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PTFE4775_EXAM1 - Q u est io n 1 a Two sampl es of pol...

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Qu e s tion 1 a) Two s a m pl e s o f polypropyl e n e , A and B , hav e t h e s a m e M n and M w . Bo t h ar e lin e ar c hain s wi t h no bran c hing (r e p e a t uni t CH 2 CHCH 3 ) . S a m pl e A i s st i ff e r and ha s high e r d e n s i t y t han B . Al s o , s a m pl e A i s opaqu e wh e r e a s B i s c l e ar e r . Wha t di ff e r e n ce in t h e ir m ol ec ular st ru c t ur e s c an e xplain t h e di ff e r e n ce s in prop e r t i e s ? The differences (stiffness, density, transparency) probably arise from difference in crystallinity. In terms of the molecular structure s, they are likely stereoisomers: A must be isotactic or syndiotactic, B must be atactic. b) L e t R t h e RM S e nd- t o- e nd di st an ce o f a rando m c oil . A poly m e r wi t h a m ol . w t . o f 100 k g / m ol ha s a m e a s ur e d R o f 10 n m . Wha t m ol ec ular w e igh t o f t h e s a m e poly m e r will giv e an R o f 30 n m ? S how your r e a s oning bri e f ly . R = <r o 2 > 1/2 = n 1/2 l. R n 1/2 M 1/2 (since n M). ’ = ‘is proportional to’ A 3-fold increase in R corresponds to a 3-fold increase in n 1/2 , or a 9-fold increase in n. Answer: 900 kg / mol . c) Fill in th e blank s . - The main difference in structure between thermoplastics and thermosets is that thermoplastics are usually un c ro ss link e d lin e ar c hain s whereas thermosets are c ro ss link e d n e twork s . - A polymer with a structure .... -A-B-A-B-A-B-A-B-... is called alt e rnating copolymer, whereas one with a structure ...-A-A-A-A-A-B-B-B-B- .... is called blo c k copolymer. - Polymers with C=C double bonds in their backbone may exhibit c i s -and-tran s isomerism. - The crystalline lamellae of polymer crystals may assemble to form larger structures called s ph e rulit e s - Faster cooling rate typically leads to high e r glass transition temperature. d) For each line below, c hoo s e on e type of reaction ( A to H ) that the statement fits best. - Makes monodisperse polymers. (D) c ontroll e d living - Involves an immiscible solvent and a surfactant. (C) e mul s ion - Reaction between diol and diacid, for example. (H) c ond e n s ation - Can be used to produce isotactic polypropylene (A) Zi e gl e r-Natta - All polymerizations that have no side product. (B) addition - Re-distributes the mol. wt. of a Nylon batch, for example. (F) amid e int e r c hang e Qu e s tion 2
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