Solution Viscosity Experimental Evidence

Experimental relations between g and g, g0, or h are discussed for various polymers in later sections Table 4.1 gives a summary. 

Berry et al. (43) Poly(vinyl acetate) (low branching) g h (high branching) 

These studies taken together indicate that none of the proposed relationships will serve to correlate [ 7] or g with the degree of branching for all types of structure, which is not surprising in view of the complexities of branched structures. If viscosity measurements are to be used for the estimation of branching, there must be some prior knowledge of the type of branching present in the material studied, or of the relation between g and structural quantities such as g0 or h. 

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An important conclusion of this discussion is the fact that at very high <)> thermodynamic stability is re-established. Restabilisation is not a kinetic effect, as suggested by Feigin and Napper (10, 11), but is a consequence of lower free energy of the dispersion as compared to the floe. This conclusion is supported by experimental evidence for soft spheres (3, 5, 23). We should add, however, that for hard spheres is so high that experimental verification is difficult for most polymer-solvent systems due to the high viscosity of the solution. 

There is very little experimental evidence relating to the energetics of dissociation of poly(dienyl)lithium species. From the temperature dependence of the flow times of the concentrated solution viscosities of hexane solutions of poly(isoprenyl)lithium, Morton and Fetters 47) reported an estimate of 37 kcal/mole for the dissociation of dimers (Eq. (5)). 

If the process measured by Roovers and Bywater is reanalyzed on the basis of a monomer-dimer-dissociation equilibrium, their results yield a value of about 11 kcal/ mole. Szwarc 76a-78> has presented, without citing or providing either theoretical or experimental evidence, various values (12, 14-15, and 15-16 kcal/mole) for this step. Meier, using the approach involving the temperature dependence of concentrated solution viscosities, reported 79) a value of 21.8 kcal/mole for the dissociation enthalpy of the poly(styryl)lithium dimers. These combined results will be discussed and compared with direct calorimetric results in a later section of this review. 

Next, we present experimental evidence for the electric-field-induced decrease of Todt in a block copolymer. Because of the high melt viscosities, temperatures close to the decomposition temperature and extremely high electric field strengths are required to achieve a measurable effect. In recent studies, we have demonstrated that concentrated block copolymer solutions in a neutral solvent act like a melt, thereby effectively circumventing the above-mentioned limitations [31, 57, 70],... 

Photochromic transformations from the para- to ana-quinone structure of these compounds occur through intermediate photoproducts in the triplet state.51 The experimental evidence of the influence of oxygen in solution and the viscosity of solvents on the lifetime of these photoproducts supports this statement. It was found that both initial and photoinduced forms have lower triplet levels of the mi -type. 

An alternative point of view is that vorticity accumulates at the rear of the body, which leads to a large recirculating eddy structure, and as a consequence, the flow in the vicinity of the body surface is forced to detach from the surface. This is quite a different mechanism from the first one because it assumes that the primary process leading to separation is the production and accumulation of vorticity rather than the local dynamics within the boundary layer.25 However, viscosity still plays a critical role for a solid body in the production of vorticity. In fact, for any finite Reynolds number, there is probably some element of truth in both explanations. Furthermore, it is unlikely that experimental evidence (or evidence based on numerical solutions of the complete Navier Stokes equations) will be able to distinguish between these ideas, because such evidence for steady flows will inevitably be limited to moderate Reynolds numbers. 

HIPS microstructure depends on the process variables used in the polymerization. There are many publications reporting experimental evidence of the effect of the process variables on polymer microstructure [7-16], For example, Soto et al. [15] analyzed the effects of the initiator type, temperature and stirring rate on the particle morphology and molecular weights of the free PS. Heuristic knowledge available indicates that the particle size depends on the ratio between the solution viscosities of the PBD- and PS-rich phases the agitation rate and the interface tension between the two phases. The interface tension in turn depends on the grafting efficiency. 

By the time COlL-2 took place in 2007, the nanostructured nature of the ionic liquids had been postulated using molecular simulation [50] and evidenced by indirect experimental data [54, 85] or by direct X-ray or neutron diffraction studies [56]. This microscopic vision of these fluids changed the way their physico-chemical properties could be explained. The concept of ionicity was supported by this microscopic vision, and indirect experimental evidence came from viscosity and conductivity measurements, as presented by Watanabe et al. [54, 86]. This molecular approach pointed towards alternative ways to probe the structure of ionic liquids, not by considering only the structure of the conponent ions but also by using external probes (e.g. neutral molecular species). Solubility experiments with selected solute molecules proved to be the most obvious experimental route different molecular solutes, according to their polarity or tendency to form associative interactions, would not only interact selectively with certain parts of the individual ions but might also be solvated in distinct local environments in the ionic liquid. 

There is little experimental evidence in the scientific literature concerning the existence of this supposed optimum. Indeed there have apparently been few published studies of the effect of oil viscosity on both the effectiveness and deactivation rates of hydrophobed silica-polydimethylsiloxane antifoams. Evidence that increase in polydimethylsiloxane oil viscosity can reduce the rate of deactivation of hydrophobed silica-oil antifoams is, for example, presented by Racz et al. [3]. These authors repeatedly pulled films, using a film frame, from surfactant solution upon which antifoam had been spread. The time for film rupture was measured. After several hundred films had been pulled, the film rupture time increased dramatically indicating partial antifoam deactivation. Increase in the viscosity of the oil in a hydrophobed silica-oil antifoam, from 200 to 1000 cSt, increased the number of films that could be pulled by more than 50%, implying a decrease in rate of deactivation [3]. 

As apparent in these measurements of the relaxation times for QH6 and CeHe in CO2 over similar pressures and temperatures, there is no experimental manifestation of a specific intermolecular interaction between CO2 and fluorine. These interactions, if prevalent, would be expected to be seen in a change in relaxation rate or mechanism at high densities where the intermolecular distance between the CO2 molecule and the fluorine group would be the smallest and their potential specific interaction the greatest. It appears that at high densities, solution viscosity dominates the relaxation process, and the relaxation mechanism for both F and are similar. Therefore, there is no experimental evidence for a specific CO2-F interaction that impacts on the relaxation of these two molecules, which supports the calculations of Diep et al. (34) and the experimental efforts of Yee et al. (35). 

As shown experimentally by Piest [75], cotton which was subjected to various operations, e.g. bleaching, treatment with alkalis or acids, strong heating prior to nitration furnishes nitrocellulose solutions of low viscosity. At the same time an increase in the solubility of the nitrocotton was also observed. This is evidence that the cellulose molecules are shortened and their content of terminal group is increased. A certain proportion of hydrocellulose and oxycellulose may result. The total effect is to bring about an increase in the reductive properties of the cellulose, i.e. an increase of the copper number. 

Another kind of wall-effect was proposed by El perin (1967). He suggested that an adsorbed layer of polymer molecules could exist at the pipe wall during flow and this could lower the viscosity, create a slip, dampen turbulence pulsations, and prevent any initiation of vortices at the wall. Later work (Little 1969), however, with a transparent pipe and dyed polymer, showed that the adsorption could in be fact an experimental artifact (a quantity of polymer solution, trapped in pressure gage piping, slowly diffused back into the solvent flow). Although polymer molecules do more or less adhere to clean surfaces in thin films, there is no interaction with the bulk of the solution which could alter the flow properties (Gyr, 1974). Thus, it is evident that adsorption of the additives on surfaces is not the reason for the drag reducing effect. 

Experimentally the increase in activation energy is quite evident, but the cause of this increase is not clear. It can be argued that the increase in activation energy is related to a strong increase in viscosity of the reactive medium or to phase separation in the reactive mass when a newly formed polymer precipitates from a solution and forms colloid particles. The experimental data described by Eqs. (2.23) - (2.25) can also be treated in ways other than those used in the original publication. For example, it is possible to linearize the exponential factor in Eq. (2.23), as was done above for other purposes. Then for the range of P from 0.35 to 0.8 we can write ... 

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