Photochemical behaviour of fire-retarded polymers

The influence of a non-halogenated intumescent fire retardant on the photooxidation of polypropylene is reported. The photooxidation of polypropylene stabilised with a phenolic antioxidant and two redox antioxidants (HALS), without and with the flame retardant has been studied. The chemical modifications resulting from UV-light exposure with wavelengths above 300 nm in the presence of oxygen were followed by IR and UV-visible spectroscopies. Special attention was given to the influence of each component on the rate of oxidation of the polymeric matrix. The photooxidation of the fire-retarded polymer can be described by two independent phenomena: the photooxidation of the intumescent agent and the photooxidation of the polymer. The results obtained offer new insight in the formulation of stabilised fire-retarded PP for outdoors applications.     

Effect of vibration in processes of structure formation in polymers

It was shown that the strength characteristics of the samples produced from thermoplastics (low density polyethylene, polyamide 6, Armamid, and polyethylene terephthalate) increase as a result of short vibration treating with a frequency close to that of natural vibrations of the structural elements.     

Photochemical control of biological processes

Photochemical regulation of biological processes offers a high level of control to study intracellular mechanisms with unprecedented spatial and temporal resolution. This report summarizes the advances made in recent years, focusing predominantly on the in vivo regulation of gene function using irradiation with UV light. The majority of the described applications entail the utilization of photocaging groups installed either on a small molecule modulator of biomolecular function or directly on a biological macromolecule itself.     

Photochemical formation of substituted pentacenes

Substituted pentacenes (8a, 8b, 14a, and 14b) were prepared by Strating-Zwanenburg photodecarbonylation of diones (7a, 7b, 13a, and 13b). The compounds are red and stable in the solid state under inert atmosphere as well as in degassed solutions, but not in air. Each is soluble in common organic solvents where, unless protected, they are oxygen sensitive.     

Photochemical formation of semiconducting nanostructures

Existing data on photochemical and photocatalytic approaches to the formation of semiconducting nanoparticles and also binary semiconducting nanostructures and nanocomposites of semiconductors with metals and polymers are reviewed. The nature of the effect of irradiation on the synthesis and properties of the obtained nanostructures and the possibility of photocatalytic control of the structural and spectral parameters of nanostructural semiconducting systems are examined. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 4, pp. 199–220, July–August, 2008.     

Effect of the chemical nature of polyoxometalate complexes on redox processes leading to the formation of metallic silver nanoparticles

It was shown that the reaction of solid polyoxometalate compounds containing tetraamine cations of nickel(II) or copper(II) with an aqueous solution of silver(I) nitrate leads to the formation of nanoclusters of the metal with sizes of 6–10 nm, localized on the surface of the crystalline matrix. It was established that the realization of the oxidation–reduction processes depends substantially both on the cationic and on the anionic component of the polyoxometalate complexes.     

Functional monomers and polymers. LXI. Photochemical reactions on the synthetic polymers containing thymine bases

Photodimerization of N-2-isobutyloxyethyl thymine (T-M), bis[2-(5-methyl-1-primidinyl)-ethyl] glutarate (T-T), poly(N-2-methacryloyloxyethyl thymine) (polyMAOT), and poly(N-2-acryloyloxyethyl thymine) (poly AOT) was studied in dimethylformamide solution. The rate of intramolecular photodimerization of polyMAOT and polyAOT was 5.7 and 7.2 times faster, respectively, than that of T-T. In T-M, however, intermolecular photodimerization did not occur under the reaction condition used. Quenching studies with isoprene suggest that the photodimerization of T-T occurs predominantly in an excited triplet state, whereas that of polyMAOT and polyAOT occur in singlet and triplet states. It was concluded that the photodimers of T-T, polyMAOT, and polyAOT are two syn-fused cyclobutane-type dimers (cis-syn and trans-syn). Quantum efficiencies of thymine base disappearance were determined by ultraviolet (UV) analysis to be 0.0012 for T-T, 0.0084 for polyMAOT, and 0.010 for polyAOT.     

Photochemical reactivities of photosensitive polymers containing chalcone derivatives

Monomeric vinyl chalcone derivatives were prepared by the condensation reaction of p-hydroxybenzaldehyde with acetophenone derivatives in the presence of sodium ethoxide as a base, followed by esterification of their hydroxy groups with methacryloyl chloride. The monomers, 4-(methacryloyloxy)chalcone derivatives, have been polymerized by a free radical technique. Copolymers containing the chalcone groups with different ratios of p-nitrophenylmethacrylate as photosensitizer group have also been prepared. The photosensitivities of the polymeric chalcone derivatives were investigated through their photochemical reaction on exposure to UV light, and by measuring the changes in their UV spectra before and after irradiation. The results obtained from the disappearance rates of the C?C bonds indicate that the photoreactivities of the polymers are largely affected by the position of the substituent in the phenyl ring of the chalcone, the photosensitizer group ratio, and the irradiation time.     

Photochemical processes of 2,6-di-tert-butyl-4-methylphenol

The photosensitized oxidation of 2,6-di-t-butyl-4-methylphenol (BHT) by anthracene (A) in CCI₄, hexane and polystyrene films with light of wavelength 365 nm has been investigated. Interestingly, BHT was found to inhibit the consumption of A in the presence of oxygen. A possible kinetic scheme for the reaction of A + BHT + CCl₄ has been devised; the rate constants for the most important processes have been evaluated.     

Fluorescent probes for sensing processes in polymers

Fluorescence spectroscopy is an important analytical technique that has been widely used in a variety of applications, such as biomedicine, biology, and science of materials, because it presents some properties which makes it unique, that is, extraordinary sensitivity and selectivity, short delay time (<10(-9) s), and it is neither invasive nor destructive, so it can be used for in situ measurements. Generally, intrinsic fluorescence of many materials, like polymers, is unspecific so it is not useful to analyse their properties or to be correlated to changes in their microenvironment. The incorporation of additives with fluorescent groups would be necessary. When the fluorescence emission of these molecules is sensitive to changes of properties, such as polarity, fluidity, order, molecular mobility, pH, or electric potential, they can be used for detecting such changes in their microenvironment, and they are called fluorescent probes. As long as these probes can follow processes of practical interest, they can be employed as sensors, if the information given by the measure of fluorescence adequately reflects the changes in the system. In addition, a sensor must fulfil some other requirements in order to make them of practical use, the most important being that the material support in which the sensor molecule is inserted. This support should permit a rapid detection of the process and should allow easy processing in a variety of forms. Polymers are well-known systems in which estimation of local parameters are possible by means of fluorimetric techniques. It allows the study of dynamic processes of interest, such as polymerization kinetics and mechanisms, thermal transitions, photodegradation, swelling morphology changes, and so forth.     

Solid electrolyte interphase formation on metallic lithium

Solutions of three salts (LiBF₄, LiNTf₂, LiPF₆) in N-methyl-2-pyrrolidone (NMP), selected arbitrarily as a reference solvent, were investigated by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy techniques. The lithium surface in contact with LiPF₆ in NMP electrolyte was covered with a protective layer (SEI) which morphology comprise small particles (of ca. 0.2 μm in radius). This salt was selected for further studies. The impedance of the Li|(LiPF₆ in NMP?+?additive)|Li system was measured immediately after cell assembly and after galvanostatic charging/discharging. Fifteen different additives (10 wt.%) were used. The efficiency of individual additives was evaluated in terms of the Li|electrolyte system resistance (ΔR) or total cell impedance reduction, both deduced from EIS. Some of the additives were able to form the SEI layer and to reduce resistance/impedance of the Li|electrolyte interphase. In such cases, the lithium surface was covered with relatively uniform conglomerates, or regions separated by cracks, of ca. 1–2 μm in dimension.