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BME 314 Midterm Review

# BME 314 Midterm Review - BME 314 MIDTERM I REVIEW Created...

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BME 314 MIDTERM I REVIEW Created By: Lonnissa Hong Nguyen 1. Given a polymer sample with the following distribution of polymer molecules. 50 molecules of molar mass 1000 100 molecules of molar mass 3000 80 molecules of molar mass 5000 200 molecules of molar mass 7000 20 molecules of molar mass 10000 A. Calculate the number average 22 . 5222 450 000 , 350 , 2 20 200 80 100 50 ) 10000 20 ( ) 7000 200 ( ) 5000 80 ( ) 3000 100 ( ) 1000 50 ( = = + + + + × + × + × + × + × = = i i i n N M N M The number average molecular weight is a way of determining the molecular weight of a polymer. Polymer molecules, even ones of the same type, come in different sizes (chain lengths, for linear polymers), so the average molecular weight will depend on the method of averaging. The number average molecular weight is the common, mean, average of the molecular weights of the individual polymers . It is determined by measuring the molecular weight of n polymer molecules, summing the weights, and dividing by n . The number average molecular weight of a polymer can be determined by vapor pressure osmometry, end-group titration, and colligative properties. B. Calculate the weight average 59 . 6276 000 , 350 , 2 000 , 000 , 750 , 14 ) 10000 20 ( ) 7000 200 ( ) 5000 80 ( ) 3000 100 ( ) 1000 50 ( ) 10000 20 ( ) 7000 200 ( ) 5000 80 ( ) 3000 100 ( ) 1000 50 ( 2 2 2 2 2 2 = = × + × + × + × + × × + × + × + × + × = = i i i i w M N M N M The weight average molecular weight is a way of describing the molecular weight of a polymer . Polymer molecules, even if of the same type, come in different sizes (chain lengths, for linear polymers), so we have to take an average of some kind. where N i is the number of molecules of molecular weight M i . Intuitively, if the weight average molecular weight is w , and you pick a random monomer, then the polymer it belongs to will have a weight of w on average. The weight average molecular weight can be determined by light scattering, small angle neutron scattering (SANS), X-ray scattering, and sedimentation velocity. C. Calculate the polydispersity Index (PI) 202 . 1 22 . 5222 59 . 6276 = = = n w M M PDI Polydispersity Index (PDI), is a measure of the distribution of molecular weights in a given polymer sample. The PDI calculated is the weight average molecular weight divided by the number - 1 -

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BME 314 MIDTERM I REVIEW Created By: Lonnissa Hong Nguyen average molecular weight. It indicates the distribution of individual molecular weights in a batch of polymers. The PDI has a value always greater than 1, but as the polymer chains approach uniform chain length, the PDI approaches unity (1). Effect of Polymerization Mechanism on PDI: Typical PDI's vary based on the mechanism of polymerization and can be affected by a variety of reaction conditions. In synthetic polymers, it can vary greatly due to reactant ratio, how close the polymerization went to completion, etc. For typical addition polymerization , values of the PDI can range around 10 to 20. For typical step polymerization, values range around 2 to 3. Living polymerization , a special case of addition polymerization, leads to values very close to 1. Such is the case also in biological polymers, where the PDI can be very close or equal to 1, indicating only one length of polymer is present.
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BME 314 Midterm Review - BME 314 MIDTERM I REVIEW Created...

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