Use of χ as a function of volume fraction of rubber to determine crosslink density by swelling

A technique is described allowing a relatively simple determination of chi as a function of v(r) from swelling in heptane. A good measure of the true v(e) of the cured elastomer at all values of v(r) was demonstrated by substituting this relationship for chi in the Flory-Rehner (F-R) equation. The relationship was established over a wide range of v(r) values by using samples that had the v(e) of the cured elastomer determined by tensile retraction (TR). Applying this function to samples treated using the thiol probe method of Campbell(10.11) gave an improved measure of the types of crosslinks present in sulfur-cured stocks. An identical equation describing chi as a function of v(r) in heptane was obtained with NR, EPDM and SBR containing up to a 0.31 volume fraction of carbon black (CB) and other filters. The presence of up to 10% of clay, talc, silica, resins or metal oxides in the CB had no noticeable effect on the relationship measured. However, when the filler contained about 50% silica a distinctly different slope in the relationship was found. The percent S-1, S-2 and S-3 distribution measured was contrasted between measurements made by C-13 NMR, swelling with chi = constant or chi a, a function of v(r).