Study on Homogeneity in Sulfur Cross-Linked Network Structures of Isoprene Rubber

By TD-NMR and AFM – Zinc Stearate System


Over the past few years, rubber materials have been increasingly applied in different fields such as automobiles and aerospace industries. To date, cross-linking raw rubbers have been widely used to produce rubber networks with desirable properties. Unfortunately, the heterogeneous nature of conventional cross-linking reactions, such as vulcanization induced by various factors such as fillers, makes it hard to control the formation of rubber networks. To this end, the production of sturdier and stable rubber materials is of great importance and requires a thorough understanding of the rubber network structures.

Previous studies have revealed some of the factors affecting the formation of isoprene rubber networks. The combination of zinc oxide (ZnO) with other reagents was found to be crucial for controlling the structural network inhomogeneity consisting of network domains and mesh network of the matrix in the N-(1,3-benzothiazol-2-ylsulfanyl)cyclohexanamine (CBS)-accelerated vulcanization system. Nevertheless, despite the extensive studies on the preparation of rubber materials, there is no clear understanding of the underlying mechanisms by which the combination of ZnO and stearic acid (StH) enables the control of mesh size in the CBS-accelerated vulcanization. Consequently, the speculations that the reaction between ZnO and StH results in activator zinc stearate (ZnSt2) that plays a significant role in the cross-linking reactions needs to be verified.

The authors found out that the intermediate I was well dispersed in the rubber matrix and was the main reason for forming highly homogenous network structures when CBS was reacted with sulfur. This was attributed to its high solubility and fast diffusion properties. Increasing the concentration of ZnSt2 led to an increase in the amount of intermediate I generated and a corresponding decrease in the molecular weight between the constraints of the network structures. In contrast, a decrease in the network uniformity and a widened distribution of the molecular weight between cross-links were observed when large amounts of ZnSt2 were used. Additionally, it was worth noting that controlling the formation of the network structures was highly influenced by the amount of intermediate I. A unique feature of this work is that, for the first time, the mesh size distributions in the networks were characterized by two entirely different techniques, the results of which were found to be in near quantitative agreement.

In summary, the authors, for the first time, reported the significant role of ZnSt2 activator in sulfur cross-linking reactions of isoprene rubber using a combination of TD-NMR and AFM. The high homogeneity of the network structures uncovered unreported insights governing the vulcanization mechanisms during rubber processing. Based on this information, it was noted that the mesh network structure of the matrix could be controlled by controlling the amount of ZnSt2, which further influences the amount of intermediate I generated. Altogether, this approach provided quantitative structural information applicable in studying complicated network structures in rubber vulcanizates. In a statement to Advances in Engineering, the authors emphasized that the ideas provided in the study would advance rubber science and technology by providing a breakthrough in understanding the vulcanization mechanism.

Study on Homogeneity in Sulfur Cross-Linked Network Structures of Isoprene Rubber by TD-NMR and AFM - Zinc Stearate System - Advances in Engineering

About the author

Kosuke Miyaji received his bachelor’s degree and master’s degree at Kyoto Institute of Technology, Japan, in 2016 and 2018, respectively. In 2021, he obtained his Doctorate in Engineering at Kyoto Institute of Technology, supervised by Prof. Dr. Yuko Ikeda. He is now working as a project researcher at Kyoto Institute of Technology. He has researched structures and properties of rubber materials using modern analytical techniques such as time-domain nuclear magnetic resonance spectroscopy and synchrotron X-ray measurements.

About the author

Takuma Sugiyama received his bachelor’s degree and master’s degree at Kyoto Institute of Technology, Japan, in 2019 and 2021, respectively. He is now working as an engineer in a company, Japan.



About the author

Takumi Ohashi received his bachelor’s degree and master’s degree at Kyoto Institute of Technology, Japan, in 2010 and 2012, respectively. His research interest is the characterization of rubber materials using atomic force microscopy and synchrotron X-ray analyses. His high computer technique has improved the accuracy of these measurements in his studies.


About the author

Kay Saalwächter received his Diploma in Chemistry in 1997 at the Institute of Macromolecular Chemistry (University of Freiburg, Germany). In 2000, he finished his Doctorate at the Max-Planck Institute for Polymer Research and the University of Mainz, Germany, supervised by Hans-Wolfgang Spiess. This was followed by his Habilitation in 2004 as part of the group of Heino Finkelmann at the Institute of Macromolecular Chemistry (University of Freiburg, Germany). Since 2005 he has been holding a Professorship of Experimental Physics at the Martin-Luther-University Halle-Wittenberg (Halle, Germany). He had his first contact with polymer solid-state NMR on an exchange visit to the University of Massachusetts (Amherst) working with Klaus Schmidt-Rohr; ever since his continuing research interest has been the development and application of NMR techniques to the study of structure and dynamics in polymeric, liquid-crystalline, biological, and other “soft” materials.

Work in his group covers diverse methodologies ranging from solution-state relaxometry and diffusometry to specialized high-resolution solid-state NMR techniques, with a certain focus on low-field low-resolution NMR methods. Complementary techniques such as rheology, dielectric spectroscopy, thermal analysis and X-ray scattering are also applied, partially in collaboration with different partners. In terms of materials, the group is looking into fundamental questions in polymer crystallization, relaxations in associative networks and other branched or entangled polymers far above the glass transition, dynamics in crowded protein solutions, and interactions of polymers with lipid membranes. These works have been published in about 170 papers so far.

About the author

Yuko Ikeda is a professor of Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Japan. She received her bachelor’s degree in 1984 and master’s degree in 1986 at Kyoto Institute of Technology, supervised by Prof. Dr. Shinzo Yamashita. In 1991, she obtained her Doctorate in Engineering at Kyoto University, Japan, supervised by Prof. Dr. Toshinobu Higashimura. In 1993, she was a visiting researcher in University of Bayreuth, Germany, studying on polyelectrolytes with Prof. Dr. Manfred Schmidt. In 1996, she was dispatched to Texas Christian University, USA, to study on calixarenes with Prof. Dr. Carl David Gutsche as a researcher in the Ministry of Culture and Foreign Affairs of Japan. In 2014, she got the 29th Oenslager award from the Society of Rubber Science and Technology, Japan as a first woman chemist. From June, 2016 to March, 2021, she was a director of Center for Rubber Science and Technology in Kyoto Institute of Technology. Her research has mainly focused on rubber science and technology.

Her current research topics are fundamental studies on the vulcanization and the reinforcement of rubbers by modern analytical approaches using 3D-TEM, AFM, TD-NMR, SANS, and in situ time-resolved FT-IR, WAXD, SAXS and XAFS measurements with theoretical chemistry. Characterizations of Hevea, guayule and dandelion natural rubbers, and their cross-linked materials are also conducted by synchrotron X-ray analyses. Her works have been published in 138 papers, 70 review articles, 47 book chapters and 5 books.


Miyaji, K., Sugiyama, T., Ohashi, T., Saalwächter, K., & Ikeda, Y. (2020). Study on Homogeneity in Sulfur Cross-Linked Network Structures of Isoprene Rubber by TD-NMR and AFM – Zinc Stearate SystemMacromolecules, 53(19), 8438-8449.

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