Dynamic size measurements

Dynamic rheological measurements have recently been used to accurately determine the gel point (79). Winter and Chambon (20) have determined that at the gel point, where a macromolecule spans the entire sample size, the elastic modulus (G ) and the viscous modulus (G") both exhibit the same power law dependence with respect to the frequency of oscillation. These expressions for the dynamic moduli at the gel point are as follows ... 

The increase of pore size with increasing amount of solvent can also be monitored with dynamic DSC-measurements. An endothermic peak at T=7 °C, corresponding to the melting point of crystalline cyclohexane, is observed in the opaque samples after the phase separation resulting from the formation of dispersed cyclohexane droplets (Fig. 53). 

Particle size measurement is one of the essential requirements in almost all uses of colloids. However, our discussion in Section 1.5 makes it clear that this is no easy task, especially since even the definition of particle size is difficult in many cases. A number of techniques have been developed for measuring particle size and are well documented in specialized monographs (e.g., Allen 1990). Optical and electron microscopy described in the previous section can be used when ex situ measurements are possible or can be acceptable, but we also touch on a few nonintrusive methods such as static and dynamic light scattering (Chapter 5) and field-flow fractionation (see Vignette II Chapter 2) in other chapters. 

One possible measure of algorithmic complexity is program size. Such a measure is related to the inherent simplicity or complexity of a method. This measure is static it is independent of the size or structure of the particular input data. Some other possible measures are dynamic they measure the amount of a resource used by the method as a function of the size of the input data. Typical dynamic measures are running time and storage space. 

By combining the results of several methods (dynamic mechanical, dielectric, NMR, etc.), it is usually possible to determine quite reliably the structural units whose motions give rise to secondary relaxations. If dynamic mechanical measurements alone are employed, the usual procedure is that the chemical constitution is systematically altered and correlated with the dynamic mechanical response spectra, i.e. with the temperature-dependence of the G" and G moduli. If the presence of a certain group in polymers is marked by the formation of a loss peak characterized by a certain temperature position, size and shape etc., then the conclusion may be drawn that the motional units responsible for the secondary relaxation are identical or related with that group. Naturally, the relations obtained in this way are empirical and qualitative. 

Less sophisticated systems tend to use white light illumination, others use gas lasers or solid-state laser diodes. Laser diodes are smaller and more robust than gas lasers, resulting in smaller instruments. Their spatial properties have increased the sensitivity of particle sensors by a factor of two, enabling dynamic size ranges of 700 1 to be measured. Fast analog to... 

Beckman Coulter LS series uses a dark-field reticule having an array of 500 pm holes for automatic alignment purposes. The LS uses reverse Fourier lens optics incorporated in a binocular lens system to optimize light scattering over the widest dynamic size range. Results are calculated from either Fraunhofer or Mie theories. An application of this instrument to the measurement of micro-silica mixtures has been presented [154]. 

There are two types of Mastersizer instruments the Mastersizer Micro and the Mastersizer E, which are low cost instruments for repetitive analyses and the modular series of Mastersizer S and Mastersizer X, the ultimate in resolution and dynamic size range, which are required when samples in the form of aerosols, suspensions and dry powders need to be measured. Mastersizer X provides a selection of small size ranges using a variety of interchangeable lenses whereas the Mastersizer S provides a wider dynamic size range covered in a single measurement. For powders which are to be suspended in a solvent, emulsions, suspensions and particles in liquids there are small volume cells which require as little as 15 ml of dispersant. Where a material is either valuable or toxic the Malvern Small Volume Flow Cell, with a sample volume of 50-80 ml and full sample recovery, can be used. The X-Y sampler is a 40-sample accessory... 

The morphology of ruber modified epoxy photopolymers was found to depend on the cure conditions as well as the nature and concentration of rubber. The commercially available acrylonitrile-butadiene copolymer rubber modifiers with varying percentages of acrylonitrile content were used. They were polymerized using a photocationic initiator involving a UV exposure followed by a thermal cure. Transmission electron micrographs of osmium tetroxide stained specimens, coupled with dynamic mechanical measurements indicated that phase separation and particle size distribution depended not only on rubber concentration and compatibility, but also on the cure conditions. 

The dynamic mechanical measurements were made using a Vibron DDV-II. The determinations were carried out using samples dried at room temperature and 1 mm mercury at frequencies of 110, 11, and 3.5 Hz. The sample size was 3.0 cm x 0.5 cm x 0.01 cm, and the temperature ranged from —170° to 200°C, with a heating rate of approximately l°C/min. 

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