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     

The influence of particle shape and size on electric conductivity of metal-polymer composites

The effective electric conductivity of metal-polymer composite is investigated theoretically as a function of the electric conductivities of the constituents, of the particle shape and size and of the volume of loading. And, in order to consider the interaction between the metal particles, a self-consistent way is used to improve this function. Firstly, the percolation threshold decreases rapidly with increasing the metal particles axial ratio and decreasing the metal particle size. Secondly, the dependence of the metal particle shape and size on the effective electric conductivity of metal-polymer composite is investigated. Finally, the theoretical results on the effective electric conductivity of tin-polypropylene composite are in good agreement with the experimental data.     

Stability of thin polymer films: influence of solvents

The interface and surface properties and the wetting behavior of polymer-solvent mixtures are investigated using Monte Carlo simulations and self-consistent field calculations. We carry out Monte Carlo simulations in the framework of a coarse-grained bead-spring model using short chains (oligomers) of N(P)=5 beads and a monomeric solvent, N(S)=1. The self-consistent field calculations are based on a simple phenomenological equation of state for compressible binary mixtures and we employ Gaussian chain model. The bulk behavior of the polymer-solvent mixture belongs to type III in the classification of van Konynenburg and Scott [Phil. Trans. R. Soc. London, Ser. A 298, 495 (1980)]. It is characterized by a triple line on which the polymer-liquid coexists with solvent-vapor and a solvent-rich liquid. The solvent is not homogeneously distributed across the dense polymer film but tends to accumulate at the surface and the polymer-vapor interface. This solvent enrichment at the interface and surface becomes more pronounced upon increasing the vapor pressure and alters the surface and interface tensions. This effect gives rise to a nonmonotonic dependence of the contact angle on the vapor pressure and one might observe reentrant wetting. The results of the Monte Carlo simulations and the self-consistent field calculations qualitatively agree. The profiles of drops are investigated by Monte Carlo simulations and a pronounced solvent enrichment is observed at the wedge formed by the substrate and the liquid-vapor interface at the three-phase contact line.     

The influence of polymer morphology on polymer film electrochemistry

The electrochemical behavior of functionalized polystyrene-coated electrodes shows a marked dependence on the nature of the electrolyte ions. Scanning electron microscope and surface profile measurements are presented which show that changes in polymer film volume and morphology accompany electrochemical oxidation. Changing polymer morphology by doping the films with soluble monomers during preparation is shown to produce large changes in electrochemical response. Diffusion coefficients were determined for a neutral organic dye dopant in each of the polymer films investigated, and these correlate very well with the oxidation overpotentials observed electrochemically. The nature of polymer film/solvent interactions and the mechanism by which counter ions penetrate the polymer phase is discussed and is related to other physical properties of amorphous polymers in terms of free volume concepts.     

The size and shape of silica particles

Negatively charged silica particles were investigated at pH 10.0. They were found to be rod-shaped (cylinder) with a diameter of 5–5.5 nm and a full length of 44–67 nm depending on the rod model used. Moreover, the particles were found to be stable against aggregation in the region 0.4–50 mM NaCl. Received: 2 December 1998 Accepted in revised form: 2 February 1999     

具有特殊相互作用的聚合物共混物在选择性溶剂中的自组装行为

研究了磺化聚苯乙烯(SPS)/聚4-乙烯基吡啶(P4VP)在选择性溶剂甲醇中的自组装行为。用动态光散射的方法考察了P4VP/甲醇初始浓度以及Cu^2^+的加入对溶液中粒子尺寸的影响,并研究了Cu^2^+的加入对聚合物溶液粘度的影响。     

Solvent self-diffusion,polymer NMR relaxation,and free volume in polymer solutions

The self-diffusion coefficient of chloroform in poly(isopropyl acrylate)—chloroform solutions has been studied as a function of concentration and temperature by using the pulsed-field-gradient spin-echo NMR method. It is found that the self-diffusion coefficient of the solvent can be adequately fitted by using a simple free-volume approach with either a concentration or temperature superposition. It was noted that the free-volume parameters derived from the self-diffusion data are the same as those derived from deuterium NMR transverse relaxation-time measurements of the polymer in the same system. The equality of these two sets of experiments suggests a fundamental relationship between the two different processes. The simplest explanation is that the free volume necessary for the local segmental motion of the polymer and the translation of the solvent are similar.     

Perfluorinated polymer colloids: controlling the size, shape, and surface charge

The first report of perfluorocyclobutyl (PFCB) aryl ether polymer colloids is presented herein. The biphenyl rings were post-functionalized with chlorosulfonic acid modification in the backbone of the perfluoropolymer, thereby changing the zeta potential (ζ) of the colloids across the pH range (2-12). Self-assembly of the colloids was observed by scanning electron microscopy (SEM) on four separate surfaces which led to different morphological changes for each surface; spherical on Al substrate, clover-like on Cr substrate, flower-like on carbon substrate, and "squalloid" on Si substrate.     

The mechanisms of porphyrin complexation in solvents and polymer matrices

Experimental data for the reactions of porphyrins with metal salts in various media are analyzed; possible mechanisms of the reactions are discussed. The experimental findings as well as quantum chemical calculations suggest that the most probable reaction mechanism should involve the formation of a SAT complex and a substantial deformation of the macrocycle.     

Hemimicelle shape and size

Based on the assumption of spherical hemimicelle with a hydrophobic core of diameter of maximum hydrocarbon chain length of surfactant, the aggregation numbers of hemimicelles have been calculated for several values ofn, the carbon atoms of surfactant alkyl chain. The results are in reasonable good agreement with the hemimicelle aggregation number from experimental data for several corresponding systems. The other possibilities of the hemimicelle shape which allow to incorporate a larger number of hydrocarbon chains in a hemimicelle have also been discussed.On leave from Department of Chemistry, Peking University, Beijing, China     

Steric effects in RNA interference: probing the influence of nucleobase size and shape

Nonpolar nucleosides with varying size and shape have been used to study the hydrogen-bonding stabilization and steric effects on RNA interference. The uracil and adenine residues of siRNA guide strands have been replaced by nonpolar isosteres of uracil and adenine and by steric variants. RNAi experiments targeting Renilla luciferase mRNA have shown close correlation between siRNA thermal stability and gene suppression. Interestingly, siRNA modified at position 7 on the guide strand does not follow this correlation, having substantial RNAi activity despite low thermal stability. Sequence-selectivity studies were carried out at this position with mutated target mRNAs and nucleobase analogues with varied size (2,4-difluoro- and 2,4-dichlorobenzene) and different shape (2,3-dichlorobenzene, 4-methylbenzimidazole). The results point out the importance of nucleobase shape and steric effects in RNA interference.     

Off-lattice Monte Carlo studies on size and shape of isolated polymer chains with isotactic and syndiotactic attached side groups

Chains containing side groups were generated with up to 102 bonds in the backbone for both iso- and syndiotactic arrangements. The mean-square end-to-end distance <h ²>, the mean-square radius of gyration <r ²> and the ratios <l _i ² >/<l ₁ ² > of the axes of inertia were investigated. It could be shown that the mean size and shape of such chains are not influenced by the kind of stereoregularity.Dedicated to Prof. Dr. K. Ueberreiter on the occasion of his 70th birthday.     

The influence of water-acetone solvents on the acid-base properties of glycylglycine

The protonation and dissociation constants of glycylglycine in variable-composition water-acetone solvents were determined potentiometrically at the ionic strength of solutions 0.1 M (NaClO₄) and 298 K. Reagent solvation contributions to shifts of acid-base equilibria are analyzed.     

The influence of polymer structure and morphology on talc wettability

Advancing water contact angles were measured on freshly cleaved talc faces as well as on talc particles. The intrinsic hydrophobicity of talc was shown to be due to the dominance of the apolar components of the work of adhesion. Polyacrylamides and polysaccharides adsorb onto the surface of talc, displaying strikingly different morphologies. Adsorbed amount, apparent layer thickness, and polymer structure control talc wettability.     

Effect of bending and torsion rigidity on self-diffusion in polymer melts: a molecular-dynamics study

Extensive molecular-dynamics simulations have been performed to study the effect of chain conformational rigidity, controlled by bending and torsion potentials, on self-diffusion in polymer melts. The polymer model employs a novel torsion potential that avoids computational singularities without the need to impose rigid constraints on the bending angles. Two power laws are traditionally used to characterize the dependence of the self-diffusion coefficient on polymer length: D proportional to N(-nu) with nu=1 for NNe (reptation regime), Ne being the entanglement length. Our simulations, at constant temperature and density, up to N=250 reveal that, as the chain rigidity increases, the exponent nu gradually increases towards nu=2.0 for NNe. The value of Ne is slightly increased from 70 for flexible chains, up to the point where the crossover becomes undefined. This behavior is confirmed also by an analysis of the bead mean-square displacement. Subsequent investigations of the Rouse modes, dynamical structure factor, and chain trajectories indicate that the pre-reptation regime, for short stiff chains, is a modified Rouse regime rather than reptation.     

The relevance of shape and size of Au55 clusters

This critical review deals with the history of Au(55)(PPh(3))(12)Cl(6) and its derivatives from the very beginning in 1981 to date. Au(55) clusters obtain their special interest from their ultimate size and their ideal cuboctahedral structure. They are part of the family of so-called full-shell clusters, particles with perfectly completed geometries, also represented by icosahedral Au(13) clusters. Bare as well as ligand protected Au(55) clusters not only exhibit special chemical and physical stability, but draw their attention particularly from their unique electronic properties. Single electron switching at room temperature becomes possible, giving rise for development of applications in future nanoelectronic devices. A predominantly size-determined property of the 1.4 nm particles becomes obvious with respect of biological response. Au(55) clusters indicate an unusual cytotoxicity which seems to be caused by the unusually strong interaction between the 1.4 nm particles and the major grooves of DNA. Only marginally smaller or larger particles show drastically reduced toxicity, whereas significantly larger gold nanoparticles are completely non-toxic. Both, the electronic perspectives as well as the relevance in toxicology are at very early stages of development (75 references).     

Effect of solvents on surface resistivity measurement of polymer films

A novel method of measuring the surface resistivity of thin polymer films has been used to assess methods of cleaning acetate sheet with solvents. It has been shown that a time-dependent process occurs which can be attributed to an evaporation process. The remanent resistivity value is found to be constant if a pure solvent is used such that no residual contamination remains. In this way a suitable method of sample preparation can be developed to determine the effect of degradation processes on polymer surfaces.