POLYMER STRUCTURES ISSUES TO ADDRESS...

Transcription

1 POLYMER STRUTURES ISSUES TO ADDRESS... What are the basic microstructural features? ow are polymer properties effected by molecular weight? ow do polymeric crystals accommodate the polymer chain? Melting temperature vs. glass transition Effects of temperature on deformation & flow hapter 14-1

2 What is a polymer? What is a polymer? Poly many mer repeat unit - May be 100 s or 1000 s of repeat units long - Strong bonds within a molecule, weak interactions between molecules 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 hapter 14-2

3 Polymer omposition Most polymers are hydrocarbons i.e. made up of and Saturated hydrocarbons Each carbon bonded to four other atoms an easily rotate around bonds n 2n+2 hapter 14-4

4 hapter 14-5

5 Unsaturated ydrocarbons Double & triple bonds relatively reactive can form new bonds Double bond ethylene or ethene - n 2n 4-bonds, but only 3 atoms bound to s Double bonds less flexible (can t rotate around bond) Tend to be planar Triple bond acetylene or ethyne - n 2n-2 hapter 14-6

6 Aromatic compounds Simplest form: 6 6 benzene ring Mixed single/double bonds Planar, bulky Phenyl groups: R = free radical reactive group (, O, 6 5,.) Example: biphenyl hapter 14-7

7 hemistry of Polymers Free radical polymerization R + free radical R monomer (ethylene) R + R Initiator: example - benzoyl peroxide dimer initiation propagation O O 2 O = 2 R hapter 14-8

8 hemistry of Polymers Adapted from Fig. 14.1, allister 7e. Note: polyethylene is just a long - paraffin is short polyethylene hapter 14-9

9 Bulk or ommodity Polymers Most common plastic Teflon hapter 14-10

10 Plexiglas hapter 14-11

11 hapter 14-12

12 MOLEULAR WEIGT Molecular weight, M i : Mass of a mole of chains. Lower M higher M M n total wt of polymer total # of molecules M M n w x M w M i i i i M w is more sensitive to higher molecular weights Adapted from Fig. 14.4, allister 7e. hapter 14-13

13 Degree of Polymerization, n n = number of repeat units per chain ( ) n i = 6 n n x i n i M n m n w w n i i M w m where m average molecularweight of repeat unit m f i m i hain fraction mol. wt of repeat unit i hapter 14-15

14 Polymers Molecular Shape onformation Molecular orientation can be changed by rotation around the bonds note: no bond breaking needed Depends on saturation of bonds, size of side chains Adapted from Fig. 14.5, allister 7e. Adapted from Fig. 14.6, allister 7e. hapter 14-17

15 Molecular Structures ovalent chain configurations and strength: secondary bonding Linear B ranched ross-linked Network Direction of increasing strength Adapted from Fig. 14.7, allister 7e. hapter 14-18

16 Isomerism Isomerism two compounds with same chemical formula can have quite different structures Ex: 8 18 n-octane = methyl-4-ethyl pentane (isooctane) 3 ( 2 ) hapter 14-19

17 Polymers Molecular Shape onfigurations to change must break bonds Stereoisomerism R R R or A A B D E E D B mirror plane hapter 14-20

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

19 cis/trans Isomerism cis cis-isoprene (natural rubber) bulky groups on same side of chain trans trans-isoprene (gutta percha) bulky groups on opposite sides of chain hapter 14-22

20 opolymers Adapted from Fig. 14.9, allister 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 hapter 14-23

21 Mechanical Properties i.e. stress-strain behavior of polymers brittle polymer FS of polymer ca. 10% that of metals elastic modulus less than metal plastic elastomer Strains deformations > 1000% possible (for metals, maximum strain ca. 10% or less) Adapted from Fig. 15.1, allister 7e. hapter 14-24

22 Tensile Response: Brittle & Plastic Near Failure Initial (MPa) x brittle failure onset of necking x unload/reload plastic failure e fibrillar structure near failure aligned, crosslinked case networked case semicrystalline case amorphous regions elongate crystalline regions align crystalline regions slide 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 ) hapter 14-25

23 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 alignment -- reverses effects of drawing. ompare to cold working in metals! Adapted from Fig , allister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-all, Inc., 1974, pp ) hapter 14-26

24 Tensile Response: Elastomer ase (MPa) x initial: amorphous chains are kinked, cross-linked. brittle failure plastic failure x elastomer Deformation is reversible! ompare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers) e 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.) hapter 14-27

25 T and Strain Rate: Thermoplastics Decreasing T increases E -- increases TS -- decreases %EL Increasing strain rate same effects as decreasing T. (MPa) Data for the semicrystalline polymer: PMMA (Plexiglas) 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.) e hapter 14-28

26 Ex: polyethylene unit cell Polymer rystallinity Adapted from Fig , allister 7e. rystals must contain the polymer chains in some way hain folded structure Adapted from Fig , allister 7e. 10 nm hapter 14-29

27 Polymer rystallinity Polymers rarely 100% crystalline Too difficult to get all those chains aligned % rystallinity: % of material that is crystalline. -- tensile strength and Young s modulus 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.) hapter 14-30

28 Polymer rystal Forms Single crystals only if slow careful growth Adapted from Fig , allister 7e. hapter 14-31

29 Polymer rystal Forms Spherulites fast growth forms lamellar (layered) structures Spherulite surface Nucleation site Adapted from Fig , allister 7e. hapter 14-32

30 Spherulites crossed polarizers Maltese cross Adapted from Fig , allister 7e. hapter 14-33

31 Melting vs. Glass Transition Temp. What factors affect T m and T g? Both T m and T g increase with increasing chain stiffness hain stiffness increased by 1. Bulky sidegroups 2. Polar groups or sidegroups 3. Double bonds or aromatic chain groups Regularity affects T m only Adapted from Fig , allister 7e. hapter 14-34

32 ontrolling T m, T g T T m, T g tend to be correlated: - omopolymers: T g ~ T m mobile liquid viscous liquid allister, rubber Fig tough plastic T m T g crystalline solid partially crystalline solid Molecular weight Adapted from Fig , allister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) hapter 14-35

33 Time Dependent Deformation Stress relaxation test: -- strain to eo and hold. -- observe decrease in stress with time. e o tensile test Relaxation modulus: E r ( t) ( t) e o strain (t) time Data: Large drop in E r for T > T g E r (10s) in MPa rigid solid (small relax) Sample T g ( ) values: PE (low density) PE (high density) PV PS P transition region viscous liquid (large relax) T( ) T g (amorphous polystyrene) Adapted from Fig. 15.7, allister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) Selected values from Table 15.2, allister 7e. hapter 14-36

34 Time dependent deformation (cont.) Amorphous polystyrene hapter 14-37

35 Properties depend on % crystallinity hapter 14-38

36 an control crystallinity through heat processing eat processing of polypropylene: hapter 14-39

37 ANNOUNEMENTS Reading: ore Problems: Self-help Problems: hapter 14-40