Neutrons for the study of dynamics in soft matter systems

Neutron scattering is a valuable tool not only in studying the structure but also the dynamics of nanoscale soft matter systems. By combining different neutron scattering methods, it is possible to observe energy changes from millielectronvolt to nanoelectronvolt. A unique advantage of neutron scattering is the possibility to highlight different parts of a system simply by changing its isotopic composition, allowing to study specific parts of complex systems. This review tries to give an overview of the possibilities of neutron scattering methods together with some recent examples, mostly from neutron spin-echo spectroscopy, which is the method allowing to measure the smallest changes in energy.     

Electronic excitations in synthetic eumelanin aggregates probed by soft X-ray spectroscopies

Electronic excitations of condensed phase eumelanin aggregates are investigated with soft X-ray spectroscopies. Resonant photoemission data indicate that mechanisms of charge delocalization may occur when electrons are excited about 3 eV above the first unoccupied electronic level. An average, lower limit value of 1.6 fs was estimated for the lifetime of the excited C 1s-pi* states.     

Ultrafast dynamics of a solution in spatially restricted environments studied by photothermal spectroscopies

The ultrafast dynamics of a solution in spatially restricted environments was studied by using the ultrafast transient lens (UTL) method. The UTL method is used to monitor the molecular dynamics of a solution by means of a change in the refractive index, which is advantageous for investigating the molecular dynamics of restricted systems. We investigated the photoisomerization of azobenzene derivatives in cyclodextrin nanocavities and revealed how the confinement affects the photoisomerization dynamics and yields. We also studied the relaxation dynamics of photo-excited auramine O (AuO) in a water/aerosol-OT/n-heptane reversed micelle. Both the perturbed properties of the included water and the interactions between AuO and the interface of the reversed micelle strongly appeared to affect the relaxation dynamics. At the same time, we observed a change in the refractive index suggesting a structural change of the micelles in the picosecond region that could not be detected by transient absorption spectroscopy. In addition, we developed the total internal reflection UTL (TIR-UTL) method to monitor the ultrafast molecular dynamics at the liquid interface. The relaxation dynamics of photoexcited AuO at the silica/water interface were observed with subpicosecond time resolution, and it was revealed that the interaction with the interface strongly inhibited the relaxation process. These results demonstrated the advantages of the UTL method for investigating the molecular dynamics of a solution in spatially restricted environments.     

Modelling charge transport in organic semiconductors: from quantum dynamics to soft matter

The charge carrier dynamics in organic semiconductors has been traditionally discussed with the models used in inorganic crystalline and amorphous solids but this analogy has severe limitations because of the more complicated role of nuclear motions in organic materials. In this perspective, we discuss how a new approach to the modelling of charge transport is emerging from the alliance between the conventional quantum chemical methods and the methods more traditionally used in soft-matter modelling. After describing the conventional limit cases of charge transport we discuss the problems arising from the comparison of the theory with the experimental and computational results. Several recent applications of numerical methods based on the propagation of the wavefunction or kinetic Monte Carlo methods on soft semiconducting materials are reviewed.     

Probing ultrafast dynamics in adenine with mid-UV four-wave mixing spectroscopies

Heterodyne-detected transient grating (TG) and two-dimensional photon echo (2DPE) spectroscopies are extended to the mid-UV spectral range in this investigation of photoinduced relaxation processes of adenine in aqueous solution. These experiments are the first to combine a new method for generating 25 fs laser pulses (at 263 nm) with the passive phase stability afforded by diffractive optics-based interferometry. We establish a set of conditions (e.g., laser power density, solute concentration) appropriate for the study of dynamics involving the neutral solute. Undesired solute photoionization is shown to take hold at higher peak powers of the laser pulses. Signatures of internal conversion and vibrational cooling dynamics are examined using TG measurements with signal-to-noise ratios as high as 350 at short delay times. In addition, 2DPE line shapes reveal correlations between excitation and emission frequencies in adenine, which reflect electronic and nuclear relaxation processes associated with particular tautomers. Overall, this study demonstrates the feasibility of techniques that will hold many advantages for the study of biomolecules whose lowest-energy electronic resonances are found in the mid-UV (e.g., DNA bases, amino acids).     

Compatibility problems of chlorpromazine HCl-stearate lubricants examined by differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to investigate the interactions between chlorpromazine HCl (CPZ) and stearate lubricants. The phase diagram was constructed for each CPZ-stearate system. An unknown endothermic peak at 145°C was found for the CPZ-stearate lubricants; this was affected by the grinding and heating processes. This unknown endothermic peak proved to relate to a polymorphism transition but was not an incompatibility problem. Three polymorphisms of CPZ were also determined by DSC analysis and IR spectrophotometry.     

Fabrication of circular microfluidic channels by combining mechanical micromilling and soft lithography

The fabrication of microfluidic channels with complex three-dimensional (3D) geometries presents a major challenge to the field of microfluidics, because conventional lithography methods are mainly limited to rectangular cross-sections. In this paper, we demonstrate the use of mechanical micromachining to fabricate microfluidic channels with complex cross-sectional geometries. Micro-scale milling tools are first used to fabricate semi-circular patterns on planar metallic surfaces to create a master mold. The micromilled pattern is then transferred to polydimethylsiloxane (PDMS) through a two-step reverse molding process. Using these semi-circular PDMS channels, circular cross-sectioned microchannels are created by aligning and adhering two channels face-to-face. Straight and serpentine-shaped microchannels were fabricated, and the channel geometry and precision of the metallic master and PDMS molds were assessed through scanning electron microscopy and non-contact profilometry. Channel functionality was tested by perfusion of liquid through the channels. This work demonstrates that micromachining enabled soft lithography is capable of fabricating non-rectangular cross-section channels for microfluidic applications. We believe that this approach will be important for many fields from biomimetics and vascular engineering to microfabrication and microreactor technologies.     

Characterization of the electronic structure of C50Cl10 by means of soft x-ray spectroscopies

The electronic structure of the last synthesized fullerene molecule, the C50Cl10, has been characterized by theoretical simulation of x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and near-edge x-ray-absorption fine structure. All the calculations were performed at the gradient-corrected and hybrid density-functional theory levels. The combination of these techniques provides detailed information about the valence band and the unoccupied molecular orbitals, as well as about the carbon core orbitals.     

Rotational dynamics of solvated carbon dioxide studied by infrared, Raman, and time-resolved infrared spectroscopies and a molecular dynamics simulation

Rotational dynamics of solvated carbon dioxide (CO(2)) has been studied. The infrared absorption band of the antisymmetric stretch mode in acetonitrile is found to show a non-Lorentzian band shape, suggesting a non-exponential decay of the vibrational and/or rotational correlation functions. A combined method of a molecular dynamics (MD) simulation and a quantum chemical calculation well reproduces the observed band shape. The analysis suggests that the band broadening is almost purely rotational, while the contribution from the vibrational dephasing is negligibly small. The non-exponential rotational correlation decay can be explained by a simple rotor model simulation, which can treat large angle rotations of a relatively small molecule. A polarized Raman study of the symmetric stretch mode in acetonitrile gives a rotational bandwidth consistent with that obtained from the infrared analysis. A sub-picosecond time-resolved infrared absorption anisotropy measurement of the antisymmetric stretch mode in ethanol also gives a decay rate that is consistent with the observed rotational bandwidths.     

Quasielastic neutron scattering in soft matter

The general trend in soft matter is to study systems of increasing complexity which are more technologically and biologically relevant. This is facilitated by the capability of quasielastic neutron scattering (QENS) to selectively probe spatially resolved dynamical modes at a molecular level. The large number of recent publications using QENS for investigating complex and multi-component soft matter systems, serves as recognition of the suitability of this technique by the scientific community. Exploiting its complementarity with molecular dynamics (MD) simulations and other experimental techniques is the basis of a successful methodology for this scientific challenge. We illustrate the potential of QENS with three kinds of soft materials whose structural units increase in size/complexity: lipids, polymers and biomolecules.     

Neutron reflectivity and soft condensed matter

During the last 10–15 years neutron reflectivity has emerged as a powerful and important technique for the study of surfaces and interfaces. The selectivity and sensitivity afforded by deuterium/hydrogen exchange makes the technique particularly attractive for application to the broad field of colloid and interface science. The development of the instrumentation, specialised sample environment equipment and analysis techniques has resulted its application to complex interfaces and environments and in the study of complex multi-component systems. This review provides a summary of those developments in the last two years.     

UV, VUV and soft X-ray photoabsorption of dimethyl ether by dipole (e,e) spectroscopies

Absolute UV and VUV photoabsorption oscillator strengths (cross-sections) for the valence shell discrete and continuum regions of dimethyl ether (CH₃OCH₃, DME) have been measured from 5 to 32 eV using high resolution (HR) (0.05 eV f.w.h.m.) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low resolution (LR) (1 eV f.w.h.m.), and this has been used to determine the absolute oscillator strength scale by employing valence shell Thomas–Reiche–Kuhn (i.e., S(0)) sum-rule normalization. The presently reported HR and LR absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for dimethyl ether in excellent agreement (within 0.5%) with previously reported polarizability values. Other dipole sums S(u), (u=−1,−3,−4,−5,−6,−8,−10), and logarithmic dipole sums L(u), (u=−1 to −6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum rules.     

Application of calorimetry and thermodynamics to critical problems in materials science

Calorimetry and thermodynamic studies have long been playing a very important role in the research fields of fundamental science and technology. Some topics and examples of thermodynamics studies are given, and the details are explained on the basis of the present author’s experience, focusing attention to application of adiabatic calorimetry and thermodynamics to solve critical problems in materials science: (1) condensed gas calorimetry and third law entropy, (2) phase transition and polymorphism in simple molecular crystals, (3) incommensurate phase transitions, (4) particle size effects on the phase transitions in ferroelectric/ferroelastic crystals, (5) relaxor ferroelectrics and multi-ferroics, and some other topics in materials science and technology.     

A Hamiltonian describing molecular orientation for use in latiice dynamics of molecular crystals and other problems

A quantum-mechanical Hamiltonian describing molecular orientation is developed for use in the lattice dynamics of molecular crystals. It is expressed in terms of three of the four quaternions as coordinates together with their conjugate momenta.     

Recent developments and applications of NSE in soft matter

Neutron Spin Echo is probably the youngest spectroscopic method in neutron scattering. After a brief introduction I will try to give a short account of recent progress on the instrumentation. Through some recent examples I will illustrate that NSE is a powerful tool in colloid science applied to both “old” and recent problems. With new instrumentation we will be able to tackle new problems and one of those areas in which there has been recently exciting progress is the field of surface science.     

Fragility of short-chain poly(lactic acid)s derivatives by combining dielectric spectroscopy and fast scanning calorimetry

Low-molecular-mass bio-polyesters, such as lactic acid oligomers, constitute a growing category of plasticizers for poly (lactic acid), since they show good compatibility and are respectful of the environment. This study focuses on the glass transition of a series of oligomers with different molecular weights, with the aim of investigating how their associated dynamics occur. Dielectric and calorimetric results are combined to study the molecular mobility in a large temperature range. In comparison with poly (lactic acid), the oligomers exhibit a lower fragility index and smaller cooperative rearranging regions. Among oligomers, the fragility is clearly dependent on the molecular weight, whereas the variations of the cooperativity are subtler.     

Flow-SANS and Rheo-SANS applied to soft matter

We review the current literature pertaining to the characterization of soft matter subject to flow utilizing small-angle neutron scattering, flow-SANS, with an emphasis on the simultaneous measurement of the rheology, Rheo-SANS. Experimental results are discussed in terms of the flow induced structure and direct connection to the bulk rheology in which we highlight the use of the contrast match method as a unique advantage to neutron scattering techniques. Finally, we discuss specific areas in each field that could benefit from focused flow-SANS experiments, and the projected evolution of specialized flow-SANS sample environments.