Nanocomposites derived from a low-color aromatic polyimide (CP2) and amine-functionalized vapor-grown carbon nanofibers: In situ polymerization and characterization

Vapor-grown carbon nanofibers (VGCNF) were functionalized with amine-containing pendants via a Friedel-Crafts acylation reaction with 4-(3-aminophenoxy)benzoic acid. The resulting H2N-VGCNF with relatively high degree of functionalization was in attendance during the synthesis of a polyimide (previously designated as CP2) from 2,2-bis(phthalic anhydride)-1,1,1,3,3,3-hexafluoroisopropane (6FDA) and 1,3-bis(3-aminophenoxy)benzene (APB) in N,N-dimethylacetamide (DMAc). Thus, a series of CP2-based nanocomposite films, which contained 0.18-9.19 wt % of H2N-VGCNF (corresponding to 0.10-5.0 wt % of basic VGCNF) were obtained using the conventional poly(amic acid) precursor method. For comparison purposes, the pristine VGCNF (0.10-5.0 wt %) was also used in the in situ polymerization of 6FDA and ABP. These two series of nanocomposite films were cast from the respective poly(amic acid)/VGCNF/DMAc solutions, followed by thermal imidization at curing temperatures up to 250 degrees C. The benefit and limitation of functionalized VGCNF on the length scale and the extent of CNF dispersion in a polyimide matrix were clear: (a) 0.18 wt % H2N-VGCNF-g-CP2 film was visually transparent whereas a similarly prepared (0.10 wt %) VGCNF/CP2 film showed the presence of large CNF aggregates throughout; (b) at 0.55 wt % H2N-VGCNF (equivalent to 0.30 wt % VGCNF) content, the nanocomposite film had become translucent, and at 9.19 wt % (equivalent to 5.0 wt % VGCNF), it was opaque. Since CP2 is very soluble in THF, the CP2-grafted VGCNF were simply separated from the free CP2 by solvent extraction. The molecular weights of the extracted CP2 were measured using gel-permeation chromatography (GPC). The effects of VGCNF on molecular weight (MW) and glass-transition (T-g) were discussed in terms of GPC and thermal analysis results, respectively. The dispersion of VGCNF in CP2 was evaluated using scanning electron microscopy (SEM). The tensile properties of these nanocomposite films were determined, showing up to 45% increase in modulus.