Chemistry Reference
In-Depth Information
where l is draw ratio. The value l is accepted equal to 1.2, that is, it corre-
sponds to point on diagram s - e immediately behind yield stress.
The calculation according to the Eq. (6.14) with s = 0.33 MPa (the Eq.
(6.16)) using gives the value h
o
= 0.66 × 10
7
Pa×s, that corresponds excel-
lently to the estimation according to the Eqs. (6.12) and (6.13). This cor-
respondence confirms clearly, that polymer cold flow behind yield stress is
possible only at loosely packed matrix devitrification condition. Actually
calculation according to the Eq. (6.12) with the value h
o
= 0.82×10
9
Pa×S
for glassy loosely packed matrix using gives the value is,
p
≈ 1.75 GPa, that is,
without having physical significance. This s
p
value exceeds PC theoretical
strength [32] and that is why polymer should fail up to cold flow realization
that is observed in the brittle fracture case.
As it was noted above, the cluster model [18, 23] explains two more fea-
tures of glassy polymers behavior on cold flow plateau. An experimentally
observed high values s
p
are due to high values n
cl
, which are about of order
larger than nl
l
[23] and glassy polymer rubber-like behavior on the indicated
plateau is due to loosely packed matrix rubber-like state.
The authors of Ref. [19] carried out the theoretical estimations s
p
for two
polymers (HDPE and PAr) according to the Eq. (6.12) at the condition h
o
=
const = 0.69×10
7
Pa×s. The parameters n
cl
and n
cl
values for these polymers
were accepted according to the data of Refs. [32] and [23], respectively. The
calculation gave the following s
p
values: 18.0 MPa for HDPE and 47.8 MPa
for PAr, that corresponds well (within the limits of 10%-th error) to experi-
mental data.
Hence, the cluster model of polymers amorphous state structure and the
model of WS aggregates friction at translational motion in viscous medium
[24] combination allows to describe solid-phase polymers behavior on cold
flow (forced high-elasticity) plateau not only qualitatively, but also quan-
titatively. In addition the cluster model explains these polymers behavior
features on the indicated part of diagram s - e, which are not responded to
explanation within the frameworks of other models [14].
As it is known [33], a macroscopic polymer sample is capable to bear
large enough stress only at definite molecular weight
MW
(
MW
cr
) reach-
ing.
MW
cr
is defined by macromolecular entanglements network formation,
which is capable to spread load over entire polymer sample. Thus, entangle-
ments network represents itself some bonds frame, by its physical signifi-
cance similar to conductive bonds frame in mixture metal-insulator [34]. At
present it is known, that in glassy polymers two types of macromolecular
