Short branch effects on the creep properties of the ultra-high strength polyethylene fibers

Short branch (methyl branch) effects on the creep properties of the ultra-high strength polyethylene fibers were investigated. The temperature and the stress dependence of creep rates of several high-strength polyethylene fibers having different branch contents, which were prepared by blending of two polymers of highly and less branching, was evaluated according to a model described by Ward and Wilding, and the activation energies and the activation volumes were calculated in terms of their methyl branch contents and tensile moduli. The creep rates of ultra high strength fibers are strongly influenced by their methyl branch contents. The typical branching sample with ca.6 CH₃ units per 1000 CH₂ units shows ca. 1/20 lower creep rate than that of the less branching (1.0 CH₃ per 1000 CH₂) fiber sample at the room temperature. The activation energies of creep rate obtained by those highly branching samples are higher than those of lesser branching samples; the difference is nearly proportioal to their branch contents. Wide-angle x-ray diffraction results showed that the dimension of a-axis of unit cell increases in proportion to their branch contents. These results imply that the creep mechanism of ultra-high strength polyethylene fibers is dominated by chain slippage in the crystalline part, and also imply that some amount of methyl branch sites can be incorporated in the crystalline part in proportion to the branch content and those incorporated branch sites hinder the slippage motion of molecular chains in crystalline part, which results in the extreme lower creep rate. © 1994 John Wiley & Sons, Inc.     

Thermal decomposition of branched silanes: a computational study on mechanisms

The initial steps of the thermal decomposition of silanes in the gas phase were examined by DFT-B3LYP calculations, with particular attention being paid to the way in which the reactivity pattern changes with the degree of branching of the silane. Besides the established pathways-1,2-hydrogen shift, H(2) elimination, and homolytic dissociation-1,3-hydrogen shift was also explored as an initial reaction step which leads to disilene structures. Subsequent silylene insertion and initial steps of radical chain reactions were also studied. To estimate the energetic changes with temperature, various reaction free energies and the corresponding activation free energies up to 650?°C were calculated. Accordingly, the leading reaction channel at room temperature is 1,2-hydrogen shift with subsequent silylene insertion; for higher degrees of branching, competing pathways (homolytic dissociation, 1,3-hydrogen shift, and radical polymerization) gain in relative importance. At high temperatures, the rate-determining step changes to homolytic dissociation, and thereby the apparent rates of decomposition become dependent on the degree of branching.     

Short-chain branching in γ-radiation-induced polymerization of ethylene

In an attempt to elucidate the mechanism of chain-branch formation in the polymerization of ethylene, the effect of reaction conditions on short-chain branching in γ-radiationinduced polymerization of ethylene was investigated by using infrared spectroscopy. The concentration of methyl groups, i.e., the frequency of short-chain branching, increases with temperature and pressure and is independent of ethylene conversion to polymer and radiation intensity. The number of methyl groups per polymer molecule increases almost proportionally with the degree of polymerization. These facts indicate that short-chain branching occurs mainly by the mechanism of intramolecular hydrogen transfer. The effect of pressure on the rate of chain branching can be postulated by considering the transition state to be six-membered rings in hydrogen transfer reactions. The activation energy of chain branching is found to exceed that of propagation by 6 kcal./mole.     

RHEOLOGICAL BEHAVIOR OF POLYPHENYLENE SULFIDE/POLYAMIDE-66 BLENDS

Blends of polyphenylene sulfide (PPS) containing trace amounts of branching and/orcross-linking in chain and Polyamide-66(PA-66) have been prepared by melt blending. Therheological behavior of PPS/PA-66 blends has been studied by means of capillary rheo-meter, and compared with PPS. The effects of shear rate, shear stress and temperature onthe flow of PPS/PA-66 blends and PPS are discussed. The non-Newtonian indexes andthe activation energies of viscous flow are obtained. The results show that the apparentviscosity of PPS/PA-66 blends is not sensitive to shear rate and stress, but decreases withthe elevation of temperature. On the contrary, the apparent viscosity of the PPS decreasesobviously with the increasing of shear rate and shear stress, but it is increased by theelevation of temperatue.     

Synthesis, structure, and properties of branched polymethacrylates

The methacrylate: branching agent: chain growth regulator optimal ratios that allow the synthesis of branched polymethacrylates via the crosslinking free-radical copolymerization under the regime of conventional or catalytic chain transfer have been estimated. Relationships between the molecular-mass characteristics of the copolymers, their content of intact C=C bonds, the composition of the starting monomer mixture, and the structure of the branching agent and polymer chain growth regulator have been established. The rheological properties of the branched MMA-based copolymers have been studied. It has been shown that the copolymers are characterized by a weaker dependence of reduced viscosity on the polymer concentration in solution than that for the linear PMMA. The diffusion-sorption behavior of the branched polymethacrylates is determined by the content of the branching agent in them.     

Vacuum and oxidative pyrolysis of poly-p-xylylene. II. Chromatographic analysis and kinetics of reaction products

Reaction products of vacuum and oxidative degradation of poly-p-xylylene have been quantitatively determined by chromatographic analysis as function of time, temperature and oxygen pressure. Respective Arrhenius parameters were also ascertained for some of the reaction products and for the sums of all products. The energies of activation for the sums agree quite satisfactorily with the energies of activation obtained previously by uninterrupted experiments in quartz-spoon reaction vessels. The results found here can be described in terms of mechanisms previously postulated on the basis of the total loss in weight (or volatile production) data. Scission of “weak” links (due to abnormal structures) takes place followed by formation of various products. The whole process is governed by the initial chain scission reaction; however, the energies of activation for each of the products do not need to be identical with that of the chain scission reaction. Each product is formed by a reaction which has its own characteristic number average kinetic chain lengths; the latter have their specific energy of activation values. Oxidative degradation produces the same organic compounds as vacuum degradation and in addition CO, CO₂, and H₂O. Oxidized intermediate compounds are apparently fairly rapidly decarboxylated and decarbonylated. Oxidative chain scission is appreciably faster than that in vacuum. Almost simultaneous “weak” link and “normal” chain scission are taking place initiating the formation of a number of products.     

Mechanisms of propagation,transfer, and short-chain branching reactions in the free-radical polymerization of ethylene

The propagation, transfer, and short-chain branching reactions in the free-radical polymerization of ethylene were studied at temperatures of 20–80°C. under pressures of 160–400 kg. cm.² by means of two-stage polymerization with the use of a specially designed reaction vessel. In the first stage, the polymerization was carried out in the presence of AIBN as the initiator, and in the second stage, the propagation occurred with living radicals in the absence of the initiator. In the second stage the polymer yield is shown to increase with reaction temperature and pressure, and the molecular weight of the polymer reached constant values which were dependent upon the temperature when the contribution of the polymer formed in the first stage was very small. It is shown that in the second stage the rate of propagation, transfer, and short-chain branching are all proportional to the second power of ethylene fugacity, and that the activation energies of these reactions are 5.7, 23.4, and 10.9 kcal./mole, respectively. The polymer has no terminal vinyl group. The mechanism of these reactions is discussed on the basis of kinetic and energetic results.     

Bioinformatics Analysis of Oligosaccharide Phosphorylation Effect on the Stabilization of the β-Amylase Ligand Complex

Starch is the most abundant storage carbohydrate produced in plants. The beginning of transitory starch degradation in plants depends mainly on day cycle, posttranslational regulation of enzyme activity, and starch phosphorylation, but the molecular mechanism of these factors' influence is not yet precisely described. The aim of our analysis was to investigate the effect of phosphorylation on the intermolecular energies for stabilization of the complexes between the set of phosphorylated and nonphosphorylated carbohydrate ligands and Solanum tuberosum (L.) β-amylase model. For performing protein-ligand docking procedures and calculating the binding energies, the DOCK6 and Glide 4.5 program suites were applied. We have observed simultaneously the effect of chain elongation, phosphorylation, and chain branching. Results of flexible ligand docking show that phosphorylation as well as chain elongation increase the stabilization of the ligand-protein complex.     

Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon

Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon are calculated by the saddle-point variation and saddle-point complex-rotation methods. Relativistic and mass polarization corrections are included using first-order perturbation theory. Calculated Auger channel energies and branching ratios are used to identify high-resolution Auger spectrum in the 300-keV C(+) → CH(4) collision experiment. It is found that Auger decay of these five-electron core-excited states gives significant contributions to Auger spectrum in the range of 238-280 eV.     

微孔晶体材料C12A7-Cl-的表面氯负离子发射性能和机理

利用飞行时间质谱(TOF-MS)观测到氯负离子从合成的微孔晶体材料C12A7-Cl-(11CaO·7Al2O3·CaCl2)表面发射出来, 详细研究了C12A7-Cl-的发射特性, 包括发射强度分支比、温度效应、电场效应和表观活化能. 在我们的检测范围内从C12A7-Cl-表面发射的离子中绝大部分是氯负离子(最大强度分支比为98%), 此外还有弱的氧负离子和电子发射. 各种离子的绝对发射电流强度都随着表面温度升高或引出电场强度的增加而显著增强, 随着引出电场强度从200增加到1200 V·cm-1, 氯负离子发射的表观活化能从180.9 kJ·mol-1减小到110.0 kJ·mol-1. 氯负离子和C12A7-Cl-表面之间的结合能大约是228 kJ·mol-1. 研究了氯负离子的发射稳定性, 并且应用一种电化学注入法, 以获得持续的氯负离子发射. 基于上述实验还讨论了氯负离子的形成和发射机理. 目前的方法可望被用于发展氯负离子储存/发生器.     

Recombination of simple molecular ions studied in storage ring: dissociative recombination of H2O+

Dissociative recombination of vibrationally relaxed H2O+ ions with electrons has been studied in the heavy-ion storage ring CRYRING. Absolute cross-sections have been measured for collision energies between 0 eV and 30 eV. The energy dependence of the cross-section below 0.1 eV is found to be much steeper than the E-1 behaviour associated with the dominance of the direct recombination mechanism. Resonant structures found at 4 eV and 11 eV have been attributed to the electron capture to Rydberg states converging to electronically excited ionic states. Complete branching fractions for all dissociation channels have been measured at a collision energy of 0 eV. The dissociation process is dominated by three-body H + H + O breakup that occurs with a branching ratio of 0.71.     

Solubility and diffusion of water in acetal polymers

The rate of water sorption at 25°C. has been determined for a number of polyacetal films differing in structure, density, and orientation induced by extrusion. The equilibrium water uptake was found to be a linear function of density only; no other effect of structure or orientation was detectable. The extrapolated density for zero sorption was 1.51 g./cc., not far from the theoretical crystalline density. The diffusivity of water in unoriented films rose with decreasing density; for linear copolymer, the trend was parallel to that of the area under the dynamic mechanical loss peak associated with long-range chain motions in the disordered regions (β-transition). Less pronounced effects of molecular weight and long chain branching on diffusivity were also noted. Films crystallized while an extruded melt was still oriented showed considerable increases in water diffusivity, but no significant changes in the apparent activation energies of permeation (about 6.6 kcal./mole) or diffusion (about 11.5 kcal./mole). On annealing these films, the diffusivity remained almost constant while the sorption coefficient and retraction on remelting decreased.     

High-performance liquid chromatography of in vitro synthesized poly(ADP-ribose) on ion-exchange columns, separation of oligomers of varying chain length and estimation of apparent branching

Separated macromolecular fractions of in vitro synthesized poly(ADP-ribose) by liver nuclei were subjected to ion-exchange chromatography in a programmed high-performance liquid chromatographic elution system. The effects of ionic strength, pH and temperature on the separation of poly(ADP-ribose) chains were determined. Short chain oligomers (up to n = 11) were fractionated into individual components by baseline separation. Each fraction was analyzed for chain length. Trace amounts of Ado(P)Rib(P)Rib(P) found in phosphodiesterase digests were taken as indication of apparent branching. In phosphodiesterase digests of the shorter oligomers, besides traces of the above component, two other digestion products were also observed, presumably representing oligomer termini, one terminal fragment being dominant in short oligomers. Medium and long chain oligomers were partly resolved to individual components, and especially the long oligomers exhibited marked temperature dependent elution patterns. Apparent branching increased with increasing chain length up to about 3% for n = 44 and components presumably indicating termini diminished to mere traces. The adenine spectra of all fractions identified individual components.     

Thermal degradation of a polyphenylquinoxaline in air and vacuum

A series of poly[2,2′-(1,4-phenylene)-6,6′-bis(3-phenylquinoxalines)] were prepared. These polymers had all the same repeating unit but differed in molecular weight and polymer chain endings. The thermal degradation characteristics in air and vacuum were determined by isothermal weight loss measurements. The temperature coefficients of thermal degradation (apparent activation energies) were also determined. Whereas the apparent activation energies for degradation in air showed a considerable dependency on the type of polymer chain endings, no such effect was observed upon pyrolysis in vacuo. A possible chain-end unzipping mechanism of degradation in air is postulated to explain these results.     

On thermal dehydrochlorination of model compounds for polyvinylchloride—I

A study has been made of the thermal stability of model compounds containing secondary and tertiary chlorine and also of unsaturated models which correspond to abnormal structures in polyvinylchloride. The semi-empirical method (Vedeneyev) was used to study the CC, CCl and CH bonds in polyvinylchloride models. In conjunction with the observation that the dehydrochlorination obeys a first order rate law, an intramolecular process with a cyclic transition state is considered. Activation energies for elimination of hydrogen chloride were calculated on the principle of the additivity of bond energies of the activated complex. The agreement between the calculated and observed activation energies is satisfactory, the differences in most cases being less than 2 kcal/mole. Serious discrepancies occur rarely (for 3-ethyl-3-chloropentane and 3-chloro-1-pentene). The activation energies of the models for saturated polyvinylchloride were found not to be greatly affected by the length of the chain. In general, double bonds inside the chain reduce the thermal stability of the model compounds more markedly than when they are located at the end of the chain. This treatment should ultimately give results of accuracy sufficient for prediction of reactivities of abnormal structures in polyvinylchloride.     

Test of the information theory of branching ratios by classical trajectory computations

Extensive classical trajectory calculations are reported for the system F + HD → HF + D or DF + H, over a wide variety of initial conditions. The results of these computations are used to assess the utility of the information theoretic approach to the prediction of the initial-state dependence of the branching ratio, Γ_HF/DF. Two distinct forms for the information theoretic branching ratio are considered, corresponding to two different choices of constraints upon the transition probability matrix. The constraints in question are the “model” constraint employed by Kaplan and Levine, and the “direct” constraint, a generalisation of that introduced by Levine and Kosloff. The two forms of information theory give satisfactory predictions under selected (differing) initial conditions. Neither form is clearly superior. In particular, both fail to predict the observed trend of Γ_HF/DF with initial rotational state J, for the case of thermally averaged initial translational energies. This failure is linked to an important “interbranch constraint” not incorporated in the theory, stemming from the different activation energies for formation of the two products.     

Extension of the bebo method for the calculation of activation energies of intraradical 1,3-, 1,4- and 1,5-hydrogen shift reactions

The bond-energy—bond-order (BEBO) method has been extended for the calculation of activation energies of the radical isomerization reactions occurring via 1,3-, 1,4- and 1,5-hydrogen atom shifts. The energy of the cyclic activated complexes comprises four contributions, i.e. the energy change in formation of the transition state due to the occurrence of fractional and strained bonds, the triplet repulsion, the deformation energy and the non-bonding interaction. The method has been applied to a set of 11 reactions. The agreement between the calculated and the experimental activation energies is satisfactory.     

A common mechanism for branching, cyclopropanation, and cyclobutanation reactions in the isoprenoid biosynthetic pathway

Four reactions--chain elongation, cyclopropanation, branching, and cyclobutanation--are used in nature to join isoprenoid units for construction of the carbon skeletons for over 55,000 naturally occurring isoprenoid compounds. Those molecules produced by chain elongation have head-to-tail (regular) carbon skeletons, while those from cyclopropanation, branching, or cyclobutanation have non-head-to-tail (irregular) skeletons. Although wild type enzymes have not been identified for the branching and cyclobutanation reactions, chimeric proteins constructed from farnesyl diphosphate synthase (chain elongation) and chrysanthemyl diphosphate synthase (cyclopropanation) catalyze all four of the known isoprenoid coupling reactions to give a mixture of geranyl diphosphate (chain elongation), chrysanthemyl diphosphate (cyclopropanation), lavandulyl diphosphate (branching), and maconelliyl and planococcyl diphosphate (cyclobutanation). Replacement of the hydrogen atoms at C1 or C2 or hydrogen atoms in the methyl groups of dimethylallyl diphosphate by deuterium alters the distribution of the cyclopropanation, branching, and cyclobutanation products through primary and secondary kinetic isotope effects on the partitioning steps of common carbocationic intermediates. These experiments establish the sequence in which the intermediates are formed and indicate that enzyme-mediated control of the carbocationic rearrangement and elimination steps determines the distribution of products.     

Chrysanthemyl diphosphate synthase. The relationship among chain elongation,branching, and cyclopropanation reactions in the isoprenoid biosynthetic pathway

The genes for chrysanthemyl diphosphate (CPP) synthase and farnesyl diphosphate (FPP) synthase from sagebrush, Artemisia tridentata spiciformis, were used to prepare a series of chimeric proteins to investigate the 1'-4 chain elongation, 1'-2 branching, and c1'-2-3 cyclopropanation reactions that join isoprenoid units to build more complex structures. The two genes were modified by site-directed mutagenesis to generate an identical set of six unique restriction sites at identical locations. The locations were selected to place a restriction site between each of the five conserved regions found in prenyltransferases that catalyze chain elongation. A series of chimeric proteins were generated by replacing amino acids in FPP synthase, beginning at the N-terminus of the enzyme, with increasing stretches of peptide from CPP synthase. An analysis of the products produced by the chimeras revealed a transition from 1'-4 chain elongation, to 1'-2 branching, and ultimately to c1'-2-3 cyclopropanation. These results demonstrate that the catalytic site for chain elongation, with minor modifications in its architecture, also catalyzes 1'-2 branching and c1'-2-3 cyclopropanation, and suggest that the branching and cyclopropanation reactions, in analogy to chain elongation, are electrophilic alkylations.