Lec17 Polymeric structure.pdf

Lec17 Polymeric structure.pdf - Polymeric Structure...

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Polymeric Structure
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Contents Concept of molecular weight Chemistry of polymers % Crystallinity Tacticity in polymers
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The molecular weight distribution is important in thermoplastics . In thermosets , a gelled network of essentially infinite molecular weight is formed, so the idea of a "molecular weight distribution" is non-sensical . Two ways:- a) Number average molecular weight (M n ): Total weight of all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample. b) Weighted average molecular weight (M w ): It's based on the fact that a bigger molecule contains more of the total mass of the polymer sample than the smaller molecules do. Molecular Weight Number average molecular weight, 𝑀 ? = σ ? 𝑖 ? 𝑖 σ ? 𝑖 Where, M i = molecular weight of i th polymer chain; N i = number of chains of that molecular weight. Weighted average molecular weight, 𝑀 𝑤 = σ ? 𝑖 2 ? 𝑖 σ ? 𝑖 ? 𝑖 More appropriate
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Degree of Polymerization ?𝑃 = 𝑀 ? ? where, 𝑀 ? = Number average molecular weight m = repeat unit molecular weight (no. of atoms x atomic wt.) Example : Suppose for Polyvinyl chloride , 𝑀 ? = 21,150 g/mol Then, repeat unit molecular weight, m = 2 x 12 + 3 x 1 + 1 x 35 = 62 g/mol DP = 21,150 62 = 341 PVC Degree of Polymerization Melting Temperature Repeat unit contains 2 carbon, 3 hydrogen and 1 chlorine atom Reference : Kalpakjian, Schmid - Manufacturing Processes for Engineering Materials, 5th ed.
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Distribution of molecular weights for a typical polymer
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