Chapter 15: Characteristics, Applications & Processing of Polymers (1)

Transcription

1 Chapter 15: Characteristics, Applications & Processing of Polymers (1) 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

2 Mechanical Properties of Polymers Stress-Strain Behaviors brittle polymer Plastic polymer Elastomer/rubber elastic moduli less than for metals Adapted from Fig. 15.1, Callister & Rethwisch 8e. Strength of polymers ~ 10% of those for metals Deformation strains for polymers > 10 times of those for metals for most metals, deformation strains < ~0.3 Chapter 15-2

3 Tensile Strength & Yield Strength for Polymers fig_15_02 Chapter 15 -

4 Influence of T and Strain Rate on Thermoplastics Mechanical Property Decreasing T: -- increases E -- increases TS -- decreases %EL Increasing strain rate: -- same effects as decreasing T. s (MPa) ºC 20ºC 40ºC Plots for semicrystalline PMMA (Plexiglas) 60ºC to 1.3 Adapted from Fig. 15.3, Callister & Rethwisch 8e. (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.) e Chapter 15-4

5 Change in Shape for Polymers during Tensile Test Different from typical plastic deformation for metals where necking concentrates locally, polymer necking section elongate significantly fig_15_04 Chapter 15 -

6 Mechanisms of Deformation for Brittle, Crosslinked or Network Polymers Initial Near Failure s (MPa) x brittle failure Initial Near Failure x plastic failure aligned, crosslinked polymer Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. e network polymer Chapter 15-6

7 Mechanisms of Deformation for Semicrystalline Polymers Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister & Rethwisch 8e. (15.12 & are from J.M. Schultz, Polymer Materials Science, Prentice- Hall, Inc., 1974, pp ) s (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 Chapter 15-7

8 Elastic Deformation in Semicrystalline Polymers fig_15_12 Chapter 15 -

9 Plastic Deformation in Semicrystalline Polymers fig_15_13 Chapter 15 -

10 Predeformation by Drawing Drawing (e.g.: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction Results of (pre-) drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL) Pre-deformation for polymer has similar strengthening effects as cold working for metals Annealing after drawing decreases chain alignment -- reverses effects of drawing (reduces E and TS, enhances %EL) Annealing for polymers has similar effect of annealing for metals Adapted from Fig , Callister & Rethwisch 8e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp ) Chapter 15-10

11 Mechanisms of Deformation of Elastomers/Rubbers s (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 8e. Inset figures along elastomer curve (green) adapted from Fig , Callister & Rethwisch 8e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) Chapter 15-11

12 Relaxation & Creep for Semicrystalline Polymers Relaxation: Creep: -- strain in tension to e o and hold. -- observe decrease in stress with time. -- stress in tension to σ o and hold. -- observe increase in strain with time. e o Relaxation modulus: modulus change (decreases) with time E tensile test r ( t) s( t) e o strain σ(t) time or σ o s t) e ( t) 0 E r ( tensile test e (t) stress time Chapter 15-12

13 Strain vs. Time Behaviors Pure Elastic (Metal/Ceramics) Viscoelastic (typical plastics above glass transition Tg) Viscous (honey, pitch) fig_15_05 Chapter 15 -

14 (Relaxation) Modulus vs. Temperature Rigid and brittle (as glass) Viscoelastic (as typical plastics) Elastomer (as rubber) fig_15_07 Viscous liquid (polymer melts) Chapter 15 -

15 Fatigue in Polymer fig_15_11 Chapter 15 -

16 Fracture of Polymers (1) Craze formation prior to cracking during crazing, plastic deformation of spherulites and formation of microvoids and fibrillar bridges aligned chains fibrillar bridges microvoids crack Adapted from Fig. 15.9, Callister & Rethwisch 8e. Chapter 15-16

17 Fracture of Polymers (2) fig_15_10 Chapter 15 -

18 Melting & Glass Transition Temps. T m Melting Temperature: For crystalline material the temperature at which phase transition between (free flowing) liquid and crystalline solid happen T g Glass transition temperature For glassy material, the temperature at which the highly viscous liquid transforms to rigid amorphous solid, accompanied by sudden change in other physical/chemical properties Tm > Tg 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 side-groups 2. Polar groups or side-groups 3. Chain double bonds and aromatic chain groups Adapted from Fig , Callister & Rethwisch 8e. Chapter 15-18

19 Examples for Polymers with Different Tg and Tm Rubbers for auto tires: Tg of about -50 o C High performance nylon for under-thehood applications Tm about 250 o C Tg about 50 o C Chapter 15-19

20 Thermoplastics vs. Thermosets Thermoplastics: -- little crosslinking -- ductile -- soften with heating easy reshaping and recycle -- polyethylene polypropylene polycarbonate polystyrene Thermosets: -- significant crosslinking (10 to 50% of repeat units) -- hard -- do NOT soften with heating usually not recycled -- vulcanized rubber, epoxies, polyester resin, phenolic resin Chapter 15-20

21 Summary Polymers mechanical properties: -- E, sy, Kc, Tapplication are generally small. -- Deformation is often time and temperature dependent. 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 Table 15.3 Callister & Rethwisch 8e: Good overview of applications and trade names of polymers. Chapter 15-21