Incubation period in the 2,2,4,4‐tetramethyl‐1‐piperidinyloxy‐mediated thermal autopolymerization of styrene: Kinetics and simulations

Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide‐mediated autopolymerization of styrene are discussed. At 120–125 °C and moderate 2,2,4,4‐tetramethyl‐1‐piperidinyloxy (TEMPO) concentrations (0.02–0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (k_h)]. At higher TEMPO concentrations ([N?] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition k_ad, also becomes relevant. From previous experimental data and simulations, initial estimates of k_h ≈ 1 and k_ad ≈ 6 × 10^?7 L mol^?1 s^?1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction‐period conditions, the simulations confirm the validity of the quasi‐steady‐state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well‐known expression k_th[M]³ (where [M] is the monomer concentration and k_th is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962‐6979, 2006     

Effect of particle geometry on phase transitions in two-dimensional liquid crystals

Using a version of density-functional theory which combines Onsager approximation and fundamental-measure theory for spatially nonuniform phases, we have studied the phase diagram of freely rotating hard rectangles and hard discorectangles. We find profound differences in the phase behavior of these models, which can be attributed to their different packing properties. Interestingly, bimodal orientational distribution functions are found in the nematic phase of hard rectangles, which cause a certain degree of biaxial order, albeit metastable with respect to spatially ordered phases. This feature is absent in discorectangles, which always show unimodal behavior. This result may be relevant in the light of recent experimental results which have confirmed the existence of biaxial phases. We expect that some perturbation of the particle shapes (either a certain degree of polydispersity or even bimodal dispersity in the aspect ratios) may actually destabilize spatially ordered phases thereby stabilizing the biaxial phase.     

Dynamical characterization of rotationally hindered species in liquids

The rotational dynamics of HCl in liquid Ar has been studied by means of molecular-dynamics simulations. We calculate the lifetimes of weakly bound HCl-Ar dimers induced by the anisotropic pair interaction. It is shown that, although lifetimes are small with respect to the reorientational decorrelation, the time interval between the breaking down and formation of the next dimer is negligibly small. Thus, with respect to the rotational dynamics of the probe, the effect is similar to that and eventually would cause a time-stable complex. This provokes a peculiar hindered rotation of the diatomic in the liquid which is macroscopically embodied in the infrared spectrum of the solution as a Q-branch nonexistent otherwise.     

Elastic waves at the (0 0 1) and (1 1 0) surfaces of AlN, GaN and InN

We have studied the acoustic waves in the (0 0 1) and (1 1 0) surfaces of AlN, GaN and InN. We have employed the surface Green function matching method and different sets of calculated elastic constants available in the literature for these materials, because no experimental values are available. Important differences are found for the velocities of the bulk and surface acoustic waves coming from these sets of elastic constants, in such a way that they could be easily measured by ultrasonic and Brillouin techniques. These results together with the expressions obtained here for the velocities of the acoustic waves in high symmetry directions could be used to determine the elastic coefficients of these materials.     

Enhanced sonochemical degradation of azure B dye by the electroFenton process

The degradation of azure B dye (C₁₅H₁₆ClN₃S; AB) has been studied by Fenton, sonolysis and sono-electroFenton processes employing ultrasound at 23 kHz and the electrogeneration of H₂O₂ at the reticulated vitreous carbon electrode. It was found that the dye degradation followed apparent first-order kinetics in all the degradation processes tested. The rate constant was affected by both the pH of the solution and initial concentration of Fe²⁺, with the highest degradation obtained at pH between 2.6 and 3. The first-order rate constant decreased in the following order: sono-electroFenton > Fenton > sonolysis. The rate constant for AB degradation by sono-electroFenton is ∼10-fold that of sonolysis and ∼2-fold the one obtained by Fenton under silent conditions. The chemical oxygen demand was abated ∼68% and ∼85% by Fenton and sono-electroFenton respectively, achieving AB concentration removal over 90% with both processes.     

Surface green function matching approach to the surface dynamics of ionic crystals: I. Equivalence with the invariant green function method

The surface Green function matching (SGFM) method is shown to be equivalent to the Green function method in its translationally and rotationally invariant formulation (IGF). Such equivalence between matching and invariance conditions makes the SGFM method easily applicable to the surface dynamics of ionic crystals, due to the existing possibility of deriving an effective short-range perturbation from the invariance conditions. An application to the inelastic scattering of He from LiF(001) in the eikonal approximation is presented.     

Anomalous NMR relaxation in cartilage matrix components and native cartilage: fractional-order models

We present a fractional-order extension of the Bloch equations to describe anomalous NMR relaxation phenomena (T(1) and T(2)). The model has solutions in the form of Mittag-Leffler and stretched exponential functions that generalize conventional exponential relaxation. Such functions have been shown by others to be useful for describing dielectric and viscoelastic relaxation in complex, heterogeneous materials. Here, we apply these fractional-order T(1) and T(2) relaxation models to experiments performed at 9.4 and 11.7 Tesla on type I collagen gels, chondroitin sulfate mixtures, and to bovine nasal cartilage (BNC), a largely isotropic and homogeneous form of cartilage. The results show that the fractional-order analysis captures important features of NMR relaxation that are typically described by multi-exponential decay models. We find that the T(2) relaxation of BNC can be described in a unique way by a single fractional-order parameter (α), in contrast to the lack of uniqueness of multi-exponential fits in the realistic setting of a finite signal-to-noise ratio. No anomalous behavior of T(1) was observed in BNC. In the single-component gels, for T(2) measurements, increasing the concentration of the largest components of cartilage matrix, collagen and chondroitin sulfate, results in a decrease in α, reflecting a more restricted aqueous environment. The quality of the curve fits obtained using Mittag-Leffler and stretched exponential functions are in some cases superior to those obtained using mono- and bi-exponential models. In both gels and BNC, α appears to account for micro-structural complexity in the setting of an altered distribution of relaxation times. This work suggests the utility of fractional-order models to describe T(2) NMR relaxation processes in biological tissues.