Rheological Percolation Research Articles

Phase transition and anomalous rheological behaviour of polylactide/graphene nanocomposites

Polylactide (PLA) nanocomposites with graphene nanoplatelet (GNP) contents of 0–15 wt% were prepared and characterised in terms of rheology and electrical conductivity. As expected for rigid nanofillers, GNP incorporation significantly enhanced the viscoelastic properties of the nanocomposites. Furthermore, above a certain GNP concentration the liquid-like melt flow behaviour changed into a solid-like behaviour. However, this rheological percolation threshold was found to decrease from near 9 wt% at 180 °C to below 6 wt% at 220 °C, detected and measured by Van Gurp-Palmen plots and Winter-Chambon method, respectively. It was also found that the viscoelastic properties of some of the nanocomposites increased with increasing temperature, which is in contrast to the rheology of ideal polymeric melts. The Electrical percolation threshold of the PLA/GNP system, which is determined by a sudden increase in DC conductivity and is indicative of establishment of interconnected conductive structures of GNPs within the matrix, was observed to be between 6 and 9 wt% GNPs. Morphological studies also showed physical contact between the platelets at 9 wt% while at 6 wt% they were still separated by the PLA matrix. These observations challenge the attribution of rheological percolation threshold to the formation of a percolating network of only filler particles within the matrix. A percolating network consisting of both polymer chains and filler particles might be the reason for the temperature dependency of rheological percolation threshold in the PLA/GNP system.

Matrix tacticity controlled tuning of microstructure, constitutive behavior and rheological percolation effect of melt‐mixed amino‐functionalized MWCNT/PP nanocomposites

Amino‐functionalized multi walled nanotube (MWCNT‐NH2) filled isotactic PP and isotactic‐syndiotactic (70:30) mixed PP based melt‐mixed nanocomposites have been comparatively evaluated with regard to morphological, rheological and thermo‐mechanical properties. The ratio of mean free space lengths (Lf) to infiltrated mean free space lengths (Linf) between nanotubes in isotactic‐syndiotactic (70:30) blended matrix based nanocomposites increased relatively indicating a dispersed‐morphology. The rheological percolation threshold increased up to a higher extent of MWCNT‐NH2 loading (from øc ∼ 2.3 × 10−4 in isotactic to øc ∼ 11 × 10−4 in iso‐syndio blend) accompanied with the formation of a mechanically responsive network structure. van‐GurpPalmen plot showed a transition in the rheological response as a consequence of network morphology getting shifted to higher concentration of MWCNT‐NH2 in the isotactic‐syndiotactic mixed PP based nanocomposites than in the isotactic based one. Constitutive modeling of complex viscosity response of the nanocomposites led to functional correlation between the percolation and relaxation dynamics of polymer chains. POLYM. ENG. SCI., 58:1115–1126, 2018. © 2017 Society of Plastics Engineers

Optimal nanomaterial concentration: harnessing percolation theory to enhance polymer nanocomposite performance

A major challenge in nanocomposite research is to predict the optimal nanomaterial concentration (ONC) yielding a maximal reinforcement in a given property. We present a simple approach to identify the ONC based on our finding that it is typically located in close proximity to an abrupt increase in polymer matrix viscosity, termed the rheological percolation threshold, and thus may be used as an indicator of the ONC. This premise was validated by rheological and fractography studies of composites loaded by nanomaterials including graphene nanoribbons or carbon or tungsten disulfide nanotubes. The correlation between in situ viscosity, the rheological percolation threshold concentration and the nanocomposite fractography demonstrates the utility of the method.

Thermoplastic elastomer nanocomposites with controlled nanoparticles dispersion for HV insulation systems: Correlation between rheological, thermal, electrical and dielectric properties

Thermoplastic elastomer nanocomposites based on respectively polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) and polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene grafted maleic anhydride (SEBS-MA) block copolymers and containing functionalized zinc oxide (ZnO) nanoparticles have been investigated as candidate materials for high voltage (HV) insulation systems. The dispersion of the organically modified ZnO nanoparticles has been successfully tuned through the MA graft and the block copolymer nanostructure. In particular, nanocomposites with signs of rheological percolation, indicating the formation of a network between individually dispersed nanoparticles and polymer chains, have been obtained at ZnO content as low as 5wt% (0.9 vol%). This behavior resulted in an enhancement of the thermal conductivity and better control of the electrical conductivity while maintaining breakdown strength and dielectric losses in the same range of the unfilled insulating matrices. Furthermore, the resistance to surface erosion by partial discharges was significantly improved: in the presence of 5wt% of individually dispersed ZnO nanoparticles, the eroded volume was reduced 10 times.

Morphology and properties of electrically and rheologically percolated PLA/PCL/CNT nanocomposites

ABSTRACTPoly(lactic acid)/poly(ɛ‐caprolactone)/carbon nanotube (PLA/PCL/CNT) nanocomposites (NCs) were melt‐processed in a conventional industrial‐like twin‐screw extruder maintaining a constant PLA/PCL 80/20 wt. ratio. CNTs located in the thermodynamically favored PCL phase and, as a result, the “sea–island” morphology of the unfilled blend was replaced by a more continuous PCL dispersed phase in the ternary NCs. Rheological and electrical percolation took place at the same CNT contents (over 1.2 wt %) that TEM images suggest continuity of the PCL phase. The electrical and the low‐strain mechanical behaviors upon CNT addition were similar in the reference binary PLA/CNT and ternary PLA/PCL/CNT NCs. In the percolated NCs, the conductivity became 106–107 times higher than in the insulating compositions, while the Young modulus increased linearly upon the addition of CNT (12% increase at 4.9 wt % loading). Moreover, all the PLA/PCL/CNT NCs showed a ductile behavior (elongation at break >130%) similar to that of the unfilled PLA/PCL blend (140%), in contrast to the brittle behavior of binary PLA/CNT NCs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45265.

Effect of Nanofiller Geometry on Network Formation in Polymeric Nanocomposites: Comparison of Rheological and Electrical Properties of Multiwalled Carbon Nanotube and Graphene Nanoribbon

The present work reveals results on the network formation of graphene nanoribbon (GNR) synthesized using a nonoxidative technique from multiwalled carbon nanotube (MWCNT). MWCNT and GNR presented comparable powder conductivity and dispersion state in a poly(vinylidene fluoride) (PVDF) matrix. This enabled us to purely discern the effect of geometrical features of the nanofillers on network formation by comparing the rheological and electrical percolation in the PVDF matrix. Unique features of the rheological response of PVDF/GNR nanocomposites, such as abrupt transition to a solid state, were interpreted according to a network structure activated by the formation of primary and secondary entanglements between adsorbed and bulk polymer chains. However, MWCNT nanocomposites presented a rheological behavior consistent with a strongly flocculated network structure formed through the direct tube–tube contacts and mechanical entanglements. Poor broadband electrical conductivity of the GNR nanocomposites compare...

Achieving Large Dielectric Property Improvement in Poly(ethylene vinyl acetate)/Thermoplastic Polyurethane/Multiwall Carbon Nanotube Nanocomposites by Tailoring Phase Morphology

In this work, multiwall carbon nanotubes (MWCNTs) were melt compounded with a poly(ethylene vinyl acetate)/thermoplastic polyurethane (EVA/TPU) blend to prepare EVA/TPU/MWCNT blend nanocomposites. The effects of the blend morphology and selective localization of MWCNTs on the dielectric properties including dielectric constant and loss were systematically investigated. The results show that when the blend exhibits sea–island morphology accompanying MWCNT selective localization in island droplets, the dielectric constant was increased to 1200@100 Hz with addition of 5 wt % MWCNTs; in addition, the growth of dielectric loss is suppressed and the values of dielectric loss always remain around 1 with varying the loading of MWCNTs. Furthermore, based on the investigations of rheological percolation testing, morphological observation, and selective localization, the microcapacitor model was evoked to explain the underlying mechanism for the improvement of the dielectric constant for the blend with sea–island mo...

Rheological and Thermo-Mechanical Properties of Poly(lactic acid)/Lignin-Coated Cellulose Nanocrystal Composites

Improving the processability and physical properties of sustainable biobased polymers using biobased fillers is essential to preserve its biodegradability and make them suitable for different end user applications. Herein, we report the use of spray-dried lignin-coated cellulose nanocrystals (L-CNCs), a biobased filler, to modify the rheological and thermo-mechanical properties of poly(lactic acid) (PLA) composites. The lignin coating on CNCs not only improved the dispersion of CNCs but also enhanced their interfacial interaction with the PLA matrix, resulting in a significant improvement in rheological and thermo-mechanical properties. The rheological percolation threshold concentration obtained by power law analysis for PLA/L-CNC composites was found to be 0.66 wt %, which is significantly lower than the reported values for other PLA/CNC composites. Such a low rheological percolation concentration of L-CNCs can be attributed to excellent dispersion of L-CNCs in the PLA matrix. Addition of only 0.5 wt % ...

Sustainable, electrically-conductive bioepoxy nanocomposites

Biobased epoxy nano-composites composed of untreated multi-wall-carbon-nanotubes (MWCNT) and diphenolic acid-derived biobased epoxy: diglycidyl ether of diphenolate n-butyl ester (DGEDP-Bu), were fabricated. Electrical, rheological, and mechanical percolation thresholds were compared between biobased and commercial bisphenol A (DGEBA) epoxy composites. For both epoxies, nanocomposites loaded with 0.05–0.2 wt % MWCNT's exhibited electrical and rheological percolation at 0.05 wt % and 0.2 wt % respectively. DMA and tensile results revealed that DGEDP-Bu composites exhibited equivalent or superior properties to DGEBA composites. With 0.2 wt % MWCNT's, DGEDP-Bu nanocomposites exhibited 68% higher electrical conductivity and a three-fold higher rheological yield stress than those made from DGEBA. Rheological characterization corroborated that continuous MWCNT networks are formed within epoxies between 0.1 and 0.2 wt % MWCNT's. Moreover, upon MWCNT loading, DGEDP-Bu demonstrates equal mechanical performance and better electrical conductivity than DGEBA.

Ultrasonically assisted compounding of CNT with polypropylenes of different molecular weights

Carbon nanotubes (CNTs) were compounded with four polypropylenes (PPs) of different molecular weights and viscosities without and with ultrasonically assisted twin-screw extrusion. The dispersion of CNT and the effect of viscosity on ultrasonically induced polymer degradation at different amplitudes were studied. Three preparation methods, differing in the sequence of ultrasonic treatment application, were applied to prepare PP/CNT composites and investigate the bubble cavitation mechanism inside and outside of CNT agglomerates in composites. In Method 1, the ultrasonic treatment was applied in the masterbatch dilution step, while in Method 2 it was applied in the masterbatch preparation step. In Method 3, direct ultrasonic compounding method was applied. Ultrasonic power consumption, small amplitude oscillatory shear (SAOS), creep and recovery, electrical conductivity were measured and morphological analysis was carried out to evaluate the dispersion of CNT in PP. The ultrasonic treatment was found to have the most prominent effect on improving the dispersion of CNT using Method 2. The ultrasonic treatment using Method 3 was more efficient than that using Method 1. Moreover, it was found that ultrasonic treatment had more notable effect on improving the dispersion of CNT in the higher viscosity PP matrix. The rheological and electrical percolation threshold was significantly reduced with ultrasonic treatment in higher viscosity PP/CNT composites. The bubble cavitation mechanism inside of CNT agglomerates was found to be dominant in breakage of the agglomerates during the ultrasonic treatment.

Exfoliated graphite nanoplatelets/poly(arylene ether nitrile) nanocomposites

In this work, we demonstrate a method for synthesis of exfoliated graphite nanoplatelets (xGnPs)/poly(arylene ether nitrile) (PEN) nanocomposites via an efficient in situ polymerization. The GnPs were treated by the ultrasonic bath to reduce the layers of the GnPs, where the PEN were intercalated subsequently. Therefore, the dispersion of xGnP in the PEN resin was enhanced through in situ polymerization, which was characterized and confirmed by scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. It was found that the tensile strength and modulus were greatly enhanced with the addition of xGnP. For 2.5 wt% of xGnP-reinforced PEN, the tensile strength and modulus were increased to 115 MPa and 3121 MPa, respectively. Owing to the well dispersion of xGnP, the low rheological percolation of 2.5 wt% for PEN nanocomposites was obtained. Besides, with 1 wt% of xGnP, the corresponding initial decomposition temperature ( Tin) increased from 451°C in pure PEN to 470°C. The addition of xGnP showed enhanced thermal stability of PEN nanocomposites, which demonstrated a promising method for preparing advanced polymer-based nanocomposites.

Improving the properties of poly(arylene ether nitrile) composites reinforced by covalently modified multi-walled carbon nanotubes

To develop high-performance carbon nanotube (CNT)-based polymer nanocomposites, both the interface control and the dispersion of CNTs within the polymer hosts need to be considered. In this study, we show an effective way to modify the surface of multi-walled CNTs (MWCNTs) by applying a cyclization reaction between nitrile-modified MWCNTs and bis-phthalonitrile. Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to examine the structure and morphology of as-prepared functional CNTs. Phthalocyanines (Pcs) were found to be evenly coated on the surface of MWCNTs, resulting in good dispersion and strong interfacial adhesion between MWNCTs and the poly(arylene ether nitrile) (PEN) matrix. Compared with neat PEN, the tensile strength and tensile modulus of PEN nanocomposites with 2 wt% MWCNTs–Pc increased from 85.6 MPa to 108 MPa and from 2300 MPa to 3350 MPa, respectively. Furthermore, surface-functionalized CNTs can also form the physical MWCNT network within the PEN matrix, as confirmed by rheological and thermal stability tests. Additionally, a low rheological percolation threshold of 0.69 wt% was obtained, and the dielectric constant of PEN nanocomposites was increased from 3.3 for neat PEN to 16.6 with 5 wt% MWCNTs–Pc.

Dual-role of phosphonium – Based ionic liquid in epoxy/MWCNT systems: Electric, rheological behavior and electromagnetic interference shielding effectiveness

Epoxy networks with improved electrical properties were prepared by using the non covalently functionalized multi-walled carbon nanotube (MWCNT) combined with phosphonium ionic liquid (IL). This IL exerts the function of dispersant aid for the MWCNT as well as curing agent of epoxy prepolymer. The influence of the mixing procedure on the dispersion of the MWCNT was evaluated by rheological behavior of the uncured mixtures and by transmission electron microscopy (TEM) of the cured epoxy systems. In fact, the dispersion of MWCNT within the IL is a key parameter to provide better dispersion, higher conductivity and storage modulus. Thus, the effect of the MWCNT content on the electrical and rheological percolation has been discussed. Moreover, the electromagnetic interference shielding effectiveness (EMI SE) of some systems have been also investigated.

A promising approach to low electrical percolation threshold in PMMA nanocomposites by using MWCNT-PEO predispersions

Electrical conductive poly(methyl methacrylate) (PMMA) nanocomposites with low percolation threshold are very challenging to be prepared. Here, we show that the miscibility between poly(ethylene oxide) (PEO) as matrix for predispersions of multi-walled carbon nanotubes (MWCNTs) and PMMA represents an efficient approach to achieve very low electrical percolation threshold. PMMA/PEO-MWCNTs nanocomposites were prepared by a two-step solution casting method involving pre-mixing of MWCNTs with PEO and then mixing of PEO-MWCNTs with PMMA, resulting in a PMMA/PEO ratio of 80/20wt%. The electrical percolation threshold (EPT) value was determined to be ~0.07wt% which is significantly lower than most of the reported EPT values in the literature for PMMA/CNT composites. The very low electrical percolation threshold was attributed to the effectual role of PEO in self-assembly of secondary structures of nanotubes into an electrically conductive network. This was further confirmed by transmission electron microscopy and by comparing the obtained EPT value with the prediction of the excluded volume model in which statistical percolation threshold is defined based on uniform distribution of high-aspect ratio sticks in a matrix. Moreover, based on UV–Vis measurements and linear viscoelastic rheological measurements, optical and rheological percolation thresholds were obtained at nearly 0.01wt% and 0.5wt%, respectively.

Multi walled carbon nanotubes induced viscoelastic response of polypropylene copolymer nanocomposites: Effect of filler loading on rheological percolation

Polypropylene random copolymer nanocomposites having 0.2–7.0 vol% multi-walled carbon nanotubes (MWCNTs) were prepared via melt processing. Transmission electron microscopy (TEM) was employed to determine the nano scale dispersion of carbon nanotubes. Linear viscoelastic behavior of these nanocomposites was investigated using parallel plate rheometry. Incorporation of carbon nanotubes in the polymer matrix resulted in higher complex viscosity (η*), storage (G′) and loss modulus (G″) as compared to neat polymer, especially in the low-frequency region, suggesting a change from liquid to solid-like behavior in the nanocomposites. By plotting storage modulus vs. carbon nanotube loading and fitting with a power law function, the rheological percolation threshold in these nanocomposites was observed at a loading of ∼0.27 vol% of MWCNTs. However, electrical percolation threshold was reported at ∼0.19 vol% of MWCNTs loading. The difference in the percolation thresholds is understood in terms of nanotube connectivity with nanotubes and polymer chain required for electrical conductivity and rheological percolation.

Electrical and rheological percolation behavior of multiwalled carbon nanotube-reinforced poly(phenylene sulfide) composites

Polyphenylene sulfide-based nanocomposites filled with unmodified multiwalled carbon nanotubes from 0.5 wt% to 8.0 wt% have been prepared by melt mixing technique with a single-screw extruder and hot press. Transmission electronic microscopy and scanning electron microscopy analysis were carried out in order to assess the multiwalled carbon nanotubes dispersion throughout the polyphenylene sulfide matrix. Electrical conductivity of the polymer was dramatically enhanced by about 11 decades between 2.0 wt% and 3.0 wt% of nanotubes, suggesting the formation of three-dimensional conductive network within the polymeric matrix. The storage modulus (G′) of neat polyphenylene sulfide presented an increase by two orders of magnitude when 2.0 wt% of pristine multiwalled carbon nanotubes was considered, with the formation of an interconnected nanotube structure, indicative of “pseudo-solid-like” behavior. In addition, percolation networks were formed when the loading levels achieve up to 1.5 wt% for multiwalled carbon nanotubes/polyphenylene sulfide composites.

Synergistic effect of multiwalled carbon nanotubes and carbon black on rheological behaviors and electrical conductivity of hybrid polypropylene nanocomposites

The monograph, electrical conductivity, and rheological behaviors of isotactic polypropylene /carbon black (CB)/multiwalled carbon nanotubes (MWCNTs) composites were studied. Ternary composites including MWCNTs and CB revealed an electrical property similar to the binary MWCNTs composites. This work showed that a synergistic effect result in preserved electrical behavior with a simultaneous effective reduced of the MWCNTs amount. Moreover, the combination of conductive fillers also resulted in a decreased rheological percolation threshold, an accelerated formation of rheological network and a delay in the destruction of the filler network. The similar value of the activation energy for CB/MWCNTs and MWCNTs verified the existence of a co‐supporting networked formed by MWCNTs and CB. Two strain‐softening processes are found in a strain sweep for the ternary composite melt with high filler loading. The low‐strain‐softening process may be an indication that the MWCNTs filler network is broken into separated aggregated structures, while the high‐strain‐softening process should be related to the breakdown of the temporary network structure. POLYM. COMPOS., 39:E723–E732, 2018. © 2016 Society of Plastics Engineers

Is electrical percolation in carbon-filled polymers reflected by rheological properties?

Abstract It is well known that the electrical conductivity of particle filled polymers exhibits a distinct percolation behavior. In the literature it is stated that this percolation is accompanied by a comparable change of rheological properties designated “rheological percolation”. Evaluating dynamic-mechanical measurements from the literature and using own results from creep experiments on polymethylmethacrylate melts filled with graphite it is demonstrated that rheological properties change continuously as a function of particle concentration in the range of the threshold of electrical conductivity. A model is proposed and discussed, which describes the electrical and rheological properties. For rendering a polymer conductive, continuous pathways of conducting fillers are necessary. The rheological behavior in the percolation regime is determined by matrix molecules that become hindered in their mobility by an attachment to the particles.

Ultrasonic twin screw compounding of polypropylene with carbon nanotubes, graphene nanoplates and carbon black

The effect of ultrasonic treatment on dispersion of carbon nanotubes (CNT), graphene nanoplates (GNP) and carbon black (CB) in polypropylene (PP) using twin screw extrusion (TSE) was investigated. The ultrasonic power consumption as a function of filler concentration was found to increase for PP/GNP, decrease for PP/CNT and slightly increase for PP/CB. The difference in behavior of the ultrasonic power consumption was related to the void (bubble) concentration in the polymer composites. The efficiency of the ultrasonic treatment was verified through measurements of rheological, electrical and mechanical properties as well as by morphological studies. Ranking of ultrasonic dispersion in composites from best to worse was PP/CB, PP/CNT and PP/GNP. Ranking of the rheological and electrical percolation threshold of composites from low to high was PP/CNT, PP/CB and PP/GNP. The storage modulus and complex viscosity of PP/CNT and PP/CB composites in the low frequency region increased with the ultrasonic treatment while little changes occurred in the PP/GNP composites. In the percolation region, the electrical resistivity of ultrasonically treated composites dropped by 8, 4 and 2 orders of magnitude for PP/CNT, PP/CB and PP/GNP, respectively, compared with the untreated ones. The Young’s modulus, yield stress and elongation at break of the PP/CNT and PP/GNP composites were not affected by ultrasonic treatment, but the elongation at break of the treated PP/CB composite at a concentration of 10wt% increased from 230% to 310%. Morphological studies showed that agglomerates size and agglomerates area ratio of PP/CNT and PP/CB composites reduced with ultrasonic treatment, but not for PP/GNP composites. The incorporation of various fillers increased the crystallization temperature, melt temperature and lamella thickness and decreased the spherulites size. The fillers acted as γ form crystal nucleation agents with the ranking of nucleation ability of CNT>GNP>CB.

Effects of TiO2, ZnO, and Fe3O4 nanofillers on rheological behavior, microstructure, and reaction kinetics of rigid polyurethane foams

ABSTRACTEffects of different types and shapes of titanium dioxide, zinc oxide, and magnetite nanofillers on the rheological behavior of polyol/nanofiller suspensions, on the rigid polyurethane foam formation reaction, and hence on the final microstructure were investigated. The rheological percolation threshold of polyol/nanofiller suspensions decreased as the aspect ratio of nonspherical nanoparticles (platelet or rod) increased, regardless of the nanofiller type. The results of reaction kinetics showed that above a critical surface area (≈30 m2), independently of nanofiller type, the reaction rate increased as the surface area increased. The introduction of oxide surfaces reduced the final cell size until a critical surface area (≈30 m2). However, above this critical value cell size distribution gets wider and the cell size can no longer be correlated with the surface area. In the latter case, an increase of the reaction rate and the polymerization reaction being exothermic may facilitate uncontrolled cell nucleation, growth, and hence coalescence which results in an uncontrolled foam structure. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43658.