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Papers by Walter Richtering

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

Bulletin of the American Physical Society, Mar 18, 2021

Research paper thumbnail of Entanglement density and crossovers of polyelectrolyte solutions

Bulletin of the American Physical Society, Mar 8, 2019

Research paper thumbnail of Diffusion and Viscosity of Unentangled Polyelectrolytes

Macromolecules, Aug 30, 2021

Research paper thumbnail of 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.

Research paper thumbnail of Influence of divalent counterions on the solution rheology and supramolecular aggregation of carboxymethyl cellulose

Research paper thumbnail of Screening lengths and osmotic compressibility of flexible polyelectrolytes in excess salt solutions

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.

Research paper thumbnail of Combined UV–Vis-absorbance and reflectance spectroscopy study of dye transfer kinetics in aqueous mixtures of surfactants

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

Research paper thumbnail of Scaling laws of entangled polysaccharides

Carbohydrate Polymers, Apr 1, 2020

Research paper thumbnail of Diffusion and viscosity of non-entangled polyelectrolytes

arXiv (Cornell University), Apr 2, 2020

Research paper thumbnail of Viscoelastic behavior of spatially confined multilamellar vesicles

Polymer Preprints, Japan, 2006

Research paper thumbnail of Cononsolvency of Microgels: Phase Behaviour, Structure and Dynamics

Research paper thumbnail of Dynamically-Crosslinked Self-Assembled Thermoresponsive Microgels with Homogeneous Internal Structures

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 ...

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

Research paper thumbnail of Viscosity of Semidilute and Concentrated Nonentangled Flexible Polyelectrolytes in Salt-Free Solution

Journal of Physical Chemistry B, May 24, 2019

Research paper thumbnail of Adaptive microgels in complexes and at interfaces

Research paper thumbnail of Controlled Synthesis and Fluorescence Tracking of Highly Uniform Poly(N-isopropylacrylamide) Microgels

Journal of visualized experiments : JoVE, Sep 8, 2016

Stimuli-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels have various prospective practic... more Stimuli-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels have various prospective practical applications and uses in fundamental research. In this work, we use single particle tracking of fluorescently labeled PNIPAM microgels as a showcase for tuning microgel size by a rapid non-stirred precipitation polymerization procedure. This approach is well suited for prototyping new reaction compositions and conditions or for applications that do not require large amounts of product. Microgel synthesis, particle size and structure determination by dynamic and static light scattering are detailed in the protocol. It is shown that the addition of functional comonomers can have a large influence on the particle nucleation and structure. Single particle tracking by wide-field fluorescence microscopy allows for an investigation of the diffusion of labeled tracer microgels in a concentrated matrix of non-labeled microgels, a system not easily investigated by other methods such as dynamic ...

Research paper thumbnail of Can the Reaction Mechanism of Radical Solution Polymerization Explain the Microgel Final Particle Volume in Precipitation Polymerization of N -Isopropylacrylamide?

Macromolecular Chemistry and Physics, 2015

Research paper thumbnail of Hollow microgels squeezed in overcrowded environments

The Journal of Chemical Physics, 2018

Research paper thumbnail of Interactions between a responsive microgel monolayer and a rigid colloid: from soft to hard interfaces

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.

Research paper thumbnail of Interfacial assembly of anisotropic core-shell and hollow microgels

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...

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

Bulletin of the American Physical Society, Mar 18, 2021

Research paper thumbnail of Entanglement density and crossovers of polyelectrolyte solutions

Bulletin of the American Physical Society, Mar 8, 2019

Research paper thumbnail of Diffusion and Viscosity of Unentangled Polyelectrolytes

Macromolecules, Aug 30, 2021

Research paper thumbnail of 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.

Research paper thumbnail of Influence of divalent counterions on the solution rheology and supramolecular aggregation of carboxymethyl cellulose

Research paper thumbnail of Screening lengths and osmotic compressibility of flexible polyelectrolytes in excess salt solutions

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.

Research paper thumbnail of Combined UV–Vis-absorbance and reflectance spectroscopy study of dye transfer kinetics in aqueous mixtures of surfactants

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

Research paper thumbnail of Scaling laws of entangled polysaccharides

Carbohydrate Polymers, Apr 1, 2020

Research paper thumbnail of Diffusion and viscosity of non-entangled polyelectrolytes

arXiv (Cornell University), Apr 2, 2020

Research paper thumbnail of Viscoelastic behavior of spatially confined multilamellar vesicles

Polymer Preprints, Japan, 2006

Research paper thumbnail of Cononsolvency of Microgels: Phase Behaviour, Structure and Dynamics

Research paper thumbnail of Dynamically-Crosslinked Self-Assembled Thermoresponsive Microgels with Homogeneous Internal Structures

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 ...

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

Research paper thumbnail of Viscosity of Semidilute and Concentrated Nonentangled Flexible Polyelectrolytes in Salt-Free Solution

Journal of Physical Chemistry B, May 24, 2019

Research paper thumbnail of Adaptive microgels in complexes and at interfaces

Research paper thumbnail of Controlled Synthesis and Fluorescence Tracking of Highly Uniform Poly(N-isopropylacrylamide) Microgels

Journal of visualized experiments : JoVE, Sep 8, 2016

Stimuli-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels have various prospective practic... more Stimuli-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels have various prospective practical applications and uses in fundamental research. In this work, we use single particle tracking of fluorescently labeled PNIPAM microgels as a showcase for tuning microgel size by a rapid non-stirred precipitation polymerization procedure. This approach is well suited for prototyping new reaction compositions and conditions or for applications that do not require large amounts of product. Microgel synthesis, particle size and structure determination by dynamic and static light scattering are detailed in the protocol. It is shown that the addition of functional comonomers can have a large influence on the particle nucleation and structure. Single particle tracking by wide-field fluorescence microscopy allows for an investigation of the diffusion of labeled tracer microgels in a concentrated matrix of non-labeled microgels, a system not easily investigated by other methods such as dynamic ...

Research paper thumbnail of Can the Reaction Mechanism of Radical Solution Polymerization Explain the Microgel Final Particle Volume in Precipitation Polymerization of N -Isopropylacrylamide?

Macromolecular Chemistry and Physics, 2015

Research paper thumbnail of Hollow microgels squeezed in overcrowded environments

The Journal of Chemical Physics, 2018

Research paper thumbnail of Interactions between a responsive microgel monolayer and a rigid colloid: from soft to hard interfaces

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.

Research paper thumbnail of Interfacial assembly of anisotropic core-shell and hollow microgels

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...