Effect of flow history on linear viscoelastic properties and the evolution of the structure of multiwalled carbon nanotube suspensions in an epoxy (original) (raw)
Related papers
The rheological modelling of carbon nanotube (CNT) suspensions in steady shear flows
International Journal of Material Forming, 2008
This paper is concerned with the rheological modelling of both chemically treated and untreated carbon nanotube (CNT) suspended in a Newtonian epoxy resin. CNT suspensions generally exhibited shear-thinning characteristic-the apparent viscosity decreases as shear rate increases-when subject to steady shear flows. Chemically treated CNT suspensions with little optical microstructure were found to exhibit a less significant shear-thinning effect compared with untreated CNT suspensions where clear optical aggregates were observed. In the case of treated CNT suspensions, the shear-thinning characteristic could be described using a Fokker-Planck based orientation model. The model assumed that the treated CNTs behaved as high aspect ratio rods and that shear flow was able to align the CNTs in the flow direction, thereby resulting in a decrease in the shear viscosity. Despite the success in describing the rheological response of treated CNT in steady shear flows, the orientation model failed to explain the more pronounced shear-thinning effect observed in untreated CNT suspensions having a hierarchy of aggregate structures. A new model called the aggregation/orientation (AO) model was formulated by modifying the Fokker-Planck equation. The AO model considered elements of aggregation as well as CNT orientation and it was capable of capturing the steady shear response of untreated CNT suspensions.
The rheology and modeling of chemically treated carbon nanotubes suspensions
Journal of Rheology, 2009
This paper reports recent experimental findings and rheological modeling on chemically treated single-walled carbon nanotubes ͑CNTs͒ suspended within an epoxy resin. When a CNT suspension was subject to a steady shear flow, it exhibited a shear-thinning characteristic, which was subsequently modeled by a Fokker-Planck ͑FP͒ based orientation model. The model assumes that the shear flow aligns CNT in the flow direction, but there are events such as Brownian motion and tube-tube interaction trying to randomize the orientation. In the FP orientation model, randomizing events were modeled with an appropriate rotary diffusion coefficient ͑D r ͒ and the shear-thinning behavior was explained in terms of progressive alignment of CNTs toward the shear direction. In terms of linear viscoelasticity ͑LVE͒, small-amplitude oscillatory measurements revealed mild elasticity for semidilute treated CNT suspensions. The exact origin for this elasticity is not clear and both tube-tube interaction and bending/stretching of CNTs have been proposed by other authors as possible origins. It is, however, clear from the current modeling that the experimental evolution of storage modulus ͑GЈ͒ cannot be described using a single-mode Maxwell model or simple Brownian rod modeling. In this paper, experimental LVE data of the treated CNT suspensions were fitted using the FP orientation model with an "effective diffusion coefficient" term and an empirical relation was subsequently identified for the effective diffusion term. Intuitively, chemical treatment has created a weakly interconnected network of CNT and it is believed that the mild elasticity originated from this weak network as well as other randomizing events ͑Brownian motion and tube-tube hydrodynamic interaction͒. Finally, step strain experiments confirmed the presence of a weak network at small strains, which at large strains was found to be destroyed. Incorporation of a strain softening factor allowed for the formulation of a a͒
Rheological modeling of carbon nanotube aggregate suspensions
Journal of Rheology, 2008
This paper reports the application of a recently developed filament stretching protocol for the study of the extensional rheology of both treated and untreated carbon nanotubes (CNT) suspended within an epoxy resin. It was experimentally observed that filaments formed by treated and untreated CNT suspensions behaved differently after initial stretching. The filament thinning process of the base epoxy was consistent with a simple Newtonian fluid, whilst the filament of treated CNT suspensions also thinned in a Newtonian way but with an enhanced extensional viscosity. Filaments formed with untreated CNT suspensions behaved in a non-uniform way with local fluctuation in filament diameter, and it was not possible to obtain reliable extensional viscosity data. Irregularity of the untreated CNT filaments was consistent with coupled optical images, where spatial variation in CNT aggregate concentration was observed. In the case of treated CNT suspensions, the enhanced extensional viscosity was modelled in terms of the alignment of CNTs in the stretching direction, and the degree of alignment was subsequently estimated using a simple orientation model.
A review of the microstructure and rheology of carbon nanotube suspensions
2008
The present review is concerned with the way that the incorporation of carbon nanotubes (CNTs) into a fluid matrix can modify the microstructure and rheology of the resulting suspensions. Some background to CNT manufacture and in particular methods of dispersing them into a suspension is presented for a range of different systems, where effective dispersion of CNTs remains a delicate and open issue. Steady shear, linear viscoelasticity, nonlinear viscoelasticity and extensional responses are classified for a range of different CNT/ matrix combinations together with their associated microstructure. The rheological modelling of certain CNT/matrix systems is reviewed, with particular attention given to the authors' work on modelling CNT suspension behaviour using Fokker-Planck advection-diffusion modelling.
Modeling interactions in carbon nanotube suspensions: Transient shear flow
Journal of Rheology, 2016
Transient shear flow data of untreated multiwalled carbon nanotubes (MWCNTs) dispersed in a Newtonian epoxy matrix are analyzed. A sequence of shearing and rest steps was applied to characterize the transient responses of the suspensions. Stress overshoots appeared at very small deformation during forward and reverse flow experiments and their intensity increased with rest time between two consecutive flows, during which the suspension structure was reconstructed. The transient behavior of the MWCNT suspensions is explained with the help of a recently proposed model [G. Natale et al., AIChE J. 60(4), 1476-1487 (2014)]. The MWCNTs are described as rigid rods dispersed in a Newtonian matrix, and the evolution of the system is controlled by hydrodynamics, rod-rod interactions, and Brownian motion. The force due to the interactions is modeled as a nonlinear lubrication force and the total stress tensor is evaluated introducing a fourth-order interaction tensor. The Fokker-Planck equation is numerically solved for transient simple shear flow using a finite volume method, avoiding the need of closure approximations. The model predictions show that interactions slow down the orientation evolution of the rods. For the first time, the effect of shear rate is directly accounted by the model, which predicts that a critical shear rate is necessary to break down the structure and let the rods orient in the flow direction. In addition, we confronted the model predictions with the rheological data of a glass fiberfilled polybutene [M. Sepehr et al., 48(5), 1023-1048 (2004)], demonstrating its ability to describe the behavior of micro and nano-scale particle suspensions. V
Rheologica Acta, 2009
This paper is concerned with several issues related to the rheological behavior of polycarbonate/ multiwalled carbon nanotube nanocomposites. The composites were prepared by diluting a masterbatch of 15 wt.% nanotubes using melt-mixing method, and the dispersion was analyzed by SEM, TEM, and AFM techniques. To understand the percolated structure, the nanocomposites were characterized via a set of rheological, electrical, and thermal conductivity measurements. The rheological measurements revealed that the structure and properties were temperature dependent; the percolation threshold was significantly lower at higher temperature suggesting stronger nanotube interactions. The nanotube networks were also sensitive to the steady shear deformation particularly at high temperature. Following preshearing, the elastic modulus decreased markedly suggesting that the nanotubes became more rigid. These results were analyzed using simple models for suspensions of rod-like particles. Finally, the rheological, electrical, and thermal conductivity percolation thresholds were compared. As expected, the rheological threshold was smaller than the thermal and electrical threshold.
Elongational and shear rheology of carbon nanotube suspensions
Rheologica Acta, 2009
Rheological behavior of concentrated suspensions of chemical vapor deposition carbon nanotubes in uniaxial elongation and simple shear is studied experimentally and theoretically. Nanotubes are suspended in viscous host liquids-castor oil or its blends with n-decane. The elongational measurements are performed by analyzing self-thinning (due to surface tension effect) liquid threads of nanotube suspensions. A quasi-one-dimensional model is used to describe the self-thinning process, whereas corrections accounting for thread nonuniformity and necking are introduced a posteriori. The effects of nanotube concentration and aspect ratio, viscosity of the suspending liquid, and initial diameter of the self-thinning thread in uniaxial elongation are elucidated. The results for uniaxial elongation are compared with those for simple shear. The correspondence in the results of the shear and elongational measurements is addressed and interpreted. The results conform to the Herschel-Bulkley rheological constitutive equation (i.e., power law fluids with yield stress). However, the yield stress in elongation is about 40% higher than in simple shear flow, which suggests that the original Herschel-Bulkley model need modification with the yield stress being a function of the second invariant of the deviatoric stress tensor. The present effort is the first to study capillary self-thinning of Herschel-Bulkley liquids, which are exemplified here by suspensions of carbon nanotubes.
Critical shear rates and structure build-up at rest in MWCNT suspensions
Journal of Non-Newtonian Fluid Mechanics, 2012
In this paper, we quantitatively analyze the extent of structure build-up at rest in carbon nanotubes (CNTs) dispersed in an epoxy by a set of transient flow measurements. Stress overshoots appeared at very small deformations during forward and reverse flow measurements by providing some rest time between the two consecutive flows in opposite directions, during which the suspensions structure was reconstructed. The rest time required to build-up the structure completely was much longer than the values reported in literature (about 1 h). Moreover, unlike fiber suspensions or some nano-composites, it was shown that the Brownian motion plays an important role in the structure build-up of the CNT suspensions in the absence of flow. We observed critical shear rates at low and intermediate concentrations above which some nanotube entanglements broke down; this resulted in lower elasticity of the suspensions and partial structure build-up at rest during transient flow reversal measurements. This phenomenon and the possible mechanisms of structure evolution during flow and rest were further investigated by comparing the experimental results with the predictions of a structural model.
Shear Relaxation Behaviour in Linear Regime of Diluted Carbon Nanotube Suspensions
2011
Carbon nanotubes (CNT) are very long cylinders formed by rolled-up graphene sheets at the nano scale. In addition to their unique structure, interest for CNTs is based on their impressive physical properties [1]. CNTs have been envisaged as component-material in a large range of applications: electrical circuits, nano-probes for near-field microscopes, high-resolution gas-molecules detectors, drug and gene delivery systems, highstrength fibres and high-performance polymer-based composites. A large number of envisaged applications require dissolving CNTs and processing them in liquid phase. Control of those forming processes needs a deep understanding of the rheology of CNT suspensions [2]. In that context, this work tackles the modelling of the rheological behaviour of dilute and low semi-dilute suspensions of treated CNTs into a Newtonian solvent. CNTs have an important influence on the viscous behaviour of their suspensions. For example, dilute and low semi-dilute suspensions of t...
Discontinuous shear thickening in confined dilute carbon nanotube suspensions
Proceedings of the …, 2011
A monotonic decrease in viscosity with increasing shear stress is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. Here we demonstrate a discontinuous shear-thickening transition on varying shear stress where the viscosity jumps sharply by four to six orders of magnitude in flocculated suspensions of multiwalled carbon nanotubes (MWNT) at very low weight fractions(approximately 0.5%). Rheooptical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shearthinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear-thickening phenomena in confined geometries is pertinent for flow-controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues.