Influence of the Surfactant Tail Length on the Viscosity of Oppositely Charged Polyelectrolyte/Surfactant Complexes

Hierarchical Polyimide Microparticles with Controllable Morphology

Hierarchical polyimides (PIs) not only show outstanding thermal stability and high mechanical strength but also have great advantages in terms of microstructure and surface area, which makes them highly valuable in various fields such as aerospace, microelectronics, adsorption, catalysis, and energy storage. However, great challenges still remain in the synthesis of hierarchical PIs with well-defined microstructure. Herein, polyamide acid salts (PAAS) with tunable ionization degree are synthesized first via the polymerization of dianhydride and diamine monomers in deionized water with 1,2-dimethylimidazole (DMIZ). Then cationic cetyltrimethylammonium chloride (CTAC) is added to the PAAS aqueous solution to induce the formation of polyelectrolyte-surfactant complexes based on electrostatic interaction. After a typical hydrothermal reaction (HTR) procedure, hierarchical PIs with different microstructures such as urchin-like PI microparticles, flower-like PI microparticles, and lamellar PI petals can be fabricated simply by changing the additive amount of DMIZ and CTAC. The nanostructure self-assemblies of PAAS are dominated by the charges on macromolecular chains and the formation of hierarchical structures of polymers is ascribed to a geometrical selection process during crystal growth. This work provides valuable insights into the self-assembly behaviors of polyelectrolyte systems for synthesizing well-defined hierarchical polymers.

Salt-Induced Transformations of Hybrid Micelles Formed by Anionic Surfactant and Poly(4-vinylpyridine)

Salt-induced structural transformation of charged hybrid surfactant/polymer micelles formed by potassium oleate and poly(4-vinylpyridine) was investigated by cryo-TEM, SANS with contrast variation, DLS, and 2D NOESY. Cryo-TEM data show, that at small salt concentration beads-on-string aggregates on polymer chains are formed. KCl induces the transformation of those aggregates into rods, which is due to the screening of the electrostatic repulsion between similarly charged beads by added salt. In a certain range of salt concentration, the beads-on-string aggregates coexist with the rodlike ones. In the presence of polymer, the sphere-to-rod transition occurs at higher salt concentration than in pure surfactant system indicating that hydrophobic polymer favors the spherical packing of potassium oleate molecules. The size of micelles was estimated by DLS. The rods that are formed in the hybrid system are much shorter than those in polymer-free surfactant solution suggesting the stabilization of the semi-spherical endcaps of the rods by embedded polymer. 2D NOESY data evidence that in the spherical aggregates the polymer penetrates deep into the core, whereas in tighter packed rodlike aggregates it is located mainly at core/corona interface. According to SANS with contrast variation, inside the rodlike aggregates the polymer adopts more compact coil conformation than in the beads-on-string aggregates. Such adaptive self-assembled polymer-surfactant nanoparticles with water-insoluble polymer are very promising for various applications including drag reduction at transportation of fluids.

Stimuli-responsive polyelectrolyte surfactant complexes for the reversible control of solution viscosity

Interactions of polyelectrolytes with oppositely charged surfactants can give rise to a large variety of self-assembled structures. Some of these systems cause a drastic increase in solution viscosity, which is related to the surfactant forming aggregates interconnecting several polyelectrolyte chains. For these aggregates to form, the surfactant needs to be sufficiently hydrophobic. Here, we present a system consisting of the anionic surfactant sodium monododecyl phosphate and the cationic cellulose-based polyelectrolyte JR 400. The hydrophobicity of the surfactant can be controlled by the solution's pH. At pH > 12, the surfactant headgroup bears two charges. As a consequence, the solution viscosity decreases drastically by up to two orders of magnitude, while it can be as high as 10 Pa s at lower pH. In this paper, we investigate the changes of the mesoscopic structure of the system which lead to such drastic changes in viscosity using small angle neutron scattering and neutron spin-echo spectroscopy. Such systems are potentially interesting as they allow for a modular design where stimuli responsiveness is introduced by relatively small amounts of surfactant reusing the same simple polyelectrolyte.

Charge Modification as a Mechanism for Tunable Properties in Polymer-Surfactant Complexes

Oppositely charged polymer-surfactant complexes are frequently explored as a function of phase space defined by the charge ratio Z, (where Z = [+polymer]/[-surfactant]), commonly accessed through the surfactant concentration. Tuning the phase behaviour and related properties of these complexes is an important tool for optimising commercial formulations; hence, understanding the relationship between Z and bulk properties is pertinent. Here, within a homologous series of cationic hydroxyethyl cellulose (cat-HEC) polymers with minor perturbations in the degree of side chain charge modification, phase space is instead explored through [+polymer] at fixed Cpolymer. The nanostructures were characterised by small-angle neutron scattering (SANS) in D2O solutions and in combination with the oppositely charged surfactant sodium dodecylsulfate (h- or d-SDS). Scattering consistent with thin rods with an average radius of ∼7.7 Å and length of ∼85 Å was observed for all cat-HEC polymers and no significant interactions were shown between the neutral HEC polymer and SDS (CSDS < CMC). For the charge-modified polymers, interactions with SDS were evident and the radius of the formed complexes grew up to ∼15 Å with increasing Z. This study demonstrates a novel approach in which the Z phase space of oppositely charged polymer-surfactant complexes can be controlled at fixed concentrations.

Determination of SLES in Personal Care Products by Colloid Titration with Light Reflection Measurements

The method of colloid titration with poly(diallyldimethylammonium) chloride has been improved to detect the endpoint with an off-vessel light reflectance sensor. The digital color sensor used measures light reflectance by means of light guides, with no immersion into the reaction solution. In such a method, the optical signal is free of disturbances caused by sticky flocs in the solution. The improved automatic titration set was applied for the determination of sodium laureth sulfate (SLES) in industrial batches and commercial personal care products. The sample color and opacity do not disturb the SLES quantification. When the SLES content lies in the range from 5% to 9%, the optimal sample weight is from 6 g to 3 g.