Weak Segregation Research Articles

Regulation of Crystallization Kinetics, Morphology, and Mechanical Properties of Olefinic Blocky Copolymers

Two olefinic blocky copolymers (OBCs) were quenched from different mixing states in the melt, and crystallization kinetics and morphology at various crystallization temperatures (Tcs) and corresponding mechanical properties were studied. It is observed that, at lower Tcs, premesophase separation in the melt accelerates crystallization of OBC-A with a weak segregation strength and a larger fraction of the crystalline hard blocks due to enrichment of the hard blocks in the hard-block-rich domains. By contrast, premesophase separation retards crystallization of OBC-B with a stronger segregation strength and lower fraction of the hard blocks because of the prevailing confinement effect at lower Tcs. Moreover, since the hard blocks dissolved in the soft-block-rich domains can crystallize at lower Tcs, which can bridge the crystals formed in different hard-block-rich domains, the crystal growth is not restricted. At higher Tcs, OBC-A crystallizes more slowly from the premesophase-separated melt than that from the homogeneous melt, which is attributed to the weaker crystallizability of the hard blocks dissolved in the soft-block-rich domains and thus the restricted crystal growth. Nevertheless, mesophase separation always takes place prior to crystallization at higher Tcs for OBC-B because of the faster rate of mesophase separation. Therefore, the mixing state in the melt has little effect on crystallization and morphology of OBC-B at higher Tcs. It is found that the mechanical properties of OBCs can be regulated in a wide range by alteration of crystallization conditions. Better mechanical properties can be achieved when OBCs crystallize from the homogeneous melt and at a lower Tc.

Block Copolymer Self Assembly during Rapid Solvent Evaporation: Insights into Cylinder Growth and Stability.

Solvent evaporation has proven to be a remarkably successful tool for directing self-assembly in block copolymers, yet the microscopic mechanisms, processing history dependence and macroscopic control parameters influencing pattern selection remain poorly understood. Here, we leverage dynamical field theory simulations to clarify how copolymer self-assembly proceeds during evaporation. We find that cylinders in the vertical orientation tend to form under modest evaporation rates and relatively weak segregation strengths, and link this behavior to nontrivial, morphology-dependent density correlations present at the ordering front.

MICROPHASE SEPARATION IN POLYDISPERSE GRAFT COPOLYMERS

The phase behavior of the polydisperse An+1Bnmultigraft copolymer melt is considered in a framework of weak segregation theory. The phase diagrams of melt are constructed within first harmonics approximation for two special cases: (i) lengths of backbone subchains between successive grafting points in each macromolecule are distributed according to the Schultz–Zimm distribution whereas lengths of side chains are fixed and (ii) both lengths of backbone subchains and side chains are distributed according to the same Schultz–Zimm distribution. The regions of stability of LAM, HEX and BCC mesophases in phase diagrams deform considerably as compared with the case of monodisperse graft copolymer. For both the cases the mean field phase diagrams indicate that order–disorder transition temperature grows as polydispersity index increases.

INFLUENCE OF INITIAL TEMPERATURES ON THE COOLING OF Ag–Pd BIMETALLIC CLUSTERS VIA MOLECULAR DYNAMICS SIMULATION

Atomic segregation in bimetallic clusters can influence the surface nucleation and also the structures of clusters. It is important to study the effect of atomic segregation on the structure. In this study, initial cooling temperatures were used to tune the atomic segregation ability. Molecular dynamics simulation with an embedded atom method was used to study the relationship between the structure and atomic segregation. It was found that the higher the initial cooling temperature, the more obvious the Ag atomic segregation. When the clusters cooled down from 800 K and 600 K, the clusters formed a mixed icosahedron due to the weak atomic segregation ability. When the initial cooling temperature is higher than 1200 K, all the Ag atoms segregated to the surface layer, the clusters formed a Pd – Ag -core–shell chemical ordering. For 1200 K initial temperature, the structure is decahedral. The clusters formed an fcc structure when the clusters cooled down from 2000 K, 1800 K, and 1500 K. When the clusters cooled down from 860 K and 900 K, not all the Ag atoms segregated to the surface layer, the clusters formed a core–shell chemical ordering with a mixed Pd – Ag core. But the structure of 860 K is twinned of icosahedron and decahedron and that of 900 K is decahedral. This means that the chemical orderings and structures were influenced by the atomic segregation.

Involvement of mitochondrial dynamics in the segregation of mitochondrial matrix proteins during stationary phase mitophagy

Mitophagy, the autophagic degradation of mitochondria, is an important housekeeping function in eukaryotic cells and defects in mitophagy correlate with ageing phenomena and with several neurodegenerative disorders. A central mechanistic question regarding mitophagy is whether mitochondria are consumed en masse, or whether an active process segregates defective molecules from functional ones within the mitochondrial network, thus allowing a more efficient culling mechanism. Here, we combine a proteomic study with a molecular genetic and cell biology approach to determine whether such a segregation process occurs in yeast mitochondria. We find that different mitochondrial matrix proteins undergo mitophagic degradation at distinctly different rates, supporting the active segregation hypothesis. These differential degradation rates depend on mitochondrial dynamics, suggesting a mechanism coupling weak physical segregation with mitochondrial dynamics to achieve a distillation-like effect. In agreement, the rates of mitophagic degradation strongly correlate with the degree of physical segregation of specific matrix proteins.

On Cylindrical PS-b-PMMA in Moderate and Weak Segregation

In recent years, there has been an explosion in the number of papers dealing with “cylindrical” PS-b-PMMA, for nanoscopic template or similar applications. This paper deals with two aspects of these systems. One is the detailed shape of the cores, i.e., the degree to which they are actually cylindrical in the hexagonal unit cell, as well as the nature of the core–corona interphase region. The second one relates to the use of fluorescence techniques to study these interphases, including whether or not one needs to consider energy transfer from one cylinder to neighboring ones. We approached these questions with numerical self-consistent field theory plus simulations of energy transfer based on them, and we examined two systems: one which mimics “typical” experimental systems at weak to moderate segregation, and one chosen to explore the limits of very weak segregation and polymer composition where the effects of interest could be maximal. In addition to providing basic information about the system characte...

English

The purpose of this study was to investigate the relationship between internal auditing and operational risk in Somalia since in my best knowledge there is no study examined the relationship between internal auditing and operational risk in Somalia. Five remittance companies were selected as a sample and self-administered questionnaire was distributed 80 employees of these companies, these employees contained internal auditors, operational managers, and financial mangers. Descriptive statistics was used firstly for the data description of the variable using mean and standard deviation, and then correlations of the variables were presented based on Pearson correlations. The findings of this study include that Somali Remittance Company’s risk identification is weak, weak segregation of duties, no identity of previous incurred risks, reporting behind the schedule and impaired internal audit independence, which highlighted broad operational risk in the remittance companies. As a whole the study found that internal audit and operational risk have significant negative relationship (r=-0.0.15 and P=0.893) with Cranach’s alpha of 0.90 or 90%. Key words: Internal auditing, internal control, Remittance firms, Somalia.

Phase separation in polydisperse rod–coil block copolymers

The stability limits of the homogeneous state of melts of rod–coil RC, RC2, and CRC polydisperse block copolymers have been investigated in the framework of the weak segregation theory. It was assumed that the number of units in either the rod-like R or the flexible C block is a random variable distributed by the Schulz–Zimm distribution. Inspection of the spinodal curves shows that the copolymer melts with polydisperse rigid blocks are less stable with respect to formation of the nematic state than melts with the monodisperse ones. If flexible C blocks are polydisperse the homogeneous state of a rod–coil melt is less stable against microphase separation than the homogeneous state of monodisperse melt of the same architecture.

Experimental investigation of solid mixing and segregation in a tetrapodal blender

Known limitations of tumbling blenders (weak diffusive axial mixing and segregation of free flowing granules) have provided the motivation to investigate the flow and mixing of granules inside a tetrapodal blender. This blender can be thought of as two V-shaped pairs of arms connected and twisted at their bottom ends. In this work, more than 100 experiments were carried out under a wide set of operating conditions and geometrical configurations. Compared to the conventional V-blender, this geometry is shown to provide shorter mixing times and better axial and radial mixing efficiency, especially when its upper or lower V-shaped part is twisted by 45° with respect to the rotation axis. Segregation of granules with different sizes and densities was investigated for varying rotational speeds (5–30RPM) and fill levels (35–65%V). It is observed that the segregation intensity is far less important in the tetrapodal blender than in the V-blender, and that it decreases significantly with an increase in rotational speed, the effect of the fill level being insignificant. It is also shown that kinetic sieving is the main governing mechanism for the segregation of granules. Finally, a criterion is proposed for the scale-up of the tetrapodal blender and the V-blender so that they may operate efficiently, without pronounced segregation.

Influence of the polydispersity of side chains on the phase behavior of multigraft copolymers

Weak segregation theory is applied to studying the phase behavior of a melt of a binary multigraft copolymer with polydisperse side chains regularly spaced along the backbone. The resulting phase diagrams of the melt show an increase of the temperature of transition to an ordered state with increasing coefficient of polydispersity of side chains. The effects of the average length of the side chains and the distance between the points of their grafting on the phase diagram of the melt are examined.

Structural and electronic properties of TM23 - pAgp (TM = Ni, Pd, and Pt) clusters in the dilute limit (p = 0–4): A density functional theory investigation

We studied the structural, energetic, and electronic properties of free-standing binary clusters in the dilute limit, TM23 − p Ag p with TM = Ni, Pd, and Pt, and p = 0–4, employing density functional theory within the generalized gradient approximation to the exchange-correlation energy functional, as implemented in the SIESTA code. For all studied systems, we found that the addition of Ag impurities decreases the binding energy of the clusters. The Ag impurities always prefer the surface sites, although not always the lowest coordination sites are occupied as it is observed in Pt, where large coordination surface sites are in general preferred. Clear Ag segregation is observed in Ni with the poly-icosahedral structure and Pt in the cubic-pyramidal structure. For Pd case a weak segregation and mixing patterns coexist depending upon which structure is shown, in the other cases in general a tendency to mixing is presented.

Site-preference and valency for rare-earth sites in (R-Ce)2Fe14B magnets

Rare-earth (R) permanent magnets of R2Fe14B have technological importance due to their high energy products, and they have two R-sites (Wyckoff 4f and 4g, with four-fold multiplicity) that affect chemistry and valence. Designing magnetic behavior and stability via alloying is technologically relevant to reduce critical (expensive) R-content while retaining key properties; cerium, an abundant (cheap) R-element, offers this potential. We calculate magnetic properties and Ce site preference in (R1−xCex) 2Fe14B [R = La,Nd] using density functional theory (DFT) methods—including a DFT + U scheme to treat localized 4f-electrons. Fe moments compare well with neutron data—almost unaffected by Hubbard U, and weakly affected by spin-orbit coupling. In La2Fe14B, Ce alloys for 0≤x≤1 and prefers smaller R(4f) sites, as observed, a trend we find unaffected by valence. Whereas, in Nd2Fe14B, Ce is predicted to have limited alloying (x≤0.3) with a preference for larger R(4g) sites, resulting in weak partial ordering and segregation. The Curie temperatures versus x for (Nd,Ce) were predicted for a typical sample processing and verified experimentally.

Self‐Assembly of Amphiphilic Block Copolymers in Ternary Solvent Mixtures: Lyotropic Liquid Crystalline Phase Behavior and Structure

AbstractThe effects of the simultaneous addition of two organic solvents (glycerol and ethanol) on the lyotropic liquid crystalline (LLC) phase behavior and structure formed by PEO–PPO–PEO amphiphilic block copolymers in water are examined. Glycerol and ethanol have been selected because of their opposite effect: structure promoting and structure dissolving, respectively. Lattice parameters of Pluronic P105 (EO37PO58EO37) and F127 (EO100PO70EO100) LLC samples in the presence of any type and amount of E+G solvents decrease with increasing volume fraction of the PPO‐rich domains. Lattice parameters for the two block copolymers fall on the same line when normalized with the square root of polymer molecular weight, suggesting weak block segregation.

Influence of the segregation effect on the magnetic and optical properties of a compensated ferronematic liquid crystal

The Freedericksz transition in splay and bend geometry in a ferronematic (a suspension of single-domain magnetic particles in a nematic liquid crystal) is considered in the framework of the continuum theory. In zero magnetic field, the ferronematic was assumed to be compensated (i.e., having equal fractions of an impurity with magnetic moments directed parallel and antiparallel to the local director). Spatial distortions of the director and the concentration redistribution of the magnetic impurity in the ferronematic layer are studied as functions of the applied magnetic field and the segregation parameter. It is shown that the magnetic-field induced Freedericksz transition from the homogeneous to the inhomogeneous state has a threshold nature. The transition field as a function of the material parameters of the ferronematic is determined analytically. Magnetization of the ferronematic and its optical properties for a strong and weak segregation as functions of the magnetic field are studied.

Anisotropic Linear Response in Block Copolymer Lamellar Phases

We examine the linear response of a lamellar block copolymer phase in the weak segregation regime and focus on those effects that arise from the uniaxial symmetry of the phase. The classical two fluid model of a polymer solution is extended to allow for anisotropic monomer diffusion as well as hydrodynamic flows. The latter include constitutive laws for the stress that also reflect the symmetry of the phase. Transverse relaxation of weakly perturbed lamellae is the slowest mode, and is subdiffusive, in contrast to diffusive decay of longitudinal perturbations. Anisotropic diffusion can both enhance or reduce the rate of relaxation of long wavelength perturbations of lamellae depending on the relative magnitude of longitudinal and transverse mobilities. Hydrodynamic flows at the scale of the lamellae are negligible for most situations of interest, but not long ranged flows as would appear in, for example, multidomain configurations. We find that such flows accelerate linear decay, and even dominate diffusive relaxation in the long wavelength limit. We finally examine anisotropic effects on defect motion as exemplified by a tilt grain boundary. The boundary velocity is significantly affected by anisotropic diffusion through the coupling between undulation and permeation diffusive modes in the defect region.

Interfacial Tension and Width of an Asymmetric Polymer Mixture

AbstractWe have used continuous space rod‐bead model and an off‐lattice Monte Carlo technique to investigate interfacial properties between two incompatible polymers of different stiffnesses. We have estimated the interfacial tension as well as interfacial width of all the systems studied. Further, by studying the interfacial tension and/or interfacial width in the weak segregation limit one can estimate the critical value of temperature at which two different kinds of polymers mix. In the present work, we have estimated the critical value of temperature at which the polymers mix by studying the interfacial width in the weak segregation limit for the different systems containing the flexible and semi‐flexible polymers of different stiffnesses.

Immature integration and segregation of emotion-related brain circuitry in young children

The human brain undergoes protracted development, with dramatic changes in expression and regulation of emotion from childhood to adulthood. The amygdala is a brain structure that plays a pivotal role in emotion-related functions. Investigating developmental characteristics of the amygdala and associated functional circuits in children is important for understanding how emotion processing matures in the developing brain. The basolateral amygdala (BLA) and centromedial amygdala (CMA) are two major amygdalar nuclei that contribute to distinct functions via their unique pattern of interactions with cortical and subcortical regions. Almost nothing is currently known about the maturation of functional circuits associated with these amygdala nuclei in the developing brain. Using intrinsic connectivity analysis of functional magnetic resonance imaging data, we investigated developmental changes in functional connectivity of the BLA and CMA in twenty-four 7- to 9-y-old typically developing children compared with twenty-four 19- to 22-y-old healthy adults. Children showed significantly weaker intrinsic functional connectivity of the amygdala with subcortical, paralimbic, and limbic structures, polymodal association, and ventromedial prefrontal cortex. Importantly, target networks associated with the BLA and CMA exhibited greater overlap and weaker dissociation in children. In line with this finding, children showed greater intraamygdala connectivity between the BLA and CMA. Critically, these developmental differences were reproducibly identified in a second independent cohort of adults and children. Taken together, our findings point toward weak integration and segregation of amygdala circuits in young children. These immature patterns of amygdala connectivity have important implications for understanding typical and atypical development of emotion-related brain circuitry.

Comparison of Low‐Frequency Spectra between Aromatic and Nonaromatic Cation Based Ionic Liquids Using Femtosecond Raman‐Induced Kerr Effect Spectroscopy

In this minireview, the features of low-frequency spectra within the frequency range 0-200 cm(-1) for aromatic and nonaromatic cation based ionic liquids obtained by femtosecond Raman-induced Kerr effect spectroscopy are reviewed. For aromatic cation based ionic liquids, the aromatic group of the cations is largely responsible for the spectral line shape due to their larger polarizability anisotropy volume and more polarizable nature than nonaromatic cations. The low-frequency Kerr spectra are also compared with the bulk properties of the ionic liquids. The differences in the relationship of the first moment of the low-frequency spectrum band and the bulk properties between the aromatic and nonaromatic cation based ionic liquids with respect to the natures of constituent ions and microscopic aspects are discussed. For aromatic cation based ionic liquids, the ionic region is more pronounced than the nonpolar region in their low-frequency Kerr spectra. In contrast, the ionic and nonpolar regions of nonaromatic cation based ionic liquids are likely to be captured due to the weaker signal of the cationic region. Also, the low-frequency Kerr spectra of the nonaromatic analogues are influenced by their weaker segregation as compared to aromatic cation based ionic liquids.

The diamond and other non-conventional morphologies in two-scale multiblock AB copolymers

We revisit the idea of the existence of the ordered block copolymer phase possessing diamond symmetry Fdm (space group No. 227), which was first put forward within the framework of the strong segregation approach. For this purpose we study the order–disorder and order–order transitions in molten two-scale multiblock copolymers AmN/2(BN/2AN/2)nBmN/2via the pseudo-spectral numerical procedure of the self-consistent field theory (SCFT). The phase diagram in the plane (fC, ), where fC = n/(m + n) and = χ N(m + n) is the effective energetic Flory–Huggins parameter, is built and some accompanying quantities are analyzed. Near the order–disorder transition line the phase diagram contains the regions where the lamellar, alternating gyroid, diamond and simple cubic phase, respectively, exist. With an increase of the degree of segregation, the diamond phase is replaced by a tetragonal array of cylinders (simple square) phase, which agrees with the preceding results obtained within the Leibler-like weak segregation theory, and with the SCFT calculations for a physically similar melt of linear ABC triblock copolymers with a non-selective middle block. Thus, the diamond morphology in the system under study is shown to exist as an essentially weak or moderately (not strongly) segregated phase. The ways to visualize the patterns of ordering in such morphologies are discussed. A new quantity (topological permeability) to characterize the transport properties in 3D bicontinuous morphologies is introduced and first calculated for real block copolymer ordered morphologies. Some implications of the results obtained for the design of the block copolymer thin films with improved permeability are discussed.

Buckling of multicomponent elastic shells with line tension

We explore the effects of line tension on the shape of two component elastic shells. Segregation between components with different elastic properties combined with the subtleties related to the spherical topology leads to a rich variety of shapes. They range from irregular polyhedra at weak segregation to Janus-like morphologies with spherical and buckled regions coexisting on the same shell for large values of the line tension. We use Monte Carlo simulations to map a detailed phase diagram as a function of the relative fraction of the two components, the ratio of their bending rigidities, and the strength of the line tension.