A. I. Mikhailov

Phenomenological description of the spontaneous formation of macroscopic strings in low-concentration chiral solutions and the formation of anisometric gels

36 Gel formation has recently been described in chiral solutions of low concentrations (10–3 to 10–2 M) [1], which are two orders of magnitude lower than the per colation threshold for the formation of an isotropic gel [2]. The forming gel has a well discernible microstruc ture [1]. Examination of the xerogel produced after solvent evaporation from the samples detected aniso metric (with a length to diameter ratio of 103–105) structural elements with observable rigidity—strings [1, 3, 4]. In this work, we identified the phenomena that lead to the solidification of low concentration chiral solu tions and studied the solidified phase by the example of solutions of trifluoroacetylated amino alcohols (TFAAA) (table). TFAAA molecules are chiral (except compound 4 in the table) and approximately isometric, which allows one to exclude the effect of nonchiral steric factors on solidification. The molecu lar design of TFAAA ensures the manifestation of the full range of weak intermolecular interactions in the formation of supramolecular structures. The solutions we used were cyclohexane, chloroform, carbon tetra chloride, benzene, ethanol, methanol, and acetone. The solution microstructure was investigated by optical microscopy. Solvent evaporation was pre vented because the samples of the studied solutions were placed in closed vessels. At a concentration on the order of 10–3 M and higher and a temperature of 300–340 K, a condensed phase separates out from homochiral TFAAA solu tions. On cooling a solution within a capillary 300 μm i.d., the phase separates out as an individual discrete anisometric structural element or elements—strings (Fig. 1). The string diameter (1–3 μm) is constant, and the string length reaches several millimeters. The strings form approximately along the capillary axis, definitely being repelled from the capillary walls and showing no signs of interacting with each other. Thus, the string formation in these experiments is a separate physical phenomenon unrelated to gel formation.

Formation of free radicals in the low-temperature fluorination of polymers

The formation of free radicals in the process of direct fluorination of natural and synthetic polymers at temperatures close to 77 K was studied by ESR. The maximum concentrations of radicals, (1017–5·1018 spin g−1) and their complete oxidation were observed in the temperature interval from 77 to 250 K at a fluorine pressure of 30 Torr. The initiation of chain halogenation, which consists of homolytic breaking of chemical bonds to yield free-radical intermediates. was examined in the framework of the multi-center synchronous transitions model.

The photo-oxidation of polymers—1. Initiation of polystyrene photo-oxidation

Abstract The initiation of polystyrene photo-oxidation has been studied and the quantum yield of initiation (ϕR·) for u.v.-irradiation at 315 K in the presence of atmospheric oxygen has been found to be 10−3. Using ESR spectra, five distinct free radicals have been found; their nature depends on the wave length of the light and the irradiation conditions (temperature and presence of oxygen).

Photo-oxidation of polymers 4. The dual mechanism of polystyrene photo-oxidation: a hydroperoxide and a photochain one

Abstract By comparing the quantum yields of initiation, O 2 absorption and formation of intermediate and final products a conclusion has been made that the polystyrene photo-oxidation process involves two mechanisms: a photochain (∼90%) and a hydroperoxide (∼1%) one. The photochain mechanism is based on the reaction of excited peroxide macroradicals dissociation (the kinetic chain length is 10 2 –10 3 units), leading directly to formation of the main final products, H 2 O and CO 2 , whose yield is comparable with the quantity of the absorbed oxygen. The ordinary hydroperoxide mechanism, by which up to 10% of the radicals formed in the initiation stage are converted, includes an RO 2 • reaction with a macromolecule and hydroperoxide formation. A subsequent ROOH decomposition results in the scissions of the polymer chains and the formation of high-molecular oxidation products (carbonyls, alcohols, ethers).

Compaction of intermolecular bonds in the macroscopic chiral phase of strings

The microstructure of a cured state characteristic of a wide variety of low-concentration homochiral solutions in comparison with that of the condensed phase formed in achiral solutions is studied using optical and atomic force microscopy and X-ray diffraction analysis.

Initiation of free-radical polymerization of acetylene monomers with molecular chlorine at low temperatures

The reactions of molecular chlorine with phenylacetylene (PA) and para-diethinylbenzene (p-DEB) at low temperatures (77–210 K) were studied by electronic paramagnetic resonance (EPR), size exclusion chromatography, double bond analysis (DBA), calorimetry, and UV, visible, and IR spectroscopy. The low-temperature (100–210 K) reactions of molecular chlorine without high-energy chemical (HEC) effects formed radicals capable of oligomerizations and polymerizations. A readily soluble polymer para-diethinylbenzene having no spatial cross-linkages (but possibly having a branched structure) was obtained. Polymerization occurred in direct contact with chlorine. It accelerated near the melting point (Tm) of chlorine and proceeded vigorously along with chlorination at Tm = 172 K. The occurrence of low-temperature polymerization without HEC treatment points to the high reactivity of the reagents. From the viewpoint of applications, low-temperature oligomerization or polymerization of monomers with molecular chlorine can afford oligomers or polymers with definite properties, which cannot be obtained by other initiation procedures.

Direct and initiated halogenation of carbon nanomaterials at low temperatures

Cryochemical reactions of the direct and initiated (by photolysis and radiolysis) halogenation of carbon nanomaterials (C60 fullerene, nanotubes, and nanofibers) at 77–240 K were investigated by the ESR, IR spectroscopy, and elemental analysis techniques. A high reactivity of C60 in reactions with fluorine and chlorine with the formation of corresponding derivatives was shown. High concentrations of radical intermediates indicating the radical chain halogenation of C60 were detected (the kinetic chain length for the chlorination process reaches 104–106 units). The amount of chlorine attached to fullerene is ∼35% and practically does not depend on the initiation mode (UV or γ-irradiation at doses up to 350 kGy). The mechanism of the cryochemical halogenation of C60 is considered within the limits of the model of multicenter synchronous transitions in a molecular complex consisting of several reactant molecules including molecular fluorine or chlorine and ensuring a net exothermic effect. The amount of chlorine added to nanotubes and nanofibers did not exceed 2.5–8%, thereby indicating a low reactivity of these materials under cryogenic conditions.

Influence of radiolysis on the yield of nanocellulose from plant biomass

Radiolysis of plant biomass with doses of 200–300 kGy prior to dispersing by chemimechanical methods increases the yield of nanocellulose from cellulosic feedstock by more than a factor of two. Another advantage of radiation pretreatment of initial samples is the effect of radiation sterilization of isolated nanocellulose hydrogels, whereas the hydrogel obtained without preliminary irradiation rapidly suffers from the attack of molds during storage in the laboratory. Photolysis of plant raw material at a wavelength of 253.7 nm has almost no effect on the yield of nanocellulose. The molecular and supramolecular structure of nanoparticles with a size of 200–300 nm remains unchanged on passing to the nanoscale and corresponds to the macromolecular structure of cellulose. Industrial testing of the hydrogel as an additive (2.5%) for an adhesive composite used in the manufacture of wood laminates (plywood) showed an enhancement of the strength characteristics of the products by 15–20%. The increase in strength is mainly due to an increase in the contact area of cohesive bonding through small coiled molecular entities composed of 10–16 Kuhn segments including up to 28 monomer units each.

On the supramolecular mechanism of cell-cell commutation

Processes of cell commutation via anisometric supramolecular structures (strings) are considered in vitro and using physicochemical models of lipids and trifluoroacetylated amino alcohols. The biological effectiveness of commutation via strings is demonstrated to be higher compared to the diffusion mechanism of transfer of biologically active molecules over distances between objects typical of populations of microorganisms. A kinetic model of strings is developed, and the rate of signal transmission in such systems is estimated. It is shown that the condition of the chirality of lipids forming the biomembranes of cells follows from the experimentally observed cell commutation by means of supramolecular anisometric structures.

Experimental investigation of anisometric chiral phase xerogel

The microstructure of a cured state (xerogel) of homochiral and achiral compounds belonging to a series of trifluoro acetylated amino alcohols has been investigated. It is shown that the substance of the cured homochiral compound consists of anisometric structures with an observed rigidity (strings) which are crystalline monophase. The xerogel of an achiral compound is represented by isometric microstructures. It is suggested that intermolecular hydrogen bonds comprising the molecular strings are apparently packed within the phase.