Chemistry Reference
In-Depth Information
As the length of the side chain increase, however, melting points decrease and are accompanied by
increases in flexibility [
7
] until the length of the side chains reached six carbons. At that point, the
minimum takes an upturn and there is an increase in the melting points and decrease in flexibility.
This phenomenon is believed to be due to crystallization of the side-chains [
9
].
Alkyl substituents on the polymers of
a
-olefins that are on the
a
-carbon yield polymers with the
highest melting points. Isomers substituted on the
-carbon, however, if symmetrical, yield polymers
with lower melting points. Unsymmetrical substitutions on the
b
-carbon, on the other hand, tend to
lower the melting points further. Additional drop in the melting points result from substitutions on the
g
b
-carbon or further out on the side-chains. Terminal branching yields rubbery polymers [
10
].
Copolymers melt at lower temperatures than do homopolymers of the individual monomers. By
increasing the amount of a comonomer the melting point decreases down to a minimum (this could
perhaps be compared to a eutectic) and then rises again.
The tightest internal arrangement of macromolecules is achieved by crystallinity. As a result, the
density of a polymer is directly proportional to the degree of crystallinity, which leads to high tensile
strength, and to stiff and hardmaterials that are poorly soluble in common solvents [
11
]. The solubility of
any polymer, however, is not a function of crystallinity alone, but also of the internal structure and of the
molecular weight. The solubility generally decreases with increases in the molecular weight. The fact
that crystalline polymers are less soluble than amorphous ones can be attributed to the binding forces of
the crystals. These binding forces must be overcome to achieve dissolution. Once in solution, however,
crystalline polymers do not exhibit different properties from the amorphous ones. One should also keep
inmind the fact that crosslinked polymers will not melt and will not dissolve in any solvent. This is due to
the fact that the crosslinks prevent the chains from separating and slipping past each other.
2.2 The Amorphous State
A number of macromolecules show little tendency to crystallize or align the chains in some form of
an order and remain disordered in solid form. This, of course, is also the condition of all molten
polymers. Some of them, however, due to structural arrangement, remain completely amorphous
upon cooling. The crystalline polymers, which crystallized from the melt, on the other hand, while
containing areas of crystallinity, also contain some amorphous material. All crystalline polymers are
also amorphous above their melting temperature. When sufficiently cooled, amorphous polymers can
resemble glass. Above this glassy state, long-range segmented motions are possible and the molecular
chains are free to move past each other. On the other hand, in the rigid, glass like state, only short-
range vibrational and rotational motions of the segments are possible. At temperatures above the
glassy state, amorphous polymers resemble rubbers, if crosslinked. If not crosslinked, amorphous
polymers resemble very viscous liquids in their properties and there is molecular disorder. Tobolsky
suggested [
12
] that polymer molecules in an amorphous state might be compared to a bowl of very
long strands of cooked spaghetti. When molten, such molecules are in a state of wriggling motion,
though the amplitude and speed depend very much on the temperature [
13
]. The important thing is to
know that the chains possess random conformations. These conformations were characterized with
the aid of statistical analysis [
14
].
2.2.1 The Glass Transition and the Glassy State
When the polymer cools and the temperature lowers, the mobility in the amorphous regions of the
polymer decreases. The lower the temperature, the stiffer the polymer becomes until a point of
