Wavelength dependence photopolymerization

Figure 7. (a) Wavelength dependency of photopolymerization of the 6-cyanouracil derivatives, (b) Wavelength dependency of photoreversal of the photodimer. 

The results are shown in Table 7 (65). The photospeeds indicate NPG is nearly twice as fast as TBBS as an activator of MB sensitized photopolymerization. The wavelength dependence... 

Photopolymerizations of the bis-pyrimidine derivatives were carried out in solid film by irradiation with UV light from a spectroirradiator. The rate of the photopolymerization was found to depend on the wavelength of light. The highest reactivity was observed at 300 nm (Figure 7a), while the highest reactivity for photoreversal of the photodimer was obtained by irradiation at 240 nm (Figure 7b). The polymers... 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

Liquid crystalline azobenzene polymers can undergo photoisomerization on irradiation at 360 or 450 nm on gold films. This permits changing the refractive index of the film depending upon the wavelength used136. A study has examined the photopolymerization of p-styrene sulphonate counteranions associated with monolayers of disulphide amphiphiles as self-assembled monolayers on gold137. 

The laser-curing experiments were performed with a continuous wave argon ion laser (Spectra Physics, Model 2000) tuned to its emission line at 363.8 ran. The radiant power at the sample position was measured to be in the range 30 K) 200 mW cm 2, depending on the selected laser output Some photopolymerization experiments were carried out with a pulsed nitrogen laser (SOPRA, mt el 804 Q which emits at a wavelength of 337.1 nm. The instantaneous radiant power was calculated to be 500 kW cm-2, based on the energy of a pulse (5 m J) and its duration (8 ns). The laser was operated in a multiple-pulse mode at a repetition rate between 2.5 and 40 Hz. An electronic shutter was used to select the desired number of pulses. 

The photopolymerization rate is dependent on the radiation wavelength and the ambient temperature and atmosphere. The properties of the polymer (e.g. glass transition temperature, T ) is also dependent on the radiation conditions (e.g. radiation time). The optimum conditions for photochemical reaction can therefore be determined by taking the above factors into full account. 

Initiation. In both thermal- and photopol5unerizations, the rate of initiation depends on two processes the dissociation of the initiator and the initiation of the propagating chain. The decomposition rate Ua) of thermal initiators strongly depends on temperature, with the half-life of many thermal initiators at the reaction temperature on the order of minutes or hours. In contrast, for photopolymerizations, the rate at which photons are absorbed at a specific wavelength will determine the decomposition rate of photoinitiators. This process is not temperature-dependent. Thus, in the classic initiation mechanism, the interaction between light of a specific wavelength and a photoinitiator molecule is considered. For a unimolecular photoinitiator, this reaction step can be written as follows ... 

The laser-induced breakdown is not necessarily related to the multiphoton absorption process [145] while photopolymerization depends on it strictly [24, 25]. Therefore, measures could be taken to increase the quantum yield of polymerization, and therefore the dynamic power range. A simple and effective method is choosing laser wavelength so that radicals are produced more efficiently. It was experimentally observed [127] that the two-... 

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