Unformatted text preview: to ammonia, you get NH4 . If this material is introduced to
Cl , what is the nature and strength of the bond formed? (This sort of bond occurs in a
type of material called an ionomer.) 46. Predict which member of each pair of materials below has a higher melting temperature.
a. C2 F4 or C2 H2 F2
b. CH4 or NH3 (predict the higher vaporization temperature)
c. C14 H30 or C5 H12
47. Secondary bonds are present and extremely important in polymer behavior. Natural
polymers are either animal- or vegetable-based. Animal ﬁbers contain @ NH bonds.
Vegetable ﬁbers contain @ OH bonds. Do you expect natural polymers to be moisture
sensitive, i.e., capable of bonding with H2 O?
48. What binds one molecule to another in a molten mass of polymer?
49. Which chemical group is capable of larger secondary bonding forces, one that contains
C N groups or one that contains CH3 groups?
50. Secondary bonding involves electronic attraction of dipoles, or partial charges. Primary
bonding also involves electronic interactions, but of whole charges. Estimate the relative
magnitude of secondary bonding forces and compare them with primary bond
strengths. In so doing you have estimated how much stronger a polymer can be when all
the covalent bonds are aligned in one direction, such as in some ﬁbers, and how weak
they are normal to that direction. SECTION 2.8
Mixed Bonding 51. Give three examples of materials that exhibit mixed bonding at the primary bond level
and three examples of materials that exhibit both primary and secondary bonding.
52. Suppose a material is partly ionic and partly covalently bonded. If an external environment can shift the balance of charge sharing and charge transfer, how will this affect
electrical conductivity? If the material were partly covalent and partly metallic, how
would your answer change? Of what use would such a material be?
53. Some covalently bonded materials can be doped with electron acceptors, allowing
charge to jump from the covalent bond to the acceptor and back, perhaps into another
covalent bond. How will the presence of dopant affect electrical conductivity.
54. Ni3 Al is an intermetallic that retains good mechanical properties at high temperature.
This characteristic is typically ascribed to ionic solids. Is this material at least partially
ionic? SECTION 2.9
The Structure of Polymer
Molecules 55. Human hair is not a thermoplastic material but rather a thermoset material. Much as in
the case of vulcanized rubber, the crosslinks are sulfur bridges. Crosslinks give hair
molecules a memory...
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