Chap 15 Solns
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ch14 (1)
| Polymer Crystallinity
14.22 Explain briefly why the tendency of a polymer to crystallize decreases with increasing molecular weight. Solution The tendency of a polymer to crystallize decreases with increasing molecular weight because as the chains become longer it is more difficult for all regions along adjacent chains to align so as to produce the ordered atomic array. 14.23 For each of the following pairs of polymers, do the following: (1) state whether or not it is possible to determine whether 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. Solution (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). 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 do random copolymers. 14.24 The density of totally crystalline polypropylene at room temperature is 0.946 g/cm3. Also, at room temperature the unit cell for this material is monoclinic with lattice parameters a = 0.666 nm α = 90 b = 2.078 nm β = 99.62 c = 0.650 nm γ = 90 If the volume of a monoclinic unit cell, Vmono, is a function of these lattice parameters as Vmono = abc sin b determine the number of repeat units per unit cell. Solution For this problem we are given the density of polypropylene (0.946 g/cm3), an expression for the volume of its unit cell, and the lattice parameters, and are asked to determine the number of repeat units per unit cell. This computation necessitates the use of Equation 3.5, in which we solve for n. Before this can be carried out we must first calculate VC, the unit cell volume, and A the repeat unit molecular weight. For VC VC = abc sin b = (0.666 nm)(2.078 nm)(0.650 nm) sin (99.62) = 0.8869 nm3 = 8.869 10-22 cm3 The repeat unit for polypropylene is shown in Table 14.3, from which the value of A may be determined as follows: A = 3(AC) + 6(AH) = 3(12.01 g/mol) + 6(1.008 g/mol) = 42.08 g/mol Finally, solving for n from Equation 3.5 leads to = 12.0 repeat unit/unit cell 14.25 The density and associated percent crystallinity for two polytetrafluoroethylene materials are as follows:
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