Rheological study of segmented polyurethane and polycaprolactone blends

The dynamic rheological properties of segmented polyurethane based on polycaprolactone diol (PU-PCL diol) and poly (ɛ-caprolactone) (PCL) blends were experimentally examined and theoretically analyzed using Palierne model. PU–PCL diol was melt blended with PCL in three different ratios of 20, 30, and 40%. Initial modeling attempts failed to fit the experimental data of these blends, as the model overpredicted their rheological data. This failure is believed to be due to partial dissolution of PCL in PU. According to our obtained results from differential scanning calorimetry and dynamic thermal mechanical analysis, pure PU–PCL diol is diluted by its homopolymer. By calculating the complex modulus of this diluted matrix using a simple mixing rule and its incorporation into the model, good fitting of the Palierne equation to the experimental data was obtained.     

Uniaxial deformation and relaxation of polymer blends: relationship between flow and morphology development

Uniaxial elongational flow followed by stress relaxation of a dilute mixture of polystyrene/polymethylmethacrylate) PS/PMMA with PS (5 wt%) as a dispersed phase was investigated. The behavior of the blend was found to be dominated by the PMMA matrix during elongation and by the interface during the relaxation at long time. Such a behavior was related to drop deformation and shape recovery during the relaxation process as was confirmed by morphological analyses on samples quenched within the rheometer just after elongation and at various times during the relaxation process. The morphology and the rheological material functions variation were compared to the Yu model (Yu W, Bousmina M, Grmela M, Palierne JF, Zhou C (2002) Quantitative relationship between rheology and morphology in emulsions. J Rheol 46(6):1381–1399).     

Manifestation of phase separation processes in oscillatory shear: droplet-matrix systems versus co-continuous morphologies

Phase separation processes in mixtures of poly-α-methylstyrene-co-acrylonitrile (PαMSAN) and poly-methylmethacrylate (PMMA) with lower critical solution temperature (LCST) behavior have been studied, focusing on the manifestation of the interface in oscillatory shear measurements. By using blends of different composition, systems with a droplet-matrix morphology or a co-continuous structure are generated during the phase separation process. The feasibility of probing this morphology development by rheological measurements has been investigated. The development of a disperse droplet phase leads to an increase in the elasticity of the blend at low frequency, showing up as a shoulder in the plot of storage modulus versus frequency. Here, the droplet growth is unaffected by the shear amplitude up to strains of 0.2; therefore the resulting dynamic data are suitable for quantitative analysis. In contrast, for blends in which phase separation leads to a co-continuous structure, the storage modulus shows a power law behavior at low frequency and its value decreases as time proceeds. For the latter systems, effects of the dynamic measurement on the morphology development have been observed, even for strain amplitudes as low as 0.01. To probe the kinetics of morphology evolution in droplet-matrix systems, measurements of the time dependence of the dynamic moduli at fixed frequency should be performed (for a whole series of frequencies). Only from such measurements, curves of the frequency dependence of the moduli at a well defined residence time can be constructed. From fitting these curves to the emulsion model of Palierne, the droplet diameter distribution at that particular stage in the phase separation and growth process can be obtained. It is not appropriate to use a simplified version of the Palierne model containing only the average droplet size, because a morphology with too broad a size distribution is generated. Received: 15 February 1999 Accepted: 20 May 1999     

Viscoelastic properties of POSS–styrene nanocomposite blended with polystyrene

Polyhedral oligomeric silsesquioxane (POSS) are hybrid nanostructures of about 1.5 nm in size. These silicon (Si)-based polyhedral nanostructures are attached to a polystyrene (PS) backbone to produce a polymer nanocomposite (POSS–styrene). We have solution blended POSS–styrene of with commercial polystyrene (PS), , and studied the rheological behavior and thermal properties of the neat polymeric components and their blends. The concentration of POSS–styrene was varied from 3 up to 20 wt.%. Thermal analysis studies suggest phase miscibility between POSS–styrene and the PS matrix. The blends displayed linear viscoelastic regime and the time–temperature superposition principle applied to all blends. The flow activation energy of the blends decreased gradually with respect to the matrix as the POSS–styrene concentration increased. Strikingly, it was found that POSS–styrene promoted a monotonic decrease of zero-shear rate viscosity, η ₀, as the concentration increased. Rheological data analyses showed that the POSS–styrene increased the fractional free volume and decreased the entanglement molecular weight in the blends. In contrast, blending the commercial PS with a PS of did not show the same lubrication effect as POSS–styrene. Therefore, it is suggested that POSS particles are responsible for the monotonic reduction of zero-shear rate viscosity in the blends.     

Temperature dependence of the interfacial tension of PS/PMMA, PS/PE, and PMMA/PE blends

The effect of temperature on the interfacial tension for PS/PMMA, PS/PE, and PMMA/PE was measured using the imbedded fiber retraction method. Interfacial tensions for PS/PMMA, PS/PE, and PMMA/PE were measured over temperature ranges of 160–250 °C, 140–220 °C, and 140–220 °C, respectively. The interfacial tension was found to follow a dependence of 3.6–0.013 T dyn/cm, 7.6–0.051 T dyn/cm and 11.8–0.017 T dyn/cm for PS/PMMA, PS/PE, and PMMA/PE, respectively. Comparison of the data with the mean field theory of Helfand and Sapse were made; however, a simple linear fit to the data described the temperature dependence in the experimental window as well as the predictions of the mean field theory. Received: 6 July 1999 Accepted: 23 March 2000     

Rheological behavior of PS-melts containing surface modified PMMA-particles

The rheological properties of polystyrene (PS) compounds containing cross-linked PMMA particles are characterized by oscillatory shear experiments, while the amount of covalently grafted carboxylic-acid terminated polystyrene (CT-PS) on their surface is varied. All samples show an additional relaxation process with a long relaxation time in the frequency range where the matrix flows. The strength of this process depends strongly on the amount of particles, i.e., on the degree of filling and, to the same extent, on the different “coverage” of the particles surface, i.e., the degree of grafting. For the highest degrees of filling and in dependence on the degree of grafting a particle network is formed which is characterized by an evolving equilibrium modulus. Moreover, compatibilization by grafting CT-PS onto the PMMA particles' surface reduces the strength of the additional relaxation process remarkably. This surprising effect is related qualitatively to the balance of the interaction between particles and the interaction between the particles and the matrix due to PS grafts. Some of these results can be understood quantitatively on the basis of the sticky hard-sphere model. Received: 3 January 2000 Accepted: 14 August 2000     

The influence of epoxy resin on the morphological and rheological properties of PET/PA66 blend

Interfacial reactions have dominant effects on the morphological and rheological properties of compatibilized polymer blends. This work aims to investigate the effect of epoxy resin, as a coupling agent, on the interface properties and subsequent influences on the morphological and rheological properties of polyethylene terephthalate/polyamide66 (PET/PA66) blend. PET/PA66 70/30 blends with different amount of bisphenol A epoxy resin (0, 1, 3, and 5 wt.%) were prepared. SEM micrographs show reduction in droplet size with increasing epoxy resin concentration, confirming the reactive compatibilizing effect of the epoxy resin. Reactions at the interface of the PET-EP-PA66 blend are confirmed by FTIR spectra. Shear viscosity results demonstrates that adding epoxy resin could suppress the interfacial slip at the blend interphase. Obtained results from storage modulus (G′) curves show the presence of one plateau for the blends at low frequency region; nevertheless, relaxation spectra indicate the presence of two more relaxation mechanisms than precursors which are related to the shape relaxation of droplets and the interface relaxation. The presence of the interface relaxation time in the blend without epoxy resin can prove the presence of reactions between two condensation polymers; however, adding the epoxy resin results in reducing both relaxation time and interfacial tension and increasing interfacial shear modulus. These observations indicate that the epoxy resin has been successful to boost the reactions at the interface between two polymers. Fitting the obtained experimental data using Palierne model indicates that the general Palierne model could describe rheological properties of the blends very well.     

Rheological and morphological properties of PA6/ECO nanocomposites

Polyamide-6/poly(epichlorohydrin - co - ethylene oxide) (PA6/ECO) nanocomposites were prepared with 6 wt.% organoclay and different ECO content from 5 to 40 wt.%, via two-step melt blending process. The effects of organoclay and rubber content on the morphological and rheological properties of samples have been studied. Samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and rheometry in small amplitude oscillatory shear. XRD results indicate that the nanoclay platelets are partially exfoliated in both PA6 and ECO phases. The higher rubber content of nanocomposite samples results in higher exfoliation degree of the nanoclay layers. SEM photomicrographs of samples show that the size of rubber droplets increases by the introducing of nanoclay. Oscillatory shear measurements show that the storage modulus of nanocomposite samples significantly increases in comparison with unfilled blends. The formation of physical network is the prime cause of such increase. Moreover, presence of nanoclay dramatically increases melt yield stress of the samples. Palierne emulsion model has been applied to predict the rheological behavior of unfilled blends. A quantitative agreement between Palierne model and those of experimental data is found for low ECO content samples.     

Interfacially active particles in droplet/matrix blends of model immiscible homopolymers: Particles can increase or decrease drop size

Particles have been shown to adsorb at the interface between immiscible homopolymer melts and to affect the morphology of blends of those homopolymers. We examined the effect of such interfacially active particles on the morphology of droplet/matrix blends of model immiscible homopolymers. Experiments were conducted on blends of polydimethylsiloxane and 1,4-polyisoprene blended in either a 20:80 or 80:20 weight ratio. The effects of three different particle types, fluoropolymer particles, iron particles, and iron oxyhydroxide particles, all at a loading of 0.5 vol.%, were examined by rheology and by direct flow visualization. Particles were found to significantly affect the strain recovery behavior of polymer blends, increasing or decreasing the ultimate recovery, slowing down or accelerating the recovery kinetics, and changing the dependence of these parameters on the applied stress prior to cessation of shear. These rheological observations were found to correlate reasonably well with particle-induced changes in drop size. The particles can both increase as well as decrease the drop size, depending on the particle type, as well as on which phase is continuous. The cases in which particles cause a decrease in drop size are analogous to the particle stabilization of “Pickering emulsions” well-known from the literature on oil/water systems. We hypothesize that cases in which particles increase drop size are analogous to the “bridging–dewetting” mechanism known in the aqueous foam literature.     

Interparticle interaction and gel characterization of PE microgels using small- and large-strain viscoelastic response

PE microgels were prepared from mechanical fragmentation and from immiscible blends of PS and PE. The surface topology of microgels obtained from mechanical fragmentation was hypothesized to consist of long linear PE chains that are capable of interparticle co-crystallization as suggested by low-strain oscillatory shear experiment results. To investigate this hypothesis, PE microgels with a smooth surface and a PS corona were prepared using immiscible blends of PE and PS, followed by removal of the PS matrix. The rheological response of suspensions of PE microgels with a PS corona in squalane was similar to suspensions of PE microgels with crystallizable surface chains whereby the system would gel and exhibit hysteresis upon a cooling and heating cycle. Suspensions of PE microgels without any surface chains, however, were reversible over multiple cooling and heating cycles. It was determined that the PS corona and the cross-link density of the microgels had an effect (p < 0.01) on the reversibility whereas the microgel concentration in the suspension did not (p = 0.82).     

Melt shear rheology of carbon nanofiber/polystyrene composites

The rheological behavior and morphology of carbon nanofiber/polystyrene (CNF/PS) composites in their melt phase have been characterized both through experimental measurements and modeling. Composites prepared in the two different processes of solvent casting and melt blending are contrasted; melt-blended and solvent-cast composites were each prepared with CNF loadings of 2, 5, and 10 wt%. A morphological study revealed that the melt blending process results in composites with shorter CNFs than in the solvent-cast composites, due to damage caused by the higher stresses the CNFs encounter in melt blending, and that both processes retain the diameter of the as-received CNFs. The addition of carbon nanofiber to the polystyrene through either melt blending or solvent casting increases the linear viscoelastic moduli, G′ and G″, and steady-state viscosity, η, in the melt phase monotonically with CNF concentration, more so in solvent cast composites with their longer CNFs. The melt phase of solvent-cast composites with higher CNF concentrations exhibit a plateau of the elastic modulus, G′, at low frequencies, an apparent yield stress, and large first normal stress difference, N ₁, at low strain rates, which can be attributed to contact-based network nanostructure formed by the long CNFs. A nanostructurally-based model for CNF/PS composites in their melt phase is presented which considers the composite system as rigid rods in a viscoelastic fluid matrix. Except for two coupling parameters, all material constants in the model for the composite systems are deduced from morphological and shear flow measurements of its separate nanofiber and polymer melt constituents of the composite. These two coupling parameters are polymer–fiber interaction parameter, σ, and interfiber interaction parameter, C _I. Through comparison with our experimental measurements of the composite systems, we deduce that σ is effectively 1 (corresponding to no polymer–fiber interaction) for all CNF/PS nanocomposites studied. The dependence of CNF orientation on strain rate which we observe in our experiments is captured in the model by considering the interfiber interaction parameter, C _I, as a function of strain rate. Applied to shear flows, the model predicts the melt-phase, steady-state viscosities, and normal stress differences of the CNF/PS composites as functions of shear rate, polymer matrix properties, fiber length, and mass concentration consistent with our experimental measurements.     

Particle-induced bridging in immiscible polymer blends

Pickering emulsions are emulsions whose drops are stabilized against coalescence by particles adsorbed at their interface. Recent research on oil/water/particle systems shows that particles can sometimes adsorb at two oil/water interfaces. Such “bridging particles” can glue together drops of oil in water or vice versa. We hypothesize that the same effect should apply in immiscible polymer blends with droplet-matrix morphologies, viz., added particles should glue together drops and give rise to particle-bridged drop clusters. We test this hypothesis in PIB-in-PDMS blends [PIB, poly(isobutylene); PDMS, poly(dimethylsiloxane)] with fumed silica particles. Direct visualization shows that the particles can indeed induce clustering of the drops, and the blends appear to show gel-like behavior. Such gel-like behavior is confirmed by dynamic oscillatory experiments. However, we are unable to conclusively attribute the gel-like behavior to droplet clustering: Association of the fumed silica particles in the bulk, which itself causes gel-like behavior, confounds the results and prevents clear analysis of the gluing effect of the particles. We conclude that PIB/PDMS/fumed silica is not a good model system, for studying particle-containing polymer blends. We instead propose that spherical monodisperse silica particles can offer a far more convenient model system, and provide direct visual evidence of gluing of PIB drops in a PDMS matrix.     

The Effect of Pore Structure on the Electrical Conductivity of Ti

Open-pore Ti foam samples with porosity in the range of 10–70% and average pore size of 150–400 μm was fabricated by powder metallurgy method using polymethyl methacrylate (PMMA) as space holder initially. The resulting foam is anisotropic: the pores are spheroidal, being shorter along the pressing direction than in the pressing plane. The pore anisotropy decreases as the size of the polymethyl methacrylate (PMMA) particles used increases and hence with pore size, which leads to a higher conductivity in the plane of the pressing. As the porosity increases, the conductivity of porous Ti decreases dramatically. The porosity e{\varepsilon} dependence of the electrical conductivity σ could be well described by Maxwell approximation, while the differential effective medium approximation is only suitable to porous Ti with finite size of 400 μm in the porosity range of 40–70%, i.e., high porosity metal with randomly oriented spheroids.     

Stress relaxation behavior of PMMA/PS polymer blends

In this work, the stress relaxation behavior of PMMA/PS blends, with or without random copolymer addition, submitted to step shear strain experiments in the linear and nonlinear regime was studied. The effect of blend composition (ranging from 10 to 30 wt.% of dispersed phase), viscosity ratio (ranging from 0.1 to 7.5), and random copolymer addition (for concentrations up to 8 wt.% with respect to the dispersed phase) was evaluated and correlated to the evolution of the morphology of the blends. All blends presented three relaxation stages: a first fast relaxation which was attributed to the relaxation of the pure phases, a second one which was characterized by the presence of a plateau, and a third fast one. The relaxation was shown to be faster for less extended and smaller droplets and to be influenced by coalescence for blends with a dispersed phase concentration larger than 20 wt.%. The relaxation of the blend was strongly influenced by the matrix viscosity. The addition of random copolymer resulted in a slower relaxation of the droplets.     

A unified model for polystyrene–nanorod and polystyrene–nanoplatelet melt composites

We present a unified constitutive model capable of predicting the steady shear rheology of polystyrene (PS)–nanoparticle melt composites, where particles can be rods, platelets, or any geometry in between, as validated against experimental measurements. The composite model incorporates the rheological properties of the polymer matrix, the aspect ratio and characteristic length scale of the nanoparticles, the orientation of the nanoparticles, hydrodynamic particle–particle interactions, the interaction between the nanoparticles and the polymer, and flow conditions of melt processing. We demonstrate that our constitutive model predicts both the steady rheology of PS–carbon nanofiber composites and the steady rheology of PS–nanoclay composites. Along with presenting the model and validating it against experimental measurements, we evaluate three different closure approximations, an important constitutive assumption in a kinetic theory model, for both polymer–nanoparticle systems. Both composite systems are most accurately modeled with a quadratic closure approximation.     

Load transfer in fibre-reinforced composites with viscoelastic matrix: an analytical study

In A fibre-reinforced 2D composite material with elastic fibres and viscoelastic, isotropic matrix is studied. Starting from the solution of a reference-problem with elastic matrix material the elastic matrix parameters are substituted by their viscoelastic correspondents in the Laplace domain. For simplification the time-dependent solution is approximated by using limiting value theorems that give information about the time-dependent solution for t → 0 and t → ∞. Then the method of asymptotically equivalent functions is used and illustrated with examples of a steel fibre in a PMMA matrix. The analytical solutions are compared with their numerical counterparts. In summary it can be stated that this paper is a further contribution to the vast literature about the application of the correspondence principle to the solution of special problems of the linear viscoelasticity.     

Electrical conductivity, phase behavior, and rheology of polypropylene/polystyrene blends with multi-walled carbon nanotube

The electrical, rheological properties and phase change behavior of polypropylene (PP)/polystyrene (PS) blends filled with multi-walled carbon nanotube (MWNT) were investigated. Two kinds of masterbatch were used to prepare ternary blends of PP, PS, and MWNT, and the effects of the kinds of masterbatch were confirmed by phase morphology of ternary blends and the distribution of MWNT. From thermodynamic analysis, MWNT is expected to locate in PS phase and it shows a good agreement with the TEM observations. The ternary composites show the lowest conductive percolation threshold and fine morphologies when most MWNT particles are located at the interface. Time sweep test were carried out to monitor the phase coalescence of the ternary blends and MWNT migration and agglomeration in the PS phase during annealing. The enhancement of thermal properties of MWNT-filled blends was also investigated by DSC and TGA.     

Rheology and morphology of polystyrene/polypropylene blends with in situ compatibilization

Rheology and flow-induced morphology were studied in immiscible polypropylene (PP)/polystyrene (PS) blends with a droplet–matrix microstructure. Two reactive precursors, maleic anhydride grafted PP and amino terminated PS, were added during the melt-mixing process to form a graft copolymer. The effects of both the amount of compatibilizer and the shear history on the rheological and morphological behavior were investigated systematically. Small amplitude oscillatory experiments and scanning electron microscopy were used to study the phase morphology. Shear history has an important effect on the morphology of the uncompatibilized blends. The droplet size refines with increasing shear rate. The decrease of this effect with increasing degrees of in situ compatibilization is mapped out. The results are discussed in terms of interfacial tension and the interfacial coverage. It turns out that most of the conclusions that were previously obtained on physically compatibilized blends are also valid for chemically compatibilized ones.     

Effect of maleic anhydride content on the rheology and phase behavior of poly(styrene-co -maleic anhydride)/ poly(methyl methacrylate) blends

 We investigated the thermo- rheological behavior of high glass transition, high molecular weight and small dynamic asymmetry blends of poly(styrene-co-maleic anhydride) (SMA) and poly (methyl methacrylate) (PMMA) with varying amounts of maleic anhydride (MA) content, namely 8 wt%, 14 wt% and 32 wt%, in the SMA component. The phase separation (binodal) temperature of each blend was determined rheologically using a combination of dynamic frequency and temperature sweeps in parallel plate geometry; it was marked by a change in slope of the elastic modulus and the occurrence of a peak in tan δ in temperature sweeps. Failure of the time-temperature superposition principle and observation of two peaks in the Cole-Cole plots corroborated these findings. The blends displayed lower critical solution temperature (LCST) behavior with the critical temperatures exhibiting a non-monotonic dependence on the MA content. From rheological and thermal measurements it was concluded that SMA/PMMA blends containing 14% MA were more miscible than those containing 8% or 32% MA, a finding attributed to the compositional dependence of the interplay between SMA-SMA and SMA-PMMA interactions in the different samples. MA also influenced the dynamic asymmetry and pretransitional concentration fluctuations. The phase diagrams corresponding to each blend were modeled using a two-parameter temperature dependent interaction parameter, based on the concept of generalized Gibbs free energy of mixing. The fitted values of interaction parameter were in good agreement with values calculated explicitly using the Flory-Huggins theory. Received: 16 February 2001 Accepted: 11 July 2001     

Effect of nanofillers on the phase separation and rheological properties of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends

The viscoelastic characteristics of the blends of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) (PMMA/SAN) were investigated at various temperatures below, near, and above the phase separation temperature. The investigated polymer system is characterized by a lower critical solution temperature. Rheological behavior of the blends in the region of a phase separation was compared with change of the light scattering intensity. The presence of nanofillers in the blend results in that the phase separation occurs at a higher temperature. At the isothermal conditions, the phase separation begins earlier and proceeds with a higher rate as compared with the same blend without filler. The results of the study show the considerable change of the viscoelastic characteristics of PMMA/SAN when the polymer system passes from the homogeneous state to the heterogeneous one. Such characteristics as the dependence of the storage modulus (G ^′) on the loss modulus (G ^″), the dependence of the loss viscosity (η ^″) on the dynamic viscosity (η ^′), the dependences of the complex viscosity (η*), and the free volume fraction (f) on the blend composition are the most sensitive to the phase separation. The phase separation affects the characteristics G ^′(ω), where ω is the frequency only in a low-frequency range. Temperatures of phase separation were estimated using dependence G ^′(T) at ω, which is the constant in the range of low frequencies.