Chapter 15: Characteristics & Applications of Polymers
Chapter 15: Characteristics & Applications of Polymers ISSUES TO ADDRESS... What are the tensile properties of polymers and how are they affected by basic microstructural features ? Hardening, anisotropy, and annealing in polymers. How does the elevated temperature mechanical response of polymers compare to ceramics and metals? Chapter 15 - 1 Mechanical Properties of Polymers Stress-Strain Behavior brittle polymer plastic elastomer elastic moduli less than for metals Adapted from Fig. 15.1, Callister & Rethwisch 9e. Fracture strengths of polymers ~ 10% of those for metals Deformation strains for polymers > 1000% for most metals, deformation strains < 10% Chapter 15 - 2
Mechanisms of DeformationBrittle Crosslinked and Network Polymers Initial Near Failure (MPa)MPa) Initial x brittle failure Near Failure x plastic failure aligned, crosslinked polymer e network polymer Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 9e.
Chapter 15 - 3 Mechanisms of Deformation Semicrystalline (Plastic) Polymers fibrillar structure (MPa)MPa) Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 9e. Inset figures along plastic response curve adapted from Figs. 15.12 & 15.13, Callister & Rethwisch 9e. (MPa)From SCHULTZ, POLYMER MATERIALS SCIENCE, 1st Edition, 1974. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.)1974, pp 500-501.) x brittle failure onset of necking plastic failure x unload/reload
e undeformed structure near failure amorphous regions elongate crystalline block segments separate crystalline regions align Chapter 15 - 4 Predeformation by Drawing Drawing(MPa)ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction Results of drawing: -- increases the elastic modulus (MPa)E) in the stretching direction -- increases the tensile strength (MPa)TS) in the stretching direction Adapted from Fig. 15.13, Callister & Rethwisch 9e.
-- decreases ductility (MPa)%EL) (MPa)From Schultz, Polymer Materials Science, 1st Edition, 1974. Reprinted by permission Annealing after drawing... of Pearson Education, Inc., Upper Saddle River, NJ.)1974, pp 500-501.) -- decreases chain alignment -- reverses effects of drawing (MPa)reduces E and TS, enhances %EL) Contrast to effects of cold working in metals! Chapter 15 - 5 Mechanisms of Deformation Elastomers (MPa)MPa) x brittle failure x plastic failure elastomer x e initial: amorphous chains are
kinked, cross-linked. Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 9e. Inset figures along elastomer curve (MPa)green) adapted from Fig. 15.15, Callister & Rethwisch 9e. final: chains are straighter, still cross-linked (MPa)Fig. 15.15 adapted from Z. D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd edition. Copyright 1987 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) deformation is reversible (MPa)elastic)! Compare elastic behavior of elastomers with the: -- brittle behavior (MPa)of aligned, crosslinked & network polymers), and
-- plastic behavior (MPa)of semicrystalline polymers) (MPa)as shown on previous slides) Chapter 15 - 6 Thermoplastics vs. Thermosets Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene Thermosets: T viscous liquid mobile liquid crystalline solid Callister, rubber
Fig. 16.9 tough plastic Tm Tg partially crystalline solid Molecular weight Adapted from Fig. 15.19, Callister & Rethwisch 9e. (MPa)From F. W. Billmeyer, Jr., Textbook of Polymer Science, 3rd edition. -- significant crosslinking Copyright 1984 by John Wiley & Sons, New York. Reprinted by (MPa)10 to 50% of repeat units) permission of John Wiley & Sons, Inc.) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin 7 Chapter 15 - 7 Influence of T and Strain Rate on Thermoplastics Decreasing T... -- increases E
-- increases TS -- decreases %EL Increasing strain rate... -- same effects as decreasing T. (MPa)MPa) 80 4C 60 20C 40 Plots for semicrystalline PMMA (MPa)Plexiglas) 40C 20 0 60C 0 0.1
0.2 e to 1.3 0.3 Adapted from Fig. 15.3, Callister & Rethwisch 9e. (MPa)Reprinted with permission from T. S. Carswell and H. K. Nason, Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics, in Symposium on Plastics. Copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.) 8 Chapter 15 - 8 Melting & Glass Transition Temps. What factors affect Tm and Tg? Both Tm and Tg increase with increasing chain stiffness Chain stiffness increased by presence of 1. Bulky sidegroups 2. Polar groups or sidegroups
3. Chain double bonds and aromatic chain groups Regularity of repeat unit arrangements affects Tm only Adapted from Fig. 15.18, Callister & Rethwisch 9e. Chapter 15 - 9 Time-Dependent Deformation Stress relaxation test: -- strain in tension to e and hold. -- observe decrease in stress with time. tensile test eo There is a large decrease in Er for T 5 > Tg. 10 Er (MPa)10 s) 3 in MPa 10 rigid solid (MPa)small relax)
transition region 1 10 10-1 strain (MPa)t) time Relaxation modulus: viscous liquid 10-3 (MPa)large relax) (MPa)amorphous polystyrene) Fig. 15.7, Callister & Rethwisch 9e. (MPa)From A. V. Tobolsky, Properties and Structures of Polymers. Copyright 1960 by John Wiley & Sons, New York. Reprinted by permission
of John Wiley & Sons, Inc.) 60 100 140 180 T(MPa)C) Tg Representative Tg values (MPa)C): PE (MPa)low density) PE (MPa)high density) PVC PS PC - 110 - 90 + 87 +100 +150 Selected values from Table 15.2, Callister & Rethwisch 9e. Chapter 15 - 10 Crazing During Fracture of Thermoplastic Polymers Craze formation prior to cracking during crazing, plastic deformation of spherulites
and formation of microvoids and fibrillar bridges aligned chains fibrillar bridges microvoids crack Fig. 15.9, Callister & Rethwisch 9e. (MPa)From J. W. S. Hearle, Polymers and Their Properties, Vol. 1, Fundamentals of Structure and Mechanics, Ellis Horwood, Ltd., Chichester, West Sussex, England, 1982.) Chapter 15 - 11 Polymer Types Fibers Fibers - length/diameter >100 Primary use is in textiles. Fiber characteristics: high tensile strengths high degrees of crystallinity structures containing polar groups Formed by spinning extrude polymer through a spinneret (MPa)a die containing many small orifices) the spun fibers are drawn under tension leads to highly aligned chains - fibrillar structure
Chapter 15 - 12 Polymer Types Miscellaneous Coatings thin polymer films applied to surfaces i.e., paints, varnishes protects from corrosion/degradation decorative improves appearance can provide electrical insulation Adhesives bonds two solid materials (MPa)adherands) bonding types: 1. Secondary van der Waals forces 2. Mechanical penetration into pores/crevices Films produced by blown film extrusion Foams gas bubbles incorporated into plastic Chapter 15 - 13 Advanced Polymers Ultrahigh Molecular Weight Polyethylene (MPa)UHMWPE) Molecular weight ca. 4 x 106 g/mol
Outstanding properties high impact strength resistance to wear/abrasion low coefficient of friction self-lubricating surface UHMWPE Important applications bullet-proof vests golf ball covers hip implants (MPa)acetabular cup) Adapted from chapteropening photograph, Chapter 22, Callister 7e. Chapter 15 - 14 Advanced Polymers Thermoplastic Elastomers Styrene-butadiene block copolymer hard component domain
styrene butadiene Fig. 15.21(MPa)a), Callister & Rethwisch 9e. soft component domain Fig. 15.22, Callister & Rethwisch 9e. Chapter 15 - 15 Summary Limitations of polymers: -- E, y, Kc, Tapplication are generally small. -- Deformation is often time and temperature dependent. Thermoplastics (MPa)PE, PS, PP, PC): -- Smaller E, y, Tapplication -- Larger Kc Table 15.3 Callister & -- Easier to form and recycle Rethwisch 9e: Elastomers (MPa)rubber): -- Large reversible strains! Good overview Thermosets (MPa)epoxies, polyesters): of applications -- Larger E, y, Tapplication
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