Multifunctional polybenzoxazine nanocomposites containing photoresponsive azobenzene units, catalytic carboxylic acid groups, and pyrene units capable of dispersing carbon nanotubes

Abstract

In this study, we synthesized a new multifunctional benzoxazine monomer Azo-COOH-Py BZ—featuring an azobenzene unit, a carboxylic acid group, and a pyrene moiety—through the reaction of 4-(4-hydroxyphenylazo)benzoic acid (Azo-COOH), paraformaldehyde, and aminopyrene (Py-NH₂) in 1,4-dioxane. Fourier transform infrared (FTIR) spectroscopy and ¹H and ¹³C nuclear magnetic resonance spectroscopy confirmed the structure of this new monomer. Using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and FTIR spectroscopy, we monitored the curing behavior of Azo-COOH-Py BZ leading to the formation of poly(Azo-COOH-Py BZ); we found that the carboxylic acid and azobenzene units acted as catalysts for the ring opening reaction of the benzoxazine unit. The pyrene moiety of Azo-COOH-Py BZ enhanced the dispersibility of carbon nanotubes (CNTs) in THF, leading to the formation of highly dispersible Azo-COOH-Py BZ/CNT nanocomposites stabilized through π–π stacking of the pyrene and CNT units, as detected through fluorescence emission spectroscopy. We also used DSC and TGA to examine the curing behavior of Azo-COOH-Py BZ/CNTs to form poly(Azopy-COOH-Py BZ)/CNTs nanocomposites. Interestingly, DSC profiles revealed that the maximum exothermic peak representing the ring opening polymerization of the benzoxazine unit of Azo-COOH-Py BZ shifted to much lower temperature upon increasing the content of single-walled CNTs (SWCNTs) or multiwalled CNTs (MWCNTs), suggesting that the CNTs acted as catalysts for the ring opening reaction of the benzoxazine. In addition, the curing temperatures for the SWCNT composites were lower than those for the MWCNT composites, suggesting that the SWCNTs were dispersed better than the MWCNTs in their composites and that the thermal stability of the SWCNT nanocomposites was higher than that of the MWCNT nanocomposites. The combination of photoresponsive azobenzene units, carboxylic acid groups, and CNTs enhanced the thermal stability and char yields of the polybenzoxazine matrixes, as determined through TGA analyses.