Materials Science andEngineering

PolymersPolymers

Outline

Classification of Polymers Addition Polymerization Condensation Polymerization Degree of Polymerization Typical Thermoplastics Structure–Property Relationships

in Thermoplastics Effect of Temperature on

Thermoplastics

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Classification of Polymers Addition Polymerization Condensation Polymerization Degree of Polymerization Typical Thermoplastics Structure–Property Relationships

in Thermoplastics Effect of Temperature on

Thermoplastics

Outline (Continued)

Mechanical Properties ofThermoplastics

Elastomers (Rubbers) Thermosetting Polymers Adhesives Additives for Plastics Polymer Processing and Recycling

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Mechanical Properties ofThermoplastics

Elastomers (Rubbers) Thermosetting Polymers Adhesives Additives for Plastics Polymer Processing and Recycling

Classification of Polymers

Linear polymer - Any polymer in which moleculesare in the form of spaghetti-like chains.

Thermoplastics - Linear or branched polymers inwhich chains of molecules are not interconnected toone another.

Thermosetting polymers - Polymers that are heavilycross-linked to produce a strong three dimensionalnetwork structure.

Elastomers - These are polymers (thermoplastics orlightly cross-linked thermosets) that have an elasticdeformation > 200%.

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Linear polymer - Any polymer in which moleculesare in the form of spaghetti-like chains.

Thermoplastics - Linear or branched polymers inwhich chains of molecules are not interconnected toone another.

Thermosetting polymers - Polymers that are heavilycross-linked to produce a strong three dimensionalnetwork structure.

Elastomers - These are polymers (thermoplastics orlightly cross-linked thermosets) that have an elasticdeformation > 200%.

Comparison of the threepolymer categories

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Three ways torepresent thestructure ofpolyethylene: (a) asolid three-dimensional model,(b) a three-dimensional “space”model, and (c) asimple two-dimensional model.

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Three ways torepresent thestructure ofpolyethylene: (a) asolid three-dimensional model,(b) a three-dimensional “space”model, and (c) asimple two-dimensional model.

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Two ways to represent the benzene ring. In this case, thebenzene ring is shown attached to a pair of carbon atoms,producing styrene.

Addition Polymerization

Addition polymerization - Process by which polymerchains are built up by adding monomers togetherwithout creating a byproduct.

Unsaturated bond - The double- or even triple-covalent bond joining two atoms together in anorganic molecule.

Functionality - The number of sites on a monomerat which polymerization can occur.

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Addition polymerization - Process by which polymerchains are built up by adding monomers togetherwithout creating a byproduct.

Unsaturated bond - The double- or even triple-covalent bond joining two atoms together in anorganic molecule.

Functionality - The number of sites on a monomerat which polymerization can occur.

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The addition reaction for producing polyethylene fromethylene molecules. The unsaturated double bond in themonomer is broken to produce active sites, which thenattract additional repeat units to either end to produce achain.

Initiation of apolyethylenechain by chain-growth mayinvolve (a)producing freeradicals frominitiators such asbenzoyl peroxide,(b) attachment ofa polyethylenerepeat unit to oneof the initiatorradicals, and (c)attachment ofadditional repeatunits to propagatethe chain.

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Initiation of apolyethylenechain by chain-growth mayinvolve (a)producing freeradicals frominitiators such asbenzoyl peroxide,(b) attachment ofa polyethylenerepeat unit to oneof the initiatorradicals, and (c)attachment ofadditional repeatunits to propagatethe chain.

Termination ofpolyethylene chaingrowth: (a) theactive ends of twochains come intoclose proximity, (b)the two chainsundergo combina-tion and becomeone large chain, and(c) rearrangementof a hydrogen atomand creation of adouble covalentbond by dispropor-tionation causetermination of twochains.

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Termination ofpolyethylene chaingrowth: (a) theactive ends of twochains come intoclose proximity, (b)the two chainsundergo combina-tion and becomeone large chain, and(c) rearrangementof a hydrogen atomand creation of adouble covalentbond by dispropor-tionation causetermination of twochains.

Condensation Polymerization

Condensation polymerization - A polymerizationmechanism in which a small molecule (e.g., water,methanol, etc.) is condensed out as a byproduct.

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The condensation reaction for polyethylene terephthalate(PET), a common polyester. The OCH3 group and a hydrogenatom are removed from the monomers, permitting the twomonomers to join and producing methyl alcohol as abyproduct.

Degree of Polymerization

Degree of polymerization - The average molecularweight of the polymer divided by the molecularweight of the monomer.

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Typical Thermoplastics

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Properties of selected thermoplastics

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Repeat Units and Applications forSelected Addition Thermoplastics

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Table (Continued)

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Repeat Units and Applications forSelected Addition Thermoplastics

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(Continued)

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Structure–Property Relationshipsin Thermoplastics

Branched polymer - Any polymer consisting ofchains that consist of a main chain and secondarychains that branch off from the main chain.

Crystallinity is important in polymers since it affectsmechanical and optical properties.

Tacticity - Describes the location in the polymerchain of atoms or atom groups in nonsymmetricalmonomers.

Liquid-crystalline polymers - Exceptionally stiffpolymer chains that act as rigid rods, even abovetheir melting point.

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Branched polymer - Any polymer consisting ofchains that consist of a main chain and secondarychains that branch off from the main chain.

Crystallinity is important in polymers since it affectsmechanical and optical properties.

Tacticity - Describes the location in the polymerchain of atoms or atom groups in nonsymmetricalmonomers.

Liquid-crystalline polymers - Exceptionally stiffpolymer chains that act as rigid rods, even abovetheir melting point.

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Head-to-tail versus head-to-head arrangement of repeat units.The head-to-tail arrangement is most typical.

Copolymerizationproduces thepolymer ABS,which is reallymade up of twocopolymers, SANand BS, graftedtogether.

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Copolymerizationproduces thepolymer ABS,which is reallymade up of twocopolymers, SANand BS, graftedtogether.

Effect of Temperature on Thermoplastics

Degradation temperature - The temperature above whicha polymer burns, chars, or decomposes.

Glass temperature - The temperature range below whichthe amorphous polymer assumes a rigid glassy structure.

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The effect oftemperature onthe structureand behaviorofthermoplastics.

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The effect oftemperature onthe structureand behaviorofthermoplastics.

The effect oftemperature onthe modulus ofelasticity for anamorphousthermoplastic.Note that Tg andTm are not fixed.

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The effect oftemperature onthe modulus ofelasticity for anamorphousthermoplastic.Note that Tg andTm are not fixed.

The relationshipbetween thedensity and thetemperature of thepolymer shows themelting and lasstemperatures.Note that Tg and Tmare not fixed;rather, they areranges oftemperatures.

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The relationshipbetween thedensity and thetemperature of thepolymer shows themelting and lasstemperatures.Note that Tg and Tmare not fixed;rather, they areranges oftemperatures.

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The folded chain ,model for crystallinity in polymers,shown in (a) two dimensions and (b) three dimensions.

Crystal Structure of Several Polymers

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Properties of Selected Thermoplastics

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Repeat Units and Applications for Selected AdditionThermoplastics

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Section Mechanical Properties ofThermoplastics

Viscoelasticity - The deformation of a material by elasticdeformation and viscous flow of the material when stressis applied.

Relaxation time - A property of a polymer that is relatedto the rate at which stress relaxation occurs.

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Viscoelasticity - The deformation of a material by elasticdeformation and viscous flow of the material when stressis applied.

Relaxation time - A property of a polymer that is relatedto the rate at which stress relaxation occurs.

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The stress-strain curve for 6,6-nylon, a typical thermoplasticpolymer.

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Necks are not stable in amorphous polymers, because localalignment strengthens the necked region and reduces its rateof deformation.

Section Elastomers (Rubbers)

Geometric isomer - A molecule that has the samecomposition as, but a structure different from, a secondmolecule.

Diene - A group of monomers that contain two double-covalent bonds. These monomers are often used inproducing elastomers.

Cross-linking - Attaching chains of polymers together toproduce a three-dimensional network polymer.

Vulcanization - Cross-linking elastomer chains byintroducing sulfur or other chemicals.

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Geometric isomer - A molecule that has the samecomposition as, but a structure different from, a secondmolecule.

Diene - A group of monomers that contain two double-covalent bonds. These monomers are often used inproducing elastomers.

Cross-linking - Attaching chains of polymers together toproduce a three-dimensional network polymer.

Vulcanization - Cross-linking elastomer chains byintroducing sulfur or other chemicals.

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The cis and trans structures of isoprene. The cis form is useful forproducing the isoprene elastomer.

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(a) When the elastomer contains no cross-links, the application of a forcecauses both elastic and plastic deformation; after the load is removed, theelastomer is permanently deformed. (b) When cross-linking occurs, theelastomer still may undergo large elastic deformation; however, when theload is removed, the elastomer returns to its original shape.

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Cross-linking of polyisoprene chains may occur introducingstrands of sulfur atoms. Sites for attachment of the sulfurstrands occur by rearrangement or loss of a hydrogen atomand the breaking of an unsaturated bond.

Repeat Units and Applications forSelected Elastomers

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(Continued)

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Properties of Selected Elastomers

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The structure of thestyrene-butadiene(SB) copolymer in athermoplasticelastomer. The glassynature of the styrenedomains provideselastic behaviorwithout cross-linkingof the butadiene.

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The structure of thestyrene-butadiene(SB) copolymer in athermoplasticelastomer. The glassynature of the styrenedomains provideselastic behaviorwithout cross-linkingof the butadiene.

Section Thermosetting Polymers

Phenolics Amines Urethanes Polyesters Epoxies Polyimides Interpenetrating Polymer Networks

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Phenolics Amines Urethanes Polyesters Epoxies Polyimides Interpenetrating Polymer Networks

Fractional Units and Applications forSelected Thermosets

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Structure of a phenolic. In (a) two phenol rings are joined by acondensation reaction through a formaldehyde molecule. Eventually,a linear chain forms. In (b), excess formaldehyde serves as thecross-linking agent, producing a network, thermosetting polymer.

Adhesives

Chemically Reactive Adhesives Evaporation or Diffusion Adhesives Hot-Melt Adhesives Pressure-Sensitive Adhesives Conductive Adhesives

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Chemically Reactive Adhesives Evaporation or Diffusion Adhesives Hot-Melt Adhesives Pressure-Sensitive Adhesives Conductive Adhesives

Additives for Plastics

Fillers Pigments Stabilizers Antistatic Agents Flame Retardants Plasticizers Reinforcements Catalysts

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Fillers Pigments Stabilizers Antistatic Agents Flame Retardants Plasticizers Reinforcements Catalysts

Polymer Processing and Recycling

Forming Processes for Thermoplastics: Extrusion Blow Molding Injection Molding Thermoforming

Forming Processes for Thermosetting polymers: Calendaring Spinning Compression Molding Transfer Molding

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Forming Processes for Thermoplastics: Extrusion Blow Molding Injection Molding Thermoforming

Forming Processes for Thermosetting polymers: Calendaring Spinning Compression Molding Transfer Molding

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Schematic of an extruder used for polymer processing. (Source:Adapted from Plastics: Materials and Processing, Second Edition, byA. Brent Strong, p. 382, Fig. 11-1. Copyright © 2000 Prentice Hall.Adapted with permission of Pearson Education, Inc., Upper SaddleRiver, NJ.)

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One technique by which polymer films (used in the manufacture of garbagebags, for example) can be produced. The film is extruded in the form of abag, which is separated by air pressure until the polymer cools. (Source:Adapted from Plastics: Materials and Processing, Second Edition, by A.Brent Strong, p. 397, Fig. 11-8. Copyright © 2000 Prentice Hall. Adaptedwith permission of Pearson Education, Inc., Upper Saddle River, NJ.)

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Typical forming processes for thermoplastic: (a) extrusion,(b) blow molding, (c) injection molding, (d) thermoforming,(e) calendaring, and (f) spinning.

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Typical forming processes for thermosetting polymers: (a)compression molding and (b) transfer molding.