Page 69 - Carrahers_Polymer_Chemistry,_Eighth_Edition

32 Carraher’s Polymer Chemistry

H O H O
N N R
R N N
O H O H
H
O H
O
N N
R N
CH 3
O
H O
FIGURE 2.9 Typical hydrogen bonding (shown as “–” between hydrogen on nitrogen and oxygen for
nylon-66.

It is of interest to note that methane, ethane, and ethylene are all gases; hexane, octane, and
nonane are all liquids (at room conditions), while low molecular weight PE is a waxy solid. This
trend is primarily due to an increase in the mass per molecule and to an increase in the London
forces per polymer chain. The London force interaction between methylene units is about 8 kcal/
mol. Thus, for methane molecules the attractive forces is 8 kJ/mol; for octane it is about 64 kJ/mol;
and for polyethylene with 1,000 ethylene (or 2,000 methylenes) it is about 2,000 methylene units ×
8 kJ/mol per methylene unit = 16,000 kJ/mol well sufficient to make PE a solid and to break back-

bone bonds before it boils. (Polymers do not boil because the energy necessary to make a chain
volatile is greater than the primary backbone bond energy.)
Polar molecules such as ethyl chloride and PVC are attracted to each other by both the London
forces, but also to dipole–dipole interactions resulting from the electrostatic attraction of a chlo-
rine atom in one molecule to a hydrogen atom in another molecule. These dipole–dipole forces are
of the order of 8–25 kJ/mol, generally greater than the London forces and they are temperature
dependent. Hard plastics, such as PVC, have dipole–dipole attractive forces present between the
chains.
Strongly polar molecules such as ethanol, poly(vinyl alcohol) (PVA), cellulose, and proteins are
attracted to each other by a special type of dipole–dipole force called hydrogen bonding. Hydrogen
bonding occurs when a hydrogen present on a highly electronegative element, such as nitrogen or
oxygen, comes close to another highly electronegative element. This force is variable but for many
molecules it is about 40 kJ/mol and for something like hydrogen fl uoride (HF), which has particu-
larly strong hydrogen bonding, it is almost as strong as primary bonding. Intermolecular hydrogen
bonding is usually present in classical fibers such as cotton, wool, silk, nylon (Figure 2.9), polyac-

rylonitrile, polyesters, and polyurethanes. Intramolecular hydrogen bonds are responsible for the
helices observed in starch and globular proteins.
It is important to note that the high melting point of nylon-66 (Figure 2.9; 265°C) is a result of
a combination of dipole–dipole, London, and hydrogen-bonding forces. The relative amount of
hydrogen bonding decreases as the number of methylene groups increases and a corresponding
decrease is seen in the melting point for nylon-6,10 in comparison to nylon-66. Polyurethanes, poly-
acrylonitrile, and polyesters are characterized by the presence of strong hydrogen and polar bond-
ing and form strong fi bers. In contrast, iPP, which has no hydrogen bonding holding the PP chains
together, is also a strong fiber but because of the ability of the similar chains to fit closely together.

Thus, both the secondary bonding between chains and the ability to tightly fit together, steric factors
are important factors in determining polymer properties.
In addition to the contribution of intermolecular forces, chain entanglement is also an impor-

tant contributory factor to the physical properties of polymers. While paraffin wax and HDPE are
homologs with relatively high molecular weights, the chain length of paraffi n is too short to permit
chain entanglement and hence it lacks the strength and many other physical characteristic properties
of HDPE.

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