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.
Ionic liquid monomers with ion pair interaction energies ranging from 73.0 to 101.4 kcal/mol were designed by using the calculated electrostatic potential (ESP) values of the component ions. The ionic liquid monomers were classified as cationic monomers with an anionic counterion and anionic monomers with a cationic counterion. We evaluated the calculated ion pair interaction energy using counterion mobility as an indicator. One component of the ion pair was fixed onto a dielectric elastomer by using alkoxysilane coupling agents, while the counterion remained free to move under the applied voltage. We then measured the relative dielectric constant at 0.01 Hz, which is an indicator of the mobility of the counterion. The results showed a good correlation between the calculated ion pair interaction energy and the relative dielectric constant. The lower the ion pair interaction energy is, the easier the dissociation of the ion pair. From this result, we were able to prove the correlation between the calculated ion pair interaction energy and the mobility of the counterion. Furthermore, classification of other ion pair compounds and polyelectrolyte polymer brushes that follow the anion Hofmeister series based on ion pair interaction energies revealed the correlation between physical properties and the ionic structure. Various ionic compounds with desired physical properties can be designed by using the calculated ion pair interaction energies.
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