Chapter 5: Structures of Polymers

Transcription

1 hapter 5: Structures of Polymers ISSUES TO ADDRESS... What are the general structural and chemical characteristics of polymer molecules? What are some of the common polymeric materials, and how do they differ chemically? ow is the crystalline state in polymers different from that in metals and ceramics? AMSE 205 Spring 2016 hapter 5-1

2 What is a Polymer? Poly many mer repeat unit repeat unit Polyethylene (PE) l repeat unit l l Poly(vinyl chloride) (PV) repeat unit Polypropylene (PP) Adapted from Fig. 5.2, allister & Rethwisch 9e. AMSE 205 Spring 2016 hapter 5-2

3 Ancient Polymers Originally natural polymers were used Wood Rubber otton Wool Leather Silk Oldest known uses Rubber balls used by Incas Noah used pitch (a natural polymer) for the ark AMSE 205 Spring 2016 hapter 5-3

4 Polymer omposition Most polymers are hydrocarbons i.e., made up of and Saturated hydrocarbons Each carbon singly bonded to four other atoms Example: Ethane, 2 6 AMSE 205 Spring 2016 hapter 5-4

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6 Unsaturated ydrocarbons Double & triple bonds somewhat unstable can form new bonds Double bond found in ethylene or ethene Triple bond found in acetylene or ethyne AMSE 205 Spring 2016 hapter 5-6

7 Isomerism Isomerism two compounds with same chemical formula can have quite different structures for example: 8 18 normal-octane = ,4-dimethylhexane 3 ( 2 ) AMSE 205 Spring 2016 hapter 5-7

8 Polymerization and Polymer hemistry Free radical polymerization R + free radical R monomer (ethylene) R + R Initiator: example - benzoyl peroxide dimer initiation propagation O O 2 O = 2 R AMSE 205 Spring 2016 hapter 5-8

9 hemistry and Structure of Polyethylene Adapted from Fig. 5.1, allister & Rethwisch 9e. Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene AMSE 205 Spring 2016 hapter 5-9

10 Bulk or ommodity Polymers AMSE 205 Spring 2016 hapter 5-10

11 Bulk or ommodity Polymers (cont) AMSE 205 Spring 2016 hapter 5-11

12 Bulk or ommodity Polymers (cont) AMSE 205 Spring 2016 hapter 5-12

13 MOLEULAR WEIGT Molecular weight, M: Mass of a mole of chains. Low M high M Not all chains in a polymer are of the same length i.e., there is a distribution of molecular weights AMSE 205 Spring 2016 hapter 5-13

14 MOLEULAR WEIGT DISTRIBUTION Fig. 5.4, allister & Rethwisch 9e. M i x i w i = mean (middle) molecular weight of size range i = number fraction of chains in size range i = weight fraction of chains in size range i AMSE 205 Spring 2016 hapter 5-14

15 Molecular Weight alculation Example: average mass of a class # of Weight Students mass (lb) What is the average weight of the students in this class: a) Based on the number fraction of students in each mass range? b) Based on the weight fraction of students in each mass range? AMSE 205 Spring 2016 hapter 5-15

16 Molecular Weight alculation (cont.) alculate the number fractions and weight fractions of students in each weight as follows: total number = 20 total weight = 3,700 lb For example: for the 180 lb students x 180 w x AMSE 205 Spring 2016 hapter 5-16

17 Molecular Weight alculation (cont.) # of Students Weight mass (lb) Number Fractions (x i ) Weight Fractions (w i ) x i M i w i M i /20=0.05 (104x1)/3,700= x104= x104= /20=0.05 (116x1)/3,700= x116= x116= /20=0.10 (140x2)/3,700= x140= x140= /20=0.05 (143x1)/3,700= x143= x143= /20=0.20 (180x4)/3,700= x180= x180= /20=0.25 (182x5)/3,700= x182= x182= /20=0.10 (191x2)/3,700= x191= x191= /20=0.10 (220x2)/3,700= x220= x220= /20=0.05 (225x1)/3,700= x225= x225= /20=0.05 (380x1)/3,700= x380= x380=39.14 Total # 20 Total weight M n xim M i w wi M 3,700 lb = 185 lb = 201 lb i M n x i M i 185 lb M w wi M i 201lb AMSE 205 Spring 2016 hapter 5-17

18 Degree of Polymerization, DP DP = average number of repeat units per chain ( ) DP = 6 hain fraction mol. wt of repeat unit i 18 AMSE 205 Spring 2016 hapter 5 -

19 Molecular Structures for Polymers secondary bonding Linear Branched ross-linked Network Adapted from Fig. 5.7, allister & Rethwisch 9e. AMSE 205 Spring 2016 hapter 5-19

20 Polymers Molecular Shape onformation chain bending and twisting are possible by rotation of carbon atoms around their chain bonds, conformation encompasses portions of a molecule which are not directly linked to the same atom note: not necessary to break chain bonds to alter molecular shape Adapted from Fig. 5.5, allister & Rethwisch 9e. Thermal energy at room temperature is sufficient to rotate some simple covalent bonds. AMSE 205 Spring 2016 hapter 5-20

21 Molecular onfigurations for Polymers onfigurations arrangements of units along the axis of the chain. Atom positions are not alterable except by breaking and re-forming primary bonds. This costs a lot of energy!! R R R R R *R: atom or side group other than (l, 3 etc.) AMSE 205 Spring 2016 hapter 5-21

22 Isomerism Different atomic configurations are possible for polymers with the same composition Stereoisomerism: Atoms are linked together in the spatial arrangement in the same order but differ in their spatial arrangement. -R groups* are situated on the same side of the chain (isotactic configuration) -R groups alternate sides of the chain (syndiotactic configuration) -R groups randomly position (atactic configuration) Geometrical Isomerism: Repeat units have a carbon double bond. A side group is bonded to each of the carbon atoms participating in the double bond, which may be situated on one side of the chain (cis) or its opposite (trans). AMSE 205 Spring 2016 hapter 5-22

23 hapter 5 - AMSE 205 Spring Stereoisomerism Tacticity stereoregularity or spatial arrangement of R units along chain isotactic all R groups on same side of chain syndiotactic R groups alternate sides R R R R R R R R R R R R R R R R

24 Tacticity (cont.) atactic R groups randomly positioned R R R R R AMSE 205 Spring 2016 hapter 5-24

25 Geometrical (cis/trans) Isomerism cis cis-isoprene (natural rubber) atom and 3 group on same side of chain trans 2 trans-isoprene (gutta percha) atom and 3 group on opposite sides of chain AMSE 205 Spring 2016 hapter 5-25

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27 opolymers Fig. 5.9, allister & Rethwisch 9e. two or more monomers polymerized together random A and B randomly positioned along chain alternating A and B alternate in polymer chain block large blocks of A units alternate with large blocks of B units graft chains of B units grafted onto A backbone A B random alternating block graft AMSE 205 Spring 2016 hapter 5-27

28 Polymer rystals rystalline regions thin platelets with chain folds at faces hain folded structure Fig. 5.11, allister & Rethwisch 9e. 10 nm AMSE 205 Spring 2016 hapter 5-28

29 Polymer rystals (cont.) Polymers rarely 100% crystalline Difficult for all regions of all chains to become aligned crystalline region Degree of crystallinity expressed as % crystallinity. -- Some physical properties depend on % crystallinity. -- eat treating causes crystalline regions to grow and % crystallinity to increase. amorphous region Fig , allister 6e. (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.) AMSE 205 Spring 2016 hapter 5-29

30 Polymer Single rystals Electron micrograph multilayered single crystals (chain-folded layers) of polyethylene Single crystals only for slow and carefully controlled growth rates Fig. 5.10, allister & Rethwisch 9e. [From A. Keller, R.. Doremus, B. W. Roberts, and D. Turnbull (Eds.), Growth and Perfection of rystals. General Electric ompany and John Wiley & Sons, Inc., 1958, p Reprinted with permission of John Wiley & Sons, Inc.] 1 μm AMSE 205 Spring 2016 hapter 5-30

31 Semicrystalline Polymers Some semicrystalline polymers form spherulite structures Alternating chain-folded crystallites and amorphous regions Spherulite structure for relatively rapid growth rates Spherulite surface Fig. 5.12, allister & Rethwisch 9e. AMSE 205 Spring 2016 hapter 5-31