Characterization of sol and gel in Hevea natural rubber

The gel content of rubber from high-ammonia latex (HA-latex) decreased significantly after deproteinization with proteolytic enzyme. The addition of 1-2% ethanol in toluene solution reduced the gel content of rubbers from HA-latex, deproteinized HA-latex (HA-DP) and pale crepe. Transesterification of the rubber in toluene solution with sodium methoxide dissolved the gel fraction. The gel fractions solubilized after transesterification showed molecular weight distribution rich in low molecular-weight fraction. The Huggins k' constant of the fractionated rubbers from solubilised-gels was in the range of 0.42-0.45, lower than that of the fractionated HA-DP of 0.5-0.8. This indicates that all the branch-points were decomposed by transesterification to form linear molecules. The (M) over bar(n) values of rubber chains assembling the gel was 5.5-8.3 x 10(5) by C-13-NMR measurements of the ratio between cis and trans-isoprene units, which were comparable to the molecular weight between crosslinks, M-c of 7-11 x 10(5) by swelling measurements. These findings suggest that the branching and crosslinks are composed of two types of branch-points, i.e. one by association or aggregation of proteins or oligopeptides at the initiating end and the other by ester linkages including phosphoric ester at the terminal end.