Numerical methods for inverse problems in electrooptics of polydisperse colloids

In this paper we propose a new method for the determination of the distribution of electrical and geometrical particle parameters based on electrooptical experimental data. The electrooptical method leads to the solution of inverse ill-posed problems. The main equations for the determination of the distribution of particles on these parameters are presented. To find out the distribution functions from the electrooptical experimental data one has to solve the first-kind Fredholm integral equation corresponding to the problem under study. The proposed method of its solution is based on the penalty functions method. The results of modelling that let us compare the various numerical methods are presented.     

Molecular weight characterization of poly[(R)-3-hydroxybutyrate] synthesized by genetically engineered strains of Escherichia coli

This study investigated the relationship of growth conditions, host strains and molecular weights of poly[(R)-3-hydroxybutyrate] [P(3HB)] synthesized by genetically engineered Escherichia coli. Various PHA synthases belonging to types I-IV enzymes were expressed in E. coli JM109 under the same experimental conditions, and the molecular weights of the polymers were characterized by gel permeation chromatography. The results demonstrate that P(3HB) polymers have varied molecular weights and polydispersities dependent on the characteristics of the individual PHA synthase employed. P(3HB) with high number-average molecular weights (M_n) [(1.5-4.0) × 10⁶] and narrow polydispersities (1.6-1.8) were synthesized by PHA synthases from Ralstonia eutropha (type I), Delftia acidovorans (type I) and Allochromatium vinosum (type III). Contrary to these, P(3HB) with relatively low M_n [(0.17-0.79) × 10⁶] and broad polydispersities (2.2-9.0) were synthesized by PHA synthases from Aeromonas caviae (type I), Pseudomonas sp. 61-3 (type II) and Bacillus sp. INT005 (type IV). Furthermore, the molecular weights of P(3HB) synthesized under various culture conditions, in various hosts of E. coli and by mutants of PHA synthase were characterized. It was found that, in addition to culture pH [Kusaka et al. Appl Microbiol Biotechnol 1997;47:140], other variances such as culture temperature, host strain and use of mutants are effective in changing polymer molecular weight.     

The influence of polymer molecular-weight distributions on pulsed field gradient nuclear magnetic resonance self-diffusion experiments

 The influence of polymer molecular-weight distributions on the outcome of pulsed field gradient (PFG) NMR self-diffusion experiments has been considered. The self-diffusion coefficient, D, of monodisperse poly(ethylene oxide) (PEO) polymers has been determined in order to accurately determine the scaling behavior of D both with molecular weight and concentration. In order to investigate the influence of polydispersity on the PFG NMR signal, a model system consisting of ten reasonably monodisperse PEO polymers was made, and the PFG NMR signal intensities were recorded at a low total concentration. The data were analyzed using both inverse Laplace transformation and nonlinear least-squares fitting to a prescribed distribution function of D. Finally, the molecular-weight distribution was obtained by use of the values of the scaling parameters. We also present some model calculations used to investigate the sensitivity of the degree of polydispersity on the NMR signal decays. Received: 27 May 1999 Accepted in revised form: 19 October 1999     

De-mixing of polydisperse fluids: experimental test of a universal relation

Colloids, emulsions, polymer blends, and other important complex fluids, are polydisperse, i.e. there are variations among their constituent particles. Polydispersity is usually regarded as an ubiquitous, uncontrollable nuisance causing experimental inconsistencies. We have varied the polydispersity of a complex fluid, whilst keeping all other parameters constant, and report the first measurements of some universal physics. At coexistence (e.g. between liquid and vapour), fractionation occurs—each phase receives a different mix of the various ingredients, e.g. with the liquid disproportionately abundant with larger particles. Theory predicts, at low polydispersity, that this de-mixing becomes universal, irrespective of the material, with chemical differences between the phases proportional to polydispersity to the power two. We have studied colloid–polymer suspensions at two-phase coexistence and, using light scattering, measured the exponent as 2.16±0.44.     

Microwave-assisted extraction of high-molecular-weight hemicelluloses from spruce wood

Microwave-assisted extraction (MAE) at atmospheric pressure has been demonstrated as an efficient technology for the extraction of polymeric hemicelluloses from spruce sawdust. This technology was shown to be more efficient than conventional extraction. MAE leads to a high solubilization of wood and a selective extraction of hemicellulose polymers with high molecular weights. To optimize MAE, different treatment powers (125–573 W) of presoaked spruce sawdust in water and 1 M sodium hydroxide solution for a period of 60 min were tested. The yield of hemicellulose extraction increased with the microwave power in both mediums, but with a clear advantage for presoaked samples in basic medium. The characterization of extracted hemicelluloses has shown high extraction selectivity depending on the medium of impregnation of sawdust before MAE: High-molecular-mass acetylated galactoglucomannans (M_w ∼ 41 kDa) were isolated after presoaking in water and higher molecular mass arabinoglucoronoxylans (M_w ∼ 66 kDa) in basic medium.     

值得商榷的聚合物分子量分布宽度指数表达式

认为张晓云在“有关聚合物平均分子量和分子量多分散性的教学点滴”一文中对聚合物分子量分布宽度指数的定义以及由此得出的结论是值得商榷的。本文介绍了具有统计意义的分子量分布宽度指数的定义，比较了四种常用平均分子量的相对大小。     

Two-dimensional crystallization of hard sphere particles at a liquid–liquid interface

A method for studying crystallization of hard sphere like particles in two dimensions is presented. The method involves trapping the particles at the interface between two immiscible liquids. Particles at the interface undergo 2D Brownian motion, and at sufficiently high densities crystallization is observed. The pseudo hard sphere nature of the particle interactions under these conditions is maintained, as demonstrated by the area density at which crystallization occurs. In contrast to established techniques for studying crystallization in pseudo 2D hard spheres, the particles trapped at the interface undergo no vertical motion, so the system is in principle closer to a true 2D system. The method is therefore amenable to the study of the effects of polydispersity on crystallization behaviour. The advantages and disadvantages of the method are discussed.     

有关聚合物平均分子量和分子量多分散性的教学点滴

从相对数均分子量和重均分子量的标准偏差角度来解释聚合物分子量分布指数与分布宽度的关系，利于学生对分子量分布指数含义的理解。     

Selective recognition of Ca ions using novel polymeric phenols

The research described here elucidates the fundamental interactions of various cations with phenolic moieties constituting the side chains of novel copolymers. The phenolic group was chosen because similar interactions of the tyrosine (Tyr) moiety in proteins with alkaline earth cations are of particular interest for some biochemical systems where both the alkaline earth cations and the aromatic compounds are abundant. The present study has revealed the preferred binding site for our polymeric systems.     

具有RAFT链转移过程的活性自由基聚合的Monte Carlo模拟

活性自由基聚合是近年来高分子合成领域中研究的热点之一 .目前主要有两种体系 ,其一是ＴＥＭＰＯ调控的自由基聚合[1],但单体选择面窄 ,聚合速率慢 ;其二是原子转移聚合 ,单体适用面较广 ,但产物中常含有难以除去的金属离子[2 ].因此寻找单体适用性广 ,产物纯净的聚合体系 ,具有十分重要的意义 .近两年来 ,Ｔｈａｎｇ等[36 ]发现用双硫酯调聚的自由基聚合具有活性特征 ,并指出其原因是这类体系中的链转移具有可逆性 ,称为可逆加成 裂解链转移过程 (ＲＡＦＴ) ,可示意如下 :Ｒｉ·+ ＣＳＺＳＲｊ(ＤＳＥ)ｋａｄｄｋｄｉｓＣ·ＳＲｉＺＳＲｊ…