Viscoelastic Synergy and Microstructure Formation ... - ACS Publications

Dec 20, 2016 - Joint Institute for Nuclear Research, 141980 Dubna, Russia. §. National Research Centre “Kurchatov Institute”, 123182 Moscow, Russ...
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Viscoelastic Synergy and Microstructure Formation in Aqueous Mixtures of Nonionic Hydrophilic Polymer and Charged Wormlike Surfactant Micelles Andrey V. Shibaev,† Ksenia A. Abrashitova,† Alexander I. Kuklin,‡ Anton S. Orekhov,§ Alexander L. Vasiliev,§ Ilias Iliopoulos,∥ and Olga E. Philippova*,† †

Physics Department, Moscow State University, 119991 Moscow, Russia Joint Institute for Nuclear Research, 141980 Dubna, Russia § National Research Centre “Kurchatov Institute”, 123182 Moscow, Russia ∥ Arkema France, 92700 Colombes, France ‡

S Supporting Information *

ABSTRACT: We studied the effect of neutral polymer poly(vinyl alcohol) on the rheological properties and microstructure of highly charged mixed wormlike micelles of anionic and cationic surfactants, potassium oleate and n-octyltrimethylammonium bromide, without adding salt. It was shown that the polymer induces a hundredfold increase of viscosity and of longest relaxation time and the appearance of well-defined plateau modulus, which was assigned to interlacing of polymer and micellar chains. When the amount of added polymer exceeds 2 wt %, the rheological characteristics (the viscosity, the longest relaxation time, and the plateau modulus) level off because of microphase separation appearing as a result of the interplay of the segregation on the microscopic scale triggered by the energetic repulsion between polymer and surfactant components, on the one hand, and the translational entropy of counterions preventing the macroscopic phase separation, on the other hand. The formation of surfactant-rich and polymer-rich microphases was evidenced by small-angle neutron scattering and cryogenic transmission electron microscopy data. The results obtained open a new way to modify the rheological properties and the microstructure of wormlike micellar solutions.

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INTRODUCTION

(ii) by the release of counterions, which increase the ionic strength of the solution.9 The efficiency of oppositely charged surfactant to induce micellar growth depends crucially on the length of its hydrophobic tail and on its content in the surfactant mixture. On an example of the mixtures of sodium oleate and alkyltrimethylammonium bromide with different nalkyl groups, it was demonstrated9 that cationic surfactant with short alkyl tail (C6) induces only weak growth of sodium oleate micelles. By contrast, cationic surfactants with long alkyl tail (C10−C12) provide too strong attraction with the anionic surfactant, which induces not only a dramatic growth of micelles but also the phase separation. The most promising results were obtained for n-octyltrimethylammonium bromide (C8TAB).9 In this case, the attractive interactions are strong enough to produce a pronounced micellar growth, but not so strong to induce phase separation.9 At a given size of surfactant tail there is an optimum content of oppositely charged surfactant, which provides the maximum effect on rheological properties.9,10 In particular, in sodium oleate/C8TAB solution

Ionic surfactants are able to self-assemble into very long wormlike micelles, which can interlace thereby imparting viscoelastic properties to solutions.1−6 Because of noncovalent links between surfactant molecules in micellar chains, these properties are very sensitive to many factors including temperature, shear, and different additives.5−7 Responsive viscoelasticity of wormlike micellar solutions is currently exploited in a variety of applications including cosmetics, heating and cooling systems, oil recovery, etc.4,8 Wormlike micelles are usually obtained by adding salt to ionic surfactant solution. Salt screening the electrostatic repulsion between charged surfactant head groups allows their tighter packing, thus favoring the transition from spherical to cylindrical micelles and further growth of cylindrical micelles in length in order to reduce the number of thermodynamically unfavorable spherical end-caps. Another way to get wormlike micelles consists in the addition of oppositely charged surfactant, which permits to obtain long micellar chains with no added salt. In this system, the electrostatic screening triggering the growth of micelles in length proceeds in two ways: (i) by ion pairing of the oppositely charged head groups, which reduces the charge density of the micellar surface, and © XXXX American Chemical Society

Received: November 3, 2016 Revised: December 8, 2016

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and structural changes, we employ a combination of rheological measurements to explore macroscopic properties (viscosity, elasticity, etc.), small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) to follow the structural changes occurring at the microscopic level. As a result of these studies, we demonstrate that polymer weakly interacting with micelles can induce a hundredfold increase of the viscosity of wormlike micellar solution without altering its phase stability. The obtained results contribute to the general understanding of the behavior of complex polymer−surfactant systems and can have important practical applications taking into account the availability of the chosen polymer.

the most pronounced enhancement of viscosity relative to the single-component solutions was observed at a weight ratio of 70/30.9 Thus, tuning the length and the content of oppositely charged surfactant permits to get a significant increase in viscosity of the surfactant mixture. A promising way to enhance the viscoelasticity of wormlike surfactant micelles consists in their mixing with polymeric chains.11−26 Many combinations of polymers and viscoelastic surfactants with different interactions between them (electrostatic, hydrophobic, etc.) were studied up to now. A very pronounced increase of viscosity by few orders of magnitude was observed upon addition of oppositely charged polyelectrolyte to anionic14,19 and cationic20 wormlike micelles. In this system, the polymer−surfactant interactions were mainly of electrostatic nature. Another approach to get a stable system of polymer and micellar chains with enhanced rheological properties is based on manipulating with hydrophobic interactions between the components. For this aim different uncharged hydrophilic polymers with few hydrophobic side11,12,18 or end15−17 groups were used. It was reported that the addition of hydrophobically modified polymers to wormlike micelles of both anionic and cationic surfactants can lead to the increase of viscosity by several orders of magnitude.11,12 This was explained by penetration of hydrophobic groups of the polymers into the wormlike micelles, thus bridging them. However, the attempts to enhance the viscosity of wormlike micelles by adding uncharged water-soluble polymers without associating hydrophobic groups were not successful.21−23,25,26 It particular, rather hydrophobic polymers like poly(propylene oxide) or poly(vinyl methyl ether) even reduced the viscosity because they destroyed the cylindrical micelles as a result of the adsorption on their surface.21,22,26 At the same time, hydrophilic polymers like poly(ethylene oxide) or poly(vinylpyrrolidone) had only a minor effect on the viscosity.21,26 As for poly(vinyl alcohol) (PVA), it was shown26 that adding its minute amounts (up to 20 mM or 0.089 wt %) to wormlike micelles of cetyltrimethylammonium bromide and sodium salicylate has no effect on viscoelasticity, if the polymer is highly hydrophilic (i.e., contains a small amount of hydrophobic vinyl acetate units (1.5%)). At the same time, PVA with rather large content of vinyl acetate units (20%) decreases the viscosity, which was assigned to hydrophobic interactions of these groups with the micelles.26 These experiments demonstrated that the interaction of polymer with micellar chains can be unfavorable for the enhancement of viscoelastic properties, especially if it leads to the destruction of micelles. We suggest that a new promising approach for the enhancement of viscoelastic properties of ionic wormlike micelles in solution can consist in the use of polymer noninteracting (or weakly interacting) with micellar chains, which is capable to form its own network in the whole volume of the solution independently of micellar network. To enhance compatibility of the weakly interacting components, the micellar chains should be strongly charged, which will make unfavorable their demixing, since it will induce a pronounced loss of translational entropy of surfactant counterions. In the present paper, we make a first step toward the creation of such system on an example of highly hydrophilic polymer PVA and charged mixed micelles of potassium oleate and C8TAB. Thus, the paper is aimed at the investigation of the viscoelasticity and the microstructure in the polymer−wormlike micellar system composed of weakly interacting polymer and micellar components. To study simultaneously the rheological

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EXPERIMENTAL SECTION

Materials. Potassium oleate (TCI, purity >98%), C8TAB (ABCR, purity >98%), and PVA Mowiol 4-98 (Aldrich) were used as received. Potassium oleate has a critical micelle concentration (cmc) of 0.02− 0.04 wt %;27 the cmc value of C8TAB is much higher3.5 wt %.9 PVA used in this study has a residual content of vinyl acetate units 1.1− 1.9% and a molecular weight of 27 000 g/mol, which corresponds to the average degree of polymerization of 600 and a contour length of a polymer chain equal to 150 nm. Its overlap concentration C* is ca. 3.2 wt % as determined by viscometric measurements. The solutions were prepared using distilled deionized water purified by the Millipore MilliQ system. The samples for SANS measurements were made in D2O (99.9% isotopic purity) supplied by Deutero GmbH. Sample Preparation. First, aqueous stock solutions of surfactants and PVA were prepared. PVA was dissolved in water at 90 °C for 1 h. The stock solutions were mixed in appropriate quantities to obtain the samples, which were then stirred using a magnetic stirrer and left to equilibrate at room temperature for 7−14 days before measurements to remove air bubbles. Phase Behavior. The partial phase diagram of aqueous PVA− potassium oleate/C8TAB solutions was constructed by visual inspection of the samples 20 days after preparation. The composition of phases was determined by elemental analysis. Rheology. Rheological measurements were performed with a stress-controlled rotational rheometer (Anton Paar Physica MCR 301). The details of the measurements are described elsewhere.28−30 Cone−plate geometry with a diameter of 50 mm and a cone angle of 1° was used. Temperature was controlled by Peltier elements and set at 20.00 ± 0.05 °C. A specially constructed vapor lock filled with solvent was used to prevent solvent evaporation from the sample. The samples were equilibrated for 10−30 min in the measurement cell prior to investigation. Highly viscous samples containing surfactants and polymer were additionally subjected to low-amplitude (0.5%) oscillation at a frequency ω of 1−10 1/s for 10−20 min before measurement. After that, the rheological curves were measured at least two times to ensure that they coincide and the sample is equilibrated in the cell. In oscillatory shear experiments, the angular frequency dependences of the storage G′(ω) and loss G″(ω) moduli were measured. All measurements were made in the linear viscoelastic regime at the deformation amplitudes of 0.5−3%, at which the storage and loss moduli were independent of deformation, as estimated previously by amplitude sweep tests performed at the frequency of 10 1/s. The plateau modulus G0 was determined from the G′(ω) dependence at the frequency where G″ reaches a minimum.31 The longest relaxation time τ was estimated from the frequency ω0, at which G′(ω) and G″(ω) intercept, as τ = 1/ω0. In steady shear experiments, the dependences of viscosity on shear rate (flow curves) were measured. From the flow curves, the values of the zero-shear viscosity η0 were extracted by fitting the data with the Carreau−Yasuda model. Small-Angle Neutron Scattering. SANS experiments were performed with the YuMO spectrometer of the high-flux pulsed reactor IBR-2 at the Frank Laboratory of Neutron Physics, Joint B

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Macromolecules Institute for Nuclear Research, Dubna, Russia. The data were recorded in the range of scattering vectors q of 0.005−0.7 Å−1 by a two-detector system and treated according to standard procedures of small-angle isotopic scattering.32−34 The intensity was calibrated at the absolute scale by using a primary beam detector with vanadium standard. The samples were put into specially constructed dismountable cells with parallel quartz plates and beam path of 2 mm. All the samples were prepared in D2O, whereas hydrogenated surfactants and polymer were used. The scattering curves were fitted by a form factor of a cylinder using the program SasView. For fitting, only a part of the curve at intermediate and high q > 0.06−0.07 Å−1 (higher than the structure peak position q*) was used, and then the fit was reconstructed in the whole q-range. Two fitting parameters were used: radius and length of the cylinder. However, in all cases, the length of the cylinder obtained from the fit was much larger than the accessible q-range (of the order of 2000−3000 Å). Cryogenic Transmission Electron Microscopy. To prepare cryo-TEM specimens, the samples were applied directly onto the Lacey carbon-coated side of the 300 mesh copper grid manually via the side port of the Vitrobot (FEI) using a pipet. The Vitrobot parameters are described elsewhere.35 After the sample deposition the grid was blotted to remove excess solution and immediately plunged into a reservoir with liquid ethane cooled by liquid nitrogen. All samples were studied in bright-field TEM in a Titan Krios 60-300 TEM/STEM (FEI) equipped with a spherical aberration corrector (image corrector), a direct detection camera Falcon II (FEI), and a postcolumn energy filter (Gatan). The TEM was operated at 300 kV. The micrographs were obtained in low dose mode with total electron dose of less than 15 e/Å2. Digital Micrograph (Gatan) and TIA (FEI) software were used for the image processing.

Figure 1. Partial phase diagram of the PVA−potassium oleate/ C8TAB−water system at 20 °C: open symbols, one phase; semifilled symbols, two phases. Molar ratio [potassium oleate]/[C8TAB] = 2.5.

A convincing estimation of polymer−surfactant interactions can be made by the NMR technique. The 1H and 13C NMR spectra obtained are presented in Figures 1S and 2S (Supporting Information). They show that there is no any chemical shift changes of the peaks of surfactants and PVA upon their mixing. Therefore, in the present system, one cannot expect pronounced attraction between polymer and surfactant. A sensitive method to reveal the polymer−surfactant interactions consists in the study of the effect of polymer on the cmc value of the surfactant.37−39 To a first approximation, the cmc permits to estimate the standard free energy of micellization, ΔG0m, per mole of surfactant: ΔG0m = RT ln(cmc).40 When there are some attractive polymer−surfactant interactions, in the presence of polymer the micellar formation proceeds at lower surfactant concentration called critical aggregation concentration (cac). The cac allows to evaluate the standard free energy of surfactant aggregation onto a polymer chain, ΔG0ag, per mole of surfactant: ΔG0ag = RT ln(cac). The difference between cmc and cac can be regarded as a measure of the strength of polymer−surfactant interaction.40 In the present system, by fluorescence spectroscopy with pyrene as a probe it was shown that the cmc value of potassium oleate/C8TAB surfactant mixture under study is equal to 0.009 wt % independently of the presence of PVA (Figure 3S, Supporting Information); i.e., the polymer does not affect the aggregation of surfactant. This indicates the absence of any attractive interactions between the mixed surfactants and PVA. This seems to be contradictory with the literature data demonstrating the interaction of PVA both with some anionic41 and cationic41−43 surfactants. However, the literature results were mainly obtained for PVA samples with rather high content of hydrophobic vinyl acetate units, while the hydrophobicity of uncharged polymers is known to be one of the most important factors governing their interaction with surfactants.44 Therefore, the absence of the interaction of PVA with mixed potassium oleate/C8TAB micelles may be due to very hydrophilic character of the studied highly deacetylated polymer. Rheology. Rheological studies were performed at different amounts of added polymer, whereas the concentrations of both surfactants were fixed (2.5 wt % potassium oleate and 0.8 wt % C8TAB, molar ratio [potassium oleate]/[C8TAB] = 2.5). Figure 2 shows the results of steady-state measurements. It is seen that all flow curves exhibit a Newtonian plateau at low shear rate and a shear-thinning region, which can be due to the orientation of polymer and micellar chains along the flow direction. The value of viscosity at the Newtonian plateau

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RESULTS AND DISCUSSION Phase Behavior. In all experiments, the molar ratio [potassium oleate]/[C8TAB] was kept at 2.5. At these conditions, the micellar chains are strongly negatively charged. The micelles (without added polymer) form homogeneous solutions in water at least up to 10 wt % surfactant. The length of the micelles depends on the concentration of surfactant and according to the literature data9 should be of the order of a micrometer. For polymer−surfactant mixtures containing so large surfactant aggregates the tendency toward phase separation should be quite pronounced, and even weak repulsive or attractive interactions should have a substantial influence on the phase behavior.36 On the other hand, the micelles are strongly charged, and the phase separation can be unfavorable because of the loss of entropy of their counterions confined within one of the phases, since the solutions contain no added low molecular weight salt. The partial phase diagram for the PVA−potassium oleate/C8TAB−water system obtained by visual inspection for surfactant concentrations varying from 0 to 10.5 wt % and polymer concentrations in the range of 0− 10 wt % is depicted on Figure 1. It is seen that at low polymer and surfactant concentrations the system is homogeneous; i.e., at these conditions the phase behavior is governed by the gain in the entropy of counterions. When at fixed surfactant content the polymer concentration is increased, the phase demixing occurs; i.e., the gain in energy becomes prevailing. At the phase transition, the gel, which previously occupied the whole volume of the system, slightly shrinks, expelling a low viscous liquid. Upon syneresis, the gel phase is enriched in surfactants, whereas the liquid phase contains almost pure polymer. Therefore, the observed phase separation is of segregative type, which implies the repulsion between polymer and surfactant components. C

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polymer solutions have very low viscosities less than 0.01 Pa·s (Figure 2). From Figure 3 it is seen that polymer produces an enormous effect on the rheological properties of surfactant solutions. First, it leads to the appearance of a plateau on the frequency dependence of storage modulus G′(ω), indicating the formation of an entangled network.3,5 Second, it induces a very large decrease in the crossover frequency, implying that the relaxation time becomes much higher. Both these effects can be due to the interlacement of polymer and micellar chains. At these conditions, the polymer/polymer entanglements can be neglected because PVA concentration is near the overlap concentration C* and well below the entanglement concentration Ce, which is usually 5−10 times higher than C*.45 Pure micelles at 3.3 wt % surfactant solution seem to be also unentangled, as can be suggested from the absence of plateau modulus in the studied frequency range (Figure 3). Note that the present systems do not show simple Maxwell behavior with a single relaxation time as follows from the strong deviation of G′ (G″) dependence (Cole−Cole plot) from an ideal semicircle (Figure 3b). In this situation, the relaxation time determined from the intersection of G′(ω) and G″(ω) curves can be regarded as a terminal one. Figure 4 summarizes the effect of added polymer on different rheological properties (the zero-shear viscosity, the terminal

Figure 2. Flow curves for 3.3 wt % potassium oleate/C8TAB aqueous solutions in the presence of increasing amounts of added poly(vinyl alcohol) PVA: 0 wt % (squares), 0.7 wt % (pentagons), 1 wt % (circles), 4 wt % (reverse triangles), and for 4 wt % aqueous solution of PVA (stars) at 20 °C. Molar ratio [potassium oleate]/[C8TAB] = 2.5.

increases significantly with increasing concentration of added polymer. Figure 3 shows dynamic rheological data for surfactant solutions with and without polymer. The data for polymer alone are not presented because at this concentration the polymer solution does not exhibit viscoelastic response in the studied frequency range (note that at these concentrations the

Figure 4. Zero-shear viscosity η0 (triangles), terminal relaxation time τ (circles), and plateau modulus G0 (squares) as a function of concentration of added poly(vinyl alcohol) PVA in 3.3 wt % potassium oleate/C8TAB aqueous solutions at 20 °C. Molar ratio [potassium oleate]/[C8TAB] = 2.5.

relaxation time, and the plateau modulus) of potassium oleate/ C8TAB surfactant solutions. It is seen that the most pronounced effect is observed at the variation of polymer concentration from 0 to 2 wt % (regime I). At these conditions, the zero-shear viscosity η0 and the relaxation time τ increase by 2 orders of magnitude, and the plateau modulus G0 appears and becomes 2-fold higher. Most probably, it is due to the formation of entanglements between polymer and micellar chains. At further increase of polymer concentration from 2 to 4.5 wt %, the values of η0, τ, and G0 level off (regime II). One can suggest that the rheological properties become insensitive to further addition of polymer because of microphase separation occurring in the system. It results in the accumulation of newly added PVA chains in the vicinity of other PVA chains, thus forming polymer-rich domains. The microphase separation is known to take place, for instance, at mixing neutral and weakly charged polymers, if their uncharged units are incompatible.46,47 A similar behavior can be expected in the mixture of uncharged polymer and charged micellar chains given that they are not attracting to each other as was evidenced by

Figure 3. (a) Frequency dependences of storage G′ (filled symbols) and loss G″ (open symbols) moduli and (b) normalized Cole−Cole plots for 3.3 wt % potassium oleate/C8TAB aqueous solutions in the presence of increasing amounts of added poly(vinyl alcohol) PVA: 0 wt % (squares), 2 wt % (triangles), 3 wt % (diamonds), 4 wt % (reverse triangles), and 4.5 wt % (hexagons) at 20 °C. Molar ratio [potassium oleate]/[C8TAB] = 2.5. D

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Macromolecules fluorescence data. Microphase separation results from the competition between the short-range segregation and longrange stabilization tendencies. In the present system, the stabilization is obviously related to the presence of a large number of counterions neutralizing the charge of micellar chains. The microdomain structure propagating through the whole system does not impede counterions to travel in the most of the volume of the solution, thus ensuring the gain in the translational entropy. At the same time, it does not hinder the local segregation of incompatible polymer and surfactant components resulting in the gain in energy. Note that the addition of salt, which diminishes the gain in the translational entropy of counterions, induces the macrophase separation at these conditions. For instance, after the addition of 6 wt % KCl to an aqueous solution containing 4 wt % polymer and 3.3 wt % potassium oleate/C8TAB (at molar ratio [potassium oleate]/ [C8TAB] = 2.5) two-phase solutions are formed, with the upper phase containing mainly surfactants and the lower phase PVA chains. It is interesting that in regime II when the values of η0, τ, and G0 are already saturated, increasing amount of added polymer approaches considerably the Cole−Cole curve to the semicircle, i.e., narrows the spectrum of relaxation times (Figure 3b). It may be due to the effective concentrating of micellar chains in surfactant-rich microphase leading to the increase of their length L, since L is known3,48 to scale with surfactant concentration C as L ∼ C0.5. Indeed, the form of a Cole− Cole plot for 3.3 wt % surfactant solution in the presence of 4.5 wt % PVA (Figure 3b) is quite close to that for 5.3 wt % pure surfactant solution in the absence of polymer (Figure 5Sb, Supporting Information). Therefore, 4.5 wt % PVA makes the surfactant micelles in 3.3 wt % potassium oleate/C8TAB solution of similar length as in 5.3 wt % pure surfactant solution. The growth of micelles does not affect the number of entanglements in the system (i.e., does not change the G0 value) probably because the micelles are already longer than the entanglement length. At the same time, growth of micelles makes the breaking time τb shorter than the reptation time τrep because longer micelles need larger time to reptate out of the tube, but they have more places for breaking. As a result, during reptation such micelles break and re-form many times, which narrows the spectrum of relaxation times. In their turn, polymer chains are concentrating inside polymer-rich domains. Since the concentration of polymer in the system is of the order of C* (and well below Ce, which is usually45 5−10 times higher than C*), this process does not result in a pronounced increase of the polymer−polymer entanglements and therefore does not affect neither G0 nor η0 values. So, the evolution of the system upon addition of polymer can be considered as follows. At low polymer concentrations (regime I), the gain in mixing entropy favors the formation of entanglements between polymer and micellar chains. When the amount of added polymer increases (regime II), the gain in entropy due to mixing of polymer and micellar chains becomes lower, and PVA macromolecules prefer to segregate into polymer-rich microphase in order to decrease the energetically unfavorable contacts with micelles. Simultaneously, the surfactant chains form surfactant-rich microphase percolating the whole volume of the system. Inside this microphase, the counterions are free to move, thus keeping a high entropy of the translation motion; at the same time, the micellar chains grow in length because of the increase of the local concentration of surfactant. When the concentration of

polymer reaches ca. 5 wt %, the macrophase separation occurs (Figure 1). Therefore, microphase separation takes place just in the vicinity of macrophase separation, when the system is still able to provide sufficient gain in entropy of micellar counterions. Note that at concentrations under study (0−4.5 wt %) the polymer solution itself (in the absence of surfactants) has quite low viscosity of ca. 0.01 Pa·s, which is by 5 orders of magnitude lower than that of PVA−potassium oleate/C8TAB solutions. Thus, the rheological properties of solutions containing both PVA and wormlike surfactant micelles are vastly superior to those of their components taken separately. Such synergistic behavior was previously observed for the mixtures of wormlike surfactant micelles and hydrophobically modified polymers, in which hydrophobic side groups of polymer incorporate into the hydrophobic core of micelles, thus linking different micellar chains with each other.11 Surprisingly, in the present paper, a similar synergy was for the first time observed in the presence of polymer, which does not link to the micelles. SANS Data. To get deeper insight into the structural origin of the rheological changes, the SANS studies of surfactant and polymer−surfactant systems were performed. For better contrast and low background in the neutron scattering experiments D2O was used as a solvent. The scattering length densities of D2O and of the alkyl tails of the surfactants are 6.38 × 10−6 and ∼−0.22 × 10−6 Å2, respectively. The scattering length density estimated for PVA is 0.65 × 10−6 Å2, but it may be slightly higher due to the substitution of its hydroxyl group protons by deuterium. Thus, in D2O the alkyl tails of surfactants are suggested to have higher contrast. They also produce larger contribution to the scattering than polymer. Indeed, at the characteristic concentrations used in this work, the scattering intensity of the polymer solution without surfactants is by 1−2 orders of magnitude lower than the intensity of the surfactants solution without polymer (Figures S6−S8, Supporting Information). Therefore, we assumed that surfactant micelles are mainly responsible for the scattering. Variation of the Content of Surfactant. Let us first consider the effect of the content of surfactant on the structure of potassium oleate/C8TAB solutions containing 4 wt % PVA. Figure 5 shows the scattering curves normalized with respect to surfactant concentration. It is seen that their high q regions superimpose for all the samples, indicating that the local structure is identical independently of surfactant concentration both in the presence (Figure 5a) and in the absence of polymer (Figure 5b). This part of the curves can be well fitted by a form factor of cylinder (solid line, Figure 5). Therefore, the local cylindrical structure of micelles is unaffected by added polymer. Fitting permits to determine the characteristics of the cylindrical structure. In particular, it gives the values of radius equal to 18.6 and 18.7 Å in the presence and in the absence of polymer, respectively. The radius of cylinders agrees fairly with the length of oleate tail (19 Å).7,29 All SANS curves (Figure 5) have a correlation peak, which arises from electrostatic repulsion between similarly charged micelles.49 More clearly the peak is seen on semilogarithmic presentation (Figure S7, Supporting Information). However, the scattering curves contain the contributions of both the structure factor S(q) and the form factor P(q). At high q the structure factor S(q) is approximately unity,9 which allowed to make a fitting with the form factor of cylinder, but at lower q values S(q) comes into play especially in the present system, where the electrostatic interactions are not screened. In E

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Figure 6. Scattering curves Iq (q) for potassium oleate/C8TAB solutions of different concentrations in D2O: 2.6 wt % (squares), 3.3 wt % (circles), 4.0 wt % (triangles), 5.3 wt % (diamonds), 6.6 wt % (hexagons), 9.9 wt % (pentagons), and 13.2 wt % (open reverse triangles) in the presence (a) and in the absence (b) of 4 wt % PVA at 20 °C. Molar ratio [potassium oleate]/[C8TAB] = 2.5.

Figure 5. (a) Normalized scattering curves I(q)/C for potassium oleate/C8TAB solutions of different concentrations in D2O: 4 wt % (triangles), 5.3 wt % (diamonds), and 6.6 wt % (hexagons) in the presence of 4 wt % PVA. Molar ratio [potassium oleate]/[C8TAB] = 2.5. Line is a fit of the scattering curves by a form factor of cylinder with radius R = 18.6 ± 0.2 Å. (b) Normalized scattering curves I(q)/C for potassium oleate/C8TAB solutions of different concentrations in D2O: 4 wt % (triangles), 7.9 wt % (pentagons), and 10 wt % (open reverse triangles). Molar ratio [potassium oleate]/[C8TAB] = 2.5. Line is a fit of the scattering curves by a form factor of persistent cylinder with radius R = 18.7 ± 0.2 Å.

particular, for this reason9 the scattering curves do not demonstrate the slope I ∼ q−1 (at qR ≪ 1) characteristic for cylindrical objects. But the form factor of a cylinder P(q) is expected to scale as q−1 in this region. Therefore, to extract the position of the structure peak, the SANS curves were plotted in the coordinates Iq vs q (Holtzer’s plots), which emphasize the contribution of the structure factor in this part of the scattering curve.50 The results obtained are shown in Figure 6. It is seen that for all the samples a clear structure peak is observed. With increasing surfactant concentration, the peak becomes higher and narrower and shifts to larger q values. The increase of the peak intensity suggests better ordering of the micelles, whereas the narrowing is related to the widening of the range of electrostatic interactions. The shift to higher q values indicates a decrease of the average distance between micelles. Such behavior was previously observed for wormlike micelles of many ionic surfactants.5,13,16,49,51 Here we show that the same situation occurs in the presence of polymer (Figure 6a). Figure 7 demonstrates the dependence of the position of the correlation peak on the concentration of surfactant. It is seen that q* scales as C0.38 in the presence of polymer and as C0.43 in the absence of polymer. The both dependences are rather close to the power law expected49 for strongly interacting cylindrical assemblies in semidilute regime: q* ∼ C0.5. The characteristic

Figure 7. Dependence of the position of the correlation peak on the surfactant concentration for potassium oleate/C8TAB solutions in D2O in the presence (circles) and in the absence (squares) of 4 wt % PVA. Molar ratio [potassium oleate]/[C8TAB] = 2.5.

distance of the medium (which is of the order of 1/q*) can be regarded as the mesh size of the semidilute regime.49 The most important observation consists in the fact that in the presence of polymer the q* values are by ca. 40% higher than in pure surfactant solution (Figure 7). Therefore, polymer stimulates micellar chains to come closer to each other, which most probably indicates the microphase separation with the formation of surfactant-enriched area. For instance, Figure 7 shows that 3.3 wt % surfactant solution in the presence of 4 wt % PVA has the same q* value (i.e., the same intermicellar distance 2π/q*) as 6.9 wt % surfactant solution without PVA. These data are consistent with rheological results (Cole−Cole F

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local cylindrical structure of micelles. At the same time, polymer changes appreciably the scattering at low and intermediate q values (i.e., at larger scale). The most pronounced effect of PVA is observed in the low q region (q < 0.015 Å−1), where the scattering intensity rises significantly with increasing amount of added polymer. This may be due to the enhancement of microphase separation as well as to the increase of the length of surfactant micelles5 (as was suggested from Cole−Cole plots (Figure 3b)). By contrast, in the intermediate q-range (0.015 Å−1 < q < 0.05 Å−1) polymer lowers the scattering intensity of the system. As was discussed above, such behavior can be attributed52,53 to the increased repulsive interactions between the components (surfactants and PVA). Note that SANS data seen in Figure 8 strongly suggest that the mixture of the two surfactants (without PVA) forms wormlike micelles with finite length, which does not exhibit entanglements and a significant influence in the structure factor. The addition of PVA increases the lengths of micelles long enough to entangle each other. Then, the viscoelastic data (Figure 3) demonstrate the presence of rubbery plateau due to the entanglement effect, and simultaneously the structure factors show substantial decreasing in an intermediate q-range (Figure 8). Figure 8b shows that added polymer affects significantly the interaction peak arising from the mutual ordering of surfactant molecules as a result of electrostatic repulsion. With increasing PVA concentration, the interaction peak shifts to higher q values. Therefore, polymer induces closer approach of micelles to each other, suggesting the increase in the local concentration of surfactant molecules. This result is consistent with our assumption about the microphase separation in the system. Cryo-TEM Visualization. Cryo-TEM was used to visualize the structural changes in the wormlike surfactant solutions upon addition of polymer. It was shown that at polymer concentrations 3−4.5 wt % corresponding to regime II the samples always contain spherical microdomains immersed in a matrix of densely packed wormlike surfactant chains often arranged parallel to each other, forming bundles and bundlelike loops (Figure 9). Similarly arranged micellar chains were previously visualized in many wormlike micellar solutions10,54−56 and in supramolecular polymers.57 Here we observe numerous microdomains included in such matrix. The microdomains are suggested to be mainly composed of polymer, but polymeric chains are not seen in cryo-TEM micrographs due to very low contrast. At the same time, some of microdomains also contain few micelles. Unlike the micellar chains in the matrix, which represent long worms, the micelles inside the microdomains have the form of short rods. This is a clear indication of the difference of surfactant concentration in the matrix and in the microdomains given that the length of wormlike micelles increases with surfactant concentration.3,48 Therefore, upon microphase separation the long micellar chains persist in zones with high surfactant concentration, whereas few short micelles are located in polymer-rich microdomains with quite low surfactant concentration. The average length of the short rods in the sample with 4.5 wt % PVA is ca. 150 nm. The length of wormlike micelles L in the area with high surfactant concentration cannot be determined from cryo-TEM micrographs because on the pictures one cannot see the ends of micellar chains, but one can state that L is larger than 1 μm. For long wormlike micelles, one can estimate the characteristic periodicity in the packing of micellar chains. For instance, for the sample with 4 wt % PVA it is about 12 nm, which perfectly coincides with the data extracted from SANS correlation peak

plots) suggesting the polymer-induced increase of micelles in length, which can take place at increasing surfactant concentration. Comparison of the SANS curves of the mixed polymer− surfactant system with the mathematical sum of the curves of polymer and surfactant solutions taken separately at the same concentration as in the mixture allows one to judge about the interactions between the components in the mixture.52,53 The results obtained (Figure 8S, Supporting Information) show that at low surfactant concentration (0.66 wt %) corresponding to regime I the sum curve coincides perfectly with the scattering pattern of the mixed system. This indicates that in the mixed system both polymer and surfactant components behave the same way as in pure polymer or surfactant solutions taken separately. So, no appreciable interactions between the polymer and micellar chains were detected. By contrast, at higher surfactant concentration (3.3 wt %) corresponding to regime II the sum diverges from the scattering curve of the mixed system especially at the intermediate wave vectors (0.015 Å−1 < q < 0.05 Å−1). Such behavior is indicative of repulsive interactions between the components of the mixture.52,53 Variation of the Content of Polymer. Figure 8 depicts the scattering curves at different PVA concentrations, whereas the content of surfactants is fixed at 3.3 wt % (including 2.5 wt % potassium oleate and 0.8 wt % C8TAB, molar ratio [potassium oleate]/[C8TAB] = 2.5). It is seen that at high q values the curves superimpose, indicating that polymer does not affect the

Figure 8. (a) Scattering curves for 3.3 wt % potassium oleate/C8TAB solutions in D2O in the presence of increasing amounts of PVA: 0 wt % (squares), 1 wt % (circles), and 4 wt % (reverse triangles) at 20 °C. Molar ratio [potassium oleate]/[C8TAB] = 2.5. Inset: the same curves in semilog representation. (b) Scattering curves Iq (q) for 3.3 wt % potassium oleate/C8TAB solutions in D2O in the presence of increasing amounts of PVA: 0 wt % (squares), 1 wt % (circles), and 4 wt % (diamonds) at 20 °C. Molar ratio [potassium oleate]/[C8TAB] = 2.5. Inset: position of the correlation peak as a function of polymer concentration for the same system. G

DOI: 10.1021/acs.macromol.6b02385 Macromolecules XXXX, XXX, XXX−XXX

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CONCLUSIONS In this paper, we propose a new approach to get synergistic enhancement of rheological properties of charged wormlike micellar solutions in a salt-free medium by adding polymer. It consists in the use of hydrophilic uncharged polymer, which almost does not interact with the micelles. Despite the incompatibility of the components, the system does not phase separate in a rather wide range of polymer and surfactant concentrations because of the presence of surfactant counterions, which need to move in a large total volume of the solution in order to gain in the translational entropy. This behavior of the mixture of polymer and micellar chains is somewhat reminiscent of the behavior of the blends of two incompatible polymers, which can become miscible upon the incorporation of charged units in one of the components.46,47 It was shown that the addition of PVA to a semidilute unentagled solution of highly charged potassium oleate/C8TAB wormlike micelles gives a synergistic gain in viscoelasticity so that the mixed system acquires the viscosity hundredfold higher than that of individual components. This behavior was attributed to interlacing of polymer and micellar chains. However, when the polymer concentration gets high enough (2−4.5 wt %), the system undergoes a microphase separation with the formation of polymer-enriched microdomains embedded in the matrix mainly composed of micellar chains, which was evidenced by SANS and cryo-TEM data. Within the structure thus formed the surfactant counterions still keep the ability to travel in almost whole volume of the system and therefore do not lose the translational entropy, whereas the local segregation of components gives the gain in interaction energy. The microphase-separated system keeps a high viscosity acquired before microphase separation. The results obtained are of obvious importance for preparing materials with a controllable and readily variable microstructure as well as for numerous practical applications of wormlike micellar solutions.

Figure 9. Cryo-TEM pictures of 3.3 wt % potassium oleate/C8TAB aqueous solutions with different concentrations of added PVA: 3 wt % (a) and 4.5 wt % (b). Molar ratio [potassium oleate]/[C8TAB] = 2.5. Some microdomains are marked by white dotted lines, and short rods are indicated by arrows.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02385. NMR data, determination of critical micelle concentration, rheology of surfactant mixture, SANS data (PDF)

for the same specimen (2π/q* = 12.6 nm). Note that the SANS data do not reveal the presence of cylindrical micelles of two very different lengths. One can suggest several reasons for that. First, the contribution of short rods to the scattering should be small because only a tiny fraction of surfactant molecules are included in the rods. Second, the both types of micelles are suggested to have the same radius; therefore, the same scattering at high q values. Third, at low q, where the difference in the lengths could be detected, the structure factor decreasing scattering can hide a possible contribution of short rods, which also should make the scattering smaller. Thus, direct visualization has some advantages over SANS technique as it permits to reveal the presence of objects present in rather small amounts. As for the spherical microdomains, when approaching macrophase separation at increasing amount of added polymer from 3 to 4.5 wt % their average diameter increases from 200 until 570 nm and their boundaries become better defined, suggesting stronger segregation of the components. So, cryoTEM pictures unambiguously demonstrate the microphaseseparated structure of PVA−potassium oleate/C8TAB systems with the formation of surfactant-poor and surfactant-rich area, thus confirming the structural model suggested from the analysis of SANS results.

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AUTHOR INFORMATION

Corresponding Author

*(O.P.) Fax +7 495 9392988; Tel +7 495 9391464; e-mail [email protected]. ORCID

Olga E. Philippova: 0000-0002-1098-0255 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation (Project No. 15-13-00114). The authors express their gratitude to T. A. Ganina (Moscow State University) for the measurement of NMR spectra. H

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