11 Structure and Electrical/Dielectric Properties of Ion-Conductive Polymer. . .
183
The similar dependences were found in various sources [36–38]. In [39] the
growth of T g from −46 ◦ C to −30 ◦ C at low salt content of LiClO 4 in PEO
(1 mol/kg) and the subsequent slow decrease to −35 ◦ C for the concentration
of 7.5 mol/kg was reported. At the same time, according to [9], the addition of
lithium salt LiPF 6 in a PEO with high molecular weight reduces both glass transition
temperature and melting point. The injection of another lithium salt LiCF 3 SO 3 in
PEO results in lowering of T g from −65 ◦ C to −71 ◦ C [40]. In these cases, the
coordination bonds were absent, and, contrary to DEG-1, the molecular mobility of
the polymer chains increases. According to [41], in the poly(acrylonitrile-co-butyl
acrylate)-LiTFSI systems, the T g is lowered with the increased salt content: from
+48 ◦ C for pure copolymer to −44 ◦ C for the electrolyte containing 95 wt.% of
the salt. That can be a result of different strength of interactions between polymer
matrices and salts with no stiffening of chains due to physical cross-linking that
occurs because the solution of salt acts as a plasticizer.
Electrical and Dielectric Properties Figure 11.4 shows the isothermal spectra of
the real part of the conductivity (σ ) and the complex permittivity (ε ) for different
concentrations of LiClO 4 in DEG-1 obtained in the temperature range from −60 ◦ C
to 200 ◦ C. One can see that values and character of the σ and ε curves depend
on two factors: the content of LiClO 4 and the temperature of measurements. At
temperatures below the glass transition, the permittivity has low values and hardly
varies with frequency indicating the “blocking effect” of free charge carriers due
to the low mobility of macromolecular chains of the polymer matrix. In the same
temperature range, the values of the real part of the complex conductivity vary
linearly with frequency, i.e., such systems are insulators. At temperatures higher
than T g , “defrosting” of the polymer chains occurs that leads to the release of lithium
cations and growth of ε values. Free lithium cations pass into the conducting band
and begin to move along the polymer chain through the interactions with oxygen
ether atoms, which exist in macromolecular chains (Fig. 11.5). This charge transfer
leads to an increase in the electrical conductivity of the systems and the appearance
of a plateau at low frequencies (the so-called DC conductivity plateau, an isotherms
area, where conductivity values are independent on frequency) on the spectra of
the real part of the complex conductivity. The σ dependence on angular frequency
ω = 2πf is described by the following equation [45, 46]:
σ
(ω) = σ 0 + Aω
s
(11.1)
where σ 0 is the conductivity that is independent on frequency, the exponent factor
s equals 0 < s ≤ 1, and £ is a numeric factor. The contribution of the second
part is insignificant at low frequencies, and the conductivity, which is independent
on frequency (plateau on the graph), is associated with the DC conductivity. At
high frequencies above the critical frequency f c , the main role is played by the
second frequency-dependent parameter σ ac (ω)∼ω s that characterizes the jumping
conductivity in a disordered solid state (AC conductivity). The critical frequency
f c and the parameters A and s in the Eq. 11.1 depend on the temperature and the
conductivity of the systems [46].
183
The similar dependences were found in various sources [36–38]. In [39] the
growth of T g from −46 ◦ C to −30 ◦ C at low salt content of LiClO 4 in PEO
(1 mol/kg) and the subsequent slow decrease to −35 ◦ C for the concentration
of 7.5 mol/kg was reported. At the same time, according to [9], the addition of
lithium salt LiPF 6 in a PEO with high molecular weight reduces both glass transition
temperature and melting point. The injection of another lithium salt LiCF 3 SO 3 in
PEO results in lowering of T g from −65 ◦ C to −71 ◦ C [40]. In these cases, the
coordination bonds were absent, and, contrary to DEG-1, the molecular mobility of
the polymer chains increases. According to [41], in the poly(acrylonitrile-co-butyl
acrylate)-LiTFSI systems, the T g is lowered with the increased salt content: from
+48 ◦ C for pure copolymer to −44 ◦ C for the electrolyte containing 95 wt.% of
the salt. That can be a result of different strength of interactions between polymer
matrices and salts with no stiffening of chains due to physical cross-linking that
occurs because the solution of salt acts as a plasticizer.
Electrical and Dielectric Properties Figure 11.4 shows the isothermal spectra of
the real part of the conductivity (σ ) and the complex permittivity (ε ) for different
concentrations of LiClO 4 in DEG-1 obtained in the temperature range from −60 ◦ C
to 200 ◦ C. One can see that values and character of the σ and ε curves depend
on two factors: the content of LiClO 4 and the temperature of measurements. At
temperatures below the glass transition, the permittivity has low values and hardly
varies with frequency indicating the “blocking effect” of free charge carriers due
to the low mobility of macromolecular chains of the polymer matrix. In the same
temperature range, the values of the real part of the complex conductivity vary
linearly with frequency, i.e., such systems are insulators. At temperatures higher
than T g , “defrosting” of the polymer chains occurs that leads to the release of lithium
cations and growth of ε values. Free lithium cations pass into the conducting band
and begin to move along the polymer chain through the interactions with oxygen
ether atoms, which exist in macromolecular chains (Fig. 11.5). This charge transfer
leads to an increase in the electrical conductivity of the systems and the appearance
of a plateau at low frequencies (the so-called DC conductivity plateau, an isotherms
area, where conductivity values are independent on frequency) on the spectra of
the real part of the complex conductivity. The σ dependence on angular frequency
ω = 2πf is described by the following equation [45, 46]:
σ
(ω) = σ 0 + Aω
s
(11.1)
where σ 0 is the conductivity that is independent on frequency, the exponent factor
s equals 0 < s ≤ 1, and £ is a numeric factor. The contribution of the second
part is insignificant at low frequencies, and the conductivity, which is independent
on frequency (plateau on the graph), is associated with the DC conductivity. At
high frequencies above the critical frequency f c , the main role is played by the
second frequency-dependent parameter σ ac (ω)∼ω s that characterizes the jumping
conductivity in a disordered solid state (AC conductivity). The critical frequency
f c and the parameters A and s in the Eq. 11.1 depend on the temperature and the
conductivity of the systems [46].