Rheology of polymer carbon nanotubes composites

High-Performance PEEK/MWCNT Nanocomposites: Combining Enhanced Electrical Conductivity and Nanotube Dispersion

High-performance engineering thermoplastics offer lightweight and excellent mechanical performance in a wide temperature range. Their composites with carbon nanotubes are expected to enhance mechanical performance, while providing thermal and electrical conductivity. These are interesting attributes that may endow additional functionalities to the nanocomposites. The present work investigates the optimal conditions to prepare polyether ether ketone (PEEK)/multi-walled carbon nanotube (MWCNT) nanocomposites, minimizing the MWCNT agglomerate size while maximizing the nanocomposite electrical conductivity. The aim is to achieve PEEK/MWCNT nanocomposites that are suitable for melt-spinning of electrically conductive multifilament's. Nanocomposites were prepared with compositions ranging from 0.5 to 7 wt.% MWCNT, showing an electrical percolation threshold between 1 and 2 wt.% MWCNT (107-102 S/cm) and a rheological percolation in the same range (1 to 2 wt.% MWCNT), confirming the formation of an MWCNT network in the nanocomposite. Considering the large drop in electrical conductivity typically observed during melt-spinning and the drawing of filaments, the composition PEEK/5 wt.% MWCNT was selected for further investigation. The effect of the melt extrusion parameters, namely screw speed, temperature, and throughput, was studied by evaluating the morphology of MWCNT agglomerates, the nanocomposite rheology, and electrical properties. It was observed that the combination of the higher values of screw speed and temperature profile leads to the smaller number of MWCNT agglomerates with smaller size, albeit at a slightly lower electrical conductivity. Generally, all processing conditions tested yielded nanocomposites with electrical conductivity in the range of 0.50-0.85 S/cm. The nanocomposite processed at higher temperature and screw speed presented the lowest value of elastic modulus, perhaps owing to higher matrix degradation and lower connectivity between the agglomerates. From all the process parameters studied, the screw speed was identified to have the higher impact on nanocomposite properties.

Electrically Conductive Adhesive Based on Thermoplastic Hot Melt Copolyamide and Multi-Walled Carbon Nanotubes

For the bonding of the lightweight composite parts, it is desired to apply electrically conductive adhesive to maintain the ability to shield electromagnetic interference. Among various solvent-based adhesives, there is a new group of thermoplastic hot melt adhesives that are easy to use, solidify quickly, and are environment-friendly. To make them electrically conductive, a copolyamide-based hot melt adhesive was mixed with 5 and 10 wt% of carbon nanotubes using a melt-blending process. Well-dispersed nanotubes, observed by a high-resolution scanning microscope, led to the formation of a percolated network at both concentrations. It resulted in the electrical conductivity of 3.38 S/m achieved for 10 wt% with a bonding strength of 4.8 MPa examined by a lap shear test. Compared to neat copolyamide, Young’s modulus increased up to 0.6 GPa and tensile strength up to 30.4 MPa. The carbon nanotubes improved the thermal stability of 20 °C and shifted the glass transition of 10 °C to a higher value. The very low viscosity of the neat adhesive increased about 5–6 orders of magnitude at both concentrations, even at elevated temperatures. With a simultaneous growth in storage and loss modulus this indicates the strong interactions between polymer and carbon nanotubes.

Nanoparticles can modulate network topological defects during multimodal elastomer formation

We experimentally show that nanoparticles (NPs) can significantly regulate the network topological defects during a molecularly controlled elastomeric synthesis. Using positron annihilation lifetime spectroscopy, we demonstrate this on well-defined model systems of poly(dimethyl siloxane) elastomers and layered silicate nanoparticles (NPs). The evolutions of topological defects in elastomeric networks prepared from unimodal, bimodal, and NP dispersed bimodal elastomers are sequentially investigated. The extent of NP induced defect regulation is identified by varying the particle concentration from moderately low to an approximate upper limit. The fraction of free volume hole defects present between packed chains in the network generated by molecular control is significantly reduced. The fraction of smaller interstitial cavities near the cross-link sites shows a moderate increase at the lowest NP concentration. However, this fraction decreases at a high NP concentration and is nearly the same as that of bimodal networks that are devoid of NP infusion. Despite the variations in their fractions with NP infusion, the sizes of both these types of defects that remain in the network are minimally affected. The collective topological defects arising from chain induced heterogeneity also show a qualitative reduction upon NP infusion.

Constitutive Modeling of Nonlinear Rheological Behavior of Carbon Nanotube-Filled Polypropylene Nanocomposites

The rheological behavior of multi-walled carbon nanotube (MWCNT)-filled polypropylene (PP) nanocomposites with different filler loadings was experimentally studied and simulated using constitutive modeling. Rheological behavior was characterized in small amplitude oscillatory shear (SAOS) flow, large amplitude oscillatory shear (LAOS) flow, startup of shear flow, steady shear flow, and stress relaxation after the imposition of a step shear strain. Virgin PP and PP with CNT loadings of 1, 3, and 5 wt% were used. The formation of a rheological percolation network was observed at these loadings. The Leonov and Simhambhatla-Leonov models were used to simulate the rheological behavior. In the linear region, the simulations provided good predictions of the experimental data for both the unfilled and filled PP. In the nonlinear region, the simulations also provided good results for the virgin PP and satisfactory results for the PP/1 wt%CNT nanocomposite under most flow conditions. However, for the other two nanocomposites the model showed mixed results.

Structure-Property Relationships in Bionanocomposites for Pipe Extrusion Applications

In this work, bionanocomposites based on different biodegradable polymers and two types of nanofillers, namely a nanosized calcium carbonate and an organomodified nanoclay, were produced through melt extrusion, with the aim to evaluate the possible applications of these materials as a potential alternative to traditional fossil fuel-derived polyolefins, for the production of irrigation pipes. The rheological behavior of the formulated systems was thoroughly evaluated by exploiting different flow regimes, and the obtained results indicated a remarkable effect of the introduced nanofillers on the low-frequency rheological response, especially in nanoclay-based bionanocomposites. Conversely, the shear viscosity at a high shear rate was almost unaffected by the presence of both types of nanofillers, as well as the rheological response under nonisothermal elongational flow. In addition, the analysis of the mechanical properties of the formulated materials indicated that the embedded nanofillers increased the elastic modulus when compared to the unfilled counterparts, notwithstanding a slight decrease of the material ductility. Finally, the processing behavior of unfilled biopolymers and bionanocomposites was evaluated, allowing for selecting the most suitable material and thus fulfilling the processability requirements for pipe extrusion applications.

Nanoparticle assembly modulated by polymer chain conformation in composite materials

Mixing nanoparticles into a strategically selected polymer matrix yields nanocomposites with well-controlled microstructures and unique properties and functions. The modulation of nanoparticle assembly by polymer chain conformation can play a dominant role in determining nanocomposite structures, yet such a physical mechanism remains largely unexplored. We hypothesize that highly ordered microdomains of rigid linear polymers provide a template for nanoparticle assembly into open fractal structures. We conducted mesoscopic computer simulations and physical experiments to elucidate how polymer chain conformation regulates the dynamic evolution of nanoparticle structures during the drying processing of polymer nanocomposite films. The evaporation of polymer-nanoparticle mixtures with varying chain stiffnesses was simulated using dissipative particle dynamics. The formation of distinguished nanoparticle assemblies as a result of matrix selection was further corroborated by probing nanoparticle aggregation in different polymer nanocomposite coatings. The results show that polymer conformation not only influences the dispersion states of individual particles (dispersed vs. aggregated), but also modulates the morphologies of large-scale assembly (globular vs. fractal). The emergence of nematically ordered polymer clusters when the chain rigidity is increased creates local solvent-rich "voids" that promote anisotropic particle aggregates, which then percolate into open fractal structures upon solvent evaporation. The nanoparticle dynamics also exhibits an intriguing non-monotonic behavior attributed to the transitions between the coupling and decoupling with polymer dynamics. The nanoparticle assembly morphologies obtained in simulations match well with the electron microscopy images taken in physical experiments.

Rheological Behavior of Polymer/Carbon Nanotube Composites: An Overview

This paper reviews the current achievements regarding the rheological behavior of polymer-based nanocomposites containing carbon nanotubes (CNTs). These systems have been the subject of a very large number of scientific investigations in the last decades, due to the outstanding characteristics of CNTs that have allowed the formulation of nanostructured polymer-based materials with superior properties. However, the exploitation of the theoretical nanocomposite properties is strictly dependent on the complete dispersion of CNTs within the host matrix and on the consequent development of a huge interfacial region. In this context, a deep knowledge of the rheological behavior of CNT-containing systems is of fundamental importance, since the evaluation of the material's viscoelastic properties allows the gaining of fundamental information as far as the microstructure of nanofilled polymers is concerned. More specifically, the understanding of the rheological response of polymer/CNT nanocomposites reveals important details about the characteristics of the interface and the extent of interaction between the two components, hence allowing the optimization of the final properties in the resulting nanocomposites. As the literature contains plenty of reviews concerning the rheological behavior of polymer/CNT nanocomposites, this review paper will summarize the most significant thermoplastic matrices in terms of availability and relevant industrial applications.

Improved Properties of Metallocene Polyethylene/Poly(ethylene terephthalate) Blends Processed by an Innovative Eccentric Rotor Extruder

* Correspondence: pmhzhe@scut [...].

Structure Evolution of Epoxidized Natural Rubber (ENR) in the Melt State by Time-Resolved Mechanical Spectroscopy

In this work, time-resolved mechanical spectroscopy (TRMS) was used to accurately characterize the rheological behavior of an epoxidized natural rubber (ENR) containing 25 mol% of epoxy groups. Conventional rheological tests are not suitable to characterize with accuracy the frequency-dependent linear viscoelastic behavior of materials, such as ENR, in a transient configurational state. For this reason, TRMS was used to determine the true rheological behavior of ENR, as well as to gain some insights into the changes of its macromolecular architecture under the dynamic conditions experienced during the measurements. The constructed master curves for the moduli revealed a gradual transition of the ENR rheological state from liquid-like to solid-like through the formation of an “elastic gel” throughout the bulk of the polymer. Furthermore, the evolution of the stress relaxation modulus revealed a slow relaxation mechanism, resulting from thermally activated reactions in the molten state attributed to the formation of crosslinks. Finally, the crosslink density evolution was estimated from the TRMS data and compared with results derived from equilibrium solvent-swelling measurements. These demonstrated the accuracy of the TRMS data in the prediction of the structural changes that can take place in polymers during processing.

Relaxation Dynamics in Polyethylene Glycol/Modified Hydrotalcite Nanocomposites

Polyethylene glycol-based nanocomposites containing an organo-modified hydrotalcite with loadings ranging from 0.5 to 5 wt.% were prepared by melt mixing performed just above the melting point of the polymer matrix. In these conditions, the dispersion of the nanofiller within the polymer matrix was quite homogeneous as revealed by TEM analyses. The effect of various thermal treatments and filler loadings was thoroughly investigated by means of rheological, morphological and gas chromatography-mass spectrometry, hyphenated to thermogravimetry analysis tests. Unfilled polyethylene glycol exhibited a continuous decrease in complex viscosity upon heating. In contrast, the complex viscosity of nanocomposites containing nanofiller loadings higher than 1 wt.% showed first a decrease, followed by an increase in the complex viscosity as the temperature increases, exhibiting a minimum between 130 and 140 °C. Annealing at 180 °C for different times further increased the viscosity of the system. This unusual behavior was explained by the occurrence of grafting reactions between the ⁻OH terminal groups of the polyethylene glycol chains and the hydroxyl groups of the organo-modified filler, thus remarkably affecting the relaxation dynamics of the system.

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

This topical review discusses the theoretical progress made in the field of polymer nanocomposites, i.e., hybrid materials created by mixing (typically inorganic) nanoparticles (NPs) with organic polymers. It primarily focuses on the outstanding issues in this field and is structured around five separate topics: (i) the synthesis of functionalized nanoparticles; (ii) their phase behavior when mixed with a homopolymer matrix and their assembly into well-defined superstructures; (iii) the role of processing on the structures realized by these hybrid materials and the role of the mobilities of the different constituents; (iv) the role of external fields (electric, magnetic) in the active assembly of the NPs; and (v) the engineering properties that result and the factors that control them. While the most is known about topic (ii), we believe that significant progress needs to be made in the other four topics before the practical promise offered by these materials can be realized. This review delineates the most pressing issues on these topics and poses specific questions that we believe need to be addressed in the immediate future.

Sonication-Induced Modification of Carbon Nanotubes: Effect on the Rheological and Thermo-Oxidative Behaviour of Polymer-Based Nanocomposites

The aim of this work is the investigation of the effect of ultrasound treatment on the structural characteristics of carbon nanotubes (CNTs) and the consequent influence that the shortening induced by sonication exerts on the morphology, rheological behaviour and thermo-oxidative resistance of ultra-high molecular weight polyethylene (UHMWPE)-based nanocomposites. First, CNTs have been subjected to sonication for different time intervals and the performed spectroscopic and morphological analyses reveal that a dramatic decrease of the CNT’s original length occurs with increased sonication time. The reduction of the initial length of CNTs strongly affects the nanocomposite rheological behaviour, which progressively changes from solid-like to liquid-like as the CNT sonication time increases. The study of the thermo-oxidative behaviour of the investigated nanocomposites reveals that the CNT sonication has a detrimental effect on the thermo-oxidative stability of nanocomposites, especially for long exposure times. The worsening of the thermo-oxidative resistance of sonicated CNT-containing nanocomposites could be attributed to the lower thermal conductivity of low-aspect-ratio CNTs, which causes the increase of the local temperature at the polymer/nanofillers interphase, with the consequent acceleration of the degradative phenomena.

Tuning the Viscoelastic-Gel Transition of Single-Wall Carbon Nanotubes Embedded in pH-Responsive Polyelectrolyte Solutions

We present the detailed rheological changes that occur when small quantities of single-wall carbon nanotubes are dispersed in a poly(acrylic acid) water solution, around the overlap polymer concentration, up to the gel point. Here, pH is used to tune the gel formation. Suspensions of nanotubes at pH ≤ 5 are exfoliated and dispersed by the polymer. Contacts between the nanotubes are mainly through polymer entangling, and the suspension is viscoelastic. At pH > 5, the polymer is charged, and the solution is not a good solvent for the nanotubes anymore. Nanotube bundles covered with polymer are formed and mechanically percolate along the fluid until they become arrested. As a consequence, the rheological behavior is dominated by a mesoscale superstructure formed by nanotubes and polymer, where viscoelasticity is lost and the suspension becomes elastic. At pH ≥ 9, the surroundings for the nanotubes are worse, bundles and flocs grow to a larger extent, and they can be observable by scanning microscopies. When the suspension becomes a critical gel, the relaxation moduli can be modeled by a power law in the frequency domain in agreement with the model developed by Winter and co-workers.

Cardiac tissue regeneration: A preliminary study on carbon-based nanotubes gelatin scaffold

The aim of this study was set-up and test of gelatin and carbon nanotubes scaffolds. Gelatin-based (5%) genipin cross-linked (0.2%) scaffolds embedding single-walled carbon nanotubes (SWCNTs, 0.3, 0.5, 0.7, 0.9, and 1.3% w/w) were prepared and mechanically/electrically characterized. For biological evaluation, H9c2 cell line was cultured for 10 days. Cytotoxicity, cell growth and differentiation, immunohistochemistry, and real-time PCR analysis were performed. Myoblast and cardiac differentiation were obtained by serum reduction to 1% (C_1% ) and stimulation with 50 nM all trans-retinoic acid (C_RA ), respectively. Immunohistochemistry showed elongated myotubes in C_1% while round and multinucleated cells in C_RA with also a significantly increased expression of natriuretic peptides (NP) and ET-1 receptors in parallel with a decreased ET-1. On scaffolds, cell viability was similar for Gel-SWCNT_0.3%/0.9% ; NP and ET systems expression decreased in both concentrations with respect to control and CX-43, mainly due to a lacking of complete differentiation in cardiac phenotype during that time. Although further analyses on novel biomaterials are necessary, these results represent a useful starting point to develop new biomaterial-based scaffolds. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2750-2762, 2018.

Unexpected thermal annealing effects on the viscosity of polymer nanocomposites

The effects of thermal annealing, 12-50 K above the glass transition temperature, on the zero-shear viscosity, η, of polymer nanocomposites (PNCs) and the corresponding host polymers were studied. For all specimens, including neat and 4 wt% dioctyl phthalate (DOP)-plasticized polystyrene (PS), neat poly(methyl methacrylate) (PMMA), and PNCs containing bare and grafted silica nanoparticles in neat and DOP-plasticized PS, the η increased with time initially, and only asymptotically approached a steady-state value after thermal annealing for ∼100 to ∼200 h. We found that this phenomenon occurred regardless of the solvent used to prepare the sample although the fractional changes in η (δη/η) are visibly bigger for tetrahydrofuran (THF). Moreover, the PNCs not plasticized by DOP showed bigger δη/η than their host polymers while the plasticized ones behave essentially the same as the neat hosts. Interestingly, some unplasticized PNCs prepared from THF exhibited smaller viscosities than the host polymer, but this anomaly disappeared on thermal annealing. By correlating the viscosity measurements with the evolution of the solvent content, average NP aggregate size and the amount of adsorbed PS on silica for samples prepared from different solvents, we infer that the temporal viscosity evolution originates from out-of-equilibrium chain conformations produced during sample preparation. Because these relaxations are limited by the rearrangement of the polymer chains adsorbed on the NP or sample substrate surface, the timescales over which η changes can be much longer than the polymer reptation time, as observed.

Water-based acrylic coatings reinforced by PISA-derived fibers

Polymerization-induced self-assembly (PISA) provides nanofibers that may be used as reinforcing fillers for all-organic aqueous coatings.

Torsional stiffness determines aggregate structure in sheared colloidal rod suspensions

We report the results of simulations of suspensions of sheared rigid rods in the presence of strong attractive inter-particle interactions, using dissipative particle dynamics for a coarse-grained representation of the suspending fluid. We find that the combined effect of the attractive interactions and shear-induced alignment generically produces aggregates of aligned bundles when the contact points between rods are free to rotate. However, the introduction of substantial torsional stiffness to the inter-particle contacts recapitulates the disordered aggregates often observed in suspensions of high aspect ratio particles. We show that the degree of alignment within the aggregates depends on the strength of the torsional stiffness, while the stability of the aggregates and their impact on system viscosity depend on the competition between the attractive interaction and the shear stresses.

Electrically conductive strain sensing polyurethane nanocomposites with synergistic carbon nanotubes and graphene bifillers

Thermoplastic polyurethane (TPU) based conductive polymer composites (CPCs) with a reduced percolation threshold and tunable resistance-strain sensing behavior were obtained through the addition of synergistic carbon nanotubes (CNT) and graphene bifillers. The percolation threshold of graphene was about 0.006 vol% when the CNT content was fixed at 0.255 vol% that is below the percolation threshold of CNT/TPU nanocomposites. The synergistic effect between graphene and CNT was identified using the excluded volume theory. Graphene acted as a 'spacer' to separate the entangled CNTs from each other and the CNT bridged the broad gap between individual graphene sheets, which was beneficial for the dispersion of CNT and formation of effective conductive paths, leading to better electrical conductivity at a lower conductive filler content. Compared with the dual-peak response pattern of the CNT/TPU based strain sensors, the CPCs with hybrid conductive fillers displayed single-peak response patterns under small strain, indicating good tunability with the synergistic effect of CNT and graphene. Under larger strain, prestraining was adopted to regulate the conductive network, and better tunable single-peak response patterns were also obtained. The CPCs also showed good reversibility and reproductivity under cyclic extension. This study paves the way for the fabrication of CPC based strain sensors with good tunability.

Gelation and mechanical response of patchy rods

We perform Brownian dynamics simulations to study the gelation of suspensions of attractive, rod-like particles. We show that in detail the rod-rod surface interactions can dramatically affect the dynamics of gelation and the structure and mechanics of the networks that form. If the attraction between the rods is perfectly smooth along their length, they will collapse into compact bundles. If the attraction is sufficiently corrugated or patchy, over time, a rigid space-spanning network will form. We study the structure and mechanical properties of the networks that form as a function of the fraction of the surface, f, that is allowed to bind. Surprisingly, the structural and mechanical properties are non-monotonic in f. At low f, there are not a sufficient number of cross-linking sites to form networks. At high f, rods bundle and form disconnected clusters. At intermediate f, robust networks form. The elastic modulus and yield stress are both non-monotonic in the surface coverage. The stiffest and strongest networks show an essentially homogeneous deformation under strain with rods re-orienting along the extensional axis. Weaker, more clumpy networks at high f re-orient relatively little with strong non-affine deformation. These results suggest design strategies for tailoring surface interactions between rods to yield rigid networks with optimal mechanical properties.

Particle-size dependent melt viscosity behavior and the properties of three-arm star polystyrene-Fe3O4 composites

The melt viscosity of three-arm star polystyrene (S3PS)-Fe(3)O(4) nanoparticle composites was studied by means of rheological measurements. The arm molecular weight (M(a)) of S3PS (or radius gyration) and the particle size of Fe(3)O(4) (radius (R(p)): 3 nm and 44 nm) showed a strong influence on the melt viscosity behavior (at low shear frequencies) of S3PS-Fe(3)O(4) composites. The reinforcement (viscosity increase) was observed in the composites where the M(a) was higher than the M(c) of PS (M(c): the critical molecular weight for chain entanglement). For M(a) < M(c), when the size of Fe(3)O(4) nanoparticles was changed, the melt viscosity of the composites exhibited either plasticization (melt viscosity reduction) or reinforcement. When the content of Fe(3)O(4) was low (1 wt%), the transformation from plasticization to reinforcement behavior could be observed, which strongly depended on the size ratio of the radius of gyration (R(g)) of S3PS to the size of nanoparticles (R(p)). In addition, the magnetic properties and thermal stability of S3PS-Fe(3)O(4) composites were studied.

Investigating linear and nonlinear viscoelastic behaviour and microstructures of gelatin-multiwalled carbon nanotube composites

The linear and nonlinear rheology of physically-crosslinked-gelatin gel-multiwalled carbon nanotube (MWNT), chemically-crosslinked-gelatin gel-MWNT, and chemically–physically-crosslinked-gelatin gel-MWNT composites, are investigated.

Kinetic Polymer Arrest in Percolated SWNT Networks

Particle-polymer attractions in nanocomposites can cause significant heterogeneities in the polymer dynamics and remarkably impact the material properties. Dynamical perturbations are generally expected to be limited to interfacial polymer segments. However, composites with highly anisotropic nanoparticles usually exhibit very low percolation thresholds. In such systems, the overlapping interfacial regions could result in a complex polymer relaxation behavior that is unanticipated from dilute nanoparticle dispersions in polymer matrices. To understand this behavior, we examine a system of percolated single-wall carbon nanotubes (SWNT) in a polymer matrix, PMMA, which is known to have strong interfacial binding. Neutron spectroscopy measurements on the composites reveal not only an interfacial polymer layer that is transiently pinned to the SWNT surface, but suggest that the percolated network forms a kinetic cage that dramatically restricts both local and cooperative relaxations of noninterfacial polymer segments. These findings should help guide theories and simulations of hierarchical polymer dynamics in nanocomposites.

Flow-induced structure and rheological properties of multiwall carbon nanotube/polydimethylsiloxane composites

The structure and normal stress differences of MWNT/polymer composites are influenced by the molecular weight of the polymer matrix and the confinement effect.