Unusual rheological effects observed in polyacrylonitrile solutions

Rheological Evidence of Gel Formation in Dilute Poly(acrylonitrile)SolutionsAlexander Malkin,*,† Sergey Ilyin,† Tatyana Roumyantseva,‡ and Valery Kulichikhin§

†A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29, Leninsky prospekt, Moscow 119991, Russia‡A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31, Leninsky prospekt, Moscow119991, Russia§M.V. Lomonosov Moscow State University, Chemical Faculty, 1/3, Leninskie Gory, Moscow 119991, Russia

ABSTRACT: Rheological studies of low concentration (down to 0.1%) poly-(acrylonitrile) (PAN)−dimethyl sulfoxide (DMSO) solutions revealed the existenceof a rather unusual effect of the gel-like structure formation, while this phenomenon isabsent at higher concentrations (17−24 wt %) of PAN in DMSO. The evidence of thegel-like structure consists in the pronounced yielding, which can be detected in the0.1−5% concentration range and plateau on the frequency dependence of the storagemodulus. In addition, rather large particles were found by the dynamic light scatteringmethod. The gel-like structure of dilute PAN solutions is similar to supramolecular gels, while the gelation of water containingsystems brings to mind a phase transition. The observed maximal Newtonian viscosity at the low shear stresses for dilutesolutions should be treated as an artifact related to the method of measuring, and at long-term loading the plateau is converted tothe yield stress. Dilute PAN/DMSO gels are thixotropic. A possibility of bifurcation (due to coexistence of different states) leadsto self-oscillations of the stress response in a wide shear rate region. The dynamic viscosity coincides with the shear apparentviscosity for high concentrations but does not in the low concentration range. Such a kind of rheological behavior is not observedif dimethylformamide or aqueous solution of sodium thiocyanate was used instead of DMSO as a PAN solvent. Possible chemicalstructures responsible for the gel structure formation are discussed.

■ INTRODUCTION

The results of long-term investigations on the rheology ofpolymer solutions are an undeniable input into our under-standing of fundamental properties of macromolecules inrelation to their structure and intermolecular interactions.Currently basic information about rheological properties ofpolymer solutions enters classical textbooks on polymers1,2 andseems quite familiar. However, continuing experimental studiesof numerous and various polymer solutions continue to amazeus with new and unexpected results. Generally speaking, theyare related to a “structure” of solutions (regardless of theuncertainty of this term) related to intermolecular interactions.According to the fundamental Flory’s concept3 and well-knowndirect experimental evidence,4 polymer solutions are treated asstatistically structureless. Meanwhile, there are a lot of examplesof structure formation in polymer solutions proven by directoptical and/or spectral methods as well as manifested in theirmacro-properties (including rheological properties). It is worthmentioning such a well-known phenomenon as assembling inblock copolymers.5−9 The other well-known case of clusteringin solution is poly(ethylene oxide)/water systems.10,11 Finally,one of the most studied cases of structure formation is worm-like micelles solutions,12,13 and the analogy between theseobjects and polymer solutions was always stressed in describingtheir behavior.14 One can easily find other examples ofstructuring and self-assembling in polymer solutions.It seems a rather natural that the effect of structure formation

and self-organization appears at some threshold concentration

and becomes more and more evident with increasing polymerconcentration in a solution. Indeed, chaos-to-order transition inrigid-chain polymer solutions and in micellar solutions becomespossible above some critical concentration of a disperse phase.Deformation-induced ordering also becomes possible abovesome concentration and at high enough stresses.15 This lookslike the general law. The more surprising and unexpectedoutcome was the reverse case of structuring at low polymerconcentration in a solution and disappearance of this effect inincreasing concentration. This phenomenon has been found forpolyacrylonitrile (PAN) solutions, and it became a subject ofthis study.The long-term interest in properties of PAN solutions is

explained by their application for producing one of the mostpopular synthetic fibers in textile industry and as a precursor forcarbon fibers as the component of numerous high-techproducts. It is also possible that the interest in PAN solutionswas limited to a rather narrow concentration range (usually,from 17 to 23%) corresponding to real solutions used intechnology.16−21

We carried out rheological investigations as a method offollowing structure transformation in solutions and correspond-ing physical experiments with PAN solutions in a much widerrange of concentrations focusing on the low concentration

Received: July 10, 2012Revised: November 19, 2012Published: December 13, 2012

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© 2012 American Chemical Society 257 dx.doi.org/10.1021/ma301423u | Macromolecules 2013, 46, 257−266

pubs.acs.org/Macromolecules

range. It allowed us to find several rather interesting andunusual features in the behavior of such solutions not typicalfor numerous polymer solutions.

■ EXPERIMENTAL SECTIONSamples. The base sample used as received was standard industrial

grade polyacrylonitrile (PAN) based random copolymer (acrylonitrile/methyl acrylate/sodium itaconate = 93.0/5.7/1.3 wt %) produced byLLC “Composite Volokno” (Saratov, Russia) and destined primarilyfor the textile and carbon fiber industry. It is an amorphous polymer,though some ordered regions can appear in a highly oriented fiber butnot in a polymer coil in a solution.The majority of experiments were carried out with DMSO as a

solvent, analytical grade purchased from Sigma-Aldrich, and distilledwater as a nonsolvent. Besides other solvents, 51.5% water solution ofsodium thiocyanate and dimethylformamide (DMF) were used forcomparison.The intrinsic viscosity for PAN solution in DMSO measured on a

standard Ubbelohde viscometer at rather high shear rates (>100 s−1) isequal to [η] = 1.78 dL/g. Exercising high flow rates in measuring [η]was essential because it allows us to destroy a possible self-organizationof PAN macromolecules and to exclude the effect of structureformation in PAN solutions which will be discussed in this work. Wecan therefore assume that this value corresponds to individualmacromolecules. The crossover (overlapping) concentration, c*,would be close to 0.5% if we dealt with a real homogeneous solution.The intrinsic viscosity vs viscometric molecular weight Mη

relationship for PAN is22

η = × η− M[ ] 2.685 10 4 0.768

Then Mη = 94.6 kDa.The same measurements were carried out in DMF solutions bearing

in mind that DMF solutions do not demonstrate any anomaly. Theintrinsic viscosity of the PAN sample measured in DMF (1.53 dL/g)was close to [η] in DMSO solutions determined at high shear rates. Itmeans that in both cases we received values corresponding toindividual macromolecular chains.The weight-average molecular weight Mw of the sample was

measured by the static light scattering method in its DMSO solutionon a Zetasizer Nano ZS (Malvern Instruments). Its value was 97.4kDa. The closeness of the Mη and Mw values shows that the MWD ofthe sample under study is narrow.The obtained value of the molecular weight is typical for industrial

grades of PAN. Specimens with practically the same molecularcharacteristics are often used in many works, e.g., in the above-citedpublications.16−21

For clearing up the role of small content of itaconate in terpolymer,a model sample of poly(acrylonitrile/butyl acrylate) copolymer (PAN/PBA) without itaconate was synthesized. The synthesis was performedin water with ammonium persulfate and sodium bisulfate as initiators.Molecular weight characteristics of this sample were measured by

the GPC method using a standard Waters Styragel instrument and N-methylpyrrolidone as a solvent. This polymer has the followingmolecular characteristics: Mw = 744.6 kDa and Mη = 211.9 kDa. Themolecular weight of this specimen was higher than our terpolymersample, but it is not important for comparison with PAN because theessential point is that PAN/PBA did not contain itaconic groups.Both solvents DMSO and DMF belong to the same class of bipolar

aprotic solvents. Their physical properties are rather similar andcollected in Table 1. This table includes such parameters as density, ρ,viscosity, η, dipole moment, m, Hildebrand solubility, δ, and Hansensolubility parameters (contributions of dispersion δd, polarity δp, andhydrogen bonding δh) at 20 °C. The same parameters forhomopolymer PAN are also included in Table 1.23

As is seen, solubility parameters for DMSO and DMF are close,though DMSO looks preferable for PAN.Solutions of PAN in DMSO in a wide concentration range (from

0.03 to 24 wt %) were studied.

We used two protocols of sample preparation. The main oneconsisted in direct dissolution of PAN fibers taken in a certain quantityin a solvent. We also prepared samples by step-by-step dilution ofmore concentrated solutions to the desirable concentration. Experi-ments showed that there were no differences in properties of solutionsprepared by this or that protocol. We will therefore give experimentaldata below not mentioning the way of sample preparation.

Ternary systems PAN−DMSO−water with the water content from1 to 13 wt % and a step of 1% were studied for several concentrationsof PAN. In all ternary compositions PAN concentration is maintainedconstant in varying the partial substitution of DMSO with water.

Rheology. Rheology was the main (but not a single) method ofPAN solutions characterization. Experiments were performed on arotational rheometer Physica MCR 301 (Anton Paar) with a cone-and-plate operating unit (cone diameter 50 mm, angle between conesurface and plate 1°).

In order to prevent water absorption, a thin layer of a nonvolatileand incompatible liquid, hexadecane, was placed on the sample surface.

The following regimes of shearing were used: (i) harmonicoscillation at constant angular frequency, ω, equal to 6.28 s−1 withvarying amplitude of deformation, γ, for estimation of the limit oflinear behavior of samples under study; (ii) low amplitude harmonicoscillations in the linear domain of viscoelasticity in the angularfrequency range from ω = 0.1 to 628 s−1; (iii) flow curvemeasurements in the constant rate regime (γ = constant) by theshear rate sweep in the range from γ = 10−5 to 104 s−1 with a step offive experimental points for one decimal order and time of shearing atany point of 60 s; (iv) evolution of apparent viscosity, η, at some givenconstant shear rate or shear stress at low values of these parameters.

Physical Methods. IR spectroscopy of PAN films as well as PAN/DMSO or PAN/DMF solutions placed between two polyethylenecovers was carried out on the FT-IR spectrometer Nicolet 380(Thermo Scientific).

Transmission spectra in the visible and infrared wavelength rangeswere measured using an Evolution 300 (Thermo Scientific)spectrophotometer.

Particle size distribution was measured by the dynamic lightscattering method on a Zetasizer Nano ZS (Malvern Instruments).

The main body of experiments was carried out at 20 ± 0.1 °C.

■ RESULTSViscoelastic Properties. Figure 1 demonstrates an example

of the amplitude dependencies of the storage modulus, G′(γ),of PAN−DMSO solutions of different concentrations at asingle frequency.We can draw two interesting conclusions from these data: a

rather long-range of linearity and shortening this range indecreasing the PAN concentration in a solution. A longamplitude range of linearity is typical for rubber-like fluids,while short limit of the linear behavior is proof of the existenceof some inherent structure destroyed by deformation. Thesepreliminary propositions will be compared with otherrheological data.Frequency dependencies of the storage modulus, G′(ω), are

presented in Figure 2 for the linear domain of viscoelasticity(deformation amplitude of 1%).The concentration effect is also rather interesting. It is seen

that the viscoelastic properties of concentrated solutions, let us

Table 1. Physical Characteristics of Solvents and PANHomopolymer23

sampleρ,

g/mLη,

mPa s m, Dδ,

MPa1/2δd,

MPa1/2δp,

MPa1/2δh,

MPa1/2

DMSO 1.10 1.98 3.957 26.7 18.4 16.4 10.2DMF 0.95 0.82 3.807 24.9 17.4 13.7 11.3PAN 1.18 1.81 27.4 21.7 14.1 9.1

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say in the range of 15−24%, are quite typical for viscoelasticsolutions, while the storage modulus for dilute solutionsbecomes independent of frequency, and it is typical for solid-like (or gel-like) media.Both sets of experimental data therefore suggest that some

anomalies in the rheology of PAN−DMSO systems exist.Viscometry. The nature of these anomalies becomes

evident from the results of measuring flow curves. Viscousproperties of PAN−DMSO systems are presented in Figure 3as the dependencies of the apparent viscosity, η, on shear stress,σ.As is shown, flow curves of moderately concentrated

solutions (with concentration exceeding approximately 10c*,namely, forming entanglements as usual for flexible macro-molecules in solutions) are typical for polymer solutions. Flowcurves of concentrated (15−24%) PAN solutions are New-tonian or almost Newtonian as was described by many authorswho only studied this concentration range. However, low-concentration solutions demonstrate the yield-like behaviorwhich allows us to treat them as weak gels. The yield stress isquite visible for solutions with polymer concentration in therange 0.1−1%. The yield stress lies in the range 0.01−1 Pa, andthis threshold increases with increasing concentration. Datacollected in Figure 3 show that a distinct yield stress is observedat a concentration of PAN as low as 0.1%. The viscosity of the0.03% solution is practically the same as the viscosity of a puresolvent.

The formation of the gel-like structure at so low aconcentration is not the exception. In our publication,24 itwas shown that the yielding behavior can be observed inmolecular gels at concentrations of the same order, as low as0.036%.In the used measuring protocol, the apparent maximum

Newtonian viscosity can be measured. For many years, it wasassumed that the initial Newtonian viscosity in “complete” flowcurves is a physical reality presenting “creep flow” related to theinteraction of structure-forming elements of a gel system, e.g.,friction of solid particles in suspensions. However, in severalrecent publications,25,26 it was shown that these values shouldbe treated as artifacts related to the transient regime ofmeasuring determined by the nonsufficient time of shearing toreach the steady-state flow conditions. Quite the same approach(the transition from the Newtonian plateau to the yield stressdue to rheopexy of emulsions) was considered in paper forhighly concentrated emulsions.27 Rather close results werereported in our papers.24,28 It has been shown for manyviscoplastic media in the cited publications and also confirmedfor our PAN/DMSO dilute systems.It becomes quite clear whether to change the sweep

deformation protocol for constant stress or shear rate regime:apparent viscosity will monotonously grow (Figure 4). There is

Figure 1. Amplitude dependencies of the storage modulus of PAN−DMSO solutions. Numbers at the curves are weight concentrations ofPAN in solution. Angular frequency equals 6.28 s−1.

Figure 2. Frequency dependencies of the storage modulus of PAN−DMSO solutions. Numbers at the curves are weight concentrations ofPAN in solution.

Figure 3. Viscosity on shear stress dependencies for PAN−DMSOsystems. Numbers at the curves are weight concentrations of PAN insolution.

Figure 4. Growth of the apparent viscosity of 5% PAN in DMSO intime at constant shear rate and constant stress. The viscosity versustime dependence in log−log scales is shown in the inset for stress 0.01Pa.



no limit in this curve, and one can suppose that the stress at γ =constant will approach the yield limit. It is especially clear inpresentation experimental data in log−log scale (Figure 4,inset). The time dependence of apparent viscosity (at σ =constant) is described by a power law with the exponent equalto 0.54. No tendency to some limit is observed. It means thatbehavior of dilute PAN−DMSO systems is equivalent to manyother gel-like soft matters. The situation observed for 21%PAN−DMSO is quite different: the apparent viscosity remainsthe same during at least 60 min at either constant shear rate orshear stress. It means that this system should be treated as atypical polymer solution which flows at any shear stress. Soagain we meet with the confirmation of the main concept ofthis study: structure is created in dilute PAM/DMSO solutionsbut not detected in semiconcentrated solutions.Another interesting concern is related to the thixotropic

behavior of dilute PAN/DMSO systems. A quasi-Newtonianplateau in Figure 3 was obtained according to the chosen sweepprotocol. It does not have physical meaning so far as thetransition to yielding instead of a Newtonian branch indownward sweep takes place.The same type of rheopectic behaviortransition from a

quasi-maximal Newtonian branch to yieldinghas beenobserved for highly concentrated emulsions.27 Thixotropiceffects related to the yielding are well-known29 and are thecause of time-dependent or “soft” yielding of differentmedia.30−32 Our system also belongs to materials of this class.So, we meet with two types of behavior of PAN/DMSO

systemslike non-Newtonian fluids at high concentrations andlike yielding viscoplastic media at low concentrationsand themost interesting things happen just at low concentrations.The other characteristic feature in deformation induced

structures is their instability at a given shear rate (in a certainrange of shear rates). This instability is expressed in periodicoscillations of stress (or apparent viscosity).33 Experimentaldata presented in Figure 5 show this type of rheologicalbehavior for gels/solutions studied in this work.

Periodic stress oscillations are a consequence of transitionsbetween two structural forms of matter. In our case, this is atransition between solid-like (gel) and fluid-like (solution)states of PAN/DMSO systems. It is reasonable to think thatthis phenomenon is inherently related to a multivalued flowcurve with periodic jumps from one branch to the other.

Indeed, a careful examination of flow curves presented intraditional coordinates of shear stress vs shear rate (Figure 6)

shows that there is a branch with negative differential viscosityor plateau on the flow curves. Such a type of flow curve is well-known. It always reflects instability of flow pattern ofmacromolecular or colloid systems13,34−36 and is consideredas a standard reason for self-oscillations.37

It is interesting to note that the maximal stress observed inthe self-oscillation regime corresponds to the yield stress in flowcurves (gel-like state) and the minimal stress corresponds tothe minimal apparent viscosity at this shear rate (solution-state). Self-oscillations are therefore due to transitions betweentwo limiting states of the gel/solution structure.There is a close similarity between the experimental data

presented in Figures 5 and 6 and the effect of shear bandingover and over again described for worm-like micellarcolloids.36,38−43 It is also interesting to mention that a similartype of instability was observed for polymer solutions.44−46

Therefore, our experimental data presented in Figures 5 and 6correspond to the peculiarities of behavior common fordifferent complex fluids. The basic reasons for this instabilityare also the samecoexistence of different structure states ofmatter reflected in multivalued flow curves.In discussing the concentration dependence of viscosity, it is

reasonable to consider minimal Newtonian values because themaximum Newtonian viscosity is uncertain and should betreated as artifact, as has been mentioned above. In the range ofhigh shear stress all PAN/DMSO systems are not gels buttypical polymer solutions. The concentration dependence ofthe minimal viscosity is almost linear at low concentrations anddescribed by the power law with a standard exponent of theorder of 5−7 in the range of high concentrations (above thecrossover point) as in many other polymer solutions.47

Experimental data presented in Figure 3 allow us to makesome quantitative characteristics of the concentration depend-encies related to the domain of the structure formation. Figure7a shows the concentration dependence of the yield stress, andFigure 7b demonstrates the viscosity jump in the yield stressrange (not forgetting that the ηmax values depend of theprotocol of measurements) as functions of the weight share ofthe polymer, w, in solution.The concentration dependence of the yield stress (Figure 7a)

is close to linear (the line in the figure has the slope equal to 1).

Figure 5. Periodic oscillations of the apparent viscosity for 1% PANcontent in DMSO. Numbers at the curves are the global preset shearrates.

Figure 6. Flow curves of PAN−DMSO systems. Double arrows markthe range of oscillations. Numbers at the curves are the weightconcentration of PAN in solutions.



In our paper,24 we found that the slope for a typicalsupramolecular hydrogel described in this paper was equal to2.0, and this value was also observed for organogels.48 Thisvalue of the slope corresponds to the model of a self-assembledfibrillar network formed by energetic interactions.49 Theoreticalanalysis predicted that the plateau modulus scales withconcentration, c, as G′ ∝ c11/5 for entangled solutions and asG′ ∝ c5/2 for densely cross-linked gels.50 The exponent value inthe G′(c) dependence for suspensions is even higher: the valuesof 4.2 and 4 ± 0.5 were reported.51,52 However, theconcentration dependence of the yield stress for emulsions ispractically linear.53,54 A medium is therefore softer; theexponent is lower. Here we obtained a low value of theexponent in a scaling relationship similar to one obtained foremulsions.It is interesting that the straight line in Figure 7a being

presented in linear coordinates goes through the zero point. Itmeans that supramolecular structure appears at very lowconcentrations below the limit of the instrumental possibilities.If we treat the observed yielding as a reflection of certainsupramolecular structures, then the nature of bonds providingthese structures nevertheless remains debatable. Below we willtry to make an assumption about this issue.Figure 7b illustrates the disappearance of the viscosity jump

with increasing concentration. At concentrations exceeding18−20%, there is no jump and we are only dealing with a weakviscosity anomaly (non-Newtonian flow).Static and Dynamic Viscosity. Another difference in the

rheology of dilute and concentrated systems is shown in Figure8. This is the kind of correlation between the steady apparentviscosity η(γ) on shear rate and frequency dependence of thecomplex dynamic viscosity |η|*(ω).The equivalence of the shear rate dependence of the

apparent viscosity, η(γ), and the frequency dependence of theabsolute dynamic viscosity, |η|*(ω), is a well-known fact formany viscoelastic liquids (the Cox−Merz rule), and its originlies in the nature of non-Newtonian behavior as theconsequence of elasticity of a medium. This equivalence reallytakes place for a 24% solution. But the situation for dilutesystems (2.5 and 7.5%) is different. It means that there is theother factor (besides elasticity) leading to the non-Newtonianbehavior. It is reasonable to think that this factor is associatedwith the structure of these system destroyed by deformation.Ternary Systems. The overall picture of rheological

behavior does not change much for the ternary systemPAN−DMSO−water as compared to a PAN−DMSO system.

The amplitude dependencies of the storage modulus arepresented in Figure 9, at which the systems with 5% PAN (a)and 18% PAN (b) are compared.Adding 11% water to the 5% PAN solution leads to a strong

increase in the linearity domain from 1 to 100% strain, whilethe addition of water to an 18% PAN solution does not lead toa noticeable change in the type of observed dependencies. Thesituation with respect to frequency dependencies of the storagemodulus in low and high PAN concentration ranges is shown inFigure 10. The increase in the loss modulus G″ is analogous tothe changes in G′ and therefore is not shown here.Addition of water increases the storage modulus in both

systems. Besides, the character of the G′(ω) dependence for a5% system remains practically the same, while G′(ω) for aconcentrated solution transforms drastically. Indeed, thestorage modulus becomes almost frequency independent fora system with 7% of water. It is a testament of the solution-to-gel transition in this system, since the independence of thefrequency characteristic of the storage modulus is a typicalfeature of a solid-like soft matter.The type of flow properties of the diluted PAN system when

water is added is presented in Figure 11.It is revealed that the addition of water results in the strong

enhance of gel-like behavior of all systems includingconcentrated solutions. In all cases, the increase in the yieldstress is expressed very clearly. Moreover, the fall of apparentviscosity at the yield stress in the 5% PAN + DMSO + 13%

Figure 7. Concentration dependencies of the rheological parameters in the yield stress range: the yield stress values σY (a) and the jump of theapparent viscosity between maximal and minimal values (b).

Figure 8. Correlation between the steady shear viscosity (blacksymbols) and dynamic viscosity (open symbols) for systems withdifferent content of PAN (shown at the curves).



water is not really fall in viscosity and reflects the emergence ofinstability in deformation up to the break of a sample. Thissystem actually becomes hard and is not able to flow. It is alsointeresting to stress that the gel-like behavior is observed inconcentrated solutions even at rather low water content.The understanding of the initial Newtonian viscosity plateau

for ternary systems is the same as for PAN−DMSO gels.Indeed, the apparent viscosity increases in time in all cases withlong-term shearing (both at σ = constant and γ = constant)when Newtonian plateau is observed in a sweep mode ofdeformation.

Other Solvents. A similar series of experiments asdescribed above were carried out for the PAN−thiocyanatesolution. To make the story shorter, we would like to say thatall the rheological data show trivial behavior of these solutionsin the whole concentration range that is indistinguishable fromconventional polymer solutions. An example is shown in Figure12.Exactly the same pattern was observed for the PAN−DMF

solutions. It also does not show any anomalies at lowconcentrations as clearly seen when comparing the viscositydata for the same concentration of PAN in DMSO or DMF

Figure 9. Amplitude dependencies of the storage modulus for 5% (a) and 18% (b) PAN in DMSO with different water content. Numbers at thecurves are the weight concentration of water.

Figure 10. Frequency dependencies of the storage modulus for 5% (a) and 18% (b) PAN in DMSO with different water content. Numbers at thecurves are the weight concentration of water.

Figure 11. Viscosity vs shear stress for PAN−DMSO 5% (a) and 18% (b) systems with different water content. Numbers at the curves are theweight concentration of water.



(Figure 13). The flow curve in the latter case is a Newtonianone, while yielding is evident for the PAN−DMSO system.

Some of the published data related to dilute solutions of PANin DMF also indicate that not much is happening in viscoelasticand viscous behavior of DMF solutions.55 Figure 13 containsanother set of experimental data obtained in measuring flowproperties of a specially prepared model samplecopolymer ofacrylonitrile and butyl acrylate. This sample does not containNa itaconate. One can see that the 5% solution of thiscopolymer also behaves as a Newtonian liquid. The differentlevel in absolute viscosity values between solutions of PAN inDMF and PAN/PBA in DMSO is related to the difference inthe molecular weights of the specimens. So we can think thatitaconate plays a determining role in the rheological behavior ofdilute solutions.The main conclusion from experimental data presented and

discussed above is to prove gel-like structure formation in dilutePAN/DMSO systems, while such structures are not observed athigher concentrations of PAN. Meanwhile, adding certainamounts of water leads to a gel-like behavior. Gel structures areclearly different for waterless diluted solutions and watercontaining concentrated solutions.Temperature Transitions. Some additional information

can be obtained in comparison viscosity evolutions intemperature up-and-down sweep for dilute and concentratedsolutions. The increase in temperature leads to gel−soltransition. This is seen from Figure 14 for waterless as well

as for water containing 5% PAN compositions. A waterless geltransforms to a low-viscosity solution above 30−40 °C. Thisprocess is completely reversible at the decrease in temperatureand happens rather quickly. The gel-to-solution transformationfor water containing systems takes place at higher temperatures(reaching 80 °C), and the reverse process continues muchlonger.This data clearly demonstrate the difference in the thermal

behavior of solutions and gels. Gels clearly demonstratethixotropic behavior with slow kinetics of structure formationwhile solutions do not. This method applied to solutions withdifferent water content allowed us to come to the conclusionsthat the gel formation in water containing concentrated PAN/DMSO solutions becomes possible at a water concentration ofabout 6−7% and that the temperature dependencies ofrheological parameters of the waterless solutions are describedby the classical exponential law.

Optical Properties. First of all, we need to mention thatPAN macromolecules tend to self-organization and formclusters. The use of dynamic light scattering techniquesshows the size distribution of nonhomogeneities in a PANsolution (Figure 15). For a dilute solution (much below thecrossover concentration), a maximum occurs at 1.2 μm. Also,we see a maximum corresponding to individual coil at 22.7 nm.However their volume share is close to 23% which correspondsonly to 3% of the scattered light. So the effect of

Figure 12. Shear rate dependencies of PAN viscosity in thiocyanatesolutions. Numbers at the curves are the weight concentration of PAN.

Figure 13. Viscosity on shear stress dependencies for the 5% solutionsof PAN in DMSO and DMF and PAN/PBA in DMSO.

Figure 14. Temperature scanning of the apparent viscosity at lowshear stress for waterless 5% PAN/DMSO system and a compositioncontaining 13% of water. Scanning carried out at a rate of 5 K/min.

Figure 15. Size distribution by volume in the 0.01% and 5% PANsolution in DMSO.



macromolecule clustering is quite evident even in very lowconcentrated solutions much below the crossover concen-tration, which is close to 0.5 g/cm3.The huge average size of clusters may point to the phase

separation. Indeed, the binodal curve for PAN/DMSO systemlies at temperatures much higher than the room temperature.The observed clustering can therefore be the nuclei of a newmicrophase. In this case the percolation structure leading to thesolid-like behavior at low concentration is similar to supra-molecular structure in low-molecular-mass systems.As an example, we can compare it with phase separation in

water−fluorinated alcohol mixtures after the addition of smallportions of DMF. The hydrophobic parts of DMF are solvatedwith fluorinated alcohol molecules while the hydrophilic oxygenatoms in DMF interact with water. It results in separation ofaqueous clusters and clusters consisted in fluorinated alcohol.In this case, DMF molecules play a role of interface layerbetween these clusters.56 The same approach is discussed basedon the other amide-containing compounds.57

Also, we can pay attention to the above-mentioned similarityin behavior of our systems and worm-like micelles inclined toclustering.However, it is interesting to mention that the self-

organization effect is depressed in moderately concentratedsolutions (5%) because in this case we have found separatemacromolecular coils with an average size of 17.4 nm and theirvolume share is equal to 73%. Rather large clusters with anaverage size above 10 μm were also seen. Besides, it is worthmentioning that lower size of individual macromolecules in aconcentrated solution can be explained by amplified screeningof charged itaconic groups with counterions (like it happens inpolyelectrolytes). We see the structure evolution of PAN/DMSO solutions in the transitions through the crossoverconcentration.Some information can be obtained from the transmission

spectroscopy in the visible and infrared wavelength range. Theresults of these experiments are shown in Figure 16.

Here the comparison of PAN solutions in pure DMSO andin a DMSO/water mixture is shown. The decrease in thetransmission in a visible wavelength range (below 700 nm) isdue to the color of a polymer. Much more interesting is theabsence of the effect of polymer presence in DMSO solutionsand a rather strong effect of the decrease in the transmission inDMSO/water solutions in the infrared range. DMSO solutions

are therefore transparent regardless of the presence of 2 μm gel-like particles, while the solution in a DMSO/water mixture isturbid. One can argue that the mechanism of the gel-particleformation of both cases is rather different: this is self-organization with cluster formation in the previous case andphase separation in the latter case.The quantitative estimation of the experimental results

showed that the phase separation in the presence of waterhappens when and if the DMSO/water molar ratio becomesless than 2. This corresponds to the situation where everyoxygen atom in a sulfanyl group in DMSO capable to thehydrogen bond formation meets one movable hydrogen atomin water. Modeling by the molecular dynamic method leads tothe conclusion that a DMSO−water hydrogen bond is morestable than such a bond between two water molecules.58

Besides, the formation of stable water-to-DMSO complexes in a1:2 ratio was reported much earlier.59 It is possible that waterbinds DMSO molecules, and it worsens or even excludes itsinteraction with a polymer. As a result, phase separation occursbecause the combination of DMSO and water becomesnonsolvent for PAN.We try to make some preliminary assumptions regarding the

possible intermolecular interactions. The IR spectrum of PANfilm is characterized by an absorption band at 1593 cm−1

(Figure 17). This peak reflects the asymmetric vibrations of R−

COO− groups. This confirms that a copolymer chain containsnot an itaconic acid but its salt. However, we did not succeed tofollow the transformation of this peak in gelation, possibly dueto the low concentration of this group. We also did not find anynoticeable shifts of nitrile or carbonyl groups. We think that thisfact is also explained by the low concentration of thesefunctional groups which presumably take part in the gelformation.

■ DISCUSSIONRheological studies have demonstrated that PAN solutions inDMF and in sodium thiocyanate/water solution are Newtonianfluids while PAN/DMSO systems are gels. A very specialfeature of these gels in opposite to regular gels is the decrease intheir non-Newtonian behavior with increase in the polymerconcentration. Besides, the nature of PAN/DMSO gels is

Figure 16. Transmission of DMSO, 5% PAN solution in DMSO, and5% PAN solution in mixture DMSO with 13% of water.

Figure 17. IR spectra of PAN film and 5% PAN solutions in DMSOand DMF. Classification of peaks: 1360 cm−1, C−H deformationvibrations (δC−H) in CH; 1454 cm−1, δC−H in CH2; 1593 cm−1,asymmetric vibrations of the R−COO− group; 1732 cm−1, COstretching absorption; 2243 cm−1, absorption due to nitrile groups.



evidently different in comparison with observed in gelation if anonsolvent is added to a polymer solution.So, it is appropriate to discuss the possible mechanisms of

the PAN/DMSO gel formation. The unusual rheology in lowconcentrations relating to structure is not frequently reported.Without discussing polyelectrolytes, direct observations haveshown that clustering phenomenon really existed in dilutepoly(ethylene oxide) (PEO) solutions.10,11 PAN is the next(new) example of cluster formation at very low concentratedpolymer solutions. At the same time, there is a fundamentaldifference between PEO, on the one hand, and PAN solutions,on the other hand. Gelation effect increases with increasing inconcentration for PEO dispersions, while yielding disappearswith increasing concentration for PAN solutions. It may indeedbe intriguing why the structuring in waterless diluted solutionsis not detected in concentrated PAN solutions. We can assumethat this is due to the competition between secondary chemicalstructures (discussed below) in dilute systems and entangle-ments in concentrated systems. The dominance of the lattermakes a major contribution to the rheology of concentratedPAM/DMSO systems and masks yielding/thixotropic effectsrelated to weak chemical bonds.In the review devoted to polymer gels,60 the authors list three

main mechanisms of physical gelation in polymers: formationof hydrogen bonds, formation of Coulomb bonds, andformation of block copolymers nodules. In the presence ofwater in our systems, the mechanism of hydrogen bonds isquite possible. However, neither hydrogen bonding nor micelleformation (like in block copolymers)61,62 from separate blocksin macromolecular chains can play any role in lowconcentration waterless systems.It is usually assumed that the structure in PAN-based systems

and its gelation (related to the high concentration range) isowed to the strong polarity of CN groups in the PAN chains.Physical gelation of PAN−solvent−water systems can beattributed to the self-organization of PAN molecular chainsby forming bonds through a nonsolvent. The role of water inthis line is rather evident because the water molecules createhydrogen bonds. Indeed, the sulfanyl group in DMSO is muchmore polar than carbonyl group in DMF. Electronegativeoxygen atoms in both solvents can form hydrogen bonds withcarboxyl groups in a polymer, and the water enhances thisbonding. This is a possible mechanism of gelation at highconcentrations under the binodal. However, this is not enoughfor the low PAN concentrations.As was said, the control experiment with a PAN copolymer

without itaconate group (Figure 13) demonstrated that acarboxylate ion is a necessary component for the networkformation. So in accordance with the above experimental data,there are two necessary conditions for gel formation: usingDMSO as a solvent and the presence of carboxylate groups in acopolymer (in our case, in the itaconate component).There is a following possibility: the interaction between

oxygen atoms in a polymer carbonyl group or nitrogen atoms ina negatively charged CN group of a polymer via a positivelycharged sulfur atom in DMSO (such a possibility is absent inthe case of DMF). Besides, there is a possibility of theinteraction between the negatively charged DMSO oxygenatom and the positive carboxylate sodium atom of a polymer.This type of double interaction can create a three-dimensionalnetwork of macromolecules joined via a solvent. A possiblescheme of such a network is shown in the left part of Scheme 1.

The version of physical cross-linking for PAN−DMSO−water system is also shown in the right part of the scheme. Inthis case of gelation, water molecules act as bridges between thecarboxylate groups of different polymer parts with/or notinvolving of DMSO molecules.The above arguments concerning the mechanism of the

structure formation represent a possible explanation for therather unusual phenomenon of the gelation in dilute PAN−DMSO systems.

■ CONCLUSIONS

The rheological investigation of PAN-based terpolymersolutions in waterless DMSO showed that a spatial structureappears in low concentrated solutions while PAN/DMSOsystems in the 17−24% concentration range behave liketraditional polymer solutions. The structure formation in dilutesolutions is confirmed by the existence of an explicit yield stresswhich disappears with increasing concentration as well as byindependence of the storage modulus on frequency. Effects ofsuch kind occur in the 0.1−5% PAN concentration range. Sucha situationstructure inhomogeneity or physical cross-linkingat low concentrations and homogeneous solutions at highconcentrationslooks rather unusual. Diluted PAN/DMSOgels are thixotropic media. A possibility of coexistence ofdifferent states (bifurcation) at low concentrations leads tostress self-oscillations in a wide shear rate region. Suchrheological behavior is absent if DMSO is switched for othergood solvents of PAN. A possible reason for the absence ofyielding in concentrated solutions can be explained by maskinga weak structural network formed via DMSO cross-linking ofnitrile and carboxylate groups (detected at low PANconcentrations) by macromolecular entanglements at highPAN content in a solution. However, gel formation also occursin concentrated solutions containing water due to specific H-bonds of a DMSO−water coupling. In this case we likely meetwith phase transition. Possible chemical structures responsiblefor the formation of the gel structure are discussed. We believethat the presence of salt of itaconic acid in the structure of apolymer plays a specific role in the gel formation.

Scheme 1. Formation of Three-Dimensional Network ofMacromolecules



■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to Dr. I. I. Konstantinov (TIPS RAS)for the synthesis of a model PAN/PBA sample made accordingto our request. The authors are also grateful to anonymousreviewers for their thorough examination of our manuscript andvaluable comments, which allowed us to improve the paper.This work was supported by the Russian Foundation for BasicResearch (Grant 13-03-00016) as well as Russian Ministry ofEducation and Science (# 8534 dated September 7, 2012).

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