Polymer Photochemistry最新文献

The flash photolysis of a low molecular weight dioxane-lignin in the presence of oxygen showed the same transient spectrum as de-aerated solutions. Experiments performed in the presence of DABCO (1,4-diazabicyclo(2,2,2)octane) (a singlet oxygen scavenger) and ascorbic acid (a radical scavenger) showed that ground state oxygen is the main species participating in these processes. Singlet oxygen, sensitized by species other than carbonyls, may play a role in later stages of the reaction.

Linear polyurethane (PUR) prepolymers were synthesized starting from diphenylmethane-4,4′-diisocyanate (MDI) and polyester diol oligomers. Two different types of polyester diols were used: poly(ethylene adipate) (PEA) and poly(ethylene/butylene adipate) (PEBA). The effects of UV irradiation on the photooxidative degradation of prepared PUR prepolymer were studied.

The changes in the chain lengths and their distribution as well as the changes in chemical structure were followed. The GPC results show that the photooxidative decomposition is followed by a decrease in molecular mass together with an increase in polydispersity. This indicates a very inhomogeneous degradation, which is a consequence of the specific course and of the intensity of photooxidative degradation. This was confirmed by means of comparative IR and NMR measurements. Conclusions on the difference in behaviour of the two different types of polyester diols used in PUR synthesis were discussed.

It has been established that the photocrosslinking of PE in the presence of 9,10-anthraquinone (AQ) and some of its derivatives was initiated not only during the photoreduction of AQ but also in some photochemical reactions of AQ photoproducts. Quantitative data including AQ photoreduction quantum yield, the number of crosslinks per AQ molecule and quantum yield of crosslinking were measured. The results enabled us to establish the mechanisms of both photochemical and dark reactions of AQ photoproducts in PE which have explained not only the spectral data but also the peculiarities of photochemical crosslinking of PE initiated by AQ.

From S—MMA copolymers, polymerized in a number of ways, plates, solutions and films have been prepared. Their luminescence spectra have been measured. It has been found that the variety of spectra obtained is the result of superposition of polystyrene (PS) and monomeric styrene (Sm) spectra. A relation has been found describing the spectral distribution of the PS-Sm system as a function of PS and Sm concentration, efficiency of electron excitation energy transfer from PS to Sm and the wavelength of excitation light.

The results suggest that the luminescent analysis may be utilized to estimate the residual contents of Sm in the polymer.

The kinetics of the photopolymerization of methyl methacrylate (MMA) sensitized by an erythrosin-ascorbic acid system in aqueous solution have been studied systematically. The system was irradiated with light of 440 nm wavelength. The induction period before the onset of polymerization was kept to a minimum by suitable experimental conditions. The systems were buffered with phosphate buffer, and the effects of monomer, dye, temperature, etc., on the reaction systems were investigated. The polymerization was followed by the measurement of monomer disappearance (gravimetrically) and the chain length of the polymer formed (viscometrically). The tentative kinetic scheme proposed is examined in the light of the experimental results. The specific rate constants for the various steps have been evaluated.

Commercial polystyrene films 0·1 mm and 0·05 mm thick were photo-irradiated in mono- and multi-layer films in air, and several analytical methods—infrared and ultraviolet spectroscopy, chemiluminescence and gel permeation chromatography—have been applied to detect the degree of photo-oxidation of the polymer.

The chemiluminescence method proved to be very sensitive and it was possible to detect very minor amounts of the oxidative defects caused by photo-irradiation of the polymer. The emission intensity increased with increasing photo-irradiation time and thickness of sample films. By analysis of each film after photo-irradiation at various depths from the surface, it was clearly demonstrated that photo-oxidation of the polymer commenced at the surface.

A study of fluorescence quenching of polystyrene and statistic styrene copolymers with methyl methacrylate, ethyl acrylate and acrylonitrile by carbon tetrachloride in 1,2-dichloroethane has been carried out. The results of the study have been interpreted on the basis of the modified Stern-Volmer relation, which takes into account the fact that isolated styrenes do not participate in the process of energy migration. The coefficients of energy migration were determined for the systems studied. It was shown that the intramolecular excimers restrict the process of energy migration in polystyrene and its copolymers. The results obtained are discussed.

The emission spectra of five low molecular ketones (benzophenone, 9-anthrone, 9,10-anthraquinone, xanthone and thioxanthone) in polymeric matrices, namely polyethylene (PE), isotactic polypropylene (iPP), atactic polypropylene (aPP), polystyrene (PS) and poly(methyl methacrylate) (PMMA), exhibit the same main features as in non-polar and polar glasses of low molecular solvents at 77 K. The emission spectrum of benzophenone is least dependent on polymer matrix. The best vibrationally resolved spectrum of anthrone and xanthone is attained in non-polar matrices such as PE, iPP and aPP at low concentration. The intensity of the short wavelength vibrational bands decreases whilst that of the long wavelength bands increases with increased concentration of ketone in the range between 0·1 and 3 wt%. Emission spectra of anthrone and xanthone are most sensitive to the polarity of the polymer matrix. No difference is observed in the emission spectra of aryl carbonyl chromophores in the microcrystalline and amorphous polymer matrices. Mono-exponential decay of the emission is observed for benzophenone, 9-anthrone and 9,10-anthraquinone in all matrices under study. The decay of xanthone emission is bi-exponential in all matrices. Thioxanthone exhibits bi-exponential decay in non-polar matrices such as PE and PP and mono-exponential in polar matrices such as PMMA and PS.

The sensitivity of the emission of 9-anthrone and xanthone can be used for evaluating the polarity of the polymer matrix. The concentration effect can substantially influence the spectra of low molecular compounds dispersed in the polymer matrix.

Assuming only the conditions for the Lambert Law, and from considerations of inner filter effects, we derive an expression which yields the net fraction of light absorbed by the bottom layer of a film having any optical density and divided into any number of hypothetical layers. We show that there is an optimum total optical density at which absorption by the bottom layer is maximized. The optimum optical density is a function of the thickness of the bottom layer probed. For an infinitesimally thin bottom layer, the optimum total optical density is 0·4343.

The photodegradation of three types of polyethylene has been studied by electron spin resonance (ESR), ultraviolet absorption (UV), Fourier transform infrared absorption (FTIR), gel fraction and mechanical properties measurements.

Various types of oxygenated products were detected by FTIR spectrometry in photoirradiated polyethylene. The rate of formation of oxygenated products is greater in the case of photoirradiated crosslinked polyethylene. A detailed photodegradation mechanism for polyethylene is proposed on the basis of the experimental results.

Tensile tests on photoirradiated polyethylene were also carried out. It was found that crosslinking improved the photostability of polyethylene. The relationship between the amount of oxidation products produced and the decrease of mechanical strength gives a measure of the photostability of polyethylene. We have tried to correlate the carbonyl content with the mechanical properties to predict the photostability of the polymer.