Chemistry Reference
In-Depth Information
As results obtained in Refs. [34, 39] have shown, the behavior of cross-
linked polymers is just slightly different from the above-described one for
linear PC and PAr. However, further progress in this field is quite difficult
due to, at least, two reasons: excessive overestimation of the chemical cross-
links role and the quantitative structural model absence. In the Ref. [39]
the yielding mechanism of cross-linked polymer has been offered, based
on the application of the cluster model and the latest developments in the
deformable solid body synergetics field [40] on the example of two already
above-mentioned epoxy polymers of amine (EP-1) and anhydrazide (EP-2)
curing type.
Figure 4.8
shows the plots s - e for EP-2 under uniaxial compression
of the sample up to failure (curve 1) and at successive loading up to strain
e exceeding the yield strain e
Y
(curves 2-4). Comparison of these plots in-
dicates consecutive lowering of the “yield tooth” under constant cold flow
stress, s
p
. High values of s
p
assume corresponding values of stable clusters
network density
c
n
, which is much higher than the chemical cross-links
network density n
c
[34]. Thus though the behavior of a cross-linked polymer
on the cold flow plateau is described within the frameworks of the rubber
high-elasticity theory, the stable clusters network in this part of s - e plots
is preserved. The only process proceeding is the decay of instable clusters,
determining the loosely packed matrix devitrification. This process begins
at the stress equal to proportionality limit that correlates with the data from
[41], where the action of this stress and temperature
T
2
=
T
g
' is assumed
analogous. The analogy between cold flow and glass transition processes is
partial only: the only one component, the loosely packed matrix, is devitrivi-
cated. Besides, complete decay of instable clusters occurs not in the point of
yielding reaching at s
Y
, but at the beginning of cold flow plateau at s
p
. This
can be observed from s - e diagrams shown in Fig. 4.8. As a consequence,
the yielding is regulated not by the loosely packed matrix devitrification, but
by other mechanism. As it is shown above, as such mechanism the stability
loss by clusters in the mechanical stress field can be assumed, which also
follows from the well-known fact of derivative
d
s/
d
e turning to zero in the
yield point [42]. According to the Ref. [40] critical shear strain g
*
leading to
the loss of shear stability by a solid is equal to:
st
1
mn
,
(4.17)
g
*
=
