Chemistry Reference
In-Depth Information
9.7.11 Mechanochemical Techniques for Formation of Block Copolymers
These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of
free radical and ionic species [
413
]. When this application of shear is done in the presence of
monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages
of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in
the visco-elastic state. Both homolytic and heterolytic scissions are possible. The first one yields free
radical and the second one ionic species. Heterolytic scissions require more energy, but should not be
written off as completely unlikely [
413
]. Early work was done with natural rubber [
413
]. It swells
when exposed to many monomers and forms a visco-elastic mass. When this swollen mass is
subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The
mastication reaction was shown to be accompanied by formation of homopolymers [
413
]. Later the
technique was applied to many different polymers with many different monomers [
414
].
9.8 Processes in Polymer Degradation
There are many causes of degradation of polymers. The chief ones among them are heat, oxidation,
light, ionic radiation, hydrolysis, and mechanical shear. The effect can be discoloration, loss of
molecular weight, cross-linking, or cyclization. The loss of molecular weight can be so severe that
the polymer is degraded to a mixture of monomers and oligomers. The other effects can be
blackening or charring and loss of useful properties. On the other hand, mechanical shearing is
often applied to some polymers like rubbers to deliberately reduce molecular weight for commer-
cial processing. In the environment, synthetic polymers generally degrade due to man-made
environmental pollutants in the atmosphere, like carbon monoxide, sulfur dioxide, nitrogen
oxide, and oxidizing smog rich in ozone.
Molecular weight loss occurs through the breaking of primary valence bonds. Such chain scissions
may occur at random points along the polymer backbone or they may take place at the terminal ends
of the polymer where monomer units are released successively. This last effect can be compared to
unzipping
. The response of any particular polymeric material to specific causes of degradation
depends upon the chemical structure.
This chapter presents the degradation processes in some typical commercial polymers. The
materials selected were those that received major attention in the literature.
9.8.1 Thermal Degradation of Common Chain-Growth Polymers
Thermal degradation of polymers is conveniently studied by pyrolytic methods. The polymer litera-
ture contains many reports on such studies conducted at various temperatures in inert atmospheres, in
air, or in vacuum. The volatile products are usually monitored with accompanying measurements of
the weight loss per unit time. The reaction rates are thus measured by:
1. Loss of molecular weight as a function of temperature and the extent of degradation
2. The quantity and the composition of the volatile and nonvolatile products of degradation
3. The activation energy of the degradation process
A general scheme for thermal degradations of chain-growth polymers by free-radical reactions can
be written as follows:
