Viscosity measurement light scattering methods

A HE DETERMINATION OF COMPOSITIONAL CHANGES acrOSS the molecular weight distribution of a polymer is of considerable interest to polymer chemists. This information allows the chemist to predict the physical properties and ultimately the performance of the polymer. Several analytical techniques are of use in determining these properties. Mass spectroscopy, NMR, viscosity measurements, light scattering, and infrared (IR) spectroscopy all can be used to provide data in one form or the other about the compositional details sought. Each method has its place in the determination of the details of the structure of a polymer. IR spectroscopy, generically known as Fourier transform IR (FTIR)... 

The most widely used molecular weight characterization method has been GPC, which separates compounds based on hydrodynamic volume. State-of-the-art GPC instruments are equipped with a concentration detector (e.g., differential refractometer, UV, and/or IR) in combination with viscosity or light scattering. A viscosity detector provides in-line solution viscosity data at each elution volume, which in combination with a concentration measurement can be converted to specific viscosity. Since the polymer concentration at each elution volume is quite dilute, the specific viscosity is considered a reasonable approximation for the dilute solution s intrinsic viscosity. The plot of log[r]]M versus elution volume (where [) ] is the intrinsic viscosity) provides a universal calibration curve from which absolute molecular weights of a variety of polymers can be obtained. Unfortunately, many reported analyses for phenolic oligomers and resins are simply based on polystyrene standards and only provide relative molecular weights instead of absolute numbers. 

Often in hyperbranched polymers obtained via SCVP, it is not possible to determine the DB directly via NMR analysis. Therefore, other methods, for example, viscosity measurements and light-scattering methods have to be used to confirm the compact structure of a hyperbranched polymer. Such characterizations of hyperbranched (meth)acrylates will be discussed in the next section. 

The radii of gyration of amylose and cellulose are calculated by a method of Lifson for different glucose conformations. It is found that the radii of gyration differ widely and it can be determinated clearly by means of viscosity and light-scattering measurements which kind of glucose conformation is present in the polysaccharides. The effects of twisting upon the linear molecule is discussed. 

These macroscopic viscosity measurements have been confirmed at the molecular level. For example, dynamic light-scattering methods show an average hydrodynamic diameter (D J of about 370 A for a 50/50 copolymer of NVP/SPE in low salt (<2%) and a of about 390 A in high-salt (20% NaCl) concentrations (18). Moreover, in solutions of water or low concentrations of salt, solvent quality (as measured by second virial coefficient, A2) decreases with increasing levels of SPE. LALLS measurements for NVP/SPE 80/20 and 10/90 copolymers in 2% NaCl solution yielded molecular weights of 1.1 X 10 and 1.4 x 10 g mol respectively. The same compositions yielded second virial coefficients of 9.0 X 10 and 0.6 X 10 , respectively. In this case, a higher virial coefficient means better solution quality. 

Methods other than GPC that have been used to measure MWD of polymers include osmosis methods, viscosity methods, light scattering methods, ultracentrifugation, precipitation fractionation and column fractionation, and others, but these methods are complicated, and in some cases lengthy in operation. 

The final method to be mentioned is Zeta potential (may also be referred to as an electrophoretic method). In this method, the diluted emulsion is placed in a measurement ceU, and a static elecflic field is applied. Emulsion droplets carrying an electrostatic charge wiU move in the field at a velocity that is a function of several factors, including viscosity of the emulsion, net charge on the particle, and particle size. The velocity of movement can be determined using a light scattering method... 

It should be mentioned that, for certain classes of liquid crystals (polymeric liquid crystals, smectic liquid crystals), the light scattering method allows the determination of the viscosity coefficients, whereas their measurement by means of mechanical methods is very difficult. 

The various physical methods in use at present involve measurements, respectively, of osmotic pressure, light scattering, sedimentation equilibrium, sedimentation velocity in conjunction with diffusion, or solution viscosity. All except the last mentioned are absolute methods. Each requires extrapolation to infinite dilution for rigorous fulfillment of the requirements of theory. These various physical methods depend basically on evaluation of the thermodynamic properties of the solution (i.e., the change in free energy due to the presence of polymer molecules) or of the kinetic behavior (i.e., frictional coefficient or viscosity increment), or of a combination of the two. Polymer solutions usually exhibit deviations from their limiting infinite dilution behavior at remarkably low concentrations. Hence one is obliged not only to conduct the experiments at low concentrations but also to extrapolate to infinite dilution from measurements made at the lowest experimentally feasible concentrations. 

Viscosity, defined as the resistance of a liquid to flow under an applied stress, is not only a property of bulk liquids but of interfacial systems as well. The viscosity of an insoluble monolayer in a fluid-like state may be measured quantitatively by the viscous traction method (Manheimer and Schechter, 1970), wave-damping (Langmuir and Schaefer, 1937), dynamic light scattering (Sauer et al, 1988) or surface canal viscometry (Harkins and Kirkwood, 1938 Washburn and Wakeham, 1938). Of these, the last is the most sensitive and experimentally feasible, and allows for the determination of Newtonian versus non-Newtonian shear flow. 

Figure 3. Critical concentration behavior of actin self-assembly. For the top diagram depicting the macroscopic critical concentration curve, one determines the total amount of polymerized actin by methods that measure the sum of addition and release processes occurring at both ends. Examples of such methods are sedimentation, light scattering, fluorescence assays with pyrene-labeled actin, and viscosity measurements. Forthe bottom curves, the polymerization behavior is typically determined by fluorescence assays conducted under conditions where one of the ends is blocked by the presence of molecules such as gelsolin (a barbed-end capping protein) or spectrin-band 4.1 -actin (a complex prepared from erythrocyte membranes, such that only barbed-end growth occurs). Note further that the barbed end (or (+)-end) has a lower critical concentration than the pointed end (or (-)-end). This differential stabilization requires the occurrence of ATP hydrolysis to supply the free energy that drives subunit addition to the (+)-end at the expense of the subunit loss from the (-)-end.

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