The quality of solvents of polymers is often described in terms of the Flory χ parameter typically assumed to depend only on the temperature, T. In certain polymer‐solvent systems fitting the experimental data enforces the replacement of (χT) by a concentration‐dependent χeff. In turn, this modifies the swelling and collapse behavior. These effects are studied, in the framework of a mean‐field theory, for isolated coils and for planar brushes. The ϕ dependence of χeff gives rise to three main consequences: (i) shift in the cross‐over between Gaussian and self‐avoidance regimes; (ii) a possibility of first‐order collapse transition for isolated flexible coils; (iii) the possibility of a first‐order phase transition leading to a vertical phase separation within the brush. The discussion relates these effects directly to thermodynamic measurements and does not involve a specific microscopic model. The implementation for the case of poly(N‐isopropylamide) (PNIPAM) brushes is discussed.
ϕ vs. z plots, for brushes with N = 300, σ/a2 = 18 (σ/R = 0.019) characterized by different χeff. 相似文献
Fluorination reactions are essential to modern medicinal chemistry, thus providing a means to block site‐selective metabolic degradation of drugs and access radiotracers for positron emission tomography imaging. Despite current sophistication in fluorination reagents and processes, the fluorination of unactivated C? H bonds remains a significant challenge. Reported herein is a convenient and economic process for direct fluorination of unactivated C? H bonds that exploits the hydrogen abstracting ability of a decatungstate photocatalyst in combination with the mild fluorine atom transfer reagent N‐fluorobenzenesulfonimide. This operationally straightforward reaction provides direct access to a wide range of fluorinated organic molecules, including structurally complex natural products, acyl fluorides, and fluorinated amino acid derivatives. 相似文献
The photoconductivity (PC) of electron irradiated semiconducting diamond was examined over the energy range 1–4 eV (0.3–1.2 μm) and over the temperature range 76–500 K. The PC response spectra exhibited a strong drop below 3 eV. In one sample (RDl) this drop was by a factor of nearly 104 from 3 to 2 eV. In the range of the Ar laser lines (2.41–2.73 eV) the PC depended nearly exponentially on photon energy. A mechanism involving competition for the absorption of the exciting light between the non-photoconductive GRl system and the photo-conductive UV absorption band gives a good account of the experimental results. This includes the photon energy dependence of the PC and its thermal dependence.The proposed mechanism is further supported by the absence of the drop in PC near 2.5 eV in a lightly colored electron irradiated diamond. 相似文献
A theoretical analysis is made of the homogeneous linewidth of the electronic Raman transition between ē and 2ā excited states of Cr3+ observed in optically pumped ruby, in comparison with the widths of the phosphorescence R1 and R2 lines. In particular, we consider the phenomenological correlation between the broadening processes of the ē and 2ā levels, implied by the experimental fact that the magnitude of the Raman linewidth is approximately equal to the average, rather than the sum, of the R1 and R2 linewidths. The origin of this correlation is shown to be the quenching of pure-dephasing processes, associated with elastic phonon scattering, of the ē ? 2ā transition, while the pure-dephasing rates of the phosphorescence transitions are comparable to the rates of population-relaxation (lifetime-broadening) processes, associated with inelastic phonon scattering. 相似文献
No evidence for spontaneous fission decay (and the implied presence of superheavy nuclides) has been found in a several kilogram precipitate gathered at the Atlantis-II site in the Red Sea. The sample was investigated with a neutron multiplicity counternon-destructively so that no chemical losses should have been sustained. Upper limits have been determined for possible superheavy element concentrations in the sample. 相似文献
From a comparison of the temperature dependent line width of the electronic Raman transition between the component levels of 2E of ruby and the width of the R phosphorescence transitions follows that a time correlation must exist in the energy variation (homogeneous broadening process) of the said levels. 相似文献
The visible emission of KBr : Tl and KCl : Tl crystals was measured in the temperature range 10–300 K, on excitation in the low-energy tails of the corresponding A-bands. The peculiar temperature dependence of the visible emission is explained in terms of absorption changes due to thermal broadening of the A1-side band itself and due to the growing overlap of A1-band with the A-band on warming. A quantitative analysis is given which enables the computation of some parameters of the A1-side band. 相似文献
We describe a software package for computing and manipulating the subdivision of a sphere by a collection of (not necessarily great) circles and for computing the boundary surface of the union of spheres. We present problems that arise in the implementation of the software and the solutions that we have found for them. At the core of the paper is a novel perturbation scheme to overcome degeneracies and precision problems in computing spherical arrangements while using floating point arithmetic. The scheme is relatively simple, it balances between the efficiency of computation and the magnitude of the perturbation, and it performs well in practice. In one O(n) time pass through the data, it perturbs the inputs necessary to insure no potential degeneracies and then passes the perturbed inputs on to the geometric algorithm. We report and discuss experimental results. Our package is a major component in a larger package aimed to support geometric queries on molecular models; it is currently employed by chemists working in “rational drug design”. The spherical subdivisions are used to construct a geometric model of a molecule where each sphere represents an atom. 相似文献
