Using supercritical carbon dioxide in preparing carbon nanotube nanocomposite: Improved dispersion and mechanical properties

Improvements in carbon nanotube (CNT) dispersion and subsequent mechanical properties of CNT/poly(phenylsulfone) (PPSF) composites were obtained by applying the supercritical CO₂ (scCO₂)‐aided melt‐blending technique that has been used in our laboratory for nanoclay/polymer composite preparation. The preparation process relied on rapid expansion of the CNTs followed by melt blending using a single‐screw extruder. Scanning electronic microscopy results revealed that the CNTs exposed to scCO₂ at certain pressures, temperatures, exposure time, and depressurization rates have a more dispersed structure. Microscopy results showed improved CNT dispersion in the polymer matrix and more uniform networks formed with the use of scCO₂, which indicated that CO₂‐expanded CNTs are easier to disperse into the polymer matrix during the blending procedure. The CNT/PPSF composites prepared with scCO₂‐aided melt blending and conventional melt blending showed similar tensile strength and elongation at break. The Young's modulus of the composite prepared by means of conventional direct melt blending failed to increase beyond the addition of 1 wt% CNT, but the scCO₂‐aided melt‐blending method provided continuous improvements in Young's modulus up to the addition of 7 wt% CNT. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Improved mechanical properties of organoclay/nylon 6 nanocomposites prepared via a supercritical carbon dioxide‐aided,melt blending method

The use of supercritical carbon dioxide (scCO₂) as a processing aid to help exfoliate nanoclays and improve their dispersion after melt blending in polymer matrices has been reported in the literature by our laboratory and others. Previous work has focused on nonpolar polymers such as polypropylene. In this work, the supercritical carbon dioxide‐aided, melt blending method with single‐screw compounding was applied to an organoclay/nylon 6 composite system with favorable interactions. Wide‐angle X‐ray diffraction, transmission electron microscopy, dynamic mechanical testing, and tensile tests are provided to investigate the effect of processing with scCO₂ on the final composite morphology and properties. It was found that properties of composites prepared with the scCO₂‐aided method were similar to or higher than those reported in the literature for samples prepared with twin‐screw compounding. At 7.6 wt% the modulus value reached 4.75 GPa which is one of the highest increases (1.7 GPa) reported for these materials processed at intermediate concentrations. Beyond 7.6 wt% the improvement due to scCO₂ processing matches that of direct blending. It is possible that with the use of a twin‐screw extruder, the modulus would continue to increase. The use of nylon illustrates the importance of having functionalities that can interact with the clay during scCO₂‐aided melt compounding. POLYM. COMPOS. 36:527–537, 2015. © 2014 Society of Plastics Engineers     

Electrical and mechanical properties of acrylonitrile‐butadiene‐styrene/multiwall carbon nanotube nanocomposites prepared by melt‐blending

The electrical properties in polymer/carbon nanotube (CNT) nanocomposites are governed not only by the degree of dispersion but also to a greater extent on the aspect ratio of the CNTs in the final composites. Melt‐mixing of polymer and CNTs at high shear rate usually breaks the CNTS that lowers the aspect ratio of the nanotubes. Thus, homogeneous dispersion of CNTs while retaining the aspect ratio is a major challenge in melt‐mixing. Here, we demonstrate a novel method that involves melt‐blending of acrylonitrile‐butadiene‐styrene (ABS) and in situ polymerized polystyrene (PS)/multiwalled CNT (MWCNT) nanocomposites, to prepare electrically conducting ABS/MWCNT nanocomposites with very low CNT loading than reported. The rationale behind choosing PS/MWCNT as blending component was that ABS is reported to form miscible blend with the PS. Thus, (80/20 w/w) ABS/(PS/MWCNT) nanocomposites obtained by melt‐blending showed electrical conductivity value ≈1.27 × 10^?6 S cm^?1 at MWCNT loading close to 0.64 wt %, which is quite lower than previously reported value for ABS/MWCNT system prepared via solution blending. Scanning electron microscopy and differential scanning calorimetry analysis indicated the formation of homogenous and miscible blend of ABS and PS. The high temperature (100°C) storage modulus of ABS (1298 MPa) in the nanocomposites was increased to 1696 MPa in presence of 0.64 wt % of the MWCNT. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Hotmelts with improved properties by integration of carbon nanotubes

With the aim of the development of conductive and mechanically improved adhesives, carbon nanotubes (CNTs) were dispersed by melt mixing into a non-reactive polyolefine based hotmelt adhesive. The composite materials, containing 0.5 to 5.0 wt% multi-walled CNTs (MWNTs), showed electrical percolation at about 0.75 wt%. Investigations of the mechanical properties using tensile tests resulted in a significant enhancement of Young's modulus up to 372% and nearly doubling of tensile strength at 5.0 wt%. Even if the hotmelt material is highly elastic compared to typical thermoplastic matrices, the melt mixing resulted in suitable CNT dispersion. The melt viscosity increased with CNT loading, however near the observed electrical percolation threshold the processability was not notably reduced. Most important, next to conductivity at low CNT loadings, also a significant enhancement in the shear strength of bonded joints of AlMg3 up to values of 250% of the pure hotmelt could be obtained. The property profile can be tailored with CNT concentration, indicating the suitability of CNT addition into these hotmelt adhesives.     

Influence of crystallinity and chain interactions on the electrical properties of polyamides/carbon nanotubes nanocomposites

This work evaluates the effects of the crystallinity degree and π–π interactions between nanoparticles and a polymeric matrix on the electrical properties of polyamides and carbon nanotubes (CNT) nanocomposites. Two polymeric matrices were chosen; polyamide (PA) 6.6, a semi-crystalline polymer, and PA 6I-6T (here called aPA), a semi-aromatic and amorphous polyamide. The PA 6.6 crystallinity degree did not significantly change. Both lamellar thicknesses, amorphous (L_a) and crystalline (L_c), were estimated through Small-angle X-ray scattering. L_a increased significantly when CNTs were added. Both nanocomposites presented almost the same percolation threshold. In aPA nanocomposites, the π–π interaction between aromatic groups of CNTs and aPA is not only responsible for a homogeneous CNT dispersion, but also creates a direct path, parallel to the electrodes, for electron conduction after the percolation limit. In the PA 6.6 nanocomposites, the CNTs preferably disperse in the amorphous regions, forming a conductive network.     

Effects of the dispersion state and aspect ratio of carbon nanotubes on their electrical percolation threshold in a polymer

The electrical percolation threshold of carbon nanotubes (CNTs) is correlated with their dispersion state and aspect ratio through modeling. An analytical percolation model based on excluded volume theory and developed for systems containing two types of fillers is used. CNTs are modeled as two types of fillers: single CNT and m‐CNT bundle, and a variable P representing the dispersion state of CNTs is introduced. An equation showing the effects of the dispersion state and aspect ratio on the electrical percolation threshold of CNTs is established and verified with some of the published experimental data. It is useful for predicting the conductive behavior of polymer/CNT composites and for the design of their processing conditions. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of annealing treatment on the structure and properties of polyurethane/multiwalled carbon nanotube nanocomposites

The thermoplastic polyurethane/multiwalled carbon nanotube (TPU/CNT) nanocomposites with high conductivity and low percolation threshold value were prepared by melting blending and annealing treatment. The effect of annealing process on the microphase structure and the properties of TPU/CNT nanocomposites was studied. It has been shown that CNT flocculation can occur in TPU/CNT nanocomposites during the annealing process. At a critical CNT content, which defined the percolation threshold, CNTs could form conductivity network. The conductive percolation threshold value of TPU/CNT nanocomposites was decreased from 10 to 4% after annealing process, and the conductivity of TPU/CNT nanocomposites with 10 vol % of CNT could reach 1.1 S/m after an annealing time of 1 h. The significant enhancement of electrical conductivity was influenced by the annealing time and the content of CNTs. The formation of CNT networks was also verified by dynamic viscoelastic characterization. The results of X‐ray diffraction and differential scanning calorimetry indicated that annealing process reinforced the microphase separation of the nanocomposites. Mechanical properties test showed that the annealing treatment was in favor of improving the mechanical properties; however, further increase in the annealing time has negative effect on the mechanical properties. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The characteristics of carbon nanotube‐reinforced poly(phenylene sulfide) nanocomposites

Multiwall carbon nanotube reinforced poly (phenylene sulfide) (PPS) nanocomposites were successfully fabricated through melt compounding. Structural, electrical, thermal, rheological, and mechanical properties of the nanocomposites were systematically studied as a function of carbon nanotube (CNT) fraction. Electrical conductivity of the polymer was dramatically enhanced at low loading level of the nanotubes; the electrical percolation threshold lay between 1 and 2 wt % of the CNTs. Rheological properties of the PPS nanocomposites also showed a sudden change with the CNT fraction; the percolation threshold was in the range of 0–0.5 wt % of CNTs. The difference in electrical and rheological percolation threshold was mainly due to the different requirements needed in the carbon nanotube network in different stages. The crystallization and melting behavior of CNT‐filled PPS nanocomposites were studied with differential scanning calorimetry; no new crystalline form of PPS was observed in the nanocomposites, but the crystallization rate was reduced. The thermal and mechanical properties of the nanocomposites were also investigated, and both of them showed significant increase with CNT fraction. For 5 wt % of CNT‐filled PPS composite, the onset of degradation temperature increased by about 13.5°C, the modulus increased by about 33%, and tensile strength increased by about 172%. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Assessment of intertube interactions in different functionalized multiwalled carbon nanotubes incorporated in a phenoxy resin

In this work, about 1 wt% of different functionalized carbon nanotubes (CNTs), namely CNT? COOH (CNT with carboxylic groups), CNT? NH₂ (CNT with amine groups) and CNT? OH (CNT with hydroxyl groups), as well as nonfunctionalized CNTs were incorporated into a phenoxy resin via a melt mixing process. The extent of intertubes and polymer–tubes interactions and their influence on state of CNTs dispersion were assessed through determination of electrical, rheological, and morphological characteristics. CNT? NH₂ showed the lowest intertubes interactions followed by CNT? OH and CNT? COOH. Nanocomposite made from CNT? COOH showed the poorest state of CNTs dispersion and the biggest CNTs agglomerates and it remained nonconductive. The acid‐functionalized CNTs were not able to form strong polymer–tube interactions because of their high cohesive energy and therefore in the melt rheological investigations they exhibited the lowest storage modulus and complex viscosity as well as the highest loss factor among all the studied CNTs. A good balance between intertubes and polymer–tube interactions is necessary through proper selection of CNTs functional groups for achieving a good state of CNTs dispersion and consequently obtaining enhanced electrical and viscoelastic properties. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Surfactant‐assisted processing of polyimide/multiwall carbon nanotube nanocomposites for microelectronics applications

The dispersion and stability of carbon nanotubes (CNTs) inside a polymer matrix, especially with a high CNT content, are still big challenges. Moreover, the interaction between CNTs and the polymer matrix should be strong enough to improve the mechanical properties. The efficient dispersion of CNTs is essential for the formation of a uniform distribution of a CNT network in a polymer composite. Polyimide/multiwall CNT nanocomposites were synthesized by in situ polymerization with the aid of a surfactant. A Fourier transform infrared spectroscopy study proved that the surfactant did not hamper the polymerization of the polyimide. The microstructure, storage modulus and electrical conductivity of the nanocomposites were improved using a particular amount of the surfactant. Environmental stability test results showed that the polyimide with 1 wt% of CNTs produced with the aid of the surfactant possessed excellent reliability in high‐temperature and high‐humidity environments. Surfactants were successfully used to obtain fine‐structure polyimide/CNT nanocomposites by in situ polymerization. The enhancement of the mechanical properties was attributed to the incorporation of the surfactant. A percolation of electrical conductivity could be achieved with 1 wt% of CNTs. Copyright © 2010 Society of Chemical Industry     

Robust and wear resistant in-situ carbon nanotube/Si3N4 nanocomposites with a high loading of nanotubes

Highly homogenous carbon nanotube (CNT)/silicon nitride (Si₃N₄) nanocomposites with high CNTs loadings, up to 22 vol.%, are developed through the in-situ synthesis of CNTs on the ceramic powders, and further densification using the spark plasma sintering technique. The CNTs dispersion degree, the composite density, and their properties, especially the tribological ones, are evaluated and compared with those obtained for nanocomposites processed by the ex-situ method based on the mixing of nanotubes and ceramic powders in a solvent media. Fully dense in-situ 12 vol.% CNTs nanocomposites are 87% and 65% more wear resistant than monolithic Si₃N₄ materials and ex-situ nanocomposites, respectively, in the latter case due to the higher nanotubes dispersion and better mechanical properties attained by the in-situ process. These new in-situ CNTs nanocomposites present multifunctionality and are promising for emerging applications, especially for gasoline direct injection systems.     

Mechanical and electrical properties of carbon nanotube buckypaper reinforced silicon carbide nanocomposites

The nanocomposite was produced via phenolic resin infiltrating into a carbon nanotube (CNT) buckypaper preform containing B₄C fillers and amorphous Si particles followed by an in-situ reaction between resin-derived carbon and Si to form SiC matrix. The buckypaper preform combined with the in-situ reaction avoided the phase segregation and increased significantly the volume fraction of CNTs. The nanocomposites prepared by this new process were dense with the open porosities less than 6%. A suitable CNT–SiC bonding was achieved by creating a B₄C modified interphase layer between CNTs and SiC. The hardness increased from 2.83 to 8.58 GPa, and the indentation fracture toughness was estimated to increase from 2.80 to 9.96 MPa m^1/2, respectively, by the reinforcing effect of B₄C. These nanocomposites became much more electrically conductive with high loading level of CNTs. The in-plane electrical resistivity decreased from 124 to 74.4 μΩ m by introducing B₄C fillers.     

Network behavior of thermosetting polyimide/multiwalled carbon nanotube composites

In previous published research, network formation has been used to understand morphology and properties in polymer nanocomposites containing carbon nanotubes (CNTs) through measurements of rheological and electrical percolation thresholds, largely in thermoplastic matrices. In this research, these tools are explored as a means to understand network transport mechanisms and changes in CNT dispersion during curing in a thermosetting matrix. Specifically, rheological and electrical measurements were performed on the uncured nanocomposites, and electrical measurements were performed on the cured nanocomposites. The resulting data were applied to a percolation model. The results showed that the uncured resin played a limited role in mediating rheological transport and that little CNT aggregation occurred during curing. The results of this initial work suggest that such a combination of techniques is applicable to understanding dispersion changes resulting from curing and provides complementary insight to that provided by electron microscopy imaging of the same phenomenon.     

Structure,permeability, and rheology of supercritical CO2 dispersed polystyrene-clay nanocomposites

Improvement in clay dispersion and clay-polymer interfacial interactions are keys to producing superior nanocomposites. A supercritical CO₂ (scCO₂) processing method was utilized to pre-disperse commercial organic clays, for further solvent mixing with polystyrene (PS) to form nanocomposites with significant dispersion and interfacial enhancement. The effect of scCO₂ processing on clay pre-dispersion, and clay dispersion and interfacial interaction in nanocomposites were investigated. SEM and WAXD of the clays indicated that after scCO₂ processing the clays lose their long region ordered layer structure appreciably, associated with reduction in particle size. WAXD and TEM of the PS/clay nanocomposites showed that the polymer penetrated into the pre-dispersed clay, leading to a disordered intercalated/exfoliated structure with improved interfacial interaction rather than a disordered intercalated structure as seen with as-received clays. Relationships between those structures, rheological and barrier properties were investigated. The scCO₂-processed nanocomposites showed a plateau in the low-frequency storage modules and increased complex viscosity, each associated with significant clay dispersion in the nanocomposite. With only 1.09% volume fraction of clay, significant reduction (∼49%) of oxygen permeation was achieved.     

Improving the exfoliation of layered silicate in a poly(ethylene terephthalate) matrix using supercritical carbon dioxide

A technique for improving the exfoliation of organically modified layered silicate (OMLS) in a poly(ethylene terephthalate) (PET) matrix using super critical carbon dioxide (scCO₂) is compared to traditional direct melt blending. The process relies on the rapid expansion of a scCO₂/OMLS mixture directly into the second stage of a single screw extruder where the clay and scCO₂ mixture is subsequently melt blended with the PET matrix. The simple, environmentally benign process results in a more highly delaminated system than traditional direct melt compounding. X‐ray diffraction analysis is used to reveal improved clay morphologies and the resulting mechanical properties are examined. Rheology is used as a tool to draw conclusions about the effect that scCO₂ has on reducing PET matrix degradation during processing. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Dispersion of carbon nanotubes through amphiphilic block copolymers: Rheological and dielectrical characterizations of poly(ethylene oxide) composites

In this article, we report on some properties of polymer nanocomposites prepared from dispersions of multiwall carbon nanotubes (CNT) in aqueous solution prepared using amphiphilic block copolymers. These nanocomposites are made of polyethylene oxide as matrix and CNT wrapped with copolymers as fillers. We investigated the rheological and electrical behavior of such composites with the objectives of underlined the effect of wrapping. Two rheological and only one electrical percolation thresholds have been observed and related to polymer–CNT and CNT–CNT networks. The low values of these percolation thresholds agree with a homogeneous dispersion of CNT in the matrix. We also demonstrated that specific wrapping may induce an increase of electrical conductivity without affecting too much the viscosity of the melt. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

A facile route to develop electrical conductivity with minimum possible multi‐wall carbon nanotube (MWCNT) loading in poly(methyl methacrylate)/MWCNT nanocomposites

Today, we stand at the threshold of exploring carbon nanotube (CNT) based conducting polymer nanocomposites as a new paradigm for the next generation multifunctional materials. However, irrespective of the reported methods of composite preparation, the use of CNTs in most polymer matrices to date has been limited by challenges in processing and insufficient dispersability of CNTs without chemical functionalization. Thus, development of an industrially feasible process for preparation of polymer/CNT conducting nanocomposites at very low CNT loading is essential prior to the commercialization of polymer/CNT nanocomposites. Here, we demonstrate a process technology that involves in situ bulk polymerization of methyl methacrylate monomer in the presence of multi‐wall carbon nanotubes (MWCNTs) and commercial poly(methyl methacrylate) (PMMA) beads, for the preparation of PMMA/MWCNT conducting nanocomposites with significantly lower (0.12 wt% MWCNT) percolation threshold than ever reported with unmodified commercial CNTs of similar qualities. Thus, a conductivity of 4.71 × 10^?5 and 2.04 × 10^?3 S cm^?1 was achieved in the PMMA/MWCNT nanocomposites through a homogeneous dispersion of 0.2 and 0.4 wt% CNT, respectively, selectively in the in situ polymerized PMMA region by using 70 wt% PMMA beads during the polymerization. At a constant CNT loading, the conductivity of the composites was increased with increasing weight percentage of PMMA beads, indicating the formation of a more continuous network structure of the CNTs in the PMMA matrix. Scanning and transmission electron microscopy studies revealed the dispersion of MWCNTs selectively in the in situ polymerized PMMA phase of the nanocomposites. Copyright © 2012 Society of Chemical Industry     

Electrophoretic deposition of carbon nanotube–ceramic nanocomposites

The purpose of this paper is to present an up-to-date comprehensive overview of current research progress in the development of carbon nanotube (CNT)–ceramic nanocomposites by electrophoretic deposition (EPD). Micron-sized and nanoscale ceramic particles have been combined with CNTs, both multiwalled and single-walled, using EPD for a variety of functional, structural and biomedical applications. Systems reviewed include SiO₂/CNT, TiO₂/CNT, MnO₂/CNT, Fe₃O₄/CNT, hydroxyapatite (HA)/CNT and bioactive glass/CNT. EPD has been shown to be a very convenient method to manipulate and arrange CNTs from well dispersed suspensions onto conductive substrates. CNT–ceramic composite layers of thickness in the range <1–50 μm have been produced. Sequential EPD of layered nanocomposites as well as electrophoretic co-deposition from diphasic suspensions have been investigated. A critical step for the success of EPD is the prior functionalization of CNTs, usually by their treatment in acid solutions, in order to create functional groups on CNT surfaces so that they can be dispersed uniformly in solvents, for example water or organic media. The preparation and characterisation of stable CNT and CNT/ceramic particle suspensions as well as relevant EPD mechanisms are discussed. Key processing stages, including functionalization of CNTs, tailoring zeta potential of CNTs and ceramic particles in suspension as well as specific EPD parameters, such as deposition voltage and time, are discussed in terms of their influence on the quality of the developed CNT/ceramic nanocomposites. The analysis of the literature confirms that EPD is the technique of choice for the development of complex CNT–ceramic nanocomposite layers and coatings of high structural homogeneity and reproducible properties. Potential and realised applications of the resulting CNT–ceramic composite coatings are highlighted, including fuel cell and supercapacitor electrodes, field emission devices, bioelectrodes, photocatalytic films, sensors as well as a wide range of functional, structural and bioactive coatings.     

Role of polymer-clay interactions and nano-clay dispersion on the viscoelastic response of supercritical CO2 dispersed polyvinylmethylether (PVME)-Clay nanocomposites

Clay dispersion and polymer-clay interactions play a key role in producing property enhancements in nanocomposites; yet characterizing them in complex polymer-clay systems is often a challenge. Rheology can offer insights into clay dispersion and clay-polymer interactions. We have investigated the viscoelastic response for a series of supercritical CO₂ (scCO₂) processed polyvinylmethylether (PVME)/clay nanocomposites with varying polymer-clay interactions and nano-clay dispersion. PVME is used in this study because it is highly swellable in scCO₂, thereby enabling processing of PVME/clay mixtures without the presence of a co-solvent. Since PVME and natural clay are water-soluble, highly dispersed PVME-clay nanocomposites were prepared using water, followed by lyophilization in the presence of polymer. In this ‘weakly interacting’, but highly dispersed systems, with clay loadings above the percolation threshold, terminal behavior was observed in the linear viscoelastic moduli (i.e. no low frequency plateau is observed). When the nanocomposites were processed in scCO₂, with 15 wt% of 30B and I.30P, the WAXD patterns of the resultant nanocomposites were largely comparable, indicating partial dispersion, with intercalation peaks. However, the rheology of these two nanocomposites were significantly different despite similar inorganic volume loading (4 vol%). Even with less dispersion compared to the water-based system, the low-frequency moduli were significantly more enhanced, accompanied by a plateau, and a cross-over frequency shift. Neglecting the small differences in the actual clay content between these clays (4-5 vol% of inorganic matter), this suggests that rheology may be sensitive to strong interactions between the clay surfactant and the polymer. Therefore, polymer-clay interactions and clay-clay interactions may both be important in the ability to sustain a “so-called” percolated network, rather than just clay dispersion.