1.6 Bionanocomposite Properties and Characterisation
27
the properties of bulk materials [175]. Therefore, only using conventional mechanical
testing is insufficient to evaluate the real reinforcement effect of nanoparticles on
polymer matrices, as well as detect localised reinforcement and deformation.
Additionally, even the results of conventional mechanical testing is directly based
on a fundamental concept that the chain mobility of soft matrices is constrained by
much stiffer nanoparticles [25]. As a consequence, an effective load transfer occurs
from matrices to fillers such as nanoparticles to carry a disproportionally high fraction
of applied loads, thus leading to an increase in load resistance [25]. This phenomenon
is well known to be associated with the level of interfacial bonding between nanoparticles and polymer matrices in terms of interphase existence, dimensions, structures
and compositions [176]. Hence, the crack initiation or propagation may take place
in a nanocomposite system, resulting from the lack of effective load transfer owing
to weak interfacial bonding.
Interfaces are described as a material boundary between two or more phases with
distinct chemical/physical properties and morphological structures. Furthermore, a
material volume influenced by the interfacial interaction can be named ‘interphase’
[177]. Interphase regions start from the interfacial boundary of nanofillers with
different properties from those of bulk nanofillers and end where they are in connection with polymer matrices whose properties also vary from bulk matrices [178].
The material performance of nanocomposites is primarily impacted by the interphase regions where structural and chemical changes such as cross-linking density
and crystalline phases result in major alteration to composite bulk properties [177].
It is also worth mentioning that the change to the mobility of polymeric chains plays
an important role in mechanical and dielectric properties of nanocomposites [175].
Due to large interfacial-area-to-volume ratios in nanocomposites, interfacial regions
consist of a significant portion of bulk nanocomposites. For instance, with the addition of 5 vol% monodispersed spherical nanoparticles (particle diameter: 10 nm and
interphase thickness: 0.5 nm), the volume fraction of interphase can be as high as 25
vol% [175]. More impressively, when particle diameter is reduced to less than 5 nm,
the volume fraction of interphases was increased by over 50 vol% as compared to
that of particles [175]. The characterisation of existing interphase and its associated
properties is generally difficult to undertake as the interphase for nanocomposites is
generally on a nanoscaled level, and thus researchers have had to make most experimental efforts in an uncrosslinked state as the indirect evidence [179]. For example,
chain mobility near interphase regions can be less than those of polymer matrices in
a nanocomposite system [179]. As a result, Litvinov and Steeman [180] employed
proton, low-resolution T2 nuclear magnetic resonance (NMR) relaxation technique
to detect existing interphases between ethylene propylene dine monomers (EPDMs)
and carbon black. It was indicative of a significant difference in the chain mobility of
EPDMs near carbon black surfaces where the generated layer sizes of immobilised
EPDMs were estimated in range of 1–2 of the unit diameter of monomers [180].
Pompe and Mäder [181] identified the interphase according to differential scanning
calorimetry (DSC) in PP/glass fibre composites, which, however, was limited to
semicrystalline polymers as matrices in composite materials at only high glass fibre
contents. Brown et al. [182] studied a relationship between nanoparticle diameter and
27
the properties of bulk materials [175]. Therefore, only using conventional mechanical
testing is insufficient to evaluate the real reinforcement effect of nanoparticles on
polymer matrices, as well as detect localised reinforcement and deformation.
Additionally, even the results of conventional mechanical testing is directly based
on a fundamental concept that the chain mobility of soft matrices is constrained by
much stiffer nanoparticles [25]. As a consequence, an effective load transfer occurs
from matrices to fillers such as nanoparticles to carry a disproportionally high fraction
of applied loads, thus leading to an increase in load resistance [25]. This phenomenon
is well known to be associated with the level of interfacial bonding between nanoparticles and polymer matrices in terms of interphase existence, dimensions, structures
and compositions [176]. Hence, the crack initiation or propagation may take place
in a nanocomposite system, resulting from the lack of effective load transfer owing
to weak interfacial bonding.
Interfaces are described as a material boundary between two or more phases with
distinct chemical/physical properties and morphological structures. Furthermore, a
material volume influenced by the interfacial interaction can be named ‘interphase’
[177]. Interphase regions start from the interfacial boundary of nanofillers with
different properties from those of bulk nanofillers and end where they are in connection with polymer matrices whose properties also vary from bulk matrices [178].
The material performance of nanocomposites is primarily impacted by the interphase regions where structural and chemical changes such as cross-linking density
and crystalline phases result in major alteration to composite bulk properties [177].
It is also worth mentioning that the change to the mobility of polymeric chains plays
an important role in mechanical and dielectric properties of nanocomposites [175].
Due to large interfacial-area-to-volume ratios in nanocomposites, interfacial regions
consist of a significant portion of bulk nanocomposites. For instance, with the addition of 5 vol% monodispersed spherical nanoparticles (particle diameter: 10 nm and
interphase thickness: 0.5 nm), the volume fraction of interphase can be as high as 25
vol% [175]. More impressively, when particle diameter is reduced to less than 5 nm,
the volume fraction of interphases was increased by over 50 vol% as compared to
that of particles [175]. The characterisation of existing interphase and its associated
properties is generally difficult to undertake as the interphase for nanocomposites is
generally on a nanoscaled level, and thus researchers have had to make most experimental efforts in an uncrosslinked state as the indirect evidence [179]. For example,
chain mobility near interphase regions can be less than those of polymer matrices in
a nanocomposite system [179]. As a result, Litvinov and Steeman [180] employed
proton, low-resolution T2 nuclear magnetic resonance (NMR) relaxation technique
to detect existing interphases between ethylene propylene dine monomers (EPDMs)
and carbon black. It was indicative of a significant difference in the chain mobility of
EPDMs near carbon black surfaces where the generated layer sizes of immobilised
EPDMs were estimated in range of 1–2 of the unit diameter of monomers [180].
Pompe and Mäder [181] identified the interphase according to differential scanning
calorimetry (DSC) in PP/glass fibre composites, which, however, was limited to
semicrystalline polymers as matrices in composite materials at only high glass fibre
contents. Brown et al. [182] studied a relationship between nanoparticle diameter and