Presentation on theme: "CHAPTER 14/15: POLYMER STUDIES"— Presentation transcript:
1 CHAPTER 14/15: POLYMER STUDIES Issues:• What are the basic microstructural features of a polymer?• How are polymer properties affected by molecular weight?• How do polymeric materials accommodate the polymer chain?• What are the tensile properties of polymers and how are they affected by basic microstructural features?• Changing Polymer Properties: Hardening, anisotropy,and annealing in polymers.• How does the elevated temperature mechanical response of polymers compare to ceramics and metals?• What are the primary polymer processing methods?
2 Chapter 14 – Polymers What is a polymer? Poly mer many repeat unit C H Polyethylene (PE)ClCHPolyvinyl chloride (PVC)HPolypropylene (PP)CCH3Adapted from Fig. 14.2, Callister 7e.
3 Ancient Polymer History Originally many natural polymers were usedWood – RubberCotton – WoolLeather – SilkOldest known uses of “Modern Polymers”Rubber balls used by IncasNoah used pitch (a natural polymer) for the ark – as had all ancient mariners!
4 Polymer Composition Most polymers are hydrocarbons – i.e. made up of H and C(we also recognize Si-H ‘silicones’)Saturated hydrocarbonsEach carbon bonded to four other atomsCnH2n+2
6 Unsaturated Hydrocarbons Double & triple bonds relatively reactive – can form new bondsDouble bond – ethylene or ethene - CnH2n4-bonds, but only 3 atoms bound to C’sTriple bond – acetylene or ethyne - CnH2n-2
7 IsomerismIsomerismtwo compounds with same chemical formula can have quite different structuresEx: C8H18n-octane2-methyl-4-ethyl pentane (isooctane)
8 Chemistry of Polymers Free radical polymerization Initiator: example - benzoyl peroxide
9 Chemistry of Polymers Initiator: example - benzoyl peroxide Free radical polymerization (addition polymerization)Initiator: example - benzoyl peroxide
10 Condensation Polymerization Water is “Condensed out” during polymerization of NylonSome of the original monomer’s materials are shed (condensed out) during polymerization processProcess is conducted in the presence of a catalystWater, CO2 are commonly condensed out but other compounds can be emitted including HCN or other acids
11 Bulk or Commodity Polymers Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers
17 MOLECULAR WEIGHT • Molecular weight, Mi: Mass of a mole of chains. Lower Mhigher MSimple for small moleculesAll the same sizeNumber of grams/molePolymers – distribution of chain sizesMw is more sensitive to higher molecular weightsAdapted from Fig. 14.4, Callister 7e.
18 Molecular Weight Calculation Example: average mass of a class
19 Degree of Polymerization, n n = number of repeat units per chainni = 6Chain fractionmol. wt of repeat unit i
20 Molecular Structures • Covalent chain configurations and strength: BranchedCross-LinkedNetworkLinearsecondarybondingDirection of increasing strengthAdapted from Fig. 14.7, Callister 7e.
21 Polymers – Molecular Shape Conformation – Molecular orientation can be changed by rotation around the bondsnote: no bond breaking neededAdapted from Fig. 14.5, Callister 7e.
22 Polymers – Molecular Shape Configurations – to change must break bondsStereoisomerismmirror plane
23 Tacticity Tacticity – stereoregularity of chain isotactic – all R groups on same side of chainsyndiotactic – R groups alternate sidesatactic – R groups random
24 cis/trans Isomerism cis trans cis-isoprene (natural rubber) bulky groups on same side of chaintranstrans-isoprene (gutta percha)bulky groups on opposite sides of chain
25 Copolymers two or more monomers polymerized together Adapted from Fig. 14.9, Callister 7e.two or more monomers polymerized togetherrandom – A and B randomly vary in chainalternating – A and B alternate in polymer chainblock – large blocks of A alternate with large blocks of Bgraft – chains of B grafted on to A backboneA – B –randomalternatingblockgraft
26 Polymer Crystallinity Adapted from Fig , Callister 7e.Ex: polyethylene unit cellCrystals must contain the polymer chains in some wayChain folded structureAdapted from Fig , Callister 7e.
27 Polymer Crystallinity amorphousregionPolymers rarely exhibit 100% crystallineToo difficult to get all those chains alignedcrystallineregion• % Crystallinity: how much is crystalline.-- TS and E often increasewith % crystallinity.-- Annealing causescrystalline regionsto grow. % crystallinityincreases.Adapted from Fig , Callister 6e.(Fig is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)
28 Mechanical Properties i.e. stress-strain behavior of polymersbrittle polymerFS of polymer ca. 10% that of metalsplasticelastomerelastic modulus – less than metalAdapted from Fig. 15.1, Callister 7e.Strains – deformations > 1000% possible (for metals, maximum strain ca. 100% or less)
29 Tensile Response: Brittle & Plastic (MPa)fibrillar structurenearfailureInitialNear Failurexbrittle failureonset ofcrystallineregions alignneckingplastic failurexcrystallineregionsslideamorphousregionselongatealigned,cross-linkedcasenetworkedunload/reloadesemi-crystallinecaseStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
30 Predeformation by Drawing • Drawing…(ex: monofilament fishline)-- stretches the polymer prior to use-- aligns chains in the stretching direction• Results of drawing:-- increases the elastic modulus (E) in thestretching direction-- increases the tensile strength (TS) in the-- decreases ductility (%EL)• Annealing after drawing...-- decreases alignment-- reverses effects of drawing.• Comparable to cold working in metals!Adapted from Fig , Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
31 Tensile Response: Elastomer Case (MPa)final: chainsare straight,stillcross-linkedxbrittle failureStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig , Callister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)plastic failurexxelastomerinitial: amorphous chains arekinked, cross-linked.Deformationis reversible!e• Compare to responses of other polymers:-- brittle response (aligned, crosslinked & networked polymer)-- plastic response (semi-crystalline polymers)
32 Thermoplastics vs. Thermosets Callister,Fig. 16.9TMolecular weightTgTmmobileliquidviscousrubbertoughplasticpartiallycrystallinesolid• Thermoplastics:-- little crosslinking-- ductile-- soften w/heating-- polyethylenepolypropylenepolycarbonatepolystyrene• Thermosets:-- large crosslinking(10 to 50% of mers)-- hard and brittle-- do NOT soften w/heating-- vulcanized rubber, epoxies,polyester resin, phenolic resinAdapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)
33 T and Strain Rate: Thermoplastics (MPa)• Decreasing T...-- increases E-- increases TS-- decreases %EL• Increasingstrain rate...-- same effectsas decreasing T.20468Data for the4°Csemicrystallinepolymer: PMMA20°C(Plexiglas)40°Cto 1.360°Ce0.10.20.3Adapted from Fig. 15.3, Callister 7e. (Fig is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.)
34 Melting vs. Glass Transition Temp. What factors affect Tm and Tg?Both Tm and Tg increase with increasing chain stiffnessChain stiffness increased byBulky sidegroupsPolar groups or sidegroupsDouble bonds or aromatic chain groupsRegularity (tacticity) – affects Tm onlyAdapted from Fig , Callister 7e.
35 Time Dependent Deformation • Stress relaxation test:• Data: Large drop in Erfor T > Tg.(amorphouspolystyrene)Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)1031-1-3560100140180rigid solid(small relax)transitionregionT(°C)TgEr (10s)in MPaviscous liquid(large relax)-- strain to eo and hold.-- observe decrease instress with time.timestraintensile testeos(t)• Relaxation modulus:• Sample Tg(C) values:PE (low density)PE (high density)PVCPSPC- 110- 90+ 87+100+150Selected values from Table 15.2, Callister 7e.
36 Polymer Fracture Crazing Griffith cracks in metals – spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges formfibrillar bridgesmicrovoidscrackalligned chainsNote: crack spreads by breaking C-C bondsDuctile plastics have large craze zones that absorb large amounts of energy as it spreadsAdapted from Fig. 15.9, Callister 7e.
37 Polymer AdditivesImprove mechanical properties, processability, durability, etc.FillersAdded to improve tensile strength & abrasion resistance, toughness & decrease costex: carbon black, silica gel, wood flour, glass, limestone, talc, etc.PlasticizersAdded to reduce the glass transitiontemperature Tgcommonly added to PVC - otherwise it is brittlePolymers are almost never used as a pure materialMigration of plasticizers can be a problem
38 Polymer Additives Stabilizers Antioxidants UV protectants Lubricants Added to allow easier processing“slides” through dies easier – ex: Na stearateColorantsDyes or pigmentsFlame RetardantsCl/F & B
39 Processing of Plastics Thermoplastic –can be reversibly cooled & reheated, i.e. recycledheat till soft, shape as desired, then coolex: polyethylene, polypropylene, polystyrene, etc.Thermosetwhen heated forms a networkdegrades (not melts) when heatedmold the prepolymer then allow further reactionex: urethane, epoxyCan be brittle or flexible & linear, branching, etc.
40 Processing Plastics - Molding Compression and transfer moldingthermoplastic or thermosetMale & female parts to moldAdapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed.,John Wiley & Sons, )
41 Processing Plastics - Molding Injection moldingthermoplastic & some thermosetsAdapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition,John Wiley & Sons, )Polymer is melted in chamber & then the molten polymer is forced under pressure into the die
43 Processing Plastics – Extrusion An extruder is a device that used a large screw to melt a polymer, compress it, & force it into a moldExtremely commonly usedAdapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)
45 Polymer Types: Fibers Fibers - length/diameter >100 Textiles are main useMust have high tensile strengthUsually highly crystalline & highly polarFormed by spinningex: extrude polymer through a spinnerettePt plate with 1000’s of holes for nylonex: rayon – dissolved in solvent then pumped through die head to make fibersthe fibers are drawnleads to highly aligned chains- fibrillar structure
46 Polymer Types Coatings – thin film on surface – i.e. paint, varnish To protect itemImprove appearanceElectrical insulationAdhesives – produce bond between two adherandsUsually bonded by:Secondary bondsMechanical bondingFilms – blown film extrusionFoams – gas bubbles in plastic
47 Blown-Film ExtrusionAdapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)
48 Advanced Polymers Ultrahigh molecular weight polyethylene (UHMWPE) Molecular weight ca. 4 x 106 g/molExcellent properties for variety of applicationsbullet-proof vest, golf ball covers, hip joints, etc.UHMWPEAdapted from chapter-opening photograph, Chapter 22, Callister 7e.
49 The Stem, femoral head, and the AC socket are made from Cobalt-chrome metal alloy or ceramic, AC cup made from polyethylene
50 ABS – A Polymerized “Alloy” ABS, Acrylonitrile-Butadiene-StyreneMade up of the 3 materials: acrylonitrile, butadiene and styrene. The material is located under the group styrene plastic. Styrene plastics are in volume one of the most used plastics.PropertiesThe mechanical properties for ABS are good for impact resistance even in low temperatures. The material is stiff, and the properties are kept over a wide temperature range. The hardness and stiffness for ABS is lower than for PS and PVC.Weather and chemical resistanceThe weather resistance for ABS is restricted, but can be drastically improved by additives as black pigments. The chemical resistance for ABS is relatively good and it is not affected by water, non organic salts, acids and basic. The material will dissolve in aldehyde, ketone, ester and some chlorinated hydrocarbons.ProcessingABS can be processed by standard mechanical tools as used for machining of metals and wood. The cutting speed need to be high and the cutting tools has to be sharp. Cooling is recommended to avoid melting of the material. If the surface finish is of importance for the product, the ABS can be treated with varnish, chromium plated or doubled by a layer of acrylic or polyester. ABS can be glued to it self by use of a glue containing dissolvent. Polyurethane based or epoxy based glue can be used for gluing to other materials.
52 Summary • General drawbacks to polymers: -- E, sy, Kc, Tapplication are generally small.-- Deformation is often T and time dependent.-- Result: polymers benefit from composite reinforcement.• Thermoplastics (PE, PS, PP, PC):-- Smaller E, sy, Tapplication-- Larger Kc-- Easier to form and recycle• Elastomers (rubber):-- Large reversible strains!• Thermosets (epoxies, polyesters):-- Larger E, sy, Tapplication-- Smaller KcAnd Remember:Table 15.3 Callister 7eIs a Good overview of applications and trade names of polymers.