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mat33hw6 - severance of the crosslink bonds and polymer...

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Mat. 33 Homework # 6 3/9/09 1) DP=M n /m 2210=100000/m m=45.24887g/mol Therefore, the other repeat unit is propylene because it has the molecular weight closest to that calculated above. 2) Thermoplastic polymers soften when heated (and eventually liquefy) and harden when cooled- processes that are totally reversible and may be repeated. As the temperature is raised secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of adjacent chains is facilitated when a stress is applied. Meanwhile, thermosetting polymers are network polymers. During formation they become permanently hard and do not soften upon heating. They also have covalent crosslinks between adjacent molecular chains. During heat treatments the bonds resist vibrational and rotational chain motions at high temperatures because they anchor the chains together. 10 to 50% of the chain repeat units are crosslinked. Only excessive temperatures can cause
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Unformatted text preview: severance of the crosslink bonds and polymer degradation. 3) A) syndiotactic B) atactic C) isotactic 4) Linear and syndiotactic poly(vinyl chloride) will have a higher melting point with these conditions because according to Figure 11.48 when the molecular weights get so high even though they are different, they do not cause the melting points to differ at such a significant amount, so I simply used the data from Table 11.3 to come to this conclusion. 5) Schematic plots: 6) Small-angle grain boundaries are not as effective in interfering with the slip process as high-angle grain boundaries because as the angle gets bigger it makes it harder for the dislocation to change direction because it is more difficult as the amount of crystallographic disorientation increases. 7) A) 50 = (sigma) + 2k y (sigma) = 25 (when measuring in mm-1/2 ) k y = 12.5 B) With this given grain diameter I would predict the yield strength to be 225 MPa....
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