Static light scattering

Abstract: Summary: In this article we present some interesting properties of rodlike cellulose microcrystals (so-called “whiskers”). These microcrystals can be obtained from different cellulose sources such as wood, cotton, or animal origin. When submitted to acid hydrolysis, the cellulose fibers yield stable aqueous suspensions because of the presence of negative charges on the surface of the microcrystallites during the hydrolysis process. The obtained microcrystals are rod-shaped particles, the dimensions of which depend on the cellulose origin. For instance, the cotton whiskers have typical dimensions varying from 100 to 300 nm in length, L, and 8 to 10 nm in diameter, d, while those of the tunicate whiskers range from 100 nm to few micrometers in length and 10 to 20 nm in diameter. At very low concentrations, these whiskers are randomly suspended in water and form an isotropic phase. When the concentration reaches a critical value, the whiskers spontaneously display ordered phases showing interesting liquid crystal properties (nematic and chiral nematic). The chiral nematic orders can be retained after evaporation of the solvent (generally water), leaving iridescent films. The reflected color can be controlled by changing either the ionic strength or by applying an electric field. These colloidal particles have been investigated using several techniques including small-angle neutron scattering (SANS), small angle X-ray scattering, rheology, and more recently dynamic and static light scattering techniques (DLS and SLS) to highlight their static and dynamic behavior. Because of their geometry, important axis ratio (L/d), and high crystallinity, these rods have been also extensively used to process nanocomposites based on polymer matrices, to reinforce their mechanical properties. All these properties are discussed in this contribution. Rodlike nanocrystals in aqueous suspension (left, Tunicate, 1 wt.-%) and film (right), observed between cross-polarizers.

Abstract: The striking properties of synthetic polymers and biological macromolecules are largely determined by their shape and the internal mobility. Both quantities are closely related to the architecture of the molecules. This article deals with branched macromolecules in dilute solution, where the individual molecules are observed. The common technique for determining the shape of macromolecules is static light scattering. Information on the internal mobility and the translational motion of the mass centre can be obtained from the more recent technique of quasi-elastic or dynamic light scattering.

Abstract: Abstract Dynamic light scattering is a new method for investigating macromolecular systems. The importance of the technique lies in its non-invasive character. It can be employed on extremely small fluid volumes, the instrumentation is relatively inexpensive and allows the rapid determination of diffusion coefficients as well as providing information on relaxation time distributions for the macromolecular components of complex systems. This volume is directed in part to the philosophy and current practice in dynamic light scattering: single photon correlation techniques are introduced, a discussion of noise on photon correlation functions is given and data analysis in dynamic light scattering to polymer structure analysis is presented and a comprehensive introduction to diffusing wave spectroscopy is given. Theoretical developments relating dynamic light scattering to the viscoelasticity of polymers in solution and in the bulk are described. The second aim is to illustrate the widely varying fields in which the technique finds application. Chapters are to be found on multicomponent mixtures, polyelectrolytes, dense polymer systems, gels, rigid rods, micellar systems and the application of dynamic light scattering to biological systems.

Abstract: Dynamic light scattering (DLS) techniques for studying sizes and shapes of nanoparticles in liquids are reviewed. In photon correlation spectroscopy (PCS), the time fluctuations in the intensity of light scattered by the particle dispersion are monitored. For dilute dispersions of spherical nanoparticles, the decay rate of the time autocorrelation function of these intensity fluctuations is used to directly measure the particle translational diffusion coefficient, which is in turn related to the particle hydrodynamic radius. For a spherical particle, the hydrodynamic radius is essentially the same as the geometric particle radius (including any possible solvation layers). PCS is one of the most commonly used methods for measuring radii of submicron size particles in liquid dispersions. Depolarized Fabry-Perot interferometry (FPI) is a less common dynamic light scattering technique that is applicable to optically anisotropic nanoparticles. In FPI the frequency broadening of laser light scattered by the particles is analyzed. This broadening is proportional to the particle rotational diffusion coefficient, which is in turn related to the particle dimensions. The translational diffusion coefficient measured by PCS and the rotational diffusion coefficient measured by depolarized FPI may be combined to obtain the dimensions of non-spherical particles. DLS studies of liquid dispersions of nanometer-sized oligonucleotides in a water-based buffer are used as examples.