Increasing viscosity in entangled polyelectrolyte solutions by the addition of salt
Nicholas B.Wyatt,Casey M.Gunther,Matthew W.Liberatore *
Chemical Engineering Department,Colorado School of Mines,Golden,CO 80401,USA
a r t i c l e i n f o
Article history:
Received 3December 2010Received in revised form 24March 2011
Accepted 27March 2011
Available online 2April 2011Keywords:Xanthan欧广
Polyelectrolyte solutions
Entangled polymer solutions
a b s t r a c t
The viscosity of several polyelectrolytes is measured in both salt free solutions and solutions in the high salt limit.At low polymer concentrations,the zero shear rate viscosity decreases as much as 100-fold upon addition of a monovalent salt,namely NaCl.However,as polymer concentration increases,the viscosity difference between polymer in salt free and in monovalent salt solution diminishes.Further,the zero shear rate viscosity becomes independent of added monovalent salt at the critical polyelectrolyte concentration c D .Above c D ,the addition of monovalent salt increases the zero shear rate viscosity of the entangled polyelectrolyte solutions.The viscosity increase agrees with viscosity scaling theory for polyelectrolytes in the entangled regime.Polyelectrolytes exhibiting an increase in viscosity above c D in the presence of monovalent salt include three natural anionic polyelectrolytes (xanthan,carrageenan,welan),one synthetic anionic polyelectrolyte (hydrolyzed polyacrylamide),and one natural cationic polyelectrolyte (chitosan).Generally,these polyelectrolytes are relatively high molecular weight (>1M Dalton),which makes c D experimentally accessible (e.g.,c D ¼0.2wt%for xanthan).The magnitude of the viscosity increase is as high as 300%for xanthan and nearly independent of monovalent salt concentration in the high salt limit.The increase in viscosity in monovalent salt solution and magnitude of c D appear to be heavily in fluenced by the molecular characteristics of the polymers such as monomer weight,molecular structure,and chain conformation.
Ó2011Elsevier Ltd.All rights reserved.
1.Introduction
Polyelectrolytes are macromolecules whose repeat units bear an ionizable group.In polar solvents,the ionization of these groups along the backbone results in a macromolecule carrying an elec-trostatic charge.Charged polymer systems are encountered daily in nature in the form of biological polymers such as DNA,poly-peptides,and proteins.Polyelectrolytes are also used extensively in many important industrial applications as food additives,floccu-lants,drilling fluids and drag reducers.Charged polymer solution rheology is complex due,in part,to the polymer ’s sensitivity to the presence of ions in solution [1e 7].Several theories have been proposed to explain the observed differences in properties between polyelectrolytes and neutral polymers [8e 15].However,the majority of the theories presented are focused on the polymer in the dilute and semidilute concentration regimes.Little theory is available for polyelectrolytes in the entangled and concentrated regimes.Since polyelectrolytes are so vital to myriad different industries,a fundamental understanding of the solution rheology in various solvent conditions and across all concentration regimes is critical.
The dynamics,and therefore rheology,of polyelectrolyte solu-tions are determined in large part by the con figuration of the polyelectrolyte chains in solution.The polymer chain con figuration is determined,in turn,by the solution conditions (i.e.,solvent quality,ionic strength,temperature,etc.)[3,9].Further rheological complexity is added by factors such as chain orientation,chain stiffness,chain overlap and entanglement,and electrostatic inter-actions between polymer chains (many of which are not well understood for polyelectrolytes).In salt free solution,strong Coulombic repulsions between like charges along the poly-electrolyte backbone stretch and elongate individual chains.The polymers are surrounded by a cloud of counterions that exactly balances the charge on the chains so the solution maintains charge neutrality.The extended con figuration of the polymer chains,together with the electrostatic interactions,accounts for some of the interesting rheological properties of polyelectrolyte solutions.Because of the charge balance in salt free solution,the poly-electrolyte is very sensitive to the presence of added salt ions.
The addition of counterions,normally in the form of ionizable salt molecules,screens the electrostatic repulsions between charges along the backbone of the polyelectrolyte chain.The screening of the electrostatic interactions,in turn,allows the chain to fold up and assume a smaller,more compact conformation.The degree to which a polymer molecule ’s conformation shrinks may
*Corresponding author.
E-mail address:mliberat@mines.edu (M.W.
Liberatore).
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Polymer 52(2011)2437e 2444
be dependent on the stiffness of the polymer chain in solution [1,16].Since the molecular con figuration changes,the solution rheology must also change.The addition of salt to polyelectrolyte solutions has been shown to have dramatic effects on the solution rheology [3e 7,16e 26].In the dilute and semidilute concentration regimes,the collapsed polymer chains lead to a large reduction in both viscosity and dynamic rheological properties [1,7,22].If enough salt is added (i.e.,high salt limit is attained),electrostatic effects are predicted to be negligible and neutral polymer behavior is predicted [9,10].The high salt limit is de fined to be the limit where the number of added salt ions is greater than the number of free counterions in solution.Despite these predictions,little experimental work has been done to examine polyelectrolyte rheology and the effect of added monovalent salt on polyelectrolyte rheology in the entangled concentration regime.
Recent publications preface the work presented here and summarize the dilute and semidilute beha
vior of the polyelectrolyte xanthan in salt free and salt solutions [1,27,28].Speci fically,three critical concentrations for xanthan were determined in salt free solution (Fig.1),namely the overlap concentration c *,the entan-glement concentration c e ,and c D [1].Interestingly,the changes in viscosity near the overlap concentration for xanthan also corre-sponded to a similar increase in the drag reducing properties of the polyelectrolyte in salt free solution [27].The scaling of the zero shear rate viscosity (h 0)in the semidilute unentangled and semidilute entangled concentration regimes is well described by theory for polyelectrolytes in a good solvent [1].Above c D ,the viscosity scaling is well described by theory for neutral polymers in a good solvent.
In 50mM NaCl (i.e.,in the high salt limit),xanthan ’s zero shear rate viscosity decreased in the dilute and semidilute concentration regimes by as much as 89%(Fig.1).Further,two critical concen-trations,c *and c e ,were determined in NaCl solution,and the viscosity scaling agreed well with theory for neutral polymer in a theta solvent [1].Also,a study of different salts,varying cation size and valency,showed larger increases in viscosity in the entangled regime for divalent cations compared to monovalent ions of the same ionic radius [28].A recent review of the rheology of dilute and semidilute polymer solutions addresses the current state of the science in this area [29],and clearly states “Entanglement in polyelectrolyte solutions is not yet well understood ”.In the present
manuscript,details of a new and interesting rheological pheno-mena in entangled polyelectrolyte solutions is presented.
The predicted viscosity-concentration scaling in the entangled regime (i.e.,above c D in salt free solution and above c e in monovalent salt solution)is stronger in monovalent salt solution than in salt free solution (14/3vs.15/4,Fig.1).Despite the signi ficantly smaller viscosity of xanthan in NaCl near c e (w 800ppm),the scaling theory predicts that the viscosity in the monovalent salt solution should exceed the salt free solution at some higher concentration.The present work con firms the prediction that viscosity of poly-electrolyte solutions can be greater in monovalent salt solution than salt free solution at the same polymer concentration.
In this manuscript,we focus on the changes in rheological behavior of polyelectrolytes in the entangled regime observed when a monovalent salt is introduced into the system.We expand upon the work summarized for xanthan discussed above,then expand the study to include several other polyelectrolytes including natural (both anionic and cationic)and synthetic polyelectrolytes.For each polymer studied,we show that the solution viscosity is measurably increased by adding a monovalent salt for polymer concentrations above c D .A detailed presentation of steady and oscillatory shear rheology for xanthan is followed by a summary of the viscosity increase for a numb
er of other polyelectrolytes.2.Materials and methods 2.1.Materials
The xanthan gum used in the current study is a commercial,food grade polymer (Keltrol T 622,Mat.#20000625)donated by CP Kelco in powder form.The xanthan has an estimated molecular weight of 2Â106Da and polydispersity index of w 2.The manu-facturer reports that the product is “clari fied ”,meaning extra steps were taken to remove residual cellular debris from the fermenta-tion process.Solutions with concentrations up to 10,000ppm xanthan (1weight%)have been made and observed to be optically clear.The powder was used as received without further puri fication or modi fication.In addition,a quantity of the xanthan was dialyzed exhaustively using standard protocols to obtain the potassium salt form for comparison purposes.The rheology of the dialyzed and the as received xanthan agreed (within experimental error)with errors expected from batch to batch variation in xanthan products (additional details available in [1]).Welan gum (Kelco-Crete K1C376Lot #8l166K)and carrageenan (Genuvisco Batch #S64396)were also donated by CP Kelco.Partially hydrolyzed polyacrylamide (HPAM)was donated by Ciba Specialty Chemicals (Magna floc 1011,Lot #00947G5).Poly(acrylic acid)(PAA,4Â106Da)was obtained from Sigma Aldrich (Product #306231).Chitosan was purchased from Aldrich with a degree of deacetylation of >75%.All of the polymers were received in powder form and,with the exception of the dialyzed xanthan,were used wi
thout further modi fication or puri fication.Monomer molecular structures and pertinent prop-erties for each polymer are given in Table 1.
Deionized water was obtained by passing house deionized water through a Barnstead NANOpure Diamond UV ultrapure water system followed by a 0.2m m filter.The resulting deionized water had a measured resistance of 18.2M U -cm.Based on [30],chitosan was mixed with deionized water and titrated with acetic acid (99.7%,Fisher Scienti fic)until the polymer dissolved (pH ¼3.0Æ0.2).Reagent grade NaCl (Mallinckrodt Chemicals Product #7581-12)was used as received without further puri fica-tion.The glassware used for mixing and storage of the poly-electrolyte solutions was carefully cleaned with either ethanol or acetone (reagent grade),then rinsed with puri fied deionized water to remove all traces of salt prior to
Fig.1.Zero shear rate viscosity scaling for xanthan in salt free solution (squares)and in 50mM NaCl (circles).Adapted from [1].The focus of this work is the concentration range above c D .
N.B.Wyatt et al./Polymer 52(2011)2437e 2444
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2.2.Sample preparation
Polyelectrolyte solutions were prepared by dissolving the polymer powder in either deionized water.For the polyelectrolytes in monovalent salt solution,NaCl was subsequently added to the dissolved polymer solution.To ensure that the differences in viscosity reported here were caused by the addition of salt rather than slight variations in concentrations or preparation method,one parent polymer solution was made,divided,and salt added to one of the daughter solutions.The end result was two solutions of identical polymer concentration,but differing only in salt concen-tration(no salt vs.salt).The polymer solutions in deionized water are referred to as salt free solutions since no salt is added.When possible,the solutions were stirred using a magnetic stir bar for approximately1h before
being allowed to rest for approximately 24h at room temperature(w23 C).The units used for polymer concentration are parts per million by mass(ppm),a commonly used unit for high molecular weight polymers and drag reducers (10,000ppm¼1weight%).For the polyelectrolyte solutions at high concentration(c>1%by weight),the polymer powder was added to the solvent and the resulting mixture was shaken vigorously for several minutes to disperse the polymer.Over the following24h, the solution was stirred by hand periodically to obtain a homoge-neous mixture.Consistent rheological results confirm that the solutions were well dispersed and the polymer was well hydrated. The shear experienced by the xanthan is unlikely to degrade the polymer at the solutions investigated as little change in xanthan’s drag reduction was observed over hours in turbulentflow[27].In all cases,measurements were made one to four days following solution preparation.In the period of one to four days following sample preparation,no measurable change in rheological proper-ties of the solutions was observed.The solutions were discarded six
Table1
Monomer molecular structure,molecular weight,and charges per monomer for polymers used in the current study.
Monomer structure Polymer Monomer weight Charges per
monomer
Welan883À
1
days following preparation as polymer degradation (especially with the polysaccharides)became evident (e.g.,signi ficant decrease in viscosity and loss of optical clarity).The viscosity change due to
the addition of NaCl to the water (solvent)is insigni ficant compared to the viscosity increases reported here (e.g.,0.0009Pa-s for deion-ized water compared to 0.00098Pa s for 1M NaCl at 25 C).2.3.Rheology
All rheological data were collected using a TA Instruments AR-G2stress controlled rheometer.Experiments were conducted using either a stainless steel cone and plate (60mm diameter,1or 2 ),parallel plate (40mm diameter,1mm gap)or a concentric cylinder geometry (of outer diameter 30.0mm and inner diameter 28.0mm)in steady or oscillatory shear.Sample evaporation was minimized by using a solvent trap in conjunction with the appropriate geo-metry.Temperature was controlled at 25.0Æ0.1 C using a Peltier plate (cone and plate and parallel plate)or a Peltier jacket (concentric cylinders).Calibration with viscosity standard oils showed agreement with an error of less than 3%.The AR-G2rheometer has a lower torque limit of 0.01m N m which is suf fi-ciently sensitive to measure zero shear rate viscosities at very low polymer concentrations (c <10ppm).
The data reported here are the averages of at least three repli-cate data sets and the associated error bars represent one standard deviation unless otherwise noted.3.Results and discussion 3.1.Xanthan
In salt free solution,xanthan exhibits shear viscosity behavior typical of many polymer solutions (Fig.2).A Newtonian plateau is observed at low shear rates followed by a region of shear thinning at higher shear rates.In the presence of 50mM NaCl,the viscosity of xanthan solutions is qualitatively similar.In both salt free solu-tion and in 50mM NaCl,the shear dependence of xanthan viscosity is well described by the Cross model [1].The magnitude of both the zero shear rate viscosity (h 0)and the shear viscosity,however,are markedly different in the presence of monovalent salt (Fig.2).For concentrations below w 100ppm (dilute regime),the shear thin-ning solutions become Newtonian (in the range of shear rates studied here)upon addition of NaCl.Dilute solution viscosities are
泰晤士报高等教育副刊about 10times lower than the zero shear rate viscosity of the salt free solution.In the semidilute regime (c <500ppm),the zero shear rate viscosity decreases by as much as 100-fold upon addition of NaCl.However,for concentrations above c w 2000ppm (entangled regime),the zero shear rate viscosity increases by as much as fourfold in the presence of NaCl (Fig.2).Similar changes in viscosity are also observed at higher shear rates.At a shear rate of 100s À1,the viscosity of 500ppm xanthan decreases by 70%while for 4000ppm xanthan,the viscosity increases by 16%.Viscosity increases at higher shear rates may be advantageous in a number of industrial applications.
The changes in viscosity in the presence of NaCl may be attributed to changes in the molecular conf
ormation of the xanthan molecules.Xanthan is known to exist in two different conforma-tional states:an ordered,helical conformation and a disordered conformation which can be described as a broken or imperfect helix [31e 38].In salt free solution,the backbone of the xanthan molecule is disordered and highly extended due to electrostatic repulsions among the charged side chains.In this state,the xanthan molecule is relatively stiff,but still retains a degree of flexibility.When NaCl is added to the solution,the electrostatic interactions between charged groups on the side chains are screened,the molecules collapse,and an ordered (helical)conformation is assumed.Further,the ordered conformation is stabilized by the salt ions in solution and the molecule becomes rigid and rodlike.Several studies report the persistence length of xanthan to be 100e 150nm in salt solution [39e 42].Double stranded DNA has a persistence length of w 50nm in salt solution [43].A recent study reports the persistence length of short chain branched poly-ethylene (a flexible polymer)in solution to be about 0.8nm [44].The overall decrease in molecular size is likely responsible for the observed decreases in h 0for concentrations below 2000ppm.
文件管理系统In 50mM NaCl solution (high salt limit),the scaling of h 0with polymer concentration is well described by predictions for a neutral polymer in a Q solvent (Fig.3b).For concentrations above w 70ppm,the viscosity scaling is much stronger with concentra-tion in the presence of NaCl than in salt free solutio n.For example,in the entangled regime xanthan zero shear rate viscosity scales to the 4.67power in the high salt limit and only to the 3.75power in salt free solution.Since the viscosity scaling in monovalent
Fig.3.An expanded view of the zero shear rate viscosity scaling in the concentration region near c D .Adapted from Fig.1.Zero shear rate viscosity scaling for xanthan in salt free solution (squares)and in 50mM NaCl
(circles).
Fig.2.Viscosity as a function of shear rate for several xanthan concentrations in both salt free solution (filled symbols)and 50mM NaCl (open symbols).
N.B.Wyatt et al./Polymer 52(2011)2437e 2444
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solution is greater than the scaling in salt free solution,one would expect that,at some concentration,the viscosity of the poly-electrolyte in monovalent salt solution would be greater than t
he viscosity of the polyelectrolyte in salt free solution.As shown in Fig.2,the zero shear rate viscosity of xanthan above c D is greater in NaCl solution than in salt free solution.Further,the concentration where the viscosity in salt free solution is nearly equal to the viscosity in 50mM NaCl is the critical concentration c D (Fig.3a).
For xanthan concentrations not equal to c D ,the addition of NaCl results in a measurable change in the zero shear rate viscosity (Fig.4).Above c D ,the normalized viscosity ðh N ¼h 0;Salt =h 0;No Salt Þincreases with NaCl concentration up to a salt concentration of w 12mM NaCl (Fig.4a).Further addition of NaCl to a concentration of 1.0M has little effect on the normalized viscosity.The normalized viscosity remains constant with salt concentration above 12mM NaCl,con firming calculations of the high salt limit based on molecular weight and charges per monomer for xanthan.The high salt limit is achieved when the number of salt ions exceeds the number of free counterions (i.e.,the number of added salt charges exceeds the number of charges on the polymer chains in solution).For xanthan,the NaCl concentration at which the number of salt ions approximately equals the number of free counterions is calculated to be w 10mM,very close to the 12mM observed experimentally.For concentrations below c D ,addition of NaCl results in a measurable decrease in viscosity (Fig.4b).The decrease in zero shear rate viscosity of almost an ord
er of magnitude is observed at even the lowest monovalent salt concentration studied (3mM NaCl).The viscosity then remains constant upon further addition of NaCl up to a concentration of 1.0M.The present study generally agrees with previous work stating xanthan solution rheology is independent of monovalent salt concentration above 10mM,although no polyelectrolyte concentration is mentioned [22].Also,the change in viscosity of the solvent upon the addition of salt (e.g.,1M NaCl has a measured viscosity of 0.00098Pa s at 25 C)is negligible for polymer solutions outside of the dilute regime.
To initiate study of polymer conformation,dynamic oscillatory shear measurements were made to quantify the viscoelasticity of xanthan at concentrations of 500ppm (c <c D )and 4000ppm
(c >c D )in both salt free and 50mM NaCl solution (Fig.5).For 500ppm xanthan,the loss modulus (G 00)is greater than the storage modulus (G 0)at low frequencies in both salt free and 50mM NaCl solution.Since G 00is a measure of the energy dissipated per cycle of deformation,its magnitude gives insight into the presence (or lack)of structure in the solution.For 500ppm xanthan,G 00is w 200%larger than G 0at low frequencies (e.g.,1rad/s),indicating the lack of a structure capable of storing the energy imparted to the solution during the oscillation.Xanthan at 500ppm also exhibits an elastic response to the oscillation (i.e.,G 0>0).As the frequency of the oscillation is increased,the mo
duli cross (i.e.,G 0becomes greater than G 00)as the elastic response of the solution dominates the viscous contribution.At higher frequencies,the frequency of the oscillation is high enough that the polymer chains do not have suf ficient time to relax between oscillations.Thus,the amount of energy stored in the polymer chains exceeds the energy that is dissipated in each oscillation.In addition,the crossover of G 0and G 00shifts from a crossover frequency of 12rad/s in salt free solution to 21rad/s in 50mM NaCl (Fig.5b).The higher crossover frequency in the high salt limit corresponds to a shorter (w 40%)relaxation time for the polyelectrolyte chains and is consistent with the decrease in viscosity discussed earlier.Further,the magnitude of both G 0and G 00decrease by about 50%in the presence of 50mM NaCl.The decrease in the magnitude of the moduli,along with the increase in the crossover frequency indicate a less structured polymer network caused by the decrease in molecular size when monovalent salt is
added.
a
b
Fig.5.Dynamic moduli for 4000ppm (top)and 500ppm (bottom)xanthan in salt free solution (black curves)and in 50mM NaCl (grey
curves).
a
b
Fig.4.Normalized zero shear rate viscosity ðh N ¼h 0;Salt =h 0;No Salt Þof 4000ppm (a)and 500ppm (b)xanthan as a function of added NaCl concentration.(Note:Salt concentration is a logarithmic scale).
N.B.Wyatt et al./Polymer 52(2011)2437e 24442441
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