Dominant role of wormlike micelles in temperature-responsive viscoelastic properties of their mixtures with polymeric chains

Dual Semi-Interpenetrating Networks of Water-Soluble Macromolecules and Supramolecular Polymer-like Chains: The Role of Component Interactions

Dual networks formed by entangled polymer chains and wormlike surfactant micelles have attracted increasing interest in their application as thickeners in various fields since they combine the advantages of both polymer- and surfactant-based fluids. In particular, such polymer-surfactant mixtures are of great interest as novel hydraulic fracturing fluids with enhanced properties. In this study, we demonstrated the effect of the chemical composition of an uncharged polymer poly(vinyl alcohol) (PVA) and pH on the rheological properties and structure of its mixtures with a cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride already exploited in fracturing operations. Using a combination of several complementary techniques (rheometry, cryo-transmission electron microscopy, small-angle neutron scattering, and nuclear magnetic resonance spectroscopy), we showed that a small number of residual acetate groups (2-12.7 mol%) in PVA could significantly reduce the viscosity of the mixed system. This result was attributed to the incorporation of acetate groups in the corona of the micellar aggregates, decreasing the molecular packing parameter and thereby inducing the shortening of worm-like micelles. When these groups are removed by hydrolysis at a pH higher than 7, viscosity increases by five orders of magnitude due to the growth of worm-like micelles in length. The findings of this study create pathways for the development of dual semi-interpenetrating polymer-micellar networks, which are highly desired by the petroleum industry.

Novel Trends in the Development of Surfactant-Based Hydraulic Fracturing Fluids: A Review

Viscoelastic surfactants (VES) are amphiphilic molecules which self-assemble into long polymer-like aggregates-wormlike micelles. Such micellar chains form an entangled network, imparting high viscosity and viscoelasticity to aqueous solutions. VES are currently attracting great attention as the main components of clean hydraulic fracturing fluids used for enhanced oil recovery (EOR). Fracturing fluids consist of proppant particles suspended in a viscoelastic medium. They are pumped into a wellbore under high pressure to create fractures, through which the oil can flow into the well. Polymer gels have been used most often for fracturing operations; however, VES solutions are advantageous as they usually require no breakers other than reservoir hydrocarbons to be cleaned from the well. Many attempts have recently been made to improve the viscoelastic properties, temperature, and salt resistance of VES fluids to make them a cost-effective alternative to polymer gels. This review aims at describing the novel concepts and advancements in the fundamental science of VES-based fracturing fluids reported in the last few years, which have not yet been widely industrially implemented, but are significant for prospective future applications. Recent achievements, reviewed in this paper, include the use of oligomeric surfactants, surfactant mixtures, hybrid nanoparticle/VES, or polymer/VES fluids. The advantages and limitations of the different VES fluids are discussed. The fundamental reasons for the different ways of improvement of VES performance for fracturing are described.

Growth of wormlike micelles of surfactant induced by embedded polymer: role of polymer chain length

Incorporation of polymer chains into wormlike surfactant micelles, which find a large range of applications, offers the opportunity to modify their structure and properties. In this paper, using spectroscopic, scattering and rheological techniques and computer simulations, we study the incorporation of poly(4-vinylpyridine) of two different molecular weights (MWs) into entangled networks of wormlike surfactant micelles of potassium oleate. Using NMR-spectroscopy we show that, independent of its MW, the polymer incorporates into the core-corona interface of the surfactant micelles. According to SANS data, the polymer does not alter the micelle structure or the micelle radius, but diminishes the packing density of the surfactant. At the same time, rheology reveals a stark difference between the surfactant networks with embedded polymers of different MWs. Networks with the higher-MW polymer possess larger viscosity and a longer relaxation time, which we attribute to the larger length of the hybrid micelles. Moreover, we demonstrate that in an intermediate concentration range the higher-MW polymer is able to link neighbouring surfactant micelles together, which has never been previously observed. However, with a further increase in polymer content the micelles become smaller due to the high breaking susceptibility of the boundaries of polymer-containing sections, leading to the stabilization of micellar end-caps by the embedded macromolecules. This process is more prominent in the case of the shorter polymer. Our finding that an increased MW of macromolecules permits the formation of longer hybrid micelles and enhances their rheological properties is of obvious importance for the fundamental understanding of polymer-surfactant interactions and the development of new industrial formulations based on hybrid polymer-wormlike surfactant micelles.

Soft magnetic nanocomposites based on adaptive matrix of wormlike surfactant micelles

A network of wormlike surfactant micelles with embedded magnetic particles demonstrates high magnetoresponsive linear viscoelastic properties due to tunable matrix.

Viscoelasticity of smart fluids based on wormlike surfactant micelles and oppositely charged magnetic particles

Novel viscoelastic smart suspensions based on cationic wormlike micelles (WLMs) of erucylbis(hydroxyethyl)methylammonium chloride and oppositely charged submicron magnetite particles in the presence of added low molecular weight salt were prepared and investigated. The suspensions demonstrate remarkable stability against sedimentation, which can be due to the incorporation of particles into the network of entangled WLMs by linking to energetically unfavorable micellar end-caps. Added particles enhance significantly the viscosity, the plateau modulus, and the relaxation time of the system, acting as additional multifunctional physical cross-links in the micellar network. The increase of plateau modulus stops when the concentration of particles reaches ca. 1.5 wt %, indicating that all micellar end-caps available in the system are linked to the particles. Further addition of particles may lead just to the redistribution of micellar ends between the particles without creation of new elastically active chains. The increase of rheological characteristics by added particles is more pronounced in suspensions with a smaller content of low molecular weight salt KCl when the WLMs are shorter in length and therefore contain a larger amount of end-caps responsible for the interaction with the particles. Magnetite particles not only enhance the rheological characteristics but also impart magnetoresponsive properties to the suspension. Upon application of magnetic field, the liquidlike system transforms into a solidlike one, demonstrating a constant value of storage modulus in all frequency range and the appearance of yield stress, which is due to the formation of field-aligned chainlike aggregates of particles opposing the flow. A combination of responsive properties inherent to both the matrix and the particles makes these smart fluids very competitive with other magnetic soft matter materials for various applications.