Direct imaging and elemental mapping of microgels in natural rubber particles

Elemental distribution maps of Hevea brasiliensis natural rubber gels have been obtained using electron energy-loss spectroscopy imaging in a low-energy (80 kV) electron spectroscopy imaging transmission electron microscope. Two types of gels have been investigated: a microgel contained within the natural rubber particle, and a macrogel prepared by equilibrating dry natural rubber in toluene. Both types of gels are found to contain a high amount of calcium. The intraparticle microgel is dense and rich in calcium but poor in nitrogen, indicating the predominant role of calcium in cross-link formation. The macroscopic gel is inhomogeneous, with dense calcium-rich microgels interspersed in a matrix of a less dense gel. The significant level of nitrogen associated with the matrix of the less dense gel supports the role of proteinaceous materials in the formation of the macroscopic gel.