Rheological Percolation Threshold Research Articles

Rheological and Electrical Properties of CB/HDPE Composites

The rheological and electrical properties of composites formed by filling high density polyethylene (HDPE) with carbon black (CB) were characterized. Nonuniversal electrical behavior was observed with critical exponent t of 2.9 for the composites. The phenomenon was interpreted in terms with tunneling-percolation model. The effects of shearing frequency and particle concentration on the rheological behavior of the composites were investigated. A second plateau of storage modulus and strong shear thinning behavior were observed at low frequencies induced by aggregation of fillers. And the rheological percolation threshold is found to coincide well with the electrical one.

MWNT‐filled PC/ABS blends: Correlation of morphology with rheological and electrical response

A systematic study was done on morphological, electrical and rheological behavior of co‐continuous or dispersed‐type polycarbonate (PC)/acrylonitrile‐styrene‐butadiene (ABS) blends, containing different amounts of multiwalled carbon nanotubes (MWNT). The MWNTs gave substantial electrical conductivities to these nanocomposites at very low concentrations, owing to the effective melt processing method. Because of selective localization of MWNTs in the PC phase, along with double percolation phenomenon, the blend with co‐continuous morphology showed a lower electrical and rheological percolation threshold, higher melt viscosity and elasticity, as compared to the system with dispersed morphology. The morphology of both the blend systems was refined as a result of MWNTs incorporation but the morphology type remained unchanged. A typical role of compatibilizer in refining blend morphology was observed in both the systems. The electrical conductivity of the system filled with MWNTs in presence of compatibilizer, was lower than the systems filled with MWNTs only, which was attributed to role of compatibilizer in directing a part of MWNTs from PC matrix toward ABS phase. With increasing compatibilizer/MWNTs ratio, the influence of compatibilizer on morphology refinement and conductivity reduction was intensified. By comparing TEM micrograph of PC/SAN/MWNTs with that of PC/ABS/MWNTs, it was revealed that small portion of MWNTs was also located on polybutadiene rubber fraction of ABS. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 739‐748, 2013

Melt rheological investigation of polylactide-nanographite platelets biopolymer composites

This study is an analytical investigation of processability of biopolymer-carbon based nanofiller composites primarily through rheological investigation of samples. The composites were fabricated via dry mixing and melt-blending of biodegradable polylactide (PLA) and nanographite platelets (NGP) in a Brabender twin screw extruder. A range of different nanofiller contents (1, 3, 5, 7, and 10 wt %) were studied for NGP containing composites. The morphology was studied with X-ray diffraction and transmission electron microscopy techniques and showed poor dispersion, with agglomerates, tactoids, and exfoliated layers present. Mechanical properties showed an optimum at 3 wt % filler. Results showed that the composites exhibited higher elastic and viscous moduli than neat PLA. The rheological percolation threshold predicted by changes in slope (α) as well as liquid–solid transition theory of samples was found around 3 wt % through the change from liquid-like behavior to pseudo-solid-like behavior at terminal region during dynamic oscillatory measurements. NGP nanofillers were found to enhance the viscoelastic and mechanical properties of PLA at low concentrations; however, an efficient dispersion of nanofillers within polymer by melt intercalation method of mixing was not achieved. POLYM. ENG. SCI., 54:175–188, 2014. © 2013 Society of Plastics Engineers

Comparison of filler percolation and mechanical properties in graphene and carbon nanotubes filled epoxy nanocomposites

This paper compares the filler percolation network of multi-walled carbon nanotubes (MWCNTs) grown by chemical vapor deposition and thermally reduced functionalized graphene sheets (FGSs) in an epoxy resin. The filler network was evaluated by the plate–plate rheological response of un-cured dispersions and the electrical properties of cured materials. We found that FGS did not raise the viscosity of the system as much as MWCNT, maintaining the Newtonian behavior even at 1.5wt.% FGS. MWCNT readily formed a filler network compared to FGS, evidenced by lower electrical and rheological percolation thresholds, presence of yield stress and higher storage modulus of the dispersions. On the other hand, the mechanical performance of the cured FGS nanocomposites outperformed the MWCNT, with enhancements of 50% and 15% of Young’s modulus and strength, respectively. This combination of good processing properties with low viscosity and enhanced mechanical properties makes FGS great candidates to develop multifunctional polymer materials.

Liquid-State and Solid-State Properties of Nanotube/Polypropylene Nanocomposites Elaborated via a Simple Procedure.

Non-modified Multiwalled Carbon Nanotubes (MWCNT) and polypropylene (PP) in absence of compatibilizer have been chosen to elaborate MWCNT/PP nanocomposites using a simple melt-mixing dispersing method. Calorimetry results indicate little effect of MWCNTs on crystallinity of PP, revealing not much interaction between nanotubes and PP chains, which is compatible with the employed manufacturing procedure. In any case, a hindering of polymer chains motion by MWCNTs is observed in the molten state, using oscillatory flow experiments, and a rheological percolation threshold is determined. The percolation limit is not noticed by Pressure-Volume-Temperature (PVT) measurements in the melt, because this technique rather detects local motions. Keeping the nanocomposites in the molten state provokes an electrical conductivity increase of several orders of magnitude, but on ulterior crystallization, the conductivity decreases, probably due to a reduction of the ionic conductivity. For a concentration of 2% MWCNTs, in the limit of percolation, the conductivity decreases considerably more, because percolation network constituted in the molten state is unstable and is destroyed during crystallization.

Permeation, antifouling and desalination performance of TiO2 nanotube incorporated PSf/CS blend membranes

Polysulfone (PSf) and chitosan (CS) blend membranes were prepared by incorporating titanium dioxide nanotubes (TiO2NT) in different compositions. The proper blending of PSf and CS in the PSf/CS/TiO2 membranes was confirmed by ATR-IR spectroscopy. The influence of nanotubes on morphology of membranes was investigated by Field Emission Scanning Electron Microscopy (FESEM). The effect of nanotubes on hydrophilicity of the membranes was studied by water swelling and contact angle measurements. The distribution of TiO2NT on the membrane surface was determined by Transmission Electron Microscope (TEM) analysis. The permeation property of PSf/CS/TiO2NT membranes was carried out by measuring the time dependent pure water flux (PWF). Bovine serum albumin (BSA) protein rejection studies were performed to know the antifouling properties. The rheological percolation threshold of PSf/CS/TiO2NT solutions was measured by viscosity studies. The nanotubes incorporated PSf/CS membranes showed enhanced permeation and antifouling properties compared to PSf/CS and nascent PSf ultrafiltration membranes. Membranes prepared well above rheological percolation threshold showed drastic reduction in pore size and acted as nanofiltration (NF) membranes.

Percolation and gel-like behavior of multiwalled carbon nanotube/polypropylene composites influenced by nanotube aspect ratio

The influence of aspect ratio of multiwalled carbon nanotube (MWCNTs) on rheological and electrical properties of MWCNT/polypropylene (PP) composites has been studied. Owing to the pre-grinding for MWCNTs and the efficient twin-extruder mixing, the dispersion of MWCNTs of different aspect ratios in the PP matrix is all uniform and homogeneous. With incorporation of 10 wt% MWCNTs with an aspect ratio of 167, an electrical conductivity of 1.2 S/cm could be achieved. A concept of “gel-like behavior” is introduced to investigate the MWCNT network formation within the composites. The rheological percolation threshold (cpG') and the gel content (cg) of different MWCNT/PP composites are derived by applying the power law scaling and the Winter–Chambon method, respectively. It is revealed that the composites containing MWCNTs with the largest aspect ratio have the lowest cpG' and cg values. Furthermore, the electrical conductivities of composites are measured and the electrical percolation thresholds (cpσ) are obtained. The difference in gel content among different aspect ratios as well as the difference between rheological and electrical percolation thresholds is discussed.

Effect of Modified and Nonmodified Carbon Nanotubes on the Rheological Behavior of High Density Polyethylene Nanocomposite

This paper reports the results of studies on the rheological behavior of nanocomposites of high density polyethylene (HDPE) with pristine multiwall carbon nanotubes (CNT) as well as phenol and 1‐octadecanol (C18) functionalized CNT at 1, 2, 3, 4, 5, and 7 wt% loading. The viscosity reduction at 1 wt% CNT follows the order, pristine CNT < phenol functionalized CNT < C18 functionalized CNT. As the filler loading increases from 1 to 2, 3, and 4 wt%, neat HDPE and filled HDPE systems show similar moduli and viscosity, particularly in the low frequency region. As the filler loading increases further to 5 and 7 wt%, the viscosity and moduli become greater than the neat HDPE. The storage modulus, tanδ, and the Cole‐Cole plots show that CNT network formation occurs at higher CNT loading. The critical CNT loading or the rheological percolation threshold, where network formation occurs is found to be strongly dependant on the functionalization of CNT. For pristine CNT, the rheological percolation threshold is around 4 wt%, but for functionalized CNT it is around 7 wt%. The surface morphologies of CNT and functionalized CNT at 1 wt% loading showed good dispersion while at 7 wt% loading, dispersion was also achieved, but there are few regions with agglomeration of CNT.

Rheological and electrical properties of EVA copolymer filled with bamboo charcoal

The electrical and rheological properties of an ethylene vinyl acetate (EVA) copolymer filled with bamboo charcoal were investigated. The composites were prepared by melt process in an internal batch mixer. Size distribution analysis showed that d(50) and d(90) values of the bamboo charcoal particles are 12.7 and 40 μm, respectively, with a mean diameter of 22 μm. Scanning electron microscopy proved that the particles of bamboo charcoal present a rectangular shape. The electrical percolation threshold was observed at 0.18 volume fraction (35 wt%) of bamboo. Beyond the percolation threshold, a considerable increase in electrical properties was observed up to a limit value of 10-2 S/m. The rheological percolation was studied from different rheological models. As a result, the rheological percolation threshold was observed at 0.3 volume fraction (50 wt%) of bamboo charcoal contents. So, the electrical percolation occurs before the rheological percolation. This is principally due to the filler’s characteristics such as the specific surface area, the aspect ratio, and the surface properties. Finally, the bamboo charcoal confers high electrical properties to the EVA composite without inducing strong changes in its viscoelastic properties.

Characterization of non-covalently, non-specifically functionalized multi-wall carbon nanotubes and their melt compounded composites with an ethylene–octene copolymer

Multi-wall carbon nanotubes (MWCNTs) were functionalized with a hyperbranched polyethylene (HBPE) using a non-covalent, non-specific functionalization approach. The adsorption behavior of HBPE on MWCNTs was characterized by means of an adsorption isotherm. HBPE adsorption reached a plateau value of 0.3:1.0 (w/w) HBPE:MWCNT, corresponding to a surface coverage of approximately 30%. The functionalized MWCNTs were better dispersed in tetrahydrofuran (THF), forming smaller aggregates compared to their unmodified counterparts. Pristine and HBPE-functionalized MWCNTs were melt compounded with a low-viscosity ethylene–octene copolymer (EOC) matrix. Electrical and rheological percolation thresholds were observed at nanotube loadings of less than 1 wt%. Functionalization did not affect significantly the electrical conductivity and rheological properties. The improved dispersion of the functionalized MWCNTs within the EOC matrix resulted in improved ductility over the non-functionalized counterpart. This study demonstrates that this method of functionalization results in partial surface coverage of the nanotubes, therefore providing an efficient means for achieving good nanotube dispersion, without compromising their surface properties.

Syndiotactic polystyrene/multi-walled carbon nanotube nanocomposites: Polymorphism, thermal properties, electrical conductivity, and rheological properties

Syndiotactic polystyrene (sPS)/multi-walled carbon nanotube (MWNT) nanocomposites were prepared using a melt mixing technique. SEM images demonstrated the fine dispersion of MWNTs through the sPS matrix. DSC results illustrated that the MWNTs facilitated the nucleation of sPS (up to ca. 12.2°C increase), but retarded the subsequent crystals growth. Based on the Avrami analysis, the dimension of sPS crystals growth in the composites decreased because of the effects of extensive nucleation and the formation of a nanoconfined/constrained environment from the MWNTs. XRD data confirmed that the presence of MWNTs facilitated the formation of β-form sPS crystals. The thermal stability of sPS improved to approximately 31°C at 3 phr of MWNT loading. A rheological percolation threshold between an MWNT loading of 0.5 and 1phr was determined by measuring the rheological properties of the samples. The incorporation of MWNTs reduced the electrical resistivity of sPS by 10 orders of magnitude.

Pyridine-Modified Polymer as a Non-Covalent Compatibilizer for Multi-Walled CNT/Poly[ethylene-co- (methacrylic acid)] Composites Fabricated by Direct Melt Mixing

Multi-wall CNT/poly[ethylene-co-(methacrylic acid)] composites were prepared by melt mixing. To improve dispersion and promote polymer/nanotube interactions, a novel non-covalent compatibilizer is synthesized by reacting the polymer with 4-(aminomethyl)pyridine. The composite based on the pristine polymer shows electrical and rheological percolation thresholds at nanotube loadings of 1.85 and 1.4 wt%, respectively. When 5 wt% of the pyridine-modified compatibilizer is added, the corresponding values are reduced to 1.44 and 0.8 wt%, respectively. The electrical resistivity decreases even further as 10 wt% of the novel dispersing agent is used. Microscopy and Raman spectroscopy confirm the improved dispersion and π-interactions established during melt mixing.

The effect of surface chemistry of graphene on rheological and electrical properties of polymethylmethacrylate composites

Graphene sheets with different oxygen contents were prepared to functionalize the electrically insulating polymethylmethacrylate (PMMA). The influences of surface chemistry of graphene on rheological, electrical and electromagnetic interference (EMI) shielding properties of its PMMA composites were investigated. The appearance of frequency-independent storage modulus at low frequency suggests a solid-like viscoelastic behavior and the formation of an interconnected network of graphene in the matrix. Due to the favorable interfacial interactions arising from polarity matching, the graphene with a C/O ratio of 13.2 (graphene-13.2) shows a better dispersion in PMMA than those with lower C/O ratios, and thus its PMMA composites exhibit lower rheological and electrical percolation thresholds. The EMI shielding properties of the graphene/PMMA composites exhibit similar dependence on the oxygen content of graphene. A high EMI shielding effectiveness of ∼30dB was obtained for the PMMA composite with 4.2vol.% of graphene-13.2 with microwave absorption as the dominant EMI shielding mechanism.

Towards the development of poly(phenylene sulphide) based nanocomposites with enhanced mechanical, electrical and tribological properties

Novel poly(phenylene sulphide) (PPS) nanocomposites reinforced with an aminated derivative (PPS-NH2) covalently attached to acid-treated single-walled carbon nanotubes (SWCNTs) were prepared via simple melt-blending technique. Their morphology, viscoelastic behaviour, electrical conductivity, mechanical and tribological properties were investigated. Scanning electron microscopy revealed that the grafting process was effective in uniformly dispersing the SWCNTs within the matrix. The storage and loss moduli as a function of frequency increased with the SWCNT content, tending to a plateau in the low-frequency regime. The electrical conductivity of the nanocomposites was considerably enhanced in the range 0.1–0.5 wt% SWCNTs; electrical and rheological percolation thresholds occurred at similar nanotube concentrations. Mechanical tests demonstrated that with only 1.0 wt% SWCNTs the Young's modulus and tensile strength of the matrix improved by 51 and 37%, respectively, without decrement in toughness, ascribed to a very efficient load transfer. A moderate decrease in the friction coefficient and a 75% reduction in wear rate were found for the abovementioned nanotube loading, indicating that PPS-NH2-g-SWCNTs are good tribological additives for thermoplastic polymers. Based on the promising results obtained in this work, it is expected that these nanofillers will be used to develop high-performance thermoplastic/CNT nanocomposites for structural applications.

Study of carbon black‐filled poly(butylene succinate)/polylactide blend

AbstractIn this study, carbon black (CB) was used to control the conductivity and the compatibility of immiscible poly(butylene succinate)/polylactide (PBS/PLA) blend. It is shown that most of the CB particles are selectively dispersed in the matrix PBS phase because of the viscosity ratio of the blend components. The increasing viscosity of PBS phase prevents the coalescence of the dispersed PLA domain during the melt mixing. The domain sizes of PLA are refined when compared with that of blank PBS/PLA blend. The ternary composite shows an onset of the electrical conductivity at low filler loadings (1.5 wt %), which is attributed to a percolation of CB in the insulating matrix polymer. Moreover, the composites exhibited remarkable improvement of rheological properties in the melt state when compared with that of blank PBS/PLA blend. According to the van Gurp‐Palmen plot, the rheological percolation threshold for ternary systems is lower than 1.5 wt %. Furthermore, the ternary composites present improved mechanical properties and thermal stability even at very low loading levels of the CB. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effect of interfacial chemistry on the linear rheology and thermal stability of poly(arylene ether nitrile) nanocomposite films filled with various functionalized graphite nanoplates

AbstractPoly(arylene ether nitrile) (PEN) nanocomposites filled with functionalized graphite nanoplates (GNs) were prepared by a simple solution‐ casting method and then characterized by rheometer and thermogravimetric analysis (TGA). This study investigates how the surface treatment of GNs affects the GN dispersion state. The linear rheological test indicated that the 4‐aminophenoxyphthalonitrile‐grafted GN (GN‐CN) presented better dispersion in PEN matrix than purified GN because the corresponding composite showed the lower rheological percolation threshold, which was further confirmed by scanning electron microscopy and solution experiments. The TGA revealed that the presence of 4‐aminophenoxyphthalonitrile‐grafted GN retarded the depolymerization evidently compared with that of purified GN, showing remarkable increase in the temperatures corresponding to a weight loss of 5 wt % (increased by 21°C) and maximum rate of decomposition (increased by 9°C). Both the dispersion state and the surface functionalization of GN are very important to the thermal stability of PEN matrix. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Rheological and electrical percolation thresholds of carbon nanotube/polymer nanocomposites

AbstractPercolation thresholds of multiwalled carbon nanotube/polystyrene (MWCNT/PS) nanocomposites (NC) were determined by rheology and electrical conductivity. The percolation threshold found by electrical conductivity was 0.5% carbon nanotubes (CNT) and that by mechanical spectroscopy and relaxation measurements was 0.9% CNT. These results together with those reported in the literature show several types of percolation thresholds, depending on the average filler–filler (CNT and/or aggregates) distance in a polymer matrix. A distance close to polymer gyration radius corresponds to a “soft” rheological threshold (PC rheosoft). Close contacts between fillers giving rise to a conductive path corresponds to an electrical threshold (PC elec). At high filler concentration, fillers form a network, corresponding to a “rigid” rheological threshold (PC rheorigid). These thresholds depend on the filler content and follow the order:PC elecsoft<PC elec<PC rheorigid. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Morphology and properties of functionalised MWNT/polypropylene composites

Multiwalled carbon nanotubes (MWNTs) were oxidised via a chemical oxidation process and grafted by the reaction of 1-bromohexadecane with the carboxyl groups bound to oxidised MWNT surface. The composites of polypropylene (PP) with modified MWNTs were prepared by solution blending method. The effects of the dispersion state of MWNTs on the rheological, mechanical and thermal conductive properties of PP/MWNT composites were investigated by comparing oxidised MWNTs and grafted MWNTs (g-MWNTs) incorporated composites. After grafting with long alkyl chains, the dispersion of g-MWNTs in the PP matrix was improved, and they could be homogeneously dispersed at nanoscale. At g-MWNT loadings higher than 3·0 wt-%, the PP/g-MWNT composites exhibited rheological percolation threshold at low frequencies. With the increase in MWNT content, the tensile yield strength of the PP/oxidised MWNT composites was decreased, while that of the PP/g-MWNT composites was increased. The thermal conductivity of the PP/MWNT composites increased with increasing MWNT content.

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.

Enhancement of dispersion and bonding of graphene-polymer through wet transfer of functionalized graphene oxide

Dispersion of nanomaterials in polymeric matrices plays an important role in determining the final properties of the composites. Dispersion in nano scale, and especially in single layers, provides best opportunity for bonding. In this study, we propose that by proper functionalization and mixing strategy of graphene its dispersion, and bonding to the polymeric matrix can be improved. We then apply this strategy to graphene-epoxy system by amino functionalization of graphene oxide (GO). The process included two phase extraction, and resulted in better dispersion and higher loading of graphene in epoxy matrix. Rheological evaluation of different graphene-epoxy dispersions showed a rheological percolation threshold of 0.2 vol% which is an indication of highly dispersed nanosheets. Observation of the samples by optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM), showed dispersion homogeneity of the sheets at micro and nano scales. Study of graphene-epoxy composites showed good bonding between graphene and epoxy. Mechanical properties of the samples were consistent with theoretical predictions for ideal composites indicating molecular level dispersion and good bonding between nanosheets and epoxy matrix.