pset1_solns

pset1_solns - Problem Set 1 solutions Issued: 02/07/06 Due:...

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Problem Set 1 solutions 20.462J/3.962J Issued: 02/07/06 Spring 2006 Due: 02/16/06 10 points/problem 1. Listed in the table below are chemical and schematic structures of poly(L-lactide) and a number of derivatives of this polymer. a. Using physicochemical criteria, predict the hierarchy of degradation rates for these materials. Provide a concise but thorough explanation of your ranking. State any assumptions you make. Experiments reveal that these materials ranked in terms of their rate of degradation have the following order: polycaprolactone (PCL) < poly(L-lactide) (PLLA) < poly(lactide-co-glycolide) (PLGA) < poly(lactide-b-ethylene oxide) (PLA-b-PEO). This ranking can be rationalized via the chemical and physicochemical factors that control degradation rates: Because all of the materials are comprised of degradable ester linkages, they cannot be differentiated based on purely labile bond differences. The two slowest-degrading materials have a number of factors lowering their degradation rate: They are both semicrystalline. PLLA has the pendant methyl groups creating a steric barrier to the approach of water to the backbone. PCL is more hydrophobic and generally is highly crystalline, because its linear backbone readily packs into extensive crystallites (relative to PLLA). PLGA breaks down more quickly than PLLA because the mixture of lactide and glycolide repeat units in the chain prevents crystallization, and glycolide, lacking the bulky methyl groups, is more readily attacked by water. Lastly, PLA-b- PEO, though crystallizable, are much more quickly degraded than the other materials due to the greatly enhanced entry of water into the hydrophilic PEO domains. b. State 2 physical properties/parameters you would measure for each polymer to make your ranking more accurate, and explain your choices. Key properties that are not immediately obvious from the given information would be the percent crystallinity in the samples and their glass transition temperatures. For the block copolymer, one would like to know whether the crystallinity is present in both PLA and PEO blocks or only the PLA blocks. 20.462J/3.962J PS 1 1 of 7 9/5/06
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Polymer Chemical structure microstructure Poly(L-lactide) O CH 3 C CH O Semicrystalline CH O C O CH 3 n Poly(lactide-co- glycolide) CH C O O CH C O O C H 2 CH 3 CH 3 C O H 2 C C O O O x y amorphous Poly(lactide-b-ethylene O CH 3 oxide) C CH O H 2 C Semicrystalline CH O C CH 2 O O CH 3 x y polycaprolactone O -((CH 2 ) 5 -C-O-) n - = semicrystalline 20.462J/3.962J PS 1 2 of 7 9/5/06
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2. Given below are data from
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This note was uploaded on 11/11/2011 for the course BIO 20.410j taught by Professor Rogerd.kamm during the Spring '03 term at MIT.

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pset1_solns - Problem Set 1 solutions Issued: 02/07/06 Due:...

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