HW9 Solutions

# HW9 Solutions - Chemical Engineering 178 SOLUTIONS(30pts...

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Chemical Engineering 178 SOLUTIONS (30pts total) Homework 9 Due 11/14 1.) 4 pts total: The following data for the T g of polystyrene as a function of molecular weight was obtained by Fox and Flory ( J. Poly Sci 14 , 315 (1954)) M (g/mol) T g (C) 85,000 100 19,300 89 13,300 86 6650 77 4980 78 3590 75 3041 65 2600 62 2085 53 1675 40 What would be the T g of a theoretically infinite molecular weight sample? The glass transition as a function of molecular weight can be modeled by: (2pts) M K T T g g + = Plot T g vs. 1/M to determine T g infinity : (1pt) Tg vs. 1/M y = -90799x + 95.732 R 2 = 0.9676 0 20 40 60 80 100 120 0 0.0002 0.0004 0.0006 0.0008 1/M (mol/g) Tg (C)

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The intercept is 95.7, so the glass transition temperature of a theoretically infinite molecular weight sample is 95.7 o C. (1pt) Alternately, approximate 85,000 g/mol as an infinite chain, and the glass transition at infinite molecular weight can be assumed to be 100 o C. (full credit) 2.) 10pts total: Discuss the reasons for the differences in glass transition temperature for the following pairs of polymers with similar chemical structures. a) Polyethylene (~150K) and polypropylene (~250K).
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## This note was uploaded on 12/09/2011 for the course CHEM ENG 178 taught by Professor Segalman during the Fall '08 term at Berkeley.

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HW9 Solutions - Chemical Engineering 178 SOLUTIONS(30pts...

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