Presence Of Carbon Nanofibers Research Articles

Effect of polyaniline surface modification of carbon nanofibers on cure kinetics of epoxy resin

AbstractPolyaniline (PANI) “nanograss” was grown on carbon nanofibers (CNFs). The cure behavior of an epoxy resin with and without unmodified CNFs or PANI modified CNFs was studied by means of non‐isothermal and isothermal differential scanning calorimetry (DSC). CNFs accelerated the reaction of epoxy and diamine. PANI surface modification further increased the reaction rate and the extent of reaction. An autocatalytic cure kinetic model was used to fit the reaction curves. It was found that activation energies of the epoxy reaction decreased in the presence of CNFs and PANI modified CNFs. The observed catalytic effect of CNF and PANI surface coating can be very useful for low temperature cure of large epoxy composite products. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Composites of carbon nanofibers and thermoplastic polyurethanes with shape‐memory properties prepared by chaotic mixing

AbstractComposites of carbon nanofibers (CNFs), oxidized carbon nanofibers (ox‐CNFs), and shape‐memory thermoplastic polyurethane (TPU) were prepared in a chaotic mixer and their shape‐memory properties evaluated. The polymer was synthesized from 4,4′‐diphenylmethane diisocyanate, 1,4‐butanediol chain extender, and semicrystalline poly(ε‐caprolactone) diol soft segments. The shape‐memory action was triggered by both conductive and resistive heating. It was found that soft segment crystallinity and mechanical reinforcement by nanofibers produced competing effects on shape‐memory properties. A large reduction in soft segment crystallinity in the presence of CNF and stronger mechanical reinforcement by well‐dispersed ox‐CNF determined the shape‐memory properties of the respective composites. It was found that the maximum shape recovery force, respectively, 3 and 4 MPa, was obtained in the cases of 5 and 1 wt% CNF and ox‐CNF, respectively, compared with ∼1.8 MPa for unfilled TPU. The degree of soft segment and hard segment phase separation and thermal stability of the composites were analyzed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers.

Microstructure of carbon nanofiber/thermotropic liquid crystalline polymer composites

AbstractThe properties and microstructure of a thermotropic liquid crystalline polymer (TLCP, Vectran V400P) were investigated in the presence of carbon nanofibers (CNF). The electrical conductivity of TLCP increased with an addition of CNFs. The thermal analysis of pure TLCP and its composites revealed that a glass transition at ∼ 110°C did not change significantly. However, a decrease of tensile modulus and strength was observed with the addition of CNFs. WAXD studies showed a decrease of Herman's orientation parameter, indicating reduction of anisotropy of TLCP. Further, the disruption of molecular orientation of TLCPs was inferred by SEM and TEM analysis. SEM micrographs revealed a fibrillar structure for pure TLCPs at a macro‐scale (2–5 μm). However, this structure was not observed in composites at the same scale even though micro‐size fibrils (0.05 μm) were found with the addition of CNFs. TEM micrographs displayed banded structures of pure TLCPs, but these structures were not significant in the vicinity of CNFs. These observations confirmed that a decrease of molecular alignment and disruption of fibrillar structure of TLCP, in the presence of nanofibers, are attributed to a significant decrease in tensile modulus and strength. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Characterization of interfaces in ZrO 2–carbon nanofiber composite

Zirconia–carbon nanofiber (CNF) composite was prepared with the aim of improving the electrical conductivity of the zirconia. Transmission electron microscopy was used to characterize the interfaces in the composite and to explain the significantly improved conductivity compared with monolithic zirconia. Three different interfaces have been found: clear ZrO 2 /ZrO 2 boundaries, ZrO 2 /ZrO 2 boundaries with CNF and ZrO 2 /ZrO 2 boundaries with disordered graphite (which results from the destruction of CNFs during processing). The modified grain boundaries together with the presence of CNFs are the reason for the improved conductivity.

Carbonaceous fillers for shape memory actuation of polyurethane composites by resistive heating

The effectiveness of carbonaceous, electrically conductive fillers in shape memory actuation of polyurethane composites by resistive heating was evaluated. Specifically, the dependence of electrical resistivity on specimen temperature and imposed tensile strains encountered in shape memory test cycles was determined for shape memory polyurethane (SMPU) composites of carbon nanofiber (CNF), oxidized carbon nanofiber (ox-CNF), and carbon black (CB). The SMPU composites with crystalline soft segments were synthesized from diphenylmethane di-isocyanate, 1,4-butanediol, and poly(caprolactone)diol in a low-shear chaotic mixer and in an internal mixer. The materials synthesized in the chaotic mixer showed higher soft segment crystallinity and lower electrical percolation threshold. A reduction in soft segment crystallinity was observed in the presence of CNF and ox-CNF; the reduction was smaller in the case of ox-CNF. Only the composites of CB showed pronounced positive temperature coefficient (PTC) effects. The observed PTC effects bore a close relationship with non-linear thermal expansion during heating. The composites of CNF and ox-CNF did not show PTC effects due to low levels of soft segment crystallinity. The resistivity of composites of CB increased by several orders of magnitude with imposed tensile strain while composites of CNF and ox-CNF showed weak dependence on strain.

Cyclization and carbonization of anionic polyacrylonitrile in the presence of carbon nanofibers

Composites based on anionic polyacrylonitrile and carbon nanofibers were studied by X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis, and electron microscopy. The effect of carbon nanofibers on the specific features of cyclization and carbonization of anionic polyacrylonitrile at temperatures of to 550°C was studied.

Evaluation of nanoparticulate fillers for development of shape memory polyurethane nanocomposites

The effects of nano-size fillers on shape memory (SM) properties of polyurethane (PU) nanocomposites were evaluated. Organoclay, carbon nanofiber (CNF), silicon carbide (SiC), and carbon black (CB) were selected as the fillers in an attempt to reinforce the PU and to obtain significantly increased shape recovery stress. The shape memory PU was synthesized from diphenylmethane diisocyanate, 1,4-butanediol, and poly(caprolactone)diol, the latter with a molecular weight of 4000g/mol. The composites were prepared by melt mixing of extended chain PU with the fillers. The shape memory behavior was triggered by heating the specimen above the melting point of the crystalline soft segment. Our results indicate that exfoliated organoclay significantly augments SM performance, while CNF and SiC diminish it by interfering with crystallization of the soft segment. CB destroys the shape memory properties beyond a certain loading. Better SM performance with organoclay can be attributed to mechanical reinforcement without much interference with the soft segment crystallinity. The reduction of soft segment crystallinity in the presence of CNF and SiC was analyzed. It was found that the extent of crystallinity, as well as the crystallization temperature, was significantly reduced in the presence of these fillers.

Shape Memory Actuation by Resistive Heating in Polyurethane Composites of Carbonaceous Conductive Fillers

AbstractThe dependence of electrical resistivity on specimen temperature and imposed tensile strains was determined for shape memory polyurethane (SMPU) composites of carbon nanofiber (CNF), oxidized carbon nanofiber (ox-CNF), and carbon black (CB). The SMPU composites with crystalline soft segments were synthesized from diphenylmethane diisocyanate, 1,4-butanediol, and poly(caprolactone)diol in a low-shear chaotic mixer and in an internal mixer. The materials synthesized in the chaotic mixer showed higher soft segment crystallinity and lower electrical percolation thresholds. The soft segment crystallinity reduced in the presence of CNF and ox-CNF; although the reduction was lower in the case of ox-CNF. The composites of CB showed pronounced positive temperature coefficient (PTC) effects which in turn showed a close relationship with non-linear thermal expansion behavior. The composites of CNF and ox-CNF did not exhibit PTC effects due to low levels of soft segment crystallinity. The resistivity of composites of CNF and ox-CNF showed weak dependence on strain, while that of composites of CB increased by several orders of magnitude with imposed tensile strain. A corollary of this study was that a high level of crystallinity may cause a PTC effect and prevent any actuation through resistive heating. However, a carefully tailored compound which has reduced crystallinity and which requires minimum amount of filler may prevent PTC phenomenon and could supply necessary electrical conductivity over the operating temperature range, while offering enough soft segment crystallinity and rubberlike properties for excellent shape memory function.

A study of the thermal, dynamic mechanical, and tribological properties of polyphenylene sulfide composites reinforced with carbon nanofibers

The thermal, dynamic mechanical, and tribological properties of polyphenylene sulfide (PPS) composites reinforced with carbon nanofiber (CNF) were studied. Dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) were used to study the viscoelastic properties and thermal transitions. In order to study the tribological properties, friction and wear tests in a pin-on-disk configuration were performed. The changes in melting point, crystallization temperature, and glass transition temperature were found to be small as a result of reinforcement. Steady state wear rates of the reinforced composites sliding against the counterface of roughness 0.13–0.15 μm Ra were significantly lower than that of the unreinforced PPS. When the composites were tested against the smoother counterface of 0.06–0.11 μm Ra, the wear rates were higher. The coefficient of friction in all the cases was not practically affected by the presence of CNF. The transfer films formed on the counterface during sliding were examined by optical microscopy and atomic force microscopy (AFM). The variation of wear is discussed in terms of the texture and topography of transfer film.

Thermal characterization of carbon‐nanofiber‐reinforced tetraglycidyl‐4,4′‐diaminodiphenylmethane/4,4′‐diaminodiphenylsulfone epoxy composites

AbstractThe thermal properties of carbon nanofibers (CNF)/epoxy composites, composed of tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM) resin and 4,4′‐diaminodiphenylsulfone (DDS) as a curing agent, were investigated with differential scanning calorimetry (DSC), thermogravimetric analysis, and dynamic mechanical thermal analysis. DSC results showed that the presence of CNF had no pronounced influence on the heat of the cure reaction. However, the incorporation of CNF slightly improved the thermal stability of the epoxy. Furthermore, the storage modulus of the TGDDM/DDS epoxy was significantly enhanced, whereas the glass‐transition temperature was not significantly affected, upon the incorporation of CNFs. The storage modulus of 5 wt % CNF/epoxy composites at 25°C was increased by 35% in comparison with that of the pure epoxy. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 295–298, 2006

Cure kinetic study of carbon nanofibers/epoxy composites by isothermal DSC

AbstractAn investigation was carried out into the cure kinetics of carbon nanofibers (CNF)/epoxy composites, composed of tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM) resin and 4,4′‐diaminodiphenylsulfone (DDS) as a curing agent. The experimental data for both neat system and CNF/epoxy composites revealed an autocatalytic behavior. Analysis of DSC data indicated that the presence of carbon nanofibers had only a negligible effect on the cure kinetics of the epoxy. Kinetic analysis was performed using the phenomenological model of Kamal and two diffusion factors were introduced to describe the cure reaction in the latter stage. Activation energies and kinetic parameters were determined by fitting experimental data. Comparison between the two diffusion factors was performed, showing that the modified factor was successfully applied to the experimental data over the whole curing temperature range. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 329–335, 2005