Chemistry Reference
In-Depth Information
As one can see, all included in the Eq. (4.59) parameters for each poly-
mer are constant, from which it follows, that
cr
cr
s
= 16.3 MPa for HDPE and 17.6 MPa - for PP. Thus, the change, namely,
increase is
Y
at
e
growth is defined by polymers noncrystalline regions con-
tribution
s
= const. The value
s
[77].
The Grist dislocation model [98] assumes the formation in polymers
crystalline regions of screw dislocation (or such dislocation pair) with Burg-
ers vector
b
and the yield process is realized at the formation of critical
nucleus domain with size
u
:
nc
Bb
*
u
=
,
(4.60)
2
pt
Y
where
B
is an elastic constant, t
Y
shear yield stress.
In its turn, the domain with size
U
*
is formed at energetic barrier D
G
*
overcoming [98]:
€
‚
2
Bl l
u
*
*
D=
G
d
ln
-
1
,
(4.61)
„
…
2
p
r
†
‡
0
where
l
d
is dislocation length, which is equal to crystalline thickness,
r
o
is
dislocation core radius.
In the model [98] it has been assumed, that nucleus domain with size
u
*
is formed in defect-free part of semicrystalline polymer, that is, in crystallite.
Within the frameworks of model [1] and in respect to these polymers amor-
phous phase structure such region is loosely packed matrix, surrounding a
local order region (cluster), whose structure is close enough to defect-free
polymer structure, postulated by the Flory “felt” model [16, 17]. In such
treatment the value
u
*
can be determined as follows [43]:
u
*
=
R
cl
-
r
cl
,
(4.62)
where
R
cl
is one half of distance between neighboring clusters centers,
r
cl
is
actually cluster radius.
The value
R
cl
is determined according to the equation [18]:
-
1/ 3
2
n
e
R
=
18
,
Å,
(4.63)
cl
F
