Localization of a multiblock copolymer at a selective interface: scaling predictions and Monte Carlo verification

We investigate the localization of a hydrophobic-polar regular copolymer at a selective solvent-solvent interface with emphasis on the impact of block length M on the copolymer behavior. The considerations are based on simple scaling arguments and use the mapping of the problem onto a homopolymer adsorption problem. The resulting scaling relations treat the gyration radius of the copolymer chain perpendicular and parallel to the interface in terms of chain length N and block size M, as well as the selectivity parameter chi. The scaling relations differ for the case of weak and strong localization. In the strong localization limit a scaling relation for the lateral diffusion coefficient D( parallel) is also derived. We implement a dynamic off-lattice Monte Carlo model to verify these scaling predictions. For chain lengths in a wide range (32相似文献   

Globular structures of a helix-coil copolymer: self-consistent treatment

A self-consistent-field theory was developed in the grand canonical ensemble formulation to study transitions in a helix-coil multiblock globule. Helical and coil parts are treated as stiff rods and self-avoiding walks of variable lengths correspondingly. The resulting field theory takes, in addition to the conventional Zimm-Bragg, [J. Chem. Phys. 31, 526 (1959)] parameters, also three-dimensional interaction terms into account. The appropriate differential equations which determine the self-consistent fields were solved numerically with finite element method. Three different phase states are found: open chain, amorphous globule, and nematic liquid-crystalline (LC) globule. The LC-globule formation is driven by the interplay between the hydrophobic helical segment attraction and the anisotropic globule surface energy of an entropic nature. The full phase diagram of the helix-coil copolymer was calculated and thoroughly discussed. The suggested theory shows a clear interplay between secondary and tertiary structures in globular homopolypeptides.     

Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: When Do Strong Covalent Bonds Break Easily?

The scission kinetics of bottle-brush molecules in solution and on an adhesive substrate is modeled by means of Molecular Dynamics simulation with Langevin thermostat. Our macromolecules comprise a long flexible polymer backbone with L segments, consisting of breakable bonds, along with two side chains of length N, tethered to segments of the backbone with grafting density σ_g. In agreement with recent experiments and theoretical predictions, we find that bond cleavage is significantly enhanced on a strongly attractive substrate even though the chemical nature of the bonds remains thereby unchanged. Our simulation results indicate that the mean life time of covalent bonds decreases by more than an order of magnitude upon adsorption even for brush molecules with comparatively short side chains . The distribution of scission probability along the bonds of the backbone is found to change significantly when the length and/or the grafting density of the side chains are varied. The tension, experienced by the covalent bonds is found to grow steadily with increasing σ_g. The mean life time declines with growing contour length L as , and also with growing side chain length N. The probability distribution of fragment lengths at different times is compatible with experimental observations and reveals a two-stage (initially fast, then slow) process with different rates. The variation of the mean length L(t) of the fragments with elapsed time characterizes the thermal degradation process as a first order reaction.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

Electrospray ionization multiple-stage tandem mass spectrometric analysis of diglycosyldiacylglycerol glycolipids from the bacteria Bacillus pumilus

Electrospray ionization (ESI) combined with multiple-stage tandem mass spectrometry (MS(n)) was used to directly analyze the glycolipid mixture from bacteria Bacillus pumilus without preliminary separation. Full scan ESI-MS revealed the composition of picomole quantities of glycerolglycolipid species containing C(14)-C(19) fatty acids, some of which were monounsaturated. Two main components were identified from their molecular masses and fragmentation pathways. The fragmentation pathway of the known compound compared with the investigated compound verified the proposed structure as 1(3)-acyl-2-pentadecanoyl-3(1)-O-[beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl]-sn-glycerols. A comparison of the multiple tandem mass spectra of the different alkali-metal cation adducts indicates that the intensity of fragments and the dissociation pathways are dependent on the alkali-metal type. The basic structures of glycerolglycolipids were reflected clearly from the fragmentation patterns of the sodium cations. The intense fragments of the sugar residue from the precursor ions were obtained from the lithiated adduct ions. ESI-MS(n) spectra of [M + K](+) ions did not provide as much fragmentation as [M + Na](+) and [M + Li](+) adducts, but their spectra allow the position of glycerol acylation to be determined. On the basis of MS(2) spectra of [M + K](+) ions, it was established that all components have a C(15:0) fatty acid at the sn-2 position of the glycerol backbone and C(14)-C(19) acids at the sn-1 position of the glycerol backbone. Copyright 1999 John Wiley & Sons, Ltd.     

Analysis of a shielded TE011 mode composite dielectric resonator for stable frequency reference

Analysis of a TE₀₁₁ mode composite sapphire-rutile dielectric resonator has been carried out to study the temperature variation of resonance frequency, close to the Cs atomic clock hyperfine frequency of 9.192 GHz. The complementary behavior of dielectric permittivity with temperature of the composite has been exploited to obtain the desired turning point in the resonant frequency. The frequency of the composite structure is found to be independent of the shield diameter beyond four times the puck diameter.     

Weak violation of universality for polyelectrolyte chains: Variational theory and simulations

A variational approach is considered to calculate the free energy and the conformational properties of a polyelectrolyte chain in d dimensions. We consider in detail the case of pure Coulombic interactions between the monomers, when screening is not present, in order to compute the end-to-end distance and the asymptotic properties of the chain as a function of the polymer chain length N. We find R≃N ^ν(log N)^γ, where ν = and λ is the exponent which characterizes the long-range interaction U∝ 1/r ^λ. The exponent γ is shown to be non-universal, depending on the strength of the Coulomb interaction. We check our findings by a direct numerical minimization of the variational energy for chains of increasing size 2⁴ < N < 2¹⁵. The electrostatic blob picture, expected for small enough values of the interaction strength, is quantitatively described by the variational approach. We perform a Monte Carlo simulation for chains of length 2⁴ < N < 2¹⁰. The non-universal behavior of the exponent γ previously derived within the variational method is also confirmed by the simulation results. Non-universal behavior is found for a polyelectrolyte chain in d = 3 dimension. Particular attention is devoted to the homopolymer chain problem, when short-range contact interactions are present. Received 8 August 2000 and Received in final form 19 December 2000     

Langevin dynamics of a π4-model with long range interactions

The Dominicis-Peliti generating functional (GF) method is used for the investigation of a Langevin dynamics of the π⁴-model: the symmetric double-well on-site potential and the infinite range interparticle interaction. We limit ourselves to the range above the temperature of the second order phase transition. The role of the 1/N-fluctuations (where N is the number of particles) is systematically investigated by using the steepest descent method. It is shown that the functional Legendre transformation directly results in the kinetic equation for the complete correlation function. Although this equation resembles the mode coupling equations used to describe the glass transition, it is qualitatively different. The solutions of this non-linear equation are investigated. It is shown that 1/N-fluctuations do not result in a breaking or ergodicity if the mean-field correlator is ergodic. On the other hand, if the mean-field correlator is nonergodic (e.g. if the time is much less than the inverse Kramers rate) then 1/N-fluctuations restore the ergodicity with characteristic relaxation time proportional to N.