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Fangfang Chu,
Miriam Siebenbürger,
Frank Polzer,
Carmen Stolze,
Julian Kaiser, Martin Hoffmann,
Nils Heptner,
Joachim Dzubiella,
Markus Drechsler,
Yan Lu,
Matthias Ballauff
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ABSTRACT: Monodisperse thermosensitive dumbbell-shaped core-shell microgels are fabricated, which consist of a polystyrene core with a cross-linked poly (N-isopropylacrylamide) shell. The morphology of the microgels was investigated through cryogenic transmission electron microscopy and depolarized dynamic light scattering. The effective volume fraction and aspect ratio of the system could be adjusted through the swelling of the thermosensitive shell. We observe a phase transition of the microgels to an ordered, crystal-like state, which is apparent through Bragg-reflections in the visible range. These observations are further supported by rheological measurements where the shear-melting of the crystal phase is clearly detected.
Macromolecular Rapid Communications 04/2012; 33(12):1042-8. · 4.60 Impact Factor
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ABSTRACT: Optical second harmonic generation (SHG) was applied to study the shell layer of spherical polyelectrolyte brushes as a function of ionic strength. The particles consist of a polystyrene core from which linear chains of a quenched polyelectrolyte are grafted. Compared to all systems studied by SHG previously, the shell layer in this study has a remarkable thickness of ca. 100 nm. Angle-resolved second harmonic (SH) scattering profiles were recorded and compared to simulations with different model geometries using a nonlinear Mie scattering model. The analysis reveals that the second harmonic signal is not generated by the polyelectrolyte molecules, but rather by oriented and polarized water molecules at the inner (polystyrene/polyelectrolyte) and at the outer (polyelectrolyte/solvent) interface of the polyelectrolyte shell. The results demonstrate that angle-resolved SH light scattering provides direct access to the structure of water and the screening of charges at the inner and outer interface of the polyelectrolyte shell. Correspondingly, this work sets the experimental and theoretical basis for the study of the extended interfacial layer of electrosterically stabilized particles by SH light scattering.
08/2011;
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ABSTRACT: We fabricated thermoresponsive colloidal molecules of ca. 250 nm size. Electron- and scanning force microscopy reveal the dumbbell-shaped morphology. The temperature dependence of the size and aspect ratio (ca. 1.4 to 1.6) is analyzed by depolarized dynamic light scattering and found to be in good agreement with microscopic evidence.
Soft Matter 05/2010; 6:1125-1128. · 4.39 Impact Factor
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ABSTRACT: We report on the translation and rotation of particle clusters made through the combination of spherical building blocks. These clusters present ideal model systems to study the motion of objects with complex shape. Since they could be separated into fractions of well-defined configurations on a sufficient scale and because their overall dimensions were below 300 nm, the translational and rotational diffusion coefficients of particle doublets, triplets, and tetrahedrons could be determined by a combination of polarized dynamic light scattering (DLS) and depolarized dynamic light scattering (DDLS). The use of colloidal clusters for DDLS experiments overcomes the limitation of earlier experiments on the diffusion of complex objects near surfaces because the true 3D diffusion can be studied. When the exact geometry of the complex assemblies is known, different hydrodynamic models for calculating the diffusion coefficients for objects with complex shapes could be applied. Because hydrodynamic friction must be restricted to the cluster surface, the so-called shell model, in which the surface is represented as a shell of small friction elements, was most suitable to describe the dynamics. A quantitative comparison of the predictions from theoretical modeling with the results obtained by DDLS showed an excellent agreement between experiment and theory.
ACS Nano 10/2009; 3(10):3326-34. · 10.77 Impact Factor
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Macromolecular Chemistry and Physics 02/2009; 210(5):377 - 386. · 2.36 Impact Factor
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ABSTRACT: We present the synthesis and comprehensive characterization of dumbbell-shaped polyelectrolyte brushes (DPB). The core of these particles consists of poly(methyl methacrylate) (PMMA) and poly(styrene) onto which a dense brush shell of poly(styrene sulfonate) is grafted. The morphology of DPB particles is studied in solution by cryogenic-transmission electron microscopy. We demonstrate that well-defined DPB are generated that react to external stimuli such as surfactant and salt concentration. The rotational diffusion and collective relaxations of the DPB particles were monitored by depolarized dynamic light scattering (DDLS). Here we found a new relaxation mode in the DDLS-signal that can be ascribed to collective fluctuations of the polyelectrolyte layer affixed to the surface of the dumbbells.
The Journal of Physical Chemistry B 11/2008; 112(47):14843-50. · 3.70 Impact Factor
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ABSTRACT: We consider the interaction of multivalent counterions with spherical polyelectrolyte brushes (SPBs). SPBs result if linear polyelectrolyte chains (contour length 60 nm) are densely grafted to colloidal spheres of 116 nm in diameter. When dispersed in water the surface layer, consisting of chains of the strong polyelectrolyte poly(styrene sulfonic acid), will swell. Recent work [Mei, Phys. Rev. Lett. 97, 158301 (2006)] has demonstrated that spherical polyelectrolyte brushes undergo a collapse in the presence of a mixture of monovalent and multivalent counterions. The collapse crossover could be well described by a mean-field approach. Here we demonstrate that the application of a mean-field approach is well founded by simulation results done with molecular dynamics (MD). MD simulations show that over a wide range of multivalent counterion concentration the effects of ion correlation and fluctuations can be neglected. Higher-valent counterions are shown to interact strongly with the polyelectrolyte chains of the SPBs and thus exhibit a much reduced osmotic activity in the system. This reduction is the driving force for the collapse.
Physical Review E 04/2008; 77(3 Pt 1):031805. · 2.26 Impact Factor
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ABSTRACT: We consider the interaction of multivalent counterions with spherical polyelectrolyte brushes (SPB). The SPB result if linear polyelectrolyte (PE) chains (contour length: 60 nm) are densely grafted to colloidal spheres of 116 nm in diameter. Dispersed in water, the surface layer consisting of chains of the strong PE poly(styrene sulfonic acid) (PSS) will swell. We demonstrate that successive addition of trivalent ions (La3+) leads to a collapse in which the surface layer is shrinking drastically. All findings are discussed on the base of a theoretical mean-field approach using the Donnan equilibrium. The ion exchange and a strong binding of trivalent ions by PE chains is followed up by a drop in the osmotic pressure inside the brush. This reduction is the driving force for the collapse. The strong ion-chain correlation is discussed with results obtained from molecular dynamics simulations.
Physical Review Letters 11/2006; 97(15):158301. · 7.37 Impact Factor
