Chemistry Reference
In-Depth Information
occur because of rotations about single bonds. Molecules with different conforma-
tions are called conformational isomers, rotamers, or conformers.
Macromolecules in solution, melt, or amorphous solid states do not have regular
conformations, except for certain very rigid polymers described in
Section 4.6
and
certain polyolefin melts mentioned in Section 1.14.2. The rate and ease of change of
conformation in amorphous zones are important in determining solution and melt
viscosities, mechanical properties, rates of crystallization, and the effect of tempera-
ture on mechanical properties.
Polymers in crystalline regions have preferred conformations which represent
the lowest free-energy balance resulting from the interplay of intramolecular and
intermolecular space requirements. The configuration of a macromolecular spe-
cies affects the intramolecular steric requirements. A regular configuration is
required if the polymer is to crystallize at all, and the nature of the configuration
determines the lowest energy conformation and hence the structure of the crystal
unit cell.
Considerations of minimum overlap of radii of nonbonded substituents on the
polymer chain are useful in understanding the preferred conformations of macro-
molecules in crystallites. The simplest example for our purposes is the polyethyl-
ene (1-3) chain in which the energy barriers to rotation can be expected to be
similar to those in n-butane.
Figure 1.6
shows sawhorse projections of the confor-
mational isomers of two adjacent carbon atoms in the polyethylene chain and the
corresponding rotational energy barriers (not to scale). The angle of rotation is
cis
skew
+
skew
−
gauche
−
gauche
+
Δ
E
Δε
0
60
120 180
Rotational angle (°)
240
300
trans
trans
FIGURE 1.6
Torsional potentials about adjacent carbon atoms in the polyethylene chain. The white
circles represent H atoms and the black circles represent segments of the polymer chain.
