Shiliang Liu,Rui Yang,Qixin Zhuang,Guozhu Zhan,Peiyuan Zuo,Xiaoyun Liu
In this study, six thermosetting benzoxazole monomers with endo-alkynyl groups are prepared, and the effect of the endo-alkynyl groups and substituent group on the curing behavior and solubility of monomers, and on the thermal properties and dielectric properties of polybenzoxazoles are investigated. Our findings reveal that incorporating endo-alkynyl groups enhances the cross-linked network in the benzoxazole monomers, thereby improving their thermal stability, hydrophobicity , and dielectric constant . However, this modification also results in decreased solubility and curing properties of the monomers. In addition, the position of the fluorine atoms can affect the curing behavior of monomer and the dielectric properties of polybenzoxazoles. When a fluorine atom is positioned ortho to the cyano group , it impedes the curing of the cyanide group, which increases the curing reaction temperature. Moreover, while it is generally believed that the addition of fluorine typically lowers the dielectric constant of a material, the placement of fluorine atoms ortho to the cyano group can lead to overlapping electronic effects. This overlap may increase the polarity of the curing network, which increases the dielectric constant and dielectric loss of the polybenzoxazoles. Among the six thermosetting benzoxazole monomers studied, poly (2 F-JQ) containing m -diethynyl groups exhibit a 5 % thermal decomposition temperature (Td 5 ) of 610 °C, and a low dielectric loss of 0.0045 at 1 MHz. Compared with other polybenzoxazoles, poly (2 F-JQ) possesses a moderate dielectric constant, excellent thermal stability, and extremely low dielectric loss. In summary, this study provides a new approach for preparing resins that feature high-temperature resistance and low dielectric loss.
In recent years, click reactions with cellulose nanocrystals (CNC) participation have gradually become a research hotspot. Carboxylamine condensation is the most used method to introduce terminal alkyne groups at the reducing end of CNC as reaction sites for click reactions. However, hydroxyl groups on CNC surface would be slightly oxidized during the carboxyamine condensation process , inducing the potential positions of introduced alkynes would be not only at the reducing end but also on CNC surface. Here, aldimine condensation was proposed to introduce terminal alkyne groups just at the reducing end of CNC, and a systematic comparison analysis was conducted with carboxylamine condensation. Firstly, the selectivity and extent of alkynylation were characterized by XPS and EA. Secondly, the end aldehyde content in these CNC samples was measured by the BCA method, which quantitatively explained the grafting efficiency of aldimine condensation and further verified its feasibility. Thirdly, the clickability of the modified CNC samples was confirmed through XPS analysis of the products after a pre-designed click reaction. In sum, aldimine condensation was proven to be a simple and effective strategy for introducing terminal alkyne groups at the reducing end of CNC, which could be used as reaction sites for further click reactions.
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