3. Some polymers can form liquid crystal mesophases with unusual physical proper

Question

3. Some polymers can form liquid crystal mesophases with unusual physical properties. For example, liquid crystal Kevlar (as following picture) is strong enough to be the material of choice for bulletproof vests and is stable at temperatures up to 600 K. What molecular interactions contribute to the formation, (5%) thermal stability, and mechanical strength of liquid crystal mesophase in Kevlar? 4. From their atomic numbers and mass numbers, decide whether the following nuclei are likely to have (10%, each 2%) zero, half-integral or integral spin: (a) 1Bo, (b) 1°B, (e) 13c, (d) 1/N, (e) 19F 5· The first generally available NMR spectrometer operated at a frequency of 60 MHz; today it is also common to use a spectrometer that operates at 600 MHz. (a) What are the relative population differences of 13C spin states in these two spectrometers at 25 °C (4%)? What are the relative values of the chemical shifts observed for nuclei in these spectrometers in terms of (b) value (3%), (c) frequencies (3%)? (10%)

Explanation / Answer

3-Kevlar is similar in structure to nylon-6,6 except that instead of the amide links joining chains of carbon atoms together, they join benzene rings. The two monomers are benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene.

Polyamides like Kevlar are polymers (huge molecules made of many identical parts joined together in long chains) made by repeating amides over and over again. Amides are simply chemical compounds in which part of an organic (carbon-based) acid replaces one of the hydrogen atoms in ammonia (NH3). So the basic way of making a polyamide is to take an ammonia-like chemical and react it with an organic acid. This is an example of what chemists call a condensation reaction because two substances fuse together into one.

Kevlar is a polymer; this means that it is made up of a large number of the same basic unit, called a monomer, which are attached to each other to form a long chain.  The monomer in this case is made up of an amide group and a phenyl group.

Kevlar is a polyamide, a type of synthetic polymer, in which the amide groups are separated by para phenylene groups, meaning that the amide groups are attached to each other on opposite sides of the phenyl group.The strength of Kevlar comes from its unusually regular internal structure; this has implications for the Hydrogen bonding which occurs between the electron dense oxygen atom and the electron deficient hydrogen. The all trans configuration, giving long straight chains, means that the hydrogen bonding can occur very regularly to form a very strong lattice, similar to those formed in crystals.

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