Polymer Photochemistry最新文献

The fading rate curves and the characteristic fading curves of seven azo dyes on cellulose diacetate (CDA) and polyamide (PA) films were examined. Fading was enhanced by prolonging the time of irradiation and decreased by increasing dye concentration on the substrate, irrespective of the nature of the latter or the dye used. However, the magnitude of fading was governed by structural characteristics of both dye and substrate. The results obtained suggested that dyes of higher ability to aggregate forming large and uniform particles were more resistant to fading than those forming smaller and/or a non-uniform size of dye aggregates. Similarly, substrates of higher polarity and porosity acted in favour of association of dye molecules, thereby impeding fading. These suggestions were, indeed, substantiated by measurements of the extinction $\text{y}\text{x}$ ratio after different irradiation periods.

Photograft copolymerization of methoxypolyethyleneglycol methacrylate (SM) and 2-(dimethylamino)ethyl methacrylate (DAEM) to poly(acrylonitrile) containing bromine atoms (PANBr) in dimethyl sulphoxide was carried out and the changes of concentration of SM and DAEM with time were measured by thin-layer chromatography. Also, the change of concentration of bromine atoms in PANBr during the photolysis of PANBr was measured spectrophotometrically to determine the overall quantum yield of the photolysis reaction.

Simulation of the photograft copolymerization was carried out by substituting the overall quantum yield and other kinetic parameters such as monomer reactivity ratios, polymerization rate constants and φ factor, which were obtained separately, into equations for the photopolymerization. Agreement between data obtained from experiment and from calculation was very good.

The photoinduced graft copolymerization of methyl methacrylate onto wool fibers was investigated using peroxydiphosphate as the initiator. Up to a certain concentration of the monomer and the initiator the graft yield increased beyond that concentration a decreasing trend was observed. The reaction was carried out at four different temperatures and the energy of activation was evaluated. The kinetic data and other evidence indicate that the overall polymerization takes place by a radical mechanism. A suitable reaction scheme has been proposed.

Pyrylium and thiapyrylium salts show interesting emission properties when incorporated in a rigid PMMA matrix. In addition to a strong prompt fluorescence emission, the 18 compounds studied show a delayed fluorescence which cannot be observed in solution, even at 77 K. The delayed fluorescence/phosphorescence ratio increases with the temperature. The activation barrier as well as the intersystem crossing from the lowest excited triplet state to the lowest excited singlet state have been measured for compounds 4, 8, 11, and 17. An absence of crossing point between the potential energy surfaces of the two excited states is put forward to explain the low value (ca. 10³ s⁻¹) of the intersystem crossing rate constant.

Photooxidative degradation of polypropylene films with different morphology (degree of crystallinity and orientation) has been followed by means of mechanical tests. The kinetics of the photooxidation are significantly depressed on increasing crystallinity and especially orientation.

This behaviour has been interpreted in terms of the reinforcing action due to the oriented crystalline fibrils.

A correlation has been attempted between the photooxidative kinetics and a morphological parameter.

The transfer of electronic energy from polystyrene (PS) to an acceptor (a small molecule) as a function of PS concentration (c) in 1,2-dichloroethane has been studied. It has been concluded that in dilute PS solutions it is necessary to consider the energy migration (EM) in the PS chain, in contrast to concentrated PS solutions (c>0·4 mol litre⁻¹) in which the EM process does not take place. This has been explained by formation of intermolecular excimers, which along with intramolecular excimers, are responsible for the total restriction of the EM process in concentrated PS solutions.

Photoinitiated grafting and graft polymerization of 1,6-hexanediol diacrylate (I) and 4-(N-ethyl,N-2-acryloxyethyl)amino-4′-nitro-azobenzene (II) as co-monomers with a fixed molar ratio I:II equal to 10, in the presence of 1,2-diphenyl-2,2-dimethoxyethanone (III) as photoinitiator with a ratio R of its molar concentration of 0.005-0.38 with respect to the sum of molar concentrations of I and II, were studied using a polypropylene ribbon as a model system of a macroporous substrate. By polychromatic irradiation at 45°C with energy inputs of 90 mW cm⁻², ultrathin layers of co-polymer, able to function as asymmetric membranes, could thus be obtained. Mean polychromatic quantum yields of grafting and graft polymerization, as a function of R, were also measured. Membrane specimens with constant values of grafted (200±20) and graft polymerized (500±80) molecules of monomer units in co-polymer per unit apparent surface (Ų) have been subjected to measurements of permeabilities (P) and diffusivities (D) at 25°C for oxygen and nitrogen, as a function of R. Results show that only an optimum R minimizes P and D, and a narrow range of R-values allows a normal membrane behaviour.

A number of polymers containing trans-1,2-bis(9-carbazolyl)cyclobutane (DCzB) were examined for their photoconductivity in the presence of 2,4,7-trinitrofluorenone (TNF) and the results were correlated with the photophysical properties (excimer formation, energy migration, fluorescence quantum yield and fluorescence lifetime). Excimer formation by DCzB groups is sterically hindered whereas energy migration occurs efficiently in the DCzB polymers. Also, charge-transfer interaction with TNF is stronger for the DCzB polymers than for poly(9-vinylcarbazole) (PVCz). Photoconductivity of the DCzB polymers was better than that of PVCz when the charge-transfer band of the TNF-carbazole pair was irradiated, which agreed well with the photophysical observation. Although carrier generation and carrier transport processes could not be separately discussed at the moment, DCzB was shown to be a promising chromophore for photoconductive polymers.