Processing of boron-nitrogen preceramic polymers

A number of new oligomeric and polymeric reagents rich in boron and nitrogen have recently been derived by several research groups. These reagents offer potential for the formation of boron nitride (BN) as controlled morphology powders as well as nonpowder forms. Specifically, it has been found that a family of 3-point and 2-point poly(borazinylamine) reagents may be processed by techniques used in organic polymer chemistry, with subsequent formation of BN ceramic fibers, coatings, xerogels, aerogels, and aerosols.     

Flow behavior of a cationic biopolymer: Chitosan

Summary The effect of ionic strength (0.1 M [Cl] 0.4M), pH (3.0 pH 5.0) and shear rate (up to 1000 sec^–1) on the rheological response of concentrated chitosan solutions have been studied. It has been observed that viscosity increases with increasing chitosan concentration and that a shear thinning behavior is present for polymer concentrations above 0.50 g/dl. Also, it has been shown that the zero shear viscosity is independent of the ionic strength of the media, but increases as pH is increased. This behavior has been related to the role of the surface charge density on the chitosan backbone upon the intermolecular entanglement which control the rheological behavior of concentrated chitosan solutions.     

Weak Gel Behaviour of Poly(vinyl alcohol)-Borax Aqueous Solutions

Thermal transition of PVA-borax aqueous gels with a PVA concentration of 60 g/L and a borax concentration of 0.28 M was investigated at temperatures ranging from 15 to 60C using static light scattering (SLS), dynamic light scattering (DLS), and dynamic viscoelasticity measurements. Three relaxation modes, i.e. two fast and one slow relaxation modes, were observed from DLS measurements. Two fast relaxation modes located around 10^–310¹ sec, with one fast mode (_f1) being scattering vector q-dependent and the other fast mode (_f2, with _f2>_f1) being q-independent. The _f1 mode was attributed to the gel mode whilst the _f2 mode could be due to the hydrodynamics of intra-molecular hydrophobic domains formed by uncharged segments of polymer backbones. The slow relaxation mode with relaxation time located around 10¹10³ sec in DLS data was due to the motion of aggregated clusters and was observed only at temperatures above 40C. The amplitude and relaxation time of slow mode decrease as temperature is increased from 40 to 60C. At temperatures below 40C, no slow relaxation mode was observed. The SLS measurements showed PVA-borax-water system had fractal dimensions D _f2.4 and D _f2.0 as temperature was below and above 40C, respectively. The simple tilting test indicated gel behaviour for the PVA-borax aqueous system at temperatures below 40C with a creep flow after a long time exposure in the gravity field. But the dynamic viscoelasticity measurements demonstrated a solution behaviour for PVA/borax/water at temperatures below 40C, the critical gel point behaviour for G() and G() was not observed in this system as those reported for chemical crosslinked gels. These results suggest that the PVA-borax aqueous system is a thermoreversible weak gel.     

Inter-versus intramolecular oligomerization of nitrile groups in polyacrylonitrile

Summary Experimental evidence is presented indicating that intermolecular oligomerization of CN groups (leading to crosslinked structures) takes place, and possibly has an even greater importance than the intramolecular reaction (leading to ladder polymer), at higher temperatures. The easy, quasi built-in crosslinking upon heat treatment is considered the most significant single feature with regard to making polyacrylonitrile and its copolymers unique among synthetic polymers as a carbon fiber precursor.     

Combustion Modes of a Strongly Diluted Ti + 2B System

Combustion modes of a Ti + 2B system with a high mass content of copper and iron (63–83%) are studied for various initial temperatures and compositions of the specimens and the particle sizes of the reagents. The parametric domains of existence of highvelocity and lowvelocity layerbylayer and spin combustion modes are found.     

Convective Regime of Filtration Combustion of Energetic Materials in a Cocurrent Flow of Their Combustion Products

The convective regime of filtration combustion of energetic materials in a cocurrent flow of their combustion products is studied using a model with extremely simplified kinetics and heat transfer, which shows instability of the process. It is shown that the more accurate twotemperature model describes a steadystate regime. In this regime, the gas temperature on the hot boundary of the heating zone is well below the combustion temperature, and the solidphase temperature is well below the temperature proposed in recent studies on this topic. It is pointed out that the twotemperature approach is unjustified and intragranular nonisothermicity must be taken into account for convective regimes. It is shown that the threetemperature model, which takes into account this effect, does not give a stable steadystate solution.     

Water vapor absorption and aqueous retention of hydrolyzed starch polyacrylonitrile graft copolymers — “superabsorbent starch”

Summary Starch copolymers with polyacrylonitrile grafts were prepared using the Mn³⁺ initiation method and different amounts of acrylonitrile. The product was hydrolyzed in aqueous alkali (nitrile groups to amide and carboxyl groups) to enhance the hydrophilic character of the copolymer and achieve superabsorbent properties. At relative humidities (RH) up to 95–97 % the modified starch shows normal water vapor absorption like cellulose and other polysaccharides. When in contact with liquid distilled water, the modified starch forms a superabsorbent gel which retains very large amounts of water (up to 800 g/g). At low and high pH and in aqueous salt solutions (NaCl, CaCl₂ and synthetic urine) the water retention is strongly reduced. This is interpreted as due to the ionic strength (salt effect) and for Ca²⁺ ions probably also crosslinking by salt formation with carboxyl groups on adjacent chains or chain segments.     

Characterization of some organic poly(aminiumphosphate)s

Summary Metachromatic reactions of MB and AO are studied with poly(aminium phosphate)s. The main band() at 665 m and 492 m for MB and AO are shifted to 580 m and 455 m respectively (-bands). The metachromatic values at different concentrations of poly(aminium phosphate)s are also reported. The dye association for MB with these polymer derivatives is investigated conductometrically as well. It is observed that MB can be at the most bind with the phosphate residue of polymer upto 11 molar ratio. The polymeric nature was further established by paper chromatographic technique in Terry's, Ebel's & Pfrengle's solvents.     

Solutions of the general equations for the radiation dose dependence of the molecular weights of irradiated polymers with an initial Schulz-Zimm distribution

Summary The general equations for the radiation dose dependence of irradiated polymer molecular weights have been solved exactly. For an initial most probable molecular weight distribution (=1), the solutions are analytical and exact. For the general case (1) the solutions are numerical and exact. The present approach has resulted in the solutions for both =1 and 1 being incorporated into a group of FORTRAN computer programs which will solve experimental data for scission and crosslinking yields by both minimization and exact treatments. Simulated data treated using these FORTRAN programs are give. The FORTRAN programs are available from the authors.     

Poly(β-malic acid) : a new polymeric drug-carrier

Summary Poly(-malic acid) is a new synthetic functional polyester of the poly(-hydroxy-acid)-type whose properties are investigated in regard to possible uses as bioresorbable polyvalent drug-carrier. Degradation of polymer chains in 0.15 N phosphate buffer at pH=7.5 is monitored by aqueous GPC on SEPHADEX gels and by enzymatic titration of ultimate degradation products. It is shown that the rate of degradation obeys first order kinetics at the begining and that poly(-malic acid) degrades to malic acid at last.Presented at the 26th IUPAC Microsymposium on Macromolecules: Polymers in Medicine and Biology, Prague, July 9–12, 1984     

Dynamics and self-organization of catalytic systems

The kinetics of a catalytic reaction is frequently formulated in terms of simple concepts of the Langmuir type. Apart from limitations arising from the non-uniformity of the catalyst's surface and from the coverage dependence of the rate constants, several other complications may come into play. These may arise on the quantum level where energy flow between the various degrees of freedom may cause failure of simple transition state theory, as well as on the continuum level where formulation of rate equations in terms of coupled non-linear differential equations may give rise to a rich scenario of spatio-temporal self-organization, including kinetic oscillations, chaos, and formation of concentration patterns. Several of these phenomena are illustrated by selected examples.     

Synthesis of nanosized platinum cluster in cubic mesoporous material via a direct introduction method

Platinumclustercontaining cubic mesoporous material (PtMCM48) has been synthesized by direct introduction of chloroplatinic acid during the synthesis of MCM48. In addition, we have also studied the incipient wetness impregnation and ionexchange method to obtain the platinumcontaining cubic mesoporous material. The nature of the platinumMCM48 catalyst has been characterized by different techniques such as XRD, N₂ adsorption, TEM, XPS, and NMR. The catalyst obtained by direct introduction of platinum in the synthesis gel shows higher activity in the hydrogenation of benzene and toluene.     

Physico-chemical studies of polymeric carriers

Summary Acid-base titration of poly/N-vinylpyrrolidone-co-maleic acid/ alternating copolymer was followed by densitometry in order to detect chain conformation transition found potentiometrically. Apparent molar volume () values of the polymer were calculated for step-by-step titrated polymer samples. The effect of copolymer concentration on the conformational transition was also determined by measuring water diluted samples of each titration step. A continous increasing of was found in the case of KOH titrant, both in titration and in dilution. When NaOH was applied as a titrant, a very different profile was obtained. A minimum in vs. concentration occured in the vicinity of the half neutralization point of the dibasic polyacid.     

Note of the estimation of the Θ temperature and Ψ parameter from the critical temperature data

Summary Critical values of the polymer volume fraction _2,c and the interaction parameter _c have been computed for the case that the equation for the chemical potential of solvent contains terms _{c 2} ³ and _{c 2} ⁴ in addition to ₂ ² . For 0 _c 1/3, the limits for infinite chain length are _2,c = 0 and _c = 0.5. Quite different results are obtained for _c > 1/3, _2,c being finite and _c lower than 1/2. Conclusions for the estimation of the temperature and the entropy-of-dilution parameter are discussed.     

Mechanism of polyacrylonitrile fibre spinning by the thiocyanate method

Conclusions 1. The process of spinning PAN fibres by the thiocyanate method into baths with various precipitating powers has been studied. The mildness and severity of the baths have been varied by varying the NaSCN concentration from 12 to 24%, and also varying the temperature from 5 to 20°C.2. A dependence of the threshold precipitation concentration on polymer content of the spinning solution has been shown. At a polymer content of 13.5%, the threshold precipitant (water) concentration at which precipitation begins is 63%.3. At NaSCN concentrations above 22–24% in the precipitation bath, a precipitant concentration is established on the surface of the forming fibre at the moment of contact between the spinning solution and the precipitation bath which is below the threshold value, and precipitation begins after a certain time, during the course of which, as a result of diffusion, the precipitant concentration is raised to the threshold level.4. On softening the spinning conditions by increasing the NaSCN concentration in the precipitation bath up to a certain limit, a fibre is formed with a more perfect fibril structure, which ensures obtaining a stronger carbon fibre.5. Extreme softening of the spinning conditions leads to an increase in fibre porosity. The suggestion is advanced that this phenomenon is connected with a change in the mechanism of spinning solution phase breakdown where, along with polymer precipitation in the form of a solid phase, partial precipitation in the form of a liquid phase takes place.Translated from Khimicheskie Volokna, No. 5, pp. 3–7, September–October, 1993.     

Kinetics and Mechanism of Separation of Water from Polybenzothiazole Fibres

The kinetics and mechanism of separation of water from polybenzothiazole fibres was investigated by mass spectrometry. It is shown that weakly bound and strongly bound water of different origins is separated from fibres during thermal drawing.     

The photochromism of nitrospiropyran included in γ-cyclodextrin

Summary The inclusion compounds of 1, 3, 3-trimethyl-6-nitrospiro-[2H, 1-benzo-pyran-2, 2-indoline] (NSP) with -cyclodextrin (-CD), 2,3,6,0-permethylated -CD (-MCD), and naphthyl modified -CD (-NCD) showed normal photochromism in the solid state with high light sensitivity comparable with that of NSP dispersed in polymer matrices. This is in sharp contrast with NSP in the crystalline state, which is much less photosensitive. These results suggest that the host cavity offers a sufficient free volume for phototransformation of the guest molecule. The colored form of the inclusion compounds were found to be more stable both chemically upon prolonged UV irradiation and thermally in the dark as compared with that in a PMMA film.     

Textural and structural studies on nickel hydroxide electrodes. II. Turbostratic nickel (II) hydroxide submitted to electrochemical redox cycling

The textural and structural modifications involved in electrochemical redox cycling of turbostratic nickel (II) hydroxide has been investigated using X-ray diffraction and electron microscopy methods. It was found that during the first cycles, different phenomena compete: redox reactions which occur in the solid state, and ageing reactions via the solution. For the first galvanostatic charge performed at the C/5 rate in 4.5 N KOH, the direct oxidation of (II) to (III) and the ageing of (II) to (II) via the solution followed by the oxidation to (III) are in competition. The study of the discharge mechanism shows that the direct reduction (III)(II) is parallel to the reduction (III)(II) and the ageing of the turbostratic hydroxide via the solution. After the first cycle it was established that the alpha-generated (II) active phase consisted of a mixture of two kinds of particles, the oxidation of which follows two paths: (II)/(III) for the thicker particles and for the thinner (II)/(III), but these latter (II) particles aged via the solution by Oswald ripening and the (II)/(III) couples swung to (II)/(III).     

Texture prediction of Bi2Te3 electroplated layers using Hartman's theory of crystal growth

An experimental study of electroplated thin films of bismuth telluride, R3m crystal structure, shows that fibre textures are observed during the growth of the layers. Several orientations 00.1, 10.10, 11.0, 10.4, 10.8, 01.5 and 10.0 are developed during the deposition process, then reach a final orientation. Hartman's theory of crystal growth based on the periodic bond chain (PBC) in a crystal lattice is usually applied. This explains the equilibrium shape of a crystal defined by F faces and the faces observed during the growth (S and K faces). It also successfully predicts the orientations observed in bismuth telluride from the beginning to the end of the layer deposition. The theory may also be applied to other deposition processes.     

Effect of composition dependent Flory interaction parameter χ on polymer adsorption theory

Theories of polymer adsorption which neglect the functional dependence of Flory interaction parameter on polymer concentration may result in inconsistent predictions for the adsorption quantities. In this paper, the effect of variation of Flory interaction parameter with concentration on polymer adsorption at solid/solution interface is discussed. Comparison with the results for constant shows that this effect is significant and cannot be ignored.