Poly quantum yield data

The Battelle group found that photoisomerization of various indigo derivatives could be used to record holograms which had low (<0.2%) scattering efficiencies (36). Their refractive-index data yield nQ = 2.9 x 10 cm for an N,N -dibenzoylindigo derivative. These systems were reversible. The quantum yields of isomerization were much lower in poly(methyl methacrylate) or polystyrene than in fluid solution, in accord with arguments presented earlier. Concentration quenching was also observed in solution. 

To study the structural sensitivity of poly silanes to ionizing radiation, a number of samples were irradiated with a calibrated Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). 

Gegiou et found only a very slight viscosity effect, both in the n-Ti and in the jt-jc absorption bands on the isomerization quantum yield. They used glycerol as a viscous solvent, but the result may also be transferred to polymer matrices. In solid matrices, several photoisomerization modes are observed (see the preceding section on the influence of temperature), A com parison between azobenzene isomerization in liquid methylmethacrylate and the slow mode in poly (methylmethacrylate) showed that the difference in quantum yields on Si (0.17) and S2 excitation (0.03) is retained in the solid matrix. The fast process is not observed in n —> n excitation. These data are important in relation to the use of the azobenzene isomerization method for the determination of the free volume in a polymer. 

Guillet et al. (52) have shown that solar photochemical reactions can be carried out using crosslinked poly(ethylenevinyl acetate) (EVA) beads as a solvent. The beads can be exposed in solar ponds and the products recovered by extraction. Figure 12 shows data on the rate of photolytic conversion of the a-(o-tolyl)aceto-phenone when exposed in the solid bead and in benzene solution. Identical rates and quantum yields were observed in both media, showing that the rate of cyclizations of this type are independent of the internal viscosity of the medium. 

Photoreactions of MA with 1,2-polybutadiene, 1,4-polybutadiene, poly(styrene-co-butadiene), poly(styrene-co-isoprene), polystyrene, and poly(styrene-co-methyl methacrylate) have been studied in air. " In homogeneous solutions, MA addition to the polymers proceeds efficiently by a chain mechanism, where the quantum yield of the photoaddition was greater than unity under irradiation at A >310 nm. From the effects of solvent and photosensitizers and spectroscopic data, a radical chain mechanism was proposed to account for addition and crosslinking of the polymers by MA molecules. The photoaddition reaction was applied to the surface of polymer films. The photoreactions were conducted at the interphase between solid polymer and acetone solution of anhydride and also at the interphase between solid polymer and gaseous anhydride. Irradiation with a 300-W high-pressure lamp brought about considerable surface modification, as shown by wettability and dyeability properties. 

Figure 104.37 shows the first and the second photodimerizations of the decyl ester (ESIO) in the thin soHd film (a, b, c) with the data in solution (e) and in polymer film (d). The rate of the photodimerization in chloroform solution was slow but proceeded completely (e). The photodimerization of thymine derivatives in solution is known to proceed by way of the triplet excited state. The short lifetime of the singlet state and the inefficient intersystem crossing are the reasons for the slow reaction rate for photodimerization of thymine in solution. The photodimerization was studied in the polymer film using poly(vinyl acetate) (PVAc) (d). The polymer film was obtained from a chloroform solution of the ester derivatives of thymine and poly(vinyl acetate) by spin coating on a quartz plate. The rate of photodimerization in poly(vinyl acetate) was faster than the rate in chloroform solution. In the polymer matrix, photodimerization occurs from associated thymine bases via the singlet state with a higher quantum yield than that from the triplet-state reaction. 

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