ps5soln - Problem Set #5 BENG 434/ENAS 534 Biomaterials...

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Problem Set #5 BENG 434/ENAS 534 Biomaterials Due: Tuesday, November 27 th , at the beginning of class 1. You are studying the effects of degradation of PLGA (50:50 G:L) on its mechanical properties. You take 5 sets of samples (3 cm by 1 cm by 500 microns) and put them in PBS in an incubator at 37 ˚C. The starting Mn ~ 70k g/mol. a. If the PDI is 3.2, what is Mw? (5 pts) Mw=Mn*PDI=70k g/mol * 3.2= 224k g/mol b. Draw the GPC curve for this showing Mn and Mw. (10 pts) This should be a Gaussian distribution with Mn and Mw appropriately spaced. The x axis should be mw and the y axis should be concentration, number, or signal. c. Draw a representative stress-strain curve for this material. (Use a reasonable modulus—justify your choice of modulus) (10 pts) At room T, where you will most likely be running an instron test, one will typically see a very glassy behavior with little plasticity with a Mn near 70k. To determine an appropriate modulus, one should look to the literature. A common modulus for PLGA with Mn ~70k g/mol is approximately: 2.75GPa according to :Deng and Uhrich Journal of Materials Science: Materials In Medicine 13 1091-1096, 2002. d. How would an increased strain rate affect the stress-strain curve? (5 pts) The material will behave more like a glassy material exhibiting less ductility and a higher modulus. e. How would a decreased strain rate affect the stress-strain curve? (5 pts) The material will behave more like a rubber elastic/plastic material with a lower modulus and greater ductility before yielding f. Draw a schematic showing how the polymer degrades. (5 pts)
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O O H + g. Draw a plot showing the change in molecular weight with time. Justify your plot focusing on the rate of degradation, and the time to degrade. (10 pts) One can determine from the literature (such as the paper cited above as well as  others) that the polymer will degrade over the course of 40-60 days  (approximately) at 37 ˚C in PBS. As the chains are cleaved, there will be more  chain ends. Therefore, the degradation rate will increase.  h. For early, mid and late times regarding degradation, draw the stress-strain curves, and explain how the degradation is affecting the mechanical properties (15 pts). Early times: The chains will start to be cleaved so the mw will quickly drop. The  modulus is not likely to be terrifically affected by the mw drop but the number of  entanglements will drop quickly leading to early yielding.
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Mid times: the modulus may drop some. Yielding will happen and there will be a longer  ductile region. Late times: the modulus will likely drop significantly and the material will yield under  low stresses. 2.
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This note was uploaded on 09/24/2009 for the course BENG 434 taught by Professor Erinlavik during the Fall '08 term at Yale.

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ps5soln - Problem Set #5 BENG 434/ENAS 534 Biomaterials...

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