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Presentation on theme: "CHAPTER 14/15: POLYMER STUDIES"— Presentation transcript:

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) Cl C H Polyvinyl chloride (PVC) H Polypropylene (PP) C CH3 Adapted from Fig. 14.2, Callister 7e.

3 Ancient Polymer History
Originally many natural polymers were used Wood – Rubber Cotton – Wool Leather – Silk Oldest known uses of “Modern Polymers” Rubber balls used by Incas Noah 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 hydrocarbons Each carbon bonded to four other atoms CnH2n+2


6 Unsaturated Hydrocarbons
Double & triple bonds relatively reactive – can form new bonds Double bond – ethylene or ethene - CnH2n 4-bonds, but only 3 atoms bound to C’s Triple bond – acetylene or ethyne - CnH2n-2

7 Isomerism Isomerism two compounds with same chemical formula can have quite different structures Ex: C8H18 n-octane 2-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 Nylon Some of the original monomer’s materials are shed (condensed out) during polymerization process Process is conducted in the presence of a catalyst Water, 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


13 NOTE: See Table 15.3 for commercially important polymers – including trade names




17 MOLECULAR WEIGHT • Molecular weight, Mi: Mass of a mole of chains.
Lower M higher M Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes Mw is more sensitive to higher molecular weights Adapted 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 chain ni = 6 Chain fraction mol. wt of repeat unit i

20 Molecular Structures • Covalent chain configurations and strength:
B ranched Cross-Linked Network Linear secondary bonding Direction of increasing strength Adapted from Fig. 14.7, Callister 7e.

21 Polymers – Molecular Shape
Conformation – Molecular orientation can be changed by rotation around the bonds note: no bond breaking needed Adapted from Fig. 14.5, Callister 7e.

22 Polymers – Molecular Shape
Configurations – to change must break bonds Stereoisomerism mirror plane

23 Tacticity Tacticity – stereoregularity of chain
isotactic – all R groups on same side of chain syndiotactic – R groups alternate sides atactic – R groups random

24 cis/trans Isomerism cis trans cis-isoprene (natural rubber)
bulky groups on same side of chain trans trans-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 together random – A and B randomly vary in chain alternating – A and B alternate in polymer chain block – large blocks of A alternate with large blocks of B graft – chains of B grafted on to A backbone A – B – random alternating block graft

26 Polymer Crystallinity
Adapted from Fig , Callister 7e. Ex: polyethylene unit cell Crystals must contain the polymer chains in some way Chain folded structure Adapted from Fig , Callister 7e.

27 Polymer Crystallinity
amorphous region Polymers rarely exhibit 100% crystalline Too difficult to get all those chains aligned crystalline region • % Crystallinity: how much is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. 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 polymers brittle polymer FS of polymer ca. 10% that of metals plastic elastomer elastic modulus – less than metal Adapted 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 structure near failure Initial Near Failure x brittle failure onset of crystalline regions align necking plastic failure x crystalline regions slide amorphous regions elongate aligned, cross- linked case networked unload/reload e semi- crystalline case Stress-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 the stretching 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: chains are straight, still cross-linked x brittle failure Stress-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 failure x x elastomer initial: amorphous chains are kinked, cross-linked. Deformation is 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. 16.9 T Molecular weight Tg Tm mobile liquid viscous rubber tough plastic partially crystalline solid • Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene • Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin Adapted 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 • Increasing strain rate... -- same effects as decreasing T. 20 4 6 8 Data for the 4°C semicrystalline polymer: PMMA 20°C (Plexiglas) 40°C to 1.3 60°C e 0.1 0.2 0.3 Adapted 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 stiffness Chain stiffness increased by Bulky sidegroups Polar groups or sidegroups Double bonds or aromatic chain groups Regularity (tacticity) – affects Tm only Adapted from Fig , Callister 7e.

35 Time Dependent Deformation
• Stress relaxation test: • Data: Large drop in Er for T > Tg. (amorphous polystyrene) Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) 10 3 1 -1 -3 5 60 100 140 180 rigid solid (small relax) transition region T(°C) Tg Er (10s) in MPa viscous liquid (large relax) -- strain to eo and hold. -- observe decrease in stress with time. time strain tensile test eo s(t) • Relaxation modulus: • Sample Tg(C) values: PE (low density) PE (high density) PVC PS PC - 110 - 90 + 87 +100 +150 Selected 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 form fibrillar bridges microvoids crack alligned chains Note: crack spreads by breaking C-C bonds Ductile plastics have large craze zones that absorb large amounts of energy as it spreads Adapted from Fig. 15.9, Callister 7e.

37 Polymer Additives Improve mechanical properties, processability, durability, etc. Fillers Added to improve tensile strength & abrasion resistance, toughness & decrease cost ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. Plasticizers Added to reduce the glass transition temperature Tg commonly added to PVC - otherwise it is brittle Polymers are almost never used as a pure material Migration 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 stearate Colorants Dyes or pigments Flame Retardants Cl/F & B

39 Processing of Plastics
Thermoplastic – can be reversibly cooled & reheated, i.e. recycled heat till soft, shape as desired, then cool ex: polyethylene, polypropylene, polystyrene, etc. Thermoset when heated forms a network degrades (not melts) when heated mold the prepolymer then allow further reaction ex: urethane, epoxy Can be brittle or flexible & linear, branching, etc.

40 Processing Plastics - Molding
Compression and transfer molding thermoplastic or thermoset Male & female parts to mold Adapted 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 molding thermoplastic & some thermosets Adapted 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 mold Extremely commonly used Adapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)

44 Polymer Types: Elastomers
Elastomers – rubber Crosslinked materials Natural rubber Synthetic rubber and thermoplastic elastomers SBR- styrene-butadiene rubber styrene butadiene – Silicone rubber

45 Polymer Types: Fibers Fibers - length/diameter >100
Textiles are main use Must have high tensile strength Usually highly crystalline & highly polar Formed by spinning ex: extrude polymer through a spinnerette Pt plate with 1000’s of holes for nylon ex: rayon – dissolved in solvent then pumped through die head to make fibers the fibers are drawn leads to highly aligned chains- fibrillar structure

46 Polymer Types Coatings – thin film on surface – i.e. paint, varnish
To protect item Improve appearance Electrical insulation Adhesives – produce bond between two adherands Usually bonded by: Secondary bonds Mechanical bonding Films – blown film extrusion Foams – gas bubbles in plastic

47 Blown-Film Extrusion Adapted 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/mol Excellent properties for variety of applications bullet-proof vest, golf ball covers, hip joints, etc. UHMWPE Adapted 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-Styrene Made 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. Properties The 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 resistance The 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. Processing ABS 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.

51 A Processing Movie: Calloway Golf:

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 Kc And Remember: Table 15.3 Callister 7e Is a Good overview of applications and trade names of polymers.

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