Preparation and characterization of multiwalled carbon nanotube dispersions in polypropylene: Melt mixing versus solid‐state shear pulverization

Dispersions of multiwalled carbon nanotubes (MWNT) in polypropylene (PP) were prepared via conventional melt batch mixing and solid‐state shear pulverization. The properties and structure of each system were assessed via linear viscoelasticity, electrical conductivity, PP crystallization kinetics, dynamic mechanical analysis, scanning electron microscopy, and small angle X‐ray scattering. Increasing either the duration or the intensity of melt mixing leads to higher degrees of dispersion of MWNT in PP, although at the cost of substantial melt degradation of PP for long mixing times. Samples prepared by pulverization exhibit faster crystallization kinetics and higher mechanical stiffness than the melt blended samples, but in contrast show no measurable low frequency elastic plateau in melt rheology, and lower electrical conductivity than melt‐mixed samples. X‐ray scattering demonstrates that neither sample has uniform dispersion down to the single MWNT level. The results illustrate that subtle differences in the size and distribution of nanotube clusters lead to differences in the nanotube networks with strong impact on bulk properties. The results also highlight distinctions between conductive networks and load transfer networks and demonstrate that a complete and comparative picture of dispersion cannot be determined by simple indirect property measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1426–1436, 2009     

Electronic structure of the photoactive yellow protein chromophore: Ab initio study of the low-lying excited singlet states

The excited electronic states of the p-coumaric acid thio-ester chromophore of the Photoactive Yellow Protein (PYP) are characterized in view of identifying the key factors determining the chromophore's isomerisation. These factors include the anionic nature of the chromophore, the presence of sulfur (rather than oxygen or nitrogen) in the ester moiety, and the presence of a hydrogen-bonding environment stabilizing the phenolate moiety. Two twisted stationary S₁ structures are identified, corresponding to a twist around the double bond conjugated with the aromatic ring, and the single bond adjacent to the ring, respectively. The latter structure is accessed directly by relaxation from the Franck–Condon (FC) geometry. These structures are shown to entail a substantial polarization effect (increasing charge separation when moving towards the twisted geometry). Further, an inversion of charge character is observed for the double-bond twisted minimum, which can be accounted for by the vicinity of an S₁–S₀ conical intersection. The S₁–S₀ gap at the minimum geometries depends in a sensitive fashion on the -carbonyl heteroatom. Based upon these observations for the intrinsic properties of the chromophore, we further address the effect of the Arg52 residue, which acts as a counter-ion in the native protein environment.     

Photoinduced isomerization of the photoactive yellow protein (PYP) chromophore: interplay of two torsions, a HOOP mode and hydrogen bonding

We report on a detailed theoretical analysis, based on extensive ab initio calculations at the CC2 level, of the S(1) potential energy surface (PES) of the photoactive yellow protein (PYP) chromophore. The chromophore's photoisomerization pathway is shown to be fairly complex, involving an intimate coupling between single-bond and double-bond torsions. Furthermore, these torsional modes are shown to couple to a third coordinate of hydrogen out-of-plane (HOOP) type whose role in the isomerization is here identified for the first time. In addition, it is demonstrated that hydrogen bonding at the phenolate moiety of the chromophore can hinder the single-bond torsion and thus facilitates double-bond isomerization. These results suggest that the interplay between intramolecular factors and H-bonding determines the isomerization in native PYP.     

X-ray scattering measurements of particle orientation in a sheared polymer/clay dispersion

We report steady and transient measurements of particle orientation in a clay dispersion subjected to shear flow. An organically modified clay is dispersed in a Newtonian polymer matrix at a volume fraction of 0.02, using methods previously reported by Mobuchon et al. (Rheol Acta 46: 1045, 2007). In accord with prior studies, mechanical rheometry shows yield stress-like behavior in steady shear, while time dependent growth of modulus is observed following flow cessation. Measurements of flow-induced orientation in the flow-gradient plane of simple shear flow using small-angle and wide-angle X-ray scattering (SAXS and WAXS) are reported. Both SAXS and WAXS reveal increasing particle orientation as shear rate is increased. Partial relaxation of nanoparticle orientation upon flow cessation is well correlated with time-dependent changes in complex modulus. SAXS and WAXS data provide qualitatively similar results; however, some quantitative differences are attributed to differences in the length scales probed by these techniques.     

Communication: Universal Markovian reduction of Brownian particle dynamics

Non-Markovian processes can often be turned Markovian by enlarging the set of variables. Here we show, by an explicit construction, how this can be done for the dynamics of a Brownian particle obeying the generalized Langevin equation. Given an arbitrary bath spectral density J(0), we introduce an orthogonal transformation of the bath variables into effective modes, leading stepwise to a semi-infinite chain with nearest-neighbor interactions. The transformation is uniquely determined by J(0) and defines a sequence {J(n)}(n∈N) of residual spectral densities describing the interaction of the terminal chain mode, at each step, with the remaining bath. We derive a simple one-term recurrence relation for this sequence and show that its limit is the quasi-Ohmic expression provided by the Rubin model of dissipation. Numerical calculations show that, irrespective of the details of J(0), convergence is fast enough to be useful in practice for an effective Ohmic reduction of the dissipative dynamics.     

Complex Formation of the Tetracycline‐Binding Aptamer Investigated by Specific Cross‐Relaxation under DNP

While dynamic nuclear polarization (DNP) under magic‐angle spinning (MAS) is generally a powerful method capable of greatly enhancing the sensitivity of solid‐state NMR spectroscopy, hyperpolarization also gives rise to peculiar spin dynamics. Here, we elucidate how specific cross‐relaxation enhancement by active motions under DNP (SCREAM‐DNP) can be utilized to selectively obtain MAS‐NMR spectra of an RNA aptamer in a tightly bound complex with a methyl‐bearing ligand (tetracycline) due to the effective CH₃‐reorientation at an optimized sample temperature of approximately 160 K. SCREAM‐DNP can spectrally isolate the complex from non‐bound species in an RNA mixture. This selectivity allows for a competition assay between the aptamer and a mutant with compromised binding affinity. Variations in molecular structure and methyl dynamics, as observed by SCREAM‐DNP, between free tetracycline and RNA‐bound tetracycline are discussed.     

Application of capillary gas chromatography in the analysis of hydrocarbons in the environment

Summary Hydrocarbons are emitted into the environment in quantity and diversity. Some of these can be detrimental in low concentration, many can be active in photochemical smog production. For combatment there is also interest in the sources of hydrocarbons in the atmosphere. For source recognition fine-structured chromatographic fingerprints are essential. This also holds for oil pollution. A complicating factor in the determination of hydrocarbon pollutants may arise from the occurrence of recently biosynthesized hydrocarbons. For distinguishing between these two types the application of capillary gas chromatography is preferred to packed column gc. It will be clear that the application of capillary gc in the analysis of hydrocarbons in the environment is of great practical value. The modified Grob method used by us for the analysis of hydrocarbons in air is discussed. For the characterization and source recognition of oil pollution in water and soil application of capillary gc fingerprinting is essential; in this way valuable information can be gathered about weathering of oil pollution. Sources of oil pollution in water areas and harbour regions can be traced in certain cases. Also the possibility of measurement of residence time is present. Typical examples are given.Publication No. 601 of the TNO Research Institute for Environmental Hygiene.