Vulcanization is the most important and conventional process in preparing rubber products. Network structure in the vulcanizates has been assumed to dominantly determine their physical properties together with network-chain density. Therefore, control of network structure in the vulcanizates is of at most importance for a fundamental design of rubber products. However, inhomogeneity of the network structure has not been much elucidated in spite of the long history of vulcanization since 1839, due to the complicated reactions among rubber and cross-linking reagents. Here, we look more closely at vulcanization and show its new role to control the network inhomogeneity on the basis of small-angle neutron scattering analysis of vulcanized rubbers. Combination and composition of the cross-linking reagents, especially those of zinc oxide with the other reagents, were found to be crucial for the control. A characteristic feature of strain-induced crystallization of the vulcanizates is also accounted for by the notion of network inhomogeneity. These results will be useful for further enhancing the technological potential of the traditional yet indispensable vulcanization.

The microscopic structures of cross-linked natural rubber (NR) were investigated by means of contrast-variation small-angle neutron scattering (CV-SANS) coupled with "visualization-by-swelling method" as a function of dicumyl peroxide (DCP; cross-linker) content, where the various types of inhomogeneities in the rubber were visualized by swelling with deuterated solvent. Detailed analyses of the partial scattering functions of each component confirm the existence of network inhomogeneities due to cluster-like structures of polyisoprene chains as well as larger inhomogeneities of protein aggregates. The observed partial scattering functions of polyisoprene with different DCP contents clearly exhibited that (1) the network inhomogeneities were strongly suppressed by DCP addition and (2) the structure of protein aggregates was not significantly influenced by the introduction of the peroxide cross-linking. These nanoscopic structural aspects with respect to the content of cross-linker provide better understanding of the elastic properties of NR.