The network structure evolution of a hexamethylenetetramine (HMTA)-cured novolac-type phenolic resin over a curing temperature range of 135–155 °C was investigated using 1H-pulse nuclear magnetic resonance spectroscopy and small-angle and wide-angle X-ray scattering techniques. The aim was to elucidate the mechanism responsible for the apparent absence of inhomogeneity after curing at 175 °C, in which the inhomogeneity was first observed at the gel point below 130 °C. The HMTA-cured phenolic resin exhibited high-cross-link and low-cross-link density domains (denoted as HXD and LXD, respectively). The LXD was a minor structure having a cross-link fraction of 0.2, which was 5–6 nm in size and comprised a few meshes. As curing proceeded, intradomain reactions in the LXD occurred, and the electron density in the domain increased, decreasing the electron density difference between the HXD and LXD. This reduction in the electron density difference decreased the cross-link inhomogeneity in the phenolic resins in terms of electron density fluctuations. This structural evolution caused the apparent absence of inhomogeneity in the fully HMTA-cured phenolic resins.
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The SAXS experiments were conducted at the second hutch of SPring-8 BL03XU (Frontier Softmaterial Beamline (FSBL)) constructed by the Consortium of Advanced Softmaterial Beamline with the proposal numbers 2017A7209 and 2017B7261. This work was supported by the Photon and Quantum Basic Research Coordinated Development Program by MEXT with the grant number 13004017. This study was conducted as part of the research activities of the Special Interest Group on Thermosetting Resins in the FSBL Consortium comprising the Asahi Kasei research group, the DENSO research group, and the Sumitomo Bakelite research group.
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The authors declare that they have no conflict of interest.
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Izumi, A., Shudo, Y. & Shibayama, M. Network structure evolution of a hexamethylenetetramine-cured phenolic resin. Polym J 51, 155–160 (2019). https://doi.org/10.1038/s41428-018-0133-8
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