Chemistry Reference
In-Depth Information
FIGURE 4.13
The dependence of yield stress s
Y
on crystallites fraction, subjecting to
partial melting c
cr
, in impact (1) and quasistatic (2) tests for HDPE [51].
It is natural to assume, that the yield stress is connected with parameter
c
cr
as follows: the greater c
cr
, the larger the energy, consumed for melting
and the higher s
Y
. The data of Fig. 4.13 confirm this assumption and the
dependence s
Y
(c
cr
) is extrapolated to finite is
Y
value, since not only crystal-
lites, but also clusters participate in yielding process. As it was to be expect-
ed [54], the crystalline regions role in yielding process realization is much
larger, than the amorphous ones.
Within the frameworks of the cluster model [55] it has been assumed,
that the segment joined to cluster means fluctuation free volume microvoid
“shrinkage” and vice versa. In this case the microvoids number D
N
h
, form-
ing in polymer dilation process, should be approximately equal to segments
number D
N
f
, subjected to partial melting process. These parameters can be
estimated by following methods. The value D
N
h
is equal to [51]:
D
D=
,
f
g
N
(4.27)
h
V
h
where
V
h
is free volume microvoid volume, which can be estimated accord-
ing to the kinetic theory of free volume [27].
Macromolecules total length
L
per polymer volume unit is estimated as
follows [56]:
1
L
=
,
(4.28)
S
