Ching Hsuan Lin

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.

Emission and surface properties of main-chain type polybenzoxazine with pyridinyl moieties

A main-chain type polybenzoxazine, PBz (1), with pyridinyl moieties was successfully synthesized from the Mannich condensation of 4-phenyl-2,6-bis(4-aminophenyl) pyridine (2), paraformaldehyde, and bisphenol A. For the purpose of property comparison, a structurally similar polybenzoxazine, PBz (2), was prepared. Unfortunately, PBz (2) is insoluble in organic solvents due to its rigid structure. In contrast, the pyridinyl group provided solubility for the processing of PBz (1). When the THF dilute solution of PBz (1) was protonated with HCl, a new absorption signal at 458 nm was observed in the UV-vis spectrum. No emission at 500–700 nm occurred in the fluorescence spectrum before protonation of PBz (1). However, upon protonation, the protonated pyridinyl became a stronger acceptor, and then, an emission at 570 nm occurred between the nitrogen of benzoxazine (donor) and protonated pyridinyl groups (acceptor) after being excited at 458 nm. After thermal curing, the thin film of PBz (1) thermoset was flexible, with a Tg value of 261 °C, a coefficient of thermal expansion of 38 ppm °C−1, and 5% decomposition temperature at 414 (N2) and 419 °C (air). The contact angle is as high as 102°, and the surface energy is as low as 19.6 mJ m−2.

The robustness of a thermoset of a main-chain type polybenzoxazine precursor prepared through a strategy of A–A and B–B polycondensation

A polybenzoxazine precursor, P(BF-bapp)-1, was prepared through a strategy of A–A and B–B polycondensation. The A–A monomer is 5,5′-methylenebis(2-hydroxybenzaldehyde), and the B–B monomer is 2,2-bis[4-(4-aminophenoxy)phenyl] propane (BAPP). The inherent viscosity (at 0.5 g dL−1 in NMP) of P(BF-bapp)-1 is as high as 0.34 dL g−1, which is higher than that (0.23 dL g−1) of a structurally similar polybenzoxazine precursor, P(BF-bapp)-2, that was prepared through traditional Mannich condensation of BAPP/bisphenol F/paraformaldehyde. A third structurally similar polybenzoxazine precursor, P(BF-bapp)-3, with an inherent viscosity of 0.09 dL g−1, and a phenol and BAPP-based benzoxazine monomer, P-bapp, were prepared for properties comparison. After thermal curing of the benzoxazines, we found that a benzoxazine thermoset based on a high molecular weight precursor has advantages in higher Tg, higher char yield, lower refractive index (low dielectric constant according to Maxwells equation), and better mechanical properties. The results demonstrate the robustness of a polybenzoxazine precursor prepared through a strategy of A–A and B–B polycondensation, which has never been reported in the benzoxazine field.

Facile, efficient synthesis of a phosphinated hydroxyl diamine and properties of its high-performance poly(hydroxyl imides) and polyimide–SiO2 hybrids

A hydroxyl diamine, 1-(4-aminophenyl)-1-(3-hydroxyl-4-aminophenyl)-1-(6-oxido-6H-dibenz oxaphosphorin-6-yl)ethane (1), was prepared via a facile, one-pot procedure. Based on 1, a series of poly(hydroxyl imides) (2a–2e), were prepared in NMP/xylene by solution polymerization, followed by thermal treatment. Experimental data showed that the poly(hydroxyl imides) displayed better thermal properties than the polyimides without hydroxyl groups. Polyimide–SiO2 hybrids were prepared based on the sol–gel reaction of tetraethoxysilane (TEOS) with 2e, which was prepared from the condensation of 1 and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA). The hydroxyl group on the backbone of 2e provided organic–inorganic bonding. Therefore, light-yellow, transparent and flexible polyimide–SiO2 films with a maximum SiO2 content of 16 wt% were obtained without the use of a coupling agent. TEM data showed that several nanorods aggregated into a cluster, and the clusters were well dispersed in the polyimide–SiO2 hybrids. The optical and thermal properties of 2e were significantly improved by the incorporation of silica.

A study on the co-reaction of benzoxazine and triazine through a triazine-containing benzoxazine

To study the co-reaction of benzoxazine and triazine, a triazine-containing benzoxazine (P-tta) was prepared through nucleophilic substitution of 4-(2H-benzo[e][1,3]oxazin-3(4H)-yl)phenol (P-ap) with 2,4,6-trichloro-1,3,5-triazine. DSC thermograms show that the exothermic temperature of P-tta is lower than that of other benzoxazines with a similar structure except for the triazine structure, so we speculate that the forward polymerization is related to the existence of the triazine structure. Through monitoring the curing process using IR, we propose that the curing reactions of P-tta include a concerted co-reaction between the triazine and benzoxazine, and a self-polymerization of benzoxazine. A thermoset with a high Tg (279 °C, DMA data), a low thermal expansion coefficient (32 ppm per °C), and high thermal stability (Td5% 417 °C) can be obtained through the curing of P-tta.

Synthesis of a phosphinated tetracyanate ester and its miscible blend with 4,4′-oxydianiline/phenol-based benzoxazine

A phosphinated tetracyanate ester (4) was prepared by a three-step procedure, including nucleophilic addition of DOPO on 5,5′-methylenebis(2-hydroxybenzaldehyde), electrophilic substitution of phenol, and nucleophilic substitution with cyanobromide. Tetracyanate ester (4) was applied to copolymerize with a 4,4′-oxydianiline/phenol-based benzoxazine (P-oda) to enhance the properties of P-oda thermoset. We discussed the miscibility, microstructure, as well as the thermal and dielectric properties of the reactive blends of (4) with a P-oda. A miscible blend can be achieved in all compositions, as judged by thermal analysis. The co-reactions between (4) and P-oda, as observed in the IR analysis, explain the single-Tg phenomenon. The experimental data show that the thermal, dielectric and flame retardant properties of polybenzoxazine are enhanced through the reactive blends.

Electron-withdrawing/donating effects of substituents on the preparation of phosphinated 4,4 '-diaminodiphenylmethane for soluble, anti-oxidative, and high-Tg polyimides

In this work, we reveal a strategy to prepare a phosphinated 4,4′-diaminodiphenylmethane, 1,1-bis(4-aminophenyl)-1-(6-oxido-6H-dibenz oxaphosphorin-6-yl)methane (3). During the preparation, it was found that the effect of the electron withdrawing/donating characteristic of substituents is crucial to the synthesis. A reaction mechanism including nucleophilic addition, reduction, and electrophilic substitution is proposed. Based on diamine (3), a series of polyimides (4a–4d) were prepared. Polyimides 4 are readily soluble in some organic solvents, and can be solution cast into flexible and foldable films. They show high T g (310–390°C), high moduli (3.34–4.63 GPa), moderate coefficient of thermal expansion (46–58 ppm °C−1), good anti-oxidative stability (T d 5%: 480–537°C (N2); 456–504°C (air)), and good flame retardancy (VTM V-0 rating).

Strategy to prepare 4-hydroxylphenyl propargyl ether-based benzoxazine from bisphenol A

4-Aminophenyl propargyl ether/phenol-based benzoxazine (P-appe) has been reported by many authors. However, to the best of our knowledge, its isomer: 4-hydroxylphenyl propargyl ether/aniline-based benzoxazine (HPPE-a) has never been prepared in the literature. The precursor for HPPE-a, the 4-hydroxylphenyl propargyl ether, is difficult to prepare from hydroquinone without tedious separation because the reactivity of the two hydroxyls in hydroquinone is the same. In this study, we report a straightforward strategy to prepare HPPE-a from bisphenol A by a four-step process including thermolysis, nucleophilic substitution, oxidation, and Mannich-type condensation. The structure was well characterized by NMR, IR, and high resolution mass spectra. Two exothermic peaks at 228 °C and 249 °C were observed in the DSC thermogram of HPPE-a. According to IR analysis, the first exotherm is related with the ring opening of the oxazine, and the second exotherm is related with Claisen rearrangement of propargyl ether. Since P-appe and HPPE-a are structural isomers, the structure–property relationship of these two benzoxazines was discussed. We found that the thermal properties of a thermoset of HPPE-a are slightly lower or comparable to those of a thermoset of P-appe, but much higher than that of phenol/aniline-based benzoxazine (P-a).

A strategy for preparing spirobichroman dianhydride from bisphenol A and its resulting polyimide with low dielectric characteristic

A spirobichroman dianhydride (SBCDA) was prepared through oxidation of an octamethyl spirobichroman (OMSBC), which was synthesized from acid-fragmentation of bisphenol A by 3,4-dimethylphenol, followed by Diels–Alder reaction. The reaction mechanism was proposed, and the optimal reaction conditions were discussed. Based on a high temperature solution polymerization of SBCDA and 4,4′-diaminodiphenylmethane (DDM), a spirobichroman-containing polyimide, SBC-DDM, was successfully prepared. Because of the contorted spiro-structure and rigid polymer backbone, SBC-DDM exhibits a large free volume, leading to outstanding organo-solubility and a low dielectric constant. In addition, the resulting film of SBC-DDM shows foldability, a high glass transition temperature, and good thermal stability.

Photo-sensitive benzoxazine II: chalcone-containing benzoxazine and its photo and thermal-cured thermoset

A chalcone-containing benzoxazine (BHP-a) was synthesized from a chalcone-containing bisphenol: 1,3-bis(4-hydroxyphenyl) propanone (BHP), aniline and paraformaldehyde in a co-solvent of xylene/1-butanol (2/1, V/V). The structure of BHP-a was successfully confirmed by FTIR, 1H and 13C-NMR spectra. After thermal treatment at a temperature higher than 240 °C, BHP becomes insoluble. This indicates that the double bond of the chalcone moiety of BHP can be thermally polymerized at elevated temperature. The UV spectrum shows that the chalcone moiety of BHP-a underwent dimerization via [2π + 2π] cycloaddition. Therefore, two procedures were applied to cure BHP-a. The first one was thermal curing of chalcone and oxazine moieties of BHP-a. The second one was photo curing of the chalcone moiety, followed by thermal curing of the oxazine moiety. The thermal properties of thermosets based on the two procedures were evaluated. Thermosets of BHP-a exhibit a Tg as high as 294 °C for curing procedure one, and 328 °C for curing procedure two. The value is much higher than that of a traditional bisphenol/aniline-based benzoxazine thermoset. We conclude that the curing of the double bond of the chalcone and photo dimerization of the chalcone contribute to the good thermal properties.