remarkably reduces the peak heights, which indicates strong reinforcement by the
organoclay. The presence of intercalated organoclays restricts the mobility of the
rubber chains due to their confinement between the layers. More precisely, it can be
stated that the polar nature of CR and XNBR intercalates or exfoliates the silicate
layers very efficiently and a strong rubber–filler interaction is established. The plots
of storage modulus with temperature also indicate a strong reinforcement offered
by the organoclay, since a considerable increase in storage modulus of moderate
temperatures is observed (Fig. 29). It is clear that two widely separated different Tg
values primarily indicate the immiscibility of the phase if a blend is prepared with
those rubbers. The damping behavior (tan δ) of the blends with increasing temperature delivers some interesting information about the miscibility of the rubber with
two different Tg values. The increase of XNBR content the Tg values of CR and
XNBR shift to a lower temperature in all composites. This decrease in Tg of the
polymers may be explained if one considers the difference in the thermal expansion
coefficient of the respective polymers in a blend, resulting in thermal stress across
the boundary and development of a negative pressure within the rubber domains.
Thus, the free volume of the rubber component increases and, consequently, the
motion of the rubber chain becomes easier [109]. It is observed that the separation
factor between the two Tg peaks is increased in all blends containing layered
silicate, and that the difference between those Tg is maximum for the 50/50 rubber
blend. For the clay-containing 50/50 blend, the separation is 26 K, whereas the
separation is 20 K for the same blend without organoclay. So, presence of 10 phr
organoclay makes the blends more heterogeneous as compared to their virgin
composition. This finding also supports the inhibition action of layered silicate to
self-crosslinking by blocking the carboxylic group from forming hydrogen bonds
with the silanol group of the clay layers, rendering the carboxylic group passive to
crosslinking.
As far as the storage modulus of the self-crosslinked blends is concerned, a
relaxation at 40
C is observed in all the blends (Fig. 30). This relaxation arises due
to melting of crystalline domains of the CR chains, and the effect is more prominent
in blends of higher CR content without any filler. In the presence of organoclay,
crystalline nature of the CR phase is reduced. It will be also interesting to discuss
the damping behavior of those composites that are crosslinked by sulfur curatives.
Figure 30 shows tan δ versus temperature curves obtained from sulfurcrosslinked CR/XNBR blends in the presence and absence of organoclay. All the
curves possess two distinct relaxation peaks at different temperatures.
It can be observed that with the increase of CR content in the blends, the Tg
values corresponding to XNBR are shifted to a higher temperature to a remarkable
extent; the Tg of XNBR in the 25 XNBR/75 CR blend is shifted from À1 to 17
C as
compared with pure XNBR.
However, the shifting of Tg of the corresponding CR portion is only a few
degrees. This may be explained by considering the polarity difference between
XNBR and CR. Migration of curatives can take place from the less polar CR part to
the more polar XNBR part and, ultimately, the XNBR phase becomes highly
crosslinked. Thus, the mobility of the XNBR chains is greatly reduced and the Tg
Non-linear Viscoelastic Behaviour of Rubber-Rubber Blend Composites and. . .
125
organoclay. The presence of intercalated organoclays restricts the mobility of the
rubber chains due to their confinement between the layers. More precisely, it can be
stated that the polar nature of CR and XNBR intercalates or exfoliates the silicate
layers very efficiently and a strong rubber–filler interaction is established. The plots
of storage modulus with temperature also indicate a strong reinforcement offered
by the organoclay, since a considerable increase in storage modulus of moderate
temperatures is observed (Fig. 29). It is clear that two widely separated different Tg
values primarily indicate the immiscibility of the phase if a blend is prepared with
those rubbers. The damping behavior (tan δ) of the blends with increasing temperature delivers some interesting information about the miscibility of the rubber with
two different Tg values. The increase of XNBR content the Tg values of CR and
XNBR shift to a lower temperature in all composites. This decrease in Tg of the
polymers may be explained if one considers the difference in the thermal expansion
coefficient of the respective polymers in a blend, resulting in thermal stress across
the boundary and development of a negative pressure within the rubber domains.
Thus, the free volume of the rubber component increases and, consequently, the
motion of the rubber chain becomes easier [109]. It is observed that the separation
factor between the two Tg peaks is increased in all blends containing layered
silicate, and that the difference between those Tg is maximum for the 50/50 rubber
blend. For the clay-containing 50/50 blend, the separation is 26 K, whereas the
separation is 20 K for the same blend without organoclay. So, presence of 10 phr
organoclay makes the blends more heterogeneous as compared to their virgin
composition. This finding also supports the inhibition action of layered silicate to
self-crosslinking by blocking the carboxylic group from forming hydrogen bonds
with the silanol group of the clay layers, rendering the carboxylic group passive to
crosslinking.
As far as the storage modulus of the self-crosslinked blends is concerned, a
relaxation at 40
C is observed in all the blends (Fig. 30). This relaxation arises due
to melting of crystalline domains of the CR chains, and the effect is more prominent
in blends of higher CR content without any filler. In the presence of organoclay,
crystalline nature of the CR phase is reduced. It will be also interesting to discuss
the damping behavior of those composites that are crosslinked by sulfur curatives.
Figure 30 shows tan δ versus temperature curves obtained from sulfurcrosslinked CR/XNBR blends in the presence and absence of organoclay. All the
curves possess two distinct relaxation peaks at different temperatures.
It can be observed that with the increase of CR content in the blends, the Tg
values corresponding to XNBR are shifted to a higher temperature to a remarkable
extent; the Tg of XNBR in the 25 XNBR/75 CR blend is shifted from À1 to 17
C as
compared with pure XNBR.
However, the shifting of Tg of the corresponding CR portion is only a few
degrees. This may be explained by considering the polarity difference between
XNBR and CR. Migration of curatives can take place from the less polar CR part to
the more polar XNBR part and, ultimately, the XNBR phase becomes highly
crosslinked. Thus, the mobility of the XNBR chains is greatly reduced and the Tg
Non-linear Viscoelastic Behaviour of Rubber-Rubber Blend Composites and. . .
125