X Ray Diffraction Diffraction gratings must have spacings comparable to the wavelength of diffracted radiation Can t resolve spacings Spacing is the distance between parallel planes of atoms 1 X Rays to Determine Crystal Structure Incoming X rays diffract from crystal planes extra distance travelled by wave 2 q q d Measurement of critical angle qc allows computation of planar spacing d reflections must be in phase for a detectable signal spacing between planes X ray intensity from detector n d 2 sin qc q qc 2 X Ray Diffraction Pattern z z z c c c Intensity relative 110 a x b y a x b y a x 211 y b 200 Diffraction angle 2q Diffraction pattern for polycrystalline a iron BCC 3 Chapter 14 Polymers What is a polymer Poly many mer repeat unit repeat unit repeat unit H H H H H H C C C C C C H H H H H H H H H H H H C C C C C C H Cl H Cl H Cl Polyethylene PE Polyvinyl chloride PVC repeat unit H C H H H C C CH3 H H H C C CH3 H H C CH3 Polypropylene PP 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 CnH2n 2 H C H H 5 6 Unsaturated Hydrocarbons Double triple bonds relatively reactive can form new bonds Double bond ethylene or ethene CnH2n H H C C H H 4 bonds but only 3 atoms bound to C s Triple bond acetylene or ethyne CnH2n 2 H C C H 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 2 methyl 4 ethyl pentane isooctane H3C CH2 6CH3 CH3 H3C CH CH2 CH CH3 CH2 CH3 8 Chemistry of Polymers Free radical polymerization R H H C C R C C H H monomer ethylene free radical H H R C C H H H H initiation H H H H H H H H C C R C C C C H H H H H H propagation dimer Initiator example benzoyl peroxide H C O O C H H H H 2 C O 2R H 9 Chemistry of Polymers Note polyethylene is just a long HC paraffin is short polyethylene 10 Bulk or Commodity Polymers 11 12 13 MOLECULAR WEIGHT Molecular weight Mi Mass of a mole of chains Lower M higher M total wt of polymer Mn total of molecules M n x i M i M w w i M i Mw is more sensitive to higher molecular weights 14 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 w i ni m where m average molecular weight of repeat unit m fi mi Chain fraction mol wt of repeat unit i 15 Molecular Structures Covalent chain configurations and strength secondary bonding Linear Branched Cross Linked Network Direction of increasing strength 16 Polymers Molecular Shape Conformation Molecular orientation can be changed by rotation around the bonds note no bond breaking needed 17 Polymers Molecular Shape Configurations to change must break bonds Stereoisomerism H H H H H C C H or C C R C C H R B R H H A A C C E E D D B mirror plane 18 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 syndiotactic R groups alternate sides H H H R H H H R 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 19 cis trans Isomerism CH3 H CH3 C C CH2 CH2 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 20 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 21 Polymer Crystallinity Ex polyethylene unit cell Crystals must contain the polymer chains in some way Chain folded structure 10 nm 22 Polymer Crystallinity Polymers rarely 100 crystalline Too difficult to get all those chains aligned crystalline region Crystallinity of material that is crystalline TS and E often increase with crystallinity Annealing causes crystalline regions to grow crystallinity increases amorphous region 23
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