Small-angle X-ray scattering and linear melt rheologyof poly(tert-butyl acrylate-g-styrene) graft copolymers

The morphology and rheological properties have been studied for poly(tert-butyl acrylate-g-styrene), PtBA-g-S, graft copolymers through the analysis of different samples that varied in the number of grafted repeat units. Small-angle X-ray scattering (SAXS) measurements confirmed phase separation in these samples, and showed the presence of disordered microdomains on those copolymers with a grafting number higher than 7. SAXS results have also shown a significant improvement of microdomain segregation with temperature manifesting considerable changes in both intensity, position and full-width half-maximum of the peak q*, which is attributed to the graft-like nature of these systems. The frequency dependence of the dynamic moduli revealed three relaxation mechanisms. The low-frequency (long time) relaxation was identified with the movement of the whole molecule, the relaxation at intermediate time is related to movement of the grafts, and the high-frequency relaxation is like that found in the transition zone of main chain of poly(tert-butyl acrylate), PtBA. The rheological measurements indicated that the introduction of a small amount of polystyrene, PS, grafts in the PtBA backbone is sufficient to modify mechanical behaviour at low frequencies. Comparison of the rheological properties of the graft copolymers with higher PS content showed that the observed changes in the viscoelastic behaviour under shear seems to be related to both the microphase separated microstructure and the number of grafts present in the copolymers.     

Rheological studies of hyaluronan solutions based on the scaling law and constitutive models

We have investigated the scaling relationship between rheological behavior and concentration for both salt-free and saline solutions of hyaluronan (HA), and adopted three viscoelastic constitutive models to predict the linear/non-linear viscoelastic behavior of these aqueous solutions of HA with different molecular weights at different concentrations up to 20 mg/ml. A series of concentration equations are obtained to describe the influence of HA concentration on solution viscosity. Corresponding to dilute and semi-dilute concentration region, salt-free HA solutions have scaling relationship between specific viscosity and HA concentration as η_sp ∼ c^1.0 and η_sp ∼ c^3.5, respectively, while for 0.15 M NaCl HA solutions, the scaling exponents are 1.5 and 4.2, respectively. Simulation results indicate that these constitutive models have good applicability to describe quantitatively the rheological properties of HA entangled solutions under either dynamic or steady shear flow. In addition, the plateau modulus scaling of HA solutions can be well described by the concentration-dependent length scale.     

Dynamic mechanical and rheological properties of metallocene-catalyzed long-chain-branched ethylene/propylene copolymers

The dynamic mechanical and rheological properties of five long-chain branched (LCB) and three linear ethylene/propylene (EP) copolymers were investigated and compared using a dynamic mechanical analyzer (DMA) and an oscillatory rheometer. The novel series of LCB EP copolymers were synthesized with a constrained geometry catalyst (CGC), [C₅Me₄(SiMe₂N^tBu)]TiMe₂, and had various propylene molar fractions of 0.01-0.11 and long-chain branch frequencies (LCBF) of 0.05-0.22. The linear EP copolymers were synthesized with an ansa-zirconocene catalyst, rac-Et(Ind)₂ZrCl₂ (EBI), and contained similar levels of propylene incorporation as the CGC copolymers, but no LCB. In dynamic mechanical analysis, the dynamic storage moduli (G′) and loss moduli (G″) of the copolymers decreased with an increase of propylene molar fraction. The α- and β-transitions of the CGC copolymers were overlaid with each other. High damping (tan δ) values were found with the CGC copolymers at temperatures below 0 °C. In oscillatory rheological analysis, compared to the linear EBI counterparts, the LCB CGC copolymer melts showed higher zero shear activation energies, broader plateaus of δ and larger elastic contributions, which are essential characteristics of LCB polymers. It was found that the long chain branching was the determining factor in controlling rheological properties of the polymer melts while the short chain branching from propylene incorporation played a decisive role in affecting dynamic mechanical properties. This work represents the first rheological evidence of LCB in EP copolymers synthesized with CGC.     

Rheological properties of guar and its methyl, hydroxypropyl and hydroxypropyl-methyl derivatives in semidilute and concentrated aqueous solutions

We report on a comparative study of the rheological properties of guar [GG], methyl guar [MG], hydroxypropyl guar [HPG] and hydroxypropyl-methyl guar [MHPG] polymers aqueous solutions in semidilute (both unentangled and entangled) and concentrated regimes, using oscillatory and steady-shear techniques. In the dilute regime, molecular weights and radii of gyration have been investigated by means of light scattering measurements.Data obtained from steady-shear rheology were satisfactorily analyzed according to Cross model and the effects of polymer concentration and temperature on the rheological behaviour of guar and guar derivatives have been investigated and discussed in terms of rheological parameters, such as the zero-shear viscosity η₀, the characteristic time τ and critical coil-overlap concentration C^∗.The storage and loss moduli of guar and guar derivatives aqueous solutions have been measured using angular frequencies in the range between 10⁻³ and 10 rad/s. The data have been analyzed using the “blob” model for semidilute solutions and the scaling approach proposed by Rubinstein, Dobrynin and Colby for concentrated solutions. These rheological parameters obey a time-concentration superposition principle, so that master curves can be constructed over a wide frequency range. Moreover, we show that, at lower temperatures, these systems behave as thermo-rheological simple systems, in that the oscillatory shear response at different temperatures can be superimposed according to the empirical time-temperature superposition principle. Although these systems can be conveniently described within a unifying scaling model, the behaviour of guar derivatives are somewhat different. At higher temperatures, relatively small deviations from the scaling behaviour of the storage modulus of MG and MHPG polymers were observed. These findings can be justified by a structural re-organization of the macromolecular network, due to the hydrophobic interactions.     

Microstructure and rheological properties of pH-responsive core-shell particles

The microstructure and rheological properties of core-shell pH-responsive microgels consisting of poly(methyl methacrylate) (PMMA) particles grafted with a soft layer of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with di-allyl phthalate (DAP) were examined using dynamic light scattering and rheological techniques. The validity and limitation of the semi-empirical approach to model charged soft microgel particles developed by our group were tested on this core-shell system. The viscosity data for three different core-shell particles showed excellent agreement with the modified Krieger-Dougherty (K-D) model. Good agreement was also observed when our semi-empirical approach was compared against a theoretical model, which confirmed the validity of the semi-empirical approach to model charged soft particles. In addition, we confirmed that the new scaling law which relates the swelling ratio Q of microgels as a function of neutralization degree, α, average number of monomers between two cross-links, N_x, molar fraction of acidic units, y and concentration of mobile counter-ions, CK⁺ and C⁺Na, represented as (Nx/c₀)(CK⁺+C⁺Na)Q+Q2/3 is proportional to yN_xα. All the core-shell data at varying ionic strength and mobile counter-ions concentrations fall onto a master curve.     

Development of Novel Amidosulfobetaine Surfactant–Polymer Systems for EOR Applications

Experimental studies were conducted to evaluate the thermal stability and rheological properties of novel surfactant–polymer (SP) systems for enhanced oil recovery applications. One in-house synthesized amphoteric amidosulfobetaine surfactant 3-(N-pentadecanamidopropyl-N,N-dimethylammonium)propanesulfonate and three different polymers were evaluated. Polymer A was a terpolymer of acrylamide, acrylamido tert-butyl sulfonate, and acrylic acid, whereas polymers B and C were terpolymers of acrylamide, N-vinylpyrrolidone, and acrylamido tert-butyl sulfonate with different anionicity. Long-term thermal stability of the surfactant was assessed using FTIR, ¹H NMR, and ¹³C NMR. The surfactant was compatible with seawater at 90 °C and no precipitation was observed. Structural analysis showed good thermal stability and no structural changes were observed after aging at 90 °C. The effects of surfactant concentration, shear rate, salinity, and polymer concentration on rheological properties of the SP systems were determined. Polymer A showed highest viscosity among the investigated polymers in deionized and seawater. The interactions between the surfactant and polymer A were assessed using rheological measurements. In the presence of salts, the viscosity of all three polymers reduced significantly as a result of charge screening. At low shear rates, the added surfactant slightly decreased the viscosity and storage modulus of polymer A. At high shear rates, the effect of the surfactant on the viscosity and storage modulus of polymer A was insignificant.     

Rheological and thermal properties of m-LLDPE blends with m-HDPE and LDPE

The dynamic and steady state behavior of metallocene linear low density polyethylene (m-LLDPE) blended with metallocene high density polyethylene (m-HDPE) and with low density polyethylene (LDPE) were measured in parallel plate rheometer at 160, 180, and 200 °C. The composition dependence of zero shear viscosity η₀, the characteristic relaxation time τ₀ and the characteristic frequency ω₀ of m-LLDPE/m-HDPE blends show a linear variation in the whole range of weight fraction, which indicates that m-LLDPE/m-HDPE blends are miscible blend. At the same time, m-HDPE showing a ‘dissident’ rheological behavior should possess a certain very low degree of LCB. Two calculation methods of LCB verify this point. In contrast, the composition dependence of zero shear viscosity η₀ of m-LLDPE/LDPE blends shows a positive deviation from the log-additivity rule, which can be well fitted by using the immiscible blend equation of Utracki. The characteristic relaxation time τ₀ and the characteristic frequency ω₀ have a sharp variation with the small amounts of LDPE in the blends, which also indicates a phase separation in the system. The thermal properties of m-LLDPE/m-HDPE blends are very similar to a single-component system. However, m-LLDPE/LDPE blends are immiscible in both melt and crystal states. DSC results are consistent with the rheological properties of these two series of blends.     

Gelation behavior of polyacrylonitrile solution in relation to aging process and gel concentration

Polyacrylonitrile (PAN) was dissolved in dimethyl sulfoxide (DMSO) to prepare PAN solutions with various concentrations, and physical gel can form in PAN solution either after a long period of aging or at a low temperature, via physical cross-linking of PAN molecules. Through rheological measurements, we investigated the viscoelastic properties of the PAN solutions both at aging process and frequency scanning (given concentration and temperature). First based on the Winter and Chambon theory, we measured the critical gel time and the corresponding loss tangent tan δ for the PAN solutions during the aging processes. Then the critical gel concentration at different temperatures was examined by tracing the dynamic rheological parameters in frequency scanning tests. Difference was found to exist between the critical n values obtained in aging processes (0.74-0.76) and frequency scanning tests (0.64-0.65). Our results also showed that the power law relationships, i.e., η₀∝?−γ and Ge∝?z, are valid within experimental error for the PAN solutions before and beyond the gel point.     

Rheological characterization of polystyrene-clay nanocomposites to compare the degree of exfoliation and dispersion

Polymer-clay nanocomposites are of great interest due to their improvement in certain material properties relative to virgin polymer or conventional composites. For example, compared to conventional materials, Nylon 6/montmorillonite nanocomposites demonstrated significant improvements, including high strength, high modulus and high heat distortion temperature. Because viscoelastic measurements are highly sensitive to the nanoscale and mesoscale structure of polymeric materials, when combined with X-ray scattering, electron microscopy, thermal analysis, and mechanical property measurements, they will provide fundamental understanding of the state and mechanism of exfoliation of the layered silicate (clay) in a polymer matrix. In addition, understanding rheological properties of polymer nanocomposites is crucial for application development and understanding polymer processability.The objective of this research is to develop a rheological technique to analyze the clay morphology in nanocomposite. Previous work has demonstrated the utility of the rheological technique to differentiate (qualify) the degree of exfoliation/dispersion. This report utilizes findings from the earlier work to further map out the structure-rheological response of polystyrene nanocomposites with various composition, clay types, and dispersion; and to quantify the key parameter that dominates the characteristic rheological response. This report explored a series of polystyrene (PS)-clay nanocomposites with 1,2-dimethyl-3-n-hexadecyl imidazolium (DMHDI) organically modified clays. These PS nanocomposites investigated here demonstrated a change of pattern in dynamic mechanical spectrum, as a function of the degree of exfoliation, from typical polymer response to a terminal response of [G′∼ω, G″∼ω], then to a pattern with double crossover frequencies, and finally to a solid-like response with G′>G″ in all frequency ranges. We showed that the number of particles per unit volume is a key factor determining the characteristic response of nanocomposites.In addition, the rheological response of PS-clays nanocomposite made from DMHDI modified clay combined with high-energy sonication (characterized as exfoliated by XRD and TEM) was compared with that of nanocomposites made by dimethyl, benzyl hydrogenated tallow (2MBHT) modified clay. We found that PS nanocomposites made by DMHDI-modified clay with high-energy sonication are better dispersed than the nanocomposites made previously using 2MBHT-modified clay. We also showed that the glass transition temperatures were not very sensitive to the degree of dispersion.The key finding of this research is that rheological measurements are complimentary to traditional polymer nanocomposite analysis techniques, and they may also serve as an analytical tool by itself (under appropriate conditions), now that some fundamental behavior has been identified.     

Rheological behavior of cement pastes under Large Amplitude Oscillatory Shear

Cement pastes exhibit virtually all the rheological features of complex fluids. Thus, several rheological methods and setups have been used in the literature to characterize these materials. In the present investigation Large Amplitude Oscillatory Shear (LAOS) is for the first time exploited for cement pastes. LAOS can be used to characterize all the rheological properties within a single procedure. This technique is tested in the case of three different cement mixes: a Portland cement paste, nanoclay blended cement paste and a cement paste containing a hydro-soluble polymer. These mixes were selected in order to get rheological properties that are different both quantitatively and qualitatively. Indeed, addition of a low amount of nanoclay increased significantly the yield stress and the shear-thinning/thixotropic aspects of the cement paste, whereas addition of cellulose ether led to the decrease of yield stress and thixotropy. These non-linear rheological properties are discussed within the framework of LAOS.     

Influence of Chemical Extraction on Rheological Behavior,Viscoelastic Properties and Functional Characteristics of Natural Heteropolysaccharide/Protein Polymer from Durio zibethinus Seed

In recent years, the demand for a natural plant-based polymer with potential functions from plant sources has increased considerably. The main objective of the current study was to study the effect of chemical extraction conditions on the rheological and functional properties of the heteropolysaccharide/protein biopolymer from durian (Durio zibethinus) seed. The efficiency of different extraction conditions was determined by assessing the extraction yield, protein content, solubility, rheological properties and viscoelastic behavior of the natural polymer from durian seed. The present study revealed that the soaking process had a more significant (p < 0.05) effect than the decolorizing process on the rheological and functional properties of the natural polymer. The considerable changes in the rheological and functional properties of the natural polymer could be due to the significant (p < 0.05) effect of the chemical extraction variables on the protein fraction present in the molecular structure of the natural polymer from durian seed. The natural polymer from durian seed had a more elastic (or gel like) behavior compared to the viscous (liquid like) behavior at low frequency. The present study revealed that the natural heteropolysaccharide/protein polymer from durian seed had a relatively low solubility ranging from 9.1% to 36.0%. This might be due to the presence of impurities, insoluble matter and large particles present in the chemical structure of the natural polymer from durian seed.     

Study on degradation and crosslinking of impact polypropylene copolymer by dynamic rheological measurement

The structural stability of impact polypropylene copolymer (IPC) melt under high temperatures was explored by dynamic rheological measurement. The structure changes including degradation and crosslinking of IPC were discussed through examining the influence of temperature and additive antioxidant on dynamic rheological functions. A plateau of dynamic storage modulus (G′) appeared in low frequency region at high temperatures under air atmosphere. Furthermore, when IPC sample was annealed at 230 °C, its viscoelasticity presented a dramatic change. The time dependences of G′ for pure IPC at different temperatures were investigated. At 190 °C, a slight drop and a succedent rise of G′ for IPC0 appeared. The decrease period of G′ generally shorted and the increase of G′ became more remarkable with the increase of temperature. The decrease of G′ was ascribed to the degradation in IPC and the G′ increase was due to the crosslink reaction. Through incorporation of antioxidant into IPC, the crosslinking in IPC could be effectively inhibited, and the degradation and crosslinking in IPC were believed to result from PP component.     

α-Tocopherol-induced radical scavenging activity in carbon nanotubes for thermo-oxidation resistant ultra-high molecular weight polyethylene-based nanocomposites

α-Tocopherol, a natural antioxidant molecule, was physically immobilized on the outer surface of multi-walled carbon nanotubes (CNTs), and the resulting functionalised particles (f-CNTs) were dispersed in ultra-high molecular weight polyethylene aiming at improving its thermo-oxidation resistance. The success of the functionalization was assessed through spectroscopic and thermal analysis, and the influence of the filler on the thermo-oxidative stability of the nanocomposites was investigated through rheological analyses and infrared spectroscopy. We found that the addition of only 1 wt.% of f-CNTs brings about a surprisingly high oxidation resistance, with a five/ten-fold increase of the induction time of the degradation phenomena. Rather than to the inherent stabilizing action of the α-tocopherol, such a notable result is believed to be due to its specific chemical interactions with the CNTs, which could exhibit a considerable radical scavenging activity due to the formation of structural defects on their outer surface. The latter represent acceptor-like localized states, which radically improve the thermo-oxidative resistance of the f-CNTs-based polymer nanocomposites.     

Mechanical and rheological behavior of pNIPAAM crosslinked macrohydrogel

In this study for the first time we investigate the most common reticulated N-isopropylacrylamide (pNIPAAM) macrohydrogel for both its mechanical response and shear rheological behavior in time and frequency domains. Hydrogels are characterized by water content volume and weight measurements, FT-IR spectroscopy, scanning electron microscopy and reflecting index. Compressive uniaxial tests on equilibrated hydrogels individuate a hookean response within a 30% strain range with E_c modulus of 12.2 kPa, and a neo-hookean response within a 79% strain range which upper limit corresponds to material rupture with G_c modulus of 3.8 kPa. Tensile experiments performed for the first time on the pure material evidence a rupture limit for a strain around 30% with hookean modulus E_t of 24.8 kPa and neo-hookean modulus G_t of 7.3 kPa. Rheological studies, carried out in linear response regime around the hydrogel swelling–deswelling transition, report relaxation times of the kinetics towards the equilibrium at different temperatures. The phase transition of pNIPAAM is monitored and the transition temperature is determined following the temperature dependence of the shear modulus. We apply different literature models to the rheological response and to the swelling–deswelling transition of the hydrogel. Finally, we analyze the results providing values for microscopic material parameters such as crosslink density and mesh size.     

Modeling rheological behavior of bentonite suspensions as Casson and Robertson-Stiff fluids using Newtonian and true shear rates in Couette viscometry

The Casson model and the Robertson-Stiff model have been used to determine whether they can describe the rheology of aqueous bentonite suspensions. The assessment utilized a total of twelve sets of experimental viscometric data from literature and from this work. Equations have been presented which allowed the determination of the true shear rates experienced by the fluids within the gap of the rotational viscometer for both rheological models. Non-linear regression has been applied to determine the two rheological parameters for the Casson model and the three rheological parameters for the Robertson-Stiff model using true shear rates and Newtonian shear rates, which are used most often in the analysis of rheometric data. The results showed that both models describe well the experimental data of these bentonite suspensions with good statistical indicators. Furthermore, analysis showed that true shear rates are always higher than Newtonian shear rates for both models. The differences depend on the particular suspension and are larger at low shear rates while they become smaller at higher shear rates indicating that the fluid behavior approaches Newtonian behavior at higher shear rates. The shapes of the rheograms remained essentially unchanged indicating that the rheological parameters determined with the use of true shear rates are very similar to the rheological parameters determined with the use of Newtonian shear rates. This was further confirmed with the computation of the rheological parameters for both models and both approaches. For the Casson model differences in the yield value computed with true shear rates were at most at 7% while for the plastic viscosity at 3%. For the Robertson-Stiff model, differences of the order of 2 to 5% were observed for the K-values, of 7% for γ˙₀-values while no differences were observed for the n-values. These small differences, however, do not justify use of Newtonian shear rates when analytical solutions exist which allow use of true shear rates without any compromise.     

Influence of selective filling on rheological properties and phase inversion of two-phase polymer blends

We investigate the phase inversion of selectively filled polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends prepared by melt mixing. As we have shown by transmission electron microscopy analysis and by the calculation of different selectivity criteria, the filler used (glass spheres of submicron dimensions) resides exclusively in the PMMA-phase and modifies its rheological properties correspondingly.Four blend series made of PS and PMMA with different filler content are analyzed for the location of phase inversion concentration, φ_PI, and the width of the cocontinuity interval, CCI, in the concentration range where cocontinuity is predominant. Upon addition of filler a shift of φ_PI to higher concentrations of the filled PMMA-phase has been observed. This shift is in agreement with the predictions of a recently proposed equation defining the phase inversion concentration as that concentration where the maximum of the blends' extra elasticity occurs. The qualitative morphological analysis of these blends confirms this result.Moreover, it was found that the cocontinuity interval is widened significantly at increased filler content. We explain the appearance of a wider CCI for filled blends on the basis of a slowdown of processes leading to morphology destruction. Corresponding Tomotika experiments (stability of fibrils) substantiate these findings.     

Evolution of morphological and rheological properties along the extruder length for blends of a commercial liquid crystalline polymer and polypropylene

The main aim of this work is to study the morphological and rheological evolution during extrusion of blends of polypropylene and a commercial liquid crystalline polymer, Rodrun LC3000. This study was performed for blends with 10, 20 and 40 wt% LCP, processed at 220 and 240 °C. For this purpose, a special collecting device was used that allows the removal of samples at different locations along the extruder length. The results of the morphological study revealed that the mean diameter of the LCP structures decrease as they proceed along the extruder axis. The rheological properties were also studied ex situ both in the linear and in the non-linear regime. From the results obtained in oscillatory shear it was observed that the elastic modulus and the complex viscosity decreases from the beginning to the end of the extrusion process, reaching a minimum for the final extrudate. FT rheology was used to study the non-linear viscoelastic behavior and the results also show marked difference between samples collected at various stages of the process, the ratio I(3ω₁)/I(ω₁) decreasing along the extruder.     

Rheological properties of ABA telechelic polyelectrolyte and ABA polyampholyte reversible hydrogels: A comparative study

Two types of reversible hydrogels formed by poly(t-butyl acrylate)-poly(2-vinyl pyridine)-poly(t-butyl acrylate) (PtBA-P2VP-PtBA) and poly(acrylic acid)-poly(2-vinyl pyridine)-poly(acrylic acid) (PAA-P2VP-PAA), named telechelic polyelectrolyte and block polyampholyte, respectively, of the same degree of polymerization were studied in aqueous solutions at pH 3.7 in terms of their rheological properties. The different structural characteristics of the formed 3D networks that arise from hydrophobic interactions of the telechelic polyelectrolyte and electrostatic interactions of the polyampholyte, lead to significant different rheological properties. The results tend to show that a thermo-sensitive weak hydrogel is formed by the polyampholyte while a stiff, but fragile, hydrogel is formed by the telechelic polyelectrolyte.     

Flow properties of homogeneously aerated,expanded emulsion phase of fine powders (quasi-solid emulsion phase viscosity)

The viscosity of quasi-solid emulsion phase (μ_e) of the “homogeneously aerated, expanded emulsion phase” (HAE emulsion phase) of fine powders within the gas velocity range of minimum fluidization (U_mf) and minimum bubbling (U_mb), was obtained at ambient and elevated temperatures by measuring the strain retardation time (τ) of the HAE emulsion phase in this study.The deformation coefficient (Y) was obtained by our previously developed experimental method [Powder Technol., 81 (1994) 177].We confirmed experimentally that the HAE emulsion phase behaves exactly as quasi-elastic body, by observing the emulsion phase volume expansion and contraction within the velocity range of U_mf and U_mb. It is to be noted that U_mb>U_mf. The volume element of the unit HAE emulsion phase could be expanded or contracted by the force balance between the excessive gas drag force (above the gravitational force) and the inter-particle force of the particles of the emulsion phase. This experimental evidence enabled us to decide using the visco-elastic rheological model of the Voigt–Kelvin model body (VK model body) in terms of mathematics to analyze our experimental data. The rheological parameters, i.e., the elastic deformation coefficient (Y) and quasi-solid viscosity (μ_e) of our experiments, were obtained by using the HAE emulsion phase.Using these experimental data together with the VK model body, we developed our experimental model equation including the measured strain retardation time (τ) to obtain the “quasi-solid viscosity” of the HAE emulsion phase (μ_e). The physical meaning of this viscosity of the HAE emulsion phase developed in this study is quite different from the “apparent quasi-liquid viscosity” defined in aggregative fluidized beds, under the condition of U_mf=U_mb.The interesting experimental data correlations of comprehensive rheological parameters (μ_e, Y, σ_f) of the HAE emulsion were obtained for fine powders at ambient and elevated temperatures.