Vapor‐induced spreading dynamics of adsorbed linear and brush‐like macromolecules as observed by environmental SFM: Polymer chain statistics and scaling exponents

Scaling exponents ν, that describe the correlation between mean square end‐to‐end distances and contour lengths of macromolecules, were determined by statistical analysis of scanning force micrographs of single linear poly(2‐vinylpyridine) and brush‐like poly(butanoate‐ethyl methacrylate)‐graft‐poly(n‐butyl acrylate) macromolecules adsorbed on mica. Using an atmosphere‐controlled scanning force microscope, single adsorbed molecules were collapsed and re‐expanded upon being exposed to alcohol and water vapor, respectively. This manipulated collapse‐unfolding was used to equilibrate the molecular structure/conformation. The in situ and real‐time scanning force microscopy analysis allows the scientist to quantitatively characterize end‐to‐end distances and contour lengths of the molecules directly on the image and to observe differences in the spreading dynamics for the two types of macromolecules. A distinct difference has been observed between the expanded two‐dimensional (2D) conformations of linear and brush‐like polymer chains. Whereas a scaling exponent ν of 0.73 was found for the expanded 2D conformation of the linear molecules, a ν‐value of 0.53 was determined for the expanded 2D conformation of the seemingly stiffer brush‐like molecules. A theoretical explanation of the differences between the 2D conformations of brush‐like and linear macromolecules is proposed here. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2368–2379, 2007     

Nematic ordering of rigid rod polyelectrolytes induced by electrostatic interactions: effect of discrete charge distribution along the chain

Similar to the Debye-Hu?ckel plasma, charged groups in solutions of rigid rod polyelectrolytes attract each other. We derive expression for the correlation free energy of electrostatic attraction of the rods within the random phase approximation. In this theory, we explicitly take into account positions of charged groups on the chains and examine both charge and polymer concentration fluctuations. The correlation free energies and the osmotic pressures are calculated for isotropic and completely ordered nematic phase. The results of the discrete model are compared with results of a continuous model. The discrete model gives rise to a stronger attraction between the charged groups both in the isotropic and nematic phases and to a stronger orienting action of the electrostatic forces.     

Why ionic liquids can possess extra solvent power

We use the Flory-Huggins theory to demonstrate conditions of extra solvent power of ionic liquids. The short-range interactions between anions, cations, and molecules of the solute are taken into account. We find that solvent power of the ionic liquids is enhanced if non-Coulomb interactions between the anions and cations are repulsive. The mechanism responsible for the extra solvent power is related to the "shielding" of the anion-cation interactions by the molecules of the solute.     

Two Types of Block Copolymer Micelles with Ion‐Containing Cores

We report a combined experimental and theoretical study of micellization of block copolymer with hydrophilic nonionic corona‐forming blocks and weak polyelectrolyte (wPE) core‐forming blocks with pH‐triggered solubility in aqueous solutions. We demonstrate that in addition to micelles with neutral cores, there exist two other types of micelles with PE‐ or ionomer‐like cores, in which monovalent counterions are released or condensed on core wPE block, respectively. The transition between the two types of micelles occurred upon changes in ionization of the PE core block and resulted in nonmonotonous changes of aggregation number as a function of pH. Such micelles with stimulus responsive cores represent promising nanocarriers for controlled delivery applications.

     

Clusters in a mixture of an "amphiphilic" ionic liquid and a nonionic liquid: theoretical study

A Flory-Huggins type lattice approach is used to describe theoretically a heterogeneous mixture composed of an ionic liquid (IL) and a nonionic liquid (nIL). It is analyzed, how the behavior of the system depends on the difference in the affinities of the cations and the anions to the neutral molecules (i.e., on the "amphiphilicity" of the IL with respect to the nIL). It is proved that if the difference in the affinities is not large, two macrophases coexist in the mixture; if the difference exceeds a certain threshold value, the mixture becomes microheterogeneous: depending on its composition, it can turn either into ion clusters dispersed over the phase having low concentration of ions, or into clusters of neutral molecules dispersed over the phase having high concentration of ions. If the system is not close to the critical point, the ion clusters can be only small: the maximal ratio of their diameter to an ion diameter is of the order of ten; however, the clusters of nonionic molecules can be large, if the difference in the affinities has a certain value. It is predicted also that cavities can nucleate inside an IL, and clusters of ions can appear in a saturated vapor of an IL.     

Thermodynamic Aspects of the Increased Thermal Stability of Silicon by Doping with Transition or Rare-earth Metals

The main reason of the degradation of silicon monocrystals at heating is a structural transformation connected with a partial transition of the diamond-like structure into the structure of white tin. The reason for this transformation being observed under high pressures is the appearance of stress zones at the boundaries of variously oriented crystal microvolumes due to heat expansion anisotropy. The high stress concentration in the microvolumes provides sufficient pressure for the indicated phase transformation which results in the observed degradation of the electrophysical properties of silicon. The prevention of the structural transformation is considered to be possible by doping of Si by transition or rare-earth metals which increases the interatomic energy and decreases the thermal expansion coefficient. The choice of the doping additions is based on the bonding energy and the charge density calculated for a system of non-polarised ionic radii. The technology to increase the thermal stability of silicon has been patented^#.     

Theoretical study of the conformations of triosmium clusters with a chiral carane ligand

A theoretical study has been performed to investigate the conformations of triosmium clusters with a chiral carane ligand . The potential curves of internal rotation have been determined for organic ligands rotating around the S-C bond in the cluster complexes. The structures of possible conformers are analyzed along with reasons for their stability. The rotation barrier relative to the S-C bond is 439.8 kJ/mole, which hinders free rotation of ligands.Original Russian Text Copyright © 2004 by V. A. Potemkin V. A. Maksakov, and V. P. KirinTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 430–434, May–June, 2004.     

A Method for Multiconformational Modeling of the Three‐Dimensional Shape of a Molecule

A method for multiconformational modeling of the threedimensional shape of a molecule is proposed that includes search for conformers, their optimum superposition, and analysis of spatial features of the resulting structure. The method allows one to determine features of various molecular conformations of compounds under study, to assess the contributions of conformers to particular properties of the substance, to evaluate the space occupied by the molecule, and to compare the average size of the multiconformational model of the molecule with the sizes of the most stable conformations. The potentials of the model are illustrated by density calculations for 137 organic liquids.