424 Fig. 2 AFM phase image showing morphology of a NS3 and b SBS3 gel particles (Scan size 2 lm 9 2 lm) Nanoscale Res Lett (2009) 4:420–430 the case of SBS3 vary from 40 to 150 nm (Fig. 2b) with most of the gel particles having *100 nm diameter, which is in line with the earlier DLS ﬁndings. These gel particles are nearly spherical in shape. In the case of NS1 (Fig. 2a), even broader distribution in particle sizes can be seen in the AFM image. The values of gel content and crosslink density for all the crosslinked gels are tabulated in Table 2. With the increase in sulfur to accelerator ratio, both gel content and crosslink density increase for SBS as well NS gel systems. SBS0.5 has a gel content of 89%, which increases up to 97% in SBS3. A similar trend is also observed for crosslink density (0.8 9 10-4gmol/cc for SB0.5 to 2.4 9 10-4gmol/cc for SB3). A comparable increase in gel content and crosslink density is observed for NR gels. The increment in gel content and crosslink density values with increasing sulfur to accelerator ratio can be attributed to the formation of sulﬁde linkages between the molecules, which lead to a three-dimensional network structure. However, as the sulfur to accelerator ratio increases from 2 to 3, the increase in the amount of crosslinking tends to level off, as evident from gel content and crosslink density values of SBS2/SBS3 and NS2/NS3 systems. This is because of the saturation of sites available for crosslinking. Although the gel content values are quite close for both SBS and NS types of gels at any given sulfur to accelerator ratio, SBR gels show almost double the amount of crosslinking than their NR gel counterparts. Because of the nano size of SBR latex parti- cles compared to the NR latex, higher available surface area in nano latex particle leads to the efﬁcient diffusion of these curing agents during prevulcanization and hence higher amount of crosslinking. The effect of sulfur crosslinking is also very pronounced on the mechanical properties of different gels as compared to their virgin counter parts. The mechanical properties of 123 the gelled lattices are reported in Table 2. The maximum tensile stress of the control SBR latex (SB), which is only 0.29 MPa, shows many fold increase after sulfur cross- linking. The elongation at break (EB) value of neat SBR is 700%, which decreases considerably upon crosslinking to 360% in SBS3. The tensile strength (TS) increases steadily, while the EB value decreases consistently with the increase in amount of sulfur in the system. Increase in T.S. and reduction in EB values are related to the introduction of greater number of crosslinks initiated by the sulﬁde link- ages. In the case of NS series of gels, TS value increases by more than 10 times from 1.86 MPa in NR to 18.9 MPa in NS3 gel with a concomitant decrease in EB from 1400% in NR to 1120% in NS3. The trend in Young’s modulus (Ey) values is very similar to that of TS. However, SBR gels have comparatively higher values of Ey than the NR gels. The dynamic mechanical properties of different gels as compared to that of neat rubber strongly reﬂect the inﬂu- ence of crosslinking. With the increase in sulfur to accelerator ratio, tan d peak (considered as Tg here) shifts toward higher temperature (Table 2). It is worth mention- ing here that the neat NR has a Tg of about -56 C and that of SBR is -39 C. Hence, considerable increase in Tg with the introduction of crosslinking in the rubber matrix can be seen along with the broadening of tan d peak height (not shown here). In the case of NR gels, Tg shifts by more than ?7 C (from NR to NS3), while for SBR gels, there is a ?8 C shift from SBR to SBS3. The increase in Tg values with the progressive increase in sulfur to accelerator ratio can be ascribed to the restriction imposed on the chain movement due to the crosslinking, as there is lesser number of free chains available to execute unrestricted segmental motion. The storage modulus (E0) values at 25 C are also reported in Table 2 for the gels used in this study. As in the case of tensile modulus, E0 also increases steadily with