Chemistry Reference
In-Depth Information
At present it is accepted to consider [1], that devitrificated at testing tem-
perature amorphous phase of semicrystalline polymers is the main source of
such polymers high impact toughness. However, this conclusion has specu-
lative enough grounds and is not supported by any quantitative estimations,
which is due to corresponding models absence. The cluster model and fractal
analysis allow to fill a gap in our knowledge in respect to semicrystalline
polymers behavior at mechanical loading. In Refs. [2, 3], this is carried out
on the example of HDPE samples, which is typical representative of a poly-
mers considered class.
The parameter c in the Eq. (4.9) characterizes a polymer fraction, which
does not participated in plastic deformation process, but subjects to elastic
deformation. For semicrystalline polymer this fraction consists of devitrifi-
cated amorphous phase and crystalline phase part, which was subjected to
partial mechanical disordering [4]. In other words, the parameter c charac-
terizes he deformed polymer structural state. For the considered in Refs. [2,
3] HDPE crystallinity degree
K
= 0.687 and, hence, amorphous phase frac-
tion j
am
, makes up 1 -
K
= 0.313. As estimations according to the Eq. (4.9),
have shown the value c for HDPE depending on notch length and testing
temperature changes within the limits of 0.40 รท 0.83, that is, exceeds j
am
.
Since the yielding process in polymers is realized in densely packed regions,
then this means the necessity of crystalline phase some part disordering, the
fraction of which c
cr
can be determined according to the Eq. (4.68). Such
disordered component of HDPE structure may also be an effective impact
energy dissipative element and therefore, as candidates on a structural com-
ponent role, defining HDPE high plasticity in impact tests, one can assume
the following: devitrificated loosely packed matrix of amorphous phase,
which fraction is equal to j
l.m.
; crystalline phase disordered part, which frac-
tion is equal to c
cr
or their sum with fraction (j
l.m.
+ c
cr
).
The fraction of dissipated in impact loading process energy h
d
can be
estimated within the frameworks of solid body synergetics according to the
equation [5]:
,
(10.1)
-
b
h
= -D
1
fr
d
i
where L
i
is a polymer structure automodelity coefficient, b
fr
is a fractional
part of fracture surface fractal dimension
d
fr
, which, in its turn, is defined
according to the Eqs. (4.50) and (4.51) for quasibrittle and quasiductile frac-
ture types, accordingly. The indicated fracture types boundary is defined by
the condition n = 0.35 [6]. As it has been shown in Ref. [7], for a poly-
