-
13376 DOI: 10.1021/la903260r Langmuir 2009, 25(23),
13376–13383Published on Web 10/23/2009
pubs.acs.org/Langmuir
© 2009 American Chemical Society
Shear-Induced Phase Separation in Polyelectrolyte/Mixed
MicelleCoacervates
Matthew W. Liberatore,* Nicholas B. Wyatt, and MiKayla Henry
Department of Chemical Engineering, Colorado School of Mines,
Golden, Colorado 80004
Paul L. Dubin and Elaine Foun
Department of Chemistry, University of Massachusetts, Amherst,
Massachusetts 01003
Received June 1, 2009. Revised Manuscript Received September 25,
2009
A quantitative study of the shear-induced phase separation of a
polycation/anionic-nonionic micelle coacervate ispresented.
Simultaneous rheology and small-angle light scattering (SALS)
measurements allow the elucidation ofmicrometer-scale phase
separation under flow in three coacervate solutions. Below 18 �C,
all three of the coacervatesolutions are optically clear Newtonian
fluids across the entire shear rate range investigated. Once a
critical temperaturerange and/or shear rate is achieved, phase
separation is observed in the small-angle light scattering images
and the fluidexhibits shear thinning. Two definitive SALS patterns
demonstrate the appearance of circular droplets at low shear
ratesnear the critical temperature and ellipsoidal droplets at
higher temperatures and shear rates. The shear-induced
dropletsrange in size from ∼1 to 4 μm. The ellipsoidal droplets
have aspect ratios as high as 4. A conceptual picture in whichshear
flow extends the polyelectrolyte chains of the clear coacervate
liquid phase is proposed. The extended chains
createinterpolyelectrolyte-micelle interactions and promote
expulsion of small ions from the complex, resulting in theformation
of micrometer-scale phase-separated droplets.
Introduction
When two oppositely charged macromolecules [such as
poly-electrolytes (PEs)] are mixed, spontaneous liquid-liquid
phaseseparation can occur with the formation of a
densemacroion-richphase in equilibrium with a dilute macroion-poor
phase. Thisprocess, called complex coacervation, converts a
metastablesuspension of coacervate droplets to separate coacervate
anddilute supernatant phases upon standing or centrifugation.1
Coacervates composedof polyelectrolytes andoppositely
chargedcolloidal particles can be found in shampoos where the
nanopar-ticles are micelles or in food formulations where the
nanoparticlesare proteins. Such coacervates form a locally
segregated environ-ment that can allow selective absorption of
apolarmolecules fromthe surrounding medium, e.g., extracting
aliphatic compoundsfrom solutions.2,3 Coacervates can also be used
to deliver drugs,nutraceuticals, or topically active
ingredients.4-6 Complex coa-cervation can be viewed as an
entropically favorable ion-exchangeprocess, which occurs when the
oppositely charged macroionsrelease some of their initially bound
counterions upon forming acomplex.When the concomitant loss of
hydration is smaller thanthat leading to precipitation,
liquid-liquid phase separation willoccur. Since soluble complexes,
or aggregates thereof, are theprecursors of coacervation, their
mutual repulsion must be
weakened for this transition to take place. Liquid-liquid
phaseseparation occurs only when the charge fraction f (the ratio
ofthe number of macroion charges of a given sign to the
totalmacroion charge) is close to 0.5. It is important to identify
f asthe charge ratio in the complex (microstoichiometry)
anddistinguish it from that of the entire system, fbulk (bulk
ormacrostoichiometry, also known as the mixing ratio) and,further,
to recognize that f itself could present some polydis-persity.
Phase separation in the region of bulk charge stoichi-ometry has
been reported formixtures of cationic chitosan withsynthetic
polyanions,7 histones with DNA or with poly(styrenesulfonate),8 and
lysozyme with poly(styrene sulfonate).9 Coa-cervation can take
place in the vicinity of an fbulk of 0.5 becauseof system
polydispersity or because of disproportionation orpolarization.10
However, in these cases, the residual charge ofthe coacervating
species will result in structure formation atsubmicrometer length
scales, the morphologies of which arecurrently being
determined.9,11-16
*To whom correspondence should be addressed. E-mail:
[email protected].(1) Stuart, M. A. C. Colloid Polym. Sci. 2008,
286(8-9), 855–864.(2) Sudbeck, E. A.; Dubin, P. L.; Curran, M. E.;
Skelton, J. J. Colloid Interface
Sci. 1991, 142(2), 512–517.(3) Luque, N.; Rubio, S.;
Perez-Bendito, D. Anal. Chim. Acta 2007, 584(1),
181–188.(4) Thimma, R. T.; Tammishetti, S. J. Microencapsulation
2003, 20(2), 203–210.(5) Zhang, L.; Liu, Y. Z.; Wu, Z. C.; Chen, H.
X. Drug Dev. Ind. Pharm. 2009
35(3), 369–378.(6) Gander, B.; Blanco-Prieto, M. J.; Thomasin,
C.; Wandrey, C.; Hunkeler, D.
Encycl. Pharm. Technol. 2006, 1–5.
(7) Mincheva, R.; Manolova, N.; Paneva, D.; Rashkov, I. Eur.
Polym. J. 2006,42(4), 858–868.
(8) Raspaud, E.; Chaperon, I.; Leforestier, A.; Livolant, F.
Biophys. J. 1999, 77(3), 1547–1555.
(9) Gummel, J.; Boue, F.; Clemens, D.; Cousin, F. SoftMatter
2008, 4(8), 1653–1664.
(10) Zhang, R.; Shklovskii, B. T. Physica A 2005, 352(1),
216–238.(11) Wang, X. Y.; Lee, J. Y.; Wang, Y. W.; Huang, Q. R.
Biomacromolecules
2007, 8(3), 992–997.(12) Chodankar, S.; Aswal, V. K.;
Kohlbrecher, J.; Vavrin, R.; Wagh, A. G.
Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip.
Top. 2008, 78, 3.(13) Singh, S. S.; Aswal, V. K.; Bohidar, H. B.
Int. J. Biol. Macromol. 2007, 41
(3), 301–307.(14) Kayitmazer, A. B.; Strand, S. P.; Tribet, C.;
Jaeger, W.; Dubin, P. L.
Biomacromolecules 2007, 8(11), 3568–3577.(15) Kayitmazer, A. B.;
Bohidar, H. B.; Mattison, K. W.; Bose, A.; Sarkar, J.;
Hashidzume, A.; Russo, P. S.; Jaeger, W.; Dubin, P. L. Soft
Matter 2007, 3(8),1064–1076.
(16) Menjoge, A. R.; Kayitmazer, A. B.; Dubin, P. L.; Jaeger,
W.; Vasenkov, S.J. Phys. Chem. B 2008, 112(16), 4961–4966.
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DOI: 10.1021/la903260r 13377Langmuir 2009, 25(23),
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Liberatore et al. Article
Coacervates formed from polyelectrolytes and proteins ofopposite
charge have been studied by static and dynamic lightscattering
(DLS),17 small-angle neutron scattering,12-14 fluores-cence
recovery after photobleaching,15 rheology,17,18 total inter-nal
reflectance microscopy, cryo-TEM,15 and pulsed-fieldgradient NMR.16
The results of these studies show that theseoptically clear,
viscous fluids have complex internal structuresthat are
heterogeneous on many length scales. While biologicaland
biotechnological objectives motivate studies with proteins,their
idiosyncratic charge anisotropies complicate elucidationof
electrostatic effects. In this regard, PE/micelle
coacervatesrepresent a simplification, even though micelle lability
is aconsideration. In particular, mixtures of polyelectrolytes
withionic-nonionic mixed micelles are good model systems becausethe
micelle surface charge density can be modulated by the molefraction
of ionic surfactant, especially when the surfactant con-centrations
are much higher than the mixed surfactant criticalmicelle
concentration.Dubin et al. performed extensive studies ona system
comprised of poly(dimethyldiallylammonium chloride)(PDADMAC),
together with mixed micelles of the anionicsurfactant sodium
dodecyl sulfate (SDS) and the nonionic sur-factant Triton X-100
(TX100), using narrow molecular weightdistribution samples of the
nonhydrophobic polycation. Theauthors showed that the micelle
surface charge density (σ) andsurface potential (j) varied directly
with the mole fraction of theanionic surfactant (“Y”).19
Consequently, gradual addition ofSDS to a mixture of polycation and
nonionic micelles resulted inprogressive changes in σ and j,
leading to transitions fromnoninteracting solutions to soluble
complexes at “Yc”, and thento liquid-liquid phase separation
(coacervation) at “Yφ”.
While Yφ appears to be a true liquid-liquid phase
transition,becoming infinitely sharp when system polydispersity is
elimi-nated,20 Yc may be more appropriately described as a
second-order phase transition.21 These transitions can be
identified bydynamic light scattering, electrophoretic mobility, or
preciseturbidimetry. Turbidimetry was used to obtain phase
behavioras a function of micelle surface charge density, PE
molecularweight, PE:surfactant stoichiometry, and ionic strength.
Thesemeasurements led to maps that identify the conditions
corre-sponding to soluble complex formation, coacervation, and
pre-cipitation.22 In contrast to PE/protein systems, the
PE/micellesolutions undergo temperature-induced coacervation, which
alsoappears to be a true liquid-liquid phase separation when
systempolydispersity is minimized.23 The critical temperature of
theliquid-liquid phase transition (Tφ) decreases with an
increasingPE molecular weight, and Tφ is highly sensitive to the
other keyvariables, including ionic strength, micelle surface
charge density,and PE:surfactant stoichiometry.23 The effects of
the micellesurface charge density and PE:surfactant stoichiometry
areclosely correlated with conditions at which the effective
(electrophoretic) charge of soluble complexes is close to
zero,20
which has also been observed for protein/PE systems.24,25
Low-speed centrifugation of the metastable droplet suspen-sions
formed when T> Tφ yields optically clear, viscous
fluids(typically 5-10 wt % surfactant and 1-2 wt % PE),
whichdisplay some interesting phenomena. DLS shows abundantspecies
with a diffusivity that is only 7 times smaller than that
ofmicelles in dilute solution (despite a coacervate viscosity that
is100 times larger) and are therefore described as “free
mi-celles”.26 The equilibrium nature of the dense
“coacervate”fluids thus formed is well-established bymultiple
studies, whichdemonstrate that their properties are reversible and
indepen-dent of both the time and route of preparation, as long
asirreversible liquid-solid separation is avoided.27 When
thiscoacervate is heated, a second phase separation
temperature(denoted as Tφ
0) is observed turbidimetrically as shown inFigure 1 from ref
26. As pointed out in ref 26, the correspond-ing transition is
essentially reversible, only showing a weakhysteresis. Of
particular interest is the appearance of a secondphase with shear
or elongational flows at temperatures slightlybelow Tφ
0.23 This flow-induced phase separation was observedwhen samples
were loaded into confined geometries as shownin Figure 2 of ref
26.
A substantial body of literature describes shear-induced
phaseseparation (SIPS)28 for wormlike micelles29-34 and selected
high-molecular weight (MW) polymers,28,35,36 but this is the
firstobservation of SIPS for a polymer-micelle complex.
Sinceneither the micelle nor the PE alone exhibits such behavior,
theimplication is that complexation with PE can cause
ellipsoidalmicelles to behave like other macromolecular systems
exhibitingSIPS. One interesting consequence of coacervation is the
possi-bility of achieving very high viscosities at relatively low
surfactantconcentrations, replacing surfactants with lower
concentrationsof less expensive polymers.
Figure 1. Turbidity as a function of temperatures for a
coacervatesolution. The transition from clear to turbid allows the
determina-tion of critical temperature Tj
0. Reproduced from ref 26. Copy-right 2008. American Chemical
Society.
(17) Bohidar, H.; Dubin, P. L.; Majhi, P. R.; Tribet, C.;
Jaeger, W. Biomacro-molecules 2005, 6(3), 1573–1585.(18) Mohanty,
B.; Bohidar, H. B. Int. J. Biol. Macromol. 2005, 36(1-2),
39–46.(19) Dubin, P. L.; The, S. S.; McQuigg, D. W.; Chew, C. H.;
Gan, L. M.
Langmuir 1989, 5(1), 89–95.(20) Wang, Y. L.; Kimura, K.; Huang,
Q. R.; Dubin, P. L.; Jaeger, W.
Macromolecules 1999, 32(21), 7128–7134.(21) McQuigg, D. W.;
Kaplan, J. I.; Dubin, P. L. J. Phys. Chem. 1992, 96(4),
1973–1978.(22) Wang, Y. L.; Kimura, K.; Dubin, P. L.; Jaeger, W.
Macromolecules 2000,
33(9), 3324–3331.(23) Kumar, A.; Dubin, P. L.; Hernon,M. J.; Li,
Y. J.; Jaeger,W. J. Phys. Chem.
B 2007, 111(29), 8468–8476.(24) Xia, J. L.; Dubin, P. L.; Kim,
Y.; Muhoberac, B. B.; Klimkowski, V. J.
J. Phys. Chem. 1993, 97(17), 4528–4534.(25) Gupta, A.; Reena;
Bohidar, H. B. J. Chem. Phys. 2006, 125, 5.
(26) Dubin, P. L.; Li, Y. J.; Jaeger, W. Langmuir 2008, 24(9),
4544–4549.(27) Kaibara, K.; Okazaki, T.; Bohidar, H. B.; Dubin, P.
L. Biomacromolecules
2000, 1(1), 100–107.(28) Larson, R. G. Rheol. Acta 1992, 31(6),
497–520.(29) Fischer, P.; Wheeler, E. K.; Fuller, G. G.Rheol. Acta
2002, 41(1-2), 35–44.(30) Oda, R.; Panizza, P.; Schmutz, M.;
Lequeux, F. Langmuir 1997, 13(24),
6407–6412.(31) Narayanan, J.; Manohar, C.; Kern, F.; Lequeux,
F.; Candau, S. J.
Langmuir 1997, 13(20), 5235–5243.(32) Liu, C. H.; Pine, D. J.
Phys. Rev. Lett. 1996, 77(10), 2121–2124.(33) Rehage, H.; Hoffmann,
H.; Wunderlich, I. Phys. Chem. Chem. Phys. 1986,
90(11), 1071–1075.(34) Hu, Y. T.; Boltenhagen, P.; Pine, D. J.
J. Rheol. 1998, 42(5), 1185–1208.(35) Migler, K.; Liu, C. H.; Pine,
D. J. Macromolecules 1996, 29(5), 1422–1432.(36) Rangelnafaile, C.;
Metzner, A. B.; Wissbrun, K. F. Macromolecules 1984,
17(6), 1187–1195.
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13378 DOI: 10.1021/la903260r Langmuir 2009, 25(23),
13376–13383
Article Liberatore et al.
In this work, we characterize the thermal and shear-inducedphase
transitions in this polyelectrolyte/micelle system by small-angle
light scattering (SALS) to determine the onset of phaseseparation
as a function of the macromolecular solute concentra-tion, shear
rate, and temperature. While previous SANS andcryo-TEM indicated
the formation of dense phases (i.e., micelle-rich phases) at length
scales of several hundred nanometers underquiescent conditions,
this work probes the formation of the densephase under shear at
larger length scales. In addition, the size andshape of the
phase-separated droplets are measured and a con-ceptual picture of
the transient structures is proposed. This workis representative of
ongoing efforts to provide a molecular basisfor understanding the
macroscopic properties of intermacroioniccoacervates.
Experimental Methods
Materials. Poly(diallyldimethylammonium chloride) (PDA-DMAC) was
prepared by free radical aqueous polymerization
ofdiallylmethylammonium chloride.37 The weight- and number-average
molecular masses of the purified lyophilized polymerwere 2.19�105
and 1.41�105 Da, determined by light
scatteringandmembraneosmometry, respectively.37TritonX-100
(TX100),a nonionic surfactant, was purchased from Fluka, and
sodiumdodecyl sulfate (SDS), an anionic surfactant, with a purityof
>99% and NaCl were purchased from Fisher. All wereused without
further purification. Milli-Q water was used in allsamples.
Coacervate Preparation. PDADMAC/TX100 solutions(containing
either 3 g/L PDADMAC with 20 mM TX100 or2 g/L PDADMAC with 10 mM
TX100) and 60 mM SDSsolutions were prepared separately in 0.4 M
NaCl. The sampleswere brought to coacervation via addition of SDS
to the mixedPDADMAC/TX100 solution to produce the desired mole
frac-tion of SDS defined as Y=[SDS]/([SDS] + [TX100]). Coacer-vates
can also be prepared by the addition, at fixed Y and ionicstrength,
of mixed micelles to PDADMAC, or PDADMAC tomicelles,38 although
systematic comparisons of the coacervates soformed are incomplete.
The equilibrium nature of this system isdemonstrated by the
reversibility under conditions correspondingto soluble complexation
or coacervation, although precipitation(e.g., induced by addition
of SDS to PDADMAC in the absence
of nonionic surfactant) is effectively irreversible. As
notedabove, coacervation could also be attained by increasing
thetemperature above a critical value Tj which depends onY, polymer
concentration (CP) andmolecular weight, and ionicstrength.23 The
value of Tj for the formation of coacervatefrom a homogeneous
polycation/mixed micelle solution istypically 5-10 �C below the
value of Tj0, the temperature atwhich the coacervate itself
undergoes additional phase transi-tion (see Figure 1). Thus, values
of Y and CP were chosento produce different values of Tj and hence
Tj
0. The turbidsample was then centrifuged for 1 h at 3500 rpm to
produce anoptically clear dilute (upper) phase and a dense (lower)
phase(“coacervate”). Preparation conditions and values of Tj
andTj
0 are given in Table 1. The progressive increase in Tj0
fromsample A to sample B to sample C indicates that these
coacer-vate samples (at, e.g., 20 �C) correspond to further
progresstoward coacervate phase separation, which is confirmed
byresults reported below. We note that values of Tj
0 (determinedby turbidimetry) in Table 1 are typically ∼2 �C
higher than theonsets of phase separation as determined by SALS as
discussedbelow. The differences in the values of Tj
0 are probably a resultof the heterogeneity of the coacervate,
arising from the hetero-geneity of TX100 and the heterogeneity
among the solublecomplexes from which coacervate forms. Because of
the het-erogeneity of several components of these samples,
relativecontributions of components may differ for turbidity
versussmall-angle scattering. The effect of system
heterogeneity,arising to a large extent from the chemical
heterogeneity ofTX100 (vis-�a-vis, for example, C12E8
23), is to broaden allobservable transitions, making it
difficult to establish theirorder.
Rheology and Small-Angle Light Scattering. Rheologyand SALS data
were collected simultaneously using anAR-G2 rheometer (TA
Instruments, New Castle, DE) withthe commercially available SALS
attachment. A transparentparallel plate configuration (50 mm
diameter) with a 1 mmgap was used for all tests. SALS images were
recorded forat least four different shear rates (from 0.1 to 32
s-1) alongthe flow curves for each of the three different samples
(A, B,and C) over a range of temperatures. All images were takenat
steady state; i.e., the viscosity and SALS image werenot changing
with time at a given shear rate. The sizescale of scattering
objects captured by this SALS setup is0.94-5.0 μm (q = 1.3-6.7
μm-1). To examine the shear-induced phase separations, the SALS
images were analyzedusing ImageJ with standard protocols for
subtracting thebackground and removing the beam stop from the
rawimages.39,40
The locations of the phase boundaries under shear weredetermined
by the normalizedmean intensity of the SALS images.The
normalizedmean intensity is a representation of the turbidityof the
solutions from the perspective of small-angle scattering.Similar
analysis has been used to determine critical shear rates
Figure 2. Photo of coacervate exhibiting flow-induced
phaseseparation in a confined sample geometry. Reproduced fromref
26. Copyright 2008. American Chemical Society.
Table 1. Preparation Conditions and Properties of the
CoacervateSolutionsa
sample Yb CPb (g/L) f-
c MWnb (kDa) Tj (�C)d Tj0 (�C)d
A 0.37 2 0.49 141 12 22B 0.35 3 0.35 141 19 24C 0.44 3 0.46 141
22 29( 2
aAll coacervates prepared in 0.40 M NaCl. b Solution from
whichcoacervate is obtained. Y is the mole fraction of anionic
surfactant,and CP and MW are the polymer concentration and
number-averagemolecular weight, respectively. cThe number of SDS
charges divided bythe sumof SDS and polycation charges. dProperty
of coacervate (see thetext). Tj and Tj
0 values are all (1 �C, except as shown.
(37) Dautzenberg, H.; Gornitz, E.; Jaeger,W.Macromol. Chem.
Phys. 1998, 199(8), 1561–1571.(38) Dubin, P. L.; Rigsbee, D. R.;
Gan, L. M.; Fallon, M. A. Macromolecules
1988, 21(8), 2555–2559.
(39) Kline, S. R. J. Appl. Crystallogr. 2006, 39, 895–900.(40)
TA Instruments.ARSeries Small Angle Light-Scattering
(SALS)Accessory
Manual; TA Instruments: Newark, DE, 2008.
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DOI: 10.1021/la903260r 13379Langmuir 2009, 25(23),
13376–13383
Liberatore et al. Article
fromSALS images.41 The normalizedmean intensity is defined asthe
ratio of the average intensity of the image (after removal ofthe
beam stop) to the intensity if all pixels in the image
weresaturated (i.e., a value of 255 for the 8-bit camera used).
Apositive value of the normalized mean intensity indicates
thepresence of micrometer size structures in the flow; however,
the magnitude of the normalized mean intensity does notcorrelate
with the size of the micrometer-scale scatteringobjects.
Following the method employed by Walker et al.,42 thecorrelation
length (ac) and aspect ratio (ar) of the droplets weredetermined
using Debye-Bueche43 plots (I-0.5 vs q2). A linearfit to the
radially averaged data plotted in the Debye-Buecheformat was
calculated in each case using the method ofleast squares (example
plot included as Figure S1 of theSupporting Information). The slope
and intercept of thelinear fit were used to calculate a
characteristic length [ac=(slope/intercept)0.5] for the droplets at
various shear rates.
Figure 3. Viscosity as a function of shear rate at several
tempera-tures for three coacervate solutions: (a) sampleA, (b)
sampleB, and(c) sample C.
Figure 4. Viscosity (either Newtonian μ or ηo from the
Crossmodel fit) as a function of temperature for three
coacervatesolutions: (a) sample A, (b) sample B, and (c) sample
C.
(41) Saito, S.; Hashimoto, T.; Morfin, I.; Lindner, P.; Boue,
F.Macromolecules2002, 35(2), 445–459.
(42) Walker, L. M.; Kernick, W. A.; Wagner, N. J.Macromolecules
1997, 30(3),508–514.
(43) Debye, P.; Bueche, A. M. J. Appl. Phys. 1949, 20,
518–525.
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13380 DOI: 10.1021/la903260r Langmuir 2009, 25(23),
13376–13383
Article Liberatore et al.
A calibration of the Debye-Bueche characteristic length
wasperformed using a Microbead NIST Traceable Particle SizeStandard
(Polysciences, Inc., Warrington, PA) of polystyrenemicrospheres (3
μm in diameter). A correction factor wasdetermined by comparing the
calculated ac with the reporteddiameter of the standard. This
correction factor was thenapplied to our calculations of ac for the
coacervate solutions.In addition, the distortion of the droplet
shape with increasingshear rate was characterized by the aspect
ratio, whichwas determined by taking the ratio of the q values in
thevorticity and flow directions for a given value of the
measuredintensity.
Results and Discussion
The rheology of the coacervate solutions shows dramaticchanges
as a function of temperature and shear rate. Thethermodynamically
stable coacervates behave as Newtonianfluids at low temperatures
(typically below Tφ
0) across the entireshear rate range studied (Figure 3). Once
the samples begin toexhibit shear-induced phase separation (as
observed by SALS),their viscosity becomes shear thinning. The Cross
rheologicalmodel (eq 1) was used to fit the non-Newtonian response
ofthe coacervates.44 The model quantifies the zero shear
viscosity,degree of shear thinning, and relaxation time of the
fluid. The fits(included as Figure S2 and Table S1 of the
SupportingInfromation) exhibit similarities in all three
coacervates. First,the shear thinning index is 1 for almost all of
the temperature-shear rate combinations exhibiting shear-induced
phase separa-tion (as quantified by SALS). Since the shear thinning
index is 1,the samples exhibit a stress plateau (i.e., shear stress
is indepen-dent of shear rate), which is one indicator of possible
shearbanding.34,45 In addition, the relaxation time increases with
anincrease in temperature. Therefore, the onset of shear
thinning,which corresponds to the appearance of SIPS, appears at
lowershear rates at higher temperatures.
η¼ η0 -η¥1 þ ð _γτRÞm þ η¥ ð1Þ
The coacervate samples exhibit similarites in the
temperaturedependence of the viscosity [either μ when Newtonian or
ηo fromthe Cross model (Figure 4)]. Before entering the phase
separatingregime, samples A and C exhibit a simple monotonic
decrease inthe viscosity as the temperature is increased; however,
theviscosity of sample B is nearly independent of temperature
belowTj. As all of the coacervate solutions approach the
phaseboundary (by temperature, shear, or the combination of the
two),the solutions begin to exhibit shear thinning. All of the
samplesexhibit an increase in the viscosity (for 5-8 �C) once in
the shearthinning regime. The increased viscosity is an indication
of thephase separation occurring; i.e., the transition from the
nanoscaledomain to micrometer-scale structures would increase the
visc-osity. The shear thinning regime begins 1-5 �C below the
criticaltemperature, Tj
0. Finally, the viscosity decreases for the highesttemperature
measured for all samples.
The turbidity (and thus the phase boundaries) of the
solutionsfrom the perspective of small-angle scattering is
quantified usingthe normalizedmean intensity (Figure 5). The beam
stop does notpermit collection of the light exiting the sample at a
scatteringangle of 0�; thus, direct comparison with the previous
turbiditymeasurements40 used to locate the phase boundaries is
notpossible. The lowest temperature with a non-zero normalized
mean intensity indicates the location of the phase boundary
atthat representative shear rate (indicated by the gray regions
inFigure 5). For example, sample A exhibits shear-induced
phaseseparation at 20 �C for all four shear rates, which is 2
�CbelowTj0(Figure 5a). Sample B also exhibits SIPS below Tj
0 (at 22 �C and0.1 s-1 in Figure 5b). The first appearance of
SIPS for sample C isat 27 �C for all measured shear rates (Figure
5c), confirming thatSIPS in all three samples is measured below
Tj
0. The anomalousappearance of Figure 5c might suggest
heterogeneity of transi-tions, and temperature-induced transitions
in quiescent samplesdo become markedly sharper when system
polydispersity isreduced via replacement of the chemically
polydisperse nonionicsurfactant TX-100 with C12E8, a monodisperse
analogue.
23 Overall,the nonmonotonic nature of the normalized mean
intensity as afunction of temperature does not directly correlate
with the size of
Figure 5. Normalized mean intensity of the SALS images as a
func-tionof temperatureat several shear rates for samplesA(a),B
(b),andC(c).Dashed lines are to guide the eye.Gray shading
indicates the regionin which the samples exhibit SIPS. The vertical
line indicates thelocation of Tj
0.
(44) Cross, M. M. J. Colloid Sci. 1965, 20, 417–437.(45) Hu, Y.
T.; Lips, A. J. Rheol. 2005, 49(5), 1001–1027.
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DOI: 10.1021/la903260r 13381Langmuir 2009, 25(23),
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Liberatore et al. Article
the phase-separated domains. The definitive information
fromdetermination of the normalized mean intensity is the location
ofthe transition from a single-phase fluid to a
phase-separatedsolution.
The shear-induced phase boundary may also be
characterizedrheologically using a temperature sweep. For example,
sample Awas heated at a rate of 1 �C/min while being held at a
constantshear rate of 10 s-1 (Figure 6). The evolution of the
viscosity of thecoacervate is continuous with three distinct
regions (i.e., onephase at low temperatures, transition to the
second phase nearTj
0, and a two-phase region at higher temperatures). At
lowtemperatures, the viscosity decreases slowly with an increase
intemperature until ∼22 �C (an average change in viscosity of-0.065
Pa s �C-1). Between 22 and 25 �C, the viscosity dropsmuch more
dramatically (-0.12 Pa s �C-1). The clear coacervatesolution at
temperatures below 20 �C undergoes a transition to acompletely
phase-separated state at 25 �C and 10 s-1. Above25 �C, the
viscosity changes very slowly with temperature(-0.0045 Pa s �C-1).
The appearance of phase-separated struc-tures via SALS (see the
SALS image at 22 �C inFigure 6) precedesthe period of strongly
decreasing viscosity. Samples B and C alsoexhibit shear-induced
phase separation before the large decreasein the viscosity of the
solution. Therefore, SALS is a moresensitive measure of SIPS than
viscosity measurements alone.
The native aggregate structure of these two samples can
bedirectly compared using cryo-TEM (Figure 7). Sample B exhibitsa
distribution of ca. 50 nm aggregates interconnected to formextended
clusters. For sample A, these clusters appear to bedisconnected and
collapsed intomore dense objects similar in size.These sizes are
identical to those seen for soluble interpolymeraggregates prior to
coacervation; the aggregates appear to arisefrom association of
smaller intrapolymer complexes. For sampleA, the cryo-TEM
“snapshot” is taken at a vitrification tempera-ture well above
Tj
0. However, the Tj0 of sample B is close to thecryo-TEM
vitrification temperature of 24 �C. The native 50 nmaggregates in
the coacervates transform into themicrometer-scaleobjects under
shear, which are characterized by SALS.
It is of interest to compare samples A and B vis-�a-vis the
valuesof f- in Table 1, inasmuch as sample A, formed from a
mixturecloser to charge stoichiometry than sample B (f- values of
0.49and 0.39, respectively), shows more collapsed aggregates(Figure
7) and shows a transition at a lower temperature(Figure 5). The
importance of charge stoichiometry has beenunderlined by several
experimental9 and theoretical10 works.
However, the correlation between the bulk or mixing
chargestoichiometry, e.g., f-, and the microscopic
stoichiometry(the value of f- in the coacervate or the complex
aggregatesthat precede them) is less clear for phase separation in
the saltysolutions studied here than in salt-free systems that
undergophase separation stoichiometrically.46,47 For example, a
coa-cervate formed at Y=0.50, at an ionic strength of 0.80 M,
wasfound to contain only 23% of the total polymer and evenless (8%)
of the total surfactant, preferentially the anionic, i.e.,aY value
of 0.61 in the coacervate;48 with a similar result from asolution
in 0.40 M NaCl and a Y of 0.35, yielding a coacervatewith aY of
0.51, and with excess polycation on a charge basis.49
The tendency toward charge neutralization and retentionof
counterions are both factors that influence coacervatecomposition.
In addition, comparisons of samples A and Cwith nearly identical
values of f- show that total macro-molecular concentrations
influence coacervate properties. Thepronounced effects of
polycation MW at fixed f- noted else-where23 also suggested caution
in interpretation of the effectsof bulk stoichiometry here.
Two-dimensional SALS images provide information about
themicrostructure of a fluid by inspection. Overall, the
coacervatesundergo a transition from a homogeneous isotropic
one-phasesolution to a heterogeneous anisotropic phase-separated
sys-tem. A set of scattering images for sample A are
representativeof the three main types of scattering measured for
the coacer-vate system as a function of temperature and shear
rate(Figure 8). An almost completely black image is observed
atlower temperatures and all shear rates. The coacervate
phaseseparation is a nanoscale phenomenon under
low-temperatureconditions and thus cannot be detected by SALS.
Next, acircular region of high scattering intensity is observed
(e.g., T=22 �C and shear rate = 0.1 s-1 in Figure 8). The
circularscattering pattern indicates a nearly circular droplet on
themicrometer scale. The larger areas of intense scattering in
theimages at 22 �C versus 20 �C (at a shear rate of 0.1 s-1)
indicatea smaller droplet size (i.e., images represent length
scalesin inverse space). The third type of scattering image asseen
at 24 �C and 1 s-1 (Figure 8, right column) is ellipsoidalwith the
long dimension of high-intensity scattering in thevorticity
direction. SALS is a more sensitive measurement ofshear-induced
phase separation than flow rheology as indi-cated by the appearance
of micrometer-scale structures viaSALS before the sample begins to
shear thin (e.g., sample A at22 �C and 1 s-1). Measured SALS images
for samples B and Care included as Supporting Information (Figures
S3 and S4 ofthe Supporting Information).
Similar transitions fromno scattering to circular and
ellipsoidalscattering are observed with increasing temperatures for
all threesamples. Micron-sized scattering objects are observed at
g20 �Cfor sample A (at all of the measured shear rates). The
phase-separated droplets are very close to circular (assumed to
bespherical if three-dimensional data were available) at a shear
rateof 0.1 s-1 and ellipsoidal at all higher shear rates. Sample
Bexhibits phase-separated droplets beginning at 24 �C
(nearlycircular 2D scattering image). Ellipsoidal droplets are
observedfor sampleB from24 �Cand 1 s-1 to 30 �Cand 32 s-1. SALS
from
Figure 6. Viscosity as a function of temperature for sample A at
ashear rateof 10 s-1 showing the transition fromaone-phase state
toa phase-separated state. Arrows correspond to the locations
ofreported SALS images.
(46) Ahmed, L. S.; Xia, J. L.; Dubin, P. L.; Kokufuta, E.
J.Macromol. Sci., PureAppl. Chem. 1994, A31(1), 17–29.
(47) Tsuboi, A.; Izumi, T.; Hirata, M.; Xia, J. L.; Dubin, P.
L.; Kokufuta, E.Langmuir 1996, 12(26), 6295–6303.
(48) Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983,
95(2), 453–461.(49) Davis, D. D. Intermacromolecular association of
polycations with oppo-
sitely charged micelles. M.S. Thesis, Purdue University, West
Lafayette, IN, 1984.
-
13382 DOI: 10.1021/la903260r Langmuir 2009, 25(23),
13376–13383
Article Liberatore et al.
sample C first appears at a higher temperature, 27 �C, than
thatfrom samples A and B and exhibits the same progression
throughcircular and ellipsoidal scattering as temperature and shear
rateincrease.
The characteristic size and aspect ratio of the
phase-separateddroplets were derived from the radially averaged
intensity (e.g.,using Debye-Bueche plots like Figure S1 of the
SupportingInformation). The droplet size ranges from ∼1 to 4 μm,
and theaspect ratio of the droplet varies from ∼1 to 4. The
continuous
transition from no small-angle scattering to circular and
finallyelongated scattering patterns is observed with an increasing
shearrate at a constant temperature. For example, sample A shows
thecontinuous progression of shapes in the shear-induced
structuresat a temperature above Tφ
0 (Figure 9). At 0.1 s-1 and 26 �C, anearly circular SALS
pattern is observed, which corresponds to adroplet with an ac of
1.0 μm and an ar of 1.0. At 0.1 s
-1,approximately 1 μm nearly circular droplets appear as the
first
Figure 7. Cryo-TEM images of (a) sample A and (b) sample B
showing a native aggregate size of∼50 nm.
Figure 8. Small-angle light scattering images as a function
oftemperature at two shear rates for sample A. SIPS occurs at
highertemperatures and shear rates.
Figure 9. (a) Viscosity as a function of shear rate at 26 �C
forsample A (top) with inset SALS patterns representing the
smoothtransition from circular to ellipsoidal droplets with an
increase inaspect ratio. (b) Characteristic length (ac) and aspect
ratio of thephase-separated droplets for sampleA corresponding to
the SALSpatterns in panel a.
-
DOI: 10.1021/la903260r 13383Langmuir 2009, 25(23),
13376–13383
Liberatore et al. Article
observation of shear-induced phase separation in all three
coa-cervate solutions. The nearly circular droplet becomes
elongatedat higher shear rates until the aspect ratio of the
droplet reaches 3.0at 25 s-1 for sample A (Figure 9). The
increasing aspect ratio withshear rate indicates the growth of the
droplets is almost exclusivelyin the flow direction (and is
observed for all three samples).
A conceptual picture of SIPS in coacervates can be derivedfrom
the experimental observations presented earlier. Ingeneral, shear
flow transforms the clear coacervate fluid withdomain sizes of
50-100 nm by transforming the polyelec-trolyte/micelle complexes
into extended chains or “necklacesof polyelectrolyte decorated with
micelle beads”50 as por-trayed in Figure 10. In a manner entirely
analogous to shear-induced phase separation of simple polymers,28
these ex-tended chains create efficient
inter(polyelectrolyte/micelle)interactions, presumably at the
expense of intra(poly-electrolyte/micelle) interactions, with
complementary spacingof bound micelles that can then interact
electrostatically withmicelle-poor domains of adjacent complexes, a
form of“polarization”.10 These more efficient interactions
promotethe expulsion of small ions from the complex, resulting in
theformation of micrometer-scale phase-separated droplets. Thephase
transition temperature Tj
0 coincides with the transitionfrom shear-independent
(Newtonian) to shear-dependentviscosity. The initial appearance of
small-angle scatteringprecedes the transition in rheology from
Newtonian to shearthinning. Further studies are needed to correlate
transitiontemperatures for quiescent phase transitions reported
fromturbidimetry and small-angle scattering with observationsmade
under shear, a task that may be facilitated by thereduction of
system polydispersity.
Conclusions
The phase behavior under flow of a polycation/mixed
micellecoacervate was investigated by rheology and rheo-SALS.
Although shear-induced phase separation has been
extensivelyreported for solutions of certain polymers and wormlike
micelles,this is the first observation of SIPS for a
polymer/micelle systemand likely the first quantitative study of
SIPS in a complexcoacervate. Under shear, the coacervate solutions
convert fromhomogeneous, isotropic, one-phase systems to
heterogeneous,anisotropic, two-phase systems. Thus, behavior
typicallyobserved for wormlike micelles is attained for small
micellesbound to a polyelectrolyte. Figure 11 provides a summary
ofthe shear- and temperature-induced phase transitions observed
inthis work. Below 18 �C, all three of the coacervate solutions
areoptically clear Newtonian fluids across the entire shear rate
rangeinvestigated. Once a critical temperature and/or shear rate
isachieved, phase separation occurs. Two definitive SALS
patternsdemonstrate the appearance of circular droplets at low
shear ratesnear the critical temperature and ellipsoidal droplets
at highertemperatures and shear rates. The shear-induced droplets
range insize from∼1 to 4 μm.The ellipsoidal droplets have aspect
ratios ashigh as∼4. Overall, the shear-induced phase separation has
beenexplored as a function of shear rate and temperature at the
steadystate. Additional insights will be gained via exploration of
thekinetics of the phase separation under flow and the possibility
ofshear banding.
Acknowledgment. Partial support of this work was receivedfrom
the donors of the Petroleum Research Fund (M.W.L.).Portions of this
work were supported by grants from ShiseidoCorp. (P.L.D.) and from
the donors of the Petroleum ResearchFund to A. Dinsmore and P.L.D.
We acknowledge assistancefrom Dr. JoAn Hudson (AMRL, Clemson
University, Clemson,SC) with cryo-TEM.
Supporting Information Available: Description of thematerial, an
example fit of the small-angle light scatteringdata to the
Debye-Bueche model, viscosity curves fit to theCross model, model
fit parameters, and 2D SALS images asa function of shear rate and
temperature for samples B andC. This material is available free of
charge via the Internet athttp://pubs.acs.org.
Figure 10. Schematic representation of shear- and
temperature-induced phase separation for PDADMAC/TX100-SDS
coacer-vate. Both processes involve loss of counterions arising
from anincreased number of polyelectrolyte-micelle interactions,
but byan intercomplex vs intracomplex mechanism for the
former.Reproduced from ref 26. Copyright 2008. American
ChemicalSociety.
Figure 11. Schematic of the relevant domain size of the
coacervatesolutions with changing temperature and shear rate.
(50) Lee, L. T.; Cabane, B. Macromolecules 1997, 30(21),
6559–6566.