Bruno Marcandalli

Photosynthetic membranes II: kinetic studies of photoinitiated grafting of some acrylic monomers onto cellulose

Abstract Photochemical grafting of 1,6-hexanediol diacrylate, diethylene glycol diacrylate and tetraethylene glycol diacrylate onto cellulose in the form of filter paper was investigated kinetically at 30 ± 2 °C with 1,2-diphenyl-2,2-dimethoxyethanone as a photoinitiator, under experimental conditions suitable for the preparation of membrane systems. Irradiations were carried out polychromatically, with incident radiation of flux (2.5 × 10−8)−(22.6 × 10−8) einsteins s−1 cm−2. The ratio R between the molar concentrations of photoinitiator and monomer were varied between 0.005 and 0.095, while the n/S value (n, moles of diacrylate monomer deposited per unit apparent surface S) was varied between 7.61 and 75.0 μmol cm−2. Two consecutive constant-rate processes were indicated by the kinetic curves. The quantum yields Φ1 and Φ2 for the first and second constant-rate periods were calculated. They are almost insensitive to the difference in diacrylate monomer structure; the Φ1 show a linear dependence on R, while the Φ2 show an inverse dependence on n/S. The relevance of photochemical parameters in the proposed reaction mechanism is discussed.

Photosynthetic membranes III: Influence of photo-cross-linking on permeation and diffusion coefficients of hydrogen and carbon monoxide through asymmetric membranes prepared by photografting of 1,6-hexanediol diacrylate onto cellulose

Abstract Asymmetric membranes were prepared by photochemical grafting of 28.1 μmol cm−2 of 1,6-hexanediol diacrylate onto cellulose in the form of filter paper, in the presence of 1,2-diphenyl-2,2-dimethoxyethanone as a photoinitiator, the molar ratio of photoinitiator to acrylated monomer being fixed at 0.086. A fixed polychromatic irradiation flux of 20.2×10−8 einsteins s−1 cm−2 was used. After 10 min irradiation, a nearly quantitative graft yield was reached. By multiple internal reflection IR analysis, a structure with one pendant acrylate group per monomer unit was assigned to the grafted polymer; at longer irradiation times, a further decrease in unsaturation resulting from photo-cross-linking between pendant acrylate groups was observed. Permeabilities P and diffusivities D at 25 °C for hydrogen and carbon monoxide were measured for membrane specimens obtained at different irradiation times. An increase in P reflects a decrease in unsaturation, while the D are independent of irradiation time. The selectivity of them embranes is not influenced by photo-cross-linking.

A simple device for quality control of polymer films, using measurements of permeation and diffusion coefficients

Abstract A simple device for the rapid determination of transport characteristics of vapors, gases or gaseous mixtures through polymer films has been constructed. This device may be useful either for routine quality control or for precise research work. It consists of two coupled permeation cells: the difference between the permeation rates of a tested specimen with respect to a standard film is measured and the resulting signal interpreted in terms of diffusion and permeation coefficients. The two coupled cells are of a differential type: the gas permeating through the film is collected by a flowing stream of an inert gas and carried to the analytical system (gas-chromatographic determinations have been used in this study). Such a system allows high pressure measurements without imposing mechanical stresses on the film and determination of transport coefficients of any gaseous mixture. A theoretical analysis of the transient signal in terms of transport parameters is reported together with some experimental results on commercial polycaprolactam films.

Photosynthetic membranes. A model study of the influence of photoinitiated grafting and graft polymerization of 1,6-hexanediol diacrylate and 4-(N-ethyl,N-2-acryloxyethyl)amino-4′-nitro-azobenzene, as co-monomers, onto polypropylene ribbon, on permeation and diffusion coefficients of oxygen and nitrogen

Abstract 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 (A2) 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.

Influence of the solvent and excited state prototropic equilibria on photochemical intramolecular acylatio of trans-3-(2-hydroxybenzylidene)-4,5-dihydrofuran-2(3H)-one

Abstract The solvent dependence of the kinetics of the geometrical isomerization of trans -3-(2-hydroxybenzylidene)-4,5-dihydrofuran-2(3 H )-one and the subsequent intramolecular acylation of the cis substrate was studied. Even if the quantum yields Φ 1 and Φ 2 of the two processes increase on increasing the polarity of the methanol solutions, addition of water is found to quench dramatically Φ 1 and Φ 2 . This behaviour is interpreted in terms of the proton exchange equilibria of the excited singlet state. A detailed analysis of the dependence of the fluorescence on the pH was carried out and a mechanistic model which is able to describe satisfactorily the experimental results is proposed.

Photoresponsive membranes incorporating 6-nitro, 1′,3′,3′-trimethylspiro-(2H-1-benzopyran-2,2′-indoline)

Abstract Photoresponsive heterogeneous membranes incorporating 6-nitro,1′,3′,3′-trimethylspiro-(2 H -1-benzopyran-2,2′-indoline) (6-NO2-BIPS), supported by cellulose 2,4-diacetate have been prepared. Membrane potential, measured in 10−2 and 10−4 m KCl, in a pH range between 2·0 and 8·5, while irradiating with visible and UV light alternatively, was found to depend not only on the photochemical equilibrium between ‘closed’ and ‘open’ forms of 6-NO2-BIPS, but also on pH. Charge density in the membrane, which is responsible for the potential signal and depends on pH, never reached values high enough to generate a Nernstian response. Fixed charge concentration values under UV irradiation, at different loadings of 6-NO2-BIPS in the membrane, were estimated by comparing experimental and theoretical curves, the latter at various combinations of the relevant parameters. Experimental behaviour was found to be consistent with the acid-base properties of 6-NO2-BIPS, and also with the shift, induced by pH, in the low pH range, of the tautomeric equilibrium between the protonated ‘open’ forms of 6-NO2-BIPS.

Thermodynamic study of solvent and substituent effects on 4-substituted aminoazobenzenes

Abstract Proton dissociation equilibria of various 4-substituted aminoazobenzenes have heen studied as a function of temperature. The corresponding thermodynamic functions Δ G o , Δ H o , and Δ S o have been calculated. Azonium-ammonium tautomeric equilibria of the monoprotonated dyes have also been investigated. The experimental data have been interpreted in terms of linear free energy relationships. Analysis of the solvatochromism of these dyes has led to the determination of some specific interactions with solvents, which, together with electronic factors, can well rationalize many aspects of the behaviour of these molecules.

Thermodynamic analysis of the reaction of barium chloride with water vapour from 1173 K to 1473 K

Equilibrium pressures of hydrogen chloride and water vapour over solid or liquid barium chloride have been measured between 1173 K and 1473 K. All other gases were excluded from the system. The corresponding equilibria were represented by the following reactions: BaCl2(s) + H2O(g) ⇄ BaO(s) + 2 HCl(g), between 1173 K and 1235 K; BaCl2(1) + H20(g) ⇄ BaO(s) + 2 HCl(g), between 1235 K and 1473 K; 32 BaCl2(1)(saturated with BaO) + H2O(g) ai 12 (BaCl2 2BaO)(s) + 2 HCl(g), between 1173 K and 1268 K; and 43 BaCl2(1) (saturated with BaO) + H2O(g) ⇄ 13 (BaCl2. 3BaO)(s) + 2 HCl(g), between 1268 K and 1473 K. The equilibrium constants Kp fit satisfactorily a linear plot of In Kp versus reciprocal absolute temperature. Mean enthalpy and entropy values of the reactions studied in the temperature range investigated were obtained from the plot and compared to literature data and to the phase diagram of the barium chloride — barium oxide system. The reactions involved and the thermodynamics of reactivity of barium chloride with water are critically examined.

Kinetic approach to the release of vinyl chloride monomer from polymer films, as related to diffusional theories

Abstract Monomer-release kinetics from polyvinyl chloride films, previously saturated by equilibration with monomer solutions in n -hexane, ethylene glycol monomethylether, methanol, and water, were measured by extraction with these model solvents, at 10–50 °C. Solute desorption rate data fitted a linear plot of total amount of solute released vs . the square root of time. Their analysis, in terms of diffusional release theory of a dissolved solute from a polymer matrix with initial solute loading given by the solubility limit, allowed one to obtain diffusivities of vinyl chloride in the polymer as a function of temperature, from which an activation energy of 11.9 ± 0.9 Kcal/mol could be calculated. Release kinetics were practically controlled by the low diffusivities, without any resistance to solute transport due to external mass transfer coefficients. The accelerating influence of solvents, relative to solvent-free experiments reported in the literature, may be compared to a plasticizing penetrant effect, which, however, does not appear to be susceptible, as to its bearing on diffusivities, to the chemical nature of the solvent used.

Electrolyte permeation through photo-cured acrylic polymers on steel as related to corrosion of the substrate

Abstract The effect of 1,2-diphenyl, 2,2-dimethoxy, ethanone as photoinitiator in some ultraviolet radiation-cured acrylated acrylic formulations on steel was studied. Correlation between corrosion rates, measured with ASTM D610 standardized method, and electrolyte permeation through coatings, measured with 36 Cl-labelled 0.25 M NaCl solutions at 25°C, was systematically investigated. The length of time necessary for the complete failure of the coatings, as observed by the extension of corrosion products over the entire surface, was evaluated by visual inspection of corrosion or by the breakthrough curve of permeation, and found to be a function of photoinitiator concentration. The slight difference between failure time obtained by the two methods may well be due to a time lag between the activation of corrosion cells over the whole surface and onset of visible corrosion products everywhere. This time lag may be correlated to the protective value of the coating, and to its permeation properties, particularly.