Screening blobs

Free, isolated, collapsed chains may be viewed as a spherical arrays of close packed collapse blobs. Collapse blobs, like screening blobs, occur because of binary monomer-monomer interactions. However, screening blobs occur in good solvents when the binary interactions are repulsive while collapse blobs resiJt from attractive monomer-monomer interactions in a poor solvent. When the temperature is not too far below the 6 temperature, the associated interaction energy is weak enough so as not to perturb the Gaussian behavior of short chain segments. Quantitatively the collapse blob is defined by the combination of vg / c ... 

Our preceding discussion focused on systems described in terms of close packed screening blobs. As was previously stressed, such a picture is inherently associated with multichain interactions. Single chain blobs may also occur in systems incorporating many chains. In such situations it is necessary to allow for the effect of chain-chain interactions. The precise method varies with the details of the system. In the following we present two illustrative examples concerned, respectively, with confinement and with Pincus blobs. 

Screening blobs can occur in a single chain upon confinement to a droplet. See F. Brochard and A. Halperin C. R. Acad. Sci. Paris 11302,1043 (1986)... 

The range of semi-dilute network solutions is characterised by (1) polymer-polymer interactions which lead to a coil shrinkage (2) each blob acts as individual unit with both hydrodynamic and excluded volume effects and (3) for blobs in the same chain all interactions are screened out (the word blob denotes the portion of chain between two entanglements points). In this concentration range the flow characteristics and therefore also the relaxation time behaviour are not solely governed by the molar mass of the sample and its concentration, but also by the thermodynamic quality of the solvent. This leads to a shift factor, hm°d, is a function of the molar mass, concentration and solvent power. 

In accord with experiments on emulsions (Husband et al., 1997), the molecular configurations deduced from SCF calculations have demonstrated the crucial role of the cluster ( blob ) of 5 charged phosphoserine residues in p-casein in maintaining the steric stabilizing layer, whilst also preventing interfacial precipitation (multilayers). The mobility of this blob was demonstrated experimentally by P NMR measurements on P-casein-stabilized emulsions (ter Beek et al., 1996). It was inferred that, when the effective charge on the blob is reduced (by dephosphorylation) or screened (by salt addition), the macromolecular spring relaxes... 

For s 1. the Tit blob is smaller than the whole chain and the blob-concept starts to make sense. For large overlap in view of screening the number of concentration blobs per chain should not be important. Thus iJ should reduce to a function of the blob concentration only. In view of Eq. (9-11) we therefore expect U to become a function of c independent of n. With this assumption the scaling law (9.2) yields... 

Similar problems are abundant as soon as we leave the region of small momenta and isolated chains. As a final example we consider the semidilute limit. Using the unrenormalized loop expansion in Sect, 5.4.3 we have calculated the first order correction to fip(n). We found a correction of order where c is the segment concentration. The form of this term is due to screening and has nothing to do with the critical behavior treated by renormalization and -expansion. It thus should not be expanded in powers of e. We can trace it back to the occurrence of the size of the concentration blobs as an additional length scale. 

Oyasu H, Yamamoto T, Sato N et al. (1994) Urinary bladder-selective action of the new antimuscarinic compound vamicamide. Arzneim Forsch/Drug Res 44 1242-1249 Peterson JS, Hanson RC, Noronha-Blob L (1989) In vivo cystometrogram studies in urethane-anesthetized and conscious guinea pigs. J Pharmacol Meth 21 231-241 Pietra C, Poggesi E, Angelico P et al. (1990) Effects of some antidepressants on the volume-induced reflex contractions of the rat urinary bladder lack of correlation with muscarinic receptors activity. Pharmacol Res 22 421-432 Postius S, Szelenyi I (1983) In vivo rat bladder a new model to screen spasmolytic compounds. J Pharmacol Meth 9 53-61... 

The statement that the terminal part of the e+ track is a blob but not a spur, is not just a question of terminology. Processes (IER, Ps formation) related to energy dissipation and screening of local electric fields, proceed there in a different way. At present, experimental data clearly indicate that e+ behavior in the blob is quite different from that of intrablob electrons and ions. e+ is rather mobile and easily escapes from the blob during its thermalization (see experiments on Ps formation in electric fields [26, 27] and measurements of e+ mobility [28]). Particularly, it implies that the multiparticle nature of the terminal part of the e+ track cannot be correctly taken into account via the factor no/(no + 1) in Eq. (11). 

Considering the diffusion-recombination stage below, we neglect an interaction between the thermalized positron and its blob. This approximation, as we discussed above, assumes that the appearance of a positive potential in the blob, caused by outdiffusion of electrons, is nearly cancelled by the negative potential caused by e+ screening inside the blob. In this case we can apply the prescribed diffusion method to obtain the solution of Eq. (17). Let us write Cj(r,t) in the following form ... 

On length scales larger than the correlation length, the excluded volume interactions are screened by the overlapping chains. The semidilute solution on these length scales behaves as a melt of chains made of correlation blobs and the polymer conformation is a random walk of correlation blobs ... 

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