426 Nanoscale Res Lett (2009) 4:420–430 Fig. 4 Nanoscale morphology of gel ﬁlled samples by AFM (height image on the left and phase image on the right) for a SBNS1/4, b NRSBS1/4, c SBNS1/16, and d NRSBS1/16 NR gels with 200–300 nm domain size and SBR nanogels with less than 200 nm can be seen clearly in the TEM images of SBNS1/4 and NRSBS1/4 samples, respectively (Fig. 5a–b). However, considerable gel particle agglomer- ation can be seen in 16 phr SBR nanogel ﬁlled NR sample (Fig. 5d). NR gels show comparatively lesser tendency to agglomerate at higher loading (Fig. 5c). The bulk mor- phology as investigated from the TEM study is completely in line with the surface morphology by AFM and compli- ments each other well. Effect of Gels on the Tensile Properties The tensile properties of NS1 gel ﬁlled SBR systems and SBS1 gel ﬁlled NR systems are listed in Table 3. Compared to neat rubbers, all the gel ﬁlled systems exhibit improvement in tensile strength (TS) or in maximum ten- sile stress, Fmax (as in the case of SBR systems due to their plastic deformation before rupture), Young’s modulus (Ey), and modulus at 300% elongation with concomitant decrease in elongation at break (EB) values. It can be seen that with the increase in gel loading, irrespective of the NR or SBR gels, TS and moduli increase, whereas EB decreases consistently. For example, in NRSBS1/4, there is an increase of about 11% in TS from neat NR, whereas it is 15% for NRSBS1/16. Similarly, SBNS1/2 shows an increase of more than 17% in modulus at 300% elongation 123 compared to SBR and the same for SBNS1/16 is more than 48%. It can be pointed out here that the NS1 gels show much greater reinforcing capability in SBR than its SBR counterpart i.e. SBS1 gels in NR. This may be because of the higher TS of the NS1 (15.8 MPa) gels than the SBS1 (2.8 MPa) that accounts for the better reinforcement. However, it is worth mentioning here that unlike in con- ventional ﬁllers and nanoclays, agglomeration of gels found in 16 phr gel ﬁlled samples do not impair the TS or moduli value to that extent. This seems to be the major difference between these viscoelastic ﬁllers and other particulate nanoﬁllers [26, 27]. This is probably due to the fact that, while the nanoﬁllers in the state of aggregation can act as stress concentration points in the rubber matrix, these viscoelastic gels act as a stress-dampening or dissi- pating medium. Due to the prevailing gradient of modulus or stiffness at the interface of particulate aggregate–poly- mer matrix compared to gel aggregate–polymer matrix, stress intensity will be higher in the former case. Thus, presence of gels in rubber matrix will lead to the increase in tensile property depending on the nature of chemically crosslinked gels used. Figure 6 shows the effect of crosslink density of the SBR nanogels on their reinforcement ability in NR matrix. In this case, at a representative loading of 4 phr, tensile strength of gel ﬁlled NR systems increase steadily with the increase in crosslinking density. This change in TS and