Nanoscale investigation of the electrical properties in semiconductor polymer-carbon nanotube hybrid materials

The morphology and electrical properties of hybrids of a semiconducting polymer (namely poly(3-hexylthiophene) P3HT) and carbon nanotubes are investigated at the nanoscale with a combination of Scanning Probe Microscopy techniques, i.e., Conductive Atomic Force Microscopy (C-AFM) and time-resolved Current Sensing Force Spectroscopy Atomic Force Microscopy (CSFS-AFM, or PeakForce TUNA?). This allows us to probe the electrical properties of the 15 nm wide P3HT nanofibers as well as the interface between the polymer and single carbon nanotubes. This is achieved by applying controlled, low forces on the tip during imaging, which allows a direct comparison between the morphology and the electrical properties at the nanometre scale.     

Micromechanical properties and ductile behavior of electrospun polystyrene nanofibers

Using electrospinning technique polystyrene (PS) nanofibers in the thickness range from 150 to 800 nm have been produced. Electron microscope inspections reveal the relatively uniform thickness of the obtained fibers. The mechanical deformation mechanisms have been studied in tension tests using micro-tensile devices for a scanning electron microscope (SEM) and a transmission electron microscope (TEM). A characteristic change in the deformation behavior from the typical craze formation of PS to a micro necking and cold drawing has been found with decreasing fiber thickness. There is a surprisingly sharp fiber thickness limit between both deformation types in the range of 220–225 nm: nanofibers thicker than ∼ 225 nm deform with formation of crazes, nanofibers thinner than ∼ 225 nm show necking and cold drawing. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation and characterization of hybrid conducting polymer-carbon nanotube yarn

Hybrid polypyrrole (PPy)-multi walled carbon nanotube (MWNT) yarns were obtained by chemical and electrochemical polymerization of pyrrole on the surface and within the porous interior of twisted MWNT yarns. The material was characterized by scanning electron microscopy, electrochemical, mechanical and electrical measurements. It was found that the hybrid PPy-MWNT yarns possessed significantly higher mechanical strength (over 740 MPa) and Young's modulus (over 54 GPa) than the pristine MWNT yarn. The hybrid yarns also exhibited substantially higher electrical conductivity (over 235 S cm(-1)) and their specific capacitance was found to be in excess of 60 F g(-1). Measurements of temperature dependence of electrical conductivity revealed semiconducting behaviour, with a large increase of band gap near 100 K. The collected low temperature data are in good agreement with a three-dimensional variable range hopping model (3D-VRH). The improved durability of the yarns is important for electrical applications. The composite yarns can be produced in commercial quantities and used for applications where the electrical conductivity and good mechanical properties are of primary importance.     

Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties

Carbon nanofibers with diameters of 200-300 nm were developed through stabilization and carbonization of aligned electrospun polyacrylonitrile (PAN) nanofiber bundles. Prior to the oxidative stabilization in air, the electrospun PAN nanofiber bundle was tightly wrapped onto a glass rod, so that tension existed during the stabilization. We also investigated several carbonization procedures by varying final carbonization temperatures in the range from 1000 to 2200 °C. The study revealed that: (1) with increase of the final carbonization temperature, the carbon nanofibers became more graphitic and structurally ordered; (2) the carbon nanofiber bundles possessed anisotropic electrical conductivities, and the differences between the parallel and perpendicular directions to the bundle axes were over 20 times; and (3) the tensile strengths and Young's moduli of the prepared carbon nanofiber bundles were in the ranges of 300-600 MPa and 40-60 GPa, respectively.     

Improvement of mechanical and electrical properties of epoxy resin with carbon nanofibers

In the present research, the reinforcement effect of vapor grown carbon nanofiber (VGCNF) was studied in relation to the mechanical properties and electrical conduction behavior of fabricated nanocomposites. Different weight fractions of nanofillers into epoxy resin, from 0.05 to 1 wt% and up to 2 wt% for mechanical and electrical properties were investigated. It was found that the optimum improvement in mechanical properties of nanocomposite is obtained at 0.25 wt% of carbon nanofibers. At this filler content, 23 % enhancement in tensile strength and 10 % in flexural strength have been observed. The degree of the VGCNF dispersion has been monitored by means of viscosity variation of the suspension during the sonication process to obtain the optimum sonication time. Finally, the quality of the dispersion for post-cured nanocomposites is characterized by fractured surfaces using the scanning electron microscopy. Agglomerates had a direct effect on the reduction of tensile and flexural strength of nanocomposites. The electrical conductivity was obtained by means of surface measuring method. The optimum amount of filler for the generation of a fine electrical conductivity was found to be around 0.5 wt% of VGCNF. After the threshold point, the electrical conductivity of nanocomposites was slightly raised in spite of adding more filler contents.     

The morphology,mechanical properties,and flammability of aligned electrospun polycarbonate (PC) nanofibers

The electrospinning of the polycarbonate (PC) solutions was performed for the variable electrospinning parameters such as polymer concentration, solvent composition, applied voltage, flow rate, and take‐up velocity in order to evaluate changes of morphology, mechanical properties, and flammability of the aligned PC nanofibers as a function of the electrospinning parameters. It was found that the ratio of THF/DMF solvent in the electrospinning parameters had a major effect on the spinnability and fiber morphology. Furthermore, it was confirmed that the mechanical properties were dependent upon the fiber morphology. The spinnability of the PC solutions with a lower THF ratio in THF/DMF solvent was poor. The aligned electrospun PC fiber with the best morphology was made in the range of polymer concentration of 22%, solvent ratio of 50:50 THF : DMF, applied voltage of 14 kV, flow rate of 0.050 ml/m, and a take‐up velocity of 7.3 m/s. The ultimate strength and initial modulus of the 80% drawn 22% PC fiber were 64 ± 2 MPa (commercial 55–75 MPa) and 1.9 ± 0.1 GPa. The heat release capacity (HRC) of the 22 and 25% PC fiber were 275 ± 27 J/g K and 198 ± 1 J/g K. It was found that the flame resistance of the electrospun PC nanofiber was superior to that of the PC raw material (HRC ～360 J/g K). POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Chain confinement in electrospun nanofibers of PET with carbon nanotubes

Composite nanofibers of poly(ethylene terephthalate), PET, with multiwalled carbon nanotubes (PET/MWCNT) were prepared by the electrospinning method. Confinement, chain conformation, and crystallization of PET electrospun (ES) fibers were analyzed as a function of the weight fraction of MWCNTs. For the first time, we have characterized the rigid amorphous fraction (RAF) in polymer electrospun fibers, with and without MWCNTs. The addition of MWCNTs causes polymer chains in the ES fibers to become more extended, impeding cold crystallization of the fibers, resulting in more confinement of PET chains and an increase in the RAF. The fraction of rigid amorphous chains greatly increased with a small amount of MWCNT loading: with addition of 2% MWCNTs, RAF increased to 0.64, compared to 0.23 in homopolymer PET ES fibers. Spatial constraints also inhibit the folding of polymer chains, resulting in a decrease in crystallinity of PET. For fully amorphous PET/MWCNT composites, MWCNTs do not affect the chain conformation of PET in the ES fibers. For cold crystallized PET/MWCNT composite nanofibers, more trans conformers were formed with the addition of MWCNTs. The increase of RAF (chain confinement) is associated with an increase of the concentration of the trans conformers in the amorphous region as the MWCNT concentration increases in the semicrystalline nanofibers.     

Characterization of electrosynthesized conjugated polymer-carbon nanotube composite: optical nonlinearity and electrical property

The effects of multi-walled carbon nanotube (MWNT) concentration on the structural, optical and electrical properties of conjugated polymer-carbon nanotube composite are discussed. Multi-walled carbon nanotube-polypyrrole nanocomposites were synthesized by electrochemical polymerization of monomers in the presence of different amounts of MWNTs using sodium dodecylbenzensulfonate (SDBS) as surfactant at room temperature and normal pressure. Field emission scanning electron microscopy (FESEM) indicates that the polymer is wrapped around the nanotubes. Measurement of the nonlinear refractive indices (n(2)) and the nonlinear absorption (β) of the samples with different MWNT concentrations measurements were performed by a single Z-scan method using continuous wave (CW) laser beam excitation wavelength of λ = 532 nm. The results show that both nonlinear optical parameters increased with increasing the concentration of MWNTs. The third order nonlinear susceptibilities were also calculated and found to follow the same trend as n(2) and β. In addition, the conductivity of the composite film was found to increase rapidly with the increase in the MWNT concentration.     

Effects of hard and soft components on the structure formation,crystallization behavior and mechanical properties of electrospun poly(l-lactic acid) nanofibers

The effects of multi-wall carbon nanotubes (MWCNTs) and poly(ethylene oxide) (PEO) on the structure formation, morphology, crystallization behavior and mechanical property of electrospun poly (l-lactic acid) (PLLA) nanofiber mats were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC) and mechanical test. If incorporate hard filler, MWCNTs into electrospun PLLA nanofiber, the crystallinity, chain orientation, and crystallization behaviors were almost not influenced by the MWCNTs content owing to the MWCNTs mainly acted as impeding the crystal growth and chain diffusion. If incorporate small content of soft and miscible component, PEO (10 wt%) into the electrospun PLLA and PLLA/MWCNTs nanofibers, the crystallinity and crystallization rate of PLLA in nanofibers were obviously enhanced. The synergistic effect of PEO and MWCNTs in PLLA nanofibers was observed during melt-crystallization behaviors of PLLA/MWCNTs fibers. Based on those results, we found that the chain mobility is an important factor to influence the structure formation and crystallization behaviors in the electrospun nanofibers. Our results indicated that the structure and properties of electrospun nanofibers could be optimized by compounding with hard inorganic filler and soft polymer components.     

Comparison of two different carbon nanotubes-based hybrid multiscale composites with respect to mechanical and electrical properties

Pristine and carboxylic acid functionalised carbon nanotubes (CNTs-P and CNTs-COOH) hybrid composites based on ultrasonically processed nanocomposite epoxy matrix have been prepared. The thermodynamic interactions between CNTs and epoxy components are evaluated through solution experiment. The measurements of mechanical properties and volume electrical resistivity are performed to characterise the structure-property interplay of CNTs hybrid composites. A 0.3?wt-% CNTs-COOH loading significantly enhances the glass transition temperatures (Tg) of the resultant hybrid composites. The re-agglomeration of CNTs-P has been ascribed to the thermodynamic and kinetic factors, which determine the morphological characteristics of CNTs within the overall composites. CNTs-P network enables a continuous conductive path to be present in the composites at lower CNTs-P concentration. An analysis of the effects of the functionalisation of CNTs on the structure and properties of their hybrid composites has been carried out.     

Effect of organic solvent on morphology and mechanical properties of electrospun syndiotactic polypropylene nanofibers

We study the stress–strain behaviors of the electrospun sPP single nanofibers as well as nonwoven mats, which were electrospun from sPP solutions using two different solvents (decalin and cyclohexane) by electrospinning. The effects of organic solvents were explored on the morphologies and the mechanical properties of the corresponding electrospun sPP single nanofibers and nonwoven mats. It was found that the nature of organic solvents dramatically affected the surface morphologies, the circular and looping deposition of the electrospun sPP fibers, and the mechanical properties. The tensile strength of both electrospun sPP single nanofibers and nonwoven mats prepared from decalin-base solution was stronger than that of cyclohexane-base solution.     

Novel electrical conductive hybrid nanostructures based on PA 6/MWCNTCOOH electrospun nanofibers and anchored MWCNTCOOH

Electrical conductive nanostructures made of nanofibers of poly (amide 6) (PA6) with carboxyl functionalized multiwall carbon nanotubes (MWCNT_COOH) and anchored MWCNT_COOH were produced. The nanotubes were surface activated with carboxyl groups, dispersed in formic acid and added to a formic acid solution of PA6. The mixture was electrospun by applying a voltage of 30 kV; afterwards, the nanofiber's mats were immersed in an aqueous dispersion of MWCNT_COOH containing a nonionic surfactant. The chemical structure, morphology, thermal stability, and electrical conductivity of the nanostructures were evaluated by UV spectroscopy, scanning (SEM), and transmission electron microscopy (TEM), thermal gravimetric analyses (TGA) and volumetric conductivity measurements. The efficiency of the functionalization was confirmed by the UV peaks in the range between 220 and 250 nm (corresponding to a carbonyl group conjugated with a carbon‐carbon double bond). SEM and TEM micrographs showed the pullout of the MWCNT_COOH from the nanofibers and the formation of a stable, percolated, and anchored MWCNT_COOH network on the nanofibers due to the anchoring of the MWCNT_COOH from the surfactant solution on the MWCNT_COOH of the nanofibers. The coated nanostructures had higher thermal stability and higher electrical conductivity than the noncoated ones, showing the efficiency of this simple procedure. POLYM. ENG. SCI., 55:1263–1272, 2015. © 2015 Society of Plastics Engineers     

Cellulose acetate/multiwalled carbon nanotube nanocomposites with improved mechanical,thermal, and electrical properties

Cellulose acetate (CA)‐based nanocomposites with various contents of neat multiwalled carbon nanotube (MWCNT) or acid‐treated one (MWCNT‐COOH) are prepared via melt‐compounding method and investigated their morphology, thermal stability, mechanical, and electrical properties. SEM microphotographs reveal that MWCNT‐COOHs are dispersed uniformly in the CA matrix, compared with neat MWCNTs. FTIR spectra support that there exists a specific interaction between carboxyl groups of MWCNT‐COOHs and ester groups of CA, indicating good interfacial adhesion between MWCNT‐COOHs and CA matrix. Accordingly, thermal stability and dynamic mechanical properties of CA/MWCNT‐COOH nanocomposites were higher than those of CA/MWCNT composites. On the contrary, electrical volume resistivities of CA/MWCNT‐COOH nanocomposites are found to be somewhat higher than those of CA/MWCNT composites, which is because of the deterioration of graphene structures for MWCNT‐COOHs and the good dispersion of MWCNT‐COOHs in the CA matrix. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Investigation of post-spinning stretching process on morphological, structural, and mechanical properties of electrospun polyacrylonitrile copolymer nanofibers

Electrospun polyacrylonitrile (PAN) copolymer nanofibers with diameters of ∼0.3 μm were prepared as highly aligned bundles. The as-electrospun nanofiber bundles were then stretched in steam at ∼100 °C into 2, 3, and 4 times of the original lengths. Subsequently, characterizations and evaluations were carried out to understand morphological, structural, and mechanical properties using SEM, 2D WAXD, polarized FT−IR, DSC, and mechanical tester; and the results were compared to those of conventional PAN copolymer microfibers. The study revealed that: (1) the macromolecules in as-electrospun nanofibers were loosely oriented along fiber axes; although such an orientation was not high, a small extent of stretching could effectively improve the orientation and increase the crystallinity; (2) most of macromolecules in the crystalline phase of as-electrospun and stretched nanofibers possessed the zig-zag conformation instead of the helical conformation; and (3) the post-spinning stretching process could substantially improve mechanical properties of the nanofiber bundles. To the best of our knowledge, this study represented the first successful attempt to stretch electrospun nanofibers; and we envisioned that the highly aligned and stretched electrospun PAN copolymer nanofibers could be an innovative type of precursor for the development of continuous nano-scale carbon fibers with superior mechanical strength.     

Morphological, mechanical, and electrical properties as a function of thermal bonding in electrospun nanocomposites

Poly lactic acid (PLA) was electrospun with various concentrations of multi-wall carbon nanotubes (MWNT) and thermal bonding was utilized as a post-processing treatment to improve the mechanical and electrical properties of the nanofibrous mats. Thermally bonded fiber-fiber junctions were observed with scanning electron microscopy. An increase in either the strength or modulus of the PLA mats both with and without MWNTs was observed; the maximum modulus and maximum strength occur at different treatment conditions. The electrical conductivity of the MWNT loaded mats showed significant improvement after treatment just below the composite melting point.     

Single‐wall carbon nanotubes dispersion behavior and its effects on the morphological and mechanical properties of the electrospun nanofibers

The dispersion behavior of single‐walled carbon nanotube (SWCNT) has important effects on morphological and mechanical properties of SWCNT composite nanofibers. The relationship of the dispersion conditions with morphological and mechanical characteristics for SWCNT / polyacrylonitrile (PAN) / polyvinylpyrrolidone (PVP) composite nanofibers have been examined. The SEM and TEM analyses of the nanofibers revealed that the deformation in the nanofiber structures increases with increasing concentration of SWCNTs. Tensile results showed that only 2 wt% SWCNT loading to the electrospun composite nanofibers gave rise to 10‐fold and 3‐fold increase in the tensile modulus and tenacity of nanofiber layers, respectively. Essentially, high mechanical properties and uniform morphology of the composite nanofibers were found at SWCNT concentration of ∼2 wt% due to their stable and individual dispersion. POLYM. COMPOS., 33:1951–1959, 2012. © 2012 Society of Plastics Engineers     

Effects of crystallization on dispersion of carbon nanofibers and electrical properties of polymer nanocomposites

Polymer nanocomposites filled with low volume fractions of carbon nanofibers (CNFs) were prepared by melt‐compounding. Three types of polymers with different crystallization behavior, i.e., weakly‐crystallized low density polyethylene (LDPE), strongly crystallized high density polyethylene (HDPE) and amorphous polystyrene (PS), were selected as matrices for the nanocomposites. The effects of polymer crystallization on the dispersion of CNFs were examined. Optical and electron microscopic examinations revealed that the dispersion of CNFs in the nanocomposite matrices was strongly depended on the crystallization behavior of polymer matrices. The CNFs were found to disperse uniformly in weakly crystallized LDPE and amorphous PS matrices, but agglomerated in HDPE due to its strong crystallization tendency. Such a distinct dispersion behavior of CNFs in polymers had a profound effect on the electrical properties of the nanocomposites investigated. The PS/CNF nanocomposites exhibited the lowest percolation threshold. The HDPE/CNF nanocomposites showed the largest percolation threshold due to the CNF agglomeration within the amorphous phase of HDPE. POLYM. ENG. SCI., 48:177–183, 2008. © 2007 Society of Plastics Engineers     

Effect of chitosan on morphology and conformation of electrospun silk fibroin nanofibers

The electrospinning of silk fibroin(SF)/chitosan(CS) blends with different composition ratios was performed with formic acid as a spinning solvent. The SF/CS blends containing up to the CS content of 30% could be electrospun into the continuous fibrous structure, although pure CS was not able to be electrospun into the fibrous structure. As-spun SF/CS blend nanofibers showed smaller diameter and narrower diameter distribution than pure SF nanofibers, and the diameter gradually decreased from 450 to 130 nm with the addition of CS in blends. However, at the blend compositions with above 40 wt% chitosan, the continuous SF nanofibers containing CS beads were produced. We also investigated the influence of the methanol treatment on the secondary structure of as-spun SF or SF/CS blend nanofibers by means of ATR-IR and solid-state CP-MAS ¹³C-NMR. Comparing with the pure SF nanofibers, the conformational change of the as-spun SF/CS blend nanofibers into β-sheet was faster because the CS with rigid backbone synergistically might promote the conformational transition of SF by an intermolecular interaction.     

Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns

The fiber spinning technique of electrospinning was optimized in order to prepare unidirectional aligned, structurally oriented, and mechanically useful carbon precursor fibers with diameters in the nanoscale range. The fiber spinning velocity and fiber draw ratio was measured to be between 140 and 160 m/s and 1:300,000, respectively, for fibers spun from 10 wt% polyacrylonitrile (PAN) solutions with dimethylformamide (DMF). A high-speed, rotating target was used to collect unidirectional tows of PAN fibers. Aligned and (+) birefringent fibers with diameters between 0.27 and 0.29 μm (FESEM) were collected from electrospinning 15 wt% PAN in DMF solutions at 16 kV onto a target rotating with a surface velocity between 3.5 and 12.3 m/s. Dichroism measurements (Polarized FTIR) of the nitrile-stretching vibration show an increase in the molecular orientation with take-up speed. Wide angle X-ray diffraction patterns (WAXD) show equatorial arcs from the reflection at and (1120) reflection at A maximum chain orientation parameter of 0.23 was determined for fibers collected between 8.1 and 9.8 m/s. Twisted yarns of highly aligned PAN nanofibers with twist angles between 1.1 and 16.8° were prepared. The ultimate strength and modulus of the twisted yarns increase with increasing angle of twist to a maximum of 162±8.5 MPa and 5.9±0.3 GPa, respectively, at an angle of 9.3°.     

High-temperature stable electrospun MgO nanofibers,formation mechanism and thermal properties

MgO nanofiber with good morphology was difficult to obtain through high temperature heat-treatment due to the widely existed pulverization phenomenon, which restricted its applications in the high-temperature field. In this work, MgO nanofibers were prepared by electrospinning through mixed precursors: magnesium acetate (MA) and magnesium citrate (MC). The fiber with MC mass content in the range of 30–50% exhibited good high-temperature stability and the original feature was preserved after heat-treated at 1000 °С. A plausible formation mechanism of the polycrystalline MgO nanofibers was provided in the present paper. Suitable confection of MA and MC brought a milder decomposition process and led to a more compact structure of MgO nanofiber, which contributed to the excellent high-temperature stability. The considerable effects of microstructure on thermal conductivity of the fiber were discussed. The method in this paper has greatly expanded the solution scope for preparing electrospun fibers with expectation of structure modification and strength enhancement.