422 and ‘c’ is the amount (phr) of prevulcanized SBR gel added into the NR latex. Characterization of Gelled Latex Samples and Measurements of Various Properties of Gel Filled Rubbers Gel fraction of the prevulcanized latex ﬁlms was measured by immersing the samples in toluene at room temperature (25 ± 2 C) for 48 h (equilibrium swelling time that was determined from the experiments), and calculated from the weight of the samples before and after swelling as follows: Gel fraction ¼ W2=W1 1 ð Þ where W1 is the initial weight of the polymer and W2, the weight of the insoluble portion of the polymer. The results reported here are the averages of three samples. Crosslink density, which is deﬁned as the number of network chains per unit volume, was determined from initial weight, equilibrium swollen weight, and ﬁnal deswollen weight of the sample swollen in toluene. The number of crosslink points, m per cm3, was calculated using the well-known Flory–Rehner equation [20]: 2 3 trÞ þ tr þ v1tr2 1 lnð1 m ¼ V 4 5 ð2Þ tr13 t2r where v1 is the polymer–solvent interaction parameter, V, the molar volume of the solvent, and tr, the volume fraction of the rubber in the swollen gel. tr was calculated using the following equation [21]: ðDS FfAwÞqr 1 tr ¼ ðDS FfAwÞqr 1 þ Asqs 1 3 ð Þ where Ds, Ff, Aw, As, qr, and qs are deswollen weight of the sample, fraction insoluble, sample weight, weight of the absorbed solvent corrected for swelling increment, density of rubber, and density of solvent, respectively. Dynamic light scattering (DLS) technique was used for the measurement of particle size of gels and their distri- bution. Before testing, the latex samples were diluted to 0.1 g/L concentration level using doubly distilled water. The DLS studies were carried out in Zetasizer Nano-ZS (Malvern Instrument Ltd, Worcestershire, UK) with a He– Ne laser of 632.8 nm wavelength. The data were analyzed by in-built machine software. The mean hydrodynamic particle diameter (Zavg) was directly obtained from the machine software (as per ISO 13321). The energy dispersive X-ray sulfur (S) mapping of the gel ﬁlled raw rubber systems was recorded in Oxford ISIS 300 EDX system (Oxford Instruments, Oxfordshire, UK) attached to the JSM 5800 (JEOL Ltd., Tokyo, Japan) scanning electron microscope operating at an accelerating 123 Nanoscale Res Lett (2009) 4:420–430 voltage of 20 kV. The scan size in all the specimens was 10 square microns with a 2009 magniﬁcation. The white points in the ﬁgures denote sulfur signals. The morphology of the gel particles, as well as the gel ﬁlled matrices was analyzed with the help of atomic force microscopy (AFM). AFM studies were carried out in air at ambient conditions (25 C, 60% RH) using multimode AFM, from Veeco Digital Instruments, Santa Barbara, CA, USA. Topographic height and phase images were recorded in the tapping mode AFM with the set point ratio of 0.9, using silicon tip having spring constant of 40 N/m. The cantilever was oscillated at it resonance frequency of *280 kHz. Scanning was done at least 3 different posi- tions of each sample and the representative images were taken. The latex gel samples were diluted several times before testing with doubly distilled water. A drop of this diluted sample was placed on a freshly cleaved mica sur- face which was allowed to dry before taking the image. In the case of gel ﬁlled matrices, very thin cast ﬁlm samples were used for morphology. Due to the difference in their elastic modulus, one of the phases appears darker (NR) and the other one brighter (SBR) in all the AFM micrographs. The gel ﬁlled rubber samples for transmission electron microscopy (TEM) analysis were prepared by ultra-cryo- microtomy using Leica Ultracut UCT, at around 30 C below the glass transition temperature of the compounds. Freshly cut glass knives with cutting edge of 45 were used to get the cryosections of 50-nm thickness. The microscopy was performed using JEM-2100 (JEOL Ltd., Tokyo, Japan) operating at an accelerating voltage of 200 kV. For the measurement of mechanical properties of the neat matrix, individual gels and gel ﬁlled matrices, tensile specimens were punched out from the cast sheets of 1 mm thickness, using ASTM Die-C. The tests were carried out as per the ASTM D 412-98 method in a universal testing machine, Zwick Roell Z010 (Zwick Roell, Ulm, Germany), at a crosshead speed of 500 mm per min at 25 ± 1 C. TestXpert II software (Zwick Roell, Ulm, Germany) was used for data acquisition and analysis. The average of three tests is reported here. The experimental error was within ±1% for tensile strength and modulus, and within ±3% for elongation at break values. Dynamic mechanical properties of gels, as well as gel ﬁlled rubbers were measured as a function of temperature using the Dynamic Mechanical Analyzer DMA Q800 (TA Instruments, Luken’s Drive, New Castle, DE, USA). The measurements were taken under ﬁlm-tension mode in the appropriate temperature range with a heating rate of 3 C/ min and at 1 Hz frequency. The peak value of Tan d curves was taken as the glass transition temperature (Tg). Thermal Advantage software (TA Instruments, Newcastle, Dela- ware) was used for data acquisition and analysis.