Solubilization of Carbon Nanofibers with a Covalently Attached Hyperbranched Poly(ether ketone)

Abstract

Because 5-phenoxyisophthalic acid, an A2B monomer (where A denotes an acid functionality and B an activated aromatic C−H), was easily polymerized via a Friedel–Crafts acylation in poly(phosphoric acid)/phosphorus pentoxide [PPA/P2O5; 1:4 (w/w)] to form a CO2H-terminated hyperbranched poly(ether ketone), HPB-PEK, it was polymerized in the presence of various amounts (1, 2, 5, 10, 20, 30, and 40 wt %) of vapor-grown carbon nanofibers (VGCNFs) under similar reaction conditions to form polymers grafted to VGCNFs. Considering the potential of cross-linking reactions during polycondensation processes because of the multifunctionality existing in each reacting species, it is remarkable that no gelation was observed for all of the in situ synthesis experiments conducted. The collective evidence based on the data from Soxhlet extraction (mass balance), elemental analysis, thermogravimetric analysis, Fourier-transform infrared spectroscopy, nuclear magnetic resonance, scanning electron microscopy, and transmission electron microscopy of the resulting materials implicates that, under our reaction conditions, most of the HPB-PEK (>93 wt % estimated) was grafted to the surfaces of VGCNFs, resulting in the formation of highly coated nanofibers. The grafted (HPB-PEK)-g-VGCNF materials were practically insoluble in dichlorobenzene or toluene but showed distinctly improved solubility in polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformaide, dimethylacetamide, and ethanol and even higher solubility in a ethanol/triethylamine mixture or in a 10% aqueous ammonia solution, apparently promoted by the numerous peripheral CO2H groups. From the intrinsic viscosity data analysis, all (HPB-PEK)-g-VGCNFs appeared to behave like an organic polymer in dilute solution and show rodlike character with increasingly shorter/smaller HPB-PEK grafts. As a way to determine both the ease in performing a chemical transformation on the periphery of the hyperbranched component of the resulting (HPB-PEK)-g-VGCNF and the end-group effect on some of their physical properties, the carboxylic acid end groups of the 10 wt % (HPB-PEK)-g-VGCNF were converted to benzothiazole, dodecyl ester, and amine end groups. As an example of how these transformations alter the physical properties, the dodecyl-terminated nanocomposite displayed an excellent solubility in chloroform and a much lower Tg than the CO2H-terminated (HPB-PEK)-g-VGCNF.