MEMS 0040 Exam 3 Spring 2022 key

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University of Pittsburgh *

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Material Science

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Apr 3, 2024

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MEMS 0040 Materials and Manufacturing Exam 3 Spring 2022 Name: ____________________________________________________ • 50 minutes • Closed book • You may use a calculator I have neither given nor received unauthorized assistance on this test: (Signature): ________________________________________________
Section 1: True/False (2 points each, 20 points total) __ T __ 1. Branching in a thermoplastic polymer decreases the density of the polymer. __ F __ 2. Polymers which exhibit tacticity can crystallize more easily than polymers which do not exhibit tacticity. Tacticity is related to asymmetry of side groups, and this always reduces crystallinity compared to polymers with identical side groups. __ F __ 3. Polymer melts usually exhibit pseudoelastic behavior, so that the viscosity increases with increasing shear rate. Polymer melts exhibit pseudoplastic behavior, not pseudoelastic behavior. __ F __ 4. Thermoset polymers are hard to recycle because different thermoset polymers usually do not mix well after they melt. Thermoset polymers do not melt. __ T __ 5. Plastic bags are commonly made by the blow film extrusion process. __ F __ 6. During injection molding, the auger moves forward while spinning to deliver a shot of molten thermoplastic polymer into the mold. The augur does not spin during its forward motion in injection molding. __ T __ 7. Polymer fibers have high elastic modulus (compared to bulk polymer) because of the high degree of molecule alignment along the axis of the fiber. __ T __ 8. The fused deposition modeling (FDM) process for additive manufacturing of polymers is essentially an extrusion process. __ F __ 9. The fracture strength of carbon fibers increases as the fiber diameter increases. Larger fibers have a higher likelihood of critical flaws, so fracture strength is lower. Strength is a measure of stress to failure, and is measured as force per unit area. __ T __ 10. The pultrusion process can be used to make a composite tube with continuous fibers oriented parallel to the axis of the tube.
Section 2: Problems and Short Answer Questions (80 points total) Polymer Synthesis (20 points total) 11. (a) Polyethylene is synthesized by addition polymerization, whereas polyethylene terephthalate is synthesized by condensation polymerization. Explain the basic difference between addition polymerization and condensation polymerization processes. (4 points) In addition polymerization, monomer molecules are simply added stepwise to the growing polymer. In condensation polymerization, each step of monomer addition also produces a small byproduct molecule, such as water. (b) Polystyrene is a thermoplastic polymer. Draw the repeat unit for this polymer, and list the number of each type of atom in the repeat unit. (4 points) The repeat unit of polystyrene is There are eight carbon atoms and eight hydrogen atoms in the repeat unit. (c) Polypropylene is a thermoplastic polymer which can show tacticity. List two types of tacticity which can be exhibited by polypropylene. Which type would be expected to have a higher degree of crystallinity? (4 points) Two types of tacticity are the isotactic configuration and the atactic configuration. The isotactic configuration is expected to have a higher degree of crystallinity. (d) If the average molecular weight of polytetrafluoroethylene is 60,000 amu, calculate the average degree of polymerization for the polymer. Assume that the atomic weight of carbon is 12 amu and the atomic weight of fluorine is 19 amu. (4 points) Repeat unit has one carbon atom and two fluorine atoms. Repeat unit weight = 12 + 2(19) = 50 amu Degree of polymerization = 60,000/50 = 1200 (e) If the average degree of polymerization of polyvinyl chloride is 3000, calculate the average molecular weight of the polymer. Assume that the atomic weight of carbon is 12 amu, the atomic weight of hydrogen is 1 amu, and the atomic weight of chlorine is 35 amu. (4 points) Repeat unit has two carbon atoms, three hydrogen atoms, and one chlorine atom. Repeat unit weight = 2(12) + 3(1) + 1(35) = 62 amu Average molecular weight = (3000)(62 amu) = 186,000 amu
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