92
5 Reinforcing Mechanism of Rubber by Nanofiller
aggregate itself. The clustering or association of the primary aggregates is always
via bound rubber, definitely not via direct contact between the primary particles.
Thirdly, Fig. 5.5 is shown a sketch of the nanofiller network formed by the intermediation of bound rubber, which is supposedly formed after the gelation or the
percolation. In the case of CB, connecting site is of rubber layer of 3 nm thickness
as suggested in A. This is a kind of cross-linking point in the filler network, and the
presence of such a provisional or temporary cross-linking point is easily recognized
to be ubiquitous in the image. It has maintained a little flexibility due to the presence
of rubber layer between the rigid CB aggregates, while the direct contact between
the nanoparticles (present only inside of the primary aggregate) does not allow such
flexibility in the filler networks. Moreover, when this provisional cross-linking point
is broken down under a deformation, it is quite possible that a new cross-linking
point may be reproduced at the site shown in B. The presence of B is ubiquitous
as well as A. This transiency between A and B is also another important factor of
semiflexibility of nanofiller network as shown in Fig. 5.5.
This scenario strongly suggests that the rubber reinforcement effect by compounding nanofiller into rubber is dual: The rigidity of nanofiller and its primary
aggregates and the flexibility of 3D nanofiller network structure produced upon mixing with rubber. In other words, rubber reinforcement is due to a cooperation or
more appropriately ‘unification through integration’ of the bound rubber absorbed
on the nanofiller surface (wetting by rubber, due to rubber-to-filler interaction) and
the 3D clustering ability of nanofillers (filler-to-filler interaction) even overcoming
the wetting by rubber. Figure 5.5 is certainly the most important result obtained by
3D-TEM studies on rubber reinforcement by nanofillers.
Lastly, in accordance with this scenario, the deformation behavior of the flexible
nanofiller network is schematically shown in Fig. 5.6. Cross-linking transitions, A to
B and B to A, result in a higher toughness, which affords an additional mechanical
toughness to the CB-loaded rubber vulcanizates. In terms of rubber reinforcement
research, further elucidation of the exact mechanism of rubber reinforcement on quite
a number of combinations of rubber and nanofiller is to be accelerated in a near future,
by taking these schemes into account. Moreover, it is notable that the deformation
mode in Fig. 5.6 is fully consistent and compatible with the pantograph-like shape
change, which has been mentioned on polymer gels [58] and particularly on rubber
vulcanizates [4, 59–61]. They are still to be substantiated, but we estimate that this
type of deformation has to be much more highlighted as a possible or even probable
mode for the deformation of rubber vulcanizates, cross-linked by a sulfur/accelerator
system in particular.
Specifically, the thickness of rubber layer in the nanofiller network is ca. 1.3 nm
for silica and ca. 3 nm for CB in NR and IR. Both figures are within the range of van
der Waals force, and hopping or quantum mechanical tunneling distances of electron
[12–14]. The observation of electrical percolation phenomena on silica (Fig. 4.9) and
on CB (Fig. 4.16) suggests that these figures are quite rational. If the distances were
of scores of nm, the observation might have been failed.
Just at the same time, Krüppel’s team has reported a viscoelastic analysis on
slip friction behaviors of CB-loaded SBR vulcanizates [62], in which is displayed a
5 Reinforcing Mechanism of Rubber by Nanofiller
aggregate itself. The clustering or association of the primary aggregates is always
via bound rubber, definitely not via direct contact between the primary particles.
Thirdly, Fig. 5.5 is shown a sketch of the nanofiller network formed by the intermediation of bound rubber, which is supposedly formed after the gelation or the
percolation. In the case of CB, connecting site is of rubber layer of 3 nm thickness
as suggested in A. This is a kind of cross-linking point in the filler network, and the
presence of such a provisional or temporary cross-linking point is easily recognized
to be ubiquitous in the image. It has maintained a little flexibility due to the presence
of rubber layer between the rigid CB aggregates, while the direct contact between
the nanoparticles (present only inside of the primary aggregate) does not allow such
flexibility in the filler networks. Moreover, when this provisional cross-linking point
is broken down under a deformation, it is quite possible that a new cross-linking
point may be reproduced at the site shown in B. The presence of B is ubiquitous
as well as A. This transiency between A and B is also another important factor of
semiflexibility of nanofiller network as shown in Fig. 5.5.
This scenario strongly suggests that the rubber reinforcement effect by compounding nanofiller into rubber is dual: The rigidity of nanofiller and its primary
aggregates and the flexibility of 3D nanofiller network structure produced upon mixing with rubber. In other words, rubber reinforcement is due to a cooperation or
more appropriately ‘unification through integration’ of the bound rubber absorbed
on the nanofiller surface (wetting by rubber, due to rubber-to-filler interaction) and
the 3D clustering ability of nanofillers (filler-to-filler interaction) even overcoming
the wetting by rubber. Figure 5.5 is certainly the most important result obtained by
3D-TEM studies on rubber reinforcement by nanofillers.
Lastly, in accordance with this scenario, the deformation behavior of the flexible
nanofiller network is schematically shown in Fig. 5.6. Cross-linking transitions, A to
B and B to A, result in a higher toughness, which affords an additional mechanical
toughness to the CB-loaded rubber vulcanizates. In terms of rubber reinforcement
research, further elucidation of the exact mechanism of rubber reinforcement on quite
a number of combinations of rubber and nanofiller is to be accelerated in a near future,
by taking these schemes into account. Moreover, it is notable that the deformation
mode in Fig. 5.6 is fully consistent and compatible with the pantograph-like shape
change, which has been mentioned on polymer gels [58] and particularly on rubber
vulcanizates [4, 59–61]. They are still to be substantiated, but we estimate that this
type of deformation has to be much more highlighted as a possible or even probable
mode for the deformation of rubber vulcanizates, cross-linked by a sulfur/accelerator
system in particular.
Specifically, the thickness of rubber layer in the nanofiller network is ca. 1.3 nm
for silica and ca. 3 nm for CB in NR and IR. Both figures are within the range of van
der Waals force, and hopping or quantum mechanical tunneling distances of electron
[12–14]. The observation of electrical percolation phenomena on silica (Fig. 4.9) and
on CB (Fig. 4.16) suggests that these figures are quite rational. If the distances were
of scores of nm, the observation might have been failed.
Just at the same time, Krüppel’s team has reported a viscoelastic analysis on
slip friction behaviors of CB-loaded SBR vulcanizates [62], in which is displayed a