Chemistry Reference
In-Depth Information
fairly regular in structure. Random copolymerization will prevent crystallization.
Thus, polyethylene would be an ideal elastomer except for the fact that its very
regular and symmetrical geometry permits the chains to pack together closely and
crystallize very quickly. To inhibit crystallization and confer elastomeric proper-
ties on this polymer, ethylene is commonly copolymerized with substantial pro-
portions of another olefin or with vinyl acetate.
A melting temperature range is observed in all semicrystalline polymers,
because of variations in the sizes and perfection of crystallites. The crystal melt-
ing point is the highest melting temperature observed in an experiment like differ-
ential scanning calorimetry. It reflects the behavior of the largest, defect-free
crystallites. For high-molecular-weight linear polyethylene this temperature,
labeled
T
m
, is about 141
C. Other regular, symmetrical polymers will have lower
or higher melting points depending on chain flexibility and interchain forces. At
equilibrium at the melting point, the Gibbs free-energy change of the melting pro-
cess,
Δ
G
m
, is zero and
T
m
5Δ
H
m
=Δ
S
m
(4-1)
The conformations of rigid chains will not be much different in the amorphous
state near
T
m
than they are in the crystal lattice. This means that the melting pro-
cess confers relatively little additional disorder on the system;
ΔS
m
is low and
T
m
is increased correspondingly. For example, ether units in poly(ethylene oxide)
(1-42) make this structure more flexible than polyethylene, and the
T
m
of high-
molecular-weight versions of the former species is only 66
C. By contrast, poly
(
p
-xylene) (4-1) is composed of stiff chains and its crystal melting point
is
375
C.
CH
2
CH
2
n
4-1
H
m
values and an increase in
T
m
. Polyamides, which are hydrogen bonded, are higher melting than polyolefins
with the same degree of polymerization, and the melting points of polyamides
decrease with increasing lengths of hydrocarbon sequences between amide group-
ings. Thus the
T
m
of nylon-6 and nylon-11 are 225 and 194
C, respectively.
Bulky side groups in vinyl polymers reduce the rate of crystallization and the
ability to crystallize by preventing the close approach of different chain segments.
Such polymers require long stereoregular configurations (Section 1.12.2) in order
to crystallize.
Crystal perfection and crystallite size are influenced by the rate of crystalliza-
tion, and
T
m
is affected by the thermal history of the sample. Crystals grow in
size by accretion of segments onto stable nuclei. These nuclei do not exist at tem-
peratures above
T
m
, and crystallization occurs at measurable rates only at
Δ
Stronger intermolecular forces result in greater
