Epoxy Molecular Structure Alteration in Graphene …
465
models for such layers are widely discussed in west literature on electrochemistry
[55, 56]. It has earlier been pointed out [57] that interfacial electric effects related
to DEL-formation may give their contribution into the overall dielectric properties
nanocomposites.
In our nanocomposites, such DELs consist of free γ -dipoles and lateral moieties
of EMCs and will be localized along both lateral facets and defect areas of basal
planes of the nanoparticles. It is clear, that γ -dipoles involved in DELs will give
negligible contribution in ε 2 .
Taking the above-mentioned mechanisms of epoxy–graphene interaction into
account, loading dependences of ε 1 can be explained in the assumption that the
first mechanism (breeding γ -dipoles) dominates at 0% < C f < 1%, whereas at 1%
< C f < 2% an intensity of the second mechanism (binding γ -dipoles) enhances as
much as both the processes compensate each other. As a consequence, ε 1 undergoes
negligible variations. At C f > 2% ASSs having occupied and DELs having formed,
the first process prevails again.
In contrast to ε 1
C f
, ε 2
C f
varies with increasing f . At 0% < C f < 2%
ε 2
C f
varies in a non-regular way with increasing f . When C f > 2% ε 2 increases
monotonously with increasing f . Such behavior can be understood as follows. Obviously that small and movable γ -dipoles contribute into ε 2 over the entire frequency
region studied. As mentioned above, γ -dipoles involved in DELs do not contribute
in ε 2 . However, the single DEL can behave as a spontaneously polarized domain,
which is capable to dissipate the energy of an electric field applied. Its contribution
will depend on its size and spatial orientation relative to the field.
To penetrate into physical background of the effect of increasing ε 1 and ε 2 of epoxy
with increasing its MLGNP-loading, loading-related evolution of minor dielectric
relaxation (MDR) processes has been studied. It is known that a number of the minor
relaxation (as compared to α-relaxation) processes take place in the vitreous state of
thermosets (i.e., at T < T g ). These are referred to as “the dipole-group relaxation,”
labeled by β, γ , δ, and so on (beginning at the high-temperature end of ε 2 (T )), and
originates mainly from local vibration modes of the molecules [53].
In pure DGEBA-epoxy resins, MDR-processes have been widely studied earlier
[58–64]. So, Mikolajczak et al. [58] have studied dynamic mechanical behavior of
an epoxy resin made with the diglicidyl ether of bisphenol A (DGEBA) and 4,4diaminodiphenylmethane (DDM) and found both γ -and β-relaxation processes in
the glassy region. They suggested that the existence of γ - and β-relaxations indicate
merely two types of local regions of mobility in the epoxy. Namely, the γ -relaxation
may indicate molecular motions in those regions where polymer chains are relatively
loosely packed, whereas β-relaxation deals with the motions in those regions where
loose packing is caused by unreacted molecular segments and/or by subtantially
lower cross-linking densities. Another possible mechanism for the γ -relaxation is
due to the crankshaft rotation [65] below the glass transition temperature.
Indeed, recent studies which have been made by using the atomic force microscope at fracture interfaces [66–70], IR-spectroscopy [66, 67, 69, 70], and differential
465
models for such layers are widely discussed in west literature on electrochemistry
[55, 56]. It has earlier been pointed out [57] that interfacial electric effects related
to DEL-formation may give their contribution into the overall dielectric properties
nanocomposites.
In our nanocomposites, such DELs consist of free γ -dipoles and lateral moieties
of EMCs and will be localized along both lateral facets and defect areas of basal
planes of the nanoparticles. It is clear, that γ -dipoles involved in DELs will give
negligible contribution in ε 2 .
Taking the above-mentioned mechanisms of epoxy–graphene interaction into
account, loading dependences of ε 1 can be explained in the assumption that the
first mechanism (breeding γ -dipoles) dominates at 0% < C f < 1%, whereas at 1%
< C f < 2% an intensity of the second mechanism (binding γ -dipoles) enhances as
much as both the processes compensate each other. As a consequence, ε 1 undergoes
negligible variations. At C f > 2% ASSs having occupied and DELs having formed,
the first process prevails again.
In contrast to ε 1
C f
, ε 2
C f
varies with increasing f . At 0% < C f < 2%
ε 2
C f
varies in a non-regular way with increasing f . When C f > 2% ε 2 increases
monotonously with increasing f . Such behavior can be understood as follows. Obviously that small and movable γ -dipoles contribute into ε 2 over the entire frequency
region studied. As mentioned above, γ -dipoles involved in DELs do not contribute
in ε 2 . However, the single DEL can behave as a spontaneously polarized domain,
which is capable to dissipate the energy of an electric field applied. Its contribution
will depend on its size and spatial orientation relative to the field.
To penetrate into physical background of the effect of increasing ε 1 and ε 2 of epoxy
with increasing its MLGNP-loading, loading-related evolution of minor dielectric
relaxation (MDR) processes has been studied. It is known that a number of the minor
relaxation (as compared to α-relaxation) processes take place in the vitreous state of
thermosets (i.e., at T < T g ). These are referred to as “the dipole-group relaxation,”
labeled by β, γ , δ, and so on (beginning at the high-temperature end of ε 2 (T )), and
originates mainly from local vibration modes of the molecules [53].
In pure DGEBA-epoxy resins, MDR-processes have been widely studied earlier
[58–64]. So, Mikolajczak et al. [58] have studied dynamic mechanical behavior of
an epoxy resin made with the diglicidyl ether of bisphenol A (DGEBA) and 4,4diaminodiphenylmethane (DDM) and found both γ -and β-relaxation processes in
the glassy region. They suggested that the existence of γ - and β-relaxations indicate
merely two types of local regions of mobility in the epoxy. Namely, the γ -relaxation
may indicate molecular motions in those regions where polymer chains are relatively
loosely packed, whereas β-relaxation deals with the motions in those regions where
loose packing is caused by unreacted molecular segments and/or by subtantially
lower cross-linking densities. Another possible mechanism for the γ -relaxation is
due to the crankshaft rotation [65] below the glass transition temperature.
Indeed, recent studies which have been made by using the atomic force microscope at fracture interfaces [66–70], IR-spectroscopy [66, 67, 69, 70], and differential