Retarders relative effects

The trend in relative effectiveness of RAFT agents with varying Z is rationalized in terms of interaction of Z with the C=S double bond to activate or deactivate that group towards free radical addition. Substituents that facilitate addition generally retard fragmentation. O-Alkyl xanthates (Z=0-alkyl, Table... 

Figure 2.23 Diffraction gratings. Each line in a grating acts as a separate source of radiation (a) but radiation transmitted at any angle 6 is retarded relative to radiation from the preceding line (b) by the distance x. In the transmitted radiation some wavelengths will undergo constructive interference while the majority will suffer destructive effects. Reflectance gratings (c) are frequently used and the principles of monochro-mation are the same as for transmission gratings.

The entry-length region is characterized by a diffusive process wherein the flow must adjust to the zero-velocity no-slip condition on the wall. A momentum boundary layer grows out from the wall, with velocities near the wall being retarded relative to the uniform inlet velocity and velocities near the centerline being accelerated to maintain mass continuity. In steady state, this behavior is described by the coupled effects of the mass continuity and axial momentum equations. For a constant-viscosity fluid,... 

Fig. 14.4. Effect of self-phase modulation. The initial pulse (top) is deformed along the propagation. The centre of the beam, which is more intense, is retarded relative to its head and trail by the non-linear refractive index. The resulting change in the envelope generates low frequencies on the rising front and high frequencies on the trailing front...

Although there is no direct evidence that the boronate anions, such as RB(OH)3, are capable of effecting the transmetafl tiQQjit is quite reasonable to assume a similar effect of the base for the transmetallation of organoboronic acids. The cross-coupling reaction of arylboronic acids with aryl halides at pH 7-8.5 is retarded relative to the reaction at pH 9.5-11 [51]. The of phenylboronic acid is 8.8, thus suggesting the formation of the hydroxyboronate anion [RB((OH)3 at pH > pA and its transmetallation to the palladium(II) halides. The formation of ArB(OH)3 at pH 11-12 has been reported [52]. 

Figure 16 shows that the best retardation effect is obtained with R22f5 among repulsive block copolymers investigated in this study. Thus, the competitive interactions of the blocks with different homopolymers are shown to promote the retardation. In other words, as the relative repulsion between block C and homopolymer B and between block D and homopolymer A increases, i.e., if the interaction of block C (D) with homopolymer B (A) is more repulsive than that of block C (D) with homopolymer A (B),the rate of phase separation is retarded more effectively. Vilgis and Noolandi [81] also predicted strong interfacial activity of block copolymers with such interactions. Therefore, it can be concluded that the interaction energy between blocks also has an important effect on the phase separation of immiscible blends. 

If resonance development is retarded relative to what is being measured by P (presumably the charge change + 8 on the base), that is, aresc < p, the intrinsic rate consant is reduced because 8 log K1res(C ) > 0 (equation 3). On the other hand, if resonance development were synchronous with charge change (aresc = p), no effect on k0 would occur if resonance development were ahead (aresc > P), k0 would be higher than that in the reference reaction. 

There are several practical difficulties in translating incremental outgassing data into diffusion coefficients (Farley 2000). The most commonly used computational models require that the distribution of diffusant be uniform within the diffusion domain. This assumption is violated in many samples by the a-ejection effect and by He diffusion in nature, both of which act to round the concentration profile at the grain surface. As a consequence, the initial rate of He release from a sample is anomalously retarded relative to later release. Fortunately this effect can be identified and greatly reduced by incremental outgassing schedules that involve cycling from low to high temperatures and back (Farley 2000). 

Fig. 9.29 (a) Simplified diagram of the experimental setup and orientations of the electric field vectors of the electromagnetic radiation, (b) Relative orientation of the electric components at half-wave retardation, (c) Effect of retardation on the polarization... 

The opposite is true along the concave segments, where the wave front is retarded relative to the planar front. Here, there is a diffusive focusing of B into the region ahead of the front, leading to a local increase in wave velocity. These local increases and decreases in propagation velocity tend to eliminate die local curvature. Thus we may say that the diffusion of the autocatalyst has a stabiliz-ing effect on the planar wave. 

Adsorption of material, for example polymer, will always have the same gross effect on the position of the effluent compared with that of an inert tracer. The effluent will be retarded relative to the tracer (i.e. it will appear later) because of the retention process, although this also depends on the magnitude of the IPV/excluded volume effect—the product of quantities / F—as discussed in Section 7.2.1. Considering the case of a linear adsorption isotherm (i.e. adsorption, T, is linear with c) when there is no IPV/excluded volume effect (/= ), the retardation factor, Fr, as defined in Equation 7.12, is greater than unity. Hence, the component velocity of the adsorbed/retained species will be less than that of an inert tracer by the factor 1/Fr whereas the tracer travels at the same average velocity as the fluid. 

PSLi to 48 was retarded relative to DPE. The addition reaction in benzene required 3-5 days for completion as monitored by UV-visible spectroscopy. The addition was accelerated in the presence of small amounts of THF ([THF]/[Li]=30) such that addition was complete in 12 h. Using the rate constant for addition of PSLi to 48 [1.7xlO M s ], the sigma value (a) for the tert-butyldimethylsiloxy substituent was calculated to be -0.3 compared to the a value of -0.46 calculated for this substituent determined from the kinetics of addition of PSLi to 24 under the same conditions [170]. The reduced effectiveness with two substituents is consistent with the X-ray structural evidence for diethyl ether solvated diphenylmethyllithiums which indicates that one of the phenyl rings is distorted by an angle of 37° from coplanarity with the other ring and the diphenylmethyl carbon [52] as discussed in Sect. 2.2. 

The relative effects of phosphonates can be evaluated by comparing the time at which the induction period is terminated at different dosages of a chemical. In Fig. 17, the time of termination of the induction period is plotted as a function of dosage.Among the acids, the most efficient retarder is DTPMP which requires 0.05% for an induction period of 21 hours, compared to periods of 13 and 10 hours respectively for acids ATMP and HEDP at the same dosage. Among the salts, Na DTMP is the most effective retarder. 

Ramachandran and LoweryP l applied conduction calorimetry to the study of the relative effects of various retarders on the hydration of cement. The retarders used were calcium gluconate, glucose, glycolic acid, molasses, sodium borate, sodium citrate, sodium heptonate, sodium hexametaphosphate, sodium pyrophosphate, sugar-free lignosulfonate, and sucrose. The dosage of the chemicals varied between 0.025 and 1.2%. 

The relative effects of superplasticizers on the hydration of C3S has been compared in the presence of 1% SMF, 1% SNF, and 0.25% LS (lignosulfonate). The rate of hydration was determined by TG technique (Fig. The amount of calcium hydroxide formed in pastes with these admixtures was distinctly lower than those in reference samples, indicating significant retardation effects. It was also found that the C-S-H formed in the presence of the admixtures had a higher C/S ratio. It is evident from the figure that the retardation effect of even 0.25% LS is significant. 

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