Structure Of Soft Materials Research Articles

Analysis of native structures of soft materials by cryo scanning probe tomography

We propose a universal tool for structural analysis of soft/frozen-hydrated materials at a macromolecular level over a wide range of temperatures (−120 to +50 °C). The cryogenic scanning probe microscope presented here allows detailed investigation and serial-section 3D tomography of soft matter under native-like conditions.

Synchrotron XRR study of soft nanofilms at the mica–water interface

We describe here the design of a liquid cell specific for synchrotron X-ray reflectometry (XRR) characterisation of soft matter nanofilms at the mica–water interface. The feature of the cell is a “bending mica” method: by slightly bending the mica substrate over an underling cylinder the rigidity of the mica sheet along the bending axis is enhanced, providing sufficient flatness along the apex of the cylinder as required by XRR measurements. Using this cell, we have performed XRR measurements for a number of systems and in this article we show example results: (1) a cationic surfactant, C16TAB; (2) a zwitterionic surfactant, C12H25PC; (3) a semi-fluorinated surfactant, F4H11(d)TAB; and (4) surface complex of an anionic fluorinated surfactant, CsPFN, and a positively charged polymer, PEI. For the data analysis we have taken into account the mica crystal truncation rod, i.e. the reflectivity from the mica substrate, and fitted the data with a custom Java™ based software package. Our results unravel detailed structural information of these soft nanofilms, indicating that this method is suitable for XRR measurements of a wide range of soft matter structures at the mica–water interface.

Dynamic size tuning of multidimensional optically bound matter

We generate and dynamically control one-, two- and three-dimensional optically bound structures of soft matter in the geometry of counter-propagating incoherent laser beams. We report results for the Bessel, Gaussian, and Laguerre-Gaussian laser modes and particularly focus on the influence of the lateral dimensions of the beam profile on the resulting self-arranged optically bound structures. Employing the transfer of the orbital angular momentum of light in the Laguerre-Gaussian beams, we show that optically bound structures can conserve their spatial arrangements even while orbiting along the beam circumference.

Analyses of Hierarchal Structures of Soft Materials by Using Combined Scattering Methods

Analyses of Hierarchal Structures of Soft Materials by Using Combined Scattering Methods

Curved Structures in Soft Materials Stabilized by the Flexoelectric Effect

Curved Structures in Soft Materials Stabilized by the Flexoelectric Effect

Analyses of Hierarchical Structures of Soft Materials by Using Combined Scattering Methods

Most of soft materials form hierarchical structures and their rich varieties of mechanical properties originate from the structures and the correlation among hierarchies. Here we discuss how we can analyze the hierarchical structures in soft materials by using the combination of various scattering methods. We describe the features of each scattering method briefly and show the examples of the analyses of soft materials with the combined scattering methods.

Volume confinement induced microstructural transitions and property enhancements of supramolecular soft materials

The rheological properties of supramolecular soft functional materials are determined by the networks within the materials. This research reveals for the first time that the volume confinement during the formation of supramolecular soft functional materials will exert a significant impact on the rheological properties of the materials. A class of small molecular organogels formed by the gelation of N-lauroyl-L-glutamic acid din-butylamide (GP-1) in ethylene glycol (EG) and propylene glycol (PG) solutions were adopted as model systems for this study. It follows that within a confined space, the elasticity of the gel can be enhanced more than 15 times compared with those under un-restricted conditions. According to our optical microscopy observations and rheological measurements, this drastic enhancement is caused by the structural transition from a multi-domain network system to a single network system once the average size of the fiber network of a given material reaches the lowest dimension of the system. The understanding acquired from this work will provide a novel strategy to manipulate the network structure of soft materials, and exert a direct impact on the micro-engineering of such supramolecular materials in micro and nano scales.

Quantitative analysis and relevant features of the scientific literature related to SAXS and SANS

We present and discuss here numerical information derived from a systematic searching of scientific papers related to SAXS and SANS published in indexed journals – from 1945 until nowadays – recorded by the Web of Science Data Bank (WoS). We have detected interesting features regarding the time dependence of the number of papers/year, N(t), indicating the existence of three well-defined periods of historical evolution with rather well-defined boundaries. All three periods exhibit a positive and approximately linear variation of N(t) but, at the two transitions between periods, the rate of growth exhibits clear and strong increases. Differences of the historical evolutions in the numbers of papers/year related to SAXS and to SANS were established. The different behaviours regarding the numbers of papers/year related to SAXS and to SANS and the existence of three different and well defined periods for N(t) can be qualitatively understood as a consequence of the progressive and increasing availability along the last three decades of very brilliant synchrotrons, last generation commercial X-ray sources, new neutron facilities, powerful computers and novel theoretical approaches for SAS data analysis. The rates of growth in the number of papers/year published by authors from a set of different countries are approximately constant along the last two decades. For other countries we have detected a slowing down effect in the number of papers/year while a clear acceleration could be noticed for the production of SAS papers by authors from several emerging countries. These opposite trends compensate in such a way that the number of SAS (SAXS+SAXS) articles published per year all around the world maintained a vigorous linear growth – during more than 20 years – at a constant rate of 60 papers/year, without any indication of eventual saturation. The observed distribution of articles among different journals indicates that a very high fraction of the volume of SAS research is focused to the structure of soft matter.

Three-dimensional structure and multistable optical switching of triple-twisted particle-like excitations in anisotropic fluids

Control of structures in soft materials with long-range order forms the basis for applications such as displays, liquid-crystal biosensors, tunable lenses, distributed feedback lasers, muscle-like actuators and beam-steering devices. Bistable, tristable and multistable switching of well-defined structures of molecular alignment is of special interest for all of these applications. Here we describe the facile optical creation and multistable switching of localized configurations in the molecular orientation field of a chiral nematic anisotropic fluid. These localized chiro-elastic particle-like excitations--dubbed 'triple-twist torons'--are generated by vortex laser beams and embed the localized three-dimensional (3D) twist into a uniform background. Confocal polarizing microscopy and computer simulations reveal their equilibrium internal structures, manifesting both skyrmion-like and Hopf fibration features. Robust generation of torons at predetermined locations combined with both optical and electrical reversible switching can lead to new ways of multistable structuring of complex photonic architectures in soft materials.

Novel Chiral Effect That Produces the Anisotropy in 3D Structured Soft Material: Chirality-Driven Cubic−Tetragonal Liquid Crystal Phase Transition

The effect of chirality on the liquid crystal organization has been systematically investigated between (S,S)- and (R,R)-isomers of a dichiral azobenzene compound. The racemic mixture was found to show the cubic phase (Im3m), indicating that the studied compound shows the cubic structure without the effect of chirality. However, with increasing optical purity of the racemic mixture, the cubic (Im3m) structure changes into the tetragonal phases (I422 and I4(1)22), which have been known as the SmQ phases, via a chiral cubic phase (I432). This is the first example of "chirality-driven" cubic-tetragonal structural change observed in liquid crystal systems, which is experimental evidence showing the novel type of chiral effect that produces the anisotropy in the structures of soft materials. The emergence of the anisotropy, i.e., the cubic-tetragonal phase transformation, is also induced by means of photoinduced phase transition of the SmQ phases based on cis-trans photoisomerization of the azobenzene compounds.

Confined polymer systems: synergies between simulations and neutron scattering experiments

Molecular simulations and neutron reflectivity are both extremely valuable tools for determining the structure of soft matter at interfaces and under confinement. The high resolution structural information provided by these techniques allows us to obtain a thorough understanding at the molecular level. Here we present examples of polymer thin films and show the advantages these two techniques offer and how we can combine the approaches in order to provide a complete structural and thermodynamic picture.

Physical aging of plastoferrites under tensile stress and its effect on microwave properties

Aging phenomena in soft viscoelastic materials have been used as an important tool to investigate the physics of complexity for both scientific and practical purposes. At ambient conditions, microwave frequency-domain spectroscopy (gigahertz-FDS) is employed to investigate the electromagnetic properties at continuum length scales for two commercial plastoferrites (PFs) subjected to a uniaxial tension during a long time t of application. PFs are an attractive prototype for investigating the interactions between polarization and magnetization phenomena, and time. For both PFs we have measured simultaneously the time evolution of the effective complex permittivity ε=ε′−jε″ and magnetic permeability μ=μ′−jμ″ spectra during tensile-loading experiments. These changes allow a determination of how physical aging affects the microwave properties of PFs with tensile elongation in the range of 3%–17% on the time scale of hundreds of hours. At short times of strain application the ε and μ measurements under stress can be explained in terms of a Gaussian molecular network model (affine behavior) in the limit of low strain. However, the large-strain mechanical response does show nonaffine behavior. The key achievement is that we present observational evidence for a three-stage aging in which (“zone I”) ε′ and ε″ increase as the ln(t) and reach a peak value at tI, then (“zone II”) there is a slower decrease in ε′ and ε″∝−ln(t) as time moves from tI up to tII, and finally, (“zone III”) as time moves to higher values, ε′ and ε″ saturate for long times. We have provided experimental evidence that, while tI is found to be constant with the initial elongation ratio λ0 that is applied to the PF sample, the data revealed that tII∝λ0. In sharp contrast, there is no significant change in μ′ and μ″ as a function of the elongation ratio. It can be argued that the decoupling of the dielectric properties from the magnetic properties may be regarded as a consequence of rigidity of the ferrite grains. To probe the influence of aging that alters the initial morphology of the PF samples, scanning electron microscopy (SEM) was used. The results of the SEM studies indicate the existence of cracks at the PF surface and disagglomeration of the ferrite grains. A tentative phenomenological model is proposed to explain the three-stage evolution of the elasticity network which has been tracked by gigahertz-FDS during the isothermal application of tension. Because the structure of soft materials is scale dependent, the response to a mechanical load also depends on the length and time scales of the probing excitation. We argued that the effective permittivity increase during stage I can be attributed to the electric dipole reorientation with respect to the direction of the polarization under the tensile stress applied to the material. The physical mechanism driving the effective permittivity decrease in stage II is thought to be a consequence of the breakup of the adsorbed ferrite particle network due to the detachment of chains from the surface of the particles. Stage III reflects the eventual stable steady state for which local stresses are redistributed in the elasticity network of the filled polymer under strain. The understanding of the physical aging mechanisms is significant because it can offer a strategy for materials selection and PF-based device performance optimization.

Ultrafast excited-state dynamics of nanoscale near-infrared emissive polymersomes.

Formed through cooperative self-assembly of amphiphilic diblock copolymers and electronically conjugated porphyrinic near-infrared (NIR) fluorophores (NIRFs), NIR-emissive polymersomes (50 nm to 50 microm diameter polymer vesicles) define a family of organic-based, soft-matter structures that are ideally suited for deep-tissue optical imaging and sensitive diagnostic applications. Here, we describe magic angle and polarized pump-probe spectroscopic experiments that: (i) probe polymersome structure and NIRF organization and (ii) connect emitter structural properties and NIRF loading with vesicle emissive output at the nanoscale. Within polymersome membrane environments, long polymer chains constrain ethyne-bridged oligo(porphinato)zinc(II) based supermolecular fluorophore (PZn n ) conformeric populations and disperse these PZn n species within the hydrophobic bilayer. Ultrafast excited-state transient absorption and anisotropy dynamical studies of NIR-emissive polymersomes, in which the PZn n fluorophore loading per nanoscale vesicle is varied between 0.1-10 mol %, enable the exploration of concentration-dependent mechanisms for nonradiative excited-state decay. These experiments correlate fluorophore structure with its gross spatial arrangement within specific nanodomains of these nanoparticles and reveal how compartmentalization of fluorophores within reduced effective dispersion volumes impacts bulk photophysical properties. As these factors play key roles in determining the energy transfer dynamics between dispersed fluorophores, this work underscores that strategies that modulate fluorophore and polymer structure to optimize dispersion volume in bilayered nanoscale vesicular environments will further enhance the emissive properties of these sensitive nanoscale probes.

The Trapezoidal Cylinder Phase: A New Mode of Self-Assembly in Liquid-Crystalline Soft Matter

A new rectangular columnar liquid crystalline phase with p2gg lattice is reported, which represents a polygonal cylinder array composed of cylinders with trapezoidal cross section. In these polygonal cylinders, one of the sides has a different length and is composed of a different material than the others. This tiling pattern was obtained in two series of T-shaped facial amphiphilic triblock molecules in which a rigid rod-like p-terphenyl core is substituted laterally by a polar and flexible oligoethylene glycol chain, terminated either by a hydrogen-bonding COOH group or by a Li carboxylate group, and having identical or different alkyl groups in the terminal positions. The trapezoidal cylinder phase provides an improved packing for relatively long and rigid alkyl chains at lower temperature and more space inside the polygonal cylinders than triangular cylinders. This combination of conformational and space-filling effects leads to different phase sequences. The trapezoidal cylinder phases pave the way to a new level of complexity in LC engineering and show the huge potential of the general concept of polyphilic tectons for the design of new complex soft matter structures.

Imaging of soft matter with tapping-mode atomic force microscopy and non-contact-modeatomic force microscopy

In this work, we take topographic images of DNA molecules and nanometre-sizedpolystyrene spheres in air as well as in water using an atomic force microscope (AFM). Wewould like to compare the applied forces and the resolution for images taken with theamplitude-modulation (AM) detection, or the tapping mode, and the frequency-modulation(FM) detection, or the non-contact (NC) mode. Experiments for these two modes arecarried out on the same area of the samples, with the same tip, under the sameenvironment, and with the same oscillation amplitude. Our experiments indicate that, inthe FM mode, the tip exerts a very gentle force on soft materials and provides a heightmeasurement close to the true value. In the AM mode, the tip exerts a stronger force onsoft materials and causes their deformation, especially in the liquid environment. Theresolution of the FM mode is about the same as that of the AM mode for operation in air,but the former is significantly superior to the latter in water. We thus concludethat the FM mode can obtain images with a higher resolution and provides asufficient sensitivity to image the true and fine structure of soft matter on surfaces.

Formation of Periodic Minimal Surface Structures in Polymeric Materials

We shall review our recent theoretical study of formation of curious mesoscopic structures in soft matter, especially in diblock copolymers. Universality of the mechanism of forming these structures is emphasized in terms of the coarse-grained approach.

Using spin polarised positive muons for studying guest molecule partitioning in soft matter structures

Fully polarised positive muons substituted for protons in organic free radicals can be used as spin labels which reveal information about the structure, dynamics and environment of these radicals. In applications via the technique of avoided-level-crossing muon spin resonance (ALC-microSR), the positive muon has been used to study the partitioning of phenyl alcohols in lamellar phase colloidal dispersions of a cationic dichain surfactant. Here we describe the experimental technique which permits highly sensitive spectroscopy as previously demonstrated for surfactant mixtures. We also demonstrate its capability in the study of partitioning of cosurfactant molecules in surfactant bilayers in order to elucidate the main factors which contribute to cosurfactant ordering at interfaces. The technique takes advantage of the positive muon combining with an electron to a hydrogen-like atom that is called muonium. This atom attaches to a phenyl group, forming a cyclohexadienyl-type radical that contains the muon as a polarised spin label, providing an excellent probe even for very low phenyl alcohol concentrations. The position of one type of resonance, which on the basis of spectroscopic selection rules is denoted as Delta(0), is related to the solvent polarity of the radicals' environment. The results derived from Delta(0) measurements reveal a systematic trend where the increasing chain length of the phenyl alcohol results in a deeper immersion of the phenyl ring of the alcohol into the surfactant bilayer with the OH group anchored at the interface. In addition, the data suggest partial penetration of water molecules into the bilayer. Furthermore, data ensuing from a second resonance (called Delta(1), which is dependent upon the degree of confinement of the radical within the surfactant aggregate structure) indicates not only that the phenyl alcohol resides in an anisotropic environment, (i.e. that the host molecule is unable to undergo full 3-D reorientation on a timescale of 50 ns), but the resonance line widths indicate that the radicals are undergoing fast rotation about a particular axis, in this instance about the first C-C substituent bond at the phenyl ring. Detailed analysis of these Delta(1) line shapes suggests that other types of motion involving reorientation of the above rotation axis are also present. At room temperature, the hydrocarbon chains of the double layers form an aggregate state commonly referred to as the L(beta) phase, where the motions of surfactant alkyl chains are effectively frozen out. These chains melt on heating over a temperature range which is solution composition dependent (ca. 51 to 67 degrees C), but in all cases leading to a liquid-like disordered hydrocarbon regime whilst retaining the overall lamellar structure (and in this state is termed L(alpha)). Above the L(alpha)/L(beta) chain ordering phase transition the tracer molecules reside within the bilayer, but below this transition (and depending on their water-oil solubility) they are completely or partly expelled. This interpretation is further supported by Heisenberg spin exchange experiments. The water-bilayer partitioning reflects both typical classical and nonclassical hydrophobic solvation depending on temperature and chain length of phenyl alcohols.

Three-dimensional nano-structure of in situ silica in natural rubber as revealed by 3D-TEM/electron tomography

Dispersion of particulate nano-fillers has been assumed to be one of the most important determining factors of physical properties of the composites. In this paper, for the first time, a three dimensional (3D) observation of nano-structure of particulate silicas in natural rubber (NR) is reported by using a 3D transmission electron microscope (3D-TEM), which is a TEM combined with electron tomography to reconstruct 3D structural images. Here, in situ silica generated in NR matrix and a conventional silica (VN-3) mechanically mixed into NR were, without any staining of the samples, subjected to elucidation of their structural characteristics such as particle size, volume fraction and aspect ratio in NR matrix at nanometer scale. 3D-TEM/electron tomography enabled us to evaluate the density of in situ silica in NR matrix. The same samples were also subjected to atomic force microscopy (AFM) measurements to suggest a difference in the nano-filler/rubber interface area between the two samples. Since the dispersion of nano-filler in the amorphous matrix significantly controls the properties of nano-composites, 3D-TEM/electron tomography together with AFM is expected to play a most important role in revealing nanometer level 3D structure of soft materials.

The scaling behavior of a highly aggregated colloidal suspension microstructure and its change in shear flow

The nature of the network structure and the evolution of structural change in shear flow were investigated for metal particle dispersions in terms of fractal aggregation of colloidal particles. Polymer-stabilized metal particle inks were prepared via a polyvinyl chloride coating dispersed in solvent. The fractal dimension of 1.74 was calculated with the scaling model based on the power law relationship between the elastic modulus and volume fraction. This scaling behavior can be explained by considering the deformable network structure of soft materials. While the elastic property of the floc was dominant, the limit of linearity was found at the inter-floc link, which is relatively weak and brittle. The steady shear results reveal two mechanisms that contribute to the breakdown of the microstructure in metal particle inks at increasing shear rate. Scaling of steady shear viscosity shows that these mechanisms are related to both inter-floc interactions and the elasticity of the floc itself. Further, these results suggest that individual flocs deform with weak inter-floc interactions and rupture into smaller flocs or aggregates at high shear stress, which is associated with the increased shear rate.

Scattering studies of large scale structures at the ultra small angle neutron scattering instrument S18

In recent years ultra small angle neutron scattering (USANS) has developed into a powerful standard method for large scale structure investigations. The upgraded instrument S18 at the ILL's 58 MW high flux reactor is operated routinely with increasing beam time demand. The performance of the instrument and its abilities will be discussed in this paper. A peak to background ratio better than 10 5 is reached using Agamalian's tail reduction method. A q-range from 2.10 −5 up to 5.10 −2 A ̊ −1 can be covered. This allows a clear overlap with standard pinhole SANS instruments. The new way collecting scattering data logarithmically equidistant in q-space saves measuring time. This allows measuring times of about 1.5 h for strong scattering specimens with reasonable statistics. We will present an overview of recent experiments which have been performed in co-operation with different groups from the international user community. This work comprises of structure investigations of petroliferous sedimentary rocks showing fractal scattering behaviour and time resolved USANS studies of the dynamics of hydration of cement paste. Concerning soft matter structures, Pirelli rubber nanocomposites have been investigated. In addition, time resolved measurement on a D 2O solution of a PPO–PEO–PPO block copolymer (Reverse Pluronic 25R5) and the dynamics of phase separation of methyl-hydroxy-propyl cellulose (MHPC) have been studied using a sample temperature control system.