Dilute polyelectrolyte solutions: recent progress and open questions.

Polyelectrolytes are a class of polymers possessing ionic groups on their repeating units. Since counterions can dissociate from the polymer backbone, polyelectrolyte chains are strongly influenced by electrostatic interactions. As a result, the physical properties of polyelectrolyte solutions are significantly different from those of electrically neutral polymers. The aim of this article is to highlight key results and some outstanding questions in the polyelectrolyte research from recent literature. We focus on the influence of electrostatics on conformational and hydrodynamic properties of polyelectrolyte chains. A compilation of experimental results from the literature reveals significant disparities with theoretical predictions. We also discuss a new class of polyelectrolytes called poly(ionic liquid)s that exhibit unique physical properties in comparison to ordinary polyelectrolytes. We conclude this review by listing some key research challenges in order to fully understand the conformation and dynamics of polyelectrolytes in solutions.

Flow properties reveal the particle-to-polymer transition of ultra-low crosslinked microgels.

Exploiting soft, adaptive microgels as building blocks for soft materials with controlled and predictable viscoelastic properties is of great interest for both industry and fundamental research. Here the flow properties of different poly(N-isopropylacrylamide) (pNIPAM) microgels are compared: regularly crosslinked versus ultra-low crosslinked (ULC) microgels. The latter are the softest microgels that can be produced via precipitation polymerization. The viscosity of ULC microgel suspensions at low concentrations can be described with models typically used for hard spheres and regularly crosslinked microgels. In contrast, at higher concentrations, ULC microgels show a much softer behavior compared to regularly crosslinked microgels. The increase of the storage modulus with concentration discloses that while for regularly crosslinked microgels the flow properties are mainly determined by the more crosslinked core, for ULC microgels the brush-like interaction is dominant at high packing fractions. Both the flow curves and the increase of the storage modulus with concentration indicates that ULC microgels can form glass and even reach an apparent jammed state despite their extreme softness. In contrast, the analysis of oscillatory frequency sweep measurements show that when approaching the glass transition the ultra-low crosslinked microgels behave as the regularly crosslinked microgels. This is consistent with a recent study showing that in this concentration range the equilibrium phase behavior of these ULC microgels is the one expected for regularly crosslinked microgels.

Effect of ionic strength on the structure and elongational kinetics of vimentin filaments.

Intermediate filaments are a major structural element in the cytoskeleton of animal cells that mechanically integrate other cytoskeletal components and absorb externally applied stress. Their role is likely to be linked to their complex molecular architecture which is the product of a multi-step assembly pathway. Intermediate filaments form tetrameric subunits which assemble in the presence of monovalent salts to form unit length filaments that subsequently elongate by end-to-end annealing. The present work characterizes this complex assembly process using reconstituted vimentin intermediate filaments with monovalent salts as an assembly trigger. A multi-scale approach is used, comprising static light scattering, dynamic light scattering and quantitative scanning transmission electron microscopy (STEM) mass measurements. Light scattering reveals the radius of gyration (Rg), molecular weight (Mw) and diffusion coefficient (D) of the assembling filaments as a function of time and salt concentration (cS) for the given protein concentration of 0.07 g L-1. At low cS (10 mM KCl) no lateral or elongational growth is observed, whereas at cS = 50-200 mM, the hydrodynamic cross-sectional radius and the elongation rate increases with cS. Rgversus Mw plots suggest that the mass per unit length increases with increasing salt content, which is confirmed by STEM mass measurements. A kinetic model based on rate equations for a two step process is able to accurately describe the variation of mass, length and diffusion coefficient of the filaments with time and provides a consistent description of the elongation accelerated by increasing cS.

Droplet microfluidic SANS.

The coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated with a range of model systems: isotopic solvent (H2O/D2O) mixtures, surfactant (sodium dodecyl sulfate, SDS) solutions and colloidal (silica) suspensions. Several droplet carrier phases are evaluated and fluorinated oil emerges as a suitable fluid with minimal neutron background scattering (commensurate with air), and excellent interfacial properties. The combined effects of flow dispersion and compositional averaging caused by the neutron beam footprint are evaluated in both continuous and droplet flows and an operational window is established. Systematic droplet-SANS dilution measurements of colloidal silica suspensions enable unprecedented quantification of form and structure factors, osmotic compressibility, enhanced by constrained global data fits. Contrast variation measurements with over 100 data points are readily carried out in 10-20 min timescales, and validated for colloidal silica of two sizes, in both continuous and droplet flows. While droplet microfluidics is established as an attractive platform for SANS, the compositional averaging imposed by large (∼1 cm) beam footprints can, under certain circumstances, make single phase, continuous flow a preferable option for low scattering systems. We propose simple guidelines to assess the suitability of either approach based on well-defined system parameters.

Microfluidic approach to the formation of internally porous polymer particles by solvent extraction.

We report the controlled formation of internally porous polyelectrolyte particles with diameters ranging from tens to hundreds of micrometers through selective solvent extraction using microfluidics. Solvent-resistant microdevices, fabricated by frontal photopolymerization, encapsulate binary polymer (P)/solvent (S1) mixtures by a carrier solvent phase (C) to form plugs with well-defined radii and low polydispersity; the suspension is then brought into contact with a selective extraction solvent (S2) that is miscible with C and S1 but not P, leading to the extraction of S1 from the droplets. The ensuing phase inversion yields polymer capsules with a smooth surface but highly porous internal structure. Depending on the liquid extraction time scale, this stage can be carried out in situ, within the chip, or ex situ, in an external S2 bath. Bimodal polymer plugs are achieved using asymmetrically inverted T junctions. For this demonstration, we form sodium poly(styrenesulfonate) (P) particles using water (S1), hexadecane (C), and methyl ethyl ketone (S2). We measure droplet extraction rates as a function of drop size and polymer concentration and propose a simple scaling model to guide particle formation. We find that the extraction time required to form particles from liquid droplets does not depend on the initial polymer concentration but is rather proportional to the initial droplet size. The resulting particle size follows a linear relationship with the initial droplet size for all polymer concentrations, allowing for the precise control of particle size. The internal particle porous structure exhibits a polymer density gradient ranging from a dense surface skin toward an essentially hollow core. Average particle porosities between 10 and 50% are achieved by varying the initial droplet compositions up to 15 wt % polymer. Such particles have potential applications in functional, optical, and coating materials.