In this activity you will compare the structures of nylon and Kevlar. The characteristic properties of polymers are directly related to their molecular structures. What are some of the uses of each of these two polymers?

Building the Models:
The structures of the polymers nylon-6,6 and Kevlar are:

	Nylon-6,6	-NH(CH2)6NHCO(CH2)4CO-
	Kevlar	-NH-C6H4-NHCO-C6H4-CO-

Use your model kit to make the structures that represent the repeating units for these polymers. Then join your nylon model together with those from the other students' kits to make a long-chain model of the polymer. Do the same for the Kevlar structure. Use your models to help you work through the tasks that follow.

Questions:

a. How do the models of the polymer chains behave when stretched? Which polymer will be more elastic?
b. Use the models to explain the following data, which were obtained in experiments to test the strengths of the two polymers:

	Elongation at fracture
i. Nylon-6,6 ii. Kevlar	18% 3%

c. The bond lengths in Kevlar are nylon-6,6 can be estimated from comparable functional groups in other organic molecules:

	Bond length (nm)
	Nylon-6,6	Kevlar
N-C (in CONH group)	0.132	0.132
N-C (attached to a hydrocarbon group)	0.147	0.135
C-C (in a hydrocarbon group)	0.154	0.139
C-C (attached to a CONH group)	0.150	0.148

There is usually a good correlation in chemistry between bond length and bond strength:  shorter bonds are stronger, longer bonds are weaker.

i. Draw the structures of nylon-6,6 and Kevlar, and mark on each of them the bond length from the table.

ii. Use the bond lengths to explain why Kevlar is more stable than nylon-6,6 when heated.