Structural mobility of molecular bottle-brushes investigated by NMR relaxation dynamics

The structural mobility of monomeric units of molecular bottle-brushes was studied by a systematic evaluation of NMR relaxation dynamics. The spin-spin relaxation time (T₂) was determined by Carr-Purcell-Meiboom-Gill (CPMG) NMR spectroscopic measurements. T₂ for protons that reside on the exterior and interior of the bottle-brush macromolecules varied with the grafting density and side chain length in bottle-brush copolymers. Poly((2-(2-bromopropionyloxy)ethyl methacrylate-stat-methyl methacrylate)-graft-butyl acrylate) (poly((BPEM-stat-MMA)-graft-PBA) was studied as a model brush copolymer. The T₂ values for protons of MMA units in the brush backbone significantly decreased with increasing side chain length and grafting density of PBA. The mobility and relaxation times T₂ for the side chain PBA protons decreased with grafting density. However, after initial increase, the relaxation times eventually decreased with PBA side chain length.     

Dependence of relaxation processes in a low-density polyethylene with different crosslink densities investigated by fluorescence spectroscopy

Relaxation processes of one type of low-density polyethylene with three different degrees of crosslinking were studied using the fluorescence of 1-pyrenyl groups attached covalently to polymer chains. The average molecular weights between crosslinks (M_c) of the polyethylenes were 125, 76.5, and 7 kg mol⁻¹. As assessed by changes of the integrated fluorescence intensity and the full width at half maximum of the 0-0 emission band with temperature, chain mobility decreases with increasing crosslinking density. Chain mobility in the polyethylene with the smallest M_c value appears to be strongly inhibited up to 220 K, while motions of small segments of the polymer chains were observed for samples with lower crosslinking degrees at temperatures similar to those of the pristine (non-crosslinked) polymer. These data are discussed in terms of local versus bulk properties of the films.     

Particle flow in a hydrocyclone investigated by positron emission particle tracking

The flow of a particle through a hydrocyclone acting on water has been studied by positron emission particle tracking (PEPT). The positron-emitting radioactive tracer was ¹⁸F. It was found that the activity on an ion-exchange resin particle labeled with ¹⁸F did not leach out into the water during the duration of an experiment. In the state-of-the-art PET camera it is shown to be possible to locate the centroid of the tracer particle with a standard deviation of only about 0.2 mm once per ms, making both the temporal and spatial resolution high enough to trace the particle in its very fast motion through the hydrocyclone. The design of the hydrocyclone was a modified Stairmand high-efficiency geometry with a long cone. The results are, among other things, shown as spatial tracks of the tracer particle as it moves through the hydrocyclone. Several interesting features were seen. The particle path, although the particle was much larger than the cut size of the cyclone, exhibited excursions into the inner, upwardly directed, part of the vortex giving rise to recirculatory loops. Moreover, at a particular position low in the cyclone, the particle exhibited a complicated flowpattern moving up and down repeatedly across this position. Careful analysis of the motion is presented, particularly of the motion low in the hydrocyclone, on basis of which it is made likely that this position represents the end of the vortex in the hydrocyclone.     

Local dynamics in epoxy coatings containing iron oxide nanoparticles by dielectric relaxation spectroscopy

Nanocomposites of photocurable epoxy resin and epoxy‐modified iron oxide magnetic nanoparticles were analyzed by dielectric relaxation spectroscopy to study the local dynamics at temperatures well below the glass‐transition temperature. Two secondary processes were detected, β and γ processes, but the second one was just detected at lower temperatures in the high‐frequency part of the spectra and moved out of the frequency range at higher temperatures. Data were fitted to the Havriliak–Negami and Arrhenius models, and the obtained parameters were analyzed. Relaxation times of the β secondary relaxation did not change with the nanoparticle content, but the relaxation strength increased. The increase could not be explained when we took into account the molecular origin of the relaxation. The presence of ferromagnetic nanoparticles enhanced the internal field and increased the relaxation strengths. Transmission electron microscopy images showed that the nanoparticles were well dispersed in the matrix, without magnetic agglomerates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Lipid--protein interactions revealed by two-photon microscopy and fluorescence correlation spectroscopy

Cellular processes involve a multitude of chemical reactions that must be kept in delicate equilibrium to maintain cellular homeostasis. Powerful biophysical techniques are needed to measure the localization and concentration of target molecules as well as to quantify complex molecular processes in model and in vivo systems. Two-photon microscopy and fluorescence correlation spectroscopy (FCS) can measure association and dynamics of appropriate molecules under equilibrium conditions. FCS provides information on motility (diffusion coefficients), concentration (number of particles), association (molecular brightness), and localization (image) of the target molecules. All of this information, in conjunction with computational modeling techniques, can help us to better understand the network of complex molecular interactions, which are at the basis of cellular processes. Fluorescence imaging techniques add the beauty of visualization to the scientific information. Photons emitted by a fluorescent dye are digitized, and the associated spatial information and intensity can be translated into different colors and shades providing the researcher not only with quantitative intensity information but also with spatial resolution and visual comprehension of two- or three-dimensional images. In this Account, we review the use of two-photon excitation microscopy and FCS in the study of lipid-protein interactions. We discuss these new methodologies and techniques, and we present examples of different complexity from qualitative to quantitative, from simple model systems to studies in living cells.     

Thermal dynamics of side-chain copolymethacrylates as studied by the dielectric spectroscopy and relaxation of second-harmonic generation

A series of copolymethacrylates with different contents of tolane-based mesogenic groups have been synthesized. The mesogenic group content was characterized with ¹H-n.m.r.. The phase behaviours were determined using a differential scanning calorimeter and optical polarizing microscopy. A smectic A phase was obtained when the mesogenic group content was increased up to 80 mol.%. Dielectric relaxation results indicated that the amplitude of the -relaxation was suppressed significantly due to the presence of the liquid crystalline phase. The reduction of the molecular motion is beneficial to the enhancement of the temporal stability of the effective second-harmonic coefficient for the polymer with a higher mesogenic group content. Moreover, the second harmonic coefficient is enhanced as the mesogenic group content increases. The self-alignment nature of the liquid crystal phase is favourable for alignment of the NLO-active mesogenic group under an applied electric field and preserving such alignment after removal of the electric field. The relationship between thermal dynamic behaviour and second-order nonlinear optical properties is also discussed.     

Particle dynamics and relaxation in bimodal suspensions during drying using multispeckle diffusing wave spectroscopy

The motion of polystyrene particles in a bimodal suspension drop during drying was characterized via multispeckle diffusing wave spectroscopy. The fast and slow dynamics of bimodal particles, which were expressed in terms of autocorrelation function data from two kinds of cameras, were well connected in short and long lag time regimes. Characteristic time of β‐relaxation, representing the rapid movement of bimodal particles, became lower in the short lag time region as the portion of small particles increased, reflecting their fast Brownian motion. The difference in the relaxation features between bimodal suspension with large and small particles and unimodal suspension with only large particles became more evident as the size ratio between particles was high. Drying temperature could encourage the particle movement at the early stages of drying, leading to lower relaxation time, and inversely retard the relaxation behavior when drying time further elapsed, due to the structural rearrangement of particles. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1114–1121, 2017     

Optical excitations in organic dendrimers investigated by time-resolved and nonlinear optical spectroscopy

Our research is concerned with the optical properties of covalently bound branched multichromophore systems. The presence of strong intramolecular interactions in dendrimers and other branched macromolecules has stimulated new approaches toward improved energy transfer and light-emitting and enhanced nonlinear optical materials, as well as the possibility of delocalized (exciton) excitations in molecular aggregates. This Account summarizes some of our investigations, which combine the use of different time-resolved techniques to examine the dynamics in organic (conjugated) branched structures and provide important structure-function correlations necessary for applications.     

The photo-oxidation of poly(vinyl chloride) investigated by a stress relaxation technique

A stress relaxation technique was used to study the kinetics of the photo-oxidation of plasticized poly(vinyl chloride) film. The effects of ultraviolet irradiation were characterized by the slope of a difference line between the stress relaxation curve of an irradiated test sample and that of a nonirradiated control. An activation energy of 12 kcal/-mole was obtained for the rate-controlling reaction, which was postulated to be hydrogen abstraction from the substrate by peroxy radicals. The rate of relaxation was dependent on the radiation energy, being greater at lower energies. It was also independent of oxygen pressure at “high” pressures and dependent at “low” pressure. The nature of the plasticizer and the presence of stabilizer and ultraviolet absorbers all affected the relaxation behavior.     

Hydrolysis and condensation of alkoxysilanes investigated by internal reflection FTIR spectroscopy

The hydrolysis of a series of alkyl-substituted alkoxysilanes and an antimicrobial quaternary ammonium silane was investigated in water-acetone solvents by Fourier transform infrared (FTIR) spectroscopy. An internal reflectance cell was employed for the investigation. Rates of sequential hydrolysis of the first, second, and in one case the third alkoxy groups were extracted by nonlinear regression of curves of methanol concentration plotted versus time using data obtained from the FTIR spectra of the reaction mixture. Acid catalysis of the hydrolysis was observed. Condensation of the silanol groups produced by the hydrolysis was investigated using similar techniques and rate data are presented.     

Stress relaxation and hysteresis of nanocomposite gel investigated by SAXS and SANS measurement

The stress relaxation phenomena of nanocomposite gel (NC gels) after uniaxial elongation was investigated by time-resolved small-angle scattering techniques of neutrons and X-rays. Nanocomposite gels consist of clay platelets and poly(N-isopropylacrylamide). It was found that clay oriented instantly and polymer chains were elongated along the stretching direction by elongation, followed by gradual process of peeling-off of adsorbed polymer chains on the clay platelets. When the specimen was held in the deformed state, stress relaxation was observed. This was mainly ascribed to peeling-off of polymer chains. When the strain of the specimen was removed, the polymer chains tended to be adsorbed again to the surface of the clay platelets. The deformation mechanism of NC gels is discussed on the emphasis of the peeling-off and peeling-on process of polymer chains.     

Adsorption of benzothiophene on Y zeolites investigated by infrared spectroscopy and flow calorimetry

Diffuse reflectance infrared spectra of benzothiophene adsorbed on different Y zeolites reveal that the cations and protons in the zeolites are the sites responsible for the adsorption of benzothiophene. On NaY, benzothiophene was molecularly adsorbed on the cations through the electrophilic interaction between the cations and the thiophenic rings. On the transition metal ion exchanged NiY and CuY zeolites, because of the presence of the d-electrons in the cations, the thiophenic rings interact with the cations to form the π-complexes through the σ–π electron donations. In the presence of hydroxyl species in the zeolites, the adsorbed sulfur compounds attach to the protons molecularly via the electrophilic interaction and undergo the opening of the thiophenic rings depending on the acidity of the zeolites and the adsorption amount. The apparent heat of adsorption of benzothiophene in normal octane on the Y zeolites determined by flow calorimetry shows that the adsorption strength based on the measured heat for each mole sulfur adsorbed on the Y zeolite is in the order of CuY > NiY > NaY  USY. For USY, due to the endothermic breakage of the thiophenic ring of benzothiophene induced by the acid sites of the zeolite, the apparent heat of adsorption is similar to that obtained from the adsorption on NaY. This work demonstrates that the transition metal ion exchanged zeolites exhibit excellent properties for sulfur adsorption because of the formation of the π-complexes and that the acidity of the zeolites is not advantageous for sulfur removal due to the strong adsorption and decomposition of the adsorbed species.     

Sorption of methanol on zeolite HBeta investigated by in situ MAS NMR spectroscopy

Methanol adsorption and desorption experiments were performed on acidic zeolites applying a new in situ MAS NMR technique. ¹H MAS NMR spectra of dehydrated zeolites were recorded during injection of nitrogen gas loaded with methanol into the MAS rotor. The spectra indicated that in zeolite HBeta also silanol groups are involved in the adsorption of methanol which is in contrast to the behaviour of SiOH groups in zeolites HZSM-5 and HY. The desorption experiments show that about 70% of the methanol molecules contributing to adsorbate complexes in zeolite HBeta are weakly hydrogen-bonded at silanol groups. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural relaxation dynamics of a silicate glass probed by refractive index and ionic conductivity

Relaxation occurs spontaneously in all glasses and is a fundamental step of important technological processes, such as annealing, crystal nucleation, and chemical strengthening by ion exchange. Despite extensive studies over the past decades, there are still conflicting results on whether the kinetics of structural relaxation depends on the analyzed property. Thus, in this study, we used a lithium disilicate glass as a model composition to determine the structural relaxation kinetics during physical aging experiments by measuring the time evolution of the refractive index and ionic conductivity down to 35 K below the initial fictive temperature. In all cases, variations in these properties were adequate to capture the structural changes throughout the aging experiments. At each temperature, the experimental relaxation data fit quite well with the classical stretched exponential relation. We also found that the relaxation process starts faster when probed by ionic conductivity than by refractive index; however, they show similar average relaxation times. These very small structural rearrangements are always the same, but ionic conductivity changes faster than refractive index at the beginning of the process. Our comprehensive results strongly indicate that relaxation dynamics is indeed dependent on the analyzed property.     

Determination of trace elements in oils by plasma emission spectroscopy

An analytical method for the determination of phosphorus and other elements at low concentrations in edible oils has been developed using a plasma emission spectrometer. The method is fast because it eliminates the ashing process in sample preparation and because the actual measurement takes less than 1 min. Reproducibility and accuracy of the measurement are good and very low detection limits have been observed, e.g., 0.5 ppm for phosphorus and 5 ppb for copper. However, the long-term stability of the instrument cannot as yet be guaranteed and a procedure that prescribes regular measurement of samples of known concentration is essential as a means of detecting any drift in the output signal.     

Dielectric loss and phase transition of sodium potassium niobate ceramic investigated by impedance spectroscopy

The dielectric permittivity of Na_0.80K_0.20NbO₃ ceramic was investigated by impedance spectroscopy. The dielectric characterization was performed from room temperature to 800 °C, in the frequency range 5 Hz–13 MHz. The bulk permittivity was derived by the variation of the imaginary part of the impedance as a function of reciprocal angular frequency. The permittivity values as a function of temperature showed two maxima. The first maximum is very similar at 200 °C and the second one positioned at around 400 °C, which was associated to Curie’s temperature. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency dispersion was investigated in terms of dielectric loss. The Na_0.80K_0.20NbO₃ showed a dissipation factor between 5 and 40 over a frequency range from 1 to 10² kHz.     

Microdomain structure and chain orientation in polypropylene/polyethylene blends investigated by micro-Raman confocal imaging spectroscopy

Compositional domain structure of blends of isotactic polypropylene with linear polyethylene and chain orientation of neat polymers and of the blends were assessed by micro-Raman confocal imaging spectroscopy and FT Raman spectroscopy. The results were correlated with polarised photoacoustic FTIR and with DSC. The surface and inner parts of compression-moulded, injection-moulded and drawn specimens were compared. Polymer domains in the blends and domains of different orientation were identified. The larger size of the polymer domains and lower chain orientation in the core of the specimens prepared by injection-moulding were explained by different cooling conditions of the melt. Possibilities and limitations of the micro-Raman confocal spectroscopy method were discussed.     

Molecular dynamics and NMR spin relaxation in proteins

Molecular dynamics simulations often play a central role in the analysis of biomolecular NMR data. The focus here is on NMR spin-relaxation, which can provide unique insights into the time-dependence of conformational fluctuations, especially on picosecond to nanosecond time scales which can be directly probed by simulations. A great deal has been learned from such simulations about the general nature of such motions and their impact on NMR observables. In principle, relaxation measurements should also provide valuable benchmarks for judging the quantitative accuracy of simulations, but there are a variety of experimental and computational obstacles to making useful direct comparisons. It seems likely that simulations on time scales that are just now becoming generally feasible may provide important new information on internal motions, overall rotational diffusion, and the coupling between internal and rotational motion. Such information could provide a sound foundation for a new generation of detailed interpretation of NMR spin-relaxation results.