Dynamic rheological study of paraffin wax and its organoclay nanocomposites

Dynamic rheological data for paraffin wax and its organoclay nanocomposites are reported. Dynamic mechanical analysis of paraffin wax for temperatures ranging from ?40 to 55°C showed a decrease of several orders of magnitude in the dynamic moduli and a significant shift toward viscous behavior, which resulted from the occurrence of two solid–solid phase transitions. In both the crystalline and mesophase regions, the dispersion of organoclay platelets in paraffin wax via ultrasonication increased the storage modulus, whereas the effect on the loss modulus was temperature‐dependent. The melt rheology data of the wax–clay nanocomposites at 70°C showed that the complex viscosities increased monotonically with clay addition and demonstrated shear‐thinning behavior for frequencies between 0.1 and 100 rad/s. The complex viscosity versus angular frequency data were well fit by a power‐law function for which the shear‐thinning exponent provides a gauge for the extent of clay exfoliation. The nanocomposites exhibited low‐frequency solid behavior, which indicated good exfoliation of the organoclay in the wax matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Shear and elongational rheology of polyethylenes with different molecular characteristics. I. Shear rheology

Four metallocene polyethylenes (PE), one conventional low density polyethylene (LDPE), and one conventional linear low density polyethylene (LLDPE) were characterized in terms of their complex viscosity, storage and loss moduli, and phase angle at different temperatures. The effects of molecular weight, breadth of molecular weight distribution, and long‐chain branching (LCB) on the shear rheological properties of PEs are studied. For the sparsely long‐chain branched metallocene PEs, LCB increases the zero‐shear viscosity. The onsets of shear thinning are shifted to lower shear rates. There is also a plateau in the phase angle, δ, for these materials. Master curves for the complex viscosity and dynamic moduli were generated for all PE samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Dynamic mechanical properties of some liquid crystalline polyacrylates

Summary The dynamic mechanical properties of some LC (nematic and smectic) polyacrylates with biphenyl mesogens with different spacers and tails are described. From plots of G and G against frequency and temperature the phase transitions could be clearly detected, in agreement with DSC analysis. Furthermore, the dynamic viscosities of the LC polyacrylates compared with an isotropic model polymer of similar stucture showed a strong shear rate and temperature dependence. Surprisingly it was found that the viscosity of the LC polymers was higher in the nematic phase than in the isotropic phase.     

Rheological behavior of low molecular weight polystyrene composites containing monodisperse crosslinked polystyrene beads

Steady shear viscosities and dynamic moduli of polymer composites, consisting of crosslinked polystyrene beads and low molecular weight polystyrene matrix, were measured in a cone-and-plate rheometer at different temperatures. Viscosities and dynamic moduli were found to be very sensitive to filler loading and measurement temperature. Steady shear viscosities of 30% and 40% loaded low molecular polystyrene composites showed a power-law behavior over the entire range of shear rates. Storage and loss moduli were initially linear with frequency on double logarithmic plots, with limiting slopes of 0.3 and 0.1. At high concentration of filler particles, they showed a flat plateau at low frequencies, indicating that these systems exhibit a yield behavior. A 20% PS composite loaded with beads of high crosslink densities resulted in poor dispersion of beads as a result of poor dispersion of particles. PS beads 1.16 μm in diameter showed a higher viscosity. It is due to the apparent increase in loading resulting from broken particles. At low measurement temperature, filler effects were suppressed by high viscosity matrix and showed a similar rheological behavior to high molecular weight by PS matrix. We suggest that rheological behavior reflects the state of dispersion of beads in the matrix.     

Rheological properties of cotton pulp cellulose dissolved in 1‐butyl‐3‐methylimidazolium chloride solutions

Rheological properties of cotton pulp dissolved in 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl) solutions were characterized using an advanced rheometer. The complex viscosity, dynamic modulus, and shear viscosity at different temperature were studied. In the steady shear measurements, all the solutions show a shear‐thinning behavior at high shear rates. The complex viscosity as a function of frequency was fitted by extended Carreau–Yasuda model. In all cotton pulp/[Bmim]Cl solutions, the complex dynamic viscosity (η*) and steady shear viscosity (η_a) followed the Cox–Merz rule only at lower frequency. The effects of tested temperature on viscosity and viscoelastic behavior of the solutions were also investigated. The value of activation energy for the dissolution of cotton pulp in ionic liquids was 65.28 kJ/mol at the concentration of 10 wt% and was comparable with the ones for the dissolution of cellulose in NMMO. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Dynamic mechanical properties of some liquid crystalline polyacrylates

Summary The dynamic mechanical properties of some LC (nematic and smectic) polyacrylates with biphenyl mesogens with different spacers and tails are described. From plots of G and G against frequency and temperature the phase transitions could be clearly detected, in agreement with DSC analysis. Furthermore, the dynamic viscosities of the LC polyacrylates compared with an isotropic model polymer of similar stucture showed a strong shear rate and temperature dependence. Surprisingly it was found that the viscosity of the LC polymers was higher in the nematic phase than in the isotropic phase.     

Model filled polymers. VII: Flow behavior of polymers containing monodisperse crosslinked polymeric beads

Steady shear viscosities and dynamic moduli of polymer composites, consisting of combinations of crosslinked beads and matrices of polystyrene (PS) and polymethacrylates (PMA), are measured in a cone and plate rheometer. Viscosities and moduli were very sensitive to chemical composition. Crosslinked beads of identical composition to the matrix exhibited the lowest viscosity enhancements at low shear rates and the lowest moduli in dynamic mechanical analysis. The effects of bead concentration on rheological behavior were compared for PS and PMMA beads in a PMMA matrix. PMMA beads produce small effects, whereas PS beads yield highly non-Newtonian systems in PMMA, showing a yield stress of 1100 Pa at 30 wt% filler loading and dynamic moduli independent of frequency. We suggest that rheological behavior reflects the state of dispersion of beads in the matrix. Beads identical in composition to the matrix yield uniform dispersions. We propose that uniform and stable bead dispersions exhibit the lowest viscosity and moduli. Beads that cluster in the matrix, such as PS beads in PMMA, exhibit highly non-Newtonian behavior.     

Influence of concentration and temperature on the flow behavior of oil-in-water emulsions stabilized by sucrose palmitate

To study the influence that concentration and temperature exert on the viscous behavior of emulsions stabilized by a sucrose ester (SE) of high hydrophilic-lipophilic balance (HLB), flow curves and droplet size distributions were determined. Flow curves of presheared emulsions always exhibited a shear-thinning behavior at intermediate shear rates, a tendency to a limiting viscosity at high shear rates, and a metastable region at low rates. This behavior can be fitted to a Carreau model. Both SE and oil concentrations increase emulsion viscosity as a result of a more structured system with a lower droplet size and polydispersity. An increase in temperature usually leads to a decrease in emulsion viscosity. However, at high oil concentration, coalescence and phase separation take place at low temperature. On the other hand, at high temperature, droplet bursting due to shear forces, leading to an increase in viscosity, may result. Despite the strong structural breakdown caused by steady shear, master flow curves may be obtained by using superposition methods.     

Rheological Study of Vegetable Oil Based Hyperbranched Polyurethane/Multi-Walled Carbon Nanotube Nanocomposites

Hyperbranched polyurethanes [HBPUs] and vegetable oil based polymer nanocomposites have been drawing an imperative attention for their plentiful advantages across a spectrum of potential applications. This study divulges the rheological behaviors of Mesua ferrea L. seed oil modified HBPU/multiwall carbon nanotube [MWCNT] nanocomposites prepared by in-situ technique. Rheological phase transition behavior was studied at 120°C in the steady shear and oscillation mode. The nanocomposites showed shear thinning behavior in both the modes. The rheological characteristics were dependent on the loading of the nanotube as confirmed from this study. The storage and loss moduli values were higher than the pure HBPU and they showed improved viscosity by nanocomposite formation. The nanocomposites revealed a pseudo–solid-like behavior at relatively low frequencies. The effects of temperature on storage and loss modulus have also been explored. The temperature dependence complex viscosity further described the ease of processibility. It has been tried to establish a structural property relationship of the systems from rheological study.     

Dynamic rheological properties of high‐density polyethylene/polystyrene blends extruded in the presence of ultrasonic oscillations

The linear rheological properties of high‐density polyethylene (HDPE), polystyrene (PS), and HDPE/PS (80/20) blends were used to characterize their structural development during extrusion in the presence of ultrasonic oscillations. The master curves of the storage shear modulus (G′) and loss shear modulus (G″) at 200°C for HDPE, PS, and HDPE/PS (80/20) blends were constructed with time–temperature superposition, and their zero shear viscosity was determined from Cole–Cole plots of the out‐of‐phase viscous component of the dynamic complex viscosity (η″) versus the dynamic shear viscosity. The experimental results showed that ultrasonic oscillations during extrusion reduced G′ and G″ as well as the zero shear viscosity of HDPE and PS because of their mechanochemical degradation in the presence of ultrasonic oscillations; this was confirmed by molecular weight measurements. Ultrasonic oscillations increased the slopes of log G′ versus log G″ for HDPE and PS in the low‐frequency terminal zone because of the increase in their molecular weight distributions. The slopes of log G′ versus log G″ for HDPE/PS (80/20) blends and an emulsion model were used to characterize the ultrasonic enhancement of the compatibility of the blends. The results showed that ultrasonic oscillations could reduce the interfacial tension and enhance the compatibility of the blends, and this was consistent with our previous work. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3153–3158, 2004     

The rheological characterization of a series of thermotropic liquid crystalline polymers

Dynamic rheological experiments were performed on a series of two copolymers and a homopolymer based on units of terephthaloyl chloride and isophthaloyl chloride at 90/10 and 75/25 mole ratios combined with 1,10-bis(4-hydroxyphenyl)-decane. Optical microscopy and wide angle X-ray diffraction (WAXD) confirmed that all of the polyesters in the present series formed nematic liquid crystals with nematic-to-isotropic temperatures in the range of 270 to 320°C with increasing terephthaloyl unit content. Broad nematic-to-isotropic transitions observed by differential scanning calorimetry (DSC) were indicative of biphasic regions where the nematic and isotropic phases coexist. The rheological behavior of each polymer was more complex in the nematic phase than in the isotropic phase with shear thinning occurring in the former but Newtonian behavior in the latter. There were also some indications that nematic flow behavior could be induced in these polymers by dynamic oscillatory shear flow above the nematic-to-isotropic transition, T_i. A form of hysteresis was observed with the homopolymer in that measurements of the dynamic viscosity, η*, taken with ascending frequency sweeps were higher than those taken with descending frequency sweeps.     

热致性液晶共聚酯(PET/60PHB)粘弹性质与温度之间的关系

聚对苯二甲酸乙二酯/60聚羟基苯甲酸(PET/60PHB)共聚酯熔体是多相体系。本文用Rheometrics Mechanical Spectrometer 605流变仪分相态研究其粘弹性质。发现在液晶、微晶共存态,熔体具有各向同性熔体的流变性质。当熔体中各向异性相占优时,熔体的流变性质与一般聚合物熔体的差别很大。当各向同性相占优时,熔体的流变行为具有两重性。一方面,随着ω增大,G′远离G″;另一方面,η_(?)=η~8(当ω=γ时)。     

Blends of poly[ethylene(vinyl acetate)] and polychloroprene: Studies on capillary and dynamic flows

Rheological properties of the blends of poly[ethylene(vinylacetate)] (EVAc; vinylacetate content 28%) and polychloroprene (CR) have been measured through capillary and dynamic uniaxial elongational flows. Capillary flow indicates their shear thinning behavior. The decrease in the out of phase viscosities with increasing frequency is in accordance with the power law equation, whereas dynamic elongational viscosities follow nonlinear relationship in log-log plot with an initial increase at 11 Hz, followed by a very sharp drop. With an increase in temperature, the viscosity for capillary flow of all blends goes down due to their positive activation energy of flow but for dynamic elongational flow of EVAc blended with CR, viscosity increases, except for 30/70 blend and pure CR, in which case the dynamic elongational viscosity decreases with an increase in temperature. This abnormal behavior in dynamic elongational viscosity is due to the process of melting and recrystallization of EVAc at low heating rate (1°C/min) beyond the melting temperature. Capillary viscosities of all blends show positive deviation from the log additive values of pure polymers. But in the case of dynamic elongational flow, all blends show positive deviation at frequencies of 3.5 and 35 Hz and at higher temperatures (80–120°C). © 1997 John Wiley & Sons, Inc.     

Structure and rheology of twin liquid crystalline polymers

Addition of short mesogenic segments at the ends of flexible chains alters the mechanical properties by orders of magnitude. Twin liquid crystalline polymers (TLCPs) were synthesized from 4-[(4′-alkoxybenzoyl)oxy]-benzoyl chloride and ,ω-dihydroxy-telechelic polytetrahydrofuran (PTHF) of different molecular weights. With increasing temperature, four equilibrium states of these TLCPs, i.e. crystalline state, phase separated state with mesogenic domains in isotropic PTHF matrix, phase separated amorphous state, and single phase isotropic state, have been observed by dynamic mechanical measurements, differential scanning calorimetry and polarizing microscopy. In the phase separated state, mesogenic domains function as physical crosslinks which give rise to unusually high viscoelastic properties at small strains. Disturbing this state by large amplitude shear resulted in very pronounced shear thinning and slow recovery of structure. At increased temperature, the mesogenic domains become isotropic and their effect as physical crosslinks was significantly reduced, as shown by lower viscoelasticity and weak shear thinning. In the single phase isotropic state above the coexistence temperature T_s, the TLCPs behaved like a common homopolymer of low molecular weight. T_s decreased as the weight ratio of PTHF spacer increased in the experimental range (50–82% PTHF).     

Effect of the particle size on the viscoelastic properties of filled polyethylene

Composites of surface treated and non-treated colloidal calcium carbonate and high-density polyethylene with different filler loading were prepared. Their viscoelastic properties were studied by dynamic strain sweep and small-amplitude oscillatory shear, and compared to those of the corresponding composites of micron-sized calcite. The specific surface area of the filler enormously increases as the average particle diameter becomes smaller than 600 nm, leading to a strong tendency to agglomeration (soft flocks) and aggregation (hard clusters that need attrition to be disintegrated). In nanocomposites, more and stronger filler clusters are formed than in microcomposites due to the large contact area between the particles. The clusters have different shapes and maximum packing than the nearly spherical primary particles, thus enhance the moduli and viscosity of the composites. The obtained results indicate that the higher moduli and viscosity of the nanocomposites is not a direct consequence of the particle size but is due to the presence of more agglomerates and aggregates. Clusters that are local structures and do not represent a space-filling filler network enhance the moduli in the low frequency region more than at high frequencies and increase the storage more than the loss modulus. The presence of strong local structures in the nanocomposites leads to weak log moduli-log frequency dependence in the low frequency (terminal) region. Polymer adsorption on the particles' surface results in a transient filler-polymer network and slow dynamics of the bound polymer, which contribute to the moduli of the complex fluid. The sum of all these factors leads to gradual increase in moduli and to a shift of the crossover frequency to lower values. Above a certain filler volume fraction, the composite responds as a viscoelastic solid (storage modulus>loss modulus over the whole frequency range and both moduli are frequency independent in the terminal zone of the log-log plot).     

Melt rheology of polypropylene reinforced with polyaniline‐coated short glass fibers

This research deals with the melt rheology of isotactic polypropylene (iPP) reinforced with short glass fibers (SGF) coated with electrically conductive polyaniline (PAn). Composites containing 10, 20, and 30 wt % PAn‐SGF were studied. Moreover, a composite of 30 wt % PAn‐SGF was also prepared with a blend of iPP and PP‐grafted‐maleic anhydride (iPP/PP‐gMA). The composites showed linear viscoelastic regime at small strain amplitudes. The onset of nonlinearity decreased as the concentration of filler increased. The time‐temperature superposition principle applied to all composites. The filler increased the shear moduli (G′, G″) and the complex viscosity η*. Steady‐state shear experiments showed yield stress for the composites with 20 and 30 wt % PAn‐SGF. Strikingly, the 10 wt % composite showed higher steady state viscosity than the 20 wt %. Rheo‐optics showed that shear induced disorder of microfibers at a concentration of 10 wt %. However, at 20 wt % concentration shear aligned the microfibers along the flow axis, this would explain the anomalous steady state viscosity values. The viscosity exhibited a shear thinning behavior at high shear rates for all composites. Creep experiments showed that the filler induced greater strain recovery in the composites and that the amount of strain recovery increased as the PAn‐SGF concentration increased. However, the enhancement of strain recovery (as well as shear viscosity) was more significant when using the iPP/PP‐gMA blend, suggesting greater adhesion between this matrix and the filler PAn‐SGF. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Dynamic viscoelastic behavior of polypropylene/polybutene-1 blends and its correlation with morphology

In this study the rheology, morphology, and interfacial interaction of polypropylene (PP)/polybutene-1 (PB-1) blends in different percentages of PB-1 are investigated. The morphology of cryo-fractured surfaces of samples was studied by scanning electron microscopy (SEM). The SEM images showed a droplet-matrix structure in all range of compositions and the size of dispersed phase increased proportionally with PB-1 content. The miscibility of blends at various compositions is evaluated by viscoelastic parameters determined by dynamic oscillation rheometry in the linear viscoelastic region. A distinct Newtonian plateau at low frequencies is observed and the variations of complex viscosity (η*) against angular frequency (ω) for all blends are in agreement with Cross model. The complex viscosity of samples at various percentages of PB-1 showed the log-additivity mixing rule behavior in low frequencies and positive-negative deviation behavior (PNDB) at high shear rates. The phenomena such as decrease in the sensitivity of storage modulus to shear rate in the terminal region, the deviation of Cole–Cole plots from the semi-circular shape, and the tail in relaxation spectrums at high relaxation times are the evidences of two phase heterogenous morphology. The effect of time–temperature on the phase behavior is studied and the interfacial tension between matrix and dispersed phase was evaluated by using emulsion theoretical models. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Percolation network formation in poly(4‐vinylpyridine)/aluminum nitride nanocomposites: Rheological,dielectric, and thermal investigations

This work is concerned with several issues related to the rheological behavior of poly(4‐vinylpyridine)/aluminum nitride (AlN) nanocomposites. The composites are prepared by solution processing combined with ultrasonication and magnetic stirring. To understand the percolated structure, the nanocomposites are characterized via a set of rheological, dielectric, and thermal conductivity analyses. The nanoparticle networks are sensitive to the steady shear deformation particularly at low shear rates, where a shear‐thinning domain is observed. The rheological measurements revealed also that the activation energy is significantly lower at high nanofiller loadings suggesting stronger AlN interactions. The changes in the terminal behavior of shear moduli are the result of variations in composite elasticity determined by the percolation network. The flocculation and percolation thresholds estimated from the rheological moduli dependence on AlN loading are correlated with the dielectric constant values. Thermal conductivity is determined from a new theoretical model involving, besides the contribution of each phase, both percolation processes and the shape of the nanofiller. POLYM. COMPOS., 35:1543–1552, 2014. © 2013 Society of Plastics Engineers     

Rheology of low nematic transition temperature thermotropic liquid crystalline copolyester HBA/HQ/SA

The rheology of a liquid crystalline copolyester of hydroxybenzoic acid, hydroquinone, and sebacic acid (HBA/HQ/SA copolyester) was studied on both a rotational and a capillary rheometer. DSC studies show that the copolyester has a crystalmesophasic and a broad mesophasic-isotropic transition at 170°C and 220°C. Optical texture observations show the mesophase is characterized by line defect textures, which are characteristic of a nematic structure. At 220°C, both isotropic and nematic phases coexist with the latter being the major. As temperature reaches 250°C, a clear dominance of isotropic phase is observed. At this temperature, the nematic phase of irregular shapes randomly disperses within the isotropic matrix. Subsequent rheological studies were thus conducted in crystal/nematic biphase, single nematic phase, nematic/isotropic biphase, and the near single isotropic phase. Dynamic strain sweep measurements show that a linear viscoelastic region exists at all temperatures tested. The maximum strain amplitude for the linear viscoelastic region is found to be highly structure dependent; it is > 100% in the nematic phase, ∼20% in the biphases, and only about 5% in the isotropic phase. The concurrence of curves obtained at different temperatures in a Cole-Cole plot of G′ vs. G″ indicates similar structures in the nematic phase and biphases. Measurements of steady shear viscosity using a rotational rheometer and a roundhole capillary rheometer show that in the nematic phase the copolyester behaves as a shear thinning fluid for a wide shear rate range of 1 ∼ 10,000 s⁻¹, in which the power law index is about 0.6 ∼ 0.8, and the viscosity is < 10 Pa.s at shear rates >1 s⁻¹.     

EFFECT OF TEMPERATURE ON THE RHEOLOGY OF POLYMERIC LIQUID CRYSTALS

As part of a more general effort to elucidate the flow of polymeric liquid crystals, the effect of temperature on the rheological properties has been investigated. A lyotropic sample of poly(γ-benzyl-L-glutamate) in m-cresol has been used throughout the measurements. Under steady state shear flow the viscosity and the first normal stress difference have been measured. Oscillatory flow after cessation of steady shear flow reveals a time effect which led to the determination of initial and final moduli; both have been measured as a function of temperature. Finally some transients have been considered as well.

The limiting zero frequency value of the dynamic viscosity and the zero shear value of the steady state viscosity have different activation energies. The dynamic moduli can be scaled for temperature effects by means of the zero frequency viscosity. Because of the difference in activation energies this scaling does not hold for the steady state properties. An alternative scaling procedure is suggested. The time scale of the transients is nearly independent of temperature. None of the available theoretical models describes the measured phenomena.