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Intramolecular and Intermolecular Association during Chemical Cross-Linking of Dilute Solutions of Different Polysaccharides under the Influence of Shear Flow Zhengjun Liu, Atoosa Maleki, Kaizheng Zhu, Anna-Lena Kjøniksen,* and Bo Nystro1 m Department of Chemistry, UniVersity of Oslo, P.O. Box 1033, Blindern, N-0315, Oslo, Norway ReceiVed: August 13, 2007; In Final Form: October 4, 2007
Effects of shear flow on intramolecular and intermolecular associations of dilute aqueous alkali solutions of dextran, hydroxyethylcellulose (HEC), and a hydrophobically modified analogue (HM-HEC) in the presence of a chemical cross-linker agent were characterized with the aid of viscometry and rheo-small-angle light scattering (rheo-SALS) methods. The picture that emerges at short times in the course of cross-linking of the polymer solutions under the influence of a constant shear rate is that HEC coils contract because of intramolecular cross-linking, whereas the HM-HEC species show an incipient association and the dextran molecules are unaffected. At longer times, interchain cross-linking of the polymers promoted the growth of large flocs, which were disrupted by shear forces when they were sufficiently large. These findings are novel, and both the building up of aggregates and disaggregation are well substantiated by the SALS results.
Introduction Intermolecular cross-linking of a semidilute polymer solution with the aid of a chemical agent is a classical example of forming an interconnected permanent network or a macroscopic chemical gel.1-3 If a dilute polymer solution is cross-linked, the overall connectivity is lost, and nonlinked aggregates of various sizes are formed and these species may be called microgels.4 These nano-objects have fundamental and practical implications in fields such as material science, molecular electronics, and biomimetic chemistry. Furthermore, in some biological and biotechnological applications, it is necessary to deal with a variety of interactions relevant to aggregation and disaggregation of particles. These phenomena are crucial for many important biological processes such as aggregation of blood platelets. In dilute solutions of a polymer that can be cross-linked in the presence a chemical agent, shear flows are expected to have a momentous impact on the aggregation and disaggregation processes in these systems.5 Shear flows tend to bring molecules close to each other faster than Brownian motion does under quiescent conditions, thus altering aggregation kinetics of the reacting polymer species by speeding up the process. Flocculation under the influence of shear flow is known as orthokinetic aggregation,6 whereas perikinetic association7 denotes the process where Brownian motion is the dominant transport mechanism. To decide which mechanism is the prevailing one, the Peclet number8 Pe is usually introduced through the expression Pe = γˆ /(Dt/R2), where γ˘ is the shear rate, R the particle radius, and Dt the translational diffusion coefficient. The Peclet number of a particle in solution determines whether its motion is governed by hydrodynamic flow or by diffusion. When Pe is large, particles follow the flow streamlines, while for low values of Pe, particles are subjected to Brownian motion. In the former case, the collision frequency of particles is higher, and the probability of forming clusters is higher in this case than for simple Brownian motion. This is called the shear* To whom correspondence should be addressed: e-mail a.l.kjoniksen@ kjemi.uio.no, Tel +47 22855508, Fax +47 22855441.
enhanced aggregation regime, which operates over a certain range of Peclet numbers. When the measuring sample is exposed to high shear rates, the clusters or flocs are expected to break up under the action of large shear stresses and in reaction mixtures containing few aggregates; the aggregate growth levels off or even ceases. When the flocs are huge, not even covalently connected clusters (as for chemical cross-linking) will sustain the hydrodynamical stresses exerted on them at high shear rates. During cross-linking of polymers in the dilute concentration regime, competition between intrachain and interchain crosslinking is expected to occur. In a recent investigation,5 intramolecular and interchain associations of dilute aqueous alkali solutions of the hydrophilic hydroxyethylcellulose (HEC) biopolymer in the presence of a chemical cross-linker agent were studied under the influence of shear flow. When a dilute solution of HEC was subjected to shear, intrapolymer cross-linking with shrinking of the molecules was found, and at moderate shear rates interpolymer cross-linking at longer times followed this behavior. In this work, the aim is to examine how the interplay between intrachain and interchain cross-linking under the influence of shear flow in the presence of a chemical agent is affected by hydrophobic modification of the polymer and polymer architecture. For this purpose, we have employed rheo-small-angle light scattering (rheo-SALS) methods to simultaneously monitor rheological and structural alterations during the chemical crosslinking reaction of dilute solutions of hydroxyethylcellulose (HEC) and a hydrophobically modified analogue (HM-HEC) at alkali conditions with divinyl sulfone (DVS) as the difunctional cross-linker agent. In addition, the cross-linking of dilute aqueous alkali solutions of dextran, which is a bacterial polysaccharide composed of R-1,6-D-glucopyranose residues with a few percent of R-1,2-, R-1,3-, and R-1,4-linked branch units, is studied. While the HEC chains in dilute aqueous solutions are rather extended and semirigid, the dextran molecules are branched and assume a compact coil-like conformation. The chemical structures of the polymers and the cross-linker agent are illustrated in Figure 1. In this scenario, the effects of polymer conformation and hydrophobicity on the
10.1021/jp076497h CCC: $40.75 © 2008 American Chemical Society Published on Web 01/05/2008
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Figure 1. Schematic illustrations of the chemical structures of HEC/HM-HEC, dextran (only R (1f3) side branches are shown in this figure), and the cross-linker divinyl sulfone.
viscosity and structure of the polymers during the cross-linking process under the influence of shear flow will be surveyed. To the best of our knowledge, this is the first work where these vital issues are investigated. Previously, shear-induced aggregation and disruption of aggregates are issues that have been addressed for systems of gelatin,9 methylhydroxypropylcellulose,10 and pectin.11 Experimental Section Materials and Solution Preparation. In this study, a hydroxyethylcellulose (HEC) sample with the trade name Natrosol 250 GR (lot no. A-0382) was obtained from Hercules, Aqualon Division, and it was utilized as a reference and as the precursor for the synthesis of the hydrophobically modified analogue. The degree of substitution of hydroxyethyl groups per repeating anhydroglucose unit of the polymer is 2.5 (given by the manufacturer). The weight-average molecular weight (Mw ) 400 000) of this HEC sample has previously been measured12 by intensity light scattering at 25 °C. The fixed polymer concentration (0.1 wt %) utilized in this study is much smaller than the overlap concentrations of these polymers. The dextran sample was purchased from ACROS Chemicals (no. 40626). The molecular weight (Mw ) 6.2 × 105) and the polydispersity index (Mw/Mn ) 1.8) of the sample were previously determined13 by means of size-exclusion chromatography coupled
to multiangle light scattering and refractive index detectors. The cross-linking agent DVS was purchased from Merck and utilized without further purification. The hydrophobically modified hydroxyethylcellulose sample (HM-HEC) was synthesized according to a standard procedure.14 The details of the preparation and characterization of the sample have been reported elsewhere,15 and only a brief summary is given here. After completion of the hydrophobization reactions, acetic acid neutralized the liquid reaction mixtures, and the products were collected by filtration. The product was washed thoroughly with acetone and dried at 70 °C for 24 h under reduced pressure to remove rests of acetone. The molar degree of substitution of hydrophobic glycidyl hexadecyl ether groups is only 2 mol %, and under this condition the hydrophobic groups prefer to be bound to the OH groups on the longer side chains rather than the OH functions of the backbone (Figure 1). The chemical structure and purity of the HM-HEC sample were ascertained by 1H NMR (deuteron-DMSO was used as the solvent) with a Bruker AVANCE DPX300 NMR spectrometer (Bruker Biospin, Fa¨llanden, Switzerland) operating at 300.13 MHz at 25.0 °C. The molar degree of substitution of the glycidyl hexadecyl ether groups determined from NMR analysis was 2 mol %, obtained from the peak ratios between
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Figure 2. Time dependence of the shear viscosity for 0.1 wt % alkali (0.05 M NaOH) solutions of HEC, HM-HEC, and dextran in the presence of 30 µL/g DVS under the influence of the shear rates indicated. The insets illustrate magnification of the data around the transition zones.
Figure 3. Time dependences of the shear viscosity for 0.1 wt % solutions (0.05 M NaOH) of HEC, HM-HEC, and dextran in the presence of 30 µL/g DVS at a fixed shear rate of 30 s-1.
the anomeric protons (4.9 ppm) and the methyl protons (0.8 ppm) of the hexadecyl chain. To remove salt and other low-molecular-weight impurities, dilute HEC, HM-HEC, and dextran solutions were dialyzed against Millipore water for 7 days and recovered by freezedrying. Regenerated cellulose with a molecular weight cutoff of ∼8000 (Spectrum Medical Industries) was used as dialyzing membrane. All solutions were prepared by weighting the components and Millipore water was always used. After freeze-drying, the polymer was usually (but in one case some other NaOH concentrations were also considered) redissolved in 0.05 M NaOH (basic condition is necessary for the cross-linker reaction to proceed), solutions with a fixed polymer concentration of 0.1 wt % were prepared, and the samples were homogenized by stirring at room temperature for 1 day. At these conditions,
Liu et al. the polymer (0.1 wt %) and the cross-linker (15 or 30 µL/g) concentrations had no effect on the pH of the solution. Prior to the commencement of measurement, prescribed amounts of the cross-linker agent were added to the solutions under vigorous stirring, and shortly afterward the experiments were started. The same procedure to prepare the samples was always employed to ensure good reproducibility of the measurements. The crosslinking reaction in solutions of the polymers occurs between the cross-linker molecules and hydroxyl groups on the polymer chains. All experiments were carried out at 25.0 °C. Rheological Experiments. Steady shear viscosity measurements were performed in a Paar-Physica MCR 300 rheometer using a cone-and-plate geometry, with a cone angle of 1° and a diameter of 75 mm. This rheometer operates effectively with this geometry even on dilute polymer solutions, and the viscosity of water can easily be measured over an extended shear rate domain. To prevent evaporation of the solvent, the free surface of the sample was always covered with a thin layer of lowviscosity silicone oil (the viscoelastic response of the sample is virtually not affected by this layer). In the steady shear measurements, a fixed shear rate was set, and the time evolution of the viscosity was monitored during the cross-linker reaction. The sample was homogenized during a short time and was then rapidly transferred to the plate, and the measurements were started immediately. The measuring unit is equipped with a temperature unit (Peltier plate) that gives an accurate temperature control ((0.05 °C) over an extended time. Rheo-Small-Angle Light Scattering Measurements (RheoSALS). Simultaneous rheological and small-angle light scattering experiments under the influence of a preset shear rate were conducted using the Paar-Physica MCR 300 rheometer, equipped with a specially designed parallel plate-plate configuration (the diameter of the plate is 43 mm) in glass. In all measurements, a 10 mW diode laser operating at a wavelength of 658 nm was used as the light source, and a polarizer is placed in the front of the laser and an analyzer below the sample, making both polarized (polarizer and analyzer parallel) and depolarized (polarizer and analyzer perpendicular) experiments possible. Only polarized measurements were conducted in this study. Utilizing a prism, the laser beam was deflected and passed through the sample placed between the transparent parallel plates. The distance between the plates is small (1.0 mm) so that the effect of possible multiple scattering is reduced when the sample becomes turbid at later times in the cross-linking reaction. The light propagated along the velocity gradient direction, thus probing the structure in the plane of flow and vorticity. The forward scattered light at small angles was collected on a flat translucent screen below the sample (distance between sample and screen is 12.3 cm). The 2D scattering patterns formed on the screen were captured using a CCD camera (driver LuCam V. 3.8), which plane is parallel to that of the screen. A Lumenera (VGA) CCD camera (Lumenera Corp., Ottawa, Canada) with a Pentax lens was utilized, and the scattered images were stored on a computer using the StreamPix (NorPix, Montreal, Quebec, Canada) application software (version 3.18.5), which enables a real-time digitalization of the images. The images were acquired via the CCD camera with an exposure time of 200 ms. Subsequently, the pictures were analyzed using the SALS software program (version 1.1) developed by the Laboratory of Applied Rheology and Polymer Processing, Department of Chemical Engineering, Katholieke Universiteit Leuven, Leuven, Belgium. The scattering functions were recorded continuously during the run. The approximate accessible scattering wave vector (q) range is
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Figure 4. Scheme for the cross-linking reaction of the polymers containing hydroxyl groups with DVS and shear-induced alterations of the structure of HEC, HM-HEC, and dextran during the cross-linker reaction. I ) polymer chain without cross-linker, II ) intramolecular cross-linking for HEC and dextran and hydrophobic intermolecular, association for HM-HEC, III ) intermolecular cross-linking and aggregation, and IV ) breakup of interaggregate chains by shear.
between q ) 4 × 10-4 nm-1 and q ) 2 × 10-3 nm-1. The wave vector q is given by q ≡ |q| ) (4πn/λ) sin(θ/2), where λ is the wavelength of the incident beam and θ is the scattering angle. The refractive index n was measured with an automatic refractometer (model PTR 46) purchased from Index Instruments Ltd., England. The temperature of the instrument is controlled electronically to a high stability by using a Peltier cell. Because of the experimental uncertainty and limited resolution of the results with the plate-plate geometry for the present dilute solutions and the fact that the shear rate varies within this type of geometry (complicated data analysis), rheological results are only presented from the cone-plate geometry. Results and Discussion Before the results are presented and discussed, it may be instructive to give a background for these experiments and some theoretical aspects. It is well established for colloidal dispersions that in early stages of aggregation shear flow can promote flocculation,16,17 while in the later stages it limits18-20 the aggregate growth. In the quiescent state, both theoretical21,22 and experimental23 studies on dilute aggregating dispersions have shown that the clusters are highly branched and resemble fractals. It has been demonstrated that the fractal dimension of
these clusters depends on the mechanism of aggregation. Depending on the stickiness of the particles, these mechanisms include slow (reaction-limited cluster aggregation, RLCA) as well as fast (diffusion-limited cluster aggregation, DLCA) aggregation.21,24 However, in shear flow DLCA cannot be considered as a dominating mechanism, but a much more probable mechanism is the cluster-cluster aggregation as was suggested by recent simulation results.25 For aggregating colloids, the connecting forces are of physical origin (weak forces), e.g., van der Waals attraction and hydrophobic interactions. In the present work, a chemical cross-linker agent is employed to connect the chains, and in this case the reaction between the cross-linker and hydroxide groups of the polymer chains creates strong chemical bonds. In this process, bonds can be formed within polymer coils (intrapolymer association) and/or between different polymer chains (interpolymer aggregation). Although the strength and nature of the chemical bonds are different from the physical attraction forces operating in dispersions, the scenario for the shear-induced aggregation of the polymer molecules in the presence of a chemical cross-linker agent is reminiscent of that of aggregating colloids through physical interactions.
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Figure 5. Time evolution of the shear viscosity for 0.1 wt % solutions (0.05 M NaOH) of HEC, HM-HEC, and dextran in the presence of the indicated cross-linker concentrations at a fixed shear rate of 30 s-1.
Figure 6. Time evolution of the shear viscosity for 0.1 wt % solutions of dextran with different concentrations of NaOH in the presence of 30 µL/g DVS at a constant shear rate of 30 s-1.
Shear Viscosity. In Figure 2, the time evolution of the shear viscosity at various shear rates is depicted for 0.1 wt % alkali (with 0.05 M NaOH) solutions of HEC, HM-HEC, and dextran in the presence of 30 µL/g DVS. For the HEC solution with added cross-linker agent, a minimum is found in the viscosity curves at intermediate times for the considered shear rates, followed by a strong rise at later times (a magnification of the transition region is shown in the inset). During the cross-linking reaction, a competition between intrachain and interchain association is continuously in progress. However, over a certain
Liu et al. shear rate range, the tendency of forming interpolymer aggregates is inhibited due to shear flow-induced rotations of the molecules. This favors intrachain cross-linking, and the minima of the curves are attributed5 to contraction of the molecules generated by the cross-linking process. In the quiescent state, no shrinking was observed from the previous dynamic light scattering measurements,5 but only aggregation. At shear rates of 300 s-1 and above, the viscosity was constant in the course of the cross-linking reaction, and neither intrachain nor interchain aggregation was detected.5 A close inspection of the inset (Figure 2a) reveals that the amplitude of the minimum of the viscosity curve becomes more pronounced with increasing shear rate, which may indicate that enhanced shear flow promotes a more efficient intrachain cross-linking. The marked rise in the viscosity noticed at longer times suggests that the many collisions of the species in the course of the cross-linking reaction lead to interpolymer cross-linking and the formation of flocs. As the shear rate increases, the inclination of building up huge structures is reduced by mechanical disturbances. When sufficiently large flocs have evolved, the mechanical stresses will breakup the giant clusters. The basic features of shear induced aggregation and rupture of flocs can be described in the framework of computer simulation19,25-27 and theoretical analysis28 of aggregating colloids. The effect of shear rate on the hydrophobically modified analogue (HM-HEC) is more complex (see Figure 2b). In this case no minimum is found in the viscosity curve, but a hump is developed at intermediate times, and the amplitude of this hump is reduced with increasing shear rate. The effect is ascribed to intermolecular hydrophobic interactions, which overshadow the intrachain compression of the entities, and at higher shear rates these associations are progressively disrupted. It should be noted that when the HM-HEC solution without cross-linker agent is exposed to the same shear rate (the result is not shown here), only a very slight increase of the viscosity is detected. This indicates that the presence of cross-linker drastically strengthened this association power. In the judgment of the behavior of HM-HEC, we should bear in mind that these species from the start are more compressed than the corresponding unmodified units because of intramolecular hydrophobic interactions (cf. Figure 3). Moreover, some of the OH groups of HEC have been consumed in connection with the hydrophobic modification of the polymer; hence, the degree of intrachain cross-linking of the hydrophobically modified analogue is reduced as well as the molecules ability to contract. At longer times, a similar behavior is observed as for the unmodified analogue. In the cross-linking process of the dilute dextran solution under shear (Figure 2c), no shrinking or intermolecular association is detected at intermediate times. The dextran coils are compact,29 and the branching and compactness of this structure will obstruct a significant condensation of the weakly interacting coils. The huge fluctuations in the experimental points at long times in the aggregation domain reflect the simultaneous building up and breaking down of clusters. A comparison of the time evolution of the viscosity for the different polymer systems at a given shear rate (30 s-1) is displayed in Figure 3. The differences in the time dependency of the viscosity for the polymers are evident in the inset, and the low viscosity for dextran in the initial state supports the hypothesis that the dextran molecules are compact. Initially, we note that the viscosity of the HM-HEC solution is lower than the corresponding one for HEC, and this finding substantiates the conjecture of intramolecular hydrophobic interactions. At
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Figure 7. 2D SALS patterns at different stages during the cross-linker reaction for 0.1 wt % solutions (0.05 M NaOH) of the polymers indicated in the presence of 30 µL/g DVS at a fixed shear rate of 30 s-1.
longer times, the growth of the intermolecular hydrophobic associations of HM-HEC yields higher values of the viscosity than the corresponding ones of HEC. The upper part of Figure 4 shows how the cross-linker operates in connecting polymer chains equipped with hydroxyl functional groups. We notice from the reaction scheme that alkali conditions favor the cross-linker reaction, and actually rather high pH values (pH > 11.8) are needed for the reaction to proceed.30-32 The lower part of Figure 4 illustrates the time evolution of the structures of the polymers under the influence of shear flow in the course of the cross-linking reaction. The general picture that emerges for all polymers is that at long times huge aggregates are built up, and when the flocs are sufficiently large, they are disintegrated by the shear forces. The prominent difference in behavior between the polymer systems is at early times (step II), where the cross-linking process does not affect the size of the dextran molecules, whereas the HEC molecules experience shrinking. This contraction effect was thoroughly analyzed and discussed in a recent study on dilute HEC solutions.5 In the case of HM-HEC, association structures are formed at intermediate times because of hydrophobic interactions. Effects of cross-linker concentration on the time evolution of the viscosity for the polymer systems during cross-linking are depicted in Figure 5 for polymer solutions of 0.1 wt % (0.05 M NaOH) at a fixed shear rate of 30 s-1. The obvious effect is that the aggregation stage is shifted toward shorter times when the cross-linker concentration is raised; otherwise, the profiles of the viscosity curves are similar. The basis for this displacement is that as the number of cross-linking molecules increases, the probability of forming links is augmented. Another effect, except the change of the cross-linker density, which may influence the cross-linking reaction, is the pH value of the solution. In Figure 6, the effect of NaOH addition on the time dependence of the viscosity for a 0.1 wt % solution of dextran with 30 µL/g DVS is shown under the influence of a constant shear rate of 30 s-1. A salient feature is the earlier start of the aggregation stage as the pH increases, which is expected (see Figure 4) because the cross-linker reaction is speeded up when the mixture becomes more basic. We also note that the amplitude of the aggregation step is higher as the value of pH rises because the growth of the flocs is faster. This does not necessary mean that the maximum size of the aggregates is larger at higher pH, but this may simply indicate that the number of big aggregates is greater. In studies of
aggregating colloids,19,28 it has been argued for a limiting growth size of the aggregates at a given shear rate, above which disaggregation occurs due to viscous stresses. Rheo-SALS. The rheo-SALS methods are powerful to characterize structural changes of the species developed during the cross-linking reaction, both in the quiescent state and under the influence of shear flow. Figure 7 shows typical 2D SALS scattered intensity patterns at different stages during the crosslinker reactions of 0.1 wt % solutions (0.05 M NaOH; 30 µL/g DVS) of HEC, HM-HEC, and dextran at a fixed shear rate of 30 s-1. In this wave vector range, all the SALS patterns are virtually isotropic, which suggests that on this dimensional scale no major structural changes occur. However, there are some general features that should be noted. For the reaction mixtures of HEC and dextran, an abrupt increase of the scattered intensity takes place in the aggregation stage, and when the flocs are disrupted at longer times, the scattered intensity is reduced. In the case of HM-HEC, a more progressive development of the intensity appears at early times, before the flocs are broken up at late times. This initial behavior is associated with the impact of the hydrophobic interactions. In Figure 8, the q dependences of the scattered intensity, obtained from the digitalization of the images along the flow direction, during the cross-linker process for 0.1 wt % solutions (0.05 M NaOH) of HEC, HM-HEC, and dextran in the presence of 30 µL/g DVS are depicted at a constant shear rate of 30 s-1. In the aggregation stages, strong upturns in the scattered intensity are observed at low q values for the three systems, which is a signature of formation of large-scale heterogeneities33 or multichain associations.34 At longer times, the q dependence of the scattered intensity becomes weaker because the association complexes are disintegrated. Figure 9 shows the scattered intensity at a fixed low q value (q ) 0.27 µm-1) for all systems at the same conditions indicated above. There are some prominent features that should be noted. At early times the scattered intensity is almost constant for the HEC and dextran systems, whereas for the HM-HEC system the scattered intensity increases and the values of the intensity are much higher than for the two unmodified polymers. This is again a signal of that the hydrophobic stickers contribute initially in building up association complexes. The aggregation transition is characterized by a sharp rise of the intensity, and in this stage the difference between the systems virtually disappears since the interpolymer cross-linking process is dictating the course of events. At longer times, the intensity gradually drops as the
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Figure 10. Time dependences of the scattered intensity (at a fixed q value of 0.27 µm-1) during the cross-linker reaction (30 µL/g DVS) for 0.1 wt % solutions (0.05 M NaOH) of HEC at the shear rates indicated.
Figure 8. SALS scattered intensity profiles (every fourth point is shown) along the flow direction for 0.1 wt % solutions (0.05 M NaOH) of the polymers indicated in the presence of 30 µL/g DVS at a fixed shear rate of 30 s-1.
imposed, a monotonous disaggregation of the association structures occurs. After a certain period of time, the cross-linker reaction is essentially completed, and the scattering results indicate that the fragmentation of association structures proceeds over a long time. Conclusions
Figure 9. Time evolution of the scattered intensity (at a fixed q value of 0.27 µm-1) in the course of the cross-linking process (30 µL/g DVS) of 0.1 wt % solutions (0.05 M NaOH) of the polymers indicated at a constant shear rate of 30 s-1.
flocs are disintegrated. However, in the case of HM-HEC the scattered intensity exhibits an anomalous behavior at long times. The reason for this may be that the hydrophobic moieties make the shear-disrupted fragments sticky, and the rate of breaking up complexes is balanced by the rate of rebuilding aggregates at this relatively low shear rate. To elucidate the impact of shear flow on the formation of structures during the cross-linking process, time dependences of the scattered intensity (at a fixed q value of 0.27 µm-1) for 0.1 wt % solutions (0.05 M NaOH) of HEC in the presence of 30 µL/g DVS at some different shear rates are displayed in Figure 10. We notice that in quiescent state a sharp transition zone is distinguished during the formation of aggregates, and at longer times the intensity levels off. This suggests that when the aggregates have grown to a limiting size through interpolymer cross-linking, the building up and breaking up (Brownian motion) rates cancel each other. When shear stresses are
In this work, some novel aspects concerning the building up and breaking down of association structures during the chemical cross-linking of dilute solutions of polysaccharides of various natures under the influence of shear flows are presented. The main findings can be summarized in the following way. In solutions of HEC, intrapolymer cross-linking under shear stresses leads to contracted coils at intermediate times, followed by aggregation at longer times due to interchain cross-linking, and when the flocs have reached a certain size, they are disintegrated. The building up of aggregates and the following disruption are both endorsed by the SALS results. A similar behavior was observed for dextran, but in this case no compression of the species could be detected prior to the aggregation stage, probably due to the inherent compactness of these coils. In the case of HM-HEC, association complexes start to build up at an early stage because of hydrophobic interactions, and in the disaggregation stage at long times the rheo-SALS methods disclose a competition between disruption and rebuilding of multichain aggregates. The processes of building up of flocs and disruption were confirmed for the three polymer systems by the SALS results. The growth of the aggregates starts at earlier times when the cross-linker concentration increases as well as when the value of pH of the reaction mixture rises. These findings have demonstrated that changing parameters such as shear rate, cross-linker density, pH, and the nature of the polymer can modulate the delicate interplay between aggregation and disaggregation. Acknowledgment. A.M. and B.N. gratefully acknowledge financial support provided by a FUNMAT Project (Novel functional polymer materials for drug delivery applications). A.K. and B.N. thank VISTA/Statoil for financial support through the project “Screening of different generic types of polymers for possible use in enhanced oil recovery processes” (no. 6338). References and Notes (1) Kinetics of Aggregation and Kinetics; Family, F., Landau, J. P., Eds.; Elsevier Science Publishers: Amsterdam, 1984.
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