LYCRA01

Polyurethanes

resource:http://pslc.ws/macrog/urethane.htm

Polyurethanes are the most well known polymers used to make foams. If you're sitting on a padded chair right now, the cushion is more than likely made of a polyurethane foam. Polyurethanes are more than foam.

Much more than foam!

Polyurethanes are the single most versatile family of polymers there is. Polyurethanes can be elastomers, and they can be paints. They can be fibers, and they can be adhesives. They just pop up everywhere. A wonderfully bizarre polyurethane is spandex.

Of course, polyurethanes are called polyurethanes because in their backbones they have a urethane linkage.

The picture shows the a simple polyurethane, but a polyurethane can be any polymer containing the urethane linkage in its backbone chain. More sophisticated polyurethanes are possible, for example:

Polyurethanes are made by reacting diisocyanates with di-alcohols. To find out how, click here.

Sometimes, the dialcohol is replaced with a diamine, and the polymer we get is a polyurea, because it contains a urea linkage, rather than a urethane linkage. But these are usually called polyurethanes, because they probably wouldn't sell well with a name like polyurea.

Polyurethanes can hydrogen bond very well, and thus can be very crystalline. For this reason they are often used to make block copolymers with soft rubbery polymers. These block copolymers have properties of thermoplastic elastomers.

Spandex

One unusual polyurethane thermoplastic elastomer is spandex, which DuPont sells under the trade name Lycra. It has both urea and urethane linkages in its backbone. What gives spandex its special properties is the fact that it has hard and soft blocks in its repeat structure. The short polymeric chain of a polyglycol, usually about forty or so repeats units long, is soft and rubbery. The rest of the repeat unit, you know, the stretch with the urethane linkages, the urea linkages, and the aromatic groups, is extremely rigid. This section is stiff enough that the rigid sections from different chains clump together and align to form fibers. Of course, they are unusual fibers, as the fibrous domains formed by the stiff blocks are linked together by the rubbery soft sections. The result is a fiber that acts like an elastomer! This allows us to make fabric that stretches for exercise clothing and the like.

LYCRA-SPANDEX

LYCRA was the first spandex fibre introduced into the clothing materials market - from fashion ware to sports ware!

Lycra is a commercial material specially designed to have special properties for a wide variety of clothing articles.

Lycra-Spandex polymer materials are strong fibres with elastic properties (polymithine?).

The above diagram is an example of the molecular structure of 'Lycra-Spandex' type molecules

n = a very large number of repeating double 'molecular segments' in the polymer chain.

x is the variable number of units in the 'elastic' molecular segment, this gives differing degrees of 'stretchiness'.

Therefore its molecules have a stretchy section that make it soft and rubbery, and a rigid section that makes it tougher than rubber.

Lycra is an artificial fibre that readily stretches but is still a strong material overall.

Parts of the long polymer molecules stretch easily i.e. elastic at the molecular level, but other sections of the polymer chain are more rigid to increase strength.

Lycra fibres can be stretched upto 500% (i.e. 5x) of their original length.

Some advanced molecular ideas

(i) I wouldn't have thought this stretching capacity could be totally explained by the zig-zag chain of carbon and oxygen atoms, i.e. -(-C-C-O-)x-, so is this polyether section coiled up prior to be stretched?

The intermolecular forces between the relatively non-polar polyether sections of adjacent Lycra molecules are the weakest Van der Waal forces, instantaneous dipole - induced dipole attractive forces.

(ii) The benzene rings are much more rigid compared to a hydrocarbon/ether saturated chain of atoms, but is the strength and rigidity of this section also enhanced by hydrogen bonding between -NH-CO- groupings in adjacent Lycra molecules? (as in protein tissue and synthetic nylon polymers).

It is used in making sports clothing such as wetsuits, and with other fibres to make comfortable clothing with a snug fit.

Lycra is a lightweight material but isn't damaged by sunlight, sweat or detergents - all of which can make other materials wear out.

http://osf1.gmu.edu/~sslayden/curr-chem/spandex/spandex-prob.htm

Spandex Structure and Properties

Notice that because Spandex is a polymer, its macromolecular structure is made up of repeating units (mers) denoted by the x and n next to the parentheses in the structure. Each Spandex fiber will differ somewhat in length and composition depending on the exact values of x and n.

Expanded Structure:

Draw in the missing hydrogens and non-bonded electrons in the abbreviated structure of Spandex.

Functional Groups:

Spandex contains some composite functional groups not mentioned in the early chapters of your text. First, circle each of the simple functional groups in the abbreviated structure of Spandex. A urethane is a composite functional group that is part ester and part amide. Circle the urethane functional group(s). You should know the structure of urea. Circle the substituted urea functional group(s).

Hybridization and Geometry:

For each of the atoms in the abbreviated Spandex structure, state its hybridization and geometry. (Remember the "amide" rule -- the N of amides is sp2 hydbridized.)

Resonance Structures:

Functional groups that have resonance contributors (and thus double-bond character) will have higher barriers to rotation. Find the urethane and urea derivative functional groups in the structure above and show how the carbonyl group and the adjacent heteroatom can be drawn as another resonance structure.

Conformational Analysis:

With respect to the resonance structures you drew, state which have higher or lower energy barries to rotation. Explain why there is a barrier to rotation for these groups.

Here is a small Spandex molecule in one possible conformation.

Dependence of Physical Properties on Structure:

With respect to your answers above, explain why one segment of Spandex is "soft and rubbery" and the other segment is "rigid."