Chemistry Reference
In-Depth Information
FIGURE 15.28
The stress - strain (σ - ε) diagrams for initial (1), extruded up to ε
e
= 0.52
(2) and annealed (3) REP samples [62]
The hydrostatic extrusion and subsequent annealing of rarely cross-
linked epoxy polymer (REP) result to very essential changes of its mechani-
cal behavior and properties, in addition unexpected ones enough. The quali-
tative changes of REP mechanical behavior can be monitored according to
the corresponding changes of the stress - strain (σ - ε) diagrams, shown in
Fig. 15.28. The initial REP shows the expected enough behavior and both
its elasticity modulus
E
and yield stress σ
Y
are typical for such polymers
at testing temperature
T
being distant from glass transition temperature
T
g
on about 40 K [51]. The small (≈ 3 MPa) stress drop beyond yield stress is
observed, that is also typical for amorphous polymers [61]. However, REP
extrusion up to
e
= 0.52 results to stress drop Δσ
Y
(“yield tooth”) disappear-
ance and to the essential
E
and σ
Y
reduction. Besides, the diagram σ - ε itself
is now more like the similar diagram for rubber, than for glassy polymer.
This specimen annealing at maximum temperature
T
an
= 353 K gives no less
strong, but diametrically opposite effect - yield stress and elasticity modu-
lus increase sharply (the latter in about twice in comparison with the initial
REP and more than one order in comparison with the extruded specimen).
Besides, the strongly pronounced “yield tooth” appears. Let us note, that
specimen shrinkage at annealing is small (≈10%), that makes up about 20%
of e
e
[62].
The common picture of parameters
E
and σ
Y
change as a function of ε
e
is presented in
Figs. 15.29
a
nd
15.30
,
accordingly. As one can see, both in-
