3D Brownian Diffusion of Submicron-Sized Particle Clusters

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. Because they could be separated into fractions of well-defined configurations on a sufficient scale and their overall dimensions were below 300 nm, the translational and rotational diffusion coefficients of particle duplets, 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 coefficient 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

Brillouin Scattering from a Homogeneous 1,4-Polyisoprene-Poly(vinylethylene) Diblock Copolymer and Its Constituent Homopolymers

Polarized Rayleigh-Brillouin Spectroscopy (BS) has been used to probe the local segmental dynamics at hypersonic frequencies in a homogeneous l,4-polyisoprene-l,2-polybutadiene diblock copolymer far from the microphase separation transition (MST) and the corresponding homopolymers. An Arrhenius temperature dependence along with an exponential decay of the structural relaxation times τ8 provides an adequate fit of the experimental longitudinal loss modulus M′′, which reveals a similar dynamics in the diblock and 1,4-polyisoprene but at variance with the dynamics in 1,2-polybutadiene (poly(vinylethylene)). On the contrary, recent photon correlation spectroscopy (PCS) data obtained in the same materials near and above Tg indicate a much broader distribution of relaxation times in the diblock than in the constituent homopolymers. Moreover, a Vogel-Fulcher-Hesse-Tammann equation cannot describe reasonably both the low-temperature (PCS) and high-temperature (BS) data. The combined results of the PCS and BS measurements in the present homogeneous diblock copolymer with a single Tg indicate that a difference in segmental mobilities and shape parameters ß (with regard to the conventional Kohlrausch-Williams-Watts equation) may explain the behavior of the diblock near and above Tg, while at high temperatures where BS applies, a fast single relaxation time process seems to be responsible for the similar hypersonic dispersion in the diblock and homopolymer. These measurements represent a first, to our knowledge, application of dynamic light scattering methods to probe the dynamic behavior of homogeneous diblocks. © 1991, American Chemical Society. All rights reserved

13C Nuclear magnetic relaxation study of segmental dynamics of hyaluronan in aqueous solutions

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