The functionalization of carbon nanotubes using a batch oscillatory flow reactor

This paper describes an efficient method for the functionalizing of multi-walled carbon nanotubes (MWCNT) using oscillatory flow mixing (OFM). A 3. l batch oscillatory flow reactor (OFR) was designed and constructed for pilot scale functionalization of MWCNT in order to potentially improve their compatibility within a thermoplastic polyphenylene sulphide (PPS) matrix. The OFM batch reactor consisted of a jacketed cylindrical vessel with a vertical axial oscillator that contained a series of baffled mixing plates. MWCNTs dispersed in dimethylformamide (DMF) were introduced into the reactor and a two stage reaction for functionalizing MWCNTs with PPS compatible groups was carried out under oscillation of baffles at elevated temperatures. Fluid mixing observations in the reactor showed that MWCNTs formed a uniform dispersion of aggregated flocs before and during the functionalization reaction. On completion of the reaction and cessation of the oscillation, the aggregated flocs of MWCNT rapidly sedimented at the bottom of the reactor; hence could be collected as a concentrated mass thereby facilitating the separation of functionalized MWCNTs from the solvent. The functionalized MWCNTs were dried and then characterized by transmission electron microscopy, infrared spectroscopy as well as thermal gravimetric analysis in order to investigate the extent of MWCNT functionalization. The characterization results confirmed the effective and relatively uniform functionalization of the MWCNTs despite formation of aggregates, indicating that OFM provides a viable approach for functionalizing MWCNTs

Cross-slot extensional rheometry and the steady-state extensional response of long chain branched polymer melts

Stress-optical measurements at a flow stagnation point in confined geometries such as the cross-slot provide an elegant way to perform extensional testing for polymer melts. This technique is especially useful for samples which have a steady-state that cannot be reached (easily) in standard elongational rheometry, for example, highly branched polymers which show a non-homogeneous deformation that occurs in stretching experiments for Hencky strains above 4. In contrast to filament stretching, the cross-slot provides one point at which steady-state extensional flow may be sustained indefinitely. In this study, a Cambridge multi-pass rheometer [ Coventry, K. D., and M. R. Mackley, J. Rheol. 52, 401–415 (2008) ] is used to generate planar elongational flow in a cross-slot geometry for different polyethylene melts. The experimental results are compared to finite element flow simulations using the multi-mode Pompom constitutive equations. The steady-state elongational viscosity at the stagnation point is computed from the flow-induced stress birefringence and the strain-rate determined from numerical calculations of the flow field. We apply this technique to a range of different branched high- and low-density polyethylene melts. This demonstrates both the effectiveness of this technique and shows how the stress distribution in a complex flow depends on molecular structure. Cross slot extensional rheometry therefore provides a very promising technique for parameterizing molecular constitutive equations for LCB melts

An experimental evaluation of the formation of an instability in monodisperse and polydisperse polystyrenes

This paper presents an experimental study on the formation of a polymer flow instability seen in a range of polystyrene melts flowing through contraction–expansion slit geometries. Seen for both monodisperse and polydisperse materials, it took the form of an oscillation in the principal stress difference (PSD) and velocity field perpendicular to the bulk flow. The instability originated downstream of the slit and overtime propagated back upstream such that both regions were unstable. For the monodisperse materials, flowrates were characterised using a theory for linear entangled polymers. This allowed a comparison of the flow regime, with respect to both chain stretch and orientation, and the onset of the instabilities. These are presented as stability maps for comparison with future numerical work. In all cases, no single flowrate, Weissenberg number or critical pressure drop was seen which triggered the instability. The variation of slit curvature and length was found to affect the onset of the flow instability, but not its general form. For all materials, an increase in polydispersity and decrease in molecular weight led to increased processability of the material, delaying the onset of the instability until higher flowrates

Fluid mechanics and design aspects of a novel oscillatory flow screening mesoreactor

A novel, continuous, oscillatory flow screening mesoreactor, composed of tubes with smooth periodic constrictions, is presented as a new option for small-scale reaction engineering applications, particularly those involving the screening of solid catalysts. Experimental studies using particle image velocimetry have demonstrated that this reactor provides good fluid mixing, and is able to suspend catalyst particles with a wide range of sedimentation velocities. Results from 2D and 3D CFD simulations exhibit semi-quantitative agreement with the experimental data, and will aid in the design of future reactors. Potential application areas for this reactor are specialist chemical manufacture and high-throughput screening. Unlike conventional screening devices, the flow patterns in this reactor can be reproduced at larger scales, therefore, results in the laboratory can be related directly to larger scale production.Engineering and Physical Sciences Research Council (EPSRC).Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/6954/2001

Effect of branching in cross-slot flow: the formation of ``W cusps''

The sensitivity of the principal stress difference (PSD) profile to molecular architecture is demonstrated for flow in a cross-slot geometry. For materials with low levels of branching, the pattern along the outlet centre line exhibited “single cusps”, while an increase in molecular branching was found to lead to “W cusps”. The formation of these W cusps was found to be independent of extensional rate for the conditions probed, and they were formed initially at the stagnation point and travelled along the outlet centre line with time. Comparison with simulations performed using a multi-mode “pom-pom” model failed to predict W cusps, although the general level of PSD was accurately captured