Chemistry Reference
In-Depth Information
Fig. 2.9
Illustration
of the helical conformation
of isotactic vinyl polymers
phase change. Equilibrium conditions, however, are seldom if ever attained and as a result complete
crystallinity, as stated above, is not reached. The tendency to crystallize depends upon the following:
1. Structural regularity of the chains that leads to establishment of identity periods.
2. Free rotational and vibrational motions in the chains that allow different conformations to be
assumed.
3. Presence of structural groups that are capable of producing lateral intermolecular bonds (van der
Wall forces) and regular, periodic arrangement of such bonds.
4. Absence of bulky substituents or space irregularity, which would prevent segments of the chains
from fitting into crystal lattices or prevent laterally bonding groups from approaching each other
close enough for best interaction.
Natta and Carradini [
30
] postulated three principles for determining the crystal structures of
polymers These are:
1. It is possible to assume that all mer units in a crystal occupy geometrically equivalent positions
with respect to the chain axis. This is known as the
Equivalence Postulate
.
2. The conformation of the chain in a crystal is assumed to approach the conformation of minimum
potential energy for an isolated chain oriented along an axis. This is the
Minimum Energy
Postulate
.
3. As many elements of symmetry of isolated chain as possible are maintained in the lattice, so
equivalent atoms of different mer units along an axis tend to assume equivalent positions with
respect to atoms of neighboring chains. This is the
Packing Postulate
.
X-ray diffraction studies of polymers crystallized from the melt show recognizable features in
some of them. The Bragg reflections, however, appear more broad and diffuse than those obtained
from well-developed single crystals. Such broadening could be the result of the crystals being small.
It could also be the result lattice defects. Because diffraction patterns are too weak, it is impossible to
reach a conclusion. The majority opinion, however, leans toward the small crystal size as being the
