Chih-Feng Wang

Benzoxazine as a reactive noncovalent dispersant for carbon nanotubes

The dispersion of carbon nanotubes (CNTs) into individual particles or small bundles has remained a challenge and, thereby, has limited their applicability. Reactive noncovalent dispersion is an attractive option for changing the interfacial properties of nanotubes; the CNTs retain their graphene structure while the reactive functionalities of the dispersant moieties allow the dispersion of the CNTs within a thermosetting polymer matrix. Nevertheless, that approach typically requires multistep syntheses and expensive reactants. In this study, we used two commercially available benzoxazine monomers as reactive noncovalent dispersants of CNTs. We used transmission electronic microscopy and thermogravimetric analysis to study the morphologies and thermal properties, respectively, of the resulting benzoxazine-modified CNTs. The presence of benzoxazine coatings improved the compatibility of the CNTs with various organic solvents; in addition, the adsorbed benzoxazines retained their reactivity. Such benzoxazine-modified CNTs have potential application in the preparation of a variety of CNT/polymer composites.

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

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.

Chapter 33 – Surface Properties of Polybenzoxazines

Publisher Summary This chapter reviews the study of surface properties such as surface free energy, antisticking application in nanoimprint lithography, and superhydrophobic surface. Polybenzoxazines present the thermosetting resin, which have some excellent properties such as near zero volumetric change upon polymerization, no release of volatiles during curing, low melting viscosity for benzoxazine monomer, high glass Tg, high thermal stability, low water absorption, good mechanical properties, and excellent dielectric properties. However, the surface property of polybenzoxazine is rarely reported. Polybenzoxazines are novel thermosetting polymers that possess many unusual physical properties that are superior to those of traditional polymers, such as epoxy and phenolic resins. The advancing contact angle is relatively less sensitive to surface roughness and heterogeneity than the receding angle; thus the advancing contact angle data are commonly used to calculate the components of surface and interfacial tension. Polybenzoxazine is discovered to be a low surface free energy polymer material since the discovery of well-known fluoropolymers and silicones. The superhydrophobic polybenzoxazine surface gives a rough surface possessing both micro- and nanoscale binary structures. Each microisland (300–700 nm) on the polybenzoxazine surface is covered with nanospheres. Such a structure dramatically increases the surface roughnesses and leads to composite interfaces in which air becomes trapped within the grooves beneath the liquid and thus induces superhydrophobicity.

Self-assembled nanostructure of polybenzoxazine resins from reaction-induced microphase separation with poly(styrene-b-4-vinylpyridine) copolymer

In this study, we synthesized the diblock copolymer poly(styrene-b-4-vinylpyridine) through nitroxide-mediated radical polymerization and then blended it with the monomer (3-phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-yl)methanol (PA-OH). Fourier transform infrared (FTIR) spectroscopy revealed evidence for intermolecular hydrogen bonding between the pyridyl groups of P4VP and the OH group of PA-OH. Thermal curing resulted in the block copolymer being incorporated into the polybenzoxazine resin, forming a nanostructure through a mechanism involving reaction-induced microphase separation, as evidenced using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). This approach also provided a variety of composition-dependent nanostructures, including disordered spherical, wormlike, and cylindrical structures through the intriguing balance between the contents of the polybenzoxazine and the diblock copolymer.

Thermal property of an aggregation-induced emission fluorophore that forms metal–ligand complexes with Zn(ClO4)2 of salicylaldehyde azine-functionalized polybenzoxazine

In this report, we designed a new and simple salicylaldehyde azine-functionalized benzoxazine (azine-BZ) monomer via Mannich condensation reaction of aniline and paraformaldehyde with 1,2-bis(2,4-dihydroxybenzylidene)hydrazine in 1,4-dioxane. Compared with 3-phenyl-3,4-dihydro-2H-benzoxazine monomer (263 °C), the maximum exothermic peak of azine-BZ shifted to a lower temperature (213 °C) based on differential scanning calorimetry (DSC) analyses because of the basicity of the phenolic group (OH) in the ortho position and the azine groups. Blending azine-BZ with different weight ratios of zinc perchlorate [Zn(ClO4)2] to form benzoxazine/zinc ion complexes not only affected the thermal properties based on thermogravimetric analysis (TGA) due to physical crosslinking through metal–ligand interactions but also expedited the ring-opening polymerization, decreasing the curing temperature from 213 to 184 °C (at 10 wt% Zn2+). Based on the fluorescence results, the azine-BZ and azine-BZ/Zn(ClO4)2 complexes were non-emissive in a THF solution. Their fluorescence increased gradually upon the addition of water. Interestingly, both the pure azine-BZ and Zn(ClO4)2-blended complex still emitted light after thermal curing at 150 °C, as determined through photoluminescence measurements, indicating that the azine group could act as a probe of the curing behavior of the benzoxazine monomer, as well as a fluorescent chemosensor for Zn2+ and, possibly, other transition metal ions through a metal–ligand charge transfer mechanism.