Polymer Photochemistry最新文献

Chemical effects of 184·9 nm, 194·2 nm and 253·7 nm ultraviolet radiation on dilute aqueous solutions of heparin-Ca salt are investigated. The degradation of heparin in dilute aqueous solutions involves two different mechanisms: (a) at 184·9 nm, an H^• abstraction process takes place: and (b) at 194·2 nm and 253·7 nm, heparin free radicals formed due to glycosidic bond dissociation are involved. These mechanisms are responsible for the depolymerisation of heparin molecules. The presence of sulphate groups renders stability to heparin, which requires long exposure to ultraviolet radiation to produce metachromasia in the cationic dye Acridine Orange, and changes in reducing-end group, hexuronic acid and hexosamine moieties in the presence of oxygen or nitrogen. The effects of three wavelengths and the existence of two degradation pathways are discussed.

This paper presents the results of a study on the photolysis of the azide group in polyesters of 3-azido-ortho-phthalic acid and linear diols. It was stated that the apparent quantum yield of the azide group photolysis depends on the number of carbons separating the azide group and on the diol type, being different for diols with even and odd numbers of carbons. The formulated kinetic equation takes into account the effect of the initial concentration of the azide group on the rate of photolysis. Investigation of all the polyesters with a constant initial concentration of azide groups showed that the number of carbons in a diol does not affect the real quantum yield. However, it is affected by whether that number is odd or even. A value for the activation energy of photolysis was also determined. It amounts to E_a = 1·6 kcal mol⁻¹ for polyesters with an odd number of carbons in the diol, and E_a = 0·34 kcal mol⁻¹ for polyesters with an even number of carbons in the diol. It can be concluded from the research that the way of introducing the light-sensitive group into the polymer chain is decisive for the primary processes in the polymer under irradiation. The real quantum yield for the light-sensitive group attached to the main chain amounts to φ = 0·61. For the light-sensitive group introduced into the polymer chain this value is smaller; for polymers with an even number of carbons in the diol φ = 0·42−0·47 and for polymers with an odd number of carbons in the diol φ = 0·51−0·54.

Significant aspects of the free radical chemistry of 2,2,6,6,-tetramethylpiperidine derivatives (HALS) which are of interest in respect to the mechanism of inhibition of polyolefin photo-oxidation have been elucidated by performing an ESR study of the reactions initiated by t-butoxyl radicals. With unsubstituted 2,2,6,6,-TMP the preferred site of the H-abstraction is the NH bond; this selectivity is preserved also when activated CH bonds adjacent to ester groups are present. The reactions of the aminyl radical products with the parent molecules and other reactive compounds have been followed by kinetic ESR and their rate constants and activation energies measured.

The absorption spectra of seven different monoazo dyes at different concentrations in water and in methanol as well as their absorption on cellulose diacetate (CDA) and polyamide (PA) films were examined. It was found that the extent of dye aggregation in methanol is greater than in water despite the higher dielectric constant of the latter, reflecting the anomalous behaviour of water. However, structural characteristics and concentration of the dye determine the state of dye molecules in water and methanol. Some of the dyes examined had greater tendency to aggregate while the others were probably present in monodispersed form. On the contrary, all the dyes used seem to have a strong tendency to aggregate on the CDA and PA films, indicating that involvement of the dye in an interaction with the substrate alters the structural characteristics of the dye and, therefore, its ability to aggregate. In accordance with this were the results of studies dealing with evaluation of the lightfastness of the same dyes on both films using daylight as well as a lightfastness tester for different exposure periods.

The sensitizing activity of thio-organotin stabilizers and some of their known or presumed by-products was studied during the photooxidation of polystyrene films in the solar UV range. All the dialkyltin dithioesters accelerated the photo-oxidation of PS noticeably. Among by-products, dialkyltin thioester chloride and alkyltin trichloride are also sensitizers. The nature of primary photochemical acts is discussed on the basis of a comparison of the electronic spectra of the compounds under study.

With a view to the manufacture of printing plates, films consisting of diethylene glycol diacrylate and poly(methyl methacrylate) acting as a binding agent were irradiated with UV light. The monomer absorbs UV radiation below 300 nm, whereas the resulting polymer as well as the poly(methyl methacrylate) only weakly contribute to total absorption.

On irradiation, UV light of wavelengths below 300 nm initiates polymerization of the monomer also in the absence of any photo-initiator and induces degradation of polymer chains: the poly(methyl methacrylate) depolymerizes to the monomer methyl methacrylate, whereas poly(diethylene glycol diacrylate) does not decompose to degradation products with CC double bonds. No degradation occurs when the films are irradiated with UV radiation of wavelength above 300 nm, but polymerization of diethylene glycol diacrylate is initiated to some extent under these conditions.

Annealing at 60°C of films irradiated at room temperature to maximum conversion results by thermal polymerization in a loss of pendant double bonds of diacrylate monomer units which are incorporated into the network only by polymerization via one double bond.

Interactions during the UV stabilization of cis-1,4-polybutadiene of a hindered piperidine (Tinuvin 770), its analogue hindered nitroxyl radical, 4-hydroxy-2,2,6,6-tetramethylpiperidino-1-oxy, with the UV-absorbers, ortho-hydroxybenzophenone and 2(2′-hydroxy-5-methylphenyl)benzotriazole (Tinuvin P) were investigated by using UV/VI, IR and ESR spectroscopy. Mechanisms for the observed antagonistic effects between the two groups of photostabilizers have been proposed.