Walter Richtering - Profile on Academia.edu (original) (raw)

Papers by Walter Richtering

Effect of counterion and solvent type on the scattering and rheology of polyelectrolyte solutions

Bulletin of the American Physical Society, Mar 18, 2021

Entanglement density and crossovers of polyelectrolyte solutions

Bulletin of the American Physical Society, Mar 8, 2019

Macromolecules, Aug 30, 2021

We report chain self-diffusion and viscosity data for sodium polystyrene sulfonate (NaPSS) in sem... more We report chain self-diffusion and viscosity data for sodium polystyrene sulfonate (NaPSS) in semidilute saltfree aqueous solutions measured by pulsed field gradient NMR and rotational rheometry respectively. The observed concentration dependence of η and D are consistent with the Rouse-Zimm scaling model with a concentration dependent monomeric friction coefficient. The concentration dependence of the monomeric friction coefficient exceeds that expected from free-volume models of diffusion, and its origin remains unclear. Correlation blobs and dilute chains with equivalent end-to-end distances exhibit nearly equal friction coefficients, in agreement with scaling. The viscosity and diffusion data are combined using the Rouse model to calculate the chain dimensions of NaPSS in salt-free solution, these agree quantitatively with SANS measurements.

Oscillatory rheology of carboxymethyl cellulose gels: Influence of concentration and pH

Carbohydrate Polymers, Sep 1, 2021

The flow properties of ionic polysaccharides are determined by the interplay of electrostatic and... more The flow properties of ionic polysaccharides are determined by the interplay of electrostatic and hydrophobic interactions, which depend on the ionic strength and pH of the solvent. We explore the LVE and LAOS rheology of carboxymethyl cellulose (CMC) in aqueous media, focusing on its gelling behaviour. We find that addition of HCl promotes gel formation and addition of NaOH suppresses it. The former effect is interpreted as being caused by a decrease of the charge density of the polymer, which facilitates interchain associations and the later effect can be assigned to solubilisation of cellulose backbone by NaOH. Our results along with a review of the literature allow us to establish the concentration regimes and associated properties of physical gels of carboxymethyl cellulose. At neutral pH, the storage modulus of NaCMC gels of varying molecular weight and DS at a given concentration does not vary by more than a factor 5.

Soft Matter, 2020

We report results of small angle neutron scattering measurements made on sodium polystyrene sulfo... more We report results of small angle neutron scattering measurements made on sodium polystyrene sulfonate in aqueous salt solutions. The correlation length (ξ) and osmotic compressibility are measured as a function of polymer (c) and added salt (cS) concentrations, and the results are compared with scaling predictions and the random-phase approximation (RPA). In Dobrynin et al.’s scaling model the osmotic pressure consists of a counter-ion contribution and a polymer contribution. The polymer contribution is found to be two orders of magnitude smaller than expected from the scaling model, in agreement with earlier observations made on neutral polymers in good solvent condition. RPA allows the determination of single-chain dimensions in semidilute solutions at high polymer and added salt concentrations, but fails for cS ≤ 2 M. The χ parameter can be modelled as the sum of an intrinsic contribution (χ0) and an electrostatic term: χ ~ χ0 + K′/√cs , where χ0 > 0.5 is consistent with the hydrophobic nature of the backbone of NaPSS. The dependence of χelec ~ 1/√cs disagrees with the random-phase approximation (χelec ~ 1/cs), but agrees with the light scattering results in dilute solution and Dobrynin et al.’s scaling treatment of electrostatic excluded volume.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Aug 1, 2018

We report an analytical approach to study the competitive processes of solubilisation in micelles... more We report an analytical approach to study the competitive processes of solubilisation in micelles and of adsorption onto hydrophobic surfaces of poorly soluble hydrophobic dyes. The method is demonstrated on model systems containing two sources of Disperse Red 60: a bulk powder and a donor red textile, with molecularly dissolved dye stabilised in an aqueous environment by mixed micelles of anionic and non-ionic surfactants. The process of dye transfer between a donor textile (red polyester), surfactant micelles and an acceptor textile (white polyamide) was quantified by a combination of colorimetric analyses. UV-Vis absorbance was used to follow the extraction of the dye and to evaluate the solubilisation capacity of the micellar solution. A calibration curve for textile reflectance versus the adsorbed dye was generated to quantify the mass of dye transfered onto the acceptor textile. A combination of both techniques allowed us to compare the amount of dye desorbed from the donor textile and adsorbed onto the acceptor textile as a function of time for systems undergoing exhaustion-solubilisation mechanisms and only solubilisation mechanism. Up to 10 min of the washing process, the released dye is predominantly solubilised in surfactant micelles. At later times, the adsorption of the dye on the hydrophobic surface is energetically favoured. The shift of the desorption equilibrium in the presence of the acceptor textile results in 30% increase in the release of the dye. The reported methodology provides direct comparative analysis between the solubilisation capacity 1

Carbohydrate Polymers, Apr 1, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

arXiv (Cornell University), Apr 2, 2020

Viscoelastic behavior of spatially confined multilamellar vesicles

Polymer Preprints, Japan, 2006

Langmuir : the ACS journal of surfaces and colloids, Jan 20, 2017

The internal morphology of temperature-responsive degradable poly(N-isopropylacrylamide) (PNIPAM)... more The internal morphology of temperature-responsive degradable poly(N-isopropylacrylamide) (PNIPAM) microgels formed via an aqueous self-assembly process based on hydrazide and aldehyde-functionalized PNIPAM oligomers is investigated. A combination of surface force measurements, small angle neutron scattering (SANS), and ultra-small angle neutron scattering (USANS) was used to demonstrate that the self-assembled microgels have a homogenously cross-linked internal structure. This result is surprising given the sequential addition process used to fabricate the microgels, which was expected to result in a densely crosslinked shell-diffuse core structure. The homogeneous internal structure identified is also significantly different than conventional microgels prepared via precipitation polymerization, which typically exhibit a diffuse shell-dense core structure. The homogenous structure is hypothesized to result from the dynamic nature of the hydrazone crosslinking chemistry used coupled ...

Aging in dense suspensions of soft thermosensitive microgel particles studied with particle-tracking microrheology

Physical Review E, 2010

Journal of Physical Chemistry B, May 24, 2019

We report viscosity data of non-entangled sodium polystyrene sulfonate (NaPSS) in salt-free aqueo... more We report viscosity data of non-entangled sodium polystyrene sulfonate (NaPSS) in salt-free aqueous solution as a function of polymer concentration (c) and degree of polymerisation (N). Different empirical equations are examined and found not describe the semidilute solution viscosity over a wide concentration range and/or to yield values of [η] that do not match dilute solution measurements. Deviations from the scaling prediction of η sp ∝ c 1/2 (Fuoss' law) are observed at high concentrations. Specifically, we find η sp ∼ N 1.26 c 1/2 e 1.4c in the semidilute regime, which agrees with the scaling prediction only for c 0.02 M. The viscosity data presented in this study and in earlier reports show a high degree of consistency. A comparison with diffusion measurements for NaPSS in salt-free solution by Oostwal and co-workers suggests that the disagreement between scaling theory and experiments does arise solely from a concentration dependence of the monomeric friction coefficient.

Microgels are macromolecular networks swollen by the solvent they are dissolved in. They are uniq... more Microgels are macromolecular networks swollen by the solvent they are dissolved in. They are unique systems that are distinctly different from common colloids, such as, e.g., rigid nanoparticles, flexible macromolecules, micelles or vesicles. When swollen, they are soft and have a fuzzy surface with dangling chains and the presence of cross-links provides structural integrity-in contrast to linear and (hyper-) branched polymers. Obviously, the cross-linker content will allow controlling whether microgels behave more "colloidal" or "macromolecular". Finally, microgels reveal interfacial activity without being amphiphilic.

Macromolecular Chemistry and Physics, 2015

Precise particle size customization is a key requirement for the applications of stimuli-sensitiv... more Precise particle size customization is a key requirement for the applications of stimuli-sensitive poly(N-isopropylacrylamide) microgels. No empirically supported mechanism has been presented to explain the microgel particle size dependence on polymerization conditions. In this work, nonstirred precipitation polymerization is employed to address this question. It is argued that the reaction proceeds by radical solution polymerization mechanism rather than as emulsion polymerization. The number of particles in the batch, and the microgel particle volume, is shown to correlate with the charge density of the particles determined by the kinetic chain length. Cross-linking effi ciency is recognized to be an important factor in the particle nucleation. Properties of radical solution polymerization, such as differences in the rate of decomposition of thermally decomposing initiator and redox initiator, are used to tune the particle number density in the batch. Contemporary synthesis approaches, including temperature-programmed synthesis, are discussed in the context of these results.

The Journal of Chemical Physics, 2018

We study how a cavity changes the response of hollow microgels with respect to regular ones in ov... more We study how a cavity changes the response of hollow microgels with respect to regular ones in overcrowded environments. The structural changes of hollow poly(N-isopropylacrylamide) microgels embedded within a matrix of regular ones are probed by small-angle neutron scattering with contrast variation. The form factors of the microgels at increasing compressions are directly measured. The decrease of the cavity size with increasing concentration shows that the hollow microgels have an alternative way with respect to regular cross-linked ones to respond to the squeezing due to their neighbors. The structural changes under compression are supported by the radial density profiles obtained with computer simulations. The presence of the cavity offers to the polymer network the possibility to expand toward the center of the microgels in response to the overcrowded environment. Furthermore, upon increasing compression, a two step transition occurs: First the microgels are compressed but the internal structure is unchanged; then, further compression causes the fuzzy shell to collapse completely and reduce the size of the cavity. Computer simulations also allow studying higher compression degrees than in the experiments leading to the microgel's faceting.

Physical Chemistry Chemical Physics

We investigated the interaction between microgel monolayers at the air–water interface and a hard... more We investigated the interaction between microgel monolayers at the air–water interface and a hard colloid in water. Our results show that microgel monolayers change from soft to hard repulsive interfaces when the VPTT is exceeded.

Microgels, cross-linked polymers with submicrometer size, are ideal soft model systems. While sph... more Microgels, cross-linked polymers with submicrometer size, are ideal soft model systems. While spherical microgels have been studied extensively, anisotropic microgels have been hardly investigated. In this study, we compare the interfacial deformation and assembly of anisotropic core-shell and hollow microgels. The core-shell microgel consists of an elliptical core of hematite covered with a thin silica layer and a thin shell made of poly(N -isopropylacrylamide) (PNiPAM). The hollow microgels were obtained after a two step etching procedure of the inorganic core. The behavior of these microgels at the oil-water interface was investigated in a Langmuir Blodgett trough combined with ex-situ atomic force microscopy. First, the influence of the architecture of anisotropic 1 ar X iv :2 20 2. 01 83 3v 1 [ co nd -m at .s of t] 3 F eb 2 02 2 microgels on their spreading at the interface was investigated experimentally and by dissipative particle dynamic simulations. Hereby, the importance o...

Langmuir, 2021

Anisotropic, submicrometer sized particles are versatile systems providing interesting features i... more Anisotropic, submicrometer sized particles are versatile systems providing interesting features in creating ordering in 2-dimensional systems. Combining hard ellipsoids with a soft shell further enhances the opportunities to trigger and control order and alignment. In this work, we report a rich 2D phase behavior and show how softness affects the ordering of anisotropic particles at fluid oil-water interfaces. Three different core-shell systems were synthesized such that they have the same elliptical hematitesilica core but differ with respect to thickness and stiffness of the soft microgel shell. Compression isotherms, the shape of individual core-shell microgel as well as their 2D order at a decane-water interface are investigated by means of the Langmuir-Blodgett technique combined with ex-situ Atomic Force Microscopy (AFM) imaging, as well