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Biomedical subjects

Walter Richtering

Publications and source records attributed to Walter Richtering.

9 recordsLinked to original sources
PubMed2006

Influence of shell thickness and cross-link density on the structure of temperature-sensitive poly-N-isopropylacrylamide-poly-N-isopropylmethacrylamide core-shell microgels investigated by small-angle neutron scattering.

Swelling properties of doubly temperature sensitive core-shell microgels consisting of two thermosensitive polymers with lower critical solution temperatures (LCTS) at, respectively, 34 degrees C in the core and 44 degrees C in the shell have been investigated by small-angle neutron scattering (SANS). A core-shell form factor has been employed to evaluate the structure, and the real space particle structure is expressed by radial density profiles. By this means, the influences of both shell/core mass composition and shell cross-linker content on the internal structure have been revealed at temperatures above, between, and below the LCSTs. Higher shell/core mass ratios lead to an increased expansion of the core at temperatures between the LCSTs, whereas a variation of cross-linker in the shell mainly effects the dimensions of the shell. The influence on the core structure was interpreted as resulting from an elastic force developed from the swollen shell. At temperatures below the core LCST, the core cannot swell to its native size (i.e., in the absence of a shell), because the maximum expanded shell network prohibits further swelling. Thus, depending on temperature, the shell either expands or compresses the core.

Journal Article↗
PubMed2005

Structure of multiresponsive "intelligent" core-shell microgels.

A doubly temperature-sensitive core-shell microgel composed of two temperature-sensitive polymers with different lower critical solution temperatures (LCSTs) in the core and shell has been studied by small-angle neutron scattering (SANS). The application of a novel universal form factor model in the analysis of the SANS data reveals that the radial density profile at temperatures above the LCSTs of both polymers can be well described by a two-box profile with narrow interfaces. At temperatures between the LCSTs, the radial density profile reveals that the core in the core-shell microgel has larger dimensions than the naked core. Thus the swollen shell pulls the core apart. At temperatures below both LCSTs, however, the shell restricts the core swelling, and the core is found to be smaller than in its native state. This clearly demonstrates the mutual influence of core and shell swelling.

Journal Article↗
PubMed2005

Structures and dynamics of thermosensitive microgel suspensions studied with three-dimensional cross-correlated light scattering.

The structure factors, short- and long-time diffusion coefficients, and hydrodynamic interactions of concentrated poly(N-isopropylacryamide) microgel suspensions were measured with simultaneous static and dynamic three-dimensional cross-correlated light scattering. The data are interpreted through comparison to hard sphere theory. The structure factors are known to be described well by the hard sphere approximation. When the structure factor is fit to an effective hard sphere volume fraction and radius, the diffusion and hydrodynamic interactions are also well described by the hard sphere model. We demonstrate that one single hard sphere volume fraction is sufficient to describe the microgel structures, hydrodynamic interactions, and long- and short-time collective diffusion coefficients. This result is surprising because the particle form of the microgels at these temperatures is not rigid, but rather "fuzzy" spheres with dangling polymer chains.

Journal Article↗
PubMed2004

Are thermoresponsive microgels model systems for concentrated colloidal suspensions? A rheology and small-angle neutron scattering study.

The structure of concentrated temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgel suspensions has been investigated employing rheology and small-angle neutron scattering (SANS). A previously described model expression for the particle form factor P(inho)(q) is extended by a model hard sphere structure factor S(q), and the average radial density profiles phi(r) are calculated from the amplitude of the form factor A(q) and the structure factor S(q). By this procedure, a direct real space description of the spatial ordering in the neighborhood of a single particle is obtained. The overall particle size and the correlation length xi of the concentration fluctuations of the internal polymer network decrease with concentration, revealing the increasing compression of the spheres. Thus, the particle form factor P(inho)(q) of the swollen PNiPAM microgels depends on concentration. The particle-particle interaction potential does not change significantly between 25 and 32 degrees C. Even approximately 1 K below the lower critical solution temperature (LCST), the experimental scattering intensity distributions I(q)/c are described very well by the hard sphere structure factor when an equivalent hard sphere particle size R(HS) and volume fraction eta(HS) are used. Microgels with different degrees of cross-linking and particle size resemble true hard sphere behavior up to effective volume fractions of phi(eff) < 0.35. At higher effective volume fractions phi(eff) > 0.35 strong deviations from true hard spheres are observed. Interpenetration of the outer, less cross-linked regions of the soft spheres as well as particle compression occurred at higher concentrations. In agreement with this, the equilibrium colloidal phase behavior and rheology also has some features of soft sphere systems. At temperatures well above the LCST, the interaction potential becomes strongly attractive and the collapsed microgel spheres form aggregates consisting of flocculated particles without significant long-range order. Hence, an attractive interaction potential in concentrated suspensions of PNiPAM microgels leads to distinctively different structures as compared to attractive hard sphere colloids. When the peculiar structural properties of the PNiPAM microgels are considered, they can be used as model systems in colloidal science.

Journal Article↗
PubMed2004

Shear-induced morphology transition and microphase separation in a lamellar phase doped with clay particles.

We report on the influence of shear on a nonionic lamellar phase of tetraethyleneglycol monododecyl ether (C12E4) in D2O containing clay particles (Laponite RD). The system was studied by means of small-angle light scattering (SALS) and small-angle neutron scattering (SANS) under shear. The SANS experiments were conducted using a H2O/D2O mixture of the respective scattering length density to selectively match the clay scattering. The rheological properties show the familiar shear thickening regime associated with the formation of multilamellar vesicles (MLVs) and a shear thinning regime at higher stresses. The variation of viscosity is less pronounced as commonly observed. In the shear thinning regime, depolarized SALS reveals an unexpectedly strong variation of the MLV size. SANS experiments using the samples with lamellar contrast reveal a change in interlamellar spacing of up to 30% at stresses that lead to MLV formation. This change is much more pronounced than the change observed, when shear suppresses thermal bilayer undulations. Microphase separation occurs, and as a consequence, the lamellar spacing decreases drastically. The coincidence of the change in lamellar spacing and the onset of MLV formation is a strong indication for a morphology-driven microphase separation.

Journal Article↗
PubMed2004

Small-angle neutron scattering study of structural changes in temperature sensitive microgel colloids.

The structure of temperature-sensitive poly(N-isopropylacrylamide) microgels in dilute suspension was investigated by means of small-angle neutron scattering. A direct modeling expression for the scattering intensity distribution was derived which describes very well the experimental data at all temperatures over an extensive q range. The overall particle form as well as the internal structure of the microgel network is described by the model. The influence of temperature, cross-linking density, and particle size on the structure was revealed by radial density profiles and clearly showed that the segment density in the swollen state is not homogeneous, but gradually decays at the surface. The density profile reveals a box profile only when the particles are collapsed at elevated temperatures. An increase of the cross-linking density resulted in both an increase of the polymer volume fraction in the inner region of the particle and a reduction of the smearing of the surface. The polymer volume fraction inside the colloid decreased with increasing particle size. The structural changes are in good agreement with the kinetics of the emulsion copolymerization used to prepare the microgel colloids.

Acrylic Resins↗
PubMedDate not supplied

Shear-induced mixing and demixing in aqueous methyl hydroxypropyl cellulose solutions.

The influence of shear flow on the phase separation of aqueous methyl hydroxypropyl cellulose solutions was investigated by means of rheoturbidity and online rheo small angle light scattering (SALS) experiments. In semidilute solution shear-induced mixing was observed and the cloud curve was shifted to higher temperatures with increasing shear rate. With higher concentrated solutions, however, shear-induced demixing was found. The shear-induced mixing is interpreted as being a disruption of slightly entangled clusters under the influence of the shear energy. The shear demixing appears in line with the observation with other systems. A characteristic butterfly pattern was observed in rheo-SALS.

Hypromellose Derivatives↗