isoprene units were cross-linked to the natural rubber
chains (as shown in Fig. 6). The interphase LENR chain that
cross-linked to the natural rubber chain could act as a
chain extender when the external stress was applied, thus
resulting in an improvement in the elongation at break of
OPA filled NR composites. This may be attributed to the
ability of ENR to strain crystallize like natural rubber [14].
Rattanasom et al. [15] also demonstrated that strain
induced crystallization could be responsible for strength
increases in rubber composites. Another more prominent
fact that accounts for the strength improvement is that the
ENR chain penetrated into the porous structure of the OPA
during the coating process and wetted the outer layer of
the OPA particles. It was, therefore, able to absorb and
transfer stress more effectively, thus leading to improved
tensile strength.
Tensile modulus at 100% (M100) and 300% (M300)
elongation at break and hardness of OPA filled NR composites,
with and without LENR coating, are shown in
Table 2. Both tensile moduli (M100 and M300) of the NR
composites increased as the OPA loading increased.
However, the LENR-coated OPA filled NR composites
showed a lower tensile modulus than that of the corresponding
raw OPA filled NR composites. As modulus is a
measure of the stiffness [16], the same trend was
observed for the hardness of the LENR-coated OPA filled
NR composites, as compared to the raw OPA ones. This
finding was attributed to the softening of the LENRcoated
OPA particles (as discussed earlier). Riley et al.
[17] and Ismail and Mathialagan [18] reported that filler
modulus and filler loading do affect the tensile modulus
of composites. This explains why the tensile modulus of
LENR-coated OPA filled NR composites increased with
increasing LENR-coated OPA loading, but showed a lower
value than that of the raw OPA filled NR composites.
3.4. Morphological studies
The SEM micrographs of the tensile fractured surface of
raw OPA filled NR composites and LENR-coated OPA filled