Polymer Concentration Regime Research Articles

Polymer concentration regimes from fractional microrheology

In this work, a framework for deriving theoretical equations for mean squared displacement (MSD) and fractional Fokker–Planck is developed for any arbitrary rheological model. The obtained general results are then specified for different fractional rheological models. To test the novel equations extracted from our framework and bridge the gap between microrheology and fractional rheological models, microrheology of polystyrene in tetrahydrofuran solutions at several polymer concentrations is measured. By comparing the experimental and theoretical MSDs, we find the fractional rheological parameters and demonstrate for the first time that the polymer concentration regimes can be distinguished using the fractional exponent and relaxation time data because of the existence of a distinct behavior in each regime. We suggest simple approximations for the critical overlap concentration and the shear viscosity of viscoelastic liquidlike solutions. This work provides a more sensitive approach for distinguishing different polymer concentration regimes and measuring the critical overlap concentration and shear viscosity of polymeric solutions, which is useful when conventional rheological characterization methods are unreliable due to the volatility and low viscosity of the samples.

Binding and stability properties of PEG2000 to globular proteins: The case of lysozyme

• Lysozyme-PEG interaction was studied using calorimetry and spectroscopic techniques. • A PEG/lysozyme interaction in the low polymer concentration regime occurs. • The interaction process is weak, and it is entropically driven. • At low concentration, PEG induces an increase of the denaturation temperature. Poly(ethylene glycol) is a linear polymer widely used in many biotechnological and industrial applications. It is thought that PEG is an inert polymer and its effect on protein stability and dynamics is essentially due to the excluded volume effect which depends on its concentration. However, increasing evidence suggests that direct interactions can be established between PEG and proteins. With the aim to shed more light on PEG-protein interactions, this work evaluated the enthalpy pair interaction coefficient and the enthalpy cross-interaction coefficient for the system PEG2000-lysozyme. In addition, using calorimetry and spectroscopy, the binding, the stability and the conformational behaviour of lysozyme were evaluated in the low polymer concentration regime. We found that a weak interaction between lysozyme and PEG occurs with a binding constant of 1600 ± 230 M −1 . The interaction process is mainly entropically driven: the release of water molecules associated to protein surface and polymer chains may account for this phenomenon. In addition, we found that the general lysozyme structure was only marginally affected by PEG interaction. A small but significant increase in the denaturation temperature was observed most likely due to weak interactions with the polymer and to the dehydration of protein surface.

Determination of a critical separation concentration for associative polymers in porous media based on quantification of dilute and semi-dilute concentration regimes

Hydrophobic interactions are an inherent property of associative polymers which results in the formation of molecular aggregates. The loss of hydrophobic interactions under reservoir conditions have been reported, and this results in increased fluid – rock interaction effect such as adsorption. However, existing adsorption studies only reports the interaction of individual molecules with rock surface without taking into account mechanically retained molecular aggregates in narrow pores. The implication of this is a reduction in associative effect between polymer molecules during transport in a porous media. In this work, the minimization of this phenomenon and sustenance of the interaction network was studied through a theoretically defined dimensionless parameter for the quantification of molecular interactions in the various polymer concentration regimes. It was established in this work that associative polymers exhibit a critical separation concentration during propagation in a porous media under given reservoir conditions. This critical separation concentration marks the concentration beyond which large molecular aggregates constituting the hydrophobic network is sterically excluded and retained in narrow pore spaces. This separation phenomenon was predicted to occur when the proportion of molecular interactions arising from hydrophobic and intramolecular interactions are equal. Thus, the balance in the molecular interactions in the semi – dilute concentration regime was identified as key in minimizing the loss of hydrophobic interactions to aggregate retention and optimizing its sustainability in a porous media. Consequently, a novel approach was developed based on this knowledge for the transport of associative polymers at which these hydrophobic interactions are sustained. It was demonstrated that propagating associative polymers using this approach ensured the sustainability at concentrations below the critical separation concentration tend to maximize the hydrophobic interaction network with minimal loss of polymer chains retention mechanisms.

Crowding and Confinement Effects in Different Polymer Concentration Regimes and Their Roles in Regulating the Growth of Nanotubes

Among numerous research studies focusing on macromolecular crowding and confinement, few are concerned with the medium at a broad range of concentrations, let alone the interplay between the crowdi...

Insights on the interaction between sodium dodecyl sulfate and partially hydrolyzed microblock hydrophobically associating polyacrylamides in different polymer concentration regimes

The partially hydrolyzed microblock hydrophobically associating polyacrylamides (HMBHAPs) with different microblock lengths (NH) and some positive charges were prepared by free radical copolymerization in aqueous solution and the HMBHAPs were characterized by FTIR, 1H-NMR, ion titration, elemental analysis and thermal gravimetric analysis. Moreover, the hydrodynamic size and microtopography of HMBHAPs were also characterized by dynamic light scattering and environment scanning electron microscope. The self-association effect of HMBHAPs and the interaction between HMBHAPs and oppositely charged sodium dodecyl sulfate (SDS) in different polymer concentration regimes were investigated through the measurements involving rheology, fluorescence, conductivity and zeta potential. The results show that HMBHAPs presented different viscosity behaviors in the concentration range concerned that could be divided into three different concentration regimes, where the viscosity contribution made by association effect was significantly discrepant and enhanced with increasing microblock length. Moreover, rheological measurements indicate that, with the addition of SDS, zero-shear viscosity (η0) of HMBHAPs presented different variation trends in three polymer concentration regimes: at lower HMBHAPs concentration, the η0 of HMBHAPs/SDS system was smaller than that in absence of SDS; at moderate HMBHAPs concentration, the system viscosity increased slightly; whereas, the viscosity distinctly increased with increasing SDS concentration at higher HMBHAPs concentration. The I3/I1 and Ie/Im also demonstrated that the aforementioned viscosity behaviors were closely related to the efficiency and number of the associations, as well as the micro-block longer the association effect stronger. In addition, η0 and Ie/Im exhibited a very interesting “double peaks” phenomenon with increasing SDS, which may be interpreted by the simultaneous influence of electrostatic interaction and the association effect between cationic hydrophobic groups and anionic surfactant. The SDS amounts binded by HMBHAPs showed a slightly increasing trend as increasing polymer concentration and decreasing NH. Zeta potentials at n-octane-water interface showed a “charge reversal” phenomenon with increasing HMBHAPs concentration, which appeared earlier for longer microblock. With the addition of SDS, Zeta potential rapidly varied from an initial smaller positive charge to a greater negative charge, and the variation presented earlier with varied polymer concentration regimes and increasing NH. This result may be ascribed to the adsorption of HMBHAPs macromolecules at the oil-water interface and different inter-molecular conformations after interacting with SDS at the interface.

Production of polyamide 11 microparticles for Additive Manufacturing by liquid-liquid phase separation and precipitation

Within this contribution liquid-liquid phase separation (LLPS) and precipitation from ethanol was studied as an approach to produce polyamide 11 (PA11) powders for selective laser sintering (SLS). To this end, the cloud point and solution temperature curve of the PA11-ethanol system was determined experimentally via turbidity measurements. The proper range of system composition and temperature for particle formation was deduced. The dependence of particle characteristics on process parameters (polymer concentration, stirring conditions and temperature regime) during LLPS and precipitation was assessed and the products were characterized with respect to their size and morphology. Furthermore, structural, rheological and thermal characteristics were analyzed and correlated with process parameters. Molecular weight distributions were determined. After removal of fines and dry coating with hydrophobic fumed silica, an optimized PA11 powder with mean particle size of 91 µm showing good flowability for SLS was obtained. SLS processability of this optimized PA11 powder was demonstrated by building multi-layered test specimens in a laser sintering machine. With this contribution, we present a comprehensive workflow for the process development, product characterization and product application of a SLS powder manufactured via precipitation.

Non-adsorbing polymers and yield stress of cement paste: Effect of depletion forces

In this work, we study the effect of non-adsorbing polymers with molar masses varying over several decades in both dilute and semi-dilute regimes on the yield stress of cement and calcite pastes. In both suspensions, we measure an increase of yield stress, the magnitude of which mainly depends on the polymer concentration regimes. From interparticle force calculations, we show that non-adsorbing polymers are at the origin of interparticle attractive depletion forces. In the absence of superplasticizers, Van der Waals attractive forces are the dominant flocculating mechanism in the system and depletion forces resulting from non-adsorbing polymers are negligible. The repulsive forces that result from superplasticizers' steric contribution increase the average interparticle distance and therefore decrease the magnitude of the attractive van der Waals forces. In this latter system, attractive depletion forces induced by non-adsorbing polymers become important and greatly contribute to the flocculation mechanism.

Conditions Leading to Polyelectrolyte Complex Overcharging in Solution: Complexation of Poly(acrylate) Anion with Poly(allylammonium) Cation

Complexation between poly(acrylate) (PA) and poly(allylammonium) (PAH) macroions at pH = 7.0 was studied by means of electrokinetics, microcalorimetry, and DLS. At low polyelectrolyte concentrations and no electrolyte present strong overcharging of primary nanocomplexes occurred. In contrast, the increase in polyelectrolyte concentration led to flocculation taking place near the equivalence. The nanocomplex charge reversal was also achieved in the high polymer concentration regime by the abrupt instead of stepwise titrant addition. The procedure was successfully used in the case of several other polyion pairs. The presence of electrolyte affected the PAH–PA interpolyelectrolyte neutralization considerably, leading to ion-specific aggregation and extrinsic charge compensation. The complexation energetics was weakly influenced by ionic conditions. To the best of our knowledge, the reported results are the first direct evidence for primary complex overcharging. Their rationalization explains why interpolyele...

Polymer concentration regulated aging in aqueous Laponite suspensions

By rheologically examining poly(vinyl alcohol)-water-Laponite multicomponent suspensions, we report evidence of concentration of adsorbing polymer strictly regulating the aging dynamics in multicomponent Laponite suspensions. The study is performed in the dilute polymer concentration regime. We analyze the observed aging pattern by fixing the non-monotonic transitions observed between temporally achieved suspension elastic moduli and polymer concentration, as the border of a sub-regime. This divides the aging pattern into three sub-regimes, across which the suspension elastic modulus shows an oscillatory relationship with polymer concentration. We also observe that suspensions belonging to first two sub-regimes remained colloidally stable during the timescale of measurement. In the third sub-regime, macrophase separation occurred, while suspensions were aging. For an explanation to this observation, we conducted three complementary measurements on select suspensions from each sub-regime, after they attained structurally arrested state. They are (a) small-angle neutron scattering, (b) structure recovery upon shear melting, and (c) zeta potential estimation. The collective evidences are analyzed to reach a possible explanation. The observation and analysis highlight the effect of concentration of adsorbing polymer on aging clay suspensions and have direct applications in stabilization, rheology modification, as well as polymer nanocomposite preparation.

Humidity-dependent compression-induced glass transition of the air–water interfacial Langmuir films of poly(d,l-lactic acid-ran-glycolic acid) (PLGA)

Constant rate compression isotherms of the air-water interfacial Langmuir films of poly(D,L-lactic acid-ran-glycolic acid) (PLGA) show a distinct feature of an exponential increase in surface pressure in the high surface polymer concentration regime. We have previously demonstrated that this abrupt increase in surface pressure is linked to the glass transition of the polymer film, but the detailed mechanism of this process is not fully understood. In order to obtain a molecular-level understanding of this behavior, we performed extensive characterizations of the surface mechanical, structural and rheological properties of Langmuir PLGA films at the air-water interface, using combined experimental techniques including the Langmuir film balance, X-ray reflectivity and double-wall-ring interfacial rheometry methods. We observed that the mechanical and structural responses of the Langmuir PLGA films are significantly dependent on the rate of film compression; the glass transition was induced in the PLGA film only at fast compression rates. Surprisingly, we found that this deformation rate dependence is also dependent on the humidity of the environment. With water acting as a plasticizer for the PLGA material, the diffusion of water molecules through the PLGA film seems to be the key factor in the determination of the glass transformation properties and thus the mechanical response of the PLGA film against lateral compression. Based on our combined results, we hypothesize the following mechanism for the compression-induced glass transformation of the Langmuir PLGA film; (1) initially, a humidified/non-glassy PLGA film is formed in the full surface-coverage region (where the surface pressure shows a plateau) during compression; (2) further compression leads to the collapse of the PLGA chains and the formation of new surfaces on the air side of the film, and this newly formed top layer of the PLGA film is transiently glassy in character because the water evaporation rate in the top surface region is momentarily faster than the humidification rate (due to the initial roughness of the newly formed surface); (3) after some time, the top layer itself becomes humidified through diffusion of water from the subphase, and thus it becomes non-glassy, leading to the relaxation of the applied compressive stress.

Structure and interaction in the polymer-dependent reentrant phase behavior of a charged nanoparticle solution.

Small-angle neutron scattering (SANS) studies have been carried out to examine the evolution of interaction and structure in a nanoparticle (silica)-polymer (polyethylene glycol) system. The nanoparticle-polymer solution interestingly shows a reentrant phase behavior where the one-phase charged stabilized nanoparticles go through a two-phase system (nanoparticle aggregation) and back to one-phase as a function of polymer concentration. Such phase behavior arises because of the nonadsorption of polymer on nanoparticles and is governed by the interplay of polymer-induced attractive depletion with repulsive nanoparticle-nanoparticle electrostatic and polymer-polymer interactions in different polymer concentration regimes. At low polymer concentrations, the electrostatic repulsion dominates over the depletion attraction. However, the increase in polymer concentration enhances the depletion attraction to give rise to the nanoparticle aggregation in the two-phase system. Further, the polymer-polymer repulsion at high polymer concentrations is believed to be responsible for the reentrance to one-phase behavior. The SANS data in polymer contrast-matched conditions have been modeled by a two-Yukawa potential accounting for both repulsive and attractive parts of total interaction potential between nanoparticles. Both of these interactions (repulsive and attractive) are found to be long range. The magnitude and the range of the depletion interaction increase with the polymer concentration leading to nanoparticle clustering. At higher polymer concentrations, the increased polymer-polymer repulsion reduces the depletion interaction leading to reentrant phase behavior. The nanoparticle clusters in the two-phase system are characterized by the surface fractal with simple cubic packing of nanoparticles within the clusters. The effect of varying ionic strength and polymer size in tuning the interaction has also been examined.

Tailoring Polyacrylonitrile Interfacial Morphological Structure by Crystallization in the Presence of Single-Wall Carbon Nanotubes

In order to improve stress transfer between polymer matrixes and nanofillers, controlling the structure development in the interphase region during composite processing is a necessity. For polyacrylonitrile (PAN)/single-wall carbon nanotubes (SWNT) composites, the formation of the PAN interphase in the presence of the SWNT as a function of processing conditions is studied. Under these conditions, three distinct interfacial coating morphologies of PAN are observed on SWNT. In the semidilute polymer concentration regime subjected to shearing, PAN extended-chain tubular coatings are formed on SWNT. Dilute PAN/SWNT quiescent solutions subjected to cooling yields hybrid periodic shish-kebab structures (first observation for PAN polymer), and dilute PAN/SWNT quiescent solutions subjected to rapid cooling results in the formation of an irregular PAN crystalline coating on the SWNT.

Flavour release study as a way to explain xanthan–galactomannan interactions

In spite of numerous studies and abundant literature, xanthan(X)/galactomannan(GM) interactions still remain controversial while such mixtures have been intensively employed for a long time. The present work aims to bring new highlights on the nature of the interactions involved between both polysaccharides through an original approach: the behaviour of small volatile solutes in water-based X/GM solutions and mixtures. To this aim, a study of limonene release from X/guar (X/G) and X/carob (X/C) water-based mixed solutions has been investigated in the dilute and semi-dilute polymer concentration regimes. The aroma compound release from both matrices was compared together with the rheological properties of the solutions. When considering the limonene release as a function of the concentration regime, a dramatic decrease of the limonene release was observed for the X/C mixtures in the neighbourhood and above the critical overlap concentration C*. This result combined with kinetic experiments allowed to state the occurrence of specific interactions between limonene and polymeric junction zones formed in the X/C matrices. The apolar nature of the aroma compounds used allows to consider the nature by itself of the interactions responsible for volatile retention to be hydrophobic. A different mechanism for synergistic interactions between X and G was observed. The synergy appears quite lower than X/C mixtures and can be explained by phase separation occurring in the X/G mixture, favoured by ordered conformation of X.

Direct Comparison of Atomic Force Microscopic and Total Internal Reflection Microscopic Measurements in the Presence of Nonadsorbing Polyelectrolytes

We have investigated the structural and depletion forces between silica glass surfaces in aqueous, salt-free solutions of sodium poly(styrene sulfonate). The interaction forces were investigated by two techniques: total internal reflectance microscopy (TIRM) and colloid probe atomic force microscopy (AFM). The TIRM technique measures the potential energy of interaction directly, while the AFM is a force balance. Comparison between the data sets was used to independently calibrate the AFM data since the separation distances cannot be unequivocally determined by this technique. Oscillatory structural forces are excellent for this work since they give multiple reference points against which to analyze. Comparison of the data from the two techniques highlighted significant uncertainties in the AFM data. At low polymer concentrations, a significant uncertainty in the apparent zero separation distance was seen as a result of the AFM cantilever reaching an apparent constant compliance region prior to any real contact between the surfaces. Further complications arising from the number and position of the measured minima were also seen in the dilute polymer concentration regime as a result of hydrodynamic drainage between the approaching surfaces in the AFM perturbing the delicate structural components in the fluid.

The effect of solution surface tension on aroma compound release from aqueous xanthan solutions

This study aimed to elucidate the mechanisms governing the release of limonene from model matrices in aqueous xanthan solutions. The aroma compound release behaviour from the matrices has been linked with both the rheological and the surface tension properties of the solutions. Considering the limonene release as a function of the polymer concentration regime, a significant rise in limonene retention is observed above the critical overlap concentration, C*, of the biopolymer. Increasing the amount of xanthan involves a marked decrease in limonene release until the concentrated regime is reached (above C**), at which the limonene release is constant whatever the polymer content. In parallel, solution surface tension properties allow us to state the establishment of intra- and intermolecular non-polar interactions of xanthan molecules above C*. As such molecular interaction may explain the change in limonene behaviour, these preliminary results contribute to a better understanding of the phenomena governing the retention/release of the volatile from such xanthan-containing media.

Phase behavior of cationic hydroxyethyl cellulose-sodium dodecyl sulfate mixtures: effects of molecular weight and ethylene oxide side chain length of polymers.

Novel cationic hydroxyethyl cellulose (HEC) polymers with different molecular weights (1.1 x 10(5) to 1.7 x 10(6) g/mol) and ethylene oxide (EO) side chain lengths (1.5-2.9 EO units) were mixed with sodium dodecyl sulfate (SDS) in aqueous solutions. The phase diagrams of cationic HEC-SDS complexes were determined in the dilute polymer concentration regime (< 0.5 wt %) with gradual addition of SDS molecules. The viscosity and structures of the complexes during the phase evolution were studied using rheometry and dynamic light scattering. The gradual addition of SDS first induced interchain associations with the bound SDS aggregates serving as cross-linkers to form an open network structure, producing a very broad size distribution and high viscosities of the complex solutions, and then condensed the network and induced a structure reorganization, resulting in globular aggregates with narrow size distributions. The growth of these globular aggregates in size eventually led to macroscopic sedimentation near charge neutralization. Further addition of SDS randomly broke the sedimentary aggregates into small particles and SDS micelles with low solution viscosities. The effects of molecular weight and EO side chain length of polymers on the phase boundary, viscosity, and structure of cationic HEC-SDS complexes were discussed.

Linear and Nonlinear Viscoelasticity of Semidilute Aqueous Mixtures of a Nonionic Cellulose Derivative and Ionic Surfactants

Linear and nonlinear viscoelasticity of aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) in the presence of a surfactant (sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB)) have been examined over an extended polymer concentration regime at different surfactant-to-polymer ratios (r). By tuning the value of r and the polymer concentration, the strength of the polymer−surfactant associations can be modulated. At moderate levels of surfactant addition, the viscosity enhancement is much more pronounced for the EHEC−SDS system than for the EHEC−CTAB system. The frequency dependencies of the dynamic moduli cannot in general be described by a simple Maxwell model, but the rheological behavior is more complex with a distribution of relaxation modes. For EHEC solutions without surfactant, the polymer concentration dependence of the zero-shear viscosity η0 can above the entanglement concentration be described by a power law (η0 ∼ Cx, with x = 4.1). By addition of CTAB, the main effect is...

Adsorption of Poly(vinylpyrrolidone)/Surfactant(s) Mixtures at the Silica/Water Interface: A Calorimetric Investigation

The adsorption of binary mixtures of poly(vinylpyrrolidone) (PVP) and sodium dodecyl sulfate (SDS) and of ternary mixture of PVP, SDS, and pentaethylene glycol monodecyl ether (C10E5) onto silica has been studied by the measurements of the adsorbed amount of PVP and the heat of adsorption. The amount of PVP adsorbed from the binary mixtures decreases as the SDS concentration is increased, whereas the enthalpy change passes through a maximum at SDS concentration of ca. 4 g/L. The chain conformation of PVP adsorbed on silica has been estimated from the adsorption data in relation to the surfactant concentration. It changes from loops and tails to trains upon increasing SDS concentration. At SDS concentrations higher than 4 g/L, a comblike conformation is observed. The adsorption of PVP/SDS/C10E5 ternary mixtures depends on the polymer concentration regime.

Enzymatic degradation of guar and substituted guar galactomannans.

Enzymatic degradation of guar galactomannan is studied using gel permeation chromatography (GPC) and steady shear viscometry. In very dilute polymer solutions, reaction rate increases with first-order kinetics with substrate concentration. In the intermediate concentration regime, the enzyme/polymer binding saturates, and the degradation kinetics is zero-order. The observations are in accord with a Michaelis-Menton kinetics model. The Michaelis-Menton parameter, Km and Vmax, were determined to be 0.6 mM and 7.8 x 10(-10) mol/(mL s) for guar at pH = 7, where the maximal velocity of the reaction, Vmax, was measured in terms of the molar concentration of glycosidic bonds broken per unit time. However, as the solution increases in concentration, the reaction rate decreases and the enzyme diffusion through the concentrated polymer gel becomes a limiting factor. A reaction-diffusion model is presented to express the competition between enzyme reaction and diffusion. The scaling theory and kinetic data are used to define the boundaries of the polymer concentration regimes between substrate (i.e., polymer strand) limited reactions, enzyme limited reactions, and hindered diffusion limited reactions. The influence of polymer derivatization on the degradation kinetics was also explored. The degradation rate was shown to be greatly affected by the type of substituent groups as well as the degree of substitution. The triggering mechanism and controlled degradation were found for the enzymatic hydrolysis of cationically derivatized guar solutions.

Linear and nonlinear viscoelastic properties of aqueous solutions of cationic polyacrylamides

Full Paper : Linear and nonlinear viscoelasticity of aqueous solutions of cationic polyacrylamides at different levels of salt addition have been examined over an extended polymer concentration regime. The polymers have different charge density and the molecular weight of the polymers is high. The rheological features of the systems depend on polymer concentration, charge density, and amount of added salt. The frequency dependences of the dynamic moduli cannot be described by a simple Maxwell model, but the rheological behavior is more complex with a distribution of relaxation modes. The power laws, describing the polymer concentration dependences of the longest relaxation time and the zero-shear viscosity, revealed a stronger concentration dependence of these parameters as the salt concentration increased. At sufficiently hight salt concentration, a behavior resembling that of entangled neutral polymers was observed. Nonlinear shear thinning behavior is found for all systems at higher shear rates. The magnititude of this effect depends on polymer concentration, charge density, and level of added salt. Thixotropy is only observed for solutions of the polymer of high charge density in the absence of added salt. Both, the linear and nonlinear rheological properties elucidate the intricate interplay betwen polyelectrolyte effects and entanglements.