Polyurethane Nanocomposites

This review describes the present state of science and technology of photopolymerizable (UV-curable) polyurethane (PU) nanocomposites which include nanosilica and organically-modified clay (organoclay). A number of documented improvements of properties of PU nanocomposites compared to the pristine PU are presented. Many data on the structure and properties of PU nanocomposites were obtained not only for UV-cured urethane acrylate oligomers, but also for nanocomposites produced in the dark reactions. These data are critically reviewed. There is an expectation in the field of dramatic improvement of properties of PU nanocomposites under low loading (1-5 wt%) of organoclay

Magnetic and spin effects in the photoinitiation of polymerization

Application of a moderate external magnetic field increases the efficiency of triplet photoinitiators of polymerization. The reasons for this observation are briefly outlined. Such well-documented magnetic and spin effects as the magnetic isotope effect (MIE) and chemically induced dynamic nuclear and electron polarization (CIDNP and CIDEP) have been observed under the photolysis of photoinitiators. CIDEP results are emphasized. The study of CIDEP under the photolysis of initiators allows the detection and identification of free radicals of initiators and the spin multiplicity of the reacting photoinitiator (triplet or singlet). The transfer of the spin polarization of the initiator free radical to monomers is a promising technique for following radical reaction pathways. The results of a CIDEP study of the sensitized decomposition of phosphine oxide photoinitiators are presented

Electronically Excited States of Free Radicals

Formation of the excited doublet (D) and quartet (Q) states of free radicals under their photoexcitation is discussed. The relative positions of the D and Q states are compared to the positions of the photoexcited states of organic molecules (Jablonsky diagram). A number of representative cases of the excited states of free radicals detected by their transient absorption or emission are presented. A special case of the population having the lowest Q state in some radicals is discussed. A spin–statistical factor in the reactions of Q and D is debated

Fast Recombination of Free Radicals in Solution and Microviscosity

Rates of fast reactions are inversely proportional to the solvent viscosity (η). However, a quantitative study demonstrates that dynamic viscosity η is often a crude reflection of a viscous drug exerted on a molecule or radical. This paper aims to present an accurate dependence of the rates of fast bi- and monomolecular reactions upon the viscous drug of a media. Different correction coefficients fmicro are discussed, which should lead to a dependence rate ∝ (fmicroη)−1. Microviscosity is viscosity, leading to the expected rate dependence upon shear viscosity. In many cases, experimentally measured diffusion coefficients of molecules of a similar structure to the reactive radicals lead to the correct prediction of radicals’ diffusion coefficients and the rate constants of radicals recombination. Microviscosity of complex non-Newtonian liquids (biological liquids, polymeric solutions) can be measured using low MW molecular probes. Usually, the measured ηmicro is much lower than the shear η of complex biological or polymeric liquids. Cis–trans isomerization of bulky groups in monomolecular reactions is often described with Kramers’ theory. An example of such isomerization of a cyanine dye studied experimentally and theoretically is presented. It is demonstrated in the selected case that Kramers’ theory adequately describes the dependence of cis–trans isomerization of organic compounds upon η

Molecular Terms of Dioxygen and Nitric Oxide

Molecular terms of dioxygen and nitic oxide are presented. Electron spin resonance spectra of diatomic molecules corresponding to these terms are discussed. Gas-phase ESR can be a convenient method of monitoring paramagnetic pollutants in the atmosphere. We ran additional calculations in molecular physics for terms of these molecules and Zeeman transitions

Cage Effect under Photolysis in Polymer Matrices

Photoinduced elementary reactions of low-MW compounds in polymers is an area of active research. Cured organic polymer coatings often undergo photodegradation by free-radical paths. Besides practical importance, such studies teach how the polymer environment controls elementary free-radical reactions. Presented here is a review of recent literature which reports such studies by product analysis and by a time-resolve technique of photochemical reaction inside the cage of a polymer and in the bulk of a polymer. It was established that application of moderate external magnetic field allows the control of the kinetics of free radicals in elastomers. Preheating and stretching of elastomers affect reactivity of photoproduced radicals

A laser flash photolysis study of magnetic field effects in photoinduced electron transfer between Ru (bpy)<SUP>2+</SUP><SUB>3</SUB> and N,N'-dimethylviologen in micellar solutions

The kinetics of electron transfer between photoexcited Ru(bpy)2+3 and N,N'-dimethylviologen (MV2+) have been studied by means of laser flash photolysis in sodium dodecyl sulfate (SDS) and sodium laurate (SL) micelles. The decay of photoexcited Ru(bpy)2+3 follows first-order kinetics with kobs 106-107 s-1, and dependences of kobs versus surfactant and quencher concentrations were obtained. Analysis of the data showed that the quenching is mainly intramicellar. Although SDS solutions shows no escape of the radicals MV+ and Ru(bpy)3+3 from the micelles, for solutions of SL micelles an escape value &#934; es 0.08 was determined. The dependences of &#934; es on the concentration of SL and SDS show a drop in the vicinity of the cmc. Radical MV+ is not incorporated into the SL micelles. An increase of &#934; es in the SL micelles up to 20-25% was observed under application of an external magnetic field (B = 0.2-0.47 T) during steady-state irradiation and laser flash photolysis studies. The magnetic field effect is mainly described according to a hyperfine coupling mechanism. Geminate recombination kinetics of the pair 3[MV+ Ru(bpy)3+3] was observed in both SDS and SL micelles. Pertinent kinetic analyses were made and the requirements for geminate recombination kinetics observations were discussed