whose formation is induced by radia- 
tion. The crosslinking profoundly af- 
fects melting and solubility. 
Polyethylene 
At room temperature polyethylene 
has a white appearance reminiscent of 
paraffin wax, to which it has a certain 
chemical analogy (apart from molecu- 
lar length). It can be cold-drawn. It 
begins to soften at about 70° C, and 
becomes a viscous transparent liquid 
at about 115° C. It is readily soluble 
in many organic compounds at tem- 
peratures of about 80° C. These 
properties are a serious disadvantage 
in many applications. 
During irradiation, polyethylene can 
be crosslinked readily, and primary 
bonds can form between adjacent 
carbon atoms (5). After a radiation 
dose of about 0.05 units or 2-million 
roentgen (amount depends on molecu- 
lar weight) it no longer melts, because 
it is transformed partly into a three- 
dimensional network. This cffect is 
shown in Fig. 1. 
After irradiation up to 1 unit there 
is no obvious change in the physical 
properties at room temperature; after 
doses up to 10 units it becomes progres- 
sively more flexible; above this it 
stiffens again and may transform into 
brittle glass-like material. In this 
region it is almost entirely amorphous, 
even at room temperature. It de- 
velops a strong yellow or brown tint 
similar to that observed in many 
polymers after irradiation. 
By irradiation of specimens under 
certain conditions for about 4 units, 
polyethylene can be converted into a 
new type of polymer that is flexible, 
amorphous and largely transparent at 
room temperature. The amorphous 
character is retained down to very low 
temperatures. This material is so very 
different from the more usual poly- 
ethylene that a new name, ‘‘setylene,”’ 
might well be used to emphasize its 
thermoset character. 
Melting. It is misleading, perhaps, 
to refer to a radiation-produced rise in 
the melting point if by this is meant 
the removal of crystallinity. In ordi- 
nary polyethylene, the molecules are 
bound together largely by Van der 
Waals forces in the crystalline regions, 
which comprise about two-thirds of the 
specimen at room temperature. When 
these melt, the solid polymer is con- 
verted to a liquid. Radiation does 
not increase the melting point of these 
170 
crystalline regions—instead there is a 
slight decrease (6). The crosslinks 
produced by radiation hold the mole- 
cules together even when crystallinity 
is destroyed above 115° C. The irra- 
diated polymer then is transformed into 
a flexible amorphous material that 
has certain rubber-like properties as 
shown in Fig. 2. 
The melting properties of slightly 
irradiated polyethylene are shown in 
Fig. 3. Small cubes were placed in 
the reactor, irradiated to varying ex- 
tents, and then were heated to 190° C 
for 90 min. The unirradiated cubes 
fused together into a liquid that subse- 
quently solidified to the shape shown. 
The same was true of polyethylene 
irradiated for varying dosages up to 
about 0.04 or 0.05 units, though to a 
less marked extent because the small 
amount of crosslinking that had oc- 
curred increased the average molecular 
weight sufficiently to produce a very 
viscous liquid that did not flow readily. 
Longer irradiations prevented the cubes 
from fusing together and they could 
be separated readily. 
The transition at about 0.05 units of 
radiation corresponds to about 0.5 
crosslinks per molecule. This figure is 
lower than the minimum that might 
be expected (namely one crosslink per 
molecule) for gel formation. The low 
transition occurs because the molecules 
are of different sizes and the longer 
molecules have a greater chance of 
having at least one crosslink. These 
longer molecules link to form a non- 
fusible network that retains the shorter 
molecules that have not been cross- 
linked. The shorter molecules can be 
‘‘Memory”’ Effect 
An interesting property of lightly-irradiated polyethylene can be 
referred to as the ‘‘memory”’ effect. 
If the polymer is crosslinked very 
slightly (abcut 1.5 crosslinks per molecule) and then heated above the 
usual melting point, it is extremely flexible and can be deformed very 
considerably. If this modified shape is cooled, crystallization occurs 
and the molecules become locked in their new position. 
then resembles polyethylene molded into this new shape. 
The specimen 
However, on 
reheating the specimen above the melting point, the crystallites melt and 
the few crosslinks present return the specimen to its original shape. 
The process can be repeated any number of times, and the time of 
storage in the distorted form is irrelevant. 
n 
Young's Modulus (108 dynes /cm®) 
07% 
(amorphous, 
crosslinked) 
0% (unirradiated )~~_ 
4 
50 100 
Temperature (°C) 
FIG. 4. 
polyethylene. Percentages give 
tions of carbons crosslinked 
Elastic properties of irradiated 
propor- 
# Rubber-like| Glass-like, 
elasticity elasticity 
E=32RT/Mc- 
Young's Modulus (10° dynes/cm?) 
5 10 20 
Radiation Dose (units) 
200 100 50 20 10 
Average Number of Corbon Atoms Between Links| 
50 
FIG. 5. Elastic properties of amorphous 
polyethylene at 150° C 
