Chemistry Reference
In-Depth Information
a
if
(a
c
) shows that local order regions of BPE noncrystalline regions are con-
centrated mainly in anisotropic interfacial regions, as it has been assumed
The common fraction of the ordered regions (clusters and crystallites)
j
ord
can be determined according to the percolation relationship [91]:
j
ord
= 0.03(
T
m
-
T
)
0.55
,
(4.66)
where
T
m
and
T
are melting and testing temperatures, respectively.
Then clusters relative fraction j
cl
is estimated from the obvious relation-
ship [102]:
j
cl
= j
ord
- a
c
.
(4.67)
The comparison of the experimental and estimated as a sum of crystal-
line and noncrystalline regions contributions in yield stress theoretical val-
ues a
Y
shows their good correspondence for considered BPE.
It has been noted earlier [104, 105], that stress decay beyond stress
(“yield tooth”) for polyethylenes is expressed the stronger the greater value
a
c
is. For amorphous polymers it has been shown that the indicated “yield
tooth” is due to instable clusters decay in yielding process and this decay is
expressed the clearer the higher instable clusters relative fraction [39]. By
analogy with the indicated mechanism the authors of Ref. [102] assume that
“yield tooth” will be the stronger the larger crystallites fraction is subjected
to mechanical disordering (partial melting) in yielding process. The indi-
cated fraction of crystallites c
cr
is determined as difference [102]:
c
cr
= c - a
am
,
(4.68)
where c is polymer fraction, subjecting to elastic deformation, a
am
is fraction
of amorphous phase.
The value c can be determined within the frameworks of polymers plas-
ticity fractal concept [35] according to the Eq. (4.9). The dependence of c
cr
on a
c
shows c
cr
increasing at a
c
growth [102]. At small a
c
values all crystal-
lites are subjected to disordering owing to that “yield tooth” in BPE curves
stress-strain is absent and these curves are acquired the form, which is typi-
cal for rubbers. Hence, stress decay beyond yield stress intensification is due
to c
cr
growth [103].
