Polymer Photochemistry最新文献

In order to search for photoradical polymerization systems that are not subject to oxygen inhibition, photoradical polymerization of the N,N-dialkylaminoethyl methacrylate—aryl ketone system has been studied. It was found that photoradical polymerization of this system, where N,N-dialkylaminoethyl methacrylate functions both as a co-initiator and as a monomer, is accelerated in the presence of oxygen. The polymerization rate was faster with N,N-diethylaminoethyl methacrylate than with the N,N-dimethyl analogue. Crosslinking occurred rapidly in the presence of a multifunctional acrylate monomer, resulting in the formation of gels. As compared with photopolymerizations of acrylate monomers using non-polymerizable, low-molecular-weight alkylamine-aryl ketone combinations as photo-initiators, the present system has an advantage that less low-molecular-weight amine remains in the exposed area because it polymerizes.

A technique involving in situ Fourier transform infrared spectroscopy in the reflection-absorbance mode (FTIR-RA) was used to study photoisomerization and the thermal back reaction of a solid solution of trans-azobenzene in a poly(methyl methacrylate) (PMMA) matrix. The FTIR-RA method, although inherently less sensitive than UV spectrophotometry, affords better resolution of specific isomer absorbances and permits simultaneous observation of changes in the matrix and chromophore vibrational bands. A change in the orientation of the acrylate side groups in isotactic sequences of the atactic PMMA matrix was observed to coincide with photoisomerization of the dissolved azobenzene. This orientational change is a direct confirmation of the matrix effect on isomerization reactions postulated by other researchers. The frequencies of the most intense vibrational absorption bands in the thermally unstable cis isomer were determined. With this information, the relative quantum yields $\text{φ}{}_{\mathrm{<mtext>t\to c</mtext>}}\text{φ}{}_{\mathrm{<mtext>c\to t</mtext>}}$ for photoisomerization of azobenzene and the half-life for thermal cis→trans isomerization were obtained and compared with literature values.

Cellulose nitrate films were photolytically degraded in air and in vacuum. Crosslinking reactions took place during the early stages of the irradiation in the presence of air, with the formation of a new chromophoric system which absorbed light at 280 nm. It is evident that oxygen is necessary to effect crosslinking, and the nitrate group also appears to participate in this reaction. Denitration occurred during the process of photoirradiation. Further irradiation led to significant degradation of the crosslinked structures as well as chain scission, restructuring, yellowing and a catastrophic loss in mechanical properties. Carbonyl and aldehyde groups were formed as major products.

Carbamoylethylated cotton samples having 0·392, 0·524, 0·725, 1·379 and 1·546% N were prepared by reacting cotton cellulose in the fabric form with different concentrations of acrylamide in the presence of NaOH. The behaviour of these samples towards gamma radiation was investigated and compared with untreated cotton and alkali-treated cotton which was prepared under conditions similar to those of carbamoylethylation but in the absence of acrylamide. Irradiation was performed at different radiation doses (3·307–29·766 Mrad). Results of copper number and carboxyl content indicated that the carbamoylethylated cotton samples exhibited higher resistance to irradiation than the alkali-treated and untreated cotton samples. Furthermore, this resistance was higher the greater the extent of carbamoylethylation, expressed as % N. With respect to degree of polymerization, results suggested that the presence of the carbamoylethyl groups in the molecular structure of cotton cellulose impeded glucosidic bond scission, provided that these groups were present in relatively small amounts. This and the indication of differences between the tensile strength and elongation at break among the substrates examined were interpreted in terms of differences in microstructural features which, in turn, determine the susceptibility of each substrate to irradiation irrespective of the radiation dose used.

The photophysical properties of poly(N-vinylcarbazole) are interpreted, based on the photophysical properties of the diastereomers of 2,4-di(N-carbazolyl)pentane and a theoretical study of the conformational distribution in the dyads of poly(N-vinylcarbazole). From the conformational distribution in the polymer it is derived that more than 95% of the excitation will occur on a chromophore belonging to an excimer site. Since excimer formation at these excimer sites is very rapid (>10¹⁰ s⁻¹), it is concluded that, if energy migration is to be important, the rate constant of energy migration from one chromophore to another should be ⪢10¹⁰ s⁻¹.

The phosphorescence emission of polymers and copolymers of p-vinylbenzophenone (VB) can be detected in chloroform and benzene solutions at room temperature, when oxygen is carefully removed. Their spectroscopic properties are close to those of benzophenone (B), but changes in the vibrational structure and an increase of E-type delayed fluorescence is observed. The measurement of absolute quantum yields of phosphorescence in poly(p-vinylbenzophenone) and the observation of quenching in p-vinylbenzophenone-vinylferrocene copolymers are in agreement with previous photochemical results: no triplet energy migration between benzophenone units in the polymer chain in solution could be inferred from these results.

The photo-induced graft copolymerization of methyl methacrylate onto nylon-6 was investigated using acriflavin as the photoinitiator. The systems were buffered with phosphate-citrate buffer (Na₂HPO₄-citric acid). Up to a certain concentration of the monomer and the initiator, the graft yield goes on increasing: thereafter a decreasing trend was observed. The reaction was carried out within the temperature range 30–55°C and from the corresponding Arrhenius plot the energy of activation was evaluated. The effect of ascorbic acid, buffer and amount of the fiber on the graft yield has been investigated. From the effect of solvent on the rate of grafting, which has been investigated, the chain transfer constant (C_s) of the solvent has been evaluated. The kinetic data and other evidence indicate that the overall polymerization takes place by a radical mechanism. A suitable mechanism has been suggested and rate expressions have been derived.

Three titration methods developed for the determination of concentrations of hydroperoxides formed in photo-oxidized polymers are reviewed. They are based on the reduction by sodium iodide, by ferrous ion and by gaseous sulfur dioxide. The advantages and the limits of these methods are described on several polymer samples studied recently in the laboratory: the iodide method can be used only when the hydroperoxides are thermally stable up to 80°C. On the other hand, the ferrous ion method is particularly well adapted to unstable hydroperoxides but this method requires the complete solubilisation of the polymer. Lastly, the sulfur dioxide method can be easily carried out, but with some polymers secondary effects are observed which perturb the measurements.

A kinetic model for the post-irradiation oxidation of isotactic polypropylene films has been obtained giving concentration profiles of some reactants and products as a function of time and of the spatial co-ordinates. The diffusion parameters for oxygen and the rate constants for the main steps in the hydroperoxidative cycle were determined for the reaction within film depth of 12–40 μm.