TEM Network Visualizations of silica-filled NR and silica-
filled DPNR vulcanizates without silane coupling agent
are depicted in Figs. 8 and 9, respectively. In both micrographs,
silica aggregates of around 50–100 nm size can
be seen as dark particles throughout the sample. The vulcanized
rubber network appears as a stained mesh structure,
while polystyrene appears as unstained regions. Some silica
aggregates in the NR vulcanizate are well-bonded to
the NR network as seen from the network strands connecting
silica aggregates to the NR network. In addition, there
are vacuoles or voids surrounding the silica aggregates.
The network visualization of the DPNR vulcanizate is
markedly different from the NR vulcanizate. There are
clearly vacuoles surrounding the silica aggregates in the
DPNR vulcanizate. The styrene polymerizes between the
rubber network and the silica particles or aggregates, and
this results in the polystyrene vacuoles surrounding the
silica particles or aggregates. Ladouce-Stelandre et al.
[15] have suggested that the formation of such vacuoles
is due to a weak interface between silica particles and rubber
chains. The formation of vacuoles in swelling behavior
is a characteristic of a composite without interactions between
filler particles and rubber matrix as has been shown
by Valentin et al. [24] using a combination of multiplequantum
(MQ) NMR and equilibrium swelling experiments.
Since silica exhibits a low dispersive component
of surface energy, cds
, the filler-to-rubber interaction is
weak and not many rubber chains adsorb on its surface.
Once a segment of a rubber chain is attached to the silica
surface, it is possible that multiple attachments can occur
due to segmental reptation of the rubber chain. Both single
and multiple attachments or configurations (a) and (b) in
Fig. 5 will lead to formation of vacuoles when the vulcanizate
is swollen in styrene. There are less vacuoles present
in the NR vulcanizate as compared to the DPNR vulcanizate
without silane, which suggests higher filler-to-rubber
interactions in the former.