Nucleation rate effect viscosity

Shear enhancement effects in foam formation can be understood through the modified cavity model. Shear force behaves as catalyst to reduce energy barrier to allow a quik path from stable gas cavity to unstable bubble phase. It can be concluded that both shear rate and viscosity contribute to foam nucleation in the continuous foam extrusion process. Therefore, proper die opening and process conditions will help to optimise the foam product. 11 refs. 

The effect of the viscosity of the crystallization medium on the nucleation rate has been described by Turnbull and Fisher. The frequency of atomic or molecular transport at the nucleus-liquid interface, v can be related to the bulk viscosity, with the Stokes-Einstein relation ... 

The rate of nucleation of particles or clusters of size x can be written as the product of the number of clusters of size x and the probability that another molecule gets to the interface by overcoming kinetic barriers which provide an activation energy barrier, Ag. This latter term includes viscosity and diffusion effects of the bulk liquid medium as well as solvent association reactions that deplete monomers. Ifx is the critical size then the nucleation rate, Jx is... 

Furthermore, the physical state of the second component at the time of matrix nucleation is of importance. It may be presumed that the mode of nucleation of a polymer in the presence of solidified domains of the second polymeric phase is heterogeneous, and therefore the nucleation rate should be higher than in the pure homopolymer. The effect of blending on the nucleation behavior is more subtie and complex in the presence of a molten second component. Factors such as miscibility, relative melt-viscosity, and inherent crystallizability all influence the formation of critical size nuclei [Nadkami and Jog, 1991]. 

The rate of nucleation, I, has been estimated by Turnbull and Fisher [25] from the shape of AG and the influence of local viscosity, governed by a free enthalpy AG,. In Fig. 3.61 this equation is listed. The rate I applies to the case that nucleation is unhindered. The first exponent of the equation expresses retardation of nucleation due to viscosity effects with the given parameters (see Sect. 5.6). It stops nucleation as the glass transition temperature is approached. The nucleation described is homogeneous nucleation and creates a continuous stream of new crystals in the remaining melt or solution. For polymers it takes a supercooling of about 50 K to overcome the... 

The persistence length is determined by the ratio of the growth rate (secondary nucleation rate (coi). The LH theory predicts a change in the growth rate because temperature dependent whereas a> is strongly dependent. Alternative explanations for changes in rate have been put forward and include effects such as the temperature dependence of the interfacial energies, viscosity effects and nucleation processes. 

Achieving steady-state operation in a continuous tank reactor system can be difficult. Particle nucleation phenomena and the decrease in termination rate caused by high viscosity within the particles (gel effect) can contribute to significant reactor instabilities. Variation in the level of inhibitors in the feed streams can also cause reactor control problems. Conversion oscillations have been observed with many different monomers. These oscillations often result from a limit cycle behavior of the particle nucleation mechanism. Such oscillations are difficult to tolerate in commercial systems. They can cause uneven heat loads and significant transients in free emulsifier concentration thus potentially causing flocculation and the formation of wall polymer. This problem may be one of the most difficult to handle in the development of commercial continuous processes. 

Temperature has a complex effect on crystallization rate. Initially, as the temperature falls below the equilibrium melting temperature, the crystallization rate increases because nucleation is favored. However, as the temperature continues to fall, the polymer s viscosity increases, which hampers crystallization. As a rule of thumb, a polymer crystallizes fastest at a temperature approximately mid-way between its glass transition temperature and its equilibrium melting temperature. 

To model this, Duncan-Hewitt and Thompson [50] developed a four-layer model for a transverse-shear mode acoustic wave sensor with one face immersed in a liquid, comprised of a solid substrate (quartz/electrode) layer, an ordered surface-adjacent layer, a thin transition layer, and the bulk liquid layer. The ordered surface-adjacent layer was assumed to be more structured than the bulk, with a greater density and viscosity. For the transition layer, based on an expansion of the analysis of Tolstoi [3] and then Blake [12], the authors developed a model based on the nucleation of vacancies in the layer caused by shear stress in the liquid. The aim of this work was to explore the concept of graded surface and liquid properties, as well as their effect on observable boundary conditions. They calculated the hrst-order rate of deformation, as the product of the rate constant of densities and the concentration of vacancies in the liquid. 

Addition of starch has a nucleating effect, which increases the rate of crystallisation. The rheology of starch/PCL blends depends on the extent of starch granule destruction and the formation of thermoplastic starch during extrusion. Increasing the heat and shear intensities can reduce the melt viscosity, but enhance the extrudate-swell properties of the polymer. 

Then, viscosity, supercooling, and cooling rate evidently determine the process of nucleation (i.e., ti). However, their effect on the free energy for TAG nucleation has not been evaluated. In the same way, the interaction among such... 

The nature and strength of solvent intermolecular interactions (especially the presence of hydrogen bonding) can greatly influence the physical and chemical outcomes of ultrasound irradiation. These interactions determine the intensity of bubble collapse and the ease with which bubble nucleation can occur (recall that nucleation occurs when the pressure amplitude of the ultrasound wave exceeds the natural cohesive forces in the liquid). Increases in solvent viscosity and surface tension reduce the rate of bubble nucleation (i.e., fewer microbubbles are formed) but increase the intensity of bubble collapse (i.e., higher temperatures and pressures). Some of the adverse effects of high surface tension can be overcome with the addition of small amounts of surfactants, which reduce the solvent surface tension and facilitate bubble nucleation. ... 

It has been noted (Scheirs, 2000) that this leads to a dramatic decrease in viscosity that renders the polymer unprocessable or, at the best, results in defects such as haze due to crystallites that nucleate more readily from the lower-molar-mass, degraded polymer. The rate of loss of properties due to hydrolysis is orders of magnitude faster than oxidative degradation at the same temperature. To avoid these effects, the moisture level in an aromatic polyester such as PET must be kept below 0.02%. 

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