L39 - SUMMARY FROM LAST CLASS Polymers The structure of...

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Dr. P. Lucas U of A MSE 110 Polymers SUMMARY FROM LAST CLASS The structure of most polymers can be seen as long flexible organic chain macromolecules entangled in the same way as a spaghetti dish. Polymer chains are formed by linking many monomer units. A macromolecule is considered a polymer above a 1000 units when chains entanglement becomes significant and solid like behavior arise. The length of the macromolecules in a synthetic polymers is distributed over a range of sizes. Different monomer produce polymers with different properties and monomers can be mixed to make copolymers . Addition or chain growth polymerization must be initiated with a free radical. Long chains are grown from the initiated monomers. Condensation or step growth polymerization can be initiated by each monomer unit. This initially produce many short chains called oligomers . The formation of protein from amino acids is a example of natural condensation polymerization reaction.
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Dr. P. Lucas U of A MSE 110 Molecular weight Since polymers are made of macromolecules with a distribution of chain length, it is necessary to define an average molecular weight to describe a polymer. POLYMERS Where x i is the fraction of chain within the size range i. And M i is the mean molecular weight of the chain range i. One way of defining an average molecular weight is obtained by dividing the distribution of chains into a series of chain size range. The number-average molecular weight is expressed as: = i i n M x M For example, in the size range 5-10 (10 3 g/mol), the fraction x i is 0.05 (5% of the chains have a size within that range) and the mean molecular weight is M i =7.5 (10 3 g/mol), the middle of the range.
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Dr. P. Lucas U of A MSE 110
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