HW7_Practice

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Jan 9, 2024

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14.22 Explain briefly why the tendency of a polymer to crystallize decreases with increasing molecular weight. 14.23 For each of the following pairs of polymers, do the following: (1) state whether it is possible to determine if one polymer is more likely to crystallize than the other; (2) if it is possible, note which is the more likely and then cite reason(s) for your choice; and (3) if it is not possible to decide, then state why. 15.5 In your own words, briefly describe the phenomenon of viscoelasticity. 15.22 For each of the following pairs of polymers, plot and label schematic stress – strain curves on the same graph [i.e., make separate plots for parts .22(a), .22(b), and .22(c)]. Solutions begin on the next page.

14.22 Explain briefly why the tendency of a polymer to crystallize decreases with increasing molecular weight. A higher molecular weight means the chains are longer. Longer chains have more difficulty become ordered and aligned. 14.23 For each of the following pairs of polymers, do the following: (1) state whether it is possible to determine if one polymer is more likely to crystallize than the other; (2) if it is possible, note which is the more likely and then cite reason(s) for your choice; and (3) if it is not possible to decide, then state why. (a) Linear and syndiotactic poly (vinyl chloride); linear and isotactic polystyrene. (b) Network phenol-formaldehyde; linear and heavily crosslinked cis-isoprene. (c) Linear polyethylene; lightly branched isotactic polypropylene. (d) Alternating poly(styrene-ethylene) copolymer; random poly (vinyl chloride- tetrafluoroethylene) copolymer. (a) Yes, for these two polymers it is possible to decide. The linear and syndiotactic poly (vinyl chloride) is more likely to crystallize; the phenyl side-group for polystyrene is bulkier than the Cl side-group for poly(vinyl chloride). Furthermore, syndiotactic and isotactic isomers are equally likely to crystallize. (b) No, it is not possible to decide for these two polymers. Both heavily crosslinked and network polymers are not likely to crystallize. (c) Yes, it is possible to decide for these two polymers. The linear polyethylene is more likely to crystallize. The repeat unit structure for polypropylene is chemically more complicated than is the repeat unit structure for polyethylene. Furthermore, branched structures are less likely to crystallize than are linear structures. (d) Yes, it is possible to decide for these two copolymers. The alternating poly(styrene-ethylene) copolymer is more likely to crystallize. Alternating copolymers crystallize more easily than random copolymers. 15.5 In your own words, briefly describe the phenomenon of viscoelasticity. Viscoelasticity is the property of a material to exhibit both elastic and viscous characteristics when undergoing deformation. An elastic response to deformation is instantaneous and recoverable while a viscous response is not instantaneous isn’t necessari ly recoverable. 15.22 For each of the following pairs of polymers, plot and label schematic stress – strain curves on the same graph [i.e., make separate plots for parts .22(a), .22(b), and .22(c)]. (a) Isotactic and linear polypropylene having a weight-average molecular weight of 120,000 g/mol; atactic and linear polypropylene having a weight-average molecular weight of 100,000 g/mol

(b) Branched poly(vinyl chloride) having a degree of polymerization of 2000; heavily crosslinked poly(vinyl chloride) having a degree of polymerization of 2000 (c) Poly(styrene-butadiene) random copolymer having a number-average molecular weight of 100,000 g/mol and 10% of the available sites crosslinked and tested at 20°C; poly(styrene- butadiene) random copolymer having a number-average molecular weight of 120,000 g/mol and 15% of the available sites crosslinked and tested at −85°C. (b) Shown below are the two stress strain curves for the PVC material. The branched PVC will probably display the stress-strain behavior of a plastic, curve B in Figure 15.1. However the heavily crosslinked PVC will have a higher tensile modulus, and, also a higher strength, and will most likely fail in a brittle manner.

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