Chemistry Reference
In-Depth Information
A polymers orientation by uniaxial tension can be accompanied by essen-
tial volume changes. These changes are realized by different mechanisms,
defined by polymeric material structure. So, at uniaxial drawing of semi-
crystalline high density polyethylene (HDPE) volume change up to 30%
was observed, which is due to cracks formation, oriented perpendicularly
to tension direction [1]. At solid-phase extrusion of polymerization-filled
compositions (componors) on the basis of ultra-high-molecular polyeth-
ylene (UHMPE) volume increase is due to interfacial boundaries polymer
matrix-filler breakdown and it can be reached of ~10% [2]. However, in the
case of melt uniaxial tension volume changes are nit observed. It has been
assumed [1] that in this case at melt orientation macromolecules high mo-
bility and structure ordered elements absence are ensured viscous medium
tension without microvoids formation and such oriented melt crystallization
results to formation of the system, not consisting of pores and other con-
tinuum interruptions.
The authors of Ref. [3] substantiated in very general terms and with frac-
tal analysis methods application the described above behavior of polymer
materials at uniaxial tension.
As Balankin shown [4], the relative change of excitation region volume
in deformed body can be presented in the form:
s
s
(
)
(
)
or
d
V
=- =- ± +
,
1
n
12
n
d
VV
d
(14.1)
e
e
rel
d
E
E
or
where d
V
e
is excitation region volume, n and n
e
are Poisson's ratio for initial
and oriented polymers, respectively; s and s
or
are fracture stress,
E
and
E
or
are elasticity modulus for initial and oriented polymers, accordingly. The
first member in right part of the Eq. (14.1) is connected with elastic strains,
the second one - with stress relaxation by plastic deformation, the third one
- with micro, meso- and macrodefects formation. If the defects accumula-
tion results always to volume increase, then volume change, coupled with
plastic deformation, has sigh, opposite one to elastic component: “minus” at
s > 0 and “plus” at s < 0 (compression stress).
The dependence of oriented polymer Poisson's ratio n
e
on the parameter
d
VV
V
D=
rel
d
(14.2)
d
e
has the following form [4]:
