Vulcanization: New Focus on a Traditional Technology by Small-Angle Neutron Scattering Yuko Ikeda,* Norihito Higashitani, Kensuke Hijikata, Yota Kokubo, and Yuichi Morita Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan Mitsuhiro Shibayama, Noboru Osaka, Takuya Suzuki, and Hitoshi Endo Institute for Solid State Physics, The UniVersity of Tokyo, Kashiwa, Chiba 277-8581, Japan Shinzo Kohjiya Department of Chemistry, Faculty of Science (SC4-303), Mahidol UniVersity, Salaya Campus Phuthamonthon, Nakorn Pathom 73170, Thailand ReceiVed December 6, 2008; ReVised Manuscript ReceiVed February 6, 2009 ABSTRACT: 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. Introduction Vulcanization is one of the most traditional chemical pro- cesses in polymer industries. 1-3 Almost all rubber products, such as pneumatic tires of automobiles and airplanes, are manufac- tured using vulcanization (cross-linking reactions by sulfur). Since the discovery of using sulfur by Goodyear in 1839, sulfur cross-linking reactions have been advanced by ceaseless in- novation of accelerators, activators, retarders, and so on, in order to improve processability and mechanical properties. 1-5 How- ever, the mechanism of vulcanization has not been conclusively clarified yet, due to complicated chemical reactions between rubber, elemental sulfur, and other cross-linking reagents at each processing step. Up to now, most vulcanization systems have been developed by skillful and elaborate techniques based on the trial-and-error method. In the community of rubber technologists, complexity of the vulcanization has intuitively been accepted to give a heteroge- neous network structure, and the inhomogeneity in cross-linked rubbers has been a difficult yet challenging problem in polymer science. 2,6,7 Homogeneous or heterogeneous: which kind of processing is better for desired properties? How does one control the heterogeneity of rubber mix during processing? Ultimately, what kind of network structure is the best for producing high- performance rubber materials? In order to answer these ques- tions, identification of the inhomogeneity in the cross-linked rubbers is a very important subject not only from the strong demand for a rational product design but also from the academic interests. Small-angle neutron scattering (SANS) has been one of the powerful tools since its first application to a structural study in polymer science. 8 Both hydrogen/deuterium labeling and hy- drogen/deuterium contrast-matching techniques allow us to determine the size, orientation, and conformation of target polymer chains, 7,9 and the elucidation of polymer network homogeneity has become an important issue for SANS more recently. 7 However, we can hardly find a systematic study on network inhomogeneity in sulfur cross-linked rubbers. In this study, isoprene rubber (IR), a synthetic analog of natural rubber (NR), was sulfur cross-linked, and the swollen IRs in deuterated toluene (d-toluene) were used for SANS measurements to elucidate the network inhomogeneity and local network structure. The reason of using not NR but IR as rubber is based on the fact that the nonrubber components in NR such as proteins and lipids 10,11 gave an additional scattering in the SANS profile, 12 which made the analysis difficult by overlapping with the scattering ascribable to inhomogeneity. 6,7,13,14 Invisible inhomogeneity in cross-linked rubbers can be visualized by swelling similarly as is the case in polymer gels. 7,15 In fact, various homogeneous and heterogeneous networks were studied by SANS using deuterated solvents. 16-19 A new role of vulcanization that controls the network inhomogeneity is shown here. Additionally, effects of the network inhomogeneity are studied on strain-induced crystallization (SIC) behaviors of the cross-linked IR as revealed by simultaneous time-resolved wide- angle X-ray diffraction (WAXD) and tensile measurements using a synchrotron radiation system at SPring-8. 20-24 Experimental Section Materials. Isoprene rubber (IR 2200) was supplied from JSR Co. Elemental sulfur (powder, 150 mesh), stearic acid (LUNAC S-25), zinc oxide (average diameter: 0.29 µm 25 ), and N-cyclohexyl- 2-benzothiazole sulfenamide (CBS, Sanceler CM-G) were com- mercial grades for rubber processing and used as received. They * Corresponding author. Telephone: +81 75 724 7558. Fax: +81 75 724 7558. E-mail: yuko@kit.ac.jp. 2741 Macromolecules 2009, 42, 2741-2748 10.1021/ma802730z CCC: $40.75  2009 American Chemical Society Published on Web 03/16/2009