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Even though the phenomenon of polymer fracture is well
know and well studied,
it is still not well understood.
If a polymer bond is put under sufficient stress, it
will break. However, a polymer chain will, if possible
relieve stresses by slippage or change of conformation.
To reach a stress high enough to break it, the chain
must have restricted movement. This is, to some extent,
the case in a highly oriented system, such as a fibre.
Orientation
A polymer chain can be regarded as a one-dimensional
system since its length is at least 1000 times its diameter.
The polymer has very different properties along the
chain as compared to across it. In a completely relaxed
state, polymer chains are coiled randomly, which means
that the total system will show the same properties
in all directions. The chains can, however, be aligned,
i.e., oriented in one direction, which means that the
total system will, just as the chains, have different
properties in different directions, known as anisotropy.
A fibre is a highly oriented polymer system.
Figura 1
Figure 1. The left part of the figure shows randomly
coiled chains. This system has the same properties in
all directions. To the right are oriented chains, which
gives the system different properties along and transverse
to the orientation axis.
Crystallinity
In addition to orientation, a polymer can also organise
in crystallites. Not all polymers can crystallise, since
not all types of polymer chains can arrange themselves
well enough to form crystals. Polystyrene is, for example,
completely amorphous. A polymer is never 100% crystalline
since it will always contain unordered parts, such as
chain ends and flaws. Therefore, polymers with crystalline
regions are called semi-crystalline. A polymer has to
be semi-crystalline in order to be drawn into a fibre.
In an unoriented polymer the crystallites are spherulitic
with the same characteristics in every direction. When
the polymer is oriented, the sperulites will be deformed
and rearrange in a different type of crystal called
a micro-fibril. This process is called cold drawing,
see Figure 2.
Figura 2
Figure 2. Stress-strain curve and reorganisation of
the crystal structure for a semi-crystalline polymer
during cold drawing.
Between the micro-fibrils there are amorphous parts.
This part contains chain ends, closed loop chains and
tie molecules. Closed loop chains starts and ends in
the same crystal and will thus not help in holding the
crystallites together. Tie molecules, on the other hand,
start and end in different crystals and thus tie the
crystallites together and are responsible for the ultimate
strength of the polymer, see Figure 3.
Figura 3
Figure 3. Crystalline and amorphous parts in a oriented
system. a) Tie chain, b) chain end, c) closed loop chain.
Fracture
The tie chains will start to take the load when the
system is stretched. They will then become taut and
eventually, if they cannot relieve the stress in any
other way, they will break. When they break they will
form radicals, a very reactive molecular intermediate.
The short tie chains will break first and then the slightly
longer chains will start to take the load, see Figure
4.
Figura 4
Figure 4. The taut tie chain in the middle circle breaks
due to the applied force F. The slightly longer tie
chains will then become taut and start to take the load.
Since there are more chains of intermediate length
than short ones, see Figure 5, the polymer will get
progressively stronger until so many chains have been
broken that a crack will start to form. This crack will
propagate and catastrophic failure will result.
Figura 5
Figure 5. Distribution of the number of tie chains
of different length.
Some researchers have reported that this crack will
start when as few as 10% of the tie chains in a region
have broken.
Measurements
When the chains are broken, radicals are formed. These
are very reactive intermediates that are difficult to
measure. There are, however, two measuring techniques,
chemiluminescence (CL) and electron spin resonance (ESR)
that have been used to study radicals formed at fracture.
It is from the result of these measurements that the
theory of fibre fracture has been derived. During the
1970’s there were in particular two major research
groups, one in Soviet Union and one in USA that worked
with the ESR technique. The major findings were that
radicals are not being detected until at approximately
60% of the ultimate load at failure of the fibre. Later
a research group in Australia repeated the same measurements
but using CL, which is known to be a more sensitive
technique, and came to the same conclusion. Figure 6
shows a schematic drawing of how a CL and stress strain
curve of a polyamide can look.
Figura 6
Figure 6. Schematic drawing of stress and CL as a function
of strain of a polyamide fibre.
What was also found was that if the load is cycled
the load of the previous cycle has to be surpassed in
order to create new radicals, see Figure 7. This supports
the theory that when stressed tie chains are broken
and form radicals they are consumed and other longer
chains will start to take the load.
Figura 7
Figure 7. Schematic drawing of CL and stress during
cyclic loading of polyamide fibres.
The radicals that are formed are believed to originate
from the fracture of either bond a or b in the following reaction scheme. These bonds are
slightly weaker than the others due to the stabilising
effect of the amide group.
a b
References
1. K.L. DeVries, Free-Radical Processes in Mechano-Chemical
Degradation of Plastics and Rubber, Journal of Polymer
Science: Applied Polymer Symposium, 1979, p. 439-453.
2. M. Igarashi, Free-Radical Identification by ESR in
Polyethylene and Nylon, Journal of Polymer Science:
Polymer Chemistry Edition, 1983, Vol 21, p. 2405-2425.
3. G.A. George, G.T. Egglestone & S.Z. Riddell,
Stress-Induced Chemiluminescence from Nylon 66 Fibres,
Journal of Applied Polymer Science, 1982, Vol 27, p.
3999-4018.
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