric material carbons are 109 to each other (tetrahedral bond angle for sp 3 hybridization zigzag structure

9 Polymers are gigantic compared to hydrocarbon molecules called macromolecules For most polymers long, flexible chains with a string of carbon atoms in the backbone remaining electrons can be involved in side bonding with atoms or groups of atoms structural entities are called mers

10 ommon Polymers Polytetrafluoroethylene (PTFE) Trade name Teflon Mer unit Polyvinylchloride (PV) Mer unit F F F F F F F F F F F F Polypropylene (PP) Mer unit l l l

14 Molecular Structure The physical characteristics of a polymer depends not only on its molecular weight and shape but also on differences in the structure of the molecular chains. Types of Molecular Structures Linear Linear polymer are those in which the mer units are joined together end to end in single chains. There is only Van der Waals bonding between chains. Examples polyethylene, nylon.

Branched Polymers where sidebranch chains are connected to the main ones. The chain packing efficiency is reduced compared to linear polymers lower density.

15 Branched Polymers where sidebranch chains are connected to the main ones. The chain packing efficiency is reduced compared to linear polymers lower density. ross linked Adjacent linear chains are joined one to another at various positions by covalent bonds. Many rubbers have this structure. Network Trifuntional mer units having three active covalent bonds, form three dimensional networks. Example: epoxy, phenolformaldehyde

17 Isomers ydrocarbons with the same composition but different atomic arrangements are called isomers (ex: Butane and Isobutane ) Butane Isobutane These isomers have different properties (e.g. b.p.) Two types of isomerism are possible: stereoisomerism and geometrical isomerism.

18 Stereoisomerism Stereoisomerism: atoms are linked together in the same order, but can have different spatial arrangement 1 Isotactic configuration: all side groups R are on the same side of the chain. 2 Syndiotactic configuration: side groups R alternate sides of the chain. 3 Atactic configuration: random orientations of groups R along the chain.

19 Geometrical isomerism Geometrical isomerism: consider two carbon atoms bonded by a double bond in a chain. atom or radical R bonded to these two atoms can be on the same side of the chain (cis structure) or on opposite sides of the chain (trans structure). is-polyisoprene Trans-polyisoprene

20 Summary: Size Shape -Structure

21 Thermoplastic and Thermosetting Polymers The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure. Thermoplast (thermoplastics: Polymer that soften when heated (and eventually liquefy) and harden when cooled processes that are totally reversible and may be repeated. Example (polyethylene, most linear polymers. Thermoset (thermosetting polymers: Polymers became permanently hard when heat is applied and do not soften upon subsequent heating. Examples: vulcanized rubbers, epoxies, phenolics, etc.

23 opolymers (composed of different mers) Random copolymer opolymers: at least two different types of mers, can differ in the way the mers are arranged: Alternating copolymer Synthetic rubbers are copolymers Block copolymer Graft copolymer

24 Polymer rystallinity The crystalline state may exist in polymeric materials. Atomic arrangement in polymer crystals is more complex than in metals or ceramics (unit cells are typically large and complex). Polyethylene Polymer molecules are often partially crystalline (semi-crystalline), with crystalline regions dispersed within amorphous material.

25 rystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density: ( ρ S ρ A ) ( ρ ρ ) 100 % rystallinity ρ = ρ x ρ c : Density of perfect crystalline polymer ρ A : Density of completely amorphous polymer S ρ s : Density of partially crystalline polymer that we are analyzing A

26 Polymer rystals Thin crystalline platelets grown from solution - chains fold back and forth: chain-folded model The average chain length is much greater than the thickness of the crystallite Polyethylene

27 Photomicrograph spherulite structure of polyethylene Spherulites: Aggregates of lamellar crystallites ~ 10 nm thick, separated by amorphous material. Aggregates approximately spherical in shape.

28 Mechanical Behavior of Polymers The mechanical properties of polymers are specified with many of the same parameters used for metals. But polymers are highly sensitive to the rate of deformation (strain rate), the temperature and the environment. A: Brittle Polymer B: Plastic Polymer : Elastomer The stress-strain behavior can be brittle (A), plastic (B), and highly elastic () Deformation shown by curve is totally elastic (rubber-like elasticity). This class of polymers - elastomers

29 Modulus of elasticity defined as for metals Ductility (%EL) defined as for metals Yield strength - For plastic polymers (B), yield strength is defined by the maximum on curve just after the elastic region (different from metals) Tensile strength is defined at the fracture point and can be lower than the yield strength (different from metals) Moduli of elasticity Polymers: ~ 10 MPa - 4 GPa Metals: ~ GPa Tensile strengths Polymers: ~ MPa Metals: 100 s s MPa Elongation Polymers: up to 1000 % in some cases Metals: < 100%

30 Temperature increase leads to: Decrease in elastic modulus Reduction in tensile strength Increase in ductility polymethyl methacrylate (PMMA)

31 Viscoelasticity Amorphous polymer: glass at low temperatures, rubber at intermediate temperatures, viscous liquid at high T. Low temperatures: elastic deformation at small strains (σ = Eε). Deformation is instantaneous when load is applied. Deformation is reversible. igh temperatures: viscous. Deformation is time dependent and not reversible. Intermediate temperatures: viscoelastic behavior. Instantaneous elastic strain followed by viscous time dependent strain. Viscoelastic behavior determined by rate of strain (elastic for rapidly applied stress, viscous for slowly applied stress)

32 Elastic Load is applied at t a and released at t r Viscoelastic Viscous

33 Forming Techniques for Plastics Various techniques are employed in the forming of polymeric materials: Injection Molding ompression Molding Transfer Molding Rotational Molding Extrusion Blow Molding Blown film extrusion Thermoforming alendaring Fibering Foaming Laminating

34 ompression Molding Process for forming thermosets by applying heat and pressure. A measured amount of thermoset powder, granules or pellets, is fed into the mold cavity. eat softens the material and pressure fills the cavity, then the material is cured. eat actually causes the polymer to transform into a highly crosslinked and networked structure. Process is of limited use for thermosets because of the cooling time required of the mold. Typical products include electrical insulators, pot handles, and some automotive parts.

36 Injection Molding Associated with processing thermoplastics. owever, with development of the reciprocating screw type equipment, thermosets can also be injection molded. The basic process includes plasticizing, injection, cooling, and ejection. Granules are feed from a hopper into to a screw that rotates to feed the material into a heated chamber to allow the material to change to a molten state. The material is then forced through a nozzle into the mold cavity. A cooling time is necessary to allow the polymer to become solid, and then is ejected from the mold by mechanical ejector pins.

38 Blown Film Extrusion Used to produce thin film hollow tubes. Somewhat of a combination of extrusion, blown molding and calendaring. As material is extruded, air is forced through the center of a die, causing the material to expand to the diameter of the mold. Mold is open at the end, and the material is continuously taken up on rollers. During the take up process, the walls on the "tube may be seamed welded and perforated such as the case with garbage bags.

39 Extrusion ontinuous flow of molten material is forced through a die. Shape of the final product is determined by the shape of the die opening. Thermoplastic material is fed from a hopper, similar to the configuration of the screw system in injection molding. The screw forces the material through a tapered opening in the die. eat and friction causes plasticizing to occur, softens the material, and forces it through the die opening. Material is cooled by either air or water. Rate of cooling can be controlled and further forming is possible. Example, PV pipe is extruded as electrical conduit. If allow to be immersed in hot water, the conduit can be bent at 90 degree angles. Products that are extruded include tubing, rods, bars, moldings, sheets and films. Extrusion is also used for coating wire and cable.

41 Making Fiber Optic able

Chapter 14 Polymers CHAPTER 7 POLYMERIC MATERIALS. Ancient Polymer History. Rubber balls used by Incas Noah used pitch (a natural polymer) for the ark

Chapter 14 Polymers CHAPTER 7 POLYMERIC MATERIALS. Ancient Polymer History. Rubber balls used by Incas Noah used pitch (a natural polymer) for the ark Chapter 14 Polymers What is a polymer? Polymers are organic materials made of very large molecules containing hundreds of thousands of unit molecules called mers linked in a chain-like structure (repeated