Polymer structures


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  • Polymer- can have various lengths depending on number of repeat units
  • Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers
  • Simple for small molecules
    All the same size
    Number of grams/mole
    Polymers – distribution of chain sizes
  • End of lecture 1
  • Polymer structures

    1. 1. POLYMER STRUCTURES Desiree Jane Enriquez Sarah Reyes Bernald Cabrera Nephtalie Cruz Arwie Mendoza 1
    2. 2. POLYMER 2
    3. 3. Polymer History Originally natural polymers were used in Wood Cotton Leather Rubber Wool Silk Synthetic Polymers Plastics Rubbers Fiber materials 3
    4. 4. Polymer Composition Most polymers are hydrocarbons – i.e. made up of H and C Saturated hydrocarbons ◦ Each carbon bonded to four other atoms H H C H H C H H CnH2n+2 4
    5. 5. 5
    6. 6. Unsaturated Hydrocarbons Double & triple bonds relatively reactive – can form new bonds  Double bond – ethylene or ethene - CnH2n H H C C H H H C C H 4-bonds, but only 3 atoms bound to C’s  Triple bond – acetylene or ethyne - CnH2n-2 6
    7. 7. Isomerism  Isomerism ◦ two compounds with same chemical formula can have quite different structures Ex: C8H18  n-octane H H H H H H H H H C C C C C C C C H = H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3 ⇓ H H H H H H H H H3C ( CH2 ) CH3 6  2-methyl-4-ethyl pentane (isooctane) CH3 H3C CH CH2 CH CH3 CH2 CH3 7
    8. 8. Chemistry of Polymers  Free radical polymerization H H C C + R R C C H H monomer (ethylene) free radical H H R C C H H initiation H H H H + H H H H H H C C R C C C C H H H H H H propagation dimer  Initiator: H example - benzoyl peroxide C O O C H H H H 2 C O =2R H 8
    9. 9. Chemistry of Polymers Note: polyethylene is just a long HC - paraffin is short polyethylene 9
    10. 10. Bulk or Commodity Polymers 10
    11. 11. 11
    12. 12. 12
    13. 13. MOLECULAR WEIGHT low M high M Not all chains in a polymer are of the same length 13
    14. 14. Molecular weight ◦ The properties of a polymer depend on its length ◦ synthesis yields polymer distribution of lengths ◦ Define “average” molecular weight ◦ Two approaches are typically taken  Number average molecular weight (Mn)  Weight-average molecular weight (Mw)
    15. 15. Molecular mass example
    16. 16. Degree of Polymerization, n n = number of repeat units per chain H H H H H H H H H H H H H C C (C C ) C C C C C C C C H ni = 6 H H H H H H H H H H H H Mn nn = ∑ xi ni = m Mw nw = ∑ wi ni = m where m = average molecular weight of repeat unit m = Σf i mi Chain fraction mol. wt of repeat unit i 16
    17. 17. End to End Distance, r 17
    18. 18. Molecular Structures • Covalent chain configurations and strength: secondary bonding Linear Branched Cross-Linked Network Direction of increasing strength 18
    19. 19. Polymers – Molecular Shape Conformation – Molecular orientation can be changed by rotation around the bonds ◦ note: no bond breaking needed 19
    20. 20. Polymers – Molecular Shape Configurations – to change must break bonds Stereoisomerism H H H H C C H H R or C C R C C H R H H A A C B E C E D D B mirror plane 20
    21. 21. Tacticity Tacticity – stereoregularity of chain isotactic – all R groups on same side of chain H H H H H H H H C C C C C C C C H R H R H R H R H H H R H H H R syndiotactic – R groups alternate sides C C C C C C C C H R H H H R H H H H H H H R H H atactic – R groups random C C C C C C C C H R H R H H H R 21
    22. 22. cis/trans Isomerism CH3 H C C CH2 CH2 CH3 C C CH2 CH2 H cis trans cis-isoprene (natural rubber) trans-isoprene (gutta percha) bulky groups on same side of chain bulky groups on opposite sides of chain 22
    23. 23. Copolymers 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– random alternating block B– graft 23
    24. 24. Polymer Crystallinity Ex: polyethylene unit cell Crystals must contain the polymer chains in some way ◦ Chain folded structure 10 nm 24
    25. 25. Polymer Crystallinity Polymers rarely 100% crystalline  Too difficult to get all those chains aligned • % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. 25
    26. 26. Polymer Crystal Forms  Single crystals – only if slow careful growth 26
    27. 27. Polymer Crystal Forms  Spherulites – fast growth – forms lamellar (layered) structures 27
    28. 28. Spherulites – crossed polarizers Maltese cross 28