Polymer Photochemistry最新文献

Polystyrene films have been exposed to 254 nm radiation under high vacuum at 25°C. The time evolution characteristics of new ultraviolet-absorbing chromophores have been examined; in particular, increased absorptions at 240 nm and 400 nm have been measured. Whilst the initial rates of increase in these absorbances are relatively rapid, they are not maintained, and decline gradually with increasing irradiation. The fall-off in rates has been attributed to an optical filter effect, the products, which have higher extinction coefficients for 254 nm radiation, absorbing some of the incident radiation in direct competition with the polymer. The results were analysed in terms of a model which allowed for an appropriately reduced rate of absorption by the polymer, and a good correlation between experimental data and theoretical prediction was observed. Other factors which could contribute to the apparent inhibition of chromophore formation are discussed, and their effects assessed.

The rate constants and the activation energy for the mutual recombination of peroxy radicals generated from atactic polypropylene (PP) and for model compounds of PP and PP autoxidation products have been determined by kinetic ESR. The type of peroxy radicals and the preferred sites of attack for the hydrogen abstraction by t-butoxyl radicals were identified. The results are of interest in respect to the mechanism and the kinetic simulation of PP autoxidation.

Kinetic parameters have been obtained for the radiation-induced graft copolymerisation of styrene and maleic anhydride onto polyethylene. Evidence of sensitisation by maleic anhydride of styrene grafting has been obtained, although in the early stages of grafting we also have an indication of the formation of grafted branches which contain both maleic anhydride and styrene in regular sequences. The production of copolymers of this type provides modified polythenes having changed physical and chemical properties. In certain situations there is evidence of diffusion control in the later stages of the grafting reactions. Also, in the early stages of grafting, no true steady-state position appears with respect to the monomer concentration dependence and the intensity dependence.

The free-radical intermediates and volatile products of the UV photolysis of five N-acetyl amino acids were studied by ESR and mass spectroscopy. At low temperatures (77 or 100 K) radicals were produced by cleavage of the CH₃CO bond in the N-acetyl group and by subsequent abstraction reactions. Photolysis of frozen aqueous solutions, in which the radicals were isolated in the matrix, showed that methyl and acyl radicals were the initial radical intermediates. Carbon dioxide was the major volatile photolysis product. A non-radical decarboxylation reaction is proposed. Comparison of the photolysis products with those reported for poly(amino acid)s showed that the N-acetyl amino acids are not appropriate models for the UV photolysis of poly(amino acid)s.

The energy transfer evaluation of poly(vinyl phenyl ketone) (PVPK) as a polymeric photosensitizer has been carried out following two different procedures: by monitoring the trans → cis isomerization of stilbene by quantitative analysis of cis-stilbene with High Performance Liquid Chromatography (HPLC) and by viscometric measurements of the course of the reaction. The Stern-Volmer constants (k_sv) afforded by the HPLC and viscometric methods, 88 and 92m⁻¹ respectively, show a good concordance.

During the photo-irradiation of polyethylene a new fluorescence emission spectrum with a peak at 420 nm and a corresponding excitation spectrum with distinguishable peaks at 298, 312 and 327 nm were observed; the intensities of these peaks increased with irradiation time. In the case of polypropylene no emission spectrum was observed in these regions. However, infrared spectra of both polymers showed formation of carbon-carbon double bonds during the photo-irradiation. These differences in the fluorescence emission spectra between two polymers indicate that the polyethylene forms polyene structures fairly easily during the photo-irradiation, while polypropylene forms only isolated double bonds or very short sequences of double bonds.

Thermal and photo- grafting of vinyl monomers on polypropylene plates of 1 mm thickness were carried out in water using benzoyl peroxide and benzophenone respectively as initiators. The resulting grafted samples were subjected to electron microscopy in a morphological observation of their surfaces. The grafted samples prepared by thermal grafting revealed a flat surface irrespective of the type of monomer used. The appearance of the surface of the samples prepared by photografting may be classified roughly into two types, flat or grainy. The flat surfaces were provided by the hydrophobic monomers methyl methacrylate and styrene. The grainy type was obtained using the hydrophilic monomers acrylic acid, methacrylic acid and N-vinylpyrrolidone.

Photografting of the hydrophilic monomers was characterized by the high susceptibility of the grafted chains to photodecomposition. It was supposed that the formation of a grainy surface is closely related to an abnormal polymerization in which grafted chains are more susceptible to photodecomposition.