Materials of Engineering ENGR 151 CHARACTERISTICS, APPLICATIONS AND PROCESSING OF POLYMERS

Transcription

1 Materials of Engineering ENGR 151 CHARACTERISTICS, APPLICATIONS AND PROCESSING OF POLYMERS

2 POLYMER CRYSTALLINITY Crystalline regions thin platelets with chain folds at faces Chain folded structure Fig , Callister & Rethwisch 9e. 10 nm

3 POLYMER CRYSTALLINITY (CONT.) Polymers rarely 100% crystalline Difficult for all regions of all chains to become aligned crystalline region Degree of crystallinity expressed as % crystallinity. -- Some physical properties depend on % crystallinity. -- Heat treating causes crystalline regions to grow and % crystallinity to increase. amorphous region Fig , Callister 6e. (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.)

4 POLYMER SINGLE CRYSTALS Electron micrograph multilayered single crystals (chain-folded layers) of polyethylene Single crystals only for slow and carefully controlled growth rates Fig , Callister & Rethwisch 9e. [From A. Keller, R. H. Doremus, B. W. Roberts, and D. Turnbull (Eds.), Growth and Perfection of Crystals. General Electric Company and John Wiley & Sons, Inc., 1958, p Reprinted with permission of John Wiley & Sons, Inc.] 1 μm

5 CHAPTER 15: CHARACTERISTICS, APPLICATIONS & PROCESSING 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? What are the primary polymer processing methods? 5

6 MECHANICAL PROPERTIES OF POLYMERS STRESS-STRAIN BEHAVIOR brittle polymer plastic elastomer highly elastic polymer 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% 6

7 MECHANICAL PROPERTIES OF POLYMERS STRESS-STRAIN BEHAVIOR Brittle polymer fractures with little strain. Plastic behavior similar to metals Initial elastic deformation followed by yielding and plastic deformation Elastomer rubberlike elasticity 7

8 MECHANICAL PROPERTIES OF POLYMERS TENSILE AND YIELD STRENGTHS 8

9 MECHANISMS OF DEFORMATION BRITTLE CROSSLINKED AND NETWORK POLYMERS Initial Near Failure σ(mpa) x brittle failure Initial Near Failure x plastic failure aligned, crosslinked polymer Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 9e. e network polymer 9

10 MECHANISMS OF DEFORMATION SEMICRYSTALLINE (PLASTIC) POLYMERS Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 9e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister & Rethwisch 9e. (From SCHULTZ, POLYMER MATERIALS SCIENCE, 1st Edition, Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.)1974, pp ) σ(mpa) x brittle failure onset of necking x unload/reload plastic failure fibrillar structure near failure undeformed structure amorphous regions elongate e crystalline regions align crystalline block segments separate 10

11 MECHANISMS OF DEFORMATION SEMICRYSTALLINE (PLASTIC) POLYMERS 11

12 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 stretching direction -- decreases ductility (%EL) Annealing after drawing decreases chain alignment -- reverses effects of drawing (reduces E and TS, enhances %EL) Adapted from Fig , Callister & Rethwisch 9e. (From Schultz, Polymer Materials Science, 1st Edition, Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.)1974, pp ) 12

13 MECHANISMS OF DEFORMATION ELASTOMERS σ(mpa) x initial: amorphous chains are kinked, cross-linked. brittle failure e plastic failure x elastomer x deformation is reversible (elastic)! final: chains are straighter, still cross-linked Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 9e. Inset figures along elastomer curve (green) adapted from Fig , Callister & Rethwisch 9e. (Fig 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.) 13

14 THERMOPLASTICS AND THERMOSETS Thermoplastic can be reversibly cooled & reheated, i.e. recycled heat until soft, shape as desired, then cool ex: polyethylene, polypropylene, polystyrene. Thermoset when heated forms a molecular network (chemical reaction) degrades (doesn t melt) when heated a prepolymer molded into desired shape, then chemical reaction occurs ex: urethane, epoxy

15 THERMOPLASTICS VS. THERMOSETS Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene Thermosets: -- significant crosslinking (10 to 50% of repeat units) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin T mobile liquid crystalline solid viscous liquid Callister, rubber Fig tough plastic partially crystalline solid Molecular weight Adapted from Fig , Callister & Rethwisch 9e. (From F. W. Billmeyer, Jr., Textbook of Polymer Science, 3rd edition. Copyright 1984 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) 15

16 INFLUENCE OF T AND STRAIN RATE ON THERMOPLASTICS Decreasing T increases E -- increases TS -- decreases %EL Increasing strain rate same effects as decreasing T. σ(mpa) 80 4ºC ºC 40ºC Plots for semicrystalline PMMA (Plexiglas) 60ºC e to 1.3 Adapted from Fig. 15.3, Callister & Rethwisch 9e. (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 ) 16

17 MELTING & GLASS TRANSITION TEMPS. What factors affect T m and T g? Both T m and T g 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 T m only Adapted from Fig , Callister & Rethwisch 9e. 17

18 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. (From J. W. S. Hearle, Polymers and Their Properties, Vol. 1, Fundamentals of Structure and Mechanics, Ellis Horwood, Ltd., Chichester, West Sussex, England, 1982.) 18