Stabilizer screening effect

Is the UV-stabilization only due to the screening effect (or more precisely light absorbing effect in a spectral region where the absorption spectra of polymer and UV-stabilizer overlap) of the UV-stabilizers and/or can it be enhanced by an energy transfer from the excited polymer to the stabilizer molecule ... 

Deactivation of the excited stabilizer molecule in its excited singlet (screening effect) and triplet state (energy transfer). 

The carbon black and amines can be used to provide synergistic stabilization [182] for EPR against radical induced oxidation. This is because the carbon black acts as a free radical trap in addition to the screening effect and amine functions... 

Figure 13. Effect of NiDBC as uv stabilizer (0.025%) and as an external screen at equivalent screening effectiveness in cumene...

The H-bond strength of the third group—nonlumlnescent molecules—is similar to that of the salicylic derivatives, and no ground state tautomers are observed. Tremendous pK changes are necessary in order to facilitate proton transfer in the excited state. The deactivation path is still a matter of speculation. It is worth noting that a substance of the fluorescent class (phenyl salicylate, Table 2) has been the first ultraviolet stabilizer (9) used on a technical scale, despite its poor absorption intensity (screening effect). This substance is photo-chemlcally converted into 2,2 -dlhydroxy benzophenone (87) in a photo-Fries reaction. This can, however, not explain the total efficiency of this stabilizer (93). 

Polyolefins - An Arrhenius model has been developed for lifetime prediction of the light stability of polypropylene . Photooxidation processes in blends of polypropylene with poly(butylene terephthalate) (PBT) are complicated by the thermal sensitivity of the polypropylene and the screening effect of the terephthalate ester . This effect is shown in Scheme 1. 

For metabolic stability screens to be most effective, the screens must be tightly linked to some means of gathering information on metabolite structure. Methods... 

As usual, though, the devil is in the details. The conditions used to run a particular microsome stability screen can radically affect the results. Microsomes are very sensitive to the presence of co-solvents, so DMSO concentrations normally need to be kept at no more than 0.2% and ACN at no more than 1%. Insoluble compounds, then, can be particularly problematic. Compound precipitation can mean that most of the material is unavailable for metabolism, but may dissolve when later diluted with ACN for analysis, resulting in a falsely high stability estimate. Ways of optimizing assays to minimize such effects have recently appeared in the literature.The concentration of compound used in the assay can make for more than a 70-fold difference in the percent remaining at the measured time point, while the concentration of the microsomes is also... 

For metabolic stability screens to be most effective, the screens must be tightly linked to some means of gathering information on metabolite structure. Methods for rapidly determining metabolite molecular weight and limited structural information have improved dramatically and allow this approach to be routinely employed (Anari and Baillie, 2005 Watt et al., 2003). The goal of this type of approach is to allow the identification of metabolic soft spots which can then be altered to produce compounds with improved metabolic stability. The literature of successful structural modification to increase stability has recently been reviewed (Thompson, 2001). 

Electrically stabihzed colloidal dispersions are very sensitive to the addition of electrolytes. If the concentration of ions in the solution increases, decreases as a result of both entropic and electrical screening effects, leading to a reduction in the repulsive potential. On the other hand, colloid particles dispersed in organic media (low dielectric constant) cannot be effectively stabilized by charges because is extremely short. In these cases, steric stabilization is recommended. Steric stabihzation is imparted by nonionic amphiphilic molecules (usually polymeric molecules). The lyophobic moiety will adsorb onto the surface of the colloidal particles, while its lyophilic moiety will be extended in the continuous phase. When two sterically stabihzed particles approach each other, the concentration of the lyophilic segments in the portion of the continuous phase between the particles is increased. This higher local concentration results in an osmotic pressure that... 

A similar screening effect is present for dipolar liquids each molecule bearing a permanent dipole tends to organize around itself other dipolar molecules with the orientation which optimizes stabilization energy and screens the field produced by the singled out molecule. 

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