weight is found to decrease rapidly 
with radiation dose as 
M, = K/(R + Ro) 
where M, is the viscosity average 
molecular weight (deduced from vis- 
cosity measurements), A is a constant, 
R is the radiation dose and Rp is a small 
correction factor that can be considered 
as the ‘‘virtual”’ radiation dose needed 
to produce the initial molecular weight 
from an infinite chain. Therefore, 
measurement of viscosity appears to 
offer an extremely convenient means 
of estimating radiation doses of from 
1-million roentgens upwards. All that 
is required is to irradiate a small piece 
of Perspex, dissolve it in a suitable 
solvent, and measure the viscosities of 
various concentrations at a given 
temperature. This method of meas- 
uring intrinsic viscosity is used often 
in polymer science and can give very 
reproducible results. The irradiated 
specimen may be kept for considerable 
periods before testing. 
The two processes of side-chain and 
main-chain degradation are closely 
related, each main chain fracture being 
associated with the decomposition of 
approximately one side chain. The 
energy required per main-chain frac- 
ture is about 60 ev. Main-chain frac- 
ture takes place uniformly throughout 
the specimen; the absence of a surface 
effect is demonstrated by viscosity 
measurements. On the other hand, 
bubbling does not take place at a 
distance of about 1 mm from the sur- 
face. It has been shown that this is 
due to the evolution of the gas produced 
by side-chain fracture and that this 
process takes place during irradiation. 
The gases produced by irradiation can 
be retained in the polymer for con- 
siderable periods prior to bubbling by 
heating. 
One can envisage a possible use for 
this irradiated polymer for heat insula- 
tion purposes. Blocks of irradiated 
polymethyl methacrylate could be 
placed around the vessel to be insu- 
lated. When heated, bubble forma- 
tion would occur and insulating mate- 
rial would be produced in situ. The 
absence of bubbles in the outer layers 
would result in a material having a 
tough skin. 
Polyisobutylene 
Polyisobutylene has methyl groups 
replacing some of the hydrogen atoms 
in polyethylene. It degrades rapidly 
8174 
when irradiated and can be converted 
readily to a viscous liquid. At first 
thought this is surprising because we 
would expect hydrogen atoms to be 
evolved and, thus, active bonds to be 
made available for crosslinking. Dif- 
fraction data show that the molecular 
chain in polyisobutylene is arranged 
very differently from that in most long- 
chain polymers, probably due to steric 
hindrance of the relatively large side 
groups. Therefore, the main chain is 
in a state of strain, and this weakness 
renders it more liable to fracture under 
radiation. 
APPLICATIONS 
The fields in which use of high- 
energy radiation can be applied to 
produce changes in long-chain mole- 
cules appear to be considerable. 
Polymer research. It is possible to 
use radiation to produce polymers 
for polyisobutylene and polymethyl 
methacrylate. 
A further field is in the quantitative 
study of the ability of various types of 
bonding to resist the effect of high- 
energy radiation. The influence of 
the benzene ring in polystyrene has 
been mentioned, and other structures 
are being studied. This is of particular 
interest in such fields as copolymeriza- 
tion and in studies of the role played 
by sulfur and carbon black in rubber 
vulcanization. 
The process is not limited to long- 
chain polymers. It can be used to 
modify the properties of shorter mole- 
cules, such as those present in oils, 
although the radiation doses needed 
are much greater. 
The method also can be applied to 
biological systems. A fuller under- 
standing can be expected only when 
studies of simpler long-chain molecules 
FIG, 11. 
Radiation-induced degradation 
of a solid rod of polytetrafluoroethylene 
into a coarse powder 
(crosslinked to any required extent) 
and study their properties as a function 
of the degree of crosslinking. By 
using small radiation doses, a known 
degree of chain branching can be 
produced, and the effect of this on 
viscosity, softening or melting points, 
elastic properties, etc., then can be 
investigated quantitatively. 
Where radiation-induced chain deg- 
radation occurs, it becomes possible to 
produce polymers of known molecular 
weight and of a given molecular weight 
distribution (namely, that correspond- 
ing to random chain fracture). This 
method has already been used to study 
the relationship between viscosity 
molecular weight and intrinsic viscosity 
FIG. 12. After the slightly irradiated rod 
of polymethyl methacrylate (left) has been 
heated to about 120° C, it is expanded by 
bubble formation 
(in which the chemical and physical 
structures are known) permit making 
useful predictions of the mechanism of 
radiation effects. 
Commercial possibilities. Apart 
from its use in research work, the possi- 
bility of producing crosslinked, de- 
graded, and foamed material under 
accurately controlled conditions by a 
purely physical process can well have 
valuable commercial possibilities. In 
the long run, these will depend on the 
cost of irradiation, the enhanced value 
of the material, and the comparative 
cost of the competitive chemical 
process (where one is available). 
There are two stages in which the 
process can be introduced. In the 
