Polymeric materials, polarizability

The nonlinear optical and dielectric properties of polymers find increasing use in devices, such as cladding and coatings for optical fibres, piezoelectric and optical fibre sensors, frequency doublers, and thin films for integrated optics applications. It is therefore important to understand the dielectric, optical and mechanical response of polymeric materials to optimize their usage. The parameters that are important to evaluate these properties of polymers are their dipole moment polarizability a, hyperpolarizabilities 0... 

As an introduction to Section 36.5, let us note that solitons, polarons and bipolarons are excitations of major importance which are inherently nonlinear and thus will favor nonlinear responses in the presence of external fields. Also, it must be realized that the quasi one-dimensionality of the polymer chains allows them to easily undergo structural distortions that result in a significant lowering of the first electronic excitation. Thus, in a perturbation scheme, the linear and nonlinear polarizabilities must be significantly enhanced, since they are inversely proportional to the energy of the electronic excitation. Since 1983, these facts have drawn our attention to the field of polarization of polymeric materials, which is described in detail in Section 36.5. 

The nature of the metal-metal interactions in 4.23 and 4.24, and in related polymeric materials, has been analyzed in terms of a published theoretical analysis [58]. The conclusion reached was that the bridging groups affect the molecular scale dielectric constant between the metal sites, which, in turn, mediates the essentially through-space interaction. Based on this concept, more polarizable atoms would be expected to lead to stronger metal-metal coupling, and this has indeed been observed. For example, the less polarizable fluorinated arene groups in 4.19 lead to no detectable Fe Fe interaction by cyclic voltammetry, whereas polarizable atoms such as sulfur (see Chapter 3, Section 3.3.7) lead to large A i/2 values. 

Intrinsic birefringence. The intrinsic birefringence of polymeric materials was first calculated by Kuhn. He considered a polymer segment whose end-to-end vector is fixed at r, and calculated its average polarizability assuming that... 

The need to develop superior polymeric materials with an excellent combination of chemical, physical, and mechanical properties with ease of processability has triggered research on semifluorinated polymers. Incorporation of fluorine in polymers imparts many interesting properties. Ruorine and perfluoro-alkyl groups in polymers enhance processability and increase thermal stability, reduce water absorption and dielectric constant, and enhance gas permeability. All of these superior properties of perfluorinated polymers result from many unique properties of fluorine. The highest electronegativity of fluorine and the high bond strength and low polarizability of C-F bonds cause many of these properties. 

Relating Macroscopic and Molecular Properties. For many or-ganic/polymeric materials, the bulk polarization is best understood through averaging of the molecular polarizabilities of the weakly-interacting constituent materials. The molecular polarizability is expressed in a series expansion, similar to equation 2 for the polarization, (28)... 

In summary, silica gel can be an excellent stationary phase for use in exclusion chromatography in the separation of high molecular weight, weakly polar or polarizable polymers. It cannot be used for separating mixtures that require an aqueous mobile phase or operate at a pH outside the range of 4-8. Examples of the type of materials that can be separated by exclusion chromatography using silica gel are the polystyrenes, polynuclear aromatics, polysiloxanes and similar polymeric mixtures that are soluble and stable in solvents such as tetrahydrofuran. 

Based on their thermal properties, polymeric and glassy molecular PR materials can be classified as high-7 and low-7), materials. The Tg values of polymer composites can be lowered by doping small molecules as plasticizer. Introduction of a long alkyl side chain can effectively reduce the Tg of fully functionalized polymers. The glassy molecular materials usually possess low Tg if they are amorphous. For the high-7 and low-7), materials, the figures of merit of the NLO chromophores are defined by Eqs. (35) and (40), respectively. The latter are obviously enhanced by contributions from the polarizability anisotropy of the NLO chromophore. 

RESS is useful for materials that are soluble in CO2. Unfortunately, CO2, with no dipole moment and very low polarizability, is a very weak solvent and dissolves very few polymers. Cosolvents such as methanol or acetone can be mixed with SCFs to increase the solvating power of SCFs during RESS. In drug delivery applications, RESS has been used to prepare polymeric films, microparticles, nanospheres, liposomes, and porous foams (Figure 1). A... 

Teflon is made in a similar way by free-radical polymerization of tetrafluoroethene. Carbon-fluorine bonds are quite strong (slightly stronger than C—H bonds), and like polyethylene. Teflon is a very stable, inert material. We are all familiar with the most characteristic property of Teflon, its nonstick surface. This can be understood by comparing Teflon and polyethylene. The high electronegativity of fluorine makes C—F bonds less polarizable than C—H bonds, causing the dispersion forces in Teflon to be less than... 

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