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Homework 4 solutions

Homework 4 solutions - ENMA300/ENME382 Homework 4 Due...

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ENMA300/ENME382 Homework 4 Due: Monday Sept. 26, 2011 1. a) What is a mer? A mer is the basic repeat unit in a polymer. b) Why do we only use mers for polymers? Polymers are composed of many repeating units so it is much easier to represent a polymer using its mer structure than to write out the entire polymer. In a sense this is like representing a crystal structure using one unit cell. Note: When I want to represent a polymer and not just the mer itself, I put square brackets around the mer and an n or a specific number (for degree of polymerization if I have one) in the lower right hand corner. c) What is the mer for PTFE (polytetrafluoroethylene)? F F C C F F d) What is the mer for polyvinylchloride (PVC)? H H C C H Cl e) What is the mer for polystyrene (PS)? H H C C H Where the ring is an aromatic ring as shown on page 101 of the text. f) Which of the polymers in c, d and e can have atactic and syndiotactic forms? Why? You need to have some ‘asymmetry’ to be able to have atactic or syndiotactic forms. Thus, PVC and PS can be atactic, isotactic or syndiotactic. Since all of the side groups for PTFE are F it can not exhibit stereoisomerism. g) What is a thermoplastic polymer? Name and briefly describe four common characteristics. 1
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Thermoplastic polymers are polymers which soften upon heating. They are relatively soft, tend to be ductile after ‘higher’ temperatures (near or above the glass transition temperature); they will melt; they are brittle at very low temperatures (well below the glass transition temperature). You can picture them as long chains. They can crystallize. h) What is a thermoset polymer? Name and briefly describe four common characteristics Thermoset polymers are polymers which decompose upon heating. They are network polymers; generally harder and stronger than thermoplastic polymers; have many covalent crosslinks between adjacent molecular chains – in some cases you can’t really
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