will retain this orientation. The mate- 
rial produced will have different 
properties parallel and perpendicular 
to the direction of stretch. 
Other Polymers 
Numerous other long-chain poly- 
mers can be crosslinked by high-energy 
radiation; these include Nylon, poly- 
vinyl acetate, polyvinyl alcohol, gutta 
percha, and Neoprene. Nylon, for 
example, becomes far more _ brittle 
and is less extensible after irradiation. 
With an adequate amount of cross- 
linking, creep largely can be suppressed. 
The property of crosslinking is not 
confined to long-chain polymers with 
carbon atoms in the main chain. 
Figure 10 shows how lubricating oil 
solidifies. Silicones have been success- 
fully crosslinked, the amount of radia- 
tion required being smaller for the 
longer molecules, as is to be expected 
if crosslinking occurs at random. The 
crosslinked polymer formed is trans- 
parent and does not flow even at fairly 
elevated temperatures. 
CHAIN DEGRADATION 
The individual molecules in poly- 
tetrafluoroethylene, polymethyl meth- 
acrylate, polyisobutylene, and cellulose 
are degraded by high-energy radiation. 
The changes have varying effects. 
Polytetrafluoroethylene 
The essential difference between this 
polymer and polyethylene consists in 
the substitution of fluorine for hydro- 
gen atoms on the carbon atoms of the 
main chain. The inert character of 
PTFE can be ascribed to the strong 
bond between the fluorine and carbon 
atoms. 
If the crosslinking of polyethylene is 
due to the breaking of C-H bonds and 
the removal of hydrogen having active 
carbon atoms that can crosslink, it is 
to be expected that the possibility of 
crosslinking in PTFE will be dimin- 
ished greatly by the high stability 
of the C-F bonds. This behavior is, in 
fact, observed. On irradiation, PTFE 
tends to break up, and gases (par- 
ticularly CF) are released (13). En- 
ergy absorbed in the molecules breaks 
the C-C bonds in the main chain 
rather than the stronger C-F bonds. 
Figure 11 shows that after 10 units 
of radiation, solid blocks of PTFE 
decompose to a coarse powder. Thin 
layers of the same material are more 
resistant. A detailed analysis (based 
fo) 
c 
o 
o 
z 
fe 
o 
oO 
= 
ra 
e 
o 
= 
ie 
e 
=) 
o 
2 
° 
= 
Crosslinks, Mc, (X 104) 
Ol 
Radiation Dose, R,(units) 
FIG. 9. Effect of radiation on crosslinking 
of rubber 
on weight changes during irradiation) 
shows that weight losses are propor- 
tional to the surface area and to the 
square of the radiation dose. 
These results are consistent with the 
hypothesis that chain fracture occurs 
at random, producing groupings the 
shortest of which are eventually 
evolved as CFy, provided they are 
produced close to the surface. Similar 
gases produced deeper within the speci- 
men cannot escape, and the internal 
pressure of these gases (together with 
the weakening due to the reduction of 
SS Bie ral ach 
FIG. 10. Radiation-induced solidification 
(by crosslinking) of lubricating oil into a 
yellow, bubbled mass 
molecular weight arising from the frac- 
ture of the main chain) breaks the 
material into fragments. 
The irradiated material softens at a 
lower temperature, and this may offer 
considerable processing advantages as 
compared with the more usual form of 
polytetrafluoroethylene. 
Polymethy! Methacrylate 
Polymethyl methacrylate shows most 
strikingly the degradative effects pro- 
duced on the main and side chains by 
exposure to high-energy radiation (14, 
15). 
When exposed to small radiation 
doses (of the order of 0.5 units or less) 
specimens of the polymer in the form 
of rods or blocks appear to be almost 
unaffected; the only obvious physical 
changes are a slight yellow coloration 
and an increased brittleness. After 
irradiations of the order ‘of one unit 
at about 80° C., the polymer swells 
into a foam or mass of bubbles con- 
sisting mainly of hydrogen, carbon 
monoxide and carbon dioxide, which 
are the main breakdown products of 
the side chain. Increases in volume 
of the order of eight times can be 
produced. 
Figure 12 shows that if irradiation 
is stopped before the foaming stage is 
reached, and the specimen is subse- 
quently heated, the same process of 
foaming takes place. The higher the 
radiation dose, the lower is the tem- 
perature needed to produce this effect, 
at least for temperatures above about 
10> G. 
The explanation is that radiation 
produces side-chain fracture at random 
throughout the specimen, and _ the 
resultant gases are retained in the solid 
until a combination of increased pres- 
sure (due to the temperature rise) and 
weakened polymer structure cause 
bubble formation. Bubbles do not 
occur near the surface; the gases pro- 
duced there can escape. 
This behavior may be of interest in 
connection with research into the 
diffusion of gases in solids and the 
formation of nuclei. The process is, in 
some respects, analogous to the forma- 
tion of bubbles in boiling liquids; but it 
has the advantage (from the point of 
view of the investigator) that it takes 
place more slowly and can be stopped 
at any stage by sudden cooling. 
Viscosity measurements of irradiated 
material give a measure of the molecu- 
lar weight of the polymer. Molecular 
173 
