We used Positron Annihilation Lifetime Spectroscopy (PALS) to investigate the effect of cross-linking on the free volume properties of Natural Rubber (NR) composites. In order to form any meaningful product for industrial purposes, NR is mixed with different compounds, then cured in a process called vulcanization. One of the most important compounds used in the mixing process is sulfur. Sulfur forms cross-links with NR restricting the mobility of the polymer chains. Addition of sulfur and vulcanization reduce the oPs lifetime and intensity. By compating un-vulcanized and vulcanized samples we show that sulfur inhibits Ps formation and that with cross-link formation during vulcanization the o-Ps lifetime and intensity both decrease with increasing sulfur concentration. Swelling measurements were also done using toluene absorption to measure cross-link density as a function of sulfur concentration.1 Introduction Physical properties of natural rubber (NR) are improved by reinforcing it with fillers such as carbon black (CB), sulfur or silica. These fillers are mixed together with natural rubber and other additives then vulcanized [1]. During vulcanization, S-S and C-S cross-links are formed improving the mechanical and physical properties of NR. Previous studies by Positron Annihilation Lifetime Spectroscopy (PALS) have focused on the effect of carbon black in rubber [2][3][4][5][6]. The motivation has been to understand how CB loading affects the free volume holes existing in the rubber. Results found by an earlier group in this lab [5,13,14] shows that the free volume hole size in rubber is not affected by CB loading, however the number of the free volume holes was found to decrease. This observation is in good agreement with other authors. [2][3][4]6] Within this context of filler loading in the rubber, we previously [7], investigated the effect of sulfur concentrations in the vulcanized NR samples. We found that as the concentration of sulfur in the sample increases, both the size and the amount of free volume holes in the sample decrease. This we interpreted as the formation of cross-links between sulfur and polymer chains restricting the chain mobility, hence reducing the free volume spaces in the polymer. This was in good agreement with other authors who have also observed that the free volume of sulfur cured NR is dependent on the cross-link densities between sulfur and NR [8][9][10][11]. Hence, in this paper, we have tried to investigate this cross-linking effect further by extending our work to un-vulcanized NR samples.We use PALS and equilibrium swelling methods with a view to understanding the nature and distribution of local free volume and its correlation with the degree of cross-linking. PALS has been extensively used in the study of free volume in amorphous polymers [12][13][14][15][16]. The basis for this technique lies in the fact that the pickoff lifetime τ 3 of ortho-positronium (o-Ps) is sensitive to its local molecular environment and in particular, increases or decreases with size of the free-...

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The free volume behavior as a function of time during curing of natural rubber-carbon black composites studied by positron annihilation lifetime spectroscopy (PALS) is reported. The lifetime and intensity of the ortho-positronium (o-Ps) pick-off annihilation, which correlate with the average size and the number of holes respectively, were measured as a function of curing time. The samples were cured at 160 °C, while all PALS measurements were made at room temperature. Results for curing D706 (Duradene-706), a synthetic rubber is also reported and compared with those found from natural rubber (NR).

The free volume behavior as a function of strain during deformation of natural rubber-carbon black composites studied by positron annihilation lifetime spectroscopy (PALS) is reported. The lifetime and intensity of the ortho-positronium (o-Ps) pick-off annihilation, which correlate with the average size and the number of holes respectively, were measured as a function of strain. The results show how PALS can detect the changes that occur in polymer composites samples and help explain the effect of carbon black on the properties of the composites when they are deformed under compression.

Carbon black (CB) is essentially carbon in the form of extremely fine particle aggregates (100-200nm) having an amorphous molecular structure. Oil absorption or compressed dibutyl phthalate (CDBP) and iodine adsorption or nitrogen surface area (N2SA) are parameters commonly used to describe CB morphology or structure and the surface area of CB available for reaction with rubber. We have used both positron lifetime and Doppler broadening spectroscopy to study these two parameters for a variety of rubber CB composite samples. The o-Ps pickoff lifetime depends only on the type of rubber and is independent of CDBP and N2SA. However, the o-Ps lifetime intensity, I3, and the S parameter decrease with both CDBP and N2SA.