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Azopyridine-functionalized benzoxazine with Zn(ClO4)2 form high-performance polybenzoxazine stabilized through metal–ligand coordination
Mohamed Gamal Mohamed,Wei-Chen Su,Yung-Chih Lin,Chih-Feng Wang,Jem-Kun Chen,Kwang-Un Jeong
RSC Advances Pub Date : 09/22/2014 00:00:00 , DOI:10.1039/C4RA08381D
Abstract

In this study, we prepared a benzoxazine monomer (Azopy-BZ) that features azobenzene and pyridine units through the reaction of paraformaldehyde, aniline, and 4-(4-hydroxphenylazo)pyridine (Azopy-OH), which is obtained through a diazonium reaction of 4-aminopyridine with phenol in the presence of sodium nitrite and NaOH. The azobenzene and pyridine groups in the benzoxazine monomer play the following two roles: (i) allowing photoisomerization between the planar trans form and the nonplanar cis form of the azobenzene unit (characterized using UV-vis spectroscopy and contact angle analyses) and (ii) serving as a catalyst that accelerated the ring opening polymerization of the benzoxazine units, which was characterized by the exothermic peak shifting to a lower temperature during differential scanning calorimetry (DSC) analyses. The curing temperature of the model benzoxazine 3-phenyl-3,4-dihydro-2H-benzoxazine (Pa-type) was 263 °C; it decreased to 208 °C for Azopy-BZ, presumably because of the basicity of the azobenzene and pyridine groups. Blending with zinc perchlorate [Zn(ClO4)2] not only improved the thermal properties, as determined through dynamic mechanical analysis (DMA), due to physical crosslinking of the pyridine units through zinc cation coordination in a metal–ligand bonding mode, but also further facilitated the ring opening polymerization to occur at a temperature of only 130 °C (DSC). Thus, the presence of Zn(ClO4)2 overcame the problem of high temperature curing (ca. 180–210 °C) required for traditional polybenzoxazines. Introducing the azobenzene and pyridine units and the zinc salt into this polybenzoxazine system provided a multifunctional material that exhibited photoisomerization-based tuning of its surface properties, accelerated ring opening polymerization of its oxazine rings, and increasing physical crosslinking density, through metal–ligand interactions, to enhance its thermal properties.

Graphical abstract: Azopyridine-functionalized benzoxazine with Zn(ClO4)2 form high-performance polybenzoxazine stabilized through metal–ligand coordination
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