C C C C C C C C C C C C C C C C C C CH 3 CH 3 H. Polyethylene (PE) Polypropylene (PP) Polyvinyl chloride (PVC) repeat unit. repeat unit.

Transcription

1 Poly many mer repeat unit repeat unit Polyethylene (PE) l repeat unit l l Polyvinyl chloride (PV) 3 Polypropylene (PP) Adapted from Fig. 14.2, allister 7e. 3 repeat unit 3 1

2 2

3 3

4 4

5 Molecular weight, M i : Mass of a mole of chains. Lower M higher M M w is more sensitive to higher molecular weights Adapted from Fig. 14.4, allister 7e. 5

6 ovalent chain configurations and strength: secondary bonding Linear B ranched ross-linked Network Adapted from Fig. 14.7, allister 7e. 6

7 7 Tacticity stereoregularity of chain R R R R R R R R R R R R

8 cis cis-isoprene (natural rubber) bulky groups on same side of chain trans trans-isoprene (gutta percha) bulky groups on opposite sides of chain 8

9 Adapted from Fig. 14.9, allister 7e. random alternating block A B graft 9

10 Adapted from Fig. 14.6, allister 7e. 10

11 Ex: polyethylene unit cell Adapted from Fig , allister 7e. rystals must contain the polymer chains in some way hain folded structure Adapted from Fig , allister 7e. 10 nm 11

12 Polymers rarely 100% crystalline Too difficult to get all those chains aligned % rystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. crystalline region amorphous region Adapted from Fig , allister 6e. (Fig is from.w. ayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.) 12

13 What factors affect T m and T g? Adapted from Fig , allister 7e. 13

14 i.e. stress-strain behavior of polymers σ FS of polymer ca. 10% that of metals elastic modulus less than metal Strains deformations > 1000% possible (for metals, maximum strain ca. 10% or less) Adapted from Fig. 15.1, allister 7e. 14

15 Near Failure Initial σ(mpa) x brittle failure onset of necking x unload/reload plastic failure ε fibrillar structure near failure aligned, crosslinked case networked case semicrystalline case amorphous regions crystalline regions slide crystalline regions align elongate Stress-strain curves adapted from Fig. 15.1, allister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, allister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice- all, Inc., 1974, pp ) 15

16 σ(mpa) x brittle failure plastic failure x elastomer ε x final: chains are straight, still cross-linked Stress-strain curves adapted from Fig. 15.1, allister 7e. Inset figures along elastomer curve (green) adapted from Fig , allister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) initial: amorphous chains are kinked, cross-linked. Deformation is reversible! ompare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers) 16

17 Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene T Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin mobile liquid crystalline solid viscous liquid allister, rubber Fig tough plastic partially crystalline solid Molecular weight T m T g Adapted from Fig , allister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) 17

18 Decreasing T increases E -- increases TS -- decreases %EL Increasing strain rate same effects as decreasing T. σ(mpa) Data for the semicrystalline polymer: PMMA (Plexiglas) ε 0.3 to 1.3 Adapted from Fig. 15.3, allister 7e. (Fig is from T.S. arswell and J.K. Nason, 'Effect of Environmental onditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.) 18

19 omposites: -- Multiphase material w/significant proportions of each phase. Matrix: -- The continuous phase -- Purpose is to: - transfer stress to other phases - protect phases from environment -- lassification: MM, M, PM metal ceramic polymer woven fibers 0.5 mm cross section view Dispersed phase: -- Purpose: enhance matrix properties. MM: increase σy, TS, creep resist. M: increase Kc PM: increase E, σy, TS, creep resist. -- lassification: Particle, fiber, structural 0.5 mm Reprinted with permission from D. ull and T.W. lyne, An Introduction to omposite Materials, 2nd ed., ambridge University Press, New York, 1996, Fig. 3.6, p

20 omposites Particle-reinforced Fiber-reinforced Structural Largeparticle Dispersionstrengthened ontinuous (aligned) Discontinuous (short) Laminates Sandwich panels Aligned Randomly oriented Adapted from Fig. 16.2, allister 7e. 20

21 Particle-reinforced Fiber-reinforced Structural Examples: - Spheroidite steel matrix: ferrite (α) (ductile) 60 µm particles: cementite ( Fe 3 ) (brittle) Adapted from Fig , allister 7e. (Fig is copyright United States Steel orporation, 1971.) - W/o cemented carbide matrix: cobalt (ductile) V m : particles: W (brittle, hard) Adapted from Fig. 16.4, allister 7e. (Fig is courtesy arboloy Systems, Department, General Electric ompany.) vol%! 600 µm - Automobile tires matrix: rubber (compliant) particles: (stiffer) Adapted from Fig. 16.5, allister 7e. (Fig is courtesy Goodyear Tire and Rubber ompany.) 0.75 µm 21

22 Particle-reinforced Fiber-reinforced Structural Elastic modulus, Ec, of composites: -- two approaches. Data: u matrix w/tungsten particles E(GPa) Adapted from Fig. 16.3, allister 7e. (Fig is from R.. Krock, ASTM Proc, Vol. 63, 1963.) (u) ( W) vol% tungsten Application to other properties: -- Electrical conductivity, σe: Replace E in equations with σe. -- Thermal conductivity, k: Replace E in equations with k. 22

23 Particle-reinforced Fiber-reinforced Structural Fiber Materials - Whiskers - Thin single crystals - large length to diameter ratio graphite, SiN, Si high crystal perfection extremely strong, strongest known very expensive Fibers polycrystalline or amorphous generally polymers or ceramics Ex: Al 2 O 3, Aramid, E-glass, Boron, UMWPE Wires Metal steel, Mo, W 23

24 Adapted from Fig. 16.8, allister 7e. 24

25 Particle-reinforced Fiber-reinforced Structural Discontinuous, random 2D fibers Example: arbon-arbon -- process: fiber/pitch, then burn out at up to 2500º. -- uses: disk brakes, gas turbine exhaust flaps, nose cones. (b) view onto plane fibers: very stiff very strong matrix: less stiff less strong Other variations: -- Discontinuous, random 3D -- Discontinuous, 1D (a) fibers lie in plane Adapted from F.L. Matthews and R.L. Rawlings, omposite Materials; Engineering and Science, Reprint ed., R Press, Boca Raton, FL, (a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p (ourtesy I.J. Davies) Reproduced with permission of R Press, Boca Raton, FL. 25

26 Particle-reinforced Fiber-reinforced Structural ritical fiber length for effective stiffening & strengthening: Ex: For fiberglass, fiber length > 15 mm needed Shorter, thicker fiber: fiber σ(x) length < 15 σfd τ c Longer, thinner fiber: σ fiber length > 15 τ σ(x) f d c Adapted from Fig. 16.7, allister 7e. Poorer fiber efficiency Better fiber efficiency 26

27 Particle-reinforced Fiber-reinforced Structural Estimate of Ec and TS for discontinuous fibers: -- valid when σ fiber length > 15 τ -- Elastic modulus in fiber direction: f d c Values from Table 16.3, allister 7e. (Source for Table 16.3 is. Krenchel, Fibre Reinforcement, openhagen: Akademisk Forlag, 1964.) -- TS in fiber direction: 27

28 Particle-reinforced Fiber-reinforced Structural Stacked and bonded fiber-reinforced sheets -- stacking sequence: e.g., 0º/90º -- benefit: balanced, in-plane stiffness Sandwich panels -- low density, honeycomb core -- benefit: small weight, large bending stiffness Adapted from Fig , allister 7e. face sheet adhesive layer honeycomb Adapted from Fig , allister 7e. (Fig is from Engineered Materials andbook, Vol. 1, omposites, ASM International, Materials Park, O, 1987.) 28

29 Ms: Increased toughness Force un-reinf particle-reinf fiber-reinf Bend displacement MMs: Increased creep resistance 10-4 ε 6061 Al ss (s -1 ) PMs: Increased E/ρ 10 3 ceramics E(GPa) 10 2 PMs 10 1 metal/ metal alloys.1 G=3E/8 polymers.01 K=E Density, ρ [mg/m 3 ] 6061 Al w/si whiskers σ(mpa) Adapted from T.G. Nieh, "reep rupture of a silicon-carbide reinforced aluminum composite", Metall. Trans. A Vol. 15(1), pp , Used with permission. 29