Computations of ESR Coupling Constants in Organic Radicals. II. Nitro derivatives of the phenyl radical

We compute the ESR coupling constants and the geometries of a set of nitro derivatives of the phenyl radical. We consider the three mononitrophenyls, the six dinitrophenyls and 2,4,6-trinitrophenyl. The computations are based on the use of the Gaussian 82 Program Package with the STO-3G basis set. There are no experimental data available.     

E.p.r. studies on poly(paraphenylene)s synthesized by two different routes

Polyphenylene has been prepared using two different polymerization procedures. We show that, if the two materials are very similar from a structural point of view, their magnetic properties as seen by e.p.r. are very different. We show that the existence of unpaired spins in one of these polymers is correlated with structural defects which are probably created during the polymerization process.     

Comment on “Self-diffusion in high-purity α-Al2O3: Comparison of Ti-doped,Mg-doped and undoped single crystals” P. Fielitz,S. Ganschow,K. Klemens,and G. Borchardt,J. Eur. Ceram. Soc., 41, (2021), 663–668”

We comment on recent observations of O and Al self-diffusion in single-crystal sapphire with variable doping concentrations of Mg and Ti by Fielitz et al [1]. The paper reports a null effect of aliovalent doping on oxygen diffusivity. We posit that the extensive heat treatment involved in their experimental protocol may have caused dopant evaporation near the surface, and therefore the null result in oxygen diffusivity, whereas an effect on Al diffusivity is still discernible due to a greater diffusion depth of Al. We propose that buffering mechanisms are ultimately responsible for modest increases of self-diffusion with respect to dopant concentrations; in the Mg-doped case, DFT calculations suggest that negatively charged Mg interstitial defects are the principal charge compensating defects for positively charged Mg substitutional ions.     

Thermal decomposition of additive polymerization initiators. I. Azobisisobutyronitrile

We measured the rate of thermal decomposition of azobisisobutyronitrile (AIBN) from the rate of loss of 2,2-diphenylpicrylhydrazyl (DPPH) on reaction of the free radicals formed by thermal decomposition of azobisisobutyronitrile (AIBN) in organic solvents with DPPH. There has been some doubt about the quantitative relation between AIBN and DPPH required to get the rate constant of thermal decomposition of AIBN. In the past, AIBN has been used in excess, and the rate constants measured by using DPPH are smaller than they ought to be. In our experiments we used as little of AIBN relative to DPPH as possible and obtained a value of 3.8 × 10^?4 min.^?1 at 50°C. as the rate constant of the first-order reaction, which was shown satisfactorily by the linearity of the graph. We also obtained 32 kcal./mole as activation energy. We were also able to make clear the meaning of the result when a large quantity of AIBN was used.     

Epoxy polymers. V. Dielectric strength

This report deals with material properties which determine dielectric strength rather than phenomena that occur during breakdown. We propose that the determining factors are the same for liquids and polymers. We report dielectric strengths calculated from measured breakdown potentials and sample thicknesses for liquids, ionic solutions, and epoxy polymers doped with ionizable materials. The breakdown field (in liquids and polymers) is hypothesized to be that which generates some critical current density value in the dielectric. The reasons for this value being critical and characteristic of the material are unknown; however, the ionic current density that the measured breakdown field produces can be calculated from the Onsager theory of high-field electrolytic conduction, and this ionic current density is assumed to be the critical threshold quantity involved. Because of field effects on the dissociative ionization and conductivity of electrolytes in organic systems, the breakdown field varies with concentration and solute species, but the ionic current density associated with the breakdown field is essentially constant for a wide range of species and molar concentrations in a given solvent.     

A Monte Carlo simulation study of the mechanical and conformational properties of networks of helical polymers. I. General concepts

We study the mechanical and conformational properties of networks of helical polymers with a combination of Monte Carlo simulations based on the Wang-Landau algorithm and the three-chain model. We find that the stress-strain behavior of these networks has novel features not observed in typical networks made of synthetic polymers. In particular, we find that as these networks are stretched they first strengthen, then soften and finally strengthen again. This non-monotonic behavior of the stress correlates with the one of the helical content and is rationalized by the elongation-induced formation and melting of the helical structure of the polymer. We complement these results with a study of other conformational properties of the polymer strands that clarify the molecular mechanisms behind the mechanical behavior of these networks. Finally, we present a qualitative comparison of some of our results with the theoretical ones recently reported by Kutter and Terentjev.     

Equilibria in the alcoholysis reactions of terephthalic esters and chemical valorization of polyethyleneterephthalate waste. I. Equilibrium constants determination

The depolymerization of polyethyleneterephthalate (PET) by alcoholysis is an easy operation and gives interesting prospects for the valorization of wastes. The reactive species being composed of esters and alcohols, all possible alcoholysis reactions happen, whether wanted or not. Finally, a complex blend of many molecules follows. Practically, two great types of reactions occur: a reaction called “interchange,” and a reaction called “polycondensation.” We have determined the values of global equilibrium constants of those two types of reaction. The values of polycondensation equilibrium constants are close to those estimated from the “equireactivity” principle. We did not observe any particular behavior of monomer species. When reactants are the di‐ or mono‐propylene glycols, the molar proportion of these glycolic radicals is higher in the free glycols than in the polyester chains. Both proportions are similar, when diethylene glycol or 2‐ethylhexanol are used. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 329–340, 1999     

Planar Brush of End-Tethered Molecular Bottle-Brushes. Scaling Mode

We present a scaling theory describing structural properties of brushes formed by molecular bottle-brushes tethered by the ends of the main chains to inert planar surface. We consider bottle-brushes where q ≥ lateral side chains (grafts) emanate from each of the branching points regularly distributed along the main chain (backbone). Such macromolecule can be envisioned as an array of polymer stars whose centres are connected by flexible spacers into a necklace. We demonstrate that such starlike mode of branching leads to novel features both in conformations of individual bottle-brushes in solution and in appearance of additional scaling regime for brushes formed by bottle-brushes tethered to the surface.     

Viscoelasticity of ionomer gels: 2. The elastic moduli

The unusual viscoelastic properties of metal sulphonate- (or metal carboxylate-) containing ionomers in semidilute solutions with non-polar solvents are reexamined. At fairly low polymer concentration of relatively short chains, these ionomer solutions show, besides the extremely high-viscosity behaviour studied in an earlier paper, an extensive plateau in the storage modulus and a cusp in the loss modulus, both characteristic of a highly entangled polymer melt or concentrated solution.We argue that, owing to the temporary crosslinks, the polymer chains are moving inside a network made up by the other crosslinked chains and hence undergo one-dimensional diffusional motions (reptation). The extensive plateau then arises from the disengagement of the chain from a deformed tube, as in the Doi-Edwards theory of melts and concentrationed solutions. We derive expressions for the storage and loss moduli which are in qualitative agreement with the experimental curves.     

Peroxide modification of polyethylene. Prediction of molecular weight distributions by probability generating functions

We present a mathematical model able to describe the complete molecular weight distributions of polyethylene during reactive modification by organic peroxides. The method is applicable to batch processes, such as modification in a press or a plug-flow extruder, and in its present form is valid up to the gel point. We apply probability generating function definitions to the mass balances of radical and polymer species in the reacting medium. Three different probability generating functions are applied, each one directly applicable either to the number, weight or chromatographic distributions. These generating functions are numerically inverted to obtain the corresponding calculated molecular weight distribution. Two different inversion methods are used, and their relative performances analyzed. Predictions are compared with qualitative experimental data obtained in a press. Model predictions on molecular weight distributions are in agreement with experimental trends.     

Some Problems of Structure Formation in Self-organizing Liquid Oligomer Systems. II. Theoretical Models for Formation of Super-molecular Structure of Oligomer Systems

Abstract

The driving force for self-organization are the fundamental laws of thermodynamics. We consider some theoretical models which qualitatively explain anomalous properties of these systems.     

Barrier and surface properties of polyurethane-epoxy interpenetrating polymer networks. II

We have measured the advancing contact angles of drops of methanol-ethylene glycol mixtures on films of previously studied polyurethane-epoxy interpenetrating polymer networks. The extrapolated critical surface tensions were in excellent agreement with those obtained from advancing contact angles of drops of water-methanol mixtures. A sharp minimum is observed in the critical surface tension at network compositions where we have previously found maxima in ultimate mechanical properties. We advance a physical explanation based on unrelieved surface strains. We have also measured the toluene vapor transmission (permeability, diffusion and sorption coefficients) in these films. These results, together with the previously obtained water vapor permeabilities, are in complete accord with the expected morphologies of these networks.     

Adsorption of polymers at the solid-liquid interface: a comparison of the e.p.r. and i.r. techniques

Several experimental techniques are available for studying the adsorption of polymers at the solid-liquid interface, though each is subject to considerable uncertainty, either in measurement or interpretation. We have used electron paramagnetic resonance (e.p.r.) to study the adsorption of PVP and PMMA on carbon and silica, and compared the results with both n.m.r. and i.r. l.r. measures the actual number of segments attached to the surface whereas magnetic resonance measures the number of segments immobilized by close association with the surface. For PMMA adsorbed onto silica from chloroform, e.p.r. indicated a flatter configuration than did i.r., a result consistent with alternative segments being bound to the surface. Differences in flexibility between PVP and PMMA may have given rise to different configurations at the silica—chloroform interface. Conditions under which e.p.r. can be used to study the adsorption of polymers were investigated.     

The use of modeling to understand the impact of screening on U.S. mortality: examples from mammography and PSA testing

Surveillance data represent a vital resource for understanding the impact of cancer control interventions on the population cancer burden. However, population cancer trends are a complex product of many factors, and estimating the contribution of any one of these factors can be challenging. Surveillance modeling is a technique for estimating the contribution of one or more interventions of interest to trends in disease incidence and mortality. In this article, we present several approaches to surveillance modeling of cancer screening interventions. We classify models as biological or epidemiological, depending on whether they model the full unobservable aspects of disease onset and progression, or models which reduce the complex process to simpler terms by summarizing portions of the disease process using mostly observed population level measures. We also describe differences between macrolevel models, microsimulation models and mechanistic models. We discuss procedures for model calibration and validation, and methods for presenting model results which are robust with respect to certain types of biased model estimates. As examples, we present several models of the impact of mammography screening on breast cancer mortality, and PSA screening on prostate cancer mortality. Both these examples are appropriate uses of surveillance modeling, even though for mammography there is extensive (although somewhat controversial) randomized trial evidence, whereas for PSA this biomarker has seen extensive use as a screening test prior to any controlled trial evidence of its efficacy. Some of the models presented here were developed as part of the National Cancer Institute's Cancer Intervention and Surveillance Modeling Network.     

Multiperiod design of azeotropic separation systems. I. An agent based solution

In this paper, the first in a two-part series, we explore an agent based solution approach to the multiperiod design of azeotropic separation systems. Asynchronous Teams (A-Teams), a software organization that has been successfully applied to combinatorial optimization problems in other disciplines, is applied to this chemical process system design problem. A-Teams is a framework in which multiple design techniques can work harmoniously in the solution of a problem too demanding for any one method. We introduce a simple architecture for constructing A-Teams that incorporates the use of persistent data. Several approaches for generating improved multiperiod azeotropic design solutions are developed and are used simultaneously in a large-scale demonstration. The software organization is found to be capable of developing and improving a population of preliminary designs for the multiperiod azeotropic design problem.     

Brain cholesterol. XI. A review of biosynthesis in adult mice

During the past decade, our laboratory has been involved in studying the biosynthesis of brain cholesterol under various conditions. As a result of these studies, we feel that the hypothesis of metabolic stability of cholesterol in adult life is untenable. Our data suggest that there are several compartments of sterol metabolism ranging from extremely fast to metabolically very slow. It is our hypothesis that (a) brain function is more nearly associated with fast turnover compartments than with metabolically slow ones; (b) these compartments can be altered by a variety of stress conditions. Current experimental work being done with monkeys and baboons, and in human fetii suggest that conclusions formed about the mouse generally are applicable to these higher evolutionary forms. What remains unsolved is the relationship between cholesterol metabolism and brain function. We continue to search for answers to physiological psychology.     

Interpreting the appearance of dispersed systems: II. A guide for surfactant systems

We first analyze how color and transparency are perceived. Drawing on principles of light scattering investigated in Part I, we suggest simple rules and procedures in a diagnostic form for visually observ-ing fluid surfactant systems to estimate sizes of dispersed particles. Rules and procedures are organized into a guide, the use of which we illustrate by observing certain important surfactant systems. We conclude that it is possible to estimate particle sizes in the Rayleigh, Rayleigh-Debye-Gans, and Mie scattering regimes from such observations alone.     

Epoxy resins based on trimethylolpropane. I. Determination of chemical structres

The chemical structure of an aliphatic epoxy resin was studied by using gel permeation chromatography (GPC), Fourier transform infrared (FTIR), FTIR-TGA (thermogravimetric analysis), mass spectrometry in fast atom bombardment (FAB) mode, and wet analysis. We found that when trimethylolpropane is used as starting monomer, only two OH groups are epoxidized. The resin and its different fractions have a functionality of two. Under the conditions of synthesis used by the supplier, three main products were found with masses of 246,339, and 430. No trifunctional product was detected. © 1995 John Wiley & Sons, Inc.     

Polymer dissolution. I. On the dissolution behavior of copolymers of methyl methacrylate and methacrylic acid

Copolymers of methyl methacrylate with methacrylic acid [P(MMA/MA)] are interesting resist materials for microlithography. At moderate baking temperatures, these copolymers undergo an intramolecular cyclization, yielding terpolymers containing anhydride moieties. This process has striking consequences for the dissolution behavior and results in higher ketone solubilities. The dissolution rates in methyl ethyl ketone (MEK) and in mixtures with ethyl glycol (EG) correlate in an Arrhenius-like manner with the reciprocal baking temperatures. Two distinct temperature ranges with different slopes between 130–180°C and 180–230°C are found. This is in good agreement with other findings, indicating a different mechanism of anhydride formation in these temperature regions. The activation energies for the dissolution of P(MMA/MA) in EG or EG/MEK mixtures are about 17 kcal/mol and those of the thermally treated material in MEK or MEK/EG or mixtures of MEK with methyl isobutyl ketone (MIBK) are in the range of 20–30 kcal/mol. For the investigation of the M_n dependence, γ-irradiated probes of the copolymer were used. We obtained the usual exponential M_n dependence with an exponent of 0.7–0.8 in MIBK and 0.3–0.4 in EG. Our findings are in agreement with a “relaxation-controlled” dissolution behavior, especially for the anhydride-containing terpolymer. No residual layers or pronounced inhibition periods indicative for gel-layer formation, however, could be found. We suggest a normal dissolution process with a very small gel layer. For the copolymer in an alcohol-containing solvent, a stress-driven dissolution behaviour is more likely. © 1994 John Wiley & Sons, Inc.     

A class of benzenoid chemicals suppresses apoptosis in C. elegans

Benzene is a human carcinogen that might act through both genotoxic and nongenotoxic mechanisms to promote tumorigenesis. The genotoxic effects of benzene are well established, however, its potential nongenotoxic roles in carcinogenesis are poorly understood. We find that benzene suppresses somatic apoptosis in C. elegans; this suggests a potential nongenotoxic mechanism by which this chemical might promote tumorigenesis. We find that two other benzenoid chemicals, biphenyl and toluene, also inhibit apoptosis in C. elegans. Notably, these chemicals are suspected carcinogens in mammals; this suggests that a subclass of benzenoid chemicals might promote tumorigenesis by suppressing apoptosis. A benzene metabolite, 1,4-benzoquinone, can directly inhibit the activity of caspase-3; this suggests a general molecular mechanism by which benzenoid chemicals might suppress apoptosis. These findings suggest that C. elegans is an excellent alternative animal model for studying the antiapoptotic activity of tumor-promoting chemicals and for identifying in vivo targets of these chemicals.