High-pressure absorption spectroscopy in polymerizing systems

Quantitative high-pressure absorption spectroscopy in the mid infrared, near infrared, and ultraviolet region enables direct and precise investigation of high-pressure ethylene polymerizations with thermal, chemical, and laser-photochemical initiation. The special advantages of kinetic studies via quantitative near infrared spectroscopy in an extended wavenumber range are outlined. Extensions of the method on copolymer systems are indicated. The spectroscopic technique is of general applicability in studies on reactions and equilibria in dense fluid states including liquids and compressed gases.     

Molecular weight distributions from measurements of polymolecular property functions

A general method for the calculation of molecular weight distributions from measurements of polymolecular property functions is given. It is based on the Hilbert-Schmidt theory of compact integral operators. Two variation problems are described, both taking the error of measurement into consideration. The method is applied to light scattering on Gaussian polymer coils (theta state) and to elution curves of Phase Distribution Chromatography (PDC).     

Biopolymer mimicry with polymeric wormlike micelles: Molecular weight scaled flexibility,locked-in curvature,and coexisting microphases

Giant and stable wormlike micelles formed in water from a series of poly(ethylene oxide) (PEO)-based diblock copolymer amphiphiles mimicked the flexibility of various cytoskeletal filaments. The worm diameter (d) was found by cryo-transmission electron microscopy to scale with the length of the hydrophobic chain (N_h) of the copolymer as d ∼ N_h^0.61. By fluorescence video imaging of worm dynamics, we also showed that the persistence length (l_P) of wormlike micelles scaled as l_P ∼ d^2.8, consistent with a fluid aggregate (∼d³) rather than a solid rod (∼d⁴). By polymerizing the unsaturated bonds of assembled copolymers, fluid worms were converted to solid-core worms, extending the bending rigidity from that of intermediate filament biopolymers to actin filaments and, in principle, microtubules. Through partial crosslinking, polymerized worms further locked in spontaneous curvature at a novel fluid-to-solid percolation point. The dynamics of distinct, branched conformations were also imaged for recently discovered Y-junctioned wormlike micelles composed of diblocks of high molecular weight (>10–15 kg/mol). Finally, block copolymers of hydrophilic weight fraction close to the transition between a vesicle- and worm-former assembled into both structures, allowing encapsulation of wormlike micelles in giant vesicles reminiscent of cytoskeletal filaments enclosed within cells. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 168–176, 2004     

Molecular weight distributions of linear polymers: Detailed analysis from GPC data

The GPC method is used widely to measure molecular weights of linear polymers. High-quality GPC data contains detailed information on many aspects of the polymer's molecular weight distribution (MWD). This information can be extracted from the data using computer analysis. Equations have been derived for the two simplest MWD functions in the GPC coordinates: the Flory function (one growing polymer chain produces one polymer molecule), and for the case when two polymer radicals combine into one polymer molecule. The equations were used to analyze MWD of two classes of polymers. The first class includes polymers with narrow MWD: polyethylene, ethylene-propylene and ethylene-hexene copolymers, syndiotactic polystyrene, and radical polystyrene. The second class includes polymers with broad MWD: ethylene copolymers and polypropylene produced with heterogeneous, Ti-based catalysts. The examples demonstrate that the resolution of complex GPC curves into their constitutents serve as an important source of information about kinetics of polymerization reactions. © 1995 John Wiley & Sons, Inc.     

Polystyrene fluorescence spectra. Molecular weight dependence in different solvents

The emission spectra from polystyrene samples of molecular weights ranging from 500 to 612,000 were measured in solvents of different thermodynamic character. The excimer to monomeric emission ratio depends upon the polymer molecular weight but is independent of the solvent. The molecular weight dependence is explained in terms of a distribution of the excimer traps among the macromolecules.     

Molecular weight distributions of industrially-produced poly-(epsilon-caprolactams) by gel permeation chromatography

Gel permeation chromatography with differential refractometry is used to obtain molecular weight distributions (MWD) of poly-(epsilon-caprolactams). Elution is carried out using an m-cresolchlorobenzene mixture (50:50, v/v) at 50 degrees C. MW values are obtained by a Hamielec-based calibration method, using broad-MWD poly-(epsilon-caprolactam) standards with the same chemical nature and similar MWD to the samples. Relative errors for the number-average MW (Mn) using this calibration method range from 0.4% (in the low polyamide MW range) to 20% (in the high polyamide MW range). These values are much lower than those obtained from narrow-MWD polystyrene calibration, which range from 39% to 78%. Similar values have been obtained for the other usual average MW parameters. The ability to obtain repeatability parameters for a given confidence interval and the utilization of statistical criteria for chromatogram rejection allow this method to be used in quality control for MWD of poly-(epsilon-caprolactams). Thus, production variables are related to polyamide-6 behavior in its ulterior treatment. Typical relative standard deviation percentages (for n=6) of a polyamide sample range from 1.9% (for Mn) to 3.3% (for M(z+1)).     

Histidine in enzyme active centers.

The presence of histidine in the active center of an enzyme can be demonstrated by kinetic measurements, chemical modification, NMR spectroscopy or X-ray structure analysis. Histidine is the only naturally occurring amino acid to contain an imidazole residue as a side chain. The role of histidine in enzyme catalysis depends, inter alia, upon the special features of the imidazole residue: it thus tends to form hydrogen bonds, combines donor and acceptor properties and can take part in either nucleophilic or base catalysis. In some of these enzymes the position of each atom is known; however, the theories as to how the catalysis proceeds at a molecular level are controversial.     

Molecular weight distributions and moments for condensation polymerizations characterized by two rate constants

Molecular weight distributions (MWD) and moments have been computed for condensation polymerizations of ARB-type monomers wherein the reactions involving the monomer are characterized by a rate constant k₁₁ and those involving other species, by k_p. For k₁₁/k_p < unity, the MWD curves are found to split into two, one for even n and one for odd values of n. Substantial amounts of unreacted monomer are found in the reaction mass for this case and so moments and the polydispersity index computed without the monomer are better representations of the distributions. For k₁₁/k_p > unit, conventional MWDs are observed with dispersions different from those representing polymerizations satisfying the equal reactivity hypothesis.     

Phase separation of a liquid coexisting with vapor in ternary systems

Model T-x ₁-x ₂ diagrams are considered for ternary systems with liquid-liquid phase separation in cases where invariant equilibria between two liquids and vapor exist in binary subsystems. Original Russian Text ? K.A. Khaldoyanidi, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 6, pp. 1052–1056.     

Dislocations as active centers in topochemical reactions

The kinetics and mechanism are discussed for the topochemical reaction of Al with Si produced by heat and localized pressure. The structure of the contact zone at the surface of the silicon is used to discuss the role of dislocations as active centers. The activations conditions for the silicon are deduced, together with the effective area of a center. The possible source of dislocations in deformed solids is discussed.     

Nucleation on active centers in confined volumes

Kinetic equations describing nucleation on active centers are solved numerically to determine the number of supercritical nuclei, nucleation rate, and the number density of nuclei for formation both of droplets from vapor and also crystalline phase from vapor, solution, and melt. Our approach follows standard nucleation model, when the exhaustion of active centers is taken into account via the boundary condition, and thus no additional equation (expressing exhaustion of active centers) is needed. Moreover, we have included into our model lowering of supersaturation of a mother phase as a consequence of the phase transition process within a confined volume. It is shown that the standard model of nucleation on active centers (Avrami approach) gives faster exhaustion of active centers as compared with our model in all systems under consideration. Nucleation rate (in difference to standard approach based on Avrami model) is equal to the time derivative of the total number of nuclei and reaches some maximum with time. At lower nucleation barrier (corresponding to higher initial supersaturation or lower wetting angle of nucleus on the surface of active center) the exhaustion of active centers is faster. Decrease in supersaturation of the mother phase is faster at higher number of active centers.     

Electrochemical preparation of polymers with multinodal molecular weight distributions

A direct electrochemical preparation of polybutadiene to a predetermined molecular weight distribution has been achieved. Butadiene is a good model compound, inasmuch as the rate of electrochemical initiation is high relative to the rate of propagation. Rate constants for the temperature interval of interest and for systems of comparable composition were reported and permitted a priori prediction of the weight fractions and the number average molecular weight of each fraction. Polymer was formed by a series of pulses which successively initiated polymerization, permitted growth in the absence of current, and electrochemically terminated polymerization. The polymers produced showed excellent agreement with calculated composition and distribution in the range of molecular weights 10,000 to 50,000.     

Heteropolyacids dispersed within a polymer matrix as a new catalytic systems with controlled oxidative-reductive and acid-base active centers

Polymer support such as polypyrrole was selected as a matrix for heteropolyacid H₅PMo₁₀V₂O₄₀ in an attempt to prepare heterogeneous catalysts containing two different active centers: protons and transition metal ions. Exchanging protons from heteropolyanions dispersed in polymer matrix into ferric or ferrous ions cause the modifications of their catalytic properties. It is manifested by decrease of activity of acid-base centers and increase of activity of oxidative-reductive centers. Oxidation state of iron in all samples before and after catalytic reaction is the same (Fe(III)), but their structure is not similar. For catalysts doped with ferric ions the structural order is much more pronounced than for these doped with ferrous ions.     

Radiative recombination rates with different statistical distributions

The isotope shift and hyperfine structure in a rhenium hollow cathode discharge was studied for transitions of the type 5d ⁵6s7s 5d ⁵6s6p and 5d ⁵6s6d 5d ⁵6s6p through Doppler-free saturation absorption laserspectroscopy and high resolution interferometry. Taking configuration mixing in the lower levels of 5d ⁵6s6p under consideration, we obtain average configuration isotope shift values for 5d ⁵6s7s of –1760(100) MHz and for 5d ⁵6s6d of –1970(200) MHz. These experimental values compare extremely well with the theoretically predicted configuration isotope shifts in rhenium, based on pseudo-relativistic Hartree-Fock calculations, of –1710 MHz and –1940 MHz, resp. In addition hyperfine structure constants for rhenium levels of 5d ⁵6s6d are reported here for the first time.Research scientist from the University of Istanbul, Turkey     

Molecular copper centers in potassium tantalate crystals

ESR spectra with resolved hyperfine and superhyperfine structures have been recorded in the 3-cm range at 4.2–800 K for copper-doped KTaO₃ crystals. The axial centers have three orientations. The principal values have been determined for the g tensor and for the hyperfine and superhyperfine interactions tensors. The g factor is explained in a simple crystalline-field model incorporating covalency effects. An MO LCAO model is used to interpret the hyperfine structure and to determine the bond covalencies along the axis of the CuO ₄ ^2– molecular centers and also for bonds perpendicular to that axis. The numbers of lines and the magnitudes and angle dependences for the superhyperfine splittings reveal details in the molecular structure of these centers. The scope for interpreting the results in terms of Jahn-Teller centers is discussed. The measurements do not indicate features characteristic of Jahn-Teller centers, so preference is given to the molecular-center model.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 562–568, September–October, 1988.