Polymeric light sources

One of the most fascinating developments in recent times concerns the generation of light with the aid of polymers. This development is characterized by two inventions, which are described in the following subsections the polymeric light-emitting diode and the polymer laser. 

The phenomenon of polymer-based electroluminescence was first demonstrated in the case of poly(p-phenylene vinylene), PPV (tz —tt energy gap 2.5 eV) [24], and was later also observed with many PPV derivatives and other fully TT-conjugated polymers. Typical representatives are shown in Tables 6.1 and 6.2. Table 6.1 relates to PPV and some of its derivatives, whereas Table 6.2 lists other classes of polymers that have been employed in LED work. 

Polymers, e.g. PPV derivatives, containing electron-withdrawing cyano groups. The latter provide for electron transport, thus complementing the hole-transport property. 

Highly luminescent polymers soluble in organic solvents. High electron affinity affords improved electron transport. 

Quatemization of nitrogen allows manipulation of the emission wavelength. 

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Polymeric light sources Conversion of electric power to light with organic light-emitting diodes. Conjugated polymers in laser materials... 

If the light source is switched on and off and held for long periods of equal duration in either light or darkness, then the radical concentration in the system will consist of an alternation between the situation described in Figs. 6.5a and b. Because we have specified that the duration of each phase is long, the net behavior is essentially a series of plateaus in which the illumination is either Iq or zero and the radical concentration is either [M], or zero, with brief transitions in between. This is illustrated in Fig. 6.5c. The concentration of radicals is consistent with Iq, but is present only half of the time hence the rate of polymerization is only half what it would be for the same illumination operating continuously. 

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

Composite sensing layers, consisting of bioactive molecule-charged beads entrapped in a polymeric structure, have been successfully used to realize multi-purpose biochips for DNA, proteins or enzymes. For all these different biochips, the chemiluminescence and electro-chemiluminescence measurements required only a CCD camera and neither light sources nor optical filters are needed. 

Photoinitiation offers several advantages. Polymerization can be spatially directed (i.e., confined to specific regions) and turned on and off by turning the light source off and on. 

With the light curing mechanism, there is a limitation to the penetration of the light. The dentist may have to place a restoration that is 6 + mm thick, whereas the light may penetrate only 2 mm [182]. Factors that affect this penetration are the translucence of the material, the color or shade used to match the tooth, the ability to place the light source close to the material being polymerized, and the intensity of the source. Under relatively ideal conditions, the mean depth of cure is approximately 4-5 mm. Thus, the dental application requires that the material be placed in layers. Due to the oxygen inhibition of the outside surface of the resin layers, additional layers can be laminated and cured with the appearance of uniformity of the final restoration. 

UV-Light Source Irradiation Wavelength (nm) Light Intensity h E s-1 cm-2 Polymerization Rate Rp (mol L-1 s1) Quantum Yield ( ) (mol E-1)... 

Finally, it must be pointed out that the close to first-order kinetic law observed in this study is by no means specific to polymerizations induced by intense laser irradiation a similar kinetic law was obtained by exposing these multiacrylic photoresists to conventional UV light sources that were operated at much lower light-intensities (27,34). This indicates that the unimolecular termination process does not depend so much on the rate and type of initiation used but rather on the monomer functionality and on the cross-link density which appear as the decisive factors. 

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