Polymers Absorption Phenomena

The close correspondence between the absorption spectrum in solution and the photocurrent spectrum of the adsorbed dye is by no means found in all cases. The adsorbed state can be different in structure from the solution state which is seen in a different photocurrent spectrum. It has been found e. g. that polymers are formed in the adsorbed state. This is a well known phenomenon for cyanine dyes 50-5b where polymer bands are found in the absorption spectrum of the adsorbed molecules. An example is given in the photocurrent spectra of Fig. 15. One sees that with increasing amount of adsorbed dye — no equilibrium adsorption was reached during this experiment — the polymer absorption band appears in the photocurrent. 

However, the theophylline-imprinted polymer beads also bound more caffeine than theophylline, pointing out the fact that nonspecific hydrophobic interactions may play a role in the absorption phenomenon. 

Because the absorption phenomenon at EUV is atomic, almost every element is opaque at EUV, except thin films of polymers with high carbon content (as in aromatic PHOST-based polymers, as well as in acrylate and ahcylic polymers) and silicon. Polymers containing high amounts of oxygen and fluorine have very high absorptivity at EUV. ... 

Nuclear magnetic resonance (NMR) is an absorption phenomenon, similar to ultraviolet (UV) and infrared (IR), but the energy of NMR is from radio-frequency radiation by nuclei exposed to a magnetic field. Since Purcell and Bloch in 1946 announced the observation of the phenomenon in bulk matter, NMR has become an indispensable tool in chemistry for the study of molecular stracture and behavior. In this chapter, first we describe the basic theory that underlies the NMR phenomenon. Then we discuss the techniques involved in its spectroscopy. Finally, we illustrate the spectra of some well-known synthetic and biological polymers. We also discuss the advances in this field since 1994. 

We have found for polypropynoic acid that this series of polymers reveals selective fluorescence spectra together with nonselective absorption. To account for this phenomenon, a scheme was proposed according to which PCSs are characterized by energy transfer from excited levels of some conjugation sections to the lower levels of other sections, followed by luminescence from the latter40 41,246,248,249,253. ... 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

Reservoir capacity is, in our view, an attempt by a polymer to dissolve. Because of cross-linking and molecular weight, the system does not fully dissociate into a true solution. Rather than dissolving in the normal sense, the polymer is said to swell in the solvent. Absorption of a solvent, water or organic, is a volumetric phenomenon controlled by the relative polarities of polymer and solvent. A nonpolar backbone is preferred for absorbing nonpolar solvents. The molecule we call polyurethane, however, is not entirely nonpolar but is close enough for use as an absorbing matrix. 

The presence of a photoconductivity peak at 610 nm at the threshold of the absorption spectrum (curve 4) is a common phenomenon in inorganic semiconductors and is explained by competition between surface and volume recombination processes of the charge carriers. The optical activation energy determined from the spectral photoconductivity threshold is equal to 1.82 + 0.02 eV. The thresholds of the photoelectromotive force and the absorption spectra are likewise in agreement with this value. It is remarkable that the same value has been found for the activation energy of the dark conductivity in this polymer... 

Complexes between chiral polymers having ionizable groups, and achiral small molecules become, under certain conditions, optically active for the absorption regions of the achiral small molecules. Dyes such as acridine orange and methyl orange have been used as achiral species, since they are in rapport with biopolymers through ionic coupling. This phenomenon has been applied to the detection of the helix chirality in poly-a-amino acids, polynucleotides, or polysaccharides when instrumental limitations prevent direct detection of the helices. 

Every polymer, no matter what the rose bengal concentration, shows a maximum absorption for the rose bengal moiety at 571-2 nn when the spectrum is taken in a non-polar solvent such as methylene chloride. RB benzyl ester, on the other hand, shows a maximum absorption at 564 nm in MeOH. This phenomenon is explained by a relatively large influence of hydrogen bonding solvents on the absorption maxima. The absorption maximum of RB benzyl ester (essentially a monomeric rose bengal polymer unit) in different solvents was measured. The positions of the absorption maxima, as a function of solvent, are shown in Thble IV. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

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