Segment polarizabilities

Table 4.1. Segmental Polarizabilities for Various Structures at Optical Frequencies...

Electrooptical properties will be covered only briefly. Fluorocarbons find widespread utility in altering electrooptical properties of coatings. These properties are to be considered as derived from bulk properties of the fluorocarbon. In that regard, fluoropolymers are the most often selected. It is known from Eq. (2) that the electrooptical properties of fluorocarbons can be linked directly to the nature of the C—F bond (a oc n and e <=< n ). It is instructive to consider some relevant values. The dielectric constants e of PTFE, PE, and nylon-6,6 have been determined to be 2.1 (60 Hz-2 GHz), 2.2-2.3 (1 kHz), and 3.6-3.0 (100 Hz-1 GHz), respectively. The dielectric constants for PE and PTFE are comparable. The explanation can be found by comparing segmental polarizabilities a for groups with C—F bonds versus those with C—H bonds, as shown in Table 4.1. They are nearly identical. As e is related to a, it is not surprising that PE and PTFE have similar dielectric constants. The value of e for nylon-6,6 is included above for comparison. 

The numbers in brackets are the section numbers where the symbol or the abbreviation is first defined or reintroduced, c/., segmental polarizability 2.2.2 ... 

So-8X Aa Ay Ap AA (8o,xi 8ijXi) 2.2.3 local 8(x) slope 2.2.3 difference of segmental polarizabilities of two species 2.2.2 surface tension difference of pure blend components 3.1.2.2 difference of chemical potentials of species (pA-pB) 2.1 difference of Hamaker constant of species 3.1.2.1 ... 

Segmental polarizability cm Sulfuric acid (includes form ( 1 - 0 2) = (34, 35)... 

Set 0X14, l-oximino-3-X-5-methyl-l,2-benzoquinones, is of interest because the substituents in this data set do not differ significantly from each other in electrical effects. As chemical reactivities in water normally do not depend on polarizability, it would seem that the variations in pXa in this data set should be a function of steric effects. Correlation with the two parameter segmental steric effect equation gave best fit with the v/ parameters. The correlation is significant at the 99.0% confidence level and is good. It seems probable that the pXa values of these compounds are dependent on steric effects. 

The low polarizability associated with the C—F bond will influence molecular interactions. Before proceeding, it is useful to examine a few of the relevant numbers (Table 4.1) and compare them to those of materials that are structurally similar. Note, in a relative sense, the low value of a for the —CF2—CF2— segment versus ethylene oxide, ester, or dimethylsiloxane segments. It is also important to point out that fluorocarbons and comparable hydrocarbons have similar polarizabilities. This fact will become important for the discussion of surface tensions. 

Polymer Melts. When a rigid, polarizable monomer forms wither a mainchtiin polymer with flexible segments in belween or a side-chain polymer with flexible segments between the rigid segments and the flexible main chain, liquid-crystal phases are usually viable. 

AF5 is due to dipole and/or polarizability effects which arise when an uncharged segment is adsorbed on the charged surface. It is represented by... 

The stress-optical coefficient, Ka, of an elastomer network is a constant, independent of extension ratio and crosslink density. It is directly proportional to the difference between the longitudinal and transverse polarizabilities of the statistical chain segment (fei — 2) ... 

Although only a1 and e are used as additional descriptors so far, some others might be useful in the future. One of the most apparent is the local electronic polarizability of the molecule in the vicinity of a surface segment i, which we may represent by a local refractive index n,. Such local polarizability or local refractive index would allow for a refinement of the electrostatic misfit term, which presently only takes into account an average electronic polarizability of organic molecules. It is also likely that the local electronic polarizability is of importance to the strength of hydrogen bonds, which so far in COSMO-RS are only a function of polarity. Finally, from a physical perspective, the vdW interactions should be a function of the local polarizability as well. 

As before, the polarizability of the ith individual segment is a. = a2I+ (o - a2) (u.u .). The polarizability of the entire chain is the summation of this polarizability over all the segments and its average over the conformation of the macromolecule. In other words,... 

Equation (7.34) is the leading term in a near field expansion of the electric field generated from a radiating dipole. Substituting equations (7.34) and (7.33) into equation (7.32), and multiplying the result by the mean polarizability ot- of segment i, the following... 

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