Article pubs.acs.org/Macromolecules
Wormlike Surfactant Micelles with Embedded Polymer Chains Alexander L. Kwiatkowski,† Hari Sharma,‡ Vyacheslav S. Molchanov,† Anton S. Orekhov,§ Alexander L. Vasiliev,§ Elena E. Dormidontova,*,‡ and Olga E. Philippova*,† †
Physics Department, Moscow State University, 119991 Moscow, Russia Polymer Program, Institute of Materials Science and Physics Department, University of Connecticut, Storrs, Connecticut 06269, United States § National Research Centre “Kurchatov Institute”, 123182 Moscow, Russia ‡
S Supporting Information *
ABSTRACT: Active use of wormlike micelle (WLM) solutions in a broad range of applications demands control of their viscoelastic properties under different external conditions that can be achieved by using hybrid materials, such as polymer− surfactant complexes. Understanding the properties of such hybrid materials remains a challenge. Using a combination of several experimental techniques with molecular dynamics simulations, we investigate the interaction of poly(4-vinylpyridine) with WLMs of the anionic surfactant potassium oleate. We find that this polymer is solubilized by the micelles at the interface between the tails and headgroups of surfactant, thus screening the hydrophobic polymer backbone from interactions with water while maintaining hydrogen bonding between the pyridine rings and water. By use of SANS with contrast variation, it was shown that the macromolecules associated with the micelles have an expanded coil-like conformation with persistence length 4-fold higher than that of a polymer chain in a good solvent. The rheological behavior of the micellar solutions shows that at low polymer concentrations (regime I) the system maintains high viscoelasticity with the plateau modulus remaining almost unchanged, while the zero-shear viscosity slightly decreases. At higher polymer concentrations (regime II), the viscosity drops by more than 4 orders of magnitude, approaching that of pure solvent. The transition from the I to II regime occurs when the number of added macromolecules is approximately equal to the number of elastically active strands in micellar network. The observed changes are attributed to polymer-induced shortening of WLMs accompanied by loop and branch formation. The saturated polymer−surfactant complex contains about one surfactant per repeat unit of the polymer and represents a best compromise when the WLM retains its general cylindrical geometry (but not the viscoelastic properties) and the polymer maintains an expanded coil-like conformation.
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INTRODUCTION Viscoelastic WLM solutions are actively used in a variety of applications, including cosmetics, oil recovery, and so on.1−4 Since WLMs are responsive to external conditions, the ability to control or modify the properties of WLM solutions is essential for their successful applications. One way to modify WLM properties is to use hybrid materials obtained by e.g. combining WLMs with polymers, which cannot only provide not only additional stability to the materials but also affect WLM dynamic equilibrium and therefore responsiveness. Furthermore, investigation of polymer−WLM interactions is also important because in many applications the WLMs come into contact with different polymers, which may affect their properties. While interactions of polymers with surfactant assemblies in aqueous solutions have been widely reported in the literature,5−30 most of the studies were devoted to spherical surfactant micelles.5−15 The research on interactions of polymers and WLMs is much more limited16−30 with many questions remaining to be answered. Understanding the interaction between covalently bonded macromolecular chains © XXXX American Chemical Society
and breakable self-assembled WLMs, as discussed in this paper, is fundamentally interesting and important for practical applications, as it provides insights into structural and dynamic reorganization of WLMs in the presence of polymer and polymer adjustment to WLMs, which can affect viscoelastic properties of the hybrid materials. The influence of various polymers such as water-soluble nonionic polymers, as well as polyelectrolytes, on the properties of WLMs has been previously investigated. It has been shown that highly hydrophilic water-soluble nonionic polymers usually do not produce a pronounced effect on WLMs, since they reside predominantly in the bulk aqueous solution and interact only slightly with the micelles. Such behavior was observed, for example, for poly(ethylene oxide) (PEO) or poly(vinylpyrrolidone) added to cetyltrimethylammonium bromide/ sodium salicylate micellar solution.16 More hydrophobic polymers like poly(propylene oxide) (PPO) or poly(vinyl Received: July 14, 2017 Revised: August 13, 2017
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DOI: 10.1021/acs.macromol.7b01500 Macromolecules XXXX, XXX, XXX−XXX
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The obtained results provide fundamental insights into polymer−surfactant interactions and properties of the hybrid system under consideration. Taking into account that such hybrid aggregates are quite promising building blocks for the construction of various self-assembled systems, our results will also offer guidance in designing new responsive materials employing WLMs and polymers.
methyl ether) are able to interact more strongly with the micelles due to their more hydrophobic repeat units, but still a part of these macromolecules reside in the aqueous phase forming loops.16,17,28 The polymer adsorbed on the micellar surface strongly perturbs the structure of WLMs inducing their transformation into spheres, ensuring the higher surface-tovolume ratio necessary for the accommodation of large macromolecules at the core−water interface. The use of amphiphilic nonionic polymers containing both hydrophobic and hydrophilic blocks (e.g., a triblock copolymer with two long hydrophilic PEO blocks bonded to a central shorter hydrophobic block of PPO) allows retention of the local cylindrical structure of WLMs, since in this case the hydrophilic blocks are exposed to water as free ends (rather than loops).19 The cylindrical structure of WLMs can be preserved when oppositely charged polyelectrolyte chains are embedded into the micelles. In such systems, both electrostatic and hydrophobic interactions are involved in the complex formation. The cylindrical aggregates in water are especially stable when the polymer contains hydrophobic entities along with the charged groups. In this case, the polymer backbones are located in the micellar interior, whereas the charged and/or hydrophobic side groups of polymer are intercalated between surfactant heads on the micelle surface. For instance, stable hybrid polymer− surfactant micelles were obtained for highly hydrophobic polyelectrolytes like poly(4-vinyl benzoate).22 This polymer, which is insoluble in water at neutral pH,31 incorporates in the core of the cylindrical micelles of alkyltrimethylammonium surfactant with 1−2 polymer chains per micelle.22 The radius of the resulting aggregates remains equal to the surfactant tail length, whereas their contour length is controlled by the polyelectrolyte and varies from 80 nm to hundreds of nanometers. The formation of stable hybrid micelles was also observed for copolymers containing both hydrophobic and charged repeat units, e.g., copolymers of styrene and sodium styrenesulfonate20,21 or copolymers of methyl methacrylate and sodium styrenesulfonate.29 Generally, stable hybrid polymer− surfactant cylindrical micelles are obtained in the case of polymers charged oppositely to the constituent surfactants20−23 under conditions when the polymers were localized at the core−corona interface of the micelles. However, such oppositely charged systems are very sensitive to the polymer composition and often have a rather limited range of phase miscibility.32 In the present paper, we propose an alternative approach to obtain stable hybrid polymer−surfactant cylindrical micelles. We use an uncharged hydrophobic polymer which is forced to reside at the core−corona interface of micelles because it has low solubility both in water (i.e., in the external solution) and in hydrocarbons (i.e., in the micellar core). Specifically we study a hybrid polymer−WLM system composed of poly(4-vinylpyridine) (P4VP) and the WLMs formed by the anionic surfactant potassium oleate in the presence of KCl. We investigate under what conditions the stable complexation between the polymer and surfactant can be achieved and what is the composition of the saturated complex. Using a combination of different experimental techniques and molecular dynamics simulations, we characterize the complex and determine the polymer conformation and location in the WLM and analyze how the presence of polymer affects both the thermodynamic and rheological properties of the WLM solutions.
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EXPERIMENTAL SECTION
Materials. Potassium oleate (>98% purity) from TCI Europe, potassium chloride (>99.5% purity) from Fluka, and ethanol (>99% purity) from Merck were used as received. Water was purified by Millipore Milli-Q system. P4VP provided by Aldrich was used without further purification. Its weight-average molecular weight M w determined by static light scattering in ethanol solution (refractive index increment dn/dc = 0.235) was shown to be equal to 77 000 g/ mol, which corresponds to a degree of polymerization of 730 and a contour length of 180 nm. The value of Mw obtained is somewhat higher than that provided by manufacturer (60 000 g/mol). Sample Preparation. Stock solutions of potassium oleate were prepared by dissolving potassium oleate and potassium chloride in distilled−deionized water followed by adjusting the pH to 11 with KOH. The concentration of potassium chloride was kept at 6 wt %. Solutions were mixed using a magnetic stirrer for 1 day and allowed to equilibrate at room temperature for one more day. To prepare surfactant−polymer solutions, an appropriate amount of 5 wt % solution of P4VP in ethanol was poured at the bottom of a vial and left at room temperature until full evaporation of ethanol was achieved, after which the stock solution of potassium oleate was added to the vial, and the resulting mixture was stirred for 1 day and then left for equilibration at room temperature for another day. In the surfactant− polymer solutions thus-prepared polymer concentrations were varied from 0.002 to 0.57 wt % (1.9 × 10−4 ÷ 0.054 mol of monomer units (monomol)/L). Phase Behavior. Aqueous solutions of potassium oleate and P4VP were prepared over a range of surfactant and polymer concentrations. The salt concentration was fixed at 6 wt % and pH at 11. The homogeneity of the solutions was verified by visual inspection. In order to determine the chemical composition of the coexisting phases, a series of solutions with fixed concentration of potassium oleate (0.047 M) and increasing concentrations of added polymer were prepared. In phase-separated samples, the precipitate and supernatant phases were carefully isolated from each other and dried in an oven at 60 °C. The powders thus obtained were characterized by elemental analysis. Dynamic and Static Light Scattering. Dynamic light scattering (DLS) and static light scattering (SLS) measurements were performed on an ALV/DLS/SLS-5022F goniometer system, consisting of an ALV6010/EPP digital correlator, a helium−neon laser with a wavelength of 632.8 nm, and a stepping-motor-driven variable-angle detection system. The temperature was controlled by a Lauda Ecoline RE 306 system and kept at 20 °C. Solutions were filtered through 0.45 μm Millipore Millex-FG filter just before the measurements. Experimental details and data treatment are described elsewhere.33 Small-Angle Neutron Scattering (SANS). SANS measurements were carried out with a two-detector system at the YuMO instrument of the high-flux pulsed reactor IBR-2M at the Frank Laboratory of Neutron Physics (Joint Institute for Nuclear Research, Dubna, Russia) at 20 °C. Scattering data were obtained in the range of scattering vector Q from 0.006 to 0.6 Å−1 and treated using the standard procedures of small-angle isotopic scattering.34,35 The contrast variation (matching) technique was used in order to obtain scattering curves separately for the surfactant and the polymer. The composition of the matching D2O/H2O mixtures was estimated using the following values of scattering length densities: 6.38 × 10−6 Å−2 for D2O, −0.56 × 10−6 Å−2 for H2O, 0.15 × 10−6 Å−2 for potassium oleate, and 2 × 10−6 Å−2 for P4VP. To obtain scattering curves of the surfactant, a D2O/ H2O 37/63 (v/v) mixture was used as a solvent (matching the neutron scattering length density of P4VP). To achieve scattering mainly from B
DOI: 10.1021/acs.macromol.7b01500 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules the polymer, a D2O/H2O 10/90 (v/v) mixture was used (matching the neutron scattering length density of potassium oleate). Cryogenic Transmission Electron Microscopy (cryo-TEM). The cryo-TEM specimens were prepared in their native condition by applying the sample via the side port of the Vitrobot (FEI) directly onto the lacey carbon-coated side of the 300 mesh copper TEM grid. The Vitrobot parameters are described elsewhere.36 After the sample deposition the grid was blotted and immediately plunged into liquid ethane. All samples were studied in bright-field TEM in a Titan Krios 60-300 TEM/STEM (FEI) operated at 300 kV equipped with a spherical aberration corrector (image corrector), a direct detection camera Falcon II (FEI), and a postcolumn energy filter (Gatan). The micrographs were obtained in low dose mode with a total electron dose of less than 15 e/Å2. Digital Micrograph (Gatan) and TIA (FEI) software were used for the image processing. Rheology. Rheological measurements (steady shear and frequency sweep) were carried out on a controlled-stress rheometer Anton Paar Physica MCR 301 at 20 °C. For these experiments, a cone−plate (diameter 50 mm, cone angle 1°) and coaxial cylinders (i.d. 24.661 mm, o.d. 26.667 mm) measurement cells were used for high- and lowviscosity samples, respectively. The steady shear experiments were performed in the shear rate range 0.05−90 s−1. In these measurements, the zero-shear viscosity η0 was determined from the Newtonian plateau at low shear rate by fitting the data with the Carreau−Yashuda model.37 Frequency sweep experiments were carried out in the linear viscoelastic regime at angular frequencies ω varying from 0.04 to 40 rad/s. The plateau modulus G0 was determined from the G′(ω) dependence at the frequency where G″(ω) reaches a minimum.38 Molecular Dynamics Simulations. To model a WLM solution of potassium oleate in the presence of 6 wt % potassium chloride in SPC (simple point charge model) water, we applied united atom molecular dynamics simulations using the GROMOS 53a6 force field.39 For the oleate molecule we employed the model discussed in ref 40 with the potassium ion force field taken from ref 41. To model the P4VP molecule (with 60 repeat units), we adopted the approach of ref 42 for the pyridine ring with the rest of the force field details given in the Supporting Information (Figure S1, Tables S1 and S2). We used a simulation box of 26 × 14 × 14 nm. NPT simulations were performed using the GPU version of GROMACS 4.6.543 at 1 bar pressure (compressibility 4.5 × 10−5 bar −1) and temperature 300 K. Isotropic pressure coupling was initially applied using the Berendsen barostat (with a time constant of 1 ps) for 2 ns and subsequently using Parrinello−Rahman barostat (with a time step of 2 fs) for the main production run. Temperature coupling was carried out separately for water and surfactants/ions using a velocity-rescale thermostat with a time constant of 1 ps. The particle-mesh Ewald (PME) method was used for the long-range electrostatic interactions with a cutoff distance of 0.9 nm for both the van der Waals and electrostatic interactions. For visualization of chain conformation and hydration, Visual Molecular Dynamics (VMD)44 has been used.
Figure 1. Phase diagram of potassium oleate/poly(4-vinylpyridine) in 6 wt % aqueous solutions of KCl at pH 11. The dashed line corresponds to the onset of phase separation. The open symbols represent homogeneous solutions; the semifilled symbols denote phase separated samples.
polymer solution just before the phase separation threshold: 0.9 potassium oleate molecules per repeat unit of P4VP. To get information about the location of polymer in the surfactant micelle, 1H NMR spectroscopy was used (Figure S2). The results obtained suggest that when associated with the micelles, the polymeric chains are predominantly located close to the headgroup area. Similar conclusions can be drawn from the computer modeling data shown in Figure 2. As is seen, the backbone of the polymer is in contact with hydrophobic surfactant tails, while the pyridine rings are oriented predominantly away from the surfactant tails in the headgroup area. A few surfactants may occasionally protrude into the headgroup area to provide better coverage of the polymer. The main reason for the pyridine rings to be located in the headgroup area is the gain in contacts with water molecules, which can form hydrogen bonds with nitrogen, as shown in Figure 2. Overall, the P4VP chain crawls and wiggles along the WLM at the tail/headgroup interface. These results seem to be quite logical taking into account that P4VP is insoluble both in water and in hydrocarbons. Thus, when P4VP is added to potassium oleate solution, it is incorporated within the existing surfactant micelles. The micelles provide a thermodynamically favored environment for the macromolecules, as being embedded in the WLMs polymer avoids unfavorable contacts with water, while maintaining some degree of water hydrogen bonding with the pyridine nitrogen and gains some conformational freedom compared to a collapsed conformation in bulk water. The incorporation of P4VP can be favorable for the micelles, too, since the polymer can partially shield the alkane tails from contacts with water and partially screen electrostatic repulsion between similarly charged surfactant headgroups. Surfactant−Polymer Complexes. When a surfactant solution is saturated with polymer, the DLS measurements show only one mode (Figure 3) with a diffusion coefficient of (2.1 ± 0.1) × 10−11 m2/s (Figure S3), which corresponds48 to a hydrodynamic radius of the effective sphere Rh equal to 11 nm. The width of the distribution of hydrodynamic radii is similar to that of polymer coils in ethanol (Figure S4). This suggests that the size distribution of potassium oleate−P4VP aggregates saturated with polymer is governed by the polydispersity of the polymer. As the single diffusion mode suggests that only surfactant− polymer aggregates of similar size (without neat surfactant
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RESULTS AND DISCUSSION Phase Behavior. The surfactant−polymer phase diagram in 6 wt % KCl is depicted in Figure 1. It covers the range of potassium oleate concentrations from 0.5 to 3 wt % (0.016− 0.094 M). In the absence of polymer, such potassium oleate solutions contain long WLMs.26,45,46 The polymer P4VP is insoluble in water at alkaline pH47 but can be dispersed in water in the presence of surfactant. The higher the amount of polymer, the higher concentration of potassium oleate needed to solubilize it. When the amount of surfactant is insufficient for the solubilization of all polymer molecules, a phase separation occurs with the formation of a solid precipitate. According to the elemental analysis data, the precipitate is composed of pure polymer (without surfactant), whereas the supernatant represents the surfactant solution saturated with polymer, which has the same composition as a homogeneous surfactant− C
DOI: 10.1021/acs.macromol.7b01500 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Computer simulation snapshot of a potassium oleate WLM with an embedded P4VP polymer of 60 repeat units in 6 wt % KCl aqueous solution. P4VP is shown in blue, the surfactant tail in olive, and oxygens of the headgroup in red. Water interacting with P4VP is shown in magenta in the enlarged snapshot. Other water molecules and ions are not shown for clarity. Zoom-in pictures (top) show a local surfactant arrangement near the P4VP chain and water hydrogen bonded to nitrogen of P4VP in an atomistic representation (left) and surfactant surface representation (shown in green, right).
SLS it was determined that the weight-average molecular weight of the aggregates is equal to 4.0 × 105 g/mol (Figure S5). Taking into account the composition of surfactant− polymer aggregate determined by elemental analysis (0.9 potassium oleate molecules per repeat unit of polymer), one can easily estimate that each aggregate contains on average 1.5 polymer chains. The second virial coefficient A2 is equal to (1.4 ± 0.1) × 10−4 cm3 mol/g2, which corresponds to good solvent conditions for the surfactant−polymer aggregates. As the aggregates under study are small (
